Nicotinamide N-methyltransferase knockdown protects against diet-induced obesity

Abstract

In obesity and type 2 diabetes, Glut4 glucose transporter expression is decreased selectively in adipocytes1. Adipose-specific knockout or overexpression of Glut4 alters systemic insulin sensitivity2. Here we show, using DNA array analyses, that nicotinamide N-methyltransferase (Nnmt) is the most strongly reciprocally regulated gene when comparing gene expression in white adipose tissue (WAT) from adipose-specific Glut4-knockout or adipose-specific Glut4-overexpressing mice with their respective controls. NNMT methylates nicotinamide (vitamin B3) using S-adenosylmethionine (SAM) as a methyl donor3,4. Nicotinamide is a precursor of NAD+, an important cofactor linking cellular redox states with energy metabolism5. SAM provides propylamine for polyamine biosynthesis and donates a methyl group for histone methylation6. Polyamine flux including synthesis, catabolism and excretion, is controlled by the rate-limiting enzymes ornithine decarboxylase (ODC) and spermidine–spermine N1-acetyltransferase (SSAT; encoded by Sat1) and by polyamine oxidase (PAO), and has a major role in energy metabolism7,8. We report that NNMT expression is increased in WAT and liver of obese and diabetic mice. Nnmt knockdown in WAT and liver protects against diet-induced obesity by augmenting cellular energy expenditure. NNMT inhibition increases adipose SAM and NAD+ levels and upregulates ODC and SSAT activity as well as expression, owing to the effects of NNMT on histone H3 lysine 4 methylation in adipose tissue. Direct evidence for increased polyamine flux resulting from NNMT inhibition includes elevated urinary excretion and adipocyte secretion of diacetylspermine, a product of polyamine metabolism. NNMT inhibition in adipocytes increases oxygen consumption in an ODC-, SSAT- and PAO-dependent manner. Thus, NNMT is a novel regulator of histone methylation, polyamine flux and NAD+-dependent SIRT1 signalling, and is a unique and attractive target for treating obesity and type 2 diabetes.

Loss of the RNA polymerase III repressor MAF1 confers obesity resistance

“…To examine this issue, we measured the total cellular concentration of NAD + in wild-type and Maf1 −/− livers and skeletal muscle by liquid chromatography-mass spectrometry (LC-MS). Consistent with the view that NAD + synthesis in the liver is not limited by the activity of the nicotinamide salvage pathway (Houtkooper et al 2010;Kraus et al 2014), the level of NAD + in Maf1 −/− livers was the same as for wild-type tissue (Fig. 6E).…”

“…S6B; Supplemental Table S1). Perturbations of polyamine synthesis have been linked to changes in adiposity (Jell et al 2007;Pirinen et al 2007), and mice expressing reduced levels of nicotinamide N-methyltransferase (NNMT), which influences polyamine synthesis, are obesity-resistant (Kraus et al 2014). Consistent with these observations and the increased level of spermidine in Maf1 −/− tissues, the expression of Nnmt mRNA was significantly reduced in the liver, as was the level of NNMT protein in the liver and muscle ( Fig.…”

“…S6A; Kraus et al 2014). Increased SAM levels in turn can lead to increased polyamine synthesis, and, indeed, the liver and muscle showed increased levels of polyamine pathway metabolites, including spermidine.…”

“…We infer that the increased energetic cost of these processes alters the balance between fuel utilization and storage and contributes to the lean phenotype. Additional factors contributing to the energy expenditure and obesity resistance of the mice include the reduced expression of NNMT in the liver, muscle, and potentially other tissues and downstream effects on NAD + metabolism and/or the polyamine pathway (Kraus et al 2014;Liu et al 2014). How the different molecular effects of the Maf1 knockout are partitioned in terms of energy expenditure and obesity resistance has yet to be determined, but our current results already establish MAF1 as a novel and unconventional therapeutic target for the treatment of obesity and related diseases.…”

“…Thus, in addition to its potential to affect SAM-dependent methylation reactions and the supply of propylamine groups for polyamine synthesis, NNMT can regulate the availability of NAD + for cellular redox metabolism (Supplemental Fig. S6A; Kraus et al 2014). To examine this issue, we measured the total cellular concentration of NAD + in wild-type and Maf1 −/− livers and skeletal muscle by liquid chromatography-mass spectrometry (LC-MS).…”