RIOK3 interacts with caspase-10 and negatively regulates the NF-kappaB signaling pathway

RIOK3 was initially characterized as a homolog of Aspergillus nidulans sudD and showed down-regulation at the invasive front of malignant melanomas, but the molecular mechanism remains elusive. Here, we report that overexpression of RIOK3 inhibits TNFalpha-induced NF-kappaB activation, but down-regulation of endogenous RIOK3 expression by siRNA potentiates it. A yeast two-hybrid experiment revealed that RIOK3 interacted with Caspase-10, and further, a GST pull-down assay and endogenous coimmunoprecipitation validated the interaction. We subsequently showed that the interaction was mediated by the RIO domain of RIOK3 and each death effector domain of Caspase-10. Interestingly, our data demonstrated that RIOK3 suppressed caspase-10-mediated NF-kappaB activation by competing RIP1 and NIK to bind to Caspase-10. Importantly, the kinase activity of RIOK3 was confirmed to be relevant to NF-kappaB signaling. Taken together, our findings strongly suggest that RIOK3 negatively regulates NF-kappaB signaling pathway activated by TNFalpha dependent on its kinase activity and NF-kappaB signaling pathway activated by Caspase-10 independent of its kinase activity.