Molecular Pharmacology, Vol 16, 215-223, Copyright © 1979 by the American Society for Pharmacology and Experimental Therapeutics

Inhibitors of Microtubule Assembly Enhance Beta-adrenergic and Prostaglandin E₁-Stimulated Cyclic AMP Accumulation in S49 Lymphoma Cells

¹ Division of Clinical Pharmacology, Departments of Medicine and Pharmacology, Cardiovascular Research Institute, University of California, San Francisco, San Francisco, California 94143

We have used wild-type S49 lymphoma cells and several variant S49 clones to characterize the several-fold enhancement of beta-adrenergic- and PGE₁-stimulated accumulation of cyclic AMP produced by colchicine and vinblastine, two inhibitors of microtubule assembly. The effect of the inhibitors was dose- and time-dependent, increased maximal response without changing the EC₅₀ for isoproterenol, did not occur with lumicolchicine, and could not be attributed to inhibition of either phosphodiesterase activity or cellular extrusion of cyclic AMP. Colchicine did not alter the extent to which S49 cells became refractory to isoproterenol. The enhancement of beta-adrenergic-stimulated cyclic AMP accumulation by colchicine in an S49 variant having a decreased number of beta-adrenergic receptors was decreased relative to wild type cells, whereas a variant lacking cyclic AMP-dependent protein kinase activity responded to colchicine as did wild-type. In S49 variants having altered coupling between adenylate cyclase and receptors for PGE₁ and beta-adrenergic amines or absent adenylate cyclase activity in response to hormonal effectors, neither isoproterenol nor PGE₁ stimulated cyclic AMP in the presence of colchicine. Assays of adenylate cyclase activity either in S49 plasma membranes incubated with colchicine or in homogenates prepared from colchicine-treated cells failed to show enhancement of hormonal stimulation of the enzyme. We conclude that vinblastine and colchicine enhance cyclic AMP accumulation by increasing activation of adenylate cyclase, but only in intact S49 cells. Our findings imply that these agents act on cellular structures, presumably microtubules, that normally inhibit adenylate cyclase and that colchicine and vinblastine do not interact directly with plasma membrane sites regulating adenylate cyclase. These data suggest that microtubules regulate events involved in coupling receptor occupancy to activation of adenylate cyclase in intact S49 cells.

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ACKNOWLEDGMENTS We would like to thank Gary Johnson, Mary Gleason, Harvey Kaslow, Shirley Clift and Nurit Kaiser for assistance and advice on various assays used in this study.

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