Activation of the transcription factor NF-κB by extracellular signals involves its release from the inhibitor protein IκBα in the cytoplasm and subsequent nuclear translocation. NF-κB 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-κB by a mechanism that is dependent on initial nuclear DNA damage followed by cytoplasmic signaling events. NF-κB 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-κB involves degradation of cytoplasmic IκBα by the ubiquitin-proteasome pathway in a manner that depends on the IκB kinase complex. Finally, inhibition of NF-κB 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 IκBα and nuclear translocation of NF- κB to attenuate the apoptotic response.
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