Alterations in vascular endothelial growth factor protein expression in experimental vein grafts

Administration of exogenous vascular endothelial growth factor (VEGF), an endothelial cell (EC)-specific mitogen, has been reported to promote angiogenesis, stimulate reendothelialization and limit intimai hyperplasia (IH) in animal models. Experimental vein grafts (VG) universally develop IH with accelerated adventitial angiogenesis following implantation into the arterial circulation, however, whether alterations in VEGF protein expression occur after bypass grafting is not known. This study examines the early changes in VEGF protein expression in VG. Male New Zealand White rabbits had right carotid interposition bypass grafting using the reversed ipsilateral external jugular vein. The VG and contralateral control external jugular veins (CV) were harvested from 12 animals at 10 min, 60 min, 6 hr, 24 hr, 3 days and 7 days after bypass grafting and processed for Western blot (Wb) and immunohistochemical (IHC) analyses (n=2 at 24 hr and earlier time-points and n=3 at 3 and 7 days). Compared to CV, there was a significant decrease in VEGF protein expression in VG, as quantified by Wb, by 10 minutes and through to day 3 postoperatively, ranging from 2.5-fold to 8.9-fold (p<0.05 by ANOVA with post hoc Tukey-Kramer multiple comparison tests). At day 7, VEGF protein expression in VG was not significantly different from CV. Compared to CV, VEGF immunostaining in VG was less intense, in fewer cells, which in particular, are localized in the adventitia and medial layer adjacent to the adventitia rather than throughout the vessel wall. EC did not stain for VEGF in both CV and VG. This study demonstrates that, in VG, VEGF protein expression is acutely reduced and confined to the SMC-adventitia interface. These findings suggest that global reduction of VEGF protein expression may alter EC-SMC interaction and contribute to the formation of IH in VG. Furthermore, the localization of VEGF predominantly in the adventitia may serve to promote ingrowth of new vasa vasorum.