The Role of the Integrated Stress Response Kinase Gcn2 in Cell Cycle Regulation and Tumorigenesis

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 the transcription factor 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 models. However, the mechanisms by which GCN2 regulates cell survival and the role GCN2 plays in tumorigenesis are not clearly understood. Here, we demonstrate through 35S labeling, luciferase reporter assays, and polysome profiling that activation of GCN2 selectively upregulates the translation of a p21Cip1 transcript variant containing 5\u27 upstream open reading frames (uORFs) through phosphorylation of eIF2α. Mutational analysis reveals that the uORFs suppress its translation under basal conditions, but promote its translation under stress. Functionally, ablation of p21 ameliorates G1/S arrest and reduces cell survival in response to GCN2 activation. We also demonstrate in a genetically engineered mouse model of soft tissue sarcoma that loss of GCN2 has no major effect on the rate of 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. Together, these results demonstrate that GCN2 exerts its pro-survival effects partially through p21-induced cell cycle arrest and that sarcomas adapt to loss of GCN2 by upregulating other components of the ISR in order to exploit the survival benefits of this signaling pathway