Structure-activity relationship of alginates to induce TNF-alpha secretion from RAW264.7 cells

Alginate is an acidic polysaccharide present in seaweed, and consists of mannuronate (M) and guluronate (G). This polysaccharide is widely used as food additive, cosmetic and pharmaceutical materials inducing thickener, humectant, and disintegrator of tablet because of its characteristic physicochemical feature and the biologically innocent property. Several bioactivities of alginates such as in vivo antitumor activity and activation of immune system have been reported. However, there is no detailed study regarding the structure-activity relationship of alginates. To clarify this point, we examined the effect of alginates with varying molecular weights and M/G ratio to induce tumor necrosis factor-alpha (TNF- alpha) release from a murine macrophage cell line, RAW264.7 cells. Range of molecular size and M/G ratio of highly purified alginate polymers used in this study were 9,000-38,000 and 1.50-3.17, respectively. Among the alginates tested, I-S, the highest molecular weight of alginate (MW 38.000, M/G 2.24), showed the most potent TNF-alpha inducing activity. Alginates with molecular weight less than 20,000, and polyguluronate (MW 4,l00) and polymannuronate (MW 4,600) had no or only a trace activity. Furthermore, among the alginates with similar molecular, alginates having higher M/G ratio tended to have higher activity. These results suggest that M/G ratio also influences the TNF-alpha inducing activity in addition to the molecular weight. By the analysis of the effects of three MAP kinase inhibitors on I-S and LPS-mediated induction of TNF-alpha, the inhibition profile of I-S by these inhibitors was different from that of LPS. Namely, I-S-mediated TNF-alpha induction was more significantly inhibited by JNK and p41 MAP kinase (ERK) inhibitors than that of lipopolysaccharide (LPS). These results suggest that the underlying mechanism of I-S-mediated TNF-alpha release may be different from that of LPS in terms of the major MAP kinases involved.