Novel proapoptotic effect of hepatocyte growth factor: synergy with palmitate to cause pancreatic {beta}-cell apoptosis

Maladaptive positive feedback production of ChREBPβ underlies glucotoxic β-cell failure

Preservation and expansion of β-cell mass is a therapeutic goal for diabetes. Here we show that the hyperactive isoform of carbohydrate response-element binding protein (ChREBPβ) is a nuclear effector of hyperglycemic stress occurring in β-cells in response to prolonged glucose exposure, high-fat diet, and diabetes. We show that transient positive feedback induction of ChREBPβ is necessary for adaptive β-cell expansion in response to metabolic challenges. Conversely, chronic excessive β-cell-specific overexpression of ChREBPβ results in loss of β-cell identity, apoptosis, loss of β-cell mass, and diabetes. Furthermore, β-cell "glucolipotoxicity" can be prevented by deletion of ChREBPβ. Moreover, ChREBPβ-mediated cell death is mitigated by overexpression of the alternate CHREBP gene product, ChREBPα, or by activation of the antioxidant Nrf2 pathway in rodent and human β-cells. We conclude that ChREBPβ, whether adaptive or maladaptive, is an important determinant of β-cell fate and a potential target for the preservation of β-cell mass in diabetes.

Protection from β-cell apoptosis by inhibition of TGF-β/Smad3 signaling

Prevailing insulin resistance and the resultant hyperglycemia elicits a compensatory response from pancreatic islet beta cells (β-cells) that involves increases in β-cell function and β-cell mass. However, the sustained metabolic stress eventually leads to β-cell failure characterized by severe β-cell dysfunction and progressive loss of β-cell mass. Whereas, β-cell dysfunction is relatively well understood at the mechanistic level, the avenues leading to loss of β-cell mass are less clear with reduced proliferation, dedifferentiation, and apoptosis all potential mechanisms. Butler and colleagues documented increased β-cell apoptosis in pancreas from lean and obese human Type 2 diabetes (T2D) subjects, with no changes in rates of β-cell replication or neogenesis, strongly suggesting a role for apoptosis in β-cell failure. Here, we describe a permissive role for TGF-β/Smad3 in β-cell apoptosis. Human islets undergoing β-cell apoptosis release increased levels of TGF-β1 ligand and phosphorylation levels of TGF-β's chief transcription factor, Smad3, are increased in human T2D islets suggestive of an autocrine role for TGF-β/Smad3 signaling in β-cell apoptosis. Smad3 phosphorylation is similarly increased in diabetic mouse islets undergoing β-cell apoptosis. In mice, β-cell-specific activation of Smad3 promotes apoptosis and loss of β-cell mass in association with β-cell dysfunction, glucose intolerance, and diabetes. In contrast, inactive Smad3 protects from apoptosis and preserves β-cell mass while improving β-cell function and glucose tolerance. At the molecular level, Smad3 associates with Foxo1 to propagate TGF-β-dependent β-cell apoptosis. Indeed, genetic or pharmacologic inhibition of TGF-β/Smad3 signals or knocking down Foxo1 protects from β-cell apoptosis. These findings reveal the importance of TGF-β/Smad3 in promoting β-cell apoptosis and demonstrate the therapeutic potential of TGF-β/Smad3 antagonism to restore β-cell mass lost in diabetes.

Hepatocyte growth factor ameliorates hyperglycemia and corrects β-cell mass in IRS2-deficient mice

Insulin resistance, when combined with decreased β-cell mass and relative insufficient insulin secretion, leads to type 2 diabetes. Mice lacking the IRS2 gene (IRS2(-/-) mice) develop diabetes due to uncompensated insulin resistance and β-cell failure. Hepatocyte growth factor (HGF) activates the phosphatidylinositol 3-kinase/Akt signaling pathway in β-cells without recruitment of IRS1 or IRS2 and increases β-cell proliferation, survival, mass, and function when overexpressed in β-cells of transgenic (TG) mice. We therefore hypothesized that HGF may protect against β-cell failure in IRS2 deficiency. For that purpose, we cross-bred TG mice overexpressing HGF in β-cells with IRS2 knockout (KO) mice. Glucose homeostasis analysis revealed significantly reduced hyperglycemia, compensatory hyperinsulinemia, and improved glucose tolerance in TG/KO mice compared with those in KO mice in the context of similar insulin resistance. HGF overexpression also increased glucose-stimulated insulin secretion in IRS2(-/-) islets. To determine whether this glucose homeostasis improvement correlated with alterations in β-cells, we measured β-cell mass, proliferation, and death in these mice. β-Cell proliferation was increased and death was decreased in TG/KO mice compared with those in KO mice. As a result, β-cell mass was significantly increased in TG/KO mice compared with that in KO mice, reaching levels similar to those in wild-type mice. Analysis of the intracellular targets involved in β-cell failure in IRS2 deficiency showed Pdx-1 up-regulation, Akt/FoxO1 phosphorylation, and p27 down-regulation in TG/KO mouse islets. Taken together, these results indicate that HGF can compensate for IRS2 deficiency and subsequent insulin resistance by normalizing β-cell mass and increasing circulating insulin. HGF may be of value as a therapeutic agent against β-cell failure.

Palmitate-induced impairments of β-cell function are linked with generation of specific ceramide species via acylation of sphingosine

Prolonged exposure to palmitate impairs β-cell function and mass. One of the proposed mechanisms is alteration in ceramide (Cer) generation. In the present study, exposure to palmitate induced the level of palmitoyl transferase and Cer synthases, enzymes of the Cer de novo and salvage pathways, and doubled total Cer levels, which was associated with decreased insulin secretion and augmented apoptosis in MIN6 cells and human islets. By inhibiting enzymes of the pathways pharmacologically with myriocin (ISP-1) or fumonisin B1 or by small interfering RNA (siRNA), we showed that Cer(14:0), Cer(16:0), Cer(20:1), and Cer(24:0) species, generated by the salvage pathway, are linked to the harmful effect of palmitate on β-cells. Oleate attenuates negative effects of palmitate on β-cells. When oleate was included during culture of MIN6 cells with palmitate, the palmitate-induced up-regulation of the enzymes of the de novo and salvage pathways was prevented resulting in normalized levels of all Cer species except Cer(20:1). Our data suggest that enhanced Cer generation in response to elevated palmitate levels involves both de novo and salvage pathways. However, the negative effects of palmitate on β-cells are attributed to generation of Cer species Cer(14:0), Cer(16:0), and Cer(24:0) via acylation of sphingosine.

Lipotoxicity in the pancreatic beta cell: not just survival and function, but proliferation as well?

Free fatty acids (FFAs) exert both positive and negative effects on beta cell survival and insulin secretory function, depending on concentration, duration, and glucose abundance. Lipid signals are mediated not only through metabolic pathways, but also through cell surface and nuclear receptors. Toxicity is modulated by positive signals arising from circulating factors such as hormones, growth factors and incretins, as well as negative signals such as inflammatory mediators and cytokines. Intracellular mechanisms of lipotoxicity include metabolic interference and cellular stress responses such as oxidative stress, endoplasmic reticulum (ER) stress, and possibly autophagy. New findings strengthen an old hypothesis that lipids may also impair compensatory beta cell proliferation. Clinical observations continue to support a role for lipid biology in the risk and progression of both type 1 (T1D) and type 2 diabetes (T2D). This review summarizes recent work in this important, rapidly evolving field.

Alteration of endoplasmic reticulum lipid rafts contributes to lipotoxicity in pancreatic β-cells

Chronic saturated fatty acid exposure causes β-cell apoptosis and, thus, contributes to type 2 diabetes. Although endoplasmic reticulum (ER) stress and reduced ER-to-Golgi protein trafficking have been implicated, the exact mechanisms whereby saturated fatty acids trigger β-cell death remain elusive. Using mass spectroscopic lipidomics and subcellular fractionation, we demonstrate that palmitate pretreatment of MIN6 β-cells promoted ER remodeling of both phospholipids and sphingolipids, but only the latter was causally linked to lipotoxic ER stress. Thus, overexpression of glucosylceramide synthase, previously shown to protect against defective protein trafficking and ER stress, partially reversed lipotoxic reductions in ER sphingomyelin (SM) content and aggregation of ER lipid rafts, as visualized using Erlin1-GFP. Using both lipidomics and a sterol response element reporter assay, we confirmed that free cholesterol in the ER was also reciprocally modulated by chronic palmitate and glucosylceramide synthase overexpression. This is consistent with the known coregulation and association of SM and free cholesterol in lipid rafts. Inhibition of SM hydrolysis partially protected against ATF4/C/EBP homology protein induction because of palmitate. Our results suggest that loss of SM in the ER is a key event for initiating β-cell lipotoxicity, which leads to disruption of ER lipid rafts, perturbation of protein trafficking, and initiation of ER stress.

Sphingolipids in obesity, type 2 diabetes, and metabolic disease

Metabolic disease, including obesity and type 2 diabetes, constitutes a major emerging health crisis in Western nations. Although the symptoms and clinical pathology and physiology of these conditions are well understood, the molecular mechanisms underlying the disease process have largely remained obscure. Sphingolipids, a lipid class with both signaling and structural properties, have recently emerged as key players in most major tissues affected by diabetes and are required components in the molecular etiology of this disease. Indeed, sphingolipids have been shown to mediate loss of insulin sensitivity, to promote the characteristic diabetic proinflammatory state, and to induce cell death and dysfunction in important organs such as the pancreas and heart. Furthermore, plasma sphingolipid levels are emerging as potential biomarkers for the decompensation of insulin resistance to frank type 2 diabetes. Despite these discoveries, the roles of specific sphingolipid species and sphingolipid metabolic pathways remain obscure, and newly developed experimental approaches must be employed to elucidate the detailed molecular mechanisms necessary for rational drug development and other clinical applications.

Ablation of dihydroceramide desaturase confers resistance to etoposide-induced apoptosis in vitro

Sphingolipid biosynthesis is potently upregulated by factors associated with cellular stress, including numerous chemotherapeutics, inflammatory cytokines, and glucocorticoids. Dihydroceramide desaturase 1 (Des1), the third enzyme in the highly conserved pathway driving sphingolipid biosynthesis, introduces the 4,5-trans-double bond that typifies most higher-order sphingolipids. Surprisingly, recent studies have shown that certain chemotherapeutics and other drugs inhibit Des1, giving rise to a number of sphingolipids that lack the characteristic double bond. In order to assess the effect of an altered sphingolipid profile (via Des1 inhibition) on cell function, we generated isogenic mouse embryonic fibroblasts lacking both Des1 alleles. Lipidomic profiling revealed that these cells contained higher levels of dihydroceramide than wild-type fibroblasts and that complex sphingolipids were comprised predominantly of the saturated backbone (e.g. sphinganine vs. sphingosine, dihydrosphingomyelin vs. sphingomyelin, etc.). Des1 ablation activated pro-survival and anabolic signaling intermediates (e.g. Akt/PKB, mTOR, MAPK, etc.) and provided protection from apoptosis caused by etoposide, a chemotherapeutic that induces sphingolipid synthesis by upregulating several sphingolipid biosynthesizing enzymes. These data reveal that the double bond present in most sphingolipids has a profound impact on cell survival pathways, and that the manipulation of Des1 could have important effects on apoptosis.

Roles of ceramide and sphingolipids in pancreatic β-cell function and dysfunction

Recent technical advances have re-invigorated the study of sphingolipid metabolism in general, and helped to highlight the varied and important roles that sphingolipids play in pancreatic β-cells. Sphingolipid metabolites such as ceramide, glycosphingolipids, sphingosine 1-phosphate and gangliosides modulate many β-cell signaling pathways and processes implicated in β-cell diabetic disease such as apoptosis, β-cell cytokine secretion, ER-to-golgi vesicular trafficking, islet autoimmunity and insulin gene expression. They are particularly relevant to lipotoxicity. Moreover, the de novo synthesis of sphingolipids occurs on many subcellular membranes, in parallel to secretory vesicle formation, traffic and granule maturation events. Indeed, the composition of the plasma membrane, determined by the activity of neutral sphingomyelinases, affects β-cell excitability and potentially insulin exocytosis while another glycosphingolipid, sulfatide, determines the stability of insulin crystals in granules. Most importantly, sphingolipid metabolism on internal membranes is also strongly implicated in regulating β-cell apoptosis.

Free fatty acids block glucose-induced β-cell proliferation in mice by inducing cell cycle inhibitors p16 and p18

Pancreatic β-cell proliferation is infrequent in adult humans and is not increased in type 2 diabetes despite obesity and insulin resistance, suggesting the existence of inhibitory factors. Free fatty acids (FFAs) may influence proliferation. In order to test whether FFAs restrict β-cell proliferation in vivo, mice were intravenously infused with saline, Liposyn II, glucose, or both, continuously for 4 days. Lipid infusion did not alter basal β-cell proliferation, but blocked glucose-stimulated proliferation, without inducing excess β-cell death. In vitro exposure to FFAs inhibited proliferation in both primary mouse β-cells and in rat insulinoma (INS-1) cells, indicating a direct effect on β-cells. Two of the fatty acids present in Liposyn II, linoleic acid and palmitic acid, both reduced proliferation. FFAs did not interfere with cyclin D2 induction or nuclear localization by glucose, but increased expression of inhibitor of cyclin dependent kinase 4 (INK4) family cell cycle inhibitors p16 and p18. Knockdown of either p16 or p18 rescued the antiproliferative effect of FFAs. These data provide evidence for a novel antiproliferative form of β-cell glucolipotoxicity: FFAs restrain glucose-stimulated β-cell proliferation in vivo and in vitro through cell cycle inhibitors p16 and p18. If FFAs reduce proliferation induced by obesity and insulin resistance, targeting this pathway may lead to new treatment approaches to prevent diabetes.

Paradoxical action of growth factors: antiproliferative and proapoptotic signaling by HGF/c-MET

Hepatocyte growth factor (HGF)/mesenchymal-epithelial transition factor (c-MET) signaling is usually associated with the promotion of cellular growth and often with progression of tumors. Nevertheless, under certain conditions HGF can also act as an antiproliferative and proapoptotic factor and can sensitize various cancer cells, treated with anticancer drugs, to apoptosis. Not only HGF but also its various truncated forms as well as intracellular fragments of its membrane receptor, c-MET, may act as antiproliferative and proapoptotic factors toward various cells. This review focuses on different mechanisms responsible for such paradoxical action of the known typical growth factor. It also points toward the possibilities of usage of this information in anticancer therapy.

Ceramide formation as a target in beta-cell survival and function

INTRODUCTION

Ceramide may be synthesized de novo or generated by sphingomyelinase-dependent hydrolysis of sphingomyelin.

AREAS COVERED

The role of ceramide, ceramide-sensitive signaling and ion channels in β-cell apoptosis, lipotoxicity and amyloid-induced β-cell death.

EXPERT OPINION

Ceramide participates in β-cell dysfunction and apoptosis after exposure to TNFα, IL-1β and IFN-γ, excessive amyloid and islet amyloid polypeptide or non-esterified fatty acids (lipotoxicity). Knockout of sphingomyelin synthase 1, which converts ceramide to sphingomyelin, leads to impairment of insulin secretion. Increased ceramidase activity or pharmacological inhibition of ceramide synthetase, inhibits β-cell apoptosis. Ceramide contributes to endoplasmatic reticulum (ER) stress, decreased mitochondrial membrane potential in insulin-secreting cells and mitochondrial release of cytochrome c into the cytosol, which are all triggers of apoptotic cell death. Ceramide-dependent signaling involves activation of extracellularly regulated kinases 1 and 2 (ERK1/2), downregulation of Period (Per)-aryl hydrocarbon receptor nuclear translocator (Arnt)-single-minded (Sim) kinase (PASK), activation of okadaic-acid-sensitive protein phosphatase 2A (PP2A) and stimulation of NADPH-oxidase with generation of superoxides and lipid peroxides. Ceramide reduces the activity of voltage gated potassium (Kv)-channels in insulin-secreting cells. The role of ceramide in β-cell survival and function may be therapeutically relevant, because ceramide formation can be suppressed by pharmacological inhibition of ceramide synthetase and/or sphingomyelinase.

A lipidomic screen of palmitate-treated MIN6 β-cells links sphingolipid metabolites with endoplasmic reticulum (ER) stress and impaired protein trafficking

Saturated fatty acids promote lipotoxic ER (endoplasmic reticulum) stress in pancreatic β-cells in association with Type 2 diabetes. To address the underlying mechanisms we employed MS in a comprehensive lipidomic screen of MIN6 β-cells treated for 48 h with palmitate. Both the overall mass and the degree of saturation of major neutral lipids and phospholipids were only modestly increased by palmitate. The mass of GlcCer (glucosylceramide) was augmented by 70% under these conditions, without any significant alteration in the amounts of either ceramide or sphingomyelin. However, flux into ceramide (measured by [3H]serine incorporation) was augmented by chronic palmitate, and inhibition of ceramide synthesis decreased both ER stress and apoptosis. ER-to-Golgi protein trafficking was also reduced by palmitate pre-treatment, but was overcome by overexpression of GlcCer synthase. This was accompanied by increased conversion of ceramide into GlcCer, and reduced ER stress and apoptosis, but no change in phospholipid desaturation. Sphingolipid alterations due to palmitate were not secondary to ER stress since they were neither reproduced by pharmacological ER stressors nor overcome using the chemical chaperone phenylbutyric acid. In conclusion, alterations in sphingolipid, rather than phospholipid, metabolism are more likely to be implicated in the defective protein trafficking and enhanced ER stress and apoptosis of lipotoxic β-cells.