High expression of IMPACT protein promotes resistance to indoleamine 2,3‐dioxygenase‐induced cell death

Abstract

Indoleamine 2,3-dioxygenase (IDO), a tryptophan degrading enzyme, is a potent immunomodulatory factor. IDO expression in fibroblasts selectively induces apoptosis in immune cells but not in primary skin cells. However, the mechanism(s) of this selective effect of IDO-induced low tryptophan environment is not elucidated. The aim of present study was to investigate whether the activity of general control non-derepressible-2(GCN2) kinase stress-responsive pathway and its known inhibitor, protein IMPACT homolog, in immune and skin cells are differentially regulated in response to IDO-induced low tryptophan environment. IDO-expressing human fibroblasts were co-cultured with Jurkat cells, human T cells, fibroblasts, or keratinocytes. Activation of GCN2 pathway was significantly higher in immune cells exposed to IDO-expressing environment relative to that of skin cells. In contrast, IMPACT was highly and constitutively expressed in skin cells while its expression was very low in stimulated T cells and undetectable in Jurkat cells. A significant IDO-induced suppressive as well as apoptotic effect was demonstrated in IMPACT knocked down fibroblasts co-cultured with IDO-expressing fibroblasts. Proliferation of Jurkat cells, stably transduced with IMPACT-expressing vector, was rescued significantly in tryptophan-deficient but not IDO-expressing environment. This may be due to the ability of IMPACT to recover the effects of IDO-mediated tryptophan depletion (GCN2 dependent) but not the effects of IDO-generated cytotoxic metabolites. These findings collectively suggest for the first time that high expression of protein IMPACT homolog in non-immune cells such as skin cells acts as a protective mechanism against IDO-induced GCN2 activation, therefore, makes them resistant to the amino acid-deprived environment caused by IDO.

Role of indoleamine 2,3-dioxygenase in health and disease

“…Thus, in more differentiated cells, such as macrophages and Tregs, GCN2 activation has a signalling function [226,259], whereas GCN2-mediated cell cycle arrest adversely affects rapidly proliferating effector T-cells [225]. Studies with human T-cells and fibroblasts have identified that the sensitivity of cells to L-Trp starvation and GCN2-mediated apoptosis relates to the expression level of the protein IMPACT, an inhibitor of GCN2 kinase activity [262]. Accordingly, compared with fibroblasts, T-cells constitutively express low levels of IMPACT and, in response to low-L-Trp environments, exhibit higher levels of GCN2 activation and apoptosis; IMPACT gene overexpression of T-cells protected against L-Trp deficiency, whereas gene knockdown of IMPACT in fibroblasts rendered these cells more sensitive to low L-Trp [262].…”

“…Studies with human T-cells and fibroblasts have identified that the sensitivity of cells to L-Trp starvation and GCN2-mediated apoptosis relates to the expression level of the protein IMPACT, an inhibitor of GCN2 kinase activity [262]. Accordingly, compared with fibroblasts, T-cells constitutively express low levels of IMPACT and, in response to low-L-Trp environments, exhibit higher levels of GCN2 activation and apoptosis; IMPACT gene overexpression of T-cells protected against L-Trp deficiency, whereas gene knockdown of IMPACT in fibroblasts rendered these cells more sensitive to low L-Trp [262]. Further studies are clearly required into how L-Trp starvation and GCN2 signalling influences cellular function and viability of other IDO1-expressing cell types and the molecular signalling mechanisms involved.…”

Correlation between frequencies of blood monocytic myeloid-derived suppressor cells, regulatory T cells and negative prognostic markers in patients with castration-resistant metastatic prostate cancer

“…Some MDSC subsets have been shown to express indoleamine 2,3 dioxygenase (IDO), depleting the microenvironment of tryptophan, and through expression of inducible nitric oxide synthase (iNOS) and arginase-1, l-arginine that serves as a substrate for both enzymes, is depleted from the microenvironment [8]. Importantly, T cells are particularly sensitive to amino acid deprivation [9]. In addition, nitric oxide (NO), a product of l-arginine catabolism by iNOS, is in itself able to suppress T-cell function.…”

Borrelidin, a small molecule nitrile-containing macrolide inhibitor of threonyl-tRNA synthetase, is a potent inducer of apoptosis in acute lymphoblastic leukemia

“…Indoleamine 2, 3 dioxygenase (IDO), similarly to ASNase and borrelidin, induces amino acid deprivationmediated stress in cells through catabolism of tryptophan. It has been shown that the apoptotic effects of IDO on immune cells are at least in part due to the selective activation of the GCN2 kinase pathway in these cells [19][20][21].…”