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

Abstract

Breast cancer is currently the leading cause of cancer death among women worldwide. AP-1 (c-Fos/c-Jun) isassociated with proliferation and survival, while cytoplasmic c-Fos activates phospholipid synthesis in cellsinduced to differentiate or grow. Estrogen receptor α 46 (ERα46) is a splice variant of full-length ERα66 and it isknown that it has an inhibitory role in cancer cell growth. We investigated c-Fos localization, its relationship toAP-1, the non genomic pathway of phospho-Tyr537-ERα66, as well as ERα46 and ERα66 isoforms in ratmammary gland development and carcinogenic transformation, and in mammary tumors. Female rats wereinjected: a) saline solution (Control mammary gland, CMG) or b) N-Nitroso-N-methyl urea (NMU), and sampleswere taken at 60, 90, 120 and 150 days of life. In addition, we analyzed hormone-dependent (HD) and in-dependent (HI) tumors in ovariectomized rats, and intact tumors (IT) in non-ovariectomized ones. Our resultsshow that, in CMG, nuclear c-Fos and proliferation decreased with age, AP-1 content was low, and nuclearERα46/ERα66 ratio was higher than 1. In NMU, nuclear c-Fos and proliferation increased with carcinogenictransformation, AP-1 content was high, and nuclear ERα46/ERα66 was below 1. As tumor grade increased,proliferation, nuclear c-Fos and AP-1 expression were negatively associated to nuclear ERα46/ERα66 in IT. InHD, nuclear ERα46/ERα66, nuclear c-Fos expression, AP-1 levels and proliferation were lower than in HI, whosegrowth is estrogen-independent. Phospho-Tyr537-ERα66 content and ERK1/2 activation were associated withAP-1 levels and cell proliferation. Collectively, our findings support the notion that variant detection andERα46/ERα66 ratio could shed light on the role of ERα isoforms in mammary gland transformation and thebehavior of ERα positive mammary tumors.