Abstract 184: Polo Like Kinase 1 Regulates Smooth Muscle Cell Proliferation in the G2-M Phase in Human Coronary Artery Bypass Conduits

Abstract

Proliferation of smooth muscle cells (SMCs) and the resultant intimal hyperplasia cause coronary artery bypass graft (CABG) failure. Internal mammary artery (IMA) graft is immune to intimal hyperplasia (IH), but a saphenous vein graft is prone to develop neointima. The fate of SMCs is determined by the balance between the mitogenic and anti-mitogenic signals. Mitosis of SMCs through the cell cycle involves crossing the two restriction (R)-points in G1 and G2 phase, respectively. Upon loss of proper G1/S control, cells will progress into the G2-phase where G2 R-point can prevent cell proliferation. In this study, we examined the effect of mitogenic PDGF-BB stimulation on the G2 R-point and its relationship with the phosphorylation of PLK1, Cdc2 and Histone H3 in isolated SMCs of human bypass graft conduits. We observed increased expression and phosphorylation of PLK1 in PDGF-stimulated SV-SMCs compared to control SMCs without the treatment. Furthermore, PLK1 was phosphorylated and activated for a longer period of time in PDGF-stimulated SV- than IMA-SMCs. PLK1 m-RNA level was higher in PDGF-stimulated SV-SMCs than IMA. An ATP-competitive inhibitor of PLK1 attenuated PDGF-BB-induced proliferation in IMA and SV-SMCs. Cell proliferation was measured using cell count and immunoblotting against phospho-Histone H3 (pHistone) at Ser-10. Treatment with PLK1 inhibitor reduced PDGF-induced Cdc2 activation. Silencing the PLK1 gene by siRNA transfection of SV- and IMA-SMCs significantly reduced the expression of pHistone. These data demonstrate differential activity of PDGF-BB-induced PLK1, which was quantitatively and temporally greater in SV-SMCs than in the IMA. PLK1 inhibitor completely blocked the phosphorylation of PLK1 and attenuated proliferation in SV-SMCs. This in part, could suggest that PLK1 plays a critical role in the development of neointimal hyperplasia in SV grafts following CABG. Thus, inhibition of PLK1 activity could be a target in developing better therapeutic approach to prevent vein-graft disease.