Mechanisms of Beta-Cell Apoptosis in Type 2 Diabetes-Prone Situations and Potential Protection by GLP-1-Based Therapies.

Type 2 diabetes (T2D) is characterized by chronic hyperglycemia secondary to the decline of functional beta-cells and is usually accompanied by a reduced sensitivity to insulin. Whereas altered beta-cell function plays a key role in T2D onset, a decreased beta-cell mass was also reported to contribute to the pathophysiology of this metabolic disease. The decreased beta-cell mass in T2D is, at least in part, attributed to beta-cell apoptosis that is triggered by diabetogenic situations such as amyloid deposits, lipotoxicity and glucotoxicity. In this review, we discussed the molecular mechanisms involved in pancreatic beta-cell apoptosis under such diabetes-prone situations. Finally, we considered the molecular signaling pathways recruited by glucagon-like peptide-1-based therapies to potentially protect beta-cells from death under diabetogenic situations.

ERK1 is dispensable for mouse pancreatic beta cell function but is necessary for glucose-induced full activation of MSK1 and CREB.

AIMS/HYPOTHESIS: Insufficient insulin secretion from pancreatic beta cells, which is associated with a decrease in beta cell mass, is a characteristic of type 2 diabetes. Extracellular signal-related kinase 1 and 2 (ERK1/2) inhibition in beta cells has been reported to affect insulin secretion, gene transcription and survival, although whether ERK1 and ERK2 play distinct roles is unknown. The aim of this study was to assess the individual roles of ERK1 and ERK2 in beta cells using ERK1 (also known as Mapk3)-knockout mice (Erk1 -/- mice) and pharmacological approaches. METHODS: NAD(P)H, free cytosolic Ca2+ concentration and insulin secretion were determined in islets. ERK1 and ERK2 subplasmalemmal translocation and activity was monitored using total internal reflection fluorescence microscopy. ERK1/2, mitogen and stress-activated kinase1 (MSK1) and cAMP-responsive element-binding protein (CREB) activation were evaluated by western blot and/or immunocytochemistry. The islet mass was determined from pancreatic sections. RESULTS: Glucose induced rapid subplasmalemmal recruitment of ERK1 and ERK2. When both ERK1 and ERK2 were inhibited simultaneously, the rapid transient peak of the first phase of glucose-induced insulin secretion was reduced by 40% (p < 0.01), although ERK1 did not appear to be involved in this process. By contrast, ERK1 was required for glucose-induced full activation of several targets involved in beta cell survival; MSK1 and CREB were less active in Erk1 -/- mouse beta cells (p < 0.01) compared with Erk1 +/+ mouse beta cells, and their phosphorylation could only be restored when ERK1 was re-expressed and not when ERK2 was overexpressed. Finally, the islet mass of Erk1 -/- mice was slightly increased in young animals (4-month-old mice) vs Erk1 +/+ mice (section occupied by islets [mean ± SEM]: 0.74% ± 0.03% vs 0.62% ± 0.04%; p < 0.05), while older mice (10 months old) were less prone to age-associated pancreatic peri-insulitis (infiltrated islets [mean ± SEM]: 7.51% ± 1.34% vs 2.03% ± 0.51%; p < 0.001). CONCLUSIONS/INTERPRETATION: ERK1 and ERK2 play specific roles in beta cells. ERK2 cannot always compensate for the lack of ERK1 but the absence of a clear-cut phenotype in Erk1 -/- mice shows that ERK1 is dispensable in normal conditions.

GLP-1 and GIP receptors signal through distinct ß-arrestin 2-dependent pathways to regulate pancreatic ß cell function.

Glucagon-like peptide 1 (GLP-1R) and glucose-dependent insulinotropic polypeptide (GIPR) receptors are G-protein-coupled receptors involved in glucose homeostasis. Diabetogenic conditions decrease ß-arrestin 2 (ARRB2) levels in human islets. In mouse ß cells, ARRB2 dampens insulin secretion by partially uncoupling cyclic AMP (cAMP)/protein kinase A (PKA) signaling at physiological doses of GLP-1, whereas at pharmacological doses, the activation of extracellular signal-related kinase (ERK)/cAMP-responsive element-binding protein (CREB) requires ARRB2. In contrast, GIP-potentiated insulin secretion needs ARRB2 in mouse and human islets. The GIPR-ARRB2 axis is not involved in cAMP/PKA or ERK signaling but does mediate GIP-induced F-actin depolymerization. Finally, the dual GLP-1/GIP agonist tirzepatide does not require ARRB2 for the potentiation of insulin secretion. Thus, ARRB2 plays distinct roles in regulating GLP-1R and GIPR signaling, and we highlight (1) its role in the physiological context and the possible functional consequences of its decreased expression in pathological situations such as diabetes and (2) the importance of assessing the signaling pathways engaged by the agonists (biased/dual) for therapeutic purposes.

The nuclear receptor REV-ERBα is implicated in the alteration of ß-cell autophagy and survival under diabetogenic conditions.

Pancreatic ß-cell failure in type 2 diabetes mellitus (T2DM) is associated with impaired regulation of autophagy which controls ß-cell development, function, and survival through clearance of misfolded proteins and damaged organelles. However, the mechanisms responsible for defective autophagy in T2DM ß-cells remain unknown. Since recent studies identified circadian clock transcriptional repressor REV-ERBα as a novel regulator of autophagy in cancer, in this study we set out to test whether REV-ERBα-mediated inhibition of autophagy contributes to the ß-cell failure in T2DM. Our study provides evidence that common diabetogenic stressors (e.g., glucotoxicity and cytokine-mediated inflammation) augment ß-cell REV-ERBα expression and impair ß-cell autophagy and survival. Notably, pharmacological activation of REV-ERBα was shown to phenocopy effects of diabetogenic stressors on the ß-cell through inhibition of autophagic flux, survival, and insulin secretion. In contrast, negative modulation of REV-ERBα was shown to provide partial protection from inflammation and glucotoxicity-induced ß-cell failure. Finally, using bioinformatic approaches, we provide further supporting evidence for augmented REV-ERBα activity in T2DM human islets associated with impaired transcriptional regulation of autophagy and protein degradation pathways. In conclusion, our study reveals a previously unexplored causative relationship between REV-ERBα expression, inhibition of autophagy, and ß-cell failure in T2DM.

Calcium-activated calpain-2 is a mediator of beta cell dysfunction and apoptosis in type 2 diabetes.

The islet in type 2 diabetes (T2DM) and the brain in neurodegenerative diseases share progressive cell dysfunction, increased apoptosis, and accumulation of locally expressed amyloidogenic proteins (islet amyloid polypeptide (IAPP) in T2DM). Excessive activation of the Ca(2+)-sensitive protease calpain-2 has been implicated as a mediator of oligomer-induced cell death and dysfunction in neurodegenerative diseases. To establish if human IAPP toxicity is mediated by a comparable mechanism, we overexpressed human IAPP in rat insulinoma cells and freshly isolated human islets. Pancreas was also obtained at autopsy from humans with T2DM and nondiabetic controls. We report that overexpression of human IAPP leads to the formation of toxic oligomers and increases beta cell apoptosis mediated by increased cytosolic Ca(2+) and hyperactivation of calpain-2. Cleavage of alpha-spectrin, a marker of calpain hyperactivation, is increased in beta cells in T2DM. We conclude that overactivation of Ca(2+)-calpain pathways contributes to beta cell dysfunction and apoptosis in T2DM.

GLP-1 mediates antiapoptotic effect by phosphorylating Bad through a beta-arrestin 1-mediated ERK1/2 activation in pancreatic beta-cells.

Strategies based on activating GLP-1 receptor (GLP-1R) are intensively developed for the treatment of type 2 diabetes. The exhaustive knowledge of the signaling pathways linked to activated GLP-1R within the beta-cells is of major importance. In beta-cells, GLP-1 activates the ERK1/2 cascade by diverse pathways dependent on either Galpha(s)/cAMP/cAMP-dependent protein kinase (PKA) or beta-arrestin 1, a scaffold protein. Using pharmacological inhibitors, beta-arrestin 1 small interfering RNA, and islets isolated from beta-arrestin 1 knock-out mice, we demonstrate that GLP-1 stimulates ERK1/2 by two temporally distinct pathways. The PKA-dependent pathway mediates rapid and transient ERK1/2 phosphorylation that leads to nuclear translocation of the activated kinases. In contrast, the beta-arrestin 1-dependent pathway produces a late ERK1/2 activity that is restricted to the beta-cell cytoplasm. We further observe that GLP-1 phosphorylates the cytoplasmic proapoptotic protein Bad at Ser-112 but not at Ser-155. We find that the beta-arrestin 1-dependent ERK1/2 activation engaged by GLP-1 mediates the Ser-112 phosphorylation of Bad, through p90RSK activation, allowing the association of Bad with the scaffold protein 14-3-3, leading to its inactivation. beta-Arrestin 1 is further found to mediate the antiapoptotic effect of GLP-1 in beta-cells through the ERK1/2-p90RSK-phosphorylation of Bad. This new regulatory mechanism engaged by activated GLP-1R involving a beta-arrestin 1-dependent spatiotemporal regulation of the ERK1/2-p90RSK activity is now suspected to participate in the protection of beta-cells against apoptosis. Such signaling mechanism may serve as a prototype to generate new therapeutic GLP-1R ligands.

Methods to Study Roles of ß-Arrestins in the Regulation of Pancreatic ß-Cell Function.

Novel findings reveal important functional roles for ß-arrestin 1 and ß-arrestin 2 in the regulation of insulin secretion, ß-cell survival, and ß-cell mass plasticity not only by glucose but also by G-protein-coupled receptors, such as the glucagon-like peptide-1 (GLP-1) and the pituitary adenylate cyclase-activating polypeptide (PACAP) receptors or GPR40, or tyrosine kinase receptors, such as the insulin receptor. Here, we describe experimental protocols to knock down ß-arrestins by small interference RNA, to follow subcellular localization of ß-arrestins in the cytosol and nucleus of the insulinoma INS-1E rat pancreatic ß-cell line, and to analyze ß-arrestin protein expression by Western blot using INS-1E cells and isolated mouse or human pancreatic islets. We also provide details on how to genotype ß-arrestin 2 knockout (Arrb2-/-) mice and to evaluate ß-arrestin-mediated roles in ß-cell mass plasticity and ß-cell signaling using immunocytochemistry on pancreatic sections or on primary dispersed ß-cells from wild-type mice and Arrb2-/- mice.

Targeting protein misfolding to protect pancreatic beta-cells in type 2 diabetes.

The islet in type 2 diabetes is characterized by beta-cell dysfunction and deficit, increased beta-cell apoptosis and amyloid deposits that derived from islet amyloid polypeptide (IAPP). In species such as humans that are vulnerable to developing type 2 diabetes, IAPP has the propensity to form toxic oligomers that contribute to beta-cell dysfunction and apoptosis, defining type 2 diabetes as a protein misfolding disorder. In this report, we review mechanisms known to contribute to protein misfolding and formation of toxic oligomers, and the deleterious consequences of these oligomers on beta-cell function and survival. Finally, we will consider approaches to prevent protein misfolding and formation of toxic oligomers as potential novel therapeutic targets for type 2 diabetes and other protein misfolding diseases.

Proteasomal degradation of the histone acetyl transferase p300 contributes to beta-cell injury in a diabetes environment.

In type 2 diabetes, amyloid oligomers, chronic hyperglycemia, lipotoxicity, and pro-inflammatory cytokines are detrimental to beta-cells, causing apoptosis and impaired insulin secretion. The histone acetyl transferase p300, involved in remodeling of chromatin structure by epigenetic mechanisms, is a key ubiquitous activator of the transcriptional machinery. In this study, we report that loss of p300 acetyl transferase activity and expression leads to beta-cell apoptosis, and most importantly, that stress situations known to be associated with diabetes alter p300 levels and functional integrity. We found that proteasomal degradation is the mechanism subserving p300 loss in beta-cells exposed to hyperglycemia or pro-inflammatory cytokines. We also report that melatonin, a hormone produced in the pineal gland and known to play key roles in beta-cell health, preserves p300 levels altered by these toxic conditions. Collectively, these data imply an important role for p300 in the pathophysiology of diabetes.

ERK1/2 control phosphorylation and protein level of cAMP-responsive element-binding protein: a key role in glucose-mediated pancreatic beta-cell survival.

cAMP-responsive element-binding protein (CREB) is required for beta-cell survival by regulating expression of crucial genes such as bcl-2 and IRS-2. Using MIN6 cells and isolated rat pancreatic islets, we investigated the signaling pathway that controls phosphorylation and protein level of CREB. We observed that 10 mmol/l glucose-induced CREB phosphorylation was totally inhibited by the protein kinase A (PKA) inhibitor H89 (2 micromol/l) and reduced by 50% with the extracellular signal-regulated kinase (ERK)1/2 inhibitor PD98059 (20 micromol/l). This indicates that ERK1/2, reported to be located downstream of PKA, participates in the PKA-mediated CREB phosphorylation elicited by glucose. In ERK1/2-downregulated MIN6 cells by siRNA, glucose-stimulated CREB phosphorylation was highly reduced and CREB protein content was decreased by 60%. In MIN6 cells and islets cultured for 24-48 h in optimal glucose concentration (10 mmol/l), which promotes survival, blockade of ERK1/2 activity with PD98059 caused a significant decrease in CREB protein level, whereas CREB mRNA remained unaffected (measured by real-time quantitative PCR). This was associated with loss of bcl-2 mRNA and protein contents, caspase-3 activation, and emergence of ultrastructural apoptotic features detected by electron microscopy. Our results indicate that ERK1 and -2 control the phosphorylation and protein level of CREB and play a key role in glucose-mediated pancreatic beta-cell survival.

The glucagon-miniglucagon interplay: a new level in the metabolic regulation.

Miniglucagon (glucagon 19-29) is the ultimate processing product of proglucagon, present in the glucagon-secreting granules of the alpha cells, at a close vicinity of the insulin-secreting beta cells. Co-released with glucagon and thanks to its original mode of action and its huge potency, it suppresses, inside the islet of Langerhans, the detrimental effect of glucagon on insulin secretion, while it leaves untouched the beneficial effect of glucagon on glucose competence of the beta cell. At the periphery, miniglucagon is processed at the surface of glucagon- and insulin-sensitive cells from circulating glucagon. At that level, it acts via a cellular pathway which uses initial molecular steps distinct from that of insulin which, when impaired, are involved in insulin resistence. This bypass allows miniglucagon to act as an insulin-like component, a characteristic which makes this peptide of particular interest from a pathophysiological and pharmacological point of views in understanding and treating metabolic diseases, such as the type 2 diabetes.

ß Cell-specific increased expression of calpastatin prevents diabetes induced by islet amyloid polypeptide toxicity.

The islet in type 2 diabetes (T2D) shares many features of the brain in protein misfolding diseases. There is a deficit of ß cells with islet amyloid derived from islet amyloid polypeptide (IAPP), a protein coexpressed with insulin. Small intracellular membrane-permeant oligomers, the most toxic form of IAPP, are more frequent in ß cells of patients with T2D and rodents expressing human IAPP. ß Cells in T2D, and affected cells in neurodegenerative diseases, share a comparable pattern of molecular pathology, including endoplasmic reticulum stress, mitochondrial dysfunction, attenuation of autophagy, and calpain hyperactivation. While this adverse functional cascade in response to toxic oligomers is well described, the sequence of events and how best to intervene is unknown. We hypothesized that calpain hyperactivation is a proximal event and tested this in vivo by ß cell-specific suppression of calpain hyperactivation with calpastatin overexpression in human IAPP transgenic mice. ß Cell-specific calpastatin overexpression was remarkably protective against ß cell dysfunction and loss and diabetes onset. The critical autophagy/lysosomal pathway for ß cell viability was protected with calpain suppression, consistent with findings in models of neurodegenerative diseases. We conclude that suppression of calpain hyperactivation is a potentially beneficial disease-modifying strategy for protein misfolding diseases, including T2D.

Extracellularly regulated kinases 1/2 (p44/42 mitogen-activated protein kinases) phosphorylate synapsin I and regulate insulin secretion in the MIN6 beta-cell line and islets of Langerhans.

The p44/p42 MAPKs (ERK1/2) cascade regulates beta-cell nuclear events, which modulates cell differentiation and gene transcription, whereas its implication in processes occurring in the cytoplasm, such as activation of the exocytotic machinery, is still unclear. Using the MIN6 beta-cell line and isolated rat islets of Langerhans, we investigated whether glucose, by activating the ERK1/2 cascade, induces phosphorylation of cytoplasmic proteins implicated in exocytosis of insulin granules such as synapsin I. We observed that the majority of ERK1/2 activity induced by glucose remains in the cytoplasm and physically interacts with synapsin I, allowing phosphorylation of the substrate. Therefore, we reexamined the potential requirement of ERK1/2 for insulin secretion. Blocking activation of ERK1/2 using MEK1/2, the MAPK kinase inhibitor PD98059 or using small interfering RNA-mediated silencing of ERK1 and ERK2 expressions resulted in partial inhibition of glucose-induced insulin release, indicating that ERK1/2 pathway participates also in the regulation of insulin secretion. Moreover, using the pancreatic islet perifusion model, we found that the ERK1/2 activity participates in the first and second phases of insulin release induced by glucose. Taken together, our results demonstrate new aspects of the glucose-dependent actions of ERK1/2 in beta-cells exerted on cytoplasmic proteins, including synapsin I, and participating in the overall glucose-induced insulin secretion.

Activation of Melatonin Signaling Promotes ß-Cell Survival and Function.

Type 2 diabetes mellitus (T2DM) is characterized by pancreatic islet failure due to loss of ß-cell secretory function and mass. Studies have identified a link between a variance in the gene encoding melatonin (MT) receptor 2, T2DM, and impaired insulin secretion. This genetic linkage raises the question whether MT signaling plays a role in regulation of ß-cell function and survival in T2DM. To address this postulate, we used INS 832/13 cells to test whether activation of MT signaling attenuates proteotoxicity-induced ß-cell apoptosis and through which molecular mechanism. We also used nondiabetic and T2DM human islets to test the potential of MT signaling to attenuate deleterious effects of glucotoxicity and T2DM on ß-cell function. MT signaling in ß-cells (with duration designed to mimic typical nightly exposure) significantly enhanced activation of the cAMP-dependent signal transduction pathway and attenuated proteotoxicity-induced ß-cell apoptosis evidenced by reduced caspase-3 cleavage (∼40%), decreased activation of stress-activated protein kinase/Jun-amino-terminal kinase (∼50%) and diminished oxidative stress response. Activation of MT signaling in human islets was shown to restore glucose-stimulated insulin secretion in islets exposed to chronic hyperglycemia as well as in T2DM islets. Our data suggest that ß-cell MT signaling is important for the regulation of ß-cell survival and function and implies a preventative and therapeutic potential for preservation of ß-cell mass and function in T2DM.

Cooperative effects between protein kinase A and p44/p42 mitogen-activated protein kinase to promote cAMP-responsive element binding protein activation after beta cell stimulation by glucose and its alteration due to glucotoxicity.

Long-term hyperglycemia, a major characteristic of the diabetic state, contributes to the deterioration of the beta cell function, a concept known as beta cell glucotoxicity. We used the MIN6 beta cell line and isolated rat islets to clarify the signaling mechanism(s) used by glucose to activate cAMP-responsive element binding protein (CREB), a transcription factor crucial for beta cell biology, and to evaluate the possible downregulation of this mechanism mediated by long-term hyperglycemia. We report that glucose (10 mM) induces an increase in cytosolic calcium concentration that leads to cAMP-induced protein kinase A (PKA) activation, promoting nuclear translocation of activated ERK1/2. The observation that glucose-induced CREB phosphorylation was totally inhibited by the PKA inhibitor H89 (2 microM) and reduced by 50% with the ERK1/2 inhibitor PD98059 (20 microM) indicates that ERK1/2, located downstream of PKA, cooperates with PKA and is responsible for half of the PKA-mediated CREB phosphorylation elicited by glucose in MIN6 beta cells. We also found that exposure of mu cells for 24 h to high glucose (25 mM) induced a 70% decrease in cellular ERK1/2 and a 50% decrease in CREB content. In high-glucose-treated, ERK1/2- and CREB-downregulated beta cells, there was a loss of glucose (10 mM, 5 min)-stimulated ERK1/2 and CREB phosphorylation that was associated with nuclear apoptotic characteristics. Since we have shown that activation of ERK1/2 is crucial for CREB phosphorylation, loss of the ERK1/2-CREB signaling pathway in beta cells due to long-term hyperglycemia is likely to exacerbate beta cell failure in diabetic states by affecting physiologically relevant gene expression and by inducing apoptosis.

Insulin-degrading enzyme inhibition, a novel therapy for type 2 diabetes?

The insulin-degrading enzyme (IDE) has been identified as a type 2 diabetes and Alzheimer's disease susceptibility gene, though its physiological function remains unclear. Maianti et al. (2014) now propose that an IDE inhibitor may be a promising therapeutic strategy for type 2 diabetes.

UCHL1 deficiency exacerbates human islet amyloid polypeptide toxicity in ß-cells: evidence of interplay between the ubiquitin/proteasome system and autophagy.

The islet in type 2 diabetes mellitus (T2DM) is characterized by a deficit in ß-cells and increased ß-cell apoptosis attributable at least in part to intracellular toxic oligomers of IAPP (islet amyloid polypeptide). ß-cells of individuals with T2DM are also characterized by accumulation of polyubiquitinated proteins and deficiency in the deubiquitinating enzyme UCHL1 (ubiquitin carboxyl-terminal esterase L1 [ubiquitin thiolesterase]), accounting for a dysfunctional ubiquitin/proteasome system. In the present study, we used mouse genetics to elucidate in vivo whether a partial deficit in UCHL1 enhances the vulnerability of ß-cells to human-IAPP (hIAPP) toxicity, and thus accelerates diabetes onset. We further investigated whether a genetically induced deficit in UCHL1 function in ß-cells exacerbates hIAPP-induced alteration of the autophagy pathway in vivo. We report that a deficit in UCHL1 accelerated the onset of diabetes in hIAPP transgenic mice, due to a decrease in ß-cell mass caused by increased ß-cell apoptosis. We report that UCHL1 dysfunction aggravated the hIAPP-induced defect in the autophagy/lysosomal pathway, illustrated by the marked accumulation of autophagosomes and cytoplasmic inclusions positive for SQSTM1/p62 and polyubiquitinated proteins with lysine 63-specific ubiquitin chains. Collectively, this study shows that defective UCHL1 function may be an early contributor to vulnerability of pancreatic ß-cells for protein misfolding and proteotoxicity, hallmark defects in islets of T2DM. Also, given that deficiency in UCHL1 exacerbated the defective autophagy/lysosomal degradation characteristic of hIAPP proteotoxicity, we demonstrate a previously unrecognized role of UCHL1 in the function of the autophagy/lysosomal pathway in ß-cells.

Autophagy defends pancreatic ß cells from human islet amyloid polypeptide-induced toxicity.

Type 2 diabetes (T2D) is characterized by a deficiency in ß cell mass, increased ß cell apoptosis, and extracellular accumulation of islet amyloid derived from islet amyloid polypeptide (IAPP), which ß cells coexpress with insulin. IAPP expression is increased in the context of insulin resistance, the major risk factor for developing T2D. Human IAPP is potentially toxic, especially as membrane-permeant oligomers, which have been observed to accumulate within ß cells of patients with T2D and rodents expressing human IAPP. Here, we determined that ß cell IAPP content is regulated by autophagy through p62-dependent lysosomal degradation. Induction of high levels of human IAPP in mouse ß cells resulted in accumulation of this amyloidogenic protein as relatively inert fibrils within cytosolic p62-positive inclusions, which temporarily averts formation of toxic oligomers. Mice hemizygous for transgenic expression of human IAPP did not develop diabetes; however, loss of ß cell-specific autophagy in these animals induced diabetes, which was attributable to accumulation of toxic human IAPP oligomers and loss of ß cell mass. In human IAPP-expressing mice that lack ß cell autophagy, increased oxidative damage and loss of an antioxidant-protective pathway appeared to contribute to increased ß cell apoptosis. These findings indicate that autophagy/lysosomal degradation defends ß cells against proteotoxicity induced by oligomerization-prone human IAPP.

ß-Cell failure in type 2 diabetes: a case of asking too much of too few?

The islet in type 2 diabetes (T2DM) is characterized by a deficit in ß-cells, increased ß-cell apoptosis, and extracellular amyloid deposits derived from islet amyloid polypeptide (IAPP). In the absence of longitudinal studies, it is unknown if the low ß-cell mass in T2DM precedes diabetes onset (is a risk factor for diabetes) or develops as a consequence of the disease process. Although insulin resistance is a risk factor for T2DM, most individuals who are insulin resistant do not develop diabetes. By inference, an increased ß-cell workload results in T2DM in some but not all individuals. We propose that the extent of the ß-cell mass that develops during childhood may underlie subsequent successful or failed adaptation to insulin resistance in later life. We propose that a low innate ß-cell mass in the face of subsequent insulin resistance may expose ß-cells to a burden of insulin and IAPP biosynthetic demand that exceeds the cellular capacity for protein folding and trafficking. If this threshold is crossed, intracellular toxic IAPP membrane permeant oligomers (cylindrins) may form, compromising ß-cell function and inducing ß-cell apoptosis.