Translesion polymerase η is upregulated by cancer therapeutics and confers anticancer drug resistance.

Maja T Tomicic,Dorthe Aasland,Christina Barckhausen,Bernd Kaina,Steffen C Naumann,Ruth Meise,Markus Christmann

doi:10.1158/0008-5472.can-14-0953

Abstract

DNA repair processes are a key determinant of the sensitivity of cancer cells to DNA-damaging chemotherapeutics, which may induce certain repair genes as a mechanism to promote resistance. Here, we report the results of a screen for repair genes induced in cancer cells treated with DNA crosslinking agents, which identified the translesion polymerase η (PolH) as a p53-regulated target acting as one defense against interstrand crosslink (ICL)-inducing agents. PolH was induced by fotemustine, mafosfamide, and lomustine in breast cancer, glioma, and melanoma cells in vitro and in vivo, with similar inductions observed in normal cells such as lymphocytes and diploid fibroblasts. PolH contributions to the protection against ICL-inducing agents were evaluated by its siRNA-mediated attenuation in cells, which elevated sensitivity to these drugs in all tumor cell models. Conversely, PolH overexpression protected cancer cells against these drugs. PolH attenuation reduced repair of ICL lesions as measured by host cell reactivation assays and enhanced persistence of γH2AX foci. Moreover, we observed a strong accumulation of PolH in the nucleus of drug-treated cells along with direct binding to damaged DNA. Taken together, our findings implicated PolH in ICL repair as a mechanism of cancer drug resistance and normal tissue protection.

Highlights

Because of the high toxicity in replicating cells, genotoxic agents are being used as anticancer drugs
Expression of polymerase h (PolH) upon fotemustine exposure In melanoma cells, toxicity induced by chloroethylating anticancer drugs like fotemustine depends on the p53 and methylguanine-DNA methyltransferase (MGMT) status [23, 37]
In case of the chloroethylating agents, ACNU [22] and fotemustine [25], this protective effect was related to the induction of the nucleotide excision repair genes xpc and ddb2, which are robustly regulated by p53

Summary

Introduction

Because of the high toxicity in replicating cells, genotoxic agents are being used as anticancer drugs. Most important are drugs that induce DNA interstrand crosslinks (ICL), which are potent killing lesions [1]. The ICL-inducing drugs include derivatives of nitrogen mustards and chloroethyl nitrosoureas. One of the most often used nitrogen mustard is cyclophosphamide, whose active metabolite alkylates the N7 position of guanine yielding cytotoxic ICL [2]. Chloroethyl nitrosoureas used in cancer therapy are carmustine (BCNU), lomustine (CCNU), nimustine (ACNU), and fotemustine (Muphoran). Upon formation of a nucleophilic chloroethylenium ion, the O6-position of guanine becomes alkylated, leading to the formation of O6-chloroethylguanine O6-ClG can be repaired by the DNA repair protein O6-methylguanine-DNA methyltransferase O6ClG undergoes intramolecular rearrangement to form N1-O6-ethenoguanine and N1-guanine-N3-cytosine ICLs [5, 6].

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Results

Conclusion