RIPK1 down-regulation promotes tumor progression while enhancing the apoptotic response to TLR3 ligand in head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC) is the most frequent malignancy of the aerodigestive tract and the limitations in current chemotherapeutic approaches have yielded a poor prognosis. Synthetic double stranded (ds) RNA, which activate toll-like receptor 3 (TLR3) and generate interferon regulatory factor 3 (IRF3)-mediated proapoptotic responses in cancer cells, are being investigated as potent adjuvants to chemotherapy. We had previously shown that cells derived from metastatic HNSCC were unable to activate prosurvival NF-κB in response to TLR3 ligand, resulting in an enhanced apoptotic response compared to cells from primary tumors. Here, we demonstrate that in metastatic tumor-derived cell lines, transcriptional downregulation of receptor interacting protein kinase 1 (RIPK1), an adaptor protein of the TLR3 pathway, enhances dsRNA-mediated apoptosis due to a loss of NF-κB activation. This is consistent with our observations supporting the reduction of RIPK1 expression during the tumor progression of HNSCC correlated with an increased promoter methylation in matched tumor samples from HNSCC patients. Treatment of metastatic tumor-derived cell lines with a hypomethylating agent rescued the expression of RIPK1, demonstrating that promoter methylation may be responsible for the downregulation of RIPK1. We show that silencing of RIPK1 enhances the tumor promoting properties in HNSCC cell lines by increasing the rate of migration, EGFR expression and anoikis resistance. As the downregulation of RIPK1 expression contributes to the metastatic phenotype of HNSCC, but is essential for TLR3-NF-κB mediated pro-survival responses, we believe the results described here may open new prospects for using synthetic dsRNA to target metastatic tumor cells. In light of these findings, RIPK1 downregulation as a treatment biomarker, or combinational therapies with NF-κB-signaling inhibitors, could be used to enhance the immunotherapeutic efficacy of synthetic dsRNA. As the downregulation of RIPK1 has been observed in additional carcinomas, our findings suggest that synthetic dsRNA could be used as a broad anti-cancer therapy and that RIPK1 expression can be a useful indicator to predict the therapy response