THROMBIN INACTIVATES MYOSIN LIGHT-CHAIN PHOSPHATASE VIA RHO AND ITS TARGET RHO-KINASE IN HUMAN ENDOTHELIAL-CELLS

The role of Rho GTPase and its downstream targets Rho kinase and myosi n light chain phosphatase in thrombin-induced endothelial cell contrac tion was investigated. The specific Rho inactivator C3-transferase fro m Clostridium botulinum as well as microinjection of the isolated Rho- binding domain of Rho kinase or active myosin light chain phosphatase abolished thrombin-stimulated endothelial cell contraction. Conversely , microinjection of constitutively active V14Rho, constitutively activ e catalytic domain of Rho kinase, or treatment with the phosphatase in hibitor tautomycin caused contraction. These data are consistent with the notion that thrombin activates Rho/Rho kinase to inactivate myosin light chain phosphatase in endothelial cells. In fact, we demonstrate that thrombin transiently inactivated myosin light chain phosphatase, and this correlated with a peak in myosin light chain phosphorylation . C3-transferase abolished the decrease in myosin light chain phosphat ase activity as well as the subsequent increase in myosin light chain phosphorylation and cell contraction. These data suggest that thrombin activates the Rho/Rho kinase pathway to inactivate myosin light chain phosphatase as part of a signaling network that controls myosin light chain phosphorylation/contraction in human endothelial cells.