Docosahexaenoic Acid Inhibits Superoxide Dismutase 1 Gene Transcription in Human Cancer Cells: The Involvement of Peroxisome Proliferator-Activated Receptor α and Hypoxia-Inducible Factor-2α Signaling
- Erin R. Tuller,
- Charles T. Beavers,
- Jessica R. Lou,
- Michael A. Ihnat,
- Doris M. Benbrook and
- Wei-Qun Ding
- Departments of Obstetrics and Gynecology (E.R.T., D.M.B.), Biochemistry and Molecular Biology (D.M.B.), Pathology (C.T.B., J.R.L., W.-Q.D.), and Cell Biology (M.A.I.), University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma
- Address correspondence to:
Dr. Wei-Qun Ding, University of Oklahoma Health Sciences Center, 975 NE 10th Street, BRC 409, Oklahoma City, OK 73104. E-mail weiqun-ding{at}ouhsc.edu
Abstract
Docosahexaenoic acid (DHA; n-3, 22:6) is known to have anticancer activity, but its mechanisms of action remain to be further elucidated. We recently demonstrated that DHA down-regulates superoxide dismutase (SOD) 1 gene expression, thereby weakening cellular antioxidant forces and enhancing cytotoxicity in various human cancer cells. The objective of this study was to investigate the mechanism of the inhibitory effect of DHA on SOD-1 gene expression in human cancer cells. A reporter gene assay indicated that DHA suppresses SOD-1 gene transcription in a time- and concentration-dependent manner in human cancer cells. Pretreatment with vitamin E did not block the inhibitory effect of DHA, indicating that this suppression does not depend on lipid peroxidation. The suppressive effect of DHA on SOD-1 gene transcription could be mimicked by the peroxisome proliferator-activator receptor (PPAR) α ligand clofibrate but not the PPARγ ligand troglitazone, suggesting the involvement of PPARα signaling. Deletion analysis of the key DNA binding elements in the SOD-1 gene promoter identified the distal hypoxia response element (HRE), but not the peroxisome proliferator response element or nuclear factor-κB element, as essential for the suppressive effects of DHA. Coimmunoprecipitation confirmed that PPARα, but not PPARγ, forms a complex with hypoxia-inducible factor (HIF)-2α in cancer cells. Chromatin immunoprecipitation analysis indicated that both DHA and clofibrate reduce HIF-2α binding to the HRE. Thus, we have identified the distal HRE in the SOD-1 gene promoter that mediates the suppression on the transcription of this gene by DHA, and we have demonstrated the involvement of PPARα and HIF-2α signaling in this event.
Footnotes
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This study was supported by the Oklahoma Center for the Advancement of Science and Technology [Grant HR04-021]; the American Cancer Society [Grant IRG-05-066-01]; and the College of Medicine, University of Oklahoma Health Sciences Center [Grant COM 22134].
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ABBREVIATIONS: DHA, docosahexaenoic acid; SOD, superoxide dismutase; PPRE, peroxisome proliferation-responsive element; PPAR, peroxisome proliferator-activated receptor; HIF, hypoxia-inducible factor; NF-κB, nuclear factor-κB; HRE, hypoxia-response element; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; 4-HNE, 4-hydroxy nonenal; LPS, lipopolysaccharide; PAGE, polyacrylamide gel electrophoresis; IP, immunoprecipitation; ChIP, coimmunoprecipitation; bp, base pair(s); ANOVA, analysis of variance; PUFA, polyunsaturated fatty acid; Lu, luciferase; EPA, eicosapentaenoic acid.
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- Accepted June 15, 2009.
- Received May 1, 2009.
- The American Society for Pharmacology and Experimental Therapeutics
