Over-expression of Reticulon 3 (RTN3) enhances TRAIL-mediated apoptosis via up-regulation of death receptor 5 (DR5) and down-regulation of c-FLIP.

Reticulons (RTNs) are a group of integral membrane proteins that have no homology to other known apoptosis-related domains. Herein, we found that RTN3 overexpressing Caki cells were sensitive to TRAIL-mediated apoptosis. RTN3-induced down-regulation of c-FLIP was recovered by pan-caspase inhibitor, z-VAD to basal levels in TRAIL-treated cells. The forced expression of c-FLIP attenuated the TRAIL-mediated apoptosis in RTN3 over-expressing cells. In addition, RTN3 over-expression provoked the enhanced protein levels in DR4 and DR5 as well as levels in DR5 surface protein but slight increase in DR4 surface protein. RTN3-mediated enhancement of TRAIL-induced apoptosis was markedly blocked by the DR5/Fc chimera or DR5 siRNA, indicating that the sensitization by RTN3 was mainly mediated through interactions of TRAIL with its receptors, DR5. Over-expression of RTN3 also enhanced TNF-alpha and Fas-mediated apoptosis. Taken together, over-expression of RTN3 might increase DR5 surface protein and concomitantly more activate caspase pathways, which cause the c-FLIP cleavage and enhancement of TRAIL-mediated apoptosis.