UNIQUE PATTERNS OF REGULATION OF NITRIC-OXIDE PRODUCTION IN FIBROBLASTS

The pathways regulating rat and mouse embryonic and lung fibroblast ni tric oxide production were analyzed in an attempt to evaluate the pote ntial role of these cells in nonspecific host defense and inflammation . Interleukin-1 beta (IL-1 beta) was found to be the strongest single activator in all types of fibroblasts examined. In addition, lipopolys accharide (LPS) was synergistic with IL-1 beta or tumor necrosis facto r-alpha (TNF-alpha) in induction of nitric oxide synthesis. These patt erns of responsiveness are not observed in macrophages and may be sign ificant in initiation of early host defense processes, before specific interferon-gamma (IFN-gamma)-mediated immune responses have become op erative. Rat and mouse fibroblasts were also found to produce nitric o xide when primed with IFN-gamma and simultaneously treated with IL-1, TNF-alpha, or LPS. The doses of IFN-gamma effective in priming fibrobl asts for nitric oxide production were as low as 1-10 U/ml. Furthermore , effective triggering doses of LPS, TNF-alpha, and IL-1 were 10 ng/ml , 100 U/ml, and 0.2 ng/ml, respectively. These results demonstrate tha t fibroblasts are activated more readily to produce nitric oxide than interstitial macrophages and may be the major source of this mediator in tissues. Immunohistochemical studies demonstrated that fibroblasts are heterogeneous with respect to inducible nitric oxide synthase expr ession with the majority of cells not involved in the response. Fibrob lasts were also found to be distinct from macrophages in their sensiti vity to the suppressive effects of transforming growth factor-beta, wh ich in fibroblasts inhibited both IFN-gamma plus LPS- and IFN-gamma pl us TNF-alpha-induced nitric oxide production. At the stage of growth c risis, a dramatic increase in nitric oxide production was observed in rat fibroblasts in response to IFN-gamma or TNF-alpha that may be dire ctly correlated with cellular senescence. Taken together, our data sug gest that mouse and rat fibroblasts are potential effecters in both IF N-gamma-dependent and -independent nitric oxide-mediated processes and that the patterns regulating nitric oxide metabolism in these cells a re distinct from those of macrophages.