Below is our coverage of the basic science talks at ENDO, including talks by Dr. Ronald Kahn (Joslin Diabetes Center, Boston, MA), Robert Lefkowitz (Duke University Medical School, Durham, NC), Domenico Accili (Columbia University, New York, NY), David Altshuler (Broad Institute, Boston, MA), Michael Schwartz (University of Washington, Seattle, WA), and Jürgen Eckel (Paul LAngerhans Group, German Diabetes, Center, Dusseldorf, Germany). Titles of the most notable presentations are highlighted in yellow, while coverage that was not included in our daily coverage is highlighted in blue.
- Executive Highlights
- Basic Science
- Presidential Plenary
- Plenary Session: Presidential Plenary
- Plenary Session: Edwin B. Astwood Award Lecture
- Plenary Session: Roy O. Greep Award Lecture
- Symposium: Novel Mechanisms of Hepatic Metabolism
- Symposium: Novel Adipokines
Insulin Action and Insulin Resistance: At the Crossroads
Ronald Kahn, MD (Joslin Diabetes Center, Boston, MA)
In a talk packed with conference attendees and basic science, Dr. Ronald Kahn reviewed insulin action and its sites of regulation and highlighted new concepts in the insulin signaling pathway. Dr. Kahn pointed out that insulin signaling is a system of checks and balances, and that receptor modulators can modify insulin signaling and insulin resistance states. Dr. Kahn continued by stating that the “receptor is more important than the hormone,” demonstrating that the unoccupied insulin receptor is still a signaling molecule on its own. Regarding new tools, Dr. Kahn highlighted strategies using mouse and cellular models for insulin signaling. He also pointed out using induced pluripotent stem cells to study human insulin resistance can dissect apart genetic and acquired defects in insulin resistance. Dr. Kahn’s review of these new developments and ideas show that it is an exciting time in the field to learn much more about insulin action and resistance.
- Dr. Kahn showed that many players in the insulin signaling pathway have feedback systems on one another. For example, P85 is linked to the process of insulin resistance, called ER stress, which also begins to feedback and turn off the insulin signaling pathway. In addition, transcription factor Fox01 has both positive and negative regulatory roles.
- Dr. Kahn discussed the Glypican-4 molecule to illustrate that insulin receptor modulators can modify insulin signaling and resistance. Biopsies of human adipose tissue showed that visceral tissues had low levels of Glypican-4 while subcutaneous tissues had high levels of Glypican-4. This insulin receptor modulator correlated with insulin resistance and BMI, in that lower levels were associated with higher BMI and insulin resistance levels.
- Regarding unoccupied insulin receptors, Dr. Kahn showed that “empty” IGF-1 receptors continued to affect signaling and miRNA expression. If this applies for the insulin receptor as well, modulating this basal level of action could potentially serve as a target for therapy.
Plenary Session: Presidential Plenary
Seven Transmebrane Receptors
Robert Lefkowitz, MD (Duke University Medical School, Durham, NC)
Dr. Robert Lefkowitz discussed his decades of work on the ubiquitous class of G-protein-coupled receptors, focusing particularly on his recent research on “biased ligands.” He explained that typically, when an agonist binds to its receptor, it leads to two responses – activation of the relevant G-protein and recruitment of another molecule that attenuates the signaling pathway after a few minutes. In some cases (like angiotensin signaling in hypertension), one of these responses is desired while the other is not, but most drugs that target these receptors block the entire receptor-ligand interaction, potentially leading to off-target effects. Dr. Lefkowitz’s group has found that it is possible to develop receptor agonist analogs that inhibit just one of the two responses by stabilizing the receptor in a particular conformation, and he believes this could lead to drugs with much more targeted, specific effects. Though this science is still in its infancy and deals with a very large class of receptors, it could be an interesting avenue to explore in the development of new diabetes drugs, as many G-protein-coupled receptors (such as GPR40) are involved in the regulation of islet cell function.
Plenary Session: Edwin B. Astwood Award Lecture
Mechanism of Insulin Action & the Pathogenesis of Diabetes with a Focus on Pancreatic Beta Cell Failure
Domenico Accili, MD (Columbia University, New York, NY)
Dr. Domenico Accili’s lecture on beta cell function in type 2 diabetes was one of many at this year’s ENDO, suggesting a growing consensus in the field that the next generation of therapies will need to address the underlying mechanism of beta cell failure. Dr. Accili explained that as we have learned more about the complex biology of type 2 diabetes, it has become evident that preserving beta cell function is the key to preventing susceptible individuals from progressing to later-stage type 2 diabetes. He presented substantial data from mice and some data from humans suggesting that beta cell de-differentiation, not apoptosis, is the main cause of beta cell failure in type 2 diabetes and that dedifferentiated beta cells often convert into other types of endocrine cells. He pointed to impaired metabolic flexibility as a harbinger of dedifferentiation, presenting evidence that these defective beta cells are unable to properly choose between glucose and lipids as a fuel source. When choosing among currently available treatments for type 2 diabetes, many leaders in the field have recommended favoring those that address insulin sensitivity over those that simply treat the symptom of hyperglycemia, but this science suggests that a therapy targeting beta cell dedifferentiation could be far more effective than any of the current options.
- Beta cell failure is the main feature that determines whether someone will progress to type 2 diabetes. Dr. Accili showed a graph of insulin secretion plotted against insulin sensitivity for patients with normal glucose tolerance over five years to demonstrate that a steep decline in insulin secretion was what distinguished those who progressed to type 2 diabetes from those who did not. Many of the “non-progressors” became just as insulin resistant as the “progressors,” but they maintained euglycemia due to an increase in compensatory beta cell function.
- Dr Accili argued that beta cell dedifferentiation, not apoptosis, is the central mechanism behind beta cell failure. Dr. Accili and others have demonstrated through lineage tracing experiments that in diabetic mice, beta cells do not die but rather trans-differentiate into other types of endocrine cells. In humans, experiments have shown that people with and without type 2 diabetes have the same total number of endocrine islet cells (as measured by synaptofizin reactivity), but people with diabetes have a smaller percentage of beta cells (as measured by reactivity to both synaptofizin and insulin), suggesting that their beta cells have converted to some other type of endocrine cell.
- Impaired metabolic flexibility may be an indication of beta cell failure. In mice induced to develop diabetes, beta cells display an inability to properly select substrates as fuel sources, as glucose oxidation is blunted and palmitate oxidation is greatly increased at low and high glucose concentrations.
Plenary Session: Roy O. Greep Award Lecture
Human Genetic Variation & the Inherited Basis of Type 2 Diabetes
David Altshuler, MD, PhD (Broad Institute, Boston, MA)
Roy O. Greep Award recipient Dr. David Altshuler delivered a stirring plenary lecture on the genetic basis of type 2 diabetes. Dr. Altshuler emphasized that studies of inheritability are incredibly cost-effective relative to clinical trials, and that prior to undertaking drug discovery, genetic assays can establish whether perturbation of an intended drug target results in the desired effect without toxicity (his message echoed his words from ADA 2012). In particular, his work using genome-wide association studies (GWAS) has identified 80 new genetic variants within the human genome that are thought to influence the development of type 2 diabetes. Provocatively, Dr. Altshuler noted that these regions include little overlap with previous “candidate” genes in the literature and suggested that some previously identified genetic risk factors are, in fact, artifacts. Instead, Dr. Altshuler offered a compelling argument in favor of more robust and reproducible genetic variants that appear to be significantly linked to the development of and protection from type 2 diabetes.
- Less than 5% of drugs that enter clinical trails make it to market, because “we don’t know how to pick targets that will be effective in mankind.” Dr. Altshuler noted that this inability to predict efficacy is responsible for the incredible costs of drug development. Identifying targets by understanding the consequences of perturbing particular targets before proceeding with drug development efforts can help reduce these enormous costs and improve the overall yield of drug development.
- Well over 500 genes have been prematurely proposed as risk factors for type 2 diabetes. Dr. Altshuler emphasized that recent population-based studies have identified 80 variant regions (“haplotypes”) of the human genome that are reliably and robustly associated with an increased risk of developing type 2 diabetes. These haplotypes do not overlap with previously identified gene candidates, leading Dr. Altshuler to imply that the majority of these risk factors may be unreliable markers.
- Sequence variations in SLC16A11 have been identified in Latin Americans populations that are significantly correlated with the prevalence of type 2 diabetes. This finding, reported by the SIGMA Type 2 Diabetes Consortium, involved a genetic study of 8,000 Mexicans and Latino Americans and identified four missense mutations within the risk haplotype, each copy of which is associated with a 25% increase in the risk of type 2 diabetes. Forced expression of SLC16A11 has been shown to increase triacylglycerol levels in liver cells (unpublished data), which is thought to influence the development of type 2 diabetes. Together, this haplotype may be able to explain part of the higher diabetes incidence in the Latino population.
- Dr. Altshuler’s group has identified multiple genes that, when inactivated, reduce the risk of developing type 2 diabetes. These genes were discovered by drawing 1,520 samples from a population of elderly, obese, euglycemic patients who, in theory, should have progressed to diabetes. Instead, Dr. Altshuler’s hypothesis was that these healthy subjects may share particular genes that mediate this protective effect. Notably, the group identified a loss-of-function, frame shift mutation in SLC30A8 that is associated with lower fasting plasma glucose levels and protection from type 2 diabetes, among other traits. This gene was one of 12 eventually identified genes that together were associated with a 65% reduction in the risk of type 2 diabetes (p < 0.000001).
- Are there more such protective mutations left to identify? Dr. Altshuler describe these early findings as “low hanging fruit,” citing that there remains great potential to identify genetic agents whose inactivation may reduce the risk of type 2 diabetes.
Symposium: Novel Mechanisms of Hepatic Metabolism
Cooperation Between Brain & Islet in Glucose Homeostasis & Diabetes
Michael Schwartz, MD (University of Washington, Seattle, WA)
Dr. Michael Schwartz emphasized the importance of the central nervous system (CNS) and insulin-independent mechanisms in the pathophysiology of type 2 diabetes, describing uncontrolled diabetes as a state of leptin deficiency as well as insulin deficiency and asserting that successful interventions must improve insulin-independent glucose disposal. He discussed data presented at this year’s ADA demonstrating that leptin infusions were sufficient to restore euglycemia in diabetic rats without any changes in insulin levels; he believes these findings will force the field to reconsider the usefulness of targeting insulin-independent mechanisms when developing treatments for type 2 diabetes. As an example, he highlighted FGF19, a hormone produced in the gut that has been shown to dramatically improve glucose tolerance when injected into the brains of diabetic mice, despite having no effect on insulin secretion or insulin resistance. Dr. Schwartz closed by presenting evidence that insulin can inhibit hepatic glucose production indirectly as well as directly, and he hypothesized that this unknown indirect mechanism is mediated by signaling in the brain. He believes that future research should focus on whether CNS control of glucose effectiveness is required for glycemic control in type 2 diabetes, what pathways are involved in the regulation of hepatic glucose production, and whether changes in CNS signals are responsible for remission of diabetes after bariatric surgery.
Symposium: Novel Adipokines
Endocrine Effects of Circulating DPP-4
Jürgen Eckel, PhD (Paul Langerhans Group, German Diabetes Center, Dusseldorf, Germany)
Dr. Jürgen Eckel presented intriguing evidence suggesting that DPP-4 could be a major link between obesity and the metabolic syndrome. It has been increasingly recognized in recent years that adipocytes are active endocrine cells with a critical role in metabolic regulation, and Dr. Eckel and his colleagues found that DPP-4, which cleaves GLP-1 and is a well-established target for type 2 diabetes drugs, is one of the major factors secreted by these cells. DPP-4 is best known for its inhibitory effects on incretins, but its ubiquitous expression and wide variety of potential substrates suggest that it may regulate metabolism via other mechanisms as well. Dr. Eckel presented data demonstrating that DPP-4 expression is very high in the visceral fat depots of obese subjects compared to non-obese subjects and that bariatric surgery leads to a significant decrease in its release from adipose tissue and concentration in the bloodstream. Dr. Eckel found that levels of DPP-4 are correlated with a subject’s risk score for developing metabolic syndrome, and additional studies have found that in obese children, whose DPP-4 levels are even higher than those of obese adults, increased DPP-4 might be a predictor of developing type 2 diabetes. Dr. Eckel also presented evidence to support his hypothesized mechanism of DPP-4 action on vascular smooth muscle – he believes it activates the extracellular signal-regulated kinase (ERK) signaling pathway via the protease-activated receptor 2 (PAR2) receptor, thereby increasing proliferation in the vessel wall and accelerating vascular damage. Overall, he concluded that DPP-4 may be a helpful new biomarker for metabolic dysfunction and a critical link between obesity and other metabolic disorders.
- DPP-4 is secreted by adipocytes and may exert widespread effects on metabolic regulation. Dr. Eckel’s group analyzed adipocytes in culture to identify potentially important proteins and were intrigued to find that DPP-4 was one of the major factors present. DPP-4 is an ubiquitously expressed protease that can cleave two specific amino acids off a variety of cell surface proteins; it is present in high amounts in plasma and body fluids and is secreted by immune cells as well as adipose tissue. It is best known for its rapid inactivation of incretin hormones, thanks to the popular class of type 2 diabetes drugs (DPP-4 inhibitors) that restores the glucose-lowering effects of GLP-1, but its wide variety of potential substrates suggest that it may regulate metabolic processes by other mechanisms as well.
- High levels of DPP-4 are correlated with obesity and may predict the development of metabolic syndrome and type 2 diabetes. Dr. Eckel presented data showing that DPP-4 expression is elevated in the visceral fat depots of obese subjects and that levels were particularly high in obese children. Additional data demonstrated that one year after bariatric surgery, DPP-4 release from adipose tissue explants was completely normalized and its concentration in the bloodstream was significantly lower, suggesting a potential mechanism behind the sustained metabolic changes, including remission of diabetes, often seen with this surgery. Levels of DPP-4 were significantly correlated with subjects’ risk score for metabolic syndrome in one clinical study, and the above analysis of obese children suggested that high DPP-4 levels may be a predictor of type 2 diabetes onset in that population.
- Evidence suggests that DPP-4 exerts direct, damaging effects on blood vessels in addition to its impairment of incretin signaling. Several studies have shown that DPP-4 activates the ERK signaling pathway, which leads to increased proliferation of smooth muscle cells in the vascular wall, and recent research by Dr. Eckel and colleagues has identified PAR2 as the key receptor that mediates this activation. Dr. Eckel suggested that this mechanism might lead to a vasoprotective effect (albeit one that was not seen to a significant extent in long-term outcomes studies), and it could also offer new targets for the development of drugs to protect against the micro- and macrovascular complications of diabetes.
Questions and Answers
Q: I’m intrigued by your observation that DPP-4 levels are so much higher in boys. Boys have more visceral fat than girls after puberty – do you think that could be the explanation?
A: No, everyone in the study was definitely pre-puberty.
Q: Were there differences between males and females in adults?
A: We haven’t really seen that, though there were slight differences in activity. Activity is definitely different with aging; in the elderly, there’s a clear decrease in activity but not abundance. The enzyme is glycosylated, etc., and it loses activity due to poorly understood mechanisms. But males and females have the same levels in the circulation.
-- by Melissa An, Adam Brown, Hannah Deming, Varun Iyengar, Emily Regier, Manu Venkat, and Kelly Close