Extracellular domain splice variants of a transforming protein tyrosine phosphatase ? mutant differentially activate Src-kinase dependent focus formation

Medical Clinic IV, Otfried-Müller Str.10, 72076 Tübingen, Germany.
Genes to Cells (Impact Factor: 2.81). 02/2007; 12(1):63-73. DOI: 10.1111/j.1365-2443.2006.01034.x
Source: PubMed


The extracellular domains of receptor-type protein-tyrosine phosphatases (PTPs) contain a diverse range of protein modules like fibronectin- or immunoglobulin-like structures. These are frequently expressed in a tissue- and development specific manner as splice variants. The extracellular domain of PTPalpha is rather short and heavily glycosylated. Two splice variants are known, which it differs by an exon encoding nine amino acids within the extracellular domain. We have analyzed the expression pattern of both variants and found that the smaller form is ubiquitously expressed while the larger form was found at an increased level only in brain, some skeletal muscle and differentiating cells like granule neurons, adipocytes and myotubes. The phosphatase activity of both forms was similar when tested in vitro using para-nitrophenylphosphate as a substrate and in a transient expression system with the substrates c-Fyn or c-Src. In a quantitative focus formation assay the capability of the larger form to activate Src-dependent focus formation in intact cells was increased more than twofold whereas the capability to dephosphorylate the insulin receptor in a BHK cell system was similar. We conclude that the two splice variants of PTPalpha are expressed differentially and regulate c-Src activity in different ways.

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Available from: Jörg B Schulz, Jan 19, 2015
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    • "on the biological system (see Supplementary data): Y789F mutation reduces but does not eliminate RPTPa-Src binding (Vacaru and den Hertog, 2010a), and some activation of Src in vivo can be detected when RPTPa(Y789F) is retrovirally expressed at high levels (Kapp et al, 2007). Additionally, third-party proteins may act in lieu of pTyr789 in some situations. "
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    ABSTRACT: Receptor protein tyrosine phosphatase α (RPTPα)-mediated Src activation is required for survival of tested human colon and oestrogen receptor-negative breast cancer cell lines. To explore whether mutated RPTPα participates in human carcinogenesis, we sequenced RPTPα cDNAs from five types of human tumours and found splice mutants in ∼30% of colon, breast, and liver tumours. RPTPα245, a mutant expressed in all three tumour types, was studied further. Although it lacks any catalytic domain, RPTPα245 expression in the tumours correlated with Src tyrosine dephosphorylation, and its expression in rodent fibroblasts activated Src by a novel mechanism. This involved RPTPα245 binding to endogenous RPTPα (eRPTPα), which decreased eRPTPα-Grb2 binding and increased eRPTPα dephosphorylation of Src without increasing non-specific eRPTPα activity. RPTPα245-eRPTPα binding was blocked by Pro210 → Leu/Pro211 → Leu mutation, consistent with the involvement of the structural 'wedge' that contributes to eRPTPα homodimerization. RPTPα245-induced fibroblast transformation was blocked by either Src or eRPTPα RNAi, indicating that this required the dephosphorylation of Src by eRPTPα. The transformed cells were tumourigenic in nude mice, suggesting that RPTPα245-induced activation of Src in the human tumours may have contributed to carcinogenesis.
    Preview · Article · Jul 2011 · The EMBO Journal
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    • "In agreement with previous results with PTPa793(Y789F) (Zheng et al. 2000), they found that overexpressed Tyr789 fi Phe mutants alone did not transform normal NIH3T3 cells (Lammers et al. 2000; Kapp et al. 2007). However, they found that both PTPa793(Y789F) and PTPa802 (Y798F) did induce foci in NIH3T3 cells that had been primed by over-expression of Src (Kapp et al. 2007). This is not surprising, because the reduced ability of the Tyr789 fi Phe mutants to activate Src (Zheng et al. 2000) was compensated by the over-expression of Src itself. "
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    ABSTRACT: Two isoforms of the transmembrane protein tyrosine phosphatase PTPalpha, which differ by nine amino acids in their extracellular regions, are expressed in a tissue-specific manner. Over-expression of the shorter isoform transforms rodent cells, and it has previously been reasonable to assume that this was a direct consequence of its dephosphorylation and activation of Src. Transformation by the longer wild-type isoform has not previously been studied. We tested the activities of both isoforms in NIH3T3 cells and found that, while both dephosphorylated and activated Src similarly, only the shorter isoform induced focus formation or anchorage-independent growth. Differences in phosphorylation of PTPalpha at its known regulatory sites, Grb2 binding to PTPalpha, phosphorylation level of focal adhesion kinase by PTPalpha, or overall localization were excluded as possible explanations for the differences in transforming activities. The results suggest that transformation by PTPalpha involves at least one function other than, or in addition to, its activation of Src and that this depends on PTPalpha's extracellular domain. Previous studies have suggested that PTPalpha might be a useful target in breast and colon cancer therapy, and the results presented here suggest that it may be advantageous to develop isoform-specific therapeutic reagents.
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