Characterization of microRNA expression in bovine adipose tissues: a potential regulatory mechanism of subcutaneous adipose tissue development

Background  

MicroRNAs (miRNAs), a family of small non-coding RNA molecules, appear to regulate animal lipid metabolism and preadipocyte conversion to form lipid-assimilating adipocytes (i.e. adipogenesis). However, no miRNA to date has been reported to modulate adipogenesis and lipid deposition in beef cattle.     

Feeding a slowly digestible carbohydrate diet during pregnancy of insulin-resistant rats prevents the excess of adipogenesis in their offspring

An obesogenic environment during pregnancy has been shown to increase the risk of dysregulation on adipogenesis and insulin resistance in the offspring. Being essential for the growing fetus, glucose supply is guaranteed by a number of modifications in the mother’s metabolism, and thus, glucose control during pregnancy especially among obese or diabetic women is paramount to prevent adverse consequences in their children.Besides the election of low-glycemic-index carbohydrates, the rate of carbohydrate digestion could be relevant to keep a good glucose control. In the present study, we compared the effects of two high-fat diets with similar glycemic load but different rates of carbohydrate digestion given to pregnant insulin-resistant rats. After birth, all animals were fed a standard diet until age 14 weeks. We analyzed offspring body composition, plasma and adipocyte lipidomics, lipid metabolism in adipose tissue and insulin sensitivity. Those animals whose mothers were fed the rapid-digesting carbohydrate diet exhibited an excessive adipogenesis. Thus, these animals showed a marked lipidemia, increased lipid synthesis in the adipose tissue and reduced glucose transporter amount in the adipose. On the contrary, those animals whose mothers were fed the slow-digesting carbohydrate diet showed a profile in the measured parameters closer to that of the offspring of healthy mothers. These results support the hypothesis that not only glycemic index but the rate of carbohydrate digestion during gestation may be critical to regulate the programming of adipogenesis in the offspring.     

Epigenetic developmental programs and adipogenesis

Psychotropic agents are notorious for their ability to increase fat mass in psychiatric patients. The two determinants of fat mass are the production of newly differentiated adipocytes (adipogenesis), and the volume of lipid accumulation. Epigenetic programs have a prominent role in cell fate commitments and differentiation required for adipogenesis. In parallel, epigenetic effects on energy metabolism are well supported by several genetic models. Consequently, a variety of psychotropics, often prescribed in combinations and for long periods, may utilize a common epigenetic effector path causing an increase in adipogenesis or reduction in energy metabolism. In particular, the recent discovery that G protein coupled signaling cascades can directly modify epigenetic regulatory enzymes implicates surface receptor activity by psychotropic medications. The potential therapeutic implications are also suggested by the effects of the clinically approved antidepressant tranylcypromine, also a histone demethylase inhibitor, which has impressive therapeutic effects on metabolism in the obese phenotype.     

Long non-coding RNAs regulation in adipogenesis and lipid metabolism: Emerging insights in obesity

Obesity is a widespread health problem that brings about various adipose tissue dysfunctions. The balance of energy storage and energy expenditure is critical for normal fat accumulation and lipid metabolism. Therefore, understanding the molecular basis of adipogenesis and thermogenesis is essential to maintain adipose development and lipid homeostasis. Increasing evidence demonstrated that lncRNAs (long non-coding RNAs), a class of non-protein coding RNAs of >200 nucleotides in length, are identified as key regulators in obesity-related biological processes through diverse regulatory mechanisms. In this review, we concentrate on recent and relevant studies on the roles of lncRNAs in regulation of white adipogenesis, brown adipocyte differentiation and lipid metabolism. In addition, the diagnostic and therapeutic potential of lncRNAs is highlighted, and that will make recommendations for the future application of lncRNAs in the treatment of obesity.     

Critical role of the peroxisomal protein PEX16 in white adipocyte development and lipid homeostasis

The importance of peroxisomes for adipocyte function is poorly understood. Herein, we provide insights into the critical role of peroxin 16 (PEX16)-mediated peroxisome biogenesis in adipocyte development and lipid metabolism. Pex16 is highly expressed in adipose tissues and upregulated during adipogenesis of murine and human cells. We demonstrate that Pex16 is a target gene of the adipogenesis “master-regulator” PPARγ. Stable silencing of Pex16 in 3T3-L1 cells strongly reduced the number of peroxisomes while mitochondrial number was unaffected. Concomitantly, peroxisomal fatty acid (FA) oxidation was reduced, thereby causing accumulation of long- and very long-chain (polyunsaturated) FAs and reduction of odd-chain FAs. Further, Pex16-silencing decreased cellular oxygen consumption and increased FA release. Additionally, silencing of Pex16 impaired adipocyte differentiation, lipogenic and adipogenic marker gene expression, and cellular triglyceride stores. Addition of PPARγ agonist rosiglitazone and peroxisome-related lipid species to Pex16-silenced 3T3-L1 cells rescued adipogenesis. These data provide evidence that PEX16 is required for peroxisome biogenesis and highlights the relevance of peroxisomes for adipogenesis and adipocyte lipid metabolism.     

miR-128-3p regulates 3T3-L1 adipogenesis and lipolysis by targeting <Emphasis Type="Italic">Pparg</Emphasis> and <Emphasis Type="Italic">Sertad2</Emphasis>

Differentiation of adipocytes and their aggregation to adipose tissue are critical for mammalian growth and development. MicroRNAs (miRNAs) are a class of endogenous small non-coding RNAs that play important roles in adipogenesis and lipid metabolism. miR-128-3p may contribute to adipose tissue development according to the previous studies. However, the role of miR-128-3p in the process of preadipocyte differentiation and lipid metabolism is not yet understood. The purpose of this research was to investigate the biological function and molecular mechanism of miR-128-3p in 3T3-L1 cells. In the present study, we found that miR-128-3p was downregulated during the process of 3T3-L1 preadipocyte differentiation. Overexpression of miR-128-3p obstructed the expressions of adipogenic marker genes as well as the lipid droplets accumulation and triglyceride content, suggesting the importance of miR-128-3p for adipogenesis. Moreover, miR-128-3p could lead to the retardation of cell proliferation in 3T3-L1 preadipocytes. Further evidences showed that, as a negative regulator of adipogenesis, miR-128-3p could directly target peroxisome proliferator-activated receptor γ (Pparg) which resulted in the suppression of 3T3-L1 preadipocyte differentiation, and miR-128-3p could also bind with SERTA domain containing 2 (Sertad2) which drove triglyceride hydrolysis and lipolysis. In addition, inhibition of Sertad2 with siRNA displayed the same effects as overexpression of miR-128-3p. Our research demonstrated that miR-128-3p impeded 3T3-L1 adipogenesis by targeting Pparg and Sertad2, resulting in the obstruction of preadipocyte differentiation and promotion of lipolysis. Taken together, this study offers profound insight into the mechanism of miRNA-mediated adipogenesis and lipid metabolism.     

Review: Animal model and the current understanding of molecule dynamics of adipogenesis

Among several potential animal models that can be used for adipogenic studies, Wagyu cattle is the one that presents unique molecular mechanisms underlying the deposit of substantial amounts of intramuscular fat. As such, this review is focused on current knowledge of such mechanisms related to adipose tissue deposition using Wagyu cattle as model. So abundant is the lipid accumulation in the skeletal muscles of these animals that in many cases, the muscle cross-sectional area appears more white (adipose tissue) than red (muscle fibers). This enhanced marbling accumulation is morphologically similar to that seen in numerous skeletal muscle dysfunctions, disease states and myopathies; this might indicate cross-similar mechanisms between such dysfunctions and fat deposition in Wagyu breed. Animal models can be used not only for a better understanding of fat deposition in livestock, but also as models to an increased comprehension on molecular mechanisms behind human conditions. This revision underlies some of the complex molecular processes of fat deposition in animals.     

The adipokine Chemerin induces lipolysis and adipogenesis in bovine intramuscular adipocytes

The adipokine Chemerin is reported to regulate adipogenesis and glucose homeostasis in vivo and in 3T3-L1 cells. Our team is focused on the role of Chemerin in metabolism and intramuscular adipocyte differentiation because intramuscular fat is the basic material for the formation of marbling in livestock and poultry meat. In this study, bovine intramuscular mature adipocytes were cultured in medium with Chemerin, and the process of lipolysis of mature adipocytes and the adipogenesis of de-differentiated preadipocytes were investigated. The results showed that Chemerin induced significant lipolytic metabolism in intramuscular mature adipocytes, indicated by increased levels of glycerol, FFA, and up-regulated expression of the lipolysis critical factors HSL, LPL, and leptin. Meanwhile, the expressions of adipogenic key factors PPARγ, C/EBPα, and A-FABP were decreased by Chemerin during lipolysis or dedifferentiation in mature adipocytes. The de-differentiated preadipocytes could re-differentiate into mature adipocytes. Intriguingly, the formation of cells’ lipid droplets was promoted by Chemerin during preadipocyte differentiation. In addition, mRNA and protein expressions of PPARγ, C/EBPα, and A-FABP were up-regulated by Chemerin during preadipocytes differentiation. These results suggest that Chemerin promotes lipolysis in mature adipocytes and induces adipogenesis during preadipocyte re-differentiation, further indicating a dual role for Chemerin in the deposition of intramuscular fat in ruminant animals.

     

Towards a mechanistic understanding of lipodystrophy and seipin functions

CGL (Congenital generalized lipodystrophy) is a genetic disorder characterized by near complete loss of adipose tissue along with increased ectopic fat storage in other organs including liver and muscle. Of the four CGL types, BSCL2 (Berardinelli–Seip Congenital lipodystrophy type 2), resulting from mutations in the BSCL2/seipin gene, exhibits the most severe lipodystrophic phenotype with loss of both metabolic and mechanical adipose depots. The majority of Seipin mutations cause C-terminal truncations, along with a handful of point mutations. Seipin localizes to the ER and is composed of a conserved region including a luminal loop and two transmembrane domains, plus cytosolic N- and C-termini. Animal models deficient in seipin recapitulate the human lipodystrophic phenotype. Cells isolated from seipin knockout mouse models also exhibit impaired adipogenesis. Mechanistically, seipin appears to function as a scaffolding protein to bring together interacting partners essential for lipid metabolism and LD (lipid droplet) formation during adipocyte development. Moreover, cell line and genetic studies indicate that seipin functions in a cell-autonomous manner. Here we will provide a brief overview of the genetic association of the CGLs, and focus on the current understanding of differential contributions of distinct seipin domains to lipid storage and adipogenesis. We will also discuss the roles of seipin-interacting partners, including lipin 1 and 14-3-3β, in mediating seipin-dependent regulation of cellular pathways such as actin cytoskeletal remodelling.     

Overexpression of glucose-6-phosphate dehydrogenase is associated with lipid dysregulation and insulin resistance in obesity

Glucose-6-phosphate dehydrogenase (G6PD) produces cellular NADPH, which is required for the biosynthesis of fatty acids and cholesterol. Although G6PD is required for lipogenesis, it is poorly understood whether G6PD in adipocytes is involved in energy homeostasis, such as lipid and glucose metabolism. We report here that G6PD plays a role in adipogenesis and that its increase is tightly associated with the dysregulation of lipid metabolism and insulin resistance in obesity. We observed that the enzymatic activity and expression levels of G6PD were significantly elevated in white adipose tissues of obese models, including db/db, ob/ob, and diet-induced obesity mice. In 3T3-L1 cells, G6PD overexpression stimulated the expression of most adipocyte marker genes and elevated the levels of cellular free fatty acids, triglyceride, and FFA release. Consistently, G6PD knockdown via small interfering RNA attenuated adipocyte differentiation with less lipid droplet accumulation. Surprisingly, the expression of certain adipocytokines such as tumor necrosis factor alpha and resistin was increased, whereas that of adiponectin was decreased in G6PD overexpressed adipocytes. In accordance with these results, overexpression of G6PD impaired insulin signaling and suppressed insulin-dependent glucose uptake in adipocytes. Taken together, these data strongly suggest that aberrant increase of G6PD in obese and/or diabetic subjects would alter lipid metabolism and adipocytokine expression, thereby resulting in failure of lipid homeostasis and insulin resistance in adipocytes.     

Antiobesity mechanisms of action of conjugated linoleic acid

Conjugated linoleic acid (CLA), a family of fatty acids found in beef, dairy foods and dietary supplements, reduces adiposity in several animal models of obesity and some human studies. However, the isomer-specific antiobesity mechanisms of action of CLA are unclear, and its use in humans is controversial. This review will summarize in vivo and in vitro findings from the literature regarding potential mechanisms by which CLA reduces adiposity, including its impact on (a) energy metabolism, (b) adipogenesis, (c) inflammation, (d) lipid metabolism and (e) apoptosis.     

MicroRNAs: Emerging roles in adipogenesis and obesity

Obesity is a serious health problem worldwide associated with an increased risk of life-threatening diseases such as type 2 diabetes, atherosclerosis, and certain types of cancer. Understanding the molecular basis of adipogenesis and fat cell development in obesity is essential to identify new biomarkers and therapeutic targets for the development of anti-obesity drugs. Recent computational and experimental studies have shown that microRNAs (miRNAs) appear to play regulatory roles in many biological processes associated with obesity, including adipocyte differentiation and lipid metabolism. In addition, many miRNAs are dysregulated in metabolic tissues from obese animals and humans, which potentially contributes to the pathogenesis of obesity-associated complications. The discovery of circulating miRNAs has highlighted their potential as both endocrine signaling molecules and disease markers. The potential of miRNA based therapeutics targeting obesity is highlighted as well as recommendations for future research which could lead to a breakthrough in the treatment of obesity.     

Dipeptidyl-peptidase-IV by cleaving neuropeptide Y induces lipid accumulation and PPAR-γ expression

We evaluated the effects of dipeptidyl peptidase-IV (DPPIV), and its inhibitor, vildagliptin, on adipogenesis and lipolysis in a pre-adipocyte murine cell line (3T3-L1). The exogenous rDPPIV increased lipid accumulation and PPAR-γ expression, whereas an inhibitor of DPPIV, the anti-diabetic drug vildagliptin, suppresses the stimulatory role of DPPIV on adipogenesis and lipid accumulation, but had no effect on lipolysis. NPY immunoneutralization or NPY Y(2) receptor blockage inhibited DPPIV stimulatory effects on lipid accumulation, collectively, indicating that DPPIV has an adipogenic effect through NPY cleavage and subsequent NPY Y(2) activation. Vildagliptin inhibits PPAR-γ expression and lipid accumulation without changing lipolysis, suggesting that this does not impair the ability of adipose tissue to store triglycerides inside lipid droplets. These data indicate that DPPIV and NPY interact on lipid metabolism to promote adipose tissue depot.     

Adipogenesis and lipid production in adipocytes subjected to sustained tensile deformations and elevated glucose concentration: a living cell-scale model system of diabesity

Adipocyte fate commitment is characterized by morphological changes of fibroblastic pre-adipocyte cells, and specifically by accumulation of lipid droplets (LDs) as part of the adipogenesis metabolism. Formation of LDs indicates the production of triglycerides from glucose through an insulin-regulated glucose internalization process. In obesity, adipocytes typically become insulin resistant, and glucose transport into the cells is impaired, resulting in type 2 diabetes. In the present study, we monitored the adipogenesis in 3T3-L1 cultured cells exposed to high (450 mg/dL hyperglycemia) and low (100 mg/dL physiological) glucose concentrations, in a novel cell culture model system of diabesity. In addition to glucose conditions, cells were concurrently exposed to different substrate tensile strains (12% and control) based on our prior work which revealed that adipogenesis is accelerated in cultures subjected to static, chronic substrate tensile deformations. Phase-contrast images were taken throughout the adipogenesis process (3 weeks) and were analyzed by an image processing algorithm which quantitatively monitors cell differentiation and lipid accumulation (number of LDs per cell and their radius as well as cell size and shape). The results indicated that high glucose concentrations and substrate tensile strains delivered to adipocytes accelerated lipid production by 1.7- and 1.4-fold, respectively. In addition, significant changes in average cell projected area and in other morphological attributes were observed during the differentiation process. The importance of this study is in characterizing the adipogenesis parameters as potential read-outs that can predict the occurrence of insulin resistance in the development of diabesity.     

Peroxisome proliferator-activated receptor gamma: its role in metabolic syndrome

Here we review PPARgamma function in relation to human adipogenesis, insulin sensitization, lipid metabolism, blood pressure regulation and prothrombotic state to perhaps provide justification for this nuclear receptor remaining a key therapeutic target for the continuing development of agents to treat human metabolic syndrome.