Monocyte-expressed urokinase inhibits vascular smooth muscle cell growth by activating Stat1

Abstract

After vascular injury, a remodeling process occurs that features leukocyte migration and infiltration. Loss of endothelial integrity allows the leukocytes to interact with vascular smooth muscle cells (VSMCs) and to elicit "marching orders"; however, the signaling processes are poorly understood. We found that human monocytes inhibit VSMC proliferation and induce a migratory potential. The monocytes signal the VSMCs through the urokinase-type plasminogen activator (uPA). The VSMC uPA receptor (uPAR) receives the signal and activates the transcription factor Stat1 that, in turn, mediates the antiproliferative effects. These results provide the first evidence that monocytes signal VSMCs by mechanisms involving the fibrinolytic system, and they imply an important link between the uPA/uPARrelated signaling machinery and human vascular disease. IntroductionRestenosis after percutaneous vascular therapeutic interventions remains unresolved. 1,2 After vascular injury, the endothelial integrity is disturbed and monocytes infiltrate the injured vessels. The monocytes promote an inflammatory response by generating migratory and proliferative signals that converge on vascular smooth muscle cells (VSMCs). The VSMCs first migrate to the normally thin intimal layer and thereafter proliferate, causing neointimal formation and restenosis. 3,4 Although VSMC proliferation was reported in most animal models of vascular remodeling, few proliferating VSMCs were detected in human lesions. This finding implies an antiproliferative mechanism for VSMC growth control, at least during the early phase of the remodeling process. 5 We showed earlier that the urokinase-type plasminogen activator (uPA) and its receptor (uPAR) are up-regulated on VSMCmonocyte interaction in a coculture model and contribute to increased VSMC motility. 6 uPA is an unusual molecule of dual function that switches from the proteolytic enzyme to the signalinducing ligand, depending on external stimuli. 7,8 The uPA/uPAR system has a nonproteolytic role in vitro and in vivo that extends beyond its role in fibrinolysis. 9,10 We investigated the mechanisms underlying changes in VSMC behavior on interaction with monocytes. We found that the monocytes express uPA to convey an antiproliferative signal to the VSMCs through their uPAR receptor. This signaling is mediated by activation of the VSMC transcription factor signal transducer and activator of transcription 1 (Stat1). Our study may explain the increased VSMC migratory potential and the absence of VSMC proliferation observed at the early step of a remodeling process of the vessel wall after vascular injury in humans. Conceivably, a useful strategy for therapeutic intervention could ensue. Materials and methodsCell culture, monocyte isolation, coculture, and cell separation after coculture Human vascular smooth muscle cells from coronary artery were obtained from Clonetics (San Diego, CA). Cells were grown in Smooth Muscle Medium-2 (SmGM2) medium (Clonetics) supplemented with 5% fetal bovine serum and were used between...

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Urokinase-induced activation of the gp130/Tyk2/Stat3 pathway mediates a pro-inflammatory effect in human mesangial cells via expression of the anaphylatoxin C5a receptor

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