TL1, a novel tumor necrosis factor-like cytokine, induces apoptosis in endothelial cells - Involvement of activation of stress protein kinases (stress-activated protein kinase and p38 mitogen-activated protein kinase) and caspase-3-like protease

TL1 is a recently discovered novel member of the tumor necrosis factor (TNF ) cytokine family. TL1 is abundantly expressed in endothelial cells, but it s function is not known. The present study was undertaken to explore whethe r TL1 induces apoptosis in endothelial cells and if so, to explore its mech anism of action. Cultured bovine pulmonary artery endothelial cells (BPAEC) exposed to TL1 showed morphological (including ultrastructural) and bioche mical features characteristic of apoptosis. TL1-induced apoptosis in BPAEC was a time- and concentration-dependent process (EC50 = 72 ng/ml). The effe ct of TL1 was not inhibited by soluble TNF receptors 1 or 2. TL1 up-regulat ed 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 a poptosis in the BPAEC was reduced by expression of a dominant-interfering m utant of c-Jun (62.8%, p < 0.05) or by a specific p38 inhibitor, SB203580 ( 1-10 mu M) dose-dependently. TL1 also activated caspases in BPAEC, and TL1- induced apoptosis in BPAEC was significantly attenuated by the caspase inhi bitor, ZVAD-fluromethyl-ketone, The major component activated by TL1 in BPA EC was caspase 3, which was based on substrate specificity and immunocytoch emical analysis. These findings suggest that TL1 may act as an autocrine fa ctor to induce apoptosis in endothelial cells via activation of multiple si gnaling pathways, including stress protein kinases as well as certain caspa ses.