Lowered responsiveness of the catalyst of adenylyl cyclase to stimulation by GS in heterologous desensitization: a role for adenosine 3',5'-monophosphate-dependent phosphorylation.

Treatment of chick hepatocytes with glucagon results in homologous and heterologous desensitization of the receptor-stimulated adenylyl cyclase. The loci of postreceptor heterologous desensitization was studied. The addition of excess purified GS to glucagon-desensitized hepatocyte membranes did not fully restore fluoride stimulation of adenylyl cyclase, even though the absolute activity was increased at least 2-fold. Treatment of chick hepatocytes with 8-bromo-cAMP resulted in a similar reduction of fluoride stimulation that could not be restored by the addition of purified GS. When membranes from control and glucagon-treated hepatocytes were treated with purified catalytic subunit of protein kinase-A (PKA), fluoride stimulation was lowered in control, but not glucagon-treated, membranes. Treatment of membranes from S49 kin- lymphoma cells with PKA also resulted in decreased fluoride- and forskolin-stimulated adenylyl cyclase activity, but activity stimulated by Mn2+ was not altered. Since previous studies from our laboratory had shown that GS and G(i) are not substrates for protein kinase-A, it appears that the catalyst of adenylyl cyclase is the likely locus of modulation. To determine if both chick hepatocytes and S49 cells contain similar types of adenylyl cyclase that could account for the similar PKA regulatory properties, we used polymerase chain reaction-based techniques to identify GS-stimulated adenylyl cyclases present in these systems. The chick liver contains both type 5 and type 6 adenylyl cyclases, while S49 cells contain the type 6 enzyme. Type 5 and 6 adenylyl cyclases are members of one widely expressed subfamily of mammalian GS-responsive adenylyl cyclases and share a predicted PKA phosphorylation site in the central cytoplasmic loop. This site is not found in other known adenylyl cyclases (types 1-4), although the olfactory-specific type 3 enzyme has a predicted site nearby. These data indicate that one component of hormone-induced desensitization of the adenylyl cyclase system can be at the level of the catalyst, where PKA-mediated phosphorylation could result in lowered responsiveness. The types 5 and 6 adenylyl cyclases are likely candidates for such regulation.