eIF2α Phosphorylation Mediates IL24-Induced Apoptosis through Inhibition of Translation.

IL24 is an immunomodulatory cytokine that also displays broad cancer-specific suppressor effects. The tumor-suppressor activities of IL24 include inhibition of angiogenesis, sensitization to chemotherapy, and cancer-specific apoptosis. Supra-physiologic activation and/or overexpression of translation initiation factors are implicated in the initiation and progression of cancer animal models as well as a subset of human cancers. Activation and/or overexpression of translation initiation factors correlate with aggressiveness of cancer and poor prognosis. Two rate-limiting translation initiation complexes, the ternary complex and the eIF4F complex, are regulated by eIF2α and 4E-BP1 phosphorylation, respectively. The work reported here provides direct evidence that IL24 induces inhibition of translation initiation leading to apoptosis in squamous cell carcinoma. A dominant constitutively active mutant of eIF2α, which is resistant to phosphorylation, was used to determine the involvement of eIF2α in IL24-induced apoptosis. Treatment with IL24 resulted in inhibition of protein synthesis, expression of downstream biomarkers of ternary complex depletion such as CHOP, and induction of apoptosis in cancer cells. The constitutively active nonphosphorylatable mutant of eIF2α, eIF2α-S51A, reversed both the IL24-mediated translational block and IL24-induced apoptosis. Intriguingly, IL24 treatment also caused hypophosphorylation of 4E-BP1, which binds to eIF4E with high affinity, thus preventing its association with eIF4G and therefore preventing elF4F complex assembly.Implications: These results demonstrate a previously unrecognized role of IL24 in inhibition of translation, mediated through both phosphorylation of eIF2α and dephosphorylation of 4E-BP1, and provide the first direct evidence for translation control of gene-specific expression by IL24. Mol Cancer Res; 15(8); 1117-24. ©2017 AACR.

Sigma 1 Receptor plays a prominent role in IL-24-induced cancer-specific apoptosis.

Interleukin-24 (IL-24), a member of the IL-10 cytokine family, is an immunomodulatory cytokine that also displays broad cancer-specific suppressor effects. The tumor suppressor activities of IL-24 include inhibition of angiogenesis, sensitization to chemotherapy, and cancer-specific apoptosis. We show that Sigma 1 Receptor (S1R), a ligand-regulated protein chaperone contributes to IL-24 induction of apoptosis. IL-24 generated from an adenovirus expressing IL-24 (Ad.IL-24) induces cancer-specific apoptosis by inducing an endoplasmic reticulum (ER) stress, reactive oxygen species production, and calcium mobilization. The present studies reveals that S1R is required for Ad.IL-24-induced cell death. We provide several lines of evidence to confirm a physical and functional interaction between IL-24 and S1R including: (a) S1R and IL-24 co-localize, as judged by immunocytochemical analysis studies; (b) S1R and IL-24 co-immunoprecipitate using either S1R or IL-24 antibody; (c) S1R agonist (+)-SKF10047 inhibits apoptosis by Ad.IL-24; (d) (+)-SKF10047-mediated inhibition of Ad.IL-24 results in: diminished ER stress protein expression; (e) Calcium mobilization; and (f) ROS production. Collectively, these data demonstrate that S1R interacts with IL-24 and suggest that IL-24:S1R interaction determines apoptosis induction by Ad.IL-24. These studies define Sigma 1 Receptor as a key initial mediator of IL-24 induction of cancer-specific killing. These findings have important implications for our understanding of IL-24 as a tumor suppressor protein as well as an immune modulating cytokine.