Ablation of Elovl6 protects pancreatic islets from high-fat diet-induced impairment of insulin secretion

Integrating genetics with single-cell multiomic measurements across disease states identifies mechanisms of beta cell dysfunction in type 2 diabetes

Dysfunctional pancreatic islet beta cells are a hallmark of type 2 diabetes (T2D), but a comprehensive understanding of the underlying mechanisms, including gene dysregulation, is lacking. Here we integrate information from measurements of chromatin accessibility, gene expression and function in single beta cells with genetic association data to nominate disease-causal gene regulatory changes in T2D. Using machine learning on chromatin accessibility data from 34 nondiabetic, pre-T2D and T2D donors, we identify two transcriptionally and functionally distinct beta cell subtypes that undergo an abundance shift during T2D progression. Subtype-defining accessible chromatin is enriched for T2D risk variants, suggesting a causal contribution of subtype identity to T2D. Both beta cell subtypes exhibit activation of a stress-response transcriptional program and functional impairment in T2D, which is probably induced by the T2D-associated metabolic environment. Our findings demonstrate the power of multimodal single-cell measurements combined with machine learning for characterizing mechanisms of complex diseases.

Integration of single-cell multiomic measurements across disease states with genetics identifies mechanisms of beta cell dysfunction in type 2 diabetes

Altered function and gene regulation of pancreatic islet beta cells is a hallmark of type 2 diabetes (T2D), but a comprehensive understanding of mechanisms driving T2D is still missing. Here we integrate information from measurements of chromatin activity, gene expression and function in single beta cells with genetic association data to identify disease-causal gene regulatory changes in T2D. Using machine learning on chromatin accessibility data from 34 non-diabetic, pre-T2D and T2D donors, we robustly identify two transcriptionally and functionally distinct beta cell subtypes that undergo an abundance shift in T2D. Subtype-defining active chromatin is enriched for T2D risk variants, suggesting a causal contribution of subtype identity to T2D. Both subtypes exhibit activation of a stress-response transcriptional program and functional impairment in T2D, which is likely induced by the T2D-associated metabolic environment. Our findings demonstrate the power of multimodal single-cell measurements combined with machine learning for identifying mechanisms of complex diseases.

ELOVL6 promoter binding sites directly targeted by sterol regulatory element binding protein 1 in fatty acid synthesis of goat mammary epithelial cells

The elongation of long-chain fatty acid family member 6 (ELOVL6) gene plays an important role in the synthesis of long-chain saturated and monounsaturated fatty acids. Although some studies have revealed that ELOVL6 is the target of sterol regulatory element binding protein 1 (SREBP1; gene name SREBF1) in rodents, the mechanism underlying ELOVL6 regulation during lactation in dairy goats remains unknown. The present study aimed to investigate the transcriptional regulation mechanism of ELOVL6 in goat mammary epithelial cells (GMEC). We used PCR to clone and sequenced a 2,370 bp fragment of the ELOVL6 5' flanking region from goat genomic DNA. Deletion analysis revealed a core promoter region located -105 to -40 bp upstream of the transcriptional start site. Mutant sterol regulatory elements (SRE) 1 and 3 significantly reduced the ELOVL6 promoter activities in GMEC. Both SRE1 and SRE3 binding sites were required for the basal transcriptional activity of ELOVL6. Luciferase reporter assays showed that SREBF1 knockdown decreased ELOVL6 promoter activities in GMEC. Furthermore, SRE1 and SRE3 sites were simultaneously mutated completely abolished the stimulatory effect of SREBF1 and the repressive effect of linoleic acid on ELOVL6 gene promoter activities. Furthermore, chromatin immunoprecipitation assays confirmed that SREBP1 directly bound to SRE sites in the ELOVL6 promoter. In conclusion, these results indicate that SREBP1 regulates ELOVL6 transcription via the SRE elements located in the ELOVL6 promoter in goat mammary gland.

Goat's Milk Intake Prevents Obesity, Hepatic Steatosis and Insulin Resistance in Mice Fed A High-Fat Diet by Reducing Inflammatory Markers and Increasing Energy Expenditure and Mitochondrial Content in Skeletal Muscle

Goat's milk is a rich source of bioactive compounds (peptides, conjugated linoleic acid, short chain fatty acids, monounsaturated and polyunsaturated fatty acids, polyphenols such as phytoestrogens and minerals among others) that exert important health benefits. However, goat's milk composition depends on the type of food provided to the animal and thus, the abundance of bioactive compounds in milk depends on the dietary sources of the goat feed. The metabolic impact of goat milk rich in bioactive compounds during metabolic challenges such as a high-fat (HF) diet has not been explored. Thus, we evaluated the effect of milk from goats fed a conventional diet, a conventional diet supplemented with 30% Acacia farnesiana (AF) pods or grazing on metabolic alterations in mice fed a HF diet. Interestingly, the incorporation of goat's milk in the diet decreased body weight and body fat mass, improved glucose tolerance, prevented adipose tissue hypertrophy and hepatic steatosis in mice fed a HF diet. These effects were associated with an increase in energy expenditure, augmented oxidative fibers in skeletal muscle, and reduced inflammatory markers. Consequently, goat's milk can be considered a non-pharmacologic strategy to improve the metabolic alterations induced by a HF diet. Using the body surface area normalization method gave a conversion equivalent daily human intake dose of 1.4 to 2.8 glasses (250 mL per glass/day) of fresh goat milk for an adult of 60 kg, which can be used as reference for future clinical studies.

Effect of Supplementation with Hydroethanolic Extract of Campomanesia xanthocarpa (Berg.) Leaves and Two Isolated Substances from the Extract on Metabolic Parameters of Mice Fed a High-Fat Diet

There are still controversies regarding the correlation between the beneficial effects for health and the administration of isolated compounds or crude extracts in therapeutic applications. Campomanesia xanthocarpa, found in the Brazilian Cerrado, demonstrated beneficial effects in metabolic disorders associated with obesity. We investigated the effects of Campomanesia xanthocarpa hydroethanolic extract and two isolated substances from the extract (S1 and S2) in a diet-induced obesity (DIO) model. Male Swiss mice were divided into five groups: (1) American Institute of Nutrition (AIN-93M) diet, (2) high-fat diet (HF), (3) HF supplemented with C. xanthocarpa hydroethanolic leaf extract at 100 mg/kg (HFE), (4) HF supplemented with S1 at 1 mg/kg (HFS1) and (5) HF supplemented with S2 at 1 mg/kg (HFS2). The HFS1, HFS2 and HFE groups did not present decreasing body weight or visceral adiposity gain. No differences in glycemic and lipid parameters, or in the expression of protein content in two cytokines, interleukin-6 (IL-6) and anti-inflammatory (IL-10), were observed. Only the HFS1 group displayed decreased food intake. Even though substantial effects such as an improvement in obesity features or the metabolic and histological parameters promoted by S1, S2 and the extract were not observed, further investigations are necessary to evaluate the principal genes and protein expressions involved in regulating food behavior promoted by S1.

Stearoyl CoA desaturase is a gatekeeper that protects human beta cells against lipotoxicity and maintains their identity

AIMS/HYPOTHESIS

During the onset of type 2 diabetes, excessive dietary intake of saturated NEFA and fructose lead to impaired insulin production and secretion by insulin-producing pancreatic beta cells. The majority of data on the deleterious effects of lipids on functional beta cell mass were obtained either in vivo in rodent models or in vitro using rodent islets and beta cell lines. Translating data from rodent to human beta cells remains challenging. Here, we used the human beta cell line EndoC-βH1 and analysed its sensitivity to a lipotoxic and glucolipotoxic (high palmitate with or without high glucose) insult, as a way to model human beta cells in a type 2 diabetes environment.

METHODS

EndoC-βH1 cells were exposed to palmitate after knockdown of genes related to saturated NEFA metabolism. We analysed whether and how palmitate induces apoptosis, stress and inflammation and modulates beta cell identity.

RESULTS

EndoC-βH1 cells were insensitive to the deleterious effects of saturated NEFA (palmitate and stearate) unless stearoyl CoA desaturase (SCD) was silenced. SCD was abundantly expressed in EndoC-βH1 cells, as well as in human islets and human induced pluripotent stem cell-derived beta cells. SCD silencing induced markers of inflammation and endoplasmic reticulum stress and also IAPP mRNA. Treatment with the SCD products oleate or palmitoleate reversed inflammation and endoplasmic reticulum stress. Upon SCD knockdown, palmitate induced expression of dedifferentiation markers such as SOX9, MYC and HES1. Interestingly, SCD knockdown by itself disrupted beta cell identity with a decrease in mature beta cell markers INS, MAFA and SLC30A8 and decreased insulin content and glucose-stimulated insulin secretion.

CONCLUSIONS/INTERPRETATION

The present study delineates an important role for SCD in the protection against lipotoxicity and in the maintenance of human beta cell identity.

DATA AVAILABILITY

Microarray data and all experimental details that support the findings of this study have been deposited in in the GEO database with the GSE130208 accession code.

Transgenic Mice Overexpressing SREBP-1a in Male ob/ob Mice Exhibit Lipodystrophy and Exacerbate Insulin Resistance

Sterol regulatory element-binding protein (SREBP)-1a is a key transcription factor that activates the expression of genes involved in the synthesis of fatty acids, triglycerides (TGs), and cholesterol. Transgenic mice that overexpress the nuclear form of SREBP-1a under the control of the phosphoenolpyruvate carboxykinase promoter (Tg-1a) were previously shown to display a lipodystrophic phenotype characterized by enlarged and fatty livers, diminished peripheral white adipose tissue (WAT), and insulin resistance. In the current study, we crossed these Tg-1a mice with genetically obese (ob/ob) mice (Tg-1a;ob/ob) and examined change in fat distribution between liver and adipose tissues in severe obesity and mechanism underlying the lipodystrophic phenotype in mice with Tg-1a. Tg-1a;ob/ob mice developed more severe steatohepatitis but had reduced WAT mass and body weight compared with ob/ob mice. The reduction of WAT mass in Tg-1a and Tg-1a;ob/ob mice was accompanied by enhanced lipogenesis and lipid uptake in the liver, reduced plasma lipid levels, impaired adipocyte differentiation, reduced food intake, enhanced energy expenditure, and extended macrophage infiltration and fibrosis in WAT. Despite the improved glucose tolerance, Tg-1a;ob/ob mice showed severe peripheral insulin resistance. Adenoviral hepatic expression of SREBP-1a mimicked these phenotypes. The "fat steal"-like lipodystrophy phenotype of the Tg-1a;ob/ob model demonstrates that hepatic SREBP-1a activation has a strong impact on the partition of TG accumulation, resulting in adipose-tissue remodeling by inflammation and fibrosis and insulin resistance.

Stearate-to-palmitate ratio modulates endoplasmic reticulum stress and cell apoptosis in non-B non-C hepatoma cells

The increased prevalence of hepatocellular carcinoma (HCC) without viral infection, namely, NHCC, is a major public health issue worldwide. NHCC is frequently derived from non‐alcoholic fatty liver (NAFL) and non‐alcoholic steatohepatitis, which exhibit dysregulated fatty acid (FA) metabolism. This raises the possibility that NHCC evolves intracellular machineries to adapt to dysregulated FA metabolism. We herein aim to identify NHCC‐specifically altered FA and key molecules to achieve the adaptation. To analyze FA, imaging mass spectrometry (IMS) was performed on 15 HCC specimens. The composition of saturated FA (SFA) in NHCC was altered from that in typical HCC. The stearate‐to‐palmitate ratio (SPR) was significantly increased in NHCC. Associated with the SPR increase, the ELOVL6 protein level was upregulated in NHCC. The knockdown of ELOVL6 reduced SPR, and enhanced endoplasmic reticulum stress, inducing apoptosis of Huh7 and HepG2 cells. In conclusion, NHCC appears to adapt to an FA‐rich environment by modulating SPR through ELOVL6.

Maternal exercise modifies body composition and energy substrates handling in male offspring fed a high-fat/high-sucrose diet

KEY POINTS

Maternal training during gestation enhances offspring body composition and energy substrates handling in early adulthood. Offspring nutrition also plays a role as some beneficial effects of maternal training during gestation disappear after consumption of a high-fat diet.

ABSTRACT

Maternal exercise during gestation has been reported to modify offspring metabolism and health. Whether these effects are exacerbated when offspring are receiving a high-fat diet remains unclear. Our purpose was to evaluate the effect of maternal exercise before and during gestation on the offspring fed a high-fat/high-sucrose diet (HF) by assessing its body composition, pancreatic function and energy substrates handling by two major glucose-utilizing tissues: liver and muscle. Fifteen-week-old nulliparous female Wistar rats exercised 4 weeks before as well as during gestation at a constant submaximal intensity (TR) or remained sedentary (CT). At weaning, pups from each group were fed either a standard diet (TRCD or CTCD) or a high-fat/high-sucrose diet (TRHF or CTHF) for 10 weeks. Offspring from TR dams gained less weight compared to those from CT dams. Selected fat depots were larger with the HF diet compared to control diet (CD) but significantly smaller in TRHF compared to CTHF. Surprisingly, the insulin secretion index was higher in islets from HF offspring compared to CD. TR offspring showed a higher muscle insulin sensitivity estimated by the ratio of phosphorylated protein kinase B to total protein kinase B compared with CT offspring (+48%, P < 0.05). With CD, permeabilized isolated muscle fibres from TR rats displayed a lower apparent affinity constant (K_m ) for pyruvate and palmitoyl coenzyme A as substrates compared to the CT group (-46% and -58%, respectively, P < 0.05). These results suggest that maternal exercise has positive effects on young adult offspring body composition and on muscle carbohydrate and lipid metabolism depending on the nutritional status.

Elovl6 Deficiency Improves Glycemic Control in Diabetic db/db Mice by Expanding β-Cell Mass and Increasing Insulin Secretory Capacity

Dysfunctional fatty acid (FA) metabolism plays an important role in the pathogenesis of β-cell dysfunction and loss of β-cell mass in type 2 diabetes (T2D). Elovl6 is a microsomal enzyme that is responsible for converting C16 saturated and monounsaturated FAs into C18 species. We previously showed that Elovl6 played a critical role in the development of obesity-induced insulin resistance by modifying FA composition. To further define its role in T2D development, we assessed the effects of Elovl6 deletion in leptin receptor-deficient C57BL/KsJ db/db mice, a model of T2D. The db/db;Elovl6^-/- mice had a markedly increased β-cell mass with increased proliferation and decreased apoptosis, an adaptive increase in insulin, and improved glycemic control. db/db islets were characterized by a prominent elevation of oleate (C18:1n-9), cell stress, and inflammation, which was completely suppressed by Elovl6 deletion. As a mechanistic ex vivo experiment, isolated islets from Elovl6^-/- mice exhibited reduced susceptibility to palmitate-induced inflammation, endoplasmic reticulum stress, and β-cell apoptosis. In contrast, oleate-treated islets resulted in impaired glucose-stimulated insulin secretion with suppressed related genes irrespective of the Elovl6 gene. Taken together, Elovl6 is a fundamental factor linking dysregulated lipid metabolism to β-cell dysfunction, islet inflammation, and β-cell apoptosis in T2D, highlighting oleate as the potential culprit of β-cell lipotoxicity.

Combined lipidomic and proteomic analysis of isolated human islets exposed to palmitate reveals time-dependent changes in insulin secretion and lipid metabolism

Studies on the pathophysiology of type 2 diabetes mellitus (T2DM) have linked the accumulation of lipid metabolites to the development of beta-cell dysfunction and impaired insulin secretion. In most in vitro models of T2DM, rodent islets or beta-cell lines are used and typically focus is on specific cellular pathways or organs. Our aim was to, firstly, develop a combined lipidomics and proteomics approach for lipotoxicity in isolated human islets and, secondly, investigate if the approach could delineate novel and/ or confirm reported mechanisms of lipotoxicity. To this end isolated human pancreatic islets, exposed to chronically elevated palmitate concentrations for 0, 2 and 7 days, were functionally characterized and their levels of multiple targeted lipid and untargeted protein species determined. Glucose-stimulated insulin secretion from the islets increased on day 2 and decreased on day 7. At day 7 islet insulin content decreased and the proinsulin to insulin content ratio doubled. Amounts of cholesterol, stearic acid, C16 dihydroceramide and C24:1 sphingomyelin, obtained from the lipidomic screen, increased time-dependently in the palmitate-exposed islets. The proteomic screen identified matching changes in proteins involved in lipid biosynthesis indicating up-regulated cholesterol and lipid biosynthesis in the islets. Furthermore, proteins associated with immature secretory granules were decreased when palmitate exposure time was increased despite their high affinity for cholesterol. Proteins associated with mature secretory granules remained unchanged. Pathway analysis based on the protein and lipid expression profiles implicated autocrine effects of insulin in lipotoxicity. Taken together the study demonstrates that combining different omics approaches has potential in mapping of multiple simultaneous cellular events. However, it also shows that challenges exist for effectively combining lipidomics and proteomics in primary cells. Our findings provide insight into how saturated fatty acids contribute to islet cell dysfunction by affecting the granule maturation process and confirmation in human islets of some previous findings from rodent islet and cell-line studies.

Aldehyde dehydrogenase 1a3 defines a subset of failing pancreatic β cells in diabetic mice

Insulin-producing β cells become dedifferentiated during diabetes progression. An impaired ability to select substrates for oxidative phosphorylation, or metabolic inflexibility, initiates progression from β-cell dysfunction to β-cell dedifferentiation. The identification of pathways involved in dedifferentiation may provide clues to its reversal. Here we isolate and functionally characterize failing β cells from various experimental models of diabetes and report a striking enrichment in the expression of aldehyde dehydrogenase 1 isoform A3 (ALDH(+)) as β cells become dedifferentiated. Flow-sorted ALDH(+) islet cells demonstrate impaired glucose-induced insulin secretion, are depleted of Foxo1 and MafA, and include a Neurogenin3-positive subset. RNA sequencing analysis demonstrates that ALDH(+) cells are characterized by: (i) impaired oxidative phosphorylation and mitochondrial complex I, IV and V; (ii) activated RICTOR; and (iii) progenitor cell markers. We propose that impaired mitochondrial function marks the progression from metabolic inflexibility to dedifferentiation in the natural history of β-cell failure.

Absence of Elovl6 attenuates steatohepatitis but promotes gallstone formation in a lithogenic diet-fed Ldlr(-/-) mouse model

Nonalcoholic steatohepatitis (NASH) is a progressive form of nonalcoholic fatty liver disease (NAFLD) that can develop into liver cirrhosis and cancer. Elongation of very long chain fatty acids (ELOVL) family member 6 (Elovl6) is a microsomal enzyme that regulates the elongation of C12–16 saturated and monounsaturated fatty acids (FAs). We have previously shown that Elovl6 plays an important role in the development of hepatic insulin resistance and NASH by modifying FA composition. Recent studies have linked altered hepatic cholesterol homeostasis and cholesterol accumulation to the pathogenesis of NASH. In the present study, we further investigated the role of Elovl6 in the progression of lithogenic diet (LD)-induced steatohepatitis. We showed that the absence of Elovl6 suppresses hepatic lipid accumulation, plasma total cholesterol and total bile acid (BA) levels in LDL receptor-deficient (Ldlr^−/−) mice challenged with a LD. The absence of Elovl6 also decreases hepatic inflammation, oxidative stress and liver injury, but increases the formation of cholesterol crystals in the less dilated gallbladder. These findings suggest that Elovl6-mediated changes in hepatic FA composition, especially oleic acid (C18:1n-9), control handling of hepatic cholesterol and BA, which protects against hepatotoxicity and steatohepatitis, but promotes gallstone formation in LD-fed Ldlr^−/− mice.