Drug-repurposing screen identifies thiostrepton as a novel regulator of the tumor suppressor DAB2IP

The tumor suppressor DAB2IP, a RasGAP and cytoplasmic adaptor protein, modulates signal transduction in response to several extracellular stimuli, negatively regulating multiple oncogenic pathways. Accordingly, the loss of DAB2IP in tumor cells fosters metastasis and enhances chemo- and radio-resistance. DAB2IP is rarely mutated in cancer but is frequently downregulated or inactivated by multiple mechanisms. Solid experimental evidence show that DAB2IP reactivation can reduce cancer aggressiveness in tumors driven by multiple different oncogenic mutations, making this protein an interesting target for anti-cancer therapy. Based on these premises, we screened a library of FDA-approved drugs to search for molecules that can increase DAB2IP protein levels. We exploited CRISPR/Cas9 gene editing to generate two prostate cancer cell models in which endogenous DAB2IP is fused to HiBiT, a peptide tag that enables luminescence-based detection of protein levels in a sensitive and quantitative manner. Using this approach, we identified drugs able to increase DAB2IP levels. We focus our attention on thiostrepton, a natural cyclic oligopeptide antibiotic that has been reported to inhibit survival of various cancer cell lines. Functional experiments revealed that the cancer inhibitory effect of thiostrepton is reduced in the absence of DAB2IP, suggesting that the observed upregulation contributes to its action. These findings encourage the further development of thiostrepton for the treatment of solid cancers, and unveil a novel molecular mechanism underlying its anti-tumoral action.