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The mitochondria-regulated death pathway mediates asbestos-induced alveolar epithelial cell apoptosis
Vijayalakshmi Panduri1, Sigmund A Weitzman, Navdeep Chandel
1Department of Medicine, Divisions of Pulmonary and Critical Care Medicine and Hematology-Oncology, Northwestern University Feinberg School of Medicine, Chicago, IL 60611, USA.
Abstract:
The mechanisms underlying asbestos-induced pulmonary toxicity are not fully understood. Alveolar epithelial cell (AEC) apoptosis by iron-derived reactive oxygen species (ROS) is one important mechanism implicated. The two major pathways regulating apoptosis include (i) the mitochondrial death (intrinsic) pathway caused by DNA damage, and (ii) the plasma-membrane death receptor (extrinsic) pathway. However, it is unknown whether asbestos activates either death pathway in AEC. We determined whether asbestos triggers AEC mitochondrial dysfunction by exposing cells (A549 and rat alveolar type II) to amosite asbestos and assessing mitochondrial membrane potential changes (deltapsi(m)) using a fluorometric technique involving tetremethylrhodamine ethyl ester (TMRE) and mitotracker green. Unlike inert particulates (titanium dioxide and glass beads), amosite asbestos caused dose- and time-dependent reductions in deltapsi(m). Asbestos-induced deltapsi(m) was associated with the release of cytochrome c from the mitochondria to the cytoplasm as well as activation of caspase 9, a mitochondrial-activated caspase. In contrast, a lower level of caspase 8, the death receptor-activated caspase, was detected in asbestos-exposed AEC. An iron chelator (phytic acid or deferoxamine) or a hydroxyl radical scavenger (sodium benzoate) each blocked asbestos-induced reductions in deltapsi(m) and caspase 9 activation, suggesting a role for iron-derived ROS. Finally, Bcl-X(L), a mitochondrial antiapoptotic protein that prevents cell death by preserving the outer mitochondrial membrane integrity, blocked asbestos-induced decreases in A549 cell deltapsi(m) and reduced apoptosis as assessed by DNA fragmentation. We conclude that asbestos-induced AEC apoptosis results from mitochondrial dysfunction, in part due to iron-derived ROS, which is followed by the release of cytochrome c and caspase 9 activation. Our findings suggest an important role for the mitochondria-regulated death pathway in the pathogenesis of asbestos-associated pulmonary toxicity.
Insights
Asbestos exposure triggers alveolar epithelial cell apoptosis via mitochondrial dysfunction, driven by iron-derived reactive oxygen species (ROS). This activates the intrinsic cell death pathway, contributing to asbestos-related lung toxicity.
Area of Science:
- Cell Biology
- Toxicology
- Pulmonary Medicine
Background:
- Asbestos-induced pulmonary toxicity mechanisms remain unclear.
- Alveolar epithelial cell (AEC) apoptosis, mediated by iron-derived reactive oxygen species (ROS), is a key implicated factor.
- Apoptosis occurs via intrinsic (mitochondrial) or extrinsic (death receptor) pathways, but asbestos's effect on these in AECs is unknown.
Purpose of the Study:
- To investigate whether asbestos exposure activates the mitochondrial (intrinsic) or death receptor (extrinsic) apoptosis pathway in AECs.
- To determine if asbestos induces mitochondrial dysfunction in AECs.
Main Methods:
- Exposed A549 and rat alveolar type II cells to amosite asbestos.
- Assessed mitochondrial membrane potential (deltapsi(m)) using TMRE and Mitotracker Green.
- Measured cytochrome c release, caspase activation (caspase 9 and 8), and apoptosis via DNA fragmentation.
- Utilized iron chelators and ROS scavengers to investigate ROS involvement.
- Tested the effect of the antiapoptotic protein Bcl-X(L).
Main Results:
- Amosite asbestos dose- and time-dependently reduced deltapsi(m) in AECs, unlike inert particles.
- Asbestos exposure led to cytochrome c release and caspase 9 activation, indicating mitochondrial pathway involvement.
- Iron chelators and ROS scavengers inhibited asbestos-induced deltapsi(m) reduction and caspase 9 activation.
- Bcl-X(L) mitigated asbestos-induced deltapsi(m) decrease and reduced apoptosis.
Conclusions:
- Asbestos induces AEC apoptosis primarily through mitochondrial dysfunction, partly mediated by iron-derived ROS.
- The intrinsic mitochondrial death pathway, involving cytochrome c release and caspase 9 activation, plays a significant role in asbestos-associated pulmonary toxicity.