Insulin Induces Upregulation of Vascular AT 1 Receptor Gene Expression by Posttranscriptional Mechanisms

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Abstract
Background —An interaction of insulin with angiotensin II effects could be pathophysiologically important for the pathogenesis of atherosclerosis and hypertension. Methods and Results —We examined the effect of insulin on AT 1 receptor gene expression in cultured vascular smooth muscle cells (VSMCs). A 24-hour incubation with insulin (100 nmol/L) produced a 2-fold increase in AT 1 receptor density on VSMCs, as assessed by radioligand binding assays. This enhanced AT 1 receptor expression was caused by a time- and concentration-dependent upregulation of the AT 1 receptor mRNA levels, as assessed by Northern analysis. The maximal effect was detected after a 24-hour incubation of cells with 100 nmol/L insulin (270±20%). AT 1 receptor upregulation was caused by a stabilization of the AT 1 receptor mRNA, because the AT 1 receptor mRNA half-life was prolonged from 5 hours under basal conditions to 10 hours after insulin stimulation. In contrast, insulin had no influence on AT 1 receptor gene transcription, as assessed by nuclear run-on assays. The insulin-induced AT 1 receptor upregulation was followed by an increased functional response, because angiotensin II evoked a significantly elevated intracellular release of calcium in cells that were preincubated with 100 nmol/L insulin for 24 hours. The insulin-induced AT 1 receptor upregulation was dependent on tyrosine kinases, as assessed by experiments with the tyrosine kinase inhibitor genistein. Furthermore, experiments using the intracellular calcium chelator bis(2-amino-5-methylphenoxy)ethane- N , N , N ′, N ′-tetraacetic acid tetraacetoxymethyl ester suggest that intracellular calcium release may be involved in AT 1 receptor regulation. Conclusions —Insulin-induced upregulation of the AT 1 receptor by posttranscriptional mechanisms may explain the association of hyperinsulinemia with hypertension and arteriosclerosis, because activation of the AT 1 receptor plays a key role in the regulation of blood pressure and fluid homeostasis.