Differences between murine arylamine N-acetyltransferase type 1 and human arylamine N-acetyltransferase type 2 defined by substrate specificity and inhibitor binding

Small Molecule Inhibitors of Arylamine N-Acetyltransferase 1 Attenuate Cellular Respiration

Arylamine N-acetyltransferase 1 (NAT1) expression has been shown to attenuate mitochondrial function, suggesting it is a promising drug target in diseases of mitochondrial dysfunction. Here, several second-generation naphthoquinones have been investigated as small molecule inhibitors of NAT1. The results show that the compounds inhibit both in vitro and in whole cells. A lead compound (Cmp350) was further investigated for its ability to alter mitochondrial metabolism in MDA-MB-231 cells. At concentrations that inhibited NAT1 by over 85%, no overt toxicity was observed. Moreover, the inhibitor decreased basal respiration and reserve respiratory capacity without affecting ATP production. Cells treated with Cmp350 were almost exclusively dependent on glucose as a fuel source. We postulate that Cmp350 is an excellent lead compound for the development of NAT1-targeted inhibitors as both experimental tools and therapeutics in the treatment of hypermetabolic diseases such as amyotrophic lateral sclerosis, cancer cachexia, and sepsis.

The Arylamine N-Acetyltransferases as Therapeutic Targets in Metabolic Diseases Associated with Mitochondrial Dysfunction

In humans, there are two arylamine N-acetyltransferase genes that encode functional enzymes (NAT1 and NAT2) as well as one pseudogene, all of which are located together on chromosome 8. Although they were first identified by their role in the acetylation of drugs and other xenobiotics, recent studies have shown strong associations for both enzymes in a variety of diseases, including cancer, cardiovascular disease, and diabetes. There is growing evidence that this association may be causal. Consistently, NAT1 and NAT2 are shown to be required for healthy mitochondria. This review discusses the current literature on the role of both NAT1 and NAT2 in mitochondrial bioenergetics. It will attempt to relate our understanding of the evolution of the two genes with biologic function and then present evidence that several major metabolic diseases are influenced by NAT1 and NAT2. Finally, it will discuss current and future approaches to inhibit or enhance NAT1 and NAT2 activity/expression using small-molecule drugs. SIGNIFICANCE STATEMENT: The arylamine N-acetyltransferases (NATs) NAT1 and NAT2 share common features in their associations with mitochondrial bioenergetics. This review discusses mitochondrial function as it relates to health and disease, and the importance of NAT in mitochondrial function and dysfunction. It also compares NAT1 and NAT2 to highlight their functional similarities and differences. Both NAT1 and NAT2 are potential drug targets for diseases where mitochondrial dysfunction is a hallmark of onset and progression.

Upregulation of cytidine deaminase in NAT1 knockout breast cancer cells

PURPOSE

Arylamine N-acetyltransferase 1 (NAT1), a phase II metabolic enzyme, is frequently upregulated in breast cancer. Inhibition or depletion of NAT1 leads to growth retardation in breast cancer cells in vitro and in vivo. A previous metabolomics study of MDA-MB-231 breast cancer cells suggests that NAT1 deletion leads to a defect in de novo pyrimidine biosynthesis. In the present study, we observed that NAT1 deletion results in upregulation of cytidine deaminase (CDA), which is involved in the pyrimidine salvage pathway, in multiple breast cancer cell lines (MDA-MB-231, MCF-7 and ZR-75-1). We hypothesized that NAT1 KO MDA-MB-231 cells show differential sensitivity to drugs that either inhibit cellular pyrimidine homeostasis or are metabolized by CDA.

METHODS

The cells were treated with (1) inhibitors of dihydroorotate dehydrogenase or CDA (e.g., teriflunomide and tetrahydrouridine); (2) pyrimidine/nucleoside analogs (e.g., gemcitabine and 5-azacytidine); and (3) naturally occurring, modified cytidines (e.g., 5-formyl-2'-deoxycytidine; 5fdC).

RESULTS

Although NAT1 KO cells failed to show differential sensitivity to nucleoside analogs that are metabolized by CDA, they were markedly more sensitive to 5fdC which induces DNA damage in the presence of high CDA activity. Co-treatment with 5fdC and a CDA inhibitor, tetrahydrouridine, abrogated the increase in 5fdC cytotoxicity in NAT1 KO cells, suggesting that the increased sensitivity of NAT1 KO cells to 5fdC is dependent on their increased CDA activity.

CONCLUSIONS

The present findings suggest a novel therapeutic strategy to treat breast cancer with elevated NAT1 expression. For instance, NAT1 inhibition may be combined with cytotoxic nucleosides (e.g., 5fdC) for breast cancer treatment.

Non-coding and intergenic genetic variants of human arylamine N-acetyltransferase 2 (NAT2) gene are associated with differential plasma lipid and cholesterol levels and cardiometabolic disorders

Arylamine N-acetyltransferase 2 (NAT2) is a phase II metabolic enzyme, best known for metabolism of aromatic amines and hydrazines. Genetic variants occurring in the NAT2 coding region have been well-defined and are known to affect the enzyme activity or protein stability. Individuals can be categorized into rapid, intermediate, and slow acetylator phenotypes that significantly alter their ability to metabolize arylamines, including drugs (e.g., isoniazid) and carcinogens (e.g., 4-aminobiphenyl). However, functional studies on non-coding or intergenic variants of NAT2 are lacking. Multiple, independent genome wide association studies (GWAS) have reported that non-coding or intergenic variants of NAT2 are associated with elevated plasma lipid and cholesterol levels, as well as cardiometabolic disorders, suggesting a novel cellular role of NAT2 in lipid and cholesterol homeostasis. The current review highlights and summarizes GWAS reports that are relevant to this association. We also present a new finding that seven, non-coding, intergenic NAT2 variants (i.e., rs4921913, rs4921914, rs4921915, rs146812806, rs35246381, rs35570672, and rs1495741), which have been associated with plasma lipid and cholesterol levels, are in linkage disequilibrium with one another, and thus form a novel haplotype. The dyslipidemia risk alleles of non-coding NAT2 variants are associated with rapid NAT2 acetylator phenotype, suggesting that differential systemic NAT2 activity might be a risk factor for developing dyslipidemia. The current review also discusses the findings of recent reports that are supportive of the role of NAT2 in lipid or cholesterol synthesis and transport. In summary, we review data suggesting that human NAT2 is a novel genetic factor that influences plasma lipid and cholesterol levels and alters the risk of cardiometabolic disorders. The proposed novel role of NAT2 merits further investigations.

Arylamine N-Acetyltransferase 1 Activity is Regulated by the Protein Acetylation Status

Arylamine N-acetyltransferase 1 (NAT1) is a drug metabolizing enzyme that influences cancer cell proliferation and survival, especially in breast cancer. Lysine-acetylation is an important Post-Translational Modification (PTM) in the regulation of diverse cellular processes. Histone deacetylases (HDACs) and Sirtuins (SIRT) may have an important role on the NAT1 acetylation status, affecting its catalytic capacity and having an impact on the downstream functions of this protein. The aim of the present work is to investigate the acetylation status of NAT1 in human breast cancer. Breast cancer cell lines MDA-MB-231 (ER-, PR-, HER2-) and ZR-75-1 (estrogen receptor+, PR+, HER2+) were cultured in the presence of HDAC inhibitors (SAHA, TSA) or Sirtuin inhibitors (AGK2, EX527, Sirtinol). Under these conditions, NAT1 protein and gene expression as well as enzymatic activity were quantified. Acetylation of NAT1 protein was evaluated following an immunoprecipitation protocol and acetyl-Lysine quantification. Sirt1 and Sirt2 knockdown were performed and NAT1 protein and NAT1 mRNA expression and catalytic activity were quantified. The treatment of MDA-MB-231 or ZR-75-1 cells with increasing HDAC inhibitors resulted in 2 to 15-fold upregulation in NAT1 message expression. Finally, the catalytic activity of NAT1 in the presence of HDAC inhibition increased 2-fold. Conversely, the inhibition of Sirtuin activity did not cause significant changes in NAT1 message but produced a significant decrease in NAT1 catalytic activity. NAT1 acetylation was higher in the cells treated with HDAC inhibitors, as well as Sirtuin inhibitors. Finally, silencing of Sirt1 and Sirt2 genes by siRNA transient knockdown of each or both genes resulted in reduction of NAT1 protein expression and catalytic activity. The use of HDAC and Sirtuin inhibitors has been demonstrated as a promising powerful therapeutic alternative in various cancers. These inhibitors can significantly attenuate tumor burden by limiting tumor growth and metastasis. These compounds can also induce DNA damage, cell cycle arrest, apoptosis, and autophagy to promote cancer cell death. Several studies have shown that NAT1 is upregulated in cancer cells. The results of the present study show that the acetylation status of NAT1 is an important factor that might have a relevant role in the progression of cancer.

Deletion of arylamine N-acetyltransferase 1 in MDA-MB-231 human breast cancer cells reduces primary and secondary tumor growth in vivo with no significant effects on metastasis

Arylamine N-acetyltransferase 1 (NAT1) is frequently upregulated in breast cancer. Previous studies showed that inhibition or depletion of NAT1 in breast cancer cells diminishes anchorage-independent growth in culture, suggesting that NAT1 contributes to breast cancer growth and metastasis. To further investigate the contribution of NAT1 to growth and cell invasive/migratory behavior, we subjected parental and NAT1 knockout (KO) breast cancer cell lines (MDA-MB-231, MCF-7, and ZR-75-1) to multiple assays. The rate of cell growth in suspension was not consistently decreased in NAT1 KO cells across the cell lines tested. Similarly, cell migration and invasion assays failed to produce reproducible differences between the parental and NAT1 KO cells. To overcome the limitations of in vitro assays, we tested parental and NAT1 KO cells in vivo in a xenograft model by injecting cells into the flank of immunocompromised mice. NAT1 KO MDA-MB-231 cells produced primary tumors smaller than those formed by parental cells, which was contributed by an increased rate of apoptosis in KO cells. The frequency of lung metastasis, however, was not altered in NAT1 KO cells. When the primary tumors of the parental and NAT1 KO cells were allowed to grow to a pre-determined size or delivered directly via tail vein, the number and size of metastatic foci in the lung did not differ between the parental and NAT1 KO cells. In conclusion, NAT1 contributes to primary and secondary tumor growth in vivo in MDA-MB-231 breast cancer cells but does not appear to affect its metastatic potential.

Functional variability of rhesus macaque (Macaca mulatta) NAT2 gene for drug-metabolising arylamine N-acetyltransferase 2

Human NAT2 is a polymorphic pharmacogene encoding for N-acetyltransferase 2, a hepatic enzyme active towards arylamine and arylhydrazine drugs, including the anti-tubercular antibiotic isoniazid. The isoenzyme also modulates susceptibility to chemical carcinogenesis, particularly of the bladder. Human NAT2 represents an ideal model for anthropological investigations into the demographic adaptation of worldwide populations to their xenobiotic environment. Its sequence appears to be subject to positive selection pressures that are population-specific and may be attributed to gene-environment interactions directly associated with exogenous chemical challenges. However, recent evidence suggests that the same evolutionary pattern may not be observed in other primates. Here, we report NAT2 polymorphism in 25 rhesus macaques (Macaca mulatta) and compare the frequencies and functional characteristics of 12 variants. Seven non-synonymous single nucleotide variations (SNVs) were identified, including one nonsense mutation. The missense SNVs were demonstrated to affect enzymatic function in a substrate-dependent manner, albeit more moderately than certain NAT1 SNVs recently characterised in the same cohort. Haplotypic and functional variability of NAT2 was comparable to that previously observed for NAT1 in the same population sample, suggesting that the two paralogues may have evolved under similar selective pressures in the rhesus macaque. This is different to the population variability distribution pattern reported for humans and chimpanzees. Recorded SNVs were also different from those found in other primates. The study contributes to further understanding of NAT2 functional polymorphism in the rhesus macaque, a non-human primate model used in biomedicine and pharmacology, indicating variability in xenobiotic acetylation that could affect drug metabolism.

Moonlighting in drug metabolism

Drug metabolizing enzymes catalyze the biotransformation of many of drugs and chemicals. The drug metabolizing enzymes are distributed among several evolutionary families and catalyze a range of detoxication reactions, including oxidation/reduction, conjugative, and hydrolytic reactions that serve to detoxify potentially toxic compounds. This detoxication function requires that drug metabolizing enzymes exhibit substrate promiscuity. In addition to their catalytic functions, many drug metabolizing enzymes possess functions unrelated to or in addition to catalysis. Such proteins are termed 'moonlighting proteins' and are defined as proteins with multiple biochemical or biophysical functions that reside in a single protein. This review discusses the diverse moonlighting functions of drug metabolizing enzymes and the roles they play in physiological functions relating to reproduction, vision, cell signaling, cancer, and transport. Further research will likely reveal new examples of moonlighting functions of drug metabolizing enzymes.

Comparative analysis of xenobiotic metabolising N-acetyltransferases from ten non-human primates as in vitro models of human homologues

Xenobiotic metabolising N-acetyltransferases (NATs) perform biotransformation of drugs and carcinogens. Human NAT1 is associated with endogenous metabolic pathways of cells and is a candidate drug target for cancer. Human NAT2 is a well-characterised polymorphic xenobiotic metabolising enzyme, modulating susceptibility to drug-induced toxicity. Human NATs are difficult to express to high purification yields, complicating large-scale production for high-throughput screens or use in sophisticated enzymology assays and crystallography. We undertake comparative functional investigation of the NAT homologues of ten non-human primates, to characterise their properties and evaluate their suitability as models of human NATs. Considering the amount of generated recombinant protein, the enzymatic activity and thermal stability, the NAT homologues of non-human primates are demonstrated to be a much more effective resource for in vitro studies compared with human NATs. Certain NAT homologues are proposed as better models, such as the NAT1 of macaques Macaca mulatta and M. sylvanus, the NAT2 of Erythrocebus patas, and both NAT proteins of the gibbon Nomascus gabriellae which show highest homology to human NATs. This comparative investigation will facilitate in vitro screens towards discovery and optimisation of candidate pharmaceutical compounds for human NAT isoenzymes, while enabling better understanding of NAT function and evolution in primates.

Treatment of Rats with Apocynin Has Considerable Inhibitory Effects on Arylamine N-Acetyltransferase Activity in the Liver

The effect of apocynin on the activity of arylamine N-acetyltransferases (NATs) in excised liver samples was examined using eighteen Sprague-Dawley rats. Three groups of six animals each were fed a normal diet alone or a treatment of 50 or 100 mg/kg/day of apocynin via gavages for eight (8) weeks. Chronic in vivo administration of apocynin led to significant (p < 0.001) reduction of in vitro liver NAT activity up to 93% as compared with untreated rats (18.80 ± 2.10 μmols p-anisidine/min/μg liver protein). In vitro exposure of untreated liver homogenates to apocynin led to a dose-dependent inhibition of NAT activity with IC₅₀ = 0.69 ± 0.02 mM. In silico modelling of apocynin tautomers and radical species into human NAT crystal structures supported the hypothesis that thiol functionalities in NAT enzymes may be crucial in apocynin binding. The involvement of human NAT enzymes in different pathological conditions, such as cancer, has encouraged the research for selective NAT inhibitors in both humans and animal models with possible chemopreventive properties.