Effect of cations on the tyrosine kinase activity of the insulin receptor: inhibition by fluoride is magnesium dependent

Hypomagnesemia and Cardiovascular Risk in Type 2 Diabetes

Hypomagnesemia is tenfold more common in individuals with type 2 diabetes (T2D), compared to the healthy population. Factors that are involved in this high prevalence are low Mg2+ intake, gut microbiome composition, medication use and presumably genetics. Hypomagnesemia is associated with insulin resistance, which subsequently increases the risk to develop T2D or deteriorates glycaemic control in existing diabetes. Mg2+ supplementation decreases T2D associated features like dyslipidaemia and inflammation; which are important risk factors for cardiovascular disease (CVD). Epidemiological studies have shown an inverse association between serum Mg2+ and the risk to develop heart failure (HF), atrial fibrillation (AF) and microvascular disease in T2D. The potential protective effect of Mg2+ on HF and AF may be explained by reduced oxidative stress, fibrosis and electrical remodeling in the heart. In microvascular disease, Mg2+ reduces the detrimental effects of hyperglycemia and improves endothelial dysfunction. Though, clinical studies assessing the effect of long-term Mg2+ supplementation on CVD incidents are lacking and gaps remain on how Mg2+ may reduce CVD risk in T2D. Despite the high prevalence of hypomagnesemia in people with T2D, routine screening of Mg2+ deficiency to provide Mg2+ supplementation when needed is not implemented in clinical care as sufficient clinical evidence is lacking. In conclusion, hypomagnesemia is common in people with T2D and is both involved as cause, probably through molecular mechanisms leading to insulin resistance, and consequence and is prospectively associated with development of HF, AF and microvascular complications. Whether long-term supplementation of Mg2+ is beneficial, however, remains to be determined.

Magnesium upregulates insulin receptor and glucose transporter-4 in streptozotocin-nicotinamide-induced type-2 diabetic rats

Objective. We investigated the effects of magnesium supplementation on glucose tolerance, insulin sensitivity, oxidative stress as well as the concentration of insulin receptor and glucose transporter-4 in streptozotocin-nicotinamide induced type-2 diabetic (T2D) rats. Methods. Rats were divided into four groups designated as: 1) control (CTR); 2) diabetic untreated (DU); 3) diabetic treated with 1 mg of Mg/kg diet (Mg1-D); and 4) diabetic treated with 2 mg of Mg/kg diet (Mg2-D). T2D was induced with a single intraperitoneal (i.p.) injection of freshly prepared streptozotocin (55 mg/kg) aft er an initial i.p. injection of nicotinamide (120 mg/kg). Glucose tolerance, insulin sensitivity, lipid profile, malondialdehyde (MAD) and glutathione content, insulin receptors (INSR) and glucose transporter-4 (GLUT4), fasting insulin and glucose levels were measured, and insulin resistance index was calculated using the homeostatic model assessment of insulin resistance (HOMA-IR). Results. Magnesium supplementation improved glucose tolerance and lowered blood glucose levels almost to the normal range. We also recorded a noticeable increase in insulin sensitivity in Mg-D groups when compared with DU rats. Lipid perturbations associated T2D were significantly attenuated by magnesium supplementation. Fasting glucose level was comparable to control values in the Mg-D groups while the HOMA-IR index was significantly lower compared with the DU rats. Magnesium reduced MDA but increased glutathione concentrations compared with DU group. Moreover, INSR and GLUT4 levels were elevated following magnesium supplementation in T2D rats. Conclusion. These findings demonstrate that magnesium may mediate effective metabolic control by stimulating the antioxidant defense, and increased levels of INSR and GLUT4 in diabetic rats.

Determinants of hypomagnesemia in patients with type 2 diabetes mellitus

BACKGROUND

Hypomagnesemia (plasma magnesium (Mg²⁺) concentration <0.7 mmol/L) has been described in patients with type 2 diabetes. Polypharmacy is inevitable when treating a complex disease such as type 2 diabetes and could explain disturbances in the plasma Mg²⁺ concentration. In this study, we aimed to establish the extent of hypomagnesemia in a cohort of type 2 diabetes patients and to identify the determinants of plasma Mg²⁺ levels.

METHODS

Patient data and samples of 395 type 2 diabetes patients were investigated. Plasma Mg²⁺ concentrations were measured using a spectrophotometric assay. Using Pearson correlation analyses, variables were correlated to plasma Mg²⁺ levels. After excluding confounding variables, all parameters correlating (P < 0.1) with plasma Mg²⁺ were included in a stepwise backward regression model.

RESULTS

The mean plasma Mg²⁺ concentration in this cohort was 0.74 ± 0.10 mmol/L. In total, 121 patients (30.6%) suffered from hypomagnesemia. Both plasma triglyceride (r = -0.273, P < 0.001) and actual glucose levels (r = -0.231, P < 0.001) negatively correlated with the plasma Mg²⁺ concentration. Patients using metformin (n = 251, 62%), proton pump inhibitors (n = 179, 45%) or β-adrenergic receptor agonists (n = 31, 8%) displayed reduced plasma Mg²⁺ levels. Insulin use (n = 299, 76%) positively correlated with plasma Mg²⁺ levels. The model predicted (R²) 20% of all variance in the plasma Mg²⁺ concentration.

CONCLUSIONS

Hypomagnesemia is highly prevalent in type 2 diabetes patients. Plasma triglycerides and glucose levels are major determinants of the plasma Mg²⁺ concentration, whereas only a minor part (<10%) of hypomagnesemia can be explained by drug intake, excluding polypharmacy as a major cause for hypomagnesemia in type 2 diabetes.

Hypomagnesemia in Type 2 Diabetes: A Vicious Circle?

Over the past decades, hypomagnesemia (serum Mg(2+) <0.7 mmol/L) has been strongly associated with type 2 diabetes mellitus (T2DM). Patients with hypomagnesemia show a more rapid disease progression and have an increased risk for diabetes complications. Clinical studies demonstrate that T2DM patients with hypomagnesemia have reduced pancreatic β-cell activity and are more insulin resistant. Moreover, dietary Mg(2+) supplementation for patients with T2DM improves glucose metabolism and insulin sensitivity. Intracellular Mg(2+) regulates glucokinase, KATP channels, and L-type Ca(2+) channels in pancreatic β-cells, preceding insulin secretion. Moreover, insulin receptor autophosphorylation is dependent on intracellular Mg(2+) concentrations, making Mg(2+) a direct factor in the development of insulin resistance. Conversely, insulin is an important regulator of Mg(2+) homeostasis. In the kidney, insulin activates the renal Mg(2+) channel transient receptor potential melastatin type 6 that determines the final urinary Mg(2+) excretion. Consequently, patients with T2DM and hypomagnesemia enter a vicious circle in which hypomagnesemia causes insulin resistance and insulin resistance reduces serum Mg(2+) concentrations. This Perspective provides a systematic overview of the molecular mechanisms underlying the effects of Mg(2+) on insulin secretion and insulin signaling. In addition to providing a review of current knowledge, we provide novel directions for future research and identify previously neglected contributors to hypomagnesemia in T2DM.

Magnesium modifies the association between serum phosphate and the risk of progression to end-stage kidney disease in patients with non-diabetic chronic kidney disease

It is known that magnesium antagonizes phosphate-induced apoptosis of vascular smooth muscle cells and prevents vascular calcification. Here we tested whether magnesium can also counteract other pathological conditions where phosphate toxicity is involved, such as progression of chronic kidney disease (CKD). We explored how the link between the risk of CKD progression and hyperphosphatemia is modified by magnesium status. A post hoc analysis was run in 311 non-diabetic CKD patients who were divided into four groups according to the median values of serum magnesium and phosphate. During a median follow-up of 44 months, 135 patients developed end-stage kidney disease (ESKD). After adjustment for relevant clinical factors, patients in the lower magnesium-higher phosphate group were at a 2.07-fold (95% CI: 1.23-3.48) risk for incident ESKD and had a significantly faster decline in estimated glomerular filtration rate compared with those in the higher magnesium-higher phosphate group. There were no significant differences in the risk of these renal outcomes among the higher magnesium-higher phosphate group and both lower phosphate groups. Incubation of tubular epithelial cells in high phosphate and low magnesium medium in vitro increased apoptosis and the expression levels of profibrotic and proinflammatory cytokine; these changes were significantly suppressed by increasing magnesium concentration. Thus, magnesium may act protectively against phosphate-induced kidney injury.

Insulin action on polyunsaturated phosphatidic acid formation in rat brain: an "in vitro" model with synaptic endings from cerebral cortex and hippocampus

The highly efficient formation of phosphatidic acid from exogenous 1-stearoyl-2-arachidonoyl-sn-glycerol (SAG) in rat brain synaptic nerve endings (synaptosomes) from cerebral cortex and hippocampus is reported. Phosphatidic acid synthesized from SAG or 1,2-dipalmitoyl-sn-glycerol (DPG) was 17.5 or 2.5 times higher, respectively, than from endogenous synaptosomal diacylglycerides. Insulin increased diacylglycerol kinase (DAGK) action on endogenous substrate in synaptic terminals from hippocampus and cerebral cortex by 199 and 97%, respectively. Insulin preferentially increased SAG phosphorylation from hippocampal membranes. In CC synaptosomes insulin increased phosphatidic acid (PA) synthesis from SAG by 100% with respect to controls. Genistein (a tyrosine kinase inhibitor) inhibited this stimulatory insulin effect. Okadaic acid or cyclosporine, used as Ser/Threo protein phosphatase inhibitors, failed to increase insulin effect on PA formation. GTP gamma S and particularly NaF were potent stimulators of PA formation from polyunsaturated diacylglycerol but failed to increase this phosphorylation when added after 5 min of insulin exposure. GTP gamma S and NaF increased phosphatidylinositol 4,5 bisphosphate (PIP2) labeling with respect to controls when SAG was present. On the contrary, they decreased polyphosphoinositide labeling with respect to controls in the presence of DPG. Our results indicate that a DAGK type 3 (DAGKepsilon) which preferentially, but not selectively, utilizes 1-acyl-2-arachidonoyl-sn-glycerol and which could be associated with polyphosphoinositide resynthesis, participates in synaptic insulin signaling. GTP gamma S and NaF appear to be G protein activators related to insulin and the insulin receptor, both affecting the signaling mechanism that augments phosphatidic acid formation.

Regulation of heart insulin receptor tyrosine kinase activity by magnesium and spermine

Insulin action and aspects of the insulin-signaling pathway have been studied in the heart although the direct regulation of the heart's insulin receptor has not been explored. This study describes the first purification and characterization of the mammalian (rabbit, rat and bovine) heart insulin receptor. The rabbit heart IR showed maximum insulin binding of 18 microg/mg (approximately 1 mole insulin/mole (alpha2beta2) receptor) and a curvilinear Scatchard plot with a high affinity KD for insulin binding of approximately 4 nM at optimal pH (7.8) and NaCl concentration (150 mM). The insulin receptor tyrosine kinase activity was stimulated by insulin, Mg2+ (half-maximum response at approximately 5.6-10.6 nM and approximately 8.5 mM, respectively) and by the physiological polyamines, spermine and spermidine. The stimulation by Mg2+ and the polyamines occurred with and without insulin. These characteristics of the heart insulin receptor provide a mechanism for regulating the activity of the receptor's tyrosine kinase activity by the intracellular free Mg2+ concentration and the polyamines in the absence and presence of insulin.

Okadaic acid-sensitive activation of Maxi Cl(-) channels by triphenylethylene antioestrogens in C1300 mouse neuroblastoma cells

1. The regulation of Maxi Cl(-) channels by 17beta-oestradiol and non-steroidal triphenylethylene antioestrogens represents a rapid, non-classical effect of these compounds. In the present study we have investigated the signalling pathways used for the regulation of Maxi Cl(-) channel activity by oestrogens and antioestrogens in C1300 neuroblastoma cells. 2. Whole-cell Maxi Cl(-) currents were readily and reversibly activated by tamoxifen, toremifene and the membrane-impermeant ethyl-bromide tamoxifen, only when applied to the extracellular medium. 3. Pre-treatment of C1300 cells with oestrogen or cAMP prevented the antioestrogen-induced activation of Maxi Cl(-) channels. The inhibitory effect of 17beta-oestradiol and cAMP was abolished by the kinase inhibitor staurosporine. 4. Current activation was unaffected by the removal of intracellular Ca(2+) and Mg(2+), but was completely abolished in the presence of okadaic acid. These results are consistent with the participation of an okadaic acid-sensitive serine/threonine protein phosphatase in the activation of Maxi Cl(-) channels. However, neither oestrogen or antioestrogen treatment modified the total activity of the two major serine/threonine phosphatases, PP1 and PP2A, in C1300 cells. 5. Although the role of these Maxi Cl(-) channels remains unknown, our findings suggest strongly that their modulation by oestrogens and antioestrogens is linked to intracellular signalling pathways.

Synergistic interaction of magnesium and vanadate on glucose metabolism in diabetic rats

The effect of vanadate (V) alone, magnesium (Mg) alone, and the combination of Mg plus V (MgV) on insulin-mediated glucose disposal and glucose tolerance was investigated in normal and streptozotocin-induced diabetic rats. MgV, magnesium sulfate (MgSO4) and sodium metavanadate (NaV) were added to the drinking water of normal or diabetic rats (approximately 300 g) for 3 weeks. After 3 weeks of V treatment (both MgV and NaV), diabetic rats demonstrated a normal meal tolerance test without any increase in the plasma insulin response. Rats also received a euglycemic insulin clamp (12 mU/kg x min for 120 minutes) with 3-3H-glucose infusion to quantify total body glucose disposal, glycolysis (3H2O production), and glycogen synthesis (total body glucose disposal minus glycolysis). Total glucose disposal was decreased in diabetic versus control rats (29 +/- 2 v 35 +/- 2 mg/kg x min, P < .01) and returned to levels greater than the nondiabetic control values after MgV (41 +/- 2, P < .01). Supersensitivity to insulin was not observed in diabetic rats treated with NaV (34 +/- 1). Glycogen synthesis was increased by both MgV and NaV treatment (23 +/- 21, P < .01 and 18 +/- 1, P < .05 v 14 +/- 2 mg/kg x min) in diabetic rats. A small increase in glycolysis was observed in MgSO4 and MgV rats (18 +/- 1 and 18 +/- 1 v 16 +/- 1, P < .05). NaV alone had no effect on glycolysis. Thus, Mg has a synergistic effect with V to increase muscle glycogen synthesis in diabetic rats. In normal rats, neither MgSO4 nor NaV had any effect on glucose utilization. However, MgV increased glucose disposal to rates that were significantly higher than the rate in untreated control rats (P < .05). Based on these results, MgV is superior to either V alone or Mg alone in improving insulin sensitivity and glycogen synthesis in diabetic rats.