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Lysophosphatidylcholine (lysoPC), an active component of oxidized low-density lipoprotein, stimulates proliferation of vascular smooth muscle cells (VSMC). We investigated the direct impact of 17β-estradiol (E2) on the proliferation of VSMC from rat aorta.VSMC derived from both female and male rats expressed estrogen receptors α and β. Treatments with 1% fetal bovine serum or 5 μM lysoPC increased the incorporation of [3H]-thymidine in VSMC obtained from female rats. 17β-E2 did not alter the response to fetal bovine serum, but significantly suppressed the enhanced deoxyribonucleic acid synthesis which had been induced by lysoPC in a dose-dependent manner (10−14–10−6 M). Estrogen also inhibited the proliferation of VSMC from male animals. ICI 182,780, a specific estrogen receptor antagonist, and 17α-E2, an inactive form of estradiol, also decreased the mitogenic response to lysoPC in VSMC. In addition, N-acetyl-l-cysteine, a potent antioxidant, inhibited the lysoPC effect. Flow cytometric analysis using the oxidation-sensitive probe 2´,7´-dichlorofluorescin diacetate revealed that elevated intracellular formation of reactive oxygen species elicited with lysoPC was depressed significantly by 17β-E2, ICI 182,780, or 17α-E2 as well as by N-acetyl-l-cysteine.17β-E2 inhibits in vitro VSMC proliferation induced by lysoPC via a nongenomic antioxidant mechanism.