MIDDLE T-ANTIGEN TRANSFORMED ENDOTHELIAL-CELLS EXHIBIT AN INCREASED ACTIVITY OF NITRIC-OXIDE SYNTHASE

Endothelioma cell lines transformed by polyoma virus middle T antigen (mTa) cause cavernous hemangiomas in syngeneic mice by recruitment of host cells. The production of nitric oxide (NO), as measured by nitrit e and citrulline production, was significantly higher: in mTa-transfor med endothelial cells in comparison with nontransformed control cells. The maximal activity of NO synthase (NOS), was about 200-fold higher in cell lysates from the tEnd.1 endothelioma cell line than in lysates from nontransformed controls, whereas the affinity for arginine did n ot differ. The biochemical characterization of NOS and the study of mR NA transcripts indicate that tEnd.1 cells express both the inducible a nd the constitutive isoforms. NOS hyperactivity is not a simple conseq uence of cell transformation but needs a tissue-specific mTa expressio n. Since tEnd.1-conditioned medium induces NOS activity in normal endo thelial cells, most likely NOS hyperactivity in endothelioma cells is attributable to the release of a soluble factor This NOS-activating fa ctor, which seems to be an anionic protein, could stimulate tEnd.1 cel ls to express NOS by an autocrine way. By the same mechanism, tEnd.1 c ells could induce NOS in the neighboring endothelial cells, and NO rel ease could play a role in the hemangioma development. Such hypothesis is confirmed by our in vivo experiments, showing that the administrati on of the NOS inhibitor L-canavanine to endothelioma-bearing mice sign ificantly reduced both the volume and the relapse time of the tumor.