Retinoid signaling and activator protein-1 expression in ferrets given beta-carotene supplements and exposed to tobacco smoke.
BACKGROUND
Epidemiologic studies have demonstrated that individuals who eat more fruits and vegetables and/or have high levels of serum beta-carotene have a lower risk of cancer, especially lung cancer. However, recent human intervention studies using beta-carotene supplements have shown an increase in the risk of lung cancer among smokers and asbestos workers. In this study, we used an animal model system to evaluate the hazard associated with a combination of high-dose beta-carotene supplementation and tobacco smoking.


METHODS
Ferrets were given a beta-carotene supplement, exposed to cigarette smoke, or both for 6 months. Cell proliferation and squamous metaplasia in lung tissue were assessed by examination of proliferating-cell nuclear antigen expression and histopathologic examination, respectively. beta-Carotene and retinoid concentrations in lung tissue and plasma samples were analyzed by high-performance liquid chromatography. Expression of genes for retinoic acid receptors (RARs) and activator protein-1 (encoded by the c-Jun and c-Fos genes) in lung tissue specimens was examined by western blotting.


RESULTS
A strong proliferative response in lung tissue and squamous metaplasia was observed in all beta-carotene-supplemented animals, and this response was enhanced by exposure to tobacco smoke. When compared with control groups, all three treatment groups had statistically significantly lower concentrations of retinoic acid in lung tissue, and they exhibited 18%-73% reductions in RARbeta gene expression; however, RARalpha and RARgamma gene expression was not reduced. Ferrets given a beta-carotene supplement and exposed to tobacco smoke had threefold to fourfold elevated expression of the c-Jun and c-Fos genes.


CONCLUSIONS
Diminished retinoid signaling, resulting from the suppression of RARbeta gene expression and overexpression of activator protein-1, could be a mechanism to enhance lung tumorigenesis after high-dose beta-carotene supplementation and exposure to tobacco smoke.
Wang
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Jean Mayer United States Department of Agriculture Human Nutrition Research Center on Aging at Tufts University and Department of Biochemistry, Tufts University School of Medicine, Boston, MA 02111, USA. Wang_CN@HNRC.TUFTS.EDU
Liu
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C
Bronson
R T
RT
Smith
D E
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Krinsky
N I
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Russell
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M
D000818:Animals; D002455:Cell Division / Q000187:drug effects; D003043:Cocarcinogenesis; D004224:Diterpenes; D015536:Down-Regulation / Q000187:drug effects; D004781:Environmental Exposure; D005289:Ferrets; D005786:Gene Expression Regulation / Q000187:drug effects*; D016762:Genes, fos; D016758:Genes, jun; D006801:Humans; D008168:Lung / Q000737:chemistry / Q000473:pathology*; D008175:Lung Neoplasms / Q000139:chemically induced*; D008297:Male; D008679:Metaplasia; D010947:Plants, Toxic; D011230:Precancerous Conditions / Q000139:chemically induced*; D018809:Proliferating Cell Nuclear Antigen / Q000096:biosynthesis / Q000235:genetics; D018168:Receptors, Retinoic Acid / Q000187:drug effects*; D000084562:Retinyl Esters; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D012906:Smoke / Q000009:adverse effects*; D014026:Nicotiana; D018808:Transcription Factor AP-1 / Q000096:biosynthesis* / Q000235:genetics / Q000502:physiology; D014212:Tretinoin / Q000032:analysis; D014801:Vitamin A / Q000031:analogs & derivatives / Q000032:analysis; D019207:beta Carotene / Q000032:analysis / Q000633:toxicity*
D004224:Diterpenes; D018809:Proliferating Cell Nuclear Antigen; D018168:Receptors, Retinoic Acid; D000084562:Retinyl Esters; D012906:Smoke; D018808:Transcription Factor AP-1; C082925:retinoic acid receptor beta; D019207:beta Carotene; D014801:Vitamin A; C014794:retinol palmitate; D014212:Tretinoin