Heme Biosynthetic Pathway is Functionally Linked to Adipogenesis via Mitochondrial Respiratory Activity

A Multi-Omics Approach to Disclose Metabolic Pathways Impacting Intestinal Permeability in Obese Patients Undergoing Very Low Calorie Ketogenic Diet

A very low calorie ketogenic diet (VLCKD) impacts host metabolism in people marked by an excess of visceral adiposity, and it affects the microbiota composition in terms of taxa presence and relative abundances. As a matter of fact, there is little available literature dealing with microbiota differences in obese patients marked by altered intestinal permeability. With the aim of inspecting consortium members and their related metabolic pathways, we inspected the microbial community profile, together with the set of volatile organic compounds (VOCs) from untargeted fecal and urine metabolomics, in a cohort made of obese patients, stratified based on both normal and altered intestinal permeability, before and after VLCKD administration. Based on the taxa relative abundances, we predicted microbiota-derived metabolic pathways whose variations were explained in light of our cohort symptom picture. A totally different number of statistically significant pathways marked samples with altered permeability, reflecting an important shift in microbiota taxa. A combined analysis of taxa, metabolic pathways, and metabolomic compounds delineates a set of markers that is useful in describing obesity dysfunctions and comorbidities.

Analysis of blood methylation quantitative trait loci in East Asians reveals ancestry-specific impacts on complex traits

Methylation quantitative trait loci (mQTLs) are essential for understanding the role of DNA methylation changes in genetic predisposition, yet they have not been fully characterized in East Asians (EAs). Here we identified mQTLs in whole blood from 3,523 Chinese individuals and replicated them in additional 1,858 Chinese individuals from two cohorts. Over 9% of mQTLs displayed specificity to EAs, facilitating the fine-mapping of EA-specific genetic associations, as shown for variants associated with height. Trans-mQTL hotspots revealed biological pathways contributing to EA-specific genetic associations, including an ERG-mediated 233 trans-mCpG network, implicated in hematopoietic cell differentiation, which likely reflects binding efficiency modulation of the ERG protein complex. More than 90% of mQTLs were shared between different blood cell lineages, with a smaller fraction of lineage-specific mQTLs displaying preferential hypomethylation in the respective lineages. Our study provides new insights into the mQTL landscape across genetic ancestries and their downstream effects on cellular processes and diseases/traits.

Iron: The silent culprit in your adipose tissue

Iron plays a vital role in essential biological processes and requires precise regulation within the body. Dysregulation of iron homeostasis, characterized by increased serum ferritin levels and excessive accumulation of iron in the liver, adipose tissue, and skeletal muscle, is associated with obesity and insulin resistance. Notably, iron excess in adipose tissue promotes adipose tissue dysfunction. As optimal adipose tissue function is crucial for maintaining a healthy phenotype in obesity, a comprehensive understanding of iron homeostasis in adipose tissue is imperative for designing new therapeutic approaches to improve and prevent adipose tissue dysfunction. Here, we conducted a review of relevant studies, focusing on and providing valuable insights into the intricate interplay between iron and adipose tissue. It sheds light on the impact of iron on adipogenesis and the physiology of both white and brown adipose tissue. Furthermore, we highlight the critical role of key modulators, such as cytosolic aconitase, mitochondria, and macrophages, in maintaining iron homeostasis within adipose tissue.

Heme biosynthesis regulates BCAA catabolism and thermogenesis in brown adipose tissue

With age, people tend to accumulate body fat and reduce energy expenditure 1 . Brown (BAT) and beige adipose tissue dissipate heat and increase energy expenditure via the activity of the uncoupling protein UCP1 and other thermogenic futile cycles 2,3 . The activity of brown and beige depots inversely correlates with BMI and age 4-11 , suggesting that promoting thermogenesis may be an effective approach for combating age-related metabolic disease 12-15 . Heme is an enzyme cofactor and signaling molecule that we recently showed to regulate BAT function 16 . Here, we show that heme biosynthesis is the primary contributor to intracellular heme levels in brown adipocytes. Inhibition of heme biosynthesis leads to mitochondrial dysfunction and reduction in UCP1. Although supplementing heme can restore mitochondrial function in heme-synthesis-deficient cells, the downregulation of UCP1 persists due to the accumulation of the heme precursors, particularly propionyl-CoA, which is a product of branched-chain amino acids (BCAA) catabolism. Cold exposure promotes BCAA uptake in BAT, and defects in BCAA catabolism in this tissue hinder thermogenesis 17 . However, BCAAs' contribution to the TCA cycle in BAT and WAT never exceeds 2% of total TCA flux 18 . Our work offers a way to integrate current literature by describing heme biosynthesis as an important metabolic sink for BCAAs.

Mitochondrial respiratory complex I deficiency inhibits brown adipogenesis by limiting heme regulation of histone demethylation

Mitochondrial functions play a crucial role in determining the metabolic and thermogenic status of brown adipocytes. Increasing evidence reveals that the mitochondrial oxidative phosphorylation (OXPHOS) system plays an important role in brown adipogenesis, but the mechanistic insights are limited. Herein, we explored the potential metabolic mechanisms leading to OXPHOS regulation of brown adipogenesis in pharmacological and genetic models of mitochondrial respiratory complex I deficiency. OXPHOS deficiency inhibits brown adipogenesis through disruption of the brown adipogenic transcription circuit without affecting ATP levels. Neither blockage of calcium signaling nor antioxidant treatment can rescue the suppressed brown adipogenesis. Metabolomics analysis revealed a decrease in levels of tricarboxylic acid cycle intermediates and heme. Heme supplementation specifically enhances respiratory complex I activity without affecting complex II and partially reverses the inhibited brown adipogenesis by OXPHOS deficiency. Moreover, the regulation of brown adipogenesis by the OXPHOS-heme axis may be due to the suppressed histone methylation status by increasing histone demethylation. In summary, our findings identified a heme-sensing retrograde signaling pathway that connects mitochondrial OXPHOS to the regulation of brown adipocyte differentiation and metabolic functions.

Nutrients and the Circadian Clock: A Partnership Controlling Adipose Tissue Function and Health

White adipose tissue (WAT) is a metabolic organ with flexibility to retract and expand based on energy storage and utilization needs, processes that are driven via the coordination of different cells within adipose tissue. WAT is comprised of mature adipocytes (MA) and cells of the stromal vascular cell fraction (SVF), which include adipose progenitor cells (APCs), adipose endothelial cells (AEC) and infiltrating immune cells. APCs have the ability to proliferate and undergo adipogenesis to form MA, the main constituents of WAT being predominantly composed of white, triglyceride-storing adipocytes with unilocular lipid droplets. While adiposity and adipose tissue health are controlled by diet and aging, the endogenous circadian (24-h) biological clock of the body is highly active in adipose tissue, from adipocyte progenitor cells to mature adipocytes, and may play a unique role in adipose tissue health and function. To some extent, 24-h rhythms in adipose tissue rely on rhythmic energy intake, but individual circadian clock proteins are also thought to be important for healthy fat. Here we discuss how and why the clock might be so important in this metabolic depot, and how temporal and qualitative aspects of energy intake play important roles in maintaining healthy fat throughout aging.

Analysis of rare coding variants in 200,000 exome-sequenced subjects reveals novel genetic risk factors for type 2 diabetes

AIMS

The study aimed to elucidate the effects of rare genetic variants on the risk of type 2 diabetes (T2D).

MATERIALS AND METHODS

Weighted burden analysis of rare variants was applied to a sample of 200,000 exome-sequenced participants in the UK Biobank project, of whom over 13,000 were identified as having T2D. Variant weights were allocated based on allele frequency and predicted effect, as informed by a previous analysis of hyperlipidaemia.

RESULTS

There was an exome-wide significant increased burden of rare, functional variants in three genes, GCK, HNF4A and GIGYF1. GIGYF1 has not previously been identified as a diabetes risk gene and its product appears to be involved in the modification of insulin signalling. A number of other genes did not attain exome-wide significance but were highly ranked and potentially of interest, including ALAD, PPARG, GYG1 and GHRL. Loss of function (LOF) variants were associated with T2D in GCK and GIGYF1 whereas nonsynonymous variants annotated as probably damaging were associated in GCK and HNF4A. Overall, fewer than 1% of T2D cases carried one of these variants. In HNF1A and HNF1B there was an excess of LOF variants among cases but the small numbers of these fell short of statistical significance.

CONCLUSIONS

Rare genetic variants make an identifiable contribution to T2D in a small number of cases but these may provide valuable insights into disease mechanisms. As larger samples become available it is likely that additional genetic factors will be identified.

Conserved immunomodulatory transcriptional networks underlie antipsychotic-induced weight gain

Although antipsychotics, such as olanzapine, are effective in the management of psychiatric conditions, some patients experience excessive antipsychotic-induced weight gain (AIWG). To illuminate pathways underlying AIWG, we compared baseline blood gene expression profiles in two cohorts of mice that were either prone (AIWG-P) or resistant (AIWG-R) to weight gain in response to olanzapine treatment for two weeks. We found that transcripts elevated in AIWG-P mice relative to AIWG-R are enriched for high-confidence transcriptional targets of numerous inflammatory and immunomodulatory signaling nodes. Moreover, these nodes are themselves enriched for genes whose disruption in mice is associated with reduced body fat mass and slow postnatal weight gain. In addition, we identified gene expression profiles in common between our mouse AIWG-P gene set and an existing human AIWG-P gene set whose regulation by immunomodulatory transcription factors is highly conserved between species. Finally, we identified striking convergence between mouse AIWG-P transcriptional regulatory networks and those associated with body weight and body mass index in humans. We propose that immunomodulatory transcriptional networks drive AIWG, and that these networks have broader conserved roles in whole body-metabolism.

Intestinal Transcriptome Analysis Reveals Enrichment of Genes Associated with Immune and Lipid Mechanisms, Favoring Soybean Meal Tolerance in High-Growth Zebrafish (Danio Rerio)

The molecular mechanisms underlying fish tolerance to soybean meal (SBM) remain unclear. Identifying these mechanisms would be beneficial, as this trait favors growth. Two fish replicates from 19 experimental families were fed fishmeal-(100FM) or SBM-based diets supplemented with saponin (50SBM + 2SPN) from juvenile to adult stages. Individuals were selected from families with a genotype-by-environment interaction higher (HG-50SBM + 2SPN, 170 ± 18 mg) or lower (LG-50SBM + 2SPN, 76 ± 10 mg) weight gain on 50SBM + 2SPN for intestinal transcriptomic analysis. A histological evaluation confirmed middle intestinal inflammation in the LG- vs. HG-50SBM + 2SPN group. Enrichment analysis of 665 differentially expressed genes (DEGs) identified pathways associated with immunity and lipid metabolism. Genes linked to intestinal immunity were downregulated in HG fish (mpx, cxcr3.2, cftr, irg1l, itln2, sgk1, nup61l, il22), likely dampening inflammatory responses. Conversely, genes involved in retinol signaling were upregulated (rbp4, stra6, nr2f5), potentially favoring growth by suppressing insulin responses. Genes associated with lipid metabolism were upregulated, including key components of the SREBP (mbtps1, elov5l, elov6l) and cholesterol catabolism (cyp46a1), as well as the downregulation of cyp7a1. These results strongly suggest that transcriptomic changes in lipid metabolism mediate SBM tolerance. Genotypic variations in DEGs may become biomarkers for improving early selection of fish tolerant to SMB or others plant-based diets.

Dysmetabolic adipose tissue in obesity: morphological and functional characteristics of adipose stem cells and mature adipocytes in healthy and unhealthy obese subjects

The way by which subcutaneous adipose tissue (SAT) expands and undergoes remodeling by storing excess lipids through expansion of adipocytes (hypertrophy) or recruitment of new precursor cells (hyperplasia) impacts the risk of developing cardiometabolic and respiratory diseases. In unhealthy obese subjects, insulin resistance, type 2 diabetes, hypertension, and obstructive sleep apnoea are typically associated with pathologic SAT remodeling characterized by adipocyte hypertrophy, as well as chronic inflammation, hypoxia, increased visceral adipose tissue (VAT), and fatty liver. In contrast, metabolically healthy obese individuals are generally associated with SAT development characterized by the presence of smaller and numerous mature adipocytes, and a lower degree of VAT inflammation and ectopic fat accumulation. The remodeling of SAT and VAT is under genetic regulation and influenced by inherent depot-specific differences of adipose tissue-derived stem cells (ASCs). ASCs have multiple functions such as cell renewal, adipogenic capacity, and angiogenic properties, and secrete a variety of bioactive molecules involved in vascular and extracellular matrix remodeling. Understanding the mechanisms regulating the proliferative and adipogenic capacity of ASCs from SAT and VAT in response to excess calorie intake has become a focus of interest over recent decades. Here, we summarize current knowledge about the biological mechanisms able to foster or impair the recruitment and adipogenic differentiation of ASCs during SAT and VAT development, which regulate body fat distribution and favorable or unfavorable metabolic responses.

Mitochondrial dysfunction and pancreatic islet β-cell failure (Review)

Pancreatic β-cells are the only source of insulin in humans. Mitochondria uses pyruvate to produce ATP as an intermediate link between glucose intake and insulin secretion in β-cells, in a process known as glucose-stimulated insulin secretion (GSIS). Previous studies have demonstrated that GSIS is negatively regulated by various factors in the mitochondria, including tRNA^Leu mutations, high p58 expression, reduced nicotinamide nucleotide transhydrogenase activity, abnormal levels of uncoupling proteins and reduced expression levels of transcription factors A, B1 and B2. Additionally, oxidative stress damages mitochondria and impairs antioxidant defense mechanisms, leading to the increased production of reactive oxygen species, which induces β-cell dysfunction. Inflammation in islets can also damage β-cell physiology. Inflammatory cytokines trigger the release of cytochrome c from the mitochondria via the NF-κB pathway. The present review examined the potential factors underlying mitochondrial dysfunction and their association with islet β-cell failure, which may offer novel insights regarding future strategies for the preservation of mitochondrial function and enhancement of antioxidant activity for individuals with diabetes mellitus.

Critical Roles of ELVOL4 and IL-33 in the Progression of Obesity-Related Cardiomyopathy via Integrated Bioinformatics Analysis

The molecular mechanisms underlying obesity-related cardiomyopathy (ORCM) progression involve multiple signaling pathways, and the pharmacological treatment for ORCM is still limited. Thus, it is necessary to explore new targets and develop novel therapies. Microarray analysis for gene expression profiles using different bioinformatics tools has been an effective strategy for identifying novel targets for various diseases. In this study, we aimed to explore the potential genes related to ORCM using the integrated bioinformatics analysis. The GSE18897 (whole blood expression profiling of obese diet-sensitive, obese diet-resistant, and lean human subjects) and GSE47022 (regular weight C57BL/6 and diet-induced obese C57BL/6 mice) were used for bioinformatics analysis. Weighted gene co-expression network analysis (WGCNA) of GSE18897 was employed to investigate gene modules that were strongly correlated with clinical phenotypes. Gene Ontology (GO) functional enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were performed on the co-expression genes. The expression levels of the hub genes were validated in the clinical samples. Yellow co-expression module of WGCNA in GSE18897 was found to be significantly related to the caloric restriction treatment. In addition, GO functional enrichment analysis and KEGG pathway analysis were performed on the co-expression genes in yellow co-expression module, which showed an association with oxygen transport and the porphyrins pathway. Overlap analysis of yellow co-expression module genes from GSE18897 andGSE47022 revealed six upregulated genes, and further experimental validation results showed that elongation of very-long-chain fatty acids protein 4 (ELOVL4), matrix metalloproteinase-8 (MMP-8), and interleukin-33 (IL-33) were upregulated in the peripheral blood from patients with ORCM compared to that in the controls. The bioinformatics analysis revealed that ELOVL4 expression levels are positively correlated with that of IL-33. Collectively, using WGCNA in combination with integrated bioinformatics analysis, the hub genes of ELVOL4 and IL-33 might serve as potential biomarkers for diagnosis and/or therapeutic targets for ORCM. The detailed roles of ELVOL4 and IL-33 in the pathophysiology of ORCM still require further investigation.

Consumption of Diet Soda Sweetened with Sucralose and Acesulfame-Potassium Alters Inflammatory Transcriptome Pathways in Females with Overweight and Obesity

SCOPE

Low-calorie sweetener (LCS) consumption is associated with metabolic disease in observational studies. However, physiologic mechanisms underlying LCS-induced metabolic impairments in humans are unclear. This study is aimed at identifying molecular pathways in adipose impacted by LCSs.

METHODS AND RESULTS

Seven females with overweight or obesity, who did not report LCS use, consumed 12 ounces of diet soda containing sucralose and acesulfame-potassium (Ace-K) three times daily for 8 weeks. A subcutaneous adipose biopsy from the left abdomen and a fasting blood sample were collected at baseline and post-intervention. Global gene expression were assessed using RNA-sequencing followed by functional pathway analysis. No differences in circulating metabolic or inflammatory biomarkers were observed. However, ANOVA detected 828 differentially expressed annotated genes after diet soda consumption (p < 0.05), including transcripts for inflammatory cytokines. Fifty-eight of 140 canonical pathways represented in pathway analyses regulated inflammation, and several key upstream regulators of inflammation (e.g., TNF-alpha) were also represented.

CONCLUSION

Consumption of diet soda with sucralose and Ace-K alters inflammatory transcriptomic pathways (e.g., NF-κB signaling) in subcutaneous adipose tissue but does not significantly alter circulating biomarkers. Findings highlight the need to examine molecular and metabolic effects of LCS exposure in a larger randomized control trial for a longer duration.

3-Amino-1,2,4-Triazole Induces Quick and Strong Fat Loss in Mice with High Fat-Induced Metabolic Syndrome

BACKGROUND

Obesity is a growing epidemic with limited effective treatments and an important risk factor for several diseases such as metabolic syndrome (MetS). In this study, we aimed to investigate the effect of 3-amino-1,2,4-triazole (ATZ), an inhibitor of catalase and heme synthesis, in a murine model for MetS.

METHODS

Male C57BL/6 mice with high-fat diet-induced MetS received ATZ (500 mg·kg-1·24 h-1) for 12 weeks.

RESULTS

The HFD group showed increased blood pressure and body weight, enhanced fat deposition accompanied by an increase in adipocyte diameter, and decreased lipolysis in white adipose tissue (WAT). The expression of genes related to inflammation was increased in WAT of the HFD group. Concurrently, these mice exhibited an increase in leptin, nonesterified fatty acid (NEFA), insulin, and glucose in plasma, coupled with glucose intolerance and insulin resistance. Strikingly, ATZ prevented the increase in blood pressure and the HFD-induced obesity as observed by lower body weight, WAT index, triglycerides, NEFA, and leptin in plasma. ATZ treatment also prevented the HFD-induced increase in adipocyte diameter and even induced marked atrophy and the accumulation of macrophages in this tissue. ATZ treatment also improved glucose metabolism by increasing glucose tolerance and insulin sensitivity, GLUT4 mRNA expression in WAT in parallel to decreased insulin levels.

CONCLUSIONS

In the context of HFD-induced obesity and metabolic syndrome, the fat loss induced by ATZ is probably due to heme synthesis inhibition, which blocks adipogenesis by probably decreased RevErbα activity, leading to apoptosis of adipocytes and the recruitment of macrophages. As a consequence of fat loss, ATZ elicits a beneficial systemic antiobesity effect and improves the metabolic status.

PGRMC2 is an intracellular haem chaperone critical for adipocyte function

Heme is an essential prosthetic group of numerous proteins and a central signaling molecule in many physiologic processes^1,2. The chemical reactivity of heme requires that a network of intracellular chaperone proteins exist to avert the cytotoxic effects of free heme, but the constituents of such trafficking pathways are unknown^3,4. Heme synthesis is completed in mitochondria, with ferrochelatase (FECH) adding iron to protoporphyrin IX. How this vital but highly reactive metabolite is delivered from mitochondria to hemoproteins throughout the cell remains poorly defined^3,4. Here, we show that PGRMC2 is required for delivery of labile, or signaling heme, to the nucleus. Deletion of PGMRC2 in brown fat, which has a high demand for heme, reduced labile heme in the nucleus and increased stability of the heme-responsive transcriptional repressors Rev-Erbα and BACH1. Ensuing alterations in gene expression spawn severe mitochondrial defects that rendered adipose-specific PGRMC2-null mice unable to activate adaptive thermogenesis and prone to greater metabolic deterioration when fed a high-fat diet. In contrast, obese-diabetic mice treated with a small-molecule PGRMC2 activator showed substantial improvement of diabetic features. These studies uncover a role for PGRMC2 in intracellular heme transport, reveal the impact of adipose tissue heme dynamics on physiology, and suggest that modulation of PGRMC2 may revert obesity-linked defects in adipocytes.

Corn dried distillers grains with solubles (cDDGS) in the diet of pigs change the expression of adipose genes that are potential therapeutic targets in metabolic and cardiovascular diseases

Background

Corn dried distillers grains with solubles (cDDGS) are a byproduct of biofuel and alcohol production. cDDGS have been used in pig feed for many years, because they are readily available and rich in protein, fiber, unsaturated fatty acids and phytosterols. However, feed mixtures too high in cDDGS result in the worsening of backfat quality. We performed RNA-sequencing analysis of backfat from crossbred pigs fed different diets. The diets were isoenergetic but contained different amounts of cDDGS and various sources of fats. The animals were divided into four dietary groups during the two months of experimentation: group I (control (-cDDGS+rapeseed oil)), group II (+cDDGS+rapeseed oil), group III (+cDDGS+beef tallow), and group IV (+cDDGS+coconut oil). The aim of the present experiment was to evaluate changes in the backfat transcriptome of pigs fed isoenergetic diets that differed in cDDGS presence.

Results

Via DESeq2 software, we identified 93 differentially expressed genes (DEGs) between groups I and II, 13 between groups I and III, and 125 between groups I and IV. DEGs identified between group I (-cDDGS+rapeseed oil) and group II (+cDDGS+rapeseed oil) were highly overrepresented in several KEGG pathways: metabolic pathways (FDR < 1.21e-06), oxidative phosphorylation (FDR < 0.00189), fatty acid biosynthesis (FDR < 0.00577), Huntington’s disease (FDR < 0.00577), fatty acid metabolism (FDR < 0.0112), Parkinson’s disease (FDR < 0.0151), non-alcoholic fatty liver disease (NAFLD) (FDR < 0.016), Alzheimer’s disease (FDR < 0.0211) and complement and coagulation cascades (FDR < 0.02).

Conclusions

We observed that the addition of cDDGS positively affects the expression of several genes that have been recently proposed as potential targets for the treatment of obesity, diabetes, cardiovascular disease, and Alzheimer’s disease (e.g., FASN, AACS, ALAS1, HMGCS1, and VSIG4). Thus, our results support the idea of including cDDGS into the diets of companion animals and humans and encourage research into the bioactive ingredients of cDDGS.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5265-x) contains supplementary material, which is available to authorized users.