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Targeting the MEK1 Cascade in Lung Epithelium Inhibits Proliferation and Fibrogenesis by Asbestos

Departments of ¹ Pathology, ² Medicine, ³ Medical Biostatistics, and ⁴ Civil and Environmental Engineering, University of Vermont, Burlington, Vermont; and ⁵ Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut

Correspondence and requests for reprints should be addressed to Brooke T. Mossman, Ph.D., Professor of Pathology, Director, Environmental Pathology Program, University of Vermont College of Medicine, Department of Pathology, 89 Beaumont Avenue, Burlington, VT 05405. E-mail: Brooke.Mossman{at}uvm.edu

The extracellular signal–regulated kinases 1 and 2 (ERK1/2) are phosphorylated after inhalation of asbestos. The effect of blocking this signaling pathway in lung epithelium is unclear. Asbestos-exposed transgenic mice expressing a dominant-negative mitogen-activated protein kinase kinase-1 (dnMEK1) (i.e., the upstream kinase necessary for phosphorylation of ERK1/2) targeted to lung epithelium exhibited morphologic and molecular changes in lung. Transgene-positive (Tg+) (i.e., dnMEK1) and transgene-negative (Tg–) littermates were exposed to crocidolite asbestos for 2, 4, 9, and 32 days or maintained in clean air (sham controls). Distal bronchiolar epithelium was isolated using laser capture microdissection and mRNA analyzed for molecular markers of proliferation and Clara cell secretory protein (CCSP). Lungs and bronchoalveolar lavage fluids were analyzed for inflammatory and proliferative changes and molecular markers of fibrogenesis. Distal bronchiolar epithelium of asbestos-exposed wild-type mice showed increased expression of c-fos at 2 days. Elevated mRNA levels of histone H3 and numbers of Ki-67–labeled proliferating bronchiolar epithelial cells were decreased at 4 days in asbestos-exposed Tg+ mice. At 32 days, distal bronchioles normally composed of Clara cells in asbestos-exposed Tg+ mouse lungs exhibited nonreplicating ciliated and mucin-secreting cells as well as decreased mRNA levels of CCSP. Gene expression (procollagen 3-a-1, procollagen 1-a-1, and IL-6) linked to fibrogenesis was also increased in lung homogenates of asbestos-exposed Tg– mice, but reduced in asbestos-exposed Tg+ mice. These results suggest a critical role of MEK1 signaling in epithelial cell proliferation and lung remodeling after toxic injury.

Key Words: mitogen-activated protein kinases • asbestosis • fibrosis • Clara cell • cell signaling

CLINICAL RELEVANCE The extracellular signal–regulated kinases (ERK1/2) have been implicated in acute lung injury by many toxic pollutants and in inflammatory lung diseases, including asthma and pulmonary fibrosis.