Antagonism of peroxisome proliferator-activated receptor γ prevents high-fat diet-induced obesity in vivo

Abstract

Peroxisome proliferator-activated receptor γ (PPARγ) has been reported to play an important role to regulate adiposity and insulin sensitivity. It is not clear whether antagonism of PPARγ using a synthetic ligand has significant effects on adipose tissue weight and glucose metabolism in vivo. The aim of this study is to examine the effects of a synthetic PPARγ antagonist (GW9662) on adiposity and glycemic control in high-fat (HF) diet-fed mice. First the properties of GW9662 as a PPARγ antagonist were estimated in vitro. GW9662 displaced [³H]rosiglitazone from PPARγ with K_i values of 13 nM, indicating that the affinity of GW9662 for PPARγ was higher than that of rosiglitazone (110 nM). GW9662 had no effect on PPARγ transactivation in cells expressing human PPARγ. Treatment of 3T3-L1 preadipocytes with GW9662 did not increase aP2 expression or [¹⁴C]acetic acid uptake. GW9662 did not recruit transcriptional cofactors to PPARγ. Limited trypsin digestion of the human PPARγ/GW9662 complex showed patterns of digestion distinct from those of rosiglitazone. This suggests that the binding characteristics between GW9662 and PPARγ are different from those of rosiglitazone. Treatment of HF diet-fed mice with GW9662 revealed that this compound prevented HF diet-induced obesity without affecting food intake. GW9662 suppressed any increase in the amount of visceral adipose tissue, but it did not change HF diet-induced glucose intolerance. These data indicate that antagonism of PPARγ using a synthetic ligand suppresses the increased adiposity observed in HF diet-induced obesity, and that a PPARγ antagonist could possibly be developed as an anti-obesity drug.

Introduction

Adipose tissue plays a central role in regulating the body's energy balance. Adipose tissue helps control energy homeostasis (including food intake), metabolic efficiency, and energy expenditure via the hormones it secretes. The quantity of body fat present in mammals varies greatly. This variability can also be easily observed between individuals of the same species, which highlights the complexity of the interplay of factors that control fat deposition. This huge range of fat mass variation is a phenomenon unlike any other seen in the body. It is determined by both an individual's genetic background and lifestyle factors such as diet and physical activity. Having a significant amount of excess body fat (obesity) is a major health problem that increases the risk of developing diabetes, hypertension, and coronary artery disease [1], [2], [3].

The cellular and molecular mechanisms behind adipocyte differentiation have been studied extensively [4]. A number of key transcription factors that participate in the complex transcriptional cascade during adipocyte differentiation have been identified, including some peroxisome proliferator-activated receptor (PPAR) family proteins (α, δ and γ) [5].

PPARγ is abundantly expressed in adipose tissue, where it is a key regulator of adipocyte differentiation. Thus, PPARγ regulates energy homeostasis in this way [6], [7], [8], [9].

Thiazolidinediones (TZDs), antidiabetics currently used as insulin sensitizers, are well-known synthetic PPARγ ligands in terms of specificity and affinity. TZDs can bind directly to activate PPARγ and stimulate adipocyte differentiation [10], [11], [12], [13]. This activity is well correlated with the ability to lower blood glucose in diabetic mice [14]; however, the physiological role of PPARγ in mature adipocytes and the regulation of insulin sensitivity in vivo remain largely unclear. In vivo deletion of PPARγ function is an effective means of investigating its physiological role. Generation of PPARγ-deficient mice by gene targeting is a useful method for investigating the role of PPARγ in vivo. Since PPARγ knockout is lethal for mouse embryos due to a defect in placental development [15], several groups have generated tissue-specific PPARγ deletion mice using the Cre/lox P system [16], [17], [18], [19], [20], [21].

One of the other methods used to analyze the function of PPARγ in vivo is the dosing of a synthetic PPARγ antagonist. Several groups have reported in vitro and in vivo studies on PPARγ antagonists, but because these studies used a partial antagonist, these reports did not sufficiently elucidate the role of PPARγ[22], [23], [24], [25], [26]. GW9662, however, is a full synthetic PPARγ antagonist. Leesnitzer et al. [27] reported previously that GW9662 suppressed adipocyte differentiation in vitro, and several other in vitro studies showed that GW9662 is a full PPARγ antagonist [28], [29], [30]. Unfortunately, there have been no reports on the in vivo use of GW9662.

In this report we analyzed high-fat (HF) diet mice treated with GW9662 in order to investigate the role of PPARγ in vivo. GW9662 treatment suppressed HF diet-induced obesity, but did not change glucose intolerance. This study provides evidence that PPARγ antagonism prevents the increased adiposity induced by a HF diet.