INSULIN AND SECRETAGOGUES DIFFERENTIALLY REGULATE FLUID-PHASE PINOCYTOSIS IN INSULIN-SECRETING BETA-CELLS

The physiological role of the beta-cell insulin receptor is unknown. T o evaluate a candidate function, the insulin regulation of fluid-phase pinocytosis was investigated in a clonal insulinoma cell line (beta T C6-F7) and, for comparison, also in Chinese hamster ovary cells transf ected with the human insulin receptor (CHO-T cells). In CHO-T cells, t he net rate of fluid-phase pinocytosis was rapidly increased 3-4-fold over the basal rate by 100 nM insulin, with half-maximal stimulation a t 2 nM insulin, as assayed by cellular uptake of horseradish peroxidas e from the medium, Wortmannin, an inhibitor of phosphatidylinositol (P I)-3-kinase, blocked insulin-stimulated pinocytosis with an IC50 of 7. 5 nM without affecting the basal rate of pinocytosis. In insulin-secre ting beta TC6-F7 cells, the secretagogues glucose and carbachol (at ma ximally effective concentrations of 15 mM and 0.5 mM respectively) aug mented fluid-phase pinocytosis 1.65-fold over the basal rate. Wortmann in also inhibited secretagogue-stimulated pinocytosis in these beta-ce lls with an IC50 of 7 nM but did not affect the basal rate of pinocyto sis measured in the absence of secretagogues. Wortmannin did not influ ence either basal or secretagogue-induced insulin secretion. Although these beta TC6-F7 cells have cell-surface insulin receptors, adding ex ogenous insulin or insulin-like growth factor 1 did not affect their r ate of fluid-phase pinocytosis, either in the absence or presence of s ecretagogues. From these observations, we conclude that: (1) in both i nsulin-secreting beta-cells and in conventional, insulin-responsive CH O-T cells, a common, wortmannin-sensitive reaction, which probably inv olves PI-3-kinase, regulates fluid-phase pinocytosis; (2) the insulin- receptor signal transduction pathway is dissociated from the regulatio n of fluid-phase pinocytosis in the insulin-secreting beta-cell line w e studied; and (3) the enhancement of fluid-phase pinocytosis associat ed with secretagogue-induced insulin release in beta TC6-F7 cells is n ot attributable to autocrine activation of beta-cell surface insulin r eceptors.