Reovirus Activates a Caspase-Independent Cell Death Pathway

Berger, Angela K.; Danthi, Pranav

Induction of necroptosis by any of the pathways described above requires the formation of the ripoptosome or a similar multiprotein complex (54) . This 2-MDa signaling complex is comprised of three core components, RIP1, caspase-8, and FADD (54) . In addition, it also contains regulators such as cFLIP, cIAP1, cIAP2, and XIAP. The decision between cell survival, apoptosis, and necroptosis occurs at the level of the ripoptosome and depends on the activity of caspase-8 (40) . When procaspase-8 is sufficiently processed and caspase-8 activity is high, apoptosis ensues. In contrast, when caspase-8 activity is lower due to heterodimerization of caspase-8 with cFLIP L , no apoptosis occurs. Under these conditions, caspase-8-cFLIP L complexes retain sufficient activity to cleave RIP1, preventing necroptosis. Thus, when Results are expressed as the mean ratios of caspase-3/7 activity from infected cell lysates to that from equivalently treated mock-infected cells for triplicate samples. We note that the DMSO-treated cells are the same as those used in Fig. 2 . Error bars indicate SD. *, P value of Ͻ0.05 as determined by Student's t test in comparison to T3Dinfected cells treated with DMSO. activity of caspase-8 is low, cells survive. If, instead, caspase-8 is blocked by binding to cFLIP L or the presence of cIAPs, RIP1 is not cleaved and the cells undergo necroptosis. Thus, we should anticipate that when caspase-8 is active, reovirus infection results in apoptosis, and when it is inhibited, by inclusion of caspase-8 or pan-caspase inhibitor, reovirus infection produces necroptosis. In our studies, reovirus-induced cell death was sensitive to Nec-1 even in the absence of treatment with caspase inhibitors. These data suggest that the activity of caspase-8 following reovirus infection is not sufficient to cleave and inactivate RIP1, thereby allowing induction of necroptosis.