Hsp90 inhibition abolishes LPS‐mediated NF‐κB transcriptional activation independent of IKKβ in human lung microvascular endothelial cells (HLMVEC) (847.7)

LPS promotes NF‐κB‐dependent inflammation by activating IKKβ which phosphorylates inhibitory IKB proteins leading to their degradation. Basally, NF‐κB exists in an inactive cytoplasmic complex with IKB. Stimulus‐dependent degradation of IKB allows NF‐κB to translocate into the nucleus and initiate transcription. Hsp90 is essential for the activation of a number of kinases, including IKKβ, and thus an important regulator of the NF‐κB pathway. Here we demonstrate that hsp90 inhibition by 17‐AAG attenuated LPS‐induced NF‐κB reporter and target gene activation in HLMVEC. Similarly, inhibition of upstream TAK‐1 and IKKβ blocked IKBα phosphorylation/degradation by LPS. However, 17‐AAG blocked the LPS‐induced phosphorylation/ubiquitination of IKBα without preventing its degradation. Indeed, in the presence of 17‐AAG, LPS degraded a phosphorylation/ubiquitination‐deficient mutant IKBα. This 17‐AAG‐mediated degradation did not involve IKKβ, since addition of excess IKKβ inhibitor, after 17‐AAG, failed to recover IKBα protein levels. Instead, 17‐AAG prevented the LPS‐induced recruitment of NF‐κB and its co‐activator CBP to NF‐κB target gene promoter. This was associated with the prevention of LPS‐induced core histone H3 eviction and N‐terminal phosphorylation of RNA polymerase II. Thus, we conclude that hsp90 inhibition does not affect IKKβ‐dependent NF‐κB signaling but blocks NF‐κB transcriptional activation via an epigenetic mechanism in HLMVEC. Grant Funding Source : HL093460