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Cytoplasmic Phospholipase A₂ Deletion Enhances Colon Tumorigenesis

¹ Program in Colorectal Cancer, Center for Molecular Medicine, University of Connecticut Health Center, Farmington, Connecticut; ² Department of Pathobiology and Veterinary Science, University of Connecticut, Storrs, Connecticut; ³ Department of Pharmacology and Toxicology, University of Kansas, Lawrence, Kansas; and ⁴ Renal Division, Brigham and Women's Hospital, Department of Medicine, Harvard School of Medicine, Harvard-Massachusetts Institute of Technology Division of Health Sciences and Technology, Boston, Massachusetts

Requests for reprints: Daniel W. Rosenberg, Program in Colorectal Cancer, Center for Molecular Medicine, University of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT 06030. Phone: 860-679-8704; Fax: 860-679-7639; E-mail: rosenberg{at}nso2.uchc.edu.

Cellular pools of free arachidonic acid are tightly controlled through enzymatic release of the fatty acid and subsequent utilization by downstream enzymes including the cyclooxygenases. Arachidonic acid cleavage from membrane phospholipids is accomplished by the actions of phospholipase A₂ (PLA₂). Upon release, free arachidonic acid provides substrate for the synthesis of eicosanoids. However, under certain conditions, arachidonic acid may participate in ceramide-mediated apoptosis. Disruption of arachidonic acid homeostasis can shift the balance of cell turnover in favor of tumorigenesis, via overproduction of tumor-promoting eicosanoids or alternatively by limiting proapoptotic signals. In the following study, we evaluated the influence of genetic deletion of a key intracellular phospholipase, cytoplasmic PLA₂ (cPLA₂), on azoxymethane-induced colon tumorigenesis. Heterozygous and null mice, upon treatment with the organotropic colon carcinogen, azoxymethane, developed a significant (P < 0.05) increase in colon tumor multiplicity (7.2-fold and 5.5-fold, respectively) relative to their wild-type littermates. This enhanced tumor sensitivity may be explained, in part, by the attenuated levels of apoptosis observed by terminal deoxynucleotidyl transferase–mediated nick end labeling staining within the colonic epithelium of heterozygous and null mice (50% of wild type). The lower frequency of apoptotic cells corresponded with reduced ceramide levels (69% and 46% of wild-type littermates, respectively). Remarkably, increased tumorigenesis resulting from cPLA₂ deletion occurred despite a significant reduction in prostaglandin E₂ production, even in cyclooxygenase-2–overexpressing tumors. These data contribute new information that supports a fundamental role of cPLA₂ in the control of arachidonic acid homeostasis and cell turnover. Our findings indicate that the proapoptotic role of cPLA₂ in the colon may supercede its contribution to eicosanoid production in tumor development.

Key Words: cPLA₂ • colon • apoptosis • arachidonic acid • ceramide

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