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Perspectives |
Authors' Affiliations: 1 Center of Excellence in Environmental Toxicology, Transdisciplinary Tobacco Use Research Center, Abramson Cancer Center, and Departments of 2 Pharmacology and 3 Psychiatry, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
Requests for reprints: Trevor M. Penning, University of Pennsylvania, 3620 Hamilton Walk, Philadelphia, PA 19104-6084. Phone: 215-898-9445; Fax: 215-573-2236; E-mail: penning{at}pharm.med.upenn.edu.
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Toward this end, a study reported by Gumus et al. (6) in this issue of the journal characterized gene expression profiles in the oral mucosa of smokers and nonsmokers and validated their findings by examining the in vitro effects of tobacco smoke condensate on gene expression in an oral leukoplakia cell line. Another related study also reported in this issue of the journal was conducted by Zhang et al. (7), who compared gene expression in cells obtained from bronchial brushes from never, current, and former smokers. Smoking effects on global gene expression were common across these studies. Certain genes (CYP1A1 and CYP1B1) that were up-regulated in buccal oral specimens of smokers were part of a larger group (CYP1A1, CYP1B1, ALDH3A1, NQO1, and AKR1C1) up-regulated following in vitro exposure to tobacco smoke condensate (6) and in the airway cells of smokers (7). Zhang and colleagues provide further in vivo evidence that many of these genes are down-regulated in persons who reported quitting smoking at least 1 year before the assessment (CYP1B1, AKR1C1, AKR1C2, AKR1B10, and ALDH3A1). Many of these same genes were previously shown to be overexpressed in oral cancer cells exposed to tobacco smoke condensate (8), in bronchial epithelial cells of smokers, and in patients' non–small cell lung carcinoma cells (9, 10). The effects are also consistent with the effects of smoking on the airway transcriptome (11, 12). Taken together, these findings identify important tobacco smoke exposure response genes. The studies of Gumus et al. and Zhang et al. in this issue of the journal provide an elegant illustration of the value of cross-model (preclinical and clinical) validation in translational cancer prevention research.
| Significance for Cancer Etiology |
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Another gene that showed altered expression was ALDH3A1. When coupled with the overexpression of AKR1C1, AKR1C2, AKR1B10, and NQO1 genes, another outcome of chronic tobacco exposure is to mount a counter-response to oxidative stress (Fig. 1B). Tobacco smoke leads to oxidative stress due to the presence of benzoquinone and heavy metals and the production of redox-active PAH o-quinones, which is AKR mediated (see above). Each of these up-regulated genes contains an antioxidant response element in their gene promoters (32, 34, 35). A consequence of oxidative stress is the formation of lipid peroxides that can break down to form reactive bifunctional electrophiles (e.g., 4-hydroxy-2-nonenal and 4-oxo-2-nonenal; refs. 36, 37). Induction of these genes provides a route to the detoxification of these reactive lipid aldehydes.
One remaining gene that deserves comment is the altered expression of AKR1B10 (small intestine like aldose reductase; Fig. 1C). Whereas this gene can protect against the toxic effects of lipid aldehydes, it has another function (38–40). It is the most catalytically efficient of all the known all-trans-retinal reductases (40). Elimination of retinal as retinol prevents the conversion of retinal to retinoic acid. Retinoic acid is a ligand for retinoid acid receptor and retinoid X receptor, and activation of these nuclear receptors leads to cell differentiation (41). If retinoid acid receptor and retinoid X receptor are deprived of their ligand, this could lead to a pro-proliferative response. In fact, retinoid acid receptor and retinoid X receptor are often lost in lung cancer cell lines, suggesting that there is a coordinated loss of retinoic acid signaling (42).
| Significance for Cancer Prevention and Treatment |
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If oxidative stress is a driving component of the lung carcinogenic process, antioxidant strategies may also be desirable. Two high profile clinical trials addressed the use of antioxidant vitamins: The Alpha-Tocopherol Beta-Carotene Prevention Study (45) and the Carotene and Retinol Efficacy Trial (46). Both studies revealed that lung cancer risk was significantly increased in heavy smokers receiving β-carotene (47). The altered expression of AKR1B10, reported to be coupled with the high overexpression of AKR1B10 seen in non–small cell lung carcinoma by Fukumoto et al. (9), suggests why chemopreventive studies that target retinoid X receptor and retinoid acid receptor are likely to fail. AKR1B10 will rapidly eliminate retinal as retinol and prevent formation of retinoic acid. By contrast, in the Alpha-Tocopherol Beta-Carotene study, high serum
-tocopherol was associated with lower lung cancer incidence, although the effect was stronger in smokers with less cumulative tobacco exposure (48).
Overexpression of CYP1B1 and AKRs may also contribute to cancer chemotherapeutic drug resistance, and therefore chronic smokers with lung cancer may have lower response rates on these therapies (Fig. 1D). CYP1B1 is implicated in the metabolic clearance of docetaxel, gefitinib, and erlotinib (49, 50). In addition, AKR1C1 is overexpressed in HT29 colon cancer cells that are resistant to the glutathione S-transferase inhibitor ethacrynic acid (51). AKR1C1 and AKR1C2 genes are overexpressed in non–small cell lung carcinoma in patients with poor response to anti-pyrimidine therapy (52), and AKR1C1 and AKR1C2 are overexpressed in ovarian cancer cell lines resistant to cisplatin (53, 54). In the latter case, cisplatin resistance could be accomplished by transfection of AKR1C1. Because cisplatin is widely used for non–small cell lung carcinoma, this raises the prospect that its efficacy could be improved by the coadministration of nonsteroidal anti-inflammatory drugs, which are known inhibitors of AKR isozymes.
To increase our understanding of the molecular mechanisms through which tobacco smoke exposure influences the efficacy of cancer prevention and therapeutic agents, a more rigorous exposure measurement, beyond the traditional smoking pack-years assessment, is needed (5). This should include ongoing assessment of tobacco exposure following diagnosis, biochemical verification of self-reported use using biomarkers (e.g., cotinine, 3'-trans-hydroxycotinine, and 3-hydroxy-B[a]P), and use of pharmacotherapy to reduce tobacco exposure.
| Conclusion |
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| Disclosure of Potential Conflicts of Interest |
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| Footnotes |
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Note: The Pennsylvania Department of Health specifically disclaims responsibility for any analyses, interpretations, or conclusions.
Received for publication February 29, 2008.
Accepted March 5, 2008
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Key Articles
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