Involvement of the Ca2+/phospholipid-dependent protein kinase in the G1 transit of T51B rat liver epithelial cells.

The G0----G1 and G1----S transitions (but not the intervening events) in the G1 phase of T51B rat liver epithelial cells in serum-stimulated confluent cultures required a high concentration of extracellular Ca2+ and were accompanied or immediately preceded by increases in the amount of EDTA-extractable protein kinase C, a Ca2+/phospholipid-dependent enzyme. Involvement of this Ca2+-dependent enzyme in the two Ca2+-dependent transitions was further indicated by the facts that 12-O-tetradecanoyl phorbol-13-acetate (TPA), a compound that stimulated protein kinase C from T51B cells even in the absence of Ca2+, enabled these cells to transit G1 in Ca2+-deficient medium, while a TPA analogue (4 alpha-phorbol-12, 13-didecanoate (4 alpha-PDD) that did not stimulate the enzyme in cell-free preparations did not promote G0----G1 or G1----S transit in Ca2+-deficient medium.