Heat shock factor 1 is a promising therapeutic target against adult T-cell leukemia.

Patients with adult T-cell leukemia (ATL), which is caused by human T-cell leukemia virus type 1 (HTLV-1), show poor prognosis because of drug resistance. Heat shock protein (HSP) 90 is reportedly essential for ATL cell survival as it regulates important signaling pathways, thereby making HSP90 inhibitors new therapeutic candidates for ATL. However, HSP90 inhibition increases the expression of other HSPs, suggesting that HSPs may contribute to drug resistance. The heat shock factor 1 (HSF1) transcription factor is the primary regulator of the expression of HSPs. Furthermore, targeting HSF1 disrupts the HSP90 chaperone function. Herein, we demonstrated that HSF1 is overexpressed in HTLV-1-infected T cells. HSF1 knockdown inhibited the proliferation of HTLV-1-infected T cells. HSF1 inhibitor KRIBB11 reduced the expression and phosphorylation of HSF1, downregulated HSP70 and HSP27 expression, and suppressed Akt, nuclear factor-κB, and AP-1 signals. KRIBB11 treatment induced DNA damage, upregulated p53 and p21, and reduced the expression of cyclin D2/E, CDK2/4, c-Myc, MDM2, and β-catenin, thereby preventing retinoblastoma protein phosphorylation and inhibiting G1-S cell cycle progression. KRIBB11 also induced caspase-mediated apoptosis concomitant with the suppression of Bcl-xL, Mcl-1, XIAP, c-IAP1/2, and survivin expression. KRIBB11 inhibited HSP70 and HSP90 upregulation through treatment with AUY922, an HSP90 inhibitor, and enhanced the cytotoxic effect of AUY922, suggesting a salvage role of HSF1-dependent HSP induction in response to drug treatment. Finally, treatment of mice with KRIBB11 reduced ATL tumor growth. Therefore, this study provides a strong rationale to target HSF1 and validates the anti-ATL activity of KRIBB11.