Ca 2+ ‐dependent cell surface protein phosphorylation may be involved in the initiation of DNA synthesis

Incubating T51B rat liver cells in Ca 2+ ‐deficient, serum‐rich medium containing only 0.02 mM Ca 2+ strikingly decreased the phosphorylation of several trypsin‐removable cell surface proteins and arrested the cells in late G 1 phase. Raising the Ca 2+ concentration in the Ca 2+ ‐deficient medium from 0.02 mM to 0.5 mM or adding 80 nM TPA (12‐O‐tetradecanoyl‐phorbol‐13‐acetate), a, protein kinase C activator, stimulated the phosphorylation of a certain set of surface proteinS'Within 5 min and the initiation of DNA replication within the next 2 hr. By contrast, incubation in the same Ca 2+ ‐deficient medium, which does not affect the proliferation of neoplastic T51B‐261B cells, did not reduce the phosphorylation of cell surface proteins. These observations suggest that the stimulation of a Ca 2+ ‐dependent protein kinase (possibly protein kinase C) directly or indirectly phosphorylates certain cell surface proteins that might be part of the mechanism that triggers the Ca 2+ ‐dependent G 1 →S transition of normal cells. They also suggest that an alteration of this Ca 2+ ‐dependent protein kinase might be the reason for neoplastic cells being able to proliferate in the face of an external Ca 2+ shortage that would stop the proliferation of normal cells.