Thesis (Ph.D.) - Indiana University, Department of Medical Sciences, 2022The ETS family of transcription factors is a large group of 28 proteins in humans, and its members regulate a wide variety of processes in both embryological and adult tissue. Four of these members, ERG, ETV1, ETV4, and ETV5, become expressed in prostate cancer cells following chromosomal rearrangements. By far, the most common ETS factor to become aberrantly expressed in prostate cancer is ERG, which occurs in approximately 50% of prostate cancers. Once expressed, ERG drives oncogenic phenotypes such as migration and invasion and cooperates with other mutations, such as activation of PI3K/AKT signaling, to drive tumorigenesis. Therefore, understanding molecular mechanisms that govern the regulation and activation of ERG is paramount for developing precision therapeutics for prostate cancer patients. As a transcription factor, ERG has three critical functions that enable it to transcriptionally regulate its target genes. ERG must bind DNA, ERG must gain an activating phosphorylation mark, and ERG must interact with its co-activator EWS. Disrupting any of these three processes has the potential to reduce ERG function, and thus be leveraged as a therapeutic approach. However, not all of these approaches have equal likelihoods of being therapeutically relevant. The ETS family of transcription factors is defined by the shared similarity between the DNA-binding domains of each of the ETS factors. Additionally, multiple non-oncogenic ETS factors are normally expressed in prostate cells, some of which have anti-tumor activities. Therefore, approaches aiming to disrupt the ETS-DNA interaction hold a distinct probability to produce side effects resulting from the disruption of non-oncogenic ETS, either in the prostate or elsewhere in the body. For this reason, the work here was focused on the mechanism of ERG’s phosphorylation and interaction with EWS. Here, I detail a mechanism by which TLR4 signaling leads to the phosphorylation of ERG via the MAPK pathway. Activated ERG then transcriptionally activates its target gene program, including TLR4 and endogenous ligands for TLR4, resulting in a positive feedback loop. I also describe an indirect interaction between ERG and EWS, which is enabled by the RNA-binding protein, PABPC1.