Regulation of stability and inhibitory activity of the tumor suppressor SEF through casein-kinase II-mediated phosphorylation.

Inflammation and cancer are intimately linked. A key mediator of inflammation is the transcription-factor NF-κB/RelA:p50. SEF (also known as IL-17RD) is a feedback antagonist of NF-κB/RelA:p50 that is emerging as an important link between inflammation and cancer. SEF acts as a buffer to prevent excessive NF-κB activity by sequestering NF-κB/RelA:p50 in the cytoplasm of unstimulated cells, and consequently attenuating the NF-κB response upon pro-inflammatory cytokine stimulation. SEF contributes to cancer progression also via modulating other signaling pathways, including those triggered by growth-factors. Despite its important role in human physiology and pathology, mechanisms that regulate SEF biochemical properties and inhibitory activity are unknown. Here we show that human SEF is an intrinsically labile protein that is stabilized via CK2-mediated phosphorylation, and identified the residues whom phosphorylation by CK2 stabilizes hSEF. Unlike endogenous SEF, ectopic SEF was rapidly degraded when overexpressed but was stabilized in the presence of excess CK2, suggesting a mechanism for limiting SEF levels depending upon CK2 processivity. Additionally, phosphorylation by CK2 potentiated hSef interaction with NF-κB in cell-free binding assays. Most importantly, we identified a CK2 phosphorylation site that was indispensable for SEF inhibition of pro-inflammatory cytokine signaling but was not required for SEF inhibition of growth-factor signaling. To our knowledge, this is the first demonstration of post-translational modifications that regulate SEF at multiple levels to optimize its inhibitory activity in a specific signaling context. These findings may facilitate the design of SEF variants for treating cytokine-dependent pathologies, including cancer and chronic inflammation.