title: Regulation of mitogen-activated protein kinases in cardiac myocytes through the small G protein Rac1.
abstract: Small guanine nucleotide-binding proteins of the Ras and Rho (Rac, Cdc42, and Rho) families have been implicated in cardiac myocyte hypertrophy, and this may involve the extracellular signal-related kinase (ERK), c-Jun N-terminal kinase (JNK), and/or p38 mitogen-activated protein kinase (MAPK) cascades. In other systems, Rac and Cdc42 have been particularly implicated in the activation of JNKs and p38-MAPKs. We examined the activation of Rho family small G proteins and the regulation of MAPKs through Rac1 in cardiac myocytes. Endothelin 1 and phenylephrine (both hypertrophic agonists) induced rapid activation of endogenous Rac1, and endothelin 1 also promoted significant activation of RhoA. Toxin B (which inactivates Rho family proteins) attenuated the activation of JNKs by hyperosmotic shock or endothelin 1 but had no effect on p38-MAPK activation. Toxin B also inhibited the activation of the ERK cascade by these stimuli. In transfection experiments, dominant-negative N17Rac1 inhibited activation of ERK by endothelin 1, whereas activated V12Rac1 cooperated with c-Raf to activate ERK. Rac1 may stimulate the ERK cascade either by promoting the phosphorylation of c-Raf or by increasing MEK1 and/or -2 association with c-Raf to facilitate MEK1 and/or -2 activation. In cardiac myocytes, toxin B attenuated c-Raf(Ser-338) phosphorylation (50 to 70% inhibition), but this had no effect on c-Raf activity. However, toxin B decreased both the association of MEK1 and/or -2 with c-Raf and c-Raf-associated ERK-activating activity. V12Rac1 cooperated with c-Raf to increase expression of atrial natriuretic factor (ANF), whereas N17Rac1 inhibited endothelin 1-stimulated ANF expression, indicating that the synergy between Rac1 and c-Raf is potentially physiologically important. We conclude that activation of Rac1 by hypertrophic stimuli contributes to the hypertrophic response by modulating the ERK and/or possibly the JNK (but not the p38-MAPK) cascades.
authors.affiliation: Division of Biomedical Sciences (Molecular Pathology Section), Imperial College School of Medicine, London SW7 2AZ, United Kingdom. a.clerk@ic.ac.uk
authors.forename: A
authors.initials: A
authors.lastname: Clerk
authors.forename: F H
authors.initials: FH
authors.lastname: Pham
authors.forename: S J
authors.initials: SJ
authors.lastname: Fuller
authors.forename: E
authors.initials: E
authors.lastname: Sahai
authors.forename: K
authors.initials: K
authors.lastname: Aktories
authors.forename: R
authors.initials: R
authors.lastname: Marais
authors.forename: C
authors.initials: C
authors.lastname: Marshall
authors.forename: P H
authors.initials: PH
authors.lastname: Sugden
mesh_terms: D000818:Animals; D009320:Atrial Natriuretic Factor / Q000235:genetics; D006332:Cardiomegaly / Q000209:etiology; D002478:Cells, Cultured; D019332:Endothelin-1 / Q000494:pharmacology; D004789:Enzyme Activation / Q000187:drug effects; D005786:Gene Expression Regulation; D006160:Guanosine Triphosphate / Q000378:metabolism; D006801:Humans; D048728:MAP Kinase Kinase Kinase 1; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism; D051381:Rats; D014162:Transfection; D020830:rac1 GTP-Binding Protein / Q000378:metabolism*; D020742:rhoA GTP-Binding Protein / Q000378:metabolism
chemical_list: D019332:Endothelin-1; D009320:Atrial Natriuretic Factor; D006160:Guanosine Triphosphate; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf; D020928:Mitogen-Activated Protein Kinases; D048728:MAP Kinase Kinase Kinase 1; C482576:MAP3K1 protein, human; D020830:rac1 GTP-Binding Protein; D020742:rhoA GTP-Binding Protein