Estrogen receptor variant ER‐α36 facilitates estrogen signaling via EGFR in glioblastoma

Glioblastoma (GBM) is a deadly and common primary brain tumor. Poor prognosis is linked to high proliferation and cell heterogeneity. Sex differences may play a role in patient outcome. Previous studies showed that ER‐α36, a variant of the estrogen receptor (ER), mediated non‐genomic estrogen signaling and is highly expressed in many ER‐negative malignant tumors. ER‐α36 also associates with epidermal growth factor receptor (EGFR). The primary purpose of this study is to investigate the cross talk between ER‐α36 and EGFR in estrogen‐mediated GBM cell proliferation. Here, we showed that ER‐α36 was highly expressed and confirmed that ER‐α36 co‐labels with EGFR in human GBM samples using immunohistochemical techniques. We also investigated the mechanisms of estrogen‐induced proliferation in ER‐α‐negative cell lines. We found that GBM cells showed varying responsive to mitogenic estrogen signaling which correlated with ER‐α36 expression, and knockdown of ER‐α36 diminished the response. Exposure to estrogen also caused upregulation of cyclin protein expression in vitro. We also found that low concentration of estrogen promoted SRC‐Y‐416 and inhibited SRC‐Y‐527 phosphorylation, corresponding with activated SRC signaling. Inhibiting SRC or EGFR abolished estrogen‐induced mitogenic signaling, including cyclin expression and MAPK phosphorylation. Cumulatively, our results demonstrate that ER‐α36 promotes non‐genomic estrogen signaling via the EGFR/SRC/MAPK pathway in GBM. This may be important for the treatment of ER‐α‐negative GBMs that retain high level of ER‐α36, since estrogen may be a viable therapeutic target for these patients.