Regulation of mitogen-activated protein kinases in cardiac myocytes through the small G protein Rac1

Small guanine nucleotide-binding proteins of the Ras and Rho (Rac, Cdc42, a nd Rho) families have been implicated in cardiac myocyte hypertrophy, and t his may involve the extracellular signal-related kinase (ERK), c-Jun N-term inal kinase (NK), and/or p38 mitogen-activated protein kinase (MAPK) cascad es. In other systems, Rac and Cdc42 have been particularly implicated in th e activation of JNKs and p38-MAPKs. We examined the activation of Rho famil y small G proteins and the regulation of MAPKs through Rad in cardiac myocy tes, Endothelin 1 and phenylephrine (both hypertrophic agonists) induced ra pid activation of endogenous Rad, and endothelin 1 also promoted significan t activation of RhoA, Toxin B (which inactivates Rho Family proteins) atten uated 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. Tn transfection experiments, dominant-neg ative N17Rac1 inhibited activation of ERK by endothelin 1, whereas activate d 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 activati on. 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-R af-associated ERK-activating activity. V12Rac1 cooperated with c-Raf to inc rease expression of atrial natriuretic factor (ANF), whereas N17Rac1 inhibi ted endothelin 1-stimulated ANF expression, indicating that the synergy bet ween Rad and c-Raf is potentially physiologically important. We conclude th at activation of Rad by hypertrophic stimuli contributes to the hypertrophi c response by modulating the ERK and/or possibly the JNK (but not the p38-M APK) cascades.