HUMAN MONOCYTES INDUCE A CARCINOMA CELL-LINE TO SECRETE HIGH AMOUNTS OF NITRIC-OXIDE

Nitric oxide (NO) is a short-lived pleiotropic mediator with a multitu de of biologic functions. The inducible form of NO synthase (iNOS) is responsible far the discontinuous production of high amounts of NO and is important for the cytotoxic capacity of macrophages in rodents, wh ereas NO production by human macrophages or monocytes (MO) is under de bate. Here we report that high amounts of NO are synthesized in cocult ures of human MO with the human carcinoma cell line RT4 without furthe r stimulation. Both cell types have to be viable and metabolically act ive for NO production, However, in contrast to reports by others, we c ould demonstrate that tumor cells and not MO are the producers of NO b y the following findings: 1) NO release was induced in RT4 cells, but not in MO, by diluted supernatants (SN) of RT4/MO cocultures; 2) SN of MO stimulated with tumor cell membrane preparations were sufficient t o induce NO release by tumor cells; and 3) iNOS mRNA expression could be detected only in tumor cells, not in MO. Separating bath cells by a cell-impermeable membrane resulted in NO amounts comparable to those in cocultures with direct cell contact, indicating one or more soluble NO-inducing factors. Considerable amounts of IL-1 beta and TNF-alpha were present in cocultures. IL-1 beta and TNF-alpha, mediators produce d by activated MO, in combination induce NO release in RT4 cells. Bloc king of TNF-alpha or IL-1 in SN inhibited NO release in RT4 cells. Thi s indicates that IL-1 beta and TNF-alpha play prominent roles in iNOS induction by MO in RT4 tumor cells.