Cyclic AMP has the ability to influence multiple events during B cell stimulation

Negative regulators of cellular proliferation are important in maintaining a balanced growth control. In this study we have examined the effects of the diterpene forskolin on various parameters of B cell activation. Forskolin is known to elevate intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels and thereby to influence B cell stimulation. We found that forskolin exerted an inhibitory effect on early as well as late events during stimulation of resting normal human B cells. Cells were activated either by antibodies to surface immunoglobulins (anti-mu), by the monoclonal antibody 1F5 reactive with the CD20 antigen or by 12-O-tetradecanoylphorbol 13-acetate. While anti-mu stimulation induces increased phosphatidylinositol (PI) turnover and Ca2+]i fluxes, the latter two reagents confer an activation of B cells independent of the PI/Ca2+ pathway. We found a clear inhibitory effect of forskolin on the anti-mu-induced PI turnover and Ca2+]i fluxes as well as on later parameters of cell activation. There was also a clear inhibition of G1 entry and DNA synthesis when PI/Ca2+-independent activation was employed, indicating that cAMP interferes with B lymphocyte stimulation in several ways. Importantly, forskolin maintained its inhibitory effect when added late after anti-mu stimulation, implying an effect also at multiple stages of activation. When examining the inhibitory effect of forskolin on neoplastic B cells, we found essentially no differences from the responses in normal cells.