RIFN-GAMMA-ACTIVATED RAT NEUTROPHILS INDUCE TUMOR-CELL APOPTOSIS BY NITRIC-OXIDE

We have previously shown that 1) neutrophils activated with various cy tokines, including rat recombinant interferon gamma (rIFN-gamma), inhi bit tumor cell growth and that 2) nitric oxide (NO) is the effector mo lecule in tumor inhibition by rIFN-gamma-stimulated rat peritoneal exu date neutrophils, In this study, we examined the nature of tumor cell death induced by rat peritoneal neutrophils activated by rIFN-gamma in order to clarify the mechanism of apoptosis in neoplastic tumor cell death, DNA of 3 syngeneic rat tumor cell lines was significantly fragm ented within 3 hr of incubation in the presence of rIFN-gamma-activate d neutrophils, and this effect was dependent on both the concentration of rIFN-gamma and the number of neutrophils. This DNA fragmentation w as inhibited by L-N-(I-iminoethyl)-ornithine (L-NIO), a NO synthase in hibitor, but not by superoxide dismutase (SOD), Tumor cells treated wi th the activated neutrophils were shown by electron microscopy to be a poptotic, exhibiting necrotic features with a longer incubation. On th e other hand, cytolysis of tumor cells, as determined by a [H-3]-uridi ne release assay, was first observed only at 24 hr of incubation with the rIFN-gamma-activated neutrophils. Taken together, our results sugg est that tumor cell apoptosis by activated neutrophils is NO-dependent and that apoptotic tumor cells undergo necrosis as a secondary proces s. We suggest that tumor cell apoptosis induced by activated neutrophi ls plays an important role in regulation of neoplastic tumor cell grow th and death in vivo. (C) 1997 Wiley-Liss, Inc.