Monochloramine inhibits etoposide-induced apoptosis with an increase in DNA aberration

Monochloramine (NH2Cl) is a physiological oxidant produced by activated neutrophils, and it affects apoptosis signaling. We studied the effects of NH2Cl on the cell death induced by etoposide, a widely used anticancer agent that is directed to DNA topoisomerase II. Jurkat T cells, a human acute T cell leukemia cell line, were pretreated with 70 μM of NH2Cl for 10 min. After 24 h, 5–30 μM of etoposide was added to the NH2Cl pretreated and control cells, and their apoptosis, caspase activity, cell morphology, and cellular DNA contents were measured. NH2Cl pretreatment significantly inhibited apoptosis and caspase activation induced by etoposide or camptothecin, a DNA topoisomerase I poison, but not by staurosporine or Fas stimulation. The apoptosis inhibition actually resulted in the proliferation of the survived cells and, notably, the survived cells showed more aberrant morphology, such as variation in nuclear size, nuclear fragments, and multinucleated cells. DNA content analysis of the survived cells showed an increase in aneuploid nuclei. Cell cycle analysis after 24 h of NH2Cl treatment showed a significant decrease in S phase cells with a concurrent increase in G0/G1 phase cells, which suggested that NH2Cl induced G1 arrest. Using synchronized Jurkat cells, etoposide and camptothecin were found to be particularly cytotoxic to S phase cells, whereas staurosporine and Fas stimulation were not. Thus NH2Cl-induced G1 arrest was a likely cause of the observed resistance to etoposide. These observations suggested that inflammation-derived oxidants may make the tumor cells more resistant to etoposide and increase the risk of tumor progression and the development of secondary tumors by increasing the survival of DNA damage-bearing cells.