CALCIUM AS A PERMISSIVE FACTOR BUT NOT AN INITIATION-FACTOR IN DNA-SYNTHESIS INDUCTION IN CULTURED RAT HEPATOCYTES BY THE PEROXISOME PROLIFERATOR CIPROFIBRATE

The non-genotoxic hepatocarcinogen and peroxisome proliferating agent, ciprofibrate, is a liver mitogen both in vivo and in cultured adult rat hepatocytes, but the mechanisms of its mitogenicity have not been elucidated. We previously observed that ciprofibrate rapidly increased hepatocyte free intracellular Ca2+ concentration ([Ca2+](i)), suggesting that this effect may play a role in the initiation of DNA synthesis. In the present study, we have identified a relationship between Ca2+ and the stimulation of hepatocyte DNA synthesis by ciprofibrate. Exposure of cultured adult rat hepatocytes to ciprofibrate (200 mu M) for 48 hr increased DNA synthesis by approximately 2-fold, and this response was attenuated in a Ca2+-deficient medium and by the Ca2+ channel blockers nicardipine and verapamil. To examine the relationship between the stimulation of hepatocyte DNA synthesis and increases in [Ca2+](i) by ciprofibrate, the intracelular Ca2+ chelator 5,5'-dimethyl-1,2-bis(2-aminophenoxyethane)-N,N,N',N'tetraacetic acid (dimethyl-BAPTA) was employed. Pretreatment of hepatocytes with dimethyl-BAPTA blocked ciprofibrate-induced [Ca2+](i) increase, but did not block ciprofibrate-induced hepatocyte DNA synthesis. Dimethyl-BAPTA was only effective in reducing ciprofibrate-induced DNA synthesis when present during the latter 24 hr of a 48-hr culture period. These data suggest that the early mobilization of hepatocyte [Ca2+](i) by ciprofibrate does not play an initiating role in the induction of hepatocyte DNA synthesis but rather may operate as a permissive factor for the entry of ciprofibrate-treated adult rat hepatocytes into S-phase.