The Durk Pearson & Sandy Shaw®
Life Extension NewsTM
Volume 6 No.
5 • November-December 2003
Thiol Antioxidants Reduce Bone Loss in Estrogen-Deficient Female Mice
There may be a number of mechanisms explaining the protective effect of estrogens on increasing or maintaining bone mass in female mammals. The hormones have direct effects on osteoclasts (bone cells that dismantle bone under the control of signals from osteoblasts, the bone-building cells). Also, inflammatory cytokines, such as TNF-alpha, IL-1, and IL-6, have been implicated in bone loss; the effects of these cytokines on bone are opposed by estrogens.
Estrogens act as powerful antioxidants. Since bone loss is stimulated by increased oxidative stress (by, for example, stimulating the secretion of TNF-alpha and other cytokines), this is a very plausible mechanism for estrogenic bone protection. The authors of a new paper investigated the effects of estrogen on antioxidant defenses in bone. They report finding substantial decreases in glutathione and thioredoxin, and in the enzymes that regenerate the reduced forms of these protective thiol antioxidants in ovariectomized rodent bone marrow, that were reversed by estrogen administration. They also found that administration of the thiol antioxidant and glutathione precursor N-acetylcysteine suppressed TNF-alpha expression in osteoclasts, thus reducing bone loss. The N-acetylcysteine was administered to the mice at 100 mg/kg•day intraperitoneally twice per day for 14 days.
The authors note that, "Although [the experiments] suggest that 17-beta-estradiol maintains bone quantity through induction of thiol antioxidants, [the experiments] do not distinguish between central and local mechanisms for this effect on bone, nor, if the action is local, do they identify the target cell in bone. We therefore tested the effects of 17-beta-estradiol on the thiol antioxidant system in osteoblastic and osteoclastic cells. We found that neither glutathione reductase nor total glutathione were [sic] influenced by incubation in 17-beta-estradiol [in osteoblasts]. . . . In contrast, 17-beta-estradiol strongly stimulated glutathione and thioredoxin reductases in osteoclast-like cells in vitro. . . . This was completely reversed by ICI 182,780 [an estrogen inhibitor]. . . . The 17-beta-estradiol also strongly stimulated glutathione levels in osteoclasts. . . ."
The authors reasoned that, if estrogen regulates osteoclasts through thiol antioxidants, they would expect 17-beta-estradiol and thiol-modulating agents to influence reactive oxygen species (ROS)-sensitive signals in osteoclastic cells. They note that the best-studied signaling target for ROS is NF-kappaB, activated by ROS, which controls apoptosis, among other things. They found that BSO (which depletes glutathione) augmented, while N-acetylcysteine suppressed, the level of activated NF-kappaB. Since TNF-alpha is a target cytokine for NF-kappaB, they tested the effects of the antioxidants and estrogen on TNF-alpha and found that both 17-beta-estradiol and N-acetylcysteine suppressed expression of the mRNA for TNF-alpha in osteoclasts.
These experiments suggest that thiol antioxidants, such as N-acetylcysteine, may be able to prevent bone loss by modulating, at least in part, signaling pathways used by estrogen to prevent bone loss. It is not clear from this paper what the appropriate dose for women would be. We therefore recommend the use of the dietary amino acid cysteine (N-acetylcysteine is rapidly broken down into cysteine in the GI tract) at amounts that could be found in a reasonably egg-rich diet (about 250 mg of cysteine is found in one egg).
- Lean et al. A crucial role for thiol antioxidants in estrogen-deficiency bone mass. J Clin Invest 112:915-23 (2003).