NNMT inhibition restores drug sensitivity in taxane-resistant prostate cancer cells
A metabolic enzyme identified in taxane-resistant cells
A study found that reducing the metabolic enzyme NNMT restored drug sensitivity in some prostate cancer cells resistant to taxane chemotherapy. The researchers investigated NNMT’s role using cell experiments and analyses of publicly available patient data. The effect varied by cell model, and the study did not test treatment efficacy in patients. The study was published in Cell Death Discovery.
Castration-resistant prostate cancer (CRPC) does not respond to androgen-deprivation therapy; taxanes such as docetaxel and cabazitaxel are important treatment options. The researchers repeatedly exposed DU145 prostate cancer cells to each drug, with recovery periods, to create models resistant to docetaxel or cabazitaxel. They then compared the transcriptomes and proteomes of the parental and resistant cells. This approach examined changes in RNA and protein expression together, rather than genomic sequence.
In both resistant models, expression of NNMT increased along with that of ABCB1, a known drug-efflux protein. NNMT is a metabolic enzyme that adds a methyl group to nicotinamide. To determine whether NNMT contributed to resistance, the researchers both reduced and increased its expression.
Drug responses changed when NNMT was manipulated
In cabazitaxel-resistant DU145 cells, reducing NNMT expression with siRNA decreased colony formation and proliferation and increased cabazitaxel sensitivity compared with controls. In docetaxel-resistant cells, siRNA treatment alone substantially reduced survival, making it difficult to assess the drug response. The researchers generated cells in which NNMT was removed using CRISPR-Cas9; in a 72-hour drug-treatment experiment comparing these cells with controls, they found increased taxane sensitivity.
Conversely, overexpressing NNMT in parental DU145 and 22Rv1 cells that responded to the drugs reduced their sensitivity to docetaxel and cabazitaxel. Their ability to form colonies after drug treatment also increased. Beyond observing that NNMT expression and resistance rose together, directly manipulating NNMT changed the drug response, providing functional evidence of its role.
The researchers also used 1-methylnicotinamide (1-MNA) as a feedback inhibitor. In cell experiments at concentrations of 1–4 mM, it increased taxane sensitivity and reduced colony formation. These results do not establish a dose or safety for use in patients.
Connection to TGFβ and EMT, and model-specific limitations
RNA analysis of docetaxel-resistant cells with NNMT removed showed reduced TGFβ signaling and gene programs related to epithelial–mesenchymal transition (EMT). EMT is a process associated with changes in cell adhesion and migration. Treatment with the TGFβ pathway inhibitor SB431542 further increased taxane sensitivity. This supports the involvement of the pathway in NNMT-associated resistance, but does not mean that every molecular step has been elucidated.
In PC3 cells, which are androgen receptor (AR)-negative, reduced proliferation and increased taxane sensitivity were also observed following NNMT targeting. By contrast, in the AR-positive 22Rv1 line, NNMT expression was not induced when resistance developed, and lowering NNMT produced no measurable change in survival or drug response. Since most clinical CRPC is AR-positive, it is difficult to generalize the results to the overall patient population.
What the patient data support
Data from metastatic prostate cancer showed correlations between NNMT expression and that of some EMT-related genes. However, the finding that NNMT was elevated in nonresponders to taxane-containing combination chemotherapy came from cohorts with endometrial serous adenocarcinoma and breast cancer. Because these data involved multiple drugs used together, they should also be distinguished from responses to taxanes alone. An analysis linking high NNMT expression and shorter overall survival in tumors with high EMT was based on gastric adenocarcinoma data. These findings cannot be directly interpreted as evidence about taxane response or survival in patients with prostate cancer.
This study was based on established cell lines and public data and included no animal experiments or interventional trials in patients. Some key functional experiments had only two independent biological replicates, so reproducibility needs to be assessed. Experimental conditions and analyses can be reviewed in the full paper and the PubMed citation and abstract.
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 pat
The study identifies NNMT as a candidate target for addressing taxane resistance in prostate cancer cells that depend on NNMT. The opposing changes in drug sensitivity when NNMT was reduced or increased provide stronger functional evidence than an expression correlation alone. At the same time, targeting NNMT had no effect in an AR-positive resistant model in which NNMT was not naturally increased. Follow-up studies should therefore account for model characteristics, including NNMT expression and AR status, and establish the range in which the effect is reproducible. Efficacy and safety in vivo, as well as an appropriate inhibition strategy, also remain to be tested. The current results cannot determine whether patients would live longer or whether a clinical combination regimen would be effective.