Downregulation of PERK activity and eIF2α serine 51 phosphorylation by mTOR complex 1 elicits pro-oxidant and pro-death effects in tuberous sclerosis-deficient cells article

Oxidative stress determines cell fate through several mechanisms, among which regulation of mRNA translation by the phosphorylation of the alpha (α) subunit of the translation initiation factor eIF2α at serine 51 (eIF2αP) plays a prominent role. Increased eIF2αP can contribute to tumor progression as well as tumor suppression. While eIF2αP is increased in most cells to promote survival and adaptation to different forms of stress, we demonstrate that eIF2αP is reduced in tuberous sclerosis complex 2 (TSC2)-deficient cells subjected to oxidative insults. Decreased eIF2αP in TSC2-deficient cells depends on reactive oxygen species (ROS) production and is associated with a reduced activity of the endoplasmic reticulum (ER)-resident kinase PERK owing to the hyper-activation of the mammalian target of rapamycin complex 1 (mTORC1). Downregulation of PERK activity and eIF2αP is accompanied by increased ROS production and enhanced susceptibility of TSC2-deficient cells to extrinsic pro-oxidant stress. The decreased levels of eIF2αP delay tumor formation of TSC2-deficient cells in immune deficient mice, an effect that is significantly alleviated in mice subjected to an anti-oxidant diet. Our findings reveal a previously unidentified connection between mTORC1 and eIF2αP in TSC2-deficient cells with potential implications in tumor suppression in response to oxidative insults. © 2018 The Author(s)

IFN-γ-response mediator GBP-1 represses human cell proliferation by inhibiting the Hippo signaling transcription factor TEAD

Interferon-gamma (IFN-γ) is a pleiotropic cytokine that exerts important functions in inflammation, infectious diseases, and cancer. The large GTPase human guanylate-binding protein 1 (GBP-1) is among the most strongly IFN-γ-induced cellular proteins. Previously, it has been shown that GBP-1 mediates manifold cellular responses to IFN-γ including the inhibition of proliferation, spreading, migration, and invasion and through this exerts anti-tumorigenic activity. However, the mechanisms of GBP-1 anti-tumorigenic activities remain poorly understood. Here, we elucidated the molecular mechanism of the human GBP-1-mediated suppression of proliferation by demonstrating for the first time a cross-talk between the anti-tumorigenic IFN-γ and Hippo pathways. The α9-helix of GBP-1 was found to be sufficient to inhibit proliferation. Protein-binding and molecular modeling studies revealed that the α9-helix binds to the DNA-binding domain of the Hippo signaling transcription factor TEA domain protein (TEAD) mediated by the 376VDHLFQK382 sequence at the N-terminus of the GBP-1-α9-helix. Mutation of this sequence resulted in abrogation of both TEAD interaction and suppression of proliferation. Further on, the interaction caused inhibition of TEAD transcriptional activity associated with the down-regulation of TEAD-target genes. In agreement with these results, IFN-γ treatment of the cells also impaired TEAD activity, and this effect was abrogated by siRNA-mediated inhibition of GBP-1 expression. Altogether, this demonstrated that the α9-helix is the proliferation inhibitory domain of GBP-1, which acts independent of the GTPase activity through the inhibition of the Hippo transcription factor TEAD in mediating the anti-proliferative cell response to IFN-γ