PT  - JOURNAL ARTICLE
AU  - Ragu Kanagasabai
AU  - Soumendrakrishna Karmahapatra
AU  - Corey A. Kientz
AU  - Yang Yu
AU  - Victor A. Hernandez
AU  - Evan E. Kania
AU  - Jack C. Yalowich
AU  - Terry S. Elton
TI  - The Novel C-terminal Truncated 90-kDa Isoform of Topoisomerase II<em>α</em> (TOP2<em>α</em>/90) Is a Determinant of Etoposide Resistance in K562 Leukemia Cells via Heterodimerization with the TOP2<em>α</em>/170 Isoform
AID  - 10.1124/mol.117.111567
DP  - 2018 May 01
TA  - Molecular Pharmacology
PG  - 515--525
VI  - 93
IP  - 5
4099  - http://molpharm.aspetjournals.org/content/93/5/515.short
4100  - http://molpharm.aspetjournals.org/content/93/5/515.full
SO  - Mol Pharmacol2018 May 01; 93
AB  - DNA topoisomerase IIα (170 kDa, TOP2α/170) is essential in proliferating cells by resolving DNA topological entanglements during chromosome condensation, replication, and segregation. We previously characterized a C-terminally truncated isoform (TOP2α/90), 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. TOP2α/90 (786 aa) is the translation product of a TOP2α mRNA that retains a processed intron 19. TOP2α/90 lacks the active-site tyrosine-805 required to generate double-strand DNA breaks as well as nuclear localization signals present in the TOP2α/170 isoform (1531 aa). Here, we found that TOP2α/90, like TOP2α/170, was detectable in the nucleus and cytoplasm of K562 and K/VP.5 cells. Coimmunoprecipitation of endogenous TOP2α/90 and TOP2α/170 demonstrated heterodimerization of these isoforms. Forced expression of TOP2α/90 in K562 cells suppressed, whereas siRNA-mediated knockdown of TOP2α/90 in K/VP.5 cells enhanced, etoposide-mediated DNA strand breaks compared with similarly treated cells transfected with empty vector or control siRNAs, respectively. In addition, forced expression of TOP2α/90 in K562 cells inhibited etoposide cytotoxicity assessed by clonogenic assays. qPCR and immunoassays demonstrated TOP2α/90 mRNA and protein expression 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 cytotoxic TOP2α/170-DNA covalent complexes in response to TOP2α-targeting agents.