Resistance to aromatase inhibitor is conferred by HER2 signaing in MCF-7 cells stably expresing aromatase

Abstract

We investigated the effect of HER2 overexpression on resistance to aromatase inhibitor letrozole in MCF-7 breast cancer cells stably expressing cellular aromatase (MCF-7/CA). Forced expression of HER2 in MCF-7/CA cells by infection with retroviral vector showed increased Akt and MAPK phosphorylation and phosphorylation of estrogen receptor (ER) a on Ser118 and Ser167. MCF-7/CA cells overexpressing HER2 (MCF-7/CA pBMN-HER2) showed more than 2 fold higher level of ER-mediated transcription activity upon androstenedione treatment as compared to vector control MCF-7/CA (MCF-7/CA pBMN-vec) cells and letrozole co-treatment did not effectively block androstenedione-induced transcription. Inhibition of Akt and MAPK by LY294002 and UO126 in MCF-7/CA pBMN-HER2 cells, respectively, resulted in the reduction of androstenedione-induced transcription activities comparable to those of MCF-7/CA pBMN-vec cells. MCF-7/CA pBMN-HER2 cells showed increased proliferation in the medium containing charcoal-stripped serum supplemented with androstenedione as compared to MCF-7/CA pBMN-vec cells. Moreover, co-treatment of letrozole could not efficiently abrogate androstenedione-mediated cell proliferation in MCF-7/CA pBMN-HER2 cells. MCF-7/CA pBMN-HER2 cells were also shown to be resistant to letrozole-induced apoptosis. Aromatase activities of both MCF-7/CA pBMN-HER2 and MCF-7/CA pBMN-vec cells were reduced to the similar basal levels by letrozole treatment, suggesting that resistance to letrozole is not conferred by the resistance of aromatase enzyme itself to letrozole. Chromatin immunopreciptation assays revealed that MCF-7/CA pBMN-HER2 cells showed ligand-independent association of estrogen receptor (ER) α, coactivators AIB1 and CBP to the promoter region of pS2 gene. Upon androstenedione treatment, there were increased associations of ERα, AIB1 and CBP with pS2 promoter in MCF-7/CA pBMN-HER2 cells as compared to MCF-7/CA pBMN-vec cells. These results suggest that recruitment of coactivator complexes to estrogen-responsive promoter by HER2 overexpression may play a role in developing letrozole resistance through upregulation of estrogen-responsive genes and increased cell survival and proliferation.