title: Loss of the SxxSS motif in a human T-cell factor-4 isoform confers hypoxia resistance to liver cancer: an oncogenic switch in Wnt signaling.
abstract: OBJECTIVE
  Aberrantly activated Wnt/β-catenin signaling is important in hepatocellular carcinoma (HCC) development. Downstream gene expressions involving the Wnt/β-catenin cascade occur through T-cell factor (TCF) proteins. Here, we show the oncogenic potential of human TCF-4 isoforms based on the expression of a single conserved SxxSS motif.
  
  
  METHODS
  We investigated the TCF-4J and K isoform pair characterized by the presence (K) or absence (J) of the SxxSS motif. The mRNA expression profiles were examined in 47 pairs of human HCCs and adjacent non-cancerous liver tissues by RT-PCR. Proliferation, sphere assays and immunoblot analysis were performed under normoxia and hypoxia conditions. The ability of HCC cells overexpressing TCF-4J (J cells) and K (K cells) to grow as solid tumors in nude mice was explored.
  
  
  RESULTS
  TCF-4J expression was significantly upregulated in HCC tumors compared to corresponding peritumor and normal liver and was preferentially expressed in poorly differentiated HCCs. In contrast, TCF-4K was downregulated in those same HCC tumors. TCF-4J-overexpressing HCC cells (J cells) revealed a survival advantage under hypoxic conditions, high proliferation rate and formation of aggregates/spheres compared to overexpression of TCF-4K (K cells). The hypoxic J cells had high expression levels of HIF-2α and EGFR as possible mechanisms to promote tumorigenesis. Increased stability of HIF-2α under hypoxia in J cells was associated with a decreased level of von Hippel-Lindau (VHL) protein, a known E3 ligase for HIF-αs. In a xenograft model, the J cells rapidly developed tumors compared to K cells. Tumor tissues derived from J cells exhibited high expression levels of HIF-2α and EGFR compared to the slow developing and small K cell derived tumors.
  
  
  CONCLUSIONS
  Our results suggest that the specific TCF-4J isoform, which lacks a regulatory SxxSS motif, has robust tumor-initiating potential under hypoxic conditions.
authors: Liver Research Center, Rhode Island Hospital and the Warren Alpert Medical School of Brown University, Providence, Rhode Island, United States of America.
authors: Hironori
authors: H
authors: Koga
authors: Orkhontuya
authors: O
authors: Tsedensodnom
authors: Yoshito
authors: Y
authors: Tomimaru
authors: Evan J
authors: EJ
authors: Walker
authors: Han Chu
authors: HC
authors: Lee
authors: Kang Mo
authors: KM
authors: Kim
authors: Hirohisa
authors: H
authors: Yano
authors: Jack R
authors: JR
authors: Wands
authors: Miran
authors: M
authors: Kim
mesh_terms: D000328:Adult; D000368:Aged; D020816:Amino Acid Motifs; D000818:Animals; D044466:Asian People; D015687:Cell Hypoxia / Q000235:genetics; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D002471:Cell Transformation, Neoplastic / Q000235:genetics / Q000473:pathology; D005260:Female; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000378:metabolism; D008113:Liver Neoplasms / Q000235:genetics / Q000473:pathology*; D008297:Male; D051379:Mice; D008875:Middle Aged; D009857:Oncogenes / Q000235:genetics*; D020033:Protein Isoforms / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D055550:Protein Stability; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D056910:Republic of Korea; D013329:Structure-Activity Relationship; D058118:Transcription Factor 7-Like 2 Protein / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D060449:Wnt Signaling Pathway / Q000235:genetics; D055815:Young Adult
chemical_list: D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D020033:Protein Isoforms; D012333:RNA, Messenger; C544894:TCF7L2 protein, human; D058118:Transcription Factor 7-Like 2 Protein