Nitric oxide synthesis and TNF-alpha secretion in RAW 264.7 macrophages - Mode of action of a fermented papaya preparation

Macrophage inducible nitric oxide synthase is able to generate massive amou nts of nitric oxide (NO) which contributes to the host immune defense again st viruses and bacteria. Monocyte-macrophages stimulated with the bacterial wall component lipopolysaccharide (LPS) and cytokines such as interferon-g amma (IFN-gamma) express the inducible form of nitric oxide synthase (iNOS) . Furthermore, tumor necrosis factor-alpha (TNF-alpha) is one of the centra l regulatory cytokines in macrophage antimicrobial activity and synergizes with IFN-gamma in the induction of NO synthesis. Because of its pivotal rol e in both antimicrobial and tumoricidal activities of macrophages, a signif icant effort has focused on developing therapeutic agents that regulate NO production. In the present study fermented papaya preparation (FPP) is show n to exert both immunomodulatory and antioxidant activity in the macrophage cell line RAW 264.7. Interestingly, a low and a high molecular weight frac tion (LMF and HMF, respectively) of FPP exhibited different activity patter ns. FPP fractions alone did not affect NO production, However in the presen ce of IFN-gamma, both LMF and HMF significantly increased iNOS activity and nitrite as well as nitrate accumulation. NO radical formation measured in real-time by electron paramagnetic resonance spectroscopy was higher in the presence of LMF and IFN-gamma. On the contrary, iNOS mRNA levels were enha nced further with HMF than with LMF Moreover, LMF displayed a stronger supe roxide anion scavenging activity than HMF. In the presence of IFN-gamma, bo th FPP fractions stimulated TNF-alpha secretion. However in non-stimulated macrophages, TNF-alpha secretion was enhanced by HMF only. Since water-solu ble FPP fractions contained no lipid A, present data indicate that FPP is a macrophage activator which augments nitric oxide synthesis and TNF-alpha s ecretion independently of lipopolysaccharides. (C) 2000 Elsevier Science In c. All rights reserved.