There is a tightly regulated interaction, which is well‐conserved in evolution, between the metabolic and immune systems that is deranged in states of over‐ or under‐nutrition. Obesity, an energy‐rich condition, is characterized by the activation of an inflammatory process in metabolically active sites such as adipose tissue, liver and immune cells. The consequence of this response is a sharp increase in circulating levels of proinflammatory cytokines, adipokines and other inflammatory markers. Activation of the immune response in obesity is mediated by specific signaling pathways, with Jun N‐terminal kinase and IκB kinase β/nuclear factor κ‐light‐chain‐enhancer of activated B cells being the most well studied. It is known that the above events modify insulin signaling and result in the development of insulin resistance. The nutrient overload characterizing obesity is a metabolic stressor associated with intracellular organelle (e.g. the endoplasmic reticulum) stress. The exact characterization of the series of events and the mechanisms that integrate the inflammatory response with metabolic homeostasis at the cellular and systemic level is a very active research field. In this minireview, we discuss the signaling pathways and molecules associated with the development of obesity‐induced inflammation, as well as the evidence that supports a critical role for the stress response in this process.

Attenuation of CRH receptor type 1 (CRH-R1) signaling activity might involve desensitization and uncoupling of CRH-R1 from intracellular effectors. We investigated the desensitization of native CRH-R in human myometrial cells from pregnant women and recombinant CRH-R1alpha stably overexpressed in human embryonic kidney (HEK) 293 cells. In both cell types, CRH-R1-mediated adenylyl cyclase activation was susceptible to homologous desensitization induced by pretreatment with high concentrations of CRH. Time course studies showed half-maximal desensitization occurring after approximately 40 min of pretreatment and full recovery of CRH-R1alpha functional response within 2 h of removal of CRH pretreatment. In HEK 293 cells, desensitization of CRH-R1alpha was associated with receptor phosphorylation and subsequent endocytosis. To analyze the mechanism leading to CRH-R1alpha desensitization, we overexpressed a truncated beta-arrestin (319-418) and performed coimmunoprecipitation and G protein-coupled receptor kinase (GRK) translocation studies. We found that GRK3 and GRK6 are the main isoforms that interact with CRH-R1alpha, and that recruitment of GRK3 requires Gbetagamma-subunits as well as beta-arrestin. Site-directed mutagenesis of Ser and Thr residues in the CRH-R1alpha C terminus, identified Thr399 as important for GRK-induced receptor phosphorylation and desensitization.We conclude that homologous desensitization of CRH-R1alpha involves the coordinated action of multiple GRK isoforms, Gbeta gamma dimers and beta-arrestin. Based on our identification of key amino acid(s) for GRK-dependent phosphorylation, we demonstrate the importance of the CRH-R1alpha carboxyl tail for regulation of receptor activity.

Interactions between Eph receptors and their membrane-bound ligands (ephrins) are of critical importance for key developmental processes such as boundary formation or vascular development. Their downstream signaling pathways are intricate and heterogeneous at several levels, the combined effect being a highly complex and flexible system. Here we demonstrate that activated EphB1 induces tyrosine phosphorylation of the focal adhesion protein paxillin at Tyr-31 and Tyr-118 and is recruited to paxillin-focal adhesion kinase (FAK) complexes. Pretreatment with the specific Src inhibitor PP2, or expression of dominant-negative, kinase-dead c-Src abrogates EphB1-induced tyrosine phosphorylation of paxillin. Cells transfected with the paxillin mutant Y31F/Y118F displayed a reduced migration in response to ephrin B2 stimulation. Furthermore, expression of an LD4 deletion mutant (paxillin ⌬LD4) significantly reduces EphB1-paxillin association, paxillin tyrosine phosphorylation, as well as EphB1-dependent cell migration. Finally, mutation of the Nck-binding site of EphB1 (Y594F) interrupts the interaction between Nck, paxillin, and EphB1. These data suggest a model in which ligand-activated EphB1 forms a signaling complex with Nck, paxillin, and focal adhesion kinase and induces tyrosine phosphorylation of paxillin in a c-Src-dependent manner to promote cell migration.The Eph family of receptor tyrosine kinases and their membrane-bound ligands, the ephrins, are key regulators of several developmental processes, including cell migration, boundary formation, axonal guidance, synaptogenesis, and angiogenesis. Both receptor-and ligand-initiated signals mediate repulsion, adhesion, and de-adhesion mechanisms involved in the motility of adherent cells. After activation by their cognate ligands, Eph receptors are phosphorylated at specific tyrosine residues in the cytoplasmic domain, thus displaying new docking sites for known signaling molecules. Indeed, a variety of Src homology domain (SH2/SH3)-containing proteins have been identified as Eph receptor-binding partners, including the adaptor proteins Nck, Grb10, Grb7, SHEP-1, SLAP, the low molecular weight phosphotyrosine phosphatase LMW-PTP, the tyrosine kinases Crk, Fyn, and Src, the phosphatidylinositol-3 kinase, and the Ras GTPase-activating protein (reviewed in Ref. 1). Furthermore, PDZ-binding motifs located at the C terminus of the Eph receptors bind PDZ domain containing proteins such as AF6, Ryk (an RTK-interacting protein), Pick1, syntenin, and the two glutamate receptor-interacting proteins, Grip1 and Grip2 (2).Importantly, many of the proteins identified in the Eph signaling pathways have been implicated in the regulation of cell morphology, adhesion, and motility. Activation of Eph receptors and ephrins has been shown to affect cell attachment by means of integrin and focal adhesion protein-dependent mechanisms, but contradictory results have been described for the interplay between these systems. In previous works, we showed that EphB1 receptor regulates integrin-de...

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Corticotropin-releasing hormone (CRH) regulates diverse biological functions in mammals, through activation of two types of specific G protein-coupled receptors that are expressed as multiple mRNA spliced variants. In most cells, the type 1␣ CRH receptor (CRH-R1␣) preferentially activates the G s -adenylyl cyclase signaling cascade. CRH-R1␣-mediated signaling activity is impaired by insertion of 29 amino acids in the first intracellular loop, a sequence modification that is characteristic of the human-specific CRH-R1␤ variant. In various tissues, CRH signaling events are regulated by protein kinase C (PKC). The CRH receptors contain multiple putative PKC phosphorylation sites that represent potential targets. To investigate this, we expressed recombinant CRH-R1␣ or CRH-R1␤ in human embryonic kidney 293 cells and analyzed signaling events after PKC activation. Agonist (oxytocin) or phorbol 12-myristate 13-acetate-induced activation of PKC led to phosphorylation of both CRH-R1 variants. However, CRH-R1␣ and CRH-R1␤ exhibited different functional responses to PKC-induced phosphorylation, with only the CRH-R1␤ susceptible to cAMP signaling desensitization. This was associated with a significant decrease of accessible CRH-R1␤ receptors expressed on the cell surface. Both CRH-R1 variants were susceptible to homologous desensitization and internalization following treatment with CRH; however, PKC activation increased internalization of CRH-R1␤ but not CRH-R1␣ in a ␤-arrestin-independent manner. Our findings indicate that CRH-R1␣ and -R1␤ exhibit differential responses to PKC-induced phosphorylation, and this might represent an important mechanism for functional regulation of CRH signaling in target cells.