Environmental Health Perspectives 105, Supplement 5, September 1997

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Analyzing the Genes and Peptide Growth Factors Expressed in Lung Cells in Vivo Consequent to Asbestos Exposure and in Vitro

Arnold R. Brody, Jing-Yao Liu, David Brass, and Miriam Corti

Lung Biology Program and Department of Pathology, Tulane University Medical Center, New Orleans, Louisana

Abstract
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 (~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 -ß), 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-ß 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-ß1 and -ß2. 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. -- Environ Health Perspect 105(Suppl 5):1165-1171 (1997)

Key words : asbestos, in situ hybridization, immunohistochemistry, platelet-derived growth factor, transforming growth factors alpha and beta, tumor necrosis factor alpha, alveolar epithelial culture

This paper is based on a presentation at The Sixth International Meeting on the Toxicology of Natural and Man-Made Fibrous and Non-Fibrous Particles held 15-18 September 1996 in Lake Placid, New York. Manuscript received at EHP 27 March 1997; accepted 14 July 1997.

This research was supported by National Institutes of Health grant RO1ES06766, the Tulane/Xavier Center for Bioenvironmental Research, and the Tulane Cancer Center.

Address correspondence to Dr. A.R. Brody, Lung Biology Program and Department of Pathology, Tulane University Medical Center, 1430 Tulane Avenue, SL-79 New Orleans, LA 70112-2699. Telephone: (504) 585-6928. Fax: (504) 587-7389. E-mail: abrody@tmc.tulane.edu

Abbreviations used: BAD, bronchiolar-alveolar duct; BrdU, bromodeoxyuridine; IHC, immunohistochemistry; ISH, in situ hybridization; PCNA, proliferating cell nuclear antigen; PDGF, platelet-derived growth factor; TGF- alpha and ß, transforming growth factors alpha and beta; TNF- alpha , tumor necrosis factor alpha.

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Last Update: October 30, 1997