The unfolded protein response (UPR) is a cellular homeostatic program initiated by an excess of unfolded/misfolded client proteins in the ER lumen, with a primarily cytoprotective effect. There are 3 ER-resident proximal sensors of unfolded proteins and ER stress: PKR-like ER stress kinase (PERK), activating transcription factor 6 (ATF6), and inositol-requiring transmembrane kinase and endonuclease 1 (IRE1) (1
). Under steady-state conditions, these stress sensors are bound by the chaperone protein GRP78 (BiP). The accumulation of unfolded proteins results in the dissociation of GRP78 and the subsequent activation of PERK, ATF6, and IRE1 (4
). Active PERK phosphorylates eukaryotic initiation factor 2-alpha (eIF2α) at Ser51, preventing the exchange of GDP for GTP, effectively blocking the initiation of protein synthesis during cellular recovery from ER stress (6
). While general protein synthesis is limited, the translation of specific mRNAs is induced, including that of the transcription factor ATF4
). Stresses in the tumor microenvironment (e.g., low oxygen, decreased amino acid availability, and low glucose) activate components of the UPR, the abrogation of which leads to inhibition of tumor progression (8
). In addition to microenvironmental stressors, the loss of tumor suppressor function, specifically of the tuberous sclerosis complex genes (TSC1
), has also been associated with UPR activation (14
). Collectively, these findings reveal a link between UPR activation and adaptation to tumor microenvironmental stress.
Another cellular adaptation mechanism induced by both normal and transformed cells is autophagy, a self-digestive cellular stress response that enables cells to catabolize and recycle proteins, cytosolic components, and organelles to promote survival (15
). Common cellular stresses known to induce autophagy include amino acid starvation, glucose withdrawal, and ER stress. The regulation of autophagy is accomplished primarily by the autophagy-related (Atg) proteins, organized into multiple complexes and acting at various stages within the pathway. Activation of a specific major complex (the ULK1 [Atg1] kinase complex) leads to a signaling cascade that culminates in the formation of the autophagosome, a double-membrane vesicle that encloses the cellular contents destined for degradation. Once fully formed, the autophagosomes fuse with lysosomes, resulting in the degradation and recycling of their contents. Although autophagy may play a complex role in tumorigenesis and tumor resistance to therapy (16
), Atg proteins have been proposed as attractive targets for cancer therapy (18
). Cells under ER stress have been shown to activate macro-autophagy, and it has been suggested that at least part of the autophagosomal membrane originates from enlarged ER membrane (20
) and is dependent on an active PERK/eIF2α pathway (22
). Recently, it was shown that in response to hypoxic stress, PERK-dependent increases in ATF4 lead to prolonged autophagic flux, contributing to hypoxic adaptation (10
Cell-autonomous insults, including oncogene activation, are known to activate cellular stress responses, including metabolic stress, apoptosis, DNA damage responses, and cellular senescence. Therefore, transformed cells often acquire a concurrent activation of pro-survival pathways (23
). In particular, the MYC
oncogene, which is the target of chromosomal translocation and gene amplification during the development of many human cancers, is known to induce both increased proliferation and apoptosis, depending on the cellular context (26
). c-Myc functions as a transcription factor and induces proliferation through the upregulation of genes required for cell cycle progression (cyclin D, cyclin E), energy metabolism (lactate dehydrogenase A, GLUT1
), and ribosome biogenesis (RNA polymerase III, ribosomal proteins) (30
). The latter has been associated with an increased rate of protein synthesis, which is required for MYC oncogenic activity and survival (31
). However, the particular pathways under Myc-dependent translational control and the underlying mechanism by which this occurs remain elusive.
Based on the ability of the Myc
oncogene to induce considerable increases in both ribosome biogenesis and rates of protein synthesis (reviewed extensively in refs. 34
), we hypothesized that oncogenic activation of c-Myc during transformation would inflict ER stress and elicit UPR activation. Moreover, we reasoned that such an induction of the UPR would be cytoprotective and thus promote Myc-dependent tumorigenesis. Using several inducible Myc cell models as well as genetic and pharmacologic tools, we show that Myc induction leads to activation of the PERK/eIF2α/ATF4 axis of the UPR, resulting in increased autophagy and protection against ER-dependent apoptosis. Moreover, we demonstrate that robust upregulation of the UPR occurs in human lymphoma samples and mouse lymphoma models, suggestive of a role for UPR activation in the pathogenesis of disease. Therefore, we have identified a cell-autonomous role for PERK as a molecular switch regulating autophagy and apoptosis in response to Myc activation, and have uncovered PERK as a therapeutic target for the treatment of Myc-associated malignancies.