Histone Deacetylation Inhibitor Sensitizes T-ALLs to Topoisomerase II Inhibitor By Attenuating G2 Cell Cycle Arrest and Blocking Nuclear Entry of Ctip during DNA Repair

Double strand DNA repair can be epigenetically modulated to sensitize the malignant cells to chemotherapy. CTV-1 and Molt-3 were segregated as T-acute lymphoblastic cells (T-ALL) according to their expression profiles. We found in these cells doxorubicin, a topoisomerase II (TOP2) inhibitor, induced expression and phosphorylation of cell cycle checkpoint kinase 1(CHK1) associated with G2 cell cycle arrest before achieving apoptosis; while a histone deacetylation inhibitor (HDACi), suberoylanilide hydroxamic acid (SAHA), synergistically enhanced cell death by attenuation of expression and phosphorylation of CHK1, and by attenuated expression and phosphorylation of a double strand DNA repair protein, C-terminal binding protein (CtBP)-interacting protein [CtIP], and was associated with shortened G2 cell cycle arrest. Very often is Doxorubicin as a topoisomerase II inhibitor enrolled in the treatment of T-ALL. We evidenced with isobologram synergistic cell-killing by doxorubicin and SAHA in CTV-1 and Molt-3 T-ALL cell lines of which expression and phosphorylation of CHK1 were negatively affected by SAHA. In the SAHA-treated T-ALL cells, the repair of doxorubicin-induced double strand DNA break (DSB) was associated with increased γH2AX. However, although SAHA increased expression of γH2AX and acetyl H2AX, apoptosis was enhanced with shortened G2/M arrest and the hampered nuclear entry of CtIP revealed by immunofluorescent confocal microscopy. HDACi thus synergistically impaired the topoisomerase II-induced DSB repair and enhanced apoptosis by eliciting DNA repair with γH2AX expression; however, aborted the DNA repair and induced apoptosis by hampering the nuclear entry of CtIP. Attenuation of activated CHK1 may be potentially a biomarkers of synergistic cytotoxicities of HDACi and DSB-inducing chemotherapeutics.