Prostate apoptosis response-4 mediates TGF-β-induced epithelial-to-mesenchymal transition

摘要

A growing body of evidence supports that the epithelial-to-mesenchymal transition (EMT), which occurs during cancer development and progression, has a crucial role in metastasis by enhancing the motility of tumor cells. Transforming growth factor- β (TGF- β ) is known to induce EMT in a number of cancer cell types; however, the mechanism underlying this transition process is not fully understood. In this study we have demonstrated that TGF- β upregulates the expression of tumor suppressor protein Par-4 (prostate apoptosis response-4) concomitant with the induction of EMT. Mechanistic investigations revealed that exogenous treatment with each TGF- β isoform upregulates Par-4 mRNA and protein levels in parallel levels of phosphorylated Smad2 and I κ B- α increase. Disruption of TGF- β signaling by using ALK5 inhibitor, neutralizing TGF- β antibody or phosphoinositide 3-kinase inhibitor reduces endogenous Par-4 levels, suggesting that both Smad and NF- κ B pathways are involved in TGF- β -mediated Par-4 upregulation. NF- κ B-binding sites in Par-4 promoter have previously been reported; however, using chromatin immunoprecipitation assay we showed that Par-4 promoter region also contains Smad4-binding site. Furthermore, TGF- β promotes nuclear localization of Par-4. Prolonged TGF- β 3 treatment disrupts epithelial cell morphology, promotes cell motility and induces upregulation of Snail, vimentin, zinc-finger E-box binding homeobox 1 and N-Cadherin and downregulation of Claudin-1 and E-Cadherin. Forced expression of Par-4, results in the upregulation of vimentin and Snail expression together with increase in cell migration. In contrast, small interfering RNA-mediated silencing of Par-4 expression results in decrease of vimentin and Snail expression and prevents TGF- β -induced EMT. We have also uncovered a role of X-linked inhibitor of apoptosis protein in the regulation of endogenous Par-4 levels through inhibition of caspase-mediated cleavage. In conclusion, our findings suggest that Par-4 is a novel and essential downstream target of TGF- β signaling and acts as an important factor during TGF- β -induced EMT.