The microRNA-34a (miR-34a), a tumor suppressive microRNA (miRNA), is implicated in epithelial-mesenchymal transition (EMT) and cancer stem cells. Lymphoid enhancer-binding factor-1 (LEF1) is a key transcription factor in the Wnt signaling pathway, and has been suggested to be involved in regulation of cell proliferation and invasion. Here, the molecular mechanism of miR-34a and LEF1 in cooperatively regulating prostate cancer (PCa) cell invasion is described. Molecular profiling analysis of miRNA levels in PCa cells revealed a negative correlation between miR-34a and LEF1 expression, and the downregulation of LEF1 by miR-34a was confirmed by luciferase assays. Further, miR-34a specifically repressed LEF1 expression through direct binding to its 3′-untranslated (3′-UTR) regions. miR-34a modulated the levels of LEF1 to regulate EMT in PCa cells. Functionally, miR-34a negatively correlated with the migration and invasion of PCa cells through LEF1. An analysis of miR-34a expression levels in matched human tumor and benign tissues demonstrated consistent and statistically significant downregulation of miR-34a in primary PCa specimens. These data strongly suggest that miR-34a/LEF1 regulation of EMT plays an important role in PCa migration and invasion. Implications The miR-34a/LEF1 axis represents a potential molecular target for novel therapeutic strategies in PCa.

Interleukin 24 (IL-24) is a tumor-suppressing protein, which inhibits angiogenesis and induces cancer cell-specific apoptosis. We have shown that IL-24 regulates apoptosis through phosphorylated eukaryotic initiation factor 2 alpha (eIF2α) during endoplasmic reticulum (ER) stress in cancer. Although multiple stresses converge on eIF2α phosphorylation, the cellular outcome is not always the same. In particular, ER stress-induced apoptosis is primarily regulated through the extent of eIF2α phosphorylation and activating transcription factor 4 (ATF4) action. Our studies show for the first time that cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) activation is required for IL-24-induced cell death in a variety of breast cancer cell lines and this event increases ATF4 activity. We demonstrate an undocumented role for PKA in regulating IL-24-induced cell death, whereby PKA stimulates phosphorylation of p38 mitogen-activated protein kinase and upregulates extrinsic apoptotic factors of the Fas/FasL signaling pathway and death receptor 4 expression. We also demonstrate that phosphorylation and nuclear import of tumor suppressor TP53 occurs downstream of IL-24-mediated PKA activation. These discoveries provide the first mechanistic insights into the function of PKA as a key regulator of the extrinsic pathway, ER stress, and TP53 activation triggered by IL-24.

Interleukin 24 (IL24) 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. 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 IL-24 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 IL-24 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 non-phosphorylatable mutant of eIF2α, eIF2α-S51A, reversed both the IL24-mediated translational block and IL24-induced apoptosis. Intriguingly, IL-24 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 IL-24 in inhibition of translation, mediated through both phosphorylation of eIF2α and de-phosphorylation of 4E-BP1, and provide the first direct evidence for translation control of gene-specific expression by IL-24.