Sex steroids and bone: current perspectives

Although the process of bone remodelling or its control has not yet been fully elucidated there is, at present, sufficient information available to conclude that ovarian steroids (estrogens, androgens, progesterone) play an essential role in skeletal homeostasis. The mechanism of action of sex steroids on the skeleton is still not entirely clear, but it has traditionally included indirect effects on systemic hormones that regulate calcium balance and a direct receptor-mediated action. More recently, changes in cytokine production within the bone marrow, as well as pro-apoptotic and anti-apoptotic effects in the osteoblastic cells, have been proposed as new perspectives on the cellular and molecular mechanisms by which sex steroids influence adult bone homeostasis. Mechanical loading, when combined with estrogens or androgens, results in a greater osteogenic response than either condition separately. Women are especially at risk for osteoporosis if they have had a premature or surgical menopause and have not received hormone replacement therapy (HRT). Other reproductive factors that can help to identify women with osteopenia and emphasize the role of sex steroids in preserving bone mass in premenopausal women include: age at menarche, menstrual history and irregularities (including those associated with excessive exercise), age at menopause, previous hysterectomy, hyperprolactinaemia, anorexia nervosa, scoliosis, ovarian dysgenesis, pregnancy and lactation, and pharmacological ovarian suppression. The prevention of osteoporosis starts with the onset of the menarche. A combination of exercise, appropriate nutrition and a healthy lifestyle all maximize bone mineral accrual and result in optimal peak bone mass; normal ovarian function is essential to this process. Unfortunately, many women actually become aware of the need for osteoporosis prevention much later in life, usually after they have already become menopausal. HRT, however, has important limitations for prevention of fractures in post-menopausal women. Future perspectives for treatment of osteoporosis include androgen therapy and anabolic agents. Specifically, synthetic ligands of the estrogen receptor that can evoke the non-genotrophic but not the genotrophic signal of the receptor may be bone anabolic agents, as opposed to natural estrogens or selective estrogen receptor modulators that are anti-resorptive agents. The same ligands may circumvent the side effects associated with conventional HRT.