Estrogen and tamoxifen metabolites protect smooth muscle cell membrane phospholipids against peroxidation and inhibit cell growth.

The goal of this study was to test the hypothesis that antioxidant estrogens, by a mechanism independent of the estrogen receptor, protect phospholipids residing in the plasma membrane of vascular smooth muscle cells from peroxidation and peroxidation-induced cell growth and migration. Peroxidation of membrane phospholipids was assessed by HPLC analysis of phospholipids extracted from rat aortic vascular smooth muscle cells prelabeled with cis-parinaric acid (a fatty acid that is susceptible to peroxidation, which quenches its fluorescent properties). Incubation of cells for 2 hours with the peroxyl radical donor 2,2'-azobis-2,4-dimethylvaleronitrile (AMVN) caused peroxidation of all measured membrane phospholipids. This effect was attenuated by pretreating cells for 15 minutes with 50 to 5000 ng/mL of 2-hydroxyestradiol (strong antioxidant but weak estrogen-receptor ligand) or 4-hydroxytamoxifen (strong antioxidant and potent estrogen-receptor ligand), but not by estrone or droloxifene (both weak antioxidants but potent estrogen-receptor ligands). Moreover, pretreatment of cells for 20 hours with physiological concentrations (0.3 ng/mL) of 2-hydroxyestradiol or pharmacologically relevant concentrations of 4-hydroxytamoxifen (40 ng/mL) also decreased AMVN-induced phospholipid peroxidation. Both 2-hydroxyestradiol and 4-hydroxytamoxifen were as effective as 2,2,5, 7,8-pentamethyl-6-hydrochromane (an antioxidant homolog of vitamin E) in attenuating AMVN-induced peroxidation of membrane phospholipids. Also, physiological concentrations of 2-hydroxyestradiol, but not estrone, and pharmacologically relevant concentrations of 4-hydroxytamoxifen attenuated AMVM-induced DNA synthesis, cell proliferation, and cell migration. These studies demonstrate in vascular smooth muscle cells that antioxidant estrogens via a non-estrogen receptor-dependent mechanism attenuate peroxidation of membrane phospholipids and peroxidation-induced cell growth and migration.