IN-VITRO EFFECT OF ESTRADIOL, PROGESTERONE, TESTOSTERONE, AND OF COMBINED ESTRADIOL PROGESTINS ON LOW-DENSITY-LIPOPROTEIN (LDL) OXIDATION IN POSTMENOPAUSAL WOMEN/

Objective:One of the mechanisms currently proposed to explain the card ioprotective effect of hormone replacement therapy (HRT) is the antiox idative property of estrogens. Considering that HRT involves the admin istration of an estrogen, usually combined with a progestin and someti mes with an androgen, we investigated the following in vitro: (1) the effect of estradiol, progesterone, and testosterone on the oxidation o f low density lipoprotein; (2) the possible pro-oxidative effect of pr ogesterone and testosterone on native low density lipoprotein; and (3) the possible modification of the antioxidant effect of estradiol on l ow density lipoprotein induced by progestins. Design:Low density lipop rotein was isolated from blood samples obtained from 20 untreated post menopausal women and divided in multiple aliquots, each containing 0.5 mg LDL protein. In Protocol 1 (n = 10) different doses of estradiol, progesterone, and testosterone ranging from 0 to 26 mu g/ml were teste d inducing oxidation with 15 mu M copper sulfate. In Protocol 2 (n = 6 ) we studied the rate of oxidation of low density lipoprotein incubate d with progesterone or testosterone without any oxidative induction. I n Protocol 3 (n = 10) we studied the concomitant effect of 15 mu M est radiol with four separate progestins (progesterone, medroxyprogesteron e acetate, norethindrone, and norgestrel) in different doses (0, 5, 15 , and 50 mu M) After incubation for 4 h at 37 degrees C, malonaldehyde was measured as a marker of low density lipoprotein oxidation. The re sults were expressed in mean +/- SD. Results: Protocol 1: Estradiol in duced a dose-dependent decrease in malonaldehyde generation, from a ba seline of 61.8 +/- 30.2 nmol/mg protein to 11.6 +/- 7.1 nmol/mg protei n at the highest dose of estradiol tested (p < 0.0001). Progesterone o r testosterone did not modify malonaldehyde generation. Protocol 2: Pr ogesterone and testosterone did not show pro-oxidative action. Protoco l 3: Estradiol 15 mu M alone induced a 35% decrease in malonaldehyde g eneration, from a baseline of 75.4 +/- 25.4 to 49.3 +/- 18.8 nmol/mg p rotein (p < 0.0001). Norgestrel and norethindrone did not modify the a ntioxidant effect of estradiol (p > 0.05). Progesterone and medroxypro gesterone acetate induced a further reduction of malonaldehyde concent ration to 37.2 +/- 20.8 and 38.6 +/- 18.2 nmol/mg protein, only at the highest dose tested (p < 0.02 and p < 0.01, respectively). Conclusion s: Our results demonstrate that, in contrast with the potent antioxida nt effect of estradiol, progesterone and testosterone did not show any pro-or antioxidant effect on low density lipoprotein in vitro. Furthe rmore, progestins did not counteract the antioxidant effect of estradi ol in vitro.