Pure apocrine carcinomas of the breast express ER-alpha 36, a novel alternatively spliced variant of human estrogen receptor-alpha (ER-alpha 66).

Abstract

Abstract Abstract #4035 Background: Pure apocrine carcinomas (PACs) are defined as tumors composed entirely of epithelium showing apocrine differentiation (large cells with prominent eosinophilic, flocculent cytoplasm with sharply defined borders and large nucleus with prominent macronucleolus). PACs do not express classic estrogen receptor alpha (ER-α66) or progesterone receptor (PR) but consistently over-expresses androgen receptor (AR). We have investigated expression of a novel alternatively spliced variant of ER-α66, named ER-α36, which we recently identified and cloned (Wang ZY et al, PNAS 2006;103: 9063) and demonstrated that it mediates membrane initiated steroid signaling (MISS). MISS is dependent on ER interaction with receptor tyrosine kinases, and we hence investigated co-expression of ER-α36 with EGFR and HER2 in PACs.
 Methods: 17 patients with breast PAC were studied for expression of ER-α66, PR, AR, HER-2, EGFR and ER-α36 using immunohistochemical methods. ER-α36-specific antibody was custom-made by the Alpha Diagnostic International (San Antonio, TX) against the unique 20 amino acids at the C-terminal of the ER-a36; other antibodies were from commercially available sources. Mammary carcinoma cell line (MDA-MB-231) that lacks expression of ER-α66 but expresses ER-α36 and EGFR, was used as a positive control.
 Results: All tumors showed characteristic steroid receptor expression profile of apocrine carcinomas (ERα66 and PR negative; AR strongly positive). ERα36 expression was detected in 16/17 cases (94%) in membranous and cytoplasmic distribution. Co-expression of ER-α36 with either one of the tyrosine kinase receptors (HER2 or EGFR) was detected in 16 cases (100% of ER-α36 positive PACs). In an experimental model (MDA-MB-231 cells), 17β-estradiol (E2) strongly induced rapid phosphorylation of the MAPK/ERK1/2, but had only a weak effect in ER-α36 siRNA knock-down MDA-MB-231 cells.
 Conclusion: Despite the lack of classic full length ER-alpha expression (ER-α66) in PACs, a novel alternatively splice variant (ER-α36) is commonly expressed in this aggressive tumor type. This receptor variant is capable of strong E2 signal transduction and responsible for an increased cell growth. This non-genomic estrogen signaling probably involves membranous interaction between a tyrosine kinase receptor (EGFR or HER2) and ER-α36, resulting in activation of mitogen activated protein kinase pathway. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 4035.