Role of tyrosyl-DNA phosphodiesterase 1 and inter-players in regulation of tumor cell sensitivity to topoisomerase I inhibition.

Tyrosyl-DNA phosphodiesterase 1 (TDP1) plays a unique function as it catalyzes the repair of topoisomerase I-mediated DNA damage. Thus, ovarian carcinoma cell lines exhibiting increased TDP1 levels and resistance to the topoisomerase I poisons campthotecins were used to clarify the role of this enzyme. The camptothecin gimatecan was employed as a tool to inhibit topoisomerase I because it produces a persistent damage. The resistant sublines displayed an increased capability to repair drug-induced single-strand breaks and a reduced amount of drug-induced double-strand breaks, which was enhanced following TDP1 silencing. In loss of function studies using U2-OS cells, we found that TDP1 knockdown did not produce a change in sensitivity to camptothecin, whereas co-silencing of other pathways cooperating with TDP1 in cell response to topoisomerase I poisons indicated that XRCC1 and BRCA1 were major regulators of sensitivity. No change in cellular sensitivity was observed when TDP1 was silenced concomitantly to RAD17, which participates in the stabilization of collapsed replication forks. The expression of dominant-negative PARP1 in cells with reduced expression of TDP1 due to a constitutively expressed TDP1 targeting microRNA did not modulate cell sensitivity to camptothecin. Mild resistance to gimatecan was observed in cells over-expressing TDP1, a feature associated with decreased levels of drug-induced single-strand breaks. In conclusion, since TDP1 alone can account for mild levels of camptothecin resistance, repair of topoisomerase I-mediated DNA damage likely occurs through redundant pathways mainly implicating BRCA1 and XRCC1, but not RAD17 and PARP1. These findings may be relevant to define novel therapeutic strategies.