Work carried out in our laboratory and by others has clearly demonstrated that the synthetic estrogens mestranol and ethinyl estgradiol (EE) promote hepatocarcino-genesis in rats and, we have been conducting experiments designed to probe the mechanisms of promotion by EE. An initial response to EE treatment in control rats is a significant increase in liver DNA synthesis which ceases by 21 days despite continuous treatment with the estrogen. Studies in progress have shown that hepatocyte growth factors appear in the serum and plasma of EE- treated rats. In vitro studies with cultured hepatocytes indicate the EE alone does not stimulate DNA synthesis. During carcinogenesis in initiated animals, upon chronic treatment with EE the initiated cells undergo clonal expansion to produce preneoplastic foci. Thus, it is likely that initiated hepatocytes are altered in their sensitivity to growth stimulators and/or the growth inhibitors involved in regulating normal hepatocyte growth in the presence of chronic EE treatment. We wish to begin to focus our studies on the nature of the alterations in growth regulation in the preneoplastic hepatocytes promoted by EE. We, therefore, wish to investigate the hypothesis that promotion by EE stimulates or alters the production of growth factors found both in the serum/plasma and in the hepatocytes themselves and that preneoplastic hepatocytes are altered in their growth responses to these factors. The specific aims of this proposal are: 1. to characterize the growth factor(s) induced by treatment of female rats with EE. 2. to determine whether transforming growth factor (TGF)-type alpha and beta growth factors are present and increased or decreased in the livers of rats during treatment with EE. 3. to determine whether isolated hepatocytes from GGT foci are altered in their response to various growth factors including those identified in Specific Aims 1 and 2 above. 4. to continue studies designed to determine whether the elevation of hepatic bile acids caused by EE treatment has a mechanistic role in promotion by EE. 5. to continue studies designed to extend our initial observations that tamoxifen (T) inhibits induction of liver DNA synthesis and promotion by EE but alone has promoting activity.