No abstract

Obesity is associated with perturbations in incretin production and whole-body glucose metabolism, but the precise underlying mechanism remains unclear. Here, we tested the hypothesis that nicotinamide phosphoribosyltransferase (NAMPT), which mediates the biosynthesis of nicotinamide adenine dinucleotide (NAD +), a key regulator of cellular energy metabolism, plays a critical role in obesity-associated intestinal pathophysiology and systemic metabolic complications. To this end, we generated a novel mouse model, namely intestinal epithelial cell-specific Nampt knockout (INKO) mice. INKO mice displayed diminished glucagon-like peptide-1 (GLP-1) production, at least partly contributing to reduced early-phase insulin secretion and postprandial hyperglycemia. Mechanistically, loss of NAMPT attenuated the Wnt signaling pathway, resulting in insufficient GLP-1 production. We also found that diet-induced obese mice had compromised intestinal NAMPT-mediated NAD + biosynthesis and the Wnt signaling pathway, associated with impaired GLP-1 production and whole-body glucose metabolism, resembling the INKO mice. Finally, administration of a key NAD + intermediate, nicotinamide mononucleotide (NMN), restored intestinal NAD + levels and obesity-associated metabolic derangements, manifested by a decrease in ileal Proglucagon expression and GLP-1 production as well as postprandial hyperglycemia in INKO and diet-induced obese mice. Collectively, our study provides mechanistic and therapeutic insights into intestinal NAD + biology related to obesity-associated dysregulation of GLP-1 production and postprandial hyperglycemia.

The intestine has garnered attention as a target organ for developing new therapies for impaired glucose tolerance. The intestine, which produces incretin hormones, is the central regulator of glucose metabolism. Glucagon-like peptide-1 (GLP-1) production, which determines postprandial glucose levels, is regulated by intestinal homeostasis. Nicotinamide phosphoribosyltransferase (NAMPT)-mediated nicotinamide adenine dinucleotide (NAD+) biosynthesis in major metabolic organs such as the liver, adipose tissue, and skeletal muscle plays a crucial role in obesity- and aging-associated organ derangements. Furthermore, NAMPT-mediated NAD+ biosynthesis in the intestines and its upstream and downstream mediators, adenosine monophosphate-activated protein kinase (AMPK) and NAD+-dependent deacetylase sirtuins (SIRTs), respectively, are critical for intestinal homeostasis, including gut microbiota composition and bile acid metabolism, and GLP-1 production. Thus, boosting the intestinal AMPK–NAMPT–NAD+–SIRT pathway to improve intestinal homeostasis, GLP-1 production, and postprandial glucose metabolism has gained significant attention as a novel strategy to improve impaired glucose tolerance. Herein, we aimed to review in detail the regulatory mechanisms and importance of intestinal NAMPT-mediated NAD+ biosynthesis in regulating intestinal homeostasis and GLP-1 secretion in obesity and aging. Furthermore, dietary and molecular factors regulating intestinal NAMPT-mediated NAD+ biosynthesis were critically explored to facilitate the development of new therapeutic strategies for postprandial glucose dysregulation.

The purpose of the present study was to test our hypothesis that intestinal nicotinamide phosphoribosyltransferase (NAMPT)-mediated nicotinamide adenine dinucleotide (NAD+) biosynthesis plays a pivotal role in whole-body energy and glucose metabolism. To this end, we generated a new mouse model, namely intestinal epithelial cell-specific Nampt knockout (INKO) mice. Under regular chow diet, deletion of intestinal epithelial NAMPT did not affect body weight gain, food intake, or whole-body energy expenditure, suggesting that intestinal epithelial NAMPT deficiency had no significant impacts on overall energy intake or expenditure. However, INKO mice displayed reduced early-phase insulin secretion and postprandial hyperglycemia, at least partly due to diminished glucagon-like peptide-1 (GLP-1) production. In addition, we found that diet-induced obese mice had impaired intestinal NAMPT-mediated NAD+ biosynthesis, associated with impaired GLP-1 production and whole-body glucose metabolism, similar to the INKO mice. Finally, administration of a key NAD+ intermediate, nicotinamide mononucleotide, to INKO and diet-induced obese mice, restored ileal NAD+ levels and obesity-associated metabolic derangements, manifested by an impairment in GLP-1 production as well as postprandial hyperglycemia. Taken together, our study provides a new understanding of the role of intestinal NAMPT-mediated NAD+ biosynthesis in energy and glucose metabolism, and therapeutic insights into intestinal NAD+ biology related to obesity-associated dysregulation of GLP-1 production and postprandial hyperglycemia (Nagahisa, Yamaguchi et al. Endocrinology. 2022). A future study using INKO mice to investigate if diminished GLP-1 production is involved in obesity-associated hypertension is warranted.

Aim: To assess the safety and efficacy of basal-supported prandial GLP-1 receptor agonist therapy (BPT)* in type 2 diabetes mellitus (T2DM). Methods: Patients with T2DM, who had previously received insulin injection therapy and who had had their treatment switched to BPT (liraglutide), were retrospectively recruited. The efficacy of BPT was assessed by determining changes in HbA1c, body weight and total daily insulin dose from baseline to 4 months after BPT initiation. Safety was assessed by comparing the frequency of hypoglycemic episodes at baseline and after 4 months. The Wilcoxon test was used to analyze changes in parameters throughout the study period. Results: Twenty-nine patients, previously treated with basal-supported oral therapy (BOT), basal-bolus insulin, or pre-mixed insulin, were recruited. When analyzed together, there was no change in HbA1c throughout the study period, but body weight decreased (baseline 68.8 ± 13.2 kg vs. month 4 67.3 ± 13.1 kg; p < 0.001). Total daily insulin dose decreased after 4 months (baseline 24.4 ± 15.5 U/day vs. month 4 14.7 ± 9.2 U/day; p < 0.001), and there was no change in the frequency of hypoglycemic episodes. Analysis was conducted within sub-groups based on previous treatment modality. In the BOT group, HbA1c decreased from baseline after 2 months and body weight did not change throughout the study period. In both the basal-bolus insulin group and the pre-mixed insulin group, HbA1c remained steady throughout and there was a decrease in body weight. No change in the frequency of hypoglycemia was observed in any of the sub-groups. Conclusion: BPT in T2DM was associated with weight loss without changes in glycemic control over 4 months, suggesting that it may be an effective and safe therapy.