The TRAIL protein is a member of the tumor necrosis factor family and can associate with two receptors (DR4 and DR5) and three decoy receptors [DcR1, DcR2, and osteoprotegerin (OPG)]. TRAIL-dependent apoptosis is dependent on DR4 or DR5 expression, yet the presence of decoy receptors does not necessarily block apoptosis. Prior observations suggested that TRAIL binds its receptors with nearly equal affinity; however, these experiments were performed under nonphysiological conditions. Truneh et al. determine the binding affinity of TRAIL for its receptors by using more physiologically relevant criteria. Using isothermal titration calorimetry and competitive inhibition, the authors found that TRAIL bound with highest affinity to DR5 and lowest affinity to OPG. TRAIL-DR5 binding increased significantly from 4oC to 37oC. Experiments using intact cells and fluorescence-activated cell sorting analyses confirmed the in vitro results. Because the affinity of TRAIL to DR5 is much greater than the interaction of TRAIL with its other receptors at 37oC, this suggests a mechanism by which decoy receptors are unable to completely inhibit TRAIL-mediated apoptosis. Also, studying receptor-ligand interactions at physiological and supraphysiological (e.g., fever) temperatures may lead to new insights about signaling mechanisms that occur in response to immune responses.
Truneh, A., Sharma, S., Silverman, C., Khandekar, S., Reddy, M.P., Deen, K.C., Mclaughlin, M.M., Srinivasula, S.M., Livi, G.P., Marshall, L.A., Alnemri, E.S., Williams, W.V., and Doyle, M.L. (2000) Temperature-sensitive differential affinity of TRAIL for its receptors: DR5 is the highest affinity receptor. J. Biol. Chem. 275: 23319-23325. [Abstract] [Full Text]