Atherosclerosis 204 (2009) 412–417 Contents lists available at ScienceDirect Atherosclerosis journal homepage: www.elsevier.com/locate/atherosclerosis Adipose tissue as a source of nicotinamide N-methyltransferase and homocysteine Monika Riederer a,c,∗ , Wolfgang Erwa b , Robert Zimmermann a , Saˇ sa Frank c , Rudolf Zechner a a Institute of Molecular Biosciences, University of Graz, Heinrichstrasse 31A, A-8010 Graz, Austria b Clinical Institute of Medical and Chemical Laboratory Diagnosis, Medical University Graz, Austria c Institute of Molecular Biology and Biochemistry, Medical University Graz, Austria article info Article history: Received 11 April 2007 Received in revised form 10 September 2008 Accepted 10 September 2008 Available online 27 September 2008 Keywords: NNMT Nicotinamide Methyltransferase Adipose tissue Homocysteine Nicotinic acid Atherosclerosis abstract Nicotinamide N-methyltransferase (NNMT) catalyses the conversion of nicotinamide to 1- methylnicotinamide and plays an important role in hepatic detoxification reactions. Here we show that, in addition to the liver, 3T3-L1 adipocytes as well as human and murine adipose tissue explants express high amounts of enzymatically active NNMT. NNMT mRNA levels and enzyme activity increased in 3T3-L1 cells in a differentiation-dependent manner. Homocysteine, the atherogenic product of the NNMT-catalyzed reaction, was secreted from 3T3-L1 cells or adipose tissue cultures. Homocysteine release increased during 3T3-L1 differentiation and was reduced when adipose tissue was treated with the NNMT inhibitor 1-methylnicotinamide. Nicotinic acid (NA), a widely used drug to lower elevated plasma lipid levels, induced NNMT enzyme activity in white adipose tissue of mice. In tissue culture nicotinamide treatment led to an increase in adipose tissue homocysteine secretion. These data support the concept that adipose tissue NNMT contributes to the increased plasma homocysteine levels in patients treated with NA. © 2008 Elsevier Ireland Ltd. All rights reserved. 1. Introduction Nicotinamide N-methyltransferase (NNMT) is an important cytosolic methyltransferase involved in detoxification reactions of the liver. The enzyme catalyzes the reaction of S-adenosyl-l- methionine with nicotinamide (and structurally related pyridines) yielding S-adenosyl-l-homocysteine and 1-methylnicotinamide. NNMT is structurally and functionally related to thioether S- methyltransferase and phenylethanolamine N-methyltransferase [1,2]. Nicotinamide (NAm), the amide of nicotinic acid (NA, vitamin B3), is the precursor of the coenzyme beta-nicotinamide adenine dinucleotide (NAD), an essential cofactor for numerous oxido- reductases. With S-adenosyl-l-methionine as the methyl group donor, the NNMT reaction yields two products: 1-methylnicotinamide, which is excreted into urine, and S-adenosyl-l-homocysteine, which is converted into homocysteine by S-adenosyl-l-homocysteine hydrolase [3]. As hyperhomocysteinemia is a suspected risk fac- ∗ Corresponding author at: Institute of Molecular Biology and Biochemistry, Med- ical University Graz, Harrachgasse 21, A-8010 Graz, Austria. Tel.: +43 316 385 4483; fax: +43 316 380 9615. E-mail address: monika.riederer@uni-graz.at (M. Riederer). tor for atherosclerosis, factors affecting the plasma concentrations of homocysteine attracted significant clinical interest. To lower plasma homocysteine levels, enzymes involved in homocysteine transformation, such as methylenetetrahydrofolate reductase, cystathionine-beta-synthase, and methionine synthase, have been targeted for increased activity by folic acid, vitamin B12, and vita- min B6 supplementation, respectively [4,5]. In a genome-wide association study, Souto et al. [6] recently identified the NNMT gene as a potential major genetic determinant of plasma homocysteine levels. NNMT is found in the liver [7] catalyzing the biotransforma- tion of many drugs and xenobiotic compounds [8]. Inter-individual variation in the level of NNMT activity in the human liver may contribute to the interindividual variability in drug metabolism, therapeutic efficacy, and drug toxicity [1,2]. Specifically, due to its homology to nicotinamide, NNMT may participate in the metabolism of NA, a widely used drug to lower elevated plasma lipid levels [9], increase the concentration of plasma HDL choles- terol [10], and reduce the progression of atherosclerosis [11]. The observation that the initial effects of NA are con- fined to processes in adipose tissue (e.g., inhibition of lipolysis [12]) and microarray-based gene expression data (http://symatlas.gnf.org/SymAtlas/) suggested the presence of murine NNMT in white adipose tissue (WAT). This prompted us 0021-9150/$ – see front matter © 2008 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.atherosclerosis.2008.09.015