Abstract
Aims/hypothesisProgressive decline in functional beta cell mass is central to the development of type 2 diabetes. Elevated serum levels of extracellular nicotinamide phosphoribosyltransferase (eNAMPT) are associated with beta cell failure in type 2 diabetes and eNAMPT immuno-neutralisation improves glucose tolerance in mouse models of diabetes. Despite this, the effects of eNAMPT on functional beta cell mass are poorly elucidated, with some studies having separately reported beta cell-protective effects of eNAMPT. eNAMPT exists in structurally and functionally distinct monomeric and dimeric forms. Dimerisation is essential for the NAD-biosynthetic capacity of NAMPT. Monomeric eNAMPT does not possess NAD-biosynthetic capacity and may exert distinct NAD-independent effects. This study aimed to fully characterise the structure-functional effects of eNAMPT on pancreatic beta cell functional mass and to relate these to beta cell failure in type 2 diabetes.MethodsCD-1 mice and serum from obese humans who were without diabetes, with impaired fasting glucose (IFG) or with type 2 diabetes (from the Body Fat, Surgery and Hormone [BodyFatS&H] study) or with or at risk of developing type 2 diabetes (from the VaSera trial) were used in this study. We generated recombinant wild-type and monomeric eNAMPT to explore the effects of eNAMPT on functional beta cell mass in isolated mouse and human islets. Beta cell function was determined by static and dynamic insulin secretion and intracellular calcium microfluorimetry. NAD-biosynthetic capacity of eNAMPT was assessed by colorimetric and fluorescent assays and by native mass spectrometry. Islet cell number was determined by immunohistochemical staining for insulin, glucagon and somatostatin, with islet apoptosis determined by caspase 3/7 activity. Markers of inflammation and beta cell identity were determined by quantitative reverse transcription PCR. Total, monomeric and dimeric eNAMPT and nicotinamide mononucleotide (NMN) were evaluated by ELISA, western blot and fluorometric assay using serum from non-diabetic, glucose intolerant and type 2 diabetic individuals.ResultseNAMPT exerts bimodal and concentration- and structure-functional-dependent effects on beta cell functional mass. At low physiological concentrations (~1 ng/ml), as seen in serum from humans without diabetes, eNAMPT enhances beta cell function through NAD-dependent mechanisms, consistent with eNAMPT being present as a dimer. However, as eNAMPT concentrations rise to ~5 ng/ml, as in type 2 diabetes, eNAMPT begins to adopt a monomeric form and mediates beta cell dysfunction, reduced beta cell identity and number, increased alpha cell number and increased apoptosis, through NAD-independent proinflammatory mechanisms.Conclusions/interpretationWe have characterised a novel mechanism of beta cell dysfunction in type 2 diabetes. At low physiological levels, eNAMPT exists in dimer form and maintains beta cell function and identity through NAD-dependent mechanisms. However, as eNAMPT levels rise, as in type 2 diabetes, structure-functional changes occur resulting in marked elevation of monomeric eNAMPT, which induces a diabetic phenotype in pancreatic islets. Strategies to selectively target monomeric eNAMPT could represent promising therapeutic strategies for the treatment of type 2 diabetes.
Highlights
ENAMPT directly affect the pancreatic beta cell are unknown
Serum extracellular nicotinamide phosphoribosyltransferase (eNAMPT) monomer concentrations are elevated in type 2 diabetes and are positively correlated with increased HbA1c We first demonstrated that serum eNAMPT concentrations increased with progression of type 2 diabetes (Fig. 1a): in obese non-diabetic individuals, serum eNAMPT concentrations were 1.7 ± 0.4 ng/ml, rising to 3.4 ± 0.7 ng/ml in individuals with impaired fasting glucose (IFG) and 4.6 ± 0.6 ng/ml in type 2 diabetes (p < 0.05 vs non-diabetic for both IFG and type 2 diabetes)
This study demonstrates that eNAMPT exerts bimodal, concentration- and structure-functional-dependent effects on pancreatic beta cell functional mass, and provides clarification of previous contradictory eNAMPT studies
Summary
ENAMPT directly affect the pancreatic beta cell are unknown. Previous studies, including our own, have examined NADboosting effects of NMN on beta cell function [6, 13, 14], or have examined supraphysiological concentrations or acute effects of eNAMPT [21, 22], neither of which accurately mimic in vivo (patho)physiology. We used isolated mouse and human islets, and serum from humans with type 2 diabetes, to characterise the effects of monomeric and dimeric eNAMPT on pancreatic beta cell functional mass. Similar to iNAMPT, eNAMPT may exert NAD-biosynthetic effects, whereby eNAMPT catalyses the conversion of nicotinamide to nicotinamide mononucleotide (NMN) in serum [5, 6]. NMN may subsequently be transported into the cell (directly or via conversion into nicotinamide riboside), where it is converted into NAD by nicotinamide mononucleotide adenylyltransferases (NMNATs) 1–3. This mechanism has been disputed and NAD-independent functions, including eNAMPT-mediated proinflammatory effects, are reported [4, 7,8,9]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
