Unique patterns of regulation of nitric oxide production in fibroblasts

The pathways regulating rat and mouse embryonic and lung fibroblast nitric oxide production were analyzed in an attempt to evaluate the potential 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, lipopolysaccharide (LPS) was synergistic with IL-1 beta or tumor necrosis factor-alpha (TNF-alpha) in induction of nitric oxide synthesis. These patterns of responsiveness are not observed in macrophages and may be significant in initiation of early host defense processes, before specific interferon-gamma (IFN-gamma)-mediated immune responses have become operative. Rat and mouse fibroblasts were also found to produce nitric oxide when primed with IFN-gamma and simultaneously treated with IL-1, TNF-alpha, or LPS. The doses of IFN-gamma effective in priming fibroblasts 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 that 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 expression with the majority of cells not involved in the response. Fibroblasts were also found to be distinct from macrophages in their sensitivity to the suppressive effects of transforming growth factor-beta, which in fibroblasts inhibited both IFN-gamma plus LPS- and IFN-gamma plus TNF-alpha-induced nitric oxide production. At the stage of growth crisis, a dramatic increase in nitric oxide production was observed in rat fibroblasts in response to IFN-gamma or TNF-alpha that may be directly correlated with cellular senescence. Taken together, our data suggest that mouse and rat fibroblasts are potential effectors in both IFN-gamma-dependent and -independent nitric oxide-mediated processes and that the patterns regulating nitric oxide metabolism in these cells are distinct from those of macrophages.