Summary
Plasminogen activator (PA) expression plays an important role in smooth muscle cell
(SMC) migration and may therefore contribute to mechanical force-induced arterialization
of vein grafts. The aim of this study was to determine whether pulse pressure due
to pulsatile flow modulates SMC migration via urokinase (u-PA)-dependent mechanisms.
Using a perfused transcapillary culture system, human umbilical vein SMC were exposed
to pulse pressures (0-56 mmHg), in the absence or presence of human umbilical vein
endothelial cells (EC) by varying pulsatile flow rates (0 ml/min to 25 ml/min). SMC
cultured in the absence of EC increased their migration following exposure to increased
pulse pressure (248 ± 14%). Both u-PA and matrix metalloproteinase 1 (MMP-1) expression
was significantly elevated in SMC exposed to pressure as compared to static controls.
The role of proteases in the pulse pressure-induced enhancement of SMC migration was
confirmed following pretreatment with aprotinin, an anti u-PA antibody and metalloproteinase
inhibitors (181 ± 14% for aprotinin vs. 256 ± 25% for control, 108 ± 4% for anti-u-PA
antibody vs. 233 ± 17% for non-immune IgG, and 114 ± 9% for BB-94, 105 ± 7% for BB-3103
vs. 222 ± 5% for control). Using SMC derived from u-PA gene knockout mice, the SMC
migratory response to increased pulse pressure was completely inhibited despite a
significant increase in MMP expression in these cells. These results suggest that
pulse pressure due to pulsatile flow induces SMC migration in vitro via u-PA and MMP-dependent mechanisms. Moreover, u-PA gene deletion results in blunting
of pressure-induced SMC migration despite the endogenous upregulation of metalloproteinase.
Modulation of u-PA expression by pressure may thus represent an important mechanism
whereby hemodynamic forces regulate smooth muscle cell migration.