MafA stability in pancreatic beta cells is regulated by glucose and is dependent on its constitutive phosphorylation at multiple sites by glycogen synthase kinase 3

Chronic consumption of imbalance diets high in sucrose or fat induces abdominal obesity with different pattern of metabolic disturbances and lost in Langerhans cells population

AIM

Obesity is a worldwide health issue, associated with development of type 2 Diabetes Mellitus. The aim of this study is to analyze the effect of consumption of two hypercaloric diets on metabolic disturbance and beta cells damage.

MAIN METHODS

Male Wistar rats were subjected to twelve months consumption of three diets: a Control balanced diet (CTD, carbohydrates 58 %, proteins 29 %, lipids 13 %) and two hypercaloric diets, high in sucrose (HSD, carbohydrates 68 %, proteins 22 %, lipids 10 %) or high in fat (HFD, carbohydrates 31 %, proteins 14 %, lipids 55 %). Serum levels of glucose, triglycerides and free fatty acids were measured after zoometric parameters determination. Antioxidant enzymes activity and oxidative stress-marker were measured in pancreas tissue among histological analysis of Langerhans islets.

KEY FINDINGS

Although diets were hypercaloric, the amount of food consumed by rats decreased, resulting in an equal caloric consumption. The HSD induced hypertriglyceridemia and hyperglycemia with higher levels in free fatty acids (FFA, lipotoxicity); whereas HFD did not increased neither the triglycerides nor FFA, nevertheless the loss of islets' cell was larger. Both diets induced obesity with hyperglycemia and significant reduction in Langerhans islets size.

SIGNIFICANCE

Our results demonstrate that consumption of HSD induces more significant metabolic disturbances that HFD, although both generated pancreas damage; as well hypercaloric diet consumption is not indispensable to becoming obese; the chronic consumption of unbalanced diets (rich in carbohydrates or lipids) may lead to abdominal obesity with metabolic and functional disturbances, although the total amount of calories are similar.

Denosumab Treatment Does Not Halt Progression of Bone Lesions in Multicentric Carpotarsal Osteolysis Syndrome

Here we report the use of denosumab, a monoclonal antibody against receptor activator of nuclear factor κB ligand (RANKL), as monotherapy for multicentric carpotarsal osteolysis syndrome (MCTO) in an 11.5-year-old male with a heterozygous missense mutation in MAFB (c.206C>T; p.Ser69Leu). We treated the subject with 0.5 mg/kg denosumab every 60-90 days for 47 months and monitored bone and mineral metabolism, kidney function, joint range of motion (ROM), and bone and joint morphology. Serum markers of bone turnover reduced rapidly, bone density increased, and renal function remained normal. Nevertheless, MCTO-related osteolysis and joint immobility progressed during denosumab treatment. Symptomatic hypercalcemia and protracted hypercalciuria occurred during weaning and after discontinuation of denosumab and required treatment with zoledronate. When expressed in vitro, the c.206C>T; p.Ser69Leu variant had increased protein stability and produced greater transactivation of a luciferase reporter under the control of the PTH gene promoter than did wild-type MafB. Based on our experience and that of others, denosumab does not appear to be efficacious for MCTO and carries a high risk of rebound hypercalcemia and/or hypercalciuria after drug discontinuation. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Generation and mutational analysis of a transgenic murine model of the human MAF mutation

Aymé-Gripp syndrome is an autosomal dominant multisystem disorder. The major clinical features of this syndrome include congenital cataracts, sensorineural hearing loss, intellectual disability, and a distinctive flat facial appearance. MAF has been identified as a causative gene of the syndrome, and heterozygous variants owing to impairment in glycogen synthase kinase 3 (GSK3)-mediated MAF phosphorylation shows related disorders. However, the underlying mechanisms of these types of disorders in affected individuals remain poorly understood. To explore the underlying mechanisms and discover new phenotypes, a murine model with a Maf mutation on a GSK3 phosphorylation motif, p.Thr58Ile, was generated using CRISPR-Cas9 gene editing. This is a homologous mutation to that in human patients. Our murine model exhibited similar phenotypes to those in humans, such as lens abnormalities, short stature, growth retardation, and abnormal skull morphology. The murine model showed decreased brain volume and malocclusion. Considering the sequencing and genotyping data, our models were successfully generated for the first time (to the best of our knowledge). Therefore, this study offers new and unique functional insights into human and murine MAF and novel clinical values of MAF pathogenic variants associated with changes in the functions of several organs based on a viable murine model.

Deciphering the multifunctional role of dual leucine zipper kinase (DLK) and its therapeutic potential in disease

Dual leucine zipper kinase (DLK, MAP3K12), a serine/threonine protein kinase, plays a key role in neuronal development, as it regulates axon regeneration and degeneration through its downstream kinase. Importantly, DLK is closely related to the pathogenesis of numerous neurodegenerative diseases and the induction of β-cell apoptosis that leads to diabetes. In this review, we summarize the current understanding of DLK function, and then discuss the role of DLK signaling in human diseases. Furthermore, various types of small molecule inhibitors of DLK that have been published so far are described in detail in this paper, providing some strategies for the design of DLK small molecule inhibitors in the future.

The role and regulation of Maf proteins in cancer

The Maf proteins (Mafs) belong to basic leucine zipper transcription factors and are members of the activator protein-1 (AP-1) superfamily. There are two subgroups of Mafs: large Mafs and small Mafs, which are involved in a wide range of biological processes, such as the cell cycle, proliferation, oxidative stress, and inflammation. Therefore, dysregulation of Mafs can affect cell fate and is closely associated with diverse diseases. Accumulating evidence has established both large and small Mafs as mediators of tumor development. In this review, we first briefly describe the structure and physiological functions of Mafs. Then we summarize the upstream regulatory mechanisms that control the expression and activity of Mafs. Furthermore, we discuss recent studies on the critical role of Mafs in cancer progression, including cancer proliferation, apoptosis, metastasis, tumor/stroma interaction and angiogenesis. We also review the clinical implications of Mafs, namely their potential possibilities and limitations as biomarkers and therapeutic targets in cancer.

Multicentric Carpotarsal Osteolysis: a Contemporary Perspective on the Unique Skeletal Phenotype

PURPOSE OF REVIEW

Multicentric carpotarsal osteolysis (MCTO) is an ultra-rare disorder characterized by osteolysis of the carpal and tarsal bones, subtle craniofacial deformities, and nephropathy. The molecular pathways underlying the pathophysiology are not well understood.

RECENT FINDINGS

MCTO is caused by heterozygous mutations in MAFB, which encodes the widely expressed transcription factor MafB. All MAFB mutations in patients with MCTO result in replacement of amino acids that cluster in a phosphorylation region of the MafB transactivation domain and account for a presumed gain-of-function for the variant protein. Since 2012, fewer than 60 patients with MCTO have been described with 20 missense mutations in MAFB. The clinical presentations are variable, and a genotype-phenotype correlation is lacking. Osteolysis, via excessive osteoclast activity, has been regarded as the primary mechanism, although anti-resorptive agents demonstrate little therapeutic benefit. This paper appraises current perspectives of MafB protein action, inflammation, and dysfunctional bone formation on the pathogenesis of the skeletal phenotype in MCTO. More research is needed to understand the pathogenesis of MCTO to develop rational therapies.

Cholesterol Redistribution in Pancreatic β-Cells: A Flexible Path to Regulate Insulin Secretion

Pancreatic β-cells, by secreting insulin, play a key role in the control of glucose homeostasis, and their dysfunction is the basis of diabetes development. The metabolic milieu created by high blood glucose and lipids is known to play a role in this process. In the last decades, cholesterol has attracted significant attention, not only because it critically controls β-cell function but also because it is the target of lipid-lowering therapies proposed for preventing the cardiovascular complications in diabetes. Despite the remarkable progress, understanding the molecular mechanisms responsible for cholesterol-mediated β-cell function remains an open and attractive area of investigation. Studies indicate that β-cells not only regulate the total cholesterol level but also its redistribution within organelles, a process mediated by vesicular and non-vesicular transport. The aim of this review is to summarize the most current view of how cholesterol homeostasis is maintained in pancreatic β-cells and to provide new insights on the mechanisms by which cholesterol is dynamically distributed among organelles to preserve their functionality. While cholesterol may affect virtually any activity of the β-cell, the intent of this review is to focus on early steps of insulin synthesis and secretion, an area still largely unexplored.

Role of the Transcription Factor MAFA in the Maintenance of Pancreatic β-Cells

Pancreatic β-cells are specialized to properly regulate blood glucose. Maintenance of the mature β-cell phenotype is critical for glucose metabolism, and β-cell failure results in diabetes mellitus. Recent studies provide strong evidence that the mature phenotype of β-cells is maintained by several transcription factors. These factors are also required for β-cell differentiation from endocrine precursors or maturation from immature β-cells during pancreatic development. Because the reduction or loss of these factors leads to β-cell failure and diabetes, inducing the upregulation or inhibiting downregulation of these transcription factors would be beneficial for studies in both diabetes and stem cell biology. Here, we discuss one such factor, i.e., the transcription factor MAFA. MAFA is a basic leucine zipper family transcription factor that can activate the expression of insulin in β-cells with PDX1 and NEUROD1. MAFA is indeed indispensable for the maintenance of not only insulin expression but also function of adult β-cells. With loss of MAFA in type 2 diabetes, β-cells cannot maintain their mature phenotype and are dedifferentiated. In this review, we first briefly summarize the functional roles of MAFA in β-cells and then mainly focus on the molecular mechanism of cell fate conversion regulated by MAFA.

Second MAFA Variant Causing a Phosphorylation Defect in the Transactivation Domain and Familial Insulinomatosis

Adult-onset familial insulinomatosis is a rare disorder with recurrent, severe hypoglycemia caused by multiple insulin-secreting pancreatic tumors. The etiology was unclear until the variant p.Ser64Phe in the transcription factor MAFA, a key coordinator of β-cell insulin secretion, was defined as the cause in two families. We here describe detailed genetic, clinical, and family analyses of two sisters with insulinomatosis, aiming to identify further disease causes. Using exome sequencing, we detected a novel, heterozygous missense variant, p.Thr57Arg, in MAFA’s highly conserved transactivation domain. The impact of the affected region is so crucial that in vitro expression studies replacing Thr57 have already been performed, demonstrating a phosphorylation defect with the impairment of transactivation activity and degradation. However, prior to our study, the link to human disease was missing. Furthermore, mild hyperglycemia was observed in six additional, heterozygote family members, indicating that not only insulinomatosis but also MODY-like symptoms co-segregate with p.Thr57Arg. The pre-described MAFA variant, p.Ser64Phe, is located in the same domain, impairs the same phosphorylation cascade, and results in the same symptoms. We confirm MAFA phosphorylation defects are important causes of a characteristic syndrome, thus complementing the pathophysiological and diagnostic disease concept. Additionally, we verify the high penetrance and autosomal dominant inheritance pattern.

MafA Regulation in β-Cells: From Transcriptional to Post-Translational Mechanisms

β-cells are insulin-producing cells in the pancreas that maintain euglycemic conditions. Pancreatic β-cell maturity and function are regulated by a variety of transcription factors that enable the adequate expression of the cellular machinery involved in nutrient sensing and commensurate insulin secretion. One of the key factors in this regulation is MAF bZIP transcription factor A (MafA). MafA expression is decreased in type 2 diabetes, contributing to β-cell dysfunction and disease progression. The molecular biology underlying MafA is complex, with numerous transcriptional and post-translational regulatory nodes. Understanding these complexities may uncover potential therapeutic targets to ameliorate β-cell dysfunction. This article will summarize the role of MafA in normal β-cell function and disease, with a special focus on known transcriptional and post-translational regulators of MafA expression.

Sex-biased islet β cell dysfunction is caused by the MODY MAFA S64F variant by inducing premature aging and senescence in males

A heterozygous missense mutation of the islet β cell-enriched MAFA transcription factor (p.Ser64Phe [S64F]) is found in patients with adult-onset β cell dysfunction (diabetes or insulinomatosis), with men more prone to diabetes than women. This mutation engenders increased stability to the unstable MAFA protein. Here, we develop a S64F MafA mouse model to determine how β cell function is affected and find sex-dependent phenotypes. Heterozygous mutant males (MafAS64F/+) display impaired glucose tolerance, while females are slightly hypoglycemic with improved blood glucose clearance. Only MafAS64F/+ males show transiently higher MafA protein levels preceding glucose intolerance and sex-dependent changes to genes involved in Ca2+ signaling, DNA damage, aging, and senescence. MAFAS64F production in male human β cells also accelerate cellular senescence and increase senescence-associated secretory proteins compared to cells expressing MAFAWT. These results implicate a conserved mechanism of accelerated islet aging and senescence in promoting diabetes in MAFAS64F carriers in a sex-biased manner.

Functional analysis of large MAF transcription factors and elucidation of their relationships with human diseases

The large MAF transcription factor group is a group of transcription factors with an acidic region, a basic region, and a leucine zipper region. Four types of MAF, MAFA, MAFB, c-MAF, and NRL, have been identified in humans and mice. In order to elucidate the functions of the large MAF transcription factor group in vivo, our research group created genetically modified MAFA-, MAFB-, and c-MAF-deficient mice and analyzed their phenotypes. MAFA is expressed in pancreatic β cells and is essential for insulin transcription and secretion. MAFB is essential for the development of pancreatic endocrine cells, formation of inner ears, podocyte function in the kidneys, and functional differentiation of macrophages. c-MAF is essential for lens formation and osteoblast differentiation. Furthermore, a single-base mutation in genes encoding the large MAF transcription factor group causes congenital renal disease, eye disease, bone disease, diabetes, and tumors in humans. This review describes the functions of large MAF transcription factors in vivo and their relationships with human diseases.

Metabolic Effects of Recurrent Genetic Aberrations in Multiple Myeloma

Oncogene activation and malignant transformation exerts energetic, biosynthetic and redox demands on cancer cells due to increased proliferation, cell growth and tumor microenvironment adaptation. As such, altered metabolism is a hallmark of cancer, which is characterized by the reprogramming of multiple metabolic pathways. Multiple myeloma (MM) is a genetically heterogeneous disease that arises from terminally differentiated B cells. MM is characterized by reciprocal chromosomal translocations that often involve the immunoglobulin loci and a restricted set of partner loci, and complex chromosomal rearrangements that are associated with disease progression. Recurrent chromosomal aberrations in MM result in the aberrant expression of MYC, cyclin D1, FGFR3/MMSET and MAF/MAFB. In recent years, the intricate mechanisms that drive cancer cell metabolism and the many metabolic functions of the aforementioned MM-associated oncogenes have been investigated. Here, we discuss the metabolic consequences of recurrent chromosomal translocations in MM and provide a framework for the identification of metabolic changes that characterize MM cells.

HRD1, an Important Player in Pancreatic β-Cell Failure and Therapeutic Target for Type 2 Diabetic Mice

Inadequate insulin secretion in response to glucose is an important factor for β-cell failure in type 2 diabetes (T2D). Although HMG-CoA reductase degradation 1 (HRD1), a subunit of the endoplasmic reticulum-associated degradation complex, plays a pivotal role in β-cell function, HRD1 elevation in a diabetic setting contributes to β-cell dysfunction. We report in this study the excessive HRD1 expression in islets from humans with T2D and T2D mice. Functional studies reveal that β-cell-specific HRD1 overexpression triggers impaired insulin secretion that will ultimately lead to severe hyperglycemia; by contrast, HRD1 knockdown improves glucose control and response in diabetic models. Proteomic analysis results reveal a large HRD1 interactome, which includes v-maf musculoaponeurotic fibrosarcoma oncogene homolog A (MafA), a master regulator of genes implicated in the maintenance of β-cell function. Furthermore, mechanistic assay results indicate that HRD1 is a novel E3 ubiquitin ligase that targets MafA for ubiquitination and degradation in diabetic β-cells, resulting in cytoplasmic accumulation of MafA and in the reduction of its biological function in the nucleus. Our results not only reveal the pathological importance of excessive HRD1 in β-cell dysfunction but also establish the therapeutic importance of targeting HRD1 in order to prevent MafA loss and suppress the development of T2D.

Kindlin-2 modulates MafA and β-catenin expression to regulate β-cell function and mass in mice

β-Cell dysfunction and reduction in β-cell mass are hallmark events of diabetes mellitus. Here we show that β-cells express abundant Kindlin-2 and deleting its expression causes severe diabetes-like phenotypes without markedly causing peripheral insulin resistance. Kindlin-2, through its C-terminal region, binds to and stabilizes MafA, which activates insulin expression. Kindlin-2 loss impairs insulin secretion in primary human and mouse islets in vitro and in mice by reducing, at least in part, Ca²⁺ release in β-cells. Kindlin-2 loss activates GSK-3β and downregulates β-catenin, leading to reduced β-cell proliferation and mass. Kindlin-2 loss reduces the percentage of β-cells and concomitantly increases that of α-cells during early pancreatic development. Genetic activation of β-catenin in β-cells restores the diabetes-like phenotypes induced by Kindlin-2 loss. Finally, the inducible deletion of β-cell Kindlin-2 causes diabetic phenotypes in adult mice. Collectively, our results establish an important function of Kindlin-2 and provide a potential therapeutic target for diabetes.

Beta cell dysfunction and reduction in beta cell mass are hallmark events in the pathogenesis of diabetes mellitus. We identify focal adhesion protein Kindlin-2 as a key factor that controls insulin synthesis and secretion and beta cell mass by modulating MafA and beta-catenin proteins in pancreatic beta cells.

FA2H Exhibits Tumor Suppressive Roles on Breast Cancers via Cancer Stemness Control

Background: Triple negative breast cancers are aggressive, enriched with cancer stem cells, and lack effective targeted therapies with little side effects. Methods: We isolated cancer stem cells from two triple negative breast cancer cell lines via cell sorting following transcriptome sequencing, bioinformatics analysis, experimental and clinical validations, as well as functional investigations to explore genes capturing triple negative breast cancer features for improved diagnosis and therapeutics in clinics. Results: We found that FA2H is under-expressed in triple negative breast cancers both in vitro and in clinics, and FA2H suppresses cancer stemness via inhibiting the STAT3/IL6 axis and NFkB signaling. Conclusions: This study reports the tumor suppressive roles of FA2H on breast cancer cells through cancer stemness control. FA2H and other candidates unveiled in this study that capture the features of cancer stem cells may contribute as diagnostic marker and/or effective therapeutic targets for improved triple negative breast cancer management.

STAT3 dictates β-cell apoptosis by modulating PTEN in streptozocin-induced hyperglycemia

Insufficient pancreatic β-cell mass or insulin-producing β-cells are implicated in all forms of diabetes mellitus. However, the molecular mechanisms underlying β-cell destruction are complex and not fully defined. Here we observed that activation of STAT3 is intensely and specifically inhibited in β-cells under hyperglycemic conditions. By knocking out STAT3 specifically in mouse β-cells, we found that the loss of STAT3 sensitized mice to three low doses of STZ stimulation resulting in hyperglycemia. Mechanistically, accumulating PTEN, induced by STAT3 deficiency, directly represses phosphorylation of AKT, which negatively modulates transcription factor activation, dysregulates β-cell function, positively promotes apoptotic signaling, and finally induces β-cell apoptosis. Notably, the defective secretion of insulin and β-cells apoptosis was completely rescued by PTEN ablation in STAT3-null islets or PTEN inhibitor bpv(phen) treatment. Thus our data suggest that STAT3 is a vital modulator of β-cell survival and function, highlighting a critical role for STAT3 in the negative regulation of PTEN-AKT signaling pathway associated with β-cell dysfunction and apoptosis.

Notch signaling dynamically regulates adult β cell proliferation and maturity

Notch signaling regulates differentiation of the pancreatic endocrine lineage during embryogenesis, but the role of Notch in mature β cells is unclear. We found that islets derived from lean mice show modest β cell Notch activity, which increases in obesity and in response to high glucose. This response appeared maladaptive, as mice with β cell-specific-deficient Notch transcriptional activity showed improved glucose tolerance when subjected to high-fat diet feeding. Conversely, mice with β cell-specific Notch gain of function (β-NICD) had a progressive loss of β cell maturity, due to proteasomal degradation of MafA, leading to impaired glucose-stimulated insulin secretion and glucose intolerance with aging or obesity. Surprisingly, Notch-active β cells had increased proliferative capacity, leading to increased but dysfunctional β cell mass. These studies demonstrate a dynamic role for Notch in developed β cells for simultaneously regulating β cell function and proliferation.

Glucose regulates MafA transcription factor abundance and insulin gene expression by inhibiting AMP-activated protein kinase in pancreatic β-cells

Insulin mRNA expression in pancreatic islet β-cells is up-regulated by extracellular glucose concentration, but the underlying mechanism remains incompletely understood. MafA is a transcriptional activator specifically enriched in β-cells that binds to the insulin gene promoter. Its expression is transcriptionally and posttranscriptionally regulated by glucose. Moreover, AMP-activated protein kinase (AMPK), a regulator of cellular energy homeostasis, is inhibited by high glucose, and this inhibition is essential for the up-regulation of insulin gene expression and glucose-stimulated insulin secretion (GSIS). Here we mutagenized the insulin promoter and found that the MafA-binding element C1/RIPE3b is required for glucose- or AMPK-induced alterations in insulin gene promoter activity. Under high-glucose conditions, pharmacological activation of AMPK in isolated mouse islets or MIN6 cells by metformin or 5-aminoimidazole-4-carboxamide riboside decreased MafA protein levels and mRNA expression of insulin and GSIS-related genes (i.e. glut2 and sur1). Overexpression of constitutively active AMPK also reduced MafA and insulin expression. Conversely, pharmacological AMPK inhibition by dorsomorphin (compound C) or expression of a dominant-negative form of AMPK increased MafA and insulin expression under low-glucose conditions. However, AMPK activation or inhibition did not change the expression levels of the β-cell-enriched transcription factors Pdx1 and Beta2/NeuroD1. AMPK activation accelerated MafA protein degradation, which is not dependent on the proteasome. We also noted that MafA overexpression prevents metformin-induced decreases in insulin and GSIS-related gene expression. These findings indicate that high glucose concentrations inhibit AMPK, thereby increasing MafA protein levels and activating the insulin promoter.

MAFA missense mutation causes familial insulinomatosis and diabetes mellitus

We report a disease-causing mutation in the β-cell–enriched MAFA transcription factor. Strikingly, the missense p.Ser64Phe MAFA mutation was associated with either of two distinct phenotypes, multiple insulin-producing neuroendocrine tumors of the pancreas—a condition known as insulinomatosis—or diabetes mellitus, recapitulating the physiological properties of MAFA both as an oncogene and as a key islet β-cell transcription factor. The implication of MAFA in these human phenotypes will provide insights into how this transcription factor regulates human β-cell activity as well as into the mechanisms of Maf-induced tumorigenesis.

The β-cell–enriched MAFA transcription factor plays a central role in regulating glucose-stimulated insulin secretion while also demonstrating oncogenic transformation potential in vitro. No disease-causing MAFA variants have been previously described. We investigated a large pedigree with autosomal dominant inheritance of diabetes mellitus or insulinomatosis, an adult-onset condition of recurrent hyperinsulinemic hypoglycemia caused by multiple insulin-secreting neuroendocrine tumors of the pancreas. Using exome sequencing, we identified a missense MAFA mutation (p.Ser64Phe, c.191C>T) segregating with both phenotypes of insulinomatosis and diabetes. This mutation was also found in a second unrelated family with the same clinical phenotype, while no germline or somatic MAFA mutations were identified in nine patients with sporadic insulinomatosis. In the two families, insulinomatosis presented more frequently in females (eight females/two males) and diabetes more often in males (12 males/four females). Four patients from the index family, including two homozygotes, had a history of congenital cataract and/or glaucoma. The p.Ser64Phe mutation was found to impair phosphorylation within the transactivation domain of MAFA and profoundly increased MAFA protein stability under both high and low glucose concentrations in β-cell lines. In addition, the transactivation potential of p.Ser64Phe MAFA in β-cell lines was enhanced compared with wild-type MAFA. In summary, the p.Ser64Phe missense MAFA mutation leads to familial insulinomatosis or diabetes by impacting MAFA protein stability and transactivation ability. The human phenotypes associated with the p.Ser64Phe MAFA missense mutation reflect both the oncogenic capacity of MAFA and its key role in islet β-cell activity.

A partnership with the proteasome; the destructive nature of GSK3

Glycogen Synthase Kinase-3 (GSK3) was originally reported as a key enzyme of glucose homeostasis through regulation of the rate of glycogen synthesis. It has subsequently been found to influence most cellular processes, including growth, differentiation and death, as part of its role in modulating response to hormonal, nutritional and cellular stress stimuli. More than 100 protein targets for GSK3 have been proposed although only a small fraction of these have been convincingly validated in physiological cell systems. The effects of GSK3 phosphorylation on substrates include alteration of enzyme activity, protein localisation, protein:protein interaction and protein stability. This latter form of regulation of GSK3 substrates is the focus of this review. There is an ever-growing list of GSK3 substrates that upon phosphorylation are targeted to the beta-transducin repeat containing protein (β-TrCP), thereby allowing ubiquitination of bound protein by cullin-1 and so initiating destruction at the proteasome. We propose the existence of a GSK3-β-TrCP ‘destruction hit-list’ that allows co-ordinated removal (or stabilisation) of a set of proteins with a common physiological purpose, through control of GSK3. We identify 29 proteins where there is relatively strong evidence for regulation by a GSK3-β-TrCP axis and note common features of regulation and pathophysiology. Furthermore, we assess the potential of pre-phosphorylation (priming) of these targets (normally a prerequisite for GSK3 recognition) to provide a second layer of regulation delineated by the priming kinase that allows GSK3 to mark them for destruction. Finally, we discuss whether this knowledge improves options for therapeutic intervention.

A Comprehensive Survey of the Roles of Highly Disordered Proteins in Type 2 Diabetes

Type 2 diabetes mellitus (T2DM) is a chronic and progressive disease that is strongly associated with hyperglycemia (high blood sugar) related to either insulin resistance or insufficient insulin production. Among the various molecular events and players implicated in the manifestation and development of diabetes mellitus, proteins play several important roles. The Kyoto Encyclopedia of Genes and Genomes (KEGG) database has information on 34 human proteins experimentally shown to be related to the T2DM pathogenesis. It is known that many proteins associated with different human maladies are intrinsically disordered as a whole, or contain intrinsically disordered regions. The presented study shows that T2DM is not an exception to this rule, and many proteins known to be associated with pathogenesis of this malady are intrinsically disordered. The multiparametric bioinformatics analysis utilizing several computational tools for the intrinsic disorder characterization revealed that IRS1, IRS2, IRS4, MAFA, PDX1, ADIPO, PIK3R2, PIK3R5, SoCS1, and SoCS3 are expected to be highly disordered, whereas VDCC, SoCS2, SoCS4, JNK9, PRKCZ, PRKCE, insulin, GCK, JNK8, JNK10, PYK, INSR, TNF-α, MAPK3, and Kir6.2 are classified as moderately disordered proteins, and GLUT2, GLUT4, mTOR, SUR1, MAPK1, IKKA, PRKCD, PIK3CB, and PIK3CA are predicted as mostly ordered. More focused computational analyses and intensive literature mining were conducted for a set of highly disordered proteins related to T2DM. The resulting work represents a comprehensive survey describing the major biological functions of these proteins and functional roles of their intrinsically disordered regions, which are frequently engaged in protein–protein interactions, and contain sites of various posttranslational modifications (PTMs). It is also shown that intrinsic disorder-associated PTMs may play important roles in controlling the functions of these proteins. Consideration of the T2DM proteins from the perspective of intrinsic disorder provides useful information that can potentially lead to future experimental studies that may uncover latent and novel pathways associated with the disease.

MAF protein mediates innate resistance to proteasome inhibition therapy in multiple myeloma

Multiple myeloma (MM) patients with the t(14;16) translocation have a poor prognosis, and unlike other molecular subgroups, their outcome has not improved with the introduction of bortezomib (Bzb). The mechanism underlying innate resistance of MM to Bzb is unknown. In the present study, we have investigated how MAF overexpression impacts resistance to proteasome inhibitor (PI) therapy (Bzb and carfilzomib). High levels of MAF protein were found in t(14;16) cell lines; cell lines from the t(4;14) subgroup had intermediate levels, whereas cell lines from the other subgroups had low levels. High expression of MAF protein in t(14;16) was associated with significantly higher PI half-maximum inhibitory concentration values compared with other molecular subgroups. PI exposure abrogated glycogen synthase kinase 3β (GSK3β)-mediated degradation of MAF protein, resulting in increased MAF protein stability and PI resistance. Subsequent studies using loss-of-function and gain-of-function models showed that silencing MAF led to increased sensitivity to PIs, enhanced apoptosis, and activation of caspase-3, -7, -8, -9, poly (ADP-ribose) polymerase, and lamin A/C. In contrast, overexpression of MAF resulted in increased resistance to PIs and reduced apoptosis. These results define the role of MAF and GSK3 in the resistance of t(14;16) MM to PIs and identifies a novel mechanism by which MAF protein levels are regulated by PIs, which in turn confers resistance to PIs.

Mechanisms of adult human β-cell in vitro dedifferentiation and redifferentiation

Recent studies in animal models and human pathological specimens suggest the involvement of β-cell dedifferentiation in β-cell dysfunction associated with type 2 diabetes. Dedifferentiated β-cells may be exploited for endogenous renewal of the β-cell mass. However, studying human β-cell dedifferentiation in diabetes presents major difficulties. We have analysed mechanisms involved in human β-cell dedifferentiation in vitro, under conditions that allow cell proliferation. Although there are important differences between the two cellular environments, β-cell dedifferentiation in the two conditions is likely to share a number of common pathways. Insights from the in vitro studies may lead to development of approaches for redifferentiation of endogenous dedifferentiated β-cells.

Phosphorylation of MafA enhances interaction with Beta2/NeuroD1

AIMS

MafA is a critical regulator of insulin expression and mature β-cell function. MafA binds to the insulin promoter through its carboxyl-terminal basic domain-leucine zipper (bZip) region and activates transcription synergistically with the β-cell-enriched transactivators Beta2 (NeuroD1) and Pdx1. MafA protein is highly phosphorylated in β-cells, and phosphorylation at multiple sites within its amino-terminal region is critical for its DNA-binding and transactivating abilities, as well as for regulation of its degradation. Here, we investigated whether phosphorylation of MafA affects its interaction with Beta2.

METHODS

By mutational analysis, we identified interaction domains of MafA and Beta2. Using in situ proximity ligation assay (PLA), we explored mechanism of phosphorylation-dependent binding of MafA with Beta2. We also searched for a pathophysiological condition that would induce lower levels of MafA phosphorylation.

RESULTS

Mutational analysis revealed that the phosphorylation sites within the amino-terminal region of MafA were not necessary for interaction with Beta2. In situ PLA suggested that phosphorylation induces conformational or configurational changes in MafA, thereby regulating the interaction with Beta2. We also found that long-term culture of the MIN6 insulinoma cell line under high-glucose conditions resulted in a decrease in β-cell-specific transcripts including insulin, along with a decrease in MafA phosphorylation and DNA binding.

CONCLUSION

Phosphorylation of MafA plays a critical role in β-cell function by regulating multiple functionalities, including binding to DNA, interaction with Beta2, and transactivation.

Regulation of Beta-Cell Function and Mass by the Dual Leucine Zipper Kinase

Diabetes mellitus is one of the most rapidly increasing diseases worldwide, whereby approximately 90-95% of patients suffer from type 2 diabetes. Considering its micro- and macrovascular complications like blindness and myocardial infarction, a reliable anti-diabetic treatment is needed. Maintaining the function and the mass of the insulin producing beta-cells despite elevated levels of beta-cell-toxic prediabetic signals represents a desirable mechanism of action of anti-diabetic drugs. The dual leucine zipper kinase (DLK) inhibits the action of two transcription factors within the beta-cell, thereby interfering with insulin secretion and production and the conservation of beta-cell mass. Furthermore, DLK action is regulated by prediabetic signals. Hence, the inhibition of this kinase might protect beta-cells against beta-cell-toxic prediabetic signals and prevent the development of diabetes. DLK might thus present a novel drug target for the treatment of diabetes mellitus type 2.

Proper activation of MafA is required for optimal differentiation and maturation of pancreatic β-cells

A key therapeutic approach for the treatment of Type 1 diabetes (T1D) is transplantation of functional islet β-cells. Despite recent advances in generating stem cell-derived glucose-responsive insulin(+) cells, their further maturation to fully functional adult β-cells still remains a daunting task. Conquering this hurdle will require a better understanding of the mechanisms driving maturation of embryonic insulin(+) cells into adult β-cells, and the implementation of that knowledge to improve current differentiation protocols. Here, we will review our current understanding of β-cell maturation, and discuss the contribution of key β-cell transcription factor MafA, to this process. The fundamental importance of MafA in regulating adult β-cell maturation and function indicates that enhancing MafA expression may improve the generation of definitive β-cells for transplantation. Additionally, we suggest that the temporal control of MafA induction at a specific stage of β-cell differentiation will be the next critical challenge for achieving optimum maturation of β-cells.

Mutations Impairing GSK3-Mediated MAF Phosphorylation Cause Cataract, Deafness, Intellectual Disability, Seizures, and a Down Syndrome-like Facies

Transcription factors operate in developmental processes to mediate inductive events and cell competence, and perturbation of their function or regulation can dramatically affect morphogenesis, organogenesis, and growth. We report that a narrow spectrum of amino-acid substitutions within the transactivation domain of the v-maf avian musculoaponeurotic fibrosarcoma oncogene homolog (MAF), a leucine zipper-containing transcription factor of the AP1 superfamily, profoundly affect development. Seven different de novo missense mutations involving conserved residues of the four GSK3 phosphorylation motifs were identified in eight unrelated individuals. The distinctive clinical phenotype, for which we propose the eponym Aymé-Gripp syndrome, is not limited to lens and eye defects as previously reported for MAF/Maf loss of function but includes sensorineural deafness, intellectual disability, seizures, brachycephaly, distinctive flat facial appearance, skeletal anomalies, mammary gland hypoplasia, and reduced growth. Disease-causing mutations were demonstrated to impair proper MAF phosphorylation, ubiquitination and proteasomal degradation, perturbed gene expression in primary skin fibroblasts, and induced neurodevelopmental defects in an in vivo model. Our findings nosologically and clinically delineate a previously poorly understood recognizable multisystem disorder, provide evidence for MAF governing a wider range of developmental programs than previously appreciated, and describe a novel instance of protein dosage effect severely perturbing development.

MiR-375 promotes redifferentiation of adult human β cells expanded in vitro

In-vitro expansion of β cells from adult human pancreatic islets could provide abundant cells for cell replacement therapy of diabetes. However, proliferation of β-cell-derived (BCD) cells is associated with dedifferentiation. Here we analyzed changes in microRNAs (miRNAs) during BCD cell dedifferentiation and identified miR-375 as one of the miRNAs greatly downregulated. We hypothesized that restoration of miR-375 expression in expanded BCD cells may contribute to their redifferentiation. Our findings demonstrate that overexpression of miR-375 alone leads to activation of β-cell gene expression, reduced cell proliferation, and a switch from N-cadherin to E-cadherin expression, which characterizes mesenchymal-epithelial transition. These effects, which are reproducible in cells derived from multiple human donors, are likely mediated by repression of PDPK1 transcripts and indirect downregulation of GSK3 activity. These findings support an important role of miR-375 in regulation of human β-cell phenotype, and suggest that miR-375 upregulation may facilitate the generation of functional insulin-producing cells following ex-vivo expansion of human islet cells.

Generation of insulin-producing cells from the mouse liver using β cell-related gene transfer including Mafa and Mafb

Recent studies on the large Maf transcription factors have shown that Mafb and Mafa have respective and distinctive roles in β-cell development and maturation. However, whether this difference in roles is due to the timing of the gene expression (roughly, expression of Mafb before birth and of Mafa after birth) or to the specific function of each gene is unclear. Our aim was to examine the functional differences between these genes that are closely related to β cells by using an in vivo model of β-like cell generation. We monitored insulin gene transcription by measuring bioluminescence emitted from the liver of insulin promoter-luciferase transgenic (MIP-Luc-VU) mice. Adenoviral gene transfers of Pdx1/Neurod/Mafa (PDA) and Pdx1/Neurod/Mafb (PDB) combinations generated intense luminescence from the liver that lasted for more than 1 week and peaked at 3 days after transduction. The peak signal intensities of PDA and PDB were comparable. However, PDA but not PDB transfer resulted in significant bioluminescence on day 10, suggesting that Mafa has a more sustainable role in insulin gene activation than does Mafb. Both PDA and PDB transfers ameliorated the glucose levels in a streptozotocin (STZ)-induced diabetic model for up to 21 days and 7 days, respectively. Furthermore, PDA transfer induced several gene expressions necessary for glucose sensing and insulin secretion in the liver on day 9. However, a glucose tolerance test and liver perfusion experiment did not show glucose-stimulated insulin secretion from intrahepatic β-like cells. These results demonstrate that bioluminescence imaging in MIP-Luc-VU mice provides a noninvasive means of detecting β-like cells in the liver. They also show that Mafa has a markedly intense and sustained role in β-like cell production in comparison with Mafb.

Loss of TRB3 alters dynamics of MLK3-JNK signaling and inhibits cytokine-activated pancreatic beta cell death

Disabling cellular defense mechanisms is essential for induction of apoptosis. We have previously shown that cytokine-mediated activation of the MAP3K MLK3 stabilizes TRB3 protein levels to inhibit AKT and compromise beta cell survival. Here, we show that genetic deletion of TRB3 results in basal activation of AKT, preserves mitochondrial integrity, and confers resistance against cytokine-induced pancreatic beta cell death. Mechanistically, we find that TRB3 stabilizes MLK3, most likely by suppressing AKT-directed phosphorylation, ubiquitination, and proteasomal degradation of MLK3. Accordingly, TRB3(-/-) islets show a decrease in both the amplitude and duration of cytokine-stimulated MLK3 induction and JNK activation. It is well known that JNK signaling is facilitated by a feed forward loop of sequential kinase phosphorylation and is reinforced by a mutual stabilization of the module components. The failure of TRB3(-/-) islets to mount an optimal JNK activation response, coupled with the ability of TRB3 to engage and maintain steady state levels of MLK3, recasts TRB3 as an integral functional component of the JNK module in pancreatic beta cells.

Loss of TRB3 alters dynamics of MLK3-JNK signaling and inhibits cytokine-activated pancreatic beta cell death

Disabling cellular defense mechanisms is essential for induction of apoptosis. We have previously shown that cytokine-mediated activation of the MAP3K MLK3 stabilizes TRB3 protein levels to inhibit AKT and compromise beta cell survival. Here, we show that genetic deletion of TRB3 results in basal activation of AKT, preserves mitochondrial integrity, and confers resistance against cytokine-induced pancreatic beta cell death. Mechanistically, we find that TRB3 stabilizes MLK3, most likely by suppressing AKT-directed phosphorylation, ubiquitination, and proteasomal degradation of MLK3. Accordingly, TRB3(-/-) islets show a decrease in both the amplitude and duration of cytokine-stimulated MLK3 induction and JNK activation. It is well known that JNK signaling is facilitated by a feed forward loop of sequential kinase phosphorylation and is reinforced by a mutual stabilization of the module components. The failure of TRB3(-/-) islets to mount an optimal JNK activation response, coupled with the ability of TRB3 to engage and maintain steady state levels of MLK3, recasts TRB3 as an integral functional component of the JNK module in pancreatic beta cells.

GSK3 protein positively regulates type I insulin-like growth factor receptor through forkhead transcription factors FOXO1/3/4

Glycogen synthase kinase-3 (GSK3) has either tumor-suppressive roles or pro-tumor roles in different types of human tumors. A number of GSK3 targets in diverse signaling pathways have been uncovered, such as tuberous sclerosis complex subunit 2 and β-catenin. The O subfamily of forkhead/winged helix transcription factors (FOXO) is known as tumor suppressors that induce apoptosis. In this study, we find that FOXO binds to type I insulin-like growth factor receptor (IGF-IR) promoter and stimulates its transcription. GSK3 positively regulates the transactivation activity of FOXO and stimulates IGF-IR expression. Although kinase-dead GSK3β cannot up-regulate IGF-IR, the constitutively active GSK3β induces IGF-IR expression in a FOXO-dependent manner. Serum starvation or Akt inhibition leads to an increase in IGF-IR expression, which could be blunted by GSK3 inhibition. GSK3β knockdown or GSK3 inhibitor suppresses IGF-I-induced IGF-IR, Akt, and ERK1/2 phosphorylation. Moreover, knockdown of GSK3β or FOXO1/3/4 leads to a decrease in cellular proliferation and abrogates IGF-I-induced hepatoma cell proliferation. These results suggest that GSK3 and FOXO may positively regulate IGF-I signaling and hepatoma cell proliferation.

Inhibition of human insulin gene transcription and MafA transcriptional activity by the dual leucine zipper kinase

Insulin biosynthesis is an essential β-cell function and inappropriate insulin secretion and biosynthesis contribute to the pathogenesis of diabetes mellitus type 2. Previous studies showed that the dual leucine zipper kinase (DLK) induces β-cell apoptosis. Since β-cell dysfunction precedes β-cell loss, in the present study the effect of DLK on insulin gene transcription was investigated in the HIT-T15 β-cell line. Downregulation of endogenous DLK increased whereas overexpression of DLK decreased human insulin gene transcription. 5'- and 3'-deletion human insulin promoter analyses resulted in the identification of a DLK responsive element that mapped to the DNA binding-site for the β-cell specific transcription factor MafA. Overexpression of DLK wild-type but not its kinase-dead mutant inhibited MafA transcriptional activity conferred by its transactivation domain. Furthermore, in the non-β-cell line JEG DLK inhibited MafA overexpression-induced human insulin promoter activity. Overexpression of MafA and DLK or its kinase-dead mutant into JEG cells revealed that DLK but not its mutant reduced MafA protein content. Inhibition of the down-stream DLK kinase c-Jun N-terminal kinase (JNK) by SP600125 attenuated DLK-induced MafA loss. Furthermore, mutation of the serine 65 to alanine, shown to confer MafA protein stability, increased MafA-dependent insulin gene transcription and prevented DLK-induced MafA loss in JEG cells. These data suggest that DLK by activating JNK triggers the phosphorylation and degradation of MafA thereby attenuating insulin gene transcription. Given the importance of MafA for β-cell function, the inhibition of DLK might preserve β-cell function and ultimately retard the development of diabetes mellitus type 2.

GSK3-mediated MAF phosphorylation in multiple myeloma as a potential therapeutic target

Multiple myeloma (MM) is an incurable haematological malignancy characterised by the proliferation of mature antibody-secreting plasma B cells in the bone marrow. MM can arise from initiating translocations, of which the musculoaponeurotic fibrosarcoma (MAF) family is implicated in ∼5%. MMs bearing Maf translocations are of poor prognosis. These translocations are associated with elevated Maf expression, including c-MAF, MAFB and MAFA, and with t(14;16) and t(14;20) translocations, involving c-MAF and MAFB, respectively. c-MAF is also overexpressed in MM through MEK/ERK activation, bringing the number of MMs driven by the deregulation of a Maf gene close to 50%. Here we demonstrate that MAFB and c-MAF are phosphorylated by the Ser/Thr kinase GSK3 in human MM cell lines. We show that LiCl-induced GSK3 inhibition targets these phosphorylations and specifically decreases proliferation and colony formation of Maf-expressing MM cell lines. Interestingly, bortezomib induced stabilisation of Maf phosphorylation, an observation that could explain, at least partially, the low efficacy of bortezomib for patients carrying Maf translocations. Thus, GSK3 inhibition could represent a new therapeutic approach for these patients.

Preventing p38 MAPK-mediated MafA degradation ameliorates β-cell dysfunction under oxidative stress

The reduction in the expression of glucose-responsive insulin gene transcription factor MafA accompanies the development of β-cell dysfunction under oxidative stress/diabetic milieu. Humans with type 2 diabetes have reduced MafA expression, and thus preventing this reduction could overcome β-cell dysfunction and diabetes. We previously showed that p38 MAPK, but not glycogen synthase kinase 3 (GSK3), is a major regulator of MafA degradation under oxidative stress. Here, we examined the mechanisms of this degradation and whether preventing MafA degradation under oxidative stress will overcome β-cell dysfunction. We show that under oxidative and nonoxidative conditions p38 MAPK directly binds to MafA and triggers MafA degradation via ubiquitin proteasomal pathway. However, unlike nonoxidative conditions, MafA degradation under oxidative stress depended on p38 MAPK-mediated phosphorylation at threonine (T) 134, and not T57. Furthermore the expression of alanine (A) 134-MafA, but not A57-MafA, reduced the oxidative stress-mediated loss of glucose-stimulated insulin secretion, which was independent of p38 MAPK action on protein kinase D, a regulator of insulin secretion. Interestingly, the expression of proteasomal activator PA28γ that degrades GSK3-phosphorylated (including T57) MafA was reduced under oxidative stress, explaining the dominance of p38 MAPK over the GSK3 pathway in regulating MafA stability under oxidative stress. These results identify two distinct pathways mediating p38 MAPK-dependent MafA degradation under oxidative and nonoxidative conditions and show that inhibiting MafA degradation under oxidative stress ameliorates β-cell dysfunction and could lead to novel therapies for diabetes.

Lithium treatment induces proteasomal degradation of over-expressed acetylcholinesterase (AChE-S) and inhibit GSK3β

Lithium is one of the most widely used mood-stabilizing agents for the treatment of bipolar disorder. Lithium is also a potent inhibitor of glycogen synthase kinase-3β (GSK3β) activity, which is linked to Alzheimer's disease (AD). In experiments with cultured HEK293T cells, we show here that GSK3β stabilizes synaptic acetylcholinesterase (AChE-S), a critical component of AD development. Cells treated with lithium exhibited rapid proteasomal degradation of AChE-S. Furthermore treatment of the cells with MG132, an inhibitor of the 26S proteasome, prevented the destabilizing effect of lithium on AChE-S. Taken together, these findings suggest that regulation of AChE-S protein stability may be an important biological target of lithium therapy.

A small molecule differentiation inducer increases insulin production by pancreatic β cells

New drugs for preserving and restoring pancreatic β-cell function are critically needed for the worldwide epidemic of type 2 diabetes and the cure for type 1 diabetes. We previously identified a family of neurogenic 3,5-disubstituted isoxazoles (Isx) that increased expression of neurogenic differentiation 1 (NeuroD1, also known as BETA2); this transcription factor functions in neuronal and pancreatic β-cell differentiation and is essential for insulin gene transcription. Here, we probed effects of Isx on human cadaveric islets and MIN6 pancreatic β cells. Isx increased the expression and secretion of insulin in islets that made little insulin after prolonged ex vivo culture and increased expression of neurogenic differentiation 1 and other regulators of islet differentiation and insulin gene transcription. Within the first few hours of exposure, Isx caused biphasic activation of ERK1/2 and increased bulk histone acetylation. Although there was little effect on histone deacetylase activity, Isx increased histone acetyl transferase activity in nuclear extracts. Reconstitution assays indicated that Isx increased the activity of the histone acetyl transferase p300 through an ERK1/2-dependent mechanism. In summary, we have identified a small molecule with antidiabetic activity, providing a tool for exploring islet function and a possible lead for therapeutic intervention in diabetes.

GSK-3: Functional Insights from Cell Biology and Animal Models

Glycogen synthase kinase-3 (GSK-3) is a widely expressed and highly conserved serine/threonine protein kinase encoded in mammals by two genes that generate two related proteins: GSK-3α and GSK-3β. GSK-3 is active in cells under resting conditions and is primarily regulated through inhibition or diversion of its activity. While GSK-3 is one of the few protein kinases that can be inactivated by phosphorylation, the mechanisms of GSK-3 regulation are more varied and not fully understood. Precise control appears to be achieved by a combination of phosphorylation, localization, and sequestration by a number of GSK-3-binding proteins. GSK-3 lies downstream of several major signaling pathways including the phosphatidylinositol 3′ kinase pathway, the Wnt pathway, Hedgehog signaling and Notch. Specific pools of GSK-3, which differ in intracellular localization, binding partner affinity, and relative amount are differentially sensitized to several distinct signaling pathways and these sequestration mechanisms contribute to pathway insulation and signal specificity. Dysregulation of signaling pathways involving GSK-3 is associated with the pathogenesis of numerous neurological and psychiatric disorders and there are data suggesting GSK-3 isoform-selective roles in several of these. Here, we review the current knowledge of GSK-3 regulation and targets and discuss the various animal models that have been employed to dissect the functions of GSK-3 in brain development and function through the use of conventional or conditional knockout mice as well as transgenic mice. These studies have revealed fundamental roles for these protein kinases in memory, behavior, and neuronal fate determination and provide insights into possible therapeutic interventions.

MafA and MafB activity in pancreatic β cells

Analyses in mouse models have revealed crucial roles for MafA (musculoaponeurotic fibrosarcoma oncogene family A) and MafB in islet β cells, with MafB being required during development and MafA in adults. These two closely related transcription factors regulate many genes essential for glucose sensing and insulin secretion in a cooperative and sequential manner. Significantly, the switch from MafB to MafA expression also appears to be vital for functional maturation of β cells produced by human embryonic stem (hES) cell differentiation. This review summarizes the discovery, distribution, and function of MafA and MafB in rodent pancreatic β cells, and describes some key questions regarding their importance to β cells.

What Are the bona fide GSK3 Substrates?

Nearly 100 proteins are proposed to be substrates for GSK3, suggesting that this enzyme is a fundamental regulator of almost every process in the cell, in every tissue in the body. However, it is not certain how many of these proposed substrates are regulated by GSK3 in vivo. Clearly, the identification of the physiological functions of GSK3 will be greatly aided by the identification of its bona fide substrates, and the development of GSK3 as a therapeutic target will be highly influenced by this range of actions, hence the need to accurately establish true GSK3 substrates in cells. In this paper the evidence that proposed GSK3 substrates are likely to be physiological targets is assessed, highlighting the key cellular processes that could be modulated by GSK3 activity and inhibition.

ATF2 interacts with beta-cell-enriched transcription factors, MafA, Pdx1, and beta2, and activates insulin gene transcription

Pancreatic β-cell-restricted expression of insulin is established through several critical cis-regulatory elements located in the insulin gene promoter region. The principal cis elements are A-boxes, E1, and C1/RIPE3b. The β-cell-enriched transcription factors Pdx1 and Beta2 bind to the A-boxes and E1 element, respectively. A β-cell-specific trans-acting factor binding to C1/RIPE3b (termed RIPE3b1 activator) was detected by electrophoretic mobility shift assay and has been identified as MafA, a member of the Maf family of basic leucine zipper (bZip) proteins. Here, ATF2, a member of the ATF/CREB family of basic leucine zipper proteins, was identified as a component of the RIPE3b1 activator. ATF2 alone was unable to bind to the C1/RIPE3b element but acquired binding capacity upon complex formation with MafA. ATF2 also interacted with Pdx1 and Beta2, and co-expression of ATF2, MafA, Pdx1, and Beta2 resulted in a synergistic activation of the insulin promoter. Immunohistochemical analysis of mouse pancreas tissue sections showed that ATF2 is enriched in islet endocrine cells, including β-cells. RNAi-mediated knockdown of MafA or ATF2 in the MIN6 β-cell line resulted in a significant decrease in endogenous levels of insulin mRNA. These data indicate that ATF2 is an essential component of the positive regulators of the insulin gene expression.

Histone deacetylase (HDAC) inhibition as a novel treatment for diabetes mellitus

Both common forms of diabetes have an inflammatory pathogenesis in which immune and metabolic factors converge on interleukin-1β as a key mediator of insulin resistance and β-cell failure. In addition to improving insulin resistance and preventing β-cell inflammatory damage, there is evidence of genetic association between diabetes and histone deacetylases (HDACs); and HDAC inhibitors (HDACi) promote β-cell development, proliferation, differentiation and function and positively affect late diabetic microvascular complications. Here we review this evidence and propose that there is a strong rationale for preclinical studies and clinical trials with the aim of testing the utility of HDACi as a novel therapy for diabetes.

Proteasome activator PA28{gamma} stimulates degradation of GSK3-phosphorylated insulin transcription activator MAFA

MAFA is a member of the MAF family of basic leucine zipper transcription factors and is a critical regulator of insulin gene expression and islet β-cell function. To be degraded by the proteasome, MAFA must be phosphorylated by GSK3 and MAP kinases at multiple serine and threonine residues (Ser49, Thr53, Thr57, Ser61, and Ser65) within its amino-terminal domain. In this study, we report that MAFA degradation is stimulated by PA28γ (REGγ and PSME3), a member of a family of proteasome activators that bind and activate the 20S proteasome. To date, only a few PA28γ-proteasome pathway substrates have been identified, including steroid receptor coactivator 3 (SRC3) and the cell cycle inhibitor p21 (CIP1). PA28γ binds to MAFA, induces its proteasomal degradation, and thereby attenuates MAFA-driven transcriptional activation of the insulin promoter. Co-expression of GSK3 enhanced the PA28γ-mediated degradation of MAFA, but mutants that contained alanine substitutions at the MAFA phosphorylation sites did not bind PA28γ and were resistant to degradation. We also found that a PA28γ mutant (N151Y) that did not stimulate p21 degradation enhanced MAFA degradation, and another mutant (K188D) that promoted greater p21 degradation did not enhance MAFA degradation. These results suggest that PA28γ stimulates MAFA degradation through a novel molecular mechanism that is distinct from that for the degradation of p21.

Targeting GSK-3 family members in the heart: a very sharp double-edged sword

The GSK-3 family of serine/threonine kinases, which is comprised of two isoforms (α and β), was initially identified as a negative regulator of glycogen synthase, the rate limiting enzyme of glycogen synthesis [1,2]. In the 30 years since its initial discovery, the family has been reported to regulate a host of additional cellular processes and, consequently, disease states such as bipolar disorders, diabetes, inflammatory diseases, cancer, and neurodegenerative diseases including Alzheimer's Disease and Parkinson's Disease [3,4]. As a result, there has been intense interest on the part of the pharmaceutical industry in developing small molecule antagonists of GSK-3. Herein, we will review the roles played by GSK-3s in the heart, focusing primarily on recent studies that have employed global and tissue-specific gene deletion. We will highlight roles in various pathologic processes, including pressure overload and ischemic injury, focusing on some striking isoform-specific effects of the family. Due to space limitations and/or the relatively limited data in gene-targeted mice, we will not be addressing the family's roles in ischemic pre-conditioning or its many interactions with various pro- and anti-apoptotic factors. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

SUMOylation negatively regulates transcriptional and oncogenic activities of MafA

Dysregulated expression of Maf proteins (namely c-Maf, MafA and MafB) leads to multiple myeloma in humans and oncogenic transformation of chicken embryonic fibroblasts. Maf proteins are transcriptional activators of tissue-specific gene expression and regulators of cell differentiation. For example, MafA is a critical regulator of crystallin genes and the lens differentiation program in chickens. In mammals, MafA is essential for the development of mature insulin-producing beta-cells of pancreas. It has been shown that MafA protein stability is regulated by phosphorylations at multiple serine and threonine residues. Here, we report that Maf proteins are also post-translationally modified by small ubiquitin-like modifier (SUMO) proteins at a conserved lysine residue in the amino-terminal transactivator domain. A SUMOylation-deficient mutant of MafA (K32R) was more potent than wild-type MafA in transactivating luciferase reporter construct driven by alphaA-crystallin or insulin gene promoter. In ovo electroporation into developing chicken embryo showed that the K32R mutant induced ectopic delta-crystallin gene expression more efficiently than the wild-type MafA. We also demonstrated that the K32R mutant had enhanced ability to induce colony formation of a chicken fibroblast cell line DF-1. Therefore, SUMOylation is a functional post-translational modification of MafA that negatively regulates its transcriptional and transforming activities.

Phosphorylation within the MafA N terminus regulates C-terminal dimerization and DNA binding

Phosphorylation regulates transcription factor activity by influencing dimerization, cellular localization, activation potential, and/or DNA binding. Nevertheless, precisely how this post-translation modification mediates these processes is poorly understood. Here, we examined the role of phosphorylation on the DNA-binding properties of MafA and MafB, closely related transcriptional activators of the basic-leucine zipper (b-Zip) family associated with cell differentiation and oncogenesis. Many common phosphorylation sites were identified by mass spectrometry. However, dephosphorylation only precluded the detection of MafA dimers and consequently dramatically reduced DNA-binding ability. Analysis of MafA/B chimeras revealed that sensitivity to the phosphorylation status of MafA was imparted by sequences spanning the C-terminal dimerization region (amino acids (aa) 279-359), whereas the homologous MafB region (aa 257-323) conveyed phosphorylation-independent DNA binding. Mutational analysis showed that formation of MafA dimers capable of DNA binding required phosphorylation within the distinct N-terminal transactivation domain (aa 1-72) and not the C-terminal b-Zip region. These results demonstrate a novel relationship between the phosphoamino acid-rich transactivation and b-Zip domains in controlling MafA DNA-binding activity.

Glucose regulates steady-state levels of PDX1 via the reciprocal actions of GSK3 and AKT kinases

The pancreatic beta cell is sensitive to even small changes in PDX1 protein levels; consequently, Pdx1 haploinsufficiency can inhibit beta cell growth and decrease insulin biosynthesis and gene expression, leading to compromised glucose-stimulated insulin secretion. Using metabolic labeling of primary islets and a cultured beta cell line, we show that glucose levels modulate PDX1 protein phosphorylation at a novel C-terminal GSK3 consensus that maps to serines 268 and 272. A decrease in glucose levels triggers increased turnover of the PDX1 protein in a GSK3-dependent manner, such that PDX1 phosphomutants are refractory to the destabilizing effect of low glucose. Glucose-stimulated activation of AKT and inhibition of GSK3 decrease PDX1 phosphorylation and delay degradation. Furthermore, direct pharmacologic inhibition of AKT destabilizes, and inhibition of GSK3 increases PDX1 protein stability. These studies define a novel functional role for the PDX1 C terminus in mediating the effects of glucose and demonstrate that glucose modulates PDX1 stability via the AKT-GSK3 axis.