Vitamin D receptor-targeted treatment to prevent pathological dedifferentiation of pancreatic β cells under hyperglycaemic stress

Targeting β-Cell Plasticity: A Promising Approach for Diabetes Treatment

The β-cells within the pancreas play a pivotal role in insulin production and secretion, responding to fluctuations in blood glucose levels. However, factors like obesity, dietary habits, and prolonged insulin resistance can compromise β-cell function, contributing to the development of Type 2 Diabetes (T2D). A critical aspect of this dysfunction involves β-cell dedifferentiation and transdifferentiation, wherein these cells lose their specialized characteristics and adopt different identities, notably transitioning towards progenitor or other pancreatic cell types like α-cells. This process significantly contributes to β-cell malfunction and the progression of T2D, often surpassing the impact of outright β-cell loss. Alterations in the expressions of specific genes and transcription factors unique to β-cells, along with epigenetic modifications and environmental factors such as inflammation, oxidative stress, and mitochondrial dysfunction, underpin the occurrence of β-cell dedifferentiation and the onset of T2D. Recent research underscores the potential therapeutic value for targeting β-cell dedifferentiation to manage T2D effectively. In this review, we aim to dissect the intricate mechanisms governing β-cell dedifferentiation and explore the therapeutic avenues stemming from these insights.

Vitamin D supplementation affects bone marrow-derived mesenchymal stem cells differentiation into insulin-producing cells

Background this study was conducted to assess the effects of vitamin D on differentiation of bone marrow- derived mesenchymal stem cells (BM-MSCs) into insulin producing cells (IPCs). Method BM-MSCs were isolated from femur and tibia of rats and incubated in low (LG) or high glucose (HG) (5mM or 25mM), or high glucose DMEM media supplemented with vitamin D (0.2nM) (HGD) for 14 days. Cells viability was analysis by MTT assay. Differentiation of SCs was confirmed using measuring genes expression level of pdx1 and insulin, and insulin secretion, glucose stimulated insulin secretion, and insulin content by ELISA method. Results Cell viability was significantly higher in HGD than LG (p < 0.05) in day 3, also, in HG and HGD than LG (p < 0.001), and HGD vs. HG (p < 0.001) in day 7. Pdx1 and insulin level was markedly higher in HGD than LG (p < 0.05 and p < 0.01). pdx1 expression was markedly higher in HGD (p < 0.05) than LG, also insulin expression the HG (p < 0.05), and HGD (p < 0.01) groups compared to the LG group. Insulin release at 5mM glucose was notably higher in the HGD group compared to LG (p < 0.05), and at 25mM glucose, both HG and HGD showed significant increases vs. LG (p < 0.05 and p < 0.01, respectively). Insulin content was significantly higher in both 5mM and 25mM glucose for HG and HGD vs. LG (p < 0.01 and p < 0.001, respectively). In conclusion, treatment BM-MSCs with vitamin D could increase their differentiation into IPCs and it can be considered as a potential supplementary agent in enhancing differentiation SCs into insulin generating cells.

Pancreatic islet cell plasticity: Pathogenic or therapeutically exploitable?

The development of pancreatic islet endocrine cells is a tightly regulated process leading to the generation of distinct cell types harbouring different hormones in response to small changes in environmental stimuli. Cell differentiation is driven by transcription factors that are also critical for the maintenance of the mature islet cell phenotype. Alteration of the insulin-secreting β-cell transcription factor set by prolonged metabolic stress, associated with the pathogenesis of diabetes, obesity or pregnancy, results in the loss of β-cell identity through de- or transdifferentiation. Importantly, the glucose-lowering effects of approved and experimental antidiabetic agents, including glucagon-like peptide-1 mimetics, novel peptides and small molecules, have been associated with preventing or reversing β-cell dedifferentiation or promoting the transdifferentiation of non-β-cells towards an insulin-positive β-cell-like phenotype. Therefore, we review the manifestations of islet cell plasticity in various experimental settings and discuss the physiological and therapeutic sides of this phenomenon, focusing on strategies for preventing β-cell loss or generating new β-cells in diabetes. A better understanding of the molecular mechanisms underpinning islet cell plasticity is a prerequisite for more targeted therapies to help prevent β-cell decline in diabetes.

25-hydroxyvitamin D levels are inversely related to metabolic syndrome risk profile in northern Chinese subjects without vitamin D supplementation

BACKGROUND

The comparatively low 25 hydroxyvitamin D [25(OH)D] levels have been reported in patients with metabolic syndrome (MetS). Herein we investigated the cross-sectional and longitudinal relationships between serum 25(OH)D levels and MetS risk profile in northern middle-aged Chinese subjects without vitamin D supplementation.

METHODS

A cohort of 211 participants including 151 MetS patients and 60 controls at 20-69 years of age were enrolled from suburban Beijing, China. The recruited MetS patients were subjected to diet and exercise counselling for 1-year. All subjects at baseline and MetS patients after intervention underwent clinical evaluations.

RESULTS

Serum 25(OH)D levels were significantly decreased in MetS patients. 25(OH)D levels were inversely related to MetS score, fasting blood glucose (FBG) and triglyceride-glucose index (TyG) after adjusting for cofounders (all P < 0.05). Participants in the lowest tertile of 25(OH)D levels had increased odds for MetS (P = 0.045), elevated FBG (P = 0.004) in all subjects, and one MetS score gain in MetS patients (P = 0.005). Longitudinally, the metabolic statuses as well as 25(OH)D levels of MetS patients were significantly improved (all P < 0.05), and the increase of 25(OH)D levels were inversely related to MetS scores, total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), FBG, and TyG, while positively related to high-density lipoprotein cholesterol (HDL-C) after adjusting for confounders.

CONCLUSIONS

25(OH)D levels were significantly decreased in MetS patients, and it was negatively associated with metabolic dysfunctions at baseline and 1-year after. Metabolic aberrations of MetS patients were significantly ameliorated with 1-year follow-up counselling accompanying by notably elevated 25(OH)D levels.

Mechanisms Linking Vitamin D Deficiency to Impaired Metabolism: An Overview

Vitamin D deficiency is a common health problem worldwide. Despite its known skeletal effects, studies have begun to explore its extra-skeletal effects, that is, in preventing metabolic diseases such as obesity, hyperlipidemia, and diabetes mellitus. The mechanisms by which vitamin D deficiency led to these unfavorable metabolic consequences have been explored. Current evidence indicates that the deficiency of vitamin D could impair the pancreatic β-cell functions, thus compromising its insulin secretion. Besides, vitamin D deficiency could also exacerbate inflammation, oxidative stress, and apoptosis in the pancreas and many organs, which leads to insulin resistance. Together, these will contribute to impairment in glucose homeostasis. This review summarizes the reported metabolic effects of vitamin D, in order to identify its potential use to prevent and overcome metabolic diseases.

Vitamin D supplementation induces CatG-mediated CD4+ T cell inactivation and restores pancreatic β-cell function in mice with type 1 diabetes

Type 1 diabetes (T1D) is a chronic autoimmune disease accompanied by the immune-mediated destruction of pancreatic β-cells. In this study, we aimed to explore the regulatory effects of vitamin D (VD) supplementation on pancreatic β-cell function by altering the expression of bioinformatically identified cathepsin G (CatG) in T1D mice. A T1D mouse model was established in nonobese diabetic (NOD) mice, and their islets were isolated and purified. Pancreatic mononuclear cells (MNCs) were collected, from which CD4⁺ T cells were isolated. The levels of interleukin (IL)-2, IL-10, tumor necrosis factor-α (TNF-α), and interferon-γ (IFN-γ) in the supernatant of mouse pancreatic tissue homogenate were assessed using ELISA. Immunohistochemistry and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labelin (TUNEL) staining were conducted to evaluate the effects of VD supplementation on pancreatic tissues of T1D mice. The pancreatic β-cell line MIN6 was used for in vitro substantiation of findings in vivo. VD supplementation reduced glucose levels and improved glucose tolerance in T1D mice. Furthermore, VD supplementation improved pancreatic β-cell function and suppressed immunological and inflammatory reactions in the T1D mice. We documented overexpression of CatG in diabetes tissue samples, and then showed that VD supplementation normalized the islet immune microenvironment through downregulating CatG expression in T1D mice. Experiments in vitro subsequently demonstrated that VD supplementation impeded CD4⁺ T activation by downregulating CatG expression and thereby enhanced pancreatic β-cell function. Results of the present study elucidated that VD supplementation can downregulate the expression of CatG and inhibit CD4⁺ T cell activation, thereby improving β-cell function in T1D.NEW & NOTEWORTHY We report that vitamin D (VD) supplementation downregulates CatG expression and inhibits CD4⁺ T cell activation, thereby improving β-cell function in type 1 diabetes (T1D). This study deepens our understanding of the pathogenesis of T1D and clarifies molecular events underlying the alleviatory effect of VD for immunotherapy against T1D.

The combination of omega-3 fatty acids with high doses of vitamin D3 elevate A1c levels: A randomized Clinical Trial in people with vitamin D deficiency

PURPOSE

This randomised clinical trial (RCT) was created to assess the influence of 1,25-dihydroxyvitamin D (VD₃ ), omega-3 fatty acids (n-3FA) and their combination (D+) on glycated haemoglobin (A1c) levels in Jordanian peoples with vitamin D deficiency (VDD).

PARTICIPANTS AND METHODS

This RCT was designed to examine the follow-up (2 months) effect of either 50 000 IU VD₃ , 300 mg n-3FA, or the combination of the two supplements on glycated Haemoglobin (A1c) levels in 146 Jordanian women and men with VDD, aged from 25 to 55 years. The eligible participants were randomised into four groups: Control (C); VD₃ supplementation (50 000 IU of VD₃ was taken weekly) (D₃ ); n-3FA supplementation (300 mg of omega-3FA was taken daily) (n-3FA); VD₃ and omega-3 supplementation group (D+) with the same protocol as the previous two groups.

RESULTS

The combination therapy (n-3FA plus VD3) for 8 weeks significantly increased A1c levels (5.79 ± 0.34 vs 5.41 ± 0.33, P < .001). Tukey test for post hoc comparisons of A1c at follow-up showed that the A1c mean levels were remarkably higher in the D+ study group comparing to the control group (5.78 vs 5.38).

CONCLUSION

The intervention of n-3FA alone or in combination with high doses of VD₃ may lead to negative effects on glycaemic control or accelerate the insulin resistance's development in susceptible people for diabetes mellitus (type 2).

Associations of Triglycerides/High-Density Lipoprotein Cholesterol Ratio With Insulin Resistance, Impaired Glucose Tolerance, and Diabetes in American Adults at Different Vitamin D3 Levels

BACKGROUND

Previous studies have shown that vitamin D3 (VD3) may be a protective factor for diabetes mellitus (DM), while triglycerides/high-density lipoprotein (TG/HDL) may be a risk factor for diabetes. However, no existing study has elucidated the interaction between TG/HDL and VD3. Therefore, this work aimed to investigate the relationships of TG/HDL with insulin resistance (IR), impaired glucose tolerance (IGT), and DM at different VD3 levels.

METHODS

With the use of the data from five National Health and Nutrition Examination Survey (NHANES) cycles, a total of 2,929 males and 3,031 females were divided into 4 groups according to their VD3 levels. Logistic regression was performed to observe the associations of TG/HDL ratio with IR, IGT, and DM in different groups.

RESULTS

The relationships of TG/HDL with IR, IGT, and DM showed a threshold effect, with the cutoff values of 1.094, 1.51, and 1.11, respectively. On both sides of the cutoff values, the correlation was first weakened and then enhanced with the increase in VD3 levels.

CONCLUSION

TG/HDL is a risk factor for IR, IGT, and DM. Both too low and too high levels of VD3 can strengthen this association, whereas keeping VD3 at a reasonable level helps to reduce the associations of TG/HDL with IR, IGT, and DM.

Vitamin D metabolites influence expression of genes concerning cellular viability and function in insulin producing β-cells (INS1E)

BACKGROUND

Studies have shown that vitamin D can enhance glucose-stimulated insulin secretion (GSIS) and change the expression of genes in pancreatic β-cells. Still the mechanisms linking vitamin D and GSIS are unknown.

MATERIAL AND METHODS

We used an established β-cell line, INS1E. INS1E cells were pre-treated with 10 nM 1,25(OH)₂vitamin D or 10 nM 25(OH)vitamin D for 72 h and stimulated with 22 mM glucose for 60 min. RNA was extracted for gene expression analysis.

RESULTS

Expression of genes affecting viability, apoptosis and GSIS changed after pre-treatment with both 1,25(OH)₂vitamin D and 25(OH)vitamin D in INS1E cells. Stimulation with glucose after pre-treatment of INS1E cells with 1,25(OH)₂vitamin D resulted in 181 differentially expressed genes, whereas 526 genes were differentially expressed after pre-treatment with 25(OH)vitamin D.

CONCLUSION

Vitamin D metabolites may affect pancreatic β-cells and GSIS through changed gene expression for genes involved in β-cell function and viability.

Bile acids in glucose metabolism and insulin signalling - mechanisms and research needs

Of all the novel glucoregulatory molecules discovered in the past 20 years, bile acids (BAs) are notable for the fact that they were hiding in plain sight. BAs were well known for their requirement in dietary lipid absorption and biliary cholesterol secretion, due to their micelle-forming properties. However, it was not until 1999 that BAs were discovered to be endogenous ligands for the nuclear receptor FXR. Since that time, BAs have been shown to act through multiple receptors (PXR, VDR, TGR5 and S1PR2), as well as to have receptor-independent mechanisms (membrane dynamics, allosteric modulation of N-acyl phosphatidylethanolamine phospholipase D). We now also have an appreciation of the range of physiological, pathophysiological and therapeutic conditions in which endogenous BAs are altered, raising the possibility that BAs contribute to the effects of these conditions on glycaemia. In this Review, we highlight the mechanisms by which BAs regulate glucose homeostasis and the settings in which endogenous BAs are altered, and provide suggestions for future research.

Claudin 4 in pancreatic β cells is involved in regulating the functional state of adult islets

The functional state (FS) of adult pancreatic islets is regulated by a large array of regulatory molecules including numerous transcription factors. Whether any islet structural molecules play such a role has not been well understood. Here, multiple technologies including bioinformatics analyses were used to explore such molecules. The tight junction family molecule claudin 4 (Cldn4) was the highest enriched amongst over 140 structural genes analysed. Cldn4 expression was ~75-fold higher in adult islets than in exocrine tissues and was mostly up-regulated during functional maturation of developing islet cells. Cldn4 was progressively down-regulated in functionally compromised, dedifferentiating insulin-secreting β cells and in db/db type 2 diabetic islets. Furthermore, the genetic deletion of Cldn4 impaired significantly the FS without apparently affecting pancreas morphology, islet architectural structure and cellular distribution, and secretion of enteroendocrine hormones. Thus, we suggest a previously unidentified role for Cldn4 in regulating the FS of islets, with implications in translational research for better diabetes therapies.

Vitamin D Increases Glucose Stimulated Insulin Secretion from Insulin Producing Beta Cells (INS1E)

BACKGROUND

Vitamin D affects the pancreatic beta cell function and in vitro studies have shown that vitamin D may influence insulin secretion, apoptosis, and gene regulation. However, the outcomes have differed and there has been uncertainty regarding the effect of different vitamin D metabolites on insulin secretion.

OBJECTIVES

We hypothesized that vitamin D could increase insulin secretion in insulin producing beta cells and investigated the effect of 25(OH) vitamin D and 1,25(OH)₂ vitamin D on insulin secretion.

METHODS

The study was conducted in INS1E cells, an established insulinoma cell line from rat. The cells were divided into three groups; a control group, a group with 1,25(OH)₂ vitamin D enriched medium (10 nM), and a group with 25(OH) vitamin D (10 nM) supplemented medium. After 72 hours of treatment, the cells underwent glucose stimulation at different concentrations (0, 5, 11, and 22 mM) for 60 minutes.

RESULTS

INS1E cells treated with 1,25(OH)₂ vitamin D showed a trend towards increased insulin secretion at all glucose concentrations compared to control cells and at 22 mM glucose, the difference was significant (18.40 +/- 1.97 vs 12.90 +/- 2.22 nmol/L, P < 0.05). However, pretreatment with 25(OH) vitamin D did not show any significant increase in insulin secretion compared to cells without vitamin D treatment. There was no difference in insulin secretion in cells not stimulated with glucose.

CONCLUSIONS

Treatment with 1,25(OH)₂ vitamin D combined with high levels of glucose increased insulin secretion in INS1E cells, whereas 25(OH) vitamin D had no effect. This suggests that glucose stimulated insulin secretion in INS1E beta cells appears to be related to the type of vitamin D metabolite treatment.

RNA-Seq Analysis of Islets to Characterise the Dedifferentiation in Type 2 Diabetes Model Mice db/db

Type 2 diabetes (T2D) is a global health issue and dedifferentiation plays underlying causes in the pathophysiology of T2D; however, there is a lack of understanding in the mechanism. Dedifferentiation results from the loss of function of pancreatic β-cells alongside a reduction in essential transcription factors under various physiological stressors. Our study aimed to establish db/db as an animal model for dedifferentiation by using RNA sequencing to compare the gene expression profile in islets isolated from wild-type, db/+ and db/db mice, and qPCR was performed to validate those significant genes. A reduction in both insulin secretion and the expression of Ins1, Ins2, Glut2, Pdx1 and MafA was indicative of dedifferentiation in db/db islets. A comparison of the db/+ and the wild-type islets indicated a reduction in insulin secretion perhaps related to the decreased Mt1. A significant reduction in both Rn45s and Mir6236 was identified in db/+ compared to wild-type islets, which may be indicative of pre-diabetic state. A further significant reduction in RasGRF1, Igf1R and Htt was also identified in dedifferentiated db/db islets. Molecular characterisation of the db/db islets was performed via Ingenuity analysis which identified highly significant genes that may represent new molecular markers of dedifferentiation.

An overview of type 2 diabetes and importance of vitamin D3-vitamin D receptor interaction in pancreatic β-cells

One significant health issue that plagues contemporary society is that of Type 2 diabetes (T2D). This disease is characterised by higher-than-average blood glucose levels as a result of a combination of insulin resistance and insufficient insulin secretions from the β-cells of pancreatic islets of Langerhans. Previous developmental research into the pancreas has identified how early precursor genes of pancreatic β-cells, such as Cpal, Ngn3, NeuroD, Ptf1a, and cMyc, play an essential role in the differentiation of these cells. Furthermore, β-cell molecular characterization has also revealed the specific role of β-cell-markers, such as Glut2, MafA, Ins1, Ins2, and Pdx1 in insulin expression. The expression of these genes appears to be suppressed in the T2D β-cells, along with the reappearance of the early endocrine marker genes. Glucose transporters transport glucose into β-cells, thereby controlling insulin release during hyperglycaemia. This stimulates glycolysis through rises in intracellular calcium (a process enhanced by vitamin D) (Norman et al., 1980), activating 2 of 4 proteinases. The rise in calcium activates half of pancreatic β-cell proinsulinases, thus releasing free insulin from granules. The synthesis of ATP from glucose by glycolysis, Krebs cycle and oxidative phosphorylation plays a role in insulin release. Some studies have found that the β-cells contain high levels of the vitamin D receptor; however, the role that this plays in maintaining the maturity of the β-cells remains unknown. Further research is required to develop a more in-depth understanding of the role VDR plays in β-cell function and the processes by which the beta cell function is preserved.

Is Hypovitaminosis D Related to Incidence of Type 2 Diabetes and High Fasting Glucose Level in Healthy Subjects: A Systematic Review and Meta-Analysis of Observational Studies

There is evidence that vitamin D status is associated with type 2 diabetes. Many observational studies have been performed investigating the relationship of vitamin D status and circulating biomarkers of glycemic regulation. To find out whether this association holds, we conducted a systematic review and meta-analysis of cross sectional and longitudinal studies. We searched Pubmed, Medline and Embase, all through June 2017. The studies were selected to determine the effect of vitamin D on the parameters of glucose metabolism in diabetic and non-diabetic subjects. Correlation coefficients from all studies were pooled in a random effects meta-analysis. The risk of bias was assessed using Grading of Recommendations Assessment, Development and Evaluation (GRADE) system. We found significant inverse relationship of vitamin D status with glycemic level in both diabetic (r = -0.223, 95% CI = -0.184 to -0.261, p = 0.000) and non-diabetic (r = -0.073, 95% CI = -0.052 to -0.093, p = 0.000) subjects. This meta-analysis concludes that hypovitaminosis D is associated with increased risk of hyperglycemia both in diabetic and non-diabetic subjects. A future strategy for the prevention of impaired glycemic regulation could be individualized supplementation of vitamin D.