title: Relative impact of androgen and estrogen receptor activation in the effects of androgens on trabecular and cortical bone in growing male mice: a study in the androgen receptor knockout mouse model.
abstract: The relative importance of AR and ER activation has been studied in pubertal male AR knockout and WT mice after orchidectomy and androgen replacement therapy, either with or without an aromatase inhibitor. AR activation dominates normal trabecular bone development and cortical bone modeling in male mice. Moreover, optimal periosteal bone expansion is only observed in the presence of both AR and ER activation.
  
  
  BACKGROUND
  Androgen receptor (AR)-mediated androgen action has traditionally been considered a key determinant of male skeletal growth. Increasing evidence, however, suggests that estrogens are also essential for normal male bone growth. Therefore, the relative importance of AR-mediated and estrogen receptor (ER)-mediated androgen action after aromatization remains to be clarified.
  
  
  METHODS
  Trabecular and cortical bone was studied in intact or orchidectomized pubertal AR knockout (ARKO) and male wildtype (WT) mice, with or without replacement therapy (3-8 weeks of age). Nonaromatizable (dihydrotestosterone [DHT]) and aromatizable (testosterone [T]) androgens and T plus an aromatase inhibitor (anastrazole) were administered to orchidectomized ARKO and WT mice. Trabecular and cortical bone modeling were evaluated by static and dynamic histomorphometry, respectively.
  
  
  RESULTS
  AR inactivation or orchidectomy induced a similar degree of trabecular bone loss (-68% and -71%, respectively). Both DHT and T prevented orchidectomy-induced bone loss in WT mice but not in ARKO mice. Administration of an aromatase inhibitor did not affect T action on trabecular bone. AR inactivation and orchidectomy had similar negative effects on cortical thickness (-13% and -8%, respectively) and periosteal bone formation (-50% and -26%, respectively). In orchidectomized WT mice, both DHT and T were found to stimulate periosteal bone formation and, as a result, to increase cortical thickness. In contrast, the periosteum of ARKO mice remained unresponsive to either DHT or T. Interestingly, administration of an aromatase inhibitor partly reduced T action on periosteal bone formation in orchidectomized WT mice (-34% versus orchidectomized WT mice on T), but not in ARKO mice. This effect was associated with a significant decrease in serum IGF-I (-21% versus orchidectomized WT mice on T).
  
  
  CONCLUSIONS
  These findings suggest a major role for AR activation in normal development of trabecular bone and periosteal bone growth in male mice. Moreover, optimal stimulation of periosteal growth is only obtained in the presence of both AR and ER activation.
authors.affiliation: Laboratory for Experimental Medicine and Endocrinology, Katholieke Universiteit Leuven, Leuven, Belgium.
authors.forename: Katrien
authors.initials: K
authors.lastname: Venken
authors.forename: Karel
authors.initials: K
authors.lastname: De Gendt
authors.forename: Steven
authors.initials: S
authors.lastname: Boonen
authors.forename: Jill
authors.initials: J
authors.lastname: Ophoff
authors.forename: Roger
authors.initials: R
authors.lastname: Bouillon
authors.forename: Johannes V
authors.initials: JV
authors.lastname: Swinnen
authors.forename: Guido
authors.initials: G
authors.lastname: Verhoeven
authors.forename: Dirk
authors.initials: D
authors.lastname: Vanderschueren
mesh_terms: D000728:Androgens / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D047072:Aromatase Inhibitors / Q000494:pharmacology; D001842:Bone and Bones / Q000187:drug effects*; D013196:Dihydrotestosterone / Q000494:pharmacology; D004789:Enzyme Activation; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D023421:Models, Animal; D009919:Orchiectomy; D010012:Osteogenesis / Q000187:drug effects; D011944:Receptors, Androgen / Q000172:deficiency* / Q000235:genetics / Q000378:metabolism*; D011960:Receptors, Estrogen / Q000819:agonists / Q000378:metabolism*; D013739:Testosterone / Q000494:pharmacology; D014129:Trabecular Meshwork / Q000187:drug effects*
chemical_list: D000728:Androgens; D047072:Aromatase Inhibitors; D011944:Receptors, Androgen; D011960:Receptors, Estrogen; D013196:Dihydrotestosterone; D013739:Testosterone