Involvement of cAMP and cAMP-dependent protein kinase in the initiation of DNA synthesis by rat liver cells.

Addition of calcium to calcium-deprived cultures of T51B rat liver cells caused brief bursts of cAMP production and cAMP-dependent protein kinase activity which were followed almost immediately by a stimulation of DNA synthesis. PKInh, a specific polypeptide inhibitor of the catalytic subunits of cAMP-dependent protein kinases, inhibited the DNA-synthetic response to calcium addition without stopping the preceding cAMP surge. Addition of cAMP to the calciumdeprived cultures increased protein kinase activity and stimulated DNA synthesis, both of which were inhibited by PKInh. DNA synthesis in these cultures was not stimulated by adding type I cAMP-dependent protein kinase holoenzyme to the calcium-deficient medium, but it was stimulated by type II cAMP-dependent protein kinase holoenzyme or the catalytic subunit from either type I or type II holoenzyme. The stimulatory actions of the type II holoenzyme or the catalytic subunits were inhibited by PKInh. Thus, a burst of cAMP-dependent protein kinase activity was ultimately responsible for the stimulation of DNA synthesis in calcium-deprived T51B cells by calcium or cAMP and it might also be involved in the events leading to initiation of DNA synthesis in many, if not all, normally cycling cells.