Regulation of glucokinase and proinsulin gene expression and insulin secretion in RIN-m5F cells by dexamethasone, retinoic acid, and thyroid hormone.

In RIN-5mF cells, treatment with either dexamethasone or retinoic acid resulted in increased glucokinase activity. Treatment with dexamethasone (50 and 500 nM) increased glucokinase activity 140% and 260%, respectively, whereas treatment with retinoic acid (100 and 1000 nM) increased glucokinase activity by 50% and 80%, respectively. An additive effect was observed when 1000 nM retinoic acid was added to either 50 or 500 nM dexamethasone. Treatment with either of these factors resulted in increased glucokinase mRNA levels. In contrast, treatment with thyroid hormone (T3; 0.2 and 10 nM) had no effect on glucokinase activity, but decreased glucokinase mRNA levels. T3 did not affect the response of either activity or mRNA levels to retinoic acid. Binding experiments revealed the presence of a single class of T3 nuclear receptors, with a Kd of 0.46 x 10(-10) and a binding capacity of 3.84 pmol/mg protein in these cells. This suggests that the changes produced by T3 may be due to a physiological effect mediated via nuclear receptors. Proinsulin mRNA levels were increased by retinoic acid (10-100 nM) and dexamethasone (5-500 nM), but decreased by T3 (0.04-10 nM). Insulin secretion in the absence or presence of glucose and potassium was increased about 50% in cells previously treated with 1000 nM retinoic acid. T3 (0.2 nM) did not affect insulin secretion in the absence or presence of glucose or in the presence of secretagogues (potassium and glyceraldehyde). Dexamethasone (50 nM) did not significantly increase insulin secretion in the absence or presence of glucose or potassium. In contrast, dexamethasone decreased glyceraldehyde-induced insulin secretion by 60% (P less than 0.05). We conclude that the glucokinase and proinsulin genes respond in parallel to dexamethasone and retinoic acid (both increased) and to T3 (both decreased).