Studies on the Mechanisms by Which Tumor Promoters Stimulate the Growth of Primary Neonatal Rat Hepatocytes

Abstract

A single exposure to a low concentration (10(-10) mol/L) of several tumor promoters, namely 12-O-tetradecanoylphorbol-13-acetate (TPA), phenobarbital (PB), nafenopin, saccharin, teleocidin, benzoyl peroxide, butylated hydroxytoluene (BHT), dichlorodiphenyltrichloroethane (DDT), lindane, clofibrate, and melittin significantly stimulated DNA synthesis of neonatal rat hepatocytes in 4-day-old primary cultures. These cultures were kept in low-calcium (0.01 mmol/L) HiWoBa2000 synthetic medium, thereby evoking a neoplastic phenotype in otherwise normal (i.e., non-initiated) cells. The simultaneous addition of a single dose of alpha-tocopherol (10(-4) mol/L) or selenous acid (10(-5) mol/L), just as that of exogenous superoxide dismutase (SOD) (4), together with each of the above agents fully suppressed the stimulation of hepatocytic DNA synthesis by the xenobiotics. Hence, these findings strengthen the view that superoxide anions (or some other oxidizing compounds) act as the common mediators of the mitogenic effects of various tumor promoters in hepatocytes. Inhibition kinetics studies, in which TPA in a single dose (10(-10) mol/L) was used as the paradigmatic compound together with several kinds of inhibitors of its activity showed that the early mitogenic effects of TPA, i.e., the commitment of quiescent (G0) hepatocytes and the reentry into active cycling of hepatocytes spontaneously poised at the G1/S boundary, required oxidizing compounds, arachidonate metabolism derivatives, and plasmalemmal calcium-binding sites and transmembrane calcium fluxes. Instead, a later TPAs effect, the flow into DNA synthesis of hepatocytes previously committed to cycle, was shown to be controlled by retinoid-modulable activities, by some product(s) of the lipoxygenase pathway, and again by plasmalemmal calcium-binding sites and transmembrane calcium fluxes. Such results reveal that in the neonatal rat hepatocyte the ability to answer to a single mitogenic stimulus and the metabolic pathways by which this answer is enacted depend upon the mitotic cycle setting of the hepatocytes at the moment of the experimental treatment.