The presence of an unusual PKC isozyme profile in rat liver cells

We have previously shown that protein kinase C (PKC) is involved in the mitogenic response of T51B cells to epidermal growth factor. In fact, epidermal growth factor was an excellent mitogen, even after prolonged pretreatment of cells with TPA, suggesting that the PKC isoform implicated in proliferation is not down-regulated by 12-O-tetradecanoyl phorbol-13-acetate (TPA). We have now determined that the PKC isozymes -alpha, -beta, -delta, -epsilon, and -zeta are present in T51B cells. All five isoforms are associated with the plasma membrane and the cytoplasm and are either in or around the nucleus. PKC-beta I has a slightly different subcellular profile from that of the other isoforms in that it is clearly and strongly associated with the nuclear membrane. Also, a unique and novel pattern is obtained from immunoblots with anti-PKC-beta I. PKC-beta I is detected as a single band of 70 kDa in the cytosolic fraction and as a doublet of 65 and 77 kDa in the membrane fraction. PKC-alpha, -delta, and -epsilon were down-regulated by pretreatment of cells with TPA, while PKC-zeta was unaffected. Of particular interest was the fact that TPA did not down-regulate PKC-beta I. In fact, the amount of this isoform associated with the plasma membrane increased. These findings indicate that it is probably PKC-beta I that is involved in the mitogenic response of T51B cells to epidermal growth factor. Since PKC-zeta is also not down-regulated by TPA, the possible involvement of this isoform needs to be resolved.