Jenny E. Gunton

Loss of ARNT/HIF1β Mediates Altered Gene Expression and Pancreatic-Islet Dysfunction in Human Type 2 Diabetes

beta cell dysfunction is a central component of the pathogenesis of type 2 diabetes. Using oligonucleotide microarrays and real-time PCR of pancreatic islets isolated from humans with type 2 diabetes versus normal glucose-tolerant controls, we identified multiple changes in expression of genes known to be important in beta cell function, including major decreases in expression of HNF4alpha, insulin receptor, IRS2, Akt2, and several glucose-metabolic-pathway genes. There was also a 90% decrease in expression of the transcription factor ARNT. Reducing ARNT levels in Min6 cells with small interfering RNA (siRNA) resulted in markedly impaired glucose-stimulated insulin release and changes in gene expression similar to those in human type 2 islets. Likewise, beta cell-specific ARNT knockout mice exhibited abnormal glucose tolerance, impaired insulin secretion, and changes in islet gene expression that mimicked those in human diabetic islets. Together, these data suggest an important role for decreased ARNT and altered gene expression in the impaired islet function of human type 2 diabetes.

Vitamin D receptor-mediated stromal reprogramming suppresses pancreatitis and enhances pancreatic cancer therapy.

The poor clinical outcome in pancreatic ductal adenocarcinoma (PDA) is attributed to intrinsic chemoresistance and a growth-permissive tumor microenvironment. Conversion of quiescent to activated pancreatic stellate cells (PSCs) drives the severe stromal reaction that characterizes PDA. Here, we reveal that the vitamin D receptor (VDR) is expressed in stroma from human pancreatic tumors and that treatment with the VDR ligand calcipotriol markedly reduced markers of inflammation and fibrosis in pancreatitis and human tumor stroma. We show that VDR acts as a master transcriptional regulator of PSCs to reprise the quiescent state, resulting in induced stromal remodeling, increased intratumoral gemcitabine, reduced tumor volume, and a 57% increase in survival compared to chemotherapy alone. This work describes a molecular strategy through which transcriptional reprogramming of tumor stroma enables chemotherapeutic response and suggests vitamin D priming as an adjunct in PDA therapy. PAPERFLICK:

A Vitamin D Receptor/SMAD Genomic Circuit Gates Hepatic Fibrotic Response

Liver fibrosis is a reversible wound-healing response involving TGFβ1/SMAD activation of hepatic stellate cells (HSCs). It results from excessive deposition of extracellular matrix components and can lead to impairment of liver function. Here, we show that vitamin D receptor (VDR) ligands inhibit HSC activation by TGFβ1 and abrogate liver fibrosis, whereas Vdr knockout mice spontaneously develop hepatic fibrosis. Mechanistically, we show that TGFβ1 signaling causes a redistribution of genome-wide VDR-binding sites (VDR cistrome) in HSCs and facilitates VDR binding at SMAD3 profibrotic target genes via TGFβ1-dependent chromatin remodeling. In the presence of VDR ligands, VDR binding to the coregulated genes reduces SMAD3 occupancy at these sites, inhibiting fibrosis. These results reveal an intersecting VDR/SMAD genomic circuit that regulates hepatic fibrogenesis and define a role for VDR as an endocrine checkpoint to modulate the wound-healing response in liver. Furthermore, the findings suggest VDR ligands as a potential therapy for liver fibrosis.

The roles of vitamin D in skeletal muscle: form, function, and metabolism.

Beyond its established role in bone and mineral homeostasis, there is emerging evidence that vitamin D exerts a range of effects in skeletal muscle. Reports of profound muscle weakness and changes in the muscle morphology of adults with vitamin D deficiency have long been described. These reports have been supplemented by numerous trials assessing the impact of vitamin D on muscle strength and mass and falls in predominantly elderly and deficient populations. At a basic level, animal models have confirmed that vitamin D deficiency and congenital aberrations in the vitamin D endocrine system may result in muscle weakness. To explain these effects, some molecular mechanisms by which vitamin D impacts on muscle cell differentiation, intracellular calcium handling, and genomic activity have been elucidated. There are also suggestions that vitamin D alters muscle metabolism, specifically its sensitivity to insulin, which is a pertinent feature in the pathophysiology of insulin resistance and type 2 diabetes. We will review the range of human clinical, animal, and cell studies that address the impact of vitamin D in skeletal muscle, and discuss the controversial issues. This is a vibrant field of research and one that continues to extend the frontiers of knowledge of vitamin Ds broad functional repertoire.

Hypoxia-inducible factor-1α regulates β cell function in mouse and human islets

Hypoxia-inducible factor-1alpha (HIF-1alpha) is a transcription factor that regulates cellular stress responses. While the levels of HIF-1alpha protein are tightly regulated, recent studies suggest that it can be active under normoxic conditions. We hypothesized that HIF-1alpha is required for normal beta cell function and reserve and that dysregulation may contribute to the pathogenesis of type 2 diabetes (T2D). Here we show that HIF-1alpha protein is present at low levels in mouse and human normoxic beta cells and islets. Decreased levels of HIF-1alpha impaired glucose-stimulated ATP generation and beta cell function. C57BL/6 mice with beta cell-specific Hif1a disruption (referred to herein as beta-Hif1a-null mice) exhibited glucose intolerance, beta cell dysfunction, and developed severe glucose intolerance on a high-fat diet. Increasing HIF-1alpha levels by inhibiting its degradation through iron chelation markedly improved insulin secretion and glucose tolerance in control mice fed a high-fat diet but not in beta-Hif1a-null mice. Increasing HIF-1alpha levels markedly increased expression of ARNT and other genes in human T2D islets and improved their function. Further analysis indicated that HIF-1alpha was bound to the Arnt promoter in a mouse beta cell line, suggesting direct regulation. Taken together, these findings suggest an important role for HIF-1alpha in beta cell reserve and regulation of ARNT expression and demonstrate that HIF-1alpha is a potential therapeutic target for the beta cell dysfunction of T2D.

Vitamin D Signaling Regulates Proliferation, Differentiation, and Myotube Size in C2C12 Skeletal Muscle Cells

Vitamin D deficiency is linked to a range of muscle disorders including myalgia, muscle weakness, and falls. Humans with severe vitamin D deficiency and mice with transgenic vitamin D receptor (VDR) ablation have muscle fiber atrophy. However, molecular mechanisms by which vitamin D influences muscle function and fiber size remain unclear. A central question is whether VDR is expressed in skeletal muscle and is able to regulate transcription at this site. To address this, we examined key molecular and morphologic changes in C2C12 cells treated with 25-hydroxyvitamin D (25OHD) and 1,25-dihydroxyvitamin D (1,25(OH)(2)D). As well as stimulating VDR expression, 25(OH)D and 1,25(OH)(2)D dose-dependently increased expression of the classic vitamin D target cytochrome P450, family 24, subfamily A, polypeptide 1 (CYP24A1), demonstrating the presence of an autoregulatory vitamin D-endocrine system in these cells. Luciferase reporter studies demonstrated that cytochrome P450, family 27, subfamily B, polypeptide 1 (CYP27B1) was functional in these cells. Both 25OHD and 1,25(OH)(2)D altered C2C12 proliferation and differentiation. These effects were related to the increased expression of genes involved in G(0)/G(1) arrest (retinoblastoma protein [Rb], 1.3-fold; ATM, 1.5-fold, both P < .05), downregulation of mRNAs involved in G(1)/S transition, including myc and cyclin-D1 (0.7- and 0.8-fold, both P < .05) and reduced phosphorylation of Rb protein (0.3-fold, P < .005). After serum depletion, 1,25(OH)(2)D (100nM) suppressed myotube formation with decreased mRNAs for key myogenic regulatory factors (myogenin, 0.5-fold; myf5, 0.4-fold, P < .005) but led to a 1.8-fold increase in cross-sectional size of individual myotubes associated with markedly decreased myostatin expression (0.2-fold, P < .005). These data show that vitamin D signaling alters gene expression in C2C12 cells, with effects on proliferation, differentiation, and myotube size.

The Vitamin D Receptor (VDR) Is Expressed in Skeletal Muscle of Male Mice and Modulates 25-Hydroxyvitamin D (25OHD) Uptake in Myofibers

Vitamin D deficiency is associated with a range of muscle disorders, including myalgia, muscle weakness, and falls. In humans, polymorphisms of the vitamin D receptor (VDR) gene are associated with variations in muscle strength, and in mice, genetic ablation of VDR results in muscle fiber atrophy and motor deficits. However, mechanisms by which VDR regulates muscle function and morphology remain unclear. A crucial question is whether VDR is expressed in skeletal muscle and directly alters muscle physiology. Using PCR, Western blotting, and immunohistochemistry (VDR-D6 antibody), we detected VDR in murine quadriceps muscle. Detection by Western blotting was dependent on the use of hyperosmolar lysis buffer. Levels of VDR in muscle were low compared with duodenum and dropped progressively with age. Two in vitro models, C2C12 and primary myotubes, displayed dose- and time-dependent increases in expression of both VDR and its target gene CYP24A1 after 1,25(OH)2D (1,25 dihydroxyvitamin D) treatment. Primary myotubes also expressed functional CYP27B1 as demonstrated by luciferase reporter studies, supporting an autoregulatory vitamin D-endocrine system in muscle. Myofibers isolated from mice retained tritiated 25-hydroxyvitamin D3, and this increased after 3 hours of pretreatment with 1,25(OH)2D (0.1nM). No such response was seen in myofibers from VDR knockout mice. In summary, VDR is expressed in skeletal muscle, and vitamin D regulates gene expression and modulates ligand-dependent uptake of 25-hydroxyvitamin D3 in primary myofibers.

Ablation of ARNT/HIF1β in Liver Alters Gluconeogenesis, Lipogenic Gene Expression, and Serum Ketones

We have previously shown that expression of the transcription factor ARNT/HIF1beta is reduced in islets of humans with type 2 diabetes. We have now found that ARNT is also reduced in livers of diabetics. To study the functional effect of its reduction, we created mice with liver-specific ablation (L-ARNT KO) using ARNT loxP mice and adenoviral-mediated delivery of Cre. L-ARNT KO mice had normal blood glucose but increased fed insulin levels. These mice also exhibited features of type 2 diabetes with increased hepatic gluconeogenesis, increased lipogenic gene expression, and low serum beta-hydroxybutyrate. These effects appear to be secondary to increased expression of CCAAT/enhancer-binding protein alpha (C/EBPalpha), farnesoid X receptor (FXR), and sterol response element-binding protein 1c (SREBP-1c) and a reduction in phosphorylation of AMPK without changes in the expression of enzymes in ketogenesis, fatty acid oxidation, or FGF21. These results demonstrate that a deficiency of ARNT action in the liver, coupled with that in beta cells, could contribute to the metabolic phenotype of human type 2 diabetes.

Effects of vitamin D in skeletal muscle: falls, strength, athletic performance and insulin sensitivity

Accompanying the high rates of vitamin D deficiency observed in many countries, there is increasing interest in the physiological functions of vitamin D. Vitamin D is recognized to exert extra‐skeletal actions in addition to its classic roles in bone and mineral homeostasis. Here, we review the evidence for vitamin Ds actions in muscle on the basis of observational studies, clinical trials and basic research. Numerous observational studies link vitamin D deficiency with muscle weakness and sarcopaenia. Randomized trials predominantly support an effect of vitamin D supplementation and the prevention of falls in older or institutionalized patients. Studies have also examined the effect of vitamin D in athletic performance, both inferentially by UV radiation and directly by vitamin D supplementation. Effects of vitamin D in muscle metabolic function, specifically insulin sensitivity, are also addressed in this review. At a mechanistic level, animal studies have evaluated the roles of vitamin D and associated minerals, calcium and phosphate, in muscle function. In vitro studies have identified molecular pathways by which vitamin D regulates muscle cell signalling and gene expression. This review evaluates evidence for the various roles of vitamin D in skeletal muscle and discusses controversies that have made this a dynamic field of research.

Altered Insulin Receptor Signalling and β-Cell Cycle Dynamics in Type 2 Diabetes Mellitus

Insulin resistance, reduced β-cell mass, and hyperglucagonemia are consistent features in type 2 diabetes mellitus (T2DM). We used pancreas and islets from humans with T2DM to examine the regulation of insulin signaling and cell-cycle control of islet cells. We observed reduced β-cell mass and increased α-cell mass in the Type 2 diabetic pancreas. Confocal microscopy, real-time PCR and western blotting analyses revealed increased expression of PCNA and down-regulation of p27-Kip1 and altered expression of insulin receptors, insulin receptor substrate-2 and phosphorylated BAD. To investigate the mechanisms underlying these findings, we examined a mouse model of insulin resistance in β-cells – which also exhibits reduced β-cell mass, the β-cell-specific insulin receptor knockout (βIRKO). Freshly isolated islets and β-cell lines derived from βIRKO mice exhibited poor cell-cycle progression, nuclear restriction of FoxO1 and reduced expression of cell-cycle proteins favoring growth arrest. Re-expression of insulin receptors in βIRKO β-cells reversed the defects and promoted cell cycle progression and proliferation implying a role for insulin-signaling in β-cell growth. These data provide evidence that human β- and α-cells can enter the cell-cycle, but proliferation of β-cells in T2DM fails due to G1-to-S phase arrest secondary to defective insulin signaling. Activation of insulin signaling, FoxO1 and proteins in β-cell-cycle progression are attractive therapeutic targets to enhance β-cell regeneration in the treatment of T2DM.