IκB kinase subunits α and γ are required for activation of NF-κB and induction of apoptosis by mammalian reovirus

Reoviruses induce apoptosis both in cultured cells and in vivo. Apoptosis plays a major role in the pathogenesis of reovirus encephalitis and myocarditis in infected mice. Reovirus-induced apoptosis is dependent on the activation of transcription factor NF-kappa B and downstream cellular genes. To better understand the mechanism of NF-kappa B activation by reovirus, NF-kappa B signaling intermediates under reovirus control were investigated at the level of Rel, I kappa B, and I kappa B kinase (IKK) proteins. We found that reovirus infection leads initially to nuclear translocation of p50 and RelA, followed by delayed mobilization of c-Rel and p52. This biphasic pattern of Rel protein activation is associated with the degradation of the NF-kappa B inhibitor I kappa B alpha but not the structurally related inhibitors I kappa B beta or I kappa B epsilon. Using IKK subunit-specific small interfering RNAs and cells deficient in individual IKK subunits, we demonstrate that IKK alpha but not IKK beta is required for reovirus-induced NF-kappa B activation and apoptosis. Despite the preferential usage of IKK alpha, both NF-kappa B activation and apoptosis were attenuated in cells lacking IKK gamma/Nemo, an essential regulatory subunit of IKK beta. Moreover, deletion of the gene encoding NF-kappa B-inducing kinase, which is known to modulate IKK alpha function, had no inhibitory effect on either response in reovirus-infected cells. Collectively, these findings indicate a novel pathway of NF-kappa B/Rel activation involving IKK alpha and IKK gamma/Nemo, which together mediate the expression of downstream proapoptotic genes in reovirus-infected cells.