| The tumor microenvironment results in several cellular stresses including hypoxia and nutrient deprivation. In order to adapt to such conditions, the cell can utilize several stress-response mechanisms, one of which is autophagy. Autophagy leads to the degradation of cellular contents, including damaged organelles and mutant proteins, which the cell can then use as an alternate energy source. Autophagy is dramatically induced by conditions found in the tumor microenvironment and many tumors depend on autophagy for survival. To explore novel mechanisms of autophagic regulation, we focused on cellular processes regulated by the tumor microenvironment. One such process is an mRNA surveillance mechanism known as the nonsense mediated RNA decay pathway (NMD), which is inhibited under conditions found in the tumor microenvironment and has been implicated in promoting tumorigenesis. Here, we identify that inhibition of NMD induces autophagy both in vitro and in vivo and that the concomitant inhibition of autophagy and NMD, either genetically or pharmacologically, leads to synergistic cell death. These studies together demonstrate that the induction of autophagy by NMD inhibition is an adaptive cellular stress response that may be therapeutically exploited. We then explored other regulators of autophagy. Amino acid deprivation leads to both GCN2 kinase activation and mTORC1 inhibition. These two processes have been viewed as independent adaptive stress responses, each of which is important for the induction of autophagy. Here we demonstrate that there is cross-talk between these two pathways. Specifically we show that inhibition of mTORC1 leads to activation of GCN2 and phosphorylation of the eukaryotic initiation factor 2alpha (eIF2alpha) in a mechanism dependent on the PP6C phosphatase. We also identify a PP6C/PP6C regulator binding protein/GCN2 complex necessary for this response. Interestingly, we have identified several PP6C mutants found in melanoma that do not bind to PP6C regulatory proteins and are rapidly degraded. Paradoxically these mutants stabilize wild-type PP6C and promote eIF2alpha phosphorylation. Therefore, we demonstrate that mTORC1 induces eIF2alpha phosphorylation in response to nutrient deprivation and is necessary for induction of autophagy. Additionally, we describe a novel function of PP6C regulating autophagy, a role that may be hijacked in melanoma to promote tumorigenesis. |
