Temporal pattern of NFκB activation influences apoptotic cell fate in a stimuli-dependent fashion

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Abstract
The transcription factor NFκB is a critical immediate early response gene involved in modulating cellular responses and apoptosis following diverse environmental injuries. The activation of NFκB is widely accepted to play an anti-apoptotic role in cellular responses to injury. Hence, enhancing NFκB activation in the setting of injury has been proposed as one potential therapeutic approach to environmental injuries. To this end, we constructed a recombinant adenoviral vector (Ad.IκBαAS) expressing antisense IκBα mRNA that is capable of augmenting NFκB activation prior to and following four types of cellular injury [TNF-α, UV, hypoxia/reoxygenation (H/R) or pervanadate treatment]. Biochemical and functional analyses of NFκB activation pathways for these injuries demonstrated two categories involving either serine (S32/36) phosphorylation (TNF-α, UV) or tyrosine (Y42) phosphorylation (H/R or PV) of IκBα. We hypothesized that activation of NFκB prior to injury using antisense IκBα mRNA would reduce apoptosis. As anticipated, recombinant adenoviral IκBα phosphorylation mutants (Ad.IκBαS32/36A or Ad.IκBαY42F) preferentially reduced NFκB activation and enhanced apoptosis following injuries associated with either serine or tyrosine phosphorylation of IκBα, respectively. These studies demonstrate for the first time that an IκBαY42F mutant can effectively modulate NFκB-mediated apoptosis in an injury-context-dependent manner. Interestingly, constitutive activation of NFκB following Ad.IκBαAS infection reduced apoptosis only following injuries associated with IκBα Y42, but not S32/36, phosphorylation. These findings demonstrate that the temporal regulation of NFκB and the apoptotic consequences of this activation are differentially influenced by the pathway mediating NFκB activation. They also provide new insight into the therapeutic potential and limitations of modulating NFκB for environmental injuries such as ischemia/reperfusion and pro-inflammatory diseases.