Curcumin suppresses the induction of indoleamine 2,3-dioxygenase by blocking the Janus-activated kinase-protein kinase Cδ-STAT1 signaling pathway in interferon-γ-stimulated murine dendritic cells

Indoleamine 2,3-dioxygenase (IDO) catalyzes the initial and rate-limiting step in the degradation of tryptophan and is strongly induced in interferon-γ (IFNγ)-stimulated dendritic cells (DCs). IDO has recently been established as a key enzyme in T-cell suppression-mediated immune tolerance to tumors. STAT1 phosphorylation appears to play an important role in the control of IDO expression by IFNγ, but the precise regulatory mechanism remains obscure. Here we present a novel mechanism of IFNγ-induced IDO expression in bone marrow-derived dendritic cells. In addition, we demonstrate that curcumin, an active component of turmeric, significantly inhibited the induction of IDO expression and activity by IFNγ. We found that curcumin suppressed STAT1 activation by directly inhibiting Janus-activated kinase 1/2 and protein kinase Cδ phosphorylation in bone marrow-derived DCs, suppressing the subsequent translocation and binding of STAT1 to the GAS element of the IRF-1 promoter. Coincident with these inhibitory effects on IFNγ-induced IDO expression, curcumin reversed IDO-mediated suppression of T-cell responses. Our results, thus, suggest that down-regulation of IDO in DCs is an important immunomodulatory property of curcumin that may be exploited therapeutically in the control of cancers.