Presentation Authors: Michael Hughes*, Mark Woodford, Mahmoud Chehab, Oleg Shapiro, Gennady Bratslavsky, Dimitra Bourboulia, Mehdi Mollapour, Syracuse, NY
Introduction: The molecular chaperone Heat Shock Protein 90 (Hsp90) is essential for the folding, stability and activity of several drivers of oncogenesis. Hsp90 inhibitors are currently under clinical evaluation in cancer patients. However, there are no biomarkers to better stratify candidate patients who would potentially respond and benefit from Hsp90 drugs. We have recently identified tumor suppressor tuberous sclerosis complex 1 (Tsc1) as a new regulator or &[Prime]co-chaperone&[Prime] of Hsp90 that decreases its ATPase activity. Tsc1 expression sensitizes cancer cells to Hsp90 inhibitors. Mutation of Tsc1 has been implicated in the development of bladder cancer. The objective of this work was to determine the underlying mechanism of the impact of Tsc1 on bladder cancer cell sensitivity to Hsp90 inhibitors.
Methods: Short-interfering RNA knockdown or transient transfection of Tsc1 was used to modulate Tsc1 levels in bladder cancer cell lines. The ATP-competitive Hsp90 inhibitors ganetespib and SNX2112 were used to challenge bladder cancer cell lines. Cell proliferation was assessed by MTT assay. Apoptosis was assessed by immunoblotting.
Results: Bladder cancer cell lines with wild type TSC1 (UM-UC-3, T24) were sensitive to the Hsp90 inhibitor ganetespib as evidenced by IC50 (33ÂµM and 7ÂµM respectively). SiRNA knockdown of TSC1 makes these cell lines resistant to Hsp90 inhibitors, with IC50s >100nM. Conversely, overexpression of wild type Tsc1-FLAG in the TSC1-null bladder cancer cell line RT4 sensitizes this cell line to apoptosis when challenged with ganetespib. Hsp90 is subject to post-translational modifications and Hsp90 isolated from TSC1-null bladder cancer cell line RT4 appears to be hypo-acetylated compared to Tsc1-containing bladder cancer cell lines UM-UC-3 and T24.
Conclusions: Tsc1 expression appears to serve as a predictive biomarker of bladder cancer response to Hsp90 inhibitors. Additionally absence of Tsc1 leads to hypo-acetylation of Hsp90 and reduce binding to its inhibitors. Thus, HDAC inhibitors alone or in combination with Hsp90 inhibitors should be explored as a potential therapeutic approach in TSC1-null bladder cancers.
Source of Funding: The National Institute of Health