Overexpression of leucocyte common antigen (LAR) P-subunit in thyroid carcinomas

The mechanism of covalent inhibition of LAR phosphatase by illudalic acid

Leukocyte antigen-related (LAR) phosphatase is a receptor-type protein tyrosine phosphatase involved in cellular signaling and associated with human disease including cancer and metabolic disorders. Selective inhibition of LAR phosphatase activity by well characterized and well validated small molecules would provide key insights into the roles of LAR phosphatase in health and disease, but identifying selective inhibitors of LAR phosphatase activity has been challenging. Recently, we described potent and selective inhibition of LAR phosphatase activity by the fungal natural product illudalic acid. Here we provide a detailed biochemical characterization of the adduct formed between LAR phosphatase and illudalic acid. A mass spectrometric analysis indicates that two cysteine residues are covalently labeled by illudalic acid and a related analog. Mutational analysis supports the hypothesis that inhibition of LAR phosphatase activity is due primarily to the adduct with the catalytic cysteine residue. A computational study suggests potential interactions between the illudalic acid moiety and the enzyme active site. Taken together, these data offer novel insights into the mechanism of inhibition of LAR phosphatase activity by illudalic acid.

Inhibition of protein tyrosine phosphatase receptor type F suppresses Wnt signaling in colorectal cancer

Wnt signaling dysregulation promotes tumorigenesis in colorectal cancer (CRC). We investigated the role of PTPRF, a receptor-type tyrosine phosphatase, in regulating Wnt signaling in CRC. Knockdown of PTPRF decreased cell proliferation in patient-derived primary colon cancer cells and established CRC cell lines. In addition, the rate of proliferation as well as colony formation ability were significantly decreased cells in tumor organoids grown in 3D, whereas the number of differentiated tumor organoids were markedly increased. Consistently, knockdown of PTPRF resulted in a decrease in the expression of genes associated cancer stem cells downstream of Wnt/β-catenin signaling. Treating PTPRF knockdown cells with GSK3 inhibitor rescued the expression of Wnt target genes suggesting that PTPRF functions upstream of the β-catenin destruction complex. PTPRF was found to interact with LRP6 and silencing PTPRF largely decreased the activation of LRP6. Interestingly, this PTPRF-mediated activation of Wnt signaling was blocked in cells treated with clathrin endocytosis inhibitor. Furthermore, knockdown of PTPRF inhibited xenograft tumor growth in vivo and decreased the expression of Wnt target genes. Taken together, our studies identify a novel role of PTPRF as an oncogenic protein phosphatase in supporting the activation of Wnt signaling in CRC.

Synthesis and PTP Inhibitory Activity of Illudalic Acid and Its Methyl Ether, with Insights into Selectivity for LAR PTP over Other Tyrosine Phosphatases under Physiologically Relevant Conditions

The protein tyrosine phosphatase (PTP) family of enzymes includes many attractive therapeutic targets, such as those in the leukocyte common antigen-related (LAR) subfamily of receptor PTPs. Synthesis and PTP inhibitory activity of illudalic acid and its methyl ether are described, with a focus on selective inhibition of LAR PTP relative to a small collection of other representative PTPs. The synthesis comprises 16 steps and provides illudalic acid in up to 12% overall yield from neopentylene-fused benzoate 1 (20 steps from commercial materials). Illudalic acid dose-dependently (measured IC₅₀ = 2.1 ± 0.2 μM) and time-dependently inhibits LAR consistent with previous reports of covalent binding. The kinetics of LAR inhibition by illudalic acid are consistent with a two-step mechanism in which the inhibitor and enzyme first interact noncovalently (K_I = 130 ± 50 μM), followed by covalent ligation at a rate k_inact = 1.3 ± 0.4 min^-1. The k_inact/K_I ratio of 10⁴ corresponds to a t^∞_1/2 of 0.5 min, as discussed herein. The phenol methyl ether of illudalic acid was found to be less potent in our dose-response assays (measured IC₅₀ = 55 ± 6 μM) but more selective for LAR, with a weaker initial noncovalent interaction and faster covalent ligation of LAR as compared to illudalic acid itself. A truncated analogue of illudalic acid that lacks the neopentylene ring fusion was found to be devoid of significant activity under our assay conditions, in contrast to previous reports. These observations collectively help inform further development of illudalic acid analogues as potent and selective inhibitors of the LAR subfamily of tyrosine phosphatases.

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.

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.

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.

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.

Function of JunB in transient amplifying cell senescence and progression of human prostate cancer

PURPOSE

Replicative senescence in cells acts as a barrier against excessive proliferation and carcinogenesis. Transient amplifying cells (TAC) are a subset of basal cell populations within the prostate from which cancers are thought to originate; therefore, we focused on prostate TAC to investigate the molecular mechanisms by which the TAC may be able to evade senescence.

EXPERIMENTAL DESIGN

TAC clones were isolated from each zone within the whole prostate and analyzed in flow cytometry. Prostate cancer cells were transfected with junB small interfering RNA (siRNA) and examined by chorioallantoic membrane assay for cancer invasion. Immunohistochemical analysis was done in primary and metastatic prostate cancer specimens.

RESULTS

TAC populations showed increased expression of p53, p21, p16, and pRb, resulting in senescence. TAC clones with reduced p16 expression successfully bypassed this phase. We further found close correlation between the levels of junB and p16 expression. Repeated transfection of junB siRNA in prostatic TAC allowed the cells to escape senescence presumably through inactivation of p16/pRb. The chorioallantoic membrane invasion assay showed much lower in invasive cancer cells with high expression of junB; conversely, silencing of junB by transfection with junB siRNA promoted invasion. We also found that metastatic prostate cancers, as well as cancers with high Gleason scores, showed significantly low junB immunopositivity.

CONCLUSIONS

JunB is an essential upstream regulator of p16 and contributes to maintain cell senescence that blocks malignant transformation of TAC. JunB thus apparently plays an important role in controlling prostate carcinogenesis and may be a new target for cancer prevention and therapy.

Prostate cancer antigen-1 contributes to cell survival and invasion though discoidin receptor 1 in human prostate cancer

A novel gene, prostate cancer antigen (PCA)-1, was recently reported to be expressed in the prostate; however, its biological roles remain unclear. Knockdown of the PCA-1 gene by small interfering RNA transfection induced apoptosis through reducing the expression of the anti-apoptotic molecule Bcl-xl and cytoplasmic release of cytochrome c in the androgen-independent prostate cancer cell line PC3. Moreover, in vitro matrigel and in vivo chorioallantoic membrane assays showed that silencing of PCA-1 significantly downregulated discoidin receptor (DDR)-1 expression, resulting in suppression of cancer-cell invasion. Transfection with PCA-1 increased the levels of both Bcl-xl and DDR1, which made the cells more invasive through the upregulation of matrix metalloproteinase 9 in DU145. Interestingly, long-term culture using androgen-free medium increased the level of PCA-1 and the related expression of Bcl-xl and DDR-1 in the androgen-sensitive cancer cell line LNCaP, suggesting that PCA-1 signaling is associated with androgen independence. Immunohistochemical analysis in a series of 169 prostate carcinomas showed that PCA-1 and DDR1 were strongly expressed in prostate cancer cells, including preneoplastic lesions, but there was little or no expression in normal epithelium. Moreover, the expression of PCA-1 and DDR-1 was associated with a hormone-independent state of prostate cancer. Taken together, we propose that PCA-1-DDR-1 signaling is a new important axis involved in malignant potential prostate cancer associated with hormone-refractory status.

c-Jun NH2 terminal kinase activation and decreased expression of mitogen-activated protein kinase phosphatase-1 play important roles in invasion and angiogenesis of urothelial carcinomas

We here examined whether c-Jun NH(2) terminal kinase (JNK) might be involved in the progression of urothelial carcinomas. In vitro and in vivo invasion assays using Matrigel and chick embryo chorioallantoic membrane approaches showed constitutive activation of JNK to significantly increase two processes, invasion and angiogenesis, in the human urothelial carcinoma cell line kU-7, this being suppressed by a JNK inhibitor, SP600125, or cell-permeable peptides. In addition, we found that mitogen-activated protein kinase phosphatase (MKP)-1 functions as an endogenous inhibitor of JNK-mediated signals in urothelial carcinoma cells: chorioallantoic membrane assays showed UMUC14 cells with low MKP-1 expression to be more invasive and have pronounced angiogenesis compared to UMUC6 cells with high MKP-1. Furthermore, knockdown of the MKP-1 gene by siRNA transfection enhanced JNK activation in UMUC6 cells to the UMUC14 level. Immunohistochemically, JNK was found to be highly phosphorylated in high-grade and invasive carcinomas (>/=pT2) as well as carcinoma in situ but not in low-grade and noninvasive phenotypes (pTa, pT1). In contrast, MKP-1 was much more expressed in low-grade/noninvasive cancers than with the high-grade/invasive phenotype, reversely correlating with phosphorylated JNK. Taken together, JNK activation and decreased expression of MKP-1 may play important roles in progression of urothelial carcinoma.

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.

High expression of a new marker PCA-1 in human prostate carcinoma

PURPOSE

Identifying the genetic factors involved in prostate carcinogenesis is critical. Novel cancer-specific markers aid in early detection, in differentiating between cancer and nonmalignant disorders, and in monitoring clinical of prostate disease. We therefore examined differential gene displays in an attempt to identify genes that may be involved in prostate carcinogenesis.

EXPERIMENTAL DESIGN

Applying fluorescent differential display analysis to human prostate carcinomas, we have identified and cloned several cDNA transcripts. Antisera were raised against synthetic peptides and used in Western blot and immunohistochemical analyses. The mRNAs were also analyzed by real-time reverse transcription-PCR. For functional analysis, we assessed methylmethane sulfonate (MMS)-induced toxicity in COS-7 cells after cDNA transfection.

RESULTS

We identified a gene, designated prostate cancer antigen-1 (pca-1), which shows high mRNA expression in prostate carcinoma. Database analysis of the deduced amino acid sequence of PCA-1 indicated high similarity to Escherichia coli AlkB, a DNA alkylation damage repair enzyme. By immunohistochemical analysis, PCA-1 was expressed in a high number of both prostate carcinoma samples and in the atypical cells within high-grade prostatic intraepithelial neoplasias but not in benign prostatic hyperplasia or normal adjacent tissues. PCA-1-transfected COS-7 cells further showed resistance against MMS-induced cell death.

CONCLUSIONS

These findings suggest that PCA-1 could be a useful diagnostic marker. Furthermore, because this human counterpart of AlkB exhibits a protective function against alkylation damage in mammalian cells, PCA-1 may also serve as a therapeutic target molecule for prostate cancer.

Phosphorylation status of Fas-associated death domain-containing protein (FADD) is associated with prostate cancer progression

It has recently been demonstrated that phosphorylation of FADD at serine 194 plays an important role in the induction of apoptosis by anti-cancer drugs in human prostate cancer cells. The present study has assessed whether this phosphorylation status is valuable as a marker for human prostate cancer progression, and has investigated its biological role in cell growth. Immunohistochemical studies revealed much higher phosphorylation of FADD at serine 194 in normal epithelial cells than in cancer cells, although FADD was found to be highly expressed to the same extent in both cases. The positivity for phosphorylated FADD was significantly lower for patients with a Gleason score greater than or equal to 7, a positive surgical margin, extracapsular or seminal vesicle invasion. In addition, a relationship was also apparent in cancer cells refractory to neoadjuvant hormonal therapy. Interestingly, in Gleason score 3 + 4 tumours, the positivity for FADD phosphorylation was statistically increased by neoadjuvant hormonal therapy, resulting in a reduced percentage of cases with a positive surgical margin and extracapsular invasion. In vitro data showed different functions of phosphorylated and non-phosphorylated FADD: in normal epithelial cells, overexpression of a phosphorylation-mimicking mutant FADD (S194E) caused G2/M cell-cycle arrest, while a non-phosphorylation-mimicking mutant (S194A) had no effect, whereas S194A overexpression resulted in cell cycle progression and enhanced colony-forming activity in cancer cells, but S194E FADD was without influence. These results clearly demonstrate that transition from phosphorylated FADD to the non-phosphorylated form might be associated with carcinogenesis and that induction of FADD phosphorylation could therefore be a target for chemohormonal therapy of human prostate cancer. Moreover, assessment of FADD phosphorylation may be useful as a new biomarker to predict cancer progression.

A proteomic approach to tumour target identification using phage display, affinity purification and mass spectrometry

Tumour-associated cell surface markers are potential targets for antibody-based therapies. We have obtained a panel of myeloid cell binding single chain variable fragments (scFv) by applying phage display selection on myeloid cell lines followed by a selection round on freshly isolated acute myeloid leukaemia (AML) blasts using flow cytometry. To identify the target antigens, the scFv were recloned and expressed in an IgG(1) format and tested for their ability to immunoprecipitate cell surface proteins. The IgGs that reacted with distinct cell membrane extractable proteins were used in large-scale affinity purification of the target antigen followed by mass-spectrometry-based identification. Well-characterised cell surface antigens, such as leukocyte antigen-related receptor protein tyrosine phosphatase (LAR PTP) and activated leukocyte adhesion molecule (ALCAM) in addition to several unknown proteins, like ATAD3A, were identified. These experiments demonstrate that phage antibody selection in combination with affinity chromatography and mass spectrometry can be exploited successfully to identify novel antibody target molecules on malignant cells.

Functional significance of the LAR receptor protein tyrosine phosphatase family in development and diseases

The protein tyrosine phosphatases (PTPs) have emerged as critical players in diverse cellular functions. The focus of this review is the leukocyte common antigen-related (LAR) subfamily of receptor PTPs (RPTPs). This subfamily is composed of three vertebrate homologs, LAR, RPTP-sigma, and RPTP-delta, as well as few invertebrates orthologs such as Dlar. LAR-RPTPs have a predominant function in nervous system development that is conserved throughout evolution. Proteolytic cleavage of LAR-RPTP proproteins results in the noncovalent association of an extracellular domain resembling cell adhesion molecules and intracellular tandem PTPs domains, which is likely regulated via dimerization. Their receptor-like structures allow them to sense the extracellular environment and transduce signals intracellularly via their cytosolic PTP domains. Although many interacting partners of the LAR-RPTPs have been identified and suggest a role for the LAR-RPTPs in actin remodeling, very little is known about the mechanisms of action of RPTPs. LAR-RPTPs recently raised a lot of interest when they were shown to regulate neurite growth and nerve regeneration in transgenic animal models. In addition, LAR-RPTPs have also been implicated in metabolic regulation and cancer. This RPTP subfamily is likely to become important as drug targets in these various human pathologies, but further understanding of their complex signal transduction cascades will be required.Key words: protein tyrosine phosphatase, LAR, signal transduction, nervous system development.