NNMT recombinant protein: an obesity regulatory hub linking histone methylation, polyamine flux, and NAD⁺ metabolism

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This article focuses on the molecular characteristics and metabolic regulatory functions of nicotinamide N-methyltransferase (NNMT), systematically elaborating its enzymatic mechanism as a cytoplasmic methyltransferase that catalyzes nicotinamide methylation using S-adenosylmethionine as the methyl donor, analyzing the association between upregulated NNMT expression in adipose tissue and liver with obesity and insulin resistance, and exploring the molecular pathways through which NNMT inhibition increases energy expenditure by elevating SAM and NAD⁺ levels, upregulating histone H3K4 methylation, and activating polyamine flux.

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NNMT Recombinant Protein: A Regulatory Hub Linking Histone Methylation, Polyamine Flux, and NAD⁺ Metabolism in Obesity

Summary: This article focuses on the molecular characteristics and metabolic regulatory functions of nicotinamide N-methyltransferase (NNMT), systematically elaborating its enzymatic mechanism as a cytoplasmic methyltransferase that catalyzes nicotinamide methylation using S-adenosylmethionine as the methyl donor, analyzing the association between upregulated NNMT expression in adipose tissue and liver with obesity and insulin resistance, and exploring the molecular pathways by which NNMT inhibition increases energy expenditure through elevating SAM and NAD⁺ levels, upregulating histone H3K4 methylation, and activating polyamine flux.

I. Molecular Characteristics and Tissue Expression Profile of NNMT.

NNMT is a cytoplasmic methyltransferase whose crystal structure has been resolved at 1.88 Å resolution in rhesus macaque and mouse, belonging to the methyltransferase superfamily. The enzyme utilizes S-adenosylmethionine as the methyl donor to catalyze the methylation of nicotinamide to 1-methylnicotinamide, thereby regulating NAD⁺ metabolic homeostasis. NNMT is highly expressed in the liver, adipose tissue, and brain, with lower expression levels in other organs, and participates in the regulation of energy expenditure, lipid metabolism, and insulin sensitivity through the NAD⁺/Sirt1 pathway. This expression distribution, predominantly in adipose tissue and liver with lower levels in other tissues, determines that the metabolic regulatory role of NNMT has clear tissue selectivity, and also provides a potential tissue-specific window for its use as a drug target.

II. Upregulation of NNMT Expression in Obesity and Insulin Resistance.

In adipose-specific Glut4 knockout mice with insulin resistance, adipose tissue Nnmt messenger ribonucleic acid levels increased twofold, whereas in adipose-specific Glut4 overexpression mice with insulin sensitivity, they decreased by 62%. Compared with lean, insulin-sensitive control mice, NNMT protein in white adipose tissue of ob/ob, db/db mice, and high-fat diet-fed mice increased 1.5 to 2-fold. Liver NNMT protein levels were increased in ob/ob and db/db mice, and liver NNMT protein levels also showed an increasing trend in high-fat diet-fed mice. Therefore, NNMT expression is upregulated in adipose tissue and liver of obese and insulin-resistant mouse models. Across 25 different mouse strains, adipose tissue Nnmt expression was highly correlated with fat mass percentage and correlated with the expression of the pro-insulin-resistant adipokine Rbp4, suggesting that elevated adipose tissue NNMT levels may play a causal role in insulin resistance and obesity.

III. Preventive Effect of NNMT Inhibition on Diet-Induced Obesity.

Knockdown of Nnmt with antisense oligonucleotides in high-fat diet-fed mice reduced Nnmt messenger ribonucleic acid and NNMT protein by 60% to 75% in white adipose tissue and by 60% to 90% in liver, with no changes in brown adipose tissue and kidney. Nnmt gene knockdown in adipose tissue and liver protected mice from diet-induced obesity, reducing relative fat mass by 47% and increasing relative lean mass by 15%. Subcutaneous and epididymal fat pad weights of Nnmt gene knockdown mice were lower than those of controls, mainly due to reduced adipocyte size. Insulin sensitivity was enhanced, serum insulin levels and the glucose-insulin product decreased by approximately 50% to 60%, glucose tolerance was improved, high-fat diet-induced hepatic steatosis was prevented, and serum triglyceride and free fatty acid levels were reduced. Therefore, Nnmt gene knockdown in white adipose tissue and liver prevents diet-induced obesity and its harmful metabolic consequences.

IV. Increased Energy Expenditure Rather Than Reduced Intake Mediates the Lean Phenotype.

Mice treated with Nnmt antisense oligonucleotides consumed the same number of calories as controls, but feed efficiency was reduced by approximately 50%. Control mice gained approximately fat per 1 kcal of food consumed, whereas treated mice gained no fat. Fecal lipid excretion was unchanged, so increased energy expenditure rather than reduced food intake or steatorrhea explained the leanness of Nnmt gene knockdown mice. This was further demonstrated by increased oxygen consumption in comprehensive laboratory animal monitoring system analysis performed before body weight differences began to appear. After overnight fasting, Nnmt gene knockdown mice lost more body weight, body fat, and lean mass than controls, with a response similar to that of lean mice fed a normal diet. Core body temperature, brown adipose tissue weight, and uncoupling protein-1 levels were unchanged, so Nnmt gene knockdown in WAT and liver increases energy expenditure, which may be independent of brown adipose tissue-induced energy dissipation or thermogenesis. In vitro, Nnmt gene knockdown in adipocytes led to a 60% increase in oxygen consumption, whereas Nnmt overexpression reduced oxygen consumption, indicating that NNMT regulates energy expenditure in a cell-autonomous manner.

V. Molecular Mechanisms by Which NNMT Regulates SAM, Histone Methylation, and Polyamine Flux.

NNMT uses SAM as a methyl donor to methylate nicotinamide and generate SAH. In mice treated with Nnmt antisense oligonucleotides, SAM and the SAM:SAH ratio in adipose tissue increased by 50%. SAM has two main functions: providing the propylamine group for polyamine biosynthesis and donating methyl groups to substrates including histones. In vivo Nnmt antisense oligonucleotide treatment enhanced ornithine decarboxylase and spermidine-spermine-N1-acetyltransferase activity and mRNA expression in adipose tissue, and the expression of adenosylmethionine decarboxylase, which provides decarboxylated SAM for spermidine and spermine synthesis, was also increased. Increased urinary diacetylspermidine provided direct evidence of enhanced polyamine flux in adipose tissue. At the level of histone modification, mono-, di-, or trimethylation levels of histone H3 lysine 4 were increased in adipose tissue of Nnmt antisense oligonucleotide-treated mice, and chromatin immunoprecipitation experiments showed enrichment of methylated H3K4 on the Odc and Ssat genes. Therefore, inhibiting NNMT alters histone methylation and increases Odc and Ssat expression, thereby leading to activation of polyamine flux. In addition, Nnmt gene knockdown increased NAD⁺ levels per milligram of adipose tissue and the expression of Nampt, Nmnat2, and Nmnat3, and increased expression of SIRT1 target genes Cd36, catalase, and succinate dehydrogenase B, consistent with SIRT1 activation. Together, these data indicate that NNMT is a novel regulator of histone methylation, polyamine flux, and NAD⁺-dependent SIRT1 signaling.

VI. Conclusion.

In NNMT-related basic research and metabolic disease drug development, high-quality recombinant NNMT protein is a core tool for dissecting its methyltransferase activity, evaluating inhibitor binding capacity, and conducting enzyme kinetics studies. Rhesus macaque-derived NNMT protein shares high sequence homology with human NNMT, and its crystal structure has been resolved at high resolution, providing an ideal model protein for structural biology research and cross-species comparison. The His tag facilitates efficient purification via immobilized metal affinity chromatography and enables detection and capture with anti-His tag antibodies. To meet the needs of NNMT functional research and targeted therapeutic development for obesity and metabolic diseases, Ulab provides NNMT His Tag Protein, Rhesus macaque. This product is suitable for applications including establishment of NNMT methyltransferase activity assay methods, in vitro screening and activity evaluation of NNMT inhibitors, research on NNMT-mediated SAM and NAD⁺ metabolic regulation mechanisms, and NNMT structure-based drug design.

This article is reviewed and published by the technical expert team of UA

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