The protein tyrosine phosphatase alpha modifies insulin secretion in INS-1E cells.
ABSTRACT Increasing evidence indicates a role of insulin signalling for insulin secretion from the pancreatic beta-cells. Therefore, regulators of insulin signalling, like protein tyrosine phosphatases, could also have an impact on insulin secretion. Here, we investigated a possible role of the negative regulator protein tyrosine phosphatase alpha (PTP alpha) for insulin secretion. RT-PCR analysis confirmed that both splice variants of the extracellular domain of PTP alpha that vary by an insert of 9 amino acids are expressed in human islets and insulinoma cells (INS-1E, RIN1046-38). Overexpression of the wild type PTP alpha splice variant containing the 9 amino acids reduced insulin secretion, as did a mutant form unable to bind Grb2 (Tyr798Phe). By contrast, overexpression of a phosphatase inactive mutant improved insulin secretion. These data reveal a functional relevance of PTP alpha for insulin secretion.
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ABSTRACT: We have identified the PTP Receptor-Type IV (PTPR4) family, including one form of PTPα and two forms of PTPε (PTPε M and PTPε C) in flounder. The existence of PTPε C was the first report in non-mammalian animals. Semi-quantitative RT-PCR showed independent expression patterns of the three forms. The sequence of PTPε C was identical to that of PTPε M except for its 5'-terminal regions. Southern blot analysis proved that there existed only one PTPε gene in the genome, indicating that the two isoforms of PTPε might have been derived from alternative splicing of this gene. Phylogenetic analysis also provided evidence that the gene duplication from the ancestor gene to PTPα and PTPε occurred before the divergence of Gnathastomata and Agnatha. These results showed that the functional evolution of protein phosphorylation is promoted by not only genome duplication, but also elaborate regulation of expression. INTRODUCTION Tyrosine phosphorylation, controlled by the coordinated actions of protein tyrosine phosphatases (PTPs) and protein tyrosine kinases (PTKs), is a critical mechanism for the regulation of numerous cell functions (Tonks and Neel, 1996; Mustelin et al., 2002). Since the first identification, PTP members have been identified in many organisms (Tonks et al., 1988; Fischer et al., 1991; Alonso et al., 2004). PTPR4 contain two cytoplasmic PTP domains (D1 and D2), one transmembrane segment and an extracellular domain. PTPα and PTPε are the only known members of PTPR4 (Sap et al., 1990; Matthews et al., 1990). The precise subcellular localization of PTPs is an important for regulating their physiological roles (Mauro and Dixon, 1994; Fischer, 1999). PTPε includes four forms of proteins coded by a single PTPε gene. The two most prevalent forms are the transmembrane form (PTPε M) and cytoplasmic form (PTPε C) derived from alternatively splicing of PTPε gene (Elson and Leder, 1995b; Elson et al., 1996). The other two forms p67 PTPε and p65 PTPε are produced by initiation of translation of PTPε mRNA, and specific proteolytic cleavage of larger PTPε proteins, respectively (Gil-Henn et al., 2001; Kraut et al., 2002). Each form possesses unique expression patterns, subcellular localizations, and functions. In teleost fish, Okubo and Aida (2003) isolated and characterized PTPα and PTPε M in medaka, and demonstrated that GnRH down regulates their expression. Van der Sar et al (2001) cloned PTPα and PTPε M in zebrafish and described their expression during development. However, none of cytoplasmic form of PTPε was identified in teleost fish. Here we report the cDNAs of PTPα and two forms of PTPε, from Japanese flounder. We proved that these two isoforms of PTPε are also from one PTPε gene by alternative mRNA splicing. The unique expression of PTPα and two isoforms of PTPε mRNAs in various tissues are described.Genes & Genetic Systems - GENES GENET SYST. 01/2008; 83(2).
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ABSTRACT: Genetic predisposition and environmental influences insidiously converge to cause glucose intolerance and hyperglycemia. Beta-cell compensates by secreting more insulin and when it fails, overt diabetes mellitus ensues. The need to understand the mechanisms involved in insulin secretion cannot be stressed enough. Phosphorylation of proteins plays an important role in regulating insulin secretion. In order to understand how a particular cellular process is regulated by protein phosphorylation the nature of the protein kinases and protein phosphatases involved and the mechanisms that determine when and where these enzymes are active should be investigated. While the actions of protein kinases have been intensely studied within the beta-cell, less emphasis has been placed on protein phosphatases even though they play an important regulatory role. This review focuses on the importance of protein phosphatase 2A in insulin secretion. Most of the present knowledge on protein phosphatase 2A originates from protein phosphatase inhibitor studies on islets and beta-cell lines. The ability of protein phosphatase 2A to change its activity in the presence of glucose and inhibitors provides clues to its role in regulating insulin secretion. An aggressive approach to elucidate the substrates and mechanisms of action of protein phosphatases is crucial to the understanding of phosphorylation events within the beta-cell. Characterizing protein phosphatase 2A within the beta-cell will certainly help us in understanding the mechanisms involved in insulin secretion and provide valuable information for drug development.JOP: Journal of the pancreas 08/2005; 6(4):303-15.
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ABSTRACT: Receptor protein tyrosine phosphatase alpha (RPTPalpha) is the mitotic activator of the protein tyrosine kinase Src. RPTPalpha serine hyperphosphorylation was proposed to mediate mitotic activation of Src. We raised phosphospecific antibodies to the two main serine phosphorylation sites, and we discovered that RPTPalpha Ser204 was almost completely dephosphorylated in mitotic NIH 3T3 and HeLa cells, whereas Ser180 and Tyr789 phosphorylation were only marginally reduced in mitosis. Concomitantly, Src pTyr527 and pTyr416 were dephosphorylated, resulting in 2.3-fold activation of Src in mitosis. Using inhibitors and knockdown experiments, we demonstrated that dephosphorylation of RPTPalpha pSer204 in mitosis was mediated by PP2A. Mutation of Ser204 to Ala did not activate RPTPalpha, and intrinsic catalytic activity of RPTPalpha was not affected in mitosis. Interestingly, binding of endogenous Src to RPTPalpha was induced in mitosis. GRB2 binding to RPTPalpha, which was proposed to compete with Src binding to RPTPalpha, was only modestly reduced in mitosis, which could not account for enhanced Src binding. Moreover, we demonstrate that Src bound to mutant RPTPalpha-Y789F, lacking the GRB2 binding site, and mutant Src with an impaired Src homology 2 (SH2) domain bound to RPTPalpha, illustrating that Src binding to RPTPalpha is not mediated by a pTyr-SH2 interaction. Mutation of RPTPalpha Ser204 to Asp, mimicking phosphorylation, reduced coimmunoprecipitation with Src, suggesting that phosphorylation of Ser204 prohibits binding to Src. Based on our results, we propose a new model for mitotic activation of Src in which PP2A-mediated dephosphorylation of RPTPalpha pSer204 facilitates Src binding, leading to RPTPalpha-mediated dephosphorylation of Src pTyr527 and pTyr416 and hence modest activation of Src.Molecular and cellular biology 04/2010; 30(12):2850-61. · 6.06 Impact Factor