Constantinos Christodoulides

Adipogenesis and WNT signalling.

An inability of adipose tissue to expand consequent to exhausted capacity to recruit new adipocytes might underlie the association between obesity and insulin resistance. Adipocytes arise from mesenchymal precursors whose commitment and differentiation along the adipocytic lineage is tightly regulated. These regulatory factors mediate cross-talk between adipose cells, ensuring that adipocyte growth and differentiation are coupled to energy storage demands. The WNT family of autocrine and paracrine growth factors regulates adult tissue maintenance and remodelling and, consequently, is well suited to mediate adipose cell communication. Indeed, several recent reports, summarized in this review, implicate WNT signalling in regulating adipogenesis. Manipulating the WNT pathway to alter adipose cellular makeup, therefore, constitutes an attractive drug-development target to combat obesity-associated metabolic complications.

Expression of the thermogenic nuclear hormone receptor coactivator PGC-1α is reduced in the adipose tissue of morbidly obese subjects

Peroxisome proliferator-activated receptor gamma coactivator 1α (PGC1α) is an accessory protein which can potentiate the transcriptional activation function of many nuclear hormone receptors. Its tissue distribution and physiological studies suggest that its principal in vivo roles are to promote cold-induced thermogenesis, mitochondrial biogenesis, hepatic gluconeogenesis, and fatty acid β-oxidation. It is expressed in the white adipose tissue of both humans and rodents, and in rodents it has been suggested to mediate in part the leptin-induced conversion of white adipocytes from fat storing to fat oxidising cells. In this study, quantitative real-time PCR has been used in human tissue to demonstrate that (1) PGC1α mRNA levels in subcutaneous fat are three-fold lower in morbidly obese than in slim subjects; (2) there are no differences in PGC1α mRNA between omental and subcutaneous mature adipocytes; (3) there is a robust induction of PGC1α expression during subcutaneous human preadipocyte differentiation ex vivo. Whether low PGC1α expression is a prelude to the development of obesity, or a consequence of that obesity, attempts to upregulate endogenous white adipose tissue expression may prove a valuable new avenue to explore in obesity therapy.

The Wnt antagonist Dickkopf-1 and its receptors are coordinately regulated during early human adipogenesis

Secretion of Wnts by adipose cells has an important role in the control of murine adipogenesis. We present the first evidence that a Wnt antagonist, Dickkopf 1 (Dkk1), is secreted by human preadipocytes and promotes adipogenesis. DKK1 mRNA increases six hours after onset of human adipogenesis and this is followed by an increase in Dkk1 protein. With further differentiation, the mRNA and protein levels progressively decline such that they are undetectable in mature adipocytes. The transient induction in DKK1 correlates with downregulation of cytoplasmic and nuclear β-catenin levels, this being a surrogate marker of canonical Wnt signalling, and Wnt/β-catenin transcriptional activity. In addition, constitutive expression of Dkk1 in 3T3-L1 preadipocytes promotes their differentiation, further supporting the functional significance of increased Dkk1 levels during human adipogenesis. Concomitant downregulation of the Dkk1 receptors LRP5 and LRP6 is likely to potentiate the ability of Dkk1 to inhibit Wnt signalling and promote differentiation. Notably, Dkk1 is not expressed in primary murine preadipocytes or cell lines. The involvement of Dkk1 in human but not murine adipogenesis indicates that inter-species differences exist in the molecular control of this process. Given the public health importance of disorders of adipose mass, further knowledge of the pathways involved specifically in human adipocyte differentiation might ultimately be of clinical relevance.

Circulating Fibroblast Growth Factor 21 Is Induced by Peroxisome Proliferator-Activated Receptor Agonists But Not Ketosis in Man

CONTEXT Murine fibroblast growth factor (FGF) 21 is a nutritionally regulated hormone secreted by the liver principally in response to peroxisome proliferator-activated receptor-alpha (PPAR alpha) activation, which plays a critical role in regulating metabolism during ketosis. FGF21 is also a PPAR gamma target gene in mouse adipose tissue. Little information is available on FGF21 functions in humans. OBJECTIVE The aim of the study was to measure plasma FGF21 during fasting, ketogenic diet, and PPAR agonist treatment in humans. DESIGN AND SETTING We conducted a prospective study involving three patient groups at two university hospitals. PATIENTS Eight healthy male volunteers underwent a 48-h period of starvation followed by 24-h refeeding (group 1); seven obese individuals were allocated to a low-carbohydrate diet for 3 months (group 2); and three groups of healthy, overweight or obese male volunteers received treatment with a PPAR alpha (20 microg/d GW590735) (n=6), PPAR delta (10 mg/d GW501516) (n=6), or PPAR gamma agonist (rosiglitazone) (n=10) for 2 wk (group 3). MAIN OUTCOME MEASURES Fasting plasma FGF21 and serum 3-hydroxybutyrate were measured. RESULTS There was no significant variation in human plasma FGF21 during fasting and refeeding. A 3-month ketogenic diet was associated with a 42% decline in plasma FGF21 levels. Circulating FGF21 increased significantly in response to treatment with PPAR alpha (39%) and PPAR delta (32%), but not PPAR gamma agonists. CONCLUSION FGF21 does not play a major role in regulating the fasting response or ketosis in man. However, plasma FGF21 is elevated in response to pharmacological activation of PPAR alpha and PPAR delta and may contribute to the beneficial metabolic effects observed in response to pharmacotherapy with these compounds.

PPARs and adipocyte function.

For long viewed as passive lipid storage depots, adipocytes are now recognised as key players in the pathogenesis of insulin resistance and metabolic disease. In parallel, the last two decades of research have seen the emergence of transcription factors of the peroxisome proliferator-activated receptor (PPAR) family as central regulators of lipid and glucose homeostasis and molecular targets for drugs to treat hyper-lipidaemia and type 2 diabetes mellitus. In this review we discuss the characteristics of PPARs and the role of the different isotypes in adipocyte biology.

The role of adiponectin in human vascular physiology

Adiponectin (ApN) is an adipose tissue-derived hormone which is involved in a wide variety of physiological processes including energy metabolism, inflammation, and vascular physiology via actions on a broad spectrum of target organs including liver, skeletal muscle, and vascular endothelium. Besides possessing insulin sensitizing and anti-inflammatory properties ApN also exerts a pivotal role in vascular protection through activation of multiple intracellular signaling cascades. Enhancement of nitric oxide generation and attenuation of reactive oxygen species production in endothelial cells along with reduced vascular smooth muscle cell proliferation and migration constitute some of ApNs vasoprotective actions. Additionally, recent data indicate that ApN has direct myocardio-protective effects. Decreased plasma ApN levels are implicated in the pathogenesis of the metabolic syndrome and atherosclerosis and may serve as a diagnostic and prognostic biomarker as well as a rational pharmaco-therapeutic target to treat these disorders. This review article summarizes recent work on the cardiovascular actions of ApN.

ETO/MTG8 Is an Inhibitor of C/EBPβ Activity and a Regulator of Early Adipogenesis

ABSTRACT The putative transcriptional corepressor ETO/MTG8 has been extensively studied due to its involvement in a chromosomal translocation causing the t(8;21) form of acute myeloid leukemia. Despite this, the role of ETO in normal physiology has remained obscure. Here we show that ETO is highly expressed in preadipocytes and acts as an inhibitor of C/EBPβ during early adipogenesis, contributing to its characteristically delayed activation. ETO prevents both the transcriptional activation of the C/EBPα promoter by C/EBPβ and its concurrent accumulation in centromeric sites during early adipogenesis. ETO expression rapidly reduces after the initiation of adipogenesis, and this is essential to the normal induction of adipogenic gene expression. These findings define, for the first time, a molecular role for ETO in normal physiology as an inhibitor of C/EBPβ and a novel regulator of early adipogenesis.

Wnt signaling in cardiovascular physiology

Wnt signaling pathways play a key role in cardiac development, angiogenesis, and cardiac hypertrophy; emerging evidence suggests that they are also involved in the pathophysiology of atherosclerosis. Specifically, an important role for Wnts has been described in the regulation of endothelial inflammation, vascular calcification, and mesenchymal stem cell differentiation. Wnt signaling also induces monocyte adhesion to endothelial cells and is crucial for the regulation of vascular smooth-muscle cell (VSMC) behavior. We discuss how the Wnt pathways are implicated in vascular biology and outline the role of Wnt signaling in atherosclerosis. Dissecting Wnt pathways involved in atherogenesis and cardiovascular disease may provide crucial insights into novel mechanisms with therapeutic potential for atherosclerosis.

Dact1, a Nutritionally Regulated Preadipocyte Gene, Controls Adipogenesis by Coordinating the Wnt/β-Catenin Signaling Network

OBJECTIVE—Wnt signaling inhibits adipogenesis, but its regulation, physiological relevance, and molecular effectors are poorly understood. Here, we identify the Wnt modulator Dapper1/Frodo1 (Dact1) as a new preadipocyte gene involved in the regulation of murine and human adipogenesis. RESEARCH DESIGN AND METHODS—Changes in Dact1 expression were investigated in three in vitro models of adipogenesis. In vitro gain- and loss-of-function studies were used to investigate the mechanism of Dact1 action during adipogenesis. The in vivo regulation of Dact1 and Wnt/β-catenin signaling were investigated in murine models of altered nutritional status, of pharmacological stimulation of in vivo adipogenesis, and during the development of dietary and genetic obesity. RESULTS—Dact1 is a preadipocyte gene that decreases during adipogenesis. However, Dact1 knockdown impairs adipogenesis through activation of the Wnt/β-catenin signaling pathway, and this is reversed by treatment with the secreted Wnt antagonist, secreted Frizzled-related protein 1 (Sfrp1). In contrast, constitutive Dact1 overexpression promotes adipogenesis and confers resistance to Wnt ligand-induced antiadipogenesis through increased expression of endogenous Sfrps and reduced expression of Wnts. In vivo, in white adipose tissue, Dact1 and Wnt/β-catenin signaling also exhibit coordinated expression profiles in response to altered nutritional status, in response to pharmacological stimulation of in vivo adipogenesis, and during the development of dietary and genetic obesity. CONCLUSIONS—Dact1 regulates adipogenesis through coordinated effects on gene expression that selectively alter intracellular and paracrine/autocrine components of the Wnt/β-catenin signaling pathway. These novel insights into the molecular mechanisms controlling adipose tissue plasticity provide a functional network with therapeutic potential against diseases, such as obesity and associated metabolic disorders.

Secreted frizzled-related protein 1 regulates adipose tissue expansion and is dysregulated in severe obesity.

Aim:The Wnt/β-catenin signaling network offers potential targets to diagnose and uncouple obesity from its metabolic complications. In this study, we investigate the role of the Wnt antagonist, secreted frizzled-related protein 1 (SFRP1), in promoting adipogenesis in vitro and adipose tissue expansion in vivo.Methods:We use a combination of human and murine, in vivo and in vitro models of adipogenesis, adipose tissue expansion and obesity-related metabolic syndrome to profile the involvement of SFRP1.Results:SFRP1 is expressed in both murine and human mature adipocytes. The expression of SFRP1 is induced during in vitro adipogenesis, and SFRP1 is preferentially expressed in mature adipocytes in human adipose tissue. Constitutive ectopic expression of SFRP1 is proadipogenic and inhibits the Wnt/β-catenin signaling pathway. In vivo endogenous levels of adipose SFRP1 are regulated in line with proadipogenic states. However, in longitudinal studies of high-fat-diet-fed mice, we observed a dynamic temporal but biphasic regulation of endogenous SFRP1. In agreement with this profile, we observed that SFRP1 expression in human tissues peaks in patients with mild obesity and gradually falls in morbidly obese subjects.Conclusions:Our results suggest that SFRP1 is an endogenous modulator of Wnt/β-catenin signaling and participates in the paracrine regulation of human adipogenesis. The reduced adipose expression of SFRP1 in morbid obesity and its knock-on effect to prevent further adipose tissue expansion may contribute to the development of metabolic complications in these individuals.