OKADAIC ACID STIMULATES GLUCOSE-TRANSPORT IN RAT ADIPOCYTES BY INCREASING THE EXTERNALIZATION RATE-CONSTANT OF GLUT4 RECYCLING

GLUT4, the major insulin-responsive glucose transporter isoform in rat adipocytes, rapidly recycles between the cell surface and an intracel lular pool with two first order rate constants, one for internalizatio n (k(in)) and the other for externalization (k(ex)). Insulin decreases k(in) by 2.8-fold and increases h(ex) by 3.3-fold, thus increasing th e steady-state cell surface GLUT4 level by approximately 8-fold (Jhun, B, H,, Rampal, k L., Liu, H., Lachaal, M., and Jung, C, (1992) J. Bio l. Chem. 267, 17710-17715), To gain an insight into the biochemical me chanisms that modulate these rate constants, we studied the effects up on them of okadaic acid (OKA), a phosphatase inhibitor that exerts a i nsulin-like effect on glucose transport in adipocytes, OKA stimulated 3-O-methylglucose transport maximally 3.1-fold and increased the cell surface GLUT4 level 3.4-fold. When adipocytes were pulse-labeled with an impermeant, covalently reactive glucose analog, [H-3]1,3-bis-(3-deo xy-D-glucopyranose-3-yloxy)-2 -propyl 4-benzoylbenzoate, and the time course of labeled CfLUT4 recycling was followed, the k(in) was found t o increase 2.8-fold upon maximal stimulation by OKA, whereas the k(in) , remained unchanged within experimental error, These findings demonst rate that ORA mimics the insulin effect on only GLUT4 externalization and suggest that insulin stimulates GLUT4 externalization by increasin g the phosphorylation state of a serine/threonine phosphoprotein, prob ably by inhibiting protein phosphatase 1 or 2A.