Sex hormone-induced mammary carcinogenesis in female Noble rats: Expression of TGF-β1 and its receptors, TGF-α, and EGF-R in mammary carcinogenesis

Abstract

We have established a Noble rat model to explore the mechanisms of hormonal mammary carcinogenesis, in which the role of androgen in promoting mammary carcinogenesis was highlighted. We have also established that stromal-epithelial interactions may be responsible for the promotional effects of testosterone in mammary carcinogenesis. Based on these understandings, in the present study we examined the expression of transforming growth factor beta-1 (TGF-β1) and its receptors (TGF-β RI, TGF-β RII), transforming growth factor alpha (TGF-α), and epidermal growth factor receptor (EGF-R) in 'pre-malignant' mammary glands treated with different protocols of sex hormones, as well as in mammary cancers. We observed that TGF-β1 was strongly expressed in most mammary tumors, whereas TGF-β RI and TGF-β RII were negative in most mammary tumor cells. The results from comparative study of 'pre-malignant' glands further showed that when the animals were treated with testosterone, either alone or in combination with 17β-estradiol, the mammary gland epithelial cells expressed high levels of TGF-β1. This over-expression of TGF-β1 can be blocked by flutamide, indicating that testosterone may be responsible for the expression of TGF-β1 in mammary glands. TGF-β RI and TGF-β RII were also expressed strongly in testosterone-treated mammary epithelial cells and only weakly detectable in 17β-estradiol treated and control mammary epithelial cells. Furthermore, TGF-β RI and TGF-β RII were also expressed in stromal cells, both in mammary tumors and in hormone-treated mammary glands. These observations indicate that the mechanism of testosterone in mammary carcinogenesis may be through its regulation of expression of TGF-β1 and its receptors. On the other hand, TGF-α was also expressed in all 39 mammary cancers, while only 81% of the cancers were EGF-R positive. TGF-α was also strongly expressed in stromal cells in all three experimental groups, but only moderately expressed in epithelial cells when treated with a combination of testosterone and 17β-estradiol. By contrast, EGF-R was strongly expressed in epithelial cells in the three experimental groups but negative in stromal cells. Flutamide or tamoxifen was unable to block the expression of TGF-α induced by the combined sex hormone treatment. However, they were effective in blocking the expression of TGF-α when the animals were treated with testosterone or 17β-estradiol alone, respectively. These results suggest that both testosterone and 17β-estradiol may be required for the over-expression of TGF-α in the mammary carcinogenesis induced by sex hormones. To our knowledge, this is the first experimental study to explore the regulation of TGF-β1, TGF-α, and their receptors by testosterone and 17β-estradiol in mammary carcinogenesis.