Abstract 481: Rationale for targeting the protein chaperone network in rhabdomyosarcoma

Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PA Cancer cells face unique proteotoxic stresses by virtue of genomic instability, increased translational output, and hypoxic environments. Coordinately, these stresses enhance dependence on cellular chaperones in order to preserve functional proteins and enable cancer cell survival. We hypothesized that perturbing specific nodes in this chaperone network may uncover unexpected vulnerabilities in certain cancer cells that can be exploited for therapeutic benefit. We found that amongst a large panel of patient-derived cancer cell lines, rhabdomyosarcomas exhibit exquisite sensitivity to the selective HSP70 inhibitor MAL3-101. Rhabdomyosarcoma is the most common soft tissue sarcoma affecting children and adolescents, and survival is exceedingly poor in patients with high risk disease. We observed that MAL3-101, at low micromolar concentrations, rapidly induces apoptosis in both fusion-positive and fusion-negative rhabdomyosarcoma cells. Unexpectedly, this was not accompanied by degradation of previously described obligate HSP70 clients or of the alveolar rhabdomyosarcoma driver oncogene PAX3-FOXO1. Instead, transcriptome profiling revealed that the predominant effector of HSP70 inhibition was activation of the unfolded protein response (UPR). We observed that combined knockdown of the constitutive HSP70 family member HSPA8 as well as the inducible members HSPA1A/B is required to phenocopy the effects of MAL3-101 treatment, confirming HSP70 as the critical target of MAL3-101. Through genetic experiments, we further showed that HSP70 inhibition with MAL3-101 induces cell death via the UPR effector CHOP, a pro-apoptotic transcription factor. Surprisingly, we found that the ER-resident UPR sensor IRE1 exhibits the greatest control on CHOP levels after MAL3-101 treatment, and appeared to do so independently of its endonuclease activity. Rhabdomyosarcoma cells grown under continuous drug exposure to generate isogenic models of resistance demonstrated a novel disconnect between HSP70 inhibition and UPR activation, which appears to be mediated in part by upregulation of another HSP70 family member, HSPA12A. Together, these data highlight a notable dependence of rhabdomyosarcoma cells on cytosolic HSP70 chaperones to silence a lethal unfolded protein response. Our data also suggest a previously underexplored link between the kinase activity of IRE1 and transcriptional control of CHOP, and identify CHOP as a potential biomarker of HSP70 inhibitor efficacy. The poor prognosis of alveolar rhabdomyosarcoma patients harboring a recurrent amplification at 12q13-14, which includes the locus encoding for CHOP, suggests an ideal target population for trials of early phase therapies that might target this vulnerability. Altogether, our findings provide a promising novel mechanism-based therapeutic strategy to improve outcomes in patients suffering from high risk rhabdomyosarcoma. Citation Format: Amit J. Sabnis, Christopher J. Guerriero, Jeffrey L. Brodsky, Trever Bivona. Rationale for targeting the protein chaperone network in rhabdomyosarcoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 481. doi:10.1158/1538-7445.AM2015-481