The unfolded protein response regulator GRP78 as a novel target of BRCA1 for inhibiting stress-induced apoptosis

Abstract

The tumor suppressor BRCA1 is mutated in a high percentage of familial breast and ovarian cancer, but our understanding of its mechanisms of action remains incomplete. We previously identified glucose-regulated protein (GRP)-78, a critical regulator of the unfolded protein response (UPR), with altered expression in ovarian surface epithelial cells derived from women with a family history of ovarian and/or breast cancer compared to those without such history. We report here that GRP78 is a novel downstream target of BRCA1. We showed that reconstitution of BRCA1-deficient cells with wild-type BRCA1 repressed GRP78 expression, whereas expression of mutant BRCA1 gene or targeted inhibition of endogenous BRCA1 using small interfering RNA (siRNA) enhanced GRP78 expression. Knockdown of BRCA1 also led to induction of other components of UPR, such as GRP94 and CHOP. Consistent with BRCA1 inactivation, overexpression of GRP78 increased proliferation and decreased apoptosis in MCF-7 (breast carcinoma) and OVCAR-3 (ovarian carcinoma) cells. GRP78 overexpression also protected cells from endoplasmic reticulum (ER) stress- and chemotherapeutic drug-induced apoptosis. Knocking down GRP78 by siRNA sensitized these cells to stress-induced apoptosis. This effect was reduced when the expression of BRCA1 was simultaneously knockdown by siRNA, indicating that BRCA1 also negatively regulates GRP78-mediated tumor cell survival. These results reveal a novel mechanism for regulating GRP78 and suggest that enhancing UPR/ER stress response may be an important component of the BRCA1-associated tumorigenesis and chemoresistance. (This work is supported by Hong Kong Research Grants Council Grants HKU7484/04M to A.S.T.W.).