Loss of insulin-induced inhibition of glucagon gene transcription in hamster pancreatic islet alpha cells by long-term insulin exposure

AIMS/HYPOTHESIS

Diabetes mellitus type 2 is characterised by hyperglucagonaemia, resulting in hepatic glucose production and hyperglycaemia. Considering that insulin inhibits glucagon secretion and gene transcription, hyperglucagonaemia in the face of hyperinsulinaemia in diabetes mellitus type 2 suggests that there is insulin resistance also at the glucagon-producing pancreatic islet alpha cells. However, the molecular mechanism of alpha cell insulin resistance is unknown. Therefore, the effect of molecules implicated in conferring insulin resistance in some other tissues was investigated on insulin-induced inhibition of glucagon gene transcription in alpha cells.

METHODS

Reporter gene assays and biochemical techniques were used in the glucagon-producing hamster pancreatic islet alpha cell line InR1-G9.

RESULTS

From among 16 agents tested, chronic insulin treatment was found to abolish insulin-induced inhibition of glucagon gene transcription. Overproduction of constitutively active protein kinase B (PKB) still inhibited glucagon gene transcription after chronic insulin treatment; together with a markedly reduced insulin-induced phosphorylation and, thus, activation of PKB, this indicates that targets upstream of PKB within the insulin signalling pathway are affected. Indeed, chronic insulin treatment markedly reduced IRS-1 phosphorylation, insulin receptor (IR) autophosphorylation and IR content. Cycloheximide and in vivo labelling experiments attributed IR downregulation to enhanced degradation.

CONCLUSIONS/INTERPRETATION

These results show that an extended exposure of alpha cells to insulin induces IR downregulation and loss of insulin-induced inhibition of glucagon gene transcription. They suggest that hyperinsulinaemia, through IR downregulation, may confer insulin resistance to pancreatic islet alpha cells in diabetes mellitus type 2.

Chronic lithium salt treatment reduces CRE/CREB-directed gene transcription and reverses its upregulation by chronic psychosocial stress in transgenic reporter gene mice

The molecular mechanism of action of the mood stabilizer lithium is assumed to involve changes in gene expression leading to neuronal adaptation. The transcription factor CREB (cAMP-responsive element binding protein) regulates the expression of many genes and has been implicated in important brain functions and the action of psychogenic agents. We here investigated the effect of lithium on cAMP-responsive element (CRE)/CREB-mediated gene transcription in the brain, using transgenic reporter mice that express the luciferase reporter gene under the control of four copies of the rat somatostatin gene promoter CRE. Chronic (21 days) but not acute (24 h) treatment with lithium (7.5 mmol/kg) significantly decreased CRE/CREB-directed gene expression in hippocampus, cortex, hypothalamus, and striatum to 60-70%, and likewise reduced CREB phosphorylation. As bipolar disorder is also considered as a stress-related disorder, the effect of lithium was determined in mice submitted to a paradigm for chronic psychosocial stress. As shown before, stress for 25 days significantly increased CRE/CREB-directed gene expression in several brain regions by 100-150%. Treatment of stressed mice with lithium decreased stress-induced CRE/CREB-directed gene expression to control levels in nearly all brain regions and likewise reduced CREB phosphorylation. Chronic lithium treatment induced beta-catenin accumulation and decreased cAMP levels, indicating an inhibitory effect of lithium on glycogen synthase kinase 3 and the adenylate cyclase/protein kinase A signalling cascade, which are known to modulate CREB activity. We here for the first time show that lithium regulates CRE/CREB-directed gene transcription in vivo and suggest CREB as a putative mediator of the neuronal adaptation after chronic lithium treatment.

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

AIMS/HYPOTHESIS

Inappropriate insulin secretion and biosynthesis are hallmarks of beta cell dysfunction and contribute to the progression from a prediabetic state to overt diabetes mellitus. During the prediabetic state, beta cells are exposed to elevated levels of proinflammatory cytokines. In the present study the effect of these cytokines and mitogen-activated protein kinase kinase kinase 1 (MEKK1), which is known to be activated by these cytokines, on human insulin gene (INS) transcription was investigated.

METHODS

Biochemical methods and reporter gene assays were used in a beta cell line and in primary pancreatic islets from transgenic mice.

RESULTS

IL-1beta and MEKK1 specifically inhibited basal and membrane depolarisation and cAMP-induced INS transcription in the beta cell line. Also, in primary islets of reporter gene mice, IL-1beta reduced glucose-stimulated INS transcription. A 5'- and 3'-deletion and internal mutation analysis revealed the rat insulin promoter element 3b (RIPE3b) to be a decisive MEKK1-responsive element of the INS. RIPE3b conferred strong transcriptional activity to a heterologous promoter, and this activity was markedly inhibited by MEKK1 and IL-1beta. RIPE3b is also known to recruit the transcription factor MafA. We found here that MafA transcription activity is markedly inhibited by MEKK1 and IL-1beta.

CONCLUSIONS/INTERPRETATION

These data suggest that IL-1beta through MEKK1 inhibits INS transcription and does so, at least in part, by decreasing MafA transcriptional activity at the RIPE3b control element. Since inappropriate insulin biosynthesis contributes to beta cell dysfunction, inhibition of MEKK1 might decelerate or prevent progression from a prediabetic state to diabetes mellitus.

CRE/CREB-driven up-regulation of gene expression by chronic social stress in CRE-luciferase transgenic mice: reversal by antidepressant treatment

Background

It has been suggested that stress provokes neuropathological changes and may thus contribute to the precipitation of affective disorders such as depression. Likewise, the pharmacological therapy of depression requires chronic treatment and is thought to induce a positive neuronal adaptation, presumably based on changes in gene transcription. The transcription factor cAMP-responsive element binding protein (CREB) and its binding site (CRE) have been suggested to play a major role in both the development of depression and antidepressive therapy.

Methodology/Principle Findings

To investigate the impact of stress and antidepressant treatment on CRE/CREB transcriptional activity, we generated a transgenic mouse line in which expression of the luciferase reporter gene is controlled by four copies of CRE. In this transgene, luciferase enzyme activity and protein were detected throughout the brain, e.g., in the hippocampal formation. Chronic social stress significantly increased (by 45 to 120%) CRE/CREB-driven gene expression measured as luciferase activity in several brain regions. This was also reflected by increased CREB-phosphorylation determined by immunoblotting. Treatment of the stressed mice with the antidepressant imipramine normalized luciferase expression to control levels in all brain regions and likewise reduced CREB-phosphorylation. In non-stressed animals, chronic (21 d) but not acute (24 h) treatment with imipramine (2×10 mg/kg/d) reduced luciferase expression in the hippocampus by 40–50%.

Conclusions/Significance

Our results emphasize a role of CREB in stress-regulated gene expression and support the view that the therapeutic actions of antidepressants are mediated via CRE/CREB-directed transcription.

Inhibition of human insulin gene transcription by the immunosuppressive drugs cyclosporin A and tacrolimus in primary, mature islets of transgenic mice

Cyclosporin A and tacrolimus are clinically important immunosuppressive drugs. They share a diabetogenic action as one of their most serious adverse effects. The underlying mechanism is unknown. Previous studies have shown that tacrolimus can inhibit insulin gene transcription at high concentrations in tumor cell lines. To study insulin gene transcription in normal, mature pancreatic islet cells, we used a novel approach in the present study. Transgenic mice that carry a human insulin promoter-reporter gene were generated. The human insulin promoter directed transcription in pancreatic islets and conferred a normal, physiological glucose response to reporter gene expression in isolated islets. After stimulation with glucose, human insulin promoter-mediated gene expression was inhibited in normal, mature islet cells by both tacrolimus and cyclosporin A to a large extent (approximately 70%) and with high potency at concentrations that are known to inhibit calcineurin phosphatase activity (IC50 values of 1 and 35 nM, respectively). Furthermore, glucose stimulated calcineurin phosphatase activity in mouse pancreatic islets, further supporting the view that calcineurin phosphatase activity is an essential part of glucose signaling to the human insulin gene. The high potency of cyclosporin A and tacrolimus in normal islets suggests that inhibition of insulin gene transcription by cyclosporin A and tacrolimus is clinically important and is one mechanism of the diabetogenic effect of these immunosuppressive drugs.

Tissue-specific transcriptional activity of a pancreatic islet cell-specific enhancer sequence/Pax6-binding site determined in normal adult tissues in vivo using transgenic mice

A pancreatic islet cell-specific enhancer sequence (PISCES) shared by the rat insulin-I, glucagon, and somatostatin genes binds the paired domain-containing transcription factor Pax6 and confers strong transcriptional activity in pancreatic islet cell lines. It was found recently that Pax6 plays a major role in islet development. In the present study, transgenic mice were used to investigate PISCES-mediated transcription in normal adult islets in vivo. In several independent mouse lines expressing a PISCES-luciferase reporter transgene, the PISCES motif directed gene expression in the adult eye, cerebellum, and discrete brain areas, consistent with the tissue distribution of Pax6. These tissues contain two Pax6 isoforms caused by alternative splicing, only one of which was found to bind the PISCES motif in electrophoretic mobility shift assays. No reporter gene expression was detected in adult pancreatic islets or in any other peripheral organ tested. RT-PCR analysis confirmed that Pax6 mRNA is present in adult islets. These results demonstrate that the PISCES motif is sufficient to direct highly tissue-specific gene expression in whole animals. The lack of PISCES-mediated transcription in adult islets indicates that the Pax6 protein(s) expressed in adult pancreatic islets function differently from the ones in the eye and cerebellum.