Nicotinamide N-methyltransferase as a therapeutic target in taxane-resistant castration-resistant prostate cancer

Drug resistance in patients remains a significant obstacle to successful treatment, even with improvements in cancer treatment strategies. Resistance to taxanes, such as docetaxel (Dtx) and cabazitaxel (Cbz), frequently emerges in castration resistant prostate cancer (CRPC). Through pulse selection of the parental cells (DU145), we established Dtx- and Cbz-resistant CRPC cell models and integrated different omic approaches, including transcriptomics and proteomics, to determine the molecular signatures underlying taxane resistance. Interestingly, several genes were regulated in the same direction (up- or down-regulation) at both the gene and protein expression levels in resistant cells compared to parental cells, suggesting that alterations primarily occur at the transcriptional level and manifest at the protein level. Among the differentially regulated genes, Cysteine Rich Protein 2 (CRIP2), a gene associated with tumor suppressor function, has been found to be the most downregulated in taxane-resistant cells. Conversely, Nicotinamide N-Methyltransferase (NNMT) exhibited a significant upregulation and has been validated in the context of taxane resistance. Its overexpression was shown to promote taxane resistance in two different CRPC cell lines, whereas depletion via siRNA or gRNA, as well as treatment with 1-methylnicotinamide (1-MNA, used as a feedback inhibitor)resensitized the resistant cells. RNA-sequencing of NNMT-knockout (CRISPR-Cas9) cells has indicated involvement of TGFβ signaling, and suppressing this pathway has further increased the taxane sensitivity. Epithelial Mesenchymal Transition (EMT) was another pathway depleted upon knockout, and subsequent analysis revealed a significant correlation between NNMT and EMT-related genes (VIM, CDH2, FN1, TGFB1, and ZEB2) in both the Cancer Cell Line Encyclopedia (CCLE) panel and patient data. Additionally, in cancers other than PC, NNMT has been observed to predict treatment outcomes, and notably, among the patients with a high EMT signature, elevated NNMT levels were associated with decreased overall survival. More importantly, NNMT-high patients were found to be non-responders to taxane-containing chemotherapy regimens. Collectively, our findings suggest that targeting NNMT and the pathways it affects, such as TGFβ, offers a viable approach for addressing taxane-resistant PC.