Suppression of human lymphocyte responsiveness by forskolin: reversal by 12-O-tetradecanoyl phorbol 13-acetate, diacylglycerol and ionomycin.

Forskolin, a potent activator of adenylate cyclase, was examined for its ability to alter human peripheral blood lymphocyte (HPBL) activation by both mitogens and antigens. We found that forskolin, at concentrations ranging from 0.04 to 25 micrograms/ml, caused a dose-dependent inhibition of HPBL responses to mitogens (concanavalin A, phytohemagglutinin, pokeweed mitogen and Staphylococcus aureus) and to recall antigens (tetanus toxoid and streptokinase/streptodornase). Inhibition was reflected in altered DNA, RNA and protein synthesis, including immunoglobulin production, and was not due to altered cell viability. Forskolin also induced a 19-fold increase in HPBL cyclic AMP levels at the same concentrations that suppressed HPBL function. To further define the mechanism(s) by which these elevations in cyclic AMP suppressed HPBL function, we tried to reverse these inhibitory effects with several agents; ascorbic acid, carbachol and levamisole had no effect. However, the phorbol ester, 12-O-tetradecanoyl phorbol 13-acetate, as well as L-alpha-1,2-dioleoyl diacylglycerol were able to completely reverse the inhibition. Furthermore, the Ca2+ ionophore, ionomycin, was also able to act synergistically with lower and less effective concentrations of 12-O-tetradecanoyl phorbol 13-acetate to reverse the inhibitory effects of forskolin. The data suggest that forskolin-induced elevations in cyclic AMP may lead to inhibition (or, more correctly, prevents the activation) of protein kinase C, presumably by inhibiting phospholipid turnover. Our studies suggest a linkage between these two opposing membrane-signal transduction systems with protein kinase C representing a pivotal point for various regulatory signals that ultimately control lymphocyte activation and function.