NNMT Silencing Activates Tumor Suppressor PP2A, Inactivates Oncogenic STKs, and Inhibits Tumor Forming Ability

Abstract

Purpose To identify potential molecular hubs that regulate oncogenic kinases and target them to improve treatment outcomes for glioblastoma (GBM) patients. Experimental Design Data mining of The Cancer Genome Atlas (TCGA) datasets identified Nicotinamide-N-methyl transferase (NNMT) as a prognostic marker for GBM, an enzyme linked to the reorganization of the methylome. We tested our hypothesis that NNMT plays a crucial role by modulating protein methylation leading to inactivation of tumor suppressors and activation of oncogenes. Further experiments were performed to understand the underlying biochemical mechanisms using GBM patient samples, established, primary, and isogenic cells. Results We demonstrate that NNMT outcompetes leucine carboxyl methyl transferase 1 (LCMT1) for methyl transfer from principal methyl donor SAM in biological systems. Inhibiting NNMT increased the availability of methyl groups for LCMT1 to methylate PP2A, resulting in the inhibition of oncogenic serine/threonine kinases (STKs). Further, NNMT inhibition retained the radiosensitizer nicotinamide and enhanced radiation sensitivity. We have provided the biochemical rationale of how NNMT plays a vital role in inhibiting tumor suppressor PP2A while concomitantly activating STKs. Conclusion We report the intricate novel mechanism in which NNMT inhibits tumor suppressor PP2A by reorganizing the methylome both at epigenome and proteome levels and concomitantly activating pro-survival STKs. In GBM tumors with NNMT expression, activation of PP2A can be accomplished by FDA approved perphenazine (PPZ) which is currently used to treat mood disorders such as schizophrenia, bipolar disorder, etc. This study forms a foundation for further GBM clinical trials using PPZ with standard of care treatment.

Systematically Dissecting the Function of RNA-Binding Proteins During Glioma Progression

“…Among the up-regulated genes, we identified NNMT, LGALS3, PDLIM4, TUBA1C and ANXA2 as top five ( Figure 5D). NNMT silencing had been shown to activate tumor suppressor PP2A and inhibits tumor forming (Palanichamy et al, 2017). This was consistent with our result that it was up-regulated in GBM.…”

Nicotinamide N -Methyltransferase: More Than a Vitamin B3 Clearance Enzyme

“…On the other hand, knockdown of NNMT in the ovarian carcinoma cell SKOV3 had the opposite effect as it increased SAM/SAH ratio, and global H3K9 and H3K27 trimethylation. Similar changes in methylation were also observed in one non-histone protein, protein phosphatase 2, suggesting that other cellular targets apart from histones might be differentially methylated by changes in NNMT expression and methyl donor levels [73,74]. These data provided a direct mechanistic link between NNMT and the regulation of epigenetic landscape of certain cancer cells.…”

Nicotinamide N-methyltransferase as a potential marker for cancer

“…These authors concluded that NNMT over-expression was related to unfavorable clinic pathological features including tumor size, differentiation and TNM stage, and that it ultimately influences patient Xu et al [39] suggested that the activation of hepatocyte nuclear factor-1beta (HNF-1beta) contributed to NNMT up-regulation in papillary thyroid cancer cells and that the depsipeptide histone deacetylase inhibitor reduced NNMT and its catalytic activity [40]. Depsipeptide inhibition of NNMT is executed at the transcription level by down-regulating the HNF-1beta transcription activator [40] ( Figure 1), and it is suggested that the methylation mediated by NNMT can not only modify histone-dependent gene expression but it can also extend beyond histones to include tumor suppressor proteins such as PP2A [66]. NNMT could inhibit PP2A both at the epigenome and proteome levels and concomitantly activate pro-survival STKs.…”