Human endothelial cells expressing polyoma middle T induce tumors

The middle T oncogene of murine polyomavirus (PymT) rapidly transforms and immortalizes murine embryonic endothelial cells (EC), leading to the format ion of vascular tumors in newborn mice, by recruitment of host, non-transfo rmed EC. These tumors are reminiscent of human vascular tumors like caverno us hemangioma, Kaposi's sarcoma or those characterizing Kasabach-Merrit syn drome. Here we investigate the in vitro and in vivo behavior of human prima ry umbilical cord vein EC expressing PymT. While PymT has been unable to tr ansform human fibroblasts in earlier experiments or controls done here, mT expressing EC (PymT-EC) derived by infection with pLX-PymT retrovirus induc e hemangiomas in nu/nu mice. These tumors contain not only human cells but also recruited mouse EC as shown by the presence of human and murine CD31 p ositive EC. In vitro analysis shows that PymT-EC retain endothelial specifi c markers like CD31, Von Willebrand factor, and VE-cadherin, and reach the confluence without signs of overgrowth. They are also responsive to vascula r endothelial growth factor-A. However, their proliferation rate is increas ed. The balance between urokinase-type plasminogen activator and plasminoge n activator inhibitor-1 is modified; RNA and catalytic activity for the for mer are elevated while PAI-1 RN4 is reduced. In contrast with murine model, where the PymT EC cells become immortal, the effects induced by PymT in hu man EC are transient. After 12-15 passages, human PymT EC stop proliferatin g, assume a senescent phenotype, and lose the ability to induce hemangiomas , At the same time both the amount of middle T protein and the level of act ivation of pp60(c-src) lower.