Anthrax Lethal Toxin Enhances IκB Kinase Activation and Differentially Regulates Pro-inflammatory Genes in Human Endothelium *

Anthrax lethal toxin (LT) was previously shown to enhance transcriptional activity of NF-κB in tumor necrosis factor-α-activated primary human endothelial cells. Here we show that this LT-mediated increase in NF-κB activation is associated with the enhanced degradation of the inhibitory proteins IκBα and IκBβ but not IκBϵ. Moreover, this was accompanied by enhanced activation of the IκB kinase complex (IKK), which is responsible for targeting IκB proteins for degradation. Importantly, LT enhancement of IκBα degradation was completely blocked by a selective IKKβ inhibitor, whereas IκBβ degradation was attenuated, suggesting a mechanistic link. Consistent with the above data, LT-cotreated cells show elevated phosphorylation of two IKK substrates, IκBα and p65, both of which were blocked by incubation with the IKKβ inhibitor. Consistent with NF-κB activation, LT increased transcription of the NF-κB regulated gene CD40. Conversely, LT inhibited transcription of another NF-κB-regulated gene, CCL2. This inhibition was linked to the LT-mediated suppression of another CCL2 -regulating transcription factor, AP-1 (activator protein-1). These data suggest that LT-mediated enhancement of NF-κB is IKK-dependent, but importantly, the net effect of LT on the transcription of proinflammatory genes is driven by the cumulative effect of LT on the particular set of transcription factors that regulate a given promoter. Together, these findings provide new mechanistic insight on how LT may disrupt the host response to anthrax.