ELL2 Is Required for the Growth and Survival of AR-Negative Prostate Cancer Cells

BACKGROUND: Elongation factor for RNA polymerase II 2 (ELL2) was reported as a putative tumor suppressor in the prostate. ELL2 is frequently down-regulated in prostatic adenocarcinoma specimens, and loss of ELL2 induced murine prostatic intraepithelial neoplasia and enhanced AR-positive prostate cancer cell proliferation. However, the ELL2 gene appears to be amplified in AR-negative neuroendocrine prostate tumors, suggesting a potential oncogenic role for ELL2 in AR-negative prostate cancer cells. In this study, we explored the potential function of ELL2 in PC-3 and DU145, two AR-negative prostate cancer cell lines. MATERIALS AND METHODS: The role of ELL2 in PC-3 and DU145 cells was studied using siRNA-mediated ELL2 knockdown. Genes regulated by ELL2 knockdown in PC-3 cells were identified and analyzed using RNA-Seq and bioinformatics. The expression of representative genes was confirmed by Western blot and/or quantitative PCR. Cell growth was determined by BrdU, MTT and colony formation assays. Cell death was analyzed by 7-AAD/Annexin V staining and trypan blue exclusion staining. Cell cycle was determined by PI staining and flow cytometry. RESULTS: ELL2 knockdown inhibited the proliferation of PC-3 and DU145 cells. RNA-Seq analysis showed an enrichment in genes associated with cell death and survival following ELL2 knockdown. The interferon-γ pathway was identified as the top canonical pathway comprising of 55.6% of the genes regulated by ELL2. ELL2 knockdown induced an increase in STAT1 and IRF1 mRNA and an induction of total STAT1 and phosphorylated STAT1 protein. Inhibition of cell proliferation by ELL2 knockdown was partly abrogated by STAT1 knockdown. ELL2 knockdown inhibited colony formation and induced apoptosis in both PC-3 and DU145 cells. Furthermore, knockdown of ELL2 caused S-phase cell cycle arrest, inhibition of CDK2 phosphorylation and cyclin D1 expression, and increased expression of cyclin E. CONCLUSION: ELL2 knockdown in PC-3 and DU145 cells induced S-phase cell cycle arrest and profound apoptosis, which was accompanied by the induction of genes associated with cell death and survival pathways. These observations suggest that ELL2 is a potential oncogenic protein required for survival and proliferation in AR-negative prostate cancer cells.