The exposure of carcinogen-initiated primary neonatal rat hepatocytes to tumor promoters modulates both the transcripts and the enzymatic activity of nuclear poly(ADP-ribose) polymerase.

Four tumor promoters, i.e. PB, TPA, NAF, and DDT, added singly to a calcium-deprived synthetic medium, elicited early and late mitogenic effects and concurrent surges of nuclear poly(ADP-ribose) polymerase (pADPRP) activity in primary neonatal rat hepatocytes mutagenized with an intra-uterine dose of DMN. These actions were fully abated by the pADPRP inhibitor 3-MBA. Conversely, EGF only acted as a full mitogen when medium's calcium was at physiological levels, and its effects could not be blocked by 3-MBA. The same tumor promoters, but not EGF, also evoked a swift and lingering amplification of pADPRP transcripts in DMN-initiated hepatocytes kept in low-calcium medium. Hence, a coordinated modulation of both pADPRP transcripts and activity by xenobiotics is likely to be involved in the clonal expansion of early preneoplastic hepatocytes.