Inhibition of MafA transcriptional activity and human insulin gene transcription by interleukin-1beta and mitogen-activated protein kinase kinase kinase in pancreatic islet beta cells

Dopamine in the Regulation of Glucose Homeostasis, Pathogenesis of Type 2 Diabetes, and Chronic Conditions of Impaired Dopamine Activity/Metabolism: Implication for Pathophysiological and Therapeutic Purposes

Dopamine regulates several functions, such as voluntary movements, spatial memory, motivation, sleep, arousal, feeding, immune function, maternal behaviors, and lactation. Less clear is the role of dopamine in the pathophysiology of type 2 diabetes mellitus (T2D) and chronic complications and conditions frequently associated with it. This review summarizes recent evidence on the role of dopamine in regulating insular metabolism and activity, the pathophysiology of traditional chronic complications associated with T2D, the pathophysiological interconnection between T2D and chronic neurological and psychiatric disorders characterized by impaired dopamine activity/metabolism, and therapeutic implications. Reinforcing dopamine signaling is therapeutic in T2D, especially in patients with dopamine-related disorders, such as Parkinson's and Huntington's diseases, addictions, and attention-deficit/hyperactivity disorder. On the other hand, although specific trials are probably needed, certain medications approved for T2D (e.g., metformin, pioglitazone, incretin-based therapy, and gliflozins) may have a therapeutic role in such dopamine-related disorders due to anti-inflammatory and anti-oxidative effects, improvement in insulin signaling, neuroinflammation, mitochondrial dysfunction, autophagy, and apoptosis, restoration of striatal dopamine synthesis, and modulation of dopamine signaling associated with reward and hedonic eating. Last, targeting dopamine metabolism could have the potential for diagnostic and therapeutic purposes in chronic diabetes-related complications, such as diabetic retinopathy.

Species-specific roles for the MAFA and MAFB transcription factors in regulating islet β cell identity

Type 2 diabetes (T2D) is associated with compromised identity of insulin-producing pancreatic islet β cells, characterized by inappropriate production of other islet cell-enriched hormones. Here, we examined how hormone misexpression was influenced by the MAFA and MAFB transcription factors, closely related proteins that maintain islet cell function. Mice specifically lacking MafA in β cells demonstrated broad, population-wide changes in hormone gene expression with an overall gene signature closely resembling islet gastrin+ (Gast+) cells generated under conditions of chronic hyperglycemia and obesity. A human β cell line deficient in MAFB, but not one lacking MAFA, also produced a GAST+ gene expression pattern. In addition, GAST was detected in human T2D β cells with low levels of MAFB. Moreover, evidence is provided that human MAFB can directly repress GAST gene transcription. These results support a potentially novel, species-specific role for MafA and MAFB in maintaining adult mouse and human β cell identity, respectively. Here, we discuss the possibility that induction of Gast/GAST and other non-β cell hormones, by reduction in the levels of these transcription factors, represents a dysfunctional β cell signature.

Increase of c-FOS promoter transcriptional activity by the dual leucine zipper kinase

The dual leucine zipper kinase (DLK) and the ubiquitously expressed transcription factor c-FOS have important roles in beta-cell proliferation and function. Some studies in neuronal cells suggest that DLK can influence c-FOS expression. Given that c-FOS is mainly regulated at the transcriptional level, the effect of DLK on c-FOS promoter activity was investigated in the beta-cell line HIT. The methods used in this study are the following: Luciferase reporter gene assays, immunoblot analysis, CRISPR-Cas9-mediated genome editing, and real-time quantitative PCR. In the beta-cell line HIT, overexpressed DLK increased c-FOS promoter activity twofold. Using 5'-,3'-promoter deletions, the promoter regions from - 348 to - 339 base pairs (bp) and from a - 284 to - 53 bp conferred basal activity, whereas the promoter region from - 711 to - 348 bp and from - 53 to + 48 bp mediated DLK responsiveness. Mutation of the cAMP response element within the promoter prevented the stimulatory effect of DLK. Treatment of HIT cells with KCl and the adenylate cyclase activator forskolin increased c-FOS promoter transcriptional activity ninefold. Since the transcriptional activity of those promoter fragments activated by KCl and forskolin was decreased by DLK, DLK might interfere with KCl/forskolin-induced signaling. In a newly generated, genome-edited HIT cell line lacking catalytically active DLK, c-Fos mRNA levels were reduced by 80% compared to the wild-type cell line. DLK increased c-FOS promoter activity but decreased stimulated transcriptional activity, suggesting that DLK fine-tunes c-FOS promoter-dependent gene transcription. Moreover, at least in HIT cells, DLK is required for FOS mRNA expression.

Hunter C. LDB1-mediated transcriptional complexes are sensitive to islet stress

Excess nutrients and proinflammatory cytokines impart stresses on pancreatic islet β-cells that, if unchecked, can lead to cellular dysfunction and/or death. Among these stress-induced effects is loss of key β-cell transcriptional regulator mRNA and protein levels required for β-cell function. Previously, our lab and others reported that LIM-domain complexes comprised the LDB1 transcriptional co-regulator and Islet-1 (ISL1) transcription factor are required for islet β-cell development, maturation, and function. The LDB1:ISL1 complex directly occupies and regulates key β-cell genes, including MafA, Pdx1, and Slc2a2, to maintain β-cell identity and function. Given the importance of LDB1:ISL1 complexes, we hypothesized that LDB1 and/or ISL1 levels, like other transcriptional regulators, are sensitive to β-cell nutrient and cytokine stresses, likely contributing to β-cell (dys)function under various stimuli. We tested this by treating β-cell lines or primary mouse islets with elevating glucose concentrations, palmitate, or a cytokine cocktail of IL-1β, TNFα, and IFNγ. We indeed observed that LDB1 mRNA and/or protein levels were reduced upon palmitate and cytokine (cocktail or singly) incubation. Conversely, acute high glucose treatment of β-cells did not impair LDB1 or ISL1 levels, but increased LDB1:ISL1 interactions. These observations suggest that LDB1:ISL1 complex formation is sensitive to β-cell stresses and that targeting and/or stabilizing this complex may rescue lost β-cell gene expression to preserve cellular function.

Regulation of dual leucine zipper kinase activity through its interaction with calcineurin

Hyperglycemia enhancing the intracellular levels of reactive oxygen species (ROS) contributes to dysfunction and progressive loss of beta cells and thereby to diabetes mellitus. The oxidation sensitive calcium/calmodulin dependent phosphatase calcineurin promotes pancreatic beta cell function and survival whereas the dual leucine zipper kinase (DLK) induces apoptosis. Therefore, it was studied whether calcineurin interferes with DLK action. In a beta cell line similar concentrations of H₂O₂ decreased calcineurin activity and activated DLK. DLK interacted via its φLxVP motif (aa 362-365) with the interface of the calcineurin subunits A and B. Mutation of the Val prevented this protein protein interaction, hinting at a distinct φLxVP motif. Indeed, mutational analysis revealed an ordered structure of DLK's φLxVP motif whereby Val mediates the interaction with calcineurin and Leu maintains an enzymatically active conformation. Overexpression of DLK wild-type but not the DLK mutant unable to bind calcineurin diminished calcineurin-induced nuclear localisation of the nuclear factor of activated T-cells (NFAT), suggesting that both, DLK and NFAT compete for the substrate binding site of calcineurin. The calcineurin binding-deficient DLK mutant exhibited increased DLK activity measured as phosphorylation of the downstream c-Jun N-terminal kinase, inhibition of CRE-dependent gene transcription and induction of apoptosis. These findings show that calcineurin interacts with DLK; and inhibition of calcineurin increases DLK activity. Hence, this study demonstrates a novel mechanism regulating DLK action. These findings suggest that ROS through inhibition of calcineurin enhance DLK activity and thereby lead to beta cell dysfunction and loss and ultimately diabetes mellitus.

Protective Effect of Peroxiredoxin 6 Against Toxic Effects of Glucose and Cytokines in Pancreatic RIN-m5F β-Cells

Taking into account a special role of pancreatic β-cells in the development of diabetes mellitus, the effects of peroxiredoxin 6 (Prx6) on the viability and functional activity of rat insulinoma RIN-m5F β-cells were studied under diabetes-simulating conditions. For this purpose, the cells were cultured at elevated glucose concentrations or in the presence of pro-inflammatory cytokines (TNF-α and IL-1) known for their special role in the cytotoxic autoimmune response in diabetes. It was found that the increased glucose concentration of 23-43 mM caused death of 20-60% β-cells. Prx6 added to cells significantly reduced the level of reactive oxygen species and protected the RIN-m5F β-cells from hyperglycemia, reducing the death of these cells by several fold. A measurement of insulin secretion by the RIN-m5F β-cells showed a significant stimulatory effect of Prx6 on the insulin-producing activity of pancreatic β-cells. It should be noted that the stimulatory activity of Prx6 was detected during culturing the cells under both normal and unfavorable conditions. The regulation of the NF-κB signaling cascade could be one of the mechanisms of Prx6 action on β-cells, in particular, through activation of RelA/p65 phosphorylation at Ser536.

The role of butyrate, a histone deacetylase inhibitor in diabetes mellitus: experimental evidence for therapeutic intervention

The contribution of epigenetic mechanisms in diabetes mellitus (DM), β-cell reprogramming and its complications is an emerging concept. Recent evidence suggests that there is a link between DM and histone deacetylases (HDACs), because HDAC inhibitors promote β-cell differentiation, proliferation, function and improve insulin resistance. Moreover, gut microbes and diet-derived products can alter the host epigenome. Furthermore, butyrate and butyrate-producing microbes are decreased in DM. Butyrate is a short-chain fatty acid produced from the fermentation of dietary fibers by microbiota and has been proven as an HDAC inhibitor. The present review provides a pragmatic interpretation of chromatin-dependent and independent complex signaling/mechanisms of butyrate for the treatment of Type 1 and Type 2 DM, with an emphasis on the promising strategies for its drugability and therapeutic implication.

IL-1 mediates amyloid-associated islet dysfunction and inflammation in human islet amyloid polypeptide transgenic mice

AIMS/HYPOTHESIS

Aggregation of islet amyloid polypeptide (IAPP) to form amyloid contributes to beta cell dysfunction in type 2 diabetes. Human but not non-amyloidogenic rodent IAPP induces islet macrophage proIL-1β synthesis. We evaluated the effect of IL-1 receptor antagonist (IL-1Ra) on islet inflammation and dysfunction in a mouse model of type 2 diabetes with amyloid formation.

METHODS

Lean and obese male mice (A/a or A(vy)/A at the agouti locus, respectively) with or without beta cell human IAPP expression (hIAPP(Tg/0)) were treated with PBS or IL-1Ra (50 mg kg(-1) day(-1)) from 16 weeks of age. Intraperitoneal glucose and insulin tolerance tests were performed after 8 weeks. Pancreases were harvested for histology and gene expression analysis.

RESULTS

Aggregation of human IAPP was associated with marked upregulation of proinflammatory gene expression in islets of obese hIAPP(Tg/0) mice, together with amyloid deposition and fasting hyperglycaemia. IL-1Ra improved glucose tolerance and reduced plasma proinsulin:insulin in both lean and obese hIAPP(Tg/0) mice with no effect on insulin sensitivity. The severity and prevalence of islet amyloid was reduced by IL-1Ra in lean hIAPP (Tg/0) mice, suggesting a feed-forward mechanism by which islet inflammation promotes islet amyloid at the early stages of disease. IL-1Ra limited Il1a, Il1b, Tnf and Ccl2 expression in islets from obese hIAPP(Tg/0) mice, suggesting an altered islet inflammatory milieu.

CONCLUSIONS/INTERPRETATION

These data provide the first in vivo evidence—using a transgenic mouse model with amyloid deposits resembling those found in human islets—that IAPP-induced beta cell dysfunction in type 2 diabetes may be mediated by IL-1. Anti-IL-1 therapies may limit islet inflammation and dysfunction associated with amyloid formation.

Something old, something new and something very old: drugs for treating type 2 diabetes

Diabetes mellitus belongs to the most rapidly increasing diseases worldwide. Approximately 90-95% of these patients suffer from type 2 diabetes mellitus, which is characterized by peripheral insulin resistance and the progressive loss of beta-cell function and mass. Considering the complications of this chronic disease, a reliable anti-diabetic treatment is indispensable. An ideal oral anti-diabetic drug should not only correct glucose homeostasis but also preserve or even augment beta-cell function and mass, ameliorate the subclinical inflammation present under insulin-resistant conditions and prevent the macro- and microvascular consequences of diabetes in order to reduce the mortality. Despite the many anti-diabetic drugs already in use, there is an ongoing research for additional drugs, guided by different concepts of the pathogenesis of type 2 diabetes. This review will briefly summarize current oral anti-diabetic drugs. In addition, emerging strategies for the treatment of diabetes will be described, among them the inhibition of glucagon action and anti-inflammatory drugs. Their suitability as 'ideal anti-diabetic drugs' will be discussed.

Could microRNAs contribute to the maintenance of β cell identity?

Normal physiology depends on defined functional output of differentiated cells. However, differentiated cells are often surprisingly fragile. As an example, phenotypic collapse and dedifferentiation of β cells were recently discovered in the pathogenesis of type 2 diabetes (T2D). These discoveries necessitate the investigation of mechanisms that function to maintain robust cell type identity. microRNAs (miRNAs), which are small non-coding RNAs, are known to impart robustness to development. miRNAs are interlaced within networks, that include also transcriptional and epigenetic regulators, for continuous control of lineage-specific gene expression. In this Opinion article, we provide a framework for conceptualizing how miRNAs might participate in adult β cell identity and suggest that miRNAs may function as important genetic components in metabolic disorders, including diabetes.

Inflammatory response in islet transplantation

Islet cell transplantation is a promising beta cell replacement therapy for patients with brittle type 1 diabetes as well as refractory chronic pancreatitis. Despite the vast advancements made in this field, challenges still remain in achieving high frequency and long-term successful transplant outcomes. Here we review recent advances in understanding the role of inflammation in islet transplantation and development of strategies to prevent damage to islets from inflammation. The inflammatory response associated with islets has been recognized as the primary cause of early damage to islets and graft loss after transplantation. Details on cell signaling pathways in islets triggered by cytokines and harmful inflammatory events during pancreas procurement, pancreas preservation, islet isolation, and islet infusion are presented. Robust control of pre- and peritransplant islet inflammation could improve posttransplant islet survival and in turn enhance the benefits of islet cell transplantation for patients who are insulin dependent. We discuss several potent anti-inflammatory strategies that show promise for improving islet engraftment. Further understanding of molecular mechanisms involved in the inflammatory response will provide the basis for developing potent therapeutic strategies for enhancing the quality and success of islet transplantation.

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.

Genome-wide association study for type 2 diabetes in Indians identifies a new susceptibility locus at 2q21

Indians undergoing socioeconomic and lifestyle transitions will be maximally affected by epidemic of type 2 diabetes (T2D). We conducted a two-stage genome-wide association study of T2D in 12,535 Indians, a less explored but high-risk group. We identified a new type 2 diabetes-associated locus at 2q21, with the lead signal being rs6723108 (odds ratio 1.31; P = 3.32 × 10⁻⁹). Imputation analysis refined the signal to rs998451 (odds ratio 1.56; P = 6.3 × 10⁻¹²) within TMEM163 that encodes a probable vesicular transporter in nerve terminals. TMEM163 variants also showed association with decreased fasting plasma insulin and homeostatic model assessment of insulin resistance, indicating a plausible effect through impaired insulin secretion. The 2q21 region also harbors RAB3GAP1 and ACMSD; those are involved in neurologic disorders. Forty-nine of 56 previously reported signals showed consistency in direction with similar effect sizes in Indians and previous studies, and 25 of them were also associated (P < 0.05). Known loci and the newly identified 2q21 locus altogether explained 7.65% variance in the risk of T2D in Indians. Our study suggests that common susceptibility variants for T2D are largely the same across populations, but also reveals a population-specific locus and provides further insights into genetic architecture and etiology of T2D.

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.

MafA promotes the reprogramming of placenta-derived multipotent stem cells into pancreatic islets-like and insulin+ cells

MafA is a pancreatic transcriptional factor that controls β-cell-specific transcription of the insulin gene. However, the role of MafA in the regulation of pancreatic transdifferentiation and reprogramming in human stem cells is still unclear. In this study, we investigate the role of MafA in placenta-derived multipotent stem cells (PDMSCs) that constitutively expressed Oct-4 and Nanog. PDMSCs were isolated and transfected with MafA using a lentivector. Our results showed that overexpression of MafA in PDMSCs significantly up-regulated the expression of pancreatic development-related genes (Sox17, Foxa2, Pdx1 and Ngn3). Microarray analysis suggested that the gene expression profile of MafA-overexpressing PDMSCs was similar to that of pancreas and islet tissues. MafA increased the expression levels of the mRNAs of NKx2.2, Glut2, insulin, glucagons and somatostatin, and further facilitated the differentiation of PDMSCs into insulin⁺ cells. The glucose-stimulated responses to insulin and c-peptide production in MafA-overexpressing PDMSCs were significantly higher than in PDMSCs with vector control. Our results indicated that MafA-overexpressing PDMSCs were more resistant to oxidative damage and oxidative damage-induced apoptosis than PDMSCs carrying the vector control were. Importantly, the expression of MafA in PDMSCs xenotransplanted into immunocompromised mice improved the restoration of blood insulin levels to control values and greatly prolonged the survival of graft cells in immunocompromised mice with STZ-induced diabetes. In summary, these data suggest that MafA plays a novel role in the reprogramming of stem cells into pancreatic β-progenitors, promotes the islet-like characteristics of PDMSCs, as well as functionally enhances insulin production to restore the regulation of blood glucose levels in transplanted grafts.

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.

IL-1beta-induced increase in intestinal epithelial tight junction permeability is mediated by MEKK-1 activation of canonical NF-kappaB pathway

IL-1β is a proinflammatory cytokine that plays a central role in the inflammatory process of the gut. IL-1β causes an increase in intestinal epithelial tight junction (TJ) permeability, but the intracellular pathways that mediate intestinal TJ permeability remain unclear. The major aims of this study were to delineate the protein kinases that regulate the IL-1β modulation of intestinal TJ barrier function and to determine the intracellular mechanisms involved, using filter-grown Caco-2 monolayers as the in vitro model system. Our results showed that IL-1β caused a rapid activation of MEKK-1 and NIK. The knockdown of MEKK-1, but not NIK, inhibited the IL-1β increase in Caco-2 TJ permeability. IL-1β caused an activation of both canonical and noncanonical NF-κB pathways; MEKK-1 regulated the activation of the canonical pathway, while NIK regulated the activation of the noncanonical pathway. Inhibition of MEKK-1 activation of the canonical pathway prevented the IL-1β increase in TJ permeability. Our data also indicated that inhibitory κB kinase was the catalytic subunit primarily involved in canonical pathway activation and TJ barrier opening. MEKK-1 also played an essential role in myosin light chain kinase gene activation. In conclusion, our data show for the first time that MEKK-1 plays an integral role in IL-1β modulation of Caco-2 TJ barrier function by regulating the activation of the canonical NF-κB pathway and the MLCK gene.

Multiple chromatin-bound protein kinases assemble factors that regulate insulin gene transcription

During the onset of diabetes, pancreatic beta cells become unable to produce sufficient insulin to maintain blood glucose within the normal range. Proinflammatory cytokines have been implicated in impaired beta cell function. To understand more about the molecular events that reduce insulin gene transcription, we examined the effects of hyperglycemia alone and together with the proinflammatory cytokine interleukin-1beta (IL-1beta) on signal transduction pathways that regulate insulin gene transcription. Exposure to IL-1beta in fasting glucose activated multiple protein kinases that associate with the insulin gene promoter and transiently increased insulin gene transcription in beta cells. In contrast, cells exposed to hyperglycemic conditions were sensitized to the inhibitory actions of IL-1beta. Under these conditions, IL-1beta caused the association of the same protein kinases, but a different combination of transcription factors with the insulin gene promoter and began to reduce transcription within 2 h; stimulatory factors were lost, RNA polymerase II was lost, and inhibitory factors were bound to the promoter in a kinase-dependent manner.

Cytokine-mediated induction of anti-apoptotic genes that are linked to nuclear factor kappa-B (NF-kappaB) signalling in human islets and in a mouse beta cell line

AIMS/HYPOTHESIS

The destruction of pancreatic beta cells leading to type 1 diabetes in humans is thought to occur mainly through apoptosis and necrosis induced by activated macrophages and T cells, and in which secreted cytokines play a significant role. The transcription factor nuclear factor kappa-B (NF-kappaB) plays an important role in mediating the apoptotic action of cytokines in beta cells. We therefore sought to determine the changes in expression of genes modulated by NF-kappaB in human islets exposed to a combination of IL1beta, TNF-alpha and IFN-gamma.

METHODS

Microarray and gene set enrichment analysis were performed to investigate the global response of gene expression and pathways modulated in cultured human islets exposed to cytokines. Validation of a panel of NF-kappaB-regulated genes was performed by quantitative RT-PCR. The mechanism of induction of BIRC3 by cytokines was examined by transient transfection of BIRC3 promoter constructs linked to a luciferase gene in MIN6 cells, a mouse beta cell line.

RESULTS

Enrichment of several metabolic and signalling pathways was observed in cytokine-treated human islets. In addition to the upregulation of known pro-apoptotic genes, a number of anti-apoptotic genes including BIRC3, BCL2A1, TNFAIP3, CFLAR and TRAF1 were induced by cytokines through NF-kappaB. Significant synergy between the cytokines was observed in NF-kappaB-mediated induction of the promoter of BIRC3 in MIN6 cells.

CONCLUSIONS/INTERPRETATION

These findings suggest that, via NF-kappaB activation, cytokines induce a concurrent anti-apoptotic pathway that may be critical for preserving islet integrity and viability during the progression of insulitis in type 1 diabetes.

Inhibition of human insulin gene transcription by peroxisome proliferator-activated receptor gamma and thiazolidinedione oral antidiabetic drugs

BACKGROUND AND PURPOSE

The transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma) is essential for glucose homeostasis. PPARgamma ligands reducing insulin levels in vivo are used as drugs to treat type 2 diabetes mellitus. Genes regulated by PPARgamma have been found in several tissues including insulin-producing pancreatic islet beta-cells. However, the role of PPARgamma at the insulin gene was unknown. Therefore, the effect of PPARgamma and PPARgamma ligands like rosiglitazone on insulin gene transcription was investigated.

EXPERIMENTAL APPROACH

Reporter gene assays were used in the beta-cell line HIT and in primary mature pancreatic islets of transgenic mice. Mapping studies and internal mutations were carried out to locate PPARgamma-responsive promoter regions.

KEY RESULTS

Rosiglitazone caused a PPARgamma-dependent inhibition of insulin gene transcription in a beta-cell line. This inhibition was concentration-dependent and had an EC(50) similar to that for the activation of a reporter gene under the control of multimerized PPAR binding sites. Also in normal primary pancreatic islets of transgenic mice, known to express high levels of PPARgamma, rosiglitazone inhibited glucose-stimulated insulin gene transcription. Transactivation and mapping experiments suggest that, in contrast to the rat glucagon gene, the inhibition of the human insulin gene promoter by PPARgamma/rosiglitazone does not depend on promoter-bound Pax6 and is attributable to the proximal insulin gene promoter region around the transcription start site from -56 to +18.

CONCLUSIONS AND IMPLICATIONS

The human insulin gene represents a novel PPARgamma target that may contribute to the action of thiazolidinediones in type 2 diabetes mellitus.

Glucose regulation of insulin gene expression in pancreatic β-cells

Production and secretion of insulin from the β-cells of the pancreas is very crucial in maintaining normoglycaemia. This is achieved by tight regulation of insulin synthesis and exocytosis from the β-cells in response to changes in blood glucose levels. The synthesis of insulin is regulated by blood glucose levels at the transcriptional and post-transcriptional levels. Although many transcription factors have been implicated in the regulation of insulin gene transcription, three β-cell-specific transcriptional regulators, Pdx-1 (pancreatic and duodenal homeobox-1), NeuroD1 (neurogenic differentiation 1) and MafA (V-maf musculoaponeurotic fibrosarcoma oncogene homologue A), have been demonstrated to play a crucial role in glucose induction of insulin gene transcription and pancreatic β-cell function. These three transcription factors activate insulin gene expression in a co-ordinated and synergistic manner in response to increasing glucose levels. It has been shown that changes in glucose concentrations modulate the function of these β-cell transcription factors at multiple levels. These include changes in expression levels, subcellular localization, DNA-binding activity, transactivation capability and interaction with other proteins. Furthermore, all three transcription factors are able to induce insulin gene expression when expressed in non-β-cells, including liver and intestinal cells. The present review summarizes the recent findings on how glucose modulates the function of the β-cell transcription factors Pdx-1, NeuroD1 and MafA, and thereby tightly regulates insulin synthesis in accordance with blood glucose levels.

MAPK kinase kinase-1 is essential for cytokine-induced c-Jun NH2-terminal kinase and nuclear factor-kappaB activation in human pancreatic islet cells

OBJECTIVE

The transcription factor nuclear factor-kappaB (NF-kappaB) and the mitogen-activated protein kinases (MAPKs) c-Jun NH(2)-terminal kinase (JNK) 1/2 are known to play decisive roles in cytokine-induced damage of rodent beta-cells. The upstream events by which these factors are activated in response to cytokines are, however, uncharacterized. The aim of the present investigation was to elucidate a putative role of the MAPK kinase kinase-1 (MEKK-1) in cytokine-induced signaling.

RESEARCH DESIGN AND METHODS

To establish a functional role of MEKK-1, the effects of transient MEKK-1 overexpression in betaTC-6 cells, achieved by lipofection and cell sorting, and MEKK-1 downregulation in betaTC-6 cells and human islet cells, achieved by diced-small interfering RNA treatment, were studied.

RESULTS

We observed that overexpression of wild-type MEKK-1, but not of a kinase dead MEKK-1 mutant, resulted in potentiation of cytokine-induced JNK activation, inhibitor of kappaB (IkappaB) degradation, and cell death. Downregulation of MEKK-1 in human islet cells provoked opposite effects, i.e., attenuation of cytokine-induced JNK and MKK4 activation, IkappaB stability, and a less pronounced NF-kappaB translocation. betaTC-6 cells with a downregulated MEKK-1 expression displayed also a weaker cytokine-induced iNOS expression and lower cell death rates. Also primary mouse islet cells with downregulated MEKK-1 expression were protected against cytokine-induced cell death.

CONCLUSIONS

MEKK-1 mediates cytokine-induced JNK- and NF-kappaB activation, and this event is necessary for iNOS expression and cell death.