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 intracellular pool with two first order rate constants, one for internalization (k(in)) and the other for externalization (k(ex)). Insulin decreases k(in) by 2.8-fold and increases k(ex) by 3.3-fold, thus increasing the steady-state cell surface GLUT4 level by approximately 8-fold (Jhun, B. H., Rampal, A. L., Liu, H., Lachaal, M., and Jung, C. (1992) J. Biol. Chem. 267, 17710-17715). To gain an insight into the biochemical mechanisms that modulate these rate constants, we studied the effects upon them of okadaic acid (OKA), a phosphatase inhibitor that exerts a insulin-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, [³H]1,3-bis-(3-deoxy-D-glucopyranose-3-yloxy)-2-propyl 4-benzoylbenzoate, and the time course of labeled GLUT4 recycling was followed, the k(ex) was found to increase 2.8-fold upon maximal stimulation by OKA, whereas the k(in) remained unchanged within experimental error. These findings demonstrate that OKA mimics the insulin effect on only GLUT4 externalization and suggest that insulin stimulates GLUT4 externalization by increasing the phosphorylation state of a serine/threonine phosphoprotein, probably by inhibiting protein phosphatase 1 or 2A.