Study On The Molecular Mechanism And Biological Function Of THAP11-PCBP1 Interaction

THAP11 belongs to the newly identified THAP family which was characterized by the presence of an evolutionarily conserved motif. Many findings suggest that THAP proteins may function as sequence-specific DNA binding factors with roles in cell proliferation, apoptosis, chromatin modification and transcriptional regulation. Our previous results have demonstrated that THAP11 represses the expression of c-Myc by binding to the promoter sites of c-Myc,and then inhibits the cell growth.To explore the biologic functions and to discover binding partners of THAP11, our labs performed a yeast two-hybrid screening of a human liver cDNA library. PolyC-binding protein 1 (PCBP1) was identified as an interacting substrate for THAP11. PCBP1 belongs to hn RNPs, and contains a highly conserved triple repeat of the KH domain, which is a distinctive feature of RNA-binding protein. PCBP1 participates in mRNA alternative splicing to hint that THAP11 may be involved in the process.Firstly,using a yeast two-hybrid screening ,we managed to find the mapping of the interaction regions of THAP11 and PCBP1.In view of regulation of alternative splicing,recognized as increasingly important in causing human disease,was studied using the CD44 gene,whose splice variants have been implicated in tumor progression,we investigated the role of the two in mRNA splicing in depth.Overexpression of PCBP1 supresses inclusion of the endogenous and exogenous CD44v, while THAP11 shows relative exon specific repression on v3, v5,v6,v8 in a PCBP1-dependent manner,which are correlating with tumor. In addition,transient overexpression of THAP11 reverses Ras-dependent activation of CD44 v5 and v6 exon inclusion,leading to the inhibition of tumor cell invasive activtity.In contrast,siRNA-mediated depletion of endogenous THAP11 upregulates the endogenous levels of CD44 isoforms ,including v5,and correlates with a increase in tumor cell invasiveness.Importantly,we detected that the expression of CD44 v6 was negatively correlated with THAP11 in clinical human liver cancer tissues by analysis of Real-Time Quantitative Polymerase Chain Reaction. In summary, the results suggest that THAP11 can prevent tumor invasion and metastasis by repressing the alternative splicing of CD44, and also the interaction with PCBP1 plays an important role in exhibiting its biological effect.In addition,THAP11/PCBP1 affects the transcriptional activity of NF-κB and the promoter activity of its downstream targeted gene IL-6.The mechanism is not related with affecting the DNA-binding activity of NF-κB.These researches reveal a novel function for THAP11 to regulate mRNA alternative splicing, and give a clue to explore the new functions of THAP proteins.Meanwhile, deepening our knows that THAP11 may be a potential candidate for tumor suppressor,can regulate the malignant phenotypes of tumor cells.