Abstract 904: The novel C-terminal truncated 90-kDa isoform of topoisomerase IIα, TOP2α/90, is a determinant of etoposide resistance in K562 leukemia cells via heterodimerization with the TOP2α/170 isoform

DNA topoisomerase IIα (170 kDa, TOP2α/170) is essential in proliferating cells since it resolves DNA topologic entanglements during chromosome condensation, replication, and segregation. We previously characterized a C-terminally truncated TOP2α/90 isoform, detectable in human leukemia K562 cells but more abundantly expressed in a clonal subline, K/VP.5, with acquired resistance to the anticancer agent etoposide (J Pharmacol Exp Ther 2017;360:152-63). TOP2α/90 (786 amino acids) is the translation product of a TOP2α mRNA that retains a processed intron 19. TOP2α/90 lacks the active-site tyrosine-805 (Tyr805) required to generate double-strand DNA breaks as well as the nuclear localization signals present in the TOP2α/170 isoform (1531 amino acids). The function of TOP2α/90 is unknown. Here, we found that TOP2α/90, like TOP2α/170, was detectable in the nucleus and cytoplasm of K562 and K/VP.5 cells. Importantly, co-immunoprecipitation of endogenous TOP2α/90 and TOP2α/170 demonstrated heterodimerization of these isoforms. Forced expression of TOP2α/90 in K562 cells suppressed, while siRNA-mediated knockdown of TOP2α/90 in K/VP.5 cells enhanced, etoposide-mediated DNA strand breaks compared with similarly treated K562 or K/VP.5 cells transfected with empty vector or control siRNAs, respectively. In addition, forced expression of TOP2α/90 in K562 cells inhibited etoposide cytotoxicity assessed by soft agar colony formation assays. qPCR and immunoassays demonstrated expression of TOP2α/90 mRNA and protein in normal human tissues/cells and in leukemia cells from patients. Together, results strongly suggest that TOP2α/90 expression decreases drug-induced TOP2α-DNA covalent complexes and is a determinant of chemoresistance through a dominant-negative effect related to heterodimerization with TOP2α/170. Alternative processing of TOP2α pre-mRNA, and subsequent synthesis of TOP2α/90, may be an important mechanism regulating the formation and/or stability of TOP2α/170-DNA covalent complexes in response to TOP2α-targeting agents.

Citation Format: Ragu Kanagasabai, Soumendra Karmahapatra, Yang Yu, Victor A. Hernandez, Corey A. Kientz, Evan E. Kania, Terry S. Elton, Jack C. Yalowich. The novel C-terminal truncated 90-kDa isoform of topoisomerase IIα, TOP2α/90, is a determinant of etoposide resistance in K562 leukemia cells via heterodimerization with the TOP2α/170 isoform [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 904.

Abstract 1660: Impact of antioxidants on myeloperoxidase (MPO)-dependent DNA damage and genotoxicity induced by etoposide (VP-16):implications for therapy-induced second malignancies

Previous work (Mol. Pharm. 79: 479-87, 2011) demonstrated that MPO, found in myeloid progenitor cells, oxidized the anticancer agent VP-16 to its phenoxyl radical (VP-O•) and led to enhanced topoisomerase II-mediated strand cleavage through redox cycling resulting in MLL translocations, implicating MPO in VP-16 leukemogenesis. Utilizing shRNA MPO in myeloid leukemia HL60 cells, MPO dependency for VP-16 activity was further established (PAACR 55: 826, 2014). In the present study, we examined the effects of dehydroascorbate (DHA) and trolox on MPO-dependent effects of VP-16, VP-16 catechol (VP-OH) and VP-16 ortho-quinone (VP-oQ, fully oxidized VP-16). In addition, the role of GSH as a mediator of the pro- or anti-oxidant effects of VP-16, its metabolites and several other phenolic agents was evaluated. Using Comet assays, VP-16-induced DNA strand breaks in MPO+ HL60 cells were significantly reduced when cells were pre-incubated with 1 mM DHA; known to act as a reductant of MPO generated VP-O•. In MPO knockdowns, VP-16-induced DNA damage was diminished compared to MPO+ cells. In these MPO knockdowns, DHA did not perturb VP-16-induced DNA damage. Similar results were demonstrated with VP-OH, consistent with MPO-mediated generation of VP-O• and DHA reduction/protection against DNA damage. For VP-oQ, 1,4-benzoquinone, and camptothecin, DNA damage was similar in MPO+ and knockdown cells with no attenuation in the presence of DHA. VP-16 and VP-OH also induced oxidative DNA damage in MPO+ cells which was reduced by DHA. This DNA damage was attenuated in MPO knockdowns with no further effect by DHA. The phenolic vitamin E analog trolox, a known MPO substrate, also reduced oxidative DNA damage induced by VP-16 and VP-OH but not by VP-oQ. Using 3′-(p-hydroxyphenyl) fluorescein (HPF), pro-oxidant activity of VP-16 was demonstrated in MPO+ cells which converted to anti-oxidant effects in MPO knockdowns. Both DHA and trolox protected cells against VP-16-induced pro-oxidant effects in MPO+ cells. When GSH levels were reduced by incubation with buthionine sulfoximine, the pro-oxidant effects of VP-16 in MPO+ cells were eliminated. N-acetyl cysteine (NAC) restored VP-16-induced pro-oxidant activity. In contrast, the pro-oxidant effects of the phenolic agents quercetin and EGCG were found to be independent of MPO and were diminished by addition of NAC. Together, results strongly suggest that MPO-catalyzed oxidation of VP-16 to redox active species leads to enhanced genotoxic events linked to the known leukemogenic action of this anticancer agent. These MPO dependent effects are also dependent on GSH likely through thiyl radical formation and cycling. The protective effects of DHA and trolox further suggest that reduction of MPO-catalyzed VP-16 free radicals may be an effective strategy to prevent drug-induced second malignancies. Support: NIH R01 CA090787.

Citation Format: Jason Goodspeed, Soumendra Karmahapatra, Ragu Kanagasabai, Alex Klausing, Jack C. Yalowich. Impact of antioxidants on myeloperoxidase (MPO)-dependent DNA damage and genotoxicity induced by etoposide (VP-16):implications for therapy-induced second malignancies. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1660. doi:10.1158/1538-7445.AM2015-1660

Abstract 4430: Acute inflammation-induced abrogation of base excision repair as a potential mechanism of cancer initiation in inflammation-mediated hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is one of the most common cancers worldwide, and chronic inflammation seems to be the most important risk factor. Since inflammation induces oxidative stress and DNA damage, base excision repair (BER) of oxidative damage in prevention of mutations and subsequent tumorigenesis in an inflammatory environment seems to be important, but a direct link between deficient DNA repair and carcinogenesis in the highly damaging environment of the inflamed liver has not been shown. Furthermore, the longstanding question about how and in which cells cancer initiation occurs and the roles that DNA damage and repair play in the process are larger problems that remain unanswered. In our study we utilize the Long Evans Cinnamon (LEC) rat, which develops spontaneous HCC after consecutive phases of acute and chronic hepatitis induced by the accumulation of copper due to a mutation in the copper transporter gene ATP7B, mimicking Wilson's disease in humans. We have previously shown that the LEC rat exhibits deficient BER during acute hepatitis, particularly in the activity of 8-oxoguanine DNA glycosylase (Ogg1), and activity returns during chronic hepatitis as preneoplastic foci begin to form. In order to study the role of BER in cancer initiation, we have generated three new cell lines from the animal. We report here that we have isolated and grown apparently spontaneously immortalized LEC hepatocytes from the acute hepatitis liver (LEC-AH) and the chronic hepatitis liver (LEC-CH) in long term culture. We have also developed a carcinoma cell line from a LEC liver tumor (LEC-T). The newly generated cell lines express albumin, and preliminary karyotyping analysis shows that the LEC-AH and LEC-CH cells are cytogenetically normal (average 42 chromosomes). As observed in the acute and chronic hepatitis tissues, LEC-AH cells exhibit decreased activity (-2.5 fold) of Ogg1 compared to LEC-CH cells. Furthermore, at 12 days post-confluency, we have observed spontaneous foci formation in the LEC-AH cells (96 foci per 106 cells) in culture while LEC-CH cells do not form foci (<1 foci per 106 cells). These preliminary results indicate a correlation between foci formation in culture and Ogg1 activity, demonstrating a potential direct link between decreased BER and preneoplastic foci formation. Additionally, the foci may represent an enriched population of initiated cells, providing a unique opportunity to study the mechanism of spontaneous cancer initiation in vitro and identify initiation markers. Experiments elucidating the specific roles of Ogg1, other BER enzymes, and oxidative damage in foci formation of LEC-AH cells are in progress, as is further molecular analysis of the foci-forming cell populations as they may reflect the biochemical and molecular characteristics of preneoplastic foci in vivo. (Supported by a grant from NIH/NCI RO1 CA113447)

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4430. doi:1538-7445.AM2012-4430