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 radioresistance. DAB2IP is rarely mutated in cancer but is frequently downregulated or inactivated by multiple mechanisms. Solid experimental evidence shows that DAB2IP reactivation reduces cancer aggressiveness in tumors driven by multiple different oncogenic mutations, making this protein an interesting target for cancer therapy. Considering this evidence, we screened a drug library to identify molecules that increase DAB2IP protein levels. We employed 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 focused our attention on thiostrepton, a natural cyclic oligopeptide antibiotic that has been reported to inhibit the 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 upregulation of this protein contributes to its action. These findings encourage further development of thiostrepton for the treatment of solid cancers and unveil a novel molecular target underlying its anti-tumoral activity.