NF-κB activation by camptothecin -: A linkage between nuclear DNA damage and cytoplasmic signaling events

Activation of the transcription factor NF-kappa B by extracellular signals involves its release from the inhibitor protein I kappa B alpha in the cytoplasm and subsequent nuclear translocation. NF-kappa B can also be activated by the anticancer agent camptothecin (CPT), which inhibits DNA topoisomerase (Topo) I activity and causes DNA double-strand breaks during DNA replication to induce S phase-dependent cytotoxicity. Here we show that CPT activates NF-kappa B by a mechanism that is dependent on initial nuclear DNA damage followed by cytoplasmic signaling events. NF-kappa B activation by CPT is dramatically diminished in cytoplasts and in CEM/C2 cells expressing a mutant Topo I protein that fails to bind CPT, This response is intensified in S phase cell populations and is prevented by the DNA polymerase inhibitor aphidicolin. In addition, CPT activation of NF-kappa B involves degradation of cytoplasmic I kappa B alpha by the ubiquitin-proteasome pathway in a manner that depends on the I kappa B kinase complex. Finally, inhibition of NF-kappa B activation augments CPT-induced apoptosis, These findings elucidate the progression of signaling events that initiates in the nucleus with CPT-Topo I interaction and continues in the cytoplasm resulting in degradation of I kappa B alpha and nuclear translocation of NF-kappa B to attenuate the apoptotic response.