MECHANISMS REGULATING MACROPHAGE-INDUCED NITRIC-OXIDE PRODUCTION BY SPONTANEOUSLY TRANSFORMED HAMSTER FIBROBLASTS

Nitric oxide has been implicated as an important effector molecule inv olved in tumor cell growth and cytotoxicity, In these studies we exami ned mechanisms regulating nitric oxide production by hamster tumor cel ls, Cocultures of hamster alveolar macrophages (HAM) and spontaneously transformed hamster embryonic fibroblasts (STHE cells) produced signi ficant quantities of nitric oxide in response to lipopolysaccharide (L PS), Culture supernatants from HAM treated with LPS also stimulated ni tric oxide production by STHE cells, whereas tumor cell culture supern atants had no effect on HAM, These data, together with the findings th at paraformaldehyde treatment of STHE cells, but not macrophages, comp letely abrogated nitric oxide production in the cocultures demonstrate that the tumor cells were the source of this mediator, In contrast to STHE cells, STHE-83/20 cells, a highly malignant variant, did not pro duce nitric oxide in response to HAM or HAM culture supernatants even in the presence of LPS, Both anti-tumor necrosis factor-alpha (TNF-alp ha) and anti-interleukin-1 alpha (IL-1 alpha) antibodies inhibited HAM -induced nitric oxide production by STHE cells, However, the kinetics of their effects were different, Moreover, although the nitric oxide s timulating activity in HAM culture supernatants was abrogated by anti- TNF-alpha antibody, it was only minimally reduced by anti-IL-1 alpha a ntibody, These data demonstrate that TNF-alpha and IL-1 alpha play dis tinct roles in induction of nitric oxide synthesis in STHE cells, HAM were also found to suppress proliferation of STHE cells, an effect tha t was inhibited by anti-TNF-alpha antibody, but not N-G-monomethyl-L-a rginine, which blocks nitric oxide synthase, Abrogation of macrophage- induced cytostasis in STHE cells by anti-TNF-alpha antibody was associ ated with decreased nitric oxide production, Thus TNF-alpha released b y macrophages may indirectly activate STHE cells for nitric oxide synt hesis by suppressing tumor cell proliferation.