**2. Insulin signaling in the liver**

*1.2.3. Regulation of magnesium transport*

100 Glucose Homeostasis

Mg2+extrusion by phosphorylating the Na+

consistent with the observation that distinct Na+

putative Na+

respectively [34,42]. The differential activation of the Na+

The specific modality of operation and regulation of the various Mg2+transport mechanisms have been extensively addressed in recent review articles [32,37-39], and we refer to those reviews for further information. For the purpose of this chapter, we will only mention that in liver cells both Mg2+entry and extrusion are under hormonal control. Hormones like catechol‐ amine and glucagon, which increase cAMP level within the hepatocyte, all promote

of α1-adrenoceptors by catecholamine also induces Mg2+extrusion. Stimulation of this class of adrenergic receptors activates PLCγ, which in turn hydrolyzes phosphatidyl-inositol bisphos‐ phate (PIP2) to diacylglycerol and inositol 1,4,5-trisphosphate (IP3). The IP3-induced Ca2+release from the endoplasmic reticulum and the subsequent capacitative Ca2+entry through the hepatocyte plasma membrane promote Mg2+extrusion from the hepatocyte, most likely by displacing Mg2+for Ca2+from organelle and cytosolic binding sites [41]. This is

nisms operate in the basolateral and the apical portion of the hepatocyte cell membrane,

and glucagon receptor) and the Ca2+-dependent Mg2+extrusion mechanism (α1-adrenergic receptors) points to the ability of the hepatocytes to activate Mg2+extrusion by different modalities and circumvent possible inhibitory mechanisms. It has to be noted, in fact, that insulin pre-treatment abolishes the Mg2+extrusion mediated by cAMP but not that mediated via α1-adrenoceptor activation [43]. Conversely, hormones or agents that maximize Ca2+release from the ER elicit a time-dependent inhibition of α1-adrenergic receptor mediated Mg2+extrusion that leaves unaffected the extrusion occurring via β-adrenergic receptors stimulation and cellular cAMP elevation [44]. In this contest, it has to be noted that cytoplasmic free [Mg2+]i modulates adenylyl cyclase activation in a variety of cell types including hepato‐ cytes [45]. Under resting conditions, cytoplasmic [Mg2+]i is insufficient to activate the adenylyl cyclase maximally. Following β-adrenoceptor or glucagon receptor stimulation the cytoplas‐ mic Mg2+pool increases markedly but transiently via the release of Mg2+from other cellular pool (namely mitochondria and endoplasmic reticulum) promoting adenylate cyclase activity and cAMP synthesis [45]. Elevation of cytoplasmic [Mg2+]i also inhibits IP3-induced Ca2+-release

[27] most likely via a direct modulatory effect of Mg2+on the IP3 receptor subunits.

/Mg2+exchanger has also been observed [47].

Cellular Mg2+accumulation is also under hormonal regulation. Among the hormones involved in the process there are insulin and vasopressin. These hormones either counteract cAMP production by acting at the level of the β-adrenergic receptor (inhibition) or the cytoplasmic phosphodiesterase that converts cAMP to AMP (stimulation), and/or activate PKC signaling, which acts as cAMP *alter ego*. Due to its ubiquitous presence and abundance, the TRPM7 channel is the mechanism most likely responsible for Mg2+accumulation in the hepatocyte [46]. It is presently unclear whether PKC activates the channel by binding RACK1 and removing this protein from a specific site near the C terminus of the channel through which RACK1 inhibits TRPM7 conductance [36], or whether phosphorylation of the channel C-terminus is also required for full activation [46]. In the case of insulin, a direct modulatory effect on the

/Mg2+exchanger mentioned above [40]. Activation

and Ca2+-dependent Mg2+extrusion mecha‐


Insulin signaling is mediated by a complex and highly integrated signaling network that controls several processes including whole body glucose homeostasis. The liver is the first organ 'seen' by insulin following its release from the β-cells into the portal vein, and is responsible for the clearance of 50% of the released insulin at the first pass. Stimulation of the insulin receptor in liver cells is a key event to regulate hepatic glucose homeostasis. In addition, insulin acts indirectly on hepatic glucose homeostasis in that insulin released from β-cells inhibits glucagon release from pancreatic α-cells thus limiting the drive on hepatic gluconeo‐ genesis. The impairment of both these processes observed in insulin resistance is linked to major health problems including type 2-diabetes.

Insulin initiates its signaling cascade by interacting with the insulin receptor on the cell surface. Binding of insulin to the extracellular α-subunits of the insulin receptor results in a confor‐ mation change that translates to the intracellular β-subunits of the receptor. The consequent activation of the kinase domain in the β-subunits of the receptor results in the autophosphor‐ ylation of specific tyrosine residues in the intracellular β-subunits. The phosphorylated insulin receptor now recruits the insulin receptor susbstrate (IRS), which upon phosphorylation on tyrosine residues acts as a docking unit for numerous cellular proteins including the phos‐ phatidyl inositol 3-kinase (PI3K) [54]. Recruitment of these proteins to the IRS results in their activation. Activation of PI3K results in the phosphorylation of PIP2 to PIP3 and in the subsequent activation of protein kinase B (PKB or Akt), which then phosphorylates Forkhead box protein O1 (FoxO1), preventing its translocation to the nucleus. In its un-phosphorylated state FoxO1 localizes in the nucleus, binds to the insulin response element sequence of gluconeogenesis-related genes, chiefly glucose 6 phosphatase and PEPCK, and increases their transcription rate, indirectly increasing the rate of hepatic glucose production. In its phos‐ phorylated state, FoxO1 is unable to translocate to the nucleus and to activate the gluconeo‐ genesis-related genes. Inhibition of FoxO1 could then improve hepatic metabolism in cases of insulin resistance and metabolic syndrome [55].

intra-hepatic Mg2+content [60]. The loss of hepatic Mg2+observed under diabetic conditions

attenuated to a significant extent by the presence of glycogen, amylopectin, or glucose within

The functional association between Mg2+and glucose is also observed for Mg2+accumulation. Insulin, one of the hormones involved in Mg2+accumulation, is also responsible for glucose accumulation and conversion to glycogen [58]. Following insulin administration, Mg2+accumulation is directly proportional to the amount of glucose present in the system [63]. Conversely, decreasing Mg2+content in the extracellular system decreased the accumulation of glucose within the cells [40,63]. In part, the limited accumulation of glucose into insulinstimulated cells in the presence of low extracellular Mg2+concentration can be explained with the reduced activation of the insulin receptor occurring in these cells as Mg2+is essential for the proper autophosphorylation of the insulin receptor and the subsequent recruitment of the insulin receptor substrate to the activated receptor [54]. All together, these pieces of evidence and observation support an essential role of Mg2+in glucose regulation and pose for the cation as an important player in the onset and development of insulin resistance and diabetes in

The physiological role of magnesium is principally related to enzyme activity. All enzymes utilizing ATP require Mg for substrate formation. Intracellular free magnesium also acts as an allosteric activator of enzyme action including critical enzyme systems such as adenylate

cyclases (adenylate cyclase, guanylate cyclase), and the kinases (hexokinase, protein kinase) [64]. Recently, our laboratory has provided evidence that Mg2+also modulates the amount of glucose 6-phosphate being routed into the endoplasmic reticulum (E.R) to be hydrolyzed to glucose plus Pi by the glucose 6-phosphatase, or to be converted to 6-phosphogluconolactone by the hexose 6-phosphate dehydrogenase, the reticular version of the G6PD. Moreover, our laboratory has provided significant evidence that both glucose and Mg2+homeostasis are altered under pathological conditions such as diabetes [61] and alcoholic liver disease [65].

Many of the enzymes of glycolytic pathway that utilizes glucose have a requirement for Mg2+ [26] and utilize MgATP2-as a cofactor [66]. The Km values for Mg2+in the glycolytic enzymes of the human erythrocyte are between 1 and 2.3 mM for hexokinase, 0.025 mM for phospho‐ fructokinase (PFK), 0.3 mM for phosphoglycerate kinase (PGK), and 1 mM for pyruvate kinase [26]. Magnesium ions (Mg2+) and MgATP2-regulate the most important glycolytic enzymes, namely hexokinase, phosphofructokinase, aldolase, phosphoglycerate kinase, and pyruvate kinase [66]. Glucokinase (Hexokinase IV or D), an enzyme expressed predominantly in liver and pancreatic β-cells of vertebrates, shows marked deviations from Michaelis-Menten kinetics when the glucose concentration is varied at a constant MgATP2-concentration, but shows no deviations from Michaelis-Menten kinetics with respect to MgATP2-[26,67]. Com‐ pared to the other hexokinase isoenzymes, this isoform has a low affinity for glucose (Table

/K+

.Mg2+exchanger [61,62], and can be

http://dx.doi.org/10.5772/57564

103

Role of Magnesium in the Regulation of Hepatic Glucose Homeostasis


ATPase, Ca2+ATPase),

, K+

depends on the enhanced phosphorylation of the Na+

**3.1. Magnesium and enzyme activation in glucose metabolism**

enzyme substrate (ATPMg, GTPMg) to enzymes such as ATPase (Na+

cyclase, phosphofructokinase, phospholipase C, and Na+

liver plasma membrane vesicle [62].

human patients.
