DDX3 in cancer: Identifying motives, targets and partners in crime

The protein DDX3 is a member of the DEAD box family of RNA helicases, which have the ability to unwind complex RNA secondary structures and hereby facilitate RNA related cellular processes, like mRNA translation. DDX3 is a DEAD box RNA helicase that plays an oncogenic role in several cancer subtypes. RK-33 is developed as a small molecule inhibitor of DDX3. In this thesis we evaluate different aspects of DDX3 and the effect of inhibition with RK-33 in cancer cells.
First, we studied why cancer cells are dependent on DDX3 for performing oncogenic tasks by using (phospho-)proteomics. We found that DDX3 inhibition reduces mitochondrial translation activity in cancer cells, which is increasingly recognized as a viable target for anti-cancer therapeutics. The mitochondria have their own translational machinery for synthesis of oxidative phosphorylation proteins. Interestingly, recent studies have shown that irradiated cancer cells increase oxidative phosphorylation to facilitate DNA repair and hereby induce resistance to radiation therapy. We found RK-33 to antagonize the increase in oxygen consumption and ATP production observed after exposure to ionizing radiation, and to reduce DNA repair in breast cancer cells. Our results explain the selective anti-cancer activity observed after RK-33 treatment, especially in combination with radiation therapy, and put inhibition of mitochondrial translation forward as a novel radiosensitization strategy. In addition, RK-33 was found to sensitize cells to treatment with PARP inhibitors.
Next, we evaluated DDX3 sensitivity in different (sub)types of cancer. DDX3 inhibition reduces oncogenic Wnt-signaling and DDX3 seems to be a promising therapeutic target in a subset of colorectal cancers. DDX3 expression is also upregulated in breast cancer metastases, when compared to the corresponding primary tumor. In addition, we found that both cytoplasmic and nuclear DDX3 expression of the tumor correlate with shorter survival of breast and colorectal cancer patients, suggesting that they may serve as biomarkers to select patients that would benefit most from DDX3 inhibition. Last, we put DDX3 inhibitors in a broader perspective by reviewing the literature on the all DEAD/H box RNA helicase family members as potential therapeutic targets in oncogenic mRNA translation. Overall we conclude that DDX3 inhibition can be a promising therapeutic strategy in cancer cells.