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High glucose and insulin inhibit VSMC MKP-1 expression by blocking iNOS via p38 MAPK activation
1Diabetes Research Laboratory, Winthrop University Hospital, Mineola, New York 11501, USA. nbegum@winthrop.org
Abstract:
Our laboratory has recently demonstrated a role for the phosphatidylinositol 3-kinase-mediated inducible NO synthase (iNOS) signaling pathway in acute regulation of insulin-induced mitogen-activated protein phosphatase-1 (MKP-1) expression in primary cultures of rat aortic vascular smooth muscle cells (VSMCs) (N. Begum, L. Ragolia, M. McCarthy, and N. Duddy. J. Biol. Chem. 273: 25164-25170, 1998). We now show that prolonged treatment of VSMCs with 100 nM insulin and high glucose (25 mM) for 12-24 h, to mimic hyperinsulinemia and hyperglycemia, completely blocked MKP-1 mRNA and protein expression in response to subsequent acute insulin treatment. To understand the mechanism of insulin resistance induced by high glucose and insulin, we studied the regulation of iNOS protein induction in these cells. Both high glucose and chronic insulin treatment caused a marked impairment of iNOS induction in response to acute insulin. Blocking of signaling via the p38 mitogen-activated protein kinase (MAPK) pathway by prior treatment for 1 h with SB-203580, a synthetic p38 MAPK inhibitor, completely prevented the inhibition of iNOS induced by high glucose and insulin and restored MKP-1 induction to levels observed with acute insulin treatment. In contrast, PD-98059, a MEK inhibitor, had no effect. Furthermore, high glucose and chronic insulin treatment caused sustained p38 MAPK activation. We conclude 1) that chronic insulin and high glucose-induced insulin resistance is accompanied by marked reductions in both iNOS and MKP-1 inductions due to p38 MAPK activation that leads to excessive cell growth and 2) that p38 MAPK/extracellular signal-regulated kinase pathways regulate iNOS induction, thereby controlling MKP-1 expression, which in turn inactivates MAPKs as a feedback mechanism and inhibits cell growth.
Insights
Chronic high glucose and insulin cause insulin resistance by activating p38 MAPK, impairing inducible NO synthase (iNOS) and mitogen-activated protein phosphatase-1 (MKP-1) expression, leading to excessive cell growth.
Area of Science:
- Vascular smooth muscle cell biology
- Signal transduction pathways
- Endocrinology and metabolism
Background:
- The phosphatidylinositol 3-kinase-mediated inducible NO synthase (iNOS) pathway is crucial for acute insulin-induced mitogen-activated protein phosphatase-1 (MKP-1) expression in vascular smooth muscle cells (VSMCs).
- Hyperinsulinemia and hyperglycemia are common conditions associated with insulin resistance.
Purpose of the Study:
- To investigate the mechanism of insulin resistance induced by prolonged exposure to high glucose and insulin in VSMCs.
- To elucidate the role of the p38 mitogen-activated protein kinase (MAPK) pathway in mediating this insulin resistance.
Main Methods:
- Primary rat aortic VSMCs were treated with high glucose (25 mM) and insulin (100 nM) for 12-24 hours to mimic chronic conditions.
- Cells were subsequently treated with acute insulin, and the expression of MKP-1 and iNOS was measured.
- The effect of p38 MAPK inhibition (using SB-203580) and MEK inhibition (using PD-98059) on iNOS and MKP-1 induction was assessed.
- Sustained p38 MAPK activation was monitored under chronic treatment conditions.
Main Results:
- Prolonged treatment with high glucose and insulin completely blocked subsequent insulin-induced MKP-1 mRNA and protein expression.
- Both high glucose and chronic insulin impaired iNOS induction in response to acute insulin.
- Inhibition of p38 MAPK signaling prevented the impairment of iNOS and restored MKP-1 induction.
- Chronic exposure to high glucose and insulin led to sustained p38 MAPK activation.
Conclusions:
- Chronic insulin and high glucose induce insulin resistance in VSMCs, characterized by reduced iNOS and MKP-1 expression, mediated by p38 MAPK activation, which promotes excessive cell growth.
- The p38 MAPK pathway plays a critical role in regulating iNOS induction and subsequent MKP-1 expression, establishing a feedback loop that controls MAPK activity and cell proliferation.