Stimulation of DNA synthesis and c-fos mRNA expression in primary rat hepatocytes by estrogens

The mechanism(s) of tumour promotion in liver by estrogens is not well understood although growth stimulation is known to be one important element of their action. As a basis for studying mechanisms of growth control by estrogens, effects of both natural and synthetic estrogens on DNA synthesis and protooncogene c-fos mRNA expression were examined in primary cultures of normal rat hepatocytes. 17beta-Estradiol (E(2)) alone was stimulatory and exhibited dramatic synergism with epidermal growth factor (EGF) in stimulating DNA synthesis. All estrogens tested (natural, synthetic, steroidal and non-steroidal) exhibited an ability to stimulate hepatocyte DNA synthesis. This appears to correlate with their ability to induce c-fos mRNA expression. In contrast to a non-estrogenic liver tumour promoter, phenobarbital, insulin is not permissive for the growth-stimulatory action of E(2). Dexamethasone, which is required for stimulation of DNA synthesis by the non-estrogenic tumour promoter alpha-hexachlorocyclohexane and tetradecanoylphorbol acetate, completely blocked E(2)-stimulated DNA synthesis. Such differential requirements for auxiliary factors suggests that estrogen and other non-estrogenic liver tumour promoters act via distinct mechanisms in stimulating hepatocyte DNA synthesis. E(2) alone had no effect, but when in combination with EGF significantly induced c-fos mRNA expression at early times in culture (maximal at 10 h in culture). Such findings, coupled with the observations that (i) E(2) and EGF were synergistic in growth stimulation, (ii) estrogen receptor levels are higher at early times in culture and (iii) the growth-stimulatory ability of E(2) is limited to 4-24 h in culture, support the notion that in hepatocytes E(2) acts via the estrogen receptor to transactivate c-fos expression (an interaction with EGF), which ultimately culminates in enhanced DNA synthesis. Dexamethasone did not block E(2)-induced c-fos gene expression, suggesting that it acts in a pathway(s) distal to activation of fos gene expression. The possible inhibitory mechanisms of action of dexamethasone on E(2)-stimulated DNA synthesis are discussed.