Combination of Rapamycin, CI-1040, and 17-AAG Inhibits Metastatic Capacity of Prostate Cancer via Slug Inhibition

Though prostate cancer (PCa) has slow progression, the hormone refractory (HRCP) and metastatic entities are substantially lethal and lack effective treatments. Transcription factor Slug is critical in regulating metastases of various tumors including PCa. Here we studied targeted therapy against Slug using combination of 3 drugs targeting 3 pathways respectively converging via Slug and further regulating PCa metastasis. Using in vitro assays we confirmed that Slug up-regulation incurred inhibition of E-cadherin that was anti-metastatic, and inhibited Bim-regulated cell apoptosis in PCa. Upstream PTEN/Akt, mTOR, Erk, and AR/Hsp90 pathways were responsible for Slug up-regulation and each of these could be targeted by rapamycin, CI-1040, and 17-AAG respectively. In 4 PCa cell lines with different traits in terms of PTEN loss and androgen sensitivity we tested the efficacy of mono- and combined therapy with the drugs. We found that metastatic capacity of the cells was maximally inhibited only when all 3 drugs were combined, due to the crosstalk between the pathways. 17-AAG decreases Slug expression via blockade of HSP90-dependent AR stability. Combination of rapamycin and CI-1040 diminishes invasiveness more potently in PCa cells that are androgen insensitive and with PTEN loss. Slug inhibited Bim-mediated apoptosis that could be rescued by mTOR/Erk/HSP90 inhibitors. Using mouse models for circulating PCa DNA quantification, we found that combination of mTOR/Erk/HSP90 inhibitors reduced circulating PCa cells in vivo significantly more potently than combination of 2 or monotherapy. Conclusively, combination of mTOR/Erk/Hsp90 inhibits metastatic capacity of prostate cancer via Slug inhibition.