There are four main cell types in the islets of Langerhans in the pancreas; α-cells which secrete glucagon, β-cells which secrete insulin, δ-cells which secrete somatostatin, and PP cells which secrete pancreatic polypeptide. Type I diabetes is an autoimmune disease in which the insulin-producing β-cells are destroyed and could potentially be treated by the creation of new β-cells, either from stem or stem-like cells or by conversion of another mature cell type. It has recently been shown that the transcription factor, Pax4, induces transdifferentiation of pancreatic α-cells into β-cells in adult mice and a team led by researchers at the Broad Institute of Harvard and MIT has now shown that a similar effect can be achieved with a small molecule.
Using a mouse α-cell line, the team screened over 30,000 compounds and found that one of them, BRD7389, induced insulin expression after 3 days treatment. Induction of insulin gene expression in α-cells peaked at 5 days (ca 50-fold at a BRD7389 concentration of 0.85µM) and the cells adopted a β-cell-like morphology. Insulin protein levels were also increased from a basal α-cell state, although were much lower than the levels produced by mature β-cells. BRD7389 also increased insulin secretion in primary human islet cells although since there are many different cells types in the human tissue samples, it is also not possible to attribute the increase in insulin levels exclusively to conversion of α-cells to β-like-cells.
Follow-up studies suggested that upregulation of insulin expression potentially involved inhibition of multiple members of the RSK family of protein kinases, but more experiments are needed to fully elucidate the mechanism of action of BRD7389. The study demonstrates, however, that a small molecule can induce insulin expression in α-cells and suggests that such a strategy could be used to increase β-cell mass by transdifferentiation in vivo. The team now want to identify other small molecules that could be used to enhance the effects of BRD7389, and boost insulin production in people with type I diabetes.
Obesity and related disorders such as diabetes have reached epidemic proportions. Although the anti-diabetic thiazolidinediones (glitazones) are effective insulin sensitizers, some members of the class have been withdrawn or had their use restricted because of safety concerns. Increased responsiveness to insulin is believed to be mediated by activation of the nuclear receptor, PPARγ but differences in clinically important side effects suggest subtle differences in pharmacology, even amongst full agonists.
Researchers at the Scripps Research Institute and the Dana-Farber Cancer Institute at Harvard University have now shown that cyclin-dependent kinase 5 (Cdk5) in adipose tissue is activated in obese mice fed a high-fat diet, resulting in phosphorylation of PPARγ. This has no effect on the adipogenic capacity of PPARγ but does alter the expression of a large number of obesity-related genes, including a reduction in expression of the insulin-sensitizing adipokine, adiponectin. Phosphorylation of PPARγ by Cdk5 was blocked both in vitro and in vivo by the full agonist, rosiglitazone, and by the partial agonist, MRL-24, leading to increased adiponectin production. The anti-diabetic effect of rosiglitazone in obese patients was also found to be closely associated with inhibition of PPARγ phosphorylation, suggesting that this may be a mechanism of insulin resistance. The authors of the study, which is published in the journal Nature, suggest that drugs that inhibit PPARγ phosphorylation by Cdk5, without necessarily activating the receptor, may provide an improved generation of anti-diabetic drugs.
The eukaryotic translation initiation factor eIF5A, which exists in two isoforms, was originally thought to be involved in formation of the first peptide bond during mRNA translation, but more recent work has implicated it as a translation elongation factor. In mammalian cells it has variously been associated with mediation of proliferation, apoptosis and inflammatory responses, although its mechanisms of action have remained vague. It has also been identified as a cofactor of the Rev trans-activator protein of HIV-1. eIF5A is unique in that it is the only known protein to contain the amino acid hypusine, formed posttranslationally via the sequential action of deoxyhypusine synthase (DHS) and deoxyhypusine hydroxylase (DOHH) acting at a specific lysine residue.
Based on the role of eIF5A in inflammation, a multi-institutional research team led by scientists at Indiana University School of Medicine has explored involvement of the protein in pancreatic islet dysfunction during the development of diabetes. In a low-dose streptozotocin mouse model of diabetes the team found that depletion of eIF5A (using siRNA) protected the mice from pancreatic β-cell loss and hyperglycemia. The depletion of eIF5A resulted in impaired translation of inducible nitric oxide synthase (iNOS)-encoding mRNA within islet cells. Further studies using an inhibitor of DHS, N1-guanyl-1,7-diaminoheptane (GC7), demonstrated a requirement for hypusination in the action of eIF5A.
The study, published in the Journal of Clinical Investigation, demonstrates a role for eIF5A in inflammation-induced damage to islet cells. Since the negative effects of eIF5A depend on hypusination, DHS may represent a viable therapeutic target for diabetes. Further work will be necessary to establish the role of this pathway in development and progression of the human disease.
A new study from a Mayo Clinic-led research team has identified novel, potent inhibitors of insulin degrading enzyme (IDE). Despite an interest in IDE for over 50 years, because of its involvement in insulin catabolism, these are the first potent and selective inhibitors of the enzyme to be described. Given their peptidic nature the current compounds are unlikely to be drugs themselves, but the team hope that their findings will enable further exploration of IDE inhibition as a therapy for diabetes.
IDE is a ubiquitously expressed, secreted enzyme belonging to a small superfamily of zinc-metalloproteases that evolved independently of conventional zinc-metalloproteases. This difference is emphasised by the team’s finding that potent, non-selective hydroxamate inhibitors of zinc metalloproteases did not inhibit IDE. A high-throughput screening campaign failed to identify useful hits, so the researchers turned to a substrate-based approach leading to identification of Ii1 (Inhibitor of IDE 1), with a Ki of 1.7nM. Additional biostructural work identified the distinctive mechanism of IDE inhibition.
In vitro studies with the inhibitors, which included equipotent retro-inverso peptide analogues, demonstrated potent inhibition of extracellular insulin catabolism. In addition, and somewhat unexpectedly, IDE inhibition also enhanced insulin signalling, suggesting IDE involvement in intracellular degradation of insulin.
As well as cleaving insulin, IDE degrades a number of other substrates including atrial natriuretic peptide, glucagon and amyloid-β protein (Aβ). Indeed there has been considerable interest in up-regulating IDE activity as a potential therapy for Alzheimer’s disease (AD). The authors of the current study, published in PLoS ONE, suggest that any concern regarding negative impacts of IDE inhibition on AD could be addressed by developing inhibitors that do not cross the blood-brain barrier. Further, in light of the recent finding that intranasal insulin improves cognition in early AD patients, and given insulin’s beneficial effects on learning and memory, it may be overly simplistic to assume that IDE’s role in AD pathogenesis is limited to its predicted effects on Aβ alone.
‘Breakfast like a king, lunch like a prince, sup like a pauper’ is an old and well known proverb but a recently published study gives new insights into why following this advice might help to fight obesity and diabetes.
Researchers at ETH have suggested that eating snacks – even healthy ones – between meals leads to a vicious circle of physical inactivity and overeating, and could ultimately lead to diabetes. The team have identified a novel mechanism by which insulin regulates both metabolic and behavioural responses to food intake. Insulin produced by the pancreas as a result of feeding inhibits the forkhead box transcription factor, Foxa2. Foxa2 regulates fat metabolism in the liver but also influences neurons in the lateral hypothalamic area of the brain which is considered to be the classic ‘feeding centre’, controlling feeding, diurnal rhythm, sleep and sexual behaviour. In the fasted state, Foxa2 is active and promotes synthesis of melanin-concentrating hormone (MCH) and orexin, proteins with roles in controlling food intake and motivated behaviour. In obese mice, Foxa2 was found to be non-functional, regardless of whether the animals were fasted or fed. Genetically modified mice with permanently active Fox2a in their brains have more MCH and orexin, eat more and have increased insulin sensitivity. The levels of physical activity after feeding are also significantly higher, and more closely resemble those of fasted animals. Conditional activation of Foxa2 in the brains of obese mice also resulted in improved glucose homeostasis, decreased fat and increased lean body mass.
The authors suggest that periods of fasting are important to ensure correct body weight since each time food is consumed, Fox2a is suppressed which reduces the motivation for physical activity and, consequently, energy expenditure. Prevention of Foxa2 phosphorylation may lead to increased levels of physical activity and could be a potential pharmacological target for the treatment of obesity and diabetes.
The link between diet and epigenetics in humans is notoriously difficult to study since people are, unsurprisingly, resistant to being fed a strictly controlled diet. Researchers at the Karolinska Institute have, however, shown that exposure to fatty acids can cause epigenetic modifications in muscle cells from healthy individuals that are the same as changes seen in people with type II diabetes.
Both hereditary and environmental factors are believed to play a role in the development of type II diabetes which is characterised by reduced sensitivity to insulin and a reduced ability to consume energy in the form of glucose. The team had previously shown that, in skeletal muscle cells from individuals with early-onset diabetes, the gene for PGC-1α (peroxisome proliferator-activated receptor-gamma coactivator 1α) is hypermethylated and shows reduced expression levels. PGC-1α is a promiscuous co-activator that plays a key role in regulating mitochondrial function and metabolism. In the present study, the team showed that DNA methylation occurs rapidly when cells from healthy individuals are exposed to factors linked to diabetes such as the fatty acid, palmitic acid, and the cytokine, TNF-α. It remains to be seen whether dietary factors substantially influence methylation of the gene for PGC-1α but the study suggests a mechanism whereby over-consumption of fatty foods could promote the development of type II diabetes. Since some epigenetic modifications are known to be inheritable, the study also raises the interesting question of whether the consequences of dietary excess can be visited on future generations.
The incretin, glucagon-like peptide-1 (GLP-1), is an intestinal hormone that stimulates production and release of insulin from pancreatic beta cells. Consequently there has been considerable interest in mimicking the activity of GLP-1 for treatment of metabolic disorders such as Type-II diabetes and obesity. There are currently two approved classes of drug that modulate GLP-1 activity: analogues of GLP-1 and inhibitors of dipeptidyl peptidase IV (DPPIV). Analogues of GLP-1, such as the 39-residue synthetic peptide, exenatide, activate the GLP-1 receptor but are resistant to proteolytic cleavage by DPPIV. The gliptins, such as sitagliptin, inhibit DPPIV, extending the half-life of the natural hormone.
Researchers at Ecole Polytechnique Fédérale de Lausanne, in collaboration with the University of Perugia and Intercept Pharmaceuticals, have now published data showing that stimulation of TGR5, a G-protein coupled receptor, leads to release of GLP-1 in obese mice. The same group had previously demonstrated that activation of TGR5 in brown adipose tissue and muscle by endogenous bile acids boosted energy expenditure and reversed diet-induced obesity in mice.
In the current work the researchers used a combination of genetic gain- and loss-of-function studies together with the TGR5 agonist, INT-777, to show the link between TGR5 signalling and GLP-1 secretion. In vitro experiments with INT-777 in enteroendocrine L-cells confirmed the induction of GLP-1 secretion and that this was linked to increased intracellular ATP/ADP ratio and a subsequent rise in intracellular calcium mobilization.
The study, published in the September 2nd edition of Cell Metabolism, opens up a potential third route to modulation of GLP-1 activity and treatment of metabolic disorders.
The nuclear receptor, PPARγ, has hitherto been regarded as the master regulator of adipogenesis, without which new adipose tissue cannot be formed. Adipogenesis plays a key role in obesity and associated metabolic diseases such as type II diabetes and the thiazolidines, which target PPARγ are widely used to treat type II diabetes. Researchers at the University of Central Florida have now discovered, however, that monocyte chemotactic protein-1 (MCP-1)-induced protein (MCPIP), can trigger adipogenesis without involvement of PPARγ. The authors had previously shown that binding of MCP-1 to its receptor, CCR2, leads to induction of the Zn-finger protein, MCPIP. In the present study, MCP-1 was found to be produced, and MCPIP to be induced, before induction of PPARγ or other transcription factors in fibroblasts undergoing differentiation into adipocytes. Knockdown of MCPIP using siRNA was found to inhibit both gene induction and adipogenesis whereas treatment with MCP-1 or forced expression of MCPIP induced adipogenesis. Forced expression of MCPIP was also shown to induce adipogenesis in PPARγ-/- mouse embryonic fibroblasts, further demonstrating that MCPIP can act independently of PPARγ.
Obesity is well known to increase MCP-1 levels and the finding that MCPIP is able to induce adipogenesis without involvement of PPARγ provides new mechanistic evidence for the role of MCP-1/CCR2 in obesity and type II diabetes. Since there is experimental evidence that MCPIP promotes angiogenesis, MCP-1/CCR2 interaction could promote the formation of new blood vessels to supply blood to growing adipose tissue as well as promoting development of the adipocytes. A drug that could block the function of MCPIP may thus have the potential to treat obesity and type II diabetes.
The physiological role of aldose reductase (AR) is still incompletely understood, although it has long been associated with the pathogenesis of diabetes-associated diseases such as cataract and neuropathy. In the last twenty years a number of AR inhibitors have entered clinical trials for the potential treatment of diabetic neuropathy. Whilst the compounds have generally been well tolerated, efficacy has not been clearly established (although one compound, Epalrestat, is approved in Japan for treatment of subjective neuropathy symptoms associated with diabetic peripheral neuropathy).
The rationale for use of AR inhibitors in diabetic complications is based on the ability of AR to reduce glucose to sorbitol, levels of which are elevated in tissues of diabetic patients. Although glucose does not have high affinity for AR, the pathway is believed to be relevant in hyperglycaemia.
AR is also known to reduce lipid aldehydes and their glutathione conjugates in response to reactive oxygen species (ROS). The products of the AR-catalysed reduction mediate activation of NFκB and the subsequent generation of inflammatory proteins. This observation led researchers at University of Texas Medical Branch and Louisiana State University Health Sciences Center to hypothesise that AR inhibitors may be useful in inflammatory diseases such as asthma. In a study published in PLoSone, the scientists stimulated primary human small airway epithelial cells (SAEC) with ragweed pollen extract (RWE). In this in vitro experiment, AR inhibition prevented RWE-induced apoptosis and expression of inflammatory mediators. Further, AR inhibition prevented allergic airway inflammation in mice sensitised with endotoxin-free RWE.
The results encourage exploration of AR inhibitors in inflammatory diseases such as asthma.
Pancreatic cancer has one of the highest fatality rates of all cancers and is the fourth leading cause of cancer-related deaths in the United States; more than 35,000 people were estimated to have died from the disease in 2008. Patients diagnosed with pancreatic cancer typically have poor prognoses because the cancer is difficult to detect in its early stages – there are few symptoms and any that do present are vague and often go unnoticed. Although the exact causes of pancreatic cancer are not understood, the disease is more common in people with type 2 diabetes, although the reasons for this link are also unclear.
A recent study by researchers at the University of Texas M.D. Anderson Cancer Center has shown, however, that diabetics who have taken the anti-diabetic drug, metformin, alone or in combination with other drugs, have a 60% lower risk of developing pancreatic cancer compared with patients who have never taken it. Metformin is the most popular anti-diabetic drug in the United States and works primarily by suppressing glucose production by the liver and by increasing insulin sensitivity.
The study, which is published in the journal Gastroenterology, also suggested some increased risk of developing pancreatic cancer in patients who received insulin or insulin secretagogues, such as sulfonylureas and meglitinides. The study population was too small to determine the effects of another class of anti-diabetic drugs, the thiazolidinones.
The study evaluated prior use of antidiabetic drugs in just over 1800 people, 973 with pancreatic adenocarcinoma (including 259 diabetic patients) and 863 controls (including 109 diabetic patients). Use of metformin was found to be associated with a significantly decreased risk of pancreatic cancer compared with metformin non-use (adjusted odds ratio 0.38) whereas long-term insulin use was associated with a moderately higher risk of pancreatic cancer compared to insulin non-use (adjusted odds ratio 2.78). The association between pancreatic cancer and long term insulin use was, however, based on only 17 cases and 9 controls and needs to be investigated further in a larger study. The impact of insulin secretagogues on risk of pancreatic cancer likewise needs to be fully assessed in a larger study. Although it was not possible to gauge the effect of severity of diabetes or effectiveness of diabetes control on the results, the study does, however, show a robust protective effect of metformin, especially after long term use, against pancreatic cancer in people with type 2 diabetes.