Activation of the integrated stress response and loss of cFLIP under glutamine limitation induce IL-8 gene expression and secretion in glutamine-dependent tumor cells.

Growing evidence suggests that the proapoptotic TNF-related apoptosis-inducing ligand receptor 2 (TRAIL-R2/DR5) signaling pathway can also trigger the production of inflammatory cytokines, thereby promoting tumor progression. We recently reported that glutamine depletion impacts the survival of glutamine-dependent tumor cells by activating the TRAIL-R2/DR5-mediated apoptotic machinery. However, it remains unclear whether glutamine limitation activates a TRAIL-R2/DR5-regulated inflammatory response. In this study, we demonstrate that glutamine starvation activates two parallel signaling pathways, leading to the gene expression and secretion of the pro-angiogenic and pro-inflammatory interleukin-8 (IL-8/CXCL8) in tumor cells. Our findings reveal that the amino acid-sensing general control nonderepressible-2 kinase (GCN2)/activating transcription factor 4 (ATF4) signaling axis contributes to the upregulation of IL-8 gene expression in glutamine-deprived tumor cells. Furthermore, our results indicate that the loss of the long isoform of cellular FLICE-inhibitory protein (cFLIP), which occurs as result of the metabolic stress induced by glutamine limitation, promotes TRAIL-independent activation of the NF-kB pathway via TRAIL-R2/DR5, a key mechanism driving the observed IL-8 upregulation under starvation conditions. Given the severe depletion of glutamine observed in growing tumors, our data suggest that IL-8 secretion, induced by this metabolic stress, may play a significant role in activating inflammatory and angiogenic responses, thereby counteracting apoptosis and ultimately promoting tumor progression.