Prolyl isomerase PIN1 negatively regulates SGK1 stability to mediate tamoxifen resistance in breast cancer cells.

Background/Aim: Endocrine therapies that inhibit oestrogen receptor (ER)-alpha signaling are the most common and effective treatment for ER-alpha-positive breast cancer. The present study aimed to elucidate the mechanisms by which down-regulation of serum-and glucocorticoid-inducible protein kinase-1 (SGK1) expression confers tamoxifen resistance in breast cancer. Materials and Methods: SGK1 expression and the cytotoxic effects of combinatorial 4-hydroxy-tamoxifen (4-OHT) treatment with SGK1 overexpression were investigated by immunoblotting, bromodeoxyuridine incorporation, and soft agar assay. Results: We showed that PIN1 down-regulates SGK1 expression through interaction with and ubiquitination of SGK1. PIN1 silencing in MCF7 cells increased SGK1 expression. In tamoxifen-resistant human breast cancer, immunohistochemical staining analysis showed an inverse correlation between SGK1 expression and severity of tamoxifen resistance. Importantly, 4-OHT in combination with overexpression of SGK1 increased cleavage of poly-(ADP-ribose) polymerase and DNA fragmentation to inhibit clonogenic growth of tamoxifen-resistant MCF7 (TAMR-MCF7) cells. Conclusion: We suggest that PIN1-mediated SGK1 ubiquitination is a major regulator of tamoxifen-resistant breast cancer cell growth and survival.