Accumulation of quinolinic acid and L-kynurenine occurs in the brain and/or blood following immune activation, and may derive from L-tryptophan following-induction of indoleamine 2,3-dioxygenase and other kynurenine-pathway enzymes. In the present study a survey of various cell lines derived from either brain or systemic tissues showed that, while all cells examined responded to interferon-γ by increased conversion of L[13C6]tryptophan into L-kynurenine (human: B-lymphocytes neuroblastoma, glioblastoma, lung, liver, kidney; rat brain microglia, astrocytes and oligodendrocytes), only macrophage-derived cells (peripheral-blood mononuclear cells; THP-1, U-937) and certain liver cells (SKHep1) synthesized [13C6]quinolinic acid. Tumour necrosis factor-α enhanced the effects of interferon-γ in THP-1 cells. Norharmane, 6-chloro-DL-tryptophan and 4-chloro-3-hydroxyanthranilate attenuated quinolinic acid formation by THP-1 cells with IC50 values of 51μM, 58μM and 0.11μM respectively. Norharmane and 6-chloro-DL-tryptophar attenuated L-kynurenine formation with IC50 values of 43μM and 51μM respectively, whereas 4-chloro-3-hydroxyanthranilate had no effect on L-kynurenine accumulation. The reductions in L-kynurenine and quinolinic acid formation are consistent with the reports that norharmane is an inhibitor of indoleamine 2,3-dioxygenase, 6-chloro-DL-tryptophan is metabolized through the kynurenine pathway, and 4-chloro-3-hydroxyanthranilate is an inhibitor of 3-hydroxyanthranilate 3,4-dioxygenase. These results suggest that many tissues may contribute to the production of L-kynurenine following indoleamine 2,3-dioxygenase induction and immune activation. Quinolinic acid may be directly synthesized from L-tryptophan in both macrophages and certain types of liver cells, although uptake of quinolinic acid precursors from blood may contribute to quinolinic acid synthesis in cells that cannot convert L-kynurenine into quinolinic acid.
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