Date of Award:

5-2004

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Chemistry and Biochemistry

Advisor/Chair:

Ann E. Aust

Abstract

Occupational exposure to asbestos has been associated with increased incidence of pulmonary interstitial fibrosis, mesothelioma of the pleura, and bronchogenic carcinoma. Although the mechanism by which asbestos causes cancer remains unknown , iron associated with asbestos is thought to play a role in the pathogenic effects of fibers.

The aim of this research was to examine and compare the asbestos-induced signaling phenomena in relevant human lung and pleural target cells, and to deter mine the role of iron from asbestos fibers in these events. Exposure of human airway epithelial (A549) cells, human pleural mesothelial (METSA) cells, and normal human small airway epithelial (SAEC) cells to asbestos resulted in a significant dephosphorylation and inactivation of epidermal growth factor receptor (EGFR). The effects of three types of asbestos, i.e. crocidolite, amosite and chrysotile, on the EGFR phosphorylation state in A549 cells appeared to be directly related to the amount of iron mobilized from these fibers. These results strongly suggest that iron plays an important role in asbestos induced inactivation of EGFR.

We observed that exposure of A549 and SAEC cells to crocidolite , but not inert titanium dioxide , led to a significant time- and dose-dependent inactivation of the main EGFR signaling pathways, including Akt and extracellular signal-regulated kinase 1/2 (ERKI/2) pathways. Crocidolite also initiated apoptosis via pathways involving activation of p38 mitogen-activated protein kinase (MAPK), caspase -3 and -9, and cleavage of poly(ADP-ribose) polymerase (PARP). Prevention of these effects with an iron chelator or endocytosis inhibitors strongly suggests that iron mobilized from fibers inside the cells initiates the observed events. Inhibition of p38 MAPK with SB203580 prevented inactivation of EGFR , inactivation of EGFR-associated survival pathways, and initiation of apoptosis. Our results also suggest that p38 MAPK-dependent protein serine/threonine phosphatase activation plays an important role in the observed phenomena. Taken together , it appears that iron-dependent p38 MAPK activation, through a serine/threonine phosphatase-mediated mechanism , regulates asbestos-induced apoptosis in human lung epithelial cells. We speculate that apoptosis of human lung target cells induced by asbestos fibers is a pathologic feature in lung injury and may account for some of the pulmonary toxicity of the fibers.

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