CELL-PROLIFERATION AND REGULATION OF NEGATIVE GROWTH-FACTORS IN MOUSE-LIVER FOCI

Foci of altered hepatocytes are preneoplastic lesions capable of progr essing to hepatocellular carcinomas, To characterize the growth of pre neoplastic hepatic lesions, size of hepatic foci was analyzed with reg ard to growth factor regulation and hepatocyte proliferation in focal and non-focal hepatocytes, Twelve-day-old female B6C3F1 mice were init iated with a single dose of the potent mutagen N-nitrosodiethylamine ( DEN) (5 mg/kg body weight), Beginning at 6 weeks of age, mice were exp osed for 16 weeks to 2038 p.p.m. unleaded gasoline (UG) vapor or 1 p.p .m. ethinyl estradiol (EE) in the diet, Analysis of hepatic foci demon strated that UG significantly increased, but EE significantly decrease d the size of DEN-initiated foci, Hepatic labeling index (LI), as meas ured by the incorporation of 5-bromo-2'-deoxyuridine, was similar in n on-focal hepatocytes at 16 weeks in all groups (0.4-0.8%) and greatly increased in hepatic foci, Hepatocyte LI was significantly increased i n DEN/UG foci (29%, n = 41) and significantly decreased in DEN/EE foci (6%, n = 23) relative to DEN/control focal hepatocytes (18%, n = 25), The mean LI of foci correlated with the focal size differences observ ed in the treatment groups, Immunohistochemical analysis with antibodi es directed to the negative growth regulator transforming growth facto r-beta1 (TGF-beta 1) demonstrated a consistent decrease of TGF-beta 1 in DEN/Ct and DEN/UG hepatic foci relative to non-lesion hepatocytes, Similar results were seen with mannose 6-phosphate/insulin-like growth factor-II receptor (M6P/IGF-II R), which facilitates activation of la tent TGF-beta 1. In contrast, only 50% of DEN/EE foci had decreased le vels of TGF-beta 1 and M6P/IGF-II R relative to non-focal hepatocytes, These data suggest that proliferative responses observed in hepatic f oci may be correlated with foci size, In contrast, chemically induced proliferative responses in non-focal hepatocytes after subchronic expo sure cannot necessarily be used to predict proliferative effects in pr eneoplastic cell populations, Furthermore, these studies suggest that hepatic foci may occur by M6P/IGF-II R enhancing activation of latent TGF-beta 1 in non-focal hepatocytes but not in the focal hepatocytes, thereby affording focal hepatocytes a selective growth advantage.