ABSTRACT
The transmembrane protein-tyrosine phosphatase (PTP) DEP-1 (density-enhanced phosphatase) is a candidate tumor suppressor in the colon epithelium. We have explored the function of DEP-1 in colon epithelial cells by inducible re-expression in a DEP-1-deficient human colon cancer cell line. Density-enhanced phosphatase-1 re-expression led to profound inhibition of cell proliferation and cell migration, and was associated with cytoskeletal rearrangements. These effects were dependent on the PTP activity of DEP-1 as they were not observed with cells expressing the catalytically inactive DEP-1 C1239S variant. shRNA-mediated suppression of DEP-1 in a colon epithelial cell line with high endogenous DEP-1 levels enhanced proliferation, further supporting the antiproliferative function of DEP-1. Nutrients, which are considered to be chemoprotective with respect to colon cancer development, including butyrate, green tea and apple polyphenols, had the capacity to elevate transcription of endogenous DEP-1 mRNA and expression of DEP-1 protein. Upregulation of DEP-1 expression, and in turn inhibition of cell growth and migration may present a previously unrecognized mechanism of chemoprevention by nutrients.
Subject(s)
Adenocarcinoma/pathology , Adenoma/pathology , Anticarcinogenic Agents/pharmacology , Colon/cytology , Colonic Neoplasms/pathology , Epithelial Cells/drug effects , Neoplasm Proteins/physiology , Protein Tyrosine Phosphatases/physiology , Adenocarcinoma/enzymology , Adenoma/enzymology , Butyrates/pharmacology , Cell Division/drug effects , Cell Line, Tumor/cytology , Cell Line, Tumor/drug effects , Cell Line, Tumor/enzymology , Cell Movement/drug effects , Cells, Cultured/cytology , Cells, Cultured/drug effects , Cells, Cultured/enzymology , Chemokine CXCL12 , Chemokines, CXC/pharmacology , Colon/enzymology , Colonic Neoplasms/enzymology , Down-Regulation , Enzyme Induction/drug effects , Epithelial Cells/cytology , Epithelial Cells/enzymology , Flavonoids/pharmacology , Humans , Lysophospholipids/pharmacology , Malus/chemistry , Neoplasm Proteins/biosynthesis , Neoplasm Proteins/genetics , Phenols/pharmacology , Plant Extracts/pharmacology , Polyphenols , Protein Phosphatase 1 , Protein Tyrosine Phosphatases/biosynthesis , Protein Tyrosine Phosphatases/genetics , RNA Interference , RNA, Messenger/biosynthesis , RNA, Messenger/genetics , RNA, Neoplasm/biosynthesis , RNA, Neoplasm/genetics , RNA, Small Interfering/pharmacology , Receptor-Like Protein Tyrosine Phosphatases, Class 3 , Recombinant Fusion Proteins/biosynthesis , Recombinant Fusion Proteins/genetics , Tea/chemistry , Transcription, Genetic/drug effects , Transfection , Up-Regulation/drug effectsABSTRACT
Ligand stimulation of PDGF beta-receptors leads to autophosphorylation of the regulatory tyrosine 857 and of tyrosine residues that in their phosphorylated form serve as docking sites for Src homology 2 domain-containing proteins. Regulation of the PDGF beta-receptor by protein-tyrosine phosphatases is poorly understood. We have investigated PDGF beta-receptor dephosphorylation by receptor-like protein-tyrosine phosphatase DEP-1 using a cell line with inducible DEP-1 expression and by characterizing in vitro dephosphorylation of the PDGF beta-receptor and of receptor-derived phosphopeptides by DEP-1. After DEP-1 induction PDGF beta-receptor.DEP-1 complexes and reduced receptor tyrosine phosphorylation were observed. Phosphopeptide analysis of the PDGF beta-receptors from DEP-1-expressing cells and of the receptors dephosphorylated in vitro by DEP-1 demonstrated that dephosphorylation of autophosphorylation sites of the receptor differed and revealed that the regulatory Tyr(P)(857) was not a preferred site for DEP-1 dephosphorylation. When dephosphorylation of synthetic receptor-derived peptides was analyzed, the selectivity was reproduced, indicating that amino acid sequence surrounding the phosphorylation sites is the major determinant of selectivity. This notion is supported by the observation that the poorly dephosphorylated Tyr(P)(562) and Tyr(P)(857), in contrast to other analyzed phosphorylation sites, are surrounded by basic amino acid residues at positions -4 and +3 relative to the tyrosine residue. Our study demonstrates that DEP-1 dephosphorylation of the PDGF beta-receptor is site-selective and may lead to modulation, rather than general attenuation, of signaling.
