A Novel Binding Factor of 14-3-3b Functions as a Transcriptional Repressor and Promotes Anchorage-independent Growth, Tumorigenicity, and Metastasis

The 14-3-3 proteins form a highly conserved family of dimeric proteins that interact with various signal transduction proteins and regulate cell cycle, apoptosis, stress response, and malignant transformation. We previously demonstrated that the b isoform of 14-3-3 proteins promotes tumorigenicity and angiogenesis of rat hepatoma K2 cells. In this study, to analyze the mechanism of 14-3-3b-induced malignant transformation, yeast two-hybrid screening was performed, and a novel 14-3-3b-binding factor, FBI1 (fourteen-three-three beta interactant 1), was identified. In vitro binding and co-immunoprecipitation analyses verified specific interaction of 14-3-3b with FBI1. The strong expression of FBI1 was observed in several tumor cell lines but not in non-tumor cell lines. Forced expression of antisense FBI1 in K2 cells inhibited anchorage-independent growth but had no significant effect on cell proliferation in monolayer culture. Down-regulation of FBI1 also inhibited tumorigenicity and metastasis accompanying a decrease in MMP-9 (matrix metalloproteinase-9) expression. In addition, the duration of ERK1/2 activation was curtailed in antisense FBI1-expressing K2 cells. A luciferase reporter assay revealed that the FBI1-14-3-3b complex could act as a transcriptional silencer, and MKP-1 (MAPK phosphatase-1) was one of the target genes of the FBI1-14-3-3b complex. Moreover, chromatin immunoprecipitation analysis demonstrated that FBI1 and 14-3-3b were presented on the MKP-1 promoter. These results indicate that FBI1 promotes sustained ERK1/2 activation through repression of MKP-1 transcription, resulting in promotion of tumorigenicity and metastasis.