Hepatocyte nuclear factor 4-alpha (HNF4α) is a well established master regulator 
of liver development and function. We identified the in vitro presence of a 
stable secondary structure, G-quadruplex (G4) in the 5' UTR of P1-HNF4A, the 
predominant HNF4α isoform(s) in adult liver. Our data suggest that the 
cooperation of G4 and the adjacent putative protein-binding sites within the 5' 
UTR was necessary and sufficient to mediate a strong translational repression. 
This was supported by analysis of deleted/mutated 5'UTRs and two native 
regulatory single-nucleotide polymorphisms in the 5'UTR. Additional results 
indicated that G4 motifs in the 5' UTRs of other liver-enriched transcription 
factors also inhibited protein expression. Moreover, pyridostatin, a G4 ligand, 
specifically potentiated the translational suppressing effect of P1-HNF4A-5' 
UTR. In summary, the present study provides the first evidence of the presence 
of G4 in human P1-HNF4A-5' UTR in vitro, and establishes a novel working model 
of strong inhibition of protein translation via interactions of G4 with 
potential RNA-binding proteins (RBPs). The protein expression of the tumor 
suppressor HNF4α may be inhibited by interactions of RBPs with the G4 motif in 
the 5' UTR to promote cell proliferation during liver development and 
carcinogenesis.