Estrogen receptor variant ER-α36 promotes tamoxifen agonist activity in glioblastoma cells

Glioblastoma (GBM) is a highly infiltrative and malignant primary brain tumor. Despite aggressive therapy, patients with GBM have a dismal prognosis with median survival of approximately 1 year. Tamoxifen (TAM), a selective estrogen receptor modulator (SERM), has been used to treat GBM for many years. ER-α36 is a novel variant of estrogen receptor-alpha66 (ER-α66) and can mediate cell proliferation through estrogen or anti-estrogen signaling in different cancer cells. Previously, we found that ER-α36 was highly expressed in GBM and was involved in the tamoxifen sensitivity of glioblastoma cells. However, the molecular mechanism responsible has not been well established. Here, we found that ER-α36 is highly expressed in glioblastoma specimens. We further found that ER-α36 knockdown increased sensitivity of glioblastoma U87 cells to TAM and decreased autophagy in these cells. However, ER-α36 overexpression decreased TAM sensitivity and induced autophagy. We also established TAM-resistant glioblastoma U251 cells by a long-term culture in TAM-containing medium and found that TAM-resistant cells showed a six-fold increase of ER-α36 mRNA expression and elevated basal autophagy. ER-α36 knockdown in these TAM-resistant cells restored TAM sensitivity. In addition, we recapitulated the physiologically relevant tumor microenvironment in an integrated microfluidic device, and U87 cells were treated with a gradient of TAM. We found that ER-α36 expression is consistent with autophagy protein P62 in a three-dimensional microenvironment. In summary, these results indicate that ER-α36 contributes to tamoxifen resistance in glioblastoma cells presumably through regulation of autophagy.