Intracellular magnesium and insulin resistance

ARTICLE

Auteur(s) : Junji Takaya, Hirohiko Higashino, Yohnosuke Kobayashi

Department of Pediatrics, Kansai Medical University, Moriguchi, Osaka 570-8506, Japan

Introduction

Magnesium (Mg), the second most abundant intracellular divalent cation, is a cofactor of many enzymes involved in glucose metabolism, especially those using high-energy phosphate bonds [1, 2]. It has been known for some time that insulin stimulates Mg uptake in insulin-sensitive tissues [3-6]. In vitro studies have shown that Mg has an important role in insulin action [7]. Impaired biological responses to insulin are referred to as insulin resistance [8, 9]. Insulin resistance occurs when normal circulating concentrations of insulin are insufficient to regulate these processes appropriately. The insulin resistance syndrome, characterized by hyperinsulinemia, obesity, hypertension, and dyslipidemia, is strongly associated with type 2 (non-insulin dependent) diabetes mellitus (DM) and atherosclerotic cardiovascular diseases [10]. The mechanisms leading to the development of insulin resistance are not fully understood. Previous observations indicate that intracellular Mg ([Mg²⁺]_i) concentrations are decreased in insulin-resistant states such as type 2 DM [11-13]. Resnick et al. reported that erythrocyte [Mg²⁺]_i was significantly reduced in type 2 DM compared with non-diabetic control subjects (184 ± 13.7 vs 223.3 ± 8.3 mmol/l, p < 0.001) [12]. It thus appears that alterations in cellular magnesium concentration contribute to the diminished cellular activities of insulin [14, 15].

This review was designed to reach a better understanding of the mechanism involved in the correlation between insulin resistance and Mg, mainly on the basis of our data. By using the MEDLINE data base (for the years from 1973 to 2002) and PubMed (for the years from 1984 to 2003), we searched the medical literature with search terms “magnesium”, “low”, and “insulin”. Articles referenced in review articles on insulin resistance were also examined.

Relationship between magnesium and insulin action

Plasma Mg and [Mg²⁺]_i concentrations are tightly regulated by several factors (figure 1). Among them, insulin is an important modulator of the cellular content of Mg [4-6, 13, 16]. In fact, in vitro and in vivo studies have demonstrated that insulin modulates the shift of Mg from extracellular to intracellular space [5-7, 16-18]. Insulin also modulates the activity of the ion transport mechanism of cells, such as erythrocytes, platelets and rat uterus cells. Insulin regulates [Mg²⁺]_i concentration by stimulating the plasma membrane adenosine triphosphate (ATPase) pump and erythrocyte Mg uptake [16]. One of the functions of Mg is to complex with ATP [19, 20]. Since Mg is a necessary cofactor in all ATP transfer reactions, this implies that [Mg²⁺]_i concentration is critical in the phosphorylation of the insulin receptor [21].

Insulin binding to specific cell surface receptors is the initial event in insulin action on target tissues. Insulin receptors are heterotetrameric glycoproteins consisting of two α-subunits and two β-subunits possessing the intrinsic tyrosine kinase activity [22]. The insulin binding activates tyrosine kinase phosphorylation at the intracellular part of the receptor, and a sequence of reactions follows [8]. It has been postulated that the activation of protein kinase by the insulin receptor is an important step in transmembrane signaling for insulin action [23, 24]. There are several examples where alteration in receptor kinase activity could explain an impairment of the insulin action [25]. Insulin receptors isolated from various tissues of type 2 diabetics or obese subjects have an impaired capacity to autophosphorylate or express the tyrosine kinase activity when exposed to insulin [26]. Suáres et al. suggested that the insulin resistance observed in the skeletal muscles of magnesium-deficient rats might be attributed to the defective tyrosine kinase activity of the insulin receptor [1]. Studies in multiple insulin resistant cell models have demonstrated that an impaired response of the tyrosine kinase to insulin stimulation is one potential mechanism causing insulin resistant-state in type 2 diabetes [22] (figure 2). The gene for the membrane glycoprotein PC-1 is considered to be a candidate for insulin resistance, since this protein has been shown to inhibit tyrosine kinase activity of the insulin receptor in cultured fibroblasts [27]. Similarly, a depletion of [Mg²⁺]_i may cause a defective tyrosine kinase function at the insulin receptor level (figure 2).

A decreased concentration of [Mg²⁺]_i is associated with a diminution in the ability of insulin to stimulate glucose uptake in insulin-sensitive tissues, such as adipose cells and skeletal muscle tissues [14]. Given in vitro evidence that low Mg concentrations can reduce tissue glucose uptake [1, 14], it seems that reduced [Mg²⁺]_i interferes with the insulin signaling mechanism involved in glucose transport [28] (figure 2). Altered [Mg²⁺]_i may also lead to decreased cellular glucose utilization, thus promoting peripheral insulin resistance with a postreceptor mechanism. However, Sebekova et al. reported that insulin resistance is not associated with a change in skeletal muscle [Mg²⁺]_i concentration in patients with reduced kidney functions [29]. The discrepancy may be induced by the grade of insulin resistance.

Insulin is an important modulator of [Mg²⁺]_i, which may regulate the insulin action to its receptor and also insulin signaling mechanisms involved in glucose transport.

Clinical manifestations

Reaven postulates “Syndrome X ” with insulin resistance as the key element linking different risk factors such as hyperinsulinemia, type 2 DM, aberrant lipoprotein metabolism, hypertension, obesity, hyperuricemia, and coronary heart disease [30]. In both human and experimental animals, dietary-induced Mg deficiency is correlated with insulin resistance/sensitivity [31-33]. Tosiello reported that Mg deficiency represents the link between the insulin resistance of hypertension, obesity, and type 2 DM, since its role in maintaining the cellular pumps necessary for peripheral vascular tone (Na⁺/K⁺ ATPase and Ca²⁺-dependent K⁺ channels) would be diminished [34]. We review several clinical manifestations induced by defective Mg metabolism.

Healthy subjects

Variations in diet Mg concentration have a relatively modest but significant effect on insulin-mediated glucose disposal in healthy subjects [31]. The induction of hypomagnesemia in healthy adults led to decreased insulin sensitivity [35]. Plasma Mg concentration may also be a nongenetic modulator of insulin action in nondiabetic healthy individuals [36, 37].

Insulin resistance is very common among Pima Indians, who have a higher risk for the development of type 2 DM [38]. Compared to Caucasians, non-diabetic Pima Indians have lower erythrocyte Mg accumulation in response to insulin infusion [39]. These results may explain the relationship between their high degree of insulin resistance and Mg metabolism disturbance.

Diabetes mellitus

Insulin resistance in subjects with type 2 DM impairs the ability of insulin to stimulate Mg and glucose uptake [40]. Diabetes mellitus may be associated with Mg depletion, which in turn may contribute to metabolic complications of diabetes such as vascular disease, osteoporosis and polyneuropathy [41]. A relationship between hypomagnesemia and insulin resistance has been reported among diabetic patients [42]. The correlation of hypomagnesemia with glycemic control is reported to be associated with hypermagnesuria [43]. Physiological concentrations of insulin induce a specific increase in the renal excretion of Mg [44]. This data may explain the Mg depletion observed in various hyperinsulinemia states. On the other hand, Mg deficit is also reported in patients with type 1 DM [41, 45, 46]. Hyperglycemia increases renal Mg clearance independent of insulin levels [47].

Studies in subjects with type 2 DM have shown that plasma Mg is inversely correlated with the degree of glycaemic control [45, 48, 49]. However, it is reported that there was no correlation between intracellular and plasma Mg levels [50, 51]. Low plasma Mg levels indicate low Mg stores, but Mg depletion must have occurred before its serum level declines. Plasma Mg has no pathophysiological impact but has more latent effects on cellular regulation.

The long-term administration of Mg has been found to improve insulin sensitivity in type 2 DM subjects [50, 52, 53]. The effects of dietary Mg supplements (3g/day for 3 weeks) were examined in type 2 DM patients [52, 54]. The results of the study showed that glucose- and arginine-induced insulin secretion as well as insulin sensitivity were significantly improved by long-term Mg supplementation [52, 54]. No difference in plasma Mg and mononuclear [Mg²⁺]_i levels was observed between the placebo and the 20.7 mmol Mg-supplemented groups [50]. But the replacement with 41.4 mmol Mg tended to increase plasma, cellular, and urine Mg in poorly controlled patients with type 2 DM [50].

Generally, an increase in plasma Mg and erythrocyte [Mg²⁺]_i was observed, but there is no consistent improvement in glycaemic control [52, 54-56]. Three months oral Mg supplementation in insulin-requiring patients with type 2 DM improved insulin sensitivity and secretion but had no effect on glycaemic control [57]. The Atherosclerosis Risk in Communities (ARIC) Study [58] reported that a low dietary Mg intake does not confer an increased risk to type 2 DM in a middle-aged population. There was a clear inverse correlation between serum total Mg and the incidence of DM in the white, but not in the black population. In an editorial comment to the ARIC study [59], doubt is expressed on the relationship between low serum Mg and the risk for DM.

Hypertension

The fasting level of [Mg²⁺]_i measured by ³¹P nuclear magnetic resonance(NMR) is significantly lower in hypertensives as compared with normotensive subjects [60, 61]. A strong inverse relationship is also present between the level of [Mg²⁺]_i and blood pressure levels. Alterations in [Mg²⁺]_i regulation may be an important contributing factor for increased vascular resistance associated with insulin-resistant status in DM patients. Hypertensive patients with left ventricular hypertrophy (LHV) have a lower [Mg²⁺]_i content compared with those without LVH. [Mg²⁺]_i is reduced in both hypertriglyceridemic normotensive patients and hypertriglyceridemic essential hypertensive patients as compared to controls [62]. Magnesium supplementation has been shown to decrease blood pressure in several [63, 64], but not in all clinical studies [65]. In experimental model rats, [Mg²⁺]_i is lower in both striated muscle cells and vascular smooth muscle cells from spontaneously hypertensive rats than in those from normotensive Wistar Kyoto rats [66]. [Mg²⁺]_i may play a key role in modulating vascular tone or resistance.

Vascular disease

Vascular disease accounts for the majority of the clinical complications in DM. With regard to the vascular oxidative stress observed in type 1 and type 2 DM, Mg protects against endothelial injury due to oxidative stress [67] and increases the production of the endothelial vasodilator, i.e. prostacyclin [68]. Thus, in addition to being an important mediator of the actions of insulin/IGF-1, Mg has a direct role in preventing exaggerated vasoconstriction and growth/remodeling in diabetes and other states associated with the abnormalities of insulin action [69].

Nadler et al. [35] showed that Mg deficiency increases angiotensin II action and thromboxane synthesis in normal human subjects. These polypeptide hormones and prostaglandins may increase vasoconstrictive actions as well as platelet aggregations and a release of growth factors from blood vessels. The depletion of Mg supplementation in type 1 DM patients decrease serum total cholesterol, serum low-density lipoprotein cholesterol, and apolipoprotein B [46]. Mg deficit is linked to the development of atherosclerosis and Mg reduces the risk of developing atherosclerosis in rabbits [70].

Barbagallo et al. [71] reported that vitamin E, i.e. antioxidant, supplementation increases glutathione levels and [Mg²⁺]_i, which may thereby mediate the effects of reduced glutathione on glucose metabolism. The long-term administration of vitamin E improves the percent change of diameter of the brachial artery and [Mg²⁺]_i in patients with type 2 DM [72]. These effects of vitamin E may be mediated by a reduction in oxidative stress and the regulation of [Mg²⁺]_i.

Blood cellular components [Mg²⁺]_i and insulin resistance

Peripheral blood cells, such as erythrocytes, lymphocytes and platelets, have been used as a model for studying the relationship between [Mg²⁺]_i and insulin action.

Erythrocytes

Insulin-resistant patients have an impaired insulin-mediated erythrocyte Mg accumulation that correlates with a decrease in insulin sensitivity [73]. Hyperinsulinemic glucose clamp studies disclosed that the severity of the defect correlates with the glucose disposal in aged non-diabetic obese patients [73] and in patients with essential hypertension [74]. Zemva and Zemva [75] reported that Mg concentration in serum and erythrocytes was lower in normotensive obese persons than non-obese persons. Paolisso and Barbagallo reported a direct relationship between [Mg²⁺]_i membrane microviscosity and total body glucose metabolism [13]. Several studies have reported that lowering Mg concentrations induces an increase in erythrocyte plasma membrane microviscosity in essential hypertension [74].

Lymphocytes

The rate constant of plasma glucose disappearance after insulin injection (insulin tolerance test) is correlated with [Mg²⁺]_i of the lymphocyte and body mass index in essential hypertension [76]. The mean lymphocyte [Mg²⁺]_i measured by a fluorescent probe in type 2 DM patients is not significantly lower than in normal subjects. However, the sudden addition of insulin caused a rapid rise in [Mg²⁺]_i in the normal subjects that was significantly greater than the rise observed in type 2 DM subjects [76]. Insulin resistance and Mg depletion may result in a vicious cycle of worsening insulin resistance in type 2 DM patients [17].

Platelets

Human platelets have insulin receptors, and insulin can mediate [Mg²⁺]_i in platelets [5, 6]. During oral glucose tolerance tests a reduction in plasma Mg and an increase in erythrocyte and platelet [Mg²⁺]_i in controls were observed, whereas its reduction in plasma, erythrocytes, and platelets was observed in both normotensive and hypertensive obese subjects [77]. The impaired ability of insulin to increase [Mg²⁺]_i in obesity could also play a role.

Under the basal condition, the platelet [Mg²⁺]_i of both type 1 and type 2 diabetic children was significantly lower than the values in nondiabetic control subjects (377 ± 62 µM, 332 ± 66 µM vs 594 ± 6 2 µM, p < 0.05) [11]. After the stimulation of platelets with insulin, the increased percentage over the resting [Mg²⁺]_i was higher in the type 2 DM than in the control (98 ± 18% vs 221 ± 51%, p < 0.05). The platelets of the type 2 DM have the capacity of reactivity for insulin.

Intracellular magnesium signaling

Lostroh and Krahl suggested Mg as a second messenger for insulin action [7]. We previously reviewed the possibility that Mg can act as a second messenger [78]. The data of a fine regulation of intracellular calcium ([Ca²⁺]_i) as well as [Mg²⁺]_i suggests that the role of Mg as a cellular regulator may be physiologically relevant [79, 80]. A decrease in [Mg²⁺]_i potentially limits the role of Mg in vital cellular processes. [Mg²⁺]_i in a millimolar range is known to fit Michaelis-Menten Km values for many cellular enzyme systems. For example, [Mg²⁺]_i is necessary for the activity of membrane-bound Na⁺/K⁺ ATPase. This enzyme is responsible for the maintenance of the transmembrane concentration gradients of both sodium and potassium and is a potential target enzyme for many hormones and growth factors [81]. The inhibition of Na⁺/K⁺ ATPase activity correlated with serum digoxin leads to a depletion of [Mg²⁺]_i and an increase in [Ca²⁺]_i [82].

It is generally known that Mg regulates the entrance and exit of Ca²⁺ in cells. Mg has been described as nature’s physiologic calcium blocker’ [83, 84]. Begum et al. reported that an “optimal intracellular free calcium level” is necessary for optimal insulin action in rat adipocytes [85]. Begum et al. [86] have shown that high [Ca²⁺]_i inhibits insulin-receptor dephosphorylation in adipocytes. Thus, in skeletal muscle and fat tissues, insulin resistance could be expected in the presence of increased [Ca²⁺]_i and suppressed [Mg²⁺]_i (figure 3). In hypertension, diabetes and obesity, the [Mg²⁺]_i deficiency is correlated with an excess of [Ca²⁺]_i [60, 87], causing a further activation of protein kinase C(PKC), which is a constitutive regulator of the insulin receptor [23, 88]. There is significant evidence that insulin resistance is induced by elevated PKC activity [89].

By stimulating calcium (Ca²⁺)-dependent potassium (K⁺) channels, [Mg²⁺]_i concentration has also been shown to be effective in modulating insulin action (mainly oxidative glucose metabolism), offsetting calcium-related excitation-contraction coupling and decreasing smooth muscle cell responsiveness to depolarizing stimuli [13]. Mg may exert a potent inhibition on Ca²⁺ channel activity and interact with Ca²⁺, which secondarily mediates insulin action. It is possible that the association between low [Mg²⁺]_i and insulin resistance is not primary but is related to abnormalities of other cations, such as Ca²⁺ [90-92].

Mg is a positive effector of inositol transport. A reduction in Mg concentration results in a significant decline in the rate of inositol transport in the promyeloid cell line HL60 [93]. Intracellular inositol depletion is a result of the reduction in the rate of innositol transport. Intracellular sorbitol with subsequent inositol depletion may develop diabetic complications (polyol theory) [94].

[Mg²⁺]_i may play a role of the second messenger for insulin action contributing to insulin resistance.

Thiazolidinediones

Thiazolidinediones, a new family of insulin-sensitizing agents, bind to the peroxisome proliferator-activated receptor gamma (PPAR-γ), which is one of the members of the steroid/thyroid hormone nuclear receptor superfamily of transcription factors involved in adipocyte differentiation and glucose and lipid homeostasis [95] (figure 2). Aside from activating the PPAR-γ receptor, these drugs have direct vascular actions (i) to block Ca²⁺ entry from the extracellular space by their effects on voltage-operated L-channels and arginine vasopressin-mediated Ca²⁺ channels [96, 97]; (ii) to block Ca²⁺ release from internal stores [2]; and (iii) to increase [Mg²⁺]_i [98, 99]. It was reported that a slightly different pattern of the subcellular distribution of [Mg²⁺]_i exists in vascular smooth muscle cells when analyzed by a fluorescent probe [79]. The nuclear area contains a higher concentration of [Mg²⁺]_i than the peripheral area. Thiazolidinediones increase [Mg²⁺]_i, activate glycolysis in hepatocytes and oppose intracellular actions of cyclic AMP [98]. Thus, the ionic effects of thiazolidinediones serve to offset the defects characteristic of the insulin resistant state [100].

Conclusion

[Mg²⁺]_i has been shown to be effective in modulating insulin action. [Mg²⁺]_i is decreased in insulin resistant states such as type 2 DM and hypertension. Suppressed [Mg²⁺]_i may result in defective tyrosine kinase and alter the function of the insulin receptor. Altered [Mg²⁺]_i may also lead to decreased cellular glucose utilization and thus promote peripheral insulin resistance with a postreceptor mechanism. [Mg²⁺]_i concentration may affect the process of insulin resistance (figure 3).

Acknowledgement

This work was supported by the Mami Mizutani Foundation. The authors thank Mr. Steven McNutt for editorial assistance.

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