Forskolin potentiation of cholera toxin-stimulated cyclic AMP accumulation in intact C6-2B cells. Evidence for enhanced Gs-C coupling.

Forskolin directly stimulates adenylate cyclase activity and acts synergistically with receptor-mediated agonists which stimulate cyclic AMP production. We have previously observed that a 3-hr incubation of C6-2B rat astrocytoma cells with 6 nM cholera toxin in the presence of 1 microM forskolin results in cyclic AMP accumulation 9-fold greater than in the absence of forskolin. Since the action of cholera toxin is mediated by the stimulatory guanine nucleotide-binding regulatory component (GS) of the adenylate cyclase complex, we proposed that the mechanism by which forskolin augments hormone responses involves an enhanced coupling of GS with the adenylate cyclase catalytic component (C). In the present communication, we report the detailed characterization of the synergistic interaction between forskolin and cholera toxin as effectors of cyclic AMP accumulation in intact C6-2B cells. After a 3-hr incubation, maximal cholera toxin-stimulated cyclic AMP accumulation was 990 +/- 34 pmol/mg of protein. In the presence of 1 microM forskolin, the response to cholera toxin increased to 13,137 +/- 1,595 pmol of cyclic AMP/mg of protein. The half-maximally effective cholera toxin concentrations estimated by nonlinear least squares regression analysis determined in the absence or presence of 0.1 mM forskolin were 56.6 and 57.5 pM, respectively. The highly reproducible lag in forskolin-stimulated cyclic AMP accumulation in C6-2B cells was abolished by cholera toxin pretreatment, indicating a possible role for GS-associated GTPase in the mechanism of forskolin action. Cholera toxin treatment markedly augmented forskolin-stimulated cyclic AMP accumulation and shifted the forskolin concentration-response curve to the left approximately 1.5 log units. When C6-2B cells were treated for 1 min with 10 nM cholera toxin, the response to forskolin was significantly potentiated by 10 min. No significant increase in cellular cyclic AMP content in the absence of a forskolin challenge was apparent for up to 45 min. It appears that prior promotion of GS-C coupling by cholera toxin treatment enhances the ability of forskolin to stimulate cyclic AMP accumulation. Whether or not forskolin interacts (i.e., binds) exclusively to C remains to be proven. However, the actions of forskolin to stimulate cyclic AMP formation and potentiate agonist-stimulated cyclic AMP formation are modulated by the activity state of GS, and at least part of the response to forskolin is mediated by GS.