Vitamin D binding protein/GC-globulin: a novel regulator of alpha cell function and glucagon secretion

Vitamin D3 Regulates Energy Homeostasis under Short-Term Fasting Condition in Zebrafish (Danio Rerio)

Vitamin D3 (VD3) is a steroid hormone that plays pivotal roles in pathophysiology, and 1,25(OH)2D3 is the most active form of VD3. In the current study, the crucial role of VD3 in maintaining energy homeostasis under short-term fasting conditions was investigated. Our results confirmed that glucose-depriving pathways were inhibited while glucose-producing pathways were strengthened in zebrafish after fasting for 24 or 48 h. Moreover, VD3 anabolism in zebrafish was significantly suppressed in a time-dependent manner under short-fasting conditions. After fasting for 24 or 48 h, zebrafish fed with VD3 displayed a higher gluconeogenesis level and lower glycolysis level in the liver, and the serum glucose was maintained at higher levels, compared to those fed without VD3. Additionally, VD3 augmented the expression of fatty acids (FAs) transporter cd36 and lipogenesis in the liver, while enhancing lipolysis in the dorsal muscle. Similar results were obtained in cyp2r1-/- zebrafish, in which VD3 metabolism is obstructed. Importantly, it was observed that VD3 induced the production of gut GLP-1, which is considered to possess a potent gluconeogenic function in zebrafish. Meanwhile, the gene expression of proprotein convertase subtilisin/kexin type 1 (pcsk1), a GLP-1 processing enzyme, was also induced in the intestine of short-term fasted zebrafish. Notably, gut microbiota and its metabolite acetate were involved in VD3-regulated pcsk1 expression and GLP-1 production under short-term fasting conditions. In summary, our study demonstrated that VD3 regulated GLP-1 production in zebrafish by influencing gut microbiota and its metabolite, contributing to energy homeostasis and ameliorating hypoglycemia under short-term fasting conditions.

25-Hydroxyvitamin D Is Associated with Islet Homeostasis in Type-2 Diabetic Patients with Abdominal Obesity

OBJECTIVE

Islet α cells input is essential for insulin secretion from β cells. The present study aims to investigate the association between 25-hydroxyvitamin D [25(OH)D] and islet function homeostasis in type-2 diabetes (T2D) patients.

METHODS

A total of 4670 T2D patients from seven communities in Shanghai, China were enrolled. The anthropometric indices, biochemical parameters, serum 25(OH)D, and islet function [including C-peptide (C-p) and glucagon] were measured.

RESULTS

The fasting plasma glucose (FPG), glycated hemoglobin (HbA1c), glucagon, and C-p levels exhibited a significantly decreasing trend in T2D patients as the 25(OH)D levels increased. Next, the population was divided into two groups: abdominal obesity and non-abdominal obesity groups. After adjustment, the 25(OH)D level was found to be associated with HbA1c, glucagon, and homeostasis model assessment of β (HOMA-β) in the non-abdominal obesity group. There was a significant relationship between 25(OH)D and HbA1c, glucagon, HOMA-IR, baseline insulin or C-p in the abdominal obesity group. In the abdominal obesity group, the ordinary least squares (OLS) regression and quantile regression revealed that 25(OH) D was obviously associated with glucagon and fasting C-p levels. In the abdominal obesity group, the moderate analysis revealed a significant interaction effect of 25(OH)D and glucagon on C-p (P=0.0124). Furthermore, the conditional indirect effect of 25(OH)D on the glucagon/C-p ratio was significantly lower at 1 standard deviation (SD) below the mean (P=0.0002), and lower at the mean of the course of diabetes (P=0.0007).

CONCLUSION

25(OH)D was found to be negatively correlated to glucagon and C-p in T2D patients with abdominal obesity. The 25(OH)D influenced C-p in part by influencing glucagon. The effect of 25(OH)D on the glucagon/C-p ratio in T2D patients with abdominal obesity, in terms of islet homeostasis, is influenced by the course of diabetes.

GC-Globulin/Vitamin D-Binding Protein Is Required for Pancreatic α-Cell Adaptation to Metabolic Stress

GC-globulin (GC), or vitamin D-binding protein, is a multifunctional protein involved in the transport of circulating vitamin 25(OH)D and fatty acids, as well as actin scavenging. In the pancreatic islets, the gene encoding GC, GC/Gc, is highly localized to glucagon-secreting α-cells. Despite this, the role of GC in α-cell function is poorly understood. We previously showed that GC is essential for α-cell morphology, electrical activity, and glucagon secretion. We now show that loss of GC exacerbates α-cell failure during metabolic stress. High-fat diet-fed GC-/- mice have basal hyperglucagonemia, which is associated with decreased α-cell size, impaired glucagon secretion and Ca2+ fluxes, and changes in glucose-dependent F-actin remodelling. Impairments in glucagon secretion can be rescued using exogenous GC to replenish α-cell GC levels, increase glucagon granule area, and restore the F-actin cytoskeleton. Lastly, GC levels decrease in α-cells of donors with type 2 diabetes, which is associated with changes in α-cell mass, morphology, and glucagon expression. Together, these data demonstrate an important role for GC in α-cell adaptation to metabolic stress.