Down regulation of phorbol diester receptors by proteolytic degradation of protein kinase C in a cultured cell line of fetal rat skin keratinocytes.

Down regulation of phorbol diester receptors was studied with respect to proteolysis of protein kinase C, which is activated by Ca2+, phospholipids, and diacylglycerols and which binds to phorbol diesters. We used FRSK cells, a cell line derived from fetal rat skin keratinocytes, because in these cells specific binding of phorbol 12,13-dibutyrate decreased rapidly (50% decrease in 30 min). This decrease (down regulation) was inhibited by some protease inhibitors, such as N-tosyl-L-phenylalanine chloromethyl ketone (TPCK), N-p-tosyl-L-lysine chloromethyl ketone (TLCK), and leupeptin, but not by inhibitors of lysosomal hydrolases. On treatment with 12-O-tetradecanoylphorbol 13-acetate, protein kinase C was rapidly translocated from the cytosol to the membranes and then decreased. This decrease in protein kinase C was also inhibited by TPCK, TLCK, and leupeptin. The decrease in membrane activity of protein kinase C was associated with increase in cytosolic activity of a protein kinase that was smaller in molecular weight (Mr 40,000-60,000) than protein kinase C, did not depend on Ca2+/phosphatidylserine/diacylglycerol, and did not bind to phorbol 12,13-dibutyrate. These results indicate that down regulation of phorbol diester receptors is probably caused by nonlysosomal proteolysis of protein kinase C. The kinase formed by cleavage may be an active catalytic site of protein kinase C.