The Sulfonylurea Drug, Glimepiride, Stimulates Glucose Transport, Glucose Transporter Translocation, and Dephosphorylation in Insulin-Resistant Rat Adipocytes In Vitro

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Abstract
Sulfonylurea drugs are widely used in the therapy of NIDDM. The improvement of glucose tolerance after long-term treatment of NIDDM patients with the drug can be explained by stimulation of glucose utilization in peripheral tissues that are characterized by insulin resistance in these patients. We studied whether the novel sulfonylurea drug, glimepiride, stimulates glucose transport into isolated insulin-resistant rat adipocytes. After long-term incubation of the cells in primary culture with high concentrations of glucose, glutamine, and insulin, stimulation of glucose transport by insulin was significantly reduced both with respect to maximal responsiveness (65% decrease of Vmax) and sensitivity (2.6-fold increase of ED50) compared with adipocytes cultured in medium containing a low concentration of glucose and no insulin. This reflects insulin resistance of glucose transport. In contrast, both responsiveness and sensitivity of glucose transport toward stimulation by glimepiride were only marginally reduced in insulin-resistant adipocytes (15% decrease of Vmax; 1.2-fold increase of ED50) versus control cells. Glimepiride, in combination with glucose and glutamine during the primary culture, caused desensitization of the glucose transport system toward stimulation by insulin, but to a lesser degree than insulin itself (50% reduction of Vmax; ninefold increase of ED50). Again, the maximal responsiveness and sensitivity of glucose transport toward stimulation by glimepiride were only slightly diminished. The presence of glimepiride during primary culture did not antagonize the induction of insulin resistance of glucose transport. The stimulation of glucose transport in insulin-resistant adipocytes by glimepiride is caused by translocation of glucose transporters from low-density microsomes to plasma membranes as demonstrated by subcellular fractionation and immunoblotting with anti-GLUT1 and anti-GLUT4 antibodies. Immunoprecipitation of GLUT4 from 32Pi- and [35S]methionine-labeled adipocytes revealed that the insulin resistance of GLUT4 translocation is accompanied by increased (three- to fourfold) phosphorylation of GLUT4 in both low-density microsomes and plasma membranes. Short-term treatment of desensitized adipocytes with glimepiride or insulin reduced GLUT4 phosphorylation by ∼70 and 25%, respectively, in both fractions. We conclude that glimepiride activates glucose transport by stimulation of GLUT1 and GLUT4 translocation in rat adipocytes via interference at a site downstream of the putative molecular defect in the signaling cascade between the insulin receptor and the glucose transport system induced by high concentrations of glucose and insulin. The molecular site of glimepiride action is related to GLUT4 phosphorylation/dephosphorylation, which may regulate glucose transporter activity and translocation. These in vitro findings implicate an additional mode of sulfonylurea action in the improvement of glucose tolerance of NIDDM patients.

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