The
small G protein RhoA plays a major role in several vascular processes and cardiovascular disorders. Here we analyze the mechanisms of RhoA regulation by
serotonin (5-HT) in arterial smooth muscle.
5-HT (0.1-10 microM) induced activation of RhoA followed by RhoA depletion at 24-72 h. Inhibition of
5-HT1 receptors reduced the early phase of RhoA activation but had no effect on 5-HT-induced delayed RhoA activation and depletion, which were suppressed by the
5-HT transporter inhibitor
fluoxetine and the
transglutaminase inhibitor monodansylcadaverin and in type 2
transglutaminase-deficient smooth muscle cells. Coimmunoprecipitations demonstrated that
5-HT associated with RhoA both in vitro and in vivo. This association was
calcium-dependent and inhibited by
fluoxetine and monodansylcadaverin.
5-HT promotes the association of RhoA with the
E3 ubiquitin ligase Smurf1, and 5-HT-induced RhoA depletion was inhibited by the
proteasome inhibitor MG132 and the RhoA inhibitor Tat-C3.
Simvastatin, the
Rho kinase inhibitor
Y-27632,
small interfering RNA-mediated RhoA gene silencing, and long-term
5-HT stimulation induced Akt activation. In contrast, inhibition of 5-HT-mediated RhoA degradation by
MG132 prevented 5-HT-induced Akt activation. Long-term
5-HT stimulation also led to the inhibition of the RhoA/
Rho kinase component of arterial contraction. Our data provide evidence that
5-HT, internalized through the
5-HT transporter, is transamidated to RhoA by
transglutaminase. Transamidation of RhoA leads to RhoA activation and enhanced proteasomal degradation, which in turn is responsible for Akt activation and contraction inhibition. The observation of transamidation of
5-HT to RhoA in pulmonary artery of hypoxic rats suggests that this process could participate in pulmonary artery remodeling and
hypertension.