ER-α36, a variant of ER-α, is the estrogen receptor that mediates mitogenic estrogen signaling in breast cancer cells.

Abstract

Abstract Abstract #18 The role of endogenous estrogens in breast cancer etiology has been well established. Dysregulated estrogen signaling increases the rate of cell proliferation and thus the risk of breast cancer. However, despite widespread agreement that estrogens are involved in breast cancer etiology, the molecular mechanisms of estrogen action, especially those by which estrogen signaling stimulates mammary tumorigenesis and breast cancer progression have not been well established.
 Recently, we have identified and cloned a 36-kDa novel isoform of estrogen receptor α (ER-α36) that is generated from a promoter located in the first intron of the original 66-kDa ER-α (ER-α66) gene. The ER-α36 differs from the ER-α66 by lacking both transcriptional activation domains (AF-1 and AF-2) but retains the DNA-binding, dimerization and most of the ligand-binding domains. ER-α36 is mainly expressed on the plasma membrane and mediates membrane-initiated estrogen signaling that activates the MAPK/ERK pathway and stimulates cell proliferation. ER-α36 inhibits the genomic estrogen signaling mediated by the AF1 and AF2 domains of ER-α66 and ligand-dependent and -independent transactivation activities of ER-β. We designed and constructed shRNA expression vectors to express shRNAs specific for ER-α36 or ER-α66. We found that ER-positive breast cancer cells MCF7 and T47D expressing ER-α36 shRNA that successfully knockdown about 90% ER-α36 expression failed to respond to estrogen stimulation. However, estrogen strongly stimulated cell growth in MCF7 and T47D cells with ER-α66 knockdown. We found that some estrogen responsive genes such as c-Myc, CyclinD1 and progesterone receptor (PR) were induced through the extra-nuclear estrogen signaling pathway while pS2 was induced by the nuclear pathway. Our results for the first time demonstrated that ER-α36, not the original ER-α36 is the estrogen receptor that actually mediates estrogen-stimulated cell proliferation in breast cancer cells.
 Among 800 cases of human breast cancer specimens examined, ER-α36 is expressed in most of the breast cancer specimens, and even in 40% of ER-negative breast cancer cases that lack expression of ER-α66. In addition, ER-α36 expressing patients responded poorly to anti-estrogen Tamoxifen treatment. In vitro study revealed that breast cancer cells that highly express ER-α36 exhibited estrogen hypersensitivity and Tamoxifen resistance, consistent with the clinical evidence that breast cancers expressing ER-α36 exhibit higher degree of malignancy and poorer survival rate.
 Taken together, our results demonstrated that ER-α36 is a novel player in mitogenic estrogen signaling that plays an important role in estrogen stimulated mammary carcinogenesis. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 18.