We have shown that reovirus-induced apoptosis requires TRAIL binding to its apoptosis-inducing receptors DR5 and/or DR4. However, exogenous TRAIL (200 ng/ml) does not induce apoptosis in uninfected HEK293 cells since these cells do not express sufficient cell surface DR4 or DR5. To induce apoptosis, reovirus must therefore up-regulate both TRAIL and a death-associated TRAIL receptor. We therefore determined that there is both an increase in the release of TRAIL and an increase in the expression of DR5, and to a lesser extent DR4, in reovirus-infected cells. It seems unlikely that the up-regulation of both DR4 and DR5 is required for TRAIL-mediated expression in reovirus-infected cells. The quicker and more dramatic increase in DR5 expression compared to DR4 expression suggests that DR5 is the major receptor involved in triggering apoptosis. Furthermore, the increased sensitivity of reovirus-infected HEK293 cells to TRAIL-induced apoptosis is detectable 12 h following infection, whereas the alteration in expression of DR4 does not occur until 24 h postinfection. These results suggest that the contribution of DR4 to reovirus-induced apoptosis may be a secondary event and that its up-regulation in reovirus-infected cells may function to amplify the effects of TRAIL/DR5 regulation.
We propose a model in which reovirus infection results in the up-regulation of DR5 and the release of TRAIL, thereby activating the TRAIL pathway of cell death (Fig. ). Similar to other DR-mediated apoptotic pathways, reovirus-induced apoptosis requires the participation of an adapter molecule (FADD) and the activation of the caspase cascade since it is reduced in the presence of inhibitors of FADD or caspase 8 activity. We have recently shown that reovirus-induced apoptosis requires the transcription factor NF-κB (3
). Future studies will be directed at examining the role of NF-κB in the up-regulation of TRAIL and DR5 in reovirus-infected cells.
Reovirus induces the TRAIL apoptotic pathway in infected cells by inducing the release of TRAIL and the up-regulation of DR5. The consequent binding of TRAIL to DR5 then promotes FADD association and the activation of caspase 8.
Our results demonstrate the involvement of the TRAIL apoptotic pathway in reovirus-induced cell death and provide the first direct evidence for the involvement of this pathway in virus-induced apoptosis. Additional support for the potential role of DR4 and DR5 in virus-induced apoptosis comes from studies suggesting that human immunodeficiency virus (HIV) infection increases the expression of TRAIL and sensitizes T cells to TRAIL-mediated apoptosis (10
). Previous studies have suggested that other members of the TNFR DR superfamily may also be involved in apoptosis induced in cells infected with a variety of viruses. Alteration of the cell surface expression of Fas may be involved in virus-induced, or viral regulation of, apoptosis in cells infected with influenza virus (21
), herpes simplex virus type 2 (19
), bovine herpesvirus 4 (30
), adenovirus (24
), and HIV type 1 (2
). Similarly, apoptosis induced by hepatitis B virus (20
), HIV type 1 (8
), bovine herpesvirus 4 (30
), and parvovirus H-1 (14
) may involve the TNFR signaling pathway. TRAIL and TRAIL receptor expression have been shown to mediate gamma interferon-induced antiviral activity (18
), although the mechanism by which this occurs is unknown.
We propose that the regulation of TRAIL and its death-promoting receptors is a primary mediator of apoptosis that is induced not only following viral infection but also as a component of apoptosis-inducing stress responses, including chemotherapy (7