Regulation of E2F-mediated transcription by FBXL-6 in HCT116 cells

Abstract

F-box proteins are components of Skp1p-Cullin-F-box protein (SCF) ubiquitin-ligase complexes, where they mediate the critical step of substrate recognition. Some of the F-box proteins are responsible for selective degradation of cell cycle regulators. The F-box and leucine-rich repeat protein 6 (FBXL-6), an isoform of F-box protein family, has been cloned but its functions are barely known. In this report we examined the effect of FBXL-6 on the E2F-mediated transcription in HCT116 colorectal cancer cells. First, we found through transient transfection of E2F-responsive reporter vectors that the E2F activity is significantly repressed in HCT116 cells. Forced expression of 5'-truncated FBXL-6 in these cells could help the repression relieved and increase the E2F-mediated transcriptional activity. RT-PCR analysis revealed that the FBXL-6 mRNA is expressed in most of the cancer cells at varying degrees. We then confirmed, using a full-length cDNA expression vector, that FBXL-6 could augment the E2F-mediated transcription in various other cancer cell lines originated from hepatocellular, gastric or colorectal cancers. These results suggest the possibility that FBXL-6 may regulate the activity or stability of a protein constituting the E2F pathway via a post-translational modification.