Osteoblast-Like Cells From Estrogen Receptor alpha Knockout Mice Have Deficient Responses to Mechanical Strain

Osteoblast-like cells in primary cultures derived from ER alpha super(-/-) mice do not proliferate in response to mechanical strain, unlike those from their ER alpha super(+/+) littermates. ER alpha super(-/-) cells also lack strain-related NO production and responsiveness to IGFs. Proliferative responsiveness to strain is rescued by transfection with functional ER alpha . ER alpha number or function in bone cells may limit bones' adaptability to mechanical loading. Introduction: 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 ER alpha . Materials and 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 ER alpha knockout (ERKO) mice (ER alpha super(-/-)) with those from their wildtype (ER alpha super(+/+)) and heterozygous (ER alpha super(+/-)) littermates and from ER beta knockout (BERKO) mice (ER beta super(+/+), ER beta super(+/-), and ER beta super(-/-)). Results: Whereas ER alpha super(+/+), ER alpha super(+/-), ER beta super(+/+), and ER beta super(-/-) cells proliferate in response to a single 10-minute period of cyclic strain, ER alpha super(-/-) cells do not. Transfection of fully functional, but not mutant, ER alpha rescues the proliferative response to strain in these cells. The strain-related response of ER alpha super(-/-) cells is also deficient in that they show no increased activity of an AP-1 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 ER alpha in the IGF axis and the ability of ER alpha super(-/-) cells to proliferate normally in response to a mitogenic stimulus that does not require functional ER alpha . Conclusions: These data indicate ER alpha '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