Expression of estrogen receptor a variants and c-Fos in rat mammary gland and tumors

Breast cancer is currently the leading cause of cancer death among women worldwide. AP-1 (c-Fos/c-Jun) is associated with proliferation and survival, while cytoplasmic c-Fos activates phospholipid synthesis in cells induced to differentiate or grow. Estrogen receptor alpha 46 (ER alpha 46) is a splice variant of full-length ER alpha 66 and it is known that it has an inhibitory role in cancer cell growth. We investigated c-Fos localization, its relationship to AP-1, the non genomic pathway of phospho-Tyr537-ER alpha 66, as well as ER alpha 46 and ER alpha 66 isoforms in rat mammary gland development and carcinogenic transformation, and in mammary tumors. Female rats were injected: a) saline solution (Control mammary gland, CMG) or b) N-Nitroso-N-methyl urea (NMU), and samples were taken at 60, 90, 120 and 150 days of life. In addition, we analyzed hormone-dependent (HD) and independent (HI) tumors in ovariectomized rats, and intact tumors (IT) in non-ovariectomized ones. Our results show that, in CMG, nuclear c-Fos and proliferation decreased with age, AP-1 content was low, and nuclear ER alpha 46/ER alpha 66 ratio was higher than 1. In NMU, nuclear c-Fos and proliferation increased with carcinogenic transformation, AP-1 content was high, and nuclear ER alpha 46/ER alpha 66 was below 1. As tumor grade increased, proliferation, nuclear c-Fos and AP-1 expression were negatively associated to nuclear ER alpha 46/ER alpha 66 in IT. In HD, nuclear ER alpha 46/ER alpha 66, nuclear c-Fos expression, AP-1 levels and proliferation were lower than in HI, whose growth is estrogen-independent. Phospho-Tyr537-ER alpha 66 content and ERK1/2 activation were associated with AP-1 levels and cell proliferation. Collectively, our findings support the notion that variant detection and ER alpha 46/ER alpha 66 ratio could shed light on the role of ER alpha isoforms in mammary gland transformation and the behavior of ER alpha positive mammary tumors.