The regulatory role of ELK-1 ubiquitination in cell proliferation and prostate cancer

Early stage Prostate Cancer (PC) is usually androgen-dependent and androgen ablation therapies have been used with good results in the destruction of androgen-dependent cells. However, anti-androgens are associated with systemic side effects and selection for tumour cells that survive in the absence of androgens, leading to androgen-independent PC. ELK-1 is a member of the ETS-domain transcription factor family. In response to mitogens, ERKs phosphorylate and activate ELK-1 to stimulate the expression of Immediate Early Genes (IEGs) such as CFOS and EGR1, thereby promoting cell cycle entry and proliferation. In PC cells, ELK-1 expression is frequently up-regulated and androgen receptor (AR) was found to function as a constitutive coactivator for ELK-1, up-regulating a major subset of its target genes involved in cell proliferation and migration. Removal or inactivation of ELK-1 could therefore serve to suppress PC tumour growth. Mono-ubiquitination modulates ELK-1 activity. De-ubiquitination of ELK-1 by USP17, a de-ubiquitinase over-expressed in multiple tumour types, increases ELK-1 target gene expression and cell proliferation, indicating that mono-ubiquitination of ELK-1 suppresses its activity (Ducker et al., 2019). This project aims to identify the ubiquitin E3 ligase responsible for ELK-1 mono-ubiquitination and evaluate its role in PC. Using a combination of gene knockdown experiments and ubiquitination assays, as well as an in silico gene expression analysis to identify ubiquitin E3 ligases that were significantly down-regulated in PC cells, I have tested candidates including FBXW7, FBXO25, SPOP and UBR5, for their ability to modify ELK-1 and suppress its activity. Moreover, using the BioID screening I was able to identify ELK-1 partners under starved and mitogen-stimulated conditions. My findings suggest that an ubiquitin E3 ligase previously reported to target ELK-1 is unlikely to promote ELK-1 ubiquitination and transactivation. Furthermore, the BioID analysis found some potential E3 ligase candidates that need to be evaluated to elucidate their role in promoting ELK-1 ubiquitination. Additionally, the BioID analysis concluded that ELK-1 interacts with transcriptional regulators to modulate ELK-1 target genes expression