Mechanism of Action

#### **Chapter 1**

## Mechanisms of Action of Metformin

*Samira Abdulla Mahmood*

#### **Abstract**

Metformin is the first-choice drug for treatment of type 2 diabetes notably those associated with obesity. It does not only reduce hyperglycemia, but also possesses pleiotropic effects opening the pave for numerous potential clinical applications. In this chapter we illustrate the various mechanisms of metformin action in reduction of hepatic glucose output, improvement of insulin action, restoration of fat metabolism and gut microbiome, reduction of inflammation, upregulation of antioxidant enzymes, and attenuation of tumor growth. Understanding of such mechanisms might propose further clinical applications for metformin.

**Keywords:** 5′ AMP-activated protein kinase (AMPK), metformin, gluconeogenesis, antioxidant, mammalian target of rapamycin (mTOR), complex 1

#### **1. Introduction**

The mechanisms underlying metformin actions appear to be complex and responsible for the pleiotropic effects of metformin. These mechanisms remain a topic of considerable debate. Actually, in the last decade we moved from a simple picture, that metformin acts via the liver 5′ AMP-activated protein kinase (AMPK), to a much more complex one, reflecting its various mechanisms in different cells and tissues.

Since the early studies have suggested that metformin acts by inhibition of complex 1 in mitochondrial electron transport chain [1] and subsequently activation of AMPK [2, 3], AMPK-independent targets have also been reported. These comprise dephosphorylation the ribosomal protein S6, suppression of mammalian target of rapamycin complex 1 (mTORC1) activation and signaling via Rag GTPase [4], attenuation of hepatic glucose 6 phosphate levels [5], suppression of redox transfer by mitochondrial glycerophosphate dehydrogenase (mGPD) [6], as well as modulation of inflammation/oxidative stress and oncogenic signaling pathways.

#### **2. Primary molecular mechanism**

Metformin, a hydrophilic drug with Pka 12.4, cannot readily be diffuse passively through the cell membrane due to its existence as cation (ionized) at physiological pH 7.4 [7]. As hydrophilic drug it needs a carrier mediated pathway to efficiently pass through the cell membrane. This is facilitated by the organic cation transporter 1(OCT 1) [8], a member of the soluble carrier family 22 (SLC22) of membrane proteins. OCT1 is mostly expressed in the liver for transferring of cations including metformin, but also facilitates the uptake of metformin from the gut lumen to the interstitium [9]. Cells express OCT1 are able to facilitate cellular uptake of metformin which is in consistence with its accumulation in particular targeted organelles. Also, other types of OCT proteins are present at apical or basolateral sites with different functions.

Within the mitochondria metformin accumulates in the matrix and inhibits complex1 electron transfer chain NADH ubiquitin oxidoreductase (NADH) [1, 10, 11], which promotes proton generation. This inhibition reduces NADH oxidation and ultimately prevents ATP production from ATP synthase. By this way, the ratios of AMP: ATP and ADP: ATP increase, **Figure 1**. Increment in these ratios, which accompanied with reduction in cellular energy activates the cellular energy sensor (house keeper enzyme) AMPK [11]. Another consequence of complex 1 inhibition is the higher levels of AMP, which in turn induces AMPK-independent effects. Moreover, metformin directly inhibits hepatic GPD2, the enzyme involves in substrate (glycerol) gluconeogenesis. Its inhibition by metformin leads to increase cytosolic redox and suppression of gluconeogenesis [12].

AMPK is a heterotrimeric protein complex that consists of α, β, and γ subunits. The α subunit represents the catalytic site that can be activated (phosphorylated) by liver kinase B1 (LKB1) [13] at Thr-172 [14] and also by calcium/ calmodulin-dependent protein kinase kinase β (CaMKKβ) at Thr-172 [15]. The β and γ denote regulatory subunits. In mammals, the γ subunits contain nucleotide-binding sites for AMP or ATP [16]. In case of cellular energy stress with low ATP, AMP or ADP directly and mutually bind to the γ subunits causing conformational change leading to AMPK activation. Metformin induces activation of AMPK by LBK1 pathway and also by AMP/ADP induced conformational changes, too. It is worth to mention that higher levels of AMP protect AMPK from dephosphorylation by phosphatases [17]. AMPK plays a role in several cellular events, including glucose metabolism, lipid metabolism, redox regulation, anti-aging and anti-inflammation [18, 19].

#### **Figure 1.**

*Primary molecular mechanism of metformin action. For explanation see text.*

### **3. Antihyperglycemic mechanisms of action**

Metformin is currently the drug of choice in treating patients with type 2 diabetes mellitus (T2DM). Its mechanisms are still elusive. Nevertheless, it lowers blood glucose through multiple mechanisms. First, it inhibits intestinal absorption of glucose. Second, it suppresses glucose production by the liver. Third, it facilitates glucose uptake into tissues, thus reducing blood glucose levels enabling better health to pancreatic beta-cells. Finally, it improves insulin sensitivity and inflammation. The most accepted action of metformin in T2DM is inhibition of gluconeogenesis and reduction in hepatic glucose output (HGO).

#### **3.1 Mechanisms to lower hepatic gluconeogenesis**

Metformin is taken up into the hepatocyte via the OTC1 [20]. Due to the difference in hepatocyte pH and pka of metformin, the drug becomes ionized and positively charge and accumulates in the cells and, further, in the mitochondria to concentrations up to 1000-fold higher than in the extracellular medium [21]. The uptake of positively charge metformin into the mitochondria is derived by the membrane potentials across the plasma membrane and mitochondrial inner membrane (positive outside) [1], Within the mitochondria, metformin inhibits complex1, which reduces ATP production and increases AMP and ADP levels. One consequence of respiratory chain inhibition is increment in ADP:ATP ratios that modestly suppress gluconeogenesis as seen experimentally in cells carrying this process [22], and hinder the hepatocytes from synthetizing the high energy requiring gluconeogenesis [23], **Figure 2**. Other consequence is changes in NAD+ :NADH ratios involving in a negative impact on gluconeogenesis [10].

Criticized comments on this mechanism are based on the higher concentration (in millimole levels) on metformin required for rapid complex 1 inhibition, although experimentation in in vitro studies have shown that inhibition of comlpex1 in rat hepatoma (H4IIE) cells does occur at lower concentrations ((50–100 μmol/l)) after long periods due to a slow transport of metformin across mitochondrial membrane [1]. This observation has been confirmed experimentally [24].

#### **3.2 Activation of hepatic AMPK**

Metformin induced reduction in cellular energy and increment in AMP:ATP ratios are indicative for activation of the energy sensor AMPK by LKB1 (see primary mechanism). Stimulation of AMPK results in repression of anabolism (fatty acid and cholesterol synthesis, gluconeogenesis) and switching on catabolism (fatty acid uptake and oxidation, glucose uptake) [25] in order to restore cell energy hemostasis and prevent cells from damage [26]. The first observation of involvement of AMPK in metformin action was reported in vitro of rat hepatocytes and rat liver in vivo [27]. Moreover, AMPK can also be activated by glucose starvation, exercise and metformin activated lysosomal mechanisms [28].

#### **3.3 AMPK dependent mechanisms**

Activated AMPK phosphorylates the cAMP specific 3′,5′-cyclic phosphodiesterase 4B (PDE4B) and activates cAMP degradation (↓cAMP) [21]. Consequently, it prevents the activation of cAMP-dependent protein kinase A (PKA), the enzyme that phosphorylates the transcription factor cAMP response element binding protein (CREB), and then activates CREB-CBP-CRTC2 (CREB:CRTC2) transcription complex involving in transcription of the genes encoding the gluconeogenic enzymes phosphoenolpyruvate carboxykinase (PEPCK) and Glucose 6-phosphatase (G6Pase) [29], **Figure 2**. On the other hand, AMPK induces phosphorylation of CREB binding protein (CBP) at serine 436 leading to dissociation of the CREB-CBP-CRTC2 transcription complex, thus repression of PEPCK and G6Pase [29]. In addition, AMPK or salt-inducible kinase 2 (SIK2) phosphorylates CREB-regulated transcriptional coactivator-2 (CRTC2), thus, inhibits its nuclear translocation and retains in cytoplasm [30]. Moreover, AMPK upregulates the orphan nuclear receptor small heterodimer partner (SHP), that functions as transcription repressor [31] through competition with CRTC2 binding in CREB–CBP complex, **Figure 2**.

Another mechanism mediated by AMPK is inhibition of fat biosynthesis and activation of fat beta-oxidation, resulting in long term enhancement of hepatic insulin sensitivity, which is clinically relevant. Metformin-induced hepatic AMPK phosphorylates the isomers of acetyl-CoA carboxylase (ACC1/ACC2) at serine residues responsible for fat beta-oxidation [32]. Phosphorylation of ACC1 and ACC2 inhibit the conversion of acetyl-CoA to malonyl-CoA resulting in reduction of liver lipogenesis and hepatosteatosis (fatty liver) and increment in fatty acids oxidation, which are factors contributing in improvement of insulin sensitivity/signaling and hyperglycemia. Likewise, activation of AMPK suppresses the expression of lipogenic genes by direct phosphorylation of transcription factors including carbohydrate response element binding protein (ChREBP), **Figure 2**, and by this means regresses the lipogenesis [33], **Figure 2**. Taken together, the role of AMPK involves in phosphorylation of key metabolic enzymes and transcription co-activators/factors modulating gene expression leading to inhibition of glucose, proteins and lipid synthesis and stimulation of glucose uptake and fatty acid oxidation.

#### **3.4 AMPK independent mechanisms**

Metformin induced a rise in AMP levels inhibits gluconeogenesis independent of AMPK. AMP allosterically inhibits fructose-1,6-bisphosphatase, a key enzyme of gluconeogenesis and AMP sensitive [34]. This action might be responsible for acute metformin action. In addition, AMP inhibits adenylate cyclase producing cAMP in response to glucagon released in starvation leading to lowering cAMP and reducing expression of gluconeogenesis enzymes [35], **Figure 2**.

#### *Mechanisms of Action of Metformin DOI: http://dx.doi.org/10.5772/intechopen.99189*

Recent proposed mechanism of increased hepatic gluconeogenesis is related to impaired white adipose tissue lipolysis with resultant increase in hepatic uptake of non-esterified fatty acids (NEFA). Hepatic beta-oxidation of NEFA can produce acetyl-coenzyme A (acetyl-CoA), the allosteric activator of the enzyme pyruvate carboxylase that is implicated in the first step f gluconeogenesis by supplying oxaloacetate [36]. Insulin regulates lipolysis of white adipose tissue, thereby, indirectly regulates hepatic gluconeogenesis [37]. Insulin resistance with inflammation in white adipose tissue increases glycerol turnover. Thus, metformin improves insulin sensitivity and reduces resistance leading to suppression of gluconeogenesis.

In addition, white adipose tissue delivers glycerol to the liver. In the liver, glycerol is phosphorylated. Through mitochondrial glycerol-3-phosphate dehydrogenase (GPD2), glycerol is converted into dihydroxyacetone phosphate (DHAP), a component included in gluconeogenesis.

Metformin inhibits GPD2, leading to suppression of DHAP and subsequently gluconeogenesis in substrate (glycerol) specific manner [12]. In context of obesity and T2DM, inhibition of gluconeogenesis from increased supply of glycerol due to dysregulation of white adipose tissue may partially benefit uncontrolled type2 diabetic patients with dysregulated white adipose tissue lipolysis [38].

As discussed above, metformin suppresses gluconeogenesis through interactions with regulatory process of gluconeogenesis as shown in inhibition of transcription (downregulation of gluconeogenic genes expression), substrate (suppression of glycerol induced DHAP formation) and increase cytosolic redox **Figure 2**.

#### **3.5 Mechanisms in skeletal muscle**

Metformin affects skeletal muscle metabolism by direct and indirect mechanisms. Emphasis has been placed on the metformin's effect to increase insulinstimulated peripheral glucose uptake and to reduce glucotoxicity, which indirectly improves muscle glucose uptake [12]. Metformin reduces gluconeogenesis and hepatic glucose output leads to reduce blood glucose levels in type 2 patients, which accompanied by improvement in insulin action. Improvement in insulin levels in circulation under metformin treatment attenuates the hyper-insulinemic pressure on insulin receptors (insensitive phosphorylated receptors) leading to upregulation and increase sensitivity of receptors to insulin [39]. Consequently, muscle glucose uptake is indirectly stimulated by metformin due to reduce insulin resistance in skeletal muscle and peripheral tissues.

In skeletal muscle, activation of AMPK by metformin increases proliferatoractivated receptor γ coactivator-1α (PGC-1α), which in turn stimulates glucose transporter 4 (GLUT4) gene transcription [40]. This mechanism induces GLUT4 and others mitochondrial genes required for catabolism. In addition, activated AMPK stimulates the translocation of GLUT4 to the plasma membrane and acutely increase skeletal glucose uptake.

Moreover, AMPK induced phosphorylation of acetyl-CoA-carboxylase-2 (ACC-2) results in reduce malonyl-COA, which is the inhibitor of carnitine *O-*palmitoyltransferase, leading to increase transport of fatty acids into mitochondria. Thus, fatty acid-oxidation is acutely increased. It worth to mention that these mechanisms are regulated by PGC-1 α, which initiates many genes involved in AMPK functions in skeletal muscle [40], **Figure 3**.

#### **3.6 Mechanism in fat tissue**

Obesity has been found to be the most crucial factor for insulin resistance (IR). In addition, insulin sensitivity decreases with age. Therefore, glucose entry into

#### **Figure 3.**

*Mechanism of increase muscle responsiveness by metformin (see text).*

tissues, including muscle and fat, decreases, since adipocytes have fewer insulin receptors. Metformin improves lipid profile in patients with T2DM. Insulin resistance (IR) means reduces tissue responsiveness to insulin with resultant elevation in insulin levels (hyperinsulinemia). Beta-cells produce more insulin, but ultimately fail to overcome IR with resultant loss of beta-cell function and development of hyperglycemia. Risk factors for IR are obesity and inactivity.

The signaling pathway connected with IR is phosphatidylinositol-3-kinase/protein kinase B protein (PI3K/PK), which also known as Akt. Akt is also important for translocation of GLUT4 onto the cell membrane surface of muscle and fat cells for glucose entry [41]. Akt inactivation or defect can lead to impairment of membrane transposition of GLUT4, which results in IR accompanied with hyperinsulinemia, hyperglycemia and cardiac impairment [42]. Metformin activates and restores the PI3K/Akt/GLUT4 signaling in rats with type 2 diabetes [43], thereby suppresses IR.

Metformin enhances disposal of blood glucose into skeletal muscle and fat, thus insulin resistance associated with diabetes is overcome. This can be translated into increase storage of glycogen in skeletal muscle, enhancement of fatty acid oxidation and reduced lipogenesis in adipose tissue, which in general reduce the body fat content.

#### **3.7 Mechanisms in the intestine**

Beside the liver, the intestine is also considered as an important target for metformin actions. Metformin lowers blood glucose not only through the action in the circulation, targeting the liver and other organs, but also through one in the intestine. The proposed actions are increase in intestinal glucose entry mainly in enterocytes with anerobic utilization, resulting in reduced net glucose uptake into blood with enhanced lactate production [44, 45], increase in glucagon like peptide-1 (GLP1) levels, increase bile acid pool within the intestine and modulation of microbiome [46].

Activated AMPK phosphorylates the glucose transporter 2 (GLUT2), which then translocated to the apical membrane of intestinal cells, mainly enterocytes, where it promotes glucose uptake into enterocytes [6]. Metformin increases uptake and utilization (anerobic) of glucose, where subsequently an increase in plasma lactate is resulted. In fact, the intestine and the liver are implicated in metformin-related

#### *Mechanisms of Action of Metformin DOI: http://dx.doi.org/10.5772/intechopen.99189*

lactate production. The effect of metformin in intestinal glucose utilization has been shown in positron emission tomography–computed tomography (PET-CT) imaging of patients treated with metformin. This imaging technique uses positronemitting **18**F-fluorodeoxyglucose (18F-FDG), that its intestinal (mainly in the colon) uptake increases in metformin treated patients confirming increase glucose uptake and metabolism in the gut [46].

Metformin inhibits mitochondrial glycerophosphate dehydrogenase, so the conversion of cytosolic pyruvates to lactate is reduced [6], thus, intracellular lactate levels are built up and then released into the plasma. This has been proved in rat studies, where the hepatic portal vein has been shown as the area with the higher peak of plasma lactate concentrations, implicating the intestine as the main site of metformin-associated anerobic glucose utilization and lactate production (estimated by 10% increase in intestinal lactate concentration) [47].

Another intestinal action of metformin directs to GLP-1, which is secreted from L cells distributed throughout the gut but concentrated in the ileum. As reported in mice studies, metformin increases the expression of the precursor proteins (pre-proglucagon and proglucagon) of GLP-1, thus potentially increasing GLP-1 production and secretion [48]. In addition, metformin affects the enzyme degrading GLP-1, dipeptidyl peptidase-4 (DPP4) by mechanisms that are not well clarified [49]. Moreover, stimulation of GLP-1 secretion can occur indirectly, via the bile acid pool alteration by metformin [46]. Metformin activated AMPK directly phosphorylates and represses bile acid sensor, the farnesoid X receptor (FXR), on ileal cells, which results in reduced FXR transcription activity and subsequently reduced sensing and ileal absorption of bile acids [50]. By its turn, the higher level of bile acid pool stimulates bile acid receptors TGR5 on L cells, inducing secretion of GLP-1 [51]. Furthermore, the consequences of reduced bile acid absorption are lower levels of cholesterol in patients taken chronic metformin [52] and diarrhea associated with metformin intolerance due to osmotic effect mediated by increased luminal bile slats levels [53].

The gut microbiome composition has been shown to contribute to the development of obesity and type 2 diabetes, which implicated in a reduction in bacteria producing short chain fatty acids (SCFAs) such as butyrate-producing bacteria and an increase in opportunistic pathogens as shown in type 2 diabetics [54]. SCFAs are considered as important signaling metabolites that impact hepatic gluconeogenesis and fatty acid metabolism [55]. Metformin modulates gut microbiota and increases SCFAs metabolizing bacteria, which lead to suppression of hepatic gluconeogenesis, reduction in FFA release from adipocytes and appetite suppression via incretin [56].

Metformin alters the microbiome composition in mice and humans, where the bacterium Akkermansia muciniphila is increased, accompanied with associated increase in mucin-producing goblet cells as demonstrated in mice model. Akkermansia muciniphila can increase endocannabinoids, which improve the thickness of gut mucous barrier and reduce inflammation [57], and so improve glucose tolerance. On the other hand, an increase in such bacteria by metformin triggers production of short chain fatty acids butyrate and propionate, which results in reduction of hepatic gluconeogenesis, appetite and weight [46]. Taken together, alteration of microbiome composition by metformin can improve metabolic disorders which needed further investigations.

#### **4. Mechanisms of antiinflammatory/antioxidant**

Beyond the glucose lowering actions, metformin can directly and indirectly modulate inflammation. Several experimental and clinical studies demonstrated the anti-inflammatory actions of metformin in endothelial cells (EC) and smooth muscle cells (SMC), monocytes, macrophages and other cell types, where it suppresses the main components of inflammation and restores cell functions [58, 59]. Since inflammation is linked to a number of clinical disorders, thus, metformin can possibly interfere with and ameliorate metabolic disorders, cardiovascular diseases, atherosclerosis, obesity cancer and aging. Although the crucial mechanisms are not well elucidated, accepted anti-inflammatory mechanisms of metformin, which are common and implicated in the before mentioned disorders are presented below.

Activation of AMPK by metformin inhibits nuclear factor kappa light-chainenhancer of activated B-cells (NF-κB) transcription [60]. NF-κB is a transcription regulator implicated in various inflammatory pathways. Metformin induced NF-κB inhibition suppresses inflammatory pathways, proinflammatory cytokines and reactive oxygen species (ROS) production [61]. Likewise, activation of AMPKphosphatase and the tensin homolog (PTEN) pathway by metformin suppresses phosphoinositide 3-kinase (PI3K)-Akt pathway that activates NF-κB in human vascular SMC. In this way, NF-κB is also inhibited, **Figure 4**. In addition, metformin suppresses Poly [ADP ribose] polymerase 1 (PARP-1), which functions as a coactivator of NF-κB transcription to stimulate pro-inflammatory pathways. Nitric oxide (NO), a mediator of in nerve, immune and CVS is decreased in oxidative stress induced by hyperglycemia. Metformin increases NO via activation of AMPK, which antagonizes inflammation and ROS production [62]. As well, metformin inhibits the differentiation of monocytes to inflammatory macrophages [63] through activation of AMPK, which reduces the phosphorylation of signal transducer and activator of transcription 3(STAT3), **Figure 4**.

Inhibition of NF-κB transcription triggers consequences in different tissues. In macrophages, inhibition of NF-κB activation by metformin can result in reduction of NO, PGE2, and proinflammatory cytokines, such as IL-1β (interleukin-1 β), TNF-α (tumor necrosis factor - α) [64], IL-6 and IL8 (responsible for calling monocytes and adhesion of endothelial cells) [65]. In human adipocyte, metformin induced inhibition of NF-κB pathway leads to suppression of proinflammatory cytokine-induced 11β-HSD1 (11 β -hydroxysteroid dehydrogenase type 1) expression [66]. 11β-HSD1 is elevated in human adipose tissue in obesity and metabolic syndrome, generates active glucocorticoids and is associated with chronic inflammation. Moreover,

*Potential mechanisms of metformin to attenuate inflammation and production of reactive oxygen species.*

#### *Mechanisms of Action of Metformin DOI: http://dx.doi.org/10.5772/intechopen.99189*

inhibition of NF-κB suppresses the expression of CXCL8, a cytokine responsible for changing the microenvironment around the tumor by attracting leukocytes and endothelial progenitors contributed in angiogenesis [67]. Metformin applies its antiinflammatory action for antifibrotic effect on heart muscle cells through activation of AMPK and inhibition of the pro-inflammatory mediators of the TRAF3 interacting protein (TRAF3IP2) molecule, which induced by aldosterone and enhances production of NF-κB [68]. Furthermore, anti-inflammatory mechanisms associated with atherosclerosis, allergic asthma, hepatic steatosis and vascular injury have been ascribed to metformin but required further elucidations.

In regard of macrophages activity, activated AMPK by metformin reduces phosphorylation of STAT3 (signal transducer and activator of transcription 3), thereby, inhibits the differentiation of monocytes into inflammatory macrophages (M1) [69], while promotes polarization into anti-inflammatory macrophages (M2). These mechanisms place metformin as potential anti-inflammatory targeting macrophages differentiations and polarization with benefits in vascular injury, atherosclerosis, certain cancer and insulin resistance [63].

Further mechanism associated with anti-inflammatory actions of metformin is the inhibition of advance glycation end-products (AGEs) [70], which are one of the crucial inflammatory factor in diabetes, promoting inflammation, ROS production and atherosclerosis [71, 72]. In fact, during hyperglycemia accumulation of glucose in cells facilitates the binding of each two closest glucose molecules with each other to form dicarbonyl compounds, which are percussors of AGEs. AGEs bind to their receptors (RAGE) in different target cells including macrophages, where they promote expression of IL1, IL6, TNF α and RAGE, and activate NF-κB pathway [68], leading to inflammation, apoptosis and fibrotic reactions, as observed in tubular cells. Metformin not only binds chemically to these precursors and renders them inactive, thereby reduces the formation of AGEs, but also suppresses RAGE via activation of AMPK [73]. Altogether, metformin suppresses RAGE/NF-κB pathway, leading to regression of RAGE effects on macrophages and change of their surface markers from inflammatory (M1) to anti-inflammatory (M2) phenotype. **Figure 4** illustrates the potential anti-inflammatory mechanisms of metformin.

Beside the direct effect on proinflammatory pathways, metformin can indirectly reduce inflammation through metabolic consequences. Reduction in hyperglycemia and subsequently the weight as well as the atherogenic LDL cholesterol levels can have favorable effect on chronic inflammation, atherosclerosis and cardiovascular disorders.

As mentioned before, metformin inhibits mitochondrial complex 1 electron transfer complex chain and reduces the production of ROS, which normally formed by synthesis of ATP from ATP synthase. Metformin can reduce ROS through activation of AMPK which inhibits TFG- β, a potent inflammatory factor stimulating the production of ROS and induce endogenous antioxidants such as glutathione reductase (GSH), superoxide dismutase (SOD) and catalase (CAT) [74]. Independent of AMPK activation, metformin can activate antioxidant SOD and clean the damaging effects of ROS in tissues. In addition, it can direct trap hydroxyl peroxide and activate antioxidant enzymes such as catalase, which decomposes H2O2. Reduction of ROS reduces IL1β [68]. Therefore, metformin has been shown to play a role in controlling and changing oxidative/inflammatory pathways in clinical and laboratory conditions through various mechanisms.

#### **5. Antineoplastic actions of metformin**

The role of metformin in treatment of cancer has been reported in various recent sophisticated publications. Clinical observational studies in liver, colon and pancreatic cancer have demonstrated that metformin prevents and decreases the risk of cancer development [75]. In addition, improvement overall survival outcomes have been reported in patients with colorectal and breast cancer [76], where metformin treated breast cancer patients showed a lower HER-2 positive rate and mortality rate than the control group [77] Besides that, metformin enhances the effects of anti-cancer drugs as shown in vitro and in vivo studies using vincristine, cisplatin, and doxorubicin [78, 79]. Altogether, the results point to involvement of metformin in chemotherapy as adjuvant or a potential anti-cancer candidate which require further experimentations.

Cancer growth and proliferation can be regressed direct and indirect by metformin. Metformin induced reduction in cancer growth has been shown to be indirect through systemic effects related to reduced blood glucose levels, improved insulin resistance and declined pro-inflammatory cytokines. This indirect action might explain the effect of metformin in several types of cancer linked to hyperinsulinemia as a risk factor. Also, metformin directly modulates several oncogenic signaling pathways described in the following text.

As mentioned in different sections of this chapter, the primary mechanism of metformin is to inhibit the oxidative phosphorylation by blockade of complex1 in mitochondria in target cells. Mitochondrial energy reduction and metabolic stress increase the endogenous levels of reactive oxygen species (ROS) which can mediate the death of cancer cells depending on oxidative phosphorylation for gaining energy [24]. Likewise, energy stress seems to hinder cancer cells from synthesis of energy requiring proteins, lipids and structural elements necessary for cancer growth and proliferation. This action can be considered as the first step of metformin induced tumor regression, and growth retardation. Furthermore, deprivation of cancer cells from ATP activates the tumor suppressor gene LKB1 which then phosphorylates AMPK [80] **Figure 5**. AMPK regulates several signaling pathways, primarily via inhibition of mammalian target of rapamycin (mTOR) signaling to suppress tumorigenesis as follows.

Metformin is taken up into cancer cells expressing OCT1, accumulates in mitochondria, blocks complex 1 and activates AMPK. AMPK phosphorylates p53 on ser15 (the tumor suppressor) which is required to start AMPK-dependent cell growth arrest and apoptosis [81], **Figure 5**. On the other hand, activated AMPK

**Figure 5.** *Mechanisms of metformin suppressing tumorigenesis.*

#### *Mechanisms of Action of Metformin DOI: http://dx.doi.org/10.5772/intechopen.99189*

phosphorylates MDMX on ser 367 leading to MDMX inactivation and p53 activation [82]. MDMX and the human MDM2 are partner proteins monitor p53 in a negative feedback fashion and restrain its function to maintain the normal development and function of different tissues [83]. Phosphorylation one of them results in inhibition of ubiquitylation (a molecular change) of p53 leading to stabilization and activation of p53.

Beyond the effect on p53, metformin inhibits mTOR. mTOR is a catalytic subunit, composes of two protein complexes, mTORC1 and mTORC2, that regulate cell growth [84]. Inhibition of mTOR attenuates cell proliferation [85]. Metformin inhibits the activation of mTOR via AMPK-dependent and -independent mechanisms. By AMPK-independent way, metformin phosphorylates directly the regulatory associated protein (raptor) that inactivates mTOR. Likewise, metformin inhibits mTOR signaling by inactivating Rag GTPase [4]. On the other hand, AMPK directly phosphorylates the tumor suppressor tuberous sclerosis complex 2 (TSC2) leading to activation of complex 1 and 2, TSC1/2. TSC1/2 inhibits Rheb, which in turn inactivates mTOR [86] and suppresses cell proliferation.

Besides AMPK and mTOR, metformin has been shown to affect other oncogenic signaling pathways. Metformin suppresses Akt (protein kinase B) expression which is associated with increased phosphatase and tensin (PTEN, a tumor suppressor gene) [87]. This is considered as main mechanism via which endometrial carcinoma is inhibited by metformin. Additionally, metformin inhibits activation of nuclear factor kappa light-chain-enhancer of activated B-cells (NF-κB) and phosphorylation of STAT3 in cancer stem cells [88]. The NF-κB and STAT3 transcription factors are involved in mediating an epigenic switch from non-transformed to cancer cells as shown in breast cancer model. This action suggests that metformin inhibits the anti-inflammatory pathway required for transformation and cancer stem cells formation [88].

Further mechanism of anticancer effect of metformin is modulation of microR-NAs (miRNAs) expression (mainly tumor suppressor miRNA) through activation and upregulation of the RNAase III endonuclease (DICER). DICER is one of the key enzymes of microRNAs biosynthesis [89]. DICER has a role in formation of miRNAs and in assembly of their machinery to target mRNAs for degradation [90]. Downregulation of DICER is oncogenic and predict poor survival in lung, breast and ovarian cancer [91, 92]. In addition, impairment of metformin effect in vitro was shown in DICER-deficient tumor cells. As shown in **Figure 5**, metformin induced upregulation of DICER leads to expression of many suppressor miRNAs that target mRNA of coding genes for degradation, thus effectively reducing gene products such as oncogenic proteins [93].

#### **6. Mechanisms of action in PCOS**

One of the pleiotropic effects of metformin is to reduce insulin resistance (IR) and secondary hyperinsulinemia in diabetes mellitus and several clinical conditions associated with hyperinsulinemia. Hyperinsulinemia is linked with the pathogenesis of polycystic ovary syndrome (PCOS), a condition of primary ovulatory disfunction associated with metabolic disturbances. PCOS is the endocrine disorder characterized by hyperandrogenism, anovulation and infertility. Obesity further exaggerates IR in obese PCOS women. Importantly, IR in PCOs women is tissue selective, which means persistence sensitivity to insulin actions on steroidogenesis in ovary and adrenal gland, in face of resistance in skeletal muscle, adipose tissue and liver to metabolic actions of insulin. Paradoxically, in PCOS women, some tissues manifest IR, while steroid-producing tissues remain insulin sensitive [94].

Mechanisms of insulin action contributing to hyperandrogenism in PCOS are various. Insulin can enhance the amplitude of luteinizing hormone (LT) pulses to increase androgen production in theca cells [95] (similarly insulin increases thecal androgen response to LH through direct binding to insulin like growth factor − 1(IGF-1) receptors in theca cells). Also, insulin may stimulate the activity of ovarian cytochrome CYP17 (P450c17) and 17β-hydroxysteroid dehydrogenase (17βHSD) to promote androgen steroidogenesis [96]. In addition, insulin can decease the hepatic synthesis of steroid hormone binding globin (SHBG), which allows more free androgen and estrogen to be available. Finally, insulin inhibits the hepatic production of IGF binding protein-1 (IGFBP-1), which increases IGF-1 in circulation and allows greater local action [97].

Furthermore, increase androgen levels may be linked to decrease adiponectin secretion by adipocyte in PCOS women, thereby further increasing insulin resistance and subsequently insulin levels [98]. In addition, insulin may affect female subcutaneous adipose tissue and generate androgen from adipocytes by increasing the activity of aldo-keto reductase IC3 (AKRIC3) [99].

Metformin can ameliorate all the above-mentioned actions of insulin in PCOS. Treatment with metformin is useful in reduction of both hyperinsulinemia and circulating androgens and also restores ovarian function with the benefits of increase ovulation, reduce serum androgen levels and improve menstrual cyclicity.

Metformin acts directly on ovarian theca cells and suppresses androgen production by inhibition the enzymatic activity of P450c17 and 17βHSD [100] or indirectly via reduction of hyperinsulinemia and IR by multiple mechanisms. It has been shown that the metabolic actions of metformin on cells include increase in tissue responsiveness to insulin action, insulin receptor numbers in skeletal muscle and adipose tissue, tyrosine kinase activity and glucose uptake. Also, metformin decreases intestinal glucose absorption, plasma glucagon levels, gluconeogenesis and glycogenolysis in the liver. Most of these actions have been mediated through activation of AMPK cascade, which result in indirect reduction of hyperinsulinemia and IR the main pathogenic component in PCOS [101].

By means of anti-inflammatory/antioxidant mechanisms contributing to PCOS, metformin inhibits NF-kB, whose activation triggers IR and inflammation in PCOS [102]. Moreover, metformin increases the activity of the antioxidant enzymes such as catalase and CuZn superoxide dismutase, thereby, it scavenges the reactive oxygen species such as hydrogen peroxide (H2O2), superoxide (O2.) and hydroxyl (OH.) radicals, where metformin also directly reacts with the latter one [103]. More other related mechanisms of metformin in PCOS are still unclear and elusive.

#### **7. Conclusion**

Based on its multiple mechanisms of action and interference with signaling pathways, metformin represents as a promising potential drug for treating various medical conditions. Furthermore, the beneficial effects arising from these mechanisms can be demonstrated and clarified by substantial basic experiments and clinical trials.

*Mechanisms of Action of Metformin DOI: http://dx.doi.org/10.5772/intechopen.99189*

#### **Author details**

Samira Abdulla Mahmood Department of Pharmacology and Toxicology, Faculty of Pharmacy, Aden University, Aden, Republic of Yemen

Address all correspondence to: samabdulla@yahoo.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 2**

## Prevention of Hyperglycemia

*Lucy A. Ochola and Eric M. Guantai*

#### **Abstract**

Hyperglycemia is the elevation of blood glucose concentrations above the normal range. Prolonged uncontrolled hyperglycemia is associated with serious lifethreatening complications. Hyperglycemia arises from an imbalance between glucose production and glucose uptake and utilization by peripheral tissues. Disorders that compromise pancreatic function or affect the glucose counter-regulatory hormones cause hyperglycemia. Acute or serious illness or injury may also bring about hyperglycemia, as can many classes of drugs. Metformin lowers blood glucose levels by inhibiting the production of glucose by the liver whilst enhancing uptake of circulating glucose and its utilization in peripheral tissues such as muscle and adipose tissue. Metformin suppresses hepatic gluconeogenesis by inhibiting mitochondrial respiration and causing a reduction of cellular ATP levels. Metformin may also modulate the gut-brain-liver axis, resulting in suppression of hepatic glucose production. Metformin also opposes the hyperglycemic action of glucagon and may ameliorate pancreatic cell dysfunction associated with hyperglycemia. Metformin is therefore recommended for use in the prevention of hyperglycemia, including drug-induced hyperglycemia, in at risk patients. The benefits of metformin in the prevention of hyperglycemia are unmatched despite its contraindications.

**Keywords:** hyperglycemia, hyperinsulinemia, insulin, metformin, glucose

#### **1. Introduction**

Chronic hyperglycemia can lead to complications involving damage to the kidneys, retina, nervous system and cardiovascular system. In this chapter, we discuss the causes of hyperglycemia, including drug-induced hyperglycemia, highlighting the importance and approaches to prevention and management of hyperglycemia. We focus on the role and rationale for the use of metformin for the prevention of hyperglycemia, presenting the evidence that supports its use for this indication.

#### **2. Hyperglycemia**

Hyperglycemia, which literally means 'high blood glucose' levels, refers to the elevation of blood glucose concentrations above the normal range. Specifically, it refers to fasting blood glucose levels greater than 7.0 mmol/L (126 mg/dl) or 2-hour postprandial blood glucose levels greater than 11.0 mmol/L (200 mg/dl) [1].

#### **2.1 Symptoms and complications**

Mild, transient hyperglycemia is largely asymptomatic. However, prolonged uncontrolled hyperglycemia is associated with various symptoms including the

classic hyperglycemic triad of polyuria, polydipsia, and polyphagia, as well as blurred vision, dehydration, weight changes (gain or loss), generalized fatigue, abdominal discomfort, nausea, vomiting and muscle cramps [1, 2]. Complications arise when the hyperglycemia is severe and/or persists over an extended period. Frequent infections, erectile dysfunction and poor wound healing are associated with prolonged hyperglycemia. Chronic hyperglycemia can also lead to many serious life-threatening complications involving damage to the kidneys (nephropathy), retina (retinopathy), nervous system (peripheral neuropathy) and cardiovascular system (myocardial infarction, stroke) [1–5].

#### **2.2 Causes of hyperglycemia**

Blood glucose levels reflect the dynamic balance between, on the one hand, dietary glucose absorption and hepatic glucose production and, on the other hand, glucose uptake and utilization by peripheral tissues. Except for dietary glucose absorption, these complex and interrelated processes are under the control of the hormone insulin and, to a lesser extent, other counter-regulatory hormones such as glucagon, catecholamines, cortisol and growth hormone [1, 6]. Hyperglycemia arises from an imbalance in these processes that determine blood glucose levels.

The greatest quantitative determinant for hyperglycemia is dysfunction in pancreatic islet cell activity which affects insulin release from the pancreas in response to. The pathophysiology of hyperglycemia also entails a resulting degree of insulin resistance and impairment in homeostatic glucose regulation. Insulin resistance results in decreased uptake of glucose by insulin-sensitive tissues as well as a consequential increase in endogenous glucose production. This all leads to hyperglycemia [7]. The elevation of blood glucose levels during the fasting state is directly proportional to the increase in hepatic glucose production while that of the postprandial state is connected to insufficient suppression of glucose output plus a defect in the stimulation of insulin hormone on recipient tissues like skeletal muscle [8].

The progression of this imbalance in blood glucose homeostasis over time leads to the development of diabetes, a chronic disease affecting glucose metabolism that occurs due to either insufficient production of insulin by the pancreas, or inadequate response by tissues to insulin [9]. The development of diabetes can be delayed or prevented by targeting the early prevention and/or reversal of hyperglycemia, as well as by inhibiting the development of hyperinsulinemia-induced insulin resistance [10]. This would also delay progression of prediabetic states to diabetes [11].

In addition to diabetes, there are a myriad of other causes of hyperglycemia, i.e., non-diabetic hyperglycemia. Disorders that compromise pancreatic function (pancreatic cancer, cystic fibrosis, chronic pancreatitis, etc.) or affect the glucose counter-regulatory hormones (pheochromocytoma, acromegaly, Cushing syndrome) cause hyperglycemia. Transient hyperglycemia may arise consequent to abnormally high carbohydrates in the diet, dextrose infusion and total parental nutrition. Acute or serious illness or injury may also bring about transient hyperglycemia referred to as stress hyperglycemia or hospital-related hyperglycemia [1, 12].

Medicines may also induce hyperglycemia [1, 6, 13].

#### **2.3 Drug-induced hyperglycemia**

Drug-induced hyperglycemia refers to the clinically relevant elevation of blood glucose levels caused by drugs [13]. Whereas drug-induced hyperglycemia is often mild and asymptomatic, severe hyperglycemia may occur particularly in predisposed patients, such as those with pre-existing pancreatic dysfunction or insulin resistance. Drug-induced hyperglycemia can occur in adults and children alike,

#### *Prevention of Hyperglycemia DOI: http://dx.doi.org/10.5772/intechopen.99342*

and certain patient factors are known to increase the risk of drug-induced hyperglycemia, such as obesity, sedentary lifestyle, stress, illness, history of gestational diabetes, or a family history of diabetes [6, 14].

Many classes of drugs have been implicated in causing hyperglycemia via various mechanisms. Some drugs cause hyperglycemia by reducing insulin production/ secretion (glucocorticoids, β-receptor antagonists, thiazide diuretics, calciumchannel blockers, phenytoin, pentamidine, calcineurin inhibitors, protease inhibitors), including by direct damage to pancreatic cells (glucocorticoids, pentamidine, statins). Glucocorticoids, β-receptor antagonists and thiazide diuretics also promote hepatic glucose production and reduce insulin sensitivity. Other classes of drugs that reduce peripheral tissue sensitivity to insulin include atypical antipsychotics, antidepressants, oral contraceptives, statins, nucleoside reverse transcriptase inhibitors and protease inhibitors [1, 6, 14–16]. Hyperglycemia is one of the common adverse effects of the anticancer agent L-asparaginase, which inhibits insulin synthesis by depleting available asparagine in pancreatic cells in addition to impairing insulin receptor activity and promoting peripheral tissue resistance to insulin [14]. Monoclonal antibodies such as nivolumab and pembrolizumab may cause severe hyperglycemia by triggering the autoimmune-mediated destruction of pancreatic cells [17, 18]. β2-receptor agonists cause hyperglycemia by promoting hepatic and

**Figure 1.** *Mechanisms of drug-induced hyperglycemia and implicated classes of drugs.*

muscle glucose production [19]. The various mechanisms of drug-induced hyperglycemia and the classes of drugs implicated are shown in **Figure 1**.

The overall occurrence of drug-induced hyperglycemia is not known and would obviously vary between individual drugs. There is a lack of data on the burden of drug-induced hyperglycemia for specific drugs, and a few studies have attempted to address this gap. For example, the incidence of corticosteroid-related hyperglycemia in patients treated with high dose corticosteroids has been estimated to be in excess 50% [20, 21]. Comparably high prevalence has been reported for clozapine [22]. These and other similar findings strongly suggest that the risk of drug-induced hyperglycemia (alongside the risk of new-onset diabetes) is real.

The onset of drug-induced hyperglycemia varies on the medication administered. At the time of or shortly after initiating corticosteroids, blood glucose levels may be altered, whereas patients on hydrochlorothiazide may not experience altered levels for weeks or longer, depending on the dose given. In regard to second generation antipsychotics (SGAs), a consensus statement developed by the American Diabetes Association (ADA) in conjunction with other medical organizations recommends monitoring fasting blood glucose for 12 weeks after initiation of therapy and annually thereafter in those without diabetes. However, cases involving hyperglycemic crises have been reported within weeks of starting SGAs [23].

#### **3. Prevention and management of hyperglycemia**

The common medical occurrence of hyperglycemic states has yet to be given the due attention it deserves, considering the numerous consequences it bears to patients and the healthcare fraternity. The existing reality of numerous patients suffering from hyperglycemia of varied cause provides an overwhelming patient load, unmatched by the number of specialized providers. However, the management of hyperglycemia has continually posed a great challenge mainly from a lack of standardized protocols [24]. Currently, lack of knowledge and consensus on strategies of management play a significant role in its mismanagement.

Insulin resistance and the resulting compensatory hyperinsulinemia is considered to preclude the development of type 2 diabetes. Hyperglycemia prophylaxis is thus highly attractive based on the numerous socio-economic benefits it confers to patients and the healthcare system. Several studies have demonstrated the advantages gained from preventing elevations of blood glucose levels across a divergent patient portfolio. Research has broadly focused on management of hyperglycemia regardless of the cause, which underlies the common pathways involved in the development of hyperglycemia.

#### **3.1 The role of insulin**

The primary strategy employed in hyperglycemia management is insulin [25]. Consensus arrived at by ADA and European Association for the Study of Diabetes (EASD) outline the management of hyperglycemia in type 2 diabetes patients. These guidelines have also been adopted in the prevention of hyperglycemia from other causes, including drug-induced hyperglycemia. The guidelines recommend the use of insulin in all hospitalized patients, with discontinuation of oral hypoglycemic medication [26, 27]. Stoppage of the drugs is on the basis that majority of hospitalized patients present with concurrent conditions and/or physiological dysfunctions that tend to contraindicate continued use of these medications if already prescribed. The pharmacokinetics of oral medication, which tend to have a slow onset of action, disallows for rapid dose adjustment to changing patient needs [28].

#### *Prevention of Hyperglycemia DOI: http://dx.doi.org/10.5772/intechopen.99342*

Therefore, it is recommended that critically ill patients be treated with a continuous insulin infusion while non-critically ill patients are initiated on subcutaneous (SC) insulin. An individualized dose adjustment for insulin is advised across major studies [26, 29]. Resumption of oral diabetic agents (ODA) when transitioning from inpatient to outpatient setting, with careful consideration given to previous insulin dosing, is advised upon successful treatment. A study involving patients without diabetes recommended the administration of intravenous (IV) insulin infusion in patients with serum blood glucose level values of greater than 10 mmol/L, with a target of achieving serum blood glucose levels of 7.8–10 mmol/L in non-critical settings and less than 7.8 mmol/L in an outpatient setting [30].

Despite numerous recommendations, challenges faced by providers during insulin administration cannot be overlooked. The biggest impediment to insulin use in management of drug-induced hyperglycemia in the affected population is the unavoidable side effect of hypoglycemia [31]. Unfortunately, insulin treatment is the most common risk factor for inpatient hypoglycemia. The incidence of hypoglycemia is approximately 30% in elderly patients, in spite of using low dose insulin and oral diabetic agents [28]. This is associated with increased mortality rate and prolonged hospital stays. Hence, constant monitoring of blood glucose levels is necessary.

Dose adjustments using patients' weight is perceived to be safe and effective as long as close monitoring is done. However, this is not always feasible, let alone practical with many patients. So too is the recommendation of individualizing glycemic targets for patients based on clinical status, risk of hypoglycemia and patient comorbidities, no matter the benefit it confers. This is because the number of patients with drug-induced hyperglycemia cannot be matched to the number of specialized health care workers required to meet this need.

Herein lies the difficulty as many patients are unable to achieve the close monitoring desired, let alone manage the expected side effects in a home-based set up. Even in hospitalized patients, lack of protocols for dose adjustment poses a hindrance in adequate control of elevated blood glucose levels. Hypoglycemia presents a consequential effect that should be carefully considered in hyperglycemia management. Any chosen medication, in addition to lifestyle interventions, should ideally be one that is safe, effective, economical and with minimal side effects.

#### **3.2 The role of oral antidiabetic medications**

Non-insulin medications provide a practical alternative to achieving glycemic control. These agents may also confer a non-glycemic benefit whilst regulating the fluctuations in blood glucose levels. Alternatives among non-insulin medication include metformin, sulphonylureas, glinides, thiazolidinediones, glucagon-like peptide-1 (GLP-1) receptor agonists, and sodium–glucose cotransporter2 (SGLT2) inhibitors.

However, the side effects of each of these agents must also be considered. For example, SGLT2 inhibitors reduce blood glucose levels by preventing proximal tubular reabsorption in the kidney. This has been shown to effectively reduce glycated hemoglobin A1c (HbA1c) levels by 0.6–1.0%. They are also associated with a low risk of hypoglycemia. However, the dehydration side effects make these agents contraindicated in renal dysfunction. They also bear an increased risk of urinary and genital tract infections and are related with the development of diabetic ketoacidosis among diabetic patients [32]. Such a profile tends to limit the use of these agents. Metformin use is contraindicated in the presence of any possible indication for iodinated contrast media and in renal insufficiency while thiazolidinediones are associated with fluid retention. On the other hand, sulfonylureas and glinides

result in hypoglycemia in most patients while GLP-1 receptor antagonists can cause nausea and hence need to be withheld in critical patients. In spite of the many side effects of oral diabetic agents and the recommendation of using insulin as first line, recent studies have leaned towards the adoption of the oral diabetic agents. The drug most endorsed based on clinical evidence has been metformin [33].

#### **4. Metformin for the prevention of hyperglycemia**

#### **4.1 Introduction and rationale**

The pathophysiology of hyperglycemia entails a degree of insulin resistance and results in decreased uptake of glucose by insulin-sensitive tissues as well as a consequential increase in endogenous glucose production [7]. Dysfunction in the activity of pancreatic islet cells affects insulin release in response to rising blood glucose levels. Targeting the prevention and/or reversal of dysglycemia and insulin resistance is the principal behind preventing the development of hyperglycemia [11]**.** Any agent used in prevention of hyperglycemia must therefore target these pathways, thereby partially or completely eliminating its development.

Metformin can rightfully be considered for hyperglycemia prevention and treatment in cases of insulin resistance. Metformin is a first-line agent in treatment of type 2 diabetes mellitus. Recent studies have shown it confers a greater benefit to patients than the other oral diabetic agents, which has led to its recommendation for use in the prevention of hyperglycemia and prediabetes in at risk patients [34–36].

#### **4.2 Mechanisms of action/pharmacodynamics**

Metformin prevents hyperglycemia by hastening the clearance of glucose [37, 38]. It causes a reduction in hyperglycemia and hyperinsulinemia [39]. This facilitates a consequent decline in high insulin and high blood glucose levels, with no effect on insulin secretion. The primary mechanism involved in lowering blood glucose levels is through improving hepatic and peripheral tissue sensitivity to insulin [40]. It inhibits the production of glucose by the liver whilst enhancing uptake of circulating glucose and its utilization in peripheral tissues such as muscle and adipose tissue.

Hepatic gluconeogenesis is an energy-demanding process in which synthesis of one molecule of glucose from lactate or pyruvate requires four molecules of ATP and two molecules of GTP. Metformin suppresses hepatic gluconeogenesis by causing a reduction of cellular ATP levels [41]. Molecularly, metformin appears to inhibit mitochondrial respiration. The resulting shift in cellular energy balance increases the activity of AMP-activated protein kinase (AMPK), which promotes the action of insulin and reduces hepatic gluconeogenesis [42]. AMPK acts as a cell energy sensor: it plays a role in energy balance at the cellular and body level by adapting to changes in the concentration of AMP/ADP relative to ATP [43]. Upon activation by a decrease in cellular energy levels, AMPK initiates a change from anabolic to catabolic pathways that consume ATP. This stimulates the uptake and use of glucose and oxidation of fatty acids, in addition to the suppression of hepatic glucose production. Metformin's' inhibition of the mitochondrial complex is the basis of its effect as observed through the change in the ratios of AMP/ATP or ADP/ATP after its administration [44]. Multiple studies have demostrated that one of the mechanisms of action of metformin is the disruption of mitochondrial complex I [45, 46].

Metformin may also modulate the gut-brain-liver axis through the activation of a duodenal AMPK-dependent pathway, as has been demonstrated in rats. This effect

#### *Prevention of Hyperglycemia DOI: http://dx.doi.org/10.5772/intechopen.99342*

involves activation of protein kinase A (Pka) by GLP-1 in duodenal enterocytes, and results in suppression of hepatic glucose production [47]. It has been shown that glucocorticoid therapy leads to changes in the activation of AMPK in Cushing's syndrome patients and in vitro in human adipocytes, effects that were reversed with metformin in human adipocytes. These indicate the likelihood of converse effects of steroids and metformin in the AMPK signaling pathway, as well as the overriding of steroid effects by metformin [44, 48]. Supporting studies demonstrate that steroidrelated increase in glucose levels can be prevented with an AMPK activator [49].

Another postulated mechanism of action for metformin is by causing an increase in circulating cyclic adenosine monophosphate (cAMP) which in turn opposes the hyperglycemic action of glucagon [42, 50]. Metformin has also been postulated to increase the concentration of Glucagon-like peptide-1 (GLP-1) by enhancing site production as well as subsequently decreasing its degradation in circulation and specific tissues via inactivation of the enzyme dipeptide peptidase-4 (DPP-4). Additionally, metformin may induce up regulation of GLP-1receptors on beta cell surfaces of the pancreas. This can aid in ameliorating the beta cell dysfunction associated with hyperglycemia via the enhancement of the role of GLP-1 on glucose dependent release of insulin [11].

#### **4.3 Metformin prevents hyperglycemia and hyperinsulinemia**

Metformin can rightfully be considered for hyperglycemia prevention and treatment in cases of insulin resistance. Metformin has been identified as a first line agent in treatment of type 2 diabetes mellitus. Recent studies have shown that it confers a greater benefit to patients than the other oral diabetic agents, which has led to its recommendation for use in the prevention of prediabetes in at risk patients [34, 35, 51]. Presently though, only a few nations have formally adopted this proposal such as Poland, Philippines and Turkey but many may adopt it in the near future based on the emerging evidence [11]. Metformin overrides most of the factors that contribute to poor glycemic management like inaccessibility to medicine and fear of developing hypoglycemia. This improves patient perception on its use regardless of the minimal side effects. In addition, it has been demonstrated to confer long term benefit to those who use it prophylactically. A study that followed up patients from a diabetes prevention program after 15 years found that the metformin treatment arm had a 17% lower incidence for developing type 2 diabetes than the placebo arm. This was determined using the HbA1c parameter, in which 36% of the patients had a risk reduction for diabetes development [34].

In a prospective observational study in persons with normal glucose tolerance and hyperinsulinemia, a dose of 2.55 ± 0.2 g/day of metformin restored physiological insulin secretion by decreasing fasting and post-glucose load hyperinsulinemia in the oral glucose tolerance test (OGTT). Over the observation period, the effect of metformin on the reduction of hyperinsulinemia increased over time, peaking after 1 year of treatment. The ability to lower fasting blood glucose levels also improved with time. Fasting blood glucose levels reached normoglycemic range at 3 months and remained so until the end of the 1 year observation period, with no development of hypoglycemia [39]**.** A substantial decrease in hyperinsulinemia from high blood glucose levels has also been reported in metformin-treated patients based on an increase in the uptake of glucose [52]. The enhancement of insulin action reduces the load on the beta cells in insulin secretion thus can aid in ameliorating the beta cell dysfunction to an extent; this confers an advantage to patients predisposed to developing hyperglycemia.

In addition, a randomized controlled study showed that there was no significant difference in blood glucose levels between critically ill patients receiving 1000 mg

of metformin daily versus a similar spectrum of patients receiving 50 International Units (IU) of regular insulin. Furthermore, metformin-treated patients had blood glucose levels subside to near-normal range [40]. The targeted desired blood glucose levels were achieved with metformin after three days while insulin failed to do the same.

#### **4.4 Metformin for drug-induced hyperglycemia**

In acute lymphoblastic leukemia patients with drug-induce hyperglycemia, metformin monotherapy controlled blood glucose in 12 out of 17 patients, without the need for insulin using a median dose of 1000 mg/day for a median of 6 days. Blood glucose levels never exceeded 11.1 mmol/L in 8 of the 12 patients. The one patient who developed hyperglycemia during relapse re-induction for leukemia treatment was effectively controlled using metformin alone [53]. Three of the patients given insulin therapy due to high blood glucose levels were eventually weaned off insulin to metformin alone. Additionally, in a controlled trial consisting of non-diabetic patients on glucocorticoids, metformin prevented an increase of 2-hour glucose AUC with, signifying glucose tolerance preservation. No changes in baseline and after 4 weeks metformin treatment was seen with the 2-hour glucose AUC whereas this parameter increased in the placebo group [54].

Similarly, the effect of metformin on prednisone-induced hyperglycemia (PIH) was observed on fasting and 2-hour post prandial glucose levels in hematological cancer patients. The fasting blood glucose readings indicated a proportion of prednisone-induced hyperglycemia of 72.7% and 14.3% in the control and treatment groups respectively. The proportion was slightly lower while using the 2-hour post prandial glucose, in which 54.5% of participants in the control group developed prednisone-induced hyperglycemia while none developed prednisoneinduced hyperglycemia in the treatment group. Patients in the control group had 16 (95% CI 1.3–194.6) times the odds of developing prednisone-induced hyperglycemia compared to patients in the treatment group. Double daily dosing (1700 mg twice daily) was more effective in preventing prednisone-induced hyperglycemia [21]. This is supported by other studies that show that that a daily dose of metformin 1500 mg contributes to 80–85% glucose lowering effects [55].

#### **4.5 Metformin for hyperglycemia: risks and benefits**

The limitations attached to the full exploitation of metformin use include its relative contraindications in many hospitalized patients who present with comorbidities like renal insufficiency or unstable hemodynamic status. Metformin is contraindicated if serum creatinine is ≥133 mmol/L in men or ≥ 124 mmol/L in women. Emerging evidence shows that the established cut-off points for renal safety may be overly restrictive [56]**.** It has been argued that there is a need to relax these cut-offs and policies to allow use of this drug to patients with stable chronic kidney disease characterized by mild–moderate renal insufficiency [57–59].

The associated risk of lactic acidosis tends to deter the use of metformin in majority of the comorbid patients on drugs that predispose to the development of hyperglycemia. However, the studies that made such recommendations used a small percentage of the patient population, thus limiting the extrapolation of these recommendations to the greater public [60]. Fortunately, the incidence of metformininduced lactic acidosis is rare and can be significantly reduced in at-risk patients by observing the necessary precautions [27, 56]. Other factors may also play a greater role in in being predictors of acidosis, such as dehydration, severe heart and renal failure. Thus, its benefits for use outweigh the potential risk of lactic acidosis.

#### *Prevention of Hyperglycemia DOI: http://dx.doi.org/10.5772/intechopen.99342*

Supporting evidence on avoidance of metformin use in certain cases is poor and inconsistent such as in patients undergoing radio-contrast imaging which theoretically predisposes patients to media-induced nephropathy, increasing the risk of lactic acidosis [56].

The benefits of metformin in the prevention of hyperglycemia are unmatched despite its list of contraindications. This has facilitated its expanded use based on its well-founded glycemic effects as well as numerous benefits conferred such as the beneficial effect on reduction of development of cardiovascular risk factors [61]. It confers good glycemic management that yields a substantial and enduring decrease in the onset and progression of micro vascular complications [60].

Moreover, large based clinical trials and systematic reviews have shown its beneficial effect of enhancing weight loss, even the weight loss associated with medicaments like antipsychotic agents [62, 63].

#### **5. Conclusions**

In summary, the suppression of glucose production by metformin's direct effect plus the enhancement of hepatic insulin signaling will curb the development of drug-induced hyperglycemia. Metformin has been shown to reduce the incidence of hyperglycemia-related complications such as diabetes and risk factors for cardiovascular disease in patients with impaired glucose tolerance and fasting blood sugar [11, 64, 65]. This has led to its endorsement of use in patients with high risk of developing the aforementioned conditions [36].

### **Author details**

Lucy A. Ochola1 \* and Eric M. Guantai<sup>2</sup>

1 Machakos Level 5 Hospital, Machakos, Kenya

2 School of Pharmacy, University of Nairobi, Nairobi, Kenya

\*Address all correspondence to: lucyochola@gmail.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Section 2
