[Show abstract][Hide abstract] ABSTRACT: We examined mouse models with altered adipocyte expression of mitoNEET, a protein residing in the mitochondrial outer membrane, to probe its impact on mitochondrial function and subsequent cellular responses. We found that overexpression of mitoNEET enhances lipid uptake and storage, leading to an expansion of the mass of adipose tissue. Despite the resulting massive obesity, benign aspects of adipose tissue expansion prevail, and insulin sensitivity is preserved. Mechanistically, we also found that mitoNEET inhibits mitochondrial iron transport into the matrix and, because iron is a rate-limiting component for electron transport, lowers the rate of β-oxidation. This effect is associated with a lower mitochondrial membrane potential and lower levels of reactive oxygen species-induced damage, along with increased production of adiponectin. Conversely, a reduction in mitoNEET expression enhances mitochondrial respiratory capacity through enhanced iron content in the matrix, ultimately corresponding to less weight gain on a high-fat diet. However, this reduction in mitoNEET expression also causes heightened oxidative stress and glucose intolerance. Thus, manipulation of mitochondrial function by varying mitoNEET expression markedly affects the dynamics of cellular and whole-body lipid homeostasis.
[Show abstract][Hide abstract] ABSTRACT: Macrophages are more abundant in adipose tissue from obese individuals than from those of normal weight and may contribute to the metabolic consequences of obesity by producing various circulating factors. One of these factors is plasminogen activator inhibitor-1 (PAI-1), which contributes to both atherosclerosis and insulin resistance. Because nutritional factors appear to regulate PAI-1 expression, we hypothesized that exposure to fatty acids and adipocyte secretory products could stimulate production of PAI-1 by adipose macrophages. Increased free fatty acid (FFA) concentrations in blood for 5 hours in nondiabetic, overweight subjects markedly suppressed insulin-stimulated glucose uptake and raised circulating PAI-1 concentrations, with a concomitant increase in the expression of the PAI-1 gene in adipose tissue. FFAs also rapidly increased PAI-1 gene expression in adipose macrophages and PAI-1 protein immunofluorescence surrounding these cells. By contrast, PAI-1 expression in circulating monocytes was very low and was not affected by raising the concentration of FFAs. Medium from cultured adipocytes stimulated PAI-1 expression in cultured macrophages and potentiated the increase in PAI-1 messenger RNA expression in response to FFAs. Together, our data suggest that adipocyte-derived factors prime adipose macrophages so that they respond to nutritional signals (FFAs) by releasing a key inflammatory adipokine, PAI-1.
No preview · Article · Feb 2010 · Science translational medicine
[Show abstract][Hide abstract] ABSTRACT: alpha1-Acid glycoprotein (alpha1-AGP) is an acute phase protein that can potentiate cytokine secretion by mononuclear cells and may induce thrombosis by stabilizing the inhibitory activity of plasminogen activator inhibitor-1. Thus, alpha1-AGP may promote pathobiologies associated with type 2 diabetes mellitus (T2DM) including insulin resistance and cardiovascular disease. Here, we demonstrate that antidiabetic peroxisome proliferator-activated receptor gamma (PPARgamma) agonists inhibited expression of 3T3-L1 adipocyte alpha1-AGP in a concentration- and time-dependent manner via an apparent PPARgamma-mediated mechanism. As a result, synthesis and secretion of the glycoprotein was reduced. While PPARgamma agonist regulation of genes with functional peroxisome proliferator response elements in their promoter such as phosphoenolpyruvate carboxykinase were unaffected when cellular protein synthesis was inhibited, downregulation of alpha1-AGP mRNA was ablated thereby supporting the proposition that PPARgamma activation inhibits alpha1-AGP expression indirectly. These results suggest a potential novel adipocytic mechanism by which PPARgamma agonists may ameliorate T2DM-associated insulin resistance and cardiovascular disease.
Preview · Article · Jul 2007 · Cell Biology International
[Show abstract][Hide abstract] ABSTRACT: Adipose tissue plays an active role in normal metabolic homeostasis as well as in the development of human disease. Beyond
its obvious role as a depot for triglycerides, adipose tissue controls energy expenditure through secretion of several factors.
Little attention has been given to the role of adipocytes in the pathogenesis of Chagas disease and the associated metabolic
alterations. Our previous studies have indicated that hyperglycemia significantly increases parasitemia and mortality in mice
infected with Trypanosoma cruzi. We determined the consequences of adipocyte infection in vitro and in vivo. Cultured 3T3-L1 adipocytes can be infected with high efficiency. Electron micrographs of infected cells revealed a large
number of intracellular parasites that cluster around lipid droplets. Furthermore, infected adipocytes exhibited changes in
expression levels of a number of different adipocyte-specific or adipocyte-enriched proteins. The adipocyte is therefore an
important target cell during acute Chagas disease. Infection of adipocytes by T. cruzi profoundly influences the pattern of adipokines. During chronic infection, adipocytes may represent an important long-term
reservoir for parasites from which relapse of infection can occur. We have demonstrated that acute infection has a unique
metabolic profile with a high degree of local inflammation in adipose tissue, hypoadiponectinemia, hypoglycemia, and hypoinsulinemia
but with relatively normal glucose disposal during an oral glucose tolerance test.
[Show abstract][Hide abstract] ABSTRACT: The interactions of transformed cells with the surrounding stromal cells are of importance for tumor progression and metastasis. The relevance of adipocyte-derived factors to breast cancer cell survival and growth is well established. However, it remains unknown which specific adipocyte-derived factors are most critical in this process. Collagen VI is abundantly expressed in adipocytes. Collagen(-/-) mice in the background of the mouse mammary tumor virus/polyoma virus middle T oncogene (MMTV-PyMT) mammary cancer model demonstrate dramatically reduced rates of early hyperplasia and primary tumor growth. Collagen VI promotes its growth-stimulatory and pro-survival effects in part by signaling through the NG2/chondroitin sulfate proteoglycan receptor expressed on the surface of malignant ductal epithelial cells to sequentially activate Akt and beta-catenin and stabilize cyclin D1. Levels of the carboxyterminal domain of collagen VIalpha3, a proteolytic product of the full-length molecule, are dramatically upregulated in murine and human breast cancer lesions. The same fragment exerts potent growth-stimulatory effects on MCF-7 cells in vitro. Therefore, adipocytes play a vital role in defining the ECM environment for normal and tumor-derived ductal epithelial cells and contribute significantly to tumor growth at early stages through secretion and processing of collagen VI.
Full-text · Article · Jun 2005 · Journal of Clinical Investigation
[Show abstract][Hide abstract] ABSTRACT: Hyperglycemia is a major independent risk factor for diabetic macrovascular disease. The consequences of exposure of endothelial
cells to hyperglycemia are well established. However, little is known about how adipocytes respond to both acute as well as
chronic exposure to physiological levels of hyperglycemia. Here, we analyze adipocytes exposed to hyperglycemia both in vitro as well as in vivo. Comparing cells differentiated at 4 mm to cells differentiated at 25 mm glucose (the standard differentiation protocol) reveals severe insulin resistance in cells exposed to 25 mm glucose. A global assessment of transcriptional changes shows an up-regulation of a number of mitochondrial proteins. Exposure
to hyperglycemia is associated with a significant induction of reactive oxygen species (ROS), both in vitro as well as in vivo in adipocytes isolated from streptozotocin-treated hyperglycemic mice. Furthermore, hyperglycemia for a few hours in a clamped
setting will trigger the induction of a pro-inflammatory response in adipose tissue from rats that can effectively be reduced
by co-infusion of N-acetylcysteine (NAC). ROS levels in 3T3-L1 adipocytes can be reduced significantly with pharmacological agents that lower
the mitochondrial membrane potential, or by overexpression of uncoupling protein 1 or superoxide dismutase. In parallel with
ROS, interleukin-6 secretion from adipocytes is significantly reduced. On the other hand, treatments that lead to a hyperpolarization
of the mitochondrial membrane, such as overexpression of the mitochondrial dicarboxylate carrier result in increased ROS formation
and decreased insulin sensitivity, even under normoglycemic conditions. Combined, these results highlight the importance ROS
production in adipocytes and the associated insulin resistance and inflammatory response.
Preview · Article · Feb 2005 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: The adipocyte exerts an important role in energy homeostasis, both as depot for energy-rich triglycerides and as a source for metabolic hormones. Adipocytes also contribute to inflammation and the innate immune response. Although it can be physiologically beneficial to combine these two functions in a single cell type under some circumstances, the proinflammatory signals emanating from adipocytes in the obese state can have local and systemic effects that promote atherosclerosis and insulin resistance. The transcriptional machinery in the adipocyte that mediates these pro-inflammatory responses has remained poorly characterized to date. In particular, no information is currently available on the NF-kappaB family of transcription factors. Here, we show that adipogenesis is associated with changes in amount and subunit composition of the NF-kappaB complexes. NF-kappaB subunits p65 (RelA), p68 (RelB), and IkappaB are upregulated during fat cell differentiation. Correspondingly, basal NF-kappaB nuclear gel shift and luciferase reporter assays are induced in parallel during differentiation. Surprisingly, endotoxin sensitivity of the classical NF-kappaB pathway is substantially delayed and attenuated despite increased overall inflammatory response in the mature adipocyte, as judged by induction of IL-6 and TNF-alpha. As a reflection of the constitutively elevated NF-kappaB activity in the mature adipocyte, adipocytes (but not preadipocytes) exert a strong inflammatory stimulus on macrophages in vitro, suggesting a cross talk between adipocytes and interstitial macrophages in adipose tissue in vivo. These effects are mediated by a secretory product of adipocytes that is unlikely to be IL-6 or TNF-alpha.
No preview · Article · Jan 2005 · AJP Endocrinology and Metabolism
[Show abstract][Hide abstract] ABSTRACT: Adiponectin is a plasma protein expressed exclusively in adipose tissue. Adiponectin levels are linked to insulin sensitivity, but a direct effect of chronically elevated adiponectin on improved insulin sensitivity has not yet been demonstrated. We identified a dominant mutation in the collagenous domain of adiponectin that elevated circulating adiponectin values in mice by 3-fold. Adiponectinemia raised lipid clearance and lipoprotein lipase activity, and suppressed insulin-mediated endogenous glucose production. The induction of adiponectin during puberty and the sexual dimorphism in adult adiponectin values were preserved in these transgenic animals. As a result of elevated adiponectin, serum PRL values and brown adipose mass both increased. The effects on carbohydrate and lipid metabolism were associated with elevated phosphorylation of 5'-AMP-activated protein kinase in liver and elevated expression of peroxisomal proliferator-activated receptor gamma2, caveolin-1, and mitochondrial markers in white adipose tissue. These studies strongly suggest that increasing endogenous adiponectin levels has direct effects on insulin sensitivity and may induce similar physiological responses as prolonged treatment with peroxisomal proliferator-activated receptor gamma agonists.
[Show abstract][Hide abstract] ABSTRACT: Adipocytes are the exclusive or predominant source of several secreted proteins that exert profound effects on systemic carbohydrate and lipid metabolism. Resistin, a 10-kDa adipose tissue specific secretory protein, has recently been implicated in exerting a negative effect on systemic insulin sensitivity. It is, however, not known how resistin mediates this insulin-desensitizing effect or what regulatory mechanisms control resistin expression. Resistin-like molecule-alpha (RELMalpha), a homolog of resistin originally identified by its upregulation in asthmatic lung, is another secreted protein expressed in adipose tissue. The regulation of RELMalpha in adipose tissue and its relationship to resistin expression has not been addressed so far. Here, we demonstrate that the expression of resistin and RELMalpha are similarly regulated in adipose tissue despite the fact that RELMalpha is exclusively expressed in the stromal vascular fraction of adipose tissue and not in adipocytes. Interestingly, this coregulation is limited to adipose tissue as the expression of RELMalpha in lung is independent of metabolic regulation. Additionally, we show that resistin and RELMalpha levels are not subject to regulation by proinflammatory stimuli. Finally, acute hyperglycemia leads to up-regulation of resistin and RELMalpha transcription in various adipose depots.
[Show abstract][Hide abstract] ABSTRACT: Acrp30 is an abundantly expressed secretory protein exclusively synthesized in adipose tissue. Due to the dysregulation in various forms of obesity in humans and mice and its strong structural similarity to TNFalpha, it is currently under study as an important molecule involved in whole body energy homeostasis. Here we describe the sequence of mouse Acrp30 locus, define the intron/exon boundaries and map the gene to the telomere of mouse chromosome 16, syntenic to the human chromosomal locus 3q27. We demonstrate that alternative polyadenylation gives rise to two distinct mRNA species. We also show that Acrp30 expression is induced only at the late stages of mouse embryonic development. Finally, we have characterized the mouse Acrp30 promoter in tissue culture cells. We propose that Acrp30 promoter has the potential to drive strong adipocyte specific heterologous gene expression in transgenic mice.
No preview · Article · Mar 2001 · Biochemical and Biophysical Research Communications
[Show abstract][Hide abstract] ABSTRACT: We report the identification of a novel mouse protein closely related to the family of mitochondrial uncoupling proteins and the oxoglutarate carrier. The cDNA encodes a protein of 287 amino acids that shares all the hallmark features of the mitochondrial transporter superfamily, including six predicted transmembrane domains. It is nearly identical to the sequence recently reported for the rat mitochondrial dicarboxylate carrier (DIC). We find that murine DIC (mDIC) is expressed at very high levels in mitochondria of white adipocytes and is strongly induced in the course of 3T3-L1 adipogenesis. To determine the consequences of the presence of mDIC on the mitochondrial membrane potential, we transiently expressed mDIC in 293-T cells. Overexpression of mDIC leads to significant mitochondrial hyperpolarization. In addition, exposure to cold down-regulates mDIC levels in vivo. In contrast, free fatty acids lead to an up-regulation of mDIC protein in 3T3-L1 adipocytes. This is the first report demonstrating preferential expression in white adipose tissue of any mitochondrial transporter. However, it remains to be determined which metabolic pathways most critically depend on high level expression of mDIC in the adipocyte.
Preview · Article · Jan 2000 · Biochemical Journal