A Hypoxia-Induced SCF(FBXL1) E3 Ligase Ubiquitinates and Degrades the MEN1 Tumor Suppressor to Promote Colorectal Cancer Tumorigenesis

PURPOSE: Emerging evidence has shown that SKP1-cullin-1-F-box-protein (SCF) E3 ligases contribute to the pathogenesis of different cancers by mediating the ubiquitination and degradation of tumor suppressors. However, the functions of SCF E3 ligases in the pathogenesis of colorectal cancer (CRC) remain obscure. MATERIALS AND METHODS: The cancerous and adjacent noncancerous tissues from CRC patients were collected, and protein levels were analyzed. Lentiviral short hairpin RNA (shRNA) and plasmid transfection were used to knock down and overexpress gene expression in CRC cell lines. Immunoprecipitation (IP), mass spectrometry, and co-IP analyses were used to determine protein interactions and the assembly of the SCF complex. Cell proliferation, migration, and tumor xenograft assays were performed to examine the effects of SCF members on CRC cell growth in vitro and in vivo. RESULTS: Hypoxia activated the docking of hypoxia-inducible factor 1α (HIF1α) onto the CUL1 promoter and induced CUL1 expression in CRC cells. CUL1 coupled with RBX1, SKP1, and FBXL1 to assemble the SCF(FBXL1) complex in CRC biopsies and cells. The SCF(FBXL1) E3 ligase specifically ubiquitinated and degraded the MEN1 tumor suppressor. Knockdown of HIF1α or SCF(FBXL1) members, or blockage of SCF(FBXL1) by two inhibitors (DT204 and SZLP1-41) caused the accumulation of MEN1 protein and led to a significant decrease in cell proliferation and migration in vitro and tumor growth in vivo. CONCLUSION: The SCF(FBXL1) E3 ligase is required for the ubiquitination of MEN1, and disruption of this complex may represent a new therapeutic strategy for the treatment of CRC.