Palmitate-induced impairment of glucose-stimulated insulin secretion precedes mitochondrial dysfunction in mouse pancreatic islets

Research advances in understanding crosstalk between organs and pancreatic β-cell dysfunction

Obesity has increased dramatically worldwide. Being overweight or obese can lead to various conditions, including dyslipidaemia, hypertension, glucose intolerance and metabolic syndrome (MetS), which may further lead to type 2 diabetes mellitus (T2DM). Previous studies have identified a link between β-cell dysfunction and the severity of MetS, with multiple organs and tissues affected. Identifying the associations between pancreatic β-cell dysfunction and organs is critical. Research has focused on the interaction between the liver, gut and pancreatic β-cells. However, the mechanisms and related core targets are still not perfectly elucidated. The aims of this review were to summarize the mechanisms of β-cell dysfunction and to explore the potential pathogenic pathways and targets that connect the liver, gut, adipose tissue, muscle, and brain to pancreatic β-cell dysfunction.

Long-term ketogenic diet causes hyperlipidemia, liver dysfunction, and glucose intolerance from impaired insulin trafficking and secretion in mice

A ketogenic diet (KD) is a very low-carbohydrate, very high-fat diet proposed to treat obesity and type 2 diabetes. While KD grows in popularity, its effects on metabolic health are understudied. Here we show that, in male and female mice, while KD protects against weight gain and induces weight loss, over long-term, mice develop hyperlipidemia, hepatic steatosis, and severe glucose intolerance. Unlike high fat diet-fed mice, KD mice are not insulin resistant and have low levels of insulin. Hyperglycemic clamp and ex vivo GSIS revealed cell-autonomous and whole-body impairments in insulin secretion. Major ER/Golgi stress and disrupted ER-Golgi protein trafficking was indicated by transcriptomic profiling of KD islets and confirmed by electron micrographs showing a dilated Golgi network likely responsible for impaired insulin granule trafficking and secretion. Overall, our results suggest long-term KD leads to multiple aberrations of metabolic parameters that caution its systematic use as a health promoting dietary intervention.

The oleic/palmitic acid imbalance in exosomes isolated from NAFLD patients induces necroptosis of liver cells via the elongase-6/RIP-1 pathway

Excessive toxic lipid accumulation in hepatocytes underlies the development of non-alcoholic fatty liver disease (NAFLD), phenotypically characterized by necrosis and steato-fibrosis, whose molecular mechanism is not yet fully understood. Patients with NAFLD display an imbalanced palmitic (PA) to oleic acid (OA) ratio. Moreover, increasing experimental evidence points out a relevant involvement of the exosomal content in disease progression. Aim of the study was to highlight the PA/OA imbalance within circulating exosomes, the subsequent intracellular alterations, and the impact on NALFD. Liver cells were challenged with exosomes isolated from both healthy subjects and NAFLD patients. The exosomal PA/OA ratio was artificially modified, and biological effects were evaluated. A NAFLD-derived exosomal PA/OA imbalance impacts liver cell cycle and cell viability. OA-modified NAFLD-derived exosomes restored cellular viability and proliferation, whereas the inclusion of PA into healthy subjects-derived exosomes negatively affected cell viability. Moreover, while OA reduced the phosphorylation and activation of the necroptosis marker, Receptor-interacting protein 1 (phospho-RIP-1), PA induced the opposite outcome, alongside increased levels of stress fibers, such as vimentin and fibronectin. Administration of NAFLD-derived exosomes led to increased expression of Elongase 6 (ELOVL6), Stearoyl-CoA desaturase 1 (SCD1), Tumor necrosis factor α (TNF-α), Mixed-lineage-kinase-domain-like-protein (MLKL) and RIP-1 in the hepatocytes, comparable to mRNA levels in the hepatocytes of NAFLD patients reported in the Gene Expression Omnibus (GEO) database. Genetic and pharmacological abrogation of ELOVL6 elicited a reduced expression of downstream molecules TNF-α, phospho-RIP-1, and phospho-MLKL upon administration of NAFLD-derived exosomes. Lastly, mice fed with high-fat diet exhibited higher phospho-RIP-1 than mice fed with control diet. Targeting the Elongase 6-RIP-1 signaling pathway offers a novel therapeutic approach for the treatment of the NALFD-induced exosomal PA/OA imbalance.

FGF-23 protects cell function and viability in murine pancreatic islets challenged by glucolipotoxicity

The fibroblast growth factor FGF-23 is a member of the FGF-15/19 subfamily with hormonal functions. Besides its well-known role for bone mineralization, FGF-23 is discussed as a marker for cardiovascular disease. We investigated whether FGF-23 has any effects on the endocrine pancreas of mice by determining insulin secretion, electrical activity, intracellular Ca2+, and apoptosis. Acute application of FGF-23 (10 to 500 ng/ml, i.e., 0.4 to 20 nM) does not affect insulin release of murine islets, while prolonged exposure leads to a 21% decrease in glucose-stimulated secretion. The present study shows for the first time that FGF-23 (100 or 500 ng/ml) partially protects against impairment of insulin secretion and apoptotic cell death induced by glucolipotoxicity. The reduction of apoptosis by FGF-23 is approximately twofold higher compared to FGF-21 or FGF-15/19. In contrast to FGF-23 and FGF-21, FGF-15/19 is clearly pro-apoptotic under control conditions. The beneficial effect of FGF-23 against glucolipotoxicity involves interactions with the stimulus-secretion cascade of beta-cells. Electrical activity and the rise in the cytosolic Ca2+ concentration of islets in response to acute glucose stimulation increase after glucolipotoxic culture (48 h). Co-culture with FGF-23 further elevates the glucose-mediated effects on both parameters. Protection against apoptosis and glucolipotoxic impairment of insulin release by FGF-23 is prevented, when calcineurin is inhibited by tacrolimus or when c-Jun N-terminal kinase (JNK) is blocked by SP600125. In conclusion, our data suggest that FGF-23 can activate compensatory mechanisms to maintain beta-cell function and integrity of islets of Langerhans during excessive glucose and lipid supply.

Cholesterol Redistribution in Pancreatic β-Cells: A Flexible Path to Regulate Insulin Secretion

Pancreatic β-cells, by secreting insulin, play a key role in the control of glucose homeostasis, and their dysfunction is the basis of diabetes development. The metabolic milieu created by high blood glucose and lipids is known to play a role in this process. In the last decades, cholesterol has attracted significant attention, not only because it critically controls β-cell function but also because it is the target of lipid-lowering therapies proposed for preventing the cardiovascular complications in diabetes. Despite the remarkable progress, understanding the molecular mechanisms responsible for cholesterol-mediated β-cell function remains an open and attractive area of investigation. Studies indicate that β-cells not only regulate the total cholesterol level but also its redistribution within organelles, a process mediated by vesicular and non-vesicular transport. The aim of this review is to summarize the most current view of how cholesterol homeostasis is maintained in pancreatic β-cells and to provide new insights on the mechanisms by which cholesterol is dynamically distributed among organelles to preserve their functionality. While cholesterol may affect virtually any activity of the β-cell, the intent of this review is to focus on early steps of insulin synthesis and secretion, an area still largely unexplored.

Pulsatile Basal Insulin Secretion Is Driven by Glycolytic Oscillations

In fasted and fed states, blood insulin levels are oscillatory. While this phenomenon is well studied at high glucose levels, comparatively little is known about its origin under basal conditions. We propose a possible mechanism for basal insulin oscillations based on oscillations in glycolysis, demonstrated using an established mathematical model. At high glucose, this is superseded by a calcium-dependent mechanism.

A Review of Current Trends with Type 2 Diabetes Epidemiology, Aetiology, Pathogenesis, Treatments and Future Perspectives

Type 2 diabetes (T2D), which has currently become a global pandemic, is a metabolic disease largely characterised by impaired insulin secretion and action. Significant progress has been made in understanding T2D aetiology and pathogenesis, which is discussed in this review. Extrapancreatic pathology is also summarised, which demonstrates the highly multifactorial nature of T2D. Glucagon-like peptide (GLP)-1 is an incretin hormone responsible for augmenting insulin secretion from pancreatic beta-cells during the postprandial period. Given that native GLP-1 has a very short half-life, GLP-1 mimetics with a much longer half-life have been developed, which are currently an effective treatment option for T2D by enhancing insulin secretion in patients. Interestingly, there is continual emerging evidence that these therapies alleviate some of the post-diagnosis complications of T2D. Additionally, these therapies have been shown to induce weight loss in patients, suggesting they could be an alternative to bariatric surgery, a procedure associated with numerous complications. Current GLP-1-based therapies all act as orthosteric agonists for the GLP-1 receptor (GLP-1R). Interestingly, it has emerged that GLP-1R also has allosteric binding sites and agonists have been developed for these sites to test their therapeutic potential. Recent studies have also demonstrated the potential of bi- and tri-agonists, which target multiple hormonal receptors including GLP-1R, to more effectively treat T2D. Improved understanding of T2D aetiology/pathogenesis, coupled with the further elucidation of both GLP-1 activity/targets and GLP-1R mechanisms of activation via different agonists, will likely provide better insight into the therapeutic potential of GLP-1-based therapies to treat T2D.

Transcriptional Profiles Reveal Deregulation of Lipid Metabolism and Inflammatory Pathways in Neurons Exposed to Palmitic Acid

The effects of the consumption of high-fat diets (HFD) have been studied to unravel the molecular pathways they are altering in order to understand the link between increased caloric intake, metabolic diseases, and the risk of cognitive dysfunction. The saturated fatty acid, palmitic acid (PA), is the main component of HFD and it has been found increased in the circulation of obese and diabetic people. In the central nervous system, PA has been associated with inflammatory responses in astrocytes, but the effects on neurons exposed to it have not been largely investigated. Given that PA affects a variety of metabolic pathways, we aimed to analyze the transcriptomic profile activated by this fatty acid to shed light on the mechanisms of neuronal dysfunction. In the current study, we profiled the transcriptome response after PA exposition at non-toxic doses in primary hippocampal neurons. Gene ontology and Reactome pathway analysis revealed a pattern of gene expression which is associated with inflammatory pathways, and importantly, with the activation of lipid metabolism that is considered not very active in neurons. Validation by quantitative RT-PCR (qRT-PCR) of Hmgcs2, Angptl4, Ugt8, and Rnf145 support the results obtained by RNAseq. Overall, these findings suggest that neurons are able to respond to saturated fatty acids changing the expression pattern of genes associated with inflammatory response and lipid utilization that may be involved in the neuronal damage associated with metabolic diseases.

Allomyrina dichotoma larva extract attenuates free fatty acid-induced lipotoxicity in pancreatic beta cells

BACKGROUD/OBJECTIVES

Allomyrina dichotoma larva (ADL), one of the many edible insects recognized as future food resources, has a range of pharmacological activities. In a previous study, an ADL extract (ADLE) reduced the hepatic insulin resistance of high-fat diet (HFD)-induced diabetic mice. On the other hand, the associated molecular mechanisms underlying pancreatic beta-cell dysfunction remain unclear. This study examined the effects of ADLE on palmitate-induced lipotoxicity in a beta cell line of a rat origin, INS-1 cells.

MATERIALS/METHODS

ADLE was administered to high-fat diet treated mice. The expression of apoptosis-related molecules was measured by Western blotting, and reactive oxidative stress generation and nitric oxide production were measured by DCH-DA fluorescence and a Griess assay, respectively.

RESULTS

The administration of ADLE to HFD-induced diabetic mice reduced the hyperplasia, 4-hydroxynonenal levels, and the number of apoptotic cells while improving the insulin levels compared to the HFD group. Treatment of INS-1 cells with palmitate reduced insulin secretion, which was attenuated by the ADLE treatment. Furthermore, the ADLE treatment prevented palmitate-induced cell death in INS-1 cells and isolated islets by reducing the apoptotic signaling molecules, including cleaved caspase-3 and PARP, and the Bax/Bcl2 ratio. ADLE also reduced the levels of reactive oxygen species generation, lipid accumulation, and nitrite production in palmitate-treated INS-1 cells while increasing the ATP levels. This effect corresponded to the decreased expression of inducible nitric oxide synthase (iNOS) mRNA and protein.

CONCLUSIONS

ADLE helps prevent lipotoxic beta-cell death in INS-1 cells and HFD-diabetic mice, suggesting that ADLE can be used to prevent or treat beta-cell damage in glucose intolerance during the development of diabetes.

Palmitate exacerbates bisphenol A toxicity via induction of ER stress and mitochondrial dysfunction

Combined exposure to dietary nutrients and environmental chemicals may elicit significantly different physiological effects than single exposures. Exposure to dietary saturated fats and environmental toxins is a physiologically-significant dual exposure that is particularly associated with lower socioeconomic status, potentially placing these individuals at heightened risk of xenobiotic toxicities. However, no prior studies have examined interactions between specific lipids and environmental xenobiotics in modulating cellular health. Using primary mouse embryonic fibroblasts, we have discovered that prior exposure to the saturated fatty acid, palmitate, exacerbates cellular toxicity associated with the industrial plasticizer, bisphenol A (BPA). Cell death upon BPA exposure following palmitate pre-treatment was greater than that occurring with either exposure alone. Mechanistically, cell death was preceded by increased endoplasmic reticulum stress and loss of mitochondrial membrane potential in palmitate plus BPA exposed cells, leading to increased caspase-3 cleavage and subsequent apoptosis. Interestingly, inclusion of the unsaturated fatty acid, oleate, along with palmitate during the pre-treatment period completely abrogated the ER stress, mitochondrial toxicity, and cell death induced by subsequent exposure to BPA. Thus, our data identify for the first time an important interaction between a fatty acid and an environmental toxin and have implications for developing nutritional interventions to mitigate the deleterious effects of such xenobiotic exposures.

lncRNA Eif4g2 improves palmitate-induced dysfunction of mouse β-cells via modulation of Nrf2 activation

Chronic oxidative stress resulting from hyperlipidemia is thought to be a key pathogenic driver of pancreatic β-cell dysfunction in leading to the onset of type 2 diabetes mellitus (T2DM). Long non-coding RNAs (lncRNAs) have been increasingly recognized to regulate dysfunction within pancreatic β-cells in the context of T2DM. In the present study, we sought to comprehensively analyze the roles of lncRNAs in dysfunctional β-cells and mouse islets. Analyses of INS-1E cells were performed by RNA-seq and qRT-PCR after treating with or without 0.5 mM palmitate for 4 days, leading us to identify the novel lncRNA Eif4g2 (lncEif4g2) as a functional regulator within these cells. When we overexpressed lncEif4g2 in INS-1E β-cells and mouse islets, this was sufficient for the reversal of palmitate-mediated reductions in cell viability, insulin production, ATP production by mitochondria, and creation of intracellular reactive oxygen species (ROS) and the dysfunction of mouse islets, with nuclear factor erythroid 2 related factor 2 (Nrf2) activation also being observed. In contrast, when lncEif4g2 was knocked down this led INS-1E cells and mouse islets to become more sensitive to palmitate-induced dysfunction, with reduced Nrf2 nuclear translocation also being detected. When antioxidants were used to treat INS-1E cells and mouse islets, however, these negative effects were reversed. Additional functional analyses revealed lncEif4g2 to be capable of directly binding to miR-3074-5p in β-cells, with the expression of lncEif4g2 and miR-3074-5p being negatively correlated with one another. We further found that cAMP-responsive element binding-protein (CREB) was a miR-3074-5p target gene in these cells, thus at least in part serving as a functional mediator of the lncEif4g2/miR-3074-5p axis within dysfunctional β-cells. In summary, our results thus reveal that lncEif4g2 is able to indirectly regulate the expression of CREB via targeting miR-3074-5p in INS-1E cells and mouse islets, thereby leading to enhanced Nrf2 activation.

Beta-aminoisobutyric acid is released by contracting human skeletal muscle and lowers insulin release from INS-1 832/3 cells by mediating mitochondrial energy metabolism

Aims/hypothesis

This study aimed to examine if beta-aminoisobutyric acid (BAIBA) is (i) secreted by skeletal muscle in humans during exercise, (ii) associated with insulin secretory function in vivo, and (iii) directly linked with acute glucose-mediated insulin release by pancreatic beta cells in vitro.

Methods

Following 2-weeks of single-leg immobilization, plasma BAIBA concentrations were measured in the brachial artery and the femoral veins of each leg in healthy male subjects, at rest and during two-legged dynamic knee-extensor exercise. During a 2-h hyperglycamic clamp, insulin secretory function and levels of plasma BAIBA were assessed in non-diabetic individuals, non-diabetic individuals following 24-h hyperglycemia and patients with type 2 diabetes. Direct effects of BAIBA on acute glucose-mediated insulin release were probed in INS-1832/3 cells under normal and ‘diabetes-like’ conditions. Finally, the effect of BAIBA on mitochondrial function was assessed in INS-1832/3 cells using extracellular flux analysis.

Results

(i) BAIBA is released from skeletal muscle at rest and during exercise under healthy conditions but is suppressed during exercise following leg immobilization, (ii) plasma BAIBA concentrations inversely associate with insulin secretory function in humans, (iii) BAIBA lowers mitochondrial energy metabolism in INS-1 832/3 cells in parallel with decreased insulin secretion

Conclusion/interpretation: BAIBA is a myokine released by skeletal muscle during exercise and indepedantly alters the triggering pathway of insulin secretion in cultured INS-1832/3 cells.

A Phenotypic Screen Identifies Calcium Overload as a Key Mechanism of β-Cell Glucolipotoxicity

Type 2 diabetes (T2D) is caused by loss of pancreatic β-cell mass and failure of the remaining β-cells to deliver sufficient insulin to meet demand. β-Cell glucolipotoxicity (GLT), which refers to combined, deleterious effects of elevated glucose and fatty acid levels on β-cell function and survival, contributes to T2D-associated β-cell failure. Drugs and mechanisms that protect β-cells from GLT stress could potentially improve metabolic control in patients with T2D. In a phenotypic screen seeking low-molecular-weight compounds that protected β-cells from GLT, we identified compound A that selectively blocked GLT-induced apoptosis in rat insulinoma cells. Compound A and its optimized analogs also improved viability and function in primary rat and human islets under GLT. We discovered that compound A analogs decreased GLT-induced cytosolic calcium influx in islet cells, and all measured β-cell-protective effects correlated with this activity. Further studies revealed that the active compound from this series largely reversed GLT-induced global transcriptional changes. Our results suggest that taming cytosolic calcium overload in pancreatic islets can improve β-cell survival and function under GLT stress and thus could be an effective strategy for T2D treatment.

PKR inhibitors suppress endoplasmic reticulum stress and subdue glucolipotoxicity-mediated impairment of insulin secretion in pancreatic beta cells

Type 2 diabetes mellitus is characterized by insulin resistance and hypersecretion of insulin from the pancreas to compensate for decreased insulin sensitivity in the peripheral tissues. In later stages of the disease insulin-secreting beta cell degeneration commences and patients require insulin replacement therapy in order to accomplish proper regulation of their blood glucose. Endoplasmic reticulum (ER) stress in the beta cells is one of the factors contributing to this detrimental effect. Protein kinase R (PKR) is a cellular stress kinase activated by ER stress and contributing to degeneration of pancreatic islets. In order to determine whether inhibition of PKR activation by specific small molecule inhibitors of PKR ameliorates pancreatic insulin secretion capacity, we treated beta cells with two imidazole/oxindole-derived inhibitors of PKR kinase, imoxin (C16) and 2-aminopurine (2-AP), in the presence of ER stress. Our results demonstrate that PKR inhibition suppresses tunicamycin-mediated ER stress without altering the insulin production capacity of the cells. Palmitic acid-mediated suppression of insulin secretion, however, was subdued significantly by PKR inhibitor treatment through an ER stress-related mechanism. We suggest that PKR inhibitor treatment may be used to increase the insulin secretion capacity of the pancreas in later stages of diabetes.

Oleic acid ameliorates palmitic acid induced hepatocellular lipotoxicity by inhibition of ER stress and pyroptosis

Background

Pyroptosis is a novel programmed cell death. It is identified as caspase-1 dependent and characterized by plasma-membrane rupture and release of proinflammatory intracellular contents inculuding IL-1 beta and IL-18. Pyroptosis is distinct from other forms of cell death, especially apoptosis that is characterized by nuclear and cytoplasmic condensation and is elicited via activation of a caspase cascade. In pyroptosis, gasdermin D (GSDMD) acts as a major executor, while NLRP3 related inflammasome is closely linked to caspase-1 activation. Given that pyroptosis has played a critical role in the progression of non-alcoholic steatohepatitis (NASH), here, we investigated whether the regulation of pyroptosis activation is responsible for the protective role of monounsaturated oleic acids in the context of hepatocellular lipotoxicity.

Methods

Human hepatoma cell line HepG2 cells were exposed to palmitic acid (PA) with or without oleic acids (OA) or/and endoplasmic reticulum (ER) stress inhibitor tauroursodeoxycholic acid (TUDCA) for 24 h. Besides, the cells were treated with the chemical ER stressor tunicamycin (TM) with or without OA for 24 h as well. The expressions of pyroptosis and ER stress related genes or proteins were determined by real-time PCR, Western blot or immunofluorescence. The morphology of pyroptosis was detected by acridine orange and ethidium bromide (AO/EB) staining. The release of IL-1 beta and tumor necrosis factor alpha (TNF-α) was determined by ELISA. Sprague–Dawley (SD) rats were fed with high fat diet (HFD) for 16 w, then, HFD was half replaced by olive oil to observe the protective effects of olive oil. The blood chemistry were analyzed, and the liver histology and the expressions of related genes and proteins were determined in the liver tissues.

Results

We demonstrated that PA impaired the cell viability and disturbed the lipid metabolism of HepG2 cells (P < 0.01), but OA robustly rescued cells from cell death (P < 0.001). More importantly, we found that instead of cell apoptosis, PA induced significant pyroptosis, evidenced by remarkably increased mRNA and protein expressions of inflammasome marker NLRP3, Caspase-1 and IL-1beta, as well as cell membrane perforation driving protein GSDMD (P < 0.05). Furthermore, we demonstrated that the PA stimulated ER stress was causally related to pyroptosis. The enhanced expressions of ER stress markers CHOP and BIP were found subcellular co-located to pyroptosis markers NLRP3 and ASC. Additionally,TM was able to induce pyroptosis like PA did, and ER stress inhibitor TUDCA was able to inhibit both PA and TM induced ER stress as well as pyroptosis. Furthermore, we demonstrated that OA substantially alleviated either PA or TM induced ER stress and pyroptosis in HepG2 cells (P < 0.01). In vivo, only olive oil supplementation did not cause significant toxicity, while HFD for 32 w obviously induced liver steatosis and inflammation in SD rats (P < 0.05). Half replacement of HFD with olive oil (a mixed diet) has remarkably ameliorated liver abnormalities, and particularly inhibited the protein expressions of either ER stress and pyroptosis markers (P < 0.05).

Conclusion

Palmitic acid induced predominant pyroptosis in HepG2 cells, and ER stress may be responsible for the induction of pyroptosis and subsequent cell death. Monounsaturated oleic acids were able to ameliorate hepatocellular lipotoxicity both in vitro and in vivo, and OA mediated inhibition of ER stress and pyroptosis may be the underlying mechanisms.

Emerging roles of β-cell mitochondria in type-2-diabetes

Type-2-Diabetes (T2D) is the most common metabolic disease in the world today. It erupts as a result of peripheral insulin resistance combined with hyperinsulinemia followed by suppression of insulin secretion from pancreatic β-cells. Mitochondria play a central role in β-cells by sensing glucose and also by mediating the suppression of insulin secretion in T2D. Here, we will summarize the evidence accumulated for the roles of β-cells mitochondria in T2D. We will present an updated view on how mitochondria in β-cells have been associated with T2D, from the genetic, bioenergetic, redox and structural points of view. The emerging picture is that mitochondrial structure and dysfunction directly contribute to β-cell function and in the pathogenesis of T2D.

Nutrient Metabolism, Subcellular Redox State, and Oxidative Stress in Pancreatic Islets and β-Cells

Insulin-secreting pancreatic β-cells play a critical role in blood glucose homeostasis and the development of type 2 diabetes (T2D) in the context of insulin resistance. Based on data obtained at the whole cell level using poorly specific chemical probes, reactive oxygen species (ROS) such as superoxide and hydrogen peroxide have been proposed to contribute to the stimulation of insulin secretion by nutrients (positive role) and to the alterations of cell survival and secretory function in T2D (negative role). This raised the controversial hypothesis that any attempt to decrease β-cell oxidative stress and apoptosis in T2D would further impair insulin secretion. Over the last decade, the development of genetically-encoded redox probes that can be targeted to cellular compartments of interest and are specific of redox couples allowed the evaluation of short- and long-term effects of nutrients on β-cell redox changes at the subcellular level. The data indicated that the nutrient regulation of β-cell redox signaling and ROS toxicity is far more complex than previously thought and that the subcellular compartmentation of these processes cannot be neglected when evaluating the mechanisms of ROS production or the efficacy of antioxidant enzymes and antioxidant drugs under glucolipotoxic conditions and in T2D. In this review, we present what is currently known about the compartmentation of redox homeostatic systems and tools to investigate it. We then review data about the effects of nutrients on β-cell subcellular redox state under normal conditions and in the context of T2D and discuss challenges and opportunities in the field.

Augmentation of Glucotoxicity, Oxidative Stress, Apoptosis and Mitochondrial Dysfunction in HepG2 Cells by Palmitic Acid

Hyperglycemia and hyperlipidemia are the hallmarks of diabetes and obesity. Experimental and epidemiological studies have suggested that dietary management and caloric restriction are beneficial in reducing the complications of diabesity. Studies have suggested that increased availability of energy metabolites like glucose and saturated fatty acids induces metabolic, oxidative, and mitochondrial stress, accompanied by inflammation that may lead to chronic complications in diabetes. In the present study, we used human hepatoma HepG2 cells to investigate the effects of high glucose (25 mM) and high palmitic acid (up to 0.3 mM) on metabolic-, inflammatory-, and redox-stress-associated alterations in these cells. Our results showed increased lipid, protein, and DNA damage, leading to caspase-dependent apoptosis and mitochondrial dysfunction. Glucolipotoxicity increased ROS production and redox stress appeared to alter mitochondrial membrane potential and bioenergetics. Our results also demonstrate the enhanced ability of cytochrome P450s-dependent drug metabolism and antioxidant adaptation in HepG2 cells treated with palmitic acid, which was further augmented with high glucose. Altered NF-kB/AMPK/mTOR-dependent cell signaling and inflammatory (IL6/TNF-α) responses were also observed. Our results suggest that the presence of high-energy metabolites enhances apoptosis while suppressing autophagy by inducing inflammatory and oxidative stress responses that may be responsible for alterations in cell signaling and metabolism.

Oleic acid protects insulin-secreting INS-1E cells against palmitic acid-induced lipotoxicity along with an amelioration of ER stress

PURPOSE

It is demonstrated that unsaturated fatty acids can counteract saturated fatty acids-induced lipotoxicity, but the molecular mechanisms are unclear. In this study, we investigated the protective effects of monounsaturated oleic acid (OA) against saturated palmitic acid (PA)-induced cytotoxicity in rat β cells as well as islets, and mechanistically focused on its regulation on endoplasmic reticulum (ER) stress.

METHODS

Rat insulinoma cell line INS-1E cells and primary islets were treated with PA with or without OA for 24 h to determine the cell viability, apoptosis, and ER stress. SD rats were fed with high-fat diet (HFD) for 16 w, then, HFD was half replaced by olive oil to observe the protective effects of monounsaturated fatty acids rich diet.

RESULTS

We demonstrated that PA impaired cell viability and insulin secretion of INS-1E cells and rat islets, but OA robustly rescued cells from cell death. OA substantially alleviated either PA or chemical ER stressors (thapsigargin or tunicamycin)-induced ER stress. Importantly, OA attenuated the activity of PERK-eIF2α-ATF4-CHOP pathway and regulated the ER Ca²⁺ homeostasis. In vivo, only olive oil supplementation did not cause significant changes, while high-fat diet (HFD) for 32 w obviously induced islets ER stress and impaired insulin sensitivity in SD rats. Half replacement of HFD with olive oil (a mixed diet) has ameliorated this effect.

CONCLUSION

OA alleviated PA-induced lipotoxicity in INS-1E cells and improved insulin sensitivity in HFD rats. The amelioration of PA triggered ER stress may be responsible for its beneficial effects in β cells.

A microRNA‑24‑to‑secretagogin regulatory pathway mediates cholesterol‑induced inhibition of insulin secretion

Hypercholesterolemia is a key factor leading to β‑cell dysfunction, but its underlying mechanisms remain unclear. Secretagogin (Scgn), a Ca2+ sensor protein that is expressed at high levels in the islets, has been shown to play a key role in regulating insulin secretion through effects on the soluble N‑ethylmaleimide‑sensitive factor attachment receptor protein complexes. However, further studies are required to determine whether Scgn plays a role in hypercholesterolemia‑associated β‑cell dysfunction. The present study investigated the involvement of a microRNA‑24 (miR‑24)‑to‑Scgn regulatory pathway in cholesterol‑induced β‑cell dysfunction. In the present study, MIN6 cells were treated with increasing concentrations of cholesterol and then, the cellular functions and changes in the miR‑24‑to‑Scgn signal pathway were observed. Excessive uptake of cholesterol in MIN6 cells increased the expression of miR‑24, resulting in a reduction in Sp1 expression by directly targeting its 3' untranslated region. As a transcriptional activator of Scgn, downregulation of Sp1 decreased Scgn levels and subsequently decreased the phosphorylation of focal adhesion kinase and paxillin, which is regulated by Scgn. Therefore, the focal adhesions in insulin granules were impaired and insulin exocytosis was reduced. The present study concluded that a miR‑24‑to‑Scgn pathway participates in the mechanism regulating cholesterol accumulation‑induced β‑cell dysfunction.

Microfluidic-enabled quantitative measurements of insulin release dynamics from single islets of Langerhans in response to 5-palmitic acid hydroxy stearic acid

Proper release of insulin from pancreatic islets of Langerhans is essential for maintaining glucose homeostasis. For full efficacy, both the pattern and the amount of hormone release are critical. It is therefore important to understand how insulin levels are secreted from single islets in both a quantitative fashion and in a manner that resolves temporal dynamics. In this study, we describe a microfluidic analytical system that can both quantitatively monitor insulin secretion from single islets while simultaneously maintaining high temporal sampling to resolve dynamics of release. We have applied this system to determine the acute and chronic effects of a recently-identified lipid, 5-palmitic acid hydroxy stearic acid (5-PAHSA), which is a member of the fatty acid hydroxy fatty acid class of lipids that are upregulated in healthy individuals. Chronic incubation (48 h) with 5-PAHSA significantly increased glucose-stimulated insulin secretion (GSIS) in murine islets compared to chronic incubation without the lipid or in the presence of palmitic acid (PA). The studies were continued in human islets from both healthy donors and donors diagnosed with type 2 diabetes mellitus (T2DM). Total amounts of GSIS were not only augmented in islets that were chronically incubated with 5-PAHSA, but the dynamic insulin release profiles also improved as noted by more pronounced insulin oscillations. With this quantitative microfluidic system, we have corroborated the anti-diabetic effects of 5-PAHSA by demonstrating improved islet function after chronic incubation with this lipid via improved oscillatory dynamics along with higher basal and peak release rates.

Mitochondrial Uncoupling Proteins: Subtle Regulators of Cellular Redox Signaling

SIGNIFICANCE

Mitochondria are the energetic, metabolic, redox, and information signaling centers of the cell. Substrate pressure, mitochondrial network dynamics, and cristae morphology state are integrated by the protonmotive force Δp or its potential component, ΔΨ, which are attenuated by proton backflux into the matrix, termed uncoupling. The mitochondrial uncoupling proteins (UCP1-5) play an eminent role in the regulation of each of the mentioned aspects, being involved in numerous physiological events including redox signaling. Recent Advances: UCP2 structure, including purine nucleotide and fatty acid (FA) binding sites, strongly support the FA cycling mechanism: UCP2 expels FA anions, whereas uncoupling is achieved by the membrane backflux of protonated FA. Nascent FAs, cleaved by phospholipases, are preferential. The resulting Δp dissipation decreases superoxide formation dependent on Δp. UCP-mediated antioxidant protection and its impairment are expected to play a major role in cell physiology and pathology. Moreover, UCP2-mediated aspartate, oxaloacetate, and malate antiport with phosphate is expected to alter metabolism of cancer cells.

CRITICAL ISSUES

A wide range of UCP antioxidant effects and participations in redox signaling have been reported; however, mechanisms of UCP activation are still debated. Switching off/on the UCP2 protonophoretic function might serve as redox signaling either by employing/releasing the extra capacity of cell antioxidant systems or by directly increasing/decreasing mitochondrial superoxide sources. Rapid UCP2 degradation, FA levels, elevation of purine nucleotides, decreased Mg²⁺, or increased pyruvate accumulation may initiate UCP-mediated redox signaling.

FUTURE DIRECTIONS

Issues such as UCP2 participation in glucose sensing, neuronal (synaptic) function, and immune cell activation should be elucidated. Antioxid. Redox Signal. 29, 667-714.

Palmitate induces nitric oxide production and inflammatory cytokine expression in zebrafish

Inflammation markers in zebrafish embryos reflect a toxic response that is common to other animal models and humans. Free fatty acids (FFAs) are known to cause damage in various tissues by inducing inflammation. In this study, we investigated whether a FFA (palmitate) induces inflammation in zebrafish embryos. Nitrous oxide (NO) production and cyclooxygenase-2 (COX-2) mRNA expression were increased in palmitate-treated zebrafish embryos in a dose-dependent manner. mRNA expression of pro-inflammatory cytokines, interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF- α), were also increased. Additionally, the mRNA expression of p65 nuclear factor-kB and I-kB-α were significantly increased after palmitate-treatment. Increased reactive oxygen species (ROS) expression was observed in palmitate-treated zebrafish embryos as well as pericardial edema. Additionally, mRNA expression of pro-inflammatory cytokines were increased in zebrafish liver and pancreas fed with palmitate-contained diet. Taken together, these results indicated that palmitate increases pro-inflammatory mediators in zebrafish embryos, suggesting that zebrafish could be an alternative animal model for inflammatory disease including diabetes.

Pro-inflammatory cytokines attenuate glucose-stimulated insulin secretion from INS-1E insulinoma cells by restricting mitochondrial pyruvate oxidation capacity - Novel mechanistic insight from real-time analysis of oxidative phosphorylation

Pro-inflammatory cytokines cause pancreatic beta cell failure during the development of type 2 diabetes. This beta cell failure associates with mitochondrial dysfunction, but the precise effects of cytokines on mitochondrial respiration remain unclear. To test the hypothesis that pro-inflammatory cytokines impair glucose-stimulated insulin secretion (GSIS) by inhibiting oxidative ATP synthesis, we probed insulin release and real-time mitochondrial respiration in rat INS-1E insulinoma cells that were exposed to a combination of 2 ng/mL interleukin-1-beta and 50 ng/mL interferon-gamma. We show that 24-h exposure to these cytokines dampens both glucose- and pyruvate-stimulated insulin secretion (P < 0.0001 and P < 0.05, respectively), but does not affect KCl-induced insulin release. Mirroring secretory defects, glucose- and pyruvate-stimulated mitochondrial respiration are lowered after cytokine exposure (P < 0.01). Further analysis confirms that cytokine-induced mitochondrial respiratory defects occur irrespective of whether fuel oxidation is coupled to, or uncoupled from, ATP synthesis. These observations demonstrate that pro-inflammatory cytokines attenuate GSIS by restricting mitochondrial pyruvate oxidation capacity. Interleukin-1-beta and interferon-gamma also increase mitochondrial superoxide levels (P < 0.05), which may reinforce the inhibition of pyruvate oxidation, and cause a modest (20%) but significant (P < 0.01) loss of INS-1E cells. Cytokine-induced INS-1E cell failure is insensitive to palmitoleate and linoleate, which is at odds with the cytoprotection offered by unsaturated fatty acids against harm caused by nutrient excess. Our data disclose a mitochondrial mechanism for cytokine-impaired GSIS in INS-1E cells, and suggest that inflammatory and nutrient-related beta cell failure emerge, at least partly, through distinct paths.

Control of pancreatic β-cell bioenergetics

The canonical model of glucose-stimulated insulin secretion (GSIS) by pancreatic β-cells predicts a glucose-induced rise in the cytosolic ATP/ADP ratio. Such bioenergetic sensitivity to metabolic fuel is unusual as it implies that ATP flux is governed, to a significant extent, by ATP supply, while it is predominantly demand-driven in other cell types. Metabolic control is generally shared between different processes, but potential control of ATP consumption over β-cell bioenergetics has been largely ignored to date. The present paper offers a brief overview of experimental evidence that demonstrates ATP flux control by glucose-fuelled oxidative phosphorylation. Based on old and new data, it is argued that ATP supply does not hold exclusive control over ATP flux, but shares it with ATP demand, and that the distribution of control is flexible. Quantification of the bioenergetic control distribution will be important from basic and clinical perspectives, but precise measurement of the cytosolic ATP/ADP ratio is complicated by adenine nucleotide compartmentalisation. Metabolic control analysis of β-cell bioenergetics will likely clarify the mechanisms by which glucose and fatty acids amplify and potentiate GSIS, respectively. Moreover, such analysis may offer hints as to how ATP flux control shifts from ATP supply to ATP demand during the development of type 2 diabetes, and why prolonged sulfonylurea treatment causes β-cell deterioration.

Do skeletal muscle-secreted factors influence the function of pancreatic β-cells?

Skeletal muscle is an endocrine organ that secretes a variety of compounds including proteins (myokines), metabolites, microRNAs (miRNAs), and exosomes, many of which are regulated by exercise and play important roles in endocrine signaling. Interorgan communication via muscle-secreted factors therefore provides a novel area for investigation and implicates the importance of skeletal muscle in the pathophysiology of metabolic diseases such as type 2 diabetes (T2D). Given that underlying molecular mechanisms of T2D are subject of ongoing research, in light of new evidence it is probable that interorgan cross-talk between skeletal muscle and pancreatic β-cells plays an important part. To date, the number of studies published in this field provide the basis of this review. Specifically, we discuss current experimental evidence in support for a role of skeletal muscle to β-cell cross-talk, paying particular attention to muscle-secreted factors including myokines, metabolites, miRNAs, and factors contained within exosomes that influence the function and/or the survival of β-cells in health and disease. In reviewing this evidence, we provide an update on the list of known muscle-secreted factors that have potential to influence the function and/or survival of β-cells under normal and diabetic conditions. We also report limitations of current cross-talk methods and discuss future directions in this growing field.

The Dynamic Effects of Isosteviol on Insulin Secretion and Its Inability to Counteract the Impaired β-Cell Function during Gluco-, Lipo-, and Aminoacidotoxicity: Studies In Vitro

Isosteviol (ISV), a diterpene molecule, is an isomer of the backbone structure of a group of substances with proven antidiabetic capabilities. The aim of this study was to investigate if ISV elicits dynamic insulin release from pancreatic islets and concomitantly is able to ameliorate gluco-, lipo-, and aminoacidotoxicity in clonal β-cell line (INS-1E) in relation to cell viability and insulin secretion. Isolated mice islets placed into perifusion chambers were perifused with 3.3 mM and 16.7 mM glucose with/without 10⁻⁷ M ISV. INS-1E cells were incubated for 72 h with either 30 mM glucose, 1 mM palmitate or 10 mM leucine with or without 10⁻⁷ M ISV. Cell viability was evaluated with a Cytotoxic Fluoro-test and insulin secretion was measured in Krebs-Ringer Buffer at 3.3 mM and 16.7 mM glucose. In the presence of 3.3 mM glucose, 10⁻⁷ M ISV did not change basal insulin secretion from perifused islets. However, at a high glucose level of 16.7 mM, 10⁻⁷ M ISV elicited a 2.5-fold increase (−ISV: 109.92 ± 18.64 ng/mL vs. +ISV: 280.15 ± 34.97 ng/mL; p < 0.01). After 72 h gluco-, lipo-, or aminoacidotoxicity in INS-1E cells, ISV treatment did not significantly affect cell viability (glucotoxicity, −ISV: 19.23 ± 0.83%, +ISV: 18.41 ± 0.90%; lipotoxicity, −ISV: 70.46 ± 3.15%, +ISV: 65.38 ± 2.81%; aminoacidotoxicity: −ISV: 8.12 ± 0.63%; +ISV: 7.75 ± 0.38%, all nonsignificant). ISV did not improve impaired insulin secretion (glucotoxicity, −ISV: 52.22 ± 2.90 ng/mL, +ISV: 47.24 ± 3.61 ng/mL; lipotoxicity, −ISV: 19.94 ± 4.10 ng/mL, +ISV: 22.12 ± 3.94 ng/mL; aminoacidotoxicity: −ISV: 32.13 ± 1.00 ng/mL; +ISV: 30.61 ± 1.54 ng/mL, all nonsignificant). In conclusion, ISV acutely stimulates insulin secretion at high but not at low glucose concentrations. However, ISV did not counteract cell viability or cell dysfunction during gluco-, lipo-, or aminoacidotoxicity in INS-1E cells.

Contribution of Adipose Tissue to Development of Cancer

Solid tumor growth and metastasis require the interaction of tumor cells with the surrounding tissue, leading to a view of tumors as tissue-level phenomena rather than exclusively cell-intrinsic anomalies. Due to the ubiquitous nature of adipose tissue, many types of solid tumors grow in proximate or direct contact with adipocytes and adipose-associated stromal and vascular components, such as fibroblasts and other connective tissue cells, stem and progenitor cells, endothelial cells, innate and adaptive immune cells, and extracellular signaling and matrix components. Excess adiposity in obesity both increases risk of cancer development and negatively influences prognosis in several cancer types, in part due to interaction with adipose tissue cell populations. Herein, we review the cellular and noncellular constituents of the adipose "organ," and discuss the mechanisms by which these varied microenvironmental components contribute to tumor development, with special emphasis on obesity. Due to the prevalence of breast and prostate cancers in the United States, their close anatomical proximity to adipose tissue depots, and their complex epidemiologic associations with obesity, we particularly highlight research addressing the contribution of adipose tissue to the initiation and progression of these cancer types. Obesity dramatically modifies the adipose tissue microenvironment in numerous ways, including induction of fibrosis and angiogenesis, increased stem cell abundance, and expansion of proinflammatory immune cells. As many of these changes also resemble shifts observed within the tumor microenvironment, proximity to adipose tissue may present a hospitable environment to developing tumors, providing a critical link between adiposity and tumorigenesis. © 2018 American Physiological Society. Compr Physiol 8:237-282, 2018.

Omega-6 fatty acid biomarkers and incident type 2 diabetes: pooled analysis of individual-level data for 39 740 adults from 20 prospective cohort studies

BACKGROUND

The metabolic effects of omega-6 polyunsaturated fatty acids (PUFAs) remain contentious, and little evidence is available regarding their potential role in primary prevention of type 2 diabetes. We aimed to assess the associations of linoleic acid and arachidonic acid biomarkers with incident type 2 diabetes.

METHODS

We did a pooled analysis of new, harmonised, individual-level analyses for the biomarkers linoleic acid and its metabolite arachidonic acid and incident type 2 diabetes. We analysed data from 20 prospective cohort studies from ten countries (Iceland, the Netherlands, the USA, Taiwan, the UK, Germany, Finland, Australia, Sweden, and France), with biomarkers sampled between 1970 and 2010. Participants included in the analyses were aged 18 years or older and had data available for linoleic acid and arachidonic acid biomarkers at baseline. We excluded participants with type 2 diabetes at baseline. The main outcome was the association between omega-6 PUFA biomarkers and incident type 2 diabetes. We assessed the relative risk of type 2 diabetes prospectively for each cohort and lipid compartment separately using a prespecified analytic plan for exposures, covariates, effect modifiers, and analysis, and the findings were then pooled using inverse-variance weighted meta-analysis.

FINDINGS

Participants were 39 740 adults, aged (range of cohort means) 49-76 years with a BMI (range of cohort means) of 23·3-28·4 kg/m2, who did not have type 2 diabetes at baseline. During a follow-up of 366 073 person-years, we identified 4347 cases of incident type 2 diabetes. In multivariable-adjusted pooled analyses, higher proportions of linoleic acid biomarkers as percentages of total fatty acid were associated with a lower risk of type 2 diabetes overall (risk ratio [RR] per interquintile range 0·65, 95% CI 0·60-0·72, p<0·0001; I2=53·9%, pheterogeneity=0·002). The associations between linoleic acid biomarkers and type 2 diabetes were generally similar in different lipid compartments, including phospholipids, plasma, cholesterol esters, and adipose tissue. Levels of arachidonic acid biomarker were not significantly associated with type 2 diabetes risk overall (RR per interquintile range 0·96, 95% CI 0·88-1·05; p=0·38; I2=63·0%, pheterogeneity<0·0001). The associations between linoleic acid and arachidonic acid biomarkers and the risk of type 2 diabetes were not significantly modified by any prespecified potential sources of heterogeneity (ie, age, BMI, sex, race, aspirin use, omega-3 PUFA levels, or variants of the FADS gene; all pheterogeneity≥0·13).

INTERPRETATION

Findings suggest that linoleic acid has long-term benefits for the prevention of type 2 diabetes and that arachidonic acid is not harmful.

FUNDING

Funders are shown in the appendix.

Palmitate-induced lipotoxicity alters acetylation of multiple proteins in clonal β cells and human pancreatic islets

Type 2 diabetes is characterized by progressive β cell dysfunction, with lipotoxicity playing a possible pathogenetic role. Palmitate is often used to examine the direct effects of lipotoxicity and it may cause mitochondrial alterations by activating protein acetylation. However, it is unknown whether palmitate influences protein acetylation in β cells. We investigated lysine acetylation in mitochondrial proteins from INS-1E β cells (INS-1E) and in proteins from human pancreatic islets (HPI) after 24 h palmitate exposure. First, we confirmed that palmitate damages β cells and demonstrated that chemical inhibition of deacetylation also impairs INS-1E function and survival. Then, by 2-D gel electrophoresis, Western Blot and Liquid Chromatography-Mass Spectrometry we evaluated the effects of palmitate on protein acetylation. In mitochondrial preparations from palmitate-treated INS-1E, 32 acetylated spots were detected, with 13 proteins resulting over-acetylated. In HPI, 136 acetylated proteins were found, of which 11 were over-acetylated upon culture with palmitate. Interestingly, three proteins, glutamate dehydrogenase, mitochondrial superoxide dismutase, and SREBP-1, were over-acetylated in both INS-1E and HPI. Therefore, prolonged exposure to palmitate induces changes in β cell protein lysine acetylation and this modification could play a role in causing β cell damage. Dysregulated acetylation may be a target to counteract palmitate-induced β cell lipotoxicity.

Into the eyes of bone marrow-derived mesenchymal stem cells therapy for myocardial infarction and other diseases

Applications of bone marrow-derived mesenchymal stem cells (BM-MSCs) have been documented for diseases occur in the sports system, the central nervous system, the cardiovascular system etc. However, poor viability of donor stem cells after transplantation limits their therapeutic efficiency. Although the autophagy theory has been reported, the underlying mechanisms are still poorly understood. Isolation and culture methods of mesenchymal stem cells are currently concentrate on four ways. Overall, BM-MSCs have both important research significance and clinical application value in cell replacement therapy, gene therapy and reconstruction of tissues as well as organs especially for myocardial infarction (MI). In this article, we review the biological characteristics of BM-MSCs and its research progress especially in MI.

The Novel Mechanisms Concerning the Inhibitions of Palmitate-Induced Proinflammatory Factor Releases and Endogenous Cellular Stress with Astaxanthin on MIN6 β-Cells

Astaxanthin, an antioxidant agent, can protect pancreatic β-cells of db/db mice from glucotoxicity and resolve chronic inflammation in adipose tissue. Nonetheless, the effects of astaxanthin on free-fatty-acid-induced inflammation and cellular stress in β-cells remain to be demonstrated. Meanwhile, palmitate enhances the secretion of pro-inflammatory adipokines monocyte chemoattractant protein-1 (MCP-1) and vascular endothelial growth factor (VEGF₁₂₀). We therefore investigated the influence of astaxanthin on palmitate-stimulated MCP-1 and VEGF₁₂₀ secretion in mouse insulinoma (MIN6) pancreatic β-cells. Furthermore, whether astaxanthin prevents cellular stress in MIN6 cells was also assessed. Pre-treatment with astaxanthin or with N-acetyl-cysteine (NAC) which is an antioxidant drug, significantly attenuated the palmitate-induced MCP-1 release through downregulation of phosphorylated c-Jun NH₂-terminal protein kinase (JNK) pathways, and suppressed VEGF₁₂₀ through the PI3K/Akt pathways relative to the cells stimulated with palmitate alone. In addition, palmitate significantly upregulated homologous protein (CHOP) and anti-glucose-regulated protein (GRP78), which are endoplasmic reticulum (ER) stress markers, in MIN6 cells. On the other hand, astaxanthin attenuated the increased CHOP content, but further up-regulated palmitate-stimulated GRP78 protein expression. By contrast, NAC had no effects on either CHOP or GRP78 enhancement induced by palmitate in MIN6 cells. In conclusion, astaxanthin diminishes the palmitate-stimulated increase in MCP-1 secretion via the downregulation of JNK pathways in MIN6 cells, and affects VEGF₁₂₀ secretion through PI3K/Akt pathways. Moreover, astaxanthin can prevent not only oxidative stress caused endogenously by palmitate but also ER stress, which NAC fails to attenuate, via upregulation of GRP78, an ER chaperon.

The impairment of glucose-stimulated insulin secretion in pancreatic β-cells caused by prolonged glucotoxicity and lipotoxicity is associated with elevated adaptive antioxidant response

Type 2 diabetes (T2D) is a progressive disease characterized by sustained hyperglycemia and is frequently accompanied by hyperlipidemia. Deterioration of β-cell function in T2D patients may be caused, in part, by long-term exposure to high concentrations of glucose and/or lipids. We developed systems to study how chronic glucotoxicity and lipotoxicity might be linked to the impairment of glucose-stimulated insulin secretion (GSIS) machinery in pancreatic β-cells. INS-1 (832/13) were exposed to glucose and/or palmitate for up to 10 weeks. Chronic high glucose and/or palmitate exposure resulted in impaired GSIS accompanied by a dramatic increase in oxidative stress, as determined by basal intracellular peroxide levels. In addition, the GSIS-associated reactive oxygen species (ROS) signals, assessed as glucose-stimulated peroxide accumulation positively correlated with GSIS in glucose- and/or palmitate-exposed cells, as well as glucose-stimulated reductions in GSH/GSSG ratios. Furthermore, the impairment of GSIS caused by chronic high glucose and/or palmitate exposures were attributed to the induction of adaptive antioxidant response and mitochondrial uncoupling, which negatively regulates glucose-derived ROS generation. Taken together, persistent glucotoxicity- and/or lipotoxicity-mediated oxidative stress and subsequent adaptive antioxidant response impair glucose-derived ROS signaling and GSIS in pancreatic β-cells.

A hypothetical model to solve the controversy over the involvement of UCP2 in palmitate-induced β-cell dysfunction

The aim of this article is to solve an existing controversy over the involvement of uncoupling protein-2 in the impairment of glucose-stimulated insulin secretion induced by chronic exposure of β-cells to palmitate. We analyzed and compared the results of studies that support and that deny the involvement of uncoupling protein-2 in this impairment. We observed that this impairment could occur in multiple stages. We provide a model in which palmitate-induced impairment of glucose-stimulated insulin secretion is proposed to occur in two stages, early stage and late stage, depending on the integrity of electron supply (glycolysis and Krebs cycle) and transport system through electron transport chain after palmitate treatment. Prolonged exposure of β-cells to palmitate can impair this system. Early-stage impairment occurs due to uncoupling by uncoupling protein-2 when this system is still intact. When this system becomes impaired, late-stage impairment occurs mainly due to reduced glucose-stimulated adenosine triphosphate production independent of uncoupling by uncoupling protein-2. The change in glucose-stimulated oxygen uptake after palmitate treatment reflects the integrity of this system and can be used to differentiate between the two stages. Some β-cells lines and islets appear to be more resistant to palmitate-induced impairment of electron supply and transport system than others, and therefore early stage is prominent in the more resistant cell lines and less prominent or absent in the less resistant cell lines. This may help to resolve the pathogenesis of diabetes and to monitor the progression of palmitate-induced β-cell dysfunction.