Aromatase Activity and Bone Homeostasis in Men

It is known that sex steroid hormones play an important role in the maintenance of bone mass in males as well as in females. Even though androgens are the major sex steroids in men, their primacy in regulating male skeletal remodeling has been increasingly questioned as direct and indirect evidence emerged suggesting that estrogens may also play a major role in male skeletal health. Recent data suggested that a threshold level of bioavailable estradiol is needed to prevent bone loss, and that with aging an increasing percentage of elderly men begin to fall below this level. The testes account for, at most, 15% of circulating estrogens in the male; the remaining 85% comes from peripheral aromatization of androgen precursors in different tissues, including bone. Human models of aromatase deficiency were the first to demonstrate the critical importance of the conversion of circulating androgens into estrogen in regulating male skeletal homeostasis. All four cases of aromatase-deficient men reported to date showed an identical skeletal phenotype, characterized by tall stature due to continued longitudinal growth, unfused epiphyses, high bone turnover, and osteopenia. Studies using knockout mice along with experimental observations in rats treated with an aromatase inhibitor provided useful information about the importance of aromatase in the male skeleton. Confirmatory evidence comes from recent interventional studies in adult men using aromatase inhibition, which confirmed that estrogens are critically important to the male skeleton by helping to control rates of bone remodeling. Intriguingly, common polymorphisms at the human aromatase (CYP19) gene have been associated with differences in aromatase activity, bone turnover, and rates of bone loss in elderly men, suggesting that variations in aromatase efficiency may also be relevant for skeletal homeostasis. Several additional mechanisms have been proposed in which aromatase activity could be modulated under certain circumstances in different tissues. Additional studies are needed to identify how these genetic, environmental, pathological, and pharmacological influences might modulate aromatase activity in vivo, increasing or reducing estrogen production in males and thereby affecting skeletal health.