Inhibitors of NAD+ Production in Cancer Treatment: State of the Art and Perspectives

The addiction of tumors to elevated nicotinamide adenine dinucleotide (NAD+) levels is a hallmark of cancer metabolism. Obstructing NAD+ biosynthesis in tumors is a new and promising antineoplastic strategy. Inhibitors developed against nicotinamide phosphoribosyltransferase (NAMPT), the main enzyme in NAD+ production from nicotinamide, elicited robust anticancer activity in preclinical models but not in patients, implying that other NAD+-biosynthetic pathways are also active in tumors and provide sufficient NAD+ amounts despite NAMPT obstruction. Recent studies show that NAD+ biosynthesis through the so-called "Preiss-Handler (PH) pathway", which utilizes nicotinate as a precursor, actively operates in many tumors and accounts for tumor resistance to NAMPT inhibitors. The PH pathway consists of three sequential enzymatic steps that are catalyzed by nicotinate phosphoribosyltransferase (NAPRT), nicotinamide mononucleotide adenylyltransferases (NMNATs), and NAD+ synthetase (NADSYN1). Here, we focus on these enzymes as emerging targets in cancer drug discovery, summarizing their reported inhibitors and describing their current or potential exploitation as anticancer agents. Finally, we also focus on additional NAD+-producing enzymes acting in alternative NAD+-producing routes that could also be relevant in tumors and thus become viable targets for drug discovery.

Corrigendum: Qualitative and Quantitative NAD+ Metabolomics Lead to Discovery of Multiple Functional Nicotinate N-Glycosyltransferase In Arabidopsis

[This corrects the article DOI: 10.3389/fpls.2019.01164.].

Qualitative and Quantitative NAD+ Metabolomics Lead to Discovery of Multiple Functional Nicotinate N-Glycosyltransferase in Arabidopsis

The Preiss-Handler pathway, which salvages nicotinate (NA) for NAD synthesis, is a conserved biochemical pathway in land plants. We previously demonstrated that various NA conjugations (mainly methylation and glycosylation) shared the NA detoxification function in all tested plants. It remains unclear whether other NA conjugates with low abundance exist in plants. In this study, we discovered at least two additional NA N-glycosides in Arabidopsis, which was tentatively elucidated as nicotinate N-pentoside (NaNP) and NA N-rhamoside (NaNRha), using liquid chromatography triple-quadrupole mass spectrometry (LC-QQQ-MS) with precursor ion-scanning (PreIS). We further quantitatively profile the NAD⁺-related metabolites in 24 tissues of Arabidopsis. Biochemical assays of UGT76C family revealed that UGT76C5 (encoded by At5g05890, previously identified as NaNGT) was a multiple functional nicotinate N-glycosyltransferase, with high preference to UDP-xylose and UDP-arabinose. The deficiency of NaNP and NaNRha in ugt76c5 mutant suggested that UGT76C5 is responsible for biosynthesis of NaNP and NaNRha in planta. We also identify one amino acid difference in PSPG (plant secondary product glycosyltransferase) motif is responsible for the divergence of NaNGT (UGT76C4) and UGT76C5. Taken together, our study not only identifies a novel nicotinate N-glycosyltransferase but also paves the way for investigations of the in planta physiological functions of various NA conjugations.