Fasting-Mimicking Diet Promotes Ngn3-Driven β-Cell Regeneration to Reverse Diabetes

As a consequence of insulin resistance, the decrease in the number of functional insulin-producing β cells contributes to the pathophysiology of T2D by eventually leading to insulin deficiency. Previously, we showed that a 4-day fasting mimicking diet (FMD) could induce metabolic changes similar to those caused by prolonged fasting and could reduce insulin and glucose levels while increasing ketone bodies and igfbp1. Although the role of periodic fasting and fasting mimicking diets on insulin secretion is unknown, the effects of intermittent fasting and chronic calorie restrictions (CR) on insulin sensitivity have been previously reported. Here, our focus is on the putative effects of the FMD in promoting β-cell regeneration, although we have also investigated the effects of the FMD on insulin resistance.

Given that β cells replicate at an extremely low rate in the adult pancreas and that β-cell neogenesis occurs rarely, depletion of β cells and the consequent loss of insulin secretion during late-stage diabetes have often been considered conditions whose reversals require islet and stem cell transplantation.

In both mouse and humans, Ngn3 expression occurs right before and during endocrine cell generation. Ngn3 mRNA expression in the developing mouse pancreas peaks around E15.5, which is roughly equivalent to week 7–8 (Carnegie stages 21–22) in human development. Expression of Ngn3 in adult mouse islets, although rare, has been demonstrated by rigorous lineage reporter analysis (Wang et al., 2009). In agreement with results from others, our data (Figures 5 and S5) indicate that Ngn3+ cells in adult pancreas islets are important for β-cell regeneration in mice. On the other hand, the role of Ngn3 in human islet development and β-cell regeneration in adulthood remains poorly understood (McKnight et al., 2010).

To investigate how the fasting mimicking conditions affect Ngn3 expression and β-cell function in human pancreatic cells, we performed ex vivo experiments using primary human pancreatic islets (Figure 6A). Briefly, the pancreatic islets from healthy and T1D subjects (HI and T1DI, respectively) were cultured according to the manufacturer’s instructions. The cultured islets were then treated with serum from subjects enrolled in a clinical trial testing the effects of a low-protein and low-calorie FMD lasting 5 days (NCT02158897). Serum samples were collected at baseline and at day 5 of the fasting mimicking diet in five subjects. We then measured IGF-1, glucose, and ketone bodies and treated human pancreatic islets with the subject-derived serum (Figure 6B and Table S1). In both healthy islets and T1D islets exposed to the serum of FMD-treated subjects, we observed a trend for glucose-dependent induction in the expression of Sox2 and Ngn3 (Figure S6A).