Overexpression of far upstream element (FUSE) binding protein (FBP)-interacting repressor (FIR) supports growth of hepatocellular carcinoma

Abstract

The far upstream element binding protein (FBP) and the FBP-interacting repressor (FIR) represent molecular tools for transcriptional fine tuning of target genes. 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 small interfering RNA (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/E2F1-dependent manner. FIR splice variants lacking or containing exon 2 and/or exon 5 are expressed in the majority of HCCs but not in normal hepatocytes. Specific inhibition of FIR isoforms with and without exon 2 revealed that both groups of FIR splice variants facilitate tumor-supporting effects. This finding was confirmed in xenograft transplantation experiments with lentiviral-infected short hairpin RNA (shRNA) targeting all FIR variants as well as FIR with and without exon 2. Conclusion: 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 with elevated FIR expression. (HEPATOLOGY 2014;60:1241-1250 A berrant activation, mutations, or dysregulation of transcriptional regulators are frequently detected in hepatocellular carcinoma (HCC). Tumor-relevant transcriptional regulators consist of DNA-interacting transcription factors (e.g., c-Myc, NF-jB) and transcriptional coactivators (e.g., b-catenin or YAP), which fine-tune initiation, progression, and termination of the transcriptional machinery to regulate the expression of target genes in a timedependent manner. Because transcriptional regulators Abbreviations: DN, dysplastic nodule; FBP, far upstream element binding protein; FIR, FBP-interacting repressor; FUSE, far upstream element; HCC, hepatocellular carcinoma.

Haploinsufficiency of thec-myctranscriptional repressorFIR, as a dominant negative-alternative splicing model, promoted p53-dependent T-cell acute lymphoblastic leukemia progression by activating Notch1

“…In addition, BLM-induced DNA damage alters FIR splicing, which contributes to the transcriptional upregulation of c-myc via dominant negative effect on endogenous FIR [12, 13]. c-Myc accelerates the cell cycle by suppressing p27Kip1 expression, eventually leading to the accumulation of DNA damage.…”

“…Markedly, a splice variant of FIR that lacks exon 2 in the transcriptional repression domain (FIRΔexon2) elevates c-Myc protein expression in vitro [11]. FIRΔexon2 mRNA is frequently upregulated in human colorectal cancers [12] as well as hepatocellular carcinoma [13], where it stimulates tumor growth by preventing FIR from suppressing c-myc [13]. FIRΔexon2 functions as a dominant negative regulator of FIR; therefore it reduces FIR function.…”

“…In fact SAP155 mutations, which potentially affect FIR/FIRΔexon2/SAP155 formation, were reported not only in myeloid lineage tumors but also lymphoid lineage tumors [20-23]. Consequently, an aberrant FIR/FIRΔexon2/SAP155 interaction is responsible for cancer development and differentiation and is a potent target for cancer screening and treatment [13, 19]. …”

Long non-coding RNA CARLo-5 expression is associated with disease progression and predicts outcome in hepatocellular carcinoma patients

“…Numerous studies have reported that genetic variants at 8q24 are associated with the risk of many kinds of tumors including hepatocellular carcinoma [17][18][19]. Interestingly, the lncRNA CARLo-5 is known to be located in the 8q24.21 gene desert [13].…”

Anti-FIRs (PUF60) auto-antibodies are detected in the sera of early-stage colon cancer patients

“…On the contrary, the far-upstream element (FUSE)-binding protein-interacting repressor (FIR), splicing variant of PUF60 lacking exon5, have been reported to be overexpressed in various malignant tumors, such as colorectal cancers [2, 3], hepatocellular carcinomas [4, 5], T-cell acute lymphoblastic leukemia [6],and non-small cell lung cancer [7]. Therefore, it is natural that anti-FIR (PUF60) antibodies could be detected in the sera of cancer patients as well as in dermatomyositis and Sjogren's syndrome.…”