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p70S6 kinase is a critical node that integrates HER-family and PI3 kinase signaling networks. Cell Signal 2014; 26:1627-35. [PMID: 24662264 DOI: 10.1016/j.cellsig.2014.03.013] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/24/2014] [Accepted: 03/10/2014] [Indexed: 12/17/2022]
Abstract
Therapies targeting oncogenic drivers rapidly induce compensatory adaptive responses that blunt drug effectiveness, contributing to therapeutic resistance. Adaptive responses are characteristic of robust cell signaling networks, and thus there is increasing interest in drug combinations that co-target the driver and the adaptive response. An alternative approach to co-inhibiting oncogenic and adaptive targets is to identify a critical node where the activities of these targets converge. Nodes of convergence between signaling modules represent potential therapeutic vulnerabilities because their inhibition could result in the collapse of the network, leading to enhanced cytotoxicity. In this report we demonstrate that p70S6 kinase (p70S6K) can function as a critical node linking HER-family and phosphoinositide-3-kinase (PI3K) pathway signaling. We used high-throughput combinatorial drug screening to identify adaptive survival responses to targeted therapies, and found that HER-family and PI3K represented compensatory signaling pathways. Co-targeting these pathways with drug combinations caused synergistic cytotoxicity in cases where inhibition of neither target was effective as a monotherapy. We utilized Reverse Phase Protein Arrays and determined that phosphorylation of ribosomal protein S6 was synergistically down-regulated upon HER-family and PI3K/mammalian target of rapamycin (mTOR) co-inhibition. Expression of constitutively active p70S6K protected against apoptosis induced by combined HER-family and PI3K/mTOR inhibition. Direct inhibition of p70S6K with small molecule inhibitors phenocopied HER-family and PI3K/mTOR co-inhibition. These data implicate p70S6K as a critical node in the HER-family/PI3K signaling network. The ability of direct inhibitors of p70S6K to phenocopy co-inhibition of two upstream signaling targets indicates that identification and targeting of critical nodes can overcome adaptive resistance to targeted therapies.
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2
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Abstract
We introduce and study a simple lattice statistical mechanics modelfor the clustering of tumor necrosis factor receptor I (TNFR1).Our model explains clustering under over-expression of the cytoplasmicsignal transducer as well as the clustering induced via extracellularligand binding; also we explain why the loss of transducer leads to arapid break-up of the clusters. The basic mechanism at work is a first-order(cooperative) phase transition caused by the multimeric binding capability ofthe receptor-transducer complex. Using cooperativity of this type, the cellsare found to have an enhanced sensitivity and robustness. In general, ourmethod can be applied to other receptor-clustering related signaling system.
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Affiliation(s)
- C Guo
- Dept. of Physics, University of California San Diego, La Jolla, CA 92093-0319 U.S.A
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3
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Swaminathan S, Arbiser JL, Hiatt KM, High W, Abul-Ezz S, Horn TD, Shah SV. Rapid improvement of nephrogenic systemic fibrosis with rapamycin therapy: Possible role of phospho-70-ribosomal-S6 kinase. J Am Acad Dermatol 2010; 62:343-5. [DOI: 10.1016/j.jaad.2009.04.022] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/24/2008] [Revised: 04/12/2009] [Accepted: 04/14/2009] [Indexed: 11/27/2022]
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Abstract
Cell cycle progression is dependent upon the action of cyclins and their partners the cyclin dependent kinases (CDKs). Each cell cycle phase has its own characteristic cyclin-CDK combination, cyclin D-CDK4,6 and cyclin E-CDK2 being responsible for progression through G(1)-phase into S-phase. Progression through G(1)-phase is regulated by signal transduction cascades activated by polypeptide growth factors and by extracellular matrix (ECM) components. Studies aiming to unravel the molecular mechanism by which these extracellular components activate the cyclin-CDK complexes in the G(1)-phase, are usually performed using serum-starved cells (G(0) cells). These cells are activated by addition of growth factors, or the cells are detached from the substratum by trypsinization and subsequently allowed to re-attach. An alternative approach, however, is to study the effects of growth factors and attachment in the ongoing cell cycle by synchronization of the cells by the mitotic shake-off method. These cells are not serum starved and not actively detached from the substratum. In this contribution it is shown that both methods yield significant different results. These observations demonstrate that data obtained with model systems should be interpreted with care, especially if the findings are used to explain cell cycle progression in cells in an intact organism.
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Affiliation(s)
- Johannes Boonstra
- Department of Molecular Cell Biology, Institute of Biomembranes, University Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands.
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5
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Yu J, Liu XW, Kim HRC. Platelet-derived growth factor (PDGF) receptor-alpha-activated c-Jun NH2-terminal kinase-1 is critical for PDGF-induced p21WAF1/CIP1 promoter activity independent of p53. J Biol Chem 2003; 278:49582-8. [PMID: 14506245 DOI: 10.1074/jbc.m309986200] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Platelet-derived growth factor (PDGF) is a potent mitogen for mesenchymal cells. PDGF AA functions as a "competent factor" that stimulates cell cycle entry but requires additional (progression) factors in serum to transit the cell cycle beyond the G1/S checkpoint. Unlike PDGF AA, PDGF B-chain (c-sis) homodimer (PDGF BB) and its viral counterpart v-sis can serve as both competent and progression factors. PDGF BB activates alpha- and beta-receptor subunits (alpha-PDGFR and beta-PDGFR) and induces phenotypic transformation in NIH 3T3 cells, whereas PDGF AA activates alpha-PDGFR only and fails to induce transformation. We showed previously that alpha-PDGFR antagonizes beta-PDGFR-mediated transformation through activation of stress-activated protein kinase-1/c-Jun NH2-terminal kinase-1, whereas both alpha-PDGFR and beta-PDGFR induce mitogenic signals. These studies revealed a striking feature of PDGF signaling; the specificity and the strength of the PDGF growth signal is modulated by alpha-PDGFR-mediated simultaneous activation of growth stimulatory and inhibitory signals, whereas beta-PDGFR mainly induces a growth-promoting signal. Here we demonstrate that PDGF BB activation of beta-PDGFR alone results in more efficient cell cycle transition from G1 to S phase than PDGF BB activation of both alpha-PDGFR and beta-PDGFR. PDGF AA activation of alpha-PDGFR or PDGF BB activation of both alpha- and beta-PDGFRs up-regulates expression of p21WAF1/CIP1, an inhibitor of cell cycle-dependent kinases and a downstream mediator of the tumor suppressor gene product p53. However, beta-PDGFR activation alone fails to induce p21WAF1/CIP1 expression. We also demonstrate that alpha-PDGFR-activated JNK-1 is a critical signaling component for PDGF induction of p21WAF1/CIP1 promoter activity. The ability of PDGF/JNK-1 to induce p21WAF1/CIP1 promoter activity is independent of p53, although the overall p21WAF1/CIP1 promoter activities are greatly reduced in the absence of p53. These results provide a molecular basis for differential regulation of the cell cycle and transformation by alpha- and beta-PDGFRs.
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Affiliation(s)
- Jiuhong Yu
- Department of Pathology, Barbara Ann Karmanos Cancer Institute, Wayne State University School of Medicine, Detroit, Michigan 48201, USA
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6
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Onimaru M, Yonemitsu Y, Tanii M, Nakagawa K, Masaki I, Okano S, Ishibashi H, Shirasuna K, Hasegawa M, Sueishi K. Fibroblast growth factor-2 gene transfer can stimulate hepatocyte growth factor expression irrespective of hypoxia-mediated downregulation in ischemic limbs. Circ Res 2002; 91:923-30. [PMID: 12433837 DOI: 10.1161/01.res.0000043281.66969.32] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Hepatocyte growth factor (HGF) is a potent angiogenic polypeptide that stimulates angiogenesis. Transcriptional regulation of HGF, however, has not been fully defined, with the exception of the hypoxia-mediated downregulation in cultured cells. In the present study, we report that angiogenic growth factors, including HGF, were upregulated in a murine model of critical limb ischemia in vivo, a finding that was in conflict with previous in vitro data. Mice deficient in basic fibroblast growth factor-2 (FGF-2) showed reduced induction of HGF protein in ischemic muscles, and overexpression of FGF-2 via gene transfer stimulated endogenous HGF, irrespective of the presence of ischemia. In culture, FGF-2 rapidly stimulated HGF mRNA, and a sustained expression was evident in the time course in vascular smooth muscle cells and fibroblasts. FGF-2-mediated induction of HGF was fully dependent on the mitogen-activated protein kinase pathway yet was not affected by either hypoxia or a protein kinase A inhibitor. In the early expression, FGF-2 directly stimulated HGF mRNA without the requirement of new protein synthesis, whereas sustained induction of HGF in the later phase was partly mediated by platelet-derived growth factor-AA. Furthermore, in vivo overexpression of FGF-2 significantly improved the blood perfusion, and the effect was abolished by systemic blockade of HGF in ischemic limbs. This is the first demonstration of a regulational mechanism of HGF expression via FGF-2 that was independent of the presence of hypoxia. The harmonized therapeutic effects of FGF-2, accompanied with the activity of endogenous HGF, may provide a beneficial effect for the treatment of limb ischemia.
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MESH Headings
- Animals
- Blood Flow Velocity/drug effects
- Cell Line
- Disease Models, Animal
- Fibroblast Growth Factor 2/biosynthesis
- Fibroblast Growth Factor 2/genetics
- Fibroblast Growth Factor 2/pharmacology
- Fibroblasts/cytology
- Fibroblasts/metabolism
- Gene Expression Regulation/drug effects
- Gene Transfer Techniques
- Growth Substances/metabolism
- Hepatocyte Growth Factor/genetics
- Hepatocyte Growth Factor/metabolism
- Hindlimb/blood supply
- Hindlimb/physiopathology
- Humans
- Ischemia/pathology
- Ischemia/physiopathology
- Laser-Doppler Flowmetry
- Male
- Mice
- Mice, Inbred BALB C
- Mice, Inbred C3H
- Mice, Inbred C57BL
- Mice, Knockout
- Microcirculation/drug effects
- Microcirculation/physiopathology
- Muscle, Skeletal/blood supply
- Muscle, Skeletal/pathology
- Muscle, Skeletal/physiopathology
- Muscle, Smooth, Vascular/cytology
- Muscle, Smooth, Vascular/metabolism
- Platelet-Derived Growth Factor/metabolism
- RNA, Messenger/metabolism
- Transcription, Genetic/drug effects
- Transcription, Genetic/physiology
- Up-Regulation/drug effects
- Up-Regulation/physiology
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Affiliation(s)
- Mitsuho Onimaru
- Division of Pathophysiological and Experimental Pathology, Department of Pathology, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan
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Huang S, Ingber DE. A discrete cell cycle checkpoint in late G(1) that is cytoskeleton-dependent and MAP kinase (Erk)-independent. Exp Cell Res 2002; 275:255-64. [PMID: 11969294 DOI: 10.1006/excr.2002.5504] [Citation(s) in RCA: 58] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Cell spreading on extracellular matrix and associated changes in the actin cytoskeleton (CSK) are necessary for progression through G(1) and entry into S phase of the cell cycle. Pharmacological disruption of CSK integrity inhibits early mitogenic signaling to the extracellular signal-regulated kinase (Erk) subfamily of the mitogen-activated protein kinases (MAPKs) and arrests the cell cycle in G(1). Here we show that this block of G(1) progression is not simply a consequence of inhibition of the MAPK/Erk pathway but instead it reveals the existence of a discrete CSK-sensitive checkpoint. Use of PD98059 to inhibit MAPK/Erk and cytochalasin D (Cyto D) to disrupt the actin CSK at progressive time points in G(1) revealed that the requirement for MAPK/Erk activation lasts only to mid-G(1), while the actin CSK must remain intact up to late G(1) restriction point, R, in order for capillary endothelial cells to enter S phase. Additional analysis using Cyto D pulses defined a narrow time window of 3 h just prior to R in which CSK integrity was shown to be critical for the G(1)/S transition. Cyto D treatment led to down-regulation of cyclin D1 protein and accumulation of the cdk inhibitor, p27(Kip1), independent of cell cycle phase, suggesting that these changes resulted directly from CSK disruption rather than from a general cell cycle block. Together, these data indicate the existence of a distinct time window in late G(1) in which signals elicited by the CSK act independently of early MAPK/Erk signals to drive the cell cycle machinery through the G(1)/S boundary and, hence, promote cell growth.
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Affiliation(s)
- Sui Huang
- Department of Surgery, Children's Hospital and Harvard Medical School, Boston, Massachusetts 02115, USA
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8
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Chen QP, Giannobile WV. Adenoviral gene transfer of PDGF downregulates gas gene product PDGFalphaR and prolongs ERK and Akt/PKB activation. Am J Physiol Cell Physiol 2002; 282:C538-44. [PMID: 11832339 PMCID: PMC2579768 DOI: 10.1152/ajpcell.00419.2001] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The delivery of platelet-derived growth factor (PDGF) for tissue engineering of skin and periodontal wounds has become an active area of interest. However, little is known regarding the extended effects of PDGF on cell signaling via gene therapy and how such an approach facilitates the exiting of cells from growth arrest and entry to competence required for cell cycling. We show in vitro expression and secretion of PDGF-AA by recombinant adenovirus encoding the PDGF-A gene (Ad-PDGF-A). The bioactive PDGF-AA protein released induces sustained downregulation of PDGFalphaR that is encoded by a growth arrest-specific (gas) gene. Ad-PDGF-A induces sustained phosphorylation of PDGFalphaR as well as prolonged phosphorylation of downstream extracellular signal-regulated kinase 1/2 and Akt signaling pathways. Furthermore, the phosphorylation of PDGFalphaR is abolished by cotransducing cells with adenovirus encoding a dominant negative mutant of the PDGF-A gene that disrupts PDGF bioactivity. These findings demonstrate the prolonged effects of adenoviral delivery of PDGF and aid in the better understanding of sustained PDGF signaling.
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Affiliation(s)
- Qi-Ping Chen
- Center for Biorestoration of Oral Health, Department of Periodontics, University of Michigan, 1011 N. University Ave., Ann Arbor, MI 48109-1078, USA
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9
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Savinska LO, Kijamova RG, Pogrebnoy PV, Ovсharenko GV, Gout IT, Filonenko VV. Comparative characterization of S6 kinase α and β isoforms expression in mammalian tissues. ACTA ACUST UNITED AC 2001. [DOI: 10.7124/bc.0005c7] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Affiliation(s)
- L. O. Savinska
- Institute of Molecular Biology and Genetics, National Academy of Sciences of Ukraine
| | - R. G. Kijamova
- Institute of Molecular Biology and Genetics, National Academy of Sciences of Ukraine
| | - P. V. Pogrebnoy
- R. E. Kavetsky Institute of Experimental Pathology, Oncology and Radiobiology, National Academy of Sciences of Ukraine
| | - G. V. Ovсharenko
- Institute of Molecular Biology and Genetics, National Academy of Sciences of Ukraine
| | - I. T. Gout
- Institute of Molecular Biology and Genetics, National Academy of Sciences of Ukraine
- Ludwig Institute for Cancer Research
| | - V. V. Filonenko
- Institute of Molecular Biology and Genetics, National Academy of Sciences of Ukraine
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10
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Gouni-Berthold I, Berthold HK, Seewald S, Seul C, Ko Y, Vetter H, Sachinidis A. Short-term stimulation of vascular smooth muscle cells with platelet-derived growth factor (PDGF)-BB and angiotensin II induces. Scand J Clin Lab Invest 2000; 60:723-31. [PMID: 11218155 DOI: 10.1080/00365510050216457] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
Abstract
Growth factors such as the platelet-derived growth factor (PDGF)-BB and angiotensin II (Ang II) have been shown to induce vascular smooth muscle cell (VSMC) proliferation after long stimulation periods. Little is known though, about the effects of PDGF-BB and Ang II on VSMC proliferation after short stimulation periods. The purpose of our study was to examine whether a short term (3-60 min) stimulation of VSMC with PDGF-BB or Ang II is sufficient to induce cell proliferation. Incubation of VSMC with Ang II (100 nM) or PDGF-BB (50 ng/ml) caused a significant increase in [3H]thymidine incorporation starting after a 3-min stimulation, while the cell counts required 32 and 8 h of stimulation, respectively. Mitogen-activated protein kinase activation reached a maximum at 5-10 min of PDGF-BB or Ang II stimulation. This study demonstrates that the growth-promoting effects of PDGF-BB and Ang II are strongly dependent on the length of the stimulation period and that while prolonged stimulation periods (>8-32 h) result in VSMC proliferation, short ones (3-60 min) result only in [3H]thymidine incorporation without an increase in cell count, a fact of considerable pathophysiological significance, considering that the time kinetics of growth factors in the VSMC microenvironment have not as yet been clarified.
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11
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Krymskaya VP, Orsini MJ, Eszterhas AJ, Brodbeck KC, Benovic JL, Panettieri RA, Penn RB. Mechanisms of proliferation synergy by receptor tyrosine kinase and G protein-coupled receptor activation in human airway smooth muscle. Am J Respir Cell Mol Biol 2000; 23:546-54. [PMID: 11017921 DOI: 10.1165/ajrcmb.23.4.4115] [Citation(s) in RCA: 93] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
Despite recent studies depicting the capacity of G protein-coupled receptors (GPCRs) to activate mitogenic signaling pathways more commonly associated with receptor tyrosine kinases (RTKs), little is known regarding the interactive effects of GPCR and RTK activation on cell growth and signal transduction. Such interactions likely mediate the physiologic growth in most cells in vivo as well as the aberrant, non-neoplastic growth that occurs in diseases such as asthma, where disruptions of the local hormonal or inflammatory state can contribute to significant GPCR activation. In this study, we show that numerous inflammatory or contractile agents, including thrombin, histamine, and carbachol, potentiate epidermal growth factor (EGF)-stimulated proliferation of human airway smooth muscle (ASM), thus demonstrating a clear synergy between RTK and GPCR activation. Alterations in promitogenic nuclear signaling were evidenced by additive or synergistic increases in Elk-1 and activator protein-1 activation, and by increases in cyclin D1 expression. Interestingly, GPCR activation did not cause EGF receptor tyrosine phosphorylation nor did it increase EGF-stimulated autophosphorylation. In the presence of EGF, histamine or carbachol did not alter the time-dependent phosphorylation of p42/p44, whereas thrombin was capable of increasing phospho-p42/p44 levels at selected time points in some, but not all, cultures. In contrast to their relative inability to alter EGF receptor-linked p42/p44 activation, thrombin, histamine, and carbachol consistently increased the late phase (> 1 h) activity of p70 S6 kinase. Collectively, these findings suggest that inflammatory and contractile agents that activate GPCRs can significantly modulate RTK-mediated ASM growth through a p70 S6 kinase-dependent, p42/p44-independent mechanism.
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Affiliation(s)
- V P Krymskaya
- Division of Pulmonary and Critical Care, Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, USA
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12
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Sachinidis A, Seul C, Gouni-Berthold I, Seewald S, Ko Y, Vetter H, Fingerle J, Hoppe J. Cholera toxin treatment of vascular smooth muscle cells decreases smooth muscle alpha-actin content and abolishes the platelet-derived growth factor-BB-stimulated DNA synthesis. Br J Pharmacol 2000; 130:1561-70. [PMID: 10928958 PMCID: PMC1572234 DOI: 10.1038/sj.bjp.0703480] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2000] [Revised: 04/11/2000] [Accepted: 05/12/2000] [Indexed: 11/08/2022] Open
Abstract
The second messenger cyclic AMP regulates diverse biological processes such as cell morphology and cell growth. We examined the role of the second messenger cyclic AMP on rat aortic vascular smooth muscle cell (VSMC) morphology and the intracellular transduction pathway mediated by platelet-derived growth factor beta-receptor (PDGF-Rbeta). The effect of PDGF-BB on VSMCs growth was assessed by [(3)H]-thymidine incorporation. Tyrosine phosphorylation of PDGF-Rbeta, PLC-gamma1, ERK1 and ERK2, p125(FAK) and paxillin as well as Sm alpha-actin was examined by the chemiluminescence Western blotting method. Actin mRNA level was quantitated by Northern blotting. Visualization of Sm alpha-actin filaments, paxillin and PDGF-Rbeta was performed by immunfluorescence microscopy. Cholera toxin (CTX; 10 nM) treatment lead to a large and sustained increase in the cyclic AMP concentration after 2 h which correlated with change of VSMC morphology including complete disruption of the Sm alpha-actin filament array and loss of focal adhesions. Treatment of VSMCs with CTX did not influence tyrosine phosphorylation of p125(FAK) and paxillin but decreased the content of a Sm alpha-actin protein. Maximal decrease of 70% was observed after 24 h of treatment. CTX also caused a 90% decrease of the actin mRNA level. CTX treatment completely abolished PDGF-BB stimulated DNA-synthesis although PDGF-Rbeta level and subcellular distribution and translocation was not altered. Furthermore CTX attenuated the PDGF-BB-induced tyrosine phosphorylation of the PDGF-Rbeta, PI 3'-K, PLC-gamma1 and ERK1/2 indicating an action of cyclic AMP on PDGF-beta receptor. We conclude that although cyclic AMP attenuates the PDGF-Rbeta mediated intracellular transduction pathway, an intact actin filament may be required for the PDGF-BB-induced DNA synthesis in VSMCs.
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Affiliation(s)
- A Sachinidis
- Medizinische Universitäts-Poliklinik, Weilhelmstr. 35-37, D-53111 Bonn, Germany.
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13
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Yakisich JS, Sidén A, Vargas VI, Eneroth P, Cruz M. Early effects of protein kinase modulators on DNA synthesis in rat cerebral cortex. Exp Neurol 1999; 159:164-76. [PMID: 10486185 DOI: 10.1006/exnr.1999.7121] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
By using tissue miniunits, protein kinase modulators, and topoisomerase inhibitors in short-term incubation (0-90 min) we studied (1) the role of protein phosphorylation in the immediate control of DNA replication in the developing rat cerebral cortex and (2) the mechanism of action for genistein-mediated DNA synthesis inhibition. Genistein decreased the DNA synthesis within less than 30 min. None of the other protein kinase inhibitors examined (herbimycin A, staurosporine, calphostin-C) or the protein phosphatase inhibitor sodium orthovanadate inhibited DNA synthesis and they did not affect the genistein-mediated inhibition. The selective topoisomerase inhibitors camptothecin and etoposide decreased the DNA synthesis to an extent similar to that of genistein and within less than 30 min. In addition, the effects of these substances on topoisomerase I and II were studied. Etoposide and genistein but not herbimycin A, staurosporine, or calphostin-C strongly inhibited the activity of topoisomerase II. Our results (1) strongly suggest that the net rate of DNA replication during the S phase of the cell cycle is independent of protein phosphorylation and (2) indicate that the early inhibitory effect of genistein on DNA synthesis is mediated by topoisomerase II inhibition rather than protein tyrosine kinase inhibition.
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Affiliation(s)
- J S Yakisich
- Applied Biochemistry, Clinical Research Center, Karolinska Institute, Novum, Huddinge University Hospital, Huddinge, S-141 86, Sweden
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