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Overexpression of Cox-2 in Human Osteosarcoma Cells Decreases Proliferation and Increases Apoptosis

¹ Department of Medicine, University of Connecticut Health Center, Farmington, Connecticut and ² Molecular Biology Institute, University of California at Los Angeles, Los Angeles, California

Requests for reprints: Carol Pilbeam, Department of Medicine, University of Connecticut Health Center, Room AM046, MC1850, 263 Farmington Avenue, Farmington, CT 06030. Phone: 860-679-3846; Fax: 860-679-1932; E-mail: pilbeam{at}nso.uchc.edu.

Overexpression of cyclooxygenase-2 (COX-2) is generally considered to promote tumorigenesis. To investigate a potential role of COX-2 in osteosarcoma, we overexpressed COX-2 in human osteosarcoma cells. Saos-2 cells deficient in COX-2 expression were retrovirally transduced or stably transfected with murine COX-2 cDNA. Functional expression of COX-2 was confirmed by Northern and Western analyses and prostaglandin production. Overexpression of COX-2 reduced cell numbers by 50% to 70% compared with controls. Decreased proliferation in COX-2-overexpressing cells was associated with cell cycle prolongation in G₂-M. Apoptosis, measured by both Annexin V binding assay and terminal deoxyribonucleotidyl transferase–mediated dUTP nick end labeling staining, was increased in cells overexpressing COX-2, and the increase was not reversed by treatment with NS-398, indicating that the effects were not mediated by prostaglandins. Retroviral COX-2 overexpression in two other human osteosarcoma cell lines, U2OS and TE85, also decreased cell viability. However, in the human colon carcinoma HCT-116 cell line, which is deficient in COX-2, retroviral overexpression of COX-2, at similar efficiency as in Saos-2 cells, increased resistance to apoptosis. Reactive oxygen species (ROS), measured by flow cytometry, were increased by COX-2 overexpression in Saos-2 cells but not in HCT-116 cells. Inhibition of peroxidase activity, but not of COX activity, blocked the ROS increase. Antioxidants blocked the increase in ROS and the increase in apoptosis due to COX-2 overexpression in Saos-2 cells. Our results suggest that (a) COX-2 overexpression in osteosarcoma cells may increase resistance to tumorigenesis by increasing ROS to levels that decrease cell viability and (b) the effects of COX-2 overexpression are cell type/tissue dependent.(Cancer Res 2006; 66(13): 6657-64)