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.

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.

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.

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.

Inhibition of membrane depolarisation-induced transcriptional activity of cyclic AMP response element binding protein (CREB) by the dual-leucine-zipper-bearing kinase in a pancreatic islet beta cell line

AIMS/HYPOTHESIS

The activation of the transcription factor cyclic AMP response element binding protein (CREB) by protein kinase A is inhibited by the human orthologue of the mitogen-activated protein kinase, dual-leucine-zipper-bearing kinase (DLK) in teratocarcinoma cells. However, pancreatic beta cells are electrically excitable and a major pathway regulating CREB in these cells is membrane depolarisation, leading to calcium influx and activation of the calcium/calmodulin-dependent protein phosphatase calcineurin. Therefore, the effect of DLK on CREB activity induced by membrane depolarisation was investigated in the beta cell line HIT.

MATERIALS AND METHODS

Reporter gene assays and biochemical techniques were used.

RESULTS

RT-PCR, Western blot analysis and immunohistochemistry demonstrated the expression of DLK in HIT cells and primary mouse islets. In transient transfection experiments, DLK inhibited both GAL4-CREB activity induced by membrane depolarisation, and transcription directed by the CREB binding site, the cyclic AMP response element. Furthermore, DLK inhibited the transcriptional activity conferred by the CREB coactivator, CREB binding protein, both under basal conditions and after membrane depolarisation. DLK was also effective in response to glucose, the most potent physiological stimulus and known to cause membrane depolarisation of beta cells. Inhibition of calcineurin enhanced DLK activity, whereas overexpression of calcineurin reduced the inhibition by DLK of transcription directed by cyclic AMP response element after membrane depolarisation.

CONCLUSIONS/INTERPRETATION

These results demonstrate a calcineurin-sensitive inhibition by DLK of CREB activity after membrane depolarisation in pancreatic islet beta cells. This inhibition may, at least partially, be mediated at the coactivator level. The results thus suggest that DLK plays a role in the regulation of beta cell function, including insulin gene transcription and beta cell apoptosis.

Insulin responsiveness of the glucagon gene conferred by interactions between proximal promoter and more distal enhancer-like elements involving the paired-domain transcription factor Pax6

Regulation of gene transcription is an important aspect of insulin's action. However, the mechanisms involved are poorly understood. Insulin inhibits glucagon gene transcription, and insulin deficiency is associated with hyperglucagonemia that contributes to hyperglycemia in diabetes mellitus. Transfecting glucagon-reporter fusion genes into a glucagon-producing pancreatic islet cell line, a 5'-, 3'-, and internal deletion analysis, and oligonucleotide cassette insertions failed in the present study to identify a single insulin-responsive element in the glucagon gene. They rather indicate that insulin responsiveness depends on the presence of both proximal promoter elements and more distal enhancer-like elements. When the paired domain transcription factor Pax6 binding sites within the proximal promoter element G1 and the enhancer-like element G3 were mutated into GAL4 binding sites, the expression of GAL4-Pax6 and GAL4-VP16 restored basal activity, whereas only GAL4-Pax6 restored also insulin responsiveness. Likewise, GAL4-CBP activity was inhibited by insulin within the glucagon promoter context. The results suggest that insulin responsiveness is conferred to the glucagon gene by the synergistic interaction of proximal promoter and more distal enhancer-like elements, with Pax6 and its potential coactivator the CREB-binding protein being critical components. These data thereby support concepts of insulin-responsive element-independent mechanisms of insulin action to inhibit gene transcription.

Inhibition of cyclic AMP response element-binding protein/cyclic AMP response element-mediated transcription by the immunosuppressive drugs cyclosporin A and FK506 depends on the promoter context

The immunosuppressants cyclosporin A and FK506 (tacrolimus) can block the phosphatase calcineurin, thereby inhibiting gene transcription directed by the cyclic AMP (cAMP)- and calcium-responsive transcription factor, cAMP response element (CRE)-binding protein, and its binding site, CRE, in various cell lines. This action is a novel molecular mechanism of cyclosporin A and FK506 action. Because inhibition of CREB/CRE-directed transcription by cyclosporin A and FK506 has previously been observed by using synthetic minienhancers, reporter fusion genes were constructed to examine the effect of cyclosporin A and FK506 on the transcriptional activity of CRE-containing natural promoters. In transient transfection experiments, cyclosporin A and FK506 inhibited the transcriptional activation by cAMP and the membrane depolarization of three CRE-containing promoters. However, cyclosporin A and FK506 failed to inhibit the activation by cAMP of another promoter, the rat insulin I gene promoter. The lack of cyclosporin A/FK506 sensitivity is not intrinsic to the insulin CRE because cyclosporin A and FK506 inhibited the activation by cAMP of the insulin CRE when isolated and used as a synthetic minienhancer. Rather, cyclosporin A/FK506 resistance may be conferred by specific promoter interactions because a mutational analysis of the insulin promoter revealed that inside this promoter, CRE activity depends on an adjacent control element. These data show that cyclosporin A and FK506 can inhibit CRE activity when the CRE resides in its natural promoter. However, the cyclosporin A/FK506 sensitivity depends on the specific promoter context. The results suggest that cyclosporin A and FK506 may alter target tissue function through the regulation of a subset of CRE-containing genes.

The hepatitis B pX protein promotes dimerization and DNA binding of cellular basic region/leucine zipper proteins by targeting the conserved basic region

The hepatitis B virus pX protein is a potent transcriptional activator of viral and cellular genes whose mechanism of action is poorly understood. Here we show that pX dramatically stimulates in vitro DNA binding of a variety of cellular proteins that contain basic region/leucine zipper (bZIP) DNA binding domains. The basis for increased DNA binding is a direct interaction between pX and the conserved bZIP basic region, which promotes bZIP dimerization and the increased concentration of the bZIP homodimer then drives the DNA binding reaction. Unexpectedly, we found that the DNA binding specificity of various pX-bZIP complexes differs from one another and from that of the bZIP itself. Thus, through recognition of the conserved basic region, pX promotes dimerization, increases DNA binding, and alters DNA recognition. These properties of pX are remarkably similar to those of the human T-cell lymphotrophic virus type I Tax protein. Although Tax and pX are not homologous, we show that the regions of the two proteins that stimulate bZIP binding contain apparent metal binding sites. Finally, consistent with this in vitro activity, we provide evidence that both Tax and pX activate transcription in vivo, at least in part, by facilitating occupancy of bZIPs on target promoters.

Identification and expression of a novel isoform of cAMP response element modulator in the human heart

In end-stage human heart failure, excessive beta-adrenergic stimulation of the cAMP-dependent signaling pathway due to enhanced endogenous catecholamines is hypothesized to contribute to expressional alterations of myocardial regulatory proteins. The cAMP response element modulator (CREM) regulates the transcription of cAMP-responsive genes and might be involved in the regulation of cardiac gene expression. Using the reverse transcription polymerase chain reaction, we identified a novel CREM mRNA, CREM-Ib deltaC-X, in the human heart. Overexpression of CREM-Ib deltaC-X decreased cAMP response element (CRE) -mediated gene transcription in HIT-T15 cells, and this activity was assigned to the part of the sequence encoding putative internally translated proteins. Two of three possible internally translated proteins were immunologically identified in cells overexpressing CREM-Ib deltaC-X tagged with the hemagglutinin epitope of the influenza virus. Both proteins were expressed in bacteria and showed CRE-specific DNA binding, formation of heterodimers with the cAMP response element binding protein (CREB), and inhibition of CREB's binding to the CRE. CREM expression was detected on the mRNA and protein levels in the human heart. We conclude that CREM-Ib deltaC-X generates internally translated repressors of CRE-mediated gene transcription, suggesting the first example for the existence and function of human cardiac CREM.

Inhibition of CREB- and cAMP response element-mediated gene transcription by the immunosuppressive drugs cyclosporin A and FK506 in T cells

The clinically important immunosuppressant drugs cyclosporin A and FK506 (tacrolimus) inhibit in T-cells calcineurin phosphatase activity and nuclear translocation of the cytosolic component of the transcription factor nuclear factor of activated T-cells (NF-ATc) that is involved in the induction of early genes during T-cell activation. This effect has been proposed to explain at least part of the immunosuppressive effect of these drugs. Previous studies in pancreatic islet cell lines have shown that cyclosporin A and FK506 through inhibition of calcineurin interfere also with the function of the transcription factor cAMP response element binding protein (CREB) that is activated by cAMP and calcium signals and binds to cAMP/calcium response elements (CRE). By transient expression of CRE-reporter genes or GAL4-CREB fusion proteins, the present study shows that inhibition of CREB/CRE-directed transcription by cyclosporin A and FK506 occurs in a great variety of cell types including in cell lines derived from tissues in which adverse effects of the immunosuppressants develop. CREB activity and CRE-mediated transcription was blocked by these drugs also in Jurkat T-cells. When taken together with recent evidence for an essential role of CREB in T-cell activation and proliferation, the present results suggest that inhibition of CREB/CRE-directed transcription may be a molecular mechanism of the immunosuppressive effect of cyclosporin A and FK506.

Involvement of the Ca(2+)-dependent phosphatase calcineurin in gene transcription that is stimulated by cAMP through cAMP response elements

Gene transcription can be induced by cAMP and Ca2+ through distinct protein kinases phosphorylating the transcription factor CREB, which binds to cAMP response elements (CREs) in various genes. Induction of gene transcription by Ca2+ has been shown recently to depend on the Ca2+/calmodulin-dependent protein phosphatase calcineurin in pancreatic islet cells. This study investigates the role of calcineurin in CRE-directed gene transcription after stimulation by cAMP. Reporter fusion genes under the transcriptional control of CREs were transiently transfected into the cell line HIT. Pharmacological evidence suggests that cAMP stimulates CRE-mediated transcription through a Ca(2+)-dependent mechanism. The immunosuppressive drugs cyclosporin A and FK506 inhibited CRE-mediated transcription stimulated by cAMP. At the same concentrations they also inhibited calcineurin phosphatase activity. Reversal of calcineurin inhibition by rapamycin or overexpression of calcineurin led to disinhibition of CRE-mediated gene transcription. Immunoblots with a phosphoCREB-specific antibody showed that cyclosporin A and FK506 do not interfere with CREB phosphorylation at serine 119 stimulated with cAMP or membrane depolarization. These results indicate that in HIT cells stimulation of CRE-mediated transcription depends not only on the activity of protein kinases phosphorylating CREB but also on the Ca2+/calmodulin-dependent protein phosphatase calcineurin that is necessary for the transcriptional competence of phosphorylated CREB.

Distinct properties of the cAMP-responsive element of the rat insulin I gene

The cAMP response element (CRE)-binding transcription factor CREB can mediate induction of gene transcription in response to calcium as well as to cAMP. Since the rat insulin I gene 5'-flanking region contains a CRE with an octamer-like motif (TGACGTCC), CREB binding and cAMP/calcium responsiveness of the insulin CRE were investigated. In an electrophoretic mobility shift assay and in Southwestern blot experiments, bacterially expressed recombinant CREB bound to the insulin CRE as it did to the rat glucagon and rat somatostatin gene CREs. However, in nuclear extracts of the pancreatic islet cell line HIT, protein complexes binding to the insulin CRE did not contain proteins with CREB-like immunoreactivity, although these bound to the glucagon and somatostatin CREs. When reporter fusion genes were transfected into HIT cells, the isolated insulin CRE increased basal activity and mediated transcriptional activation by cAMP. However, cAMP stimulation of transcription through the insulin CRE was weak when compared with the response through the glucagon and somatostatin CREs. Furthermore, the insulin CRE did not confer responsiveness to membrane depolarization and calcium influx, in contrast to the glucagon and somatostatin CREs. These results demonstrate that the functional properties of the rat insulin I gene CRE are different from those of the rat glucagon and somatostatin CREs which may be explained by a distinct pattern of nuclear protein binding and suggest the existence of post-translational mechanisms that decrease the binding of cellular CREB to the insulin CRE.

Inhibition of cAMP-responsive element-mediated gene transcription by cyclosporin A and FK506 after membrane depolarization

The cAMP-responsive element (CRE) and its cognate transcription factor CREB can mediate induction of gene transcription in response to membrane depolarization and calcium influx. In this study, the effect of cyclosporin A (CsA) and FK506 on depolarization-induced glucagon gene transcription was investigated in a pancreatic islet cell line by transfection of reporter fusion genes. CsA and FK506 inhibited depolarization-induced glucagon gene transcription, FK506 being more potent than CsA. CsA/FK506 responsiveness was mediated by the glucagon CRE and also by well characterized CREs of the choriogonadotropin and somatostatin genes. Rapamycin antagonized the inhibitory effect of FK506 but not CsA, suggesting that FK506 and CsA may act through complex formation with distinct intracellular immunophilins. Overexpression of calcineurin, which is known to be inhibited by drug-immunophilin complexes, rendered pancreatic islet cells more resistant to the inhibitory effects of CsA and FK506. These results demonstrate an inhibition by CsA and FK506 of CRE-mediated, calcium-induced transcription and suggest that membrane depolarization relies on calcineurin phosphatase activity for activation of CREB/CRE-mediated gene transcription. The interference with CRE-mediated gene transcription represents a novel mechanism of CsA/FK506 action, which may underlie pharmacological effects and toxic manifestations of these potent immunosuppressive drugs.

The immunosuppressive drugs cyclosporin A and FK506 inhibit calcineurin phosphatase activity and gene transcription mediated through the cAMP-responsive element in a nonimmune cell line

Cyclosporin A and the macrolide tacrolimus (FK506) are powerful immunosuppressive drugs that in T cells inhibit the calcium/calmodulin-dependent phosphatase calcineurin thereby preventing the activation of T-cell-specific transcription factors, such as NF-AT, involved in lymphokine gene expression. While this may explain, at least in part, the mechanism of cyclosporin A/FK506 immunosuppression, additional mechanisms have to be invoked in order to explain the pharmacological properties and toxic effects of these drugs, such as nephrotoxicity and neurotoxicity. We have studied the effects of cyclosporin A and FK506 on calcineurin phosphatase activity and gene transcription mediated by the cAMP-responsive element (CRE), a binding site of the ubiquitous transcription factor CREB. A reporter gene was placed under the transcriptional control of the CRE of the rat glucagon gene and transiently transfected into the glucagon-expressing cell line alpha TC2. Cyclosporin A and FK506 inhibited depolarization-induced gene transcription in a concentration-dependent manner (IC50 of about 1 nM and 30 nM for FK506 and cyclosporin A, respectively). Both cyclosporin A and FK506 inhibited calcineurin phosphatase activity at drug concentrations that inhibited gene transcription. The FK506 analogue rapamycin had no effect on calcineurin activity and gene transcription, but excess concentrations of rapamycin prevented the effects of FK506 on both calcineurin activity and gene transcription. These results support the notion that the interaction of drug-immunophilin complexes with calcineurin may be the molecular basis of cyclosporin A/FK506-induced inhibition of CREB/CRE-mediated gene transcription. The ability to interfere with CREB/CRE-mediated gene transcription represents a novel mechanism of cyclosporin A/FK506 action which may underlie pharmacological effects and toxic manifestations of these potent immunuosuppressive drugs.

Membrane depolarization and calcium influx induce glucagon gene transcription in pancreatic islet cells through the cyclic AMP-responsive element

Glucagon-producing pancreatic islet cells generate calcium-dependent action potentials. By the control of calcium influx through voltage-gated calcium channels, calcium is a tightly regulated second messenger in these cells. It is unknown whether calcium is a signal for glucagon gene transcription. Therefore, rat glucagon reporter fusion genes were transiently transfected into pancreatic islet cell lines. High potassium-induced membrane depolarization activated glucagon gene transcription. The effects of a calcium chelator, calcium channel blockers, calmodulin antagonists, and an inhibitor of calcium/calmodulin-dependent protein kinase II (CaM kinase II) indicate that depolarization-induced glucagon gene transcription depends on calcium influx and CaM kinase II. The depolarization-responsive element was mapped to the glucagon cAMP-responsive element (CRE). The CRE-binding protein CREB was shown, by using GAL4-CREB fusion proteins, to function as a depolarization-regulated transcription factor in pancreatic islet cells. Membrane depolarization and cAMP had synergistic effects on glucagon gene transcription. These results suggest that rat glucagon gene transcription is regulated by membrane electrical activity and calcium influx in pancreatic islet cells. This signal may be transmitted via CaM kinase II and CREB to the glucagon CRE.

Stimulation of glucose production from glycogen by glucagon, noradrenaline and non-degradable adenosine analogues is counteracted by adenosine and ATP in cultured rat hepatocytes

The glycogenolytic potency of adenosine and ATP was studied in adult rat hepatocytes and compared with the action of glucagon and noradrenaline. In cells cultured for 48 h, adenosine and ATP as well as their analogues 2-chloroadenosine, phenylisopropyladenosine, N-ethylcarboxamidoadenosine and beta-gamma-methylene-substituted ATP (p[CH2]ppA) increased glycogen phosphorylase alpha to levels indistinguishable from those obtained by the addition of glucagon or noradrenaline. The P1 receptor antagonist 8-phenyltheophylline abolished the activation of phosphorylase by adenosine and by p[CH2]ppA, but not that by ATP. Protein kinase A was activated by p[CH2]ppA and ATP via their breakdown to adenosine. [14C]Glucose production from glycogen was stimulated only 3-fold by ATP and adenosine, compared with a 7-fold increase produced by the hormones. Stimulation of glucose production by glucagon or noradrenaline was almost completely abolished by ATP or adenosine, with half-maximal effects at around 10 microM. The non-degradable adenosine analogues were equipotent with glucagon with respect to stimulation of glucose production, and their action was also inhibited by adenosine. ATP and p[CH2]ppA, which were both degraded to adenosine, showed comparable metabolic effects, whereas the alpha, beta-methylene analogue was without biological action and also was not degraded to adenosine. In the presence of the adenosine transport inhibitor nitrobenzyl thioinosine (NBTI), adenosine exerted an increased glycogenolytic potency, reaching 80% of the maximal stimulation obtained by glucagon. The glucagon-antagonistic effect of adenosine could be completely abolished by NBTI, but was not affected by phenyltheophylline. It is concluded that, in the hepatocyte culture system, adenosine and ATP decrease the catalytic efficiency of phosphorylase alpha through signals arising from their uptake into the cell.