Skeletal effects of estrogen are mediated by opposing actions of classical and nonclassical estrogen receptor pathways.
ER alpha acts either through classical (ERE-mediated) or nonclassical (non-ERE) pathways. The generation of mice carrying a mutation that eliminates classical ER alpha signaling presents a unique opportunity to study the relative roles of these pathways in bone. This study defines the skeletal phenotype and responses to ovariectomy and estrogen replacement in these mice.


BACKGROUND
Estrogen receptor alpha (ER alpha) can act either through classical estrogen response elements (EREs) or through non-ERE (nonclassical) pathways. To unravel these in bone, we crossed mice heterozygous for a knock-in mutation abolishing ERE binding (nonclassical ER alpha knock-in [NERKI]) with heterozygote ER alpha knockout mice and studied the resulting female ER alpha(+/+), ER alpha(+/NERKI), and ER alpha(-/NERKI) mice. The only ER alpha present in ER alpha(-/NERKI) mice is incapable of activating EREs but can signal through nonclassical pathways, whereas ER alpha(+/NERKI) mice may have a less drastic alteration in the balance between classical and nonclassical estrogen signaling pathways.


METHODS
BMD was measured using DXA and pQCT at 3 months of age (n = 46-48/genotype). The mice were randomly assigned to sham surgery, ovariectomy, ovariectomy + estradiol (0.25 microg/day), or ovariectomy + estradiol (1.0 microg/day; n = 10-12/group) and restudied 60 days later.


CONCLUSIONS
At 3 months of age, both the ER alpha(+/NERKI) and ER alpha(-/NERKI) mice had deficits in cortical, but not in trabecular, bone. Remarkably, changes in cortical bone after ovariectomy and estrogen replacement in ER alpha(-/NERKI) mice were the opposite of those in ER alpha(+/+) mice. Relative to sham mice, ovariectomized ER alpha(-/NERKI) mice gained more bone (not less, as in ER alpha(+/+) mice), and estrogen suppressed this increase (whereas augmenting it in ER alpha(+/+) mice). Estrogen also had opposite effects on bone formation and resorption parameters on endocortical surfaces in ER alpha(-/NERKI) versus ER alpha(+/+) mice. Collectively, these data show that alteration of the balance between classical and nonclassical ER alpha signaling pathways leads to deficits in cortical bone and also represent the first demonstration, in any tissue, that complete loss of classical ERE signaling can lead to paradoxical responses to estrogen. Our findings strongly support the hypothesis that there exists a balance between classical and nonclassical ER alpha signaling pathways, which, when altered, can result in a markedly aberrant response to estrogen.
Syed
Farhan A
FA
Endocrine Research Unit, Mayo Clinic College of Medicine, Rochester, Minnesota 55905, USA.
Mödder
Ulrike I L
UI
Fraser
Daniel G
DG
Spelsberg
Thomas C
TC
Rosen
Clifford J
CJ
Krust
Andree
A
Chambon
Pierre
P
Jameson
J Larry
JL
Khosla
Sundeep
S
D000818:Animals; D015519:Bone Density / Q000187:drug effects; D001842:Bone and Bones / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D018483:Diaphyses / Q000187:drug effects / Q000378:metabolism; D004958:Estradiol / Q000097:blood / Q000494:pharmacology; D047628:Estrogen Receptor alpha / Q000235:genetics / Q000502:physiology; D004967:Estrogens / Q000494:pharmacology / Q000502:physiology*; D005260:Female; D005269:Femur / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D005838:Genotype; D007334:Insulin-Like Growth Factor I / Q000378:metabolism; D008159:Lumbar Vertebrae / Q000187:drug effects / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D008815:Mice, Inbred Strains; D018345:Mice, Knockout; D008817:Mice, Mutant Strains; D009929:Organ Size / Q000187:drug effects; D010052:Ovariectomy; D010641:Phenotype; D011897:Random Allocation; D011960:Receptors, Estrogen / Q000235:genetics / Q000502:physiology*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D013977:Tibia / Q000187:drug effects / Q000378:metabolism; D014599:Uterus / Q000033:anatomy & histology / Q000187:drug effects
D047628:Estrogen Receptor alpha; D004967:Estrogens; D011960:Receptors, Estrogen; D004958:Estradiol; D007334:Insulin-Like Growth Factor I