Chronic administration of 17 beta-estradiol to male Syrian hamsters for at least six months induces estrogen-dependent kidney tumors, which express high levels of c-fos mRNA compared to surrounding renal or control tissues. We have investigated the cellular localization of c-Fos oncoprotein in these tissues and studied the estrogenic regulation of c-fos mRNA in kidney and in H-301 cells, a cell line derived from primary hamster kidney tumors. Immunocytochemical analyses showed that levels of c-Fos protein were high in estrogen-dependent tumors. To study the early events in this model, hamsters were treated with 17 beta-estradiol for only 15 days. At this time, well before the appearance of tumors, c-Fos protein was concentrated in interstitial capillaries, arteries, and podocytes of the glomerulus, but not in renal tubular epithelium, and this pattern was not appreciably changed from controls. To study the regulation of c-fos that led to the altered expression in tumor vs. kidney, total RNA was isolated from kidneys of Syrian hamsters 3-48 h after treatment with single injections of either 17 beta- or 17 alpha-estradiol, 17 alpha-ethinylestradiol, the steroidal antiestrogen ICI 182,780, or 17 beta-estradiol plus ICI 182,780. Similar studies were carried out on H-301 cells grown in D-MEM/F-12 medium and charcoal-stripped serum. The increases in c-fos mRNA levels in H-301 cells but not kidney were elicited by classical estrogen receptor-mediated processes. In H-301 cells, c-fos levels were increased four-fold over controls after 3 h of 17 beta-estradiol treatment and this induction was suppressed by antiestrogen treatment. In hamster kidneys, c-fos levels were increased about two-fold by 17 beta-estradiol, but this induction was not affected by antiestrogen treatment. In H-301 cells but not in hamster kidneys, 17 alpha-ethinylestradiol induced c-fos. 17 alpha-estradiol was more potent than 17 beta-estradiol in the induction of c-fos in hamster kidney but was a poor inducer in H-301 cells. These studies are consistent with regulation of c-fos expression in H-301 tumor cells by an estrogen receptor-mediated mechanism. But in hamster kidney, c-fos expression does not appear to be regulated by receptor-mediated pathways. We propose that it may be induced by estrogen metabolites.

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