Exposure to hydrogen peroxide (H₂O₂), one of the reactive oxidants in the gas phase of cigarette smoke (CS), induces aberrant phosphorylation of the epidermal growth factor receptor (EGFR), resulting ...in the lack of ubiquitination by c-Cbl, and impaired degradation. EGFR activation without the feedback regulation of normal degradation leads to uncontrolled cell growth and tumor promotion. Using immunoprecipitation, immunoblotting, and confocal microscopy, we now demonstrate that the pattern of EGFR activation by CS is similar to H₂O₂. We found that exposure of human airway epithelial cells to CS, as with exposure to H₂O₂, not only results in an increase in EGFR activation over time, but the EGFR activated by H₂O₂ or CS is neither ubiquitinated nor subsequently degraded due to its inability to bind the E3 ubiquitin ligase, c-Cbl, either directly or indirectly via the Grb2 adapter protein. Moreover, the stabilized H₂O₂- and CS-activated EGFR remains plasma membrane-bound, while a population of the receptor is trafficked to a perinuclear region. Concomitantly, CS exposure results in the activation of downstream Akt and ERK1/2 survival and proliferation pathways. Therefore, exposure to CS, like exposure to H₂O₂, results in prolonged signaling by the EGFR and may contribute to uncontrolled lung cell growth.--Khan, E. M., Lanir, R., Danielson, A. R., Goldkorn, T. Epidermal growth factor receptor exposed to cigarette smoke is aberrantly activated and undergoes perinuclear trafficking.
Scavenger Receptor B1 has been shown to play a prominent role in the uptake and delivery of vitamin E from HDL and is likely involved in regulating vitamin E in the lung. We have previously ...demonstrated that lung Scavenger Receptor B1 levels (protein and mRNA) are modulated by cigarette smoke in mice and this was accompanied by changes in lung vitamin E. To further characterize the molecular mechanism(s) involved in this process, human alveolar epitheliall cells were exposed to cigarette smoke and Scavenger Receptor B1 cellular levels and distribution were assessed. Results demonstrated that Scavenger Receptor B1 localizes in patches on the cellular membrane and in the perinuclear area of control cells. Upon cigarette smoke exposure, Scavenger Receptor B1 first translocated to the cell surface (within the first 12
h of exposure) and then cell levels (protein and mRNA levels) decreased significantly at 24
h. This decline was accompanied by increased Scavenger Receptor B1 ubiquitination which may explain the decrease in the protein levels. Cigarette smoke induced changes in both sub-cellular redistribution and ubiquitination of Scavenger Receptor B1 together with our previous
in vivo data provides evidence that cigarette smoke exposure may alter lung's ability to control its tocopherol levels.
Cigarette smoking (CS) is the main cause for emphysema, a pathologic state in the lung characterized by the destruction of alveolar walls. Ceramide, a second messenger lipid, is a critical mediator ...of alveolar destruction in lung diseases such as chronic obstructive pulmonary diseases (COPD), which consist of emphysema and chronic bronchitis. We have shown that CS induces rapid, sustained ceramide up‐regulation in human airway epithelial (HAE) cells in a dose dependent manner. We have also observed that CS induced ceramide to a similar extent as hydrogen peroxide (H2O2), a classical stimulus of ceramide generation in HAE cells and an oxidant known to be elevated in the breath or serum of patients with COPD. We showed that the amount of H2O2 found in cell culture medium exposed to CS increases with time. We demonstrated by loss‐of‐function analysis or by overexpression that nSMase2, but not aSMase, activity is required for CS‐mediated ceramide generation and apoptosis. Furthermore we observed that nSMase2 is trafficking similarly under both CS exposure and under oxidative‐stress, suggesting that the regulation of nSMase2 may mainly occur at the level of subcellular localization. Following either CS or H2O2 exposure, nSMase2 was found localized to the plasma membrane, where it can convert sphingomyelin to ceramide.
These studies suggest that at the molecular level, there is direct coupling between CS oxidative stress and the ceramide pathway, via a sphingomyelinase (SMase). Under CS exposure a lung SMase, nSMase2, is activated and displays continuous ceramide generation and pro‐apoptotic signaling, thus leading to the pathological apoptosis that causes lung injury.
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