Brody A R, Liu J Y, Brass D, Corti M
Lung Biology Program, Tulane University Medical Center, New Orleans, LA 70112-2699, USA.
Environ Health Perspect. 1997 Sep;105 Suppl 5(Suppl 5):1165-71. doi: 10.1289/ehp.97105s51165.
Inhalation of fibrogenic particles causes injury to the bronchiolar-alveolar epithelium. Consequently, there is a rapid proliferative response as the epithelium recovers and interstitial mesenchymal cells divide and produce connective tissue. In our model of brief (5-hr) exposure to chrysotile asbestos (approximately 1000 fibers/cc) in rats and mice, these events result in focal scarring at the bronchiolar-alveolar duct junctions in a histopathologic pattern identical to that seen in asbestos-exposed individuals. After 3 consecutive days of exposure, these lesions persist for at least 6 months postexposure. We postulate that cell proliferation and production of extracellular matrix is mediated in large part by three peptide growth factors, transforming growth factors alpha and beta (TGF-alpha and -beta), and platelet-derived growth factor (PDGF) A- and B-chains. To test this hypothesis in part, we have asked whether the genes that code for these growth factor proteins are activated at sites of asbestos-induced lung injury. If these genes were not activated, it would be reasonable to suspect that other potent growth factors and cytokines released during lung injury could be the primary mediators of fibroproliferative lung disease. In the studies reported here, we show, by in situ hybridization (ISH) and immunohistochemistry, that the four genes and their concomitant proteins are expressed within 24 hr in the bronchiolar-alveolar epithelium and underlying mesenchymal cells. RNase protection assay and ISH showed that the PDGF gene was upregulated during the first 5 hr of exposure and all the gene products remained above control levels for at least 2 weeks postexposure. TGF-alpha is a potent mitogen for epithelial cells, whereas the PDGF isoforms are potent growth factors for mesenchymal cells. TGF-beta retards fibroblast growth but stimulates extracellular matrix synthesis. Further studies using gene knockouts, appropriate antibodies, or antisense technology will be necessary to prove whether any of the growth factors are playing a significant role in fibrogenic lung disease. In addition, we have carried out a series of studies using type II alveolar epithelial cells purified from adult mouse lungs and maintained for up to 8 weeks in serum-free culture. These cells exhibit high transepithelial resistance values and they release TGF-beta 1 and -beta 2. This cell type also has been cultured from TGF-alpha knockout mice, resulting in monolayers with increased transepithelial resistance. This combination of studies in vivo and in vitro will allow us to pursue the mechanisms through which growth factors mediate lung fibrosis.
吸入致纤维化颗粒会导致细支气管-肺泡上皮损伤。因此，随着上皮细胞恢复，间质间充质细胞分裂并产生结缔组织，会出现快速的增殖反应。在我们对大鼠和小鼠进行短时间（5小时）暴露于温石棉（约1000根纤维/立方厘米）的模型中，这些事件会导致细支气管-肺泡导管连接处出现局灶性瘢痕，其组织病理学模式与石棉暴露个体中所见相同。连续暴露3天后，这些损伤在暴露后至少持续6个月。我们推测细胞增殖和细胞外基质的产生在很大程度上是由三种肽生长因子介导的，即转化生长因子α和β（TGF-α和-β）以及血小板衍生生长因子（PDGF）A链和B链。为了部分验证这一假设，我们询问了编码这些生长因子蛋白的基因在石棉诱导的肺损伤部位是否被激活。如果这些基因未被激活，那么有理由怀疑肺损伤期间释放的其他强效生长因子和细胞因子可能是纤维增生性肺病的主要介质。在本文报道的研究中，我们通过原位杂交（ISH）和免疫组织化学表明，这四种基因及其相应蛋白在24小时内在细支气管-肺泡上皮和下方的间充质细胞中表达。核糖核酸酶保护试验和ISH表明，PDGF基因在暴露的前5小时内上调，并且所有基因产物在暴露后至少2周内保持在对照水平之上。TGF-α是上皮细胞的强效有丝分裂原，而PDGF异构体是间充质细胞的强效生长因子。TGF-β会抑制成纤维细胞生长，但会刺激细胞外基质合成。使用基因敲除、合适的抗体或反义技术进行进一步研究，对于证明任何一种生长因子是否在纤维化肺病中发挥重要作用是必要的。此外，我们使用从成年小鼠肺中纯化并在无血清培养中维持长达8周的II型肺泡上皮细胞进行了一系列研究。这些细胞表现出高跨上皮电阻值，并且它们释放TGF-β1和-β2。这种细胞类型也已从TGF-α基因敲除小鼠中培养出来，形成了跨上皮电阻增加的单层细胞。体内和体外研究的这种结合将使我们能够探究生长因子介导肺纤维化的机制。
