Protein tyrosine phosphatase activity as a diagnostic parameter in breast cancer

Immunohistochemical Detection of Hematopoietic Cell-specific Protein-Tyrosine Phosphatase (Tyrosine Phosphatase SHP-1) in a Series of Endometrioid and Serous Endometrial Carcinoma

In this study, we evaluated the expression of SHP-1 (PTPN6) in endometrioid (Ec) and serous (Sc) subtypes of endometrial carcinoma by immunohistochemical analysis. In total, 114 patients with Ec carcinoma and 48 patients with Sc carcinoma were enrolled in this study. The correlation between the type of histology, the grade of tumor, the stage of development, and immunoreactivity to SHP-1 was evaluated. Kaplan-Meier and multivariate survival analyses, using a Cox regression model, were performed to establish whether this marker has prognostic value in these malignancies, on the basis of follow-up and stratification of the patients according to their SHP-1 immunoreactivity. A significantly higher SHP-1 expression was observed in the Ec group compared with the Sc group (P=0.0005, Fisher exact test). In the Ec group, SHP-1 immunoreactivity was correlated with grading, demonstrating that more differentiated lesions expressed SHP-1 more frequently than less differentiated neoplasms (G1 vs. G2, P=0.0243, statistically significant value, Fisher exact test; G1 vs. G3, P=0.0088, extremely significant value, Fisher exact test). Instead, in the Sc group, SHP-1 expression was not correlated with grading, as Sc is now defined as a high-grade carcinoma. SHP-1 expression did not change with neoplastic progression in Ec and Sc groups. From both univariate and multivariate analysis in the Ec group, expression of SHP-1 remained a positive prognostic factor (P=0.004, log-rank test) [HR=0.32 (0.11 to 0.94), P=0.039]. In contrast, in the Sc group, no correlation between SHP-1 expression and survival was noted (P=0.77, log-rank test). In this study, we observed that the absence of SHP-1 in immunohistochemical analysis might serve as a marker of poor prognosis for a subset of high-grade endometrial cancer.

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 4A2 expression predicts overall and disease-free survival of human breast cancer and is associated with estrogen and progestin receptor status

Expression of protein tyrosine phosphatase PTP4A2 (also known as PRL2) has been examined in a variety of human carcinomas, although its role in breast cancer remains inconclusive. Since the majority of previous breast cancer studies utilized tissue biopsies composed of heterogeneous cell populations, we hypothesized that an examination of PTP4A2 expression in carcinoma cells isolated by laser capture microdissection (LCM) would provide a more accurate means of assessing its predictive value. From investigations of 247 human breast cancer biopsies collected under standardized, stringent conditions, total RNA was extracted from LCM-procured carcinoma cells to perform microarray analyses to identify gene signatures associated with breast cancer behavior. Expression of PTP4A2 was corroborated by real-time quantitative polymerase chain reaction (qPCR) and referenced to estrogen and progesterone receptor levels. Patient outcomes for overall and disease-free survival were more favorable (p = 0.004 and p = 0.001, respectively) when the expression of PTP4A2 in breast carcinomas was increased compared to patients with biopsies with decreased PTP4A2 levels. PTP4A2 expression determined either by microarray or qPCR was elevated in either estrogen receptor (ER)-positive or progestin receptor (PR)-positive breast cancer biopsies compared to ER-negative or PR-negative biopsies. However, PTP4A2 expression was only correlated with overall survival in PR-positive breast carcinomas. These data suggest that PTP4A2 mRNA expression alone may serve as a biomarker for prediction of a breast cancer patient's risk of recurrence and overall survival.

Src and FAK mediate cell-matrix adhesion-dependent activation of Met during transformation of breast epithelial cells

Cell-matrix adhesion has been shown to promote activation of the hepatocyte growth factor receptor, Met, in a ligand-independent manner. This process has been linked to transformation and tumorigenesis in a variety of cancer types. In the present report, we describe a key role of integrin signaling via the Src/FAK axis in the activation of Met in breast epithelial and carcinoma cells. Expression of an activated Src mutant in non-neoplastic breast epithelial cells or in carcinoma cells was found to increase phosphorylation of Met at regulatory tyrosines in the auto-activation loop domain, correlating with increased cell spreading and filopodia extensions. Furthermore, phosphorylated Met is complexed with beta1 integrins and is co-localized with vinculin and FAK at focal adhesions in epithelial cells expressing activated Src. Conversely, genetic or pharmacological inhibition of Src abrogates constitutive Met phosphorylation in carcinoma cells or epithelial cells expressing activated Src, and inhibits filopodia formation. Interestingly, Src-dependent phosphorylation of Met requires cell-matrix adhesion, as well as actin stress fiber assembly. Phosphorylation of FAK by Src is also required for Src-induced Met phosphorylation, emphasizing the importance of the Src/FAK signaling pathway. However, stimulation of Met phosphorylation by addition of exogenous HGF in epithelial cells is refractory to inhibition of Src family kinases, indicating that HGF-dependent and Src/integrin-dependent Met activation occur via distinct mechanisms. Together these findings demonstrate a novel mechanism by which the Src/FAK axis links signals from the integrin adhesion complex to promote Met activation in breast epithelial cells.

Protein tyrosine phosphatases and breast cancer

Protein tyrosine phosphatases (PTPs) consist of a large family of related enzymes, including the group of classical PTPs with its two main subgroups, the transmembrane receptor-type (RPTPs) and the intracellular or non-transmembrane PTPs. Published data on the expression and function of a panel of these enzymes in normal and cancerous breast tissues are discussed in this review. While most studies, albeit on different enzymes, have tended to agree on the evidence for an increased PTP expression in breast cancer, any connection between PTP expression and the enzymes' role in cancer development and progression remains largely open to interpretation. Concomitant increases of protein tyrosine kinase (PTK) and PTP activities in many cancers further indicate that a complex dysregulation in the balance of tyrosine phosphorylation could be responsible for major alterations in various cellular processes controlling tissue homeostasis. In particular, any relationship between the expression of PTPs and their specific diverse roles in the regulation of cell growth and apoptosis in breast cancer needs to be addressed in major fundamental, preclinical and clinical studies.

Prognostic assessment of PTK activity in T1-T2, N0-N1, M0 breast cancer: a multicentric retrospective study

Protein tyrosine kinases (PTKs) play a major role in the transduction of intracellular mitogenic signal. PTKs are also involved in the process of cellular transformation. A number of studies have reported increased PTK activities in cytosolic fractions from human breast carcinoma. However, the possible pronostic value of these activities is difficult to establish from these studies, mostly conducted on limited numbers of patients. In order to clear up the issue, we have investigated a large series of patients with a long follow-up, using a retrospective multicentric study (894 breast cancers T1-T2, N0-N1, M0; median follow-up: 67 months). PTKs were measured using a radioenzymatic assay as described in our previously report. We confirmed the already observed correlation between PTK activities and Scarff-Bloom grading (p < 10(-5)), negative estrogen receptor (ER), and progesterone receptor (PR) status. By contrast, we found in this study a correlation between PTK values and clinical nodal status (p = 0.00027) not showed in our precedent analysis. In Cox multivariate analysis, PTK activity does not emerge as a significant pronostic parameter. On the other hand, tumor PTK activity assay may prove of great interest in clinical research using newly developed tyrosine kinase inhibitors in order to assess their biological impact and eventually to predict the responsiveness to these new therapeutic agents.

PTP LAR expression compared to prognostic indices in metastatic and non-metastatic breast cancer

Several prognostic indices in breast cancer, including c-erbB2, epithelial growth factor receptors (EGFR), estrogen and progesterone receptors are signal transduction molecules. Recently, expression of another signal transduction molecule, the protein tyrosine phosphatase LAR, has been suggested to be increased in breast cancer. The objective of the current investigation was to examine the relationship between LAR expression and prognostic parameters in breast cancer. LAR expression was associated with metastatic potential in the well-characterized 13762NF rat mammary adenocarcinoma clones. The metastatic MTLn3 and MTLn2 clones expressed sizable amounts of LAR. The essentially non-metastatic MTC clone had little LAR expression. C-erbB2 had highest expression in the highly metastatic MTLn3 clone, but c-erbB2 levels were sizeable in the weakly metastatic MTLn2 and non-metastatic MTC clone. EGFR expression had the strongest association with a clone's metastatic potential, being very high in MTLn3, weak in MTLn2, and undetectable in MTC. In human breast cancer specimens, LAR expression was strongly positive in 50% of metastatic cases but in only 21% of 'non-metastatic' cases. As with the 13762NF-derived clones, c-erbB2 expression was strongly positive independent of metastatic phenotype. However, 46% (6/13) of cases that were strongly positive for c-erbB2 were strongly positive for LAR. Only 17% (2/11) of negative or weakly c-erbB2 positive samples were strongly positive for LAR. All ER+ positive tumors (n = 15) were positive for LAR and 53% of these tumors were strongly positive for LAR. In ER negative cases, only 1 of 11 was strongly positive for LAR. While the current data indicate a strong association between ER and LAR expression in breast cancer tissue (p = 0.003), additional studies are warranted to further explore the relationship between LAR and prognostic indices of breast cancer progression.

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

Tyrosine phosphorylation is controlled by a balance of tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Whereas the contribution of PTKs to breast tumorigenesis is the subject of intense scrutiny, the potential role of PTPs is poorly known. RPTPalpha is implicated in the activation of Src family kinases, and regulation of integrin signaling, cell adhesion, and growth factor responsiveness. To explore its potential contribution to human neoplasia, we surveyed RPTPalpha protein levels in primary human breast cancer. We found RPTPalpha levels to vary widely among tumors, with 29% of cases manifesting significant overexpression. High RPTPalpha protein levels correlated significantly with low tumor grade and positive estrogen receptor status. Expression of RPTPalpha in breast carcinoma cells led to growth inhibition, associated with increased accumulation in G0 and G1, and delayed tumor growth and metastasis. To our knowledge, this is the first example of a study correlating expression level of a specific bona fide PTP with neoplastic disease status in humans.

Protein-tyrosine phosphatases: structure, mechanism, and inhibitor discovery

Protein-tyrosine kinases (PTKs) and their associated signaling pathways are crucial for the regulation of numerous cell functions including growth, mitogenesis, motility, cell-cell interactions, metabolism, gene transcription, and the immune response. Since tyrosine phosphorylation is reversible and dynamic in vivo, the phosphorylation states of proteins are governed by the opposing actions of PTKs and protein-tyrosine phosphatases (PTPs). In this light, both PTKs and PTPs play equally important roles in signal transduction in eukaryotic cells, and comprehension of mechanisms behind the reversible pTyr-dependent modulation of protein function and cell physiology must necessarily encompass the characterization of PTPs as well as PTKs. In spite of the large number of PTPs identified to date and the emerging role played by PTPs in disease, a detailed understanding of the role played by PTPs in signaling pathways has been hampered by the absence of PTP-specific agents. Such PTP-specific inhibitors could potentially serve as useful tools in determining the physiological significance of protein tyrosine phosphorylation in complex cellular signal transduction pathways and may constitute valuable therapeutics in the treatment of several human diseases. The goal of this review is therefore to summarize current understandings of PTP structure and mechanism of catalysis and the relationship of these to PTP inhibitor development. The review is organized such that enzyme structure is covered first, followed by mechanisms of catalysis then PTP inhibitor development. In discussing PTP inhibitor development, nonspecific inhibitors and those obtained by screening methods are initially presented with the focus then shifting to inhibitors that utilize a more structure-based rationale.

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.

Altered distribution and expression of protein tyrosine phosphatases in normal human skin as compared to squamous cell carcinomas

Amounts and subcellular localizations of 4 protein tyrosine phosphatases (PTPs) were compared in cultured normal human keratinocytes, an immortalized keratinocyte cell line, and 2 squamous cell carcinoma (SCC) lines. Cellular localizations for PTPs were determined in biopsies of normal human skin and SCCs. Compared to normal keratinocytes, SCC cell lines had higher levels of PTP-1B and T-cell PTP and comparable levels of PTP-1C or PTP-1D. The subcellular localization of each PTP was similar in the 3 types of keratinocytes with PTP-1B localizing to the endoplasmic reticulum, T-cell PTP exclusively found in the nucleus, PTP-1C localized to the plasma membrane, cytosol and nucleus, and PTP-1D present in both cytosol and nucleus. Compared to normal skin, immunoreactive PTP-1B was markedly increased in the invasive margins of SCCs while T-cell PTP was generally increased in tumors. PTP-1C immunostaining varied between cells with no obvious difference between normal and neoplastic tissues. The intensity and distribution of immunoreactive PTP-1D varied greatly between cells within tumors. These differences in amounts and in cellular and subcellular localization of these PTPs, especially those differences in invasive margins of SCCs, may reflect the diverse roles these PTPs play in the proliferation and invasive potential of neoplastic keratinocytes.

Novel alternative splicing predicts a secreted extracellular isoform of the human receptor-like protein tyrosine phosphatase LAR

RT-PCR was used to examine the expression of LAR (encoding the leukocyte-common antigen-related protein tyrosine phosphatase) in normal human colon mucosa, and colon polyps and tumors. Although the LAR protein was not detected in the colon in a previous immunohistochemical study, amplification of a region of LAR between the most membrane proximal (eighth) fibronectin type-III (FN-III) repeat and the transmembrane domain demonstrated LAR expression in all samples, but showed no difference in expression within matched samples from each patient examined. An additional minor fragment amplified in all reactions was consistently observed in colon and various cell line samples using this and two other LAR-specific sets of primers. Cloning and sequencing of the fragment identified it as deriving from a novel alternatively spliced form of LAR containing a retained intron of 85 bp. This intron encodes an additional 13 amino acids followed by an in-frame stop codon, thus its retention is predicted to give rise to a secreted LAR extracellular region isoform(s). LAR transcripts containing the intron were detected by RNase protection assay of colon samples and were present in most human tissues examined by Northern analysis. A protein in colon tumor extract was recognized by antiserum raised to the intron-encoded sequence. Soluble isoforms of the LAR extracellular immunoglobulin (Ig)-like/FN-III repeat-containing region could have a biological function distinct from those isoforms localized at the cell surface and/or coupled to intracellular phosphatase activity.

An enzyme-linked immunosorbent assay for the determination of src-family tyrosine kinase activity in breast cancer

An enzyme-linked immunosorbent assay is described for the determination of protein tyrosine kinase activity originating from the presence of src-like tyrosine kinases in biological samples. In this assay a peptide derived from p34cdc2, cdc2(6-20)NH2, is coupled to the wells of a maleic anhydride-activated microtiter plate. This particular peptide has been described as an efficient and specific substrate for protein tyrosine kinases belonging to the src family kinases (Cheng et al., J.Biol.Chem. 267 (1992) 9248-9256). After incubation of the coated substrate with sample and ATP, the amount of phosphorylated tyrosyl residues is determined with phosphotyrosine specific antibodies and a secondary peroxidase-labeled antibody. The assay appears to be very sensitive and is linear with sample protein concentration and phosphorylation time. Intra-assay variation is < 5%, whereas day-to-day variation is < 10%. The results of the assay have been compared with an ELISA in which the broad-specificity tyrosine kinase substrate poly(GluNa,Tyr)4:1 was coated. The results of both assays in 27 cytosolic breast cancer samples correlated very well (r = 0.94), in accordance with the predominant expression of src kinase activity in breast cancers (Ottenhoff-Kalff et al., Cancer Res. 52 (1992), 4773-4778). The present assay provides an easy, reproducible, and quick alternative for the usual radioactive methods used for the determination of src-kinase activities including immunecomplex kinase assay and TCA-precipitation assays. It allows the determination of src-like activities in human tumors for routine diagnostic purposes.