TRANSFORMING GROWTH-FACTOR-BETA STIMULATES ARGINASE ACTIVITY IN MACROPHAGES - IMPLICATIONS FOR THE REGULATION OF MACROPHAGE CYTOTOXICITY

Macrophage arginine metabolism via nitric oxide (NO) synthase and argi nase pathways reduces and enhances tumor cell proliferation, respectiv ely. Transforming growth factor-beta (TGF-beta) has been shown to down -regulate the NO synthase pathway. The present study describes the eff ect of TGF-beta on the arginase pathway. TGF-beta upregulated arginase activity in rat peritoneal macrophages as assessed by measuring the g eneration of [C-14]urea from [C-14]-L-arginine in the presence of N-G- monomethyl-L-arginine (L-NMMA). The stimulation, which reached fivefol d after a 48-h exposure of macrophages to 10 ng/ml TGF-beta, was due t o reduction in Km value of arginase. TGF-beta-induced up-regulation of arginase activity led to the release of more polyamines, mainly putre scine. The role of this up-regulation on macrophage cytotoxicity towar d L-929 tumor cells was analyzed in coculture experiments. Macrophages blunted DNA synthesis by L-929 cells as assessed by measuring the inc orporation of [H-3]TdR into the cells and the proportion of cells in t he G2 phase. Addition of TGF-beta in the presence of L-NMMA permitted L-929 cel Is cocultured with macrophages to resume DNA synthesis. The mechanism responsible for this restoration was the up-regulation of ar ginase activity rather than the down-regulation of NO synthase activit y since TGF-beta in the presence of L-NMMA failed to further reduce NO synthase activity whereas it sti II enhanced arginase activity; synth etic putrescine (1-10 mu M) also blunted macrophage cytotoxicity towar d L-929 cells. This is the first evidence that TGF-beta up-regulates a rginase activity in macrophages and, hence, limits macrophage-dependen t cytostasis.