Functional Study Of Receptor-like Protein Tyrosine Phosphatase PCP-2 And Signal Regulatory Protein α1 In Tumor Signal Transduction

The regulated phosphorylation of proteins on specific tyrosyl residues is involved in an increasing number of cellular signaling events including growth, differentiation, migration, and adhesion. In vivo, tyrosine phosphorylation is reversible and dynamic. Phosphorylaiton states are determined by the balance of opposing activities of protein tyrosine kinases (PTKs), which catalyze phosphate transfer, and protein tyrosine phosphotases (PTPs), which catalyze phosphate hydrolysis. Because many PTKs are oncogenes and growth factor receptors, most studies on tyrosine phosphorylation are focused on PTKs and the importance of PTPs has not been recognized until recent ten years.PCP-2, first identified by Wang H-Y et al in 1996, is a type IIb RPTP characterized by the presence of one Ig-like domain, four FNIII domains and an N-terminal MAM domain. Until now, four types of B phosphotases (PTP, PTP, PCP-2 and PTPp) have been reported. The intracellular segment of PCP-2 has two phosphatase domains and a relatively long juxtamembrane segment that is homologous to the conserved cytoplasmic domain of cadherins, which is essential for the complex formation with the intracellular catenins. In adherent junctions, the cytoplasmic domain of E-cadherin binds directly to P-catenin that, in turn, associates with a-catenin, which is thought to link cadherin complex to the actin filament network and mediates stable cell adhesion. In addition to its adhesive functions, P-catenin has also been found to serve as a key component in Wnt signaling. Signal transduction via p-catenin involves its posttranslational stabilization and passage into the neucleus, where it interacts with transcription factors of T cell factor/Iymphoid enhancer factor family to activate target genes involved in cell growth control and apoptosis such as c-myc and cyclin D1.There is increasing evidence to suggest that phosphorylation of tyrosyl residues in some components of the cadherin/catenin complex leads to loss of adhesive function and breakdown of adherens junction. Phosphorylation-dependent release of P-catenin from the cadherin complex not only regulates the integrity and function of the adhesion complex, but may also be an alternative mechanism for activating P-catenin signaling.In this study, we have detected a specific complex formation between PCP-2 and p-catenin using an in vivo binding assay. The association was independent of the tyrosine phophorylation state of p-catenin. Mutational analysis revealed that the juxtamembrane region of PCP-2 was sufficient for P-catenin binding. Co-transfection experiments with activated SrcY527F, p-catenin and PCP-2 or inactive, mutant PCP-2 indicate that PCP-2dephosphorylated p-catenin in vivo. In addition, PCP-2 can not only maintain the dephosphorylation state of P-catenin but also antagonize p-catenin/T cell factor (TCP) dependent transcriptional activity. This antagonistic effect is irrespective of the proteasome sensitivity of p-catenin but requires the p-catenin-binding sites and catalytic domains on PCP-2. Expression of PCP-2 in SW480 colon cancer cells results in decreased free pool of P-catenin and increased cadherin-catenin complex stability, with a consequent reduction in p-catenin/TCF reporter gene activity and c-myc cellular levels. SW480-PCP-2 cells also have a reduced proliferation rate, a reduced ability of migration and do not grow tumors in nude mice. Thus PCP-2 appears to play an important role in the maintenance of epithelial integrity, and a loss of its regulatory function may be an alternative mechanism for activating p-catenin signaling.