Estrogens and menopause: pharmacology of conjugated equine estrogens and their potential role in the prevention of neurodegenerative diseases such as Alzheimer's.

Menopause marks the start of a new phase in a woman's life that is associated with a decrease in circulating estrogen levels. Although the average age of women has increased from 50 to nearly 85 years, the average age at menopause has remained essentially constant at 50 years. Thus, women now spend nearly a third of their lives in an estrogen deficient state. This normal aging process in women is associated with increasing health problems such as osteoporosis, cardiovascular disease, neurodegenerative diseases, and cancer. Estrogen replacement therapy (ERT) has been shown to play an important beneficial role in the health and well being of postmenopausal women. Several estrogen preparations are available and among these conjugated equine estrogens (CEE) are most frequently used. The drug CEE, is a complex natural urinary extract of pregnant mare's urine and contains at least 10 estrogens in their sulfate ester form and these are the ring B saturated estrogens: estrone (E(1)), 17beta-estradiol (17beta-E(2)), 17alpha-estradiol (17alpha-E(2)), and the ring B unsaturated estrogens equilin (Eq), 17beta-dihydroequilin (17beta-Eq), 17alpha-dihydroequilin (17alpha-Eq), equilenin (Eqn), 17beta-dihydroequilenin (17beta-Eqn), 17alpha-dihydroequilenin (17alpha-Eqn), and Delta(8)-estrone (Delta(8)-E(1)). All of these estrogens in their unconjugated form are biologically active and can interact with recombinant human estrogen receptor alpha (ERalpha) and beta (ERbeta) with 17beta-estradiol and 17beta-dihydroequilin having the highest affinity for both receptors. A number of the ring B unsaturated estrogens had nearly twofold higher affinity for the ERbeta. The pharmacokinetics of these estrogens in postmenopausal women indicate that the unconjugated estrogens compared to their sulfated forms are cleared more rapidly. The 17-keto estrogens are metabolized to the more potent 17beta-reduced products which are cleared at a slower rate. In postmenopausal women, the extent of 17beta-activation is much higher with the ring B unsaturated estrogens than with ring B saturated estrogens. Oxidized LDL and oxidative stress are thought to contribute to both atherosclerosis and neurodegenerative disorders. Neurons in particular are at a high risk from damage resulting from oxidative stress. In vivo and in vitro studies indicate that the oxidation of LDL isolated from postmenopausal women was inhibited differently by various estrogens and other antioxidants. The unique ring B unsaturated estrogens were the most potent while the red wine component t-resveratrol was the least potent. Studies were designed to explore the cellular and molecular mechanisms that may be involved in the neuroprotective effects of CEE components. The data indicate that the neurotoxic effects of oxidized LDL and glutamate can be inhibited by various estrogens, with the ring B unsaturated estrogens being the most active. These effects are involved in the inhibition of DNA fragmentation and up-regulation of anti-apoptotic protein Bcl-2 and down-regulation of pro-apoptotic protein Bax. These combined data suggest that some of the neuroprotective benefits associated with long-term estrogen therapy may occur by the above mechanism(s). Because estrogens such as the Delta(8)-estrogens are relatively less feminizing than the classical estrogen 17beta-estradiol, they may be important in the development of more neuro-specific estrogens that will be useful in the prevention of neurodegenerative diseases, such as Alzheimer's and Parkinson disease, in both men and women.