Bovine Papillomavirus Type 1 Oncoprotein E5 Stimulates the Utilization of Superoxide Radicals in the Mouse Fibroblast Cell Line C127

The major transforming protein of bovine papillomavirus type 1 (BPV-1) is a small hydrophobic polypeptide, the E5 gene product, localized in the cellular membranes and modulating various pathways in the cell. Many studies have shown that reactive oxygen species (ROS) are essential in several biological processes, including cell transformation by oncogenes, but unregulated ROS are highly toxic to cells. We studied the effect of the bovine papillomavirus protein E5 and its mutants on the level of the superoxide radicals in the mouse fibroblast cell line C127. The superoxide level in C127 cells transfected with the E5-expressing plasmids were measured by nitroblue tetrazolium reduction. Relative concentrations of intracellular peroxide were determined by using 2,7-dichlorofluorescin diacetate. Our results showed that all transforming mutants of E5 reduced the level of superoxide in C127 cells, besides the activity of superoxide dismutase (SOD) and level of peroxides was not altered. In the presence of neopterin, an inhibitor of the superoxide-producing enzymes, the reduction of superoxide level correlated with the transforming ability of the E5-mutants. The inhibitor of the protein tyrosine kinase, tyrphostin 25 and inhibitors of oxygenases of the arachidonic acid metabolism, aspirin and nordihydroguaiaretic acid, blocked the effect of BPV-1 E5. We conclude that BPV-1 E5 and its transforming mutants are able to modulate the level of superoxide and stimulate the utilization of superoxide through protein tyrosine kinases and oxygenases of the arachidonic acid metabolism.