Heat‐shock protein 27 (HSP27, HSPB1) is synthetic lethal to cells with oncogenic activation of MET, EGFR and BRAF

The small heat‐shock protein of 27 kDa (HSP27) is highly expressed in many cancers and is associated with aggressive tumour behaviour, metastasis, poor prognosis and resistance to chemotherapy. We aimed at assessing the role of HSP27 in modulating responses to target therapies. We selected several oncogene‐addicted cancer cell lines, which undergo either cell cycle blockade or cell death in response to agents that target the specific oncogene. Surprisingly, HSP27 suppression alone resulted in the apoptotic death of MET‐addicted EBC‐1 lung cancer cells, epidermal growth factor receptor (EGFR)‐addicted colorectal carcinoma (CRC) DiFi cells and BRAF‐addicted CRC COLO205 and OXCO‐1 and melanoma COLO741 cells, all of which also undergo death when treated with the specific targeted agent. In other cell lines, such as MET‐addicted gastric carcinoma MKN45 and EGFR‐addicted CRC SW48 lines, where oncogene inhibition only blocked proliferation, HSP27 knockdown made targeted agents switch from cytostatic to cytotoxic activity. Mechanistically, the more the cells were susceptible to HSP27 suppression, the more they were primed for death, as demonstrated by increased levels of mitochondrial outer membrane permeabilization. Priming for death was accompanied by the increase in pro‐apoptotic proteins of the BCL2 family and of active caspase‐3 and lamin B. Together, these data suggest that oncogene‐addicted cells require HSP27 for survival and that HSP27 might interfere with the effectiveness of targeted agents.