Cell signaling in mesothelial cells by asbestos: Evidence for the involvement of oxidative stress in the regulation of the epidermal growth factor receptor

Asbestos has been shown to stimulate the mitogen-activated protein kinase s ignaling cascade after autophosphorylation of the epidermal growth factor r eceptor (EGF-R), an event important in regulating the response of cells to extracellular signals. in studies reported here, we have examined whether m ineral fibers with known carcinogenicity can be discriminated from nonpatho genic fibers by their ability to upregulate expression of EGF-R protein in mesothelial cells. Crocidolite and erionite, two fibrous preparations known to induce mesothelioma, increased expression of EGF-R protein in a time- a nd dose-dependent manner, whereas milled (nonfibrous) crocidolite and chrys otile asbestos, two preparations with much less or no ability to induce mes othelioma, did not. intense patterns of EGF-R protein expression were linke d to mesothelial cells phagocytosing long fibers as observed by phase-contr ast microscopy. To determine the importance of EGF-R expression in these ce lls, we assessed downstream signaling events in rat pleural mesothelial (RP M) cells by looking at the induction of activator protein-1 (AP-1), a trans cription factor that controls the transition to S phase in the cell cycle, leading to cell proliferation. Crocidolite induced, AP-1 in RPM cells in a dose-dependent manner, and this induction of AP-1 in RPM cells was inhibite d by coincubation with tryphostin AC 1478, a potent inhibitor of the EGF-R. To examine the mechanism of induction of EGF-R in RPM cells by asbestos, R PM cells were treated with crocidolite in the presence and absence of the a ntioxidant N-acetylcysteine (NAC). Reduced glutathione (GSH) was examined a s a marker of oxidative stress and the expression of EGF-R protein was meas ured. Crocidolite asbestos caused a dose-dependent depletion of GSH in RPM cells, and the presence of NAC ameliorated the expression of EGF-R protein by crocidolite. Our data suggest that carcinogenic fibers induce EGF-R via a mechanism involving oxidative stress initiating cell signaling cascades i n mesothelial cells leading to cell proliferation and carcinogenesis.