Insulin and secretagogues differentially regulate fluid-phase pinocytosis in insulin-secreting beta-cells.
The physiological role of the beta-cell insulin receptor is unknown. To evaluate a candidate function, the insulin regulation of fluid-phase pinocytosis was investigated in a clonal insulinoma cell line (beta TC6-F7) and, for comparison, also in Chinese hamster ovary cells transfected with the human insulin receptor (CHO-T cells). In CHO-T cells, the net rate of fluid-phase pinocytosis was rapidly increased 3-4-fold over the basal rate by 100 nM insulin, with half-maximal stimulation at 2 nM insulin, as assayed by cellular uptake of horseradish peroxidase from the medium. Wortmannin, an inhibitor of phosphatidylinositol (PI)-3-kinase, blocked insulin-stimulated pinocytosis with an IC50 of 7.5 nM without affecting the basal rate of pinocytosis. In insulin-secreting beta TC6-F7 cells, the secretagogues glucose and carbachol (at maximally effective concentrations of 15 mM and 0.5 mM respectively) augmented fluid-phase pinocytosis 1.65-fold over the basal rate. Wortmannin also inhibited secretagogue-stimulated pinocytosis in these beta-cells with an IC50 of 7 nM but did not affect the basal rate of pinocytosis measured in the absence of secretagogues. Wortmannin did not influence either basal or secretagogue-induced insulin secretion. Although these beta TC6-F7 cells have cell-surface insulin receptors, adding exogenous insulin or insulin-like growth factor 1 did not affect their rate of fluid-phase pinocytosis, either in the absence or presence of secretagogues. From these observations, we conclude that: (1) in both insulin-secreting beta-cells and in conventional, insulin-responsive CHO-T cells, a common, wortmannin-sensitive reaction, which probably involves PI-3-kinase, regulates fluid-phase pinocytosis; (2) the insulin-receptor signal transduction pathway is dissociated from the regulation of fluid-phase pinocytosis in the insulin-secreting beta-cell line we studied; and (3) the enhancement of fluid-phase pinocytosis associated with secretagogue-induced insulin release in beta TC6-F7 cells is not attributable to autocrine activation of beta-cell surface insulin receptors.
Xu
G
G
Department of Pathology and Laboratory Medicine, University of Pennsylvania School of Medicine, Philadelphia 19104, U.S.A.
Howland
J
J
Rothenberg
P L
PL
D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D016466:CHO Cells; D002217:Carbachol / Q000494:pharmacology; D002999:Clone Cells; D006224:Cricetinae; D005947:Glucose / Q000494:pharmacology; D006801:Humans; D007328:Insulin / Q000378:metabolism* / Q000494:pharmacology*; D007329:Insulin Antagonists / Q000494:pharmacology; D000078790:Insulin Secretion; D007340:Insulinoma; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D007700:Kinetics; D010190:Pancreatic Neoplasms; D010873:Pinocytosis / Q000187:drug effects / Q000502:physiology*; D011972:Receptor, Insulin / Q000096:biosynthesis / Q000502:physiology*; D011994:Recombinant Proteins / Q000378:metabolism; D014162:Transfection; D000077191:Wortmannin
D000730:Androstadienes; D007328:Insulin; D007329:Insulin Antagonists; D011994:Recombinant Proteins; D002217:Carbachol; D011972:Receptor, Insulin; D005947:Glucose; D000077191:Wortmannin