MafK overexpression in pancreatic beta-cells caused impairment of glucose-stimulated insulin secretion

miR-503 targets MafK to inhibit subcutaneous preadipocyte adipogenesis causing a decrease of backfat thickness in Guanzhong Black pigs

Reducing backfat thickness (BFT), determined by subcutaneous fat deposition, is vital in Chinese developed pig breeds. The level of miR-503 in the backfat of Guanzhong Black pigs was found to be lower than that in Large White pigs, implying that miR-503 may be related to BFT. However, the effect and mechanism of miR-503 on adipogenic differentiation in subcutaneous preadipocytes remain unknown. Compared with Large White pigs, the BFT and body fat content of Guanzhong Black pigs were greater, but the level of miR-503 was lower in subcutaneous adipose tissue (SAT) at 180 days of age. Furthermore, miR-503 promoted preadipocyte proliferation by increasing the proportion of S-phase and EdU-positive cells. However, miR-503 inhibited preadipocyte differentiation by downregulating adipogenic gene expression. Mechanistically, miR-503 directly targeted musculoaponeurotic fibrosarcoma oncogene homolog K (MafK) in both proliferating and differentiating preadipocytes to repress adipogenesis. Our findings provide a novel miRNA biomarker for reducing pig BFT levels to improve carcass quality.

Glucose and oleic acid mediate cellular alterations in GLP-1-induced insulin-positive differentiating UCBMSCs

Coordinated effects of glucose and oleic acid on glucagon-like peptide-1 (GLP-1) mediated differentiation of insulin-positive differentiating umbilical cord mesenchymal stromal cells (dUCBMSCs) was studied using a co-culture of NCI-H716 (GLP-1+) and UCBMSCs (insulin+). The addition of 2.5 mM glucose increased the proliferation of NCI-H716 cells by 30% and induced transformation of UCBMSCs into insulin-secreting cells in 18 days as compared to 22 days in control cells. Oleic acid (25 μM) showed decrease in cell proliferation, autophagy, and apoptosis in NCI-H716 cells while no effect was observed in dUCBMSCs. Prolonged glucose and oleic acid resulted in apoptosis and cell cycle changes in dUCBMSCs after day 18 while higher concentrations resulted in cell death. Additionally, the expression of FAS and ACC mRNA was observed in NCI-H716 and dUCBMSCs post 24-hr addition of glucose and/or oleic acid. Absorption of oleic acid was high in NCI-H716 compared to dUCBMSCs. Taken together, optimal concentrations of glucose and oleic acid could be a key factor in stimulating intrinsic GLP-1, which in turn stimulates differentiating MSCs in a glucose-dependent manner. PRACTICAL APPLICATIONS: The aim of this article was to study whether differentiating or differentiated MSCs after mobilization or post-transplant would require optimal glucose and oleic acid to naturally stimulate intrinsic GLP-1, or otherwise, the high or long-term overload of glucose or oleic acid could result in inhibition of differentiated cells resulting in failure of insulin secretion.

β-Cell-Specific Mafk Overexpression Impairs Pancreatic Endocrine Cell Development

The MAF family transcription factors are homologs of v-Maf, the oncogenic component of the avian retrovirus AS42. They are subdivided into 2 groups, small and large MAF proteins, according to their structure, function, and molecular size. MAFK is a member of the small MAF family and acts as a dominant negative form of large MAFs. In previous research we generated transgenic mice that overexpress MAFK in order to suppress the function of large MAF proteins in pancreatic β-cells. These mice developed hyperglycemia in adulthood due to impairment of glucose-stimulated insulin secretion. The aim of the current study is to examine the effects of β-cell-specific Mafk overexpression in endocrine cell development. The developing islets of Mafk-transgenic embryos appeared to be disorganized with an inversion of total numbers of insulin+ and glucagon+ cells due to reduced β-cell proliferation. Gene expression analysis by quantitative RT-PCR revealed decreased levels of β-cell-related genes whose expressions are known to be controlled by large MAF proteins. Additionally, these changes were accompanied with a significant increase in key β-cell transcription factors likely due to compensatory mechanisms that might have been activated in response to the β-cell loss. Finally, microarray comparison of gene expression profiles between wild-type and transgenic pancreata revealed alteration of some uncharacterized genes including Pcbd1, Fam132a, Cryba2, and Npy, which might play important roles during pancreatic endocrine development. Taken together, these results suggest that Mafk overexpression impairs endocrine development through a regulation of numerous β-cell-related genes. The microarray analysis provided a unique data set of differentially expressed genes that might contribute to a better understanding of the molecular basis that governs the development and function of endocrine pancreas.

Inhibition of Small Maf Function in Pancreatic β-Cells Improves Glucose Tolerance Through the Enhancement of Insulin Gene Transcription and Insulin Secretion

The large-Maf transcription factor v-maf musculoaponeurotic fibrosarcoma oncogene homolog A (MafA) has been found to be crucial for insulin transcription and synthesis and for pancreatic β-cell function and maturation. However, insights about the effects of small Maf factors on β-cells are limited. Our goal was to elucidate the function of small-Maf factors on β-cells using an animal model of endogenous small-Maf dysfunction. Transgenic (Tg) mice with β-cell-specific expression of dominant-negative MafK (DN-MafK) experiments, which can suppress the function of all endogenous small-Mafs, were fed a high-fat diet, and their in vivo phenotypes were evaluated. Phenotypic analysis, glucose tolerance tests, morphologic examination of β-cells, and islet experiments were performed. DN-MafK-expressed MIN6 cells were also used for in vitro analysis. The results showed that DN-MafK expression inhibited endogenous small-Maf binding to insulin promoter while increasing MafA binding. DN-MafK Tg mice under high-fat diet conditions showed improved glucose metabolism compared with control mice via incremental insulin secretion, without causing changes in insulin sensitivity or MafA expression. Moreover, up-regulation of insulin and glucokinase gene expression was observed both in vivo and in vitro under DN-MafK expression. We concluded that endogenous small-Maf factors negatively regulates β-cell function by competing for MafA binding, and thus, the inhibition of small-Maf activity can improve β-cell function.

Rodent animal models: from mild to advanced stages of diabetic nephropathy

Diabetic nephropathy (DN) is a secondary complication of both type 1 and type 2 diabetes, resulting from uncontrolled high blood sugar. 30-40% of diabetic patients develop DN associated with a poor life expectancy and end-stage renal disease, causing serious socioeconomic problems. Although an exact pathogenesis of DN is still unknown, several factors such as hyperglycemia, hyperlipidemia, hypertension and proteinuria may contribute to the progression of renal damage in diabetic nephropathy. DN is confirmed by measuring blood urea nitrogen, serum creatinine, creatinine clearance and proteinuria. Clinical studies show that intensive control of hyperglycemia and blood pressure could successfully reduce proteinuria, which is the main sign of glomerular lesions in DN, and improve the renal prognosis in patients with DN. Diabetic rodent models have traditionally been used for doing research on pathogenesis and developing novel therapeutic strategies, but have limitations for translational research. Diabetes in animal models such as rodents are induced either spontaneously or by using chemical, surgical, genetic, or other techniques and depicts many clinical features or related phenotypes of the disease. This review discusses the merits and demerits of the models, which are used for many reasons in the research of diabetes and diabetic complications.

Noninvasive monitoring of β-cell mass and fetal β-cell genesis in mice using bioluminescence imaging

Bioluminescence imaging (BLI) has been applied in gene therapy and research to screen for transgene expression, progression of infection, tumor growth and metastasis, and transplantation. It enables real-time and relatively noninvasive localization and serial quantification of biological processes in experimental animals. In diabetes research, BLI has been employed for the quantification of β-cell mass, monitoring of islet graft survival after transplantation, and detection of reporter gene expression. Here, we explore the use of BLI in a transgenic mouse expressing luciferase under the control of the mouse insulin 1 promoter (MIP-Luc-VU). A previous report on MIP-Luc-VU mice showed luminescence intensities emitted from the islets correlated well with the number of islets in vitro and in vivo. In this study, we showed MIP-Luc-VU mice fed a high fat diet for 8 weeks gave rise to a greater bioluminescent signal than mice fed a regular diet for the same period of time. Conversely, there was a strong reduction in the signal observed in diabetic Mafa-deficient/Mafk-transgenic mutant mice and streptozotocin-treated mice, reflecting the loss of β-cells. Furthermore, we were able to monitor fetal β-cell genesis in MIP-Luc-VU mice during the late gestational stage in a noninvasive and repetitive manner. In summary, we show that bioluminescence imaging of mice expressing a β-cell specific reporter allows detection of changes in β-cell mass and visualization of fetal β-cell neogenesis in uteri.

Superior beta cell proliferation, function and gene expression in a subpopulation of rat islets identified by high blood perfusion

AIMS/HYPOTHESIS

The blood perfusion of individual pancreatic islets is highly variable, with a subgroup of islets having high perfusion and blood vessels responsive to further blood flow increase induced by glucose. This study tested the hypothesis that there is heterogeneity between islets with regard to beta cell proliferation, function and gene expression based on differences in their blood perfusion.

METHODS

Fluorescent microspheres were injected into the ascending aorta, and then microsphere-containing and non-microsphere-containing pancreatic islets were isolated for investigation. By this procedure, the 5% of islets with the greatest blood perfusion were identified for study. Islet endothelial cells were isolated separately to investigate the role of improved vascular support in the observed differences.

RESULTS

The vascular network was found to be more dense and tortuous in microsphere-containing than other islets. The most highly blood-perfused islets also had a higher rate of beta cell proliferation, superior beta cell function and a markedly different gene expression from other islets. Cultured islets exposed to islet endothelial cell products had a similarly increased beta cell proliferation rate, yet significantly fewer changes in gene expression than observed in the most highly blood-perfused islets.

CONCLUSIONS/INTERPRETATION

A novel heterogeneity between islets was observed, with superior beta cell proliferation, function and gene expression in a subpopulation of islets identified by high blood perfusion. In contrast with a previously described population of low-oxygenated, sleeping islets, which are recruited into functionality when needed, the presently described heterogeneity is shown to remain in vitro after islet isolation.

MafA-deficient and beta cell-specific MafK-overexpressing hybrid transgenic mice develop human-like severe diabetic nephropathy

Transcription factor MafA is a key molecule in insulin secretion and the development of pancreatic islets. Previously, we demonstrated that some of the MafA-deficient mice develop overt diabetes mellitus, and the phenotype of these mice seems to be mild probably because of redundant functions of other Maf proteins. In this study, we generated hybrid transgenic mice that were MafA-deficient and also over-expressed MafK specifically in beta cells (MafA(-/-)MafK(+)). MafA(-/-)MafK(+) mice developed severe overt diabetes mellitus within 5weeks old, and showed higher levels of proteinuria and serum creatinine. Histological analysis revealed that embryonic development of beta cells in the MafA(-/-)MafK(+) mice was significantly suppressed and the reduced number of beta cells was responsible for the early onset of diabetes. Furthermore, after uninephrectomy, these mice demonstrated three characteristics of human diabetic nephropathy: diffuse, nodular, and exudative lesions. MafA(-/-)MafK(+) mice might be a useful model for the analysis of human diabetic nephropathy.

Small Maf proteins in mammalian gene control: mere dimerization partners or dynamic transcriptional regulators?

The small Maf basic leucine zipper (bZIP) proteins MafF, MafG and MafK, while modest in size, have emerged as crucial regulators of mammalian gene expression. Intriguingly, small Mafs do not contain an obvious transcriptional activation domain. However, previously perceived as "mere" partner molecules conferring DNA binding specificity to complexes with larger bZIP proteins, such as the CNC family member Nrf2, it has become clear that small Maf proteins are essential and dynamically regulated transcription factors. Current data suggest stringent control of small Maf protein function through transcriptional and post-translational mechanisms. Initial gene targeting experiments revealed considerable functional redundancy among small Maf proteins in vivo. This was not unexpected, due to the high level of homology among the three small Mafs. Nevertheless, further studies showed that these transcription factors have critical roles in various cellular processes, including stress signaling, hematopoiesis, CNS function and oncogenesis. Recent data provide a possible link between small Maf-mediated transcription and the inflammatory response.