Targeting the Hsp40/Hsp70 chaperone axis as a novel strategy to treat castration-resistant prostate cancer

Castration-resistant prostate cancer (CRPC) is characterized by reactivation of androgen receptor (AR) signaling in part by elevated expression of AR splice variants (ARv) including ARv7, a constitutively active, ligand binding domain (LBD)-deficient variant whose expression has been correlated with therapeutic resistance and poor prognosis. In a screen to identify small molecule dual inhibitors of both androgen-dependent and androgen-independent AR gene signatures, we identified the chalcone C86. Binding studies using purified proteins and CRPC cell lysates revealed C86 to interact with heat shock protein 40 (Hsp40). Pulldown studies using biotinylated-C86 found Hsp40 present in a multi-protein complex with full-length (FL-) AR, ARv7 and Hsp70 in CRPC cells. Treatment of CRPC cells with C86 or the allosteric heat shock protein 70 (Hsp70) inhibitor JG98 resulted in rapid protein destabilization of both FL-AR and ARv, including ARv7, concomitant with reduced FL-AR- and ARv7-mediated transcriptional activity. The glucocorticoid receptor (GR), whose elevated expression in a subset of CRPC also leads to androgen-independent AR target gene transcription, was also destabilized by inhibition of Hsp40 or Hsp70. In vivo, Hsp40 or Hsp70 inhibition demonstrated single agent and combinatorial activity in a 22Rv1 CRPC xenograft model. These data reveal that, in addition to recognized roles of Hsp40 and Hsp70 in FL-AR LBD remodeling, ARv lacking the LBD remain dependent on molecular chaperones for stability and function. Our findings highlight the feasibility and potential benefit of targeting the Hsp40/Hsp70 chaperone axis to treat prostate cancer that has become resistant to standard anti-androgen therapy.

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