Distinct effects of loss of classical estrogen receptor signaling versus complete deletion of estrogen receptor alpha on bone

Estrogen receptor (ER) alpha is a major regulator of bone metabolism which can modulate gene expression via a "classical" pathway involving direct DNA binding to estrogen-response elements (EREs) or via "non-classical" pathways involving protein-protein interactions. While the skeletal consequences of loss of ERE binding by ER alpha have been described, a significant unresolved question is how loss of ERE binding differs from complete loss of ER alpha. Thus, we compared the skeletal phenotype of wild-type (ER alpha(+/+)) and ERa knock out (ER alpha(-/-)) mice with that of mice in which the only ERa present had a knock-in mutation abolishing ERE binding (non-classical ER alpha knock-in [NERKI], ER alpha(-/NERKI)). All three groups were in the same genetic background (C57BL/6). As compared to both ER alpha(+/+) and ER alpha(-/-) mice, ER alpha(-/NERKI) mice had significantly reduced cortical volumetric bone mineral density and thickness at the tibial diaphysis; this was accompanied by significant decreases in periosteal and endocortical mineral apposition rates. Colony forming unit (CFU)-fibroblast, CFU-alkaline phosphatase, and CFU-osteoblast numbers were all increased in ER alpha(-/-) compared to ER alpha(+/+) mice, but reduced in ER alpha(-/NERKI) mice compared to the two other groups. Thus, using mice in identical genetic backgrounds, our data indicate that the presence of an ER alpha that cannot bind DNA but can function through protein-protein interactions may have more deleterious skeletal effects than complete loss of ER alpha. These findings suggest that shifting the balance of classical versus non-classical ER alpha signaling triggers pathways that impair bone formation. Further studies defining these pathways may lead to novel approaches to selectively modulate ER signaling for beneficial skeletal effects. (C) 2011 Elsevier Inc. All rights reserved.