Abstract
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 Ca 2+ and were accompanied or immediately preceded by increases in the amount of EDTA-extractable protein kinase C, a Ca 2+/phospholipid-dependent enzyme. Involvement of this Ca 2+-dependent enzyme in the two Ca 2+-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 Ca 2+, enabled these cells to transit G1 in Ca 2+-deficient medium, while a TPA analogue (4α-phorbol-12, 13-didecanoate (4α-PDD) that did not stimulate the enzyme in cell-free preparations did not promote G0 → G1 or G1 → S transit in Ca 2+-deficient medium.
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