The migration of vascular smooth muscle cells (SMCs) is a hallmark of the pathogenesis of atherosclerosis and restenosis after angioplasty. Plasma low-density lipoprotein (LDL) but not high-density lipoprotein (HDL) induced the migration of human coronary artery SMCs (CASMCs). Among bioactive lipids postulated to be present in LDL, lysophosphatidic acid (LPA) appreciably mimicked the LDL action. In fact, the LDL-induced migration was markedly inhibited by pertussis toxin, an LPA receptor antagonist Ki16425, and a small interfering RNA (siRNA) targeted for LPA₁ receptors. Moreover, LDL contains a higher amount of LPA than HDL does. HDL markedly inhibited LPA- and platelet-derived growth factor (PDGF)-induced migration, and sphingosine 1-phosphate (S1P), the content of which is about 4-fold higher in HDL than in LDL, mimicked the HDL action. The inhibitory actions of HDL and S1P were suppressed by S1P₂-receptor-specific siRNA. On the other hand, the degradation of the LPA component of LDL by MG lipase or the antagonism of LPA receptors by Ki16425 allowed LDL to inhibit the PDGF-induced migration. The inhibitory effect of LDL was again suppressed by S1P₂-receptor-specific siRNA. In conclusion, LPA/LPA₁ receptors and S1P/S1P₂ receptors mediate the stimulatory and inhibitory migration response to LDL and HDL, respectively. The balance of not only the content of LPA and S1P in lipoproteins but also the signaling activity between LPA₁ and S1P₂ receptors in the cells may be critical in determining whether the lipoprotein is a positive or negative regulator of CASMC migration.