Zayzafoon Majd, Gathings William E, McDonald Jay M
The University of Alabama at Birmingham, 220 West Pavilion, 619 South 19th Street, Birmingham, Alabama 35233-7331, USA.
Endocrinology. 2004 May;145(5):2421-32. doi: 10.1210/en.2003-1156. Epub 2004 Jan 28.
Space flight-induced bone loss has been attributed to a decrease in osteoblast function, without a significant change in bone resorption. To determine the effect of microgravity (MG) on bone, we used the Rotary Cell Culture System [developed by the National Aeronautics and Space Administration (NASA)] to model MG. Cultured mouse calvariae demonstrated a 3-fold decrease in alkaline phosphatase (ALP) activity and failed to mineralize after 7 d of MG. ALP and osteocalcin gene expression were also decreased. To determine the effects of MG on osteoblastogenesis, we cultured human mesenchymal stem cells (hMSC) on plastic microcarriers, and osteogenic differentiation was induced immediately before the initiation of modeled MG. A marked suppression of hMSC differentiation into osteoblasts was observed because the cells failed to express ALP, collagen 1, and osteonectin. The expression of runt-related transcription factor 2 was also inhibited. Interestingly, we found that peroxisome proliferator-activated receptor gamma (PPARgamma2), which is known to be important for adipocyte differentiation, adipsin, leptin, and glucose transporter-4 are highly expressed in response to MG. These changes were not corrected after 35 d of readaptation to normal gravity. In addition, MG decreased ERK- and increased p38-phosphorylation. These pathways are known to regulate the activity of runt-related transcription factor 2 and PPARgamma2, respectively. Taken together, our findings indicate that modeled MG inhibits the osteoblastic differentiation of hMSC and induces the development of an adipocytic lineage phenotype. This work will increase understanding and aid in the prevention of bone loss, not only in MG but also potentially in age-and disuse-related osteoporosis.
太空飞行引起的骨质流失被认为是由于成骨细胞功能下降，而骨吸收没有显著变化。为了确定微重力（MG）对骨骼的影响，我们使用了旋转细胞培养系统[由美国国家航空航天局（NASA）开发]来模拟MG。培养的小鼠颅骨在MG处理7天后碱性磷酸酶（ALP）活性降低了3倍，并且未能矿化。ALP和骨钙素基因表达也降低。为了确定MG对成骨细胞生成的影响，我们将人骨髓间充质干细胞（hMSC）培养在塑料微载体上，并在模拟MG开始前立即诱导成骨分化。观察到hMSC向成骨细胞的分化受到显著抑制，因为细胞未能表达ALP、胶原蛋白1和骨连接蛋白。与 runt 相关的转录因子2的表达也受到抑制。有趣的是，我们发现过氧化物酶体增殖物激活受体γ（PPARγ2），已知其对脂肪细胞分化很重要，脂肪酶、瘦素和葡萄糖转运蛋白4在对MG的反应中高度表达。在重新适应正常重力35天后，这些变化并未得到纠正。此外，MG降低了ERK磷酸化并增加了p38磷酸化。已知这些信号通路分别调节与runt相关的转录因子2和PPARγ2的活性。综上所述，我们的研究结果表明，模拟MG抑制了hMSC的成骨细胞分化，并诱导了脂肪细胞系表型的发展。这项工作将增进对骨质流失的理解，并有助于预防骨质流失，不仅在微重力环境中，而且可能在与年龄和废用相关的骨质疏松症中。
