Signaling through alternative Integrated Stress Response pathways compensates for GCN2 loss in a mouse model of soft tissue sarcoma

Abstract

The tumor microenvironment is characterized by deficiencies in oxygen and nutrients, such as glucose and amino acids. Activation of the GCN2 arm of the Integrated Stress Response (ISR) in response to amino acid deprivation is one mechanism by which tumor cells cope with nutrient stress. GCN2 phosphorylates the alpha subunit of the eukaryotic translation initiation factor eIF2, leading to global downregulation of translation to conserve amino acids and initiation of a transcriptional program through ATF4 to promote recovery from nutrient deprivation. Loss of GCN2 results in decreased tumor cell survival in vitro under amino acid deprivation and attenuated tumor growth in xenograft tumor models. However, it is not known what effects GCN2 loss has on the growth of autochthonous tumors that arise in their native microenvironment. Here, we demonstrate in a genetically engineered mouse model of soft tissue sarcoma that loss of GCN2 has no effect on tumor growth or animal survival. The sarcomas displayed compensatory activation of PERK or phospho-eIF2α independent upregulation of ATF4 in order to maintain ISR signaling, indicating that this pathway is critical for tumorigenesis. These results have important implications for the development and testing of small molecule inhibitors of ISR kinases as cancer therapeutics.

The integrated stress response

“…For example, cells growing in vitro or as xenografts in mice are not subjected to physiological nonautonomous tumor microenvironment conditions. Knockdown of GCN2 in human HT1080 sarcoma cells results in reduced tumor growth in xenograft mice model, while in a genetically engineered mouse model of soft tissue sarcoma, Gcn2 knockdown has no effect on tumor progression due to a compensatory activation of PERK . Thus, both PERK and GCN2 might need to be inhibited to reduce the phosphorylation of eIF2α, and indeed, this has been shown to be the case in mouse fibroblasts .…”

“…For example, GCN2 contributes to ER stress‐induced eIF2α phosphorylation in Perk −/− MEF cells . Conversely, PERK can compensate for Gcn2 loss in a genetically engineered mouse model of soft tissue sarcoma and in a Gcn2 ‐knockout 5XFAD mouse model of Alzheimer's disease , as well as for Hri loss in mouse hepatocytes . Further support for signaling redundancy between PERK and GCN2 comes from experiments on HeLa cells stressed by protein overload where knockdown of either PERK or GCN2 was compensated by a rapid upregulation of the other kinase .…”

Triazolo[4,5-d]pyrimidines as Validated General Control Nonderepressible 2 (GCN2) Protein Kinase Inhibitors Reduce Growth of Leukemia Cells

“…The role of HRI in cancer is still unclear. Interestingly, GCN2 appears to be able to compensate for loss of PERK by phosphorylating eIF2α [ [20] , [21] , [22] , [23] ].…”

Tumor cells rely on the thiol oxidoreductase PDI for PERK signaling in order to survive ER stress

“…Activation of the UPR and especially the PERK pathway are present in almost all tumors and support tumor growth and adaptation to nutrient deprivation and hypoxia 13 , 15 , 17 , 20 , 33 . It is noteworthy that loss of PERK can be compensated by GCN2 as well as GCN2 loss can be compensated by PERK in some tumor models, which in part negotiates the antitumor effects 34 , 35 . We could not observe any compensation after loss of PDI regarding to ATF4 induction and further PERK downstream signaling.…”