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SUMMARY Obese adipose tissue is characterized by infiltration of macrophages. We and others recently showed that a specific subset of macrophages is recruited to obese adipose and muscle tissue. This subset expresses CD11c and produces high levels of pro-inflammatory cytokines that are linked to the development of obesity-associated insulin resistance. We used a conditional cell ablation system, based on transgenic expression of the diphtheria toxin receptor under the control of the CD11c promoter, to study the effects of depletion of CD11c+ cells in obese mouse models. Our results show that CD11c+ cell depletion results in rapid normalization of insulin sensitivity. Furthermore, CD11c+ cell ablation leads to a marked decrease in inflammatory markers, both locally and systemically, at the level of gene expression and protein levels. Together, these results indicate that these CD11c+ cells are a potential therapeutic target for treatment of obesity-related insulin resistance and type II diabetes.

Obesity-induced insulin resistance is a major factor in the etiology of type 2 diabetes, and Jun kinases (JNKs) are key negative regulators of insulin sensitivity in the obese state. Activation of JNKs (mainly JNK1) in insulin target cells results in phosphorylation of insulin receptor substrates (IRSs) at serine and threonine residues that inhibit insulin signaling. JNK1 activation is also required for accumulation of visceral fat. Here we used reciprocal adoptive transfer experiments to determine whether JNK1 in myeloid cells, such as macrophages, also contributes to insulin resistance and central adiposity. Our results show that deletion of Jnk1 in the nonhematopoietic compartment protects mice from high-fat diet (HFD)-induced insulin resistance, in part through decreased adiposity. By contrast, Jnk1 removal from hematopoietic cells has no effect on adiposity but confers protection against HFD-induced insulin resistance by decreasing obesity-induced inflammation.

Obesity is associated with increased macrophage infiltration of adipose tissue, and these macrophages may be an important component of the chronic inflammatory response playing a crucial role in the development of insulin resistance. This prompts the question as to how macrophages infiltrate obese adipose tissue. In this issue of the JCI, Weisberg et al. show the importance of C-C motif chemokine receptor 2 (CCR2) in macrophage recruitment to adipose tissue and the development of obesity and its complications (see the related article beginning on page 115).

In the present study, the interaction between the endocytic receptor low density lipoprotein receptor-related protein (LRP) and coagulation factor VIII (FVIII) was investigated. Using purified components, FVIII was found to bind to LRP in a reversible and dose-dependent manner (K d Ϸ 60 nM). The interaction appeared to be specific because the LRP antagonist receptor-associated protein readily inhibited binding of FVIII to LRP (IC 50 ; K d Ϸ 50 nM). Furthermore, experiments using recombinant FVIII C2 domain showed that this domain contributes to the interaction with LRP. In contrast, no association of FVIII heavy chain to LRP could be detected under the same experimental conditions. Collectively, our data demonstrate that in vitro LRP is able to bind FVIII at the cell surface and to mediate its transport to the intracellular degradation pathway. FVIII-LRP interaction involves the FVIII light chain, and FVIII-vWF complex formation plays a regulatory role in LRP binding. Our findings may explain the beneficial effect of vWF on the in vivo survival of FVIII.Low density lipoprotein receptor-related protein (LRP), 1 also known as ␣2-macroglobulin receptor, is a member of the low density lipoprotein receptor family of endocytic receptors (for a review, see Refs. 1 and 2). It consists of a heavy chain and a light chain, which are associated in a noncovalent manner. The 85-kDa light chain comprises the transmembrane and cytoplasmic domains, whereas the ligand binding regions are located within the 515-kDa heavy chain (3). LRP is abundantly present in various tissues such as the liver, placenta, lung, and brain (4) and is expressed in an array of cell types: parenchymal cells, neurons and astrocytes, Leydig cells, smooth muscle cells, monocytes, and fibroblasts (4). Commonly used cell lines such as monkey kidney COS cells and Chinese hamster ovary (CHO) cells also express LRP (5, 6). The function of LRP is to mediate the binding and transport of ligands from the cell surface to the endosomal degradation pathway (1, 2). Binding and internalization of ligands is antagonized by a 39-kDa chaperone protein designated receptor-associated protein (RAP) (7,8). Currently, a wide spectrum of structurally and functionally unrelated ligands involved in a variety of processes such as lipoprotein metabolism, cell growth and migration, and neuronal regeneration (1, 2) has been identified. Furthermore, LRP seems to be linked to the process of blood coagulation. This is apparent from the observations that LRP recognizes thrombin/ antithrombin and factor Xa/␣2-macroglobulin complexes and the Kunitz-type inhibitor tissue factor pathway inhibitor (9 -11). In addition, LRP contributes to down-regulation of tissue factor expression at the surface of monocytes (12). Coagulation factor VIII (FVIII) is the precursor of its activated derivative, which stimulates factor IXa-mediated activation of factor X (for recent reviews, see Refs. 13 and 14). The fact that deficiency or dysfunction of FVIII is associated with severe bleeding tendencies demon...

Although plasminogen activator inhibitor-1 (PAI-1) is known to stimulate cell migration, little is known about underlying mechanisms. We show that both active and inactive (e.g. cleaved) PAI-1 can activate the Jak/Stat signaling system and stimulate cell migration in chemotaxis, haptotaxis, chemokinesis, and wound healing assays. Moreover, antibodies to the LDL receptor-related protein (LRP) and an LRP antagonist (RAP) blocked these motogenic effects of PAI-1, while a PAI-1 mutant that did not bind to LRP failed to activate the Jak/Stat signaling pathway or to stimulate cell migration. PAI-1 had no chemotactic effect on LRP-deficient cells. These results indicate that LRP is a signaling molecule, that it mediates the migration-promoting activity of PAI-1, and that this activity does not require intact, biologically active PAI-1. Activation of this LRP-dependent signaling pathway by PAI-1 may begin to explain how the inhibitor stimulates cell migration in a variety of normal and pathological processes.

The low density lipoprotein receptor-related protein (LRP) is a multifunctional endocytic cell-surface receptor that binds and internalizes a diverse array of ligands. The receptor contains four putative ligand-binding domains, generally referred to as clusters I, II, III, and IV. In this study, soluble recombinant receptor fragments, representing each of the four individual clusters, were used to map the binding sites of a set of structurally and functionally distinct ligands. Using surface plasmon resonance, we studied the binding of these fragments to methylamine-activated ␣ 2 -macroglobulin, pro-urokinase-type plasminogen activator, tissue-type plasminogen activator (t-PA), plasminogen activator inhibitor-1, t-PA⅐plasminogen activator inhibitor-1 complexes, lipoprotein lipase, apolipoprotein E, tissue factor pathway inhibitor, lactoferrin, the light chain of blood coagulation factor VIII, and the intracellular chaperone receptor-associated protein (RAP). No binding of the cluster I fragment to any of the tested ligands was observed. The cluster III fragment only bound to the anti-LRP monoclonal antibody ␣ 2 MR␣3 and weakly to RAP. Except for t-PA, we found that each of the ligands tested binds both to cluster II and to cluster IV. The affinity rate constants of ligand binding to clusters II and IV and to LRP were measured, showing that clusters II and IV display only minor differences in ligandbinding kinetics. Furthermore, we demonstrate that the subdomains C3-C7 of cluster II are essential for binding of ligands and that this segment partially overlaps with a RAP-binding site on cluster II. Finally, we show that one RAP molecule can bind to different clusters simultaneously, supporting a model in which RAP binding to LRP induces a conformational change in the receptor that is incompatible with ligand binding.The low density lipoprotein receptor-related protein (LRP) 1 is a 600-kDa membrane glycoprotein that is a member of the low density lipoprotein (LDL) receptor family of endocytic receptors (reviewed in Refs. 1 and 2). LRP can bind and internalize a diverse spectrum of structurally unrelated ligands in a calcium-dependent manner including apolipoproteins, lipases, proteinases, proteinase-inhibitor complexes, Kunitz-type inhibitors, matrix proteins, and other proteins such as lactoferrin, Pseudomonas exotoxin A, and malaria circumsporozoite protein (1, 2). In addition, the blood coagulation factor VIII was recently identified as a ligand of LRP (3). The broad range of ligands suggests a role for the receptor in distinct physiological and pathophysiological processes, ranging from lipoprotein metabolism, cell growth and cell migration, fibrinolysis, and thrombosis to atherosclerosis and Alzheimer's disease.LRP is synthesized as a single polypeptide chain and is cleaved in the trans-Golgi network by the endopeptidase furin into two subunits, resulting in a 515-kDa fragment that contains the ligand binding domains and an 85-kDa fragment comprising the transmembrane and cytoplasmic domains. The subunits rema...

The peroxisome proliferator-activated receptors, PPARα, PPARβ, and PPARγ, are a family of transcription factors activated by a diversity of molecules including fatty acids and fatty acid metabolites. PPARs regulate the transcription of a large variety of genes implicated in metabolism, inflammation, proliferation, and differentiation in different cell types. These transcriptional regulations involve both direct transactivation and interaction with other transcriptional regulatory pathways. The functions of PPARα and PPARγ have been extensively documented mainly because these isoforms are activated by molecules clinically used as hypolipidemic and antidiabetic compounds. The physiological functions of PPARβ remained for a while less investigated, but the finding that specific synthetic agonists exert beneficial actions in obese subjects uplifted the studies aimed to elucidate the roles of this PPAR isoform. Intensive work based on pharmacological and genetic approaches and on the use of both in vitro and in vivo models has considerably improved our knowledge on the physiological roles of PPARβ in various cell types. This review will summarize the accumulated evidence for the implication of PPARβ in the regulation of development, metabolism, and inflammation in several tissues, including skeletal muscle, heart, skin, and intestine. Some of these findings indicate that pharmacological activation of PPARβ could be envisioned as a therapeutic option for the correction of metabolic disorders and a variety of inflammatory conditions. However, other experimental data suggesting that activation of PPARβ could result in serious adverse effects, such as carcinogenesis and psoriasis, raise concerns about the clinical use of potent PPARβ agonists.