Nicotinamide N-methyltransferase catalyses the N-methylation of the endogenous β-carboline norharman: evidence for a novel detoxification pathway

Proximity-Dependent Labeling of Cysteines

Mapping protein-protein interactions is crucial for understanding various signaling pathways in living cells, and developing new techniques for this purpose has attracted significant interest. Classic methods (e.g., the yeast two-hybrid) have been supplanted by more sophisticated chemical approaches that label proximal proteins (e.g., BioID, APEX). Herein we describe a proximity-based approach that uniquely labels cysteines. Our approach exploits the nicotinamide N-methyltransferase (NNMT)-catalyzed methylation of an alkyne-substituted 4-chloropyridine (SS6). Upon methylation of the pyridinium nitrogen, this latent electrophile diffuses out of the active site and labels proximal proteins on short time scales (≤5 min). We validated this approach by identifying known (and novel) interacting partners of protein arginine deiminase 2 (PAD2) and pyruvate dehydrogenase kinase 1 (PDK1). To our knowledge, this technology uniquely exploits a suicide substrate to label proximal cysteines in live cells.

Nicotinamide N-Methyltransferase: An Emerging Protagonist in Cancer Macro(r)evolution

Nicotinamide N-methyltransferase (NNMT) has progressed from being considered merely a Phase II metabolic enzyme to one with a central role in cell function and energy metabolism. Over the last three decades, a significant body of evidence has accumulated which clearly demonstrates a central role for NNMT in cancer survival, metastasis, and drug resistance. In this review, we discuss the evidence supporting a role for NNMT in the progression of the cancer phenotype and how it achieves this by driving the activity of pro-oncogenic NAD+-consuming enzymes. We also describe how increased NNMT activity supports the Warburg effect and how it promotes oncogenic changes in gene expression. We discuss the regulation of NNMT activity in cancer cells by both post-translational modification of the enzyme and transcription factor binding to the NNMT gene, and describe for the first time three long non-coding RNAs which may play a role in the regulation of NNMT transcription. We complete the review by discussing the development of novel anti-cancer therapeutics which target NNMT and provide insight into how NNMT-based therapies may be best employed clinically.

A new underlying mechanism for the neuroprotective effect of bosutinib: Reverting toxicity-induced PARylation in SIN1-mediated neurotoxicity

Increased levels of reactive oxygen and nitrogen species play an important role in the development and progression of neurodegenerative diseases, such as Alzheimer's and Parkinson's disease. The overproduction of these highly reactive chemical species leads to DNA damage and subsequent activation of the poly(ADP-ribose)polymerase (PARP) enzyme. Several studies have demonstrated the potential use of PARP inhibitors for neuroprotection. We previously reported that the dual Src/Abl kinase inhibitor bosutinib (BOS) decreases PARP activity and acts as a chemosensitizer in cancer cells. In this study, we evaluated the neuroprotective potential of BOS with respect to its inhibitory effect on cellular poly(ADP-ribos)ylation (PARylation) using a 3-morpholinosydnonimine (SIN1)-mediated cellular toxicity model. Our data suggest that pretreatment with BOS, especially at lower doses, significantly decreased the level of SIN1-induced cellular PARylation. This regulation pattern of PARylation was found to be associated with the protective effect of BOS against SIN1 on the viability of retinoic acid-differentiated SH-SY5Y cells. Furthermore, while PARP-1 expression was decreased, phosphorylation of SAPK/JNK was not reverted at the observed neuroprotective doses of BOS. In conclusion, we suggest a novel mechanism for the neuroprotective effect of BOS involving the inhibition of cellular PARylation.

The Utility of Nicotinamide N-Methyltransferase as a Potential Biomarker to Predict the Oncological Outcomes for Urological Cancers: An Update

Nicotinamide N-methyltransferase (NNMT) catalyzes the N-methylation reaction of nicotinamide, using S-adenosyl-L-methionine as the methyl donor. Enzyme overexpression has been described in many non-neoplastic diseases, as well as in a wide range of solid malignancies. This review aims to report and discuss evidence available in scientific literature, dealing with NNMT expression and the potential involvement in main urologic neoplasms, namely, renal, bladder and prostate cancers. Data illustrated in the cited studies clearly demonstrated NNMT upregulation (pathological vs. normal tissue) in association with these aforementioned tumors. In addition to this, enzyme levels were also found to correlate with key prognostic parameters and patient survival. Interestingly, NNMT overexpression also emerged in peripheral body fluids, such as blood and urine, thus leading to candidate the enzyme as promising biomarker for the early and non-invasive detection of these cancers. Examined results undoubtedly showed NNMT as having the capacity to promote cell proliferation, migration and invasiveness, as well as its potential participation in fundamental events highlighting cancer progression, metastasis and resistance to chemo- and radiotherapy. In the light of this evidence, it is reasonable to attribute to NNMT a promising role as a potential biomarker for the diagnosis and prognosis of urologic neoplasms, as well as a molecular target for effective anti-cancer treatment.

Macrocyclic Peptides as a Novel Class of NNMT Inhibitors: A SAR Study Aimed at Inhibitory Activity in the Cell

Nicotinamide N-methyltransferase (NNMT), which catalyzes the methylation of nicotinamide, is a cytosolic enzyme that has attracted much attention as a therapeutic target for a variety of diseases. However, despite the considerable interest in this target, reports of NNMT inhibitors have still been limited to date. In this work, utilizing in vitro translated macrocyclic peptide libraries, we identified peptide 1 as a novel class of NNMT inhibitors. Further exploration based on the X-ray cocrystal structures of the peptides with NNMT provided a dramatic improvement in inhibitory activity (peptide 23: IC₅₀ = 0.15 nM). Furthermore, by balance of the peptides' lipophilicity and biological activity, inhibitory activity against NNMT in cell-based assay was successfully achieved (peptide 26: cell-based IC₅₀ = 770 nM). These findings illuminate the potential of cyclic peptides as a relatively new drug discovery modality even for intracellular targets.

Transcriptional Profiles Reveal Deregulation of Lipid Metabolism and Inflammatory Pathways in Neurons Exposed to Palmitic Acid

The effects of the consumption of high-fat diets (HFD) have been studied to unravel the molecular pathways they are altering in order to understand the link between increased caloric intake, metabolic diseases, and the risk of cognitive dysfunction. The saturated fatty acid, palmitic acid (PA), is the main component of HFD and it has been found increased in the circulation of obese and diabetic people. In the central nervous system, PA has been associated with inflammatory responses in astrocytes, but the effects on neurons exposed to it have not been largely investigated. Given that PA affects a variety of metabolic pathways, we aimed to analyze the transcriptomic profile activated by this fatty acid to shed light on the mechanisms of neuronal dysfunction. In the current study, we profiled the transcriptome response after PA exposition at non-toxic doses in primary hippocampal neurons. Gene ontology and Reactome pathway analysis revealed a pattern of gene expression which is associated with inflammatory pathways, and importantly, with the activation of lipid metabolism that is considered not very active in neurons. Validation by quantitative RT-PCR (qRT-PCR) of Hmgcs2, Angptl4, Ugt8, and Rnf145 support the results obtained by RNAseq. Overall, these findings suggest that neurons are able to respond to saturated fatty acids changing the expression pattern of genes associated with inflammatory response and lipid utilization that may be involved in the neuronal damage associated with metabolic diseases.

N-Methyl-β-carboline alkaloids: structure-dependent photosensitizing properties and localization in subcellular domains

N-Methyl-β-carboline (βC) alkaloids, including normelinonine F (1b) and melinonine F (2b), have been found in a vast range of living species playing different biological, biomedical and/or pharmacological roles. Despite this, molecular bases of the mechanisms through which these alkaloids would exert their effect still remain unknown. Fundamental aspects including the photosensitizing properties and intracellular internalization of a selected group of N-methyl-βC alkaloids were investigated herein. Data reveal that methylation of the βC main ring enhances its photosensitizing properties either by increasing its binding affinity with DNA as a biomolecular target and/or by increasing its oxidation potential, in a structure-dependent manner. As a general rule, N(9)-substituted βCs showed the highest photosensitizing efficiency. With the exception of 2-methyl-harminium, all the N-methyl-βCs investigated herein induce a similar DNA photodamage profile, dominated largely by oxidized purines. This fact represents a distinctive behavior when comparing with N-unsubstituted-βCs. On the other hand, although all the investigated compounds might accumulate mainly into the mitochondria of HeLa cells, methylation provides a distinctive dynamic pattern for mitochondrial uptake. While rapid (passive) diffusion is most probably reponsible for the prompt uptake/release of neutral βCs, an active transport appears to mediate the (reatively slow) uptake of the quaternary cationic βCs. This might be a consequence of a distinctive subcellular localization (mitochondrial membrane and/or matrix) or interaction with intracellular components. Biomedical and biotechnological implications are also discussed herein.

Metabonomic study of the intervention effects of Parthenolide on anti-thyroid cancer activity

Thyroid cancer is the most common endocrine malignant tumor in the world, and its incidence is increasing. Although the mortality rate of thyroid cancer is low, its persistence/recurrence rate is high. In addition, some patients with thyroid cancer fail to respond to radiation. Therefore, it is urgent need to develop a novel treatment for thyroid cancer. Parthenolide (PTL), a traditional Chinese medicine Tanacetum parthenium extract, has shown encouraging effects in anti-tumor, anti-inflammatory and anti-malaria. However, it is unclear whether PTL has an anti-thyroid cancer effect and its possible mechanism of action. In the recent years, metabonomics has been widely used in tumors research to explore the pharmacological mechanism of drugs, but few studies used metabonomics to investigate the pharmacological effects of PTL in thyroid tumors. In order to comprehensively reveal the mechanism and effects of PTL on anti-thyroid tumors, metabonomics combined cell biological research methods were conducted. The results showed that PTL promote apoptosis of thyroid cancer cells (TPC-1) in a concentration-dependent manner. The metabolic differences between the PTL group and the control group were compared by metabonomics, and 31 potential metabolites were identified. These metabolites were mainly involved in the tricarboxylic acid cycle, amino acid metabolism, choline metabolism and lipid metabolism. These results implied that PTL may inhibit the proliferation and development of thyroid carcinoma by accelerating oxidation emergency response, inhibiting adenosine triphosphate (ATP) synthesis and metabolic imbalance. The results of this study revealed that PTL can be an effective and potential drug for the treatment of thyroid cancer.

Nicotinamide N-methyltransferase expression in SH-SY5Y human neuroblastoma cells decreases oxidative stress

Nicotinamide N-methyltransferase (NNMT) plays a central role in cellular metabolism, regulating pathways including epigenetic regulation, cell signalling, and energy production. Our previous studies have shown that the expression of NNMT in the human neuroblastoma cell line SH-SY5Y increased complex I activity and subsequent ATP synthesis. This increase in ATP synthesis was lower than the increase in complex I activity, suggesting uncoupling of the mitochondrial respiratory chain. We, therefore, hypothesised that pathways that reduce oxidative stress are also increased in NNMT-expressing SH-Y5Y cells. The expression of uncoupling protein-2 messenger RNA and protein were significantly increased in NNMT-expressing cells (57% ± 5.2% and 20.1% ± 1.5%, respectively; P = .001 for both). Total GSH (22 ± 0.3 vs 35.6 ± 1.1 nmol/mg protein), free GSH (21.9 ± 0.2 vs 33.5 ± 1 nmol/mg protein), and GSSG (0.6 ± 0.02 vs 1 ± 0.05 nmol/mg protein; P = .001 for all) concentrations were significantly increased in NNMT-expressing cells, whereas the GSH:GSSG ratio was decreased (39.4 ± 1.8 vs 32.3 ± 2.5; P = .02). Finally, reactive oxygen species (ROS) content was decreased in NNMT-expressing cells (0.3 ± 0.08 vs 0.12 ± 0.03; P = .039), as was the concentration of 8-isoprostane F2α (200 ± 11.5 vs 45 ± 2.6 pg/mg protein; P = .0012). Taken together, these results suggest that NNMT expression reduced ROS generation and subsequent lipid peroxidation by uncoupling the mitochondrial membrane potential and increasing GSH buffering capacity, most likely to compensate for increased complex I activity and ATP production.

Bisubstrate Inhibitors of Nicotinamide N-Methyltransferase (NNMT) with Enhanced Activity

Nicotinamide N-methyltransferase (NNMT) catalyzes the methylation of nicotinamide to form N-methylnicotinamide. Overexpression of NNMT is associated with a variety of diseases, including a number of cancers and metabolic disorders, suggesting a role for NNMT as a potential therapeutic target. By structural modification of a lead NNMT inhibitor previously developed in our group, we prepared a diverse library of inhibitors to probe the different regions of the enzyme's active site. This investigation revealed that incorporation of a naphthalene moiety, intended to bind the hydrophobic nicotinamide binding pocket via π-π stacking interactions, significantly increases the activity of bisubstrate-like NNMT inhibitors (half-maximal inhibitory concentration 1.41 μM). These findings are further supported by isothermal titration calorimetry binding assays as well as modeling studies. The most active NNMT inhibitor identified in the present study demonstrated a dose-dependent inhibitory effect on the cell proliferation of the HSC-2 human oral cancer cell line.

Development of a Suicide Inhibition-Based Protein Labeling Strategy for Nicotinamide N-Methyltransferase

Nicotinamide N-methyltransferase (NNMT) catalyzes the S-adenosyl-l-methionine-dependent methylation of nicotinamide to form N-methylnicotinamide. This enzyme detoxifies xenobiotics and regulates NAD⁺ biosynthesis. Additionally, NNMT is overexpressed in various cancers. Herein, we describe the first NNMT-targeted suicide substrates. These compounds, which include 4-chloropyridine and 4-chloronicotinamide, exploit the broad substrate scope of NNMT; methylation of the pyridine nitrogen enhances the electrophilicity of the C4 position, thereby promoting an aromatic nucleophilic substitution by C159, a noncatalytic cysteine. On the basis of this activity, we developed a suicide inhibition-based protein labeling strategy using an alkyne-substituted 4-chloropyridine that selectively labels NNMT in vitro and in cells. In total, this study describes the first NNMT-directed activity-based probes.

Nicotinamide N-methyltransferase: potential involvement in cutaneous malignant melanoma

Nicotinamide N-methyltransferase (NNMT) is an enzyme that catalyzes the N-methylation of nicotinamide and pyridine compounds, participating in xenobiotic and drug metabolism. Data on literature have evidenced a possible role of NNMT in many solid cancers, but no data are currently available in cutaneous melanoma. Recent important advances have been achieved in the treatment of advanced melanoma with targeted therapy and immunotherapy. However, the identification of biomarkers that can be used for the detection of early stage disease as well as for monitoring the therapeutic response during treatment is of utmost importance. The aim of this study was to study the possible role of NNMT in melanoma. In the present study, we carried out immunohistochemical analyses to evaluate the expression of the enzyme NNMT in 34 melanomas and 34 nevi. Moreover, we explored the relationship between NNMT levels and the prognostic parameters of patients with melanoma. The results obtained showed significantly (P<0.0001) higher NNMT expression in melanoma compared with that detected in nevi. In addition, a significant (P<0.05) inverse relationship was found between enzyme levels and Breslow thickness, Clark level, the presence/number of mitoses, and ulceration. Taken together, these data seem to suggest that NNMT could represent a molecular biomarker for melanoma, thus highlighting its potential for both diagnosis and prognosis of this neoplasm.

The kinetic analysis of the N-methylation of 4-phenylpyridine by nicotinamide N-methyltransferase: Evidence for a novel mechanism of substrate inhibition

The N-methylation of 4-phenylpyridine produces the neurotoxin 1-methyl-4-phenylpyridinium ion (MPP+). We investigated the kinetics of 4-phenylpyridine N-methylation by nicotinamide N-methyltransferase (NNMT) and its effect upon 4-phenylpyridine toxicity in vitro. Human recombinant NNMT possessed 4-phenylpyridine N-methyltransferase activity, with a specific activity of 1.7 ± 0.03 nmol MPP+ produced/h/mg NNMT. Although the K_m for 4-phenylpyridine was similar to that reported for nicotinamide, its k_cat of 9.3 × 10^-5 ± 2 × 10^-5 s^-1 and specificity constant, k_cat/K_m, of 0.8 ± 0.8 s^-1 M^-1 were less than 0.15% of the respective values for nicotinamide, demonstrating that 4-phenylpyridine is a poor substrate for NNMT. At low (<2.5 mM) substrate concentration, 4-phenylpyridine N-methylation was competitively inhibited by dimethylsulphoxide, with a K_i of 34 ± 8 mM. At high (>2.5 mM) substrate concentration, enzyme activity followed substrate inhibition kinetics, with a K_i of 4 ± 1 mM. In silico molecular docking suggested that 4-phenylpyridine binds to the active site of NNMT in two non-redundant poses, one a substrate binding mode and the other an inhibitory mode. Finally, the expression of NNMT in the SH-SY5Y cell-line had no effect cell death, viability, ATP content or mitochondrial membrane potential. These data demonstrate that 4-phenylpyridine N-methylation by NNMT is unlikely to serve as a source of MPP+. The possibility for competitive inhibition by dimethylsulphoxide should be considered in NNMT-based drug discovery studies. The potential for 4-phenylpyridine to bind to the active site in two binding orientations using the same active site residues is a novel mechanism of substrate inhibition.

Inhibitors of nicotinamide N-methyltransferase designed to mimic the methylation reaction transition state

Nicotinamide N-methyltransferase (NNMT) is an enzyme that catalyses the methylation of nicotinamide to form N'-methylnicotinamide. Both NNMT and its methylated product have recently been linked to a variety of diseases, suggesting a role for the enzyme as a therapeutic target beyond its previously ascribed metabolic function in detoxification. We here describe the systematic development of NNMT inhibitors derived from the structures of the substrates involved in the methylation reaction. By covalently linking fragments of the NNMT substrates a diverse library of bisubstrate-like compounds was prepared. The ability of these compounds to inhibit NNMT was evaluated providing valuable insights into the structural tolerances of the enzyme active site. These studies led to the identification of new NNMT inhibitors that mimic the transition state of the methylation reaction and inhibit the enzyme with activity on par with established methyltransferase inhibitors.

Nicotinamide N-Methyltransferase in Health and Cancer

Over the past decade, the roles of nicotinamide N-methyltransferase and its product 1-methyl nicotinamide have emerged from playing merely minor roles in phase 2 xenobiotic metabolism as actors in some of the most important scenes of human life. In this review, the structures of the gene, messenger RNA, and protein are discussed, together with the role of the enzyme in many of the common cancers that afflict people today.