Promotion of estrogen-induced mammary gland carcinogenesis by androgen in the male Noble rat: probable mediation by steroid receptors

Both endogenous and exogenous estrogen exposure is associated with an increased breast cancer risk. In some studies, elevated serum testosterone levels have also been linked to an increased breast cancer risk. Estrogen alone or combined with progesterone induces high mammary tumor incidences in various strains of both male and female rats. Mammary gland ductal adenocarcinomas were induced after 17β-estradiol (E₂) and testosterone propionate (TP) treatment in male Noble rats. Tumor incidence was 100% after 8-9 months of treatment. Such neoplasms were not detected after either estrogen or androgen exposure alone within this time period. TP alone caused disruption of mammary gland ducts and proliferation of stromal tissue, while E₂ treatment alone induced both ductal epithelial growth and nodular atypical hyperplasia. To study the interaction of these hormones in mammary tumorigenesis, sex hormone receptors were characterized in mammary glands of Noble rats. Estrogen receptor-α (ER) was detected in age-matched, untreated mammary gland epithelium; in most early atypical hyperplastic lesions appearing after E₂ and E₂ + TP treatment and in E₂ + TP-induced mammary tumors. Two major ER putative isoforms, 116 and 120 kDa, were detected in E₂- and E₂ + TP-treated mammary glands, and in the induced tumors. A 54 kDa ER protein was found in untreated and TP-treated mammary glands and in the induced tumors. Both progesterone receptor-B (PR-B) and PR-A2, as well as androgen receptor-B (AR-B) and AR-A isoforms were markedly elevated in all E₂ + TP-induced mammary tumors. However, the levels of both PR and AR were very low in mammary glands of E₂- and E₂ + TP-treated male rats. Low and moderate levels of AR and PR, respectively, were detected in most atypical hyperplastic lesions induced by E₂- and E₂ + TP-treated mammary glands. These results suggest that androgens may interact with either AR or PR, and perhaps both receptors, in E₂ + TP-induced mammary glands and the induced tumors to effect the reduction in latency period, enhance tumor size, and increase incidence to 100%.