It is prevailingly thought that estrogen signaling is not involved in development of estrogen receptor (ER)-negative breast cancer. However, there is evidence indicating that ovariectomy prevents the development of both ER-positive and -negative breast cancer, suggesting that estrogen signaling is involved in the development of ER-negative breast cancer. Previously, our laboratory cloned a variant of ER-α, ER-α36, and found that ER-α36 mediated nongenomic estrogen signaling and is highly expressed in ER-negative breast cancer cells. In this study, we found that ER-α36 was highly expressed in 10/12 cases of triple-negative breast cancer. We investigated the role of mitogenic estrogen signaling mediated by ER-α36 in malignant growth of triple-negative breast cancer MDA-MB-231 and MDA-MB-436 cells that express high levels of ER-α36 and found that these cells strongly responded to mitogenic estrogen signaling both in vitro and in vivo. Knockdown of ER-α36 expression in these cells using the small hairpin RNA method diminished their responsiveness to estrogen. ER-α36 physically interacted with the EGFR/Src/Shc complex and mediated estrogen-induced phosphorylation of epidermal growth factor receptor (EGFR) and Src. EGFR signaling activated ER-α36 transcription through an AP1 site in the ER-α36 promoter, and ER-α36 expression was able to stabilize EGFR protein. Our results, thus demonstrated that ER-α36 mediates nongenomic estrogen signaling through the EGFR/Src/ERK signaling pathway in ER-negative breast cancer cells and suggested that a subset of ER-negative breast tumors that expresses ER-α36, retains responsiveness to mitogenic estrogen signaling.

Abstract:  Management of patients at high risk for hereditary breast cancer (HBC) must critically assess its phenotypic and genotypic heterogeneity, particularly evidenced by the varying spectra of cancer sites that are integral to the respective HBC syndromes. Targeted management must consider their biology, pathology, and molecular genetics, all in concert with their respective carcinogenic pathways, as they may differ significantly from one breast cancer syndrome to the next. A striking example of management differences pertains to BRCA1 and BRCA2 mutation‐positive breast cancers wherein those with BRCA1 mutations are frequently estrogen receptor (ER)‐negative in contrast to BRCA2 mutations which are more frequently ER‐positive; therein, significant differences exist with respect to anti‐estrogen therapy which will be more amenable to BRCA2 versus BRCA1 mutation carriers manifesting breast cancer. In turn, tumors that are negative for ER, PR, and Her2‐neu, often referred to as “triple negative” tumors, may also harbor a unique basal‐like gene expression profile and are characterized by poor prognosis wherein endocrine and/or Her2‐neu‐targeted therapies are not effective treatment options. A further confounder pertains to the lifetime risk for ovarian cancer, which differs strikingly between BRCA1 mutation carriers, who show a 40–60% lifetime risk, and their BRCA2 counterparts, who carry a lifetime risk of approximately 12–15% for ovarian cancer. It is clear that as we learn more about the biology and the molecular aspects of hereditary forms of breast cancer, it will be compelling for the clinician to integrate this knowledge with pharmacologic, radiologic, and surgical treatment options for these high‐risk patients.