LIGHT, a Member of the Tumor Necrosis Factor Ligand Superfamily, Prevents Tumor Necrosis Factor-α-mediated Human Primary Hepatocyte Apoptosis, but Not Fas-mediated Apoptosis

LIGHT is a member of tumor necrosis factor (TNF) superfamily, and its receptors have been identified as lymphotoxin-beta receptor (LTbetaR) and the herpesvirus entry mediator (HVEM)/ATAR/TR2, both of which lack the cytoplasmic sequence termed the "death domain." The present study has demonstrated that LIGHT inhibits TNFalpha-mediated apoptosis of human primary hepatocytes sensitized by actinomycin D (ActD), but not Fas- or TRAIL-mediated apoptosis. Furthermore, LIGHT does not prevent some cell lines such as HepG2 or HeLa from undergoing ActD/TNFalpha-induced apoptosis. This protective effect requires LIGHT pretreatment at least 3 h prior to ActD sensitization. LIGHT stimulates nuclear factor-kappaB (NF-kappaB)-dependent transcriptional activity in human hepatocytes like TNFalpha. The time course of NF-kappaB activation after LIGHT administration is similar to that of the pretreatment required for the anti-apoptotic effect of LIGHT. LIGHT inhibits caspase-3 processing on the apoptotic protease cascade in TNFalpha-mediated apoptosis but not Fas-mediated apoptosis. In addition, increased caspase-3 and caspase-8 activities in ActD/TNFalpha-treated cells are effectively blocked by LIGHT pretreatment. However, LIGHT does not change the expression of TNFRp55, TNFRp75, and Fas. These results indicate that LIGHT may act as an anti-apoptotic agent against TNFalpha-mediated liver injury by blocking the activation of both caspase-3 and caspase-8.