Differential regulation of prostaglandin F-2 alpha receptor isoforms by protein kinase C

Prostaglandin F-2 alpha receptors (FP) are G protein-coupled receptors that bind prostaglandin F-2 alpha (PGF(2 alpha)), resulting in the activation o f an inositol phosphate (IP) second messenger pathway. Alternative mRNA spl icing generates two FP receptor isoforms. These isoforms, designated FPA an d FPB, are otherwise identical except for their carboxyl termini. FPB is es sentially a truncated version of FPA that lacks the 46 carboxyl-terminal am ino acids, including four putative protein kinase C (PKC) phosphorylation s ites. Until now, functional differences between these FP receptor isoforms have not been identified. We now report that pretreatment with the PKC inhi bitor bisindolylmaleimide I enhanced PGF(2 alpha)-stimulated IP accumulatio n in transfected cells stably expressing the FPA isoform but not in cells s tably expressing the FPB isoform. Whole-cell phosphorylation experiments sh owed a strong agonist-dependent phosphorylation of the FPA isoform but litt le or no phosphorylation of the FPB. Pretreatment of cells with bisindolylm aleimide I decreased PGF(2 alpha)-stimulated phosphorylation of the FPA iso form consistent with a PKC-dependent phosphorylation. In vitro phosphorylat ion of an FPA carboxyl-terminal fusion protein by recombinant PKC alpha sho wed that the carboxyl terminus of the FPA is a substrate for PKC. These res ults suggest that PKC-dependent phosphorylation is responsible for differen tial regulation of second messenger signaling by FP prostanoid receptor iso forms.