Inhibition of indoleamine 2,3-dioxygenase activity in IFN-y stimulated astroglioma cells decreases intracellular NAD levels

Astroglia provide essential metabolic and neurotropic support to cells within the CNS and participate in the cellular immune response with microglia/macrophages following activation by the pro-inflammatory cytokine IFN-γ. Activation of glial cells results in local oxidative stress and induction of a number of proteins including the enzyme indoleamine 2,3-dioxygenase (IDO). As a rate-limiting enzyme, IDO regulates tryptophan catabolism via the kynurenine pathway producing a series of metabolic precursors (some of which are neurotoxic) before complete oxidation to the essential pyridine nucleotide NAD. Inhibition of this pathway may therefore prove therapeutic in neuroinflammatory disease by reducing production of cell toxins. However, kynurenine metabolism may also be cytoprotective through de novo synthesis of cellular NAD levels. We investigated the hypothesis that IDO activity is directly involved in maintenance of intracellular [NAD] in activated astroglial cells through control of de novo synthesis. Exposure to IFN-γ increased IDO activity from 7±1 nmol to 129±11 nmol kynurenine/hr/mg protein. Inhibition of IDO activity with either 6-chloro--tryptophan (competitive inhibition), or 3-ethoxy β-carboline (non-competitive inhibition) resulted in a dose-dependent decrease in IDO activity that correlated directly with decreasing [NAD] (R2=0.92 and 0.81, respectively). These results support the hypothesis that one important consequence of increasing IDO activity in astroglial cells during inflammation is to maintain NAD levels through de novo synthesis from tryptophan. Inhibition of kynurenine pathway metabolism under these conditions may significantly decrease cell viability and CNS functions unless alternate precursors for NAD synthesis are available.