Differential expression of a novel murine non-receptor protein tyrosine phosphatase during differentiation of P19 embryonal carcinoma cells

The optimization of extraction process of white mulberry leaves and the characteristic bioactive properties its powder extract

Introduction: Powder extracts from white mulberry leaves (Morus alba L.) are desirable due to their simplicity of use. Powders intended for use in food and dietary supplements can be obtained by various methods, including water extraction and spray drying.

Objective: The aim of the work was to optimize the aqueous process of extraction of white mulberry leaves and characterization of bioactive properties of the obtained extract powder.

Methods: The DNJ and flavonoids (quercetin, kaempferol) contents and were analysed by high performance liquid chromatography (HPLC). Total phenolic contents were determined using the Folin-Ciocalteu phenol reagent and antioxidant activity by ABTS and DPPH assays.

Results: The mulberry leaf extraction process has been optimized. The obtained powdered mulberry leaf extract proved to be a good source of bioactive compounds. Characteristic phenolic compounds detected in mulberry leaves and their powder extract were quercetin and kaempferol. The sum of polyphenols was 10.9 mg GAE/g dry matter in the raw material, whereas in the powder extract 42.6 mg GAE/g dry matter. In addition, it was found that the obtained powdered extract is characterized by a five-fold higher, in comparison to the raw material, the content of antioxidant activity measured by ABTS and DPPH tests. The content of 1-deoxynojirimycin (DNJ) in the powder extract was nearly four times higher than in dry mulberry leaves.

Conclusion: The optimized process of water extraction of white mulberry leaves allows to keep valuable bioactive components and to obtain their high concentration.

Expression of receptor protein tyrosine phosphatase alpha mRNA in human prostate cancer cell lines

Receptor protein tyrosine phosphatase alpha (RPTPalpha) is a transmembrane protein phosphatase, and has been proposed to be involved in the differentiation of the neuronal system. In the present study, we demonstrated the expression of RPTPalpha mRNA in several normal human tissues. We further investigated the regulation of expression of RPTPalpha mRNA in epithelial cells utilizing three commercially available human prostate cancer cell lines LNCaP, PC-3 and DU145. This is because these cells exhibit different levels of differentiation, defined by the expression of a tissue-specific differentiation antigen, prostatic acid phosphatase (PAcP), and their androgen sensitivity. LNCaP cells express PAcP and are androgen-sensitive cells, while PC-3 and DU145 cells do not express PAcP and are androgen-insensitive cells. Northern blot analyses revealed that, in LNCaP cells, fetal bovine serum (FBS) and 5alpha-dihydrotestosterone (DHT) down-regulates RPTPalpha mRNA expression, similar to the effect on PAcP. Contrarily, FBS up-regulated the RPTPalpha mRNA level in PC-3 and DU145 cells. In LNCaP cells, sodium butyrate inhibited cell growth and up-regulated RPTPalpha as well as PAcP mRNA expression. Although, sodium butyrate also inhibited the growth of PC-3 and DU145 cells, the level of RPTPalpha mRNA was decreased in PC-3, while increased in DU145 cells. Thus, data taken together indicate that the expression of RPTPalpha is apparently regulated by a similar mechanism to that of PAcP in LNCaP cells.

Hic-5, a paxillin homologue, binds to the protein-tyrosine phosphatase PEST (PTP-PEST) through its LIM 3 domain

The Hic-5 protein is encoded by a transforming growth factor-beta1- and hydrogen peroxide-inducible gene, hic-5, and has striking similarity to paxillin, especially in their C-terminal LIM domains. Like paxillin, Hic-5 is localized in focal adhesion plaques in association with focal adhesion kinase in cultured fibroblasts. We carried out yeast two-hybrid screening to identify cellular factors that form a complex with Hic-5 using its LIM domains as a bait, and we identified a cytoplasmic tyrosine phosphatase (PTP-PEST) as one of the partners of Hic-5. These two proteins are associated in mammalian cells. From in vitro binding experiments using deletion and point mutations, it was demonstrated that the essential domain in Hic-5 for the binding was LIM 3. As for PTP-PEST, one of the five proline-rich sequences found on PTP-PEST, Pro-2, was identified as the binding site for Hic-5 in in vitro binding assays. Paxillin also binds to the Pro-2 domain of PTP-PEST. In conclusion, Hic-5 may participate in the regulation of signaling cascade through its interaction with distinct tyrosine kinases and phosphatases.

Regulation of fibroblast motility by the protein tyrosine phosphatase PTP-PEST

The protein tyrosine phosphatase PTP-PEST is a cytosolic enzyme that displays a remarkable degree of selectivity for tyrosine-phosphorylated p130(Cas) as a substrate, both in vitro and in intact cells. We have investigated the physiological role of PTP-PEST using Rat1 fibroblast-derived stable cell lines that we have engineered to overexpress PTP-PEST. These cell lines exhibit normal levels of tyrosine phosphorylation of the majority of proteins but have significantly lower levels of tyrosine phosphorylation of p130(Cas) than control cells. Initial cellular events occurring following integrin-mediated attachment to fibronectin (cell attachment and spreading) are essentially unchanged in cells overexpressing PTP-PEST; similarly, the extent and time course of mitogen-activated protein kinase activation in response to integrin engagement is unchanged. In contrast, the reduced phosphorylation state of p130(Cas) is associated with a considerably reduced rate of cell migration and a failure of cells overexpressing PTP-PEST to accomplish the normally observed redistribution of p130(Cas) to the leading edge of migrating cells. Furthermore, cells overexpressing PTP-PEST demonstrate significantly reduced levels of association of p130(Cas) with the Crk adaptor protein. Our results suggest that one physiological role of PTP-PEST is to dephosphorylate p130(Cas), thereby controlling tyrosine phosphorylation-dependent signaling events downstream of p130(Cas) and regulating cell migration.

A novel putative protein-tyrosine phosphatase contains a BRO1-like domain and suppresses Ha-ras-mediated transformation

To investigate a potential role of protein-tyrosine phosphatases (PTPases) in myocardial growth and signaling, a degenerate primer-based reverse transcription-polymerase chain reaction approach was used to isolate cDNAs for proteins that contain a PTPase catalytic domain. Among the 16 cDNA clones isolated by reverse transcription-polymerase chain reaction from total neonatal rat cardiomyocyte RNA, one, designated PTP-TD14, was unique. Subsequent isolation and sequencing of a full-length PTP-TD14 cDNA confirmed that it encodes a novel 164-kDa protein, p164(PTP-TD14). The C-terminal region contains the PTP-like domain, whereas the N-terminal region shows no homology to any known mammalian protein. However, this region is homologous to a yeast protein, BRO1, that is involved in the mitogen-activated protein kinase signaling pathway. Like BRO1, p164(PTP-TD14) contains a proline-rich region with two putative SH3-domain binding sites. By Northern blot analysis, PTP-TD14 is expressed as a 5.3-kilobase pair transcript, not only in neonatal heart but also in many adult rat tissues. When expressed in either COS-7 or NIH-3T3 cells, p164(PTP-TD14) localizes to the cytoplasm in association with vesicle-like structures. Expression of p164(PTP-TD14) in NIH-3T3 cells inhibits Ha-ras-mediated transformation more than 3-fold. This inhibitory activity is localized to the C-terminal PTPase homology domain, since no inhibition of Ha-ras-mediated focus formation was observed with a PTP-TD14 mutant, in which the putative catalytic activity was presumably inactivated by a point mutation. These findings indicate that PTP-TD14 encodes a novel protein that may be critically involved in regulating Ha-ras-dependent cell growth.

Functional involvement of PTP-U2L in apoptosis subsequent to terminal differentiation of monoblastoid leukemia cells

A large family of protein tyrosine phosphatases (PTPs) bidirectionally regulate intracellular signaling pathways by reversing agonistic or antagonistic phosphorylation events derived from the action of protein tyrosine kinases. Receptor-like PTP PTP-U2 is expressed during phorbol ester-induced differentiation of monoblastoid leukemia U937 cells. We found that the shorter isoform, PTP-U2S, was expressed at an earlier phase in the course of differentiation and the longer isoform, PTP-U2L, was induced at a later phase. In the presence of 12-O-tetradecanoylphorbol-13-acetate, ectopic expression of PTP-U2L in U937 cells enhanced several characteristics of terminally differentiated cells. Most striking was that PTP-U2L enhanced apoptosis of the differentiated cells, which was only partially inhibited by caspase inhibitor Z-Asp-CH2-DCB. The catalytically inactive mutant PTP-U2L(C --> S) still retained the ability to enhance the differentiation but retained the ability to enhance the following apoptosis of the cells to a lesser extent. These data indicate a functional involvement of PTP-U2L in apoptosis subsequent to terminal differentiation of U937 cells. Since terminally differentiated blood cells often undergo apoptosis, the data also suggest that PTP-U2L might be involved in physiological turnover of hematopoietic cells in vivo.

SHP-1 is involved in neuronal differentiation of P19 embryonic carcinoma cells

Accumulating evidence suggests that tyrosine phosphorylation plays an important role in the development of the central nervous system and in the differentiation of neuronal cells. To identify protein tyrosine phosphatases (PTPs) that might regulate signaling events leading to neuronal cell differentiation, we cloned PTP genes from the murine P19 embryonic carcinoma cell line and examined the change of their expression during differentiation. P19 cells are known to be pluripotent and the aggregate formation and subsequent replating in the presence of retinoic acid (RA) induce growth arrest and neuronal differentiation. The results demonstrated that among several PTP genes expressed in P19 cells, a cytosolic Src homology region 2 domain-containing PTP, SHP-1, is expressed highly in undifferentiated P19 cells, but is reduced to an undetectable level at day 3 after replating in the presence of RA. Further, SHP-1 was tyrosine-phosphorylated and activated at day 1 after replating. When ectopic SHP-1 was constitutively expressed, P19 cells continued to proliferate and failed to differentiate upon stimulation with RA. Collectively, these results suggest that the regulated expression and activity of SHP-1 may be involved in the neuronal differentiation of P19 cells.

Identification of p130(cas) as a substrate for the cytosolic protein tyrosine phosphatase PTP-PEST

PTP-PEST is a ubiquitously expressed, cytosolic, mammalian protein tyrosine phosphatase (PTP) which exhibits high specific activity in vitro. We have investigated the substrate specificity of PTP-PEST by a novel substrate-trapping approach in combination with in vitro dephosphorylation experiments. We initially identified a prominent 130-kDa tyrosine-phosphorylated protein in pervanadate-treated HeLa cell lysates which was preferentially dephosphorylated by PTP-PEST in vitro. In order to identify this potential substrate, mutant (substrate-trapping) forms of PTP-PEST were generated which lack catalytic activity but retain the ability to bind substrates. These mutant proteins associated in stable complexes exclusively with the same 130-kDa protein, which was identified as p130(cas) by immunoblotting. This exclusive association was observed in lysates from several cell lines and in transfected COS cells, but was not observed with other members of the PTP family, strongly suggesting that p130(cas) represents a major physiologically relevant substrate for PTP-PEST. Our studies suggest potential roles for PTP-PEST in regulation of p130(cas) function. These functions include mitogen- and cell adhesion-induced signalling events and probable roles in transformation by various oncogenes. These results provide the first demonstration of a PTP having an inherently restricted substrate specificity in vitro and in vivo. The methods used to identify p130(cas) as a specific substrate for PTP-PEST are potentially applicable to any PTP and should therefore prove useful in determining the physiological substrates of other members of the PTP family.

The purification and characterization of the catalytic domain of Src expressed in Schizosaccharomyces pombe. Comparison of unphosphorylated and tyrosine phosphorylated species

The catalytic domain of chicken Src including the C-terminal tail (Src-CD), has been expressed in Schizosaccharomyces pombe and purified to homogeneity. The expressed protein is a mixture of unphosphorylated (80%) and mono-phosphorylated (20%) species, that can be separated from each other by Mono Q chromatography. By a novel mass spectrometric method that utilizes parent ion scans of unseparated peptide mixtures, we found that the mono-phosphorylated form is phosphorylated either at Tyr416 or at Tyr436. The stability of Src-CD is comparable to the wild-type protein. Src-CD auto-phosphorylates and efficiently phosphorylates substrate peptides and proteins. Auto-phosphorylation occurs by an intermolecular mechanism and is completely inhibited by an excess of substrate peptide. Kinetic measurements for two exogenous substrates, the Src substrate peptide (AEEEIYGEFEAKKKK) and denatured enolase, showed that the overall activity (kcat) of the Src-CD molecule is about 10 times higher than that of wild-type Src. The kcat values for phosphorylation of the Src substrate peptide are similar for the unphosphorylated and monophosphorylated Src-CD (50 min-1), but the apparent K(m) values differ significantly (approximately 3 microM and 10 microM, respectively). Therefore, at low substrate concentrations in vitro the mono-phosphorylated form is more active, in agreement with the importance of Tyr416 for in vivo activity. The apparent K(m) values of the mono-phosphorylated Src-CD and wild-type Src for the Src substrate peptide and enolase are similar, indicating that, under these conditions, the kinase domain is mainly responsible for substrate binding.

Expression of receptor protein-tyrosine phosphatase alpha mRNA and protein during mouse embryogenesis

Receptor protein-tyrosine phosphatase alpha (RPTP alpha) is a transmembrane member of the family of protein-tyrosine phosphatases (PTPs) that has been implicated in neuronal differentiation in vitro. Here we demonstrate that RPTP alpha is differentially expressed during mouse embryogenesis in a spatio-temporal manner. RPTP alpha expression was detectable in 6 days post coitum (dpc) embryos, but not in 7.5 dpc embryos. From 10.5 dpc onwards a striking RPTP alpha expression pattern was observed with elevated levels in the dorsal root ganglia, cranial ganglia and adrenal gland, suggesting that RPTP alpha levels are specifically enhanced in neural crest derivatives. Marked differences between RPTP alpha mRNA and protein levels indicated that RPTP alpha expression is regulated by transcriptional and (post-) translational mechanisms. The expression pattern of RPTP alpha suggests that RPTP alpha may play a role in neural crest cell differentiation in vivo.

A functional screen in yeast for regulators and antagonizers of heterologous protein tyrosine kinases

Tyrosine phosphorylation exerts a pivotal role in cell regulation processes of higher eukaryotes. Tight control of the activity of protein tyrosine kinases is crucial for ordered phosphorylation to occur. We have developed a functional screen for tyrosine kinase regulators using c-Src, the first cellular protein tyrosine kinase described, as a prototype; and fission yeast, Schizosaccharomyces pombe, as a genetically amenable host system. Inducible expression of c-Src in fission yeast is lethal. We have screened human cDNA libraries for clones able to counteract the lethal effect of Src. Two different classes of cDNAs, which we called SAS for sequences antagonizing Src, were obtained. The first class encodes for the protein tyrosine kinase Csk, known to regulate Src activity through phosphorylation of the C-terminal tyrosine. The second class consists of clones encoding three different tyrosine phosphatases, counteracting Src action by dephosphorylation of Src substrates and by dephosphorylation of Src itself. The system described here can be applied to identify regulators of other heterologous tyrosine kinases, including receptor-type tyrosine kinases, which impair growth of S. pombe.

Protein-tyrosine phosphatases expressed in mouse epidermal keratinocytes

The importance of growth factors acting via receptor-type protein-tyrosine kinases in the continuous renewal of the epidermis from the keratinocyte stem cell population has been well established. Protein-tyrosine phosphatases (PTPases), which dephosphorylate phosphotyrosine-containing proteins, may therefore be expected to play an equally important role in the control of epidermal growth and differentiation. In this study, we have made an inventory of the various PTPases that are expressed during mouse keratinocyte proliferation and maturation. A panel of 13 different PTPases probes was obtained by combining a set of PTPase cDNAs previously cloned from mouse brain and a set of PTPase probes obtained from a normalized keratinocyte PTPase cDNA library. This PTPase cDNA panel, spanning probes for receptor-type as well as cytoplasmic-type family members, was used to monitor RNA expression levels in keratinocyte fractions isolated from murine epidermis and in keratinocyte cell cultures. No overt changes were observed in PTPase mRNA levels in all strata of mouse epidermis, but comparison of cultured cells with freshly isolated keratinocytes revealed several conspicuous differences. In the cultured Balb/MK cell line, absence of PTP delta expression and upregulation of PTP kappa and, to a lesser extent, PTP gamma mRNA ratios were observed compared to the freshly isolated cells. These results provide a basis for further research on the impact of PTPase activity on epidermal growth control.

Gene expressions of protein tyrosine phosphatases in regenerating rat liver and rat ascites hepatoma cells

mRNA levels for ten protein tyrosine phosphatases (PTPs), PTP-S, PTPH1, PTP-1, GLEPP1, LRP, PTP1D, PTPG1, PTP gamma, PTP delta, and LAR, were determined during regeneration of rat liver, and mRNA levels for 5 PTPs, PTP-S, PTP-1, PTP gamma, PTP delta, and LRP, were determined in three lines of rat ascites hepatoma cells. In regenerating rat liver, the expression patterns of PTP genes after partial hepatectomy could be classified into four groups. In group 1 (PTP-S and PTPH1), the mRNA levels increased rapidly, reached a maximum 7 h after partial hepatectomy, remained at a plateau for 1-2 days and then decreased gradually. In group 2 (PTP-1, GLEPP1, and LRP), the mRNA levels showed two peaks on days 1 and 5, and then decreased gradually. In group 3 (PTP1D and PTPG1), the mRNA levels increased rapidly, reached a maximum at 7 h, remained high for several days, and then did not decrease but rather increased after day 7. In group 4 (PTP gamma, PTP delta, and LAR), the mRNA levels remained constant for the first 5 days and increased over the control levels after day 7. In rat ascites hepatomas, gene expression of non-receptor-like PTPs (PTP-S and PTP-1) showed various neoplastic alterations, whereas mRNAs of receptor-like PTPs (PTP gamma, PTP delta, and LRP) were lost or drastically decreased.

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.

Murine protein tyrosine phosphatase-PEST, a stable cytosolic protein tyrosine phosphatase

We have isolated the murine cDNA homologue of the human protein tyrosine phosphatase PTP-PEST (MPTP-PEST) from an 18.5-day mouse embryonic kidney library. The cDNA isolated has a single open reading frame predicting a protein of 775 amino acids. When expressed in vitro as a glutathione S-transferase fusion protein, the catalytic domain (residues 1-453) shows intrinsic phosphatase activity. Reverse transcriptase PCR and Northern-blot analysis show that MPTP-PEST mRNA is expressed throughout murine development. Indirect immunofluorescence in COS-1 cells against a heterologous epitope tag attached to the N-terminus of MPTP-PEST, together with cellular fractionation and Western-blot experiments from different murine cell lines, indicate that MPTP-PEST is a free cytosolic protein of 112 kDa. Finally, sequence analysis indicates that the C-terminal portion of the protein contains four regions rich in proline, glutamate, serine and threonine, otherwise known as PEST sequences. These are characteristic of proteins that display very short intracellular half-lives. Despite the presence of these motifs, pulse-chase labelling experiments demonstrate that MPTP-PEST has a half-life of more than 4 h.

Seven protein tyrosine phosphatases are differentially expressed in the developing rat brain

Regulation of protein function through tyrosine phosphorylation is critical in the control of many developmental processes, such as cellular proliferation and differentiation. Growing evidence suggests that tyrosine phosphorylation also regulates key events in neural development. Although a large body of data has demonstrated that protein tyrosine kinases play an important role in neural development, much less is known about their counterparts, protein tyrosine phosphatases (PTPases). Using polymerase chain reaction (PCR) with degenerate primers and a neonatal rat cortex cDNA library, we have identified seven PTPases expressed in the developing rat brain. Four of these are transmembrane PTPases: LAR, LRP, RPTP gamma, and CPTP1. Three are nonreceptor PTPases: PTP-1, P19-PTP, and SHP. Northern hybridization analysis demonstrates that only CPTP1 is preferentially expressed in neural tissues, whereas the others are found abundantly in nonneural tissues as well as in the brain. Within the embryonic and early postnatal brain, the seven PTPases have overlapping, yet unique, distributions. For example, LAR mRNA is highly expressed by both proliferating and postmitotic cells in the cerebral cortex at embryonic day 17 and in all layers of the cortex at postnatal day 4. In contrast, RPTP gamma mRNA is expressed by postmitotic neurons in the embryo and predominantly by neurons in the superficial layers of the postnatal cortex. Several of the PTPases examined here are expressed at very high levels in the embryonic cortical plate and postnatal neocortex, including the subplate and subventricular zone. The spatial and temporal regulation of PTPase gene expression suggests that these PTPases have important roles in signal transduction during early neuronal differentiation and neural development.

Nuclear localization of the PEP protein tyrosine phosphatase

PEP is an intracellular protein tyrosine phosphatase expressed primarily by cells of hematopoietic origin that can be divided structurally into a catalytic domain and a large carboxy-terminal domain. The carboxy-terminal domain is enriched in proline, glutamic acid, serine, and threonine residues (PEST sequences) and contains a nonperfect tandem repeat sequence enriched in proline residues and a carboxy terminus enriched in basic amino acids. Here we show that PEP is diffusely expressed in lymphoid tissues, consistent with expression by many different cell types. Analysis of the PEP protein identifies a nuclear localization sequence within the extreme carboxy terminus. Transfer of 18 amino acids from the carboxy terminus of PEP to beta-galactosidase conferred nuclear localization, indicating that this sequence was sufficient for nuclear localization. Proteins enriched in PEST sequences are often rapidly degraded. However, pulse-chase analysis indicates that PEP has a half-life of greater than 5 h.

Nuclear localization of the PEP protein tyrosine phosphatase

PEP is an intracellular protein tyrosine phosphatase expressed primarily by cells of hematopoietic origin that can be divided structurally into a catalytic domain and a large carboxy-terminal domain. The carboxy-terminal domain is enriched in proline, glutamic acid, serine, and threonine residues (PEST sequences) and contains a nonperfect tandem repeat sequence enriched in proline residues and a carboxy terminus enriched in basic amino acids. Here we show that PEP is diffusely expressed in lymphoid tissues, consistent with expression by many different cell types. Analysis of the PEP protein identifies a nuclear localization sequence within the extreme carboxy terminus. Transfer of 18 amino acids from the carboxy terminus of PEP to beta-galactosidase conferred nuclear localization, indicating that this sequence was sufficient for nuclear localization. Proteins enriched in PEST sequences are often rapidly degraded. However, pulse-chase analysis indicates that PEP has a half-life of greater than 5 h.

Rat osteoblasts and ROS 17/2.8 cells contain a similar protein tyrosine phosphatase

Tyrosine phosphorylation plays a central role in intracellular signaling by many hormones and growth factors. Termination of the signal is thought to involve dephosphorylation of target proteins by phosphotyrosine phosphatases (PTPase). Soluble protein PTPases from neonatal rat osteoblasts (ROBs) and rat osteosarcoma (ROS 17/2.8) cells were chromatographically distinguished and characterized using 32P-labelled glutamate/tyrosine co-polymer as substrate. Two activities from both cell types were chromatographically separable. The dominant PTPase activity in the presence of 60-125 mM salt (E1), was eluted from phosphocellulose by 180-280 mM NaCl, bound weakly to a strong anion exchange column (QAE-trisacryl), had an apparent Km for [32P]glutamate/tyrosine copolymer of 52 micrograms/ml, was enhanced (5-10-fold, ROS; 1.5-3-fold, ROB) by assay in 125 mM NaCl, had no significant alkaline, acid, or serine phosphatase activity and had an M(r) of 53,000. A second activity (E2) was not retained by phosphocellulose but eluted from QAE-trisacryl in a single peak at 90-130 mM NaCl. It had an apparent Km for [32P]glutamate/tyrosine copolymer of 30 micrograms/ml (ROS) and its activity was not enhanced by NaCl in the assay. Activity E1 from both cells was 50% inhibited by 0.05 microM Na3VO4, 20 microM ZnCl2, or 5-10 microM CoCl2, but not by 1 mM NaF; activity E2 had a similar inhibition profile, but was more sensitive to ZnCl2 (IC50, 5 microM). Co2+ is a relatively non-toxic metal which may be a useful tool for investigating the role of phosphotyrosine in osteoblast proliferation and function. The similarity between the E1 activity from ROS cells and ROBs suggests that ROS cells may be useful in studying PTPase regulation by hormones, but molecular approaches will be required to establish the identity of PTPases in ROBs and ROS cells.

Receptor protein tyrosine phosphatase alpha activates pp60c-src and is involved in neuronal differentiation

Here we report that protein tyrosine phosphatases (PTPases), like their enzymatic counterpart the protein tyrosine kinases, can play an important role in cell differentiation. Expression of the transmembrane PTPase receptor protein tyrosine phosphatase alpha (RPTP alpha) is transiently enhanced during neuronal differentiation of embryonal carcinoma (EC) and neuroblastoma cells. Retinoic acid induces wild type P19 cells to differentiate into endoderm- and mesoderm-like cells. By contrast, retinoic acid treatment leads to neuronal differentiation of P19 cells, ectopically expressing functional RPTP alpha, as illustrated by their ability to generate action potentials. Endogenous pp60c-src kinase activity is enhanced in the RPTP alpha-transfected cells, which may be due to direct dephosphorylation of the regulatory Tyr residue at position 527 in pp60c-src by RPTP alpha. Our results demonstrate that RPTP alpha is involved in neuronal differentiation and imply a role for pp60c-src in the differentiation process.

Cloning and characterization of a human cDNA encoding a novel putative cytoplasmic protein-tyrosine-phosphatase

We have cloned and characterized a human cDNA encoding a new member of the family of cytosolic type protein-tyrosine-phosphatase (PTP), designated as PTPG1, from an adult colon tissue cDNA library by using the PCR product as probe. We obtained 5 cDNA clones, which cover the predicted open reading frame encoding a 88-kDa protein composed of 780 amino acids, and it had no apparent signal or transmembrane sequences, suggesting that it is a cytosolic protein. The N-terminal region had a PTP catalytic domain that is 30-40% identical to previously reported human PTPs. This revealed that the enzyme composes an additional family of human PTPs. PTPG1 was characterized by a long non-enzymatic domain located at the C-terminus, including PEST sequences which are characteristic for short half-life proteins in eukaryotes. Northern blot analysis of PTPG1 mRNA showed a 4.6-kb transcript that was detected in a wide variety of cell lines to suggest its extensive expression.