The CK1ε/SIAH1 axis regulates AXIN1 stability in colorectal cancer cells.

Casein kinase 1ε (CK1ε) and axis inhibitor 1 (AXIN1) are crucial components of the ß-catenin destruction complex in canonical Wnt signaling. CK1ε has been shown to interact with AXIN1, but its physiological function and role in tumorigenesis remain unknown. In this study, we found that CK1δ/ε inhibitors significantly enhanced AXIN1 protein level in colorectal cancer (CRC) cells through targeting CK1ε. Mechanistically, CK1ε promoted AXIN1 degradation by the ubiquitin-proteasome pathway by promoting the interaction of E3 ubiquitin-protein ligase SIAH1 with AXIN1. Genetic or pharmacological inhibition of CK1ε and knockdown of SIAH1 downregulated the expression of Wnt/ß-catenin-dependent genes, suppressed the viability of CRC cells, and restrained tumorigenesis and progression of CRC in vitro and in vivo. In summary, our results demonstrate that CK1ε exerted its oncogenic role in CRC occurrence and progression by regulating the stability of AXIN1. These findings reveal a novel mechanism by which CK1ε regulates the Wnt/ß-catenin signaling pathway and highlight the therapeutic potential of targeting the CK1ε/SIAH1 axis in CRC.

R-spondin-1 induces Axin degradation via the LRP6-CK1ε axis.

R-spondins (RSPOs) are secreted signaling molecules that potentiate the Wnt/ß-catenin pathway by cooperating with Wnt ligands. RSPO1 is crucial in tissue development and tissue homeostasis. However, the molecular mechanism by which RSPOs activate Wnt/ß-catenin signaling remains elusive. In this study, we found that RSPOs could mediate the degradation of Axin through the ubiquitin-proteasome pathway. The results of Co-IP showed that the recombinant RSPO1 protein promoted the interaction between Axin1 and CK1ε. Either knockout of the CK1ε gene or treatment with the CK1δ/CK1ε inhibitor SR3029 caused an increase in Axin1 protein levels and attenuated RSPO1-induced degradation of the Axin1 protein. Moreover, we observed an increase in the number of associations of LRP6 with CK1ε and Axin1 following RSPO1 stimulation. Overexpression of LRP6 further potentiated Axin1 degradation mediated by RSPO1 or CK1ε. In addition, recombinant RSPO1 and Wnt3A proteins synergistically downregulated the protein expression of Axin1 and enhanced the transcriptional activity of the SuperTOPFlash reporter. Taken together, these results uncover the novel mechanism by which RSPOs activate Wnt/ß-catenin signaling through LRP6/CK1ε-mediated degradation of Axin.