Targeting cell surface GRP78-CD44v interaction suppresses cell migration in triple-negative breast cancer cells.

Triple-negative breast cancer (TNBC) is characterized by the absence of estrogen receptor (ER), progesterone receptor (PR), and HER-2 amplification, rendering it unresponsive to endocrine and HER2-targeted therapies. GRP78 (78 kDa glucose-regulated protein), a key endoplasmic reticulum (ER)-resident chaperone involved in protein folding and stress response, has been observed atypically localized on the cell surface of various cancer and stressed cell types, where it engages in non-canonical signaling and cellular functions. Cell surface GRP78 (csGRP78) is preferentially expressed in malignant cells relative to normal tissue, making it an attractive therapeutic target. In this study, we report that over 70% of MDA-MB-231 TNBC cells express csGRP78. Interestingly, MDA-MB-231 cells predominantly exhibit a distinct unipolar morphology, with csGRP78 prominently co-localizing with the variant isoform of CD44 (CD44v, containing variable exon 3) at the anterior region of the cell. Co-localizations of csGRP78 and CD44v were also observed in MDA-MB-231 tumor xenografts, supporting its relevance in vivo. Importantly, targeting csGRP78 with the monoclonal antibody 76-E6 downregulated CD44v expression, inhibited Src kinase signaling, disrupted cell morphology, and suppressed cell motility. We further mapped the epitope of GRP78 targeted by 76-E6. Together, our findings identify csGRP78 as a functional regulator of cell morphology and migration at least in part via a csGRP78-CD44v axis and underscore its potential as a therapeutic target in TNBC.