Low extracellular calcium enhances beta cell sensitivity to the stimulatory influence of 1,25-dihydroxyvitamin D3 on insulin release by islets from vitamin D3-deficient rats

Vitamin D3 and calcium cosupplementation alleviates cadmium hepatotoxicity in the rat: Enhanced antioxidative and anti-inflammatory actions by remodeling cellular calcium pathways

Although vitamin D (VD) and calcium (Ca) attenuate cadmium (Cd) metabolism, their combined antioxidant and anti-inflammatory actions against Cd toxicity have not been previously explored. Hence, this study measured the protective effects of VD ± Ca supplements against Cd hepatotoxicity. Forty adult male rats were distributed to: negative controls (NCs), positive controls (PCs), VD, Ca, and VD₃ and Ca (VDC) groups. All groups, except NC, received CdCl₂ in drinking water (44 mg/L) for 4 weeks individually or concurrently with intramuscular VD₃ (600 IU/kg; three times per week) and/or oral Ca (100 mg/kg; five times per week). The PC group showed abnormal hepatic biochemical parameters and increase in cellular cytochrome C, caspase-9, and caspase-3 alongside the apoptotic/necrotic cell numbers by terminal deoxynucleotidyl transferase dUTP nick end labeling technique. The PC hepatic tissue also had substantially elevated pro-oxidants (malondialdehyde [MDA]/H₂ O₂ /protein carbonyls) and inflammatory cytokines (interleukin 1β [IL-1β]/IL-6/IL17A/tumor necrosis factor-α), whereas the anti-inflammatory (IL-10/IL-22) and antioxidants (glutathione [GSH]/GPx/catalase enzyme [CAT]) markers declined. Hypovitaminosis D, low hepatic tissue Ca, aberrant hepatic expression of VD-metabolizing enzymes (Cyp2R1/Cyp27a1/cyp24a1), receptor and binding protein alongside Ca-membrane (Ca_V 1.1/Ca_V 3.1), and store-operated (RyR1/ITPR1) channels, and Ca-binding proteins (CAM/CAMKIIA/S100A1/S100B) were observed in the PC group. Both monotherapies decreased serum, but not tissue Cd levels, restored the targeted hepatic VD/Ca molecules' expression. However, these effects were more prominent in the VD group than the Ca group. The VDC group, contrariwise, disclosed the greatest alleviations on serum and tissue Cd, inflammatory and oxidative markers, the VD/Ca molecules and tissue integrity. In conclusion, this report is the first to reveal boosted protection for cosupplementing VD and Ca against Cd hepatotoxicity that could be due to enhanced antioxidative, anti-inflammatory, and modulation of the Ca pathways.

The Protective Role of Calbindin-D9k on Endoplasmic Reticulum Stress-Induced Beta Cell Death

Intracellular calcium ion content is tightly regulated for the maintenance of cellular functions and cell survival. Calbindin-D_9k (CaBP-9k) is responsible for regulating the distribution of cytosolic free-calcium ions. In this study, we aimed to investigate the effect of CaBP-9k on cell survival in pancreatic beta cells. Six-month-old wildtype CaBP-9k, CaBP-28k, and CaBP-9k/28k knockout (KO) mice were used to compare the pathological phenotypes of calcium-binding protein-deleted mice. Subsequently, the endoplasmic reticulum (ER) stress reducer tauroursodeoxycholic acid (TUDCA) was administered to wildtype and CaBP-9k KO mice. In vitro assessment of the role of CaBP-9k was performed following CaBP-9k overexpression and treatment with the ER stress inducer thapsigargin. Six-month-old CaBP-9k KO mice showed reduced islet volume and up-regulation of cell death markers resulting from ER stress, which led to pancreatic beta cell death. TUDCA treatment recovered islet volume, serum insulin level, and abdominal fat storage by CaBP-9k ablation. CaBP-9k overexpression elevated insulin secretion and recovered thapsigargin-induced ER stress in the INS-1E cell line. The results of this study show that CaBP-9k can protect pancreatic beta cell survival from ER stress and contribute to glucose homeostasis, which can reduce the risk of type 1 diabetes and provide the molecular basis for calcium supplementation to diabetic patients.

Sex-Specific Association of Serum 25-Hydroxyvitamin D3 with Insulin Resistance in Chinese Han Patients with Newly Diagnosed Type 2 Diabetes Mellitus

This study aimed to explore the association between serum 25-hydroxyvitamin D (25(OH)D) and insulin resistance as well as β-cell function in Chinese Han patients with newly diagnosed type 2 diabetes mellitus (T2DM). A total of 264 patients was included in this study. Serum 25(OH)D, plasma glucose, serum insulin and other biochemical parameters were assayed. Postprandial venous blood was collected after a mixed-nutrient load. Insulin resistance was assessed by the homeostasis model assessment for insulin resistance (HOMA-IR) and Matsuda insulin sensitivity index (Matsuda ISI). The β-cell function was assessed by the homeostasis model assessment for insulin secretion (HOMA-β) and the change in insulin divided by change in glucose from 0 to 30 min (ΔI_0-30/ΔG_0-30). Patients were divided into three groups according to tertiles of serum 25(OH)D levels. There were significant differences in HOMA-IR and Matsuda ISI among the three groups (HOMA-IR, p=0.005; Matsuda ISI, p=0.009). Pearson correlation analyses showed that serum 25(OH)D was negatively correlated with fasting serum insulin (FIns) (r=-0.209, p=0.012) and HOMA-IR (r=-0.273, p=0.001), and positively correlated with Matsuda ISI (r=0.219, p=0.009) only in the male population. Multiple stepwise regression analyses showed that in the male population, serum 25(OH)D was an independent predictor for both HOMA-IR and Matsuda ISI before and after adjustment for confounding factors, respectively (p<0.05 for both). This study indicates the association of vitamin D with insulin resistance in male patients with newly diagnosed T2DM, which may contribute to the understanding of the mechanism underlying the onset of T2DM in the Chinese Han population.

Calbindin-D9k Ablation Disrupt Glucose/Pancreatic Insulin Homeostasis

It has been proposed that cellular Ca²⁺ signals activate hormone secretion. In pancreatic β cells, which produce insulin, Ca²⁺ signals have been known to contribute to insulin secretion. Prior to this study, we confirmed that insulin-secreting β cells express CaBP-9k, and assumed that CaBP-9k play a role in β cell insulin synthesis or secretion. Using CaBP-9k knock out (KO) mice, we demonstrated that ablation of CaBP-9k causes reducing insulin secretion and increasing serum glucose. To compare the role of CaBP-9k with pathophysiological conditions, we exposed wild-type and CaBP-9k KO mice to hypoxic conditions for 10 days. Hypoxia induced endoplasmic reticulum (ER) stress, increasing both insulin signaling and insulin resistance. By exposing hypoxia, CaBP-9k KO mice showed an increased level of ER stress marker protein relative to wild type mice. Without hypoxic conditions, CaBP-9K ablation regulates calcium channels and causes ER stress in a CaBP-9K specific manner. Ablation of CaBP-9k also showed decreased levels of sulfonylurea receptor1 (SUR1) and inward-rectifier potassium ion channel 6.2 (K_ir6.2), which are insulin secretion marker genes. Overall, the results of the present study demonstrated that CaBP-9k regulates synthesis of insulin and is part of the insulin-secreting calcium signaling.

The vitamin D system is deregulated in pancreatic diseases

The vitamin D system is deregulated during development and progression of several cancer types. Data on the expression of the vitamin D system in the diseased pancreas are missing. The aim of this study was to investigate the expression of the vitamin D receptor (VDR), 1,25-dihydroxyvitamin D3 24-hydroxylase (CYP24A1), and the calcium-sensing receptor (CaSR), a vitamin D target gene, in the different regions of the pancreas in patients with chronic pancreatitis (n=6) and pancreatic ductal adenocarcinomas (PDAC) (n=17). We analyzed the expression of these genes at mRNA and protein level with quantitative real-time RT-PCR and immunostaining. mRNA expression of CYP24A1 and VDR was significantly increased in tumors compared with the adjacent non-tumorous tissue (p<0.01), while CaSR mRNA expression decreased. Both the VDR and the CaSR protein were highly expressed in the endocrine compared with the exocrine pancreas. In CP the CYP24A1 expression was highest in the endocrine pancreas, while in PDACs in the transformed ducts. In the PDAC patients CYP24A1 expression in the islets was significantly lower than in CP patients. Our data suggest that during ductal adenocarcinoma development the vitamin D system in the pancreas becomes deregulated on two levels: in the islets CYP24A1 expression decreases weakening the negative feedback regulation of the vitamin D-dependent insulin synthesis/secretion. In the transformed ducts CYP24A1 expression increases, impairing the antiproliferative effect of vitamin D in these cells.