What lipodystrophies teach us about the metabolic syndrome

Seipin Deficiency as a Model of Severe Adipocyte Dysfunction: Lessons from Rodent Models and Teaching for Human Disease

Obesity prevalence is increasing worldwide, leading to cardiometabolic morbidities. Adipocyte dysfunction, impairing white adipose tissue (WAT) expandability and metabolic flexibility, is central in the development of obesity-related metabolic complications. Rare syndromes of lipodystrophy characterized by an extreme paucity of functional adipose tissue should be considered as primary adipocyte dysfunction diseases. Berardinelli-Seip congenital lipodystrophy (BSCL) is the most severe form with a near absence of WAT associated with cardiometabolic complications such as insulin resistance, liver steatosis, dyslipidemia, and cardiomyopathy. Twenty years ago, mutations in the BSCL2 gene have been identified as the cause of BSCL in human. BSCL2 encodes seipin, an endoplasmic reticulum (ER) anchored protein whose function was unknown back then. Studies of seipin knockout mice or rats demonstrated how seipin deficiency leads to severe lipodystrophy and to cardiometabolic complications. At the cellular levels, seipin is organized in multimers that are particularly enriched at ER/lipid droplet and ER/mitochondria contact sites. Seipin deficiency impairs both adipocyte differentiation and mature adipocyte maintenance. Experiments using adipose tissue transplantation in seipin knockout mice and tissue-specific deletion of seipin have provided a large body of evidence that liver steatosis, cardiomyopathy, and renal injury, classical diabetic complications, are all consequences of lipodystrophy. Rare adipocyte dysfunctions such as in BSCL are the key paradigm to unravel the pathways that control adipocyte homeostasis. The knowledge gathered through the study of these pathologies may bring new strategies to maintain and improve adipose tissue expandability.

Not Enough Fat: Mouse Models of Inherited Lipodystrophy

Lipodystrophies belong to the heterogenous group of syndromes in which the primary defect is a generalized or partial absence of adipose tissue, which may be congenital or acquired in origin. Lipodystrophy should be considered in patients manifesting the combination of insulin resistance (with or without overt diabetes), dyslipidemia and fatty liver. Lipodystrophies are classified according to the etiology of the disease (genetic or acquired) and to the anatomical distribution of adipose tissue (generalized or partial). The mechanism of adipose tissue loss is specific to each syndrome, depending on the biological function of the mutated gene. Mice models, together with cellular studies have permitted clarification of the mechanisms by which human mutations deeply compromise adipocyte homeostasis. In addition, rodent models have proven to be crucial in deciphering the cardiometabolic consequences of the lack of adipose tissue such as NAFLD, muscle insulin resistance and cardiomyopathy. More precisely, tissue-specific transgenic and knockout mice have brought new tools to distinguish phenotypic traits that are the consequences of lipodystrophy from those that are cell-autonomous. In this review, we discuss the mice models of lipodystrophy including those of inherited human syndromes of generalized and partial lipodystrophy. We present how these models have demonstrated the central role of white adipose tissue in energetic homeostasis in general, including insulin sensitivity and lipid handling in particular. We underscore the differences reported with the human phenotype and discuss the limit of rodent models in recapitulating adipose tissue primary default. Finally, we present how these mice models have highlighted the function of the causative-genes and brought new insights into the pathophysiology of the cardiometabolic complications associated with lipodystrophy.

Insulin Resistance: From Mechanisms to Therapeutic Strategies

Insulin resistance is the pivotal pathogenic component of many metabolic diseases, including type 2 diabetes mellitus, and is defined as a state of reduced responsiveness of insulin-targeting tissues to physiological levels of insulin. Although the underlying mechanism of insulin resistance is not fully understood, several credible theories have been proposed. In this review, we summarize the functions of insulin in glucose metabolism in typical metabolic tissues and describe the mechanisms proposed to underlie insulin resistance, that is, ectopic lipid accumulation in liver and skeletal muscle, endoplasmic reticulum stress, and inflammation. In addition, we suggest potential therapeutic strategies for addressing insulin resistance.

Adipocyte-specific tribbles pseudokinase 1 regulates plasma adiponectin and plasma lipids in mice

OBJECTIVE

Multiple genome-wide association studies (GWAS) have identified SNPs in the 8q24 locus near TRIB1 that are significantly associated with plasma lipids and other markers of cardiometabolic health, and prior studies have revealed the roles of hepatic and myeloid Trib1 in plasma lipid regulation and atherosclerosis. The same 8q24 SNPs are additionally associated with plasma adiponectin levels in humans, implicating TRIB1 in adipocyte biology. Here, we hypothesize that TRIB1 in adipose tissue regulates plasma adiponectin, lipids, and metabolic health.

METHODS

We investigate the metabolic phenotype of adipocyte-specific Trib1 knockout mice (Trib1_ASKO) fed on chow and high-fat diet (HFD). Through secretomics of adipose tissue explants and RNA-seq of adipocytes and livers from these mice, we further investigate the mechanism of TRIB1 in adipose tissue.

RESULTS

Trib1_ASKO mice have an improved metabolic phenotype with increased plasma adiponectin levels, improved glucose tolerance, and decreased plasma lipids. Trib1_ASKO adipocytes have increased adiponectin production and secretion independent of the known TRIB1 function of regulating proteasomal degradation. RNA-seq analysis of adipocytes and livers from Trib1_ASKO mice indicates that alterations in adipocyte function underlie the observed plasma lipid changes. Adipose tissue explant secretomics further reveals that Trib1_ASKO adipose tissue has decreased ANGPTL4 production, and we demonstrate an accompanying increase in the lipoprotein lipase (LPL) activity that likely underlies the triglyceride phenotype.

CONCLUSIONS

This study shows that adipocyte Trib1 regulates multiple aspects of metabolic health, confirming previously observed genetic associations in humans and shedding light on the further mechanisms by which TRIB1 regulates plasma lipids and metabolic health.

Is type 2 diabetes an adiposity-based metabolic disease? From the origin of insulin resistance to the concept of dysfunctional adipose tissue

In the last decades of the past century, a remarkable amount of research efforts, money and hopes was generated to unveil the basis of insulin resistance that was believed to be the primary etiological factor in the development of type 2 diabetes. From the Reaven's insulin resistance syndrome to the DeFronzo's triumvirate (skeletal muscle, liver and beta-cell) and to Kahn's discovery (among many others) of insulin receptor downregulation and autophosphorylation, an enthusiastic age of metabolic in vivo and in vitro research took place, making the promise of a resolutory ending. However, from many published data (those of insulin receptoropathies and lipodystrophies, the genome-wide association studies results, the data on reversibility of type 2 diabetes after bariatric surgery or very-low-calorie diets, and many others) it appears that insulin resistance is not a primary defect but it develops secondarily to increased fat mass. In particular, it develops from a mismatch between the surplus caloric intake and the storage capacity of adipose tissue. On this basis, we propose to change the today's definition of type 2 diabetes in adiposity-based diabetes.Level of Evidence as a narrative review a vast array of studies have been included in the analysis, ranging from properly designed randomized controlled trials to case studies; however, the overall conclusion may be regarded as level IV.

Assessing the Causal Relationships Between Insulin Resistance and Hyperuricemia and Gout Using Bidirectional Mendelian Randomization

OBJECTIVE

Hyperuricemia is closely associated with insulin resistance syndrome (and its many cardiometabolic sequelae); however, whether they are causally related has long been debated. We undertook this study to investigate the potential causal nature and direction between insulin resistance and hyperuricemia, along with gout, by using bidirectional Mendelian randomization (MR) analyses.

METHODS

We used genome-wide association data (n = 288,649 for serum urate [SU] concentration; n = 763,813 for gout risk; n = 153,525 for fasting insulin) to select genetic instruments for 2-sample MR analyses, using multiple MR methods to address potential pleiotropic associations. We then used individual-level, electronic medical record-linked data from the UK Biobank (n = 360,453 persons of European ancestry) to replicate our analyses via single-sample MR analysis.

RESULTS

Genetically determined SU levels, whether inferred from a polygenic score or strong individual loci, were not associated with fasting insulin concentrations. In contrast, genetically determined fasting insulin concentrations were positively associated with SU levels (0.37 mg/dl per log-unit increase in fasting insulin [95% confidence interval (95% CI) 0.15, 0.58]; P = 0.001). This persisted in outlier-corrected (β = 0.56 mg/dl [95% CI 0.45, 0.67]) and multivariable MR analyses adjusted for BMI (β = 0.69 mg/dl [95% CI 0.53, 0.85]) (P < 0.001 for both). Polygenic scores for fasting insulin were also positively associated with SU level among individuals in the UK Biobank (P < 0.001). Findings for gout risk were bidirectionally consistent with those for SU level.

CONCLUSION

These findings provide evidence to clarify core questions about the close association between hyperuricemia and insulin resistance syndrome: hyperinsulinemia leads to hyperuricemia but not the other way around. Reducing insulin resistance could lower the SU level and gout risk, whereas lowering the SU level (e.g., allopurinol treatment) is unlikely to mitigate insulin resistance and its cardiometabolic sequelae.

Defining paediatric metabolic (dysfunction)-associated fatty liver disease: an international expert consensus statement

The term non-alcoholic fatty liver disease (NAFLD), and its definition, have limitations for both adults and children. The definition is most problematic for children, for whom alcohol consumption is usually not a concern. This problematic definition has prompted a consensus to rename and redefine adult NAFLD associated with metabolic dysregulation to metabolic (dysfunction)-associated fatty liver disease (MAFLD). Similarities, distinctions, and differences exist in the causes, natural history, and prognosis of fatty liver diseases in children compared with adults. In this Viewpoint we, an international panel, propose an overarching framework for paediatric fatty liver diseases and an age-appropriate MAFLD definition based on sex and age percentiles. The framework recognises the possibility of other coexisting systemic fatty liver diseases in children. The new MAFLD diagnostic criteria provide paediatricians with a conceptual scaffold for disease diagnosis, risk stratification, and improved clinical and multidisciplinary care, and they align with a definition that is valid across the lifespan.

A Systematic Review of Animal Models of NAFLD Finds High-Fat, High-Fructose Diets Most Closely Resemble Human NAFLD

BACKGROUND AND AIMS

Animal models of human disease are a key component of translational hepatology research, yet there is no consensus on which model is optimal for NAFLD.

APPROACH AND RESULTS

We generated a database of 3,920 rodent models of NAFLD. Study designs were highly heterogeneous, and therefore, few models had been cited more than once. Analysis of genetic models supported the current evidence for the role of adipose dysfunction and suggested a role for innate immunity in the progression of NAFLD. We identified that high-fat, high-fructose diets most closely recapitulate the human phenotype of NAFLD. There was substantial variability in the nomenclature of animal models: a consensus on terminology of specialist diets is needed. More broadly, this analysis demonstrates the variability in preclinical study design, which has wider implications for the reproducibility of in vivo experiments both in the field of hepatology and beyond.

CONCLUSIONS

This systematic analysis provides a framework for phenotypic assessment of NAFLD models and highlights the need for increased standardization and replication.

Dyslipidemia and diabetes mellitus: Role of lipoprotein species and interrelated pathways of lipid metabolism in diabetes mellitus

Diabetes mellitus is a complex disease. We are increasingly gaining a better understanding of its mechanisms at the molecular level. From these new insights, better therapeutic approaches should emerge. Diabetes mellitus is a syndrome with many associated subphenotypes. These include mitochondrial disorders, lipodystrophies, and inflammatory disorders involving cytokines. Levels of sphingosine-1-phosphate, which has recently been shown to play a role in glucose homeostasis, are low in diabetics, whereas levels of ceramides are increased. Major phenotypes associated with diabetes mellitus are dyslipidemias, notably hypertriglyceridemia and low high-density lipoprotein cholesterol levels. Both diabetes and dyslipidemia are strongly associated with increased risk for atherosclerotic vascular disease.

Adipocyte function and the development of cardiometabolic disease

Obesity is a medical disorder caused by multiple mechanisms of dysregulated energy balance. A major consequence of obesity is an increased risk to develop diabetes, diabetic complications and cardiovascular disease. While a better understanding of the molecular mechanisms linking obesity, insulin resistance and cardiovascular disease is needed, translational research of the human pathology is hampered by the available cellular and rodent model systems. Major barriers are the species-specific differences in energy balance, vascular biology and adipose tissue physiology, especially related to white and brown adipocytes, and adipose tissue browning. In rodents, non-shivering thermogenesis is responsible for a large part of energy expenditure, but humans possess much less thermogenic fat, which means temperature is an important variable in translational research. Mouse models with predisposition to dyslipidaemia housed at thermoneutrality and fed a high-fat diet more closely reflect human physiology. Also, adipocytes play a key role in the endocrine regulation of cardiovascular function. Adipocytes secrete a variety of hormones, lipid mediators and other metabolites that directly influence the local microenvironment as well as distant tissues. This is specifically apparent in perivascular depots, where adipocytes modulate vascular function and inflammation. Altogether, these mechanisms highlight the critical role of adipocytes in the development of cardiometabolic disease.

Lipin 1 modulates mRNA splicing during fasting adaptation in liver

Lipin 1 regulates cellular lipid homeostasis through roles in glycerolipid synthesis (through phosphatidic acid phosphatase activity) and transcriptional coactivation. Lipin 1-deficient individuals exhibit episodic disease symptoms that are triggered by metabolic stress, such as stress caused by prolonged fasting. We sought to identify critical lipin 1 activities during fasting. We determined that lipin 1 deficiency induces widespread alternative mRNA splicing in liver during fasting, much of which is normalized by refeeding. The role of lipin 1 in mRNA splicing was largely independent of its enzymatic function. We identified interactions between lipin 1 and spliceosome proteins, as well as a requirement for lipin 1 to maintain homeostatic levels of spliceosome small nuclear RNAs and specific RNA splicing factors. In fasted Lpin1-/- liver, we identified a correspondence between alternative splicing of phospholipid biosynthetic enzymes and dysregulated phospholipid levels; splicing patterns and phospholipid levels were partly normalized by feeding. Thus, lipin 1 influences hepatic lipid metabolism through mRNA splicing, as well as through enzymatic and transcriptional activities, and fasting exacerbates the deleterious effects of lipin 1 deficiency on metabolic homeostasis.

PPARs-Orchestrated Metabolic Homeostasis in the Adipose Tissue

It has been more than three decades since peroxisome proliferator-activated receptors (PPARs) were first discovered. Many investigations have revealed the central regulators of PPARs in lipid and glucose homeostasis in response to different nutrient conditions. PPARs have attracted much attention due to their ability to improve metabolic syndromes, and they have also been proposed as classical drug targets for the treatment of hyperlipidemia and type 2 diabetes (T2D) mellitus. In parallel, adipose tissue is known to play a unique role in the pathogenesis of insulin resistance and metabolic syndromes due to its ability to “safely” store lipids and secrete cytokines that regulate whole-body metabolism. Adipose tissue relies on a complex and subtle network of transcription factors to maintain its normal physiological function, by coordinating various molecular events, among which PPARs play distinctive and indispensable roles in adipocyte differentiation, lipid metabolism, adipokine secretion, and insulin sensitivity. In this review, we discuss the characteristics of PPARs with special emphasis on the roles of the different isotypes in adipocyte biology.

Management of Obesity in Cardiovascular Practice: JACC Focus Seminar

Obesity contributes to reduced life expectancy because of its link with type 2 diabetes and cardiovascular disease. Yet, targeting this poorly diagnosed, ill-defined, and underaddressed modifiable risk factor remains a challenge. In this review, we emphasize that the tendency among health care professionals to amalgam all forms of obesity altogether as a single entity may contribute to such difficulties and discrepancies. Obesity is a heterogeneous condition both in terms of causes and health consequences. Attention should be given to 2 prevalent subgroups of individuals: 1) patients who are overweight or moderately obese with excess visceral adipose tissue; and 2) patients with severe obesity, the latter group having distinct additional health issues related to their large body fat mass. The challenge of tackling high-cardiovascular-risk forms of obesity through a combination of personalized clinical approaches and population-based solutions is compounded by the current obesogenic environment and economy.

Telomere length and metabolic syndrome traits: A Mendelian randomisation study

Observational studies have revealed associations between short leucocyte telomere length (LTL), a TL marker in somatic tissues and multiple Metabolic Syndrome (MetS) traits. Animal studies have supported these findings by showing that increased telomere attrition leads to adipose tissue dysfunction and insulin resistance. We investigated the associations between genetically instrumented LTL and MetS traits using Mendelian Randomisation (MR). Fifty-two independent variants identified at FDR<0.05 from a genome-wide association study (GWAS) including 78,592 Europeans and collectively accounting for 2.93% of LTL variance were selected as genetic instruments for LTL. Summary-level data for MetS traits and for the MetS as a binary phenotype were obtained from the largest publicly available GWAS and two-sample MR analyses were used to estimate the associations of LTL with these traits. The combined effect of the genetic instruments was modelled using inverse variance weighted regression and sensitivity analyses with MR-Egger, weighted-median and MR-PRESSO were performed to test for and correct horizonal pleiotropy. Genetically instrumented longer LTL was associated with higher waist-to-hip ratio adjusted for body mass index (β = 0.045 SD, SE = 0.018, p = 0.01), raised systolic (β = 1.529 mmHg, SE = 0.332, p = 4x10^-6 ) and diastolic (β = 0.633 mmHg, SE = 0.222, p = 0.004) blood pressure, and increased MetS risk (OR = 1.133, 95% CI 1.057-1.215). Consistent results were obtained in sensitivity analyses, which provided no evidence of unbalanced horizontal pleiotropy. Telomere shortening might not be a major driver of cellular senescence and dysfunction in human adipose tissue. Future experimental studies should examine the mechanistic bases for the links between longer LTL and increased upper-body fat distribution and raised blood pressure.

Collagen β(1-O) galactosyltransferase 2 deficiency contributes to lipodystrophy and aggravates NAFLD related to HMW adiponectin in mice

AIM

Our previous results showed that Colgalt1 knock-out resulted in fetal death on day E11.5, and collagen secretion was retarded. This study aimed to elucidate the role of Collagen β(1-O) galactosyltransferase 2 (Colgalt2) in the pathogenesis of non-alcoholic fatty liver disease (NAFLD).

METHODS

Colgalt2^-/- mice were fed a high-fat diet (HFD) or methionine-and choline-deficient diet (MCD). Nanopore long-read RNA-Seq analysis of liver tissues was used to profile genomic variation. In vitro, hepatocyte steatosis and differentiation of primary pre-adipocytes were induced.

RESULTS

Colgalt2^-/- mice exhibited lipodystrophy, increased body weight, and hepatic lipid accumulation at 6 weeks of age. Colgalt2 deficiency aggravated hepatic steatosis in mice fed an HFD or a standard laboratory chow diet. Colgalt2 deficiency promotes steatohepatitis in MCD-fed mice. In HFD mice, Colgalt2 deficiency caused lipodystrophy and decreased plasma HMW, total adiponectin, and leptin levels. Colgalt2 deficiency also reduced circulating HMW/Total adiponectin in mice fed a HFD diet without differences of adiponectin mRNA and protein level in WT and Colgalt2^-/- mice. The nanopore long-read RNA-Seq analysis results revealed transcriptional changes in the adiponectin receptor downstream signaling pathway and lipogenic genes, including the AMPK signaling pathway, adipocytokine signaling pathway, and lipid metabolism (Cidea, Cidec, CD36, and PPARγ). Colgalt2 deficiency did not promote lipid accumulation in OA-induced HepG2 cells or primary hepatocytes. However, Colgalt2 deficiency inhibited adipogenesis and reduced PPARγ, adipogenesis-related transcription factors, and expression during adipocyte differentiation.

CONCLUSIONS

In mice, Colgalt2 deficiency contributes to lipodystrophy and promotes NAFLD related to HMW adiponectin. These results suggest that Colgalt2 could be a novel and promising therapeutic strategy for the treatment of NAFLD.

Sympathetic Innervation of White Adipose Tissue: to Beige or Not to Beige?

Obesity research progresses in understanding neuronal circuits and adipocyte biology to regulate metabolism. However, the interface of neuro-adipocyte interaction is less studied. We summarize the current knowledge of adipose tissue innervation and interaction with adipocytes and emphasize adipocyte transitions from white to brown adipocytes and vice versa. We further highlight emerging concepts for the differential neuronal regulation of brown/beige versus white adipocyte and the interdependence of both for metabolic regulation.

Bio-inspired intestinal scavenger from microfluidic electrospray for detoxifying lipopolysaccharide

Lipopolysaccharide (LPS) plays an important role in metabolic syndrome (MetS) and other gut-derived diseases, and detoxifying LPS is considered to be a fundamental approach to prevent and treat these diseases. Here, inspired by the feeding behaviour of scavenger, novel microfluidic microcapsules with alkaline phosphatase (ALP) encapsulation and the scavenger-like molecular sieve shell are presented for cleaning intestinal LPS. Benefiting from the precisely controlled of the pore size and microfluidic electrospray, the generated microcapsules were imparted with porous molecular-sieve shells and ALP encapsulated active cores. These microcapsules could continuously work as an intestinal scavenger after colonized in intestine. It has been demonstrated that the microcapsules could englobe LPS while inhibit the permeation of digestive enzyme, and this ability contributes to promising ALP's activity, protecting cells at the presence of LPS and reducing inflammation. In addition, this scavenger inspired microcapsule could effectively decrease the LPS in organs, reduce inflammation and regulating fat metabolism in vivo. These features make the ALP encapsulated microcapsules an ideal candidate for treating MetS and other LPS related diseases.

Lipid and glucose metabolism in white adipocytes: pathways, dysfunction and therapeutics

In mammals, the white adipocyte is a cell type that is specialized for storage of energy (in the form of triacylglycerols) and for energy mobilization (as fatty acids). White adipocyte metabolism confers an essential role to adipose tissue in whole-body homeostasis. Dysfunction in white adipocyte metabolism is a cardinal event in the development of insulin resistance and associated disorders. This Review focuses on our current understanding of lipid and glucose metabolic pathways in the white adipocyte. We survey recent advances in humans on the importance of adipocyte hypertrophy and on the in vivo turnover of adipocytes and stored lipids. At the molecular level, the identification of novel regulators and of the interplay between metabolic pathways explains the fine-tuning between the anabolic and catabolic fates of fatty acids and glucose in different physiological states. We also examine the metabolic alterations involved in the genesis of obesity-associated metabolic disorders, lipodystrophic states, cancers and cancer-associated cachexia. New challenges include defining the heterogeneity of white adipocytes in different anatomical locations throughout the lifespan and investigating the importance of rhythmic processes. Targeting white fat metabolism offers opportunities for improved patient stratification and a wide, yet unexploited, range of therapeutic opportunities.

Mitochondrial Unfolded Protein Responses in White Adipose Tissue: Lipoatrophy, Whole-Body Metabolism and Lifespan

The mitochondrial unfolded protein response (UPR^mt) is a stress response mediated by the expression of genes such as chaperones, proteases, and mitokines to maintain mitochondrial proteostasis. Certain genetically modified mice, which defect mitochondrial proteins specifically in adipocytes, developed atrophy of the white adipose tissue, resisted diet-induced obesity, and had altered whole-body metabolism. UPR^mt, which has beneficial functions for living organisms, is termed "mitohormesis", but its specific characteristics and detailed regulatory mechanism have not been elucidated to date. In this review, we discuss the function of UPR^mt in adipose atrophy (lipoatrophy), whole-body metabolism, and lifespan based on the concept of mitohormesis.

Endoplasmic reticulum stress and muscle dysfunction in congenital lipodystrophies

Lipodystrophy syndromes are a group of rare diseases related to the pathological impairment of adipose tissue and metabolic comorbidities, including dyslipidemia, diabetes, insulin resistance, hypoleptinemia, and hypoadiponectinemia. They can be categorized as partial or generalized according to the degree of fat loss, and inherited or acquired disorders, if they are associated with genetic mutations or are related to autoimmunity, respectively. Some types of lipodystrophies have been associated with changes in both redox and endoplasmic reticulum (ER) homeostasis as well as muscle dysfunction (MD). Although ER stress (ERS) has been related to muscle dysfunction (MD) in many diseases, there is no data concerning its role in lipodystrophies' muscle physiopathology. Here we focused on congenital lipodystrophies associated with ERS and MD. We also described recent advances in our understanding of the relationships among ERS, MD, and genetic lipodystrophies, highlighting the adiponectin-protective roles.

COVID-19 severity in obese patients: Potential mechanisms and molecular targets for clinical intervention

With the global spread of SARS-CoV-2, millions of people have been affected leading to the declaration of coronavirus disease 2019 (COVID-19) as a pandemic by the WHO. Several studies have linked the severity of COVID-19 cases and increased fatality in patients with obesity and other comorbid conditions such as diabetes, cardiovascular diseases, hypertension, and kidney disease. Obesity, a metabolically deranged condition, establishes a low-grade chronic inflammation in the body, which affects different organs and promotes the development of several other diseases. The ways in which SARS-CoV-2 infection aggravates the already overloaded body organs with inflammation or vice versa has perplexed the researchers. As a result, there is an intensified search for the clear-cut mechanism to understand the link of obesity with the increased severity of COVID-19 in obese patients. In this article we have discussed various mechanisms linking obesity, inflammation, and COVID-19 to enhance the understanding of the disease process and help the clinicians and scientists develop potential cellular, molecular and metabolic targets for clinical intervention and management of COVID-19 severity in obese patients.

Adipocyte-specific deletion of zinc finger protein 407 results in lipodystrophy and insulin resistance in mice

PPARγ deficiency in humans and model organisms impairs the transcriptional control of adipogenesis and mature adipocyte function resulting in lipodystrophy and insulin resistance. Zinc finger protein 407 (ZFP407) positively regulates PPARγ target gene expression and insulin-stimulated glucose uptake in cultured adipocytes. The in vivo physiological role of ZFP407 in mature adipocytes, however, remains to be elucidated. Here we generated adipocyte-specific ZFP407 knockout (AZKO) mice and discovered a partial lipodystrophic phenotype with reduced fat mass, hypertrophic adipocytes in inguinal and brown adipose tissue, and reduced adipogenic gene expression. The lipodystrophy was further exacerbated in AZKO mice fed a high-fat diet. Glucose and insulin tolerance tests revealed decreased insulin sensitivity in AZKO mice compared to control littermates. Cell-based assays demonstrated that ZFP407 is also required for adipogenesis, which may also contribute to the lipodystrophic phenotype. These results demonstrate an essential in vivo role of ZFP407 in brown and white adipose tissue formation and organismal insulin sensitivity.

Cigarette smoking blocks the benefit from reduced weight gain for insulin action by shifting lipids deposition to muscle

Cigarette smoking (CS) is known to reduce body weight and this often masks its real effect on insulin action. The present study tested the hypothesis that CS can divert lipid deposition to muscles to offset the supposed benefit of reduced body weight gain on insulin signalling in this major site for glucose tolerance (or insulin action). The study was conducted in mice exposed to chronic CS followed by either a chow (CH) diet or a high-fat (HF) diet. CS increased triglyceride (TG) levels in both plasma and muscle despite a reduced body weight gain and adiposity. CS led to glucose intolerance in CH-fed mice and they retained the glucose intolerance that was induced by the HF diet. In adipose tissue, CS increased macrophage infiltration and the mRNA expression of TNFα but suppressed the protein expression of adipose triglyceride lipase and PPARγ. While CS increased hormone-sensitive lipase and suppressed the mRNA expression of leptin, these effects were blunted in HF-fed mice. These results imply that CS impairs insulin signalling in skeletal muscle via accumulated intramuscular lipids from lipolysis and lipodystrophy of adipose tissues. This may explain why smokers may not benefit from insulin sensitising effects of reduced body weight gain.

Case Report: Metreleptin and SGLT2 Inhibitor Combination Therapy Is Effective for Acquired Incomplete Lipodystrophy

Childhood cancer survivors (CCSs) who have undergone bone marrow transplantation with systemic chemotherapy and whole-body irradiation often experience impaired glucose tolerance with marked insulin resistance. Incomplete acquired diabetic lipodystrophy should be considered as a late complication of bone marrow transplantation. A 24-year-old Japanese female patient with incomplete acquired lipodystrophy, a CCS of acute lymphocytic leukemia at the age of 3 years, was treated for diabetes mellitus and dyslipidemia at our hospital. Administration of multiple daily insulin injections (70 units/day), and oral administration of 500 mg/day metformin, 15 mg/day pioglitazone, and 200 mg/day bezafibrate had proven ineffective for her metabolic disorders. Subcutaneous administration of metreleptin improved her insulin resistance and hypertriglyceridemia within a month; however, it failed to maintain adequate plasma glucose levels in the long term. When oral administration of 10 mg/day empagliflozin was added to the metreleptin supplementation, her HbA1c value (National Glycohemoglobin Standardization Program) improved from 11% to 8%, which was maintained for an additional 18 months. This is the first case report of incomplete lipodystrophy that shows efficacy of a combination therapy with metreleptin and a sodium glucose cotransporter 2 (SGLT2) inhibitor for the treatment of diabetes and dyslipidemia. An SGLT2 inhibitor attenuates hyperglycemia through urinary glucose excretion and has been suggested to enhance lipid catabolism in the extra-adipose tissues, especially in the liver and skeletal muscles. Furthermore, metreleptin supplementation could enhance the action of the SGLT2 inhibitor by promoting satiety and lipolysis through the central nervous system. Combination therapy with metreleptin and an SGLT2 inhibitor was suggested to recover the volume of adipose tissue, possibly through improvement of insulin resistance in the adipose tissue. This report highlights the pathophysiological mechanism of an SGLT2 inhibitor in the improvement of glucose metabolism in non-healthy lean CCSs with insulin resistance. Administration of SGLT2 inhibitor, along with metreleptin supplementation, could be a good alternative therapy for diabetic lipodystrophy observed in CCSs.

Adipose Tissue: An Emerging Target for Adeno-associated Viral Vectors

Adipose tissue is one of the largest organs, playing important roles in physiology and pathologies of multiple diseases. However, research related to adeno-associated virus (AAV) targeting adipose tissue has been left far behind studies carried out in the liver, brain, heart, and muscle. Despite initial reports indicating poor performance, AAV-mediated gene delivery to adipose tissue has continued to rise during the past two decades. AAV8 and a novel engineered hybrid serotype, Rec2, have been shown to transduce adipose tissue more efficiently than other serotypes so far tested and have been applied in most of the in vivo studies. The Rec2 serotype displays high efficacy of gene transfer to both brown and white fat via local and systemic administration. This review summarizes the advances in developing AAV vectors with enhanced adipose tropism and restricting off-target transgene expression. We discuss the challenges and strategies to search for and generate novel serotypes with tropism tailoring for adipose tissue and develop AAV vector systems to improve adipose transgene expression for basic research and translational studies.

Truncation of Pik3r1 causes severe insulin resistance uncoupled from obesity and dyslipidaemia by increased energy expenditure

OBJECTIVE

Insulin signalling via phosphoinositide 3-kinase (PI3K) requires PIK3R1-encoded regulatory subunits. C-terminal PIK3R1 mutations cause SHORT syndrome, as well as lipodystrophy and insulin resistance (IR), surprisingly without fatty liver or metabolic dyslipidaemia. We sought to investigate this discordance.

METHODS

The human pathogenic Pik3r1 Y657∗ mutation was knocked into mice by homologous recombination. Growth, body composition, bioenergetic and metabolic profiles were investigated on chow and high-fat diet (HFD). We examined adipose and liver histology, and assessed liver responses to fasting and refeeding transcriptomically.

RESULTS

Like humans with SHORT syndrome, Pik3r1WT/Y657∗ mice were small with severe IR, and adipose expansion on HFD was markedly reduced. Also as in humans, plasma lipid concentrations were low, and insulin-stimulated hepatic lipogenesis was not increased despite hyperinsulinemia. At odds with lipodystrophy, however, no adipocyte hypertrophy nor adipose inflammation was found. Liver lipogenic gene expression was not significantly altered, and unbiased transcriptomics showed only minor changes, including evidence of reduced endoplasmic reticulum stress in the fed state and diminished Rictor-dependent transcription on fasting. Increased energy expenditure, which was not explained by hyperglycaemia nor intestinal malabsorption, provided an alternative explanation for the uncoupling of IR from dyslipidaemia.

CONCLUSIONS

Pik3r1 dysfunction in mice phenocopies the IR and reduced adiposity without lipotoxicity of human SHORT syndrome. Decreased adiposity may not reflect bona fide lipodystrophy, but rather, increased energy expenditure, and we suggest that further study of brown adipose tissue in both humans and mice is warranted.

Metabolically Healthy and Unhealthy Normal Weight and Obesity

Increased fat mass is an established risk factor for the cardiometabolic diseases type 2 diabetes and cardiovascular disease (CVD) and is associated with increased risk of all-cause and CVD mortality. However, also very low fat mass associates with such an increased risk. Whether impaired metabolic health, characterized by hypertension, dyslipidemia, hyperglycemia, insulin resistance, and subclinical inflammation, may explain part of the elevated risk of cardiometabolic diseases that is found in many subjects with very low fat mass, as it does in many obese subjects, is unknown. An important pathomechanism of impaired metabolic health is disproportionate fat distribution. In this article the risk of cardiometabolic diseases and mortality in subjects with metabolically healthy and unhealthy normal weight and obesity is summarized. Furthermore, the change of metabolic health during a longer period of follow-up and its impact on cardiometabolic diseases is being discussed. Finally, the implementation of the concept of metabolic health in daily clinical practice is being highlighted.

Causes, consequences, and treatment of metabolically unhealthy fat distribution

An increase in fat mass is considered to be an important risk factor for the worldwide increase in type 2 diabetes and cardiovascular disease. However, for a given fat mass, there is a large variability in the risk prediction of these cardiometabolic diseases. For example, some lean people unexpectedly have a risk of type 2 diabetes and cardiovascular disease that is similar to the increased risk that is observed in most people who have obesity. What both of these phenotypes have in common is a very characteristic fat distribution. As a result, much focus has been given on the strong predictive power of increased visceral fat mass. However, an analysis of the causes of type 2 diabetes and cardiovascular disease, as well as comparisons to rare diseases such as lipodystrophy and studying genetically determined fat distribution in the general population, suggest that an impaired ability to expand subcutaneous fat in the lower part of the body is also important for predicting the incidence of these cardiometabolic diseases. This Review, first, addresses the identification of distinct fat distribution phenotypes and their risk of cardiometabolic diseases by discussing findings from published studies that have applied precise quantification of different fat depots. Second, this Review provides support for the theory that a lower amount of lower-body fat mass is equally important to a high amount of visceral fat mass as a determinant of cardiometabolic diseases. Third, this Review discusses the genetic and lifestyle-related causes of metabolically healthy and unhealthy fat distribution. Finally, this Review summarises and appraises the effectiveness of lifestyle-related interventions and pharmacological interventions for reducing visceral adiposity and maintaining lower-body fat mass to prevent and treat cardiometabolic diseases.

Altered adipose tissue and adipocyte function in the pathogenesis of metabolic syndrome

Over the past decade, great progress has been made in understanding the complexity of adipose tissue biology and its role in metabolism. This includes new insights into the multiple layers of adipose tissue heterogeneity, not only differences between white and brown adipocytes, but also differences in white adipose tissue at the depot level and even heterogeneity of white adipocytes within a single depot. These inter- and intra-depot differences in adipocytes are developmentally programmed and contribute to the wide range of effects observed in disorders with fat excess (overweight/obesity) or fat loss (lipodystrophy). Recent studies also highlight the underappreciated dynamic nature of adipose tissue, including potential to undergo rapid turnover and dedifferentiation and as a source of stem cells. Finally, we explore the rapidly expanding field of adipose tissue as an endocrine organ, and how adipose tissue communicates with other tissues to regulate systemic metabolism both centrally and peripherally through secretion of adipocyte-derived peptide hormones, inflammatory mediators, signaling lipids, and miRNAs packaged in exosomes. Together these attributes and complexities create a robust, multidimensional signaling network that is central to metabolic homeostasis.

Review article: the emerging role of genetics in precision medicine for patients with non-alcoholic steatohepatitis

BACKGROUND

Non-alcoholic steatohepatitis (NASH) is a severe form of non-alcoholic fatty liver disease (NAFLD) characterised by liver fat accumulation, inflammation and progressive fibrosis. Emerging data indicate that genetic susceptibility increases risks of NAFLD, NASH and NASH-related cirrhosis.

AIMS

To review NASH genetics and discuss the potential for precision medicine approaches to treatment.

METHOD

PubMed search and inclusion of relevant literature.

RESULTS

Single-nucleotide polymorphisms in PNPLA3, TM6SF2, GCKR, MBOAT7 and HSD17B13 are clearly associated with NASH development or progression. These genetic variants are common and have moderate-to-large effect sizes for development of NAFLD, NASH and hepatocellular carcinoma (HCC). The genes play roles in lipid remodelling in lipid droplets, hepatic very low-density lipoprotein (VLDL) secretion and de novo lipogenesis. The PNPLA3 I148M variant (rs738409) has large effects, with approximately twofold increased odds of NAFLD and threefold increased odds of NASH and HCC per allele. Obesity interacts with PNPLA3 I148M to elevate liver fat content and increase rates of NASH. Although the isoleucine-to-methionine substitution at amino acid position 148 of the PNPLA3 enzyme inactivates its lipid remodelling activity, the effect of PNPLA3 I148M results from trans-repression of another lipase (ATGL/PNPLA2) by sequestration of a shared cofactor (CGI-58/ABHD5), leading to decreased hepatic lipolysis and VLDL secretion. In homozygous Pnpla3 I148M knock-in rodent models of NAFLD, targeted PNPLA3 mRNA knockdown reduces hepatic steatosis, inflammation and fibrosis.

CONCLUSION

The emerging genetic and molecular understanding of NASH paves the way for novel interventions, including precision medicines that can modulate the activity of specific genes associated with NASH.

Mechanisms of insulin resistance related to white, beige, and brown adipocytes

BACKGROUND

The diminished glucose lowering effect of insulin in obesity, called "insulin resistance," is associated with glucose intolerance, type 2 diabetes, and other serious maladies. Many publications on this topic have suggested numerous hypotheses on the molecular and cellular disruptions that contribute to the syndrome. However, significant uncertainty remains on the mechanisms of its initiation and long-term maintenance.

SCOPE OF REVIEW

To simplify insulin resistance analysis, this review focuses on the unifying concept that adipose tissue is a central regulator of systemic glucose homeostasis by controlling liver and skeletal muscle metabolism. Key aspects of adipose function related to insulin resistance reviewed are: 1) the modes by which specific adipose tissues control hepatic glucose output and systemic glucose disposal, 2) recently acquired understanding of the underlying mechanisms of these modes of regulation, and 3) the steps in these pathways adversely affected by obesity that cause insulin resistance.

MAJOR CONCLUSIONS

Adipocyte heterogeneity is required to mediate the multiple pathways that control systemic glucose tolerance. White adipocytes specialize in sequestering triglycerides away from the liver, muscle, and other tissues to limit toxicity. In contrast, brown/beige adipocytes are very active in directly taking up glucose in response to β adrenergic signaling and insulin and enhancing energy expenditure. Nonetheless, white, beige, and brown adipocytes all share the common feature of secreting factors and possibly exosomes that act on distant tissues to control glucose homeostasis. Obesity exerts deleterious effects on each of these adipocyte functions to cause insulin resistance.

Clinical and Molecular Prevalence of Lipodystrophy in an Unascertained Large Clinical Care Cohort

Lipodystrophies are a group of disorders characterized by absence or loss of adipose tissue and abnormal fat distribution, commonly accompanied by metabolic dysregulation. Although considered rare disorders, their prevalence in the general population is not well understood. We aimed to evaluate the clinical and genetic prevalence of lipodystrophy disorders in a large clinical care cohort. We interrogated the electronic health record (EHR) information of >1.3 million adults from the Geisinger Health System for lipodystrophy diagnostic codes. We estimate a clinical prevalence of disease of 1 in 20,000 individuals. We performed genetic analyses in individuals with available genomic data to identify variants associated with inherited lipodystrophies and examined their EHR for comorbidities associated with lipodystrophy. We identified 16 individuals carrying the p.R482Q pathogenic variant in LMNA associated with Dunnigan familial partial lipodystrophy. Four had a clinical diagnosis of lipodystrophy, whereas the remaining had no documented clinical diagnosis despite having accompanying metabolic abnormalities. We observed a lipodystrophy-associated variant carrier frequency of 1 in 3,082 individuals in our cohort with substantial burden of metabolic dysregulation. We estimate a genetic prevalence of disease of ∼1 in 7,000 in the general population. Partial lipodystrophy is an underdiagnosed condition. and its prevalence, as defined molecularly, is higher than previously reported. Genetically guided stratification of patients with common metabolic disorders, like diabetes and dyslipidemia, is an important step toward precision medicine.

The common pathophysiologic threads between Asian Indian diabetic's 'Thin Fat Phenotype' and partial lipodystrophy: the peripheral adipose tissue transcriptomic evidences

T2D is a complex disease with poorly understood mechanisms. In Asian Indians, it is associated with “thin fat” phenotype which resembles with partial lipodystrophy. We hypothesized that disturbed expression of lipodystrophy genes might play a role in T2D pathogenesis. Therefore, we attempted to establish a link between these two diseases by studying the overlap between the network of lipodystrophy genes and the differentially expressed genes (DEGs) in the peripheral subcutaneous adipose tissue of Asian Indians diabetics. We found that 16, out of 138 lipodystrophy genes were differentially regulated in diabetics and around 18% overlap between their network and the DEGs; the expression level of lipodystrophy genes showed an association with disease-related intermediate phenotypic traits among diabetics but not in the control group. We also attempted to individualize the diabetic patients based on ±2 fold altered expression of lipodystrophy genes as compared to their average expression in the control group. In conclusion, significant overlap exists between some of the lipodystrophy genes and their network with DEGs in the peripheral adipose tissue in diabetics. They possibly play a role in the pathogenesis of diabetes and individualization of diabetics is possible based on their altered expression in their peripheral adipose tissue.