**2. Regulation of β-cell function and insulin secretion**

Control of energy metabolism is essential in maintaining cellular homeostasis in all animals across the metazoan (all animals with differentiated tissues). Insulin and glucagon are hormones produced by vertebrate organisms to regulate glycaemic homeostasis. In addition, insulin-like and glucagon-like peptide genes have been detected in invertebrate organisms including, insects, molluscs and nematodes, thus inferring a similar metabolic control that is conserved among most species [1,2]. However, in the case of vertebrates, insulin and glucagon are produced by cells located in the islets of Langerhans of the animal pancreas. Under normal physiological conditions, blood glucose concentration is maintained within narrow limits by an alternate release of these powerful proteins, regardless of nutrient intake or expenditure (*e.g.* exercise). There are four main cell types that contribute to the regulation of this pancreatic function and they include, α-cells, β-cells, δ-cells and pancreatic peptide (PP)-cells [3]. The role of α-cells is to synthesise and secrete glucagon in response to low extracellular glucose concentrations, thus replenishing the plasma carbohydrate level [3]. δ-Cells secrete somatos‐ tatin that has an inhibitory effect on insulin and glucagon release, while PP-cells secrete pancreatic peptide whose physiological function has not been fully elucidated [3]. Conversely, the function of β-cells has been extensively studied and they are responsible for the biosyn‐ thesis and release of insulin in response to elevated plasma glucose, amino acid and saturated fatty acid levels [3]. These cells represent the most abundant cell type in pancreatic islets and are the primary source of dysfunction in DM.

β-Cell responsiveness and subsequent insulin secretion is subject to a plethora of cellular regulatory mechanisms. Insulin biosynthesis and secretion is a highly controlled system that has many influencing extracellular and intracellular factors including, glucose, fatty acids, amino acids, nucleotides, calcium/potassium electrochemical gradient, metabolic coupling factors (MCFs), and level of ROS and RNS. Furthermore, the fact that cellular insulin secretion is achieved by the physical release of vesicles or granules containing the protein, suggests that the process acquires a greater degree of complexity and control, and is subject to vesicle manufacture, recruitment and finally plasma membrane docking.

Glucose-Stimulated Insulin Secretion (GSIS) is fundamental to insulin exocytosis as glucose is the most potent insulin secretagogue [4]. In an environment of excess extracellular glucose, β-cell plasma membrane transporter proteins GLUT1 and GLUT2, actively transport free glucose molecules inside the cell where glycolysis can be initiated to create the nucleotide ATP (Fig. 1). Consequently, intracellular metabolism of glucose by glycolysis, and further metab‐ olism of pyruvate via the downstream tricarboxylic acid (TCA) cycle, leads to elevated NADH, FADH2 and ultimately ATP levels [4]. The increased intracellular ATP:ADP ratio closes membrane-bound ATP-sensitive K+ channels, resulting in plasma membrane depolarisation and a subsequent opening of membrane-bound voltage activated Ca2+ channels. A rapid influx of calcium ions is promoted, causing the exocytosis of insulin through fusion of the insulin containing vesicles with the plasma membrane via VAMP (vesicle-associated membrane protein) and SNARE (soluble NH2-ethylmaleimide-sensitive fusion protein attachment protein receptor) association [5]. This specific process of insulin secretion is known as KATPdependent GSIS, and since ATP generation is critical, the metabolic control points of glycolysis, the TCA cycle and oxidative phosphorylation (*i.e.* activity of metabolic enzymes such as hexokinase, phosphofructokinase, pyruvate kinase, pyruvate dehydrogenase, pyruvate carboxylase, glutamate dehydrogenase and mitochondrial redox-shuttles) have a significant impact on regulation of insulin release.

levels of pro-inflammatory proteins such as nuclear transcription factor κB (NFκB), inducible nitric oxide synthase (iNOS), NADPH oxidase (NOX), Toll-like receptors (TLR) and other proteins in response to immune signals, but also to metabolic challenge. However and in contrast to professional immunoinflammatory cells, such as macrophages or neutrophils, the β-cell is fragile when subjected to immune attack and is highly vulnerable to oxidative stress. In this chapter, we intend to review the mechanisms of insulin secretion in response to a wide variety of metabolic stimuli, the 'immune-like' characteristics of the pancreatic β-cells with respect to metabolism, secretion and cell defence, the similarities between β-cell failure/death in T1DM and T2DM and finally, to suggest novel targets for the treatment of diabetes.

Control of energy metabolism is essential in maintaining cellular homeostasis in all animals across the metazoan (all animals with differentiated tissues). Insulin and glucagon are hormones produced by vertebrate organisms to regulate glycaemic homeostasis. In addition, insulin-like and glucagon-like peptide genes have been detected in invertebrate organisms including, insects, molluscs and nematodes, thus inferring a similar metabolic control that is conserved among most species [1,2]. However, in the case of vertebrates, insulin and glucagon are produced by cells located in the islets of Langerhans of the animal pancreas. Under normal physiological conditions, blood glucose concentration is maintained within narrow limits by an alternate release of these powerful proteins, regardless of nutrient intake or expenditure (*e.g.* exercise). There are four main cell types that contribute to the regulation of this pancreatic function and they include, α-cells, β-cells, δ-cells and pancreatic peptide (PP)-cells [3]. The role of α-cells is to synthesise and secrete glucagon in response to low extracellular glucose concentrations, thus replenishing the plasma carbohydrate level [3]. δ-Cells secrete somatos‐ tatin that has an inhibitory effect on insulin and glucagon release, while PP-cells secrete pancreatic peptide whose physiological function has not been fully elucidated [3]. Conversely, the function of β-cells has been extensively studied and they are responsible for the biosyn‐ thesis and release of insulin in response to elevated plasma glucose, amino acid and saturated fatty acid levels [3]. These cells represent the most abundant cell type in pancreatic islets and

β-Cell responsiveness and subsequent insulin secretion is subject to a plethora of cellular regulatory mechanisms. Insulin biosynthesis and secretion is a highly controlled system that has many influencing extracellular and intracellular factors including, glucose, fatty acids, amino acids, nucleotides, calcium/potassium electrochemical gradient, metabolic coupling factors (MCFs), and level of ROS and RNS. Furthermore, the fact that cellular insulin secretion is achieved by the physical release of vesicles or granules containing the protein, suggests that the process acquires a greater degree of complexity and control, and is subject to vesicle

Glucose-Stimulated Insulin Secretion (GSIS) is fundamental to insulin exocytosis as glucose is the most potent insulin secretagogue [4]. In an environment of excess extracellular glucose,

**2. Regulation of β-cell function and insulin secretion**

128 Type 1 Diabetes

are the primary source of dysfunction in DM.

manufacture, recruitment and finally plasma membrane docking.

**Figure 1.** Mechanisms of nutrient and amino acid stimulated insulin secretion. Glucose metabolism is essential for stimulation of insulin secretion. The mechanisms by which amino acids enhance insulin secretion are understood to primarily rely on (a) direct depolarization of the plasma membrane (e.g. cationic amino acid, L-arginine); (b) metabo‐ lism (e.g. alanine, glutamine, leucine); and (c) co-transport with Na+ and cell membrane depolarization (e.g. alanine). Notably, rapid partial oxidation may also initially increase both the cellular content of ATP (impacting on K+ATP chan‐ nel closure prompting membrane depolarization) and other stimulus secretion coupling factors. In the absence of glu‐ cose, fatty acids may be metabolised to generate ATP and maintain basal levels of insulin secretion. Adapted from [3].

However, there also remains the possibility that KATP-independent GSIS can occur in the βcell, although the exact methodology is still not fully understood. KATP-independent GSIS has been illustrated in studies utilising diazoxide to maintain K+ channels in the open position [6] and in mice with disrupted/deleted K+ channels [7, 8]. GSIS was subsequently shown to be possible in a KATP-independent manner and it is believed that these two co-ordinate mecha‐ nisms of insulin secretion (*i.e.* KATP-dependent & KATP-independent GSIS), are responsible for the bi-phasic insulin response in animals. It is thought that the initial rise in insulin secretion is KATP-dependent, while the second phase is mediated through KATP-independent interactions dependent on mitochondrial activity [4,9].

Mitochondrial, lipid and amino acid metabolism plays a significant role in regulation of insulin secretion and GSIS. Lipid and amino acid metabolites can generate, or can directly become MCFs that enhance or inhibit GSIS. While individual amino acids alone at physiological concentrations do not enhance GSIS, some specific amino acids at higher concentrations, or in combination with others, can cause increments in GSIS [10]. Arginine, alanine, leucine and glutamine can increase GSIS, while homocysteine and cysteine at elevated concentration can inhibit GSIS [10]. The effect of amino acids is also dependent on whether β-cells are exposed acutely or chronically, as chronic exposure may influence the expression of genes involved in the control of insulin secretion [10,11]. In addition, another nutrient source, fatty acids, can also regulate GSIS in both a positive or negative manner depending on the level of saturation, carbon chain length, and whether exposure is under acute or chronic conditions. Saturated fatty acids like palmitic and stearic acid are known to chronically decrease GSIS *in vitro*, but palmitic acid can acutely enhance GSIS [12-14]. Conversely, chronic exposure to monounsa‐ turated oleic acid and polyunsaturated arachidonic acid can increase insulin production in βcells [13,15]. Fatty acids can amplify β-cell GSIS, and it is likely that they elevate insulin levels by causing changes in calcium influx and proteins associated with ion channel activity [16]. Mitochondrial metabolism of amino and fatty acid is at the hub of the reported effects on insulin secretion and GSIS, mainly because TCA-mediated metabolism of both leads to increased ATP production and protein biosynthesis, which is a prerequisite for insulin secretion (Fig. 1). The intricacies of mitochondrial-mediated metabolism of amino and fatty acids will be discussed below.

**Figure 2.** Schematic diagram representing the metabolism of selected amino acids, highlighting related metabolic stimulus-secretion coupling factors involved in insulin release. The pathway of glutamine metabolism via glutaminase, GDH, and entry into the TCA cycle (glutaminolysis) is shown along with key points of amino acid interaction with glu‐ tamine and glucose metabolism. KG, -ketoglutarate; ALT, alanine aminotransferase; AST, aspartate aminotransferase; AT, aminotransferase; BCKDH, branched-chain-keto-acid dehydrogenase; PC, pyruvate carboxylase; PDH, pyruvate de‐

The Impact of Inflammation on Pancreatic β-Cell Metabolism, Function and Failure in T1DM and T2DM…

mitochondrial NADH shuttles like glycerol-3-phosphate and the malate/aspartate shuttle (Fig. 3), for specific details refer to [11,22]. Briefly, the glycerol-3-phosphate shuttle consists of cytosolic and mitochondrial glycerol-3-phosphate dehydrogenase that operate in unison to

shuttle responsible for transferring glycolytic reducing equivalents to the mitochondria in the β-cell [11]. Here, cytosolic malate dehydrogenase reduces oxaloacetate to malate and NAD+

with a subsequent generation of NADH inside the mitochondria. Using an amino group provided by glutamate, mitochondrial oxaloacetate can be converted back to aspartate maintaining this cyclic event. The malate/aspartate shuttle is dominantly expressed in β-cells,

As alluded to previously, amino acid metabolism is essential for nutrient- and glucose-stimulat‐ ed insulin secretion, and the effects of several amino acids have been reviewed extensively [3,10, 11]. To summarise these findings briefly, both arginine and alanine have been shown to promote insulin release through changes in electrogenic transport, progressing to activation of Ca2+ ion channels [10,23,24]. It has also been demonstrated that they enhance glutamate production and consequently may play a role in malate/aspartate shuttle-mediated generation of NADH, and/or in glutathione synthesis and antioxidant defence [25]. Therefore, both arginine and alanine may

convert dihydroxyacetone phosphate to glycerol-3-phosphate and NAD+

eloquently linking glycolysis to mitochondrial & amino acid metabolism.

for glycolysis primarily through high expression of

[4]. In contrast, the malate/aspartate shuttle is the main

, with a subsequent

http://dx.doi.org/10.5772/55349

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,

hydrogenase; KIC, ketoisocaproic acid. Adapted from [21].

Pancreatic β-cells regenerate NAD+

generation of FADH2 from NAD+
