Polypyrimidine Tract-Binding Protein Enhances the Internal Ribosomal Entry Site-Dependent Translation of p27Kip1 mRNA and Modulates Transition from G1 to S Phase.

The p27Kip1 protein plays a critical role in the regulation of cell proliferation through the inhibition of cyclin-dependent kinase activity. Translation of p27Kipl is directed by an internal ribosomal entry site (IRES) in the 5' nontranslated region of p27Kipl mRNA. Here, we report that polypyrimidine tract-binding protein (PTB) specifically enhances the IRES activity of p27Kipl mRNA through an interaction with the IRES element. We found that addition of PTB to an in vitro translation system and overexpression of PTB in 293T cells augmented the IRES activity of p27Kipl mRNA but that knockdown of PTB by introduction of PTB-specific small interfering RNAs (siRNAs) diminished the IRES activity of p27Kipl mRNA. Moreover, the G1 phase in the cell cycle (which is maintained in part by p27Kipl was shortened in cells depleted of PTB by siRNA knockdown. 12-O-Tetradecanoylphorbol-13oacetate (TPA)-induced differentiation in HL60 cells was used to examine PTB-induced modulation of p27Kipl protein synthesis during differentiation. The IRES activity of p27Kipl mRNA in HL60 cells was increased by TPA treatment (with a concomitant increase in PTB protein levels), but the levels of p27Kipl mRNA remained unchanged. Together, these data suggest that PTB modulates cell cycle and differentiation, at least in part, by enhancing the IRES activity of p27Kipl mRNA. [ABSTRACT FROM AUTHOR]