Down-modulation of keratin 8 phosphorylation levels by PRL-3 contributes to colorectal carcinoma progression

Phosphatase of regenerating liver-3 (PRL-3) is a member of the PRL protein tyrosine phosphatase family and has been proposed to promote the invasiveness and metastastic capability of colorectal cancers (CRCs); however, the underlying mechanisms and target molecules of PRL-3 protein remain unknown. On the basis of the biological significance of PRL-3 phosphatase activity confirmed by the catalytically inactive PRL-3 mutant (C104S) and a PRL-3 inhibitor in CRC-derived SW480 cells, we performed protein expression profiling to search for PRL-3-mediated effector proteins. By a comparative study of phosphorylated proteins that differentially expressed in wild type and C104S mutant PRL-3-transfected SW480 cells; the cytoskeletal intermediate filament keratin 8 (KRT8) was identified as a physiological PRL-3-interacting protein. Indeed, treatment with the PRL-3 inhibitor effectively suppressed the phosphorylation of KRT8 at S73 and S431. Moreover, we detected the physiological interaction between PRL-3 and KRT8 and their colocalization at cellular lamellipodias and ruffles in vivo. In CRC tissue samples, tumor cells with high PRL3 expression showed reduction or loss of phosphorylated KRT8 expression, particularly at the invasive front and in the liver metastases. In conclusion, our results indicate that PRL-3 may play an important role for the promotion of CRC cell migration and metastatic potential through direct KRT8 dephosphorylation. (C) 2008 Wiley-Liss, Inc.