Deletion of ARNT/HIF1β in pancreatic beta cells does not impair glucose homeostasis in mice, but is associated with defective glucose sensing ex vivo

Roles of β-Cell Hypoxia in the Progression of Type 2 Diabetes

Type 2 diabetes is a chronic disease marked by hyperglycemia; impaired insulin secretion by pancreatic β-cells is a hallmark of this disease. Recent studies have shown that hypoxia occurs in the β-cells of patients with type 2 diabetes and hypoxia, in turn, contributes to the insulin secretion defect and β-cell loss through various mechanisms, including the activation of hypoxia-inducible factors, induction of transcriptional repressors, and activation of AMP-activated protein kinase. This review focuses on advances in our understanding of the contribution of β-cell hypoxia to the development of β-cell dysfunction in type 2 diabetes. A better understanding of β-cell hypoxia might be useful in the development of new strategies for treating type 2 diabetes.

Renal tubule-specific Atgl deletion links kidney lipid metabolism to glucagon-like peptide 1 and insulin secretion independent of renal inflammation or lipotoxicity

OBJECTIVE

Lipotoxic injury from renal lipid accumulation in obesity and type 2 diabetes (T2D) is implicated in associated kidney damage. However, models examining effects of renal ectopic lipid accumulation independent of obesity or T2D are lacking. We generated renal tubule-specific adipose triglyceride lipase knockout (RT-SAKO) mice to determine if this targeted triacylglycerol (TAG) over-storage affects glycemic control and kidney health.

METHODS

Male and female RT-SAKO mice and their control littermates were tested for changes in glycemic control at 10-12 and 16-18 weeks of age. Markers of kidney health and blood lipid and hormone concentrations were analyzed. Kidney and blood lysophosphatidic acid (LPA) levels were measured, and a role for LPA in mediating impaired glycemic control was evaluated using the LPA receptor 1/3 inhibitor Ki-16425.

RESULTS

All groups remained insulin sensitive, but 16- to 18-week-old male RT-SAKO mice became glucose intolerant, without developing kidney inflammation or fibrosis. Rather, these mice displayed lower circulating insulin and glucagon-like peptide 1 (GLP-1) levels. Impaired first-phase glucose-stimulated insulin secretion was detected and restored by Exendin-4. Kidney and blood LPA levels were elevated in older male but not female RT-SAKO mice, associated with increased kidney diacylglycerol kinase epsilon. Inhibition of LPA-mediated signaling restored serum GLP-1 levels, first-phase insulin secretion, and glucose tolerance.

CONCLUSIONS

TAG over-storage alone is insufficient to cause renal tubule lipotoxicity. This work is the first to show that endogenously derived LPA modulates GLP-1 levels in vivo, demonstrating a new mechanism of kidney-gut-pancreas crosstalk to regulate insulin secretion and glucose homeostasis.

Isoform-specific Roles of Prolyl Hydroxylases in the Regulation of Pancreatic β-Cell Function

Pancreatic β-cells can secrete insulin via 2 pathways characterized as KATP channel -dependent and -independent. The KATP channel-independent pathway is characterized by a rise in several potential metabolic signaling molecules, including the NADPH/NADP+ ratio and α-ketoglutarate (αKG). Prolyl hydroxylases (PHDs), which belong to the αKG-dependent dioxygenase superfamily, are known to regulate the stability of hypoxia-inducible factor α. In the current study, we assess the role of PHDs in vivo using the pharmacological inhibitor dimethyloxalylglycine (DMOG) and generated β-cell-specific knockout (KO) mice for all 3 isoforms of PHD (β-PHD1 KO, β-PHD2 KO, and β-PHD3 KO mice). DMOG inhibited in vivo insulin secretion in response to glucose challenge and inhibited the first phase of insulin secretion but enhanced the second phase of insulin secretion in isolated islets. None of the β-PHD KO mice showed any significant in vivo defects associated with glucose tolerance and insulin resistance except for β-PHD2 KO mice which had significantly increased plasma insulin during a glucose challenge. Islets from both β-PHD1 KO and β-PHD3 KO had elevated β-cell apoptosis and reduced β-cell mass. Isolated islets from β-PHD1 KO and β-PHD3 KO had impaired glucose-stimulated insulin secretion and glucose-stimulated increases in the ATP/ADP and NADPH/NADP+ ratio. All 3 PHD isoforms are expressed in β-cells, with PHD3 showing the most distinct expression pattern. The lack of each PHD protein did not significantly impair in vivo glucose homeostasis. However, β-PHD1 KO and β-PHD3 KO mice had defective β-cell mass and islet insulin secretion, suggesting that these mice may be predisposed to developing diabetes.

INFIMA leverages multi-omics model organism data to identify effector genes of human GWAS variants

Genome-wide association studies reveal many non-coding variants associated with complex traits. However, model organism studies largely remain as an untapped resource for unveiling the effector genes of non-coding variants. We develop INFIMA, Integrative Fine-Mapping, to pinpoint causal SNPs for diversity outbred (DO) mice eQTL by integrating founder mice multi-omics data including ATAC-seq, RNA-seq, footprinting, and in silico mutation analysis. We demonstrate INFIMA's superior performance compared to alternatives with human and mouse chromatin conformation capture datasets. We apply INFIMA to identify novel effector genes for GWAS variants associated with diabetes. The results of the application are available at http://www.statlab.wisc.edu/shiny/INFIMA/ .

Transcriptomic and Quantitative Proteomic Profiling Reveals Signaling Pathways Critical for Pancreatic Islet Maturation

Pancreatic β-cell dysfunction and reduced insulin secretion play a key role in the pathogenesis of diabetes. Fetal and neonatal islets are functionally immature and have blunted glucose responsiveness and decreased insulin secretion in response to stimuli and are far more proliferative. However, the mechanisms underlying functional immaturity are not well understood. Pancreatic islets are composed of a mixture of different cell types, and the microenvironment of islets and interactions between these cell types are critical for β-cell development and maturation. RNA sequencing and quantitative proteomic data from intact islets isolated from fetal (embryonic day 19) and 2-week-old Sprague-Dawley rats were integrated to compare their gene and protein expression profiles. Ingenuity Pathway Analysis (IPA) was also applied to elucidate pathways and upstream regulators modulating functional maturation of islets. By integrating transcriptome and proteomic data, 917 differentially expressed genes/proteins were identified with a false discovery rate of less than 0.05. A total of 411 and 506 of them were upregulated and downregulated in the 2-week-old islets, respectively. IPA revealed novel critical pathways associated with functional maturation of islets, such as AMPK (adenosine monophosphate-activated protein kinase) and aryl hydrocarbon receptor signaling, as well as the importance of lipid homeostasis/signaling and neuronal function. Furthermore, we also identified many proteins enriched either in fetal or 2-week-old islets related to extracellular matrix and cell communication, suggesting that these pathways play critical roles in islet maturation. Our present study identified novel pathways for mature islet function in addition to confirming previously reported mechanisms, and provided new mechanistic insights for future research on diabetes prevention and treatment.

The AHR Signaling Attenuates Autoimmune Responses During the Development of Type 1 Diabetes

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcriptional factor widely expressed in immune cells. Its ligands range from xenobiotics and natural substances to metabolites, which renders it capable of sensing and responding to a variety of environmental cues. Although AHR signaling has long been recognized to be implicated in the pathogenesis of autoimmune disorders, such as rheumatoid arthritis (RA), colitis, and systemic lupus erythematosus (SLE), its effect on the pathogenesis of type 1 diabetes (T1D) remains less understood. In this review, we intend to summarize its potential implication in T1D pathogenesis and to sort out the related regulatory mechanisms in different types of immune cells. Emerging evidence supports that β cell destruction caused by autoimmune responses can be rectified by AHR signaling. Upon activation by its ligands, AHR not only modulates the development and functionality of immune cells, but also suppresses the expression of inflammatory cytokines, through which AHR attenuates autoimmune responses during the course of T1D development. Since AHR-initiated biological effects vary between different types of ligands, additional studies would be necessary to characterize or de novo synthesize effective and safe ligands aimed to replenish our arsenal in fighting autoimmune responses and β mass loss in a T1D setting.

Integrative Analysis of HNF1B mRNA in Human Cancers Based on Data Mining

Cancer incidence is rapidly growing, and cancer is the leading cause of death worldwide in the 21st century. Hepatocyte nuclear factor 1B (HNF1B) is a transcription factor that involves the growth and development of multiple organs. The aim of this study was to explore the significance of HNF1B in human cancer by an integrative analysis of online databases. The UALCAN database, cBio cancer genomics portal, Cancer Regulome tools, Kaplan-Meier plotter and Tumor IMmune Estimation Resource (TIMER) website were used to perform the corresponding analysis. The results showed that HNF1B is dysregulated in various cancers and associated with the differential overall survival of cancer patients. HNF1B showed many mutation forms and high mutation levels in different cancer types. In addition, we found that HNF1B interacted with different genes in multiple aspects. Moreover, HNF1B expression is associated with many immune cell infiltration levels and influences the prognostic prediction of immune cells in some kinds of cancers. In conclusion, HNF1B plays a significant role in cancer and may be a potential target for cancer immunotherapy.

The Loss of ARNT/HIF1β in Male Pancreatic β-Cells Is Protective Against High-Fat Diet-Induced Diabetes

The transcription factor aryl hydrocarbon receptor nuclear translocator (ARNT)/hypoxia-inducible factor (HIF)-1β (ARNT/HIF1β) plays a key role in maintaining β-cell function and has been shown to be one of the most downregulated transcription factors in islets from patients with type 2 diabetes. We have shown a role for ARNT/HIF1β in glucose sensing and insulin secretion in vitro and no defects in in vivo glucose homeostasis. To gain a better understanding of the role of ARNT/HIF1β in the development of diabetes, we placed control (+/+/Cre) and β-cell-specific ARNT/HIF1β knockout (fl/fl/Cre) mice on a high-fat diet (HFD). Unlike the control (+/+/Cre) mice, HFD-fed fl/fl/Cre mice had no impairment in in vivo glucose tolerance. The lack of impairment in HFD-fed fl/fl/Cre mice was partly due to an improved islet glucose-stimulated NADPH/NADP+ ratio and glucose-stimulated insulin secretion. The effects of the HFD-rescued insulin secretion in fl/fl/Cre islets could be reproduced by treating low-fat diet (LFD)-fed fl/fl/Cre islets with the lipid signaling molecule 1-monoacylglcyerol. This suggests that the defects seen in LFD-fed fl/fl/Cre islet insulin secretion involve lipid signaling molecules. Overall, mice lacking ARNT/HIF1β in β-cells have altered lipid signaling in vivo and are resistant to an HFD's ability to induce diabetes.

Hormetic and regulatory effects of lipid peroxidation mediators in pancreatic beta cells

Nutrient sensing mechanisms of carbohydrates, amino acids and lipids operate distinct pathways that are essential for the adaptation to varying metabolic conditions. The role of nutrient-induced biosynthesis of hormones is paramount for attaining metabolic homeostasis in the organism. Nutrient overload attenuate key metabolic cellular functions and interfere with hormonal-regulated inter- and intra-organ communication, which may ultimately lead to metabolic derangements. Hyperglycemia and high levels of saturated free fatty acids induce excessive production of oxygen free radicals in tissues and cells. This phenomenon, which is accentuated in both type-1 and type-2 diabetic patients, has been associated with the development of impaired glucose tolerance and the etiology of peripheral complications. However, low levels of the same free radicals also induce hormetic responses that protect cells against deleterious effects of the same radicals. Of interest is the role of hydroxyl radicals in initiating peroxidation of polyunsaturated fatty acids (PUFA) and generation of α,β-unsaturated reactive 4-hydroxyalkenals that avidly form covalent adducts with nucleophilic moieties in proteins, phospholipids and nucleic acids. Numerous studies have linked the lipid peroxidation product 4-hydroxy-2E-nonenal (4-HNE) to different pathological and cytotoxic processes. Similarly, two other members of the family, 4-hydroxyl-2E-hexenal (4-HHE) and 4-hydroxy-2E,6Z-dodecadienal (4-HDDE), have also been identified as potential cytotoxic agents. It has been suggested that 4-HNE-induced modifications in macromolecules in cells may alter their cellular functions and modify signaling properties. Yet, it has also been acknowledged that these bioactive aldehydes also function as signaling molecules that directly modify cell functions in a hormetic fashion to enable cells adapt to various stressful stimuli. Recent studies have shown that 4-HNE and 4-HDDE, which activate peroxisome proliferator-activated receptor δ (PPARδ) in vascular endothelial cells and insulin secreting beta cells, promote such adaptive responses to ameliorate detrimental effects of high glucose and diabetes-like conditions. In addition, due to the electrophilic nature of these reactive aldehydes they form covalent adducts with electronegative moieties in proteins, phosphatidylethanolamine and nucleotides. Normally these non-enzymatic modifications are maintained below the cytotoxic range due to efficient cellular neutralization processes of 4-hydroxyalkenals. The major neutralizing enzymes include fatty aldehyde dehydrogenase (FALDH), aldose reductase (AR) and alcohol dehydrogenase (ADH), which transform the aldehyde to the corresponding carboxylic acid or alcohols, respectively, or by biding to the thiol group in glutathione (GSH) by the action of glutathione-S-transferase (GST). This review describes the hormetic and cytotoxic roles of oxygen free radicals and 4-hydroxyalkenals in beta cells exposed to nutritional challenges and the cellular mechanisms they employ to maintain their level at functional range below the cytotoxic threshold.