Ahlbrecht Katrin, Schmitz Judith, Seay Ulrike, Schwarz Christine, Mittnacht-Kraus Rita, Gaumann Andreas, Haberberger Rainer V, Herold Susanne, Breier Georg, Grimminger Friedrich, Seeger Werner, Voswinckel Robert
University of Giessen Lung Center, Department of Internal Medicine, University Hospital Giessen, Giessen, Germany.
Am J Respir Cell Mol Biol. 2008 Aug;39(2):163-70. doi: 10.1165/rcmb.2007-0231OC. Epub 2008 Mar 6.
Vascular endothelial growth factor-A (VEGF-A) responsive effects mediated via the receptors fetal liver kinase-1 (flk-1) and fms-like tyrosine kinase (flt-1), are key processes of pulmonary vascular development. Flk-1 has been shown to be involved in early embryonic lung epithelial to endothelial crosstalk and branching morphogenesis. Recent reports suggested a role of VEGF-A in lung epithelial cell function. Based on these observations, we hypothesize that epithelial flk-1 has a unique function in pulmonary development. Thus, the aim of this study is to elucidate spatiotemporal expression of flk-1 during lung development with respect to the epithelial system. Embryonic lungs were screened for flk-1 messenger RNA and protein at daily intervals, including postnatal stages. From Embryonic Day (ED) 12.5 through ED 15.5, flk-1 expression was restricted to the early vascular primitive network, while from ED 16.5 on flk-1 was detectable in the epithelial system and persisted there postnatally. At postnatal stages, flk-1 expression was increasingly restricted to individual cells in the alveolar septa. Isolation and in vitro cultivation of alveolar epithelial cells confirmed flk-1 expression and showed VEGF secretion into the supernatant. To our knowledge, this is the first murine study characterizing epithelial flk-1 expression at different stages throughout lung organogenesis until birth and at postnatal stages. To confirm epithelial flk-1 expression, we performed reporter gene analysis of the flk-1 promoter in vivo. Investigations on transgenic mouse strains, containing either a complete or incomplete flk-1 promoter driving expression of the lacZ reporter gene, suggested differential flk-1 regulation in endothelial and epithelial cells.
血管内皮生长因子 A（VEGF-A）通过受体胎儿肝激酶-1（flk-1）和 fms 样酪氨酸激酶（flt-1）介导的反应效应是肺血管发育的关键过程。已表明 Flk-1 参与早期胚胎肺上皮细胞与内皮细胞的相互作用及分支形态发生。最近的报道提示 VEGF-A 在肺上皮细胞功能中发挥作用。基于这些观察结果，我们推测上皮细胞 flk-1 在肺发育中具有独特功能。因此，本研究的目的是阐明 flk-1 在肺发育过程中相对于上皮系统的时空表达情况。对胚胎肺每天进行 flk-1 信使核糖核酸和蛋白质筛查，包括出生后阶段。从胚胎第（ED）12.5 天到 ED 15.5 天，flk-1 表达局限于早期血管原始网络，而从 ED 16.5 天起，flk-1 在上皮系统中可检测到，并在出生后持续存在。在出生后阶段，flk-1 表达越来越局限于肺泡隔中的单个细胞。肺泡上皮细胞的分离和体外培养证实了 flk-1 的表达，并显示有 VEGF 分泌到上清液中。据我们所知，这是第一项在小鼠中描述从肺器官发生直至出生及出生后不同阶段上皮细胞 flk-1 表达特征的研究。为证实上皮细胞 flk-1 的表达，我们在体内对 flk-1 启动子进行了报告基因分析。对含有驱动 lacZ 报告基因表达的完整或不完整 flk-1 启动子的转基因小鼠品系的研究表明，内皮细胞和上皮细胞中 flk-1 的调控存在差异。
