Lack of macrophage fatty-acid-binding protein aP2 protects mice deficient in apolipoprotein E against atherosclerosis.

The adipocyte fatty-acid-binding protein, aP2, has an important role in regulating systemic insulin resistance and lipid metabolism. Here we demonstrate that aP2 is also expressed in macrophages, has a significant role in their biological responses and contributes to the development of atherosclerosis. Apolipoprotein E (ApoE)-deficient mice also deficient for aP2 showed protection from atherosclerosis in the absence of significant differences in serum lipids or insulin sensitivity. aP2-deficient macrophages showed alterations in inflammatory cytokine production and a reduced ability to accumulate cholesterol esters when exposed to modified lipoproteins. Apoe-/- mice with Ap2+/+ adipocytes and Ap2-/- macrophages generated by bone-marrow transplantation showed a comparable reduction in atherosclerotic lesions to those with total aP2 deficiency, indicating an independent role for macrophage aP2 in atherogenesis. Through its distinct actions in adipocytes and macrophages, aP2 provides a link between features of the metabolic syndrome and could be a new therapeutic target for the prevention of atherosclerosis.

Altered tumor necrosis factor-alpha (TNF-alpha) processing in adipocytes and increased expression of transmembrane TNF-alpha in obesity.

Tumor necrosis factor-alpha (TNF-alpha) is synthesized as a 26-kDa transmembrane protein (mTNF-alpha), which may present on the cell surface or be processed to release the 17-kDa soluble form (sTNF-alpha). Because regulation of this ectodomain shedding might be critical in the generation of systemic versus local cytokine responses, we examined the rate of mTNF-alpha processing in adipocytes and its regulation in obesity. Here, we demonstrate that the 26-kDa mTNF-alpha is present in adipose tissue and that its production is significantly increased in different rodent obesity models as well as in obese humans. There was no apparent deficiency in the level of the major TNF-alpha converting enzyme in adipose tissue to account for the excess amount of mTNF-alpha produced in obesity. However, experiments in cultured fat cells stably expressing TNF-alpha demonstrated a significantly decreased rate of TNF-alpha cleavage in differentiated adipocytes compared with preadipocytes. Thus, a decreased processing rate of mTNF-alpha in mature adipocytes combined with an increase in TNF-alpha production may be a potential mechanism resulting in elevated membrane-associated TNF-alpha in adipose tissue in obesity.

Role of the fatty acid binding protein mal1 in obesity and insulin resistance.

The metabolic syndrome is a cluster of metabolic and inflammatory abnormalities including obesity, insulin resistance, type 2 diabetes, hypertension, dyslipidemia, and atherosclerosis. The fatty acid binding proteins aP2 (fatty acid binding protein [FABP]-4) and mal1 (FABP5) are closely related and both are expressed in adipocytes. Previous studies in aP2-deficient mice have indicated a significant role for aP2 in obesity-related insulin resistance, type 2 diabetes, and atherosclerosis. However, the biological functions of mal1 are not known. Here, we report the generation of mice with targeted null mutations in the mal1 gene as well as transgenic mice overexpressing mal1 from the aP2 promoter/enhancer to address the role of this FABP in metabolic regulation in the presence or absence of obesity. To address the role of the second adipocyte FABP in metabolic regulation in the presence and deficiency of obesity, absence of mal1 resulted in increased systemic insulin sensitivity in two models of obesity and insulin resistance. Adipocytes isolated from mal1-deficient mice also exhibited enhanced insulin-stimulated glucose transport capacity. In contrast, mice expressing high levels of mal1 in adipose tissue display reduced systemic insulin sensitivity. Hence, our results demonstrate that mal1 modulates adipose tissue function and contributes to systemic glucose metabolism and constitutes a potential therapeutic target in insulin resistance.

Interaction of tumor necrosis factor-alpha- and thiazolidinedione-regulated pathways in obesity.

Thiazolidinediones (TZDs) are potent insulin-sensitizing compounds and high-affinity ligands for the transcription factor peroxisomal proliferator-activated receptor gamma. The mechanism through which TZDs improve insulin sensitivity, however, is not clear. In this study, we asked whether the ability of TZD to suppress and antagonize TNF alpha is an underlying mechanism for its molecular and physiological effects, using obese (ob/ob) mice lacking TNF alpha function. We found that the lipid-lowering effects of TZD are completely independent of TNF alpha suppression, and the insulin-sensitizing effects of TZD are partially independent. TZD treatment improved insulin sensitivity in ob/ob mice both with and without functional TNF alpha, albeit with different absolute potency. To characterize the potential interdependency of TZD- and TNF alpha-regulated pathways at the molecular level, we also performed four-way transcriptional profiling of white adipose tissue of TZD- and vehicle-treated ob/ob mice, with and without TNF alpha function. The majority of metabolic genes identified were regulated independent of the presence of TNF alpha, whereas most effects on inflammatory mediators were dependent on TNF alpha. This study demonstrates that the insulin-sensitizing action of TZD occurs partially through TNF-independent mechanisms, although a subset of the molecular effects of TZD treatment in adipose tissue depends on TNF alpha.

Exclusive action of transmembrane TNF alpha in adipose tissue leads to reduced adipose mass and local but not systemic insulin resistance.

Aberrant TNF alpha expression in adipocytes is a molecular mechanism by which insulin action is modulated in adipose tissue. While this might be a compensatory response to limit adipose expansion, neither the mechanisms underlying this local effect nor its systemic biological consequences have been studied. It is also not clear whether TNF alpha-induced insulin resistance in adipocyte alone is responsible for systemic insulin resistance in the absence of obesity. In a transgenic mouse model deficient in endogenous TNF alpha, we demonstrate that specific expression of the transmembrane TNF alpha (mTNF alpha) in adipocytes leads to decreased whole body adipose mass, and local, but not systemic insulin resistance. These data demonstrate that exclusive action of TNF alpha in adipose tissue strongly inhibits insulin action at this site and leads to reduced adiposity in mice. However, this isolated adipocyte insulin resistance in the context of reduced fat mass and/or the absence of obesity is insufficient to alter systemic glucose homeostasis.

A central role for JNK in obesity and insulin resistance.

Obesity is closely associated with insulin resistance and establishes the leading risk factor for type 2 diabetes mellitus, yet the molecular mechanisms of this association are poorly understood. The c-Jun amino-terminal kinases (JNKs) can interfere with insulin action in cultured cells and are activated by inflammatory cytokines and free fatty acids, molecules that have been implicated in the development of type 2 diabetes. Here we show that JNK activity is abnormally elevated in obesity. Furthermore, an absence of JNK1 results in decreased adiposity, significantly improved insulin sensitivity and enhanced insulin receptor signalling capacity in two different models of mouse obesity. Thus, JNK is a crucial mediator of obesity and insulin resistance and a potential target for therapeutics.