The hitchhiker's guide to PGC-1α isoform structure and biological functions.

Proteins of the peroxisome proliferator-activated receptor γ (PPARγ) coactivator 1 (PGC-1) family of transcriptional coactivators coordinate physiological adaptations in many tissues, usually in response to demands for higher nutrient and energy supply. Of the founding members of the family, PGC-1α (also known as PPARGC1A) is the most highly regulated gene, using multiple promoters and alternative splicing to produce a growing number of coactivator variants. PGC-1α promoters are selectively active in distinct tissues in response to specific stimuli. To date, more than ten novel PGC-1α isoforms have been reported to be expressed from a novel promoter (PGC-1α-b, PGC-1α-c), to undergo alternative splicing (NT-PGC-1α) or both (PGC-1α2, PGC-1α3, PGC-1α4). The resulting proteins display differential regulation and tissue distribution and, most importantly, exert specific biological functions. In this review we discuss the structural and functional characteristics of the novel PGC-1α isoforms, aiming to provide an integrative view of this constantly expanding system of transcriptional coactivators.

Skeletal muscle PGC-1α1 modulates kynurenine metabolism and mediates resilience to stress-induced depression.

Depression is a debilitating condition with a profound impact on quality of life for millions of people worldwide. Physical exercise is used as a treatment strategy for many patients, but the mechanisms that underlie its beneficial effects remain unknown. Here, we describe a mechanism by which skeletal muscle PGC-1α1 induced by exercise training changes kynurenine metabolism and protects from stress-induced depression. Activation of the PGC-1α1-PPARα/δ pathway increases skeletal muscle expression of kynurenine aminotransferases, thus enhancing the conversion of kynurenine into kynurenic acid, a metabolite unable to cross the blood-brain barrier. Reducing plasma kynurenine protects the brain from stress-induced changes associated with depression and renders skeletal muscle-specific PGC-1α1 transgenic mice resistant to depression induced by chronic mild stress or direct kynurenine administration. This study opens therapeutic avenues for the treatment of depression by targeting the PGC-1α1-PPAR axis in skeletal muscle, without the need to cross the blood-brain barrier.

Liver X receptors regulate de novo lipogenesis in a tissue-specific manner in C57BL/6 female mice.

The liver X receptors (LXRs) play a key role in cholesterol and bile acid metabolism but are also important regulators of glucose metabolism. Recently, LXRs have been proposed as a glucose sensor affecting LXR-dependent gene expression. We challenged wild-type (WT) and LXRαß(-/-) mice with a normal diet (ND) or a high-carbohydrate diet (HCD). Magnetic resonance imaging showed different fat distribution between WT and LXRαß(-/-) mice. Surprisingly, gonadal (GL) adipocyte volume decreased on HCD compared with ND in WT mice, whereas it slightly increased in LXRαß(-/-) mice. Interestingly, insulin-stimulated lipogenesis of isolated GL fat cells was reduced on HCD compared with ND in LXRαß(-/-) mice, whereas no changes were observed in WT mice. Net de novo lipogenesis (DNL) calculated from Vo(2) and Vco(2) was significantly higher in LXRαß(-/-) than in WT mice on HCD. Histology of HCD-fed livers showed hepatic steatosis in WT mice but not in LXRαß(-/-) mice. Glucose tolerance was not different between groups, but insulin sensitivity was decreased by the HCD in WT but not in LXRαß(-/-) mice. Finally, gene expression analysis of adipose tissue showed induced expression of genes involved in DNL in LXRαß(-/-) mice compared with WT animals as opposed to the liver, where expression of DNL genes was repressed in LXRαß(-/-) mice. We thus conclude that absence of LXRs stimulates DNL in adipose tissue, but suppresses DNL in the liver, demonstrating opposite roles of LXR in DNL regulation in these two tissues. These results show tissue-specific regulation of LXR activity, a crucial finding for drug development.

CIDEA interacts with liver X receptors in white fat cells.

Cell death-inducing DNA fragmentation factor alpha-like effector A (CIDEA) is endogenously expressed in human but not rodent white adipocytes. We performed a bioinformatic analysis of the human CIDEA sequence and found conserved amino-acid motifs involved in binding to nuclear receptors. Protein-protein binding experiments and transactivation assays confirmed that CIDEA binds to liver X receptors and regulates their activity in vitro. Cell fractionation demonstrated that CIDEA localizes to both the cytoplasm and the nucleus in human white adipocytes. The interaction between CIDEA and nuclear receptors could therefore be of importance for the regulation of metabolic processes in human adipose tissue.

Twist1 in human white adipose tissue and obesity.

CONTEXT: Twist1 is a transcription factor implicated in the regulation of TNFα signaling and was recently shown to be highly expressed in both human and murine adipose tissue, but its role in obesity is unknown. OBJECTIVE: Our objective was to assess the expression of twist1 in human white adipose tissue (WAT), its relationship to obesity and insulin sensitivity, and how it modifies TNFα-mediated inflammation in adipocytes. PROCEDURE: Twist mRNA levels were measured in WAT from 130 nonobese and obese subjects, and its relation to clinical parameters was assessed. Twist1 expression was measured before and after weight loss as well as in different adipose regions. Human in vitro differentiated adipocytes were treated with TNFα under control conditions or after twist1 gene silencing by RNA interference. Gene expression and secretion of proinflammatory proteins were measured. RESULTS: Twist1 expression was low in obese subjects and increased after weight loss. Twist1 mRNA levels correlated with adiponectin levels and inversely with insulin resistance as well as adipocyte volume (P < 0.001 for all). Low twist1 expression associated with a hypertrophic adipose tissue and high secretion of TNFα and monocyte chemoattractant protein-1 from WAT. Finally, twist1 silencing in human adipocytes enhanced TNFα-induced monocyte chemoattractant protein-1 expression and secretion, which was paralleled by an increase in the mRNA expression of the nuclear factor-κB gene RelA. CONCLUSIONS: Low twist1 expression in human WAT correlates with obesity and an insulin-resistant phenotype, which may be mediated by an increased sensitivity to the proinflammatory effect of TNFα.

Mapping of the fibroblast growth factors in human white adipose tissue.

CONTEXT: Fibroblast growth factors (FGFs) regulate the development of white adipose tissue (WAT). However, the secretion and cellular origin of individual FGFs in WAT as well as the influence of obesity are unknown. OBJECTIVE: Our objective was to map FGFs in human sc WAT, the cellular source, and association with obesity. DESIGN: Secretion, mRNA, and circulatory levels of FGFs in human abdominal sc WAT from nonobese and obese donors were examined by microarray, real-time quantitative PCR, and ELISA. The activity of FGFs in cultured human adipocytes was determined by phosphorylation assays. RESULTS: Expression of five FGFs (FGF1, FGF2, FGF7, FGF9, and FGF18) and FGF homologous factor (FHF2) was identified in WAT. Only FGF1 was released in a time-dependent manner from sc WAT, and fat cells were the major source of FGF1 secretion. FGF1 expression increased and FGF2 decreased during adipocyte differentiation. Furthermore, FGF1 was not secreted into the circulation. Although FGF1 levels were 2-fold increased in obesity, they were unaltered by weight reduction. Only FGF1 and FGF2 induced a marked concentration-dependent phosphorylation of p44/42 in cultured human adipocytes. CONCLUSIONS: Of the investigated FGFs, only FGF1 is secreted from sc WAT and predominantly so from the adipocyte fraction. The activity in adipocyte cultures and lack of secretion into the circulation suggest that FGF1 acts as an auto- or paracrine factor. FGF1 levels are increased in obesity but unaffected by weight reduction, suggesting a primary defect in obese individuals. In conclusion, FGF1 may play a superior role among the FGFs in sc WAT and obesity development.

Role of Receptor-Interacting Protein 140 in human fat cells.

BACKGROUND: Mice lacking Receptor-interacting protein 140 (RIP140) have reduced body fat which at least partly is mediated through increased lipid and glucose metabolism in adipose tissue. In humans, RIP140 is lower expressed in visceral white adipose tissue (WAT) of obese versus lean subjects. We investigated the role of RIP140 in human subcutaneous WAT, which is the major fat depot of the body. METHODS: Messenger RNA levels of RIP140 were measured in samples of subcutaneous WAT from women with a wide variation in BMI and in different human WAT preparations. RIP140 mRNA was knocked down with siRNA in in vitro differentiated adipocytes and the impact on glucose transport and mRNA levels of target genes determined. RESULTS: RIP140 mRNA levels in subcutaneous WAT were decreased among obese compared to lean women and increased by weight-loss, but did not associate with mitochondrial DNA copy number. RIP140 expression increased during adipocyte differentiation in vitro and was higher in isolated adipocytes compared to corresponding pieces of WAT. Knock down of RIP140 increased basal glucose transport and mRNA levels of glucose transporter 4 and uncoupling protein-1. CONCLUSIONS: Human RIP140 inhibits glucose uptake and the expression of genes promoting energy expenditure in the same fashion as the murine orthologue. Increased levels of human RIP140 in subcutaneous WAT of lean subjects may contribute to economize on energy stores. By contrast, the function and expression pattern does not support that RIP140 regulate human obesity.

A possible inflammatory role of twist1 in human white adipocytes.

OBJECTIVE: Twist1 is a transcription factor that is highly expressed in murine brown and white adipose tissue (WAT) and negatively regulates fatty acid oxidation in mice. The role of twist1 in WAT is not known and was therefore examined. RESEARCH DESIGN AND METHODS: The expression of twist1 was determined by quantitative real-time PCR in different tissues and in different cell types within adipose tissue. The effect of twist1 small interfering RNA on fatty acid oxidation, lipolysis, adipokine secretion, and mRNA expression was determined in human adipocytes. The interaction between twist1 and specific promoters in human adipocytes was investigated by chromatin immunoprecipitation (ChIP) and reporter assays. RESULTS: Twist1 was highly expressed in human WAT compared with a set of other tissues and found predominantly in adipocytes. Twist1 levels increased during in vitro differentiation of human preadipocytes. Gene silencing of twist1 in human white adipocytes had no effect on lipolysis or glucose transport. Unexpectedly, and in contrast with results in mice, twist1 RNA interference reduced fatty acid oxidation. Furthermore, the expression and secretion of the inflammatory factors tumor necrosis factor-alpha, interleukin-6, and monocyte chemoattractant protein-1 were downregulated by twist1 silencing. ChIP and reporter assays confirmed twist1 interaction with the promoters of these genes. CONCLUSIONS: Twist1 may play a role in inflammation of human WAT because it can regulate the expression and secretion of inflammatory adipokines via direct transcriptional effects in white adipocytes. Furthermore, twist1 may, in contrast to findings in mice, be a positive regulator of fatty acid oxidation in human white adipocytes.

Activation of liver X receptor regulates substrate oxidation in white adipocytes.

Liver X receptors (LXRs) are nuclear receptors with established roles in cholesterol, lipid, and carbohydrate metabolism, although their function in adipocytes is not well characterized. Increased adipose tissue mass in obesity is associated with increased adipocyte lipolysis. Fatty acids (FA) generated by lipolysis can be oxidized by mitochondrial beta-oxidation, reesterified, or released from the adipocyte. The latter results in higher circulating levels of free FAs, in turn causing obesity-related metabolic complications. However, mitochondrial beta-oxidation can at least in part counteract an increased output of FA into circulation. In this study, we provide evidence that activation of LXRs up-regulates mitochondrial beta-oxidation in both human and murine white adipocytes. We also show that the expression of a kinase regulating the cellular fuel switch, pyruvate dehydrogenase kinase 4 (PDK4), is up-regulated by the LXR agonist GW3965 in both in vitro differentiated human primary adipocytes and differentiated murine 3T3-L1 cells. Moreover, activation of LXR causes PDK4-dependent phosphorylation of the pyruvate dehydrogenase complex, thereby decreasing its activity and attenuating glucose oxidation. The specificity of the GW3965 effect on oxidation was confirmed by RNA interference targeting LXRs. We propose that LXR has an important role in the regulation of substrate oxidation and the switch between lipids and carbohydrates as cellular fuel in both human and murine white adipocytes.