β‐Cell research – A decade of rapid growth (2009)

This Supplement to Diabetes, Obesity and Metabolism presents the proceedings of the 10th Servier – IGIS Symposium entitled “A Decade of Islet Research: Implications for Understanding and Treating Type 2 Diabetes”, held on 26-29 March 2009, in St Jean Cap Ferrat, France. It was a somewhat special symposium, since IGIS (International Group on Insulin Secretion) was celebrating its 10th anniversary of symposia; therefore, we feel that a short glance back is justified. IGIS was formed in the late 1990's as a reaction to the perceived lack of interest in research on the β-cell dysfunction of type 2 diabetes (T2D). Indeed, the 1980s and 1990s were the times when T2D was regarded as a purely insulin resistance disease, and few among the leading diabetes research groups thought the β-cell a worthy study object, let alone a prime target for diabetes drug development. It is extraordinary to see the radical change brought about by the past decade; even the staunchest proponents of insulin resistance agree that T2D does not develop unless the β-cell fails. More importantly, many leading research groups that in the past exclusively worked on insulin action have now extended their efforts to include the β-cell. As a result, the number of publications on the role of the β-cell in T2D has rapidly increased: a search for “β-cell and type 2 diabetes” for the decade August 1989–July 1999 gave 734 titles, while for the following decade the number of publications was 3321, i.e. a 353% increase over the former. Perish the thought that IGIS has been a critical instrument for this development! Nevertheless, we do claim a small portion of the credit via the interest that all Servier-IGIS Symposia have generated so far. These symposia are conceived by the IGIS Board and materialised through the generous unrestricted educational grant and logistical support of Servier. The IGIS Board is entering its second decade by partially refreshing itself: Suad Efendic and Don Steiner are leaving (Ele Ferannini left earlier), and they are being replaced by Domenico Accili, Bo Ahrén and Susumu Seino, while Christian Boitard, Erol Cerasi and Jean-Claude Henquin continue their mandate. We certainly hope that the new Board will be able to maintain the high standards of the Servier-IGIS Symposia, in the face of an ever more sophisticated and vast biomedical research production. The past decade has indeed been formidable from the viewpoint of biomedical research in general, and islet research in particular. Just think what the field of genomics alone has brought, from “simple” differential displays to whole genome association studies and sequencing. Also ten years ago we were all convinced that T2D is a polygenic disease, but who among us would have bet on TCF7L2 then? We were expecting to find mutations in our regular pet genes, the insulin gene, the insulin receptor gene or the like… It has not been easy to liberate oneself from “candidate-directed” hypothesis generation and research planning; many of us discovered in this decade that our open-mindedness had serious limits! But we are not “negationists” either: classical cell biology and clinical investigation have more than ever their place in creative diabetes research. IGIS 10 is an excellent example of the fine balance between “old” and “new” schools of research as applied to the β-cell. Indeed, much of our present thinking on the pathophysiology of T2D is based on dated work performed with much less sophisticated methodology. Thus, β-cell dysfunction as the key mechanism underlying T2D was already suggested in the 1960's, when it was shown that subjects with diabetes have a low insulin response to glucose infusion [1]. Furthermore, in the 1980's it became apparent that β-cell sensitivity to glucose is severely reduced, further supporting the β-cell dysfunction hypothesis for T2D [2]. An important further contribution was the demonstration of the reciprocal relationship between insulin sensitivity and insulin secretion [3]. This shows that if the β-cells respond and adapt normally to insulin resistance, diabetes is avoided, whereas an inadequate adaptation results in glucose intolerance. A consequence of this finding is that β-cell insufficiency must be crucial for the development of glucose intolerance, and that even when insulin secretion appears high, β-cells may be failing because the insulin secretion may still be insufficient for the demand imposed by the insulin resistance. This in turn necessitates assessment of the insulin secretion in relation to ambient insulin sensitivity for an accurate estimation of β-cell function in clinical studies [4]. Doing this, it has been shown that reduced β-cell function is a hallmark for the dysregulated glucose metabolism of T2D [5], gestational diabetes [6] and impaired glucose tolerance (IGT) [7]. It has also been shown that β-cell dysfunction is an early phenomenon and antedates the onset of T2D and IGT, and may thus be a marker of diabetes risk, as demonstrated in Pima Indians [8] and Caucasians [9;10]. During recent years, particular attention has been paid towards the regulation of β-cell mass. It is indeed crucial to determine the relative contribution of changes in β-cell mass and individual β-cell function, both during adaptation to insulin resistance by increased insulin production and during the insufficient insulin production characteristic of all stages of T2D [11;12]. Today in 2009 we have passed the era of questioning whether β-cell dysfunction is the key defect in the development of T2D. Rather, focus of current research is on clarifying the mechanisms both of the β-cell adaptation to insulin resistance and of the β-cell failure in T2D. Understanding these mechanisms will provide solutions to an important “grand challenge”: development of a treatment based on the pathophysiology of T2D which might prevent progression of the disease and hopefully revert it. In the 10th Servier-IGIS Symposium, several new findings on β-cell mechanisms whose derangement may lead to T2D were presented; these are the focus of the 21 papers within this Supplement. They all present novel findings on β-cell biology, be it on the physiology of the islet or more directly related to understanding the mechanisms of β-cell dysfunction during the evolution of T2D. It is not the purpose of this Editorial to summarise here all the papers of this supplement; we intend to cite a few salient aspects to emphasise what has been said above. It must be reminded that the β-cell usually adapts its function (and to a certain extent its mass) quite well to insulin resistance, since the vast majority of obese subjects and persons with other insulin-resistant states do not develop T2D. In this Supplement the hypothesis is presented that the mTOR system plays a key role for β-cell adaptation to insulin resistance. It was thus demonstrated that while inhibition of mTOR did not modify blood glucose in normal animals, it caused progression of the diabetic state in Psammomys obesus fed a high calorie diet, a model of nutrition-dependent T2D, by reducing plasma insulin due to loss of β-cells. On the other hand, under exposure to high glucose and FFA (gluco-lipotoxicity), blocking mTOR in vitro reduced ER stress and β-cell death, both in P. obesus islets and INS-1 β-cells, which suggests a dual role of the mTOR system. This certainly requires further studies, especially in vivo, given the availability of drugs that affect mTOR activity. In this context it is of interest to remind that FFAs, branded as the culprit-partner of gluco-lipotoxicity, as in the case of glucose itself, if kept within the physiological time- and concentration-limits of exposure are beneficial to β-cell function (enhancement of glucose-induced insulin secretion). Data is presented here to indicate that the positive effects of FFA are transmitted to an equal extent by the metabolism of the FFA and its interaction with several plasma membrane G-protein coupled receptors (GPRs) in the β-cell, mainly GPR40, though others are also implicated to a certain degree. What about lipotoxicity? There is controversy as to whether GPR40 also conveys the long-term negative effects of FFA; nevertheless, the dominating view, expressed in this Supplement, is that GPR40 is not involved in lipotoxicity since GPR40-overexpressing transgenic mice are not more sensitive to high-fat diet-induced diabetes, and reduction of GPR40 expression by siRNA technology does not protect β-cells from dysfunction when exposed chronically to high FFA concentrations. Adaptation to insulin resistance requires not only augmented function but probably also increased β-cell mass. The role of telomerases in β-cell regeneration is discussed in this Supplement. Telomeres are specialized chromatin structures that cap the distal ends of eukaryotic chromosomes and prevent the recognition of chromosomal ends as double-stranded DNA breaks, thus protecting these regions; telomerases are involved in their synthesis. Telomeres also allow the attachment of the chromosome to the nuclear envelope during meiosis; it is known that they play a critical role in the regulation of cell proliferation, differentiation and survival in several cell systems. It is shown here that the islet expresses the gene coding for telomerase (mTERT), and that mTERT expression in islets from LIRKO mice, where β-cells show a high degree of proliferation, was 2-fold increased whereas mTERT expression was reduced in hypo-proliferative cyclin D2−/− islets; telomerase activity parallelled these changes. These data indicate that telomerase is expressed and active in pancreatic β-cells and that its expression level, as well as telomere length, correlates with the replicative capability of β-cells. Another factor of major importance for β-cell regeneration, discussed here, is the transcription factor PDX-1. PDX-1 is a master-regulator of a long list of β-cell functions: it is a sine qua non factor for the embryonic development of the pancreas and the islet, for development of β-cells, and for normal β-cell function in post-natal life, including mediation of glucose-regulated proinsulin biosythesis. In addition, PDX-1 is a critical regulator of β-cell replication during the compensatory response to insulin resistance, as was demonstrated in IRS1-deficient mice. These insulin resistant animals develop diabetes only when crossed with PDX-1-deficient mice in which β-cell mass does not augment in response to insulin resistance. The role of PDX-1 in β-cell pathophysiology is not restricted to this adaptive regeneration, since all the known noxia of the diabetic state, e.g. chronic elevation of glucose or FFA, reduce the expression of PDX-1 and thus lead to a “functional de-differentiation” of the β-cell. A further key-player transcription factor involved in β-cell regeneration is Foxo1. As discussed in this Volume, in β-cells Foxo1 regulates cell replication downstream of IRS2 and Akt. Indeed, it has been demonstrated that Foxo1 is a negative regulator of β-cell mass both physiologically, and in response to metabolic cues such as insulin resistance, or pharmacological agents like GLP-1 receptor agonists. Thus, reduced Foxo1 function is needed to achieve β-cell compensation in insulin resistance. However, Foxo1 has an important protective function against the stress of diabetes, since it translocates to the nucleus in response to oxidative stress, and activates a transcriptional program termed “metabolic diapause”. This allows preserved insulin secretion, but decreased proliferation and intracellular glucose metabolism, with increased FFA oxidation. Again we see an example of the duality of effect found in many factors in the β-cell that, according to the environmental/cell metabolic context, can be beneficial or deleterious for β-cell function or survival. The finding that β-cell mass is diminished in overt T2DM has stimulated intensive research to understand its mechanisms, as well as to assess the potential for developing novel therapies aimed at increasing the β-cell mass. The mechanisms of β-cell regeneration are, however, far from clear. There are two major pathways to β-cell regeneration, cell replication and β-cell neogenesis; both processes seem to be of importance in different models. While β-cell replication clearly occurs during development and early in life, the potential for replication appears to decline substantially with age. It is discussed here that the choice of β-cell mass increase by neogenesis or by replication depends on the intensity of different stimuli or stressors. This statement is based on findings from in vivo models of pancreatic injury and pancreatitis, where clearly neogenesis from stem or progenitor cells does occur, provided a potent stimulus is present to drive a quiescent endocrine progenitor into an activated one. The possibility is envisaged that in vitro monolayer cultures of adult human pancreatic exocrine cells may produce the stimulus required to activate quiescent adult pancreatic progenitor cells to differentiate towards an endocrine fate. Development of therapies based on this suggestion will be contingent on unravelling the nature of the stimuli and the mechanisms by which they promote endocrine differentiation, as well as identifying the nature of progenitor cells; this is central for success in therapeutic approaches aimed to induce significant endogenous β-cell neogenesis. In the evolution of T2D a slow but progressive decline in β-cell function is usually seen, presumably caused by the toxic action of glucose and lipids on the β-cell (gluco-lipotoxicity). In this Supplement, a study of the mechanisms contributing to the slow deterioration of the functionalβ-cell mass in T2D focused on altered β-cell gene expression induced by glucose. Glucotoxic alterations were found to be dependent on oxidative stress, endoplasmic reticulum stress, cytokine-activated apoptosis, and hypoxia. In particular, it was shown that oxidative stress- and integrated stress-response genes in cultured rat islets paralleled β-cell apoptosis, which may suggest that these types of stresses play a role in reducing the β-cell mass in hyperglycaemia. However, β-cell hypoxia, and inflammatory cytokines released locally by non-endocrine islet cells may also contribute. It is of interest that a low glucose concentration may be as or even more deleterious to the β-cell than a hyperglycaemic condition. Certainly, a better understanding of the relative contribution of these mechanisms is necessary to fully characterize mechanistically the slow deterioration of the functional β-cell mass in T2D. However, there exist many difficulties in all present studies where islets isolated from rodents, or human islets obtained from cadavers are utilised. The isolation procedures exert a substantial stress on the β-cell, and in addition, what is studied in reality is the composite response of all cells present in an islet, the heterogeneous endocrine cell population as well as non-endocrine cells like endothelial cells, fibroblasts etc. The latter drawback can be circumvented, e.g. for gene array studies, by using β-cell dissection with laser capture microscopy. It is shown here that gene array results obtained with this technique may be quite different from those in whole islets exposed to glucotoxic conditions. Nevertheless, the ideal for understanding alterations in the human β-cell should be to apply laser capture microscopy to specimens freshly obtained during pancreatic surgery. This is not an insurmountable task. What are the implications of these progresses for the treatment of our patients? It all depends whether we look at the empty or full part of the half-filled glass. On the one hand, we have impressive and sophisticated promises of novel therapies; take for instance stem- or progenitor cell-derived unlimited β-cell generation for transplantation, or better, stimulation of endogenous progenitor cells for β-cell regeneration, or miRNA-based therapies; these are hardly in the pipeline of pharmaceutical industry and therefore presently irrelevant for T2D patients. The latter, miRNAs, are really novel. These are short RNA molecules that can modulate the expression of the majority of genes by repressing translation of the encoded protein. As described in extenso in the present Supplement, also in the β-cell miRNAs play an important role in physiology and in T2D. Thus, miR-375, which is abundant in β-cells, reduces glucose-stimulated insulin secretion, but is a positive regulator of β-cell growth, since genetic deletion of miR-375 in ob/ob mice prevented the compensatory β-cell mass expansion in response to insulin resistance, and hence led to severe diabetes. Several other miRNAs are also expressed in the β-cell; gluco-lipotoxic conditions seem to augment their expression and thus impair β-cell function. Attempts have been initiated to generate pharmacological and genetic means of controlling miRNA expression in order to counteract the β-cell dysfunction; a sincere wish is that coming years surprise us with novel classes of antidiabetic drugs acting on miRNA-based with the are presently approaches to these are described in in this Supplement. The results of are especially by far to all other therapies It is probably the of the that the of in diabetes There are two that may be from this that it is to T2D in the of severe and insulin resistance, and that following blood glucose is long reduction of due to β-cell function, which if as a of the central role of β-cell dysfunction in the pathophysiology of T2D. In insulin resistance also within if not These effects of the different procedures seem to to the of the GLP-1 and as well as of less and other that insulin This is an of research to be of what has been discussed in the to in β-cell function and survival the deleterious metabolic of T2D is to the of chronic with high FFA on the is this in other does T2D in a possibility is that with the of insulin resistance due to all the mechanisms envisaged at a low level, but over years or by β-cell survival. 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