How Estrogen Protects Bones

It is well known that estrogen is essential for healthy bone, and that when the production of estrogen is reduced, as occurs normally in postmenopausal women and pathogenically after exposure to radiation or chemotherapeutic drugs, bones become brittle and break easily. However, the mechanisms involved aren't clearly understood.

The new study found that one way estradiol helps to maintain bone density is by stopping the activation of an enzyme known as caspase-3. Also called the executioner caspase, caspase-3 is the central player in initiating the process of apoptosis, or programmed cell death of osteoblasts, the bone cells that aid in the growth and development of new bone and teeth.

Results of the study will be presented  at the International Association of Dental Research meeting in New Orleans.

Peter G. Bradford, Ph.D., senior author on the study said of the results: "Basic and clinical studies have shown that estrogens can prevent both bone loss and reduce the incidence of bone fractures. Our research at the molecular and cellular level suggests that the underlying basis of this protective effect of estrogens involves the prevention of apoptosis in osteoblasts and that the key event in this prevention is the inhibition of caspase-3 activity."

Bradford is an associate professor of pharmacology and toxicology in the UB School of Medicine and Biomedical Sciences and associate professor of oral biology in the UB School of Dental Medicine. Kenneth V. Gerace, a third-year dental student in his laboratory, is first author on the study.

To determine the effect of estradiol on caspase-3 activity, one group of human osteoblasts was treated with estradiol for 24 hours and another group was not. Both groups then were exposed for 24 hours to a drug called etoposide, a cancer chemotherapeutic drug that promotes apoptosis.

Results showed that caspace-3 activity decreased in cells treated with estrogen, but increased in cells not treated with estrogen.

"These findings support our hypotheses that the anti-osteoporotic effects of estradiol may result in part from its anti-apoptotic effects on osteoblasts," said Bradford.

"We now are investigating the biochemical mechanisms that mediate the estrogen-dependent inhibition of caspase-3 activity in osteoblasts and whether other pharmacological or nutritional agents might mimic or aid this action of estradiol."

Brian G. Chrzan, a UB orthodontic resident and oral biology doctoral candidate, also contributed to the research.

Fellowship support for Gerace was provided by a research training grant from the National Institutes of Health.