Polypyrimidine Tract-Binding Protein Induces p19(Ink4d) Expression and Inhibits the Proliferation of H1299 Cells

The expression of polypyrimidine tract-binding protein (PTB) is up-regulated in many types of cancer. Here, we studied the role of PTB in the growth of non small cell lung cancer cells. Data showed that PTB overexpression inhibited the growth of H1299 cells at least by inhibiting DNA synthesis. Quantitative real-time PCR and Western blot analyses showed that PTB overexpression in H1299 cells specifically induced the expression of p19(Ink4d), an inhibitor of cyclin-dependent kinase 4. Repression of p19(Ink4d) expression partially rescued PTB-caused proliferation inhibition. PTB overexpression also inhibited the growth and induced the expression of p19(Ink4d) mRNA in A549 cells. However, Western blot analyses failed to detect the presence of p19(Ink4d) protein in A549 cells. To address how PTB induced p19(Ink4d) in H1299 cells, we showed that PTB might up-regulate the activity of p19(Ink4d) gene (CDKN2D) promoter. Besides, PTB lacking the RNA recognition motif 3 (RRM3) was less effective in growth inhibition and p19(Ink4d) induction, suggesting that RNA-binding activity of PTB plays an important role in p19(Ink4d) induction. However, immunoprecipitation of ribonuclearprotein complexes plus quantitative real-time PCR analyses showed that PTB might not bind p19(Ink4d) mRNA, suggesting that PTB overexpression might trigger the other RNA-binding protein(s) to bind p19(Ink4d) mRNA. Subsequently, RNA electrophoretic mobility-shift assays revealed a 300-base segment (designated as B2) within the 3′UTR of p19(Ink4d) mRNA, with which the cytoplasmic lysates of PTB-overexpressing cells formed more prominent complexes than did control cell lysates. Insertion of B2 into a reporter construct increased the expression of the chimeric luciferase transcripts in transfected PTB-overexpressing cells but not in control cells; conversely, overexpression of B2-containing reporter construct in PTB-overexpressing cells abolished the induction of p19(Ink4d) mRNA. In sum, we have shown that PTB plays as a negative regulator in H1299 cell proliferation at least by inducing p19(Ink4d) expression at transcriptional and post-transcriptional levels.