Transfected platelet-activating factor receptor activates mitogen-activated protein (MAP) kinase and MAP kinase kinase in Chinese hamster ovary cells

Identification of two isoforms of mouse neuropeptide Y-Y1 receptor generated by alternative splicing. Isolation, genomic structure, and functional expression of the receptors

Two cDNA clones homologous with human neuropeptide (NP) Y-Y1 receptor have been isolated from a mouse bone marrow cDNA library. One was thought to be the cognate of the human NPY-Y1 receptor, termed Y1 alpha receptor, and the other form, termed Y1 beta receptor, differed from the Y1 alpha receptor in the seventh transmembrane domain and C-terminal tail. Analysis of the mouse genomic DNA showed that both receptors originated from a single gene. The different peptide sequences of the Y1 beta receptor were encoded by separate exons, hence, these receptors were generated by differential RNA splicing. High affinity binding of [125I]NPY to each receptor expressed in Chinese hamster ovary (CHO) cells and sequestration of [125I]NPY after binding to each receptor were observed. In the CHO cells expressing the Y1 alpha receptor, intracellular Ca2+ increase, inhibition of forskolin-induced cAMP accumulation, and mitogen-activated protein kinase (MAPK) activation were observed by stimulation of NPY, and these responses were abolished by pretreatment with pertussis toxin. Since wortmannin completely inhibited NPY-elicited MAPK activation, we speculate that wortmannin-sensitive signaling molecule(s) such as phosphoinositide 3-kinase may lie between pertussis toxin-sensitive G-protein and MAPK. In contrast, these intracellular signals were not detected in CHO cells expressing the Y1 beta receptor. Northern blots and reverse transcriptase-polymerase chain reaction analyses indicated that the Y1 alpha receptor was highly expressed in the brain, heart, kidney, spleen, skeletal muscle, and lung, whereas the Y1 beta receptor mRNA was not detected in these tissues. However, the Y1 beta receptor was expressed in mouse embryonic developmental stage (7 and 11 days), bone marrow cells and several hematopoietic cell lines. These results suggest that the Y1 beta receptor is an embryonic and a bone marrow form of the NPY-Y1 receptor, which decreases in the expression during development and differentiation.

Participation of reactive oxygen species in the lysophosphatidic acid-stimulated mitogen-activated protein kinase kinase activation pathway

Recent evidence suggests that reactive oxygen species (ROS) may function as second messengers in intracellular signal transduction pathways. We explored the possibility that ROS were involved in lysophosphatidic acid (LPA)-induced mitogen-activated protein (MAP) kinase signaling pathway in HeLa cells. Antioxidant N-acetylcysteine inhibited the LPA-stimulated MAP kinase kinase activity. Direct exposure of HeLa cells to hydrogen peroxide resulted in a concentration- and time-dependent activation of MAP kinase kinase. Inhibition of catalase with aminotriazole enhanced the effect of LPA on induction of MAP kinase kinase. Further, LPA stimulated ROS production in HeLa cells. These findings suggest that ROS participate in the LPA-elicited MAP kinase signaling pathway.

Distinct pathways of Gi- and Gq-mediated mitogen-activated protein kinase activation

Receptors that couple to the heterotrimeric G proteins, Gi or Gq, can stimulate phosphoinositide (PI) hydrolysis and mitogen-activated protein kinase (MAPK) activation. PI hydrolysis produces inositol 1,4,5-trisphosphate and diacylglycerol, leading to activation of protein kinase C (PKC), which can stimulate increased MAPK activity. However, the relationship between PI hydrolysis and MAPK activation in Gi and Gq signaling has not been clearly defined and is the subject of this study. The effects of several signaling inhibitors are assessed including expression of a peptide derived from the carboxyl terminus of the beta adrenergic receptor kinase 1 (beta ARKct), which specifically blocks signaling mediated by the beta gamma subunits of G proteins (G beta gamma), expression of dominant negative mutants of p21ras (RasN17) and p74raf-1 (N delta Raf), protein-tyrosine kinase (PTK) inhibitors and cellular depletion of PKC. The Gi-coupled alpha 2A adrenergic receptor (AR) stimulates MAPK activation which is blocked by expression of beta ARKct, RasN17, or N delta Raf, or by PTK inhibitors, but unaffected by cellular depletion of PKC. In contrast, MAPK activation stimulated by the Gq-coupled alpha 1B AR or M1 muscarinic cholinergic receptor is unaffected by expression of beta ARKct or RasN17 expression or by PTK inhibitors, but is blocked by expression of N delta Raf or by PKC depletion. These data demonstrate that Gi- and Gq-coupled receptors stimulate MAPK activation via distinct signaling pathways. G beta gamma is responsible for mediating Gi-coupled receptor-stimulated MAPK activation through a mechanism utilizing p21ras and p74raf independent of PKC. In contrast, G alpha mediates Gq-coupled receptor-stimulated MAPK activation using a p21ras-independent mechanism employing PKC and p74raf. To define the role of G beta gamma in Gi-coupled receptor-mediated PI hydrolysis and MAPK activation, direct stimulation with G beta gamma was used. Expression of G beta gamma resulted in MAPK activation that was sensitive to inhibition by expression of beta ARKct, RasN17, or N delta Raf or by PTK inhibitors, but insensitive to PKC depletion. By comparison, G beta gamma-mediated PI hydrolysis was not affected by beta ARKct, RasN17, or N delta Raf expression or by PTK inhibitors. Together, these results demonstrate that G beta gamma mediates MAPK activation and PI hydrolysis via independent signaling pathways.

Platelet-activating factor induces NF-kappa B activation through a G protein-coupled pathway

The capability of platelet-activating factor (PAF) to induce transcription factor activation was examined. In stably transfected Chinese hamster ovary cells expressing the PAF receptor (CHO-PAFR), PAF stimulation resulted in the nuclear expression of a DNA binding activity with specificity to the kappa B sequence. The p50 and p65 proteins, constituents of the prototypic nuclear factor kappa B (NF-kappa B), were identified as components of the DNA protein complexes by antipeptide antibodies in gel supershift as well as UV cross-linking experiments. PAF induced an initial decrease and subsequent increase of cytoplasmic I kappa B alpha levels, accompanied by up-regulation of the I kappa B alpha messenger RNA, a feature of NF-kappa B activation. PAF-induced kappa B binding activity was detected within 15 min after agonist stimulation, peaked at 30-40 min, and remained detectable by 2.5 h. SR 27417, a PAF receptor antagonist, blocked PAF-induced kappa B binding activity but not that induced by tumor necrosis factor-alpha (TNF alpha). Cholera toxin treatment markedly reduced PAF-induced kappa B binding activity, whereas pertussis toxin had no significant inhibitory effect. Neither of the two toxins affected the kappa B binding activity induced by TNF alpha in the same cells. In addition to the CHO-PAFR cells, PAF stimulated kappa B binding activity in the murine P388D1 macrophage and the human ASK.0 B cell lines that express endogenous PAF receptors. These results imply a potential role of PAF in the regulation of gene expression through a G protein-coupled transcription factor activation pathway.

A direct interaction between G-protein beta gamma subunits and the Raf-1 protein kinase

Raf-1 is a serine/threonine protein kinase positioned downstream of Ras in the mitogen-activated protein kinase cascade. Using a yeast two-hybrid strategy to identify other proteins that interact with and potentially regulate Raf-1, we isolated a clone encoding the carboxyl-terminal half of the G beta 2 subunit of heterotrimeric G-proteins. In vitro, purified G beta gamma subunits specifically bound to a GST fusion protein encoding amino acids 1-330 of Raf-1 (Raf/330). Binding assays with truncation mutants of GST-Raf indicate that the region located between amino acids 136 and 239 is a primary determinant for interaction with G beta gamma. In competition experiments, the carboxyl terminus of beta-adrenergic receptor kinase (beta ARK) blocked the binding of G beta gamma to Raf/330; however, the Raf-1-binding proteins, Ras and 14-3-3, had no effect. Scatchard analysis of in vitro binding between Raf/330 and G beta gamma revealed an affinity of interaction (Kd = 163 +/- 36 nM), similar to that seen between G beta gamma and beta ARK (Kd = 87 +/- 24 nM). The formation of native heterotrimeric G alpha beta gamma complexes, as measured by pertussis toxin ADP-ribosylation of G alpha, could be disrupted by increasing amounts of Raf/330, with an EC50 of approximately 200 nM, in close agreement with the estimated binding affinity. In vivo complexes of Raf-1 and G beta gamma were isolated from human embryonic kidney 293-T cells transfected with epitope-tagged G beta 2. The identification and characterization of this novel interaction raises several possibilities for signaling cross-talk between growth factor receptors and those receptors coupled to heterotrimeric G-proteins.

Prostaglandin F2 alpha stimulates formation of p21ras-GTP complex and mitogen-activated protein kinase in NIH-3T3 cells via Gq-protein-coupled pathway

Prostaglandin (PG) F2 alpha activated mitogen-activated protein (MAP) kinase and MAP kinase kinase in NIH-3T3 cells by a mechanism that was completely inhibited by protein kinase inhibitors, staurosporine (20 nM) or H-7 (20 microM), but was insensitive to pretreatment with islet-activating protein (100 ng/ml; 24 h) or 12-O-tetradecanoylphorbol 13-acetate (2.5 microM; 24 h). PGF2 alpha stimulation also led to a significant increase in Ras.GTP complex. Transfection of a cDNA encoding a constitutively active mutant of Gq alpha-subunit (Q209L) mimicked PGF2 alpha-induced MAP kinase activation, increase in Ras.GTP complex, and DNA synthesis in these cells, suggesting that activation of Gq mediates the PGF2 alpha-activation of Ras-MAP kinase pathway and mitogenesis in NIH-3T3 cells. These data provide a new insight into regulatory mechanisms of Ras-MAP kinase pathway through heterotrimeric G-protein-mediated pathways.

Gq pathway desensitizes chemotactic receptor-induced calcium signaling via inositol trisphosphate receptor down-regulation

Desensitization of a chemotactic receptor is an adaptive process that terminates inflammation. Although homologous desensitization can be well explained by the action of specific receptor kinases, the mechanisms of heterologous desensitization remain elusive. As an approach to evaluate the roles of Gq pathway in desensitization of calcium signaling, we expressed a constitutively active Gq alpha mutant (Gq alpha Q-L) together with platelet-activating factor (PAF) receptor in Xenopus laevis oocytes. Gq alpha Q-L expression completely attenuated the calcium-sensitive chloride current and the 45Ca release elicited by PAF. The Gq-mediated desensitization could not be ascribed to G protein/receptor uncoupling via receptor phosphorylation, because (i) PAF-induced inositol 1,4,5-trisphosphate (IP3) synthesis was only partially suppressed and (ii) a mutated PAF receptor devoid of all Ser and Thr in the third cytoplasmic loop and in the C-terminal tail was also completely desensitized by Gq alpha Q-L. In Gq alpha Q-L expressing oocytes, microinjection of IP3 failed to elicit the calcium response, and the IP3 receptor, detected by a specific antibody, disappeared. Thus, the Gq-mediated desensitization can be most likely explained by IP3 receptor down-regulation. These novel mechanisms may explain in part heterologous desensitization in chemotactic factor-stimulated inflammatory cells.

Alpha 2 adrenergic receptor subtypes expressed in Chinese hamster ovary cells activate differentially mitogen-activated protein kinase by a p21ras independent pathway

Epinephrine stimulation of rat alpha 2D, alpha 2B, and alpha 2C adrenergic receptor subtypes, expressed stably in Chinese hamster ovary (CHO) cells, caused a rapid, transient activation of mitogen-activated protein kinase (MAPK), with subtype-specific different efficiencies. The order of activation was CHO-2B approximately CHO-2D much greater than CHO-2C. Pertussis toxin blocked the stimulation of MAPK enzymatic activity and the parallel MAPK phosphorylation, demonstrating that these responses are mediated by pertussis toxin-sensitive Gi proteins. Contrary to what has been reported for the alpha 2A subtype expressed in rat-1 fibroblasts, epinephrine did not cause any detectable activation of p21ras in the CHO transfectants. Furthermore, combined application of epinephrine and phorbol myristate acetate had a potent cooperative but not additive effect in clones CHO-2D and CHO-2B but not in CHO-2C, suggesting that protein kinase C is probably differently involved in the signaling by the three alpha 2 receptor subtypes. These results show that in CHO cells, the different alpha 2 adrenergic receptor subtypes utilize differential pathways to activate MAPK in a p21ras-independent way.

Activation of mitogen-activated protein kinase and arachidonate release via two G protein-coupled receptors expressed in the rat hippocampus

Platelet-activating factor and somatostatin receptors, two G protein-coupled receptors expressed in the rat hippocampus, were analyzed for the downstream signaling pathways in Chinese hamster ovary cells stably expressing each receptor. Ligand stimulation to each CHO cell line induced (1) inhibition of forskolin-induced accumulation of cAMP, (2) arachidonate release, and (3) activation of mitogen-activated protein kinase and MAP kinase kinase. In contrast, inositol phosphate breakdown was seen only in the PAF-stimulated CHO cells. The induction of these signals accompanied no detectable Ras activation. Suppression of the signals by pertussis toxin was almost complete for the somatostatin receptor but partial for the PAF receptor, suggesting that the somatostatin receptor couples only with PTX-sensitive G protein, while the PAF receptor couples with both PTX-sensitive and -insensitive G proteins. A model of G protein-mediated signaling pathways was proposed in which the signals from Gi and those from Gq converge at MAP kinase kinase and lead to arachidonate release. The present system using CHO cells is useful for analyzing signaling pathways from G proteins to MAP kinase kinase and will thereby provide clues for understanding the mechanisms underlying the physiological and pathological events mediated by PAF, somatostatin, and other G protein-coupled receptors in the central nervous system and other tissues.

Deciphering the MAP kinase pathway

MAP kinases (MAPK) are serine/threonine kinases which are activated by a dual phosphorylation on threonine and tyrosine residues. Their specific upstream activators, called MAP kinase kinases (MAPKK), constitute a new family of dual-specific threonine/tyrosine kinases, which in turn are activated by upstream MAP kinase kinase kinases (MAPKKK). These three kinase families are successively stimulated in a cascade of activation described in various species such as mammals, frog, fly, worm or yeast. In mammals, the MAP kinase module lies on the signaling pathway triggered by numerous agonists such as growth factors, hormones, lymphokines, tumor promoters, stress factors, etc. Targets of MAP kinase have been characterized in all subcellular compartments. In yeast, genetic epistasis helped to characterize the presence of several MAP kinase modules in the same system. By complementation tests, the relationships existing between phylogenetically distant members of each kinase family have been described. The roles of the MAP kinase cascade have been analyzed by engineering various mutations in the kinases of the module. The MAP kinase cascade has thus been implicated in higher eukaryotes in cell growth, cell fate and differentiation, and in low eukaryotes, in conjugation, osmotic stress, cell wall construct and mitosis.