Expression of protein tyrosine phosphatase alpha (RPTPalpha) in human breast cancer correlates with low tumor grade, and inhibits tumor cell growth in vitro and in vivo

High Density of N- and O-Glycosylation Shields and Defines the Structural Dynamics of the Intrinsically Disordered Ectodomain of Receptor-type Protein Tyrosine Phosphatase Alpha

The intracellular phosphatase domain of the receptor-type protein tyrosine phosphatase alpha (PTPRA) is known to regulate various signaling pathways related to cell adhesion through c-Src kinase activation. In contrast, the functional significance of its relatively short, intrinsically disordered, and heavily glycosylated ectodomain remains unclear. Through detailed mass spectrometry analyses of a combination of protease and glycosidase digests, we now provide the first experimental evidence for its site-specific glycosylation pattern. This includes the occurrence of O-glycan at the N-glycosylation sequon among the more than 30 O-glycosylation sites confidently identified beside the 7 N-glycosylation sites. The closely spaced N- and O-glycans appear to have mutually limited the extent of further galactosylation and sialylation. An immature smaller form of full-length PTPRA was found to be deficient in O-glycosylation, most likely due to failure to transit the Golgi. N-glycosylation, on the other hand, is dispensable for cell surface expression and contributes less than the extensive O-glycosylation to the overall solution structure of the ectodomain. The glycosylation information is combined with the overall structural features of the ectodomain derived from small-angle X-ray scattering and high-speed atomic force microscopy monitoring to establish a dynamic structural model of the densely glycosylated PTPRA ectodomain. The observed high structural flexibility, as manifested by continuous transitioning from fully to partially extended and fold-back conformations, suggests that the receptor-type phosphatase is anchored to the membrane and kept mostly at a monomeric state through an ectodomain shaped and fully shielded by glycosylation.

Deep sequencing of pre-translational mRNPs reveals hidden flux through evolutionarily conserved alternative splicing nonsense-mediated decay pathways

BACKGROUND

Alternative splicing, which generates multiple mRNA isoforms from single genes, is crucial for the regulation of eukaryotic gene expression. The flux through competing splicing pathways cannot be determined by traditional RNA-Seq, however, because different mRNA isoforms can have widely differing decay rates. Indeed, some mRNA isoforms with extremely short half-lives, such as those subject to translation-dependent nonsense-mediated decay (AS-NMD), may be completely overlooked in even the most extensive RNA-Seq analyses.

RESULTS

RNA immunoprecipitation in tandem (RIPiT) of exon junction complex components allows for purification of post-splicing mRNA-protein particles (mRNPs) not yet subject to translation (pre-translational mRNPs) and, therefore, translation-dependent mRNA decay. Here we compare exon junction complex RIPiT-Seq to whole cell RNA-Seq data from HEK293 cells. Consistent with expectation, the flux through known AS-NMD pathways is substantially higher than that captured by RNA-Seq. Our RIPiT-Seq also definitively demonstrates that the splicing machinery itself has no ability to detect reading frame. We identify thousands of previously unannotated splicing events; while many can be attributed to splicing noise, others are evolutionarily conserved events that produce new AS-NMD isoforms likely involved in maintenance of protein homeostasis. Several of these occur in genes whose overexpression has been linked to poor cancer prognosis.

CONCLUSIONS

Deep sequencing of RNAs in post-splicing, pre-translational mRNPs provides a means to identify and quantify splicing events without the confounding influence of differential mRNA decay. For many known AS-NMD targets, the nonsense-mediated decay-linked alternative splicing pathway predominates. Exon junction complex RIPiT-Seq also revealed numerous conserved but previously unannotated AS-NMD events.

Receptor-type protein tyrosine phosphatase alpha (PTPα) mediates MMP14 localization and facilitates triple-negative breast cancer cell invasion

The ability of cancer cells to invade surrounding tissues requires degradation of the extracellular matrix (ECM). Invasive structures, such as invadopodia, form on the plasma membranes of cancer cells and secrete ECM-degrading proteases that play crucial roles in cancer cell invasion. We have previously shown that the protein tyrosine phosphatase alpha (PTPα) regulates focal adhesion formation and migration of normal cells. Here we report a novel role for PTPα in promoting triple-negative breast cancer cell invasion in vitro and in vivo. We show that PTPα knockdown reduces ECM degradation and cellular invasion of MDA-MB-231 cells through Matrigel. PTPα is not a component of TKS5-positive structures resembling invadopodia; rather, PTPα localizes with endosomal structures positive for MMP14, caveolin-1, and early endosome antigen 1. Furthermore, PTPα regulates MMP14 localization to plasma membrane protrusions, suggesting a role for PTPα in intracellular trafficking of MMP14. Importantly, we show that orthotopic MDA-MB-231 tumors depleted in PTPα exhibit reduced invasion into the surrounding mammary fat pad. These findings suggest a novel role for PTPα in regulating the invasion of triple-negative breast cancer cells.

PTPRA facilitates cancer growth and migration via the TNF-α-mediated PTPRA-NF-κB pathway in MCF-7 breast cancer cells

Protein tyrosine phosphatase receptor type A (PTPRA), one of the classic protein tyrosine phosphatases, is crucial for modulating tumorigenesis and metastasis in breast cancer; however, its functional mechanism has not fully elucidated. The present study assessed PTPRA expression and estimated its clinical impact on survival using the Gene Expression Profiling Interactive Analysis database (GEPIA). Growth curves, colony formations and Transwell assays were utilized to examine cell proliferation and migration. Additionally, luciferase reporter assays were used to examine the potential tumor signaling pathways targeted by PTPRA in HEK293T cells. Furthermore, quantitative PCR (qPCR) was utilized to confirm the transcriptional regulation of PTPRA expression. Bioinformatic analyses of data from GEPIA identified PTPRA overexpression in patients with breast cancer. The growth curve, colony formation and transwell experiments demonstrated that PTPRA upregulation significantly promoted the cell proliferation and migration of MCF-7 breast cancer cells. In contrast, PTPRA knockdown significantly attenuated cell proliferation and migration. Mechanistic experiments revealed that the transcriptional activity of NF-κB was higher compared with other classic tumor pathways when they were activated by PTPRA in HEK293T cells. Furthermore, the transcriptional activity of NF-κB was altered in a PTPRA-dose-dependent manner. Additionally, following exposure to TNF-α, PTPRA-deficient MCF-7 cells exhibited lower NF-κB transcriptional activity compared with normal control cells. The results of the present study demonstrate that PTPRA overexpression accelerates inflammatory tumor phenotypes in breast cancer and that the TNF-α-mediated PTPRA-NF-κB pathway may offer novel insight into early diagnosis and optimum treatment for breast cancer.

Protein tyrosine phosphatases: promising targets in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is the most common type of pancreatic cancer. It is the fourth leading cause of cancer-related death and is associated with a very poor prognosis. KRAS driver mutations occur in approximately 95% of PDAC cases and cause the activation of several signaling pathways such as mitogen-activated protein kinase (MAPK) pathways. Regulation of these signaling pathways is orchestrated by feedback loops mediated by the balance between protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs), leading to activation or inhibition of its downstream targets. The human PTPome comprises 125 members, and these proteins are classified into three distinct families according to their structure. Since PTP activity description, it has become clear that they have both inhibitory and stimulatory effects on cancer-associated signaling processes and that deregulation of PTP function is closely associated with tumorigenesis. Several PTPs have displayed either tumor suppressor or oncogenic characteristics during the development and progression of PDAC. In this sense, PTPs have been presented as promising candidates for the treatment of human pancreatic cancer, and many PTP inhibitors have been developed since these proteins were first associated with cancer. Nevertheless, some challenges persist regarding the development of effective and safe methods to target these molecules and deliver these drugs. In this review, we discuss the role of PTPs in tumorigenesis as tumor suppressor and oncogenic proteins. We have focused on the differential expression of these proteins in PDAC, as well as their clinical implications and possible targeting for pharmacological inhibition in cancer therapy.

High Expression of PTPN3 Predicts Progression and Unfavorable Prognosis of Glioblastoma

Background

PTPN3 was demonstrated to be involved in the progression of several types of cancers, such as gastric adenocarcinoma, lung cancer, and intrahepatic cholangiocarcinoma. However, its clinical significance in glioblastoma (GBM) has not been elucidated.

Material/Methods

We investigated the expression of PTPN3 in 95 cases of GBM with immunohistochemistry and in 8 pairs of fresh GBMs and their adjacent tissues with qualitative polymerase chain reaction. Moreover, the correlation between PTPN3 and clinicopathological factors was evaluated by chi-square test. The prognostic value of PTPN3 was investigated with univariate analysis and multivariate analysis. With MTT assay and Transwell assay, the oncogenic functions of PTPN3 in GBM proliferation and invasion were further investigated.

Results

Expression of PTPN3 in GBM tissues was significantly higher than in their corresponding adjacent tissues. High expression of PTPN3 was significantly associated with unfavorable prognosis of GBM. Moreover, in GBM cell lines, PTPN3 promoted cell proliferation and invasion, and the PTP common inhibitor pervanadate suppressed GBM proliferation and invasion.

Conclusions

Our experiments show that PTPN3 is an independent prognostic factor in GBM and indicated that postoperative detection of PTPN3 can be used to identify high-risk patients and guide individual treatment.

Recent Advances in ADAM17 Research: A Promising Target for Cancer and Inflammation

Since its discovery, ADAM17, also known as TNFα converting enzyme or TACE, is now known to process over 80 different substrates. Many of these substrates are mediators of cancer and inflammation. The field of ADAM metalloproteinases is at a crossroad with many of the new potential therapeutic agents for ADAM17 advancing into the clinic. Researchers have now developed potential drugs for ADAM17 that are selective and do not have the side effects which were seen in earlier chemical entities that targeted this enzyme. ADAM17 inhibitors have broad therapeutic potential, with properties ranging from tumor immunosurveillance and overcoming drug and radiation resistance in cancer, as treatments for cardiac hypertrophy and inflammatory conditions such as inflammatory bowel disease and rheumatoid arthritis. This review focuses on substrates and inhibitors identified more recently for ADAM17 and their role in cancer and inflammation.

Suppression of protein tyrosine phosphatase N23 predisposes to breast tumorigenesis via activation of FYN kinase

Zhang et al. identified PTPN23 as a suppressor of cell motility and invasion in mammary epithelial and breast cancer cells. They validated the underlying mechanism of PTPN23 function in breast tumorigenesis as that of a key phosphatase that normally suppresses the activity of FYN in two different models.

Disruption of the balanced modulation of reversible tyrosine phosphorylation has been implicated in the etiology of various human cancers, including breast cancer. Protein Tyrosine Phosphatase N23 (PTPN23) resides in chromosomal region 3p21.3, which is hemizygously or homozygously lost in some breast cancer patients. In a loss-of-function PTPome screen, our laboratory identified PTPN23 as a suppressor of cell motility and invasion in mammary epithelial and breast cancer cells. Now, our TCGA (The Cancer Genome Atlas) database analyses illustrate a correlation between low PTPN23 expression and poor survival in breast cancers of various subtypes. Therefore, we investigated the tumor-suppressive function of PTPN23 in an orthotopic transplantation mouse model. Suppression of PTPN23 in Comma 1Dβ cells induced breast tumors within 56 wk. In PTPN23-depleted tumors, we detected hyperphosphorylation of the autophosphorylation site tyrosine in the SRC family kinase (SFK) FYN as well as Tyr142 in β-catenin. We validated the underlying mechanism of PTPN23 function in breast tumorigenesis as that of a key phosphatase that normally suppresses the activity of FYN in two different models. We demonstrated that tumor outgrowth from PTPN23-deficient BT474 cells was suppressed in a xenograft model in vivo upon treatment with AZD0530, an SFK inhibitor. Furthermore, double knockout of FYN and PTPN23 via CRISPR/CAS9 also attenuated tumor outgrowth from PTPN23 knockout Cal51 cells. Overall, this mechanistic analysis of the tumor-suppressive function of PTPN23 in breast cancer supports the identification of FYN as a therapeutic target for breast tumors with heterozygous or homozygous loss of PTPN23.

Tumor suppressive protein phosphatases in human cancer: Emerging targets for therapeutic intervention and tumor stratification

Aberrant protein phosphorylation is one of the hallmarks of cancer cells, and in many cases a prerequisite to sustain tumor development and progression. Like protein kinases, protein phosphatases are key regulators of cell signaling. However, their contribution to aberrant signaling in cancer cells is overall less well appreciated, and therefore, their clinical potential remains largely unexploited. In this review, we provide an overview of tumor suppressive protein phosphatases in human cancer. Along their mechanisms of inactivation in defined cancer contexts, we give an overview of their functional roles in diverse signaling pathways that contribute to their tumor suppressive abilities. Finally, we discuss their emerging roles as predictive or prognostic markers, their potential as synthetic lethality targets, and the current feasibility of their reactivation with pharmacologic compounds as promising new cancer therapies. We conclude that their inclusion in clinical practice has obvious potential to significantly improve therapeutic outcome in various ways, and should now definitely be pushed forward.

Stepping out of the shadows: Oncogenic and tumor-promoting protein tyrosine phosphatases

Protein tyrosine phosphorylation is critical for proper function of cells and organisms. Phosphorylation is regulated by the concerted but generically opposing activities of tyrosine kinases (PTKs) and tyrosine phosphatases (PTPs), which ensure its proper regulation, reversibility, and ability to respond to changing physiological situations. Historically, PTKs have been associated mainly with oncogenic and pro-tumorigenic activities, leading to the generalization that protein dephosphorylation is anti-oncogenic and hence that PTPs are tumor-suppressors. In many cases PTPs do suppress tumorigenesis. However, a growing body of evidence indicates that PTPs act as dominant oncogenes and drive cell transformation in a number of contexts, while in others PTPs support transformation that is driven by other oncogenes. This review summarizes the known transforming and tumor-promoting activities of the classical, tyrosine specific PTPs and highlights their potential as drug targets for cancer therapy.

Increased PTPRA expression leads to poor prognosis through c-Src activation and G1 phase progression in squamous cell lung cancer

PTPRA is reported to be involved in cancer development and progression through activating the Src family kinase (SFK) signaling pathways, however, the roles of PTPRA in the squamous cell lung cancer (SCC) development are unclear. The purpose of this study was to clarify the clinical relevance and biological roles of PTPRA in SCC. We found that PTPRA was upregulated in squamous cell lung cancer compared to matched normal tissues at the mRNA (N=20, P=0.004) and protein expression levels (N=75, P<0.001). Notably, high mRNA level of PTPRA was significantly correlated with poorer prognosis in 675 SCC patients from the Kaplan-Meier plotter database. With 75 cases, we found that PTPRA protein expression was significantly correlated with tumor size (P=0.002), lymph node metastasis (P=0.008), depth of tumor invasion (P<0.001) and clinical stage (P<0.001). The Kaplan-Meier plot suggested that high expression of PTPRA had poorer overall survival in SCC patients (P=0.009). Multivariate Cox regression analysis suggested that PTPRA expression was an independent prognostic factor in SCC patients. In the cellular models, PTPRA promotes SCC cell proliferation through modulating Src activation as well as cell cycle progression. In conclusion, higher PTPRA level was associated with worse prognosis of SCC patients and PTPRA could promote the cell cycle progression through stimulating the c-Src signaling pathways.

Phosphatases: Their Roles in Cancer and Their Chemical Modulators

Phosphatases are involved in basically all cellular processes by dephosphorylating cellular components such as proteins, phospholipids and second messengers. They counteract kinases of which many are established oncogenes, and therefore kinases are one of the most important drug targets for targeted cancer therapy. Due to this relationship between kinases and phosphatases, phosphatases are traditionally assumed to be tumour suppressors. However, research findings over the last years prove that this simplification is incorrect, as bona-fide and putative phosphatase oncogenes have been identified. We describe here the role of phosphatases in cancer, tumour suppressors and oncogenes, and their chemical modulators, and discuss new approaches and opportunities for phosphatases as drug targets.

Regulation of RPTPα-c-Src signalling pathway by miR-218

Receptor protein tyrosine phosphatase alpha (RPTPα), an activator of Src family kinases, is found significantly overexpressed in human cancer tissues. However, little is known about the regulation of RPTPα expression. miRNAs target multiple genes and play important roles in many cancer processes. Here, we identified a miRNA, miR-218 that binds directly to the 3'-UTR of RPTPα. Ectopic overexpression of miR-218 decreased RPTPα protein leading to decreased dephosphorylation of c-Src and decreased tumour growth in vitro and in vivo. A feedback loop between c-Src and miR-218 was revealed where c-Src inhibits transcription of SLIT2, which intronically hosts miR-218. These results show a novel regulatory pathway for RPTPα-c-Src signalling.

The interaction of protein-tyrosine phosphatase α (PTPα) and RACK1 protein enables insulin-like growth factor 1 (IGF-1)-stimulated Abl-dependent and -independent tyrosine phosphorylation of PTPα

Protein tyrosine phosphatase α (PTPα) promotes integrin-stimulated cell migration in part through the role of Src-phosphorylated PTPα-Tyr(P)-789 in recruiting and localizing p130Cas to focal adhesions. The growth factor IGF-1 also stimulates PTPα-Tyr-789 phosphorylation to positively regulate cell movement. This is in contrast to integrin-induced PTPα phosphorylation, that induced by IGF-1 can occur in cells lacking Src family kinases (SFKs), indicating that an unknown kinase distinct from SFKs can target PTPα. We show that this IGF-1-stimulated tyrosine kinase is Abl. We found that PTPα binds to the scaffold protein RACK1 and that RACK1 coordinates the IGF-1 receptor, PTPα, and Abl in a complex to enable IGF-1-stimulated and Abl-dependent PTPα-Tyr-789 phosphorylation. In cells expressing SFKs, IGF-1-stimulated phosphorylation of PTPα is mediated by RACK1 but is Abl-independent. Furthermore, expressing the SFKs Src and Fyn in SFK-deficient cells switches IGF-1-induced PTPα phosphorylation to occur in an Abl-independent manner, suggesting that SFK activity dominantly regulates IGF-1/IGF-1 receptor signaling to PTPα. RACK1 is a molecular scaffold that integrates growth factor and integrin signaling, and our identification of PTPα as a RACK1 binding protein suggests that RACK1 may coordinate PTPα-Tyr-789 phosphorylation in these signaling networks to promote cell migration.

Receptor-type protein tyrosine phosphatases in cancer

Protein tyrosine phosphatases (PTPs) play an important role in regulating cell signaling events in coordination with tyrosine kinases to control cell proliferation, apoptosis, survival, migration, and invasion. Receptor-type protein tyrosine phosphatases (PTPRs) are a subgroup of PTPs that share a transmembrane domain with resulting similarities in function and target specificity. In this review, we summarize genetic and epigenetic alterations including mutation, deletion, amplification, and promoter methylation of PTPRs in cancer and consider the consequences of PTPR alterations in different types of cancers. We also summarize recent developments using PTPRs as prognostic or predictive biomarkers and/or direct targets. Increased understanding of the role of PTPRs in cancer may provide opportunities to improve therapeutic approaches.

RPTPα controls epithelial adherens junctions, linking E-cadherin engagement to c-Src-mediated phosphorylation of cortactin

Epithelial junctions are fundamental determinants of tissue organization, subject to regulation by tyrosine phosphorylation. Homophilic binding of E-cadherin activates tyrosine kinases, such as Src, that control junctional integrity. Protein tyrosine phosphatases (PTPs) also contribute to cadherin-based adhesion and signaling, but little is known about their specific identity or functions at epithelial junctions. Here, we report that the receptor PTP RPTPα (human gene name PTPRA) is recruited to epithelial adherens junctions at the time of cell-cell contact, where it is in molecular proximity to E-cadherin. RPTPα is required for appropriate cadherin-dependent adhesion and for cyst architecture in three-dimensional culture. Loss of RPTPα impairs adherens junction integrity, as manifested by defective E-cadherin accumulation and peri-junctional F-actin density. These effects correlate with a role for RPTPα in cellular (c)-Src activation at sites of E-cadherin engagement. Mechanistically, RPTPα is required for appropriate tyrosine phosphorylation of cortactin, a major Src substrate and a cytoskeletal actin organizer. Expression of a phosphomimetic cortactin mutant in RPTPα-depleted cells partially rescues F-actin and E-cadherin accumulation at intercellular contacts. These findings indicate that RPTPα controls cadherin-mediated signaling by linking homophilic E-cadherin engagement to cortactin tyrosine phosphorylation through c-Src.

Protein tyrosine phosphatase α mediates profibrotic signaling in lung fibroblasts through TGF-β responsiveness

Fibrotic lung diseases represent a diverse group of progressive and often fatal disorders with limited treatment options. Although the pathogenesis of these conditions remains incompletely understood, receptor type protein tyrosine phosphatase α (PTP-α encoded by PTPRA) has emerged as a key regulator of fibroblast signaling. We previously reported that PTP-α regulates cellular responses to cytokines and growth factors through integrin-mediated signaling and that PTP-α promotes fibroblast expression of matrix metalloproteinase 3, a matrix-degrading proteinase linked to pulmonary fibrosis. Here, we sought to determine more directly the role of PTP-α in pulmonary fibrosis. Mice genetically deficient in PTP-α (Ptpra(-/-)) were protected from pulmonary fibrosis induced by intratracheal bleomycin, with minimal alterations in the early inflammatory response or production of TGF-β. Ptpra(-/-) mice were also protected from pulmonary fibrosis induced by adenoviral-mediated expression of active TGF-β1. In reciprocal bone marrow chimera experiments, the protective phenotype tracked with lung parenchymal cells but not bone marrow-derived cells. Because fibroblasts are key contributors to tissue fibrosis, we compared profibrotic responses in wild-type and Ptpra(-/-) mouse embryonic and lung fibroblasts. Ptpra(-/-) fibroblasts exhibited hyporesponsiveness to TGF-β, manifested by diminished expression of αSMA, EDA-fibronectin, collagen 1A, and CTGF. Ptpra(-/-) fibroblasts exhibited markedly attenuated TGF-β-induced Smad2/3 transcriptional activity. We conclude that PTP-α promotes profibrotic signaling pathways in fibroblasts through control of cellular responsiveness to TGF-β.

Receptor protein-tyrosine phosphatase α regulates focal adhesion kinase phosphorylation and ErbB2 oncoprotein-mediated mammary epithelial cell motility

We investigated the role of protein-tyrosine phosphatase α (PTPα) in regulating signaling by the ErbB2 oncoprotein in mammary epithelial cells. Using this model, we demonstrated that activation of ErbB2 led to the transient inactivation of PTPα, suggesting that attenuation of PTPα activity may contribute to enhanced ErbB2 signaling. Furthermore, RNAi-induced suppression of PTPα led to increased cell migration in an ErbB2-dependent manner. The ability of ErbB2 to increase cell motility in the absence of PTPα was characterized by prolonged interaction of GRB7 with ErbB2 and increased association of ErbB2 with a β1-integrin-rich complex, which depended on GRB7-SH2 domain interactions. Finally, suppression of PTPα resulted in increased phosphorylation of focal adhesion kinase on Tyr-407, which induced the recruitment of vinculin and the formation of a novel focal adhesion kinase complex in response to ErbB2 activation in mammary epithelial cells. Collectively, these results reveal a new role for PTPα in the regulation of motility of mammary epithelial cells in response to ErbB2 activation.

Protein tyrosine phosphatases as novel targets in breast cancer therapy

Breast cancer is linked to hyperactivation of protein tyrosine kinases (PTKs), and recent studies have unveiled that selective tyrosine dephosphorylation by protein tyrosine phosphatases (PTPs) of specific substrates, including PTKs, may activate or inactivate oncogenic pathways in human breast cancer cell growth-related processes. Here, we review the current knowledge on the involvement of PTPs in breast cancer, as major regulators of breast cancer therapy-targeted PTKs, such as HER1/EGFR, HER2/Neu, and Src. The functional interplay between PTKs and PTK-activating or -inactivating PTPs, and its implications in novel breast cancer therapies based on targeting of specific PTPs, are discussed.

Protein tyrosine phosphatase kappa (PTPRK) is a negative regulator of adhesion and invasion of breast cancer cells, and associates with poor prognosis of breast cancer

PURPOSE

Receptor-like protein tyrosine phosphatase kappa (PTPRK) has been shown to exhibit homophilic binding. It is a putative tumour suppressor in primary central nervous system lymphomas and colorectal cancer. The present study investigated the expression of PTPRK in breast cancer and the biological impact of PTPRK on breast cancer cells.

METHODS

Expression of PTPRK protein and gene transcript was examined in a cohort of breast cancer patients. The association of PTPRK transcript level and pathological and clinical aspects was then analysed. Knockdown of PTPRK in breast cancer cells was performed using a specific anti-PTPRK transgene. The impact of PTPRK knockdown on breast cancer cells was investigated using in vitro cell function assays.

RESULTS

Lower levels of PTPRK transcripts were seen in the advanced breast cancer. The reduced PTPRK transcript levels were associated with poor prognosis of the disease. PTPRK transcript levels were decreased in the primary tumours of patients who died from breast cancer or had metastases. Patients with lower expression of PTPRK had shorter survival compared with those higher expression levels of PTPRK. Knockdown of PTPRK resulted in increased proliferation, adhesion, invasion, and migration of breast cancer cells in vitro.

CONCLUSIONS

Decreased expression of PTPRK in breast cancer is correlated with poor prognosis. PTPRK is a negative regulator of adhesion, invasion, migration, and proliferation of breast cancer cells. This suggests that PTPRK is a potential tumour suppressor in breast cancer.

PTPα-mediated Src activation by EGF in human breast cancer cells

Protein tyrosine phosphatase alpha (PTPα) functions as an activator of Src by dephosphorylating Tyr527/530, a critical negative regulatory site. The increase of PTPα catalytic activity requires its phosphorylation at Ser180 and/or Ser204 and its dissociation from PTPα/Grb2 complex. Here, we show that epidermal growth factor (EGF) stimulation increases the ability of PTPα to activate Src by dephosphorylating Tyr530 in BT-20 and SKBR3 breast cancer cell lines. Treatment of these cells with EGF transiently decreased the association of PTPα with Grb2 and enhanced PTPα catalytic activity via Ser180 and Ser204 phosphorylation that was blocked by the protein kinase C delta (PKCδ) inhibitor rottlerin or knockdown of PKCδ by siRNA or by the overexpression of PTPαS180A/S204A mutant. PTPα siRNA blocked EGF-mediated Src activation in cancer cells and inhibited on colony formation, whereas control siRNA did not. These results suggested that PTPα links activation of epidermal growth factor receptor (EGFR) signaling with Src activation and may provide a novel therapeutic target for treatment of breast cancer.

PTPN14 is required for the density-dependent control of YAP1

Through an shRNA-mediated loss-of-function screen, we identified PTPN14 as a potential tumor suppressor. PTPN14 interacts with yes-associated protein 1 (YAP1), a member of the hippo signaling pathway. We showed that PTPN14 promotes the nucleus-to-cytoplasm translocation of YAP1 during contact inhibition and thus inhibits YAP1 transactivation activity. Interestingly, PTPN14 protein stability was positively controlled by cell density. We identified the CRL2(LRR1) (cullin2 RING ubiquitin ligase complex/leucine-rich repeat protein 1) complex as the E3 ligase that targets PTPN14 for degradation at low cell density. Collectively, these data suggest that PTPN14 acts to suppress cell proliferation by promoting cell density-dependent cytoplasmic translocation of YAP1.

Receptor type protein tyrosine phosphatases (RPTPs) - roles in signal transduction and human disease

Protein tyrosine phosphorylation is a fundamental regulatory mechanism controlling cell proliferation, differentiation, communication, and adhesion. Disruption of this key regulatory mechanism contributes to a variety of human diseases including cancer, diabetes, and auto-immune diseases. Net protein tyrosine phosphorylation is determined by the dynamic balance of the activity of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Mammals express many distinct PTKs and PTPs. Both of these families can be sub-divided into non-receptor and receptor subtypes. Receptor protein tyrosine kinases (RPTKs) comprise a large family of cell surface proteins that initiate intracellular tyrosine phosphorylation-dependent signal transduction in response to binding of extracellular ligands, such as growth factors and cytokines. Receptor-type protein tyrosine phosphatases (RPTPs) are enzymatic and functional counterparts of RPTKs. RPTPs are a family of integral cell surface proteins that possess intracellular PTP activity, and extracellular domains that have sequence homology to cell adhesion molecules. In comparison to extensively studied RPTKs, much less is known about RPTPs, especially regarding their substrate specificities, regulatory mechanisms, biological functions, and their roles in human diseases. Based on the structure of their extracellular domains, the RPTP family can be grouped into eight sub-families. This article will review one representative member from each RPTP sub-family.

Protein tyrosine phosphatases in cancer: friends and foes!

Tyrosine phosphorylation of proteins serves as an exquisite switch in controlling several key oncogenic signaling pathways involved in cell proliferation, apoptosis, migration, and invasion. Since protein tyrosine phosphatases (PTPs) counteract protein kinases by removing phosphate moieties on target proteins, one may intuitively think that PTPs would act as tumor suppressors. Indeed, one of the most described PTPs, namely, the phosphatase and tensin homolog (PTEN), is a tumor suppressor. However, a growing body of evidence suggests that PTPs can also function as potent oncoproteins. In this chapter, we provide a broad historical overview of the PTPs, their mechanism of action, and posttranslational modifications. Then, we focus on the dual properties of classical PTPs (receptor and nonreceptor) and dual-specificity phosphatases in cancer and summarize the current knowledge of the signaling pathways regulated by key PTPs in human cancer. In conclusion, we present our perspective on the potential of these PTPs to serve as therapeutic targets in cancer.

Proteolytic processing of the protein tyrosine phosphatase α extracellular domain is mediated by ADAM17/TACE

The receptor protein tyrosine phosphatase alpha (PTPα) is involved in the regulation of tyrosine kinases like the Src kinase and the insulin receptor. As with other PTPs, its function is determined by alternative splicing, dimerisation, phosphorylation and proteolytical processing. PTPα is cleaved by calpain in its intracellular domain, which decreases its potential to dephosphorylate Src kinase. Here, we demonstrate that PTPα is also processed in the extracellular domain. Extracellular processing was exclusively found for a splice variant containing an extra nine amino acid insert three residues amino-terminal from the transmembrane domain. Processing was sensitive to the metalloprotease-inhibitor Batimastat, and CHO-M2 cells lacking a disintegrin and metalloproteinase 17 (ADAM17; tumor-necrosis-factor α converting enzyme) activity were not able to cleave PTPα. After transient overexpression of ADAM17 and PTPα in these cells, processing was restored, proving that ADAM17 is involved in this process. Further characterization of the consequences of processing revealed that dephosphorylation of the insulin receptor or activation of Src was not affected but focus formation was reduced. We conclude that extracellular proteolytic processing is a novel mechanism for PTPα regulation.

Loss of protein-tyrosine phosphatase α (PTPα) increases proliferation and delays maturation of oligodendrocyte progenitor cells

Tightly controlled termination of proliferation determines when oligodendrocyte progenitor cells (OPCs) can initiate differentiation and mature into myelin-forming cells. Protein-tyrosine phosphatase α (PTPα) promotes OPC differentiation, but its role in proliferation is unknown. Here we report that loss of PTPα enhanced in vitro proliferation and survival and decreased cell cycle exit and growth factor dependence of OPCs but not neural stem/progenitor cells. PTPα(-/-) mice have more oligodendrocyte lineage cells in embryonic forebrain and delayed OPC maturation. On the molecular level, PTPα-deficient mouse OPCs and rat CG4 cells have decreased Fyn and increased Ras, Cdc42, Rac1, and Rho activities, and reduced expression of the Cdk inhibitor p27Kip1. Moreover, Fyn was required to suppress Ras and Rho and for p27Kip1 accumulation, and Rho inhibition in PTPα-deficient cells restored expression of p27Kip1. We propose that PTPα-Fyn signaling negatively regulates OPC proliferation by down-regulating Ras and Rho, leading to p27Kip1 accumulation and cell cycle exit. Thus, PTPα acts in OPCs to limit self-renewal and facilitate differentiation.

Protein tyrosine phosphatase alpha regulates cell detachment and cell death profiles induced by nitric oxide donors in the A431 human carcinoma cell line

We investigated the role of protein tyrosine phosphatase-alpha (PTPα) expression in the cell death profile of the A431 human carcinoma cell line that was induced by cytotoxic concentrations of the nitric oxide (NO) donors sodium nitroprusside (SNP) and 3,3-bis-(aminoethyl)-1-hydroxy-2-oxo-1-triazene (NOC-18). Both NO donors promoted extensive cell detachment in A431 parental cells as compared to the detachment observed for A431 cells that ectopically expressed PTPα (A431 (A27B(PTPα)) cells). The NO-induced cell death characteristics for both cell lines were examined. After incubation for 10 hours with 2.0 mM SNP, attached or detached A431 cells underwent apoptosis. Cells were highly positive for Annexin-V, featured increased cleavage of procaspase-8, activation of downstream caspase-3, and activation of poly-ADP-ribose polymerase 1 (PARP-1). In contrast, exposure of A431 (A27B(PTPα)) cells to 2.0 mM SNP produced an increase in the release of lactate dehydrogenase and enhanced incorporation of propidium iodide. In addition, A431 (A27B(PTPα)) cells showed partial inhibition of the activities of caspase-8, caspase-3, and PARP-1 upon detachment and cell death induced by SNP treatment. Results indicate that necrotic cell damage was induced, characterized by cellular swelling and lysis. We conclude from these results that PTPα regulates the A431 tumor cell death profile mediated by NO donors. Expression of PTPα or its absence may determine the occurrence of NO-induced cell death with necrotic or apoptotic features, respectively.

Identification of PTPN23 as a novel regulator of cell invasion in mammary epithelial cells from a loss-of-function screen of the 'PTP-ome'

We used an RNAi-mediated loss-of-function screen to study systematically the role of the protein tyrosine phosphatase (PTP) superfamily of enzymes in mammary epithelial cell motility in the absence or presence of the oncoprotein tyrosine kinase ERBB2. We report that although shRNAs directed against most of the PTP family were without effect, suppression of three PTPs-PRPN23, PTPRG, and PTPRR-enhanced cell motility. Furthermore, we found that suppression of PTPN23, but not PTPRG or PTPRR, induced cell invasion. Suppression of PTPN23 increased E-cadherin internalization, impaired early endosome trafficking of E-cadherin, induced the expression of mesenchymal proteins, and caused cell scattering. The activity of SRC and β-catenin was elevated when PTPN23 was suppressed. Moreover, we identified SRC, E-cadherin, and β-catenin as direct substrates of PTPN23. Inhibition of SRC with the small molecular inhibitor SU6656 blocked the effects of PTPN23 depletion. These findings suggest that loss of PTPN23 may increase the activity of SRC and the phosphorylation status of the E-cadherin/β-catenin signaling complex to promote tumor growth and invasive behavior in breast cancer. In addition, our studies highlight functional specificity among PTPs and reveal new roles for PTPs in mammary epithelial cell biology.

Activation of Src and transformation by an RPTPα splice mutant found in human tumours

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.

Inside the human cancer tyrosine phosphatome

Members of the protein tyrosine phosphatase (Ptp) family dephosphorylate target proteins and counter the activities of protein tyrosine kinases that are involved in cellular phosphorylation and signalling. As such, certain PTPs might be tumour suppressors. Indeed, PTPs play an important part in the inhibition or control of growth, but accumulating evidence indicates that some PTPs may exert oncogenic functions. Recent large-scale genetic analyses of various human tumours have highlighted the relevance of PTPs either as putative tumour suppressors or as candidate oncoproteins. Progress in understanding the regulation and function of PTPs has provided insights into which PTPs might be potential therapeutic targets in human cancer.

PTPH1 cooperates with vitamin D receptor to stimulate breast cancer growth through their mutual stabilization

Tyrosine phosphorylation is tightly regulated by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs), and plays a critical role in malignant transformation and progression. While PTKs have a well-established role in regulating breast cancer growth, contribution of PTPs remains mostly unknown. Here, we report that the tyrosine phosphatase PTPH1 stimulates breast cancer growth through regulating vitamin D receptor (VDR) expression. PTPH1 was shown to be over-expressed in 49% of primary breast cancer and levels of its protein expression positively correlate with the clinic metastasis, suggesting its oncogenic activity. Indeed, PTPH1 promotes breast cancer growth by a mechanism independent of its phosphatase activity but dependent of its stimulatory effect on the nuclear receptor VDR protein expression and depletion of induced VDR abolishes the PTPH1 oncogenic activity. Additional analyses showed that PTPH1 binds VDR and increases its cytoplasmic accumulation leading to their mutual stabilization and stable expression of a nuclear localization deficient VDR abolishes the growth-inhibitory activity of the receptor independent of 1, 25-dihydroxyvitamin D3 (vitamin D3). These results reveal a new paradigm in which a protein tyrosine phosphatase may stimulate breast cancer growth through increasing cytoplasmic translocation of a nuclear receptor leading to their mutual stabilization.

Regulatory effects of nitric oxide on Src kinase, FAK, p130Cas, and receptor protein tyrosine phosphatase alpha (PTP-alpha): a role for the cellular redox environment

The role of NO in regulating the focal adhesion proteins, Src, FAK, p130 Cas, and PTP-alpha, was investigated. Fibroblasts expressing PTP-alpha (PTP-alpha(WT) cells), fibroblasts "knockout" for PTP-alpha (PTP-alpha(-/-) cells), and "rescued" "knockout" fibroblasts (PTP-alpha A5/3 cells) were stimulated with either S-nitroso-N-acetylpenicillamine (SNAP) or fetal bovine serum (FBS). FBS increased inducible NO synthase in both cell lines. Activation of Src mediated either by SNAP or by FBS occurred independent of dephosphorylation of Tyr527 in PTP-alpha(-/-) cells. Both stimuli promoted dephosphorylation of Tyr527 and activation of Src kinase in PTP-alpha(WT) cells. NO-mediated activation of Src kinase affected the activities of FAK and p130Cas and was dependent on the expression of PTP-alpha. Analogous to tyrosine phosphorylation, SNAP and FBS stimulated differential generation of NO and S-nitrosylation of Src kinase in both cell lines. Incubation with SNAP resulted in higher levels of NO and S-nitrosylation of immunoprecipitated Src in PTP-alpha(-/-) cells (oxidizing redox environment) as compared with the levels of NO and S-nitrosylated Src in PTP-alpha(WT) cells (reducing redox environment). SNAP differentially stimulated cell proliferation of both cell lines is dependent on the intracellular redox environment, Src activity, and PTP-alpha expression. This dependence also is observed with FBS-stimulated cell migration.

Extracellular domain dependence of PTPalpha transforming activity

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.

Pathway analysis using random forests with bivariate node-split for survival outcomes

MOTIVATION

There is great interest in pathway-based methods for genomics data analysis in the research community. Although machine learning methods, such as random forests, have been developed to correlate survival outcomes with a set of genes, no study has assessed the abilities of these methods in incorporating pathway information for analyzing microarray data. In general, genes that are identified without incorporating biological knowledge are more difficult to interpret. Correlating pathway-based gene expression with survival outcomes may lead to biologically more meaningful prognosis biomarkers. Thus, a comprehensive study on how these methods perform in a pathway-based setting is warranted.

RESULTS

In this article, we describe a pathway-based method using random forests to correlate gene expression data with survival outcomes and introduce a novel bivariate node-splitting random survival forests. The proposed method allows researchers to identify important pathways for predicting patient prognosis and time to disease progression, and discover important genes within those pathways. We compared different implementations of random forests with different split criteria and found that bivariate node-splitting random survival forests with log-rank test is among the best. We also performed simulation studies that showed random forests outperforms several other machine learning algorithms and has comparable results with a newly developed component-wise Cox boosting model. Thus, pathway-based survival analysis using machine learning tools represents a promising approach in dissecting pathways and for generating new biological hypothesis from microarray studies.

AVAILABILITY

R package Pwayrfsurvival is available from URL: http://www.duke.edu/~hp44/pwayrfsurvival.htm.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Estrogen-mediated suppression of the gene encoding protein tyrosine phosphatase PTPRO in human breast cancer: mechanism and role in tamoxifen sensitivity

We have previously demonstrated the tumor suppressor characteristics of protein tyrosine phosphatase receptor-type O (PTPRO) in leukemia and lung cancer, including its suppression by promoter methylation. Here, we show tumor-specific methylation of the PTPRO CpG island in primary human breast cancer. PTPRO expression was significantly reduced in established breast cancer cell lines MCF-7 and MDA-MB-231 due to promoter methylation compared with its expression in normal human mammary epithelial cells (48R and 184). Further, the silenced gene could be demethylated and reactivated in MCF-7 and MDA-MB-231 cells upon treatment with 5-Azacytidine, a DNA hypomethylating agent. Because PTPRO promoter harbors estrogen-responsive elements and 17beta-estradiol (E2) plays a role in breast carcinogenesis, we examined the effect of E2 and its antagonist tamoxifen on PTPRO expression in human mammary epithelial cells and PTPRO-expressing breast cancer cell line Hs578t. Treatment with E2 significantly curtailed PTPRO expression in 48R and Hs578t cells, which was facilitated by ectopic expression of estrogen receptor (ER)beta but not ERalpha. On the contrary, treatment with tamoxifen increased PTPRO expression. Further, knockdown of ERbeta by small interfering RNA abolished these effects of E2 and tamoxifen. Chromatin immunoprecipitation assay showed association of c-Fos and c-Jun with PTPRO promoter in untreated cells, which was augmented by tamoxifen-mediated recruitment of ERbeta to the promoter. Estradiol treatment resulted in dissociation of c-Fos and c-Jun from the promoter. Ectopic expression of PTPRO in the nonexpressing MCF-7 cells sensitized them to growth-suppressive effects of tamoxifen. These data suggest that estrogen-mediated suppression of PTPRO is probably one of the early events in estrogen-induced tumorigenesis and that expression of PTPRO could facilitate endocrine therapy of breast cancer.

Apoptosis of estrogen-receptor negative breast cancer and colon cancer cell lines by PTP alpha and src RNAi

We show that siRNA-mediated suppression of protein tyrosine phosphatase alpha (PTP alpha) reduces Src activity 2 to 4-fold in breast, colon and other human cancer cell lines. Src and PTP alpha RNAi induced apoptosis in estrogen receptor (ER)-negative breast cancer and colon cancer cells, but not in immortalized noncancerous breast cells, ER-positive breast cancer cells or other cancer cell types tested. RNAi of other Src family members (Fyn and Yes) or of PTP1B, a phosphatase previously suggested to be an activator of Src in breast cancer, had no effect. Although further tests with primary tumor tissues are required, the unexpected correlation between ER status and Src/PTP alpha dependence in breast cancer cell lines may be important for planning therapeutic strategies, and the insensitivity of normal breast cells to the RNAi highlights the potential of PTP alpha, which may be easier to target than Src, as a therapeutic target in ER-negative breast cancer.

Natural compounds as a source of protein tyrosine phosphatase inhibitors: application to the rational design of small-molecule derivatives

Reversible phosphorylation of tyrosine residues is a key regulatory mechanism for numerous cellular events. Protein tyrosine kinases and protein tyrosine phosphatases (PTPs) have a pivotal role in regulating both normal cell physiology and pathophysiology. Accordingly, deregulated activity of both protein tyrosine kinases and PTPs is involved in the development of numerous congenitically inherited and acquired human diseases, prompting obvious pharmaceutical and academic research interest. The development of compound libraries with higher selective PTP inhibitory activity has been bolstered by the realization that many natural products have such activity and thus are interesting biologically lead compounds, which properties are widely exploited. In addition, more rational approaches have focused on the incorporation of phosphotyrosine mimetics into specific peptide templates (peptidomimetic backbones). Additional factors furthering discovery as well as therapeutic application of new bioactive molecules are the integration of functional genomics, cell biology, structural biology, drug design, molecular screening and chemical diversity. Together, all these factors will lead to new avenues to treat clinical disease based on PTP inhibition.

Crystallization and preliminary X-ray diffraction analysis of rat protein tyrosine phosphatase eta

The rat protein tyrosine phosphatase eta (rPTPeta) is a cysteine-dependent phosphatase which hydrolyzes phosphoester bonds in proteins and other molecules. rPTPeta and its human homologue DEP-1 are involved in neoplastic transformations. Thus, expression of the protein is reduced in all oncogene-transformed thyroid cell lines and is absent in highly malignant thyroid cells. Moreover, consistent with the suggested tumour suppression role of PTPeta, inhibition of the tumorigenic process occurs after its exogenous reconstitution, suggesting that PTPeta might be important for gene therapy of cancers. In this study, the catalytic domain of rPTPeta was produced in Escherichia coli in soluble form and purified to homogeneity. Crystals were obtained by the hanging-drop vapour-diffusion method. Diffraction data were collected to 1.87 A resolution. The crystal belongs to space group P2(1)2(1)2(1), with unit-cell parameters a = 46.46, b = 63.07, c = 111.64 A, and contains one molecule per asymmetric unit.

Targeting the PTPome in human disease

Protein tyrosine phosphatases (PTPs) play vital roles in numerous cellular processes and are implicated in a growing number of human diseases, ranging from cancer to cardiovascular, immunological, infectious, neurological and metabolic diseases. There are at least 107 genes in the human genome, collectively referred to as the human 'PTPome'. Here the authors review the involvement of PTPs in human disease, discuss their potential as drug targets, and current efforts to develop PTP inhibitors for the treatment of human disease. Finally, the authors present their view of the future for PTPs as drug targets.

Fibronectin rigidity response through Fyn and p130Cas recruitment to the leading edge

Cell motility on extracellular matrices critically depends on matrix rigidity, which affects cell adhesion and formation of focal contacts. Receptor-like protein tyrosine phosphatase alpha (RPTPalpha) and the alphavbeta3 integrin form a rigidity-responsive complex at the leading edge. Here we show that the rigidity response through increased spreading and growth correlates with leading edge recruitment of Fyn, but not endogenous c-Src. Recruitment of Fyn requires the palmitoylation site near the N-terminus and addition of that site to c-Src enables it to support a rigidity response. In all cases, the rigidity response correlates with the recruitment of the Src family kinase to early adhesions. The stretch-activated substrate of Fyn and c-Src, p130Cas, is also required for a rigidity response and it is phosphorylated at the leading edge in a Fyn-dependent process. A possible mechanism for the fibronectin rigidity response involves force-dependent Fyn phosphorylation of p130Cas with rigidity-dependent displacement. With the greater displacement of Fyn from p130Cas on softer surfaces, there will be less phosphorylation. These studies emphasize the importance of force and nanometer-level movements in cell growth and function.

Protein-tyrosine phosphatases and cancer

Tyrosine phosphorylation is an important signalling mechanism in eukaryotic cells. In cancer, oncogenic activation of tyrosine kinases is a common feature, and novel anticancer drugs have been introduced that target these enzymes. Tyrosine phosphorylation is also controlled by protein-tyrosine phosphatases (PTPs). Recent evidence has shown that PTPs can function as tumour suppressors. In addition, some PTPs, including SHP2, positively regulate the signalling of growth-factor receptors, and can be oncogenic. An improved understanding of how these enzymes function and how they are regulated might aid the development of new anticancer agents.

Expression analysis and modulation by HIV-Tat of the tyrosine phosphatase HD-PTP

The human immunodeficiency virus type 1 Tat transactivates viral proteins and also affects the expression of eukaryotic genes. Since Tat is angiogenic, we assumed that the isolation of differentially expressed genes in Tat-treated endothelial cells would yield insights into the molecular mechanisms of the angiogenic process. By RNA fingerprinting, we found that Tat upregulates the tyrosine phosphatase HD-PTP mRNA in a human endothelial cell line. At the moment, little is known about HD-PTP. We here show that HD-PTP is highly conserved through evolution from yeast to man, and is ubiquitously distributed in adult and fetal tissues. HD-PTP is expressed in human cell lines derived from different tumors, but the mRNA levels do not appear to correlate with the malignant phenotype of the cells. HD-PTP mRNA was also detected in cell lines derived from tumors that develop in BKV/Tat-transgenic mice. Interestingly, a relation exists between the amounts of secreted Tat and the levels of HD-PTP mRNA. HD-PTP encodes a 185-kDa protein which is expressed in human endothelial from the umbilical cord and in human Kaposi-spindle cells. Tat-induction of HD-PTP mRNA parallels only with a slight increase of the protein, which occurs after 24 and 48 h of incubation in the presence of Tat. These results suggest that HD-PTP amounts might be regulated both at the transcriptional and post-transcriptional levels.

Epigenetic regulation of protein tyrosine phosphatases: potential molecular targets for cancer therapy

Promoter methylation-mediated silencing is a hallmark of many established tumor suppressor genes. This review focuses on the methylation and suppression of a receptor-type tyrosine phosphatase gene, PTPRO, in a variety of solid and liquid tumors. In addition, PTPRO exhibits many other characteristics of a bona fide tumor suppressor. Reactivation of genes silenced by methylation using inhibitors of DNA methyltransferases and histone deacetylases, and the potential application of PTPRO as a molecular target for cancer therapy have been discussed.