L233P mutation in the bovine leukemia virus Tax protein depressesendothelial cell recruitment and tumorigenesis in athymic nude mice

AbstractBovine leukemia virus (BLV) can be divided into two categories based on the amino acid at position 233 in the Tax protein,which probably plays a crucial role in leukemogenesis. We show here that a rat fibroblast cell line stably expressing L233-Taxformed significantly larger tumors than P233-Tax-expressing cells in a murine xenograft study. Although the microvesseldensity was comparable in both tumors, visible blood vessel invasion was observed only on tumors from L233-Tax-expressingcells. Endothelial cell tube formation assays using human umbilical vein endothelial cells showed no significant differencein angiogenic activity between conditioned medium from L233- and P233-Tax-expressing cells, whereas in vitro chemotaxisassays revealed that only L233-Tax-expressing cells produced a chemoattractant for endothelial cells. Since pathological neovascularizationcan occur from the recruitment of endothelial progenitor cells, these results suggest that L233-Tax-expressingcells recruit murine endothelial progenitor cells and promote neovascularization to support tumor growth. BLV-infectedlymphoma cells may also recruit bovine endothelial progenitor cells to promote neovascularization. The findings of this studyare consistent with our previous observation that BLV carrying P233-Tax has a significantly longer incubation period fordeveloping tumors than the virus carrying L233-Tax and provide insight into the function of Tax in leukemogenesis by BLV