Ciprofloxacin-Induced G₂ Arrest and Apoptosis in TK6 Lymphoblastoid Cells Is Not Dependent on Double-Strand Break Formation.

Drugs indicated for conditions other than tumors have shown promise in the potential treatment of leukemia and lymphoma. The fluoroquinolone antibiotic ciprofloxacin (CPFX) is known to modulate cycle cell progression and apoptosis in cancer cells and thought to induce DNA double-strand breaks (DSBs) via topoisomerase II (topo II) inhibition. DSBs trigger the phosphorylation of histone H2AX on serine 139 (γH2AX) by PI-3-like kinases including ATM; γH2AX can serve as a marker of DNA damage when measured in situ using immunocytochemistry and flow cytometry. The aim of the present study was to investigate the relationship between CPFX-mediated DNA damage and the induction of apoptosis in human lymphoblastoid cells and phytohaemagglutinin (PHA)-stimulated normal lymphocytes using γH2AX and multiparameter flow cytometry. Treatment of TK6 cells (wild-type p53) with 100 mg/ml CPFX for 2–10 hours produced no increase in γH2AX compared to controls; levels were in fact reduced in S phase cells at 6 and 10 hours. Phosphorylated ATM and G₂ arrest was also observed in TK6 cells exposed to CPFX for 4 hours. γH2AX was dramatically increased after 24 hours treatment, indicating the onset of apoptosis (confirmed by presence of activated caspase-3). CPFX had a similar lack of effect on γH2AX at early time points in WTK-1 and NH32 lymphoblastoid cells (devoid of functional p53) and proliferating lymphocytes, however, induction of apoptosis was not as pronounced in these cells compared to TK6. The data suggest that, at early time points, CPFX may act a radical scavenger to reduce the level of endogenous oxidant-induced DSBs. However, phosphorylation of ATM (but lack of γH2AX induction) does indicate the presence of a non-DSB DNA modification. At later time points, induction of apoptosis appears to be facilitated by a functional p53 signaling pathway. CPFX may therefore have a potential use as a novel chemotherapeutic agent in the treatment of lymphoblast-derived cancer.