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

NNMT depletion contributes to liver cancer cell survival by enhancing autophagy under nutrient starvation

Nicotinamide N-methyl transferase (NNMT) transfers a methyl group from S-adenosyl-L-methionine (SAM) to nicotinamide (NAM), producing 1-methylnicotinamide (1MNA). NNMT has been implicated in several cancer types and recently in metabolism, but its role in autophagy regulation has not yet been investigated. In this study, we determined that NNMT negatively regulated autophagy at the stage of ULK1 activation through protein phosphatase 2A (PP2A) activity. Specifically, NNMT knockdown increased PP2A methylation and subsequently enhanced phosphatase activity. Consequent p-ULK1 (S638) dephosphorylation derepressed ULK1 activity, resulting in autophagy induction. Accordingly, NNMT downregulation rescued tumor cells under nutrient deficiency in vivo, which was alleviated by ULK1 inhibitor treatment. In summary, our results suggest a novel mechanism by which tumor cells protect themselves against nutrient deprivation through NNMT suppression to accelerate autophagy.

Metabolic signature of the aging eye in mice

Aging is a major risk factor for age-related ocular diseases including age-related macular degeneration in the retina and retinal pigment epithelium (RPE), cataracts in the lens, glaucoma in the optic nerve, and dry eye syndrome in the cornea. We used targeted metabolomics to analyze metabolites from young (6 weeks) and old (73 weeks) eyes in C57 BL6/J mice. Old mice had diminished electroretinogram responses and decreased number of photoreceptors in their retinas. Among the 297 detected metabolites, 45-114 metabolites are significantly altered in aged eye tissues, mostly in the neuronal tissues (retina and optic nerve) and less in cornea, RPE/choroid, and lens. We noted that changes of metabolites in mitochondrial metabolism and glucose metabolism are common features in the aged retina, RPE/choroid, and optic nerve. The aging retina, cornea, and optic nerve also share similar changes in Nicotinamide adenine dinucleotide (NAD), 1-methylnicotinamides, 3-methylhistidine, and other methylated metabolites. Metabolites in taurine metabolism are strikingly influenced by aging in the cornea and lens. In conclusion, the aging eye has both common and tissue-specific metabolic signatures. These changes may be attributed to dysregulated mitochondrial metabolism, reprogrammed glucose metabolism and impaired methylation in the aging eye. Our findings provide biochemical insights into the mechanisms of age-related ocular changes.

An Epigenetic Signature in Adipose Tissue Is Linked to Nicotinamide N-Methyltransferase Gene Expression

SCOPE

The enzyme nicotinamide N-methyltransferase (NNMT) is a major methyltransferase in adipose tissue. We hypothesized an epigenetic signature in association with NNMT gene expression in adipose tissue.

METHODS AND RESULTS

The global human methylome was analyzed in visceral adipose tissue (VAT) from morbidly obese patients using the Infinium Human Methylation 450 BeadChip array (discovery cohort: n = 11). The findings were confirmed in two additional independent cohorts (cohort 1: n = 60; BMI 20-60 kg m^-2 and cohort 2: n = 40; BMI > 40 kg m^-2 ) and validated after weight loss (using microarray data). Among the genes associated with the largest methylation fold change were genes related to metabolic processes, proliferation, inflammation, and extracellular matrix remodeling, such as COL23A1, PLEC1, FBXO21, STEAP3, RGS12, IGDCC3, FOXK2, and ORAI2. In fact, the results showed 577 differentially methylated CpG sites (DMCpGs) associated with the NNMT expression levels, with low methylation levels paralleling high NNMT expression. The expression of FBXO21 and FOXK2 was specifically modified after weight loss concomitantly with a decrease in NNMT expression and inflammation-related genes. Interestingly, the adipose tissue NNMT gene expression correlated with markers of adipose tissue inflammation.

CONCLUSIONS

The expression of NNMT in VAT is associated with a specific methylome signature involving genes linked to adipose tissue metabolic pathophysiology.

NNMT activation can contribute to the development of fatty liver disease by modulating the NAD + metabolism

Nicotinamide N-methyltransferase (NNMT) catalyses the reaction between nicotinamide (NAM) and S-adenosylmethionine to produce 1-methylnicotinamide and S-adenosylhomocysteine. Recently, this enzyme has also been reported to modulate hepatic nutrient metabolism, but its role in the liver has not been fully elucidated. We developed transgenic mice overexpressing NNMT to elucidate its role in hepatic nutrient metabolism. When fed a high fat diet containing NAM, a precursor for nicotinamide adenine dinucleotide (NAD)+, these NNMT-overexpressing mice exhibit fatty liver deterioration following increased expression of the genes mediating fatty acid uptake and decreased very low-density lipoprotein secretion. NNMT overactivation decreased the NAD+ content in the liver and also decreased gene activity related to fatty acid oxidation by inhibiting NAD+-dependent deacetylase Sirt3 function. Moreover, the transgenic mice showed liver fibrosis, with the induction of inflammatory and fibrosis genes. Induced NNMT expression decreased the tissue methylation capacity, thereby reducing methylation of the connective tissue growth factor (CTGF) gene promoter, resulting in increased CTGF expression. These data indicate that NNMT links the NAD+ and methionine metabolic pathways and promotes liver steatosis and fibrosis. Therefore, targeting NNMT may serve as a therapeutic strategy for treating fatty liver and fibrosis.

N1-methylnicotinamide (MNAM) as a guardian of cardiovascular system

Atherosclerosis is identified as the formation of atherosclerotic plaques, which could initiate the formation of a blood clot in which its growth to coronary artery can lead to a heart attack. N-methyltransferase (NNMT) is an enzyme that converts the NAM (nicotinamide) to its methylated form, N1-methylnicotinamide (MNAM). Higher levels of MNAM have been reported in cases with coronary artery disease (CAD). Further, MNAM increases endothelial prostacyclin (PGI2) and nitric oxide (NO) and thereby causes vasorelaxation. The vasoprotective, anti-inflammatory and anti-thrombotic roles of MNAM have been well documented; however, the exact underlying mechanisms remain to be clarified. Due to potential role of MNAM in the formation of lipid droplets (LDs), it might exert its function in coordination with lipids, and their targets. In this study, we summarized the roles of MNAM in cardiovascular system and highlighted its possible mode of actions.

Nicotinamide N-methyltransferase enhances the progression of prostate cancer by stabilizing sirtuin 1

A previous study demonstrated that nicotinamide N-methyltransferase (NNMT) is upregulated in the tissues of patients with prostate cancer (PCa); however, the specific underlying mechanism of this remains unclear. To begin with, the expression of NNMT was investigated in the peripheral blood of patients with PCa and of healthy control subjects. The results indicated that the expression level of NNMT was elevated in the peripheral blood and tissues of patients with PCa. Furthermore, the overexpression of NNMT enhanced PC-3 cell viability, invasion and migration capacity. Additionally, the overexpression of NNMT significantly increased the mRNA level of sirtuin 1 (SIRT1) in PC-3 cells. In addition, nicotinamide treatment significantly suppressed the expression of SIRT1 even in PC-3 cells transfected with adeno-associated virus-NNMT. Furthermore, the PC-3 cell invasion capacity was notably decreased by the nicotinamide treatment; however, such effects were largely abolished by the overexpression of NNMT in PC-3 cells. These data indicated that NNMT enhanced PC-3 cell migration and invasion mainly by regulating SIRT1 expression. In summary, the present study indicated that NNMT is an important regulator of SIRT1 expression in PC-3 cells and may be a potential therapeutic target for PCa.

The Rheumatoid Arthritis-Associated Citrullinome

Increased protein citrullination is linked to various diseases including rheumatoid arthritis (RA), lupus, and cancer. Citrullinated autoantigens, a hallmark of RA, are recognized by anti-citrullinated protein antibodies (ACPAs) which are used to diagnose RA. ACPA-recognizing citrullinated enolase, vimentin, keratin, and filaggrin are also pathogenic. Here, we used a chemoproteomic approach to define the RA-associated citrullinome. The identified proteins include numerous serine protease inhibitors (Serpins), proteases and metabolic enzymes. We demonstrate that citrullination of antiplasmin, antithrombin, t-PAI, and C1 inhibitor (P1-Arg-containing Serpins) abolishes their ability to inhibit their cognate proteases. Citrullination of nicotinamide N-methyl transferase (NNMT) also abolished its methyltransferase activity. Overall, these data advance our understanding of the roles of citrullination in RA and suggest that extracellular protein arginine deiminase (PAD) activity can modulate protease activity with consequent effects on Serpin-regulated pathways. Moreover, our data suggest that inhibition of extracellular PAD activity will be therapeutically relevant.

Nicotinamide N-methyltransferase promotes epithelial-mesenchymal transition in gastric cancer cells by activating transforming growth factor-β1 expression

Previous studies have demonstrated that nicotinamide N-methyltransferase (NNMT) is aberrantly expressed in a number of tumors. In the present study, it was demonstrated that the gene and protein levels of NNMT were significantly increased in gastric cancer cells. Furthermore, upregulation of NNMT significantly increased the expression of mesenchymal markers, including α-smooth muscle actin (SMA), vimentin and fibronectin, but decreased the levels of epithelial cadherin. Since transforming growth factor (TGF)-β1 may serve a key function in epithelial-mesenchymal transition (EMT), the effects of NNMT on the expression of TGF-β1 were investigated in BGC-823 cells. The results demonstrated that overexpression of NNMT significantly induced the expression of TGF-β1. However, knockdown of NNMT inhibited the expression of TGF-β1, mothers against decapentaplegic homolog (Smad)2 and α-SMA. Additionally, pre-incubation with TGF-β1 partially eliminated NNMT-mediated changes in EMT. Collectively, the results demonstrated that upregulation of NNMT in gastric cancer cells may increase the expression of TGF-β1, therefore activating TGF-β1/Smad signaling, which in turn promotes EMT.

A small molecule inhibitor of Nicotinamide N-methyltransferase for the treatment of metabolic disorders

Nicotinamide N-methyltransferase (NNMT) is a cytosolic enzyme that catalyzes the transfer of a methyl group from the co-factor S-adenosyl-L-methionine (SAM) onto the substrate, nicotinamide (NA) to form 1-methyl-nicotinamide (MNA). Higher NNMT expression and MNA concentrations have been associated with obesity and type-2 diabetes. Here we report a small molecule analog of NA, JBSNF-000088, that inhibits NNMT activity, reduces MNA levels and drives insulin sensitization, glucose modulation and body weight reduction in animal models of metabolic disease. In mice with high fat diet (HFD)-induced obesity, JBSNF-000088 treatment caused a reduction in body weight, improved insulin sensitivity and normalized glucose tolerance to the level of lean control mice. These effects were not seen in NNMT knockout mice on HFD, confirming specificity of JBSNF-000088. The compound also improved glucose handling in ob/ob and db/db mice albeit to a lesser extent and in the absence of weight loss. Co-crystal structure analysis revealed the presence of the N-methylated product of JBSNF-000088 bound to the NNMT protein. The N-methylated product was also detected in the plasma of mice treated with JBSNF-000088. Hence, JBSNF-000088 may act as a slow-turnover substrate analog, driving the observed metabolic benefits.

Discovery of Bisubstrate Inhibitors of Nicotinamide N-Methyltransferase (NNMT)

Nicotinamide N-methyltransferase (NNMT) catalyzes the N-methylation of pyridine-containing compounds using the cofactor S-5'-adenosyl-l-methionine (SAM) as the methyl group donor. Through the regulation of the levels of its substrates, cofactor, and products, NNMT plays an important role in physiology and pathophysiology. Overexpression of NNMT has been implicated in various human diseases. Potent and selective small-molecule NNMT inhibitors are valuable chemical tools for testing biological and therapeutic hypotheses. However, very few NNMT inhibitors have been reported. Here, we describe the discovery of a bisubstrate NNMT inhibitor MS2734 (6) and characterization of this inhibitor in biochemical, biophysical, kinetic, and structural studies. Importantly, we obtained the first crystal structure of human NNMT in complex with a small-molecule inhibitor. The structure of the NNMT-6 complex has unambiguously demonstrated that 6 occupied both substrate and cofactor binding sites. The findings paved the way for developing more potent and selective NNMT inhibitors in the future.

Selective and membrane-permeable small molecule inhibitors of nicotinamide N-methyltransferase reverse high fat diet-induced obesity in mice

There is a critical need for new mechanism-of-action drugs that reduce the burden of obesity and associated chronic metabolic comorbidities. A potentially novel target to treat obesity and type 2 diabetes is nicotinamide-N-methyltransferase (NNMT), a cytosolic enzyme with newly identified roles in cellular metabolism and energy homeostasis. To validate NNMT as an anti-obesity drug target, we investigated the permeability, selectivity, mechanistic, and physiological properties of a series of small molecule NNMT inhibitors. Membrane permeability of NNMT inhibitors was characterized using parallel artificial membrane permeability and Caco-2 cell assays. Selectivity was tested against structurally-related methyltransferases and nicotinamide adenine dinucleotide (NAD+) salvage pathway enzymes. Effects of NNMT inhibitors on lipogenesis and intracellular levels of metabolites, including NNMT reaction product 1-methylnicotianamide (1-MNA) were evaluated in cultured adipocytes. Effects of a potent NNMT inhibitor on obesity measures and plasma lipid were assessed in diet-induced obese mice fed a high-fat diet. Methylquinolinium scaffolds with primary amine substitutions displayed high permeability from passive and active transport across membranes. Importantly, methylquinolinium analogues displayed high selectivity, not inhibiting related SAM-dependent methyltransferases or enzymes in the NAD+ salvage pathway. NNMT inhibitors reduced intracellular 1-MNA, increased intracellular NAD+ and S-(5'-adenosyl)-l-methionine (SAM), and suppressed lipogenesis in adipocytes. Treatment of diet-induced obese mice systemically with a potent NNMT inhibitor significantly reduced body weight and white adipose mass, decreased adipocyte size, and lowered plasma total cholesterol levels. Notably, administration of NNMT inhibitors did not impact total food intake nor produce any observable adverse effects. These results support development of small molecule NNMT inhibitors as therapeutics to reverse diet-induced obesity and validate NNMT as a viable target to treat obesity and related metabolic conditions. Increased flux of key cellular energy regulators, including NAD+ and SAM, may potentially define the therapeutic mechanism-of-action of NNMT inhibitors.

Nicotinamide N-methyltransferase expression decreases in iron overload, exacerbating toxicity in mouse hepatocytes

Iron overload causes the generation of reactive oxygen species that can lead to lasting damage to the liver and other organs. The goal of this study was to identify genes that modify the toxicity of iron overload. We studied the effect of iron overload on the hepatic transcriptional and metabolomic profile in mouse models using a dietary model of iron overload and a genetic model, the hemojuvelin knockout mouse. We then evaluated the correlation of nicotinamide N-methyltransferase (NNMT) expression with body iron stores in human patients and the effect of NNMT knockdown on gene expression and viability in primary mouse hepatocytes. We found that iron overload induced significant changes in the expression of genes and metabolites involved in glucose and nicotinamide metabolism and that NNMT, an enzyme that methylates nicotinamide and regulates hepatic glucose and cholesterol metabolism, is one of the most strongly down-regulated genes in the liver in both genetic and dietary iron overload. We found that hepatic NNMT expression is inversely correlated with serum ferritin levels and serum transferrin saturation in patients who are obese, suggesting that body iron stores regulate human liver NNMT expression. Furthermore, we demonstrated that adenoviral knockdown of NNMT in primary mouse hepatocytes exacerbates iron-induced hepatocyte toxicity and increases expression of transcriptional markers of oxidative and endoplasmic reticulum stress, while overexpression of NNMT partially reversed these effects. Conclusion: Iron overload alters glucose and nicotinamide transcriptional and metabolic pathways in mouse hepatocytes and decreases NNMT expression, while NNMT deficiency worsens the toxic effect of iron overload. For these reasons, NNMT may be a drug target for the prevention of iron-induced hepatotoxicity. (Hepatology Communications 2017;1:803-815).

Association between Nicotinamide N-Methyltransferase Gene Polymorphisms and Obesity in Chinese Han Male College Students

Some reports have shown that nicotinamide N-methyltransferase (NNMT) is associated with the body mass index (BMI) and energy metabolism. Here we explored the association between NNMT gene polymorphisms and obesity. The subjects were recruited from male Chinese Han college student. 289 of them (19 ≤ body fat percentage (BF%)) were selected as the high body fat group (HBFG), 494 of them (3 ≤ BF% < 13.5) were selected as the low body fat group (LBFG), and then a case-control study (fat versus thin) was carried out to explore the association between the NNMT gene polymorphism and the body composition using tagSNPs method. A tagSNP (rs10891644) in NNMT gene was found significantly associated with the body composition (P < 0.0026). At this locus, the BF% for the genotype GT, TT, and GG were 14.56 ± 8.35, 13.47 ± 8.11, and 12.42 ± 7.50, respectively, and the differences between the GT and the GG + TT were highly significant (P < 0.01); the OR_adjusted value of the GT versus (GG + TT) was 1.716 (P_adjusted = 0.002, 95% CI = 1.240–2.235). Therefore, the variation of the tagSNP, rs10891644, is significantly associated with obesity and the GT carriers are the susceptible population.