Rutter Meilan M, Markoff Edith, Clayton Lisa, Akeno Nagako, Zhao Guisheng, Clemens Thomas L, Chernausek Steven D
Division of Endocrinology, Cincinnati Children's Hospital Medical Center, 3333 Burnet Avenue, Cincinnati, OH 45229, USA.
Bone. 2005 Feb;36(2):224-31. doi: 10.1016/j.bone.2004.10.005.
The activities of insulin-like growth factors (IGFs) in bone are modulated by a family of binding proteins (IGFBPs) whose physiological roles remain poorly understood. We have previously shown that targeted overexpression of IGF-I in osteoblasts of transgenic (OC-IGF-I) mice stimulates bone formation. In this model, bone formation is markedly but transiently increased in an age-dependent manner, raising the possibility that IGF-I may be influencing IGFBPs to in turn modulate its paracrine actions within bone. We sought to characterize the IGFBPs in normal mouse bone during development and to determine whether osteoblast-targeted overexpression of IGF-I influenced bone IGFBP abundance in vivo. Femoral bone IGFBP content was assessed in control nontransgenic and OC-IGF-I mice by I125-IGF-I ligand and immunoblotting. Bone IGFBP-5 and IGF-I mRNA abundance was determined using real-time reverse transcription (RT)-PCR. Ligand blot of bone extract showed a 30-kDa band, identified as IGFBP-5 by immunoblot, predominated. The abundance of IGFBP-5 declined with age in both control and transgenic bone. Ligand and immunoblot analysis revealed a 5-fold increase in IGFBP-5 protein levels at 3 weeks in transgenic bone (P<0.0001). The elevated IGFBP-5 protein levels were associated with a similar increase in IGF-I mRNA abundance (4-fold, P<0.01) and a significant increase in IGFBP-5 mRNA abundance (1.5-fold). Despite the age-related decline at 6 weeks, IGFBP-5 remained significantly (P<0.01) more abundant in transgenic bone compared to controls. In contrast, bone IGFBP-4 abundance was relatively unchanged by either age or IGF-I overexpression. These studies demonstrate a distinctive developmental pattern of IGFBP-5 content in mouse bone and show that osteoblast-derived IGF-I determines skeletal IGFBP-5 abundance, at least in part by inducing its synthesis. In that IGFBP-5 is thought to stimulate bone formation, directly or via IGF-I action, such changes in bone IGFBP-5 may be important to ensure robust bone acquisition in the early postnatal period.
胰岛素样生长因子（IGFs）在骨骼中的活性受到一类结合蛋白（IGFBPs）的调节，但其生理作用仍知之甚少。我们之前已经表明，在转基因（OC-IGF-I）小鼠的成骨细胞中靶向过表达IGF-I会刺激骨形成。在这个模型中，骨形成以年龄依赖性方式显著但短暂地增加，这增加了IGF-I可能影响IGFBPs进而调节其在骨内旁分泌作用的可能性。我们试图表征正常小鼠骨骼发育过程中的IGFBPs，并确定成骨细胞靶向过表达IGF-I是否会影响体内骨IGFBP的丰度。通过I125-IGF-I配体和免疫印迹法评估对照非转基因小鼠和OC-IGF-I小鼠的股骨IGFBP含量。使用实时逆转录（RT）-PCR测定骨IGFBP-5和IGF-I mRNA丰度。骨提取物的配体印迹显示一条30 kDa的条带，经免疫印迹鉴定为IGFBP-5，占主导地位。对照和转基因骨中IGFBP-5的丰度均随年龄下降。配体和免疫印迹分析显示，转基因骨在3周时IGFBP-5蛋白水平增加了5倍（P<0.0001）。IGFBP-5蛋白水平的升高与IGF-I mRNA丰度的类似增加（4倍，P<0.01）以及IGFBP-5 mRNA丰度的显著增加（1.5倍）相关。尽管在6周时与年龄相关的下降，但与对照相比，转基因骨中的IGFBP-5仍然显著更丰富（P<0.01）。相比之下，骨IGFBP-4的丰度在年龄或IGF-I过表达时相对不变。这些研究证明了小鼠骨骼中IGFBP-5含量独特的发育模式，并表明成骨细胞衍生的IGF-I至少部分通过诱导其合成来决定骨骼中IGFBP-5的丰度。鉴于IGFBP-5被认为直接或通过IGF-I作用刺激骨形成，骨IGFBP-5的这种变化对于确保出生后早期强健的骨获取可能很重要。
