SUMMARY Pro-inflammatory cytokines produced in the tumor microenvironment lead to eradication of anti-tumor immunity and enhanced tumor cell survival. In the current study, we identified tumor necrosis factor alpha (TNF-α) as a major factor triggering cancer cell immunosuppression against T cell surveillance via stabilization of programmed cell death-ligand 1 (PD-L1). We demonstrated that COP9 signalosome 5 (CSN5), induced by NF-κB p65, is required for TNF-α-mediated PD-L1 stabilization in cancer cells. CSN5 inhibits the ubiquitination and degradation of PD-L1. Inhibition of CSN5 by curcumin diminished cancer cell PD-L1 expression and sensitized cancer cells to anti-CTLA4 therapy.

Summary Esophageal adenocarcinoma (EAC) is the most prevalent esophageal cancer type in the United States. TNFα/mTOR pathway is known to mediate the development of EAC. Additionally, aberrant activation of Gli1, downstream effector of hedgehog pathway, has been observed in EAC. In this study, we found that activated mTOR/S6K1 pathway promotes Gli1 transcriptional activity and oncogenic function through S6K1-mediated Gli1 phosphorylation at Ser84, which releases Gli1 from its endogenous inhibitor, SuFu. Moreover, elimination of S6K1 activation by mTOR pathway inhibitor enhances the killing effects of the hedgehog pathway inhibitor. Together, our results established a crosstalk between mTOR/S6K1 and the hedgehog pathways, which provides not only a mechanism for SMO-independent Gli1 activation but also a rationale for combination therapy for EAC.

MicroRNAs (miRNAs) are generated by two-step processing to yield small RNAs that negatively regulate target gene expression at the post-transcriptional level1. Deregulation of miRNAs has been linked to diverse pathological processes, including cancer2,3. Recent studies have also implicated miRNAs in the regulation of cellular response to a spectrum of stresses4, such as hypoxia, which is frequently encountered in the poorly angiogenic core of a solid tumour5. However, the upstream regulators of miRNA biogenesis machineries remain obscure, raising the question of how tumour cells efficiently coordinate and impose specificity on miRNA expression and function in response to stresses. Here we show that epidermal growth factor receptor (EGFR), which is the product of a well-characterized oncogene in human cancers, suppresses the maturation of specific tumour-suppressor-like miRNAs in response to hypoxic stress through phosphorylation of argonaute 2 (AGO2) at Tyr 393. The association between EGFR and AGO2 is enhanced by hypoxia, leading to elevated AGO2-Y393 phosphorylation, which in turn reduces the binding of Dicer to AGO2 and inhibits miRNA processing from precursor miRNAs to mature miRNAs. We also identify a long-loop structure in precursor miRNAs as a critical regulatory element in phospho-Y393-AGO2-mediated miRNA maturation. Furthermore, AGO2-Y393 phosphorylation mediates EGFR-enhanced cell survival and invasiveness under hypoxia, and correlates with poorer overall survival in breast cancer patients. Our study reveals a previously unrecognized function of EGFR in miRNA maturation and demonstrates how EGFR is likely to function as a regulator of AGO2 through novel post-translational modification. These findings suggest that modulation of miRNA biogenesis is important for stress response in tumour cells and has potential clinical implications.