By Max Bingham, PhD
MondoA in Human β-Cells as a Glucose-Responsive Transcription Factor
MondoA is likely to be an essential glucose-responsive transcription factor in human β-cells, according to Richards et al. (p. ), leading the authors to suggest that it might be an interesting target to modify the handling of glucose in diabetes. The conclusions come from a series of studies that focused on the exposure of the human pancreatic β-EndoC-βH1 cell line and donated pancreatic human islets to different levels of glucose. Initially using microarray analysis, the authors report that exposure of the cell line to high levels of glucose results in the upregulation of a very limited set of genes, with the most prominent being TXNIP. Notably, there was very limited upregulation of ChREBP targets. Previous data from rodent experiments had suggested the two were linked during exposure to glucose, with ChREBP regulating the expression of TXNIP. This pointed to something else regulating TXNIP, with the suspicion falling on the paralogue of ChREBP—MondoA. In subsequent experiments, the authors go on to show how MondoA apparently shuttles to the β-cell nucleus under high-glucose conditions, where it induces the glucose-responsive genes ARRDC4 and TXNIP. Also, by ramping up cAMP signaling either via exposure to forskolin or the GLP-1 mimetic exendin-4, they could inhibit MondoA moving to the nucleus and prevent TXNIP and ARRDC4 expression. Silencing MondoA expression in the EndoC-βH1 cells also reportedly improved glucose uptake. Author Raphael Scharfmann told Diabetes: “I really believe that a lot of information is missing on rodent and also human β-cells both in physiological and pathological conditions. New tools (in this case human β-cell lines) should help to fill this gap. Our work on MondoA in β-cells represents a type of new discovery, as to the best of my knowledge, information on MondoA in β-cells had been scarce. It will now, for example, be interesting to determine whether there is a cross talk between the GLP-1 signaling pathway, which plays a major role in β-cells, and MondoA, as suggested by our data.”
MondoA, but not ChREBP, localizes in the nucleus under high-glucose conditions in EndoC-βH1 cells. Immunocytochemistry of EndoC-βH1 cells exposed to low or high glucose for 3 h, stained with an antibody against MondoA (green) and Hoechst stain (blue). Scale bars, 20 μm.
MondoA, but not ChREBP, localizes in the nucleus under high-glucose conditions in EndoC-βH1 cells. Immunocytochemistry of EndoC-βH1 cells exposed to low or high glucose for 3 h, stained with an antibody against MondoA (green) and Hoechst stain (blue). Scale bars, 20 μm.
SRp55: A Key Splicing Factor That Controls Human Pancreatic β-Cell Function/Survival
SRp55, a splicing factor that is controlled by the diabetes susceptibility gene GLIS3, might play a major role in maintaining the function and survival of human β-cells, according to Juan-Mateu et al. (p. ). As such, they say that SRp55 may act as an important downstream mediator of GLIS3 function that has previously been implicated in both type 1 and type 2 diabetes and may be crucial to human β-cell survival. The conclusions come from a series of experiments that focused on both human islets and the EndoC-βH1 cell line and a combination of microscopy, RNA sequencing analyses, and gene/splice variant silencing and overexpression approaches. According to the authors, SRp55 is highly expressed in pancreatic β-cells and its silencing resulted in significant β-cell dysfunction and death. RNA sequencing of SRp55 knockdown cells and controls then revealed that SRp55 regulates the splicing of numerous genes involved in cell survival and death, insulin secretion, and a type of kinase signaling called JNK. Further experiments then revealed that SRp55-mediated splicing changes appear to modulate the function of two proapoptotic proteins called BIM and BAX. There were also reportedly changes in JNK signaling and endoplasmic reticulum stress that, taken together, likely explain why SRp55 depletion triggers β-cell apoptosis. Author Décio L. Eizirik commented: “The regulation and role of alternative splicing in β-cells is still terra incognita. Our paper, and other ongoing studies by our group and others, suggests that specific splicing networks (such as SRp55) regulated by inflammation or diabetes susceptibility genes (such as Glis3) control key pathways and processes for the correct function and survival of β-cells. Additional studies are now required to identify the precise networks, splicing regulators, and key splice variants that contribute to β-cell dysfunction and death during immune-induced stress in type 1 diabetes.”
SRp55 is highly expressed in human pancreatic β-cells, and its depletion leads to increased β-cell apoptosis. Fluorescence microscopy of insulin and SRp55 in human islets (left panel) and in the human EndoC-βH1 cell line (right panel) shows staining of SRp55 (red), insulin (green), and nuclei (blue). Scale bars = 1 μm.
SRp55 is highly expressed in human pancreatic β-cells, and its depletion leads to increased β-cell apoptosis. Fluorescence microscopy of insulin and SRp55 in human islets (left panel) and in the human EndoC-βH1 cell line (right panel) shows staining of SRp55 (red), insulin (green), and nuclei (blue). Scale bars = 1 μm.
DPP-4 Inhibitors for Diabetes Wound Healing: In Vitro, Animal, and Human Data
DPP-4 inhibitors may be able to improve wound healing in diabetes according to Long et al. (p. ), who now say that the drug class could be considered an option for treating diabetic ulcers. The authors conducted a series of in vitro, animal, and human studies that looked at the effects of mainly saxagliptin (a DPP-4 inhibitor) on wound healing and also underlying mechanisms. Initially focusing on a human keratinocyte cell line, they report that a range of different DPP-4 inhibitors did appear to result in reductions in simulated wounds in comparison to control subjects. Focusing then on saxagliptin, they go on to demonstrate improved wound healing in mice with streptozotocin-induced diabetes. In terms of understanding the mechanisms involved, they report that DPP-4 inhibitors largely do not promote wound healing via the NRF2 pathway as expected. Some of their previous research, as well as other studies, had suggested DPP-4 inhibitors might activate the pathway and promote wound healing. Instead, experiments with NRF2 knockdown cells and Nrf2−/− mice resulted in delayed wound healing, and yet, saxagliptin administration still resulted in effective wound healing. The authors go on to show that DPP-4 inhibitors likely promote the migration and epithelial-mesenchymal transition of keratinocytes and that they achieve this directly and indirectly through the production of fibroblasts. Finally, they report improved wound healing and recapitulation of many of the mechanisms in both mice studies and a clinical trial involving patients with type 2 diabetes with diabetic ulcers. Author Hongting Zheng said: “Most patients with diabetes experience multiple disease complications. Current research has revealed that some hypoglycemic agents possess both positive and negative effects on these complications, which inspired us to explore the additional effects of hypoglycemic drugs. Our result demonstrated the additional benefits of DPP-4 inhibitors on diabetic wound healing and provided some evidence for the development of individualized treatment strategies for patients with diabetes and diabetic ulcers.”
DPP-4 inhibitor induces epithelial-mesenchymal transition of keratinocytes directly. HaCaT cells were treated with compounds 10 mmol/L Sax, 100 mmol/L Sit, or 5 ng/mL TGF-b1 for 24 h, and cell morphology was assessed under a phase contrast microscope. Con, control, equivalent dose of DMSO; Sax, saxagliptin; Sit, sitagliptin.
DPP-4 inhibitor induces epithelial-mesenchymal transition of keratinocytes directly. HaCaT cells were treated with compounds 10 mmol/L Sax, 100 mmol/L Sit, or 5 ng/mL TGF-b1 for 24 h, and cell morphology was assessed under a phase contrast microscope. Con, control, equivalent dose of DMSO; Sax, saxagliptin; Sit, sitagliptin.
A Perspective on the Mechanisms of Insulin Secretion and Its Pulsatility
The current state of knowledge relating to the mechanisms behind pulsatile insulin secretion is reviewed by Bertram et al. (p. ). They suggest that the time has come for the accumulated knowledge gained from studies of mouse islets to be translated into studies with human islets in a bid to uncover therapeutic targets to restore normal pulsatile patterns that are apparently disrupted in diabetes. The authors say that although insulin pulsatility in mouse islets has been the subject of over four decades of research, various theories and models have emerged that propose different explanations and mechanisms behind the phenomenon. They describe two main classes of models where oscillations in intracellular Ca2+ concentrations drive concurrent oscillations in metabolism and the opposite situation where fluctuations in metabolism drive changes in Ca2+ and electrical activity. They say that there are nine key pieces of evidence that exist from experimental studies that explain insulin oscillations and describe each one in detail. However, apparently most existing models do not account for all these strands of evidence simultaneously. The authors list exactly what each model can account for, describing in detail the experimental evidence and the consequences. Finally, they detail a relatively new model, the Integrated Oscillator Model, that reportedly does now account for all nine strands of evidence. They say the key feature is the hypothesis that intracellular Ca2+ acts on the glycolytic pathway and that this ultimately results in insulin oscillations. As a result, they say the new model should serve as a good starting point for future research into insulin pulsatility in humans.
