Jana Staňková

IL-13 and IL-4 Up-Regulate Cysteinyl Leukotriene 1 Receptor Expression in Human Monocytes and Macrophages

The cysteinyl (Cys) leukotrienes (LT)C4, LTD4, and LTE4, are lipid mediators that have been implicated in the pathogenesis of asthma. The human LTD4 receptor (CysLT1R) was recently cloned and characterized. The present work was undertaken to study the potential modulation of CysLT1R expression by the Th2 cytokines IL-13 and IL-4. In this study, we report that IL-13 up-regulates CysLT1R mRNA levels, with consequently enhanced CysLT1R protein expression and function in human monocytes and monocyte-derived macrophages. CysLT1R mRNA expression was augmented 2- to 5-fold following treatment with IL-13 and was due to enhanced transcriptional activity. The effect was observed after 4 h, was maximal by 8 h, and maintained at 24 h. IL-4, but not IFN-γ, induced a similar pattern of CysLT1R up-regulation. Monocytes pretreated with IL-13 or IL-4 for 24 h showed enhanced CysLT1R protein expression, as assessed by flow cytometry using a polyclonal anti-CysLT1R Ab. They also showed enhanced responsiveness to LTD4, but not to LTB4, in terms of Ca2+ mobilization, as well as augmented chemotactic activity. Our findings suggest a possible mechanism by which IL-13 and IL-4 can modulate CysLT1R expression on monocytes and macrophages, and consequently their responsiveness to LTD4, and thus contribute to the pathogenesis of asthma and allergic diseases.

IL-5 Up-Regulates Cysteinyl Leukotriene 1 Receptor Expression in HL-60 Cells Differentiated into Eosinophils

The cysteinyl leukotrienes, leukotriene (LT) C4, LTD4, and LTE4, are lipid mediators that have been implicated in the pathogenesis of several inflammatory processes, including asthma. The human LTD4 receptor, CysLT1R, was recently cloned and characterized. We had previously shown that HL-60 cells differentiated toward the eosinophilic lineage (HL-60/eos) developed specific functional LTD4 receptors. The present work was undertaken to study the potential modulation of CysLT1R expression in HL-60/eos by IL-5, an important regulator of eosinophil function. Here, we report that IL-5 rapidly up-regulates CysLT1R mRNA expression, with consequently enhanced CysLT1R protein expression and function in HL-60/eos. CysLT1R mRNA expression was augmented 2- to 15-fold following treatment with IL-5 (1–20 ng/ml). The effect was seen after 2 h, was maximal by 4 h, and maintained at 8 h. Although CysLT1R mRNA was constitutively expressed in undifferentiated HL-60 cells, its expression was not modulated by IL-5 in the absence of differentiation. Differentiated HL-60/eos cells pretreated with IL-5 (10 ng/ml) for 24 h showed enhanced CysLT1R expression on the cell surface, as assessed by flow cytometry using a polyclonal anti-CysLT1R Ab. They also showed enhanced responsiveness to LTD4, but not to LTB4 or platelet-activating factor, in terms of Ca2+ mobilization, and augmented the chemotactic response to LTD4. Our findings suggest a possible mechanism by which IL-5 can modulate eosinophil functions and particularly their responsiveness to LTD4, and thus contribute to the pathogenesis of asthma and allergic diseases.

Structural and Functional Requirements for Agonist-induced Internalization of the Human Platelet-activating Factor Receptor

The receptor for platelet-activating factor (PAF) is a member of the G-protein-coupled receptor family. To study the structural elements and mechanisms involved in the internalization of human PAF receptor (hPAFR), we used the following mutants: a truncated mutant in the C-terminal tail of the receptor (Cys317→ Stop) and mutations in the (D/N)P(X)2,3Y motif (Asp289 → Asn,Ala and Tyr293 → Phe,Ala). Chinese hamster ovary cells expressing the Cys317→ Stop mutant exhibited a marked reduction in their capacity to internalize PAF, suggesting the existence of determinants important for endocytosis in the last 26 amino acids of the cytoplasmic tail. Substitution of Asp289 to alanine abolished both internalization and G-protein coupling, whereas substitution of Tyr293 to alanine abolished coupling but not internalization. Inhibition or activation of protein kinase C did not significantly affect the internalization process. Receptor sequestration and ligand uptake was, at least in part, blocked by concanavalin A and blockers of endocytosis mediated by clathrin-coated pits. Our data suggest that the internalization of a G-protein-coupled receptor and coupling to a G-protein can be two independent events. Moreover, the C terminus tail of hPAFR, but not the putative internalization motifs, may be involved in the internalization of hPAFR.

Selective modulation of wild type receptor functions by mutants of G-protein-coupled receptors.

Members of the G-protein-coupled receptor (GPCR) family are involved in most aspects of higher eukaryote biology, and mutations in their coding sequence have been linked to several diseases. In the present study, we report that mutant GPCR can affect the functional properties of the co-expressed wild type (WT) receptor. Mutants of the human platelet-activating factor receptor that fail to show any detectable ligand binding (N285I and K298stop) or coupling to a G-protein (D63N, D289A, and Y293A) were co-expressed with the WT receptor in Chinese hamster ovary and COS-7 cells. In this context, N285I and K298stop mutant receptors inhibited3H-WEB2086 binding and surface expression. Co-transfection with D63N resulted in a constitutively active receptor phenotype. Platelet-activating factor-induced inositol phosphate production in cells transfected with a 1:1 ratio of WT:D63N was higher than with the WT cDNA alone but was abolished with a 1:3 ratio. We confirmed that these findings could be extended to other GPCRs by showing that co-expression of the WT C-C chemokine receptor 2b with a carboxyl-terminal deletion mutant (K311stop), resulted in a decreased affinity and responsiveness to MCP-1. A better understanding of this phenomenon could lead to important tools for the prevention or treatment of certain diseases.

Janus Kinase 2 Activation by the Platelet-Activating Factor Receptor (PAFR): Roles of Tyk2 and PAFR C Terminus

Platelet-activating factor (PAF) is a phospholipid with multiple physiological and pathological actions. The PAF receptor (PAFR) belongs to the G protein-coupled, heptahelical receptor superfamily. Recently, we have shown that PAF signals through the Janus kinase (Jak)/STAT pathway and that Tyk2 plays an essential role in PAF-induced PAFR promoter 1 activation. In the present study we found that PAF stimulated Jak2 tyrosine phosphorylation in the monocytic cell line MonoMac-1 as well as in COS-7 cells transfected with PAFR and Jak2 cDNAs. The use of a G protein-uncoupled PAFR (D289A) mutant indicated that Jak2 activation was G protein independent. Interestingly, following PAF stimulation, Jak2 coimmunoprecipitated with PAFR in the presence of active Tyk2, but not with a kinase-inactive Tyk2 mutant, K930I. Moreover, Tyk2-K930I completely blocked PAF-stimulated Jak2 phosphorylation. Gradual deletion of C-terminal residues of the PAFR resulted in progressively decreased Jak2 activation. Deletion of 12 C-terminal residues in mutant V330Stop diminished Jak2 tyrosine phosphorylation by 17%. Further deletions of 25–37 residues from the PAFR C-tail (C317Stop, M311Stop, and T305Stop) resulted in a 50% decrease in Jak2 phosphorylation compared with the wild-type receptor. Complete removal of the C tail resulted in a mutant (K298Stop) that failed to activate Jak2, suggesting that the receptor C-terminal region contains important domains for Jak2 activation. Finally, the coexpression of a minigene encoding the C terminus of PAFR partially inhibited PAF-induced kinase activation. Taken together, our results indicate that PAF activates Jak2 and that Tyk2 and the C-terminal tail of PAFR are of critical importance for PAF-induced Jak2 activation.

Platelet-activating factor receptor: differential regulation and signaling by agonists and inverse agonists

Abstract The receptor for platelet-activating factor (PAFR) is a member of the G-protein-coupled receptor (GPCR) family. Due to the association of platelet-activating factor (PAF) with diverse physiological and pathological processes, considerable efforts have been invested in the development of PAFR antagonists. A large number of these molecules have been shown to specifically interact with the PAFR, but surprisingly, little is known on their impact on receptor activity or conformation. We used three different models of PAFR with high basal activity to study the inverse agonist effects of diverse PAFR antagonists. First, we co-expressed the wild-type (WT) PAFR with the Gαq protein, second, we used a point substitution mutant of PAFR L231R, which is constitutively active, and thirdly, we co-expressed the WT PAFR with another substitution mutant D63N: the co-expression of these two receptors in a 1:3 ratio (WT/D63N) produces a receptor complex with a constitutively active phenotype. Our results indicate that inverse agonists elicit distinct effects depending on the receptor model used. These results may have implications for eventual treatments based on inverse agonists.