PREPUBERTAL GENISTEIN TREATMENT MODULATES TGF-ALPHA, EGF AND EGF-RECEPTOR MESSENGER-RNAS AND PROTEINS IN THE RAT MAMMARY-GLAND

We have previously demonstrated that exposure to genistein early in li fe protects against chemically-induced mammary cancer in rats. To gain insight into the mechanism of action, we have investigated the expres sion of the EGF-signaling pathway in the mammary glands of 21 and 50 d ay old rats treated on days 16, 18, and 20 postpartum with 500 mu g ge nistein/g body weight (B.W.) or an equivalent volume of the vehicle, d imethylsulfoxide (DMSO). This prepubertal genistein treatment up-regul ated TGF-alpha and the EGF-receptor (EGFR), but not EGF, in mammary te rminal ductal structures at day 21 postpartum. TGF-alpha, EGF and EGFR mRNA levels were similar in 21 day old control- and genistein-treated animals. At day 50 postpartum, mammary glands of genistein treated ra ts had more lobules and fewer terminal end buds (TEBs) and terminal du cts (TDs), i.e. they were more differentiated. TGF-alpha mRNA levels w ere down-regulated in TEB of proestrous and estrous females; EGF mRNA levels were down-regulated in TDs of proestrous, but not in estrous fe males; and EGFR mRNA levels were not altered in 50 day old proestrous or estrous female rats. EGFR immunostaining intensity was decreased in TEBs, but not in the total gland. EGF was increased in TEBs and TDs. TGF-alpha, EGF and EGFR were also observed in the stroma and fat pad, but genistein treatment did not alter the expression of these proteins in those locations. TGF-alpha, but not EGF and EGFR, immunostaining w as observed in cell nuclei (not modulated by genistein), suggesting th at this growth factor may act directly on nuclear events such as trans cription and DNA replication. For comparative purposes, prepubertal di ethylstilbestrol treatment was investigated and found to decrease EGFR immunostaining intensity and total IHC staining in all terminal ducta l structures. We conclude that prepubertal genistein treatment directl y stimulates TGF-alpha and EGFR to enhance mammary gland differentiati on. This programs the differentiated cells for a down-regulated EGF-si gnaling pathway in TEBs and TDs of adult mammary glands. Reduced EGFR expression at time of carcinogen exposure may account for genistein pr ogramming against mammary cancer. (C) 1998 Elsevier Science Ireland Lt d. All rights reserved.