The Duration of Nuclear Extracellular Signal-Regulated Kinase 1 and 2 Signaling during Cell Cycle Reentry Distinguishes Proliferation from Apoptosis in Response to Asbestos

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Abstract
Asbestos exposure causes activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) in lung epithelial cells, the targets of asbestos-associated lung carcinomas. The functional significance of ERK1/2 activation in pulmonary epithelial and mesothelial cells is unclear. Using serum-stimulated mouse alveolar type II epithelial cells as a model for cell cycle reentry, we show that the duration of phospho-ERK1/2 in the nucleus determines cell fate in response to crocidolite asbestos. In response to 10% serum, a proliferative stimulus, phosphorylated ERK1/2 initially accumulated in the nucleus, and reduction of nuclear phospho-ERK1/2 after 2 to 4 hours was followed by expression of cyclin D1 and S-phase entry. Low levels of asbestos (2) promoted S-phase entry in low (2%) serum through an epidermal growth factor receptor-dependent pathway but did not promote cell cycle progression or induce apoptosis in the presence of high (10%) serum-containing medium. Higher levels of asbestos (1.0 to 5.0 μg/cm2) prolonged the localization of phospho-ERK1/2 in the nucleus in the presence of high serum, impeded S-phase entry, and induced apoptosis in a dose-dependent manner. Immunofluorescence microscopy indicated that the duration of signaling by phospho-ERK1/2 in the nucleus was predictive of cell fate at any concentration of asbestos. After 8 hours of exposure, cells with nuclear phospho-ERK1/2 also were positive for nuclear localization of apoptosis-inducing factor (AIF), an early event in apoptosis. In contrast, asbestos-exposed cells that displayed cytoplasmic phospho-ERK1/2 at 8 hours expressed cyclin D1 and proceeded to S phase. Our studies show that prolonged localization of phospho-ERK1/2 in the nucleus is incompatible with expression of cyclin D1 and is predictive of asbestos-associated cell death by AIF, thereby providing an approach for determining cell fate in asbestos-induced tumorigenesis.

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