GROWTH-STIMULATION FOLLOWED BY GROWTH-INHIBITION IN LIVERS OF FEMALE RATS TREATED WITH ETHINYL ESTRADIOL

Ethinyl estradiol (EE) is a strong promoter of hepatocarcinogenesis in female rats. A common effect shared by many non-genotoxic hepatic pro moters is the stimulation of hyperplastic growth. However, studies by others with several hepatic promoters have shown that following an ini tial, transient increase in liver growth, continued exposure causes an inhibition in basal and/or induced growth. We have shown that EE also causes an initial, transient increase in hepatocyte proliferation (Ca rcinogenesis, 7, 2007-20014, 1986). The objective of the investigation reported here was to determine whether chronic EE treatment also beco mes inhibitory to basal and/or induced liver growth. Female Lewis rats were treated with EE at 2.5 and 5.0 mu g/rat/day using time-release t ablets. Seven days prior to sacrifice, the rats were implanted with os motic minipumps containing bromodeoxyuridine (BrdU) to allow the cumul ative labeling of replicating hepatocytes, At sacrifice, liver tissue was fixed, sectioned and the percent labeled hepatocyte nuclei determi ned by immunohistochemistry. The results of these experiments revealed that, as expected, during the first 7 days of treatment, EE increased hepatocyte proliferation. However, after 28 and 42 days of EE treatme nt, the basal level of liver growth was dramatically inhibited. Thus, hepatocyte nuclear labeling indices, compared to controls, were reduce d by 72 and 88% after 28 and 42 days of treatment respectively. An ana lysis of I-125-labeled EGF binding to isolated liver membranes reveale d that EGF receptor levels decreased during the initial period of grow th, but had returned to control levels by day 21 when replication had ceased. In another experiment, rats were treated with EE at 5.0 mu g/r at/day for 21 days; control rats received placebo time-release tablets . At the end of that time, the rats were surgically partially hepatect omized and the level of subsequent regenerative DNA synthesis was dete rmined using [H-3]thymidine administered 2 h prior to sacrifice 24, 48 , 72 and 96 h later. The results showed that EE caused an inhibition o f regenerative growth. While at each time point the level of DNA synth esis in EE-treated rats was not significantly less than in correspondi ng controls, statistical analysis indicated that overall, EE caused a significant reduction in liver growth. The results of these studies de monstrate that chronic EE treatment leads to the appearance of a mitos uppressed state in the liver which is characterized by reduced cell tu rnover and decreased growth responsiveness. These results indicate tha t ethinyl estradiol should be added to the list of promoters of hepato carcinogenesis which, upon continued treatment, subsequently cause mit osuppressive effects, Furthermore, together with the findings of other s with other hepatic promoters, our results illustrate that initial pr oliferative effects may have little bearing on promoting potential. It now becomes important to determine whether promotion by EE actually i nvolves differential mitoinhibitory effects in non-initiated versus in itiated hepatocytes.