We have measured nuclear protein kinase activity during the prereplicative phase of rat liver regeneration. Total nuclear protein kinase activity increased significantly 15-18 h after partial hepatectomy, with the peak of activity occurring at 16 h. DEAE-Sephacel chromatography resolved nuclear protein kinase activity into two cAMP-independent (Ib and II) and two cAMP-dependent (Ia and III) protein kinases. Sixteen h after partial hepatectomy, there was a marked increase in the activities of

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... nuclear cAMP-dependent protein kinases and a decrease in the activity of nuclear cAMP-independent protein kinase II. Characterization of the two nuclear cAMP-dependent protein kinases revealed them to be identical with the cytosolic type I and II isozymes. Immunotitration of nuclear catalytic subunit and densitometric analysis of autoradiographs from 8-azido-[32P]cAMP-labeled nuclear RI revealed increases in both subunits 16 h afer partial hepatectomy. Concomitantly with the observed increase in nuclear protein kinase activity, we have observed an increase in the phosphorylation of histone H1 subspecies. Administration of the beta-adrenergic antagonist DL-propranolol, which has been shown to cause delays of equal duration in both the second phase of increased intracellular cAMP levels and the initiation of DNA synthesis (MacManus, J. P., Braceland, B. M., Youdale, T., and Whitfield, J. F. (1973) J. Cell. Physiol. 82, 157-164), results in an equivalent delay of increased nuclear protein kinase activity. Colchicine, which has previously been shown to prevent the onset of DNA synthesis (Walker, P. R., and Whitfield, J. F. (1978) Proc. Natl. Acad. Sci. U. S. A. 75, 1394-1398), also prevents the increased protein kinase activity normally observed 16 h after partial hepatectomy. We conclude that the onset of DNA synthesis in the regenerating rat liver is preceded by a cAMP-mediated translocation of type I and type II cAMP-dependent protein kinase to the nucleus and phosphorylative modification of histone H1 subspecies. The inhibitory effects of propranolol and colchicine suggest a common cAMP-mediated, colchicine-sensitive link between protein kinase translocation and the initiation of DNA synthesis.