Modulation of the β-Adrenergic Receptor-Coupled Adenylate Cyclase by Chemical Inducers of Differentiation: Effects on β Receptors and the Inhibitory Regulatory Protein G_i

Abstract

Several drugs known to induce differentiation in tumor cells were analyzed for their effects on the β-adrenergic receptor-coupled adenylate cyclase system in two human carcinoma cell lines, HeLa and A431. Each of the drugs was tested alone or in combination with sodium butyrate (NaBu), a known inducer of this signal transduction system. Puromycine amino nucleoside (PMAN) caused the largest increase in β-adrenergic receptors in HeLa cells followed by hexamethylenebisacetamide (HMBA) whereas 5′-azacytidine (5AZC) was ineffective. In addition, PMAN but not the others acted together with NaBu to elevate receptor levels 12-fold over control values. In contrast, HMBA and 5AZC were much more effective on A431 cells, PMAN caused only a slight increase in β receptors and none of the drugs acted in concert with NaBu. The increase in β receptors was usually accompanied by a corresponding increase in isoproterenol-stimulated adenylate cyclase activity. These effects of the drugs appeared to require protein synthesis as they were blocked by cycloheximide. In addition, some of the drugs caused a substantial decrease in basal adenylate cyclase activity. This effect on basal activity was abolished in cells treated with pertussis toxin, which ADP-ribosylates the inhibitory GTP-binding protein, G_i. Both HeLa and A431 cells contained a 41 kDalton substrate for the toxin which corresponds to the α; subunit of G_i. The G_i subunit was ADP-ribosylated by the toxin to a similar extent in membranes from control and drug-treated cells. Thus, the drugs appear to induce quantitative changes in β-adrenergic receptors and qualitative changes in G_i which results in a highly responsive β-adrenergic-stimulated adenylate cyclase.