Revealing GRP75-Mediated Mitochondrial Stability That Promotes Breast Cancer Survival through Subcellular Anchoring of 70 kDa Heat Shock Protein Inhibitors

The 70 kDa heat shock protein (HSP70) isoforms play distinct roles in cancer, but their structural similarity limits isoform-specific inhibition. Here, we modified nonselective HSP70 inhibitor S1g-10 via a subcellular anchoring strategy to generate compounds 5 and 8, which selectively target mitochondrial-localized GRP75 and ER-localized GRP78, respectively. Both compounds modulated their intended targets in vivo. Additionally, GRP75, but not GRP78, was identified as a key regulator of mitochondrial membrane stability in breast cancer cells and maintains the stemness of breast cancer stem cells (BCSCs). Compared with normal cells, compound 5 exhibited selective toxicity against breast cancer cells and effectively suppressed the properties of BCSCs. This study provides novel chemical probes for studying specific isoforms of HSP70 and introduces a strategy to develop GRP75-targeted therapeutics for breast cancer.