Ca2+‐dependent cell surface protein phosphorylation may be involved in the initiation of DNA synthesis

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Abstract
Incubating T51B rat liver cells in Ca2+-deficient, serum-rich medium containing only 0.02 mM Ca2+ strikingly decreased the phosphorylation of several trypsin-removable cell surface proteins and arrested the cells in late G1 phase. Raising the Ca2+ concentration in the Ca2+ -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 Ca2+ -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 Ca2+ -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 Ca2+-dependent G1→S transition of normal cells. They also suggest that an alteration of this Ca2+-dependent protein kinase might be the reason for neoplastic cells being able to proliferate in the face of an external Ca2+ shortage that would stop the proliferation of normal cells.

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