Acquired resistance to HSP90 inhibition in triple-negative breast cancer

Heat shock protein 90 (HSP90), a conserved molecular chaperone, has become a potential molecular target for cancer therapeutics. HSP90 inhibition (HSP90i) causes inhibition of several oncogenic pathways simultaneously and leads to anti-cancer activities in multiple cancers including in triple-negative breast cancer (TNBC). TNBC is a subtype of breast cancer with poor prognosis and lack of approved targeted therapies. Although HSP90i has shown promising initial clinical data, resistance to HSP90i can still arise in TNBC patients and its resistance mechanisms are not yet understood. In this study, using an in vitro system, we report for the first time the isolation of TNBC cells with acquired resistance to HSP90i. Proteome and whole transcriptome profiling, and a bioactive small molecule screen were performed to identify the molecular basis of resistance to HSP90i and potential therapeutic approaches to overcome acquired resistance to HSP90i in TNBC cells. Two independent HSP90i-resistant clones were acquired through prolonged exposure of a TNBC cell line (Hs578T) to HSP90i. The clones showed significant resistance to HSP90i, notably to resorcinol-based HSP90i. The HSP90i-resistant clones also shared genomic sequence variants, suggesting a pre-existing population of resistant cells within the parental cells. We demonstrate that upregulated expression of UGT1A9, possibly due to an increased intrinsic oxidative stress, is associated with acquired resistance to resorcinol-based HSP90i in TNBC cells, and sensitivity to HSP90i can be restored with a competitive inhibitor of UGT1A9. The HSP90i-resistant clones also exhibited slower growth and upregulated IL6- mediated JAK2-STAT3 survival signalling pathway, which might contribute to the crossresistance to chemotherapeutics and other targeted therapies seen in the clones. Finally, we demonstrate that inhibition of JAK2-STAT3 signalling pathway is able to increase the cytotoxic effects of HSP90i to TNBC cells. We conclude that by using in vitro assays, we are able to identify potential mechanisms of acquired resistance to HSP90i in TNBC cells. We propose that expression of UGT1A9 or STAT3 might be a potential biomarker of sensitivity to HSP90i in TNBC cells. A combined inhibition of HSP90 and JAK2 might be a potential therapeutic approach for the development of effective targeted therapies in TNBC patients.