Caffeine blocks activation of cyclic AMP synthesis in Dictyostelium discoideum

Cyclic AMP (cAMP) appears to play multiple roles in the development of the social ameba Dictyostelium discoideum, serving as the chemoattractant mediating aggregation, and perhaps also regulating gene transcription in both early and late stages of differentiation. Progress in understanding the mechanism of activation of the adenylate cyclase in D. discoideum has been frustrated by the inability to obtain its activation in vitro. Also, the lack of defined cAMP-defective mutants has prevented a causal relationship from being established between cAMP levels and gene expression. As an alternative approach to studying the role of cAMP in D. discoideum development, we have sought a compound which inhibits cAMP synthesis in a reasonably specific manner. Here we identify caffeine as a compound which rapidly and reversibly inhibits cAMP-dependent activation of the adenylate cyclase without affecting either cell viability or intracellular levels of ATP or GTP. Using this drug, we show that cAMP synthesis is not required for the cAMP-stimulated decrease in lightscattering, the increase in cyclic GMP synthesis, or for chemotaxis toward cAMP. Studies of the mechanism of action of caffeine show that the drug does not act by inhibiting a cAMP phosphodiesterase, by inhibiting binding of cAMP to its receptor, by itself binding to a physiological adenosine receptor, or by directly inhibiting the adenylate cyclase. Instead, caffeine blocks the cAMP-dependent activation of the adenylate cyclase. Since similar effects are obtained with the cation ionophore A23187, it is possible that caffeine exerts its effect by altering intracellular calcium distribution.