Jessop Helen L, Suswillo Rosemary F L, Rawlinson Simon C F, Zaman Gul, Lee Karla, Das-Gupta Vicky, Pitsillides Andrew A, Lanyon Lance E
Department of Veterinary Basic Sciences, The Royal Veterinary College, London, United Kingdom.
J Bone Miner Res. 2004 Jun;19(6):938-46. doi: 10.1359/jbmr.2004.19.6.938.
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.
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-/-).
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.
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.
在体内，骨骼对机械负荷的成骨反应涉及表面成骨细胞的增殖。这种反应在体外可被复制，且涉及细胞外信号调节激酶（ERK）介导的雌激素受体（ER）α激活以及雌激素反应元件活性的上调。这种增殖反应可被选择性雌激素受体调节剂阻断，并通过转染额外的ERα而增强。
我们现在通过比较源自纯合ERα基因敲除（ERKO）小鼠（ERα-/-）的这些细胞原代培养物与来自其野生型（ERα+/+）和杂合子（ERα+/-）同窝小鼠以及ER/β基因敲除（BERKO）小鼠（ERβ+/+、ERβ+/-和ERβ-/-）的细胞原代培养物的反应，研究了ER参与成骨样细胞对应变早期反应的机制。
虽然ERα+/+、ERα+/-、ERβ+/+和ERβ-/-细胞在单次10分钟的周期性应变作用下会增殖，但ERα-/-细胞不会。转染完全功能性而非突变型的ERα可挽救这些细胞对应变的增殖反应。ERα-/-细胞与应变相关的反应也存在缺陷，因为它们显示AP-1驱动的报告载体活性没有增加，且一氧化氮（NO）产生没有与应变相关的增加。它们与应变相关的前列环素产生增加得以保留。它们对成纤维细胞生长因子-2有增殖反应，但对胰岛素样生长因子（IGF）-I或IGF-II没有反应，这表明ERα在IGF轴中的重要性以及ERα-/-细胞在对不需要功能性ERα的促有丝分裂刺激做出反应时正常增殖的能力。
这些数据表明ERα在成骨样细胞对机械应变的许多早期反应中起关键作用，包括那些导致增殖的反应。它们支持这样的假设，即雌激素撤药后ERα表达或活性的降低导致对负荷的成骨反应减弱。这在绝经后骨质疏松症的病因学中可能很重要。
