FIR inhibition in HCC cells (Hep3B)

siRNA-mediated inhibition compared to untreated cells and cells transfected with nonsense siRNA Together with far upstream element binding protein (FBP), the FBP-interacting repressor (FIR) represents a molecular tool for the transcriptional fine tune regulation of target genes. Previous evidence indicates that strong overexpression of FBP in human hepatocellular carcinoma (HCC) supports tumor growth and correlates with poor patient prognosis. However, the role of the transcriptional repressor FIR in hepatocarcinogenesis remains poorly delineated. We show that overexpression of FIR correlates with tumor dedifferentiation and tumor cell proliferation in about 60% of primary HCCs. Elevated FIR levels are associated with genomic gains of the fir gene locus at chromosome 8q24.3 in human HCC specimens. In vitro, nuclear enrichment of FIR supports HCC cell proliferation and migration. Expression profiling of HCC cells after siRNA-mediated silencing of FIR identified the transcription factor DP-1 (TFDP1) as a transcriptional target of FIR. Surprisingly, FIR stimulates the expression of FBP in a TFDP1/E2F-dependent manner. FIR splice variants lacking or containing exon 2 and/or exon 5 are expressed in the majority of HCCs. Specific inhibition of FIR isoforms with and without exon 2 revealed that all FIR splice variants facilitate tumor-supporting effects. This finding was confirmed in xenograft transplantation experiments with lentiviral-infected shRNA targeting all FIR variants as well as FIR with and without exon 2. In summary, high-level nuclear FIR does not facilitate repressor properties but supports tumor growth in HCC cells. Thus, the pharmacological inhibition of FIR might represent a promising therapeutic strategy for HCC patients. Expression profiling of cells transfected with scrambled/nonsense siRNA (control), and after siRNA-mediated FIR inhibition.