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AIM:To study the protein C activation system in human liver myofibroblasts,and the effects of activated protein C (APC) on these cells.METHODS:Human liver myofibroblasts were obtained by outgrowth.Expression of protease activated receptor 1 (PAR-1),endothelial protein C receptor (EPCR) and thrombomodulin (TM) was analyzed by flow cytometry.Extracellular signal-regulated kinase (ERK)1/2 activation was assessed by West blotting using anti-phospho-ERK antibodies.Collagen synthesis was studied with real-time reverse transcription-polymerase chain reaction (RT-PCR).Activation of protein C was studied by incubating liver myofibroblasts with zymogen protein C in the presence of thrombin and detecting the generation of APC with a colorimetric assay using a peptide substrate.RESULTS:Primary cultures of human liver myofibroblasts expressed EPCR on their surface,together with PAR-1 and TM.This receptor system was functional since exposure of myofibroblasts to APC induced ERK1/2 phosphorylation in a dose- and time-dependent manner.Furthermore,APC significantly upregulated the expression of collagen mRNA,as shown byreal-time RT-PCR.Collagen upregulation was controlledthrough the ERK pathway as it was inhibited when using the mitogen-activated protein/extracellular signal-regulated kinase kinase inhibitor PD98059.Finally,using a cell-based colorimetric assay,we showedthat intact myofibroblasts converted protein C into APCin the presence of thrombin.CONCLUSION:These data suggest that APC is a new modulator of liver myofibroblast activity and contributes to the pathophysiology of chronic liver diseases.