Geldanamycin inhibits tyrosine phosphorylation-dependent NF-κB activation

Hsp90 is a protein chaperone regulating the stability and activity of many signalling molecules. The requirement of Hsp90 activity in the NF-κB pathway has been recently reported by several authors using the Hsp90 ATPase inhibitor geldanamycin (GA), an anti-tumor drug. Hsp90 inhibition blocks the synthesis and activation of the IKK complex, the major kinases complex responsible for IκBα phosphorylation on serine 32 and 36, a key step for its degradation and the nuclear translocation of NF-κB. However, the effect of GA on other IκBα kinases, including tyrosine kinases, is unknown. In the present study, we investigated the effect of GA on NF-κB activation induced by sodium pervanadate (PV), a tyrosine phosphatase inhibitor triggering c-Src-mediated tyrosine phosphorylation of IκBα. We report for the first time that GA inhibits PV-induced IκBα tyrosine phosphorylation and degradation. Using an in vitro kinase assay, we demonstrated that GA inhibits the activity of c-Src as an IκBα tyrosine kinase, but not its cellular expression. As a result, GA blocked PV-induced NF-κB DNA-binding activity on an exogenous κB element and on the endogenous iκbα promoter, thereby inhibiting iκbα transcription. Finally, we demonstrated that, despite NF-κB inhibition, pre-treatment with GA does not potentiate PV-induced apoptosis. We conclude that c-Src requires Hsp90 for its tyrosine kinase activity, and its inhibition by GA blocks c-Src-dependent signalling pathways, such as NF-κB activation induced by sodium pervanadate. The effect of GA on PV-induced apoptosis is discussed in the light of recent publications in the literature.