Differential transcriptional regulation by the α- and γ-catalytic subunit isoforms of cAMP-dependent protein kinase

The Cγ and Cα isoforms of the cAMP-dependent protein kinase (PKA) share 83% identity including all critical catalytic and substrate-binding residues defined to date. Compared to Cα, Cγ has a different substrate specificity and a selective pseudosubstrate specificity, exhibiting inhibition by regulatory subunits, but not by the protein kinase inhibitor. In these studies, Cγ-mediated gene transcription regulation was compared with that of Cα in four cell lines using transient transfection/dual luciferase assays. As compared to Cγ, Cα more efficiently activated a cAMP-response element (CRE)-regulated fragment of the human α-glycoprotein hormone promoter which was coupled to a firefly luciferase reporter gene (pGHα-fluc). This occurred in Cos7, Y1, and Kin8 adrenal cells by 23-, 6.5-, and 1.4-fold, respectively. In contrast, Cγ, but not Cα, activated the Sp1RE-regulated herpes simplex virus thymidine kinase promoter which was coupled to a Renilla luciferase reporter (pTK-rluc). In Sp1-deficient Sf9 cells, pGHα-fluc expression was maintained for both isoforms, but cotransfection with an Sp1 expression plasmid was necessary and sufficient for activation of pTK-rluc expression by Cγ. In all cell lines, cotransfection with a PDK1 expression plasmid enhanced the transcriptional activation of both Cα and Cγ (1.5- to 3-fold), while a catalytically inactive PDK1 mutant (PDK·KD) did not. These results suggest that both Cα and Cγ can activate CRE-responsive genes; however, Cα does so with better efficiency than Cγ. In contrast to Cα, Cγ activates transcription of genes containing pTK-like Sp1RE sites. Activation of different C subunit isoforms can provide a means to diversify cAMP-mediated transcription, possibly affecting cell phenotype.