Abstract 1308: Functional inhibition of HSP90 induces G0/G1 arrest and downregulates thymidylate synthase in colorectal cancer

Abstract

Abstract Purpose Objectives: Cell cycle progression and DNA synthesis are essential steps in cancer cell growth and resistance. Thymidylate synthase (TS) is a therapeutic target for 5FU. Ganetespib is a potent functional inhibitor of HSP90 currently under evaluation in clinical trials. We tested the hypothesis that HSP90 functional inhibition by ganetespib can inhibit cell cycle progression and downregulate TS levels in colorectal cell lines. Methods: Two colorectal cancer (CRC) cell lines were used, HCT-116 and HT-29. Cells were either untreated (control) or treated with ganetespib at 50 nM for 24 hours. Western blot and Real time-PCR analyses were carried out to determine the effect of ganetespib on different signal molecules involved in cell cycle arrest and DNA synthesis. A subcutaneous xenograft mouse model was used to evaluate the in vivo effects of ganetespib. Pre and post ganetespib treatment tumor biopsies from patients with rectal cancer on a phase I trial of ganetespib were analyzed using RT-PCR. Results: Treatment with ganetespib inhibited cyclin D1, pRb and E2F-1 at the transcriptional and translational level and induced p21, leading to G0/G1 cell cycle arrest. HSP90 knockdown inhibited E2F-1 and TS protein and mRNA levels in both cell lines. Arrest in the G0/G1 phase was associated with downregulation of the transcription factor E2F-1 and its target gene TS. Similar effects were observed in tumors from animals treated with ganetespib. Further, ganetespib treatment significantly inhibited the tumor growth. Inhibition of cyclin D1, Rb, E2F-1, TS and upregulation of p21 at transcriptional levels were observed in post treatment rectal tumor biopsies obtained from patients receiving ganetespib. Conclusion: In this study, we present preclinical and clinical data to support the use of HSP90 inhibitors in colorectal cancer based on their ability to reduce expression of proteins important for DNA repair and cell cycle progression leading to cell cycle arrest and tumor shrinkage. Accordingly, combining HSP90 inhibitors with chemotherapy is a rational approach for future drug development in colorectal cancer. Citation Format: Ganji Purnachandra Nagaraju, Field F. Willingham, Kevin E. Woods, Patrick Sullivan, Jerome C. Landry, Roberto Diaz, Bassel F. El-Rayes. Functional inhibition of HSP90 induces G0/G1 arrest and downregulates thymidylate synthase in colorectal cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1308. doi:10.1158/1538-7445.AM2014-1308