Nicotinamide phosphoribosyltransferase

Reaction

iNAMPT catalyzes the following chemical reaction:

:nicotinamide + 5-phosphoribosyl-1-pyrophosphate (PRPP) \rightleftharpoons nicotinamide mononucleotide (NMN) + pyrophosphate (PPi)

Thus, the two substrates of this enzyme are nicotinamide and 5-phosphoribosyl-1-pyrophosphate (PRRP), whereas its two products are nicotinamide mononucleotide and pyrophosphate.

This enzyme belongs to the family of glycosyltransferases, to be specific, the pentosyltransferases. This enzyme participates in nicotinate and nicotinamide metabolism.

Expression and regulation

The liver has the highest iNAMPT activity of any organ, about 10-20 times greater activity than kidney, spleen, heart, muscle, brain or lung. iNAMPT is downregulated by an increase of miR-34a in obesity via a 3'UTR functional binding site of iNAMPT mRNA resulting in a reduction of NAD(+) and decreased SIRT1 activity.

Endurance-trained athletes have twice the expression of iNAMPT in skeletal muscle compared with sedentary type 2 diabetic persons. In a six-week study comparing legs trained by endurance exercise with untrained legs, iNAMPT was increased in the endurance-trained legs. A study of 21 young (under 36) and 22 old (over 54) adults subject to 12 weeks of aerobic and resistance exercise showed aerobic exercise to increase skeletal muscle iNAMPT 12% and 28% in young and old (respectively) and resistance exercise to increase skeletal muscle iNAMPT 25% and 30% in young and old (respectively).

Aging, obesity, and chronic inflammation all reduce iNAMPT (and consequently NAD+) in multiple tissues, and NAMPT activity was shown to promote a proinflammatory transcriptional reprogramming of immune cells (e.g. macrophages) and brain-resident astrocytes.

Function

iNAMPT catalyzes the condensation of nicotinamide (NAM) with 5-phosphoribosyl-1-pyrophosphate to yield nicotinamide mononucleotide (NMN), the first step in the biosynthesis of nicotinamide adenine dinucleotide (NAD+). This salvage pathway, reusing NAM from enzymes using NAD+ (sirtuins, PARPs, CD38) and producing NAM as a waste product, is the major source of NAD+ production in the body. De novo synthesis of NAD+ from tryptophan occurs only in the liver and kidney, overwhelmingly in the liver.

Nomenclature

The systematic name of this enzyme class is nicotinamide-nucleotide:diphosphate phospho-alpha-D-ribosyltransferase. Other names in common use include:

• NMN pyrophosphorylase,
• nicotinamide mononucleotide pyrophosphorylase,
• nicotinamide mononucleotide synthetase, and
• NMN synthetase.

Extracellular NAMPT

Extracellular NAMPT (eNAMPT) is functionally different from intracellular NAMPT (iNAMPT), and less well understood (which is why the enzyme has been given so many names: NAMPT, PBEF and visfatin). iNAMPT is secreted by many cell types (nobably adipocytes) to become eNAMPT. The sirtuin 1 (SIRT1) enzyme is required for eNAMPT secretion from adipose tissue. eNAMPT may act more as a cytokine, although its receptor (possibly TLR4) has not been proven. It has been demonstrated that eNAMPT could bind to and activate TLR4.

eNAMPT can exist as a dimer or as a monomer, but is normally a circulating dimer. As a monomer, eNAMPT has pro-inflammatory effects that are independent of NAD+, whereas the dimeric form of eNAMPT protects against these effects.

eNAMPT/PBEF/visfatin was originally cloned as a putative cytokine shown to enhance the maturation of B cell precursors in the presence of Interleukin-7 (IL-7) and stem cell factor, it was therefore named "pre-B cell colony-enhancing factor" (PBEF). When the gene encoding the bacterial nicotinamide phosphoribosyltransferase (nadV) was first isolated in Haemophilus ducreyi, it was found to exhibit significant homology to the mammalian PBEF gene. Rongvaux et al. demonstrated genetically that the mouse PBEF gene conferred Nampt enzymatic activity and NAD-independent growth to bacteria lacking nadV. Revollo et al. determined biochemically that the mouse PBEF gene product encodes an eNAMPT enzyme, capable of modulating intracellular NAD levels. Others have since confirmed these findings. More recently, several groups have reported the crystal structure of Nampt/PBEF/visfatin and they all show that this protein is a dimeric type II phosphoribosyltransferase enzyme involved in NAD biosynthesis.

eNAMPT has been shown to be more enzymatically active than iNAMPT, supporting the proposal that eNAMPT from adipose tissue enhances NAD+ in tissues with low levels of iNAMPT, notably pancreatic beta cells and brain neurons.

Hormone claim retracted

Although the original cytokine function of PBEF has not been confirmed to date, others have since reported or suggested a cytokine-like function for this protein. In particular, Nampt/PBEF was recently re-identified as a "new visceral fat-derived hormone" named visfatin. It is reported that visfatin is enriched in the visceral fat of both humans and mice and that its plasma levels increase during the development of obesity. Noteworthy is that visfatin is reported to exert insulin-mimetic effects in cultured cells and to lower plasma glucose levels in mice by binding to and activating the insulin receptor. However, the physiological relevance of visfatin is still in question because its plasma concentration is 40 to 100-fold lower than that of insulin despite having similar receptor-binding affinity. In addition, the ability of visfatin to bind and activate the insulin-receptor has yet to be confirmed by other groups.

On 26 October 2007, A. Fukuhara (first author), I.Shimomura (senior author) and the other co-authors of the paper, who first described Visfatin as a visceral-fat derived hormone that acts by binding and activating the insulin receptor, retracted the entire paper at the suggestion of the editor of the journal 'Science' and recommendation of the Faculty Council of Osaka University Medical School after a report of the Committee for Research Integrity.

As a drug target

NAMPT has increasingly been researched as a potential drug target, with activators of NAMPT having potential applications as anti-aging drugs, while inhibitors of NAMPT may be useful for the treatment of certain forms of cancer.

Because cancer cells utilize increased glycolysis, and because NAD enhances glycolysis, iNAMPT is often amplified in cancer cells. APO866 (FK866) is an experimental drug that inhibits this enzyme. It was tested for treatment of advanced melanoma, cutaneous T-cell lymphoma (CTL), and refractory or relapsed B-chronic lymphocytic leukemia., but was dropped from development due to disappointing efficacy results in trials. However, it has been shown to inhibit epithelial–mesenchymal transition (EMT) and inhibit tumor-associated angiogenesis, and may be useful for other medical indications.

Anti-aging biomedical company Calico has licensed the experimental P7C3 analogs involved in enhancing iNAMPT activity. P7C3 compounds have been shown in a number of publications to be beneficial in animal models for age-related neurodegeneration.

Ligands

; Activators

• C8 (NAMPT activator)
• JGB-1-155 (positive allosteric modulator)
• Myricanol
• P7C3
• SBI-797812

; Inhibitors

• A-1293201
• CHS-828
• Daporinad (FK866/APO866)
• GNE-617
• Padnarsertib (KPT-9274)
• STF-118804

References

References

1. (February 1994). "Cloning and characterization of the cDNA encoding a novel human pre-B-cell colony-enhancing factor". Molecular and Cellular Biology.
2. (March 2007). "The regulation of nicotinamide adenine dinucleotide biosynthesis by Nampt/PBEF/visfatin in mammals". Current Opinion in Gastroenterology.
3. (2020). "Recent Advances in NAMPT Inhibitors: A Novel Immunotherapic Strategy". Frontiers in Pharmacology.
4. (2020). "Recent Advances in NAMPT Inhibitors: A Novel Immunotherapic Strategy". Frontiers in Pharmacology.
5. (September 2017). "Nicotinamide is an inhibitor of SIRT1 in vitro, but can be a stimulator in cells". Cellular and Molecular Life Sciences.
6. (December 2013). "Elevated microRNA-34a in obesity reduces NAD+ levels and SIRT1 activity by directly targeting NAMPT". Aging Cell.
7. (2020). "Implications of NAD⁺ Metabolism in the Aging Retina and Retinal Degeneration". Oxidative Medicine and Cellular Longevity.
8. (July 2019). "Aerobic and resistance exercise training reverses age-dependent decline in NAD⁺ salvage capacity in human skeletal muscle". Physiological Reports.
9. (June 2018). "NAMPT-Mediated NAD Biosynthesis as the Internal Timing Mechanism: In NAD+ World, Time Is Running in Its Own Way". Rejuvenation Research.
10. (April 2019). "Inflammatory macrophage dependence on NAD⁺ salvage is a consequence of reactive oxygen species-mediated DNA damage". Nature Immunology.
11. (August 2022). "NAD⁺ metabolism drives astrocyte proinflammatory reprogramming in central nervous system autoimmunity". Proceedings of the National Academy of Sciences of the United States of America.
12. (May 2018). "Quantitative Analysis of NAD Synthesis-Breakdown Fluxes". Cell Metabolism.
13. (July 2016). "Extracellular nicotinamide phosphoribosyltransferase, a new cancer metabokine". British Journal of Pharmacology.
14. (August 2015). "Unique Toll-Like Receptor 4 Activation by NAMPT/PBEF Induces NFκB Signaling and Inflammatory Lung Injury". Scientific Reports.
15. (March 2018). "NAD⁺ Intermediates: The Biology and Therapeutic Potential of NMN and NR". Cell Metabolism.
16. (February 2001). "Identification of a plasmid-encoded gene from Haemophilus ducreyi which confers NAD independence". Journal of Bacteriology.
17. (November 2002). "Pre-B-cell colony-enhancing factor, whose expression is up-regulated in activated lymphocytes, is a nicotinamide phosphoribosyltransferase, a cytosolic enzyme involved in NAD biosynthesis". European Journal of Immunology.
18. (December 2004). "The NAD biosynthesis pathway mediated by nicotinamide phosphoribosyltransferase regulates Sir2 activity in mammalian cells". The Journal of Biological Chemistry.
19. (July 2005). "Pre-B-cell colony-enhancing factor regulates NAD+-dependent protein deacetylase activity and promotes vascular smooth muscle cell maturation". Circulation Research.
20. (July 2006). "Structure of Nampt/PBEF/visfatin, a mammalian NAD+ biosynthetic enzyme". Nature Structural & Molecular Biology.
21. (September 2006). "Crystal structure of visfatin/pre-B cell colony-enhancing factor 1/nicotinamide phosphoribosyltransferase, free and in complex with the anti-cancer agent FK-866". Journal of Molecular Biology.
22. (July 2006). "Molecular basis for the inhibition of human NMPRTase, a novel target for anticancer agents". Nature Structural & Molecular Biology.
23. (2016). "The NAD World 2.0: the importance of the inter-tissue communication mediated by NAMPT/NAD⁺/SIRT1 in mammalian aging and longevity control". npj Systems Biology and Applications.
24. (May 2004). "Pre-B cell colony-enhancing factor inhibits neutrophil apoptosis in experimental inflammation and clinical sepsis". The Journal of Clinical Investigation.
25. (January 2005). "Visfatin: a protein secreted by visceral fat that mimics the effects of insulin". Science.
26. (April 2006). "An update on visfatin/pre-B cell colony-enhancing factor, an ubiquitously expressed, illusive cytokine that is regulated in obesity". Current Opinion in Lipidology.
27. (January 2006). "Visfatin--a true or false trail to type 2 diabetes mellitus". The Journal of Clinical Endocrinology and Metabolism.
28. (October 2007). "Retraction". Science.
29. (January 2018). "Rho GTPase effectors and NAD metabolism in cancer immune suppression". Expert Opinion on Therapeutic Targets.
30. (December 2019). "High expression of NAMPT in adult T-cell leukemia/lymphoma and anti-tumor activity of a NAMPT inhibitor". European Journal of Pharmacology.
31. (June 2022). "Discovery of small-molecule activators of nicotinamide phosphoribosyltransferase (NAMPT) and their preclinical neuroprotective activity". Cell Research.
32. (January 2023). "Properly Substituted Benzimidazoles as a New Promising Class of Nicotinate Phosphoribosyltransferase (NAPRT) Modulators". Pharmaceuticals.
33. (February 2024). "Drug discovery targeting nicotinamide phosphoribosyltransferase (NAMPT): Updated progress and perspectives". Bioorganic & Medicinal Chemistry.
34. {{doi. 10.1016/j.molcel.2025.05.022 {{PMID. 40505662
35. (July 2025). "Nampt: a new therapeutic target for modulating NAD⁺ levels in metabolic, cardiovascular, and neurodegenerative diseases". Canadian Journal of Physiology and Pharmacology.
36. (2018). "NAD Metabolism in Cancer Therapeutics". Frontiers in Oncology.
37. (February 2020). "NAD- and NADPH-Contributing Enzymes as Therapeutic Targets in Cancer: An Overview". Biomolecules.
38. [http://www.cancernetwork.com/news/apo866-not-effective-cutaneous-t-cell-lymphoma APO866 Not Effective for Cutaneous T-Cell Lymphoma. March 2016]
39. "UT Southwestern researchers discover novel class of NAMPT activators for neurodegenerative disease; Calico enters into exclusive collaboration with 2M to develop UTSW technology".
40. (2014). "NAMPT neuroprotection". Science-Business EXchange.
41. (September 2014). "P7C3 neuroprotective chemicals function by activating the rate-limiting enzyme in NAD salvage". Cell.
42. Pinkerton AB, Sessions EH, Hershberger P, Maloney PR, Peddibhotla S, Hopf M, Sergienko E, Ma CT, Smith LH, Jackson MR, Tanaka J, Tsuji T, Akiu M, Cohen SE, Nakamura T, Gardell SJ. Optimization of a urea-containing series of nicotinamide phosphoribosyltransferase (NAMPT) activators. ''Bioorg Med Chem Lett''. 2021 Jun 1;41:128007. {{doi. 10.1016/j.bmcl.2021.128007 {{PMID. 33798699
43. Akiu M, Tsuji T, Sogawa Y, Terayama K, Yokoyama M, Tanaka J, Asano D, Sakurai K, Sergienko E, Sessions EH, Gardell SJ, Pinkerton AB, Nakamura T. Discovery of 1-[2-(1-methyl-1H-pyrazol-5-yl)-[1,2,4]triazolo[1,5-a]pyridin-6-yl]-3-(pyridin-4-ylmethyl)urea as a potent NAMPT (nicotinamide phosphoribosyltransferase) activator with attenuated CYP inhibition. ''Bioorg Med Chem Lett''. 2021 Jul 1;43:128048. {{doi. 10.1016/j.bmcl.2021.128048 {{PMID. 33887438
44. Wang L, Liu M, Zu Y, Yao H, Wu C, Zhang R, Ma W, Lu H, Xi S, Liu Y, Hua L, Wang G, Tang Y. Optimization of NAMPT activators to achieve in vivo neuroprotective efficacy. ''Eur J Med Chem''. 2022 Jun 5;236:114260. {{doi. 10.1016/j.ejmech.2022.114260 {{PMID. 35385807