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Programmed cell death in cigarette-induced lung disease

Institution: University of California, San Francisco
Investigator(s): V. Courtney Broaddus, M.D.
Award Cycle: 1998 (Cycle 7) Grant #: 7RT-0051 Award: $371,556
Subject Area: Pulmonary Disease
Award Type: Research Project Awards

Initial Award Abstract
In the last five to ten years, large clinical trials involving tens of thousands of cigarette smokers have attempted to show that antioxidants (such as -carotene, found in many vegetables) would reduce the likelihood of cancer. This expectation seemed reasonable considering that diets rich in fruits and vegetables, and therefore rich in substances like -carotene, reduce cancer risks. To almost everyone's surprise, the results showed the opposite; namely that -carotene supplements actually increased the numbers of lung cancers in the cigarette smokers. There has been no detailed explanation of these puzzling results. As part of this project, we hope to learn why an antioxidant like -carotene could have a seemingly negative health effect. Such knowledge would have a great impact on understanding the toxic products of cigarette smoke and devising future public health measures to counter those toxic effects. Our hypothesis is outlined as follows. Oxidants are reactive, toxic products of oxygen that can damage DNA, the genetic “program” that controls cellular functions. These oxidants are found in cigarette smoke and are thought to be a major cause of cigarette induced disease, although there are literally hundreds of other carcinogenic and/or toxic substances in cigarette smoke. Oxidants can also trigger the cell to commit a type of suicide called “apoptosis” (i.e., “programmed” cell death that functions as a safety valve, providing a normal way to rid the body of cells with excessive DNA damage). Therefore, if antioxidants are given, the apoptosis caused by oxidants could be prevented and cells that would otherwise be eliminated would be allowed to survive. These cells could then be exposed to toxins (other than oxidants) in the cigarette smoke that could damage DNA without causing cell death by apoptosis, thus leading to genetic mutations and finally to cancer. We propose to prove that an extract of cigarette smoke causes apoptosis in cultured human lung cells (tracheal epithelial cells; isolated from the surfaces of airways), and confirm that antioxidants, particularly the same -carotene used in the human clinical trials, will block the apoptosis induced by cigarette smoke. Subsequently, we will study the epithelial cells in long-term cultures to learn if, in the presence of cigarette smoke extract, cells fed -carotene have less apoptosis and accumulate more DNA damage than control cells. The potentially harmful consequence of inhibiting normal apoptosis has been discussed widely in the literature but has never been directly tested. These studies may have an enormous impact on public health by showing why the effort to reduce cancer in cigarette smokers failed and whether another approach is likely to be effective.

Final Report
Introduction - This study was designed to show whether beta carotene, a chemopreventive agent, decreased apoptosis and thereby increased DNA or chromosomal damage in cells exposed to asbestos or cigarette smoke extract. The question was raised by several clinical studies of recent years, in which beta-carotene given daily to smokers increased the risk of lung cancer formation. The risk also appeared greater when beta-carotene was given to asbestos-exposed individuals, although the numbers of patients in this category were too small to reach significance.

Progress toward specific aims: Our aims were 1) To determine the major mechanisms by which asbestos/cigarette-smoke extract induces apoptosis in airway epithelial cells and 2) To determine whether chronic inhibition of apoptosis in cigarette smoke extract-exposed epithelial cells leads to an accumulation of DNA or chromosomal damage. We were able to show several key elements of our hypothesis.

1) For one, beta carotene did inhibit apoptosis in asbestos-exposed cells significantly. Interestingly, this decrease was not associated with antioxidant activity or with a decrease in asbestos uptake by the cells. The decrease in apoptosis led to an increase in the numbers of cells although the increase was found to be transient, that is, the asbestos-exposed cells failed to proliferate and eventually died by other mechanisms.

2) Exposure of the cells to beta carotene was associated with an increase in biomarkers of DNA and chromosomal damage. There was an increase in micronuclei (small fragments of nuclei that are found in the cytoplasm), in aneuploidy (by FISH of chromosome 1 and 9) and in DNA stran d breaks. However, we could not show whether the cells with evidence of damage were specifically the same cells that failed to undergo -apoptosis. The overall damage found in the cells during the 3-5 days of exposure did not further increase and when beta carotene was not maintained, the damage receded.

Future direction: Our lab is continuing to study the role of apoptosis in cellular responses to toxic environmental agents and also, more recently, to cancer cell responses to DNA damaging agents such as chemotherapy. Our interest in cigarette-smoke will be pursued along with asbestos, as two agents most associated with environmental damage to human lungs.

Impact: Along with other researchers, we hope that our work will lead to understanding of the complex role of apoptosis in the cellular response to injury. In the case of chemopreventives such as beta-carotene, we hope that our investigation into the inhibition of apoptosis and the increase in DNA/chromosomal damage will provide one possible explanation for the unexpected harm done by beta carotene treatment. As such, future studies may include similar sorts of biologic assays to assess potential harm of long term supplements.

Mesothelial cell apoptosis is confirmed in vivo by morphologic changes in cytokeratin distribution
Periodical: American Journal of Physiology Index Medicus:
Authors: Marchi E, Liu W, Broaddus VC ART
Yr: 2000 Vol: 278 Nbr: Abs: Pg: L528-L535

DNA breakage in asbestos-treated normal and transformed (TSV40) rat pleural mesothelial cells
Periodical: Mutagenesis Index Medicus:
Authors: Levresse V, Renier A, Levy F, Broaddus VC, Jaurand M-C ART
Yr: 2000 Vol: 15 Nbr: 3 Abs: Pg: 239-244

Phagocytosis of crocidolite asbestos induces oxidative stress, DNA damage and apoptosis in mesothelial cells
Periodical: American Journal of Respiratory Cell and Molecular Biology Index Medicus:
Authors: Liu W, Ernst JD, Broaddus VC ART
Yr: 2000 Vol: 23 Nbr: 3 Abs: Pg: 371-378

Vitronectin absorption to chrysotile asbestos and increases phagocytosis and toxicity for mesothelial cells
Periodical: American Journal of Physiology Index Medicus:
Authors: Wu J, Liu W, Koenig K, Idell SI, Broaddus VC ART
Yr: 2000 Vol: 279 Nbr: Abs: Pg: L916-L923

The integrin alphavbeta8 negatively regulates growth of human airway eptithelium
Periodical: Cancer Research Index Medicus:
Authors: Cambier S, Mu D, Boylen K, et al ART
Yr: 2000 Vol: 60 Nbr: 24 Abs: Pg: 7084-7093

TNF-related apoptosis inducing ligand (TRAIL) and chemotherapy cooperate to induce apoptosis in human mesothelioma cell lines
Periodical: American Journal of Respiratory Cell and Molecular Biology Index Medicus:
Authors: Liu W, Bodle E, Chen JY, Rosen GD, Broaddus VC ART
Yr: 2001 Vol: 25 Nbr: 1 Abs: Pg: 111 - 118

Apoptosis and asbestos-induced disease - is there a connection?
Periodical: Journal of Laboratory Clinical Medicine Index Medicus:
Authors: Broaddus VC ART
Yr: 0 Vol: Nbr: Abs: Pg: