Biological role, clinical significance, and therapeutic possibilities of the recently discovered metabolic hormone fibroblastic growth factor 21

Additive protection by LDR and FGF21 treatment against diabetic nephropathy in type 2 diabetes model

The onset of diabetic nephropathy (DN) is associated with both systemic and renal changes. Fibroblast growth factor (FGF)-21 prevents diabetic complications mainly by improving systemic metabolism. In addition, low-dose radiation (LDR) protects mice from DN directly by preventing renal oxidative stress and inflammation. In the present study, we tried to define whether the combination of FGF21 and LDR could further prevent DN by blocking its systemic and renal pathogeneses. To this end, type 2 diabetes was induced by feeding a high-fat diet for 12 wk followed by a single dose injection of streptozotocin. Diabetic mice were exposed to 50 mGy LDR every other day for 4 wk with and without 1.5 mg/kg FGF21 daily for 8 wk. The changes in systemic parameters, including blood glucose levels, lipid profiles, and insulin resistance, as well as renal pathology, were examined. Diabetic mice exhibited renal dysfunction and pathological abnormalities, all of which were prevented significantly by LDR and/or FGF21; the best effects were observed in the group that received the combination treatment. Our studies revealed that the additive renal protection conferred by the combined treatment against diabetes-induced renal fibrosis, inflammation, and oxidative damage was associated with the systemic improvement of hyperglycemia, hyperlipidemia, and insulin resistance. These results suggest that the combination treatment with LDR and FGF21 prevented DN more efficiently than did either treatment alone. The mechanism behind these protective effects could be attributed to the suppression of both systemic and renal pathways.

Fibroblast growth factor 21 deletion aggravates diabetes-induced pathogenic changes in the aorta in type 1 diabetic mice

Fibroblast growth factor 21 (FGF21) is an important regulator in glucose and lipid metabolism, and has been considered as a potential therapy for diabetes. The effect of FGF21 on the development and progression of diabetes-induced pathogenic changes in the aorta has not currently been addressed. To characterize these effects, type 1 diabetes was induced in both FGF21 knockout (FGF21KO) and C57BL/6 J wild type (WT) mice via multiple-dose streptozotocin injection. FGF21KO diabetic mice showed both earlier and more severe aortic remodeling indicated by aortic thickening, collagen accumulation and fibrotic mediator connective tissue growth factor expression. This was accompanied by significant aortic cell apoptosis than in WT diabetic mice. Further investigation found that FGF21 deletion exacerbated aortic inflammation and oxidative stress reflected by elevated expression of tumor necrosis factor α and transforming growth factor β, and the accumulation of 3-nitrotyrocine and 4-Hydroxynonenal. FGF21 administration can reverse the pathologic changes in FGF21KO diabetic mice. These findings demonstrate that FGF21 deletion aggravates aortic remodeling and cell death probably via exacerbation of aortic inflammation and oxidative stress. This marks FGF21 as a potential therapy for the treatment of aortic damage due to diabetes.

Biomarkers of insulin sensitivity and insulin resistance: Past, present and future

Insulin resistance in insulin target tissues including liver, skeletal muscle and adipose tissue is an early step in the progression towards type 2 diabetes. Accurate diagnostic parameters reflective of insulin resistance are essential. Longstanding tests for fasting blood glucose and HbA1c are useful and although the hyperinsulinemic euglycemic clamp remains a "gold standard" for accurately determining insulin resistance, it cannot be implemented on a routine basis. The study of adipokines, and more recently myokines and hepatokines, as potential biomarkers for insulin sensitivity is now an attractive and relatively straightforward approach. This review discusses potential biomarkers including adiponectin, RBP4, chemerin, A-FABP, FGF21, fetuin-A, myostatin, IL-6, and irisin, all of which may play significant roles in determining insulin sensitivity. We also review potential future directions of new biological markers for measuring insulin resistance, including metabolomics and gut microbiome. Collectively, these approaches will provide clinicians with the tools for more accurate, and perhaps personalized, diagnosis of insulin resistance.

The role of fibroblast growth factor 21 in diabetes and its complications: A review from clinical perspective

Fibroblast growth factor 21 (FGF21) has been well-recognized as a metabolic hormone and a promising target for treatment of metabolic diseases. The level of endogenous FGF21 is elevated in patients with impaired glucose tolerance and progressively increased from patients with overt type 2 diabetes to those with micro- and macro-vascular complications, presumably as a compensation or response to the deterioration of metabolic imbalance. A few exploratory in vivo studies, including a recent clinical trial, showed that exogenous FGF21 mimetics targeting FGF21 signaling can attain beneficial metabolic effects not with-standing the already elevated ambient FGF21 levels. In addition, some clinically available pharmacologic agents such as fenofibrates and metformin may modulate energy and macronutrients metabolism by acting through FGF21. This review mainly focuses on the role of FGF21 in development, progression and treatment of type 2 diabetes from a clinical perspective.

Mice lacking neutral amino acid transporter B(0)AT1 (Slc6a19) have elevated levels of FGF21 and GLP-1 and improved glycaemic control

OBJECTIVE

Type 2 diabetes arises from insulin resistance of peripheral tissues followed by dysfunction of β-cells in the pancreas due to metabolic stress. Both depletion and supplementation of neutral amino acids have been discussed as strategies to improve insulin sensitivity. Here we characterise mice lacking the intestinal and renal neutral amino acid transporter B(0)AT1 (Slc6a19) as a model to study the consequences of selective depletion of neutral amino acids.

METHODS

Metabolic tests, analysis of metabolite levels and signalling pathways were used to characterise mice lacking the intestinal and renal neutral amino acid transporter B(0)AT1 (Slc6a19).

RESULTS

Reduced uptake of neutral amino acids in the intestine and loss of neutral amino acids in the urine causes an overload of amino acids in the lumen of the intestine and reduced systemic amino acid availability. As a result, higher levels of glucagon-like peptide 1 (GLP-1) are produced by the intestine after a meal, while the liver releases the starvation hormone fibroblast growth factor 21 (FGF21). The combination of these hormones generates a metabolic phenotype that is characterised by efficient removal of glucose, particularly by the heart, reduced adipose tissue mass, browning of subcutaneous white adipose tissue, enhanced production of ketone bodies and reduced hepatic glucose output.

CONCLUSIONS

Reduced neutral amino acid availability improves glycaemic control. The epithelial neutral amino acid transporter B(0)AT1 could be a suitable target to treat type 2 diabetes.

Fibroblast growth factor 21 is elevated in metabolically unhealthy obesity and affects lipid deposition, adipogenesis, and adipokine secretion of human abdominal subcutaneous adipocytes

OBJECTIVE

Serum concentrations of the hepatokine fibroblast growth factor (FGF) 21 are elevated in obesity, type-2 diabetes, and the metabolic syndrome. We asked whether FGF21 levels differ between subjects with metabolically healthy vs. unhealthy obesity (MHO vs. MUHO), opening the possibility that FGF21 is a cross-talker between liver and adipose tissue in MUHO. Furthermore, we studied the effects of chronic FGF21 treatment on adipocyte differentiation, lipid storage, and adipokine secretion.

METHODS

In 20 morbidly obese donors of abdominal subcutaneous fat biopsies discordant for their whole-body insulin sensitivity (hereby classified as MHO or MUHO subjects), serum FGF21 was quantified. The impact of chronic FGF21 treatment on differentiation, lipid accumulation, and adipokine release was assessed in isolated preadipocytes differentiated in vitro.

RESULTS

Serum FGF21 concentrations were more than two-fold higher in MUHO as compared to MHO subjects (457 ± 378 vs. 211 ± 123 pg/mL; p < 0.05). FGF21 treatment of human preadipocytes for the entire differentiation period was modestly lipogenic (+15%; p < 0.05), reduced the expression of key adipogenic transcription factors (PPARG and CEBPA, -15% and -40%, respectively; p < 0.01 both), reduced adiponectin expression (-20%; p < 0.05), markedly reduced adiponectin release (-60%; p < 0.01), and substantially increased leptin (+60%; p < 0.01) and interleukin-6 (+50%; p < 0.001) release.

CONCLUSIONS

The hepatokine FGF21 exerts weak lipogenic and anti-adipogenic actions and marked adiponectin-suppressive and leptin and interleukin-6 release-promoting effects in human differentiating preadipocytes. Together with the higher serum concentrations in MUHO subjects, our findings reveal FGF21 as a circulating factor promoting the development of metabolically unhealthy adipocytes.

Effects of lifestyle modification and metformin on irisin and FGF21 among HIV-infected subjects with the metabolic syndrome

OBJECTIVE

Few studies have investigated irisin and FGF21 to elucidate the role of these hormones to regulate 'beiging' in HIV-infected patients.

DESIGN

Fifty HIV-infected subjects with the metabolic syndrome were previously recruited and randomized to receive lifestyle modification (LSM) and/or metformin over 12 months. In the current study, we assessed FGF21 and irisin at baseline and after intervention. In addition, we assessed circulating FGF21 and irisin in relationship to brown adipose tissue (BAT) gene expression in dorsocervical subcutaneous fat biopsies from 13 HIV-infected subjects.

RESULTS

At baseline, prior to intervention, HIV-infected subjects demonstrated increased log FGF21 (2·13 ± 0·06 vs 1·98 ± 0·05 pg/ml, P = 0·05) and log irisin (0·33 ± 0·02 vs 0·17 ± 0·04 μg/ml, P = 0·003) compared with healthy controls well matched based on waist circumference. After 12 months, HIV-infected subjects randomized to LSM demonstrated a relative reduction in FGF21 compared with those not randomized to LSM (-10 [-35,22] vs 40 [0,94] %change, P = 0·01). Changes in FGF21 were inversely associated with improved parameters of energy homoeostasis, including increased REE (ρ = -0·34, P = 0·046) and max VO2 (ρ = -0·38, P = 0·02), and reduced RQ (ρ = 0·40, P = 0·02) among all HIV-infected subjects. Increased UCP-1 (r = 0·75, P = 0·003), DIO2 (r = 0·58, P = 0·04) and CideA (r = 0·73, P = 0·01) gene expression in dorsocervical fat was significantly associated with FGF21 in HIV-infected subjects.

CONCLUSION

HIV-infected subjects with metabolic complications demonstrate increases in FGF21 in relationship to BAT gene expression. Relative reductions in FGF21 in those receiving long-term LSM relate to overall improvements in energy expenditure parameters. In contrast, irisin levels are elevated in HIV-infected subjects, but are not influenced by LSM nor associated with BAT gene expression.

FGF21 deletion exacerbates diabetic cardiomyopathy by aggravating cardiac lipid accumulation

Fibroblast growth factor 21 (FGF21) plays an important role in energy homoeostasis. The unaddressed question of FGF21’s effect on the development and progression of diabetic cardiomyopathy (DCM) is investigated here with FGF21 knockout (FGF21KO) diabetic mice. Type 1 diabetes was induced in both FGF21KO and C57BL/6J wild-type (WT) mice via streptozotocin. At 1, 2 and 4 months after diabetes onset, the plasma FGF21 levels were significantly decreased in WT diabetic mice compared to controls. There was no significant difference between FGF21KO and WT diabetic mice in blood glucose and triglyceride levels. FGF21KO diabetic mice showed earlier and more severe cardiac dysfunction, remodelling and oxidative stress, as well as greater increase in cardiac lipid accumulation than WT diabetic mice. Western blots showed that increased cardiac lipid accumulation was accompanied by further increases in the expression of nuclear factor (erythroid-derived 2)-like 2 (Nrf2) and its target protein CD36, along with decreases in the phosphorylation of AMP-activated protein kinase and the expression of hexokinase II and peroxisome proliferator-activated receptor gamma co-activator 1α in the heart of FGF21KO diabetic mice compared to WT diabetic mice. Our results demonstrate that FGF21 deletion-aggravated cardiac lipid accumulation is likely mediated by cardiac Nrf2-driven CD36 up-regulation, which may contribute to the increased cardiac oxidative stress and remodelling, and the eventual development of DCM. These findings suggest that FGF21 may be a therapeutic target for the treatment of DCM.

Activation and recruitment of brown adipose tissue as anti-obesity regimens in humans

Brown adipose tissue (BAT) is the site of sympathetically activated adaptive thermogenesis during cold exposure and after hyperphagia, thereby controlling whole-body energy expenditure (EE) and body fat. BAT thermogenesis is primarily dependent on the energy-dissipating activity of uncoupling protein 1 (UCP1). There are two types of UCP1-expressing adipocyte, classical brown and beige/brite adipocytes. Recent radionuclide studies have demonstrated the existence of metabolically active BAT composed of mainly beige/brite adipocytes in adult humans. Human BAT is activated by acute cold exposure, being positively correlated to cold-induced increases in EE. The inverse relationship between the BAT activity and body fatness suggests that BAT, because of its energy-dissipating activity, is protective against body fat accumulation. In fact, either repeated cold exposure or daily ingestion of some food ingredients acting on transient receptor potential channels recruited BAT in association with increased EE and decreased body fat. Moreover, possible contribution of BAT to glucose tolerance has been suggested. In addition to the sympathetic nervous system, some endocrine factors also have potential for activation/recruitment of BAT. Thus, BAT is a promising therapeutic target for combating human obesity and related metabolic disorders.

Thyroid hormone receptor regulates most genes independently of fibroblast growth factor 21 in liver

Thyroid hormone (TH) acts through specific receptors (TRs), which are conditional transcription factors, to induce fibroblast growth factor 21 (FGF21), a peptide hormone that is usually induced by fasting and that influences lipid and carbohydrate metabolism via local hepatic and systemic endocrine effects. While TH and FGF21 display overlapping actions when administered, including reductions in serum lipids, according to the current models these hormones act independently in vivo. In this study, we examined mechanisms of regulation of FGF21 expression by TH and tested the possibility that FGF21 is required for induction of hepatic TH-responsive genes. We confirm that active TH (triiodothyronine (T3)) and the TRβ-selective thyromimetic GC1 increase FGF21 transcript and peptide levels in mouse liver and that this effect requires TRβ. T3 also induces FGF21 in cultured hepatocytes and this effect involves direct actions of TRβ1, which binds a TRE within intron 2 of FGF21. Gene expression profiles of WT and Fgf21-knockout mice are very similar, indicating that FGF21 is dispensable for the majority of hepatic T3 gene responses. A small subset of genes displays diminished T3 response in the absence of FGF21. However, most of these are not obviously directly involved in T3-dependent hepatic metabolic processes. Consistent with these results, T3-dependent effects on serum cholesterol are maintained in the Fgf21(-/-) background and we observe no effect of the Fgf21-knockout background on serum triglycerides and glucose. Our findings indicate that T3 regulates the genes involved in classical hepatic metabolic responses independently of FGF21.

Milk consumption during pregnancy increases birth weight, a risk factor for the development of diseases of civilization

Antenatal dietary lifestyle intervention and nutrition during pregnancy and early postnatal life are important for appropriate lifelong metabolic programming. Epidemiological evidence underlines the crucial role of increased birth weight as a risk factor for the development of chronic diseases of civilization such as obesity, diabetes and cancer. Obstetricians and general practitioners usually recommend milk consumption during pregnancy as a nutrient enriched in valuable proteins and calcium for bone growth. However, milk is not just a simple nutrient, but has been recognized to function as an endocrine signaling system promoting anabolism and postnatal growth by activating the nutrient-sensitive kinase mTORC1. Moreover, pasteurized cow’s milk transfers biologically active exosomal microRNAs into the systemic circulation of the milk consumer apparently affecting more than 11 000 human genes including the mTORC1-signaling pathway. This review provides literature evidence and evidence derived from translational research that milk consumption during pregnancy increases gestational, placental, fetal and birth weight. Increased birth weight is a risk factor for the development of diseases of civilization thus involving key disciplines of medicine. With regard to the presented evidence we suggest that dietary recommendations promoting milk consumption during pregnancy have to be re-evaluated.

Addressing unmet medical needs in type 2 diabetes: a narrative review of drugs under development

The global burden of type 2 diabetes is increasing worldwide, and successful treatment of this disease needs constant provision of new drugs. Twelve classes of antidiabetic drugs are currently available, and many new drugs are under clinical development. These include compounds with known mechanisms of action but unique properties, such as once-weekly DPP4 inhibitors or oral insulin. They also include drugs with new mechanisms of action, the focus of this review. Most of these compounds are in Phase 1 and 2, with only a small number having made it to Phase 3 at this time. The new drug classes described include PPAR agonists/modulators, glucokinase activators, glucagon receptor antagonists, anti-inflammatory compounds, G-protein coupled receptor agonists, gastrointestinal peptide agonists other than GLP-1, apical sodium-dependent bile acid transporter (ASBT) inhibitors, SGLT1 and dual SGLT1/SGLT2 inhibitors, and 11beta- HSD1 inhibitors.

Recently discovered adipokines and cardio-metabolic comorbidities in childhood obesity

White adipose tissue (WAT) asset, in terms of cell number, fat storage capacity and endocrine function, is largely determined in early stages of life and is pivotal for shaping the WAT pro-inflammatory behavior. WAT derived adipokines have been shown to play a main role in several cardio-metabolic abnormalities of obesity. This review focuses on the most recently identified adipokines, namely adipocyte-fatty acid-binding protein, chemerin, fibroblast growth factor-21, lipocalin-2, omentin-1 and vaspin; their role in the pathogenesis of obesity and associated cardio-metabolic abnormalities; and on their adaptive response to body weight change. Evidence consistently suggests a pathogenic role for A-FABP, chemerin and FGF-21. Nevertheless, large population studies are needed to verify whether they can be useful to predict the risk of cardio-metabolic abnormalities in adulthood and/or monitor the clinical response to therapeutic interventions.

Metabolic characteristics and therapeutic potential of brown and ?beige? adipose tissues

According to the International Diabetes Federation, 10.9 million people have diabetes mellitus (DM) in Russia; however, only up to 4 million are registered. In addition, 11.9 million people have impaired glucose tolerance and impaired fasting glucose levels [1]. One of the significant risk factors for type 2 DM (T2DM) is obesity, which increases insulin resistance (IR). IR is the major pathogenetic link to T2DM. According to current concepts, there are three types of adipose tissue: white adipose tissue (WAT), brown adipose tissue (BAT) and ?beige?, of which the last two types have a thermogenic function. Some research results have revealed the main stages in the development of adipocytes; however, there is no general consensus regarding the development of ?beige? adipocytes. Furthermore, the biology of BAT and ?beige? adipose tissue is currently being intensively investigated, and some key transcription factors, signalling pathways and hormones that promote the development and activation of these tissues have been identified. The most discussed hormones are irisin and fibroblast growth factor 21, which have established positive effects on BAT and ?beige? adipose tissue with regard to carbohydrate, lipid and energy metabolism. The primary imaging techniques used to investigate BAT are PET-CT with 18F-fluorodeoxyglucose and magnetic resonance spectroscopy. With respect to the current obesity epidemic and associated diseases, including T2DM, there is a growing interest in investigating adipogenesis and the possibility of altering this process. BAT and ?beige? adipose tissue may be targets for developing drugs directed against obesity and T2DM.

Progress in the search for circulating biomarkers of nonalcoholic fatty liver disease

CONTEXT

The definitive standard for the diagnosis of nonalcoholic fatty liver disease (NAFLD) is clinico-pathological correlation, but frequently the only laboratory abnormality is an elevation of serum aminotransferases.

OBJECTIVE

This has resulted in the search for more specific laboratory biomarkers.

METHODS

The literature was searched for novel plasma/serum markers of NAFLD.

RESULTS

Studies reviewed here included histologically-confirmed patients presenting some stage of NAFLD and monitored one or more novel serum/plasma biomarkers.

CONCLUSION

The most promising application of some of these novel biomarkers for the detection and quantification of NAFLD and particularly NASH appears to be in the combination of several into diagnostic panels.

Adipokines in gestational diabetes

Gestational diabetes is characterised by glucose intolerance with onset or first recognition during pregnancy. The disease shows facets of the metabolic syndrome including obesity, insulin resistance, and dyslipidaemia. Adipokines are a group of proteins secreted from adipocytes, which are dysregulated in obesity and contribute to metabolic and vascular complications. Recent studies have assessed the role of various adipokines including leptin, adiponectin, tumour necrosis factor α (TNFα), adipocyte fatty acid-binding protein (AFABP), retinol-binding protein 4 (RBP4), resistin, NAMPT, SERPINA12, chemerin, progranulin, FGF-21, TIMP1, LCN2, AZGP1, apelin (APLN), and omentin in gestational diabetes. This Review provides an overview of these key adipokines, their regulation in, and potential contribution to gestational diabetes. Based on the evidence so far, the adipokines adiponectin, leptin, TNFα, and AFABP seem to be the most probable candidates involved in the pathophysiology of gestational diabetes.

Evaluation of Total Adiponectin, Adipocyte Fatty Acid Binding Protein and Fibroblast Growth Factor 21 Levels in Individuals With Metabolic Syndrome

Although many studies have investigated the relationships of several adipokines to metabolic syndrome (MetS), the interrelationships of adiponectin (ADP), adipocyte fatty acid binding protein (A-FABP) and fibroblast growth factor 21 (FGF 21) have not been described in detail. We examined 209 asymptomatic dyslipidemic patients divided into MetS+ (n=73) and MetS- (n=136) groups. The aim of study was to evaluate the relationships between observed adipokines, to compare the levels of total ADP, A-FABP and FGF 21 in individuals with and without MetS, and to elucidate the relationships of individual adipokines to lipid parameters, markers of insulin resistance and endothelial hemostatic markers in these groups. In MetS+ group, we found the independent positive association ADP with A-FABP (beta=0.4888, p=0.0382), A-FABP with FGF 21 (beta=0.3811, p=0.0002) and von Willebrand factor (beta=0.4502, p=0.0013), and FGF 21 with A-FABP (beta=0.4422, p=0.0002). Our study has confirmed the well-established risk profile of subjects with MetS, although clinically asymptomatic. MetS+ patients had also lower levels of ADP and higher levels of A-FABP and FGF 21. Our study evaluated the interrelationships of ADP, A-FABP and FGF 21 in asymptomatic dyslipidemic subjects with diagnosis of MetS. Especially strong association between A-FABP and FGF 21 needs to be clarified in further studies.

What we talk about when we talk about fat

There has been an upsurge of interest in the adipocyte coincident with the onset of the obesity epidemic and the realization that adipose tissue plays a major role in the regulation of metabolic function. The past few years, in particular, have seen significant changes in the way that we classify adipocytes and how we view adipose development and differentiation. We have new perspective on the roles played by adipocytes in a variety of homeostatic processes and on the mechanisms used by adipocytes to communicate with other tissues. Finally, there has been significant progress in understanding how these relationships are altered during metabolic disease and how they might be manipulated to restore metabolic health.

The nonhuman primate as a model for type 2 diabetes

PURPOSE OF THE REVIEW

Although rodent models provide insight into the mechanisms underlying type 2 diabetes mellitus (T2DM), they are limited in their translatability to humans. The nonhuman primate (NHP) shares important metabolic similarities with the human, making it an ideal model for the investigation of type 2 diabetes and use in preclinical trials. This review highlights the key contributions in the field over the last year using the NHP model.

RECENT FINDINGS

The NHP has not only provided novel insight into the normal and pathological processes that occur within the islet, but has also allowed for the preclinical testing of novel pharmaceutical targets for obesity and T2DM. Particularly, administration of fibroblast growth factor-21 in the NHP resulted in weight loss and improvements in metabolic health, supporting rodent studies and recent clinical trials. In addition, the NHP was used to demonstrate that a novel melanocortin-4 receptor agonist did not cause adverse cardiovascular effects. Finally, this model has been used to provide evidence that glucagon-like peptide-1-based therapies do not induce pancreatitis in the healthy NHP.

SUMMARY

The insight gained from studies using the NHP model has allowed for a better understanding of the processes driving T2DM and has promoted the development of well tolerated and effective treatments.

Diabetes as a complication of adipose tissue dysfunction. Is there a role for potential new biomarkers?

Increasing incidence of type 2 diabetes is a major health problem of the modern world and requires new diagnostic tools to assess early metabolic disorders, particularly insulin resistance. The link between obesity, inflammation and insulin resistance indicates the important secretory role of adipose tissue. Proinflammatory factors (cytokines, adipokines) produced by enlarged adipose tissue are related to impaired glucose metabolism. Adipokines act as paracrine factors in adipose tissue and as endocrine hormones in the liver, muscles and central nervous system. Novel adipokines secreted from adipocytes such as retinol binding protein-4 (RBP-4), vaspin, omentin, chemerin, fibroblast growth factor 21 (FGF21), adipocyte fatty acid-binding protein (A-FABP) and dipeptidyl peptidase 4 (DPP4) demonstrate pleiotropic activity and their insulin-sensitizing or enhancing insulin resistance properties have not been clearly confirmed yet. In spite of the lack of standardized automated assay methods currently available for these novel biomarkers, promising results from several studies emphasize that they might potentially be useful prognostic factors for diabetes and its complications, especially in individuals without the typical symptoms of metabolic syndrome.

Skeletal muscle mitochondrial uncoupling drives endocrine cross-talk through the induction of FGF21 as a myokine

UCP1-Tg mice with ectopic expression of uncoupling protein 1 (UCP1) in skeletal muscle (SM) are a model of improved substrate metabolism and increased longevity. Analysis of myokine expression showed an induction of fibroblast growth factor 21 (FGF21) in SM, resulting in approximately fivefold elevated circulating FGF21 in UCP1-Tg mice. Despite a reduced muscle mass, UCP1-Tg mice showed no evidence for a myopathy or muscle autophagy deficiency but an activation of integrated stress response (ISR; eIF2α/ATF4) in SM. Targeting mitochondrial function in vitro by treating C2C12 myoblasts with the uncoupler FCCP resulted in a dose-dependent activation of ISR, which was associated with increased expression of FGF21, which was also observed by treatment with respiratory chain inhibitors antimycin A and myxothiazol. The cofactor required for FGF21 action, β-klotho, was expressed in white adipose tissue (WAT) of UCP1-Tg mice, which showed an increased browning of WAT similar to what occurred in altered adipocyte morphology, increased brown adipocyte markers (UCP1, CIDEA), lipolysis (HSL phosphorylation), and respiratory capacity. Importantly, treatment of primary white adipocytes with serum of transgenic mice resulted in increased UCP1 expression. Additionally, UCP1-Tg mice showed reduced body length through the suppressed IGF-I-GH axis and decreased bone mass. We conclude that the induction of FGF21 as a myokine is coupled to disturbance of mitochondrial function and ISR activation in SM. FGF21 released from SM has endocrine effects leading to increased browning of WAT and can explain the healthy metabolic phenotype of UCP1-Tg mice. These results confirm muscle as an important endocrine regulator of whole body metabolism.

Human brown adipose tissue: regulation and anti-obesity potential

Brown adipose tissue (BAT) is the site of sympathetically activated adaptive thermognenesis during cold exposure and after hyperphagia, thereby controlling whole-body energy expenditure (EE) and body fat. Radionuclide imaging studies have demonstrated that adult humans have metabolically active BAT composed of mainly beige/brite adipocytes, recently identified brown-like adipocytes. The inverse relationship between the BAT activity and body fatness suggests that BAT is, because of its energy dissipating activity, protective against body fat accumulation in humans as it is in small rodents. In fact, either repeated cold exposure or daily ingestion of some food ingredients acting on transient receptor potential channels recruits BAT in parallel with increased EE and decreased body fat. In addition to the sympathetic nervous system, several endocrine factors are also shown to recruit BAT. Thus, BAT is a promising therapeutic target for combating human obesity and related metabolic disorders.

Additive relationship between serum fibroblast growth factor 21 level and coronary artery disease

Background

Expression and activity of the fibroblast growth factor (FGF) 21 hormone-like protein are associated with development of several metabolic disorders. This study was designed to investigate whether serum FGF21 level was also associated with the metabolic syndrome-related cardiovascular disease, atherosclerosis, and its clinical features in a Chinese cohort.

Methods

Two-hundred-and-fifty-three subjects visiting the Cardiology Department (Sixth People's Hospital affiliated to Shanghai JiaoTong University) were examined by coronary arteriography (to diagnose coronary artery disease (CAD)) and hepatic ultrasonography (to diagnose non-alcoholic fatty liver disease (NAFLD)). Serum FGF21 level was measured by enzyme-linked immunosorbent assay and analyzed for correlation to subject and clinical characteristics. The independent factors of CAD were determined by multivariate logistic regression analysis.

Results

Subjects with NAFLD showed significantly higher serum FGF21 than those without NAFLD (388.0 pg/mL (253.0-655.4) vs. 273.3 pg/mL (164.9-383.7), P < 0.01). Subjects with CAD showed significantly higher serum FGF21, regardless of NAFLD diagnosis (P < 0.05). Serum FGF21 level significantly elevated with the increasing number of metabolic disorders (P for trend < 0.01). After adjustment of age, sex, and BMI, FGF21 was positively correlated with total cholesterol (P < 0.05) and triglyceride (P < 0.01). FGF21 was identified as an independent factor of CAD (odds ratio = 2.984, 95% confidence interval: 1.014-8.786, P < 0.05).

Conclusions

Increased level of serum FGF21 is associated with NAFLD, metabolic disorders and CAD.

Adiposity and insulin resistance in humans: the role of the different tissue and cellular lipid depots

Human adiposity has long been associated with insulin resistance and increased cardiovascular risk, and abdominal adiposity is considered particularly adverse. Intra-abdominal fat is associated with insulin resistance, possibly mediated by greater lipolytic activity, lower adiponectin levels, resistance to leptin, and increased inflammatory cytokines, although the latter contribution is less clear. Liver lipid is also closely associated with, and likely to be an important contributor to, insulin resistance, but it may also be in part the consequence of the lipogenic pathway of insulin action being up-regulated by hyperinsulinemia and unimpaired signaling. Again, intramyocellular triglyceride is associated with muscle insulin resistance, but anomalies include higher intramyocellular triglyceride in insulin-sensitive athletes and women (vs men). Such issues could be explained if the "culprits" were active lipid moieties such as diacylglycerol and ceramide species, dependent more on lipid metabolism and partitioning than triglyceride amount. Subcutaneous fat, especially gluteofemoral, appears metabolically protective, illustrated by insulin resistance and dyslipidemia in patients with lipodystrophy. However, some studies suggest that deep sc abdominal fat may have adverse properties. Pericardial and perivascular fat relate to atheromatous disease, but not clearly to insulin resistance. There has been recent interest in recognizable brown adipose tissue in adult humans and its possible augmentation by a hormone, irisin, from exercising muscle. Brown adipose tissue is metabolically active, oxidizes fatty acids, and generates heat but, because of its small and variable quantities, its metabolic importance in humans under usual living conditions is still unclear. Further understanding of specific roles of different lipid depots may help new approaches to control obesity and its metabolic sequelae.

Mitochondrial fatty acid oxidation in obesity

SIGNIFICANCE

Current lifestyles with high-energy diets and little exercise are triggering an alarming growth in obesity. Excess of adiposity is leading to severe increases in associated pathologies, such as insulin resistance, type 2 diabetes, atherosclerosis, cancer, arthritis, asthma, and hypertension. This, together with the lack of efficient obesity drugs, is the driving force behind much research.

RECENT ADVANCES

Traditional anti-obesity strategies focused on reducing food intake and increasing physical activity. However, recent results suggest that enhancing cellular energy expenditure may be an attractive alternative therapy.

CRITICAL ISSUES

This review evaluates recent discoveries regarding mitochondrial fatty acid oxidation (FAO) and its potential as a therapy for obesity. We focus on the still controversial beneficial effects of increased FAO in liver and muscle, recent studies on how to potentiate adipose tissue energy expenditure, and the different hypotheses involving FAO and the reactive oxygen species production in the hypothalamic control of food intake.

FUTURE DIRECTIONS

The present review aims to provide an overview of novel anti-obesity strategies that target mitochondrial FAO and that will definitively be of high interest in the future research to fight against obesity-related disorders.

Pharmacological strategies for targeting BAT thermogenesis

Biopsies following positron emission tomography coupled to computer tomography (PET-CT) imaging have confirmed the presence of thermogenically active brown adipose tissue (BAT) in adult humans, leading to suggestions that it could be stimulated to treat obesity and its associated morbidities. The mechanisms regulating thermogenesis in BAT are better understood than ever before, and many new hypotheses for increasing the amount of brown fat or its activity are currently being explored. The challenge now is to identify safe ways to manipulate specific aspects of the physiological regulation of thermogenesis, in a manner that will be bioenergetically effective. This review outlines the nature of these regulatory mechanisms both in terms of their cellular specificity and probable effectiveness given the physiological paradigms in which thermogenesis is activated. Similarly, their potential for being targeted by new or existing drugs is discussed, drawing on the known mechanisms of action of various pharmacological agents and some probable limitations that should be considered.

Fibroblast growth factor-21 serum concentrations are associated with metabolic and hepatic markers in humans

Rather than a traditional growth factor, fibroblast growth factor-21 (FGF21) is considered to be a metabolic hormone. In the current study, we investigated serum FGF21 levels in the self-contained population of Sorbs. Serum FGF21 concentrations were quantified by ELISA and correlated with IGF1 as well as metabolic, renal, hepatic, inflammatory, and cardiovascular parameters in 913 Sorbs from Germany. Moreover, human IGF1 protein secretion was investigated in FGF21-stimulated HepG2 cells. Median FGF21 serum concentrations were 2.1-fold higher in subjects with type 2 diabetes mellitus (141.8 ng/l) compared with controls (66.7 ng/l). Furthermore, nondiabetic subjects with FGF21 levels below the detection limit of the ELISA showed a more beneficial metabolic profile compared with subjects with measurable FGF21. Moreover, FGF21 was significantly lower in female compared with male subjects after adjustment for age and BMI. In multiple regression analyses, circulating FGF21 concentrations remained independently and positively associated with gender, systolic blood pressure, triglycerides, and γ glutamyl transferase whereas a negative association was observed with IGF1 in nondiabetic subjects. Notably, FGF21 significantly inhibited IGF1 secretion into HepG2 cell culture supernatants in preliminary in vitro experiments. FGF21 serum concentrations are associated with facets of the metabolic syndrome, hepatocellular function, as well as GH status.