RhoA influences the nuclear localization of extracellular signal-regulated kinases to modulate p21Waf/Cip1 expression.
BACKGROUND
The 42/44-kD mitogen-activated protein kinases (extracellular signal-regulated kinases, ERKs) regulate smooth muscle cell (SMC) cell-cycle progression and can either promote or inhibit proliferation depending on the activation status of the small GTPase RhoA. RhoA is involved in the regulation of the actin cytoskeleton and converges on multiple signaling pathways. However, the mechanism by which RhoA modulates ERK signaling is not well defined. The purpose of this investigation was to examine whether RhoA regulates ERK downstream signaling and cellular proliferation through its effects on the cytoskeleton and the nuclear localization of ERK.


RESULTS
Treatment of SMCs with Clostridia botulinum C3 exoenzyme, which inhibits RhoA activation, decreased SMC proliferation to 24+/-7% of that of controls and increased p21Waf1/Cip1 transcription and protein levels. These effects of RhoA were reversed by inhibition of ERK phosphorylation. However, inactivation of RhoA did not alter levels of ERK phosphorylation but did increase nuclear localization of phosphorylated ERK. In addition, immunostaining demonstrated that phosphorylated ERK associated with the actin cytoskeleton, which was disrupted by C3 exoenzyme. Leptomycin B, an inhibitor of Crm1 that results in ERK nuclear accumulation, similarly increased p21Waf1/Cip1.


CONCLUSIONS
RhoA inhibition increased levels of phosphorylated ERK in the cell nucleus. Inhibition of RhoA or pharmacological inhibition of nuclear export resulted in increased p21Waf1/Cip1 expression and decreased SMC proliferation, effects that were partially dependent on ERK. RhoA regulation of the actin cytoskeleton may determine ERK subcellular localization and its subsequent effects on SMC proliferation.
Zuckerbraun
Brian S
BS
University of Pittsburgh, Department of Surgery, Pittsburgh, Pa, USA. zuckerbraunbs@msx.upmc.edu
Shapiro
Richard A
RA
Billiar
Timothy R
TR
Tzeng
Edith
E
D036002:ADP Ribose Transferases / Q000494:pharmacology; D021581:Active Transport, Cell Nucleus / Q000187:drug effects / Q000502:physiology; D000818:Animals; D001905:Botulinum Toxins / Q000494:pharmacology; D002455:Cell Division / Q000187:drug effects / Q000502:physiology; D002467:Cell Nucleus / Q000378:metabolism*; D002478:Cells, Cultured; D050759:Cyclin-Dependent Kinase Inhibitor p21; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors; D016213:Cyclins / Q000235:genetics / Q000378:metabolism*; D003599:Cytoskeleton / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005231:Fatty Acids, Unsaturated / Q000494:pharmacology; D017930:Genes, Reporter; D028884:Karyopherins / Q000037:antagonists & inhibitors; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D018160:Receptors, Cytoplasmic and Nuclear; D015398:Signal Transduction / Q000502:physiology; D020742:rhoA GTP-Binding Protein / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000097606:Exportin 1 Protein
D036002:ADP Ribose Transferases; D001905:Botulinum Toxins; D050759:Cyclin-Dependent Kinase Inhibitor p21; D018844:Cyclin-Dependent Kinases; D016213:Cyclins; D004791:Enzyme Inhibitors; D005231:Fatty Acids, Unsaturated; D028884:Karyopherins; D020928:Mitogen-Activated Protein Kinases; D018160:Receptors, Cytoplasmic and Nuclear; D020742:rhoA GTP-Binding Protein; D000097606:Exportin 1 Protein; C493958:Cdkn1a protein, rat; C063007:exoenzyme C3, Clostridium botulinum; C038753:leptomycin B