Objective: Smooth muscle cell (SMC) proliferation, invasion, and matrix metalloproteinase (MMP) secretion are key events in the development of intimal hyperplasia, the lesion that causes coronary artery bypass graft (CABG) failure. Saphenous vein (SV) grafts are the most commonly used bypass conduits but are markedly more susceptible to intimal hyperplasia than internal mammary artery (IMA) grafts. We hypothesized that this may be due to inherent functional differences between SV-SMCs and IMA-SMCs. In this study we used paired cultures of SV-SMCs and IMA-SMCs from the same patients and compared their rates of proliferation, invasion, migration, and MMP-2 and MMP-9 secretion.
Methods: SMCs were cultured from explants of paired SV and IMA from 22 patients undergoing CABG. SMC populations of equivalent passage were used to determine proliferation in response to 10% fetal calf serum (FCS), 10 ng/mL platelet-derived growth factor (PDGF), and 10 ng/mL basic fibroblast growth factor (bFGF) by counting cells during a 7-day period. Immunoblotting was used to quantify phosphorylation of p44/42-mitogen-activated protein kinase (MAPK). Invasion and migration rates of paired SMCs were quantified using a modified Boyden chamber technique in the presence or absence of a Matrigel basement membrane barrier (BD Biosciences, Oxford, UK). Conditioned media from invasion assays were analyzed for secretion of MMP-2 and MMP-9 by gelatin zymography.
Results: Analysis of areas under curves for 7-day proliferation assays revealed that the number of SV-SMCs in response to FCS, PDGF, and bFGF was 2.1, 2.0, and 2.3 times higher, respectively, than that of paired IMA-SMCs. Basal MAPK activation in SV-SMCs was approximately double that of paired IMA-SMCs. SV-SMCs exhibited a 2.1-fold increase in invasion rate (Matrigel barrier) compared with IMA-SMCs, but migration rates (no Matrigel barrier) and MMP-2 and MMP-9 secretion were similar for the two cell types.
Conclusions: Human SV-SMCs are inherently more proliferative and invasive than paired IMA-SMCs, likely due to a relative increase in p44/42-MAPK activation. These inherent functional differences between SMC of different origins may contribute to the increased prevalence of intimal hyperplasia in SV grafts compared with IMA grafts.