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

2002, Archives of Biochemistry and Biophysics

The Cc and Ca isoforms of the cAMP-dependent protein kinase (PKA) share 83% identity including all critical catalytic and substrate-binding residues defined to date. Compared to Ca, Cc 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, Cc-mediated gene transcription regulation was compared with that of Ca in four cell lines using transient transfection/dual luciferase assays. As compared to Cc, Ca more efficiently activated a cAMP-response element (CRE)-regulated fragment of the human a-glycoprotein hormone promoter which was coupled to a firefly luciferase reporter gene (pGHa-fluc). This occurred in Cos7, Y1, and Kin8 adrenal cells by 23-, 6.5-, and 1.4-fold, respectively. In contrast, Cc, but not Ca, 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, pGHa-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 Cc. In all cell lines, cotransfection with a PDK1 expression plasmid enhanced the transcriptional activation of both Ca and Cc (1.5-to 3-fold), while a catalytically inactive PDK1 mutant (PDKÁKD) did not. These results suggest that both Ca and Cc can activate CRE-responsive genes; however, Ca does so with better efficiency than Cc. In contrast to Ca, Cc 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.