Retinol binding protein 4 stimulates hepatic sterol regulatory element-binding protein 1 and increases lipogenesis through the peroxisome proliferator-activated receptor-γ coactivator 1β-dependent pathway

The brown fat-specific overexpression of RBP4 improves thermoregulation and systemic metabolism by activating the canonical adrenergic signaling pathway

Retinol-binding protein 4 (RBP4), the sole specific carrier for retinol (vitamin A) in circulation, is highly expressed in liver and adipose tissues. Previous studies have demonstrated that RBP4 plays a role in cold-mediated adipose tissue browning and thermogenesis. However, the role of RBP4 in brown adipose tissue and its metabolic significance remain unclear. Here we generated and studied transgenic mice that express human RBP4 (hRBP4), specifically in brown adipocytes (UCP1-RBP4 mice), to better understand these uncertainties. When fed a chow diet, these mice presented significantly lower body weights and fat mass than their littermate controls. The UCP1-RBP4 mice also showed significant improvements in glucose clearance, enhanced energy expenditure and increased thermogenesis in response to a cold challenge. This was associated with increased lipolysis and fatty acid oxidation in brown adipose tissue, which was attributed to the activation of canonical adrenergic signaling pathways. In addition, high-performance liquid chromatography analysis revealed that plasma RBP4 and retinol levels were elevated in the UCP1-RBP4 mice, whereas their hepatic retinol levels decreased in parallel with a chow diet. Steady-state brown fat levels of total retinol were significantly elevated in the UCP1-RBP4 mice, suggesting that their retinol uptake was increased in RBP4-expressing brown adipocytes when fed a chow diet. These findings reveal a critical role for RBP4 in canonical adrenergic signaling that promotes lipid mobilization and oxidation in brown adipocytes, where the harnessed energy is dissipated as heat by adaptive thermogenesis.

Adipokines regulate the development and progression of MASLD through organellar oxidative stress

The prevalence of metabolic dysfunction-associated steatotic liver disease (MASLD), which is increasingly being recognized as a leading cause of chronic liver pathology globally, is increasing. The pathophysiological underpinnings of its progression, which is currently under active investigation, involve oxidative stress. Human adipose tissue, an integral endocrine organ, secretes an array of adipokines that are modulated by dietary patterns and lifestyle choices. These adipokines intricately orchestrate regulatory pathways that impact glucose and lipid metabolism, oxidative stress, and mitochondrial function, thereby influencing the evolution of hepatic steatosis and progression to metabolic dysfunction-associated steatohepatitis (MASH). This review examines recent data, underscoring the critical interplay of oxidative stress, reactive oxygen species, and redox signaling in adipokine-mediated mechanisms. The role of various adipokines in regulating the onset and progression of MASLD/MASH through mitochondrial dysfunction and endoplasmic reticulum stress and the underlying mechanisms are discussed. Due to the emerging correlation between adipokines and the development of MASLD positions, these adipokines are potential targets for the development of innovative therapeutic interventions for MASLD management. A comprehensive understanding of the pathogenesis of MASLD/MASH is instrumental for identifying therapies for MASH.

Proteomic and metabolomic analyses of the human adult myocardium reveal ventricle-specific regulation in end-stage cardiomyopathies

The left and right ventricles of the human heart are functionally and developmentally distinct such that genetic or acquired insults can cause dysfunction in one or both ventricles resulting in heart failure. To better understand ventricle-specific molecular changes influencing heart failure development, we first performed unbiased quantitative mass spectrometry on pre-mortem non-diseased human myocardium to compare the metabolome and proteome between the normal left and right ventricles. Constituents of gluconeogenesis, glycolysis, lipogenesis, lipolysis, fatty acid catabolism, the citrate cycle and oxidative phosphorylation were down-regulated in the left ventricle, while glycogenesis, pyruvate and ketone metabolism were up-regulated. Inter-ventricular significance of these metabolic pathways was then found to be diminished within end-stage dilated cardiomyopathy and ischaemic cardiomyopathy, while heart failure-associated pathways were increased in the left ventricle relative to the right within ischaemic cardiomyopathy, such as fluid sheer-stress, increased glutamine-glutamate ratio, and down-regulation of contractile proteins, indicating a left ventricular pathological bias.

Retinoid X receptor heterodimers in hepatic function: structural insights and therapeutic potential

Nuclear receptors (NRs) are key regulators of multiple physiological functions and pathological changes in the liver in response to a variety of extracellular signaling changes. Retinoid X receptor (RXR) is a special member of the NRs, which not only responds to cellular signaling independently, but also regulates multiple signaling pathways by forming heterodimers with various other NR. Therefore, RXR is widely involved in hepatic glucose metabolism, lipid metabolism, cholesterol metabolism and bile acid homeostasis as well as hepatic fibrosis. Specific activation of particular dimers regulating physiological and pathological processes may serve as important pharmacological targets. So here we describe the basic information and structural features of the RXR protein and its heterodimers, focusing on the role of RXR heterodimers in a number of physiological processes and pathological imbalances in the liver, to provide a theoretical basis for RXR as a promising drug target.

Peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) family in physiological and pathophysiological process and diseases

Peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) family (PGC-1s), consisting of three members encompassing PGC-1α, PGC-1β, and PGC-1-related coactivator (PRC), was discovered more than a quarter-century ago. PGC-1s are essential coordinators of many vital cellular events, including mitochondrial functions, oxidative stress, endoplasmic reticulum homeostasis, and inflammation. Accumulating evidence has shown that PGC-1s are implicated in many diseases, such as cancers, cardiac diseases and cardiovascular diseases, neurological disorders, kidney diseases, motor system diseases, and metabolic disorders. Examining the upstream modulators and co-activated partners of PGC-1s and identifying critical biological events modulated by downstream effectors of PGC-1s contribute to the presentation of the elaborate network of PGC-1s. Furthermore, discussing the correlation between PGC-1s and diseases as well as summarizing the therapy targeting PGC-1s helps make individualized and precise intervention methods. In this review, we summarize basic knowledge regarding the PGC-1s family as well as the molecular regulatory network, discuss the physio-pathological roles of PGC-1s in human diseases, review the application of PGC-1s, including the diagnostic and prognostic value of PGC-1s and several therapies in pre-clinical studies, and suggest several directions for future investigations. This review presents the immense potential of targeting PGC-1s in the treatment of diseases and hopefully facilitates the promotion of PGC-1s as new therapeutic targets.

The regulatory role of metabolic organ-secreted factors in the nonalcoholic fatty liver disease and cardiovascular disease

Nonalcoholic fatty liver disease (NAFLD) is a chronic metabolic disease characterized by an excessive accumulation of fat in the liver, which is becoming a major global health problem, affecting about a quarter of the population. In the past decade, mounting studies have found that 25%-40% of NAFLD patients have cardiovascular disease (CVD), and CVD is one of the leading causes of death in these subjects. However, it has not attracted enough awareness and emphasis from clinicians, and the underlying mechanisms of CVD in NAFLD patients remain unclear. Available research reveals that inflammation, insulin resistance, oxidative stress, and glucose and lipid metabolism disorders play indispensable roles in the pathogenesis of CVD in NAFLD. Notably, emerging evidence indicates that metabolic organ-secreted factors, including hepatokines, adipokines, cytokines, extracellular vesicles, and gut-derived factors, are also involved in the occurrence and development of metabolic disease and CVD. Nevertheless, few studies have focused on the role of metabolic organ-secreted factors in NAFLD and CVD. Therefore, in this review, we summarize the relationship between metabolic organ-secreted factors and NAFLD as well as CVD, which is beneficial for clinicians to comprehensive and detailed understanding of the association between both diseases and strengthen management to improve adverse cardiovascular prognosis and survival.

Critical Overview of Hepatic Factors That Link Non-Alcoholic Fatty Liver Disease and Acute Kidney Injury: Physiology and Therapeutic Implications

Non-alcoholic fatty liver disease (NAFLD) is defined as a combination of a group of progressive diseases, presenting different structural features of the liver at different stages of the disease. According to epidemiological surveys, as living standards improve, the global prevalence of NAFLD increases. Acute kidney injury (AKI) is a class of clinical conditions characterized by a rapid decline in kidney function. NAFLD and AKI, as major public health diseases with high prevalence and mortality, respectively, worldwide, place a heavy burden on societal healthcare systems. Clinical observations of patients with NAFLD with AKI suggest a possible association between the two diseases. However, little is known about the pathogenic mechanisms linking NAFLD and AKI, and the combination of the diseases is poorly treated. Previous studies have revealed that liver-derived factors are transported to distal organs via circulation, such as the kidney, where they elicit specific effects. Of note, while NAFLD affects the expression of many hepatic factors, studies on the mechanisms whereby NAFLD mediates the generation of hepatic factors that lead to AKI are lacking. Considering the unique positioning of hepatic factors in coordinating systemic energy metabolism and maintaining energy homeostasis, we hypothesize that the effects of NAFLD are not only limited to the structural and functional changes in the liver but may also involve the entire body via the hepatic factors, e.g., playing an important role in the development of AKI. This raises the question of whether analogs of beneficial hepatic factors or inhibitors of detrimental hepatic factors could be used as a treatment for NAFLD-mediated and hepatic factor-driven AKI or other metabolic disorders. Accordingly, in this review, we describe the systemic effects of several types of hepatic factors, with a particular focus on the possible link between hepatic factors whose expression is altered under NAFLD and AKI. We also summarize the role of some key hepatic factors in metabolic control mechanisms and discuss their possible use as a preventive treatment for the progression of metabolic diseases.

Retinol-binding protein-4 and nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is becoming increasingly common as the global economy grows and living standards improve. Timely and effective preventions and treatments for NAFLD are urgently needed. Retinol-binding protein-4 (RBP4), the protein that transports retinol through the circulation, was found to be positively related to diabetes, obesity, cardiovascular disease, and other metabolic diseases. Observational studies on the association between serum RBP4 level and the prevalence of NAFLD found contradictory results. Some of the underlying mechanisms responsible for this association have been revealed, and the possible clinical implications of treating NAFLD by targeting RBP4 have been demonstrated. Future studies should focus on the predictive value of RBP4 on NAFLD development and its potential as a therapeutic target in NAFLD.

Non-alcoholic fatty liver disease: a multi-system disease influenced by ageing and sex, and affected by adipose tissue and intestinal function

In recent years, a wealth of factors are associated with increased risk of developing non-alcoholic fatty liver disease (NAFLD) and NAFLD is now thought to increase the risk of multiple extra-hepatic diseases. The aim of this review is first to focus on the role of ageing and sex as key, poorly understood risk factors in the development and progression of NAFLD. Secondly, we aim to discuss the roles of white adipose tissue (WAT) and intestinal dysfunction, as producers of extra-hepatic factors known to further contribute to the pathogenesis of NAFLD. Finally, we aim to summarise the role of NAFLD as a multi-system disease affecting other organ systems beyond the liver. Both increased age and male sex increase the risk of NAFLD and this may be partly driven by alterations in the distribution and function of WAT. Similarly, changes in gut microbiota composition and intestinal function with ageing and chronic overnutrition are likely to contribute to the development of NAFLD both directly (i.e. by affecting hepatic function) and indirectly via exacerbating WAT dysfunction. Consequently, the presence of NAFLD significantly increases the risk of various extra-hepatic diseases including CVD, type 2 diabetes mellitus, chronic kidney disease and certain extra-hepatic cancers. Thus changes in WAT and intestinal function with ageing and chronic overnutrition contribute to the development of NAFLD – a multi-system disease that subsequently contributes to the development of other chronic cardiometabolic diseases.

Molecular and functional characterization of the retinol-binding protein 4 (RBP4) in hepatocytes of Schizothorax prenanti in response to palmitic acid

Retinol-binding protein 4 (RBP4) protein is a kind of adipokines synthesized and secreted by the liver, which has been verified to play important roles in liver metabolism and energy homeostasis. However, the effects of RBP4 on hepatic lipid accumulation are still elusive in fish. In the present study, we cloned and characterized the RBP4 gene in Schizothorax prenanti (S. prenanti). RBP4 gene was specifically expressed in the liver and abdominal adipose tissue. Palmitic acid (PA; 400 μM) can significantly increase lipid deposition in primary hepatocytes after 12 h of treatment. Furthermore, RBP4 knockdown can relieve the excessive lipid deposition and endoplasmic reticulum stress in the hepatocytes caused by PA. The inhibition of RBP4 abolished the ability of PA to induce the expression of genes involved in lipogenesis and endoplasmic reticulum stress. These results demonstrate that RBP4 inhibition attenuated PA-induced lipid deposition and endoplasmic reticulum stress in hepatocytes of S. prenanti. This study could contribute to improve the understanding of RBP4 functions in the PA-induced lipid deposition in hepatocytes of fish.

Serum Retinol-Binding Protein Levels Are Associated with Nonalcoholic Fatty Liver Disease in Chinese Patients with Type 2 Diabetes Mellitus: A Real-World Study

BACKGROUND

The association of serum retinol-binding protein (RBP) levels with nonalcoholic fatty liver disease (NAFLD) remains controversial. Furthermore, few studies have investigated their relationship in type 2 diabetes mellitus (T2DM) patients. Therefore, the aim of the present study was to explore the association between serum RBP levels and NAFLD in Chinese inpatients with T2DM.

METHODS

This cross-sectional, real-world study included 2,263 Chinese T2DM inpatients. NAFLD was diagnosed by abdominal ultrasonography. The subjects were divided into four groups based on RBP quartiles, and clinical characteristics were compared among the four groups. The associations of both RBP levels and quartiles with the presence of NAFLD were also analyzed.

RESULTS

After adjustment for sex, age, and diabetes duration, there was a significant increase in the prevalence of NAFLD from the lowest to the highest RBP quartiles (30.4%, 40.0%, 42.4%, and 44.7% for the first, second, third, and fourth quartiles, respectively, P<0.001 for trend). Fully adjusted multiple logistic regression analysis revealed that both increased RBP levels (odds ratio, 1.155; 95% confidence interval, 1.012 to 1.318; P=0.033) and quartiles (P=0.014 for trend) were independently associated with the presence of NAFLD in T2DM patients.

CONCLUSION

Increased serum RBP levels were independently associated with the presence of NAFLD in Chinese T2DM inpatients. Serum RBP levels may be used as one of the indicators to assess the risk of NAFLD in T2DM patients.

The effect of combined exercises on the plasma levels of retinol-binding protein 4 and its relationship with insulin resistance and hepatic fat content in postmenopausal women with nonalcoholic fatty liver disease

BACKGROUND

Evidence suggests increased serum levels of retinol-binding protein 4 (RBP4) in postmenopausal women with nonalcoholic fatty liver disease (NAFLD). This study investigated the effect of combined exercises on the plasma levels of RBP4 and its relationship with insulin resistance and hepatic fat content in these women.

METHODS

This quasi-experimental study randomly assigned 24 women with fatty liver and a mean age of 56.18±4.58 years to an experimental group and a control group. The experimental group participated in ten-week incremental endurance-resistance training as combined exercises three sessions a week. Fatty liver was diagnosed with ultrasound, RBP4 levels were measured and insulin resistance was determined using the Homeostatic Model Assessment for Insulin Resistance (HOMA-IR). Between-group data were analyzed using ANCOVA, within-group data using the dependent t-test and Wilcoxon test and relationships between RBP4 and variables using Spearman and Pearson correlation coefficients.

RESULTS

Ten weeks of combined exercises significantly decreased RBP4 levels (P=0.000), HOMA-IR (P=0.011) and hepatic fat levels (P=0.000) in the experimental group compared to in the controls. The posttest showed significant correlations between RBP4 levels and hepatic fat levels (P=0.002) and no correlations between RBP4 levels and insulin resistance (P=0.116).

CONCLUSIONS

Combined exercises significantly reduced serum levels of RBP4 in postmenopausal women with fatty liver. RBP4 was not related to insulin resistance and effects of RBP4 on hepatic fat regulation were independent of the effects of insulin resistance.

Retinol-Binding Protein 4 Activates STRA6, Provoking Pancreatic β-Cell Dysfunction in Type 2 Diabetes

Pancreatic β-cell dysfunction plays a decisive role in the progression of type 2 diabetes. Retinol-binding protein 4 (RBP4) is a prominent adipokine in type 2 diabetes, although its effect on β-cell function remains elusive, and the underlying mechanisms are unknown. Here, we found that elevated circulating RBP4 levels were inversely correlated with pancreatic β-cell function in db/db mice across different glycemic stages. RBP4 directly suppressed glucose-stimulated insulin secretion (GSIS) in primary isolated islets and INS-1E cells in a dose- and time-dependent manner. RBP4 transgenic (RBP4-Tg) overexpressing mice showed a dynamic decrease of GSIS, which appeared as early as 8 weeks old, preceding the impairment of insulin sensitivity and glucose tolerance. Islets isolated from RBP4-Tg mice showed a significant decrease of GSIS. Mechanistically, we demonstrated that the stimulated by retinoic acid 6 (STRA6), RBP4's only known specific membrane receptor, is expressed in β-cells and mediates the inhibitory effect of RBP4 on insulin synthesis through the Janus kinase 2/STAT1/ISL-1 pathway. Moreover, decreasing circulating RBP4 level could effectively restore β-cell dysfunction and ameliorate hyperglycemia in db/db mice. These observations revealed a role of RBP4 in pancreatic β-cell dysfunction, which provides new insight into the diabetogenic effect of RBP4.

Influence of irisin on diet-induced metabolic syndrome in experimental rat model

OBJECTIVE

To evaluate the influence of irisin on the experimental paradigm of non-alcoholic fatty liver (NAFL) as a part of MetS cluster.

METHODS

Forty male albino rats were divided into four groups; normal control, standard diet + irisin, high carbohydrate and fat diet (HCHF), and HCHF + irisin. After the experimental period, levels of fasting blood sugar (FBS), insulin, lipid profile, kidney functions, salusin-alpha (Sal-α), adropin, and retinol-binding protein-4 (RBP-4) were evaluated. Peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1α) expression in skeletal muscle was evaluated by quantitative real-time PCR. Aorta, liver, pancreas, and skeletal muscle tissue samples were prepared for histopathological examination.

RESULTS

Rats administrated HCHF showed elevated levels of FBS, lipid profile, kidney functions, RBP-4, and downregulation of PGC-1α expression along with a decline in levels of insulin, Sal-α, and adropin while administration of irisin significantly attenuated these levels.

CONCLUSIONS

Irisin as based therapy could emerge as a new line of treatment against MetS and its related diseases.

Disruption of retinoid homeostasis induces RBP4 overproduction in diabetes: O-GlcNAcylation involved

BACKGROUND

Retinol-binding protein 4 (RBP4) is elevated and associated with inflammation in metabolic diseases. Disruption of the retinol cascade and O-GlcNAcylation of the RBP4 receptor (STRA6) are found in diabetic kidneys.

OBJECTIVES

We investigated whether the disruption of the retinol cascade induces RBP4 overproduction and if O-linked GlcNAc modification targets RBPR2 and contributes to the disruption of retinol cascades in diabetic livers.

METHODS

Western blot or immunohistochemistry for RBPR2, CRBP1, LRAT, RALDH, RARα, RARγ, RXRα, RBP4, GFAT, OGT, OGA and inflammatory markers, as well as ELISA for RBP4, were performed in livers of db/db and ob/ob mice and high glucose-cultured hepatocytes. Immunoprecipitation and dual fluorescence staining were used to explore O-GlcNAc-modified RBPR2 and RBP4 binding activity on RBPR2. Transfection of the CRBP1 gene was done to verify whether a disrupted retinol cascade induces RBP4 overproduction. OGT silencing was done to investigate the association of O-GlcNAcylation with the disruption of retinol cascade.

RESULTS

Disruption of retinol cascade, RBP4 overproduction, O-GlcNAcylation of RBPR2, decreased RBP4 binding activity on RBPR2 and inflammation were found in livers of db/db and ob/ob mice and high glucose-cultured hepatocytes. CRBP1 gene transfection reversed the suppression of the cellular retinol cascade and simultaneously attenuated the RBP4 overproduction and inflammation in high glucose-treated hepatocytes. The silencing of OGT reversed the disruption of the cellular retinol cascade, RBP4 overproduction and inflammation induced by high glucose in hepatocytes.

CONCLUSIONS

This study indicates that the disruption of cellular retinol cascade is strongly associated with RBP4 overproduction and inflammation in diabetic livers. RBPR2 is one target for high glucose-mediated O-linked GlcNAc modification, which causes liver retinol dyshomeostasis.

Functional changes of the liver in the absence of growth hormone (GH) action - Proteomic and metabolomic insights from a GH receptor deficient pig model

OBJECTIVE

The liver is a central target organ of growth hormone (GH), which stimulates the synthesis of insulin-like growth factor 1 (IGF1) and affects multiple biochemical pathways. A systematic multi-omics analysis of GH effects in the liver has not been performed. GH receptor (GHR) deficiency is a unique model for studying the consequences of lacking GH action. In this study, we used molecular profiling techniques to capture a broad spectrum of these effects in the liver of a clinically relevant large animal model for Laron syndrome.

METHODS

We performed holistic proteome and targeted metabolome analyses of liver samples from 6-month-old GHR-deficient (GHR-KO) pigs and GHR-expressing controls (four males, four females per group).

RESULTS

GHR deficiency resulted in an increased abundance of enzymes involved in amino acid degradation, in the urea cycle, and in the tricarboxylic acid cycle. A decreased ratio of long-chain acylcarnitines to free carnitine suggested reduced activity of carnitine palmitoyltransferase 1A and thus reduced mitochondrial import of fatty acids for beta-oxidation. Increased levels of short-chain acylcarnitines in the liver and in the circulation of GHR-KO pigs may result from impaired beta-oxidation of short-chain fatty acids or from increased degradation of specific amino acids. The concentration of mono-unsaturated glycerophosphocholines was significantly increased in the liver of GHR-KO pigs without morphological signs of steatosis, although the abundances of several proteins functionally linked to non-alcoholic fatty liver disease (fetuin B, retinol binding protein 4, several mitochondrial proteins) were increased. Moreover, GHR-deficient liver samples revealed distinct changes in the methionine and glutathione metabolic pathways, in particular, a significantly increased level of glycine N-methyltransferase and increased levels of total and free glutathione. Several proteins revealed a sex-related abundance difference in the control group but not in the GHR-KO group.

CONCLUSIONS

Our integrated proteomics/targeted metabolomics study of GHR-deficient and control liver samples from a clinically relevant large animal model identified a spectrum of biological pathways that are significantly altered in the absence of GH action. Moreover, new insights into the role of GH in the sex-related specification of liver functions were provided.

An intronic variant in the GCKR gene is associated with multiple lipids

Previous studies have shown that an intronic variant rs780094 of the GCKR gene (glucokinase regulatory protein) is significantly associated with several metabolites, but the associations of this genetic variant with different lipids is largely unknown. Therefore, we applied metabolomics approach to measure metabolites in a large Finnish population sample (METSIM study) to investigate their associations with rs780094 of GCKR. We measured metabolites by mass spectrometry from 5,181 participants. P < 5.8 × 10-5 was considered as statistically significant given 857 metabolites included in statistical analyses. We found novel negative associations of the T allele of GCKR rs780094 with serine and threonine, and positive associations with two metabolites of tryptophan, indolelactate and N-acetyltryptophan. Additionally, we found novel significant positive associations of this genetic variant with 12 glycerolipids and 19 glycerophospholipids. Significant negative associations were found for three glycerophospholipids (all plasmalogen-cholines), and two sphingolipids. Significant novel associations were also found with gamma-glutamylthreonine, taurocholenate sulfate, and retinol. Our study adds new information about the pleiotropy of the GCKR gene, and shows the associations of the T allele of GCKR rs780094 with lipids.

Role of exercise-induced hepatokines in metabolic disorders

The health-promoting effects of physical activity to prevent and treat metabolic disorders are numerous. However, the underlying molecular mechanisms are not yet completely deciphered. In recent years, studies have referred to the liver as an endocrine organ, since it releases specific proteins called hepatokines. Some of these hepatokines are involved in whole body metabolic homeostasis and are theorized to participate in the development of metabolic disease. In this regard, the present review describes the role of Fibroblast Growth Factor 21, Fetuin-A, Angiopoietin-like protein 4, and Follistatin in metabolic disease and their production in response to acute exercise. Also, we discuss the potential role of hepatokines in mediating the beneficial effects of regular exercise and the future challenges to the discovery of new exercise-induced hepatokines.

Vitamin A signaling and homeostasis in obesity, diabetes, and metabolic disorders

Much evidence has accumulated in the literature over the last fifteen years that indicates vitamin A has a role in metabolic disease prevention and causation. This literature proposes that vitamin A can affect obesity development and the development of obesity-related diseases including insulin resistance, type 2 diabetes, hepatic steatosis and steatohepatitis, and cardiovascular disease. Retinoic acid, the transcriptionally active form of vitamin A, accounts for many of the reported associations. However, a number of proteins involved in vitamin A metabolism, including retinol-binding protein 4 (RBP4) and aldehyde dehydrogenase 1A1 (ALDH1A1, alternatively known as retinaldehyde dehydrogenase 1 or RALDH1), have also been identified as being associated with metabolic disease. Some of the reported effects of these vitamin A-related proteins are proposed to be independent of their roles in assuring normal retinoic acid homeostasis. This review will consider both human observational data as well as published data from molecular studies undertaken in rodent models and in cells in culture. The primary focus of the review will be on the effects that vitamin A per se and proteins involved in vitamin A metabolism have on adipocytes, adipose tissue biology, and adipose-related disease, as well as on early stage liver disease, including non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH).

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Components of metabolic syndrome in relation to plasma levels of retinol binding protein 4 (RBP4) in a cohort of people aged 65 years and older

PURPOSE

Elevated plasma concentration of retinol binding protein 4 (RBP4) has recently emerged as a potential risk factor as a component of developing metabolic syndrome (MS). Therefore, this study aimed to analyse the relationship between components of MS and concentrations of plasma RBP4 in a population of subjects 65 years and older.

METHODS

The study sample consisted of 3038 (1591 male) participants of the PolSenior study, aged 65 years and older. Serum lipid profile, concentrations of RBP4, glucose, insulin, C-reactive protein, IL-6, and activity of aminotransferases were measured. Nutritional status (BMI/waist circumference) and treatment with statins and fibrates were evaluated. Glomerular filtration rate (eGFR), de Ritis ratio, and fatty liver index (FLI), as well as HOMA-IR were calculated.

RESULTS

Our study revealed a strong relationship between components of MS and RBP4 in both sexes: plasma RBP4 levels were increased in men by at least 3×, and in women by at least 4×. Hypertriglyceridemia was most strongly associated with elevated plasma RBP4 levels. Multivariate, sex-adjusted regression analysis demonstrated that chronic kidney disease [OR 1.86 (95% CI 1.78-1.94)], hypertriglyceridemia [OR 1.52 (1.24-1.87)], hypertension [OR 1.15 (1.12-1.19)], low serum HDL cholesterol [OR 0.94 (0.92-0.97)], and age > 80 years [OR 0.86 (0.81-0.90)] were each independently associated with RBP4 concentration (all p < 0.001).

CONCLUSIONS

In Caucasians 65 years and older, RBP4 serum levels are associated with a number of components of MS, independent of sex and kidney function. Hypertriglyceridemia as a component of MS is most significantly related to RBP4 concentration.

Adipocytokine Regulation and Antiangiogenic Activity Underlie the Molecular Mechanisms of Therapeutic Effects of Phyllanthus niruri against Non-Alcoholic Fatty Liver Disease

The growth of adipose tissues is considered angiogenesis-dependent during non-alcoholic fatty liver disease (NAFLD). We have recently reported that our standardized 50% methanolic extract (ME) of Phyllanthus niruri (50% ME of P. niruri) has alleviated NAFLD in Sprague⁻Dawley rats. This study aimed to assess the molecular mechanisms of action, and to further evaluate the antiangiogenic effect of this extract. NAFLD was induced by eight weeks of high-fat diet, and treatment was applied for four weeks. Antiangiogenic activity was assessed by aortic ring assay and by in vitro tests. Our findings demonstrated that the therapeutic effects of 50% ME among NAFLD rats, were associated with a significant increase in serum adiponectin, reduction in the serum levels of RBP4, vaspin, progranulin, TNF-α, IL-6, and significant downregulation of the hepatic gene expression of PPARγ, SLC10A2, and Collα1. Concomitantly, 50% ME of P. niruri has exhibited a potent antiangiogenic activity on ring assay, cell migration, vascular endothelial growth factor (VEGF), and tube formation, without any cytotoxic effect. Together, our findings revealed that the protective effects of P. niruri against NAFLD might be attributed to its antiangiogenic effect, as well as to the regulation of adipocytokines and reducing the expression of adipogenic genes.

Effects of insulin resistance on the association between the circulating retinol-binding protein 4 level and clustering of pediatric cardiometabolic risk factors

OBJECTIVES

Retinol-binding protein 4 (RBP4) and insulin resistance (IR) are clinical parameters associated with cardiometabolic diseases. The mediating and modifying roles of IR on children's susceptibility to cardiometabolic disorders are undetermined. This study investigated the mediating and modifying effects of the homeostatic model assessment of IR (HOMA-IR) on the relationship between the serum RBP4 level and clustering of pediatric cardiometabolic risk factors.

METHODS

We assessed the diet, physical activity, cardiometabolic risk factors, and clinical parameters of 272 randomly selected adolescents from a large-scale cross-sectional study (n = 2727). Two HOMA-IRs (HOMA1-IR and HOMA2-IR) were used to evaluate the designated effects.

RESULTS

Levels of serum RBP4 positively correlated with the levels of the 2 HOMA-based-IRs, and HOMA-IR correlated to all components of pediatric metabolic syndrome (MetS), the number of abnormal components, and a body-weight-weighted principal component score extracted from 12 cardiometabolic risk factors. Increased RBP4 levels had positive effects on waist circumference (WC), triglyceride, and the number of abnormal MetS components (0.310 cm, 1.384 μg/dL, and 0.021 item elevations, respectively), and the HOMA-IRs explained 17.7% to 21.9%, 11.8% to 27.6%, and 23.8% to 25.0% of these effects. The association of WC and the number of abnormal MetS components with the serum RBP4 level was enhanced by higher HOMA-IR (β for interaction, 0.13 and 0.01 for HOMA1-IR, and 0.32 and 0.02 for HOMA2-IR, respectively).

CONCLUSIONS

HOMA-IR is associated with the circulating RBP4 level and cardiometabolic risk factors in adolescents. Pediatric HOMA-IR may have mediating and modifying effects on the positive correlations between RBP4 and the clustering of MetS components.

Retinol Binding Protein-4 Levels and Non-alcoholic Fatty Liver Disease: A community-based cross-sectional study

Previous reports on the association between retinol binding protein 4 (RBP4) and nonalcoholic fatty liver disease (NAFLD) were controversial. This study aimed to investigate the association between the serum RBP4 levels and occurrence of NAFLD in Chinese population. In total, 2938 participants aged 40–75 years were involved in this community-based cross-sectional study. General information, lifestyle factors, serum levels of RBP4 and the presence of NAFLD were determined. Patients with NAFLD had significantly higher concentrations of RBP4 (37.9 ± 6.8 μg/ml) than did non-NAFLD controls (35.0 ± 6.7 μg/ml) (P < 0.001). The odds ratios (ORs) of NAFLD for the highest (vs. lowest) quartile of RBP4 were 1.884 (95% CI: 1.391, 2.551) for females (P < 0.001), and 2.107 (95% CI: 1.357, 3.273) for male participants (P < 0.01) after adjusting for related factors. The serum RBP4 levels were positively associated with the prevalence of NAFLD in middle-aged and elderly Chinese people, and Homeostatic model assessment-insulin resistance (HOMA-IR), trunk fat, the waist-to-hip ratio (WHR), systolic blood pressure (SBP), fasting insulin, high density lipoprotein cholesterol (HDL-C) and triglycerides (TG) might be implicated in the pathogenesis of RBP4 in NAFLD.

Association of RBP4 genetic variants with childhood obesity and cardiovascular risk factors

BACKGROUND

Recent data suggest that retinol-binding protein 4 (RBP4) gene variants could be associated with a risk of obesity and its co-morbidities, such as metabolic syndrome, which increases the risk of developing type 2 diabetes mellitus and cardiovascular disease.

OBJECTIVES

The present study examined the potential association of RBP4 single nucleotide polymorphisms (SNPs) with childhood obesity and its metabolic complications.

METHODS

Four RBP4 SNPs, rs3758538 (3944A>C), rs3758539 (4406G>A), rs12265684 (12177G>C) and rs34571439 (14684T>G), were genotyped in a population of 180 Spanish Caucasian children (97 obese and 83 normal-weight children). Association of RBP4 SNPs with obesity, metabolic risk factors (blood pressure, triglycerides, high-density lipoprotein cholesterol, insulin resistance) and markers of vascular inflammation, such as high-sensitive C-reactive protein (hs-CRP), was tested.

RESULTS

We found SNP rs3758538 to be associated with obesity (p = 0.007). Specifically, each copy of the minor allele C was associated with an increased risk of obesity, by more than twofold, in respect of being homozygous for the major allele A (odds ratio = 2.4; 95% confidence interval = 1.2-4.8). The rs3758538 and rs34571439 RBP4 SNPs correlated with plasma RBP4 levels. The SNPs rs12265684 and rs34571439 correlated with plasma triglyceride levels. The rs34571439 was also associated to hs-CRP levels. Marginal association of RBP4 SNPs with plasma high-density lipoprotein levels (rs34571439), blood pressure (rs12265684) and insulin resistance (rs3758539) was also observed.

CONCLUSIONS

These findings suggest that childhood obesity may be associated with variations in RBP4 gene. The presence of selective SNPs in the RBP4 gene may account for metabolic complications.

Adipocyte-specific overexpression of retinol-binding protein 4 causes hepatic steatosis in mice

There is considerable evidence that both retinoids and retinol-binding protein 4 (RBP4) contribute to the development of liver disease. To understand the basis for this, we generated and studied transgenic mice that express human RBP4 (hRBP4) specifically in adipocytes. When fed a chow diet, these mice show an elevation in adipose total RBP4 (mouse RBP4 + hRBP4) protein levels. However, no significant differences in plasma RBP4 or retinol levels or in hepatic or adipose retinoid (retinol, retinyl ester, and all-trans-retinoic acid) levels were observed. Strikingly, male adipocyte-specific hRBP4 mice fed a standard chow diet display significantly elevated hepatic triglyceride levels at 3-4 months of age compared to matched littermate controls. When mice were fed a high-fat diet, this hepatic phenotype, as well as other metabolic phenotypes (obesity and glucose intolerance), worsened. Because adipocyte-specific hRBP4 mice have increased tumor necrosis factor-α and leptin expression and crown-like structures in adipose tissue, our data are consistent with the notion that adipose tissue is experiencing RBP4-induced inflammation that stimulates increased lipolysis within adipocytes. Our data further establish that elevated hepatic triglyceride levels result from increased hepatic uptake of adipose-derived circulating free fatty acids. We obtained no evidence that elevated hepatic triglyceride levels arise from increased hepatic de novo lipogenesis, decreased hepatic free fatty acid oxidation, or decreased very-low-density lipoprotein secretion.

CONCLUSION

Our investigations establish that RBP4 expressed in adipocytes induces hepatic steatosis arising from primary effects occurring in adipose tissue. (Hepatology 2016;64:1534-1546).

Lactate's effect on human neuroblastoma cell bioenergetic fluxes

Lactate, once considered a metabolic dead-end, has been recently proposed to support neuron bioenergetics. To better understand how lactate specifically influences cell energy metabolism, we studied the effects of lactate supplementation on SH-SY5Y human neuroblastoma cell bioenergetic fluxes. Lactate supplementation increased cell respiration, there was no change in respiratory coupling efficiency, and lactate itself appeared to directly support the respiratory flux increase. Conversely, lactate supplementation reduced the glycolysis flux. This apparent pro-aerobic shift in the respiration:glycolysis ratio was accompanied by post-translational modifications and compartmental redistributions of proteins that respond to and modify bioenergetic fluxes, including cAMP-response element binding protein (CREB), p38 mitogen-activated protein kinases (p38 MAPK), AMP-activated protein kinase (AMPK), peroxisome-proliferator activated receptor gamma coactivator 1 β (PGC-1β), Akt, mammalian target of rapamycin (mTOR), and forkhead box protein O1 (FOXO1). mRNA levels for PGC-1β, nuclear respiratory factor 1 (NRF1), and cytochrome c oxidase subunit 1 (COX1) increased. Some effects depended on the direct presence of lactate, while others were durable and evident several hours after lactate was removed. We conclude lactate can be used to manipulate cell bioenergetics.

Retinoid roles in blocking hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a major health issue in many countries. The prognosis of HCC is still poor due to its high recurrence rate and its resistance to chemotherapy. Retinoids have long been known to have a connection with liver diseases, including HCC. Many experimental and clinical investigations have demonstrated associations between retinoids and hepatic disease, including the loss of retinoid activity in HCC cell lines and decreased hepatic retinoid stores and altered retinoid signaling in patients with cirrhosis and HCC. Based on these findings, preclinical and clinical investigations of retinoid effects on HCC have been undertaken. Recently, clinical trial results for the use of a synthetic retinoid, acyclic retinoid (ACR), to prevent HCC recurrence were published. In addition, extensive experimental studies on the action of retinoids in liver disease, including chronic viral hepatitis and non-alcoholic fatty liver disease (NAFLD), which lead to HCC have been performed. In the first section of this review, we will summarize the effectiveness and roles of retinoid for treating liver disease and blocking HCC. Subsequently, we will focus on ACR actions in blocking HCC.

DGAT1-deficiency affects the cellular distribution of hepatic retinoid and attenuates the progression of CCl4-induced liver fibrosis

BACKGROUND

Diacylglycerol O-acyltransferase 1 (DGAT1) catalyzes the final step of triglyceride synthesis, transferring an acyl group from acyl-CoA to diacylglycerol. DGAT1 also catalyzes the acyl-CoA-dependent formation of retinyl esters in vitro and in mouse intestine and skin. Although DGAT1 is expressed in both hepatocytes and hepatic stellate cells (HSCs), we reported genetic and nutritional studies that established that DGAT1 does not contribute to retinyl ester formation in the liver.

METHODS

We now have explored in more depth the role(s) of DGAT1 in hepatic retinoid metabolism and storage.

RESULTS

Our data show that DGAT1 affects the cellular distribution between hepatocytes and HSCs of stored and newly absorbed dietary retinol. For livers of Dgat1-deficient mice, a greater percentage of stored retinyl ester is present in HSCs at the expense of hepatocytes. This is also true for newly absorbed oral [(3)H]retinol. These differences are associated with significantly increased expression, by 2.8-fold, of cellular retinol-binding protein, type I (RBP1) in freshly isolated HSCs from Dgat1-deficient mice, raising the possibility that RBP1, which contributes to retinol uptake into cells and retinyl ester synthesis, accounts for the differences. We further show that the retinyl ester-containing lipid droplets in HSCs are affected in Dgat1-null mice, being fewer in number but, on average, larger than in wild type (WT) HSCs. Finally, we demonstrate that DGAT1 affects experimentally induced HSC activation in vivo but that this effect is independent of altered retinoic acid availability or effects on gene expression.

CONCLUSIONS

Our studies establish that DGAT1 has a role in hepatic retinoid storage and metabolism, but this does not involve direct actions of DGAT1 in retinyl ester synthesis.

Adipocytokines and hepatic fibrosis

Obesity and metabolic syndrome pose significant risk for the progression of many types of chronic illness, including liver disease. Hormones released from adipocytes, adipocytokines, associated with obesity and metabolic syndrome, have been shown to control hepatic inflammation and fibrosis. Hepatic fibrosis is the final common pathway that can result in cirrhosis, and can ultimately require liver transplantation. Initially, two key adipocytokines, leptin and adiponectin, appeared to control many fundamental aspects of the cell and molecular biology related to hepatic fibrosis and its resolution. Leptin appears to act as a profibrogenic molecule, while adiponectin has strong-antifibrotic properties. In this review, we emphasize pertinent data associated with these and other recently discovered adipocytokines that may drive or halt the fibrogenic response in the liver.

Actions of β-apo-carotenoids in differentiating cells: differential effects in P19 cells and 3T3-L1 adipocytes

β-Apo-carotenoids, including β-apo-13-carotenone and β-apo-14'-carotenal, are potent retinoic acid receptor (RAR) antagonists in transactivation assays. We asked how these influence RAR-dependent processes in living cells. Initially, we explored the effects of β-apo-13-carotenone and β-apo-14'-carotenal on P19 cells, a mouse embryonal carcinoma cell line that differentiates into neurons when treated with all-trans-retinoic acid. Treatment of P19 cells with either compound failed to block all-trans-retinoic acid induced differentiation. Liquid chromatography tandem mass spectrometry studies, however, established that neither of these β-apo-carotenoids accumulates in P19 cells. All-trans-retinoic acid accumulated to high levels in P19 cells. This suggests that the uptake and metabolism of β-apo-carotenoids by some cells does not involve the same processes used for retinoids and that these may be cell type specific. We also investigated the effects of two β-apo-carotenoids on 3T3-L1 adipocyte marker gene expression during adipocyte differentiation. Treatment of 3T3-L1 adipocytes with either β-apo-13-carotenone or β-apo-10'-carotenoic acid, which lacks RAR antagonist activity, stimulated adipocyte marker gene expression. Neither blocked the inhibitory effects of a relatively large dose of exogenous all-trans-retinoic acid on adipocyte differentiation. Our data suggest that in addition to acting as transcriptional antagonists, some β-apo-carotenoids act through other mechanisms to influence 3T3-L1 adipocyte differentiation.

Serum proteomics of hepatitis C virus infection reveals retinol-binding protein 4 as a novel regulator

Persistent infection of hepatitis C virus (HCV) can lead to liver cirrhosis and hepatocellular carcinoma, which are currently diagnosed by invasive liver biopsy. Approximately 15-20 % of cases of chronic liver diseases in India are caused by HCV infection. In North India, genotype 3 is predominant, whereas genotype 1 is predominant in southern parts of India. The aim of this study was to identify differentially regulated serum proteins in HCV-infected Indian patients (genotypes 1 and 3) using a two-dimensional electrophoresis approach. We identified eight differentially expressed proteins by MS. Expression levels of one of the highly upregulated proteins, retinol-binding protein 4 (RBP4), was validated by ELISA and Western blotting in two independent cohorts. We also confirmed our observation in the JFH1 infectious cell culture system. Interestingly, the HCV core protein enhanced RBP4 levels and partial knockdown of RBP4 had a positive impact on HCV replication, suggesting a possible role for this cellular protein in regulating HCV infection. Analysis of RBP4-interacting partners using a bioinformatic approach revealed novel insights into the possible involvement of RBP4 in HCV-induced pathogenesis. Taken together, this study provided information on the proteome profile of the HCV-infected Indian population, and revealed a link between HCV infection, RBP4 and insulin resistance.

Associations of retinol-binding protein 4 with oxidative stress, inflammatory markers, and metabolic syndrome in a middle-aged and elderly Chinese population

BACKGROUND

Retinol-binding protein 4 (RBP4), a novel adipokine secreted by adipocytes and the liver, has elevated levels in type 2 diabetes mellitus (T2DM). However, its association with human metabolic diseases remains controversial. The present study was designed to investigate the associations of plasma RBP4 levels with oxidative stress, inflammatory markers, and metabolic syndrome (MetS) in a Chinese population.

METHOD

We evaluated plasma RBP4 levels in a cross-sectional sample of 1748 Chinese men and women aged 50 to 70 years in Guangzhou using an in-house developed and validated sandwich ELISA. Plasma glucose, insulin, lipid profile, serum adiponectin, adipocyte fatty acid-binding protein (A-FABP), 8-iso-prostaglandin F2α (8-iso PGF2α), 13-(S)-hydroxyoctadecadienoic acid (13-HODE), high-sensitivity C-reactive protein (hsCRP), interleukin 6 (IL6), monocyte chemotactic protein 1 (MCP1) and tumor necrosis factor α (TNFα) were all measured. MetS was defined according to the updated National Cholesterol Education Program Adult Treatment Panel III criteria for Asian Americans.

RESULTS

Circulating RBP4 levels were positively correlated with A-FABP (r = 0.104, P < 0.001), 8-iso PGF2α (0.236, P < 0.001), and 13-HODE (0.204, P < 0.001) and were inversely correlated with HDL cholesterol (r = -0.072, P = 0.004). After multivariable adjustment, the RBP4 levels were strongly associated with MetS and its components. The ORs (95% CIs) for the comparisons of the extreme quartiles of RBP4 were 3.46 (2.87, 4.42) for MetS, 5.92 (4.47, 8.02) for hypertriglyceridemia, 1.42 (1.11, 1.68) for reduced HDL cholesterol, 1.87 (1.48, 2.36) for central obesity and 2.74 (2.15, 3.36) for hyperglycemia (all P < 0.001). When we further controlled for adipokines, markers of oxidative stress and proinflammatory response, the association of RBP4 with central obesity was abolished but not the association with other MetS components.

CONCLUSIONS

Plasma RBP4 levels are associated with an adverse profile of oxidative stress and inflammatory markers and an increased risk of MetS in this Chinese population. These associations are independent of conventional risk factors.

Elevated serum triglyceride and retinol-binding protein 4 levels associated with fructose-sweetened beverages in adolescents

Background

The metabolic effect of fructose in sugar-sweetened beverages (SSB) has been linked to de novo lipogenesis and uric acid (UA) production.

Objectives

This study investigated the biological effects of SSB consumption on serum lipid profiles and retinol-binding protein 4 (RBP4) among Taiwanese adolescents.

Methods

We evaluated the anthropometric parameters and biochemical outcomes of 200 representative adolescents (98 boys and 102 girls) who were randomly selected from a large-scale cross-sectional study. Data were analyzed using multiple regression models adjusted for covariates.

Results

Increased SSB consumption was associated with increased waist and hip circumferences, body mass index (BMI) values and serum UA, triglyceride (TG) and RBP4 levels. Adolescents who consumed >500 ml/day of beverages half-to-heavily sweetened with high-fructose corn syrup (HFCS) exhibited TG and RBP4 levels 22.7 mg/dl and 13.92 ng/ml higher than non-drinkers, respectively. HFCS drinkers with hyperuricemia had higher TG levels than HFCS drinkers with normal UA levels (98.6 vs. 81.6 mg/dl). The intake of HFCS-rich SSBs and high value of BMI (≥24) interactively reinforced RBP4 levels among overweight/obese adolescents. Circulating RBP4 levels were significantly correlated with weight-related outcomes and TG and UA concentration among HFCS drinkers (r = 0.253 to 0.404), but not among non-drinkers.

Conclusions

High-quantity HFCS-rich beverage consumption is associated with higher TG and RBP4 levels. Hyperuricemia is likely to intensify the influence of HFCS-rich SSB intake on elevated TG levels, and in overweight and obese adolescents, high BMI may modify the action of fructose on higher circulating levels of RBP4.

Human plasmacytoid dendritic cells sense lymphocytic choriomeningitis virus-infected cells in vitro

We previously reported that exosomal transfer of hepatitis C virus (HCV) positive-strand RNA from human Huh-7 hepatoma cells to human plasmacytoid dendritic cells (pDCs) triggers pDC alpha/beta interferon (IFN-α/β) production in a Toll-like receptor 7 (TLR7)-dependent, virus-independent manner. Here we show that human pDCs are also activated by a TLR7-dependent, virus-independent, exosomal RNA transfer mechanism by human and mouse hepatoma and nonhepatoma cells that replicate the negative-strand lymphocytic choriomeningitis virus (LCMV).

Impact of sterol regulatory element-binding factor-1c polymorphism on incidence of nonalcoholic fatty liver disease and on the severity of liver disease and of glucose and lipid dysmetabolism

BACKGROUND

Genetic factors that predispose individuals to nonalcoholic fatty liver disease (NAFLD) and associated diabetes and cardiovascular disease are unclear. The transcription factor sterol regulatory element-binding protein-1c (SREBP-1c) modulates lipogenesis and insulin sensitivity and was experimentally connected to NAFLD.

OBJECTIVE

We assessed the impact of a common SREBF-1c polymorphism on the incidence and severity of NAFLD and on associated glucose and lipoprotein dysmetabolism.

DESIGN

We followed up 212 randomly selected, nonobese, nondiabetic, insulin-sensitive participants in a population-based study without NAFLD or metabolic syndrome at baseline who were characterized for the common SREBF-1c gene rs11868035 A/G polymorphism, dietary habits, physical activity, adipokine profile, C-reactive protein (CRP), and circulating markers of endothelial dysfunction. A comparable cohort of NAFLD patients underwent a liver biopsy, an oral-glucose-tolerance test with minimal model analysis of glucose homeostasis variables, and an oral-fat-tolerance test with measurement of plasma lipoproteins, adipokines, and cytokeratin-18 fragments.

RESULTS

SREBF-1c predicted the 7-y incidence of NAFLD (OR: 1.71; 95% CI: 1.15, 2.53) and diabetes and the 7-y elevation in CRP and endothelial dysfunction markers. In biopsy-proven NAFLD patients, the SREBF-1c A allele conferred increased risk of severe steatosis and nonalcoholic steatohepatitis; more-severe hepatic, muscle, and adipose tissue insulin resistance; and pancreatic β cell dysfunction. SREBF-1c A allele carriers also had an impaired oral fat tolerance with a postprandial accumulation of large triglyceride-rich lipoproteins and oxidized LDLs, lower HDL cholesterol and adiponectin concentrations, and cytokeratin-18 fragment elevation.

CONCLUSION

SREBF-1c polymorphism is associated with increased risk of developing NAFLD with more severe liver histology and derangement in glucose and lipoprotein metabolism, which contribute to the presentation and natural history of NAFLD.

Retinol-binding protein 4 and its membrane receptor STRA6 control adipogenesis by regulating cellular retinoid homeostasis and retinoic acid receptor α activity

Retinoids are vitamin A (retinol) derivatives and complex regulators of adipogenesis by activating specific nuclear receptors, including the retinoic acid receptor (RAR) and retinoid X receptor (RXR). Circulating retinol-binding protein 4 (RBP4) and its membrane receptor STRA6 coordinate cellular retinol uptake. It is unknown whether retinol levels and the activity of RAR and RXR in adipocyte precursors are linked via RBP4/STRA6. Here, we show that STRA6 is expressed in precursor cells and, dictated by the apo- and holo-RBP4 isoforms, mediates bidirectional retinol transport that controls RARα activity and subsequent adipocyte differentiation. Mobilization of retinoid stores in mice by inducing RBP4 secretion from the liver activated RARα signaling in the precursor cell containing the stromal-vascular fraction of adipose tissue. Retinol-loaded holo-RBP4 blocked adipocyte differentiation of cultured precursors by activating RARα. Remarkably, retinol-free apo-RBP4 triggered retinol efflux that reduced cellular retinoids, RARα activity, and target gene expression and enhanced adipogenesis synergistically with ectopic STRA6. Thus, STRA6 in adipocyte precursor cells links nuclear RARα activity to the circulating RBP4 isoforms, whose ratio in obese mice was shifted toward limiting the adipogenic potential of their precursors. This novel cross talk identifies a retinol-dependent metabolic function of RBP4 that may have important implications for the treatment of obesity.

Retinol and retinyl esters: biochemistry and physiology

By definition, a vitamin is a substance that must be obtained regularly from the diet. Vitamin A must be acquired from the diet, but unlike most vitamins, it can also be stored within the body in relatively high levels. For humans living in developed nations or animals living in present-day vivariums, stored vitamin A concentrations can become relatively high, reaching levels that can protect against the adverse effects of insufficient vitamin A dietary intake for six months, or even much longer. The ability to accumulate vitamin A stores lessens the need for routinely consuming vitamin A in the diet, and this provides a selective advantage to the organism. The molecular processes that underlie this selective advantage include efficient mechanisms to acquire vitamin A from the diet, efficient and overlapping mechanisms for the transport of vitamin A in the circulation, a specific mechanism allowing for vitamin A storage, and a mechanism for mobilizing vitamin A from these stores in response to tissue needs. These processes are considered in this review.

A pathway approach to investigate the function and regulation of SREBPs

The essential function of sterol regulatory element-binding proteins (SREBPs) in cellular lipid metabolism and homeostasis has been recognized for a long time, and the basic biological pathway involving SREBPs has been well described; however, a rapidly growing number of studies reveal the complex regulation of these SREBP transcription factors at multiple levels. This regulation allows the integration of signals of diverse pathways involving nutrients, contributing to cellular lipid and energy homeostasis. This review attempts to integrate this knowledge. The description of the SREBP pathway is Web-linked as it refers to the online version of the pathway on wikipathways.org , which is interactively linked to genomics databases and literature. This allows a more extensive study of the pathway through reviewing these links.