Oxidized LDL Induces Proliferation and Hypertrophy in Human Umbilical Vein Endothelial Cells via Regulation of p27Kip1 Expression

Oxidized LDL (OxLDL) induces proliferation in human umbilical vein endothelial cells (HUVEC). The influence of OxLDL on the cyclin-dependent kinase inhibitor p27(Kip1), on the activity of the small GTPase RhoA as a known regulator of p27(Kip1), and on resulting cell proliferation and hypertrophy was studied. HUVEC were stimulated with OxLDL (1 to 50 mug/ml). Proliferation was quantified by (3)H-thymidine incorporation, colorimetric 3-(4,5-dimethyl-2-thiazyl)-2,5-diphenyl-2h-tetrazolium bromide assay, and cell count and was compared with proliferation of HUVEC that were transfected with dominant negative RhoA or treated with the Rho-kinase inhibitor Y27632. Hypertrophy was quantified by (3)H-leucine incorporation and by planimetry. p27(Kip1) expression was determined by Western blot analysis. p27(Kip1) was downregulated by transient transfection with antisense oligonucleotides. Low concentrations of OxLDL induced proliferation of HUVEC, paralleled by a persistent decrease of p27(Kip1) expression. With the use of antisense oligonucleotides, further downregulation of p27(Kip1) expression enhanced the OxLDL-induced proliferative response. High concentrations of OxLDL resulted in cellular hypertrophy and caused a delayed increase in p27(Kip1) expression after initial downregulation. Concomitant, OxLDL caused a significant activation of the small GTPase RhoA. In cells that were transfected with dominant negative RhoA, the effect of OxLDL on p27(Kip1) expression and on cellular proliferation was abolished. HUVEC that were preincubated with the Rho-kinase inhibitor Y27632 also showed a significantly decreased proliferative response to OxLDL stimulation. In summary, OxLDL has a dual effect on cell-cycle progression via regulation of p27(Kip1) expression, resulting in cellular proliferation and hypertrophy, involving activation of RhoA. OxLDL may importantly contribute to vascular hyperplasia in atherosclerosis and other diseases associated with increased levels of OxLDL.