Adverse physicochemical properties of tripalmitin in beta cells lead to morphological changes and lipotoxicity in vitro

The role of DGAT1 and DGAT2 in tumor progression via fatty acid metabolism: A comprehensive review

Fatty acid metabolism is a complex biochemical process, including the production, breakdown and application of fatty acids. Not only is it an important component of lipid metabolism, fatty acid metabolism is also connected to the energy metabolism pathways of cells and plays a vital role in maintaining the energy balance of organisms. Diacylglycerol-O-acyltransferase 1 (DGAT1) and Diacylglycerol-O-acyltransferase 2 (DGAT2) are key components in regulating lipid metabolism, which provide energy for cell proliferation and growth. Recent studies have shown that DGAT1 and DGAT2 influence tumor progression through fatty acid metabolism in cancer. Although DGAT1 and DGAT2 have similar names, they differ significantly in various aspects and play distinct roles in individual tumors. A comparative analysis of the physiological roles of these enzymes and their differential expressions in different types of tumors will enhance our understanding of their unique characteristics. This article summarizes the characteristics of tumor fatty acid metabolism and explains how DGAT1 and DGAT2 specifically promote tumor progression. In addition, this review discusses the potential of lipid-lowering drugs in tumor treatment, providing a new perspective on targeting fatty acid metabolism to inhibit tumor progression in the future, while emphasizing the importance of DGAT1 and DGAT2 as potential targets for tumor treatment.

The lipid side of unfolded protein response

Although our current knowledge of the molecular crosstalk between the ER stress, the unfolded protein response (UPR), and lipid homeostasis remains limited, there is increasing evidence that dysregulation of either protein or lipid homeostasis profoundly affects the other. Most research regarding UPR signaling in human diseases has focused on the causes and consequences of disrupted protein folding. The UPR itself consists of very complex pathways that function to not only maintain protein homeostasis, but just as importantly, modulate lipid biogenesis to allow the ER to adjust and promote cell survival. Lipid dysregulation is known to activate many aspects of the UPR, but the complexity of this crosstalk remains a major research barrier. ER lipid disequilibrium and lipotoxicity are known to be important contributors to numerous human pathologies, including insulin resistance, liver disease, cardiovascular diseases, neurodegenerative diseases, and cancer. Despite their medical significance and continuous research, however, the molecular mechanisms that modulate lipid synthesis during ER stress conditions, and their impact on cell fate decisions, remain poorly understood. Here we summarize the current view on crosstalk and connections between altered lipid metabolism, ER stress, and the UPR.

Elevated Adipocyte Membrane Phospholipid Saturation Does Not Compromise Insulin Signaling

Increased saturated fatty acid (SFA) levels in membrane phospholipids have been implicated in the development of metabolic disease. Here, we tested the hypothesis that increased SFA content in cell membranes negatively impacts adipocyte insulin signaling. Preadipocyte cell models with elevated SFA levels in phospholipids were generated by disrupting the ADIPOR2 locus, which resulted in a striking twofold increase in SFA-containing phosphatidylcholines and phosphatidylethanolamines, which persisted in differentiated adipocytes. Similar changes in phospholipid composition were observed in white adipose tissues isolated from the ADIPOR2-knockout mice. The SFA levels in phospholipids could be further increased by treating ADIPOR2-deficient cells with palmitic acid and resulted in reduced membrane fluidity and endoplasmic reticulum stress in mouse and human preadipocytes. Strikingly, increased SFA levels in differentiated adipocyte phospholipids had no effect on adipocyte gene expression or insulin signaling in vitro. Similarly, increased adipocyte phospholipid saturation did not impair white adipose tissue function in vivo, even in mice fed a high-saturated fat diet at thermoneutrality. We conclude that increasing SFA levels in adipocyte phospholipids is well tolerated and does not affect adipocyte insulin signaling in vitro and in vivo.

Liposomes trigger bone marrow niche macrophage "foam" cell formation and affect hematopoiesis in mice

Liposomes are the most widely used nanocarrier platform for the delivery of therapeutic and diagnostic agents, and a number of liposomes have been approved for use in clinical practice. After systemic administration, most liposomes are cleared by macrophages in the mononuclear phagocyte system, such as the liver and bone marrow (BM). However, the majority of studies have focused on investigating the therapeutic results of liposomal drugs, and too few studies have evaluated the potential side effects of empty nanocarriers on the functions of macrophages in the mononuclear phagocyte system. Here, we evaluate the potential effects of empty liposomes on the functions of BM niche macrophages. Following liposome administration, we observed lipid droplet (LD) accumulation in cultured primary macrophages and BM niche macrophages. We found that these LD-accumulating macrophages, similar to foam cells, exhibited increased expression of inflammatory cytokines, such as IL-1β and IL-6. We further provided evidence that liposome deposition and degradation induced LD biogenesis on the endoplasmic reticulum membrane and subsequently disturbed endoplasmic reticulum homeostasis and activated the inositol-requiring transmembrane kinase/endoribonuclease 1α/NF-κB signaling pathway, which is responsible for the inflammatory activation of macrophages after liposome engulfment. Finally, we also showed the side effects of dysfunctional BM niche macrophages on hematopoiesis in mice, such as the promotion of myeloid-biased output and impairment of erythropoiesis. This study not only draws attention to the safety of liposomal drugs in clinical practice but also provides new directions for the design of lipid-based drug carriers in preclinical studies.

Dietary intervention preserves β cell function in mice through CTCF-mediated transcriptional reprogramming

Pancreatic β cell plasticity is the primary determinant of disease progression and remission of type 2 diabetes (T2D). However, the dynamic nature of β cell adaptation remains elusive. Here, we establish a mouse model exhibiting the compensation-to-decompensation adaptation of β cell function in response to increasing duration of high-fat diet (HFD) feeding. Comprehensive islet functional and transcriptome analyses reveal a dynamic orchestration of transcriptional networks featuring temporal alteration of chromatin remodeling. Interestingly, prediabetic dietary intervention completely rescues β cell dysfunction, accompanied by a remarkable reversal of HFD-induced reprogramming of islet chromatin accessibility and transcriptome. Mechanistically, ATAC-based motif analysis identifies CTCF as the top candidate driving dietary intervention-induced preservation of β cell function. CTCF expression is markedly decreased in β cells from obese and diabetic mice and humans. Both dietary intervention and AAV-mediated restoration of CTCF expression ameliorate β cell dysfunction ex vivo and in vivo, through transducing the lipid toxicity and inflammatory signals to transcriptional reprogramming of genes critical for β cell glucose metabolism and stress response.

Lipidomic Analyses Uncover Apoptotic and Inhibitory Effects of Pyrvinium Pamoate on Cholangiocarcinoma Cells via Mitochondrial Membrane Potential Dysfunction

Pyrvinium pamoate (PP), an FDA-approved anthelmintic drug, has been validated as a highly potent anti-cancer agent and patented recently as a potential chemotherapeutic drug for various cancers. The aims of this study were, therefore, to investigate the ability of PP in anti-proliferative activity and focused on the lipid profiles revealing the alteration of specific lipid species in the liver fluke Opisthorchis viverrini (Ov)-associated cholangiocarcinoma (CCA) cells. PP inhibited CCA cell viability through suppressing mitochondrial membrane potential (MMP) and ATP productions, leading to apoptotic cell death. Liquid chromatography-mass spectrometry combined with chemometrics was performed to investigate lipid alteration during PP-induced apoptosis. The lipidomic analyses showed the altered lipid signatures of CCA cell types including S-acetyldihydrolipoamide, methylselenopyruvate, and triglycerides that were increased in PP-treated CCA cells. In contrast, the levels of sphinganine and phosphatidylinositol were lower in the PP-treated group compared with its counterpart. The orthogonal partial-least squares regression analysis revealed that PP-induced MMP dysfunction, leading to remarkably reduced ATP level, was significantly associated with triglyceride (TG) accumulation observed in PP-treated CCA cells. Our findings indicate that PP could suppress the MMP function, which causes inhibition of CCA cell viability through lipid production, resulting in apoptotic induction in CCA cells. These findings provide an anti-cancer mechanism of PP under apoptotic induction ability that may serve as the alternative approach for CCA treatment.

Perilipin 2 downregulation in β cells impairs insulin secretion under nutritional stress and damages mitochondria

Perilipin 2 (PLIN2) is a lipid droplet (LD) protein in β cells that increases under nutritional stress. Downregulation of PLIN2 is often sufficient to reduce LD accumulation. To determine whether PLIN2 positively or negatively affects β cell function under nutritional stress, PLIN2 was downregulated in mouse β cells, INS1 cells, and human islet cells. β Cell–specific deletion of PLIN2 in mice on a high-fat diet reduced glucose-stimulated insulin secretion (GSIS) in vivo and in vitro. Downregulation of PLIN2 in INS1 cells blunted GSIS after 24-hour incubation with 0.2 mM palmitic acid. Downregulation of PLIN2 in human pseudoislets cultured at 5.6 mM glucose impaired both phases of GSIS, indicating that PLIN2 is critical for GSIS. Downregulation of PLIN2 decreased specific OXPHOS proteins in all 3 models and reduced oxygen consumption rates in INS1 cells and mouse islets. Moreover, we found that PLIN2-deficient INS1 cells increased the distribution of a fluorescent oleic acid analog to mitochondria and showed signs of mitochondrial stress, as indicated by susceptibility to fragmentation and alterations of acyl-carnitines and glucose metabolites. Collectively, PLIN2 in β cells has an important role in preserving insulin secretion, β cell metabolism, and mitochondrial function under nutritional stress.

Molecular Mechanisms of Apoptosis Induction and Its Regulation by Fatty Acids in Pancreatic β-Cells

Pancreatic β-cell failure and death contribute significantly to the pathogenesis of type 2 diabetes. One of the main factors responsible for β-cell dysfunction and subsequent cell death is chronic exposure to increased concentrations of FAs (fatty acids). The effect of FAs seems to depend particularly on the degree of their saturation. Saturated FAs induce apoptosis in pancreatic β-cells, whereas unsaturated FAs are well tolerated and are even capable of inhibiting the pro-apoptotic effect of saturated FAs. Molecular mechanisms of apoptosis induction by saturated FAs in β-cells are not completely elucidated. Saturated FAs induce ER stress, which in turn leads to activation of all ER stress pathways. When ER stress is severe or prolonged, apoptosis is induced. The main mediator seems to be the CHOP transcription factor. Via regulation of expression/activity of pro- and anti-apoptotic Bcl-2 family members, and potentially also through the increase in ROS production, CHOP switches on the mitochondrial pathway of apoptosis induction. ER stress signalling also possibly leads to autophagy signalling, which may activate caspase-8. Saturated FAs activate or inhibit various signalling pathways, i.e., p38 MAPK signalling, ERK signalling, ceramide signalling, Akt signalling and PKCδ signalling. This may lead to the activation of the mitochondrial pathway of apoptosis, as well. Particularly, the inhibition of the pro-survival Akt signalling seems to play an important role. This inhibition may be mediated by multiple pathways (e.g., ER stress signalling, PKCδ and ceramide) and could also consequence in autophagy signalling. Experimental evidence indicates the involvement of certain miRNAs in mechanisms of FA-induced β-cell apoptosis, as well. In the rather rare situations when unsaturated FAs are also shown to be pro-apoptotic, the mechanisms mediating this effect in β-cells seem to be the same as for saturated FAs. To conclude, FA-induced apoptosis rather appears to be preceded by complex cross talks of multiple signalling pathways. Some of these pathways may be regulated by decreased membrane fluidity due to saturated FA incorporation. Few data are available concerning molecular mechanisms mediating the protective effect of unsaturated FAs on the effect of saturated FAs. It seems that the main possible mechanism represents a rather inhibitory intervention into saturated FA-induced pro-apoptotic signalling than activation of some pro-survival signalling pathway(s) or metabolic interference in β-cells. This inhibitory intervention may be due to an increase of membrane fluidity.

DGAT1 inhibitors protect pancreatic β-cells from palmitic acid-induced apoptosis

Previous studies demonstrated that prolonged exposure to elevated levels of free fatty acids (FFA), especially saturated fatty acids, could lead to pancreatic β-cell apoptosis, which plays an important role in the progression of type 2 diabetes (T2D). Diacylglycerol acyltransferase 1 (DGAT1), an enzyme that catalyzes the final step of triglyceride (TG) synthesis, has been reported as a novel target for the treatment of multiple metabolic diseases. In this study we evaluated the potential beneficial effects of DGAT1 inhibitors on pancreatic β-cells, and further verified their antidiabetic effects in db/db mice. We showed that DGAT1 inhibitors (4a and LCQ908) at the concentration of 1 μM significantly ameliorated palmitic acid (PA)-induced apoptosis in MIN6 pancreatic β-cells and primary cultured mouse islets; oral administration of a DGAT1 inhibitor (4a) (100 mg/kg) for 4 weeks significantly reduced the apoptosis of pancreatic islets in db/db mice. Meanwhile, 4a administration significantly decreased fasting blood glucose and TG levels, and improved glucose tolerance and insulin tolerance in db/db mice. Furthermore, we revealed that pretreatment with 4a (1 μM) significantly alleviated PA-induced intracellular lipid accumulation, endoplasmic reticulum (ER) stress, and proinflammatory responses in MIN6 cells, which might contribute to the protective effects of DGAT1 inhibitors on pancreatic β-cells. These findings provided a better understanding of the antidiabetic effects of DGAT1 inhibitors.

The Influence of Obesity and Associated Fatty Acids on Placental Inflammation

PURPOSE

Maternal obesity, affecting nearly 1 in 4 pregnancies, is associated with increased circulating saturated fatty acids, such as palmitate. These fatty acids are implicated in placental inflammation, which may in turn exacerbate both maternal-fetal tolerance and responses to pathogens, such as group B Streptococcus. In this review, we address the question, "How do obesity and associated fatty acids influence placental inflammation?"

METHODS

In this narrative review, we searched PubMed and Google Scholar using combinations of the key words placental inflammation or pregnancy and lipids, fatty acids, obesity, palmitate, or other closely related search terms. We also used references found within these articles that may have been absent from our original search queries. We analyzed methods and key results of these articles to compare and contrast their findings, which were occasionally at odds with each other.

FINDINGS

Although obesity can be studied as a whole, complex phenomena with in vivo mouse models and human samples from patients with obesity, in vitro modeling often relies on the treatment of cells or tissues with ≥1 fatty acids and occasionally other compounds (eg, glucose and insulin). We found that palmitate, most commonly used in vitro to recreate hallmarks of obesity, induces apoptosis, oxidative stress, mitochondrial dysfunction, autophagy defects, and inflammasome activation in many placental cell types. We compare this to in vivo models of obesity wherever possible. We found that obesity as a whole may have more complex regulation of these phenomena (apoptosis, oxidative stress, mitochondrial dysfunction, autophagy defects, and inflammasome activation) compared with in vitro models of fatty acid treatment (primarily palmitate) because of the presence of unsaturated fatty acids (ie, oleate), which may have anti-inflammatory effects.

IMPLICATIONS

The interaction of unsaturated fatty acids with saturated fatty acids may ameliorate many inflammatory effects of saturated fatty acids alone, which complicates interpretation of in vitro studies that focus on a particular fatty acid in isolation. This complication may explain why certain studies of obesity in vivo have differing outcomes from studies of specific fatty acids in vitro.

Beyond the cell factory: Homeostatic regulation of and by the UPRER

The endoplasmic reticulum (ER) is commonly referred to as the factory of the cell, as it is responsible for a large amount of protein and lipid synthesis. As a membrane-bound organelle, the ER has a distinct environment that is ideal for its functions in synthesizing these primary cellular components. Many different quality control machineries exist to maintain ER stability under the stresses associated with synthesizing, folding, and modifying complex proteins and lipids. The best understood of these mechanisms is the unfolded protein response of the ER (UPR^ER), in which transmembrane proteins serve as sensors, which trigger a coordinated transcriptional response of genes dedicated for mitigating the stress. As the name suggests, the UPR^ER is most well described as a functional response to protein misfolding stress. Here, we focus on recent findings and emerging themes in additional roles of the UPR^ER outside of protein homeostasis, including lipid homeostasis, autophagy, apoptosis, and immunity.

Short-term inhibition of autophagy benefits pancreatic β-cells by augmenting ether lipids and peroxisomal function, and by countering depletion of n-3 polyunsaturated fatty acids after fat-feeding

OBJECTIVE

Investigations of autophagy in β-cells have usually focused on its homeostatic function. More dynamic roles in inhibiting glucose-stimulated insulin secretion (GSIS), potentially involving remodelling of cellular lipids, have been suggested from in vitro studies but not evaluated in vivo.

METHODS

We employed temporally-regulated deletion of the essential autophagy gene, Atg7, in β-cells. Mice were fed chow or high-fat diets (HFD), in conjunction with deletion of Atg7 for the last 3 weeks (short-term model) or 9 weeks (long-term model). Standard in vivo metabolic phenotyping was undertaken, and 450 lipid species in islets quantified ex vivo using mass spectroscopy (MS). MIN6 cells were also employed for lipidomics and secretory interventions.

RESULTS

β-cell function was impaired by inhibiting autophagy in the longer-term, but conversely improved by 3-week deletion of Atg7, specifically under HFD conditions. This was accompanied by augmented GSIS ex vivo. Surprisingly, the HFD had minimal effect on sphingolipid and neutral lipid species, but modulated >100 phospholipids and ether lipids, and markedly shifted the profile of polyunsaturated fatty acid (PUFA) sidechains from n3 to n6 forms. These changes were partially countered by Atg7 deletion, consistent with an accompanying upregulation of the PUFA elongase enzyme, Elovl5. Loss of Atg7 separately augmented plasmalogens and alkyl lipids, in association with increased expression of Lonp2, a peroxisomal chaperone/protease that facilitates maturation of ether lipid synthetic enzymes. Depletion of PUFAs and ether lipids was also observed in MIN6 cells chronically exposed to oleate (more so than palmitate). GSIS was inhibited by knocking down Dhrs7b, which encodes an enzyme of peroxisomal ether lipid synthesis. Conversely, impaired GSIS due to oleate pre-treatment was selectively reverted by Dhrs7b overexpression.

CONCLUSIONS

A detrimental increase in n6:n3 PUFA ratios in ether lipids and phospholipids is revealed as a major response of β-cells to high-fat feeding. This is partially reversed by short-term inhibition of autophagy, which results in compensatory changes in peroxisomal lipid metabolism. The short-term phenotype is linked to improved GSIS, in contrast to the impairment seen with the longer-term inhibition of autophagy. The balance between these positive and negative inputs could help determine whether β-cells adapt or fail in response to obesity.

Glucolipotoxicity, β-Cells, and Diabetes: The Emperor Has No Clothes

Reduction of β-cell mass and function is central to the pathogenesis of type 2 diabetes. The terms glucotoxicity, lipotoxicity, and glucolipotoxicity are used to describe potentially responsible processes. The premise is that chronically elevated glucose levels are toxic to β-cells, that elevated lipid levels in the form of circulating free fatty acids (FFA) also have toxic effects, and that the combination of the two, glucolipotoxicity, is particularly harmful. Much work has shown that high concentrations of FFA can be very damaging to β-cells when used for in vitro experiments, and when infused in large amounts in humans and rodents they produce suppression of insulin secretion. The purpose of this Perspective is to raise doubts about whether the FFA levels found in real-life situations are ever high enough to cause problems. Evidence supporting the importance of glucotoxicity is strong because there is such a tight correlation between defective insulin secretion and rising glucose levels. However, there is virtually no convincing evidence that the alterations in FFA levels occurring during progression to diabetes are pathogenic. Thus, the terms lipotoxicity and glucolipotoxicity should be used with great caution, if at all, because evidence supporting their importance has not yet emerged.

ABCA12 regulates insulin secretion from β-cells

Dysregulation of lipid homeostasis is intimately associated with defects in insulin secretion, a key feature of type 2 diabetes. Here, we explore the role of the putative lipid transporter ABCA12 in regulating insulin secretion from β-cells. Mice with β-cell-specific deletion of Abca12 display impaired glucose-stimulated insulin secretion and eventual islet inflammation and β-cell death. ABCA12's action in the pancreas is independent of changes in the abundance of two other cholesterol transporters, ABCA1 and ABCG1, or of changes in cellular cholesterol or ceramide content. Instead, loss of ABCA12 results in defects in the genesis and fusion of insulin secretory granules and increases in the abundance of lipid rafts at the cell membrane. These changes are associated with dysregulation of the small GTPase CDC42 and with decreased actin polymerisation. Our findings establish a new, pleiotropic role for ABCA12 in regulating pancreatic lipid homeostasis and insulin secretion.

Induction of Stearoyl-CoA 9-Desaturase 1 Protects Human Mesenchymal Stromal Cells Against Palmitic Acid-Induced Lipotoxicity and Inflammation

In bone diseases such as osteonecrosis and osteoporosis, a shift toward a preferential differentiation of mesenchymal stromal cells (MSC) into adipocytes at the expense of the osteoblastic lineage is described, leading to excessive accumulation of adipocytes in the bone marrow of the patients. The influence of cytokines and adipokines secreted by adipocytes on skeletal health is already well-documented but the impact of free fatty acids release on bone cell biology and viability is an emerging concept. We have previously demonstrated that the saturated fatty acid (SFA) palmitate (Palm) is cytotoxic for human MSC (hMSC) and osteoblasts whereas oleate (Ole), a monounsaturated fatty acid (MUFA), has no toxic effect. Moreover, Ole protects cells against lipotoxicity. Our observations led us to propose that the toxicity of the SFA is not correlated to its intracellular accumulation but could rather be related to the intracellular SFA/MUFA ratio, which finally determines the toxic effect of SFA. Therefore, in the present study, we have investigated the potential protective role of the enzyme stearoyl-CoA 9-desaturase 1 (SCD1) against the deleterious effects of Palm. SCD1 is an enzyme responsible for desaturation of SFA to MUFA; its activation could therefore lead to modifications of the intracellular SFA/MUFA ratio. In the present study, we showed that hMSC express SCD1 and liver X receptors (LXRs), transcription factors regulating SCD1 expression. Human MSC treatment with a LXRs agonist triggered SCD1 expression and drastically reduced Palm-induced cell mortality, caspases 3/7 activation, endoplasmic reticulum stress and inflammation. We also observed that, in the presence of Palm, the LXRs agonist provoked lipid droplets formation, augmented the total cellular neutral lipid content but decreased the SFA/MUFA ratio when compared to Palm treatment alone. Addition of an inhibitor of SCD1 activity abrogated the positive effects of the LXRs agonist, suggesting that SCD1 could play a key role in protecting hMSC against lipotoxicity.

Protein translation associated to PERK arm is a new target for regulation of metainflammation: A connection with hepatocyte cholesterol

Endoplasmic reticulum stress is a cellular phenomenon that has been associated with metabolic disorders, contributing to the development of obesity, fatty liver disease, and dyslipidemias. Under metabolic overload conditions, in cells with a high protein-secretory activity, such as hepatocytes and Langerhans β cells, the unfolded protein response (UPR) is critical in to maintain protein homeostasis (proteostasis). UPR integrated by a tripartite signaling system, through activating transcription factor 6, protein kinase R-like endoplasmic reticulum kinase (PERK), and inositol-requiring enzyme 1, regulates gene transcription and translation to resolve stress and conserve proteostasis. In the current study, we demonstrated in hepatocytes under metabolic overload by saturated palmitic and stearic fatty acids, through activation of PERK signaling and CCAAT-enhancer-binding protein homologous protein (CHOP) transcription factor, an association with the expression of cyclooxygenase 2. More important, isolated exosomes from supernatants of macrophages exposed to lipopolysaccharides can also induce a metainflammation phenomenon, and when treated on hepatocytes, induced a rearrangement in cholesterol metabolism through sterol regulatory element-binding protein 2 (SREBP2), low-density lipoprotein receptor (LDLR), apolipoprotein A-I, and ABCA1. Moreover, we demonstrate the cellular effect of terpene-derived molecules, such as cryptotanshinone, isolated of plant Salvia brandegeei, regulating metainflammatory conditions through PERK pathway in both hepatocytes and β cells. Our data suggest the presence of a modulatory mechanism on specific protein translation process. This effect could be mediated by eukaryotic initiation factor-4A, evaluating salubrinal as a control molecule. Likewise, the protective mechanisms of unsaturated fatty acids, such as oleic and palmitoleic acid were confirmed. Therefore, modulation of metainflammation suggests a new target through PERK signaling in cells with a high secretory activity, and possibly the regulation of cholesterol in hepatocytes is promoted via exosomes.

Effects of Dietary Fatty Acids in Pancreatic Beta Cell Metabolism, Implications in Homeostasis

Fatty acids are involved in several metabolic processes, including the development of metabolic and cardiovascular diseases. In recent years a disease that has received escalated interest is type 2 diabetes (T2D). Many contributing factors including a high-caloric diet rich in dietary saturated fats have been broadly characterized as triggers of T2D. Insulin resistance resulting from a high saturated fat diet leads to alterations in lipid cellular intake and accumulation which generate lipotoxic conditions, a key phenomenon in the metabolism of β-cells. Alternatively, unsaturated fatty acids have been described to show opposite effects in pancreatic β-cells. The purpose of this work is to perform a critical analysis of the complex role of saturated and unsaturated fatty acids in β-cell metabolism. We discuss the diverse effects main dietary fatty acids have upon pancreatic β-cell metabolism as a key factor to maintain homeostasis by focusing in the cellular and molecular mechanisms involved in the development and progression of T2D. For instance, modifications in protein homeostasis as well as the intracellular management of lipid metabolism which are associated with inflammatory pathways. These conditions initiate critical metabolic rearrangements, that in turn have repercussions on insulin β-cell metabolism. This review allows an integral and broad understanding of different functions of fatty acids inside β-cells, being important metabolites for novel therapeutic targets in T2D treatment.

Calorie restriction and reversal of type 2 diabetes

Type 2 diabetes causes major global health problems and has been believed to be a lifelong condition with inevitable worsening. Steadily increasing numbers of drugs appeared to be required to achieve even modest control. Early type 2 diabetes has now been shown to be reversed by substantial weight loss and this has allowed temporal tracking of the underlying pathophysiological changes. Areas covered: In early type 2 diabetes, negative calorie balance decreases liver fat within days, and allows return of normal control of hepatic glucose production. Over 8 weeks, the negative calorie balance allows the raised levels of intra-pancreatic fat and simultaneously first phase insulin secretion to normalise. These findings are consistent with the 2008 Twin Cycle Hypothesis of the etiology and pathogenesis of type 2 diabetes. Individuals develop type 2 diabetes when they exceed their personal fat threshold for safe storage of fat and there is no difference in pathophysiology between those with BMI above or below 30 kg/m². Expert commentary: Type 2 diabetes can now be understood as a state of excess fat in liver and pancreas, and remains reversible for at least 10 years in most individuals.

Exploring the metabolic syndrome: Nonalcoholic fatty pancreas disease

After the first description of fatty pancreas in 1933, the effects of pancreatic steatosis have been poorly investigated, compared with that of the liver. However, the interest of research is increasing. Fat accumulation, associated with obesity and the metabolic syndrome (MetS), has been defined as “fatty infiltration” or “nonalcoholic fatty pancreas disease” (NAFPD). The term “fatty replacement” describes a distinct phenomenon characterized by death of acinar cells and replacement by adipose tissue. Risk factors for developing NAFPD include obesity, increasing age, male sex, hypertension, dyslipidemia, alcohol and hyperferritinemia. Increasing evidence support the role of pancreatic fat in the development of type 2 diabetes mellitus, MetS, atherosclerosis, severe acute pancreatitis and even pancreatic cancer. Evidence exists that fatty pancreas could be used as the initial indicator of “ectopic fat deposition”, which is a key element of nonalcoholic fatty liver disease and/or MetS. Moreover, in patients with fatty pancreas, pancreaticoduodenectomy is associated with an increased risk of intraoperative blood loss and post-operative pancreatic fistula.

Endoplasmic reticulum stress in obesity and obesity-related disorders: An expanded view

The endoplasmic reticulum (ER) is most notable for its central roles in calcium ion storage, lipid biosynthesis, and protein sorting and processing. By virtue of its extensive membrane contact sites that connect the ER to most other organelles and to the plasma membrane, the ER can also regulate diverse cellular processes including inflammatory and insulin signaling, nutrient metabolism, and cell proliferation and death via a signaling pathway called the unfolded protein response (UPR). Chronic UPR activation has been observed in liver and/or adipose tissue of dietary and genetic murine models of obesity, and in human obesity and non-alcoholic fatty liver disease (NAFLD). Activation of the UPR in obesity and obesity-related disorders likely has two origins. One linked to classic ER stress involving the ER lumen and one linked to alterations to the ER membrane environment. This review discusses both of these origins and also considers the role of post-translational protein modifications, such as acetylation and palmitoylation, and ER-mitochondrial interactions to obesity-mediated impairments in the ER and activation of the UPR.

The role of ER stress in lipid metabolism and lipotoxicity

The endoplasmic reticulum (ER) is a cellular organelle important for regulating calcium homeostasis, lipid metabolism, protein synthesis, and posttranslational modification and trafficking. Numerous environmental, physiological, and pathological insults disturb ER homeostasis, referred to as ER stress, in which a collection of conserved intracellular signaling pathways, termed the unfolded protein response (UPR), are activated to maintain ER function for cell survival. However, excessive and/or prolonged UPR activation leads to initiation of self-destruction through apoptosis. Excessive accumulation of lipids and their intermediate products causes metabolic abnormalities and cell death, called lipotoxicity, in peripheral organs, including the pancreatic islets, liver, muscle, and heart. Because accumulating evidence links chronic ER stress and defects in UPR signaling to lipotoxicity in peripheral tissues, understanding the role of ER stress in cell physiology is a topic under intense investigation. In this review, we highlight recent findings that link ER stress and UPR signaling to the pathogenesis of peripheral organs due to lipotoxicity.

The role of lipid droplet formation in the protection of unsaturated fatty acids against palmitic acid induced lipotoxicity to rat insulin-producing cells

Background

Type 2 diabetes is associated with increased plasma concentrations of non-esterified fatty acids (NEFAs), which trigger pancreatic β-cell dysfunction and apoptosis. Only long-chain saturated NEFAs induced lipotoxicity in rat insulin-producing cells in in vitro experiments, whereas unsaturated NEFAs were not toxic. Some unsaturated NEFAs even protected against lipotoxicity. In former studies it was suggested that long-chain unsaturated NEFAs, which induce the formation of lipid droplets, can cause sequestration of palmitic acid into lipid droplets. In the present structure-activity-relationship study the correlation between lipid droplet formation and the protection against palmitic acid induced lipotoxicity by unsaturated NEFAs in rat insulin-producing cells was examined.

Methods

Rat insulin-producing RINm5F and INS-1E tissue culture cells were incubated in the presence of palmitic acid and unsaturated NEFAs with different chain lengths and different numbers of double bonds. The expression of the lipid droplet associated proteins perilipin 1 and 2 was repressed by the shRNA technique and the expression analyzed by qRT-PCR and Western blotting. Viability was measured by MTT assay and the accumulation of lipid droplets was quantified by fluorescence microscopy after Oil Red O staining.

Results

Long-chain unsaturated NEFAs strongly induce the formation of lipid droplets in rat insulin-producing RINm5F and INS-1E cells. In RINm5F cells incubated with 11-eicosenoic acid (C20:1) 27 % of the cell area was covered by lipid droplets corresponding to a 25-fold increase in comparison with control cells. On the other hand the saturated NEFA palmitic acid only induced minor lipid droplet formation. Viability analyses revealed only a minor toxicity of unsaturated NEFAs, whereas the cells were markedly sensitive to palmitic acid. Long-chain unsaturated NEFAs antagonized palmitic acid induced lipotoxicity during co-incubation, whereby no correlation existed between protection and the ability of lipid droplet formation. Perilipin 1 and 2 expression was decreased after incubation with C20:1 to about 80 % by shRNA. For the protective effect of long-chain unsaturated NEFAs against lipotoxicity of saturated NEFAs repression of perilipin was not of crucial importance.

Conclusions

Long-chain unsaturated fatty acids protected rat insulin-producing cells against lipotoxicity of saturated fatty acids. This protective effect was not dependent on lipid droplet formation. Thus lipid droplet formation is apparently not essential for the protective effect of unsaturated NEFAs against palmitic acid toxicity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12986-016-0076-z) contains supplementary material, which is available to authorized users.

The Transcription Factor p8 Regulates Autophagy in Response to Palmitic Acid Stress via a Mammalian Target of Rapamycin (mTOR)-independent Signaling Pathway

Autophagy is an evolutionarily conserved degradative process that allows cells to maintain homoeostasis in numerous physiological situations. This process also functions as an essential protective response to endoplasmic reticulum (ER) stress, which promotes the removal and degradation of unfolded proteins. However, little is known regarding the mechanism by which autophagy is initiated and regulated in response to ER stress. In this study, different types of autophagy were identified in human gastric cancer MKN45 cells in response to the stress induced by nutrient starvation or lipotoxicity in which the regulation of these pathways is mammalian target of rapamycin (mTOR)-dependent or -independent, respectively. Interestingly, we found that p8, a stress-inducible transcription factor, was enhanced in MKN45 cells treated with palmitic acid to induce lipotoxicity. Furthermore, an increase in autophagy was observed in MKN45 cells stably overexpressing p8 using a lentivirus system, and autophagy induced by palmitic acid was blocked by p8 RNAi compared with the control. Western blotting analyses showed that autophagy was regulated by p8 or mTOR in response to the protein kinase-like endoplasmic reticulum kinase/activating transcription factor 6-mediated ER stress of lipotoxicity or the parkin-mediated mitochondrial stress of nutrient starvation, respectively. Furthermore, our results indicated that autophagy induced by palmitic acid is mTOR-independent, but this autophagy pathway was regulated by p8 via p53- and PKCα-mediated signaling in MKN45 cells. Our findings provide insights into the role of p8 in regulating autophagy induced by the lipotoxic effects of excess fat accumulation in cells.

Lipid mixtures containing a very high proportion of saturated fatty acids only modestly impair insulin signaling in cultured muscle cells

In vitro examinations of the effect of saturated fatty acids on skeletal muscle insulin action often use only one or two different fatty acid species, which does not resemble the human plasma fatty acid profile. We compared graded concentrations (0.1-0.8 mM) of 3 different lipid mixtures: 1) a physiologic fatty acid mixture (NORM; 40% saturated fatty acids), 2) a physiologic mixture high in saturated fatty acids (HSFA; 60% saturated fatty acids), and 3) 100% palmitate (PALM) on insulin signaling and fatty acid partitioning into triacylglycerol (TAG) and diacylglycerol (DAG) in cultured muscle cells. As expected, PALM readily impaired insulin-stimulated pAktThr308/Akt and markedly increased intracellular DAG content. In contrast, the fatty acid mixtures only modestly impaired insulin-stimulated pAktThr308M/Akt, and we found no differences between NORM and HSFA. Importantly, NORM and HSFA did not increase DAG content, but instead dose-dependently increased TAG accumulation. Therefore, the robust impairment in insulin signaling found with palmitate exposure was attenuated with physiologic mixtures of fatty acids, even with a very high proportion of saturated fatty acids. This may be explained in part by selective partitioning of fatty acids into neutral lipid (i.e., TAG) when muscle cells were exposed to physiologic lipid mixtures.

Inhibition of macrophage fatty acid β-oxidation exacerbates palmitate-induced inflammatory and endoplasmic reticulum stress responses

AIMS/HYPOTHESIS

Saturated fatty acids (SFAs) such as palmitate activate inflammatory pathways and elicit an endoplasmic reticulum (ER) stress response in macrophages, thereby contributing to the development of insulin resistance linked to the metabolic syndrome. This study addressed the question of whether or not mitochondrial fatty acid β-oxidation (FAO) affects macrophage responses to SFA.

METHODS

We modulated the activity of carnitine palmitoyl transferase 1A (CPT1A) in macrophage-differentiated THP-1 monocytic cells using genetic or pharmacological approaches, treated the cells with palmitate and analysed the proinflammatory and ER stress signatures.

RESULTS

To inhibit FAO, we created THP-1 cells with a stable knockdown (KD) of CPT1A and differentiated them to macrophages. Consequently, in CPT1A-silenced cells FAO was reduced. CPT1A KD in THP-1 macrophages increased proinflammatory signalling, cytokine expression and ER stress responses after palmitate treatment. In addition, in human primary macrophages CPT1A KD elevated palmitate-induced inflammatory gene expression. Pharmacological inhibition of FAO with etomoxir recapitulated the CPT1A KD phenotype. Conversely, overexpression of a malonyl-CoA-insensitive CPT1A M593S mutant reduced inflammatory and ER stress responses to palmitate in THP-1 macrophages. Macrophages with a CPT1A KD accumulated diacylglycerols and triacylglycerols after palmitate treatment, while ceramide accumulation remained unaltered. Moreover, lipidomic analysis of ER phospholipids revealed increased palmitate incorporation into phosphatidylethanolamine and phosphatidylserine classes associated with the CPT1A KD.

CONCLUSIONS/INTERPRETATION

Our data indicate that FAO attenuates inflammatory and ER stress responses in SFA-exposed macrophages, suggesting an anti-inflammatory impact of drugs that activate FAO.

A functional, genome-wide evaluation of liposensitive yeast identifies the "ARE2 required for viability" (ARV1) gene product as a major component of eukaryotic fatty acid resistance

The toxic subcellular accumulation of lipids predisposes several human metabolic syndromes, including obesity, type 2 diabetes, and some forms of neurodegeneration. To identify pathways that prevent lipid-induced cell death, we performed a genome-wide fatty acid sensitivity screen in Saccharomyces cerevisiae. We identified 167 yeast mutants as sensitive to 0.5 mm palmitoleate, 45% of which define pathways that were conserved in humans. 63 lesions also impacted the status of the lipid droplet; however, this was not correlated to the degree of fatty acid sensitivity. The most liposensitive yeast strain arose due to deletion of the "ARE2 required for viability" (ARV1) gene, encoding an evolutionarily conserved, potential lipid transporter that localizes to the endoplasmic reticulum membrane. Down-regulation of mammalian ARV1 in MIN6 pancreatic β-cells or HEK293 cells resulted in decreased neutral lipid synthesis, increased fatty acid sensitivity, and lipoapoptosis. Conversely, elevated expression of human ARV1 in HEK293 cells or mouse liver significantly increased triglyceride mass and lipid droplet number. The ARV1-induced hepatic triglyceride accumulation was accompanied by up-regulation of DGAT1, a triglyceride synthesis gene, and the fatty acid transporter, CD36. Furthermore, ARV1 was identified as a transcriptional of the protein peroxisome proliferator-activated receptor α (PPARα), a key regulator of lipid homeostasis whose transcriptional targets include DGAT1 and CD36. These results implicate ARV1 as a protective factor in lipotoxic diseases due to modulation of fatty acid metabolism. In conclusion, a lipotoxicity-based genetic screen in a model microorganism has identified 75 human genes that may play key roles in neutral lipid metabolism and disease.

Oleate rescues INS-1E β-cells from palmitate-induced apoptosis by preventing activation of the unfolded protein response

BACKGROUND

Saturated free fatty acids (FFAs), such as palmitate, cause β-cell apoptosis whereas unsaturated FFAs, e.g. oleate, are not harmful. The toxicity of palmitate could be mediated through endoplasmic reticulum (ER) stress which triggers the activation of a signal responding cascade also called unfolded protein response (UPR). We investigated whether or not palmitate induced β-cell apoptosis through UPR activation and whether or not oleate as a monounsaturated fatty acid could counteract these effects.

METHODS

INS-1E β-cells were incubated with palmitate [0.5mM], oleate [1mM] or the combination [0.5/1mM] for 1, 6 and 24h. Viability and induction of apoptosis were measured by WST-1 assay and FITC-Annexin/PI-staining, respectively. Western blot analyses were performed for UPR specific proteins and mRNA expression of target molecules was determined by qPCR.

RESULTS

Palmitate significantly decreased viability (29±8.8%) of INS-1E β-cells compared to controls after 24h. Stimulation with oleate showed no effect on viability but the combination of oleate and palmitate improved viability compared to palmitate treated cells (55±9.3%) or controls (26±5.3%). The number of apoptotic cells was increased 2-fold after 24h incubation with palmitate compared to controls. Again, oleate showed no effect but in combination ameliorated the effect of palmitate to control level. Phosphorylation of eIF2α was increased after 6 and 24h incubation with palmitate. In contrast, oleate had no effect and in combination prevented phosphorylation of eIF2α. Increased Xbp1 splicing was visible already 6h after palmitate treatment and remained elevated at 24h. The combination with oleate abolished Xbp1 splicing. Interestingly, mRNA expression of the chaperones Bip, Pdi, Calnexin and Grp94 was not altered by FFA treatment. Only the proapoptotic transcription factor Chop was significantly enhanced by palmitate incubation. In accordance with sustained cell survival the combination as well as oleate alone, did not result in increased Chop levels compared to controls. In summary, we showed that oleate protects INS-1E β-cells from palmitate-induced apoptosis by the suppression of ER stress which was independent of chaperone activation.

FFAR1 is involved in both the acute and chronic effects of palmitate on insulin secretion

Free fatty acids (FFAs) have pleiotropic effects on the pancreatic β-cell. Although acute exposure to FFAs stimulates glucose-stimulated insulin secretion (GSIS), prolonged exposure impairs GSIS and causes apoptosis. FFAs exert their effects both via intracellular metabolism and interaction with the FFA receptor 1 (FFAR1/GPR40). Here we studied the role of FFAR1 in acute and long-term effects of palmitate on GSIS and insulin content in isolated human islets by using the FFAR1 agonist TAK-875 and the antagonist ANT203. Acute palmitate exposure potentiated GSIS approximately 3-fold, whereas addition of the antagonist decreased this potentiation to approximately 2-fold. In the absence of palmitate, the agonist caused a 40% increase in GSIS. Treatment with palmitate for 7 days decreased GSIS to 70% and insulin content to 25% of control level. These negative effects of long-term exposure to palmitate were ameliorated by FFAR1 inhibition and further aggravated by additional stimulation of the receptor. In the absence of extracellularly applied palmitate, long-term treatment with the agonist caused a modest increase in GSIS. The protective effect of FFAR1 inhibition was verified by using FFAR1-deficient MIN6 cells. Improved β-cell function by the antagonist was paralleled by the decreased apoptosis and lowered oxidation of palmitate, which may represent the potential mechanisms of protection. We conclude that FFAR1 in the pancreatic β-cell plays a substantial role not only in acute potentiation of GSIS by palmitate but also in the negative long-term effects of palmitate on GSIS and insulin content.

Fatty acid-mediated endoplasmic reticulum stress in vivo: differential response to the infusion of Soybean and Lard Oil in rats

BACKGROUND

In cell systems, saturated fatty acids, compared to unsaturated fatty acids, induce a greater degree of ER stress and inflammatory signaling in a number of cell types, including hepatocytes and adipocytes. The aim of the present study was to determine the effects of infusions of lard oil (enriched in saturated fatty acids) and soybean oil (enriched in unsaturated fatty acids) on liver and adipose tissue ER stress and inflammatory signaling in vivo.

METHODS

Lipid emulsions containing glycerol, phosphatidylcholine, antibiotics (Control, n=7) and either soybean oil (Soybean, n=7) or lard oil (Lard, n=7) were infused intravenously into rats over a 4 h period.

RESULTS

Plasma free fatty acid levels were 0.5±0.1 mmol/L (mean±SD) in Control and were increased to 1.0±0.3 mmol/L and 1.1±0.3 mmol/L in Soybean and Lard, respectively. Glucose and insulin levels were not different among groups. Markers of endoplasmic reticulum (ER) stress and activation of inflammatory pathway signaling were increased in liver and adipose tissue from Soybean and Lard compared to Control, but were increased to a greater extent in Lard compared to Soybean.

CONCLUSIONS

These data suggest that elevated plasma free fatty acids can induce hepatic and adipose tissue ER stress and inflammation in vivo. In addition, saturated fatty acids appear to be more cytotoxic than unsaturated fatty acids in vivo.

Islet β cell mass in diabetes and how it relates to function, birth, and death

In type 1 diabetes (T1D) β cell mass is markedly reduced by autoimmunity. Type 2 diabetes (T2D) results from inadequate β cell mass and function that can no longer compensate for insulin resistance. The reduction of β cell mass in T2D may result from increased cell death and/or inadequate birth through replication and neogenesis. Reduction in mass allows glucose levels to rise, which places β cells in an unfamiliar hyperglycemic environment, leading to marked changes in their phenotype and a dramatic loss of glucose-stimulated insulin secretion (GSIS), which worsens as glucose levels climb. Toxic effects of glucose on β cells (glucotoxicity) appear to be the culprit. This dysfunctional insulin secretion can be reversed when glucose levels are lowered by treatment, a finding with therapeutic significance. Restoration of β cell mass in both types of diabetes could be accomplished by either β cell regeneration or transplantation. Learning more about the relationships between β cell mass, turnover, and function and finding ways to restore β cell mass are among the most urgent priorities for diabetes research.

Cell death-inducing DFF45-like effector b (Cideb) is present in pancreatic beta-cells and involved in palmitate induced beta-cell apoptosis

BACKGROUND

Excessive accumulation of long-chain fatty acids in the pancreatic islets is associated with beta cell dysfunction and ultimately contributes to the pathogenesis of type 2 diabetes. It has been well proved that the cell death-inducing DFF45-like effector b (Cideb) is involved in cell apoptosis and lipid metabolism. However, the expression and function of Cideb in endocrine pancreas remain to be investigated.

METHODS

By using reverse transcript polymerase chain reaction, immunohistochemistry and Western blot, we observed the expression of Cideb in pancreas tissues and clonal beta-cell lines. The physiological role of Cideb was examined under the free fatty acid (FFA) administration and Cideb ribonucleic acid interference, and further analysis on apoptosis was measured by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labelling assay and caspase-3 activity. Nile red staining and quantitative evaluation of triglyceride were used to detect the lipid accumulation. The changes in esterification of FFA were traced by radiolabelled palmitate.

RESULTS

Cideb was abundantly expressed in pancreas and mainly localized in beta cells. FFAs, especially palmitate, induced an obvious increase of Cideb expression in beta cell lines. Adenoviral-mediated overexpression of Cideb increased the apoptosis, whereas ribonucleic acid interference-based Cideb depletion in beta-TC3 cells had no effect on apoptosis in normal condition. Palmitate supplementation led to beta cell lipoapoptosis, and Cideb silencing exacerbated the apoptosis induced by palmitate, reduced intracellular triglyceride content and aggravated FFA overload in beta cells.

CONCLUSIONS

The present results suggest that increased Cideb expression upon palmitate exposure may be involved in beta cell lipoapoptosis through its influence on conversion of FFAs to lipid esters in lipid droplets.

Transcriptional analysis reveals a high impact of conjugated linoleic acid on stearoyl-Coenzyme A desaturase 1 mRNA expression in mice gastrocnemius muscle

We examined the potential implication of skeletal muscle in the fat-lowering effect observed in mice treated with moderate doses of CLA. In experiment 1, mice fed with a standard-fat diet were orally treated with sunflower oil (control) and 3 or 10 mg CLA mixture/day for 37 days. In experiment 2, mice were fed with a high-fat diet for 65 days. For the first 30 days, they received the same doses as in experiment 1 and, from that time onwards, animals received double doses. Gene expression of key proteins involved in fatty acid transport, oxidation, regulation of lipid and carbohydrate utilization, composition of muscle fiber, and thermogenesis were determined and, in most of them, no major impact of CLA was seen. Therefore, enhancement of fatty acid oxidation in muscle did not seem to contribute to the antiobesity effect of CLA as seen in other studies with higher CLA doses. However, a strong induction of classically associated lipogenic genes such as Fasn (up to twofold) and, particularly, Scd1 (up to ninefold) was found. This activation could contribute to a protective role in muscle cells, since expression of ER stress markers was decreased and inversely correlated with the induction of Scd1.

Is fat so bad? Modulation of endoplasmic reticulum stress by lipid droplet formation

LDs (lipid droplets) have long been considered as inert particles used by the cells to store fatty acids and sterols as esterified non-toxic lipid species (i.e. triacylglycerols and steryl esters). However, accumulating evidence suggests that LDs behave as a dynamic compartment, which is involved in the regulation of several aspects of the homoeostasis of their originating organelle, namely the ER (endoplasmic reticulum). The ER is particularly sensitive to physiological/pathological stimuli, which can ultimately induce ER stress. In the present review, after considering the basic mechanisms of LD formation and the signal cascades leading to ER stress, we focus on the connections between these two pathways. Taking into consideration recent data from the literature, we will try to draw possible mechanisms for the role of LDs in the regulation of ER homoeostasis and in ER-stress-related diseases.

UPR in palmitate-treated pancreatic beta-cells is not affected by altering oxidation of the fatty acid

Background

Elevated levels of lipids are detrimental for beta-cell function and mass. One of the mechanisms of how fatty acids induce apoptosis is development of the unfolded protein response (UPR). It is still far from understood how fatty acids activate the UPR, however.

Methods

We examined how palmitate-induced activation of the UPR was affected by altering the metabolism of the fatty acid in insulin-secreting INS-1E and MIN6 cell lines and intact human islets. To increase oxidation, we used low glucose (5.5 mM) or AICAR; and to reduce oxidation, we used high glucose (25 mM) or etomoxir. UPR was measured after 3, 24 and 48 hours of palmitate treatment.

Results

Modulation of palmitate oxidation by either glucose or the pharmacological agents did not affect palmitate-induced UPR activation.

Conclusion

Our finding suggests that other factors than oxidation of palmitate play a role in the activation of UPR in fatty acid-treated beta-cells.

Effect of α-linolenic acid on endoplasmic reticulum stress-mediated apoptosis of palmitic acid lipotoxicity in primary rat hepatocytes

Background

Hepatic inflammation and degeneration induced by lipid depositions may be the major cause of nonalcoholic fatty liver disease (NAFLD). In this study, we investigated the effects of saturated and unsaturated fatty acids (FA) on apoptosis in primary rat hepatocytes.

Methods

The primary rat hepatocytes were treated with palmitic acid and/or α-linolenic acid in vitro. The expression of proteins associated with endoplasmic reticulum (ER) stress, apoptosis, caspase-3 levels were detected after the treatment.

Results

The treatment with palmitic acid produced a significant increase in cell death. The unfolded protein response (UPR)-associated genes CHOP, GRP78, and GRP94 were induced to higher expression levels by palmitic acid. Co-treatment with α-linolenic acid reversed the apoptotic effect and levels of all three indicators of ER stress exerted by palmitic acid. Tunicamycin, which induces ER stress produced similar effects to those obtained using palmitic acid; its effects were also reversed by α-linolenic acid.

Conclusions

α-Linolenic acid may provide a useful strategy to avoid the lipotoxicity of dietary palmitic acid and nutrient overload accompanied with obesity and NAFLD.

Endoplasmic reticulum stress and the unfolded protein response in nonalcoholic fatty liver disease

The underlying causes of nonalcoholic fatty liver disease (NAFLD) are unclear, although recent evidence has implicated the endoplasmic reticulum (ER) in both the development of steatosis and progression to nonalcoholic steatohepatitis. Disruption of ER homeostasis, often termed "ER stress," has been observed in liver and adipose tissue of humans with NAFLD and/or obesity. Importantly, the signaling pathway activated by disruption of ER homeostasis, the unfolded protein response, has been linked to lipid biosynthesis, insulin action, inflammation, and apoptosis. Therefore, understanding the mechanisms that disrupt ER homeostasis in NAFLD and the role of ER-mediated signaling have become topics of intense investigation. The present review will examine the ER and the unfolded protein response in the context of NAFLD.

De novo lipogenesis in the differentiating human adipocyte can provide all fatty acids necessary for maturation

The primary products of de novo lipogenesis (DNL) are saturated fatty acids, which confer adverse cellular effects. Human adipocytes differentiated with no exogenous fat accumulated triacylglycerol (TG) in lipid droplets and differentiated normally. TG composition showed the products of DNL (saturated fatty acids from 12:0 to 18:0) together with unsaturated fatty acids (particularly 16:1n-7 and 18:1n-9) produced by elongation/desaturation. There was parallel upregulation of expression of genes involved in DNL and in fatty acid elongation and desaturation, suggesting coordinated control of expression. Enzyme products (desaturation ratios, elongation ratios, and total pathway flux) were also correlated with mRNA levels. We used (13)C-labeled substrates to study the pathway of DNL. Glucose (5 mM or 17.5 mM in the medium) provided less than half the carbon used for DNL (42% and 47%, respectively). Glutamine (2 mM) provided 9-10%, depending upon glucose concentration. In contrast, glucose provided most (72%) of the carbon of TG-glycerol. Pathway analysis using mass isotopomer distribution analysis (MIDA) revealed that the pathway for conversion of glucose to palmitate is complex. DNL in human fat cells is tightly coupled with further modification of fatty acids to produce a range of saturated and unsaturated fatty acids consistent with normal maturation.

α-Linolenic acid prevents endoplasmic reticulum stress-mediated apoptosis of stearic acid lipotoxicity on primary rat hepatocytes

Aims

Lipid accumulation in non-adipose tissues leads to cell dysfunction and apoptosis, a phenomenon known as lipotoxicity. Unsaturated fatty acids may offset the lipotoxicity associated with saturated fatty acids. Stearic acid induced endoplasmic reticulum (ER) stress and caused apoptotic and necrotic cell death in the primary rat hepatocytes.

Methods

Cell viability was investigated using MTT assay, and apoptosis was evaluated with Hoechst 33342 staining. Western blot analysis was used to examine the changes in the expression levels of glucose regulated protein 78 (GRP78), glucose regulated protein 94 (GRP94), and C/EBP homologous protein (CHOP). Caspase-3 activity was evaluated using a Caspase-3 substrate kit.

Results

We have studied the ability of α-linolenic acid to prevent endoplasmic reticulum stress-mediated apoptosis of rat hepatocytes elicited by stearic acid and thapsigargin. Incubation of primary rat hepatocytes for 16 h with stearic acid produced a significant increase in cell death. Stearic acid also increased levels of three indicators of ER stress -- GRP78, CHOP, and GRP94. α-Linolenic acid distinctly reduced cell death and levels of all three indicators of ER stress brought about by stearic acid. Thapsigargin, which induces ER stress produced similar effects to those obtained using stearic acid; its effects were partly reversed by α-linolenic acid.

Conclusion

These results suggest that α-linolenic acid prevents ER stress-mediated apoptosis of stearic acid lipotoxicity on primary rat hepatocytes might become a target to develop new antiapoptotic compounds in nonalcoholic fatty liver disease (NAFLD).

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.

Effects of human serum albumin complexed with free fatty acids on cell viability and insulin secretion in the hamster pancreatic β-cell line HIT-T15

AIMS

The effects of human serum albumin (HSA) complexed with various free fatty acids (FFAs) on ß-cells have not been studied in detail. In this study, we examined the effects of HSA and its mutants on FFA-induced cell viability changes and insulin secretion from the hamster pancreatic insulinoma cell line, HIT-TI5.

MAIN METHODS

Cells were exposed to different FFAs in the presence of HSA or its mutants and/or bovine serum albumin (BSA) for 24h. Cell viability, apoptosis, insulin secretion, and unbound FFA (FFA(u)) levels were determined.

KEY FINDINGS

In the presence of 0.1mM HSA, palmitate and stearate induced significant cell death at 0.1mM or higher, whereas myristate, palmitoleate, oleate, elaidate, linoleate, linoelaidate, and conjugated linoleate showed minimal changes on cell viability. Furthermore, oleate and linoleate were clearly cytoprotective against palmitate-induced cell death. The apoptosis inhibitors, cyclosporin A (csA) and the caspase inhibitor ZVAD-FMK, did not completely prevent FFA-induced cell death, although ZVAD-FMK blocked apoptosis with no differences in the presence of either HSA or BSA. In addition, insulin secretion from the cells was significantly reduced in the presence of HSA/oleate complexes. We also found differential effects of HSA mutants complexed with FFAs on cell viability.

SIGNIFICANCE

In summary, our results showed that saturated FFAs induced more cell death than unsaturated FFAs. Furthermore, modified HSA/FFA interactions caused by mutations of key amino acids involved in the binding of FFA to HSA resulted in changes in cell viability, suggesting a possible role of HSA polymorphism on FFA-induced changes in cellular functions.

Short-term effects of dietary fatty acids on muscle lipid composition and serum acylcarnitine profile in human subjects

In cultured cells, palmitic acid (PA) and oleic acid (OA) confer distinct metabolic effects, yet, unclear, is whether changes in dietary fat intake impact cellular fatty acid (FA) composition. We hypothesized that short-term increases in dietary PA or OA would result in corresponding changes in the FA composition of skeletal muscle diacylglycerol (DAG) and triacylglycerol (TAG) and/or the specific FA selected for β-oxidation. Healthy males (N = 12) and females (N = 12) ingested a low-PA diet for 7 days. After fasting measurements of the serum acylcarnitine (AC) profile, subjects were randomized to either high-PA (HI PA) or low-PA/high-OA (HI OA) diets. After 7 days, the fasting AC measurement was repeated and a muscle/fat biopsy obtained. FA composition of intramyocellular DAG and TAG and serum AC was measured. HI PA increased, whereas HI OA decreased, serum concentration of 16:0 AC (P < 0.001). HI OA increased 18:1 AC (P = 0.005). HI PA was associated with a higher PA/OA ratio in muscle DAG and TAG (DAG: 1.03 ± 0.24 vs. 0.46 ± 0.08, P = 0.04; TAG: 0.63 ± 0.07 vs. 0.41 ± 0.03, P = 0.01). The PA concentration in the adipose tissue DAG (µg/mg adipose tissue) was 0.17 ± 0.02 in those receiving the HI PA diet (n = 6), compared to 0.11 ± 0.02 in the HI OA group (n = 4) (P = 0.067). The relative PA concentration in muscle DAG and TAG and the serum palmitoylcarnitine concentration was higher in those fed the high-PA diet.

Fatty acid-induced oxidation and triglyceride formation is higher in insulin-producing MIN6 cells exposed to oleate compared to palmitate

Palmitate negatively affects insulin secretion and apoptosis in the pancreatic β-cell. The detrimental effects are abolished by elongating and desaturating the fatty acid into oleate. To investigate mechanisms of how the two fatty acids differently affect β-cell function and apoptosis, lipid handling was determined in MIN6 cells cultured in the presence of the fatty acids palmitate (16:0) and oleate (18:1) and also corresponding monounsaturated fatty acid palmitoleate (16:1) and saturated fatty acid stearate (18:0). Insulin secretion was impaired and apoptosis accentuated in palmitate-, and to some extent, stearate-treated cells. Small or no changes in secretion or apoptosis were observed in cells exposed to palmitoleate or oleate. Expressions of genes associated with fatty acid esterification (SCD1, DGAT1, DGAT2, and FAS) were augmented in cells exposed to palmitate or stearate but only partially (DGAT2) in palmitoleate- or oleate-treated cells. Nevertheless, levels of triglycerides were highest in cells exposed to oleate. Similarly, fatty acid oxidation was most pronounced in oleate-treated cells despite comparable up-regulation of CPT1 after treatment of cells with the four different fatty acids. The difference in apoptosis between palmitate and stearate was paralleled by similar differences in levels of markers of endoplasmic reticulum (ER) stress in cells exposed to the two fatty acids. Palmitate-induced ER stress was not accounted for by ceramide de novo synthesis. In conclusion, although palmitate initiated stronger expression changes consistent with lipid accumulation and combustion in MIN6 cells, rise in triglyceride levels and fatty acid oxidation was favored specifically in cells exposed to oleate.

Fatty acids and the endoplasmic reticulum in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) represents a burgeoning public health concern in westernized nations. The obesity-related disorder is associated with an increased risk of cardiovascular disease, type 2 diabetes and liver failure. Although the underlying pathogenesis of NAFLD is unclear, increasing evidence suggests that excess saturated fatty acids presented to or stored within the liver may play a role in both the development and progression of the disorder. A putative mechanism linking saturated fatty acids to NAFLD may be endoplasmic reticulum (ER) stress. Specifically, excess saturated fatty acids may induce an ER stress response that, if left unabated, can activate stress signaling pathways, cause hepatocyte cell death, and eventually lead to liver dysfunction. In the current review we discuss the involvement of saturated fatty acids in the pathogenesis of NAFLD with particular emphasis on the role of ER stress.

Interruption of triacylglycerol synthesis in the endoplasmic reticulum is the initiating event for saturated fatty acid-induced lipotoxicity in liver cells

The aim of the present study was to investigate in detail the molecular mechanisms by which free fatty acids induce liver toxicity in liver cells. HepG2 and Huh7 human liver cell lines were exposed to varying concentrations of stearate (18:0), oleate (18:1), or mixtures of the two fatty acids, and the effects on cell proliferation, lipid droplet accumulation and induction of endoplasmic reticulum stress and apoptosis were evaluated. It was observed that: (a) stearate, but not oleate, inhibited cell proliferation and induced cell death; (b) stearate-induced cell death had the characteristics of endoplasmic reticulum stress-mediated and mitochondrial-mediated apoptosis; (c) the activation of stearate in the form of stearoyl-CoA was a necessary step for the lipotoxic effect; (d) the capacity of cells to produce and accumulate triacylglycerols in the form of lipid droplets was interrupted following exposure to stearate, whereas it proceeded normally in oleate-treated cells; and (e) the presence of relatively low amounts of oleate protected cells from stearate-induced toxicity and restored the ability of the cells to accumulate triacylglycerols. Our data suggest that interruption of triacylglycerol synthesis in the endoplasmic reticulum, apparently because of the formation of a pool of oversaturated intermediates, represents the key initiating event in the mechanism of saturated fatty acid-induced lipotoxicity.

Diabetes as a disease of endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress is an integral part of life for all professional secretory cells, but it has been studied to greatest depth in the pancreatic β-cell. This reflects both the crucial role played by ER stress in the pathogenesis of diabetes and also the exquisite vulnerability of these cells to ER dysfunction. The adaptive cellular response to ER stress, the unfolded protein response, comprises mechanisms to both regulate new protein translation and a transcriptional program to allow adaptation to the stress. The core of this response is a triad of stress-sensing proteins: protein kinase R-like endoplasmic reticulum kinase (PERK), inositol-requiring enzyme 1 (IRE1) and activating transcription factor 6. All three regulate portions of the transcriptional unfolded protein response, while PERK also attenuates protein synthesis during ER stress and IRE1 interacts directly with the c-Jun amino-terminal kinase stress kinase pathway. In this review we shall discuss these processes in detail, with emphasis given to their impact on diabetes and how recent findings indicate that ER stress may be responsible for the loss of β-cell mass in the disease.

Reversibility of metabolic and morphological changes associated with chronic exposure of pancreatic islet beta-cells to fatty acids

Pancreatic beta-cells metabolise both lipid and glucose nutrients but chronic exposure (>24 h) to elevated fatty acid (FA) concentrations results in deleterious metabolic and morphological changes. The aims of this study were to assess the adaptive morphological, metabolic and secretory responses of islet beta-cells to exposure and removal of FA. Isolated mouse islets and INS-1 beta-cells were exposed to oleate or palmitate (0.5 mM) or a 1:1 mixture of both FA for 48 h prior to a 24 h period without FA. Subsequent changes in lipid storage and composition (triglycerides, TG and phospholipids, PL), gene expression, beta-cell morphology and glucose-stimulated insulin secretion (GSIS) were determined. Intracellular TG content increased during exposure to FA and was lower in cells subsequently incubated in FA-free media (P < 0.05); TG storage was visible as oil red O positive droplets (oleate) by light microscopy or 'splits' (palmitate) by electron microscopy. Significant desaturation of beta-cell FA occurred after exposure to oleate and palmitate. After incubation in FA-free media, there was differential handling of specific FA in TG, resulting in a profile that tended to revert to that of control cells. FA treatment resulted in elevated lipolysis of intracellular TG, increased FA oxidation and reduced GSIS. After incubation in FA-free media, oxidation remained elevated but inhibition of FA oxidation with etomoxir (10 microM) had no effect on the improvement in GSIS. The beta-cell demonstrates metabolic flexibility as an adaptive response to ambient concentrations of FA.

Palmitate induces insulin resistance without significant intracellular triglyceride accumulation in HepG2 cells

Previous studies showed that increased release of free fatty acids from adipocytes leads to insulin resistance and triglyceride (TG) accumulation in the liver, which may progress into hepatic steatohepatitis. We and other investigators have previously reported that palmitate induces endoplasmic reticulum stress-mediated toxicity in several tissues. This work investigated whether palmitate could induce insulin resistance and steatosis in HepG2 cells. We treated cells with either saturated fatty acid (palmitate) or unsaturated fatty acid (oleate), and observed that palmitate significantly activated c-jun N-terminal kinase and inactivated protein kinase B. Both 4-phenylbutyric acid and glycerol significantly activated protein kinase B, confirming the involvement of endoplasmic reticulum stress in palmitate-mediated insulin resistance. Oleate, but not palmitate, significantly induced intracellular TG deposition and activated sterol regulatory element binding protein-1. Instead, diacylglycerol level and protein kinase C epsilon activity were significantly increased by palmitate, suggesting the possible role of diacylglycerol in palmitate-mediated lipotoxicity. Therefore, the present study clearly showed that palmitate impairs insulin resistance, but does not induce significant TG accumulation in HepG2 cells.