Paracrine and endocrine control of the growth hormone axis by estrogen

There is a strong biological link between the growth hormone (G H) and gonadal systems in growth, development and metabolism; however, regulatory interactions are poorly understood. Advances in estrogen biology and endocrine physiology have provided insights into mechanistic links betwee n the two systems. Estrogens are synthesized from androgens by aromatase which is widely distributed in extragona dal tissues. Local generation of estrogens raise the possibility of paracrine control as an additional level to clas sical endocrine regulation of the GH system. To explore the mechanistic links, we review the pharmacology of estrogen, the effects of estrogen replacement, antagonism, and the impact of aromatase inhibition on the GH system as well as the metabolic sequelae. In men, estrogens derived from androgens drive the central secretion of GH, independent o f the androgen receptor. In hypogonadal women, physiological replacement via a parenteral route evokes no effec t while estrogen receptor antagonism and estrogen deprivation induce disparate effects, providing no consistent ev idence that estrogens regulate the central secretion of GH via paracrine or endocrine mechanisms. However, delivery of estrogen by the oral route inhibits hepatic IGF-1 production, in turn increasing GH secretion via reduced feedbac k inhibition. This endocrine route-dependent effect of oral estrogen compounds on hepatic function induces detrimen tal metabolic effects on hypogonadal women. In conclusion, estrogens regulate the secretion and action of GH v ia complex paracrine and endocrine interactions and impart metabolic effects in a route- and gender-dependent manner . The metabolic sequelae of compounds mimicking, antagonizing, or depleting estrogens, should be considered in t ailoring and optimizing their use.