Inhibition of tumor cell growth by interferon-gamma is mediated by two distinct mechanisms dependent upon oxygen tension: induction of tryptophan degradation and depletion of intracellular nicotinamide adenine dinucleotide.

Abstract

Growth of a variety of human tumor cell lines is inhibited by interferon-y (IFN-,y) in vitro. This mechanism is not well understood. The present experiments identify two separate mechanisms which account for the growth inhibitory activity of IFN-'y. Cell lines most sensitive to IFN-'y (inhibited by 10-30 U/ml IFN-y in 3 d) were stimulated by IFN-'y to oxidize tryptophan in media to kynurenine and completely eliminated tryptophan from the culture media after 48-72 h. Addition of L-tryptophan, but not other aromatic amino acids, other essential amino acids, or D-tryptophan, prevented inhibition of cell growth by IFN-'y. The amount of IFN-'y required to yield 50% inhibition of cell growth was directly related to the concentration of L-tryptophan in culture media and increased from -3 to 600 U/ml as the concentration of tryptophan in the media was increased from 25 to 1,000 MM. By contrast, inhibition of growth of the cell lines, BT20 and HT29, was not prevented by addition of tryptophan. Inhibition by IFN-y (100-300 U/ml after 5-6 d) was, however, completely prevented by addition of two inhibitors of adenosine diphosphate-ribosyl transferase (ADP-RT), 3-aminobenzamide or nicotinamide. Activity of ADP-RT was increased in these cell lines after addition of IFN-y. ADP-RT catalyzes the incorporation of the ADP moiety of nicotinamide adenine dinucleotide (NAD) into proteins and causes depletion of intracellular NAD. All tumor cell lines tested had reduced levels of intracellular NAD after treatment with IFN-'y and loss of NAD preceded inhibition of cell growth by 12-24 h. Inhibitors of IFN-y-mediated inhibition of cell growth prevented loss of levels of intracellular NAD. Generation of reactive oxygen species lead to DNA strand breaks which result in activation of ADP-RT. Increased DNA strand breaks were induced in BT20 and HT29 cells but not ME180 and A549 celIs after culture with IFN-'y. The two enzymes known to catalyze the decyclization of tryptophan to kynurenine require superoxide anion for activity.

Identification of a novel serine/threonine kinase and a novel 15-kD protein as potential mediators of the gamma interferon-induced cell death.

“…The ability of IFN-7 to induce programmed cell death is consistent with previous reports that suggested a role for this cytokine in the negative selection by apoptosis of the peripheral T-and B-lymphocyte repertoire {Liu and Janeway 1990; Grawunder et al 1993). IFN-~/has been long known as a cytokine causing the pathological type of cell death (termed necrosis) (Aune and Pogue 1989). So as with TNF-a (Laster et al 1988), IFN-7 can induce either pathological or physiological cell death depending on the target cells.…”

Stem cells ameliorate EAE via an indoleamine 2,3-dioxygenase (IDO) mechanism

“…IDO activation can result from lipopolysaccharide and cytokine stimulation, particularly interferon-γ (IFN-γ) (Byrne et al, 1986). Low tryptophan concentrations induced by IDO are associated with inhibited proliferation of viruses, protozoan parasites and other pathogens and also with decreased proliferation of tumor cells (Gupta et al, 1994;Aune and Pogue, 1989). Recent studies have suggested a role for IDO in the regulation of T cell responses, either by lymphocyte deprivation of tryptophan or by induction of the tryptophan metabolites, 3-OH-kynurenine and 3-OHanthranilic acid which inhibit T-cell reactivity (Terness et al, 2006;Terness et al, 2002;Frumento et al, 2002).…”

In Vivo Uptake and Metabolism of α-[11C]Methyl-l-Tryptophan in Human Brain Tumors

“…Under some pathologic conditions, induction of the initial and rate-limiting enzyme of the kynurenine pathway, indoleamine 2,3-dioxygenase (IDO), also leads to a significantly increased metabolism of tryptophan by the kynurenine pathway (Saito et al, 1992). Recently, interest in tryptophan metabolism via this pathway has grown due to the recognition that induction of IDO by interferon gamma leads to local tryptophan depletion, thus inhibiting cell growth in malignant tumors (Ozaki et al, 1988;Aune and Pogue, 1989;Taylor and Feng, 1991;Burke et al, 1995). This observation has led to the proposal that pharmacological induction of IDO might be an effective way to slow down tumor cell growth.…”