Prognostic and therapeutic significance of ribonucleotide reductase small subunit M2 in prostate cancer.

Abstract

240 Background: DNA-repair defects are common in advanced prostate cancer (PC) and high-risk localized tumors. Besides deficiency of DNA repair, overexpression of DNA repair genes could also contribute to poorer outcomes for PC patients. The nucleotide metabolism enzyme ribonucleotide reductase (RNR) plays the key role in DNA synthesis and repair. RRM2, the rate-limiting RNR subunit, is frequently up-regulated in cancer, and its overexpression leads to chemoresistance. Although targeting RRM2 by siRNA and small molecules has been applied in clinical trials in multiple cancers, limited knowledge of RRM2 function in PC delays potential clinical application of RRM2 inhibition. Methods: We leveraged publically available PC clinical cohorts to examine RRM2 levels and clinical outcomes. siRNAs was applied to knockdown of RRM2 in multiple PC cell lines. Cell growth, cell cycle and apoptosis were analyzed to determine siRRM2 or RRM2 inhibitor (COH29)-induced phenotypes. RNA-seq and protein array were performed to identify downstream targets of RRM2. Immunohistochemistry staining was applied to determine prevalence of RRM2 protein expression in PC tissues microarrays (TMAs). Results: In PC cohorts, increased RRM2 expression was associated with a higher likelihood of metastasis, poorer disease-free survival, and increased risk of development of lethal disease (N = 1200, PHS/HPFS cohorts). In PC cells, Inhibition of RRM2 induced remarkable cell growth inhibition, cell cycle arrest (at S phase) and apoptosis. DNA damage was observed in siRRM2/COH29-treated PC cells with increased activation of DNA damage markers. GSEA analysis of the RNA-seq dataset revealed multiple biological processes were affected by inhibition of RRM2, such as cell cycle, apoptosis, and DNA damage response. Intriguingly, MYC oncogenic signaling is the major downstream targets of RRM2. Furthermore, inhibition of RRM2 can block multiple oncogenic signaling including mTOR/AKT, SFK, and STAT signaling by repressing the key phospho-kinases in PC cells. Among 121 cases on the PC TMAs, 20% showed strong RRM2 protein expression. Conclusions: RRM2 may serve as a prognostic biomarker and novel therapeutic target in PC.