Absence of mitochondrial SLC25A51 enhances PARP1-dependent DNA repair by increasing nuclear NAD+ levels

NAD metabolism-related genes provide prognostic value and potential therapeutic insights for acute myeloid leukemia

Introduction

Acute myeloid leukemia (AML) is an aggressive blood cancer with high heterogeneity and poor prognosis. Although the metabolic reprogramming of nicotinamide adenine dinucleotide (NAD) has been reported to play a pivotal role in the pathogenesis of acute myeloid leukemia (AML), the prognostic value of NAD metabolism and its correlation with the immune microenvironment in AML remains unclear.

Methods

We utilized our large-scale RNA-seq data on 655 patients with AML and the NAD metabolism-related genes to establish a prognostic NAD metabolism score based on the sparse regression analysis. The signature was validated across three independent datasets including a total of 1,215 AML patients. ssGSEA and ESTIMATE algorithms were employed to dissect the tumor immune microenvironment. Ex vivo drug screening and in vitro experimental validation were performed to identify potential therapeutic approaches for the high-risk patients. In vitro knockdown and functional experiments were employed to investigate the role of SLC25A51, a mitochondrial NAD+ transporter gene implicated in the signature.

Results

An 8-gene NAD metabolism signature (NADM8) was generated and demonstrated a robust prognostic value in more than 1,800 patients with AML. High NADM8 score could efficiently discriminate AML patients with adverse clinical characteristics and genetic lesions and serve as an independent factor predicting a poor prognosis. Immune microenvironment analysis revealed significant enrichment of distinct tumor-infiltrating immune cells and activation of immune checkpoints in patients with high NADM8 scores, acting as a potential biomarker for immune response evaluation in AML. Furthermore, ex vivo drug screening and in vitro experimental validation in a panel of 9 AML cell lines demonstrated that the patients with high NADM8 scores were more sensitive to the PI3K inhibitor, GDC-0914. Finally, functional experiments also substantiated the critical pathogenic role of the SLC25A51 in AML, which could be a promising therapeutic target.

Conclusion

Our study demonstrated that NAD metabolism-related signature can facilitate risk stratification and prognosis prediction in AML and guide therapeutic decisions including both immunotherapy and targeted therapies.

Channeling Nicotinamide Phosphoribosyltransferase (NAMPT) to Address Life and Death

Nicotinamide phosphoribosyltransferase (NAMPT) catalyzes the rate-limiting step in NAD+ biosynthesis via salvage of NAM formed from catabolism of NAD+ by proteins with NADase activity (e.g., PARPs, SIRTs, CD38). Depletion of NAD+ in aging, neurodegeneration, and metabolic disorders is addressed by NAD+ supplementation. Conversely, NAMPT inhibitors have been developed for cancer therapy: many discovered by phenotypic screening for cancer cell death have low nanomolar potency in cellular models. No NAMPT inhibitor is yet FDA-approved. The ability of inhibitors to act as NAMPT substrates may be associated with efficacy and toxicity. Some 3-pyridyl inhibitors become 4-pyridyl activators or "NAD+ boosters". NAMPT positive allosteric modulators (N-PAMs) and boosters may increase enzyme activity by relieving substrate/product inhibition. Binding to a "rear channel" extending from the NAMPT active site is key for inhibitors, boosters, and N-PAMs. A deeper understanding may fulfill the potential of NAMPT ligands to regulate cellular life and death.

ADP-ribosylation: An emerging direction for disease treatment

ADP-ribosylation (ADPr) is a dynamically reversible post-translational modification (PTM) driven primarily by ADP-ribosyltransferases (ADPRTs or ARTs), which have ADP-ribosyl transfer activity. ADPr modification is involved in signaling pathways, DNA damage repair, metabolism, immunity, and inflammation. In recent years, several studies have revealed that new targets or treatments for tumors, cardiovascular diseases, neuromuscular diseases and infectious diseases can be explored by regulating ADPr. Here, we review the recent research progress on ART-mediated ADP-ribosylation and the latest findings in the diagnosis and treatment of related diseases.

NMNATs expression inhibition mediated NAD+ deficiency plays a critical role in doxorubicin-induced hepatotoxicity in mice

Doxorubicin (DOX) is one of the most widely used antineoplastic drugs with known cardiotoxicity while other organ toxicity, such as hepatotoxicity is not well defined. This study was to explore the role of nicotinamide adenine dinucleotide (NAD+) in DOX-induced hepatotoxicity. DOX (20 mg/kg) induced acute liver injury and oxidative stress in C57BL/6 J mice at 48 h. Notably, the expression of nuclear factor erythroid 2-related factor 2 (Nrf2) and NAD(P)H dehydrogenase quinone 1 (NQO1) were downregulated. NAD+ deficiency was confirmed due to DOX exposure. Mechanistically, the downregulation of nicotinamide mononucleotide adenylyl transferase 1 (NMNAT1), NMNAT2 and NMNAT3, while no alteration of nicotinamide phosphoribosyl transferase was proved. As a consequence of NAD+ deficiency, the expression of poly-ADP-ribose polymerase1 (PARP1), CD38 and Sirtuin1 (SIRT1) were reduced. Furthermore, supplementation of NAD+ (200 mg/kg/day) or its precursor nicotinamide mononucleotide (NMN) (500 mg/kg/day) alleviated liver injury, attenuated oxidative stress, and elevated the downregulation of Nrf2 and NQO1. More importantly, compromised expression of NMNAT1-3, PARP1, CD38 and SIRT1 were improved by NAD+ and NMN. In conclusion, NAD+ deficiency due to NMNATs expression inhibition may attribute to the pathogenesis of DOX-induced hepatotoxicity, thus providing new insights for mitigating DOX side effects.

Layered mechanisms regulating the human mitochondrial NAD+ transporter SLC25A51

SLC25A51 is the primary mitochondrial NAD+ transporter in humans and controls many local reactions by mediating the influx of oxidized NAD+. Intriguingly, SLC25A51 lacks several key features compared with other members in the mitochondrial carrier family, thus its molecular mechanism has been unclear. A deeper understanding would shed light on the control of cellular respiration, the citric acid cycle, and free NAD+ concentrations in mammalian mitochondria. This review discusses recent insights into the transport mechanism of SLC25A51, and in the process highlights a multitiered regulation that governs NAD+ transport. The aspects regulating SLC25A51 import activity can be categorized as contributions from (1) structural characteristics of the transporter itself, (2) its microenvironment, and (3) distinctive properties of the transported ligand. These unique mechanisms further evoke compelling new ideas for modulating the activity of this transporter, as well as new mechanistic models for the mitochondrial carrier family.