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The mechanism by which chronic treatment with HIV (human immunodeficiency virus)-1 protease inhibitors leads to a deterioration of glucose metabolism appears to involve insulin resistance, and may also involve impaired insulin secretion. Here we investigated the long-term effects of HIV-1 protease inhibitors on glucose-stimulated insulin secretion from beta cells and explored whether altered insulin secretion might be related to altered insulin signaling. INS-1 cells were incubated for 48 h with different concentrations of amprenavir, indinavir, nelfinavir, ritonavir or saquinavir, stimulated with 20 mM d-glucose, and insulin determined in the supernatant. To evaluate insulin signaling, cells were stimulated with 100 nM insulin for 2 min, and insulin-receptor substrate (IRS)-1, -2 and Akt phosphorylation determined. Incubation for 48 h with ritonavir, nelfinavir and saquinavir resulted in impaired glucose-induced insulin secretion at 2.5, 5 and 5 μM respectively, whereas amprenavir or indinavir had no effects even at 20 and 100 μM respectively. The impaired insulin secretion by ritonavir, nelfinavir and saquinavir was associated with decreased insulin-stimulated IRS-2 phosphorylation, and, for nelfinavir and saquinavir, with decreased insulin-stimulated IRS-1 and Thr³⁰⁸-Akt phosphorylation. No such effects on signaling were observed with amprenavir or indinavir. In conclusion, certain HIV-1 protease inhibitors, such as ritonavir, nelfinavir and saquinavir, not only induce peripheral insulin resistance, but also impair glucose-stimulated insulin secretion from beta cells. With respect to the long-term effect on beta-cell function there appear to be differences between the protease inhibitors that may be clinically relevant. Finally, these effects on insulin secretion after a 48 h incubation with protease inhibitor were associated with a reduction of the insulin-stimulated phosphorylation of insulin signaling parameters, particularly IRS-2, suggesting that protease inhibitor-induced alterations in the insulin signaling pathway may contribute to the impaired beta-cell function.

Recent studies suggest that high glucose concentrations impair insulin receptor phosphorylation and kinase activation in certain cell models. To examine whether such an effect of glucose can also be demonstrated in vivo, insulin receptor kinase activation was studied in erythrocytes from 11 patients with non-insulin-dependent diabetes (NIDDM), before and after reduction of hyperglycemia (from 14.6+/-1.6 to 6.6+/-0.5 mmol/l fasting plasma glucose within 8.6+/-0.6 days). For the measurement of receptor kinase activation, cells were incubated with insulin (0-400 nmol/l), solubilized and insulin receptors immobilized to microwells coated with anti-insulin receptor antibody. Kinase activity towards insulin receptor substrate-1 and insulin binding were then measured in these wells. Kinase activities (expressed as amol phosphate transferred per min and per fmol insulin binding activity) were similar before (2.4+/-0.4 and 32.2+/-2.0 amol/min per fmol with 0 and 400 nmol/l insulin, respectively) and after improvement of metabolic control (2.4+/-0.5 and 32.0+/-2.3 amol/min per fmol with 0 and 400 nmol/l insulin, respectively). Moreover, activities were also similar in 22 hyperglycemic patients with NIDDM (2.1+/-0.3 and 35.1+/-1.4 amol/min per fmol with 0 and 400 nmol/l insulin, respectively) compared with those in 21 non-diabetic control individuals (2.1+/-0.3 and 34.2+/-1.2 amol/min per fmol with 0 and 400 nmol/l insulin, respectively). We conclude that insulin activation of erythrocyte insulin receptor kinase is not impaired in NIDDM and is not influenced by hyperglycemia.