Osteoblast-like cells from estrogen receptor alpha knockout mice have deficient responses to mechanical strain.
BACKGROUND
In vivo, bones' osteogenic response to mechanical loading involves proliferation of surface osteoblasts. This response is replicated in vitro and involves ERK-mediated activation of the estrogen receptor (ER) alpha and upregulation of estrogen response element activity. This proliferative response can be blocked by selective estrogen receptor modulators and increased by transfection of additional ERalpha.


METHODS
We have now investigated the mechanisms of ER involvement in osteoblast-like cells' early responses to strain by comparing the responses of primary cultures of these cells derived from homozygous ERalpha knockout (ERKO) mice (ERalpha-/-) with those from their wildtype (ERalpha+/+) and heterozygous (ERalpha+/-) littermates and from ER/beta knockout (BERKO) mice (ERbeta+/+, ERbeta+/-, and ERbeta-/-).


RESULTS
Whereas ERalpha+/+, ERalpha+/-, ERbeta+/+, and ERbeta-/- cells proliferate in response to a single 10-minute period of cyclic strain, ERalpha-/- cells do not. Transfection of fully functional, but not mutant, ERalpha rescues the proliferative response to strain in these cells. The strain-related response of ERalpha-/- cells is also deficient in that they show no increased activity of an AP-I driven reporter vector and no strain-related increases in NO production. Their strain-related increase in prostacyclin production is retained. They proliferate in response to fibroblast growth factor-2 but not insulin-like growth factor (IGF)-I or IGF-II, showing the importance of ERalpha in the IGF axis and the ability of ERalpha-/- cells to proliferate normally in response to a mitogenic stimulus that does not require functional ERalpha.


CONCLUSIONS
These data indicate ERalpha's obligatory involvement in a number of early responses to mechanical strain in osteoblast-like cells, including those that result in proliferation. They support the hypothesis that reduction in ERalpha expression or activity after estrogen withdrawal results in a less osteogenic response to loading. This could be important in the etiology of postmenopausal osteoporosis.
Jessop
Helen L
HL
Department of Veterinary Basic Sciences, The Royal Veterinary College, London, United Kingdom.
Suswillo
Rosemary F L
RF
Rawlinson
Simon C F
SC
Zaman
Gul
G
Lee
Karla
K
Das-Gupta
Vicky
V
Pitsillides
Andrew A
AA
Lanyon
Lance E
LE
D000818:Animals; D002455:Cell Division / Q000187:drug effects; D047628:Estrogen Receptor alpha / Q000235:genetics / Q000502:physiology*; D005260:Female; D016222:Fibroblast Growth Factor 2 / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D007335:Insulin-Like Growth Factor II / Q000494:pharmacology; D008297:Male; D040542:Mechanotransduction, Cellular; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009569:Nitric Oxide / Q000096:biosynthesis; D010006:Osteoblasts / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D011453:Prostaglandins / Q000096:biosynthesis; D014162:Transfection
D047628:Estrogen Receptor alpha; D011453:Prostaglandins; D016222:Fibroblast Growth Factor 2; D009569:Nitric Oxide; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II