  Increase in vascular permeability occurs under many physiological conditions
and is central in diverse human pathologies. Thrombin is a pro-coagulant serine
protease, which causes the local loss of endothelial barrier integrity thereby
enabling the rapid extravasation of plasma proteins and the local formation of
fibrin-containing clots. Available information suggests that thrombin induces
endothelial permeability by promoting actomyosin contractility through the
Rho/ROCK signaling pathway. Here we took advantage of pharmacological
inhibitors, knockdown approaches, and the emerging knowledge on how
permeability factors affect endothelial junctions to investigate in detail the
mechanism underlying thrombin-induced endothelial permeability. We show that
thrombin signals through PAR-1 and its coupled G proteins G(12/13) and G(11/q)
to induce RhoA activation and intracellular calcium elevation, and that these
events are interrelated. In turn, this leads to the stimulation of ROCK, which
causes actin stress-fiber formation. However, this alone is not sufficient to
account for thrombin-induced permeability. Instead, we found that protein
kinase C-related kinase, a Rho-dependent S/T kinase, is activated in
endothelial cells upon thrombin stimulation and that its expression is required
for endothelial permeability and remodeling of cell-extracellular matrix and
cell-cell adhesions. Our results demonstrate that the signal initiated by
thrombin bifurcates at the level of RhoA to promote changes in the cytoskeletal
architecture through ROCK and the remodeling of focal adhesion components
through PRK. Ultimately, both pathways converge to cause cell-cell junction
disruption and provoke vascular leakage.
