Identification and verification of PTPN3 as a novel biomarker in predicting cancer prognosis, immunity, and immunotherapeutic efficacy

BACKGROUND

The importance of protein tyrosine phosphatase non-receptor type 3 (PTPN3) in controlling multifaceted tumor cell behaviors throughout cancer development has received widespread attention. Nevertheless, little is known about the biological roles of PTPN3 in drug sensitivity, immunotherapeutic effectiveness, tumor immune microenvironment, and cancer prognosis.

METHODS

The Cancer Genome Atlas (TCGA) database's RNAseq data were used to examine the expression of PTPN3 in 33 different cancer types. In addition, immunohistochemistry (IHC) was performed to validate the expression of PTPN3 across various cancer types within our clinical cohorts. The features of PTPN3 alterations were demonstrated throughout the cBioPortal database. This study focused on examining the prognostic and clinicopathological importance of PTPN3 through the acquisition of clinical data from the TCGA database. The investigation of PTPN3's probable role in the tumor immune microenvironment was demonstrated by the application of CIBERSORT, ESTIMATE algorithms, and the TISIDB database. Using Spearman's rank correlation coefficient, the relationships between PTPN3 expression and tumor mutation burden (TMB) and microsatellite instability (MSI) were evaluated. To further investigate the putative biological activities and downstream pathways of PTPN3 in various cancers in humans, Gene Set Enrichment Analysis (GSEA) was carried out. In addition, an examination was conducted to explore the associations between PTPN3 and the effectiveness of PD-1/PD-L1 inhibitors, utilizing data extracted from the GEO database.

RESULTS

PTPN3 was abnormally expressed in multiple cancer types and was also strictly associated with the prognosis of cancer patients. IHC was used to investigate and confirm the various expression levels of PTPN3 in various malignancies, including breast cancer, lung cancer, sarcoma, and kidney renal clear cell carcinoma in our clinical cohorts. There is a high correlation between the levels of PTPN3 expression in different cancers and infiltrating immune cells, including mast cells, B cells, regulatory T cells, CD8 + T cells, macrophages, and dendritic cells. Infiltrating immune cells, such as regulatory T cells, CD8 + T cells, macrophages, B cells, dendritic cells, and mast cells, are strongly correlated with PTPN3 expression levels in various tumors. The expression of PTPN3 exhibited a substantial correlation with many immune-related biomolecules and the expression of TMB and MSI in multiple types of cancer. In addition, PTPN3 has demonstrated promise in predicting the therapeutic benefits of PD-1/PD-L1 inhibitors and the susceptibility to anti-cancer medications in the treatment of clinical cancer.

CONCLUSIONS

Our findings highlight the importance of PTPN3 as a prognostic biomarker and predictor of immunotherapy success in various forms of cancer. Furthermore, PTPN3 appears to have an important role in modifying the tumor immune microenvironment, highlighting its potential as a promising biomarker for prognosis prediction, immunotherapeutic efficacy evaluation, and identification of immune-related characteristics in diverse cancer types.

PTPH1 promotes tumor growth and metastasis in human glioma

OBJECTIVE

Glioma is the most common form of brain tumor, accounting for over 50% of all primary tumors. Despite progress in the treatment of glioma, the prognosis is still poor. In this study, we examined protein-tyrosine phosphatase H1 (PTPH1) in human gliomas.

MATERIALS AND METHODS

Cell growth potential was measured by CCK-8 assay and colony formation. Cell cycle distribution was measured by flow cytometry. Transwell assay was used to detect the motility of tumor cells. Real-time PCR and Western blot were used to measure the mRNA and protein expression of indicated genes. Xenograft model was established to measure the role of PTPH1 in vivo.

RESULTS

The expression of PTPH1 was significantly higher in the tumor tissues as compared with that in the adjacent normal tissues. Knockdown of PTPH1 significantly slowed cell proliferation and reduced colony formation abilities in glioma cell lines U87 and U251. Additionally, knockdown of PTPH1 caused cell cycle arrest in the S-phase. Furthermore, depletion of PTPH1 in glioma U87 cells significantly limited tumor growth in a xenograft model. Interestingly, knockdown of PTPH1 also decreased cell migration abilities in both U87 and U251 cells. Accordingly, matrix metalloproteinase 9 (MMP9) was also decreased upon knockdown of PTPH1 in both cell lines. Moreover, we found that phosphorylated MEK (p-MEK) and phosphorylated MAPK (p-MAPK) were both decreased, whereas the total levels of MEK and MAPK remained unchanged after depletion of PTPH1 in both cell lines.

CONCLUSIONS

Our data suggest that PTPH1 may be a novel biomarker that indicates the aggressiveness of gliomas. Targeting PTPH1 might be a promising strategy for the treatment of gliomas.

Generation of inhibitor-sensitive protein tyrosine phosphatases via active-site mutations

Protein tyrosine phosphatases (PTPs) catalyze the dephosphorylation of phosphotyrosine, a central control element in mammalian signal transduction. Small-molecule inhibitors that are specific for each cellular PTP would be valuable tools in dissecting phosphorylation networks and for validating PTPs as therapeutic targets. However, the common architecture of PTP active sites impedes the discovery of selective PTP inhibitors. Our laboratory has recently used enzyme/inhibitor-interface engineering to generate selective PTP inhibitors. The crux of the strategy resides in the design of "inhibitor-sensitized" PTPs through protein engineering of a novel binding pocket in the target PTP. "Allele-specific" inhibitors that selectively target the sensitized PTP can be synthesized by modifying broad-specificity inhibitors with bulky chemical groups that are incompatible with wild-type PTP active sites; alternatively, specific inhibitors that serendipitously recognize the sensitized PTP's non-natural pocket may be discovered from panels of "non-rationally" designed compounds. In this review, we describe the current state of the PTP-sensitization strategy, with emphases on the methodology of identifying PTP-sensitizing mutations and synthesizing the compounds that have been found to target PTPs in an allele-specific manner. Moreover, we discuss the scope of PTP sensitization in regard to the potential application of the approach across the family of classical PTPs.

Normal TCR signal transduction in mice that lack catalytically active PTPN3 protein tyrosine phosphatase

PTPN3 (PTPH1) is a cytoskeletal protein tyrosine phosphatase that has been implicated as a negative regulator of early TCR signal transduction and T cell activation. To determine whether PTPN3 functions as a physiological negative regulator of TCR signaling in primary T cells, we generated gene-trapped and gene-targeted mouse strains that lack expression of catalytically active PTPN3. PTPN3 phosphatase-negative mice were born in expected Mendelian ratios and exhibited normal growth and development. Furthermore, numbers and ratios of T cells in primary and secondary lymphoid organs were unaffected by the PTPN3 mutations and there were no signs of spontaneous T cell activation in the mutant mice with increasing age. TCR-induced signal transduction, cytokine production, and proliferation was normal in PTPN3 phosphatase-negative mice. This was observed using both quiescent T cells and recently stimulated T cells where expression of PTPN3 is substantially up-regulated. We conclude, therefore, that the phosphatase activity of PTPN3 is dispensable for negative regulation of TCR signal transduction and T cell activation.

Degradation of tyrosine phosphatase PTPN3 (PTPH1) by association with oncogenic human papillomavirus E6 proteins

Oncoproteins from DNA tumor viruses associate with critical cellular proteins to regulate cell proliferation, survival, and differentiation. Human papillomavirus (HPV) E6 oncoproteins have been previously shown to associate with a cellular HECT domain ubiquitin ligase termed E6AP (UBE3A). Here we show that the E6-E6AP complex associates with and targets the degradation of the protein tyrosine phosphatase PTPN3 (PTPH1) in vitro and in living cells. PTPN3 is a membrane-associated tyrosine phosphatase with FERM, PDZ, and PTP domains previously implicated in regulating tyrosine phosphorylation of growth factor receptors and p97 VCP (valosin-containing protein, termed Cdc48 in Saccharomyces cerevisiae) and is mutated in a subset of colon cancers. Degradation of PTPN3 by E6 requires E6AP, the proteasome, and an interaction between the carboxy terminus of E6 and the PDZ domain of PTPN3. In transduced keratinocytes, E6 confers reduced growth factor requirements, a function that requires the PDZ ligand of E6 and that can in part be replicated by inhibiting the expression of PTPN3. This report demonstrates the potential of E6 to regulate phosphotyrosine metabolism through the targeted degradation of a tyrosine phosphatase.

Cardiac sodium channel Na(v)1.5 interacts with and is regulated by the protein tyrosine phosphatase PTPH1

In order to identify proteins interacting with the cardiac voltage-gated sodium channel Na(v)1.5, we used the last 66 amino acids of the C-terminus of the channel as bait to screen a human cardiac cDNA library. We identified the protein tyrosine phosphatase PTPH1 as an interacting protein. Pull-down experiments confirmed the interaction, and indicated that it depends on the PDZ-domain binding motif of Na(v)1.5. Co-expression experiments in HEK293 cells showed that PTPH1 shifts the Na(v)1.5 availability relationship toward hyperpolarized potentials, whereas an inactive PTPH1 or the tyrosine kinase Fyn does the opposite. The results of this study suggest that tyrosine phosphorylation destabilizes the inactivated state of Na(v)1.5.

PTPN3 and PTPN4 tyrosine phosphatase expression in human gastric adenocarcinoma

BACKGROUND

Degenerated PCR primers, designed according to the consensus tyrosine phosphatase catalytic motifs, were used in order to amplify expressed protein-tyrosine phosphatase molecules from human gastric cancer-derived cells. From such profiles, more than twenty different types of tyrosine phosphatase were identified from gastric cancer tissue. A non-receptor tyrosine phosphatase, PTPN4, was found to be expressed in a tumor-tissue profile with only low frequency. The most closely-related gene to tyrosine phosphatase, PTPN3, has been shown to be mutated in cases of human colorectal cancer, but its expression is cases of gastric cancer is not known.

MATERIALS AND METHODS

The mRNA expression of PTPN3 and PTPN4 by RT-PCR was investigated, and the protein expression status of PTPN3 was examined, using immunohistochemistry, to elucidate clinicopathological associations of the PTPN3 and PTPN4 family within human stomach cancer.

RESULTS

PTPN3 and PTPN4 were expressed in all gastric cancer cell lines and clinical cancer tissue specimens examined. Following the examination of 92 gastric cancer patients' pathological specimens, PTPN3 expression showed no statistical significance with respect to the patients' survival. A statistically significant correlation between PTPN3 staining and the differentiation status of gastric cancer tissue was, however, observed.

CONCLUSION

This finding indicates that both PTPN3 and PTPN4 are expressed within human gastric cancer cells and that PTPN3 seems to play an important role in gastric cancer differentiation and the progression of malignancy.

Mutational analysis of the tyrosine phosphatome in colorectal cancers

Tyrosine phosphorylation, regulated by protein tyrosine phosphatases (PTPs) and kinases (PTKs), is important in signaling pathways underlying tumorigenesis. A mutational analysis of the tyrosine phosphatase gene superfamily in human cancers identified 83 somatic mutations in six PTPs (PTPRF, PTPRG, PTPRT, PTPN3, PTPN13, PTPN14), affecting 26% of colorectal cancers and a smaller fraction of lung, breast, and gastric cancers. Fifteen mutations were nonsense, frameshift, or splice-site alterations predicted to result in truncated proteins lacking phosphatase activity. Five missense mutations in the most commonly altered PTP (PTPRT) were biochemically examined and found to reduce phosphatase activity. Expression of wild-type but not a mutant PTPRT in human cancer cells inhibited cell growth. These observations suggest that the mutated tyrosine phosphatases are tumor suppressor genes, regulating cellular pathways that may be amenable to therapeutic intervention.

Residue 182 influences the second step of protein-tyrosine phosphatase-mediated catalysis

Previous enzyme kinetic and structural studies have revealed a critical role for Asp181 (PTP1B numbering) in PTP (protein-tyrosine phosphatase)-mediated catalysis. In the E-P (phosphoenzyme) formation step, Asp181 functions as a general acid, while in the E-P hydrolysis step it acts as a general base. Most of our understanding of the role of Asp181 is derived from studies with the Yersinia PTP and the mammalian PTP1B, and to some extent also TC (T-cell)-PTP and the related PTPa and PTPe. The neighbouring residue 182 is a phenylalanine in these four mammalian enzymes and a glutamine in Yersinia PTP. Surprisingly, little attention has been paid to the fact that this residue is a histidine in most other mammalian PTPs. Using a reciprocal single-point mutational approach with introduction of His182 in PTP1B and Phe182 in PTPH1, we demonstrate here that His182-PTPs, in comparison with Phe182-PTPs, have significantly decreased kcat values, and to a lesser degree, decreased kcat/Km values. Combined enzyme kinetic, X-ray crystallographic and molecular dynamics studies indicate that the effect of His182 is due to interactions with Asp181 and with Gln262. We conclude that residue 182 can modulate the functionality of both Asp181 and Gln262 and therefore affect the E-P hydrolysis step of PTP-mediated catalysis.

PTPH1 is a predominant protein-tyrosine phosphatase capable of interacting with and dephosphorylating the T cell receptor zeta subunit

Protein-tyrosine phosphatases (PTPases) play key roles in regulating tyrosine phosphorylation levels in cells, yet the identity of their substrates remains limited. We report here on the identification of PTPases capable of dephosphorylating the phosphorylated immune tyrosine-based activation motifs present in the T cell receptor zeta subunit. To characterize these PTPases, we purified enzyme activities directed against the phosphorylated T cell receptor zeta subunit by a combination of anion and cation chromatography procedures. A novel ELISA-based PTPase assay was developed to rapidly screen protein fractions for enzyme activity following the various chromatography steps. We present data that SHP-1 and PTPH1 are present in highly enriched protein fractions that exhibit PTPase activities toward a tyrosine-phosphorylated TCR zeta substrate (specific activity ranging from 0.23 to 40 pmol/min/microg). We also used a protein-tyrosine phosphatase substrate-trapping library comprising the catalytic domains of 47 distinct protein-tyrosine phosphatases, representing almost all the tyrosine phosphatases identified in the human genome. PTPH1 was the predominant phosphatase capable of complexing phospho-zeta. Subsequent transfection assays indicated that SHP-1 and PTPH1 are the two principal PTPases capable of regulating the phosphorylation state of the TCR zeta ITAMs, with PTPH1 directly dephosphorylating zeta. This is the first reported demonstration that PTPH1 is a candidate PTPase capable of interacting with and dephosphorylating TCR zeta.

Evidence for regulation of the tumor necrosis factor alpha-convertase (TACE) by protein-tyrosine phosphatase PTPH1

Tumor necrosis factor alpha-convertase (TACE) is a metalloprotease-disintegrin involved in the ectodomain shedding of several proteins and is critical for proper murine development. TACE-mediated ectodomain shedding is regulated, and the cytoplasmic domain of TACE contains several potential signaling motifs, suggesting that this domain may play a role in regulating the metalloprotease activity. Here we report that the protein-tyrosine phosphatase PTPH1, which contains both a band 4.1 domain and a single PDZ domain, can interact with the cytoplasmic domain of TACE. The interaction was initially observed in a yeast two-hybrid screen and was confirmed using an in vitro binding assay and co-immunoprecipitations from eukaryotic cell extracts. The interaction is mediated via binding of the PDZ domain of PTPH1 to the COOH terminus of TACE. The latter represents a novel group I PDZ binding sequence characterized by a terminal cysteine residue. In co-expression experiments, significantly lower levels of TACE were observed in the presence of catalytically active forms of PTPH1 compared with catalytically inactive forms of PTPH1. Furthermore, phorbol ester-stimulated shedding of the TACE substrate tumor necrosis factor-alpha was decreased in cells expressing catalytically active PTPH1 compared with inactive PTPH1. Taken together, these results suggest that PTPH1 may be a negative regulator of TACE levels and function, and thus provide the first evidence for the regulation of TACE through a cytoplasmic protein.

Expression of protein tyrosine phosphatases and its significance in esophageal cancer

Expression of mRNA protein tyrosine phosphatases (PTPs) was surveyed in an esophageal cancer cell line by RT-PCR using degenerate primers. The mRNAs for eight kinds of PTPs were expressed in the cell line. We examined mRNA expression of these PTPs in 12 cases of esophageal cancer by Northern analysis. Significant signals were obtained for three kinds of PTPs, PTP1B, PTPH1, and PTPD1. The magnitude of expression of each PTP was measured as the ratio of the signal intensity of each PTP to that of a control gene (NADPH), and the ratio was then compared to normal mucosa around the cancer lesion. Among the three kinds of PTPs, the expression of PTP1B mRNA was significantly depressed in cancer lesions compared with that in the surrounding normal mucosa. In contrast, the expression of PTPH1 mRNA was significantly increased in cancer lesions compared with that in normal mucosa. PTPD1 did not show any significant trend in comparisons of cancer and surrounding normal mucosa. The results suggest that PTP1B and PTPH1 are engaged in opposing signaling pathways, the tumor-suppressive and tumor-promoting pathways, respectively, in esophageal carcinogenesis.

Cytoskeletal protein tyrosine phosphatase PTPH1 reduces T cell antigen receptor signaling

The subgroup of protein tyrosine phosphatases that contain an N-terminal ezrin-, radixin- and moesin homology (ERM) domain and a C-terminal catalytic domain is represented by three enzymes in Jurkat T cells, PTPH1, PTP-MEG1 and PTP36. These enzymes are located at the cytoplasmic face of the plasma membrane and may be involved in regulation of the membrane cytoskeleton, signal transduction, or both. Here we report that expression of PTPH1 in Jurkat T cells reduced the TCR-induced activation of reporter genes encompassing parts of the IL-2 gene promoter and driven by nuclear factor of activated T cells plus activator protein-1. PTP-MEG1 had a weaker inhibitory effect, while PTP36 had none. The catalytically inactive mutants PTPH1-CS and PTP-MEG1-CS lacked effects on gene transcription. Expression of active PTPH1 also reduced receptor-induced activation of Erk2 MAP kinase, its upstream activator, Mek, and the Jnk kinases. The effect of PTPH1 was reduced by deletion of its N-terminal ERM domain. We suggest that PTPH1 inhibits T cell activation by dephosphorylating membrane-associated targets involved in TCR signaling.

Identification of the cell cycle regulator VCP (p97/CDC48) as a substrate of the band 4.1-related protein-tyrosine phosphatase PTPH1

The human band 4.1-related protein-tyrosine phosphatase PTPH1 was introduced into NIH3T3 cells under the control of a tetracycline-repressible promoter. Ectopic expression of wild type PTPH1 dramatically inhibited cell growth, whereas a catalytically impaired mutant showed no effect. To identify the direct target of PTPH1 in the cell, we generated a substrate-trapping mutant, in which an invariant aspartate residue was changed to alanine (D811A in PTPH1). The PTPH1-D811A mutant trapped primarily a 97-kDa tyrosine-phosphorylated protein, which was determined to be VCP (also named p97 or yeast CDC48), from various cell lysates in vitro. However, when expressed in mammalian cells, the D811A mutant was observed to contain high levels of phosphotyrosine and did not trap substrates. Mutation of tyrosine 676 to phenylalanine (Y676F) in the PTPH1-D811A mutant led to a marked reduction in phosphotyrosine content. Furthermore, this double mutant specifically trapped VCP in vivo and recognized the C-terminal tyrosines of VCP, whose phosphorylation is important for cell cycle progression in yeast. Like wild type PTPH1, this double mutant also inhibited cell proliferation. Moreover, induction of wild type PTPH1 resulted in specific dephosphorylation of VCP without changing the overall phosphotyrosine profile of the cells. VCP has been implicated in control of a variety of membrane functions, including membrane fusions, and is a regulator of the cell cycle. Our results suggest that PTPH1 may exert its effects on cell growth through dephosphorylation of VCP, thus implicating tyrosine phosphorylation as an important regulator of VCP function.

Serine phosphorylation-dependent association of the band 4.1-related protein-tyrosine phosphatase PTPH1 with 14-3-3beta protein

PTPH1 is a human protein-tyrosine phosphatase with homology to the band 4.1 superfamily of cytoskeletal-associated proteins. PTPH1 was found to associate with 14-3-3beta using a yeast two-hybrid screen, and its interaction could be reconstituted in vitro using recombinant proteins. Examination of the interaction between 14-3-3beta and various deletion mutants of PTPH1 by two-hybrid tests suggested that the integrity of the PTP is important for this binding. Although both PTPH1 and Raf-1 form complexes with 14-3-3beta, they appear to do so independently. Binding of 14-3-3beta to PTPH1 in vitro was abolished by pretreating PTPH1 with potato acid phosphatase and was greatly enhanced by pretreating with Cdc25C-associated protein kinase. Thus the association between PTPH1 and 14-3-3beta is phosphorylation-dependent. Two novel motifs RSLS359VE and RVDS853EP in PTPH1 were identified as major 14-3-3beta-binding sites, both of which are distinct from the consensus binding motif RSXSXP recently found in Raf-1. Mutation of Ser359 and Ser853 to alanine significantly reduced the association between 14-3-3beta and PTPH1. Furthermore, association of PTPH1 and 14-3-3beta was detected in several cell lines and was regulated in response to extracellular signals. These results raise the possibility that 14-3-3beta may function as an adaptor molecule in the regulation of PTPH1 and may provide a link between serine/threonine and tyrosine phosphorylation-dependent signaling pathways.

Biochemical characterization of a human band 4.1-related protein-tyrosine phosphatase, PTPH1

PTPH1 is a human protein-tyrosine phosphatase with homology to the band 4.1 superfamily of cytoskeleton-associated proteins. Here, we report the purification and biochemical characterization of this enzyme from baculovirus-infected insect cells. The purified protein exhibited an apparent M(r) of 120,000 on SDS gels. The native enzyme dephosphorylated both myelin basic protein (MBP) and reduced, carboxamidomethylated, and maleylated lysozyme (RCML) but was over 5-fold more active on MBP. The Km values for the two substrates were similar (1.45 microM for MBP and 1.6 microM for RCML). Phosphorylation of PTPH1 by protein kinase C in vitro resulted in a decrease in Km but had no effect on Vmax. Removal of the NH2-terminal band 4.1 homology domain of PTPH1 by limited trypsin cleavage stimulated dephosphorylation of RCML but inhibited its activity toward MBP. The dephosphorylation of RCML by full-length PTPH1 was enhanced up to 6-fold by unphosphorylated MBP and increasing ionic strength up to 0.2 M NaCl, whereas trypsinized preparations of PTPH1 containing the isolated catalytic domain were unaffected. These results suggest that in addition to a potential role in controlling subcellular localization, the NH2-terminal band 4.1 homology domain of PTPH1 may exert a direct effect on catalytic function.

Expression of PTPH1, a rat protein tyrosine phosphatase, is restricted to the derivatives of a specific diencephalic segment

Studies to date have identified only a few proteins that are expressed in a segment-specific manner within the mammalian brain. Here we report that a nonreceptor protein tyrosine phosphatase, PTPH1, is selectively expressed in the adult thalamus. Expression of PTPH1 mRNA is detected in most, but not all, thalamic nuclei. Nuclei that are derived embryonically from the dorsal thalamus and project to the neocortex express this gene, whereas those derived from the ventral thalamus do not. PTPH1 mRNA expression is also restricted to the dorsal thalamus during development and, thus, can serve as a specific marker for the dorsal thalamic nuclei. Since the subcellular localization of PTPH1 protein is not known, its functional role is not clear. However, the restriction of its expression to the thalamic nuclei that have thalamocortical connections suggests that PTPH1 may play a role in the maintenance of these connections or in determining the physiological properties of thalamic relay nuclei.

Expression of cytoskeletal-associated protein tyrosine phosphatase PTPH1 mRNA in human hepatocellular carcinoma

We investigated the mRNA expression of cytosolic protein tyrosine phosphatase (PTPH1), which has a homologous domain to cytoskeletal-associated proteins, in human hepatocellular carcinomas (HCCs) by using reverse transcriptase-polymerase chain reaction (RT-PCR). PTPH1 mRNA was detected in all HCC cell lines (n = 6), and HCC and adjacent noncancerous tissues (n = 8) examined, indicating that PTPH1 was expressed in HCCs and hepatocytes. There was no remarkable difference in the level expression of PTPH1 mRNA between HCC and adjacent noncancerous tissues. We also performed RT-PCR single-strand conformation polymorphism (SSCP) analysis in HCC cell lines and tissues in the C-terminal region of the catalytic domain of PTPH1. In the cHc4 cell line and a HCC tissue specimen, a shifted band was detected, although it was not found in the non-cancerous tissue of the HCC specimen. Nucleotide sequence analysis showed a common mutation from T to C at the third letter of codon 919 which did not lead to amino acid substitution. These results suggest that another mutation leading to the development of HCC could occur in some region of PTPH1 other than that investigated in this study.

Src kinase associates with a member of a distinct subfamily of protein-tyrosine phosphatases containing an ezrin-like domain

A 6.2-kb full-length clone encoding a distinct protein-tyrosine phosphatase (PTP; EC 3.1.3.48), PTPD1, was isolated from a human skeletal muscle cDNA library. The cDNA encodes a protein of 1174 amino acids with N-terminal sequence homology to the ezrin-band 4.1-merlin-radixin protein family, which also includes the two PTPs H1 and MEG1. The PTP domain is positioned in the extreme C-terminal part of PTPD1, and there is an intervening sequence of about 580 residues without any apparent homology to known proteins separating the ezrin-like and the PTP domains. Thus, PTPD1 and the closely related, partially characterized, PTPD2 belong to the same family as PTPH1 and PTPMEG1, but because of distinct features constitute a different PTP subfamily. Northern blot analyses indicate that PTPD1 and PTPD2 are expressed in a variety of tissues. In transient coexpression experiments PTPD1 was found to be efficiently phosphorylated by and associated with the src kinase pp60src.

Expression and chromosomal assignment of PTPH1 gene encoding a cytosolic protein tyrosine phosphatase homologous to cytoskeletal-associated proteins

We have investigated the mRNA expression of 2 human protein tyrosine phosphatases with sequence homology to cytoskeletal proteins, PTPH1 and PTPMEG. Northern-blot analysis of PTPH1 using poly (A)+ RNA from normal human colon tissue showed a low-abundance message of 4.3 kb. Reverse-transcriptase/polymerase-chain reaction (RT-PCR) was therefore used to detect it in a wide variety of cell lines including 9 colorectal, 5 gastric, 5 hepatic and 6 hematopoietic tumor cells. PTPH1 mRNA was not detected only in Colo 320 cells over-expressing c-myc mRNA, among the colorectal cancer cell lines examined. When Colo 320 cells were incubated with 5 mM sodium butyrate for 5 days, PTPH1 mRNA became detectable, concomitant with the marked decrease in the expression level of c-myc mRNA. Moreover, the chromosomal localization of PTPH1 gene was investigated by fluorescence in situ hybridization. Interestingly, PTPH1 gene was mapped to 9q31 where the gene for Gorlin syndrome, a putative tumor suppressor gene, exists.

Isolation of a cDNA clone encoding a human protein-tyrosine phosphatase with homology to the cytoskeletal-associated proteins band 4.1, ezrin, and talin

The polymerase chain reaction (PCR), from primers corresponding to conserved sequences within the catalytic domains of the protein-tyrosine phosphatases, was used to amplify protein-tyrosine phosphatase-related cDNAs from a HeLa cell library. After probing the same cDNA library with one of the PCR products, 10 positive clones were identified. The longest of these clones (3984 base pairs) contained 2739 base pairs of open reading frame and, after a stop codon, a 3' nontranslated segment of 1222 base pairs. A 4.3-kilobase transcript was detected by Northern blot analysis of HeLa cell poly(A)+ RNA. The open reading frame predicts a protein of 913 amino acids (approximately 104 kDa), termed PTPH1. The sequence of PTPH1 can be described in terms of three segments. (i) The N-terminal segment displays homology to the domains in the cytoskeletal-associated proteins band 4.1, ezrin, and talin that direct their association with proteins at the interface between the plasma membrane and the cytoskeleton in structures such as focal adhesions. (ii) There is a central segment bearing putative phosphorylation sites for protein-serine/threonine kinases. (iii) A segment that is homologous to the members of the protein-tyrosine phosphatase family is located at the C terminus. The structure is discussed in the light of the potential role of PTPH1 in controlling cytoskeletal integrity and the possibility that overexpression of PTPH1 may reverse transformation induced by oncogenic protein-tyrosine kinases, such as the members of the src family.