TL1, a Novel Tumor Necrosis Factor-like Cytokine, Induces Apoptosis in Endothelial Cells

TL1 is a recently discovered novel member of the tu- mor necrosis factor (TNF) cytokine family. TL1 is abun- dantly expressed in endothelial cells, but its function is not known. The present study was undertaken to ex- plore whether TL1 induces apoptosis in endothelial cells and, if so, to explore its mechanism of action. Cultured bovine pulmonary artery endothelial cells (BPAEC) ex- posed to TL1 showed morphological (including ultra- structural) and biochemical features characteristic of apoptosis. TL1-induced apoptosis in BPAEC was a time- and concentration-dependent process (EC50 5 72 ng/ml). The effect of TL1 was not inhibited by soluble TNF re- ceptors 1 or 2. TL1 up-regulated Fas expression in BPAEC at 8 and 24 h after treatment, and significantly activated stress-activated protein kinase (SAPK) and p38 mitogen-activated protein kinase (p38 MAPK). The peak activities of SAPK and p38 MAPK in TL1-treated BPAEC were increased by 9- and 4-fold, respectively. TL1-induced apoptosis in the BPAEC was reduced by expression of a dominant-interfering mutant of c-Jun (62.8%, p < 0.05) or by a specific p38 inhibitor, SB203580 (1-10 mM) dose-dependently. TL1 also activated caspases in BPAEC, and TL1-induced apoptosis in BPAEC was significantly attenuated by the caspase inhibitor, ZVAD- fluromethyl-ketone. The major component activated by TL1 in BPAEC was caspase-3, which was based on sub- strate specificity and immunocytochemical analysis. These findings suggest that TL1 may act as an autocrine factor to induce apoptosis in endothelial cells via acti- vation of multiple signaling pathways, including stress protein kinases as well as certain caspases.