Antagonistic Signaling Pathways Research Articles

Epidermal Growth Factor Receptor (EGFR) in primary breast cancer – protein expression, but not gene copy number, gives important prognostic information in tamoxifen treated patients

Recently, the receptor for epidermal growth factor (EGF) was identified as a downstream element in different signaling pathways. This expanded its classical function as a receptor for EGF-like ligands to a role as mediator of diverse signaling systems and as a switch point of a cellular communication network. In addition, several downstream targets, (e.g. Smad proteins and STATs) into which signals from synergistic and antagonistic signaling pathways converge, were identified.

Cell Signalling: The Merry Lives of BAK1

Plant receptor-like kinases characterised by leucine-rich repeats have been shown to play dual roles in seemingly unrelated biological processes, inviting comparison with TOLL-like receptors of animals.

BMPs regulate multiple aspects of growth-plate chondrogenesis through opposing actions on FGF pathways

Bone morphogenetic protein (BMP) signaling pathways are essential regulators of chondrogenesis. However, the roles of these pathways in vivo are not well understood. Limb-culture studies have provided a number of essential insights, including the demonstration that BMP pathways are required for chondrocyte proliferation and differentiation. However, limb-culture studies have yielded contradictory results; some studies indicate that BMPs exert stimulatory effects on differentiation, whereas others support inhibitory effects. Therefore, we characterized the skeletal phenotypes of mice lacking Bmpr1a in chondrocytes (Bmpr1a(CKO)) and Bmpr1a(CKO);Bmpr1b+/- (Bmpr1a(CKO);1b+/-) in order to test the roles of BMP pathways in the growth plate in vivo. These mice reveal requirements for BMP signaling in multiple aspects of chondrogenesis. They also demonstrate that the balance between signaling outputs from BMP and fibroblast growth factor (FGF) pathways plays a crucial role in the growth plate. These studies indicate that BMP signaling is required to promote Ihh expression, and to inhibit activation of STAT and ERK1/2 MAPK, key effectors of FGF signaling. BMP pathways inhibit FGF signaling, at least in part, by inhibiting the expression of FGFR1. These results provide a genetic in vivo demonstration that the progression of chondrocytes through the growth plate is controlled by antagonistic BMP and FGF signaling pathways.

Attenuation of Colonic Inflammation by PPARγ in Intestinal Epithelial Cells: Effect on Toll-like Receptor Pathway

The peroxisome proliferator-activated receptor gamma (PPARgamma) is a nuclear receptor highly expressed in the colon and playing an anti-inflammatory role through inhibition of the NF-kappaB pathway. Toll-like receptor 4 (TLR4) has been known to mediate LPS-induced cellular signaling through activation of NF-kappaB pathway in intestinal epithelial cells. The aims of this study were to evaluate attenuation of inflammation by PPARgamma in intestinal epithelial cells and to study the possible relation between PPARgamma and TLR4. HT-29 human epithelial cells were stimulated with LPS (20 microg/ml) and PPARgamma ligand, 15d-PGJ2 (10 microM), or with LPS (20 microg/ml) alone for 24 hr. COX-2, IL-8, TLR4, and PPARgamma mRNA expression was assessed by RT-PCR. IL-8 protein levels and TLR4 protein expression were analyzed by ELISA and Western blot, respectively. To evaluate the action mechanisms of PPARgamma ligand, Western blot analysis for IkappaBalpha degradation was performed. Costimulation with LPS and PPARgamma ligand in comparison to LPS stimulation alone (1) decreased COX-2, IL-8 mRNA expression and IL-8 protein secretion, (2) decreased TLR4 mRNA and protein expression, and (3) decreased PPARgamma mRNA expression. PPARgamma ligand delayed LPS-induced IkappaBalpha degradation. These findings suggest that PPAR-gamma ligands suppress inflammation in intestinal epithelial cells. PPARgamma and TLR, these two antagonistic signaling pathways in intestinal epithelial cells may be partially cross-linked.

Parietal-Eye Phototransduction Components and Their Potential Evolutionary Implications

The parietal-eye photoreceptor is unique because it has two antagonistic light signaling pathways in the same cell-a hyperpolarizing pathway maximally sensitive to blue light and a depolarizing pathway maximally sensitive to green light. Here, we report the molecular components of these two pathways. We found two opsins in the same cell: the blue-sensitive pinopsin and a previously unidentified green-sensitive opsin, which we name parietopsin. Signaling components included gustducin-alpha and Galphao, but not rod or cone transducin-alpha. Single-cell recordings demonstrated that Go mediates the depolarizing response. Gustducin-alpha resembles transducin-alpha functionally and likely mediates the hyperpolarizing response. The parietopsin-Go signaling pair provides clues about how rod and cone phototransduction might have evolved.

Differential transactivation of sphingosine-1-phosphate receptors modulates NGF-induced neurite extension.

The process of neurite extension after activation of the TrkA tyrosine kinase receptor by nerve growth factor (NGF) involves complex signaling pathways. Stimulation of sphingosine kinase 1 (SphK1), the enzyme that phosphorylates sphingosine to form sphingosine-1-phosphate (S1P), is part of the functional TrkA signaling repertoire. In this paper, we report that in PC12 cells and dorsal root ganglion neurons, NGF translocates SphK1 to the plasma membrane and differentially activates the S1P receptors S1P1 and S1P2 in a SphK1-dependent manner, as determined with specific inhibitors and small interfering RNA targeted to SphK1. NGF-induced neurite extension was suppressed by down-regulation of S1P1 expression with antisense RNA. Conversely, when overexpressed in PC12 cells, transactivation of S1P1 by NGF markedly enhanced neurite extension and stimulation of the small GTPase Rac, important for the cytoskeletal changes required for neurite extension. Concomitantly, differentiation down-regulated expression of S1P2 whose activation would stimulate Rho and inhibit neurite extension. Thus, differential transactivation of S1P receptors by NGF regulates antagonistic signaling pathways that modulate neurite extension.

Nicotinamide Relaxes Vascular Smooth Muscle by Inhibiting Myosin Light Chain Kinase-Dependent Signaling Pathways: Implications for Anticancer Efficacy

Nicotinamide has been shown to be an effective tumor oxygenator in preclinical studies and is part of a promising clinical protocol for the treatment of cancer of the larynx. It has been known for some time that nicotinamide sensitizes tumors, at least in part, by modulating vascular smooth muscle contraction; however, the cellular target within the smooth muscle cell has yet to be identified. Our previous studies have eliminated targets within several agonist and antagonist signaling pathways in smooth muscle, suggesting that it must act at a point close to the contractile machinery of the cell. The present study investigated the effect of nicotinamide on four key steps responsible for force generation via actin/myosin interaction in the smooth muscle cell: calcium binding to calmodulin, calcium-calmodulin binding to smooth muscle myosin light chain kinase (MLCK) inhibitor peptide 480-501 (smMLCIP), modulation of MLCK-dependent signaling, and MLCK-induced phosphorylation of the regulatory myosin light chain, MLC20. Nicotinamide abolished the phosphorylation of MLC20, but had no significant effect on the other endpoints tested. We conclude that the vasorelaxant effects of nicotinamide are mediated mainly through inhibition of MLC20 phosphorylation, and that this could be a promising target for the development of novel tumor oxygenators to enhance radio- and chemotherapy.

Antagonistic signaling pathways regulate alternative splicing of CD44 in T cells.

Annals of the New York Academy of SciencesVolume 973, Issue 1 p. 112-115 Antagonistic Signaling Pathways Regulate Alternative Splicing of CD44 in T Cells S. WEG-REMERS, Corresponding Author S. WEG-REMERS Forschungszentrum Karlsruhe, Germany, Institute of Toxicology and Genetics, 76021 Karlsruhe, GermanyAddress for correspondence: S. Weg-Remers, Forschungszentrum Karlsruhe, Germany, Institute of Toxicology and Genetics, P.O. Box 3640, 76021 Karlsruhe, Germany. Voice: +49-7247-82-3945; fax: +49-7247-82-3354; [email protected].Search for more papers by this authorH. PONTA, H. PONTA Forschungszentrum Karlsruhe, Germany, Institute of Toxicology and Genetics, 76021 Karlsruhe, GermanySearch for more papers by this authorP. HERRLICH, P. HERRLICH Forschungszentrum Karlsruhe, Germany, Institute of Toxicology and Genetics, 76021 Karlsruhe, GermanySearch for more papers by this authorH. KÖNIG, H. KÖNIG Forschungszentrum Karlsruhe, Germany, Institute of Toxicology and Genetics, 76021 Karlsruhe, GermanySearch for more papers by this author S. WEG-REMERS, Corresponding Author S. WEG-REMERS Forschungszentrum Karlsruhe, Germany, Institute of Toxicology and Genetics, 76021 Karlsruhe, GermanyAddress for correspondence: S. Weg-Remers, Forschungszentrum Karlsruhe, Germany, Institute of Toxicology and Genetics, P.O. Box 3640, 76021 Karlsruhe, Germany. Voice: +49-7247-82-3945; fax: +49-7247-82-3354; [email protected].Search for more papers by this authorH. PONTA, H. PONTA Forschungszentrum Karlsruhe, Germany, Institute of Toxicology and Genetics, 76021 Karlsruhe, GermanySearch for more papers by this authorP. HERRLICH, P. HERRLICH Forschungszentrum Karlsruhe, Germany, Institute of Toxicology and Genetics, 76021 Karlsruhe, GermanySearch for more papers by this authorH. KÖNIG, H. KÖNIG Forschungszentrum Karlsruhe, Germany, Institute of Toxicology and Genetics, 76021 Karlsruhe, GermanySearch for more papers by this author First published: 24 January 2006 https://doi.org/10.1111/j.1749-6632.2002.tb04617.xCitations: 5Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume973, Issue1CELL SIGNALING, TRANSCRIPTION, AND TRANSLATION AS THERAPEUTIC TARGETSNovember 2002Pages 112-115 RelatedInformation

Suppressors of Cytokine Signaling Proteins Are Differentially Expressed in Th1 and Th2 Cells: Implications for Th Cell Lineage Commitment and Maintenance

Positive regulatory factors induced by IL-12/STAT4 and IL-4/STAT6 signaling during T cell development contribute to polarized patterns of cytokine expression manifested by differentiated Th cells. These two critical and antagonistic signaling pathways are under negative feedback regulation by a multimember family of intracellular proteins called suppressor of cytokine signaling (SOCS). However, it is not known whether these negative regulatory factors also modulate Th1/Th2 lineage commitment and maintenance. We show here that CD4(+) naive T cells constitutively express low levels of SOCS1, SOCS2, and SOCS3 mRNAs. These mRNAs and their proteins increase significantly in nonpolarized Th cells after activation by TCR signaling. We further show that differentiation into Th1 or Th2 phenotype is accompanied by preferential expression of distinct SOCS mRNA transcripts and proteins. SOCS1 expression is 5-fold higher in Th1 than in Th2 cells, whereas Th2 cells contain 23-fold higher levels of SOCS3. We also demonstrate that IL-12-induced STAT4 activation is inhibited in Th2 cells that express high levels of SOCS3 whereas IL-4/STAT6 signaling is constitutively activated in Th2 cells, but not Th1 cells, with high SOCS1 expression. These results suggest that mutually exclusive use of STAT4 and STAT6 signaling pathways by differentiated Th cells may derive in part, from SOCS3- or SOCS1-mediated repression of IL-12/STAT4- or IL-4/STAT6 signaling in Th2 and Th1 cells, respectively. Given the strong correlation between distinct patterns of SOCS expression and differentiation into the Th1 or Th2 phenotype, SOCS1 and SOCS3 proteins are therefore Th lineage markers that can serve as therapeutic targets for immune modulation therapy.

Sealing Your Neighbor's Fate

During vulval development in Caenorhabditis elegans , a signal arises from an anchor cell that activates the mitogen-activated protein kinase (MAPK) pathway to specify cell fate of three vulval precursor cells, with primary fate affected in the P6.p cell and secondary fate in the adjacent P5.p and P7.p cells. Berset et al. show that two different vulval cell fates are produced in adjacent cells through the action of two antagonistic signaling pathways. Upon activating the MAPK pathway, the primary fate of P6.p is induced. A lateral signal moves out from P6.p that increases expression of NOTCH in P5.p and P7.p. NOTCH turns on the MAPK phosphatase gene lip-1 . LIP-1, in turn, blocks MAPK activity in P5.p and P7.p and allows the secondary cell fate to develop. T. Berset, E. F. Hoier, G. Battu, S. Canevascini, A. Hajnal, Notch inhibition of RAS signaling through MAP kinase phosphatase LIP-1 during C . elegans vulval development. Science 291 , 1055-1058 (2001). [Abstract] [Full Text]

BMP3: To Be or Not To Be a BMP

Bone morphogenetic proteins (BMPs) are osteogenic but also have diverse functions during development. BMP3 is a major component of osteogenin, which has osteogenic activity. However, recombinant BMP3 (rhBMP3) has no apparent osteogenic function, raising the possibility that BMP3 has no bone-inducing activity or that the recombinant material is not properly processed. To resolve this apparent discrepancy, we utilized a retroviral system to study the effects of BMP3 in vitro. In addition, we generated Bmp3-deficient mice to elucidate the function of BMP3 in vivo. Retroviral as well as mammalian expression constructs were utilized to express BMP3 and to create BMP3 conditioned medium. Alkaline phosphatase (ALP) activity and transcriptional response assays were used to monitor the ability of BMP3 to elicit either a BMP-like or a transforming growth factor beta (TGF-beta)/activin-like response in osteoblastic cell lines. Finally, mice deficient in BMP3 were generated to investigate BMP3 function in vivo. BMP3 was unable to induce an osteogenic response in W-20-17, MC3T3-E1, or C3H10T1/2 cells, although all three cell lines were responsive to BMP2. However, BMP3 inhibited responsiveness to BMP2 in these assays, suggesting that BMP3 antagonizes BMP2 signaling. This inhibition did not occur through inhibition of binding of BMP2 to its receptors. BMP3 activated the TGF-beta/activin-pathway in these cells, suggesting that BMP3 exerts its inhibiting effects by activating a signaling pathway that antagonizes the BMP pathway. To examine the potential functional consequences of BMP3 action, Bmp3-/- mice, which lack BMP3, were generated. On an outbred genetic background, Bmp3-/- mice are viable and show no obvious skeletal phenotype as embryos or neonates. However, adult mice exhibit twice as much trabecular bone as do their wild-type littermates. This observation is consistent with our in vitro observations suggesting that BMP3 is an inhibitor of osteogenesis in vitro and of bone formation in vivo. BMP3 is an inhibitor of osteogenic BMPs and can signal through a TGF-beta/activin pathway. The ability of BMP3 to antagonize BMP2 activity may thus be a consequence of competition for signaling components common to TGF-beta/activin and BMP pathways. BMP3, the most abundant BMP in demineralized bone, may therefore play an essential role as a modulator of the activity of osteogenic BMPs in vivo. Therapies to accelerate bone healing usually utilize administration of exogenous BMP either in recombinant form or by gene therapy approaches. It is conceivable that the potency of osteogenic BMPs would be increased by inhibiting the activation of antagonistic signaling pathways or by increasing levels of rate-limiting signaling components shared by both BMP and TGF-beta/activin pathways.

The Role of Phosphatidylinositol 3-Kinase, Src Kinase, and Protein Kinase A Signaling Pathways in Insulin and Glucagon Regulation of CYP2E1 Expression

The signaling pathways involved in insulin and glucagon regulation of CYP2E1 expression were examined in primary cultured rat hepatocytes. Insulin addition to primary cultured rat hepatocytes for 24 h resulted in an ∼80% and >90% decrease in CYP2E1 mRNA levels at 1 and 10 nM insulin, respectively, relative to untreated cells. Addition of the phosphatidylinositol 3-kinase inhibitor wortmannin, or the Src kinase inhibitor geldanamycin, prior to insulin addition, inhibited the insulin-mediated decline in CYP2E1 mRNA. In contrast, treatment of cells with glucagon (100 nM), or the cAMP analogue dibutyryl-cAMP (50 μM), for 24 h increased CYP2E1 mRNA levels by ∼7-fold. Addition of the protein kinase A inhibitor H89 prior to glucagon treatment attenuated the glucagon-mediated increase in CYP2E1 mRNA by ∼70%. Glucagon (100 nM) opposed the effects of insulin (1 nM) on CYP2E1 mRNA expression and conversely, insulin blocked the effects of glucagon. These data provide compelling evidence for the regulation of CYP2E1 expression via mutually antagonistic signaling pathways involving insulin and glucagon.

Epidermal growth factor receptors: critical mediators of multiple receptor pathways

Recently, the receptor for epidermal growth factor (EGF) was identified as a downstream element in different signaling pathways. This expanded its classical function as a receptor for EGF-like ligands to a role as mediator of diverse signaling systems and as a switch point of a cellular communication network. In addition, several downstream targets, (e.g. Smad proteins and STATs) into which signals from synergistic and antagonistic signaling pathways converge, were identified.

Suppression of auxin signal transduction by a MAPK cascade in higher plants.

The plant hormone auxin activates many early response genes that are thought to be responsible for diverse aspects of plant growth and development. It has been proposed that auxin signal transduction is mediated by a conserved signalling cascade consisting of three protein kinases: the mitogen-activated protein kinase (MAPK), MAPK kinase (MAPKK) and MAPKK kinase (MAPKKK). Here we show that a specific plant MAPKKK, NPK1, activates a MAPK cascade that leads to the suppression of early auxin response gene transcription. A mutation in the kinase domain abolishes NPK1 activity, and the presence of the carboxy-terminal domain diminishes the kinase activity. Moreover, the effects of NPK1 on the activation of a MAPK and the repression of early auxin response gene transcription are specifically eliminated by a MAPK phosphatase. Transgenic tobacco plants overexpressing the NPK1 kinase domain produced seeds defective in embryo and endosperm development. These results suggest that auxin sensitivity may be balanced by antagonistic signalling pathways that use a distinct MAPK cascade in higher plants.

Asexual Sporulation in Aspergillus nidulans

The formation of mitotically derived spores, called conidia, is a common reproductive mode in filamentous fungi, particularly among the large fungal class Ascomycetes. Asexual sporulation strategies are nearly as varied as fungal species; however, the formation of conidiophores, specialized multicellular reproductive structures, by the filamentous fungus Aspergillus nidulans has emerged as the leading model for understanding the mechanisms that control fungal sporulation. Initiation of A. nidulans conidiophore formation can occur either as a programmed event in the life cycle in response to intrinsic signals or to environmental stresses such as nutrient deprivation. In either case, a development-specific set of transcription factors is activated and these control the expression of each other as well as genes required for conidiophore morphogenesis. Recent progress has identified many of the earliest-acting genes needed for initiating conidiophore development and shown that there are at least two antagonistic signaling pathways that control this process. One pathway is modulated by a heterotrimeric G protein that when activated stimulates growth and represses both asexual and sexual sporulation as well as production of the toxic secondary metabolite, sterigmatocystin. The second pathway apparently requires an extracellular signal to induce sporulation-specific events and to direct the inactivation of the first pathway, removing developmental repression. A working model is presented in which the regulatory interactions between these two pathways during the fungal life cycle determine whether cells grow or develop.

Asexual sporulation in Aspergillus nidulans.

The formation of mitotically derived spores, called conidia, is a common reproductive mode in filamentous fungi, particularly among the large fungal class Ascomycetes. Asexual sporulation strategies are nearly as varied as fungal species; however, the formation of conidiophores, specialized multicellular reproductive structures, by the filamentous fungus Aspergillus nidulans has emerged as the leading model for understanding the mechanisms that control fungal sporulation. Initiation of A. nidulans conidiophore formation can occur either as a programmed event in the life cycle in response to intrinsic signals or to environmental stresses such as nutrient deprivation. In either case, a development-specific set of transcription factors is activated and these control the expression of each other as well as genes required for conidiophore morphogenesis. Recent progress has identified many of the earliest-acting genes needed for initiating conidiophore development and shown that there are at least two antagonistic signaling pathways that control this process. One pathway is modulated by a heterotrimeric G protein that when activated stimulates growth and represses both asexual and sexual sporulation as well as production of the toxic secondary metabolite, sterigmatocystin. The second pathway apparently requires an extracellular signal to induce sporulation-specific events and to direct the inactivation of the first pathway, removing developmental repression. A working model is presented in which the regulatory interactions between these two pathways during the fungal life cycle determine whether cells grow or develop.

Signaling by hemolytically inactive C5b67, an agonist of polymorphonuclear leukocytes.

The hemolytically inactive complement component complex C5b67, designated iC5b67, can signal human polymorphonuclear leukocytes (PMN) both as a pertussis toxin-inhibitable agonist for chemotaxis and as an antagonist for C5a- and FMLP-stimulated chemotaxis and superoxide production. The signaling pathways utilized by iC5b67 have been further investigated. In contrast to mastoparan, iC5b67 failed to directly activate G proteins to stimulate inositol phosphate formation in COS cells that had been transfected with G alpha 16. In COS cells co-transfected with both G alpha 16 and the C5a receptor, iC5b67 could neither activate phospholipase C nor inhibit C5a receptor-mediated activation of phospholipase C. iC5b67 stimulated GTPase activity in a membrane-enriched fraction from PMN. These data support the hypothesis that iC5b67 signals through a unique receptor, likely G protein linked, but distinct from the C5a receptor. iC5b67 was able to mobilize intracellular stores to elicit increases in intracellular Ca2+. Based on the effects of herbimycin A, wortmannin, and chelerythrine on iC5b67-induced PMN chemotaxis, iC5b67 signaling involved activation of tyrosine and phosphatidylinositol 3-kinases, but not protein kinase C. Relevant to the capacity of iC5b67 to antagonize PMN superoxide production, iC5b67 induced rapid and sustained increases in intracellular cAMP, which others have shown can inhibit superoxide formation. Although iC5b67 antagonizes C5a and FMLP receptor-mediated superoxide generation, iC5b67 had no effect on PMA-induced superoxide formation. The distinct agonist and antagonist signaling pathways activated by iC5b67 in the PMN diverge soon after initial iC5b67 receptor-mediated transduction steps.

Coupling of antagonistic signalling pathways in modulation of neutrophil function

Modulation of neutrophil activation by catecholamines reflects a fine-tuning by coupling inhibitory and stimulatory receptor pathways. The catecholamine isoproterenol (ISO) binds to beta-adrenergic cell surface receptors and thereby inhibits cell responses such as O2- production stimulated by formyl peptides. However, ISO did not inhibit O2- generation activated by 1 microM ionophore A23187, the protein kinase C activators phorbol ester (PMA, 100 ng/ml) and oleoylacetylglycerol (OAG, 50 microM), and the G-protein activator NaF (40 mM). Furthermore, the overall kinetics of oxidant production in the presence of ISO were unchanged when cells were stimulated with PMA, OAG, A23187, and NaF. These results would imply that neither intracellular calcium, the activation of protein kinase C, nor the activation of G-protein are the primary target of the inhibitory pathway. Accordingly, pertussis toxin did not block PMA or NaF-stimulated superoxide generation. In contrast, formyl peptide-dependent GTPase activity is inhibited by ISO in sonicated cell preparations. Since ISO increases the cAMP concentration in the cell, the possibility is raised that a cAMP-dependent kinase inhibits signal transduction in part by blocking the interaction of this receptor with its G-protein.