Skeletal effects of androgen withdrawal.

Hypogonadism is considered to be one of the major risk factors for osteoporosis in men. Therefore, it is an important goal for skeletal research to improve our understanding of the skeletal effects of androgens. Androgen deficiency during growth is associated with a failure to acquire normal peak bone mass, and there is good evidence that the effects of androgens on skeletal growth and the development of a male skeletal phenotype are mediated through the androgen receptor. In adult men, acute withdrawal of androgens by surgical or chemical castration induces high turnover bone loss. Similarly, orchidectomy of aged, non-growing male rats is associated with a pronounced and sustained increase in bone turnover and with true loss of cancellous and cortical bone. Interestingly, the changes in bone turnover induced by orchidectomy are paralleled by a concomitant increase in B lymphopoiesis in bone marrow of rats and mice. Although there is firm evidence that male bone metabolism can be influenced by androgens and estrogen, a variety of clinical and animal experimental data have strongly suggested that, under physiological circumstances, the maintenance of cancellous bone mass in males involves the skeletal action of estrogen derived from aromatization of androgens. Aged male rats appear to closely mimic the conditions induced by androgen withdrawal in adult humans, and this animal model may be used 1) to elucidate further the role of muscle as a mediator of the actions of androgens on bone, 2) to explore the regulatory functions of androgens and estrogens in the male skeleton and the immune system, and 3) to find new treatment strategies for the prevention and treatment of osteoporosis in men.