title: Uracil-DNA glycosylase expression determines human lung cancer cell sensitivity to pemetrexed.
abstract: Uracil misincorporation into DNA is a consequence of pemetrexed inhibition of thymidylate synthase. The base excision repair (BER) enzyme uracil-DNA glycosylase (UNG) is the major glycosylase responsible for removal of misincorporated uracil. We previously illustrated hypersensitivity to pemetrexed in UNG(-/-) human colon cancer cells. Here, we examined the relationship between UNG expression and pemetrexed sensitivity in human lung cancer. We observed a spectrum of UNG expression in human lung cancer cells. Higher levels of UNG are associated with pemetrexed resistance and are present in cell lines derived from pemetrexed-resistant histologic subtypes (small cell and squamous cell carcinoma). Acute pemetrexed exposure induces UNG protein and mRNA, consistent with upregulation of uracil-DNA repair machinery. Chronic exposure of H1299 adenocarcinoma cells to increasing pemetrexed concentrations established drug-resistant sublines. Significant induction of UNG protein confirmed upregulation of BER as a feature of acquired pemetrexed resistance. Cotreatment with the BER inhibitor methoxyamine overrides pemetrexed resistance in chronically exposed cells, underscoring the use of BER-directed therapeutics to offset acquired drug resistance. Expression of UNG-directed siRNA and shRNA enhanced sensitivity in A549 and H1975 cells, and in drug-resistant sublines, confirming that UNG upregulation is protective. In human lung cancer, UNG deficiency is associated with pemetrexed-induced retention of uracil in DNA that destabilizes DNA replication forks resulting in DNA double-strand breaks and cell death. Thus, in experimental models, UNG is a critical mediator of pemetrexed sensitivity that warrants evaluation to determine clinical value.
authors.affiliation: Corresponding Author: Stanton L. Gerson, Case Comprehensive Cancer Center, Wearn 151, 111000 Euclid Avenue, Cleveland, OH 44106. stanton.gerson@case.edu.
authors.forename: Lachelle D
authors.initials: LD
authors.lastname: Weeks
authors.forename: Pingfu
authors.initials: P
authors.lastname: Fu
authors.forename: Stanton L
authors.initials: SL
authors.lastname: Gerson
mesh_terms: D045744:Cell Line, Tumor; D053903:DNA Breaks, Double-Stranded / Q000187:drug effects; D004249:DNA Damage / Q000187:drug effects; D004261:DNA Replication / Q000187:drug effects; D019008:Drug Resistance, Neoplasm / Q000235:genetics*; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D005971:Glutamates / Q000008:administration & dosage*; D006147:Guanine / Q000008:administration & dosage / Q000031:analogs & derivatives*; D006801:Humans; D008175:Lung Neoplasms / Q000188:drug therapy* / Q000201:enzymology / Q000473:pathology; D000068437:Pemetrexed; D034741:RNA, Small Interfering; D051981:Uracil-DNA Glycosidase / Q000096:biosynthesis*
chemical_list: D005971:Glutamates; D034741:RNA, Small Interfering; D000068437:Pemetrexed; D006147:Guanine; D051981:Uracil-DNA Glycosidase