Liu Y, Tolbert E M, Lin L, Thursby M A, Sun A M, Nakamura T, Dworkin L D
Department of Medicine, Rhode Island Hospital, Brown University School of Medicine, Providence, Rhode Island, USA.
Kidney Int. 1999 Feb;55(2):442-53. doi: 10.1046/j.1523-1755.1999.00267.x.
Hepatocyte growth factor (HGF) and its c-met receptor comprise a signaling system that has been implicated in tissue repair and regeneration. HGF action is specifically targeted to the damaged organ following injury; however, the mechanism underlying this important targeting process remains to be elucidated. We reasoned that induction of c-met expression might be a critical factor in determining the site specificity of this receptor-ligand system. To test this hypothesis, we examined changes in activity of the HGF/c-met system in the folic acid model of acute tubular injury and repair.
Tissue HGF and c-met mRNA levels were detected by RNase protection assay and Northern blot analysis following acute renal injury induced by a single injection of folic acid. HGF and c-met proteins were examined by a specific enzyme immunoassay and Western blotting, respectively. C-met expression and trans-activation were investigated by exposing renal epithelial mIMCD-3 cells to various cytokines in vitro.
Extremely rapid induction of renal HGF and c-met mRNA was observed beginning one hour following injection of folic acid. Circulating plasma HGF protein level rose dramatically (approximately 16-fold), peaking first at two hours and again at 24 hours following injection. Despite elevated HGF mRNA in the kidney, total kidney HGF protein actually decreased significantly at 24 hours following injury. On the other hand, both c-met mRNA and c-met protein were markedly increased in the kidney, where active renal tubule repair and regeneration take place. In vitro studies suggested that increased levels of HGF, as well as other cytokines, might account for enhanced c-met expression in renal tubular epithelial cells. Pretreatment of the cells with actinomycin D totally blocked c-met induction, suggesting that induced c-met expression occurs primarily at the transcriptional level. Using a cloned region of the c-met promoter coupled to a reporter gene, we demonstrated that HGF directly stimulated c-met promoter transactivation in renal epithelial cells.
These results suggest that local up-regulation of c-met transcription in the kidney is crucial to renal tubule repair and regeneration, not only because it increases overall activity of this receptor-ligand system, but also as a mechanism targeting HGF action specifically to renal epithelia.
肝细胞生长因子（HGF）及其c-met受体构成了一个与组织修复和再生相关的信号系统。损伤后，HGF的作用特异性地靶向受损器官；然而，这一重要靶向过程的潜在机制仍有待阐明。我们推测，c-met表达的诱导可能是决定该受体-配体系统位点特异性的关键因素。为了验证这一假设，我们在急性肾小管损伤和修复的叶酸模型中研究了HGF/c-met系统的活性变化。
单次注射叶酸诱导急性肾损伤后，通过核糖核酸酶保护分析和Northern印迹分析检测组织中HGF和c-met mRNA水平。分别采用特异性酶免疫分析和蛋白质印迹法检测HGF和c-met蛋白。通过体外将肾上皮mIMCD-3细胞暴露于各种细胞因子来研究c-met的表达和反式激活。
注射叶酸后1小时开始观察到肾脏中HGF和c-met mRNA的极快速诱导。循环血浆HGF蛋白水平显著升高（约16倍），在注射后2小时首次达到峰值，并在24小时再次达到峰值。尽管肾脏中HGF mRNA升高，但损伤后24小时肾脏总HGF蛋白实际上显著下降。另一方面，在发生活跃肾小管修复和再生的肾脏中，c-met mRNA和c-met蛋白均显著增加。体外研究表明，HGF以及其他细胞因子水平的升高可能是肾小管上皮细胞中c-met表达增强的原因。用放线菌素D预处理细胞完全阻断了c-met的诱导，表明诱导的c-met表达主要发生在转录水平。使用与报告基因偶联的c-met启动子的克隆区域，我们证明HGF直接刺激肾上皮细胞中c-met启动子的反式激活。
这些结果表明，肾脏中c-met转录的局部上调对肾小管修复和再生至关重要，这不仅是因为它增加了该受体-配体系统的整体活性，也是将HGF作用特异性靶向肾上皮的一种机制。
