Cellular and molecular responses to topoisomerase I poisons: Exploiting synergy for improved radiotherapy

The efficacy of topoisomerase (Topo) I-active drugs may be improved by better understanding the molecular and cellular responses of tumor compared to normal cells after genotoxic insults. Ionizing radiation (IR) + Topo I-active drugs (e.g., Topotecan) caused synergistic cell killing in various human cancer cells, even in cells from highly radioresistant tumors. Topo I poisons had to be added either during or immediately after IR. Synergy was caused by DNA lesion modification mechanisms as well as by concomitant stimulation of two pathways of cell death: necrosis (IR) + apoptosis (Topo I poisons). Cumulative data favor a mechanism of synergistic cell killing caused by altered DNA lesion modification and enhanced apoptosis. However, alterations in cell cycle regulation may also play a role in the synergy between these two agents in certain human cancers. We recently showed that NF-κB, a known anti-apoptotic factor, was activated in various cancer cells after poisoning Topo I using clinically active drugs. NF-κB activation was dependent on initial nuclear DNA damage followed by cytoplasmic signaling events. Cytoplasmic signaling leading to NF-κB activation after Topo I poisons was diminished in cytoplasts (lacking nuclei) and in CEM/C2 cells that expressed a mutant Topo I protein that did not interact with Topo I-active drugs. NF-κB activation was intensified in S-phase and blocked by aphidicolin, suggesting that activation was a result of double-strand break formation due to Topo I poisoning and DNA replication. Dominant-negative IκB expression augmented Topo I poison-mediated apoptosis. Elucidation of molecular signal transduction pathways after Topo I drug-IR combinations may lead to improved radiotherapy by blocking anti-apoptotic NF-κB responses. Recent data also indicate that synergy caused by IR + Topo I poisons is different from radiosensitization by β-lapachone (β-lap), a "reported" Topo I and II-α poison in vitro. In fact, β-lap does not kill cells by poisoning either Topo I or II-α in vivo. Instead, the compound is "activated" by an IR (damage)-inducible enzyme, NAD(P)H:quinone oxidoreductase (NQO1), a gene cloned as x-ray-inducible transcript #3, xip3. Unlike the lesion modification pathway induced by IR + Topo I drugs, β-lap kills cells via NQO1 futile cycle metabolism. Downstream apoptosis caused by β-lap appears to be noncaspase-mediated, involving calpain or a calpain-like protease. Thus, although Topo I poisons or β-lap in combination with IR both synergistically kill cancer cells, the mechanisms are very different.