Phosphatase of regenerating liver-3 localizes to cyto-membrane and is required for B16F1 melanoma cell metastasis in vitro and in vivo

Targeting PRL phosphatases in hematological malignancies

INTRODUCTION

Phosphatase of regenerating liver (PRL) family proteins, also known as protein tyrosine phosphatase 4A (PTP4A), have been implicated in many types of cancers. The PRL family of phosphatases consists of three members, PRL1, PRL2, and PRL3. PRLs have been shown to harbor oncogenic potentials and are highly expressed in a variety of cancers. Given their roles in cancer progression and metastasis, PRLs are potential targets for anticancer therapies. However, additional studies are needed to be performed to fully understand the roles of PRLs in blood cancers.

AREAS COVERED

In this review, we will summarize recent studies of PRLs in normal and malignant hematopoiesis, the role of PRLs in regulating various signaling pathways, and the therapeutic potentials of targeting PRLs in hematological malignancies. We will also discuss how to improve current PRL inhibitors for cancer treatment.

EXPERT OPINION

Although PRL inhibitors show promising therapeutic effects in preclinical studies of different types of cancers, moving PRL inhibitors from bench to bedside is still challenging. More potent and selective PRL inhibitors are needed to target PRLs in hematological malignancies and improve treatment outcomes.

Development and characterization of nanobodies that specifically target the oncogenic Phosphatase of Regenerating Liver-3 (PRL-3) and impact its interaction with a known binding partner, CNNM3

Phosphatase of Regenerating Liver-3 (PRL-3) is associated with cancer progression and metastasis. The mechanisms that drive PRL-3's oncogenic functions are not well understood, partly due to a lack of research tools available to study this protein. We have begun to address these issues by developing alpaca-derived single domain antibodies, or nanobodies, targeting PRL-3 with a KD of 30-300 nM and no activity towards highly homologous family members PRL-1 and PRL-2. We found that longer and charged N-terminal tags on PRL-3, such as GFP and FLAG, changed PRL-3 localization compared to untagged protein, indicating that the nanobodies may provide new insights into PRL-3 trafficking and function. The nanobodies perform equally, if not better, than commercially available antibodies in immunofluorescence and immunoprecipitation. Finally, hydrogen-deuterium exchange mass spectrometry (HDX-MS) showed that the nanobodies bind partially within the PRL-3 active site and can interfere with PRL-3 phosphatase activity. Co-immunoprecipitation with a known PRL-3 active site binding partner, the CBS domain of metal transporter CNNM3, showed that the nanobodies reduced the amount of PRL-3:CBS inter-action. The potential of blocking this interaction is highly relevant in cancer, as multiple research groups have shown that PRL-3 binding to CNNM proteins is sufficient to promote metastatic growth in mouse models. The anti-PRL-3 nanobodies represent an important expansion of the research tools available to study PRL-3 function and can be used to define the role of PRL-3 in cancer progression.

PRL3 as a therapeutic target for novel cancer immunotherapy in multiple cancer types

Phosphatase of Regenerating Liver-3 (PRL3) was discovered in 1998 and was subsequently found to be correlated with cancer progression and metastasis in 2001. Extensive research in the past two decades has produced significant findings on PRL3-mediated cancer signaling and functions, as well as its clinical relevance in diverse types of cancer. PRL3 has been established to play a role in many cancer-related functions, including but not limited to metastasis, proliferation, and angiogenesis. Importantly, the tumor-specific expression of PRL3 protein in multiple cancer types has made it an attractive therapeutic target. Much effort has been made in developing PRL3-targeted therapy with small chemical inhibitors against intracellular PRL3, and notably, the development of PRL3-zumab as a novel cancer immunotherapy against PRL3. In this review, we summarize the current understanding of the role of PRL3 in cancer-related cellular functions, its prognostic value, as well as perspectives on PRL3 as a target for unconventional immunotherapy in the clinic with PRL3-zumab.

Phosphatase-independent role of phosphatase of regenerating liver in cancer progression

Phosphatase of regenerating liver (PRL) is a family of protein tyrosine phosphatases (PTPs) that are anchored to the plasma membrane by prenylation. They are frequently overexpressed in various types of malignant cancers and their roles in cancer progression have received considerable attention. Mutational analyses of PRLs have shown that their intrinsic phosphatase activity is dispensable for tumor formation induced by PRL overexpression in a lung metastasis model using melanoma cells. Instead, PRLs directly bind to cyclin M (CNNM) Mg²⁺ exporters in the plasma membrane and potently inhibit their Mg²⁺ export activity, resulting in an increase in intracellular Mg²⁺ levels. Experiments using mammalian culture cells, mice, and C. elegans have collectively revealed that dysregulation of Mg²⁺ levels severely affects ATP and reactive oxygen species (ROS) levels as well as the function of Ca²⁺ -permeable channels. Moreover, PRL overexpression altered the optimal pH for cell proliferation from normal 7.5 to acidic 6.5, which is typically observed in malignant tumors. Here, we review the phosphatase-independent biological functions of PRLs, focusing on their interactions with CNNM Mg²⁺ exporters in cancer progression.

Association of PTP4A3 expression and tumour size in functioning pituitary adenoma: a descriptive study

BACKGROUND

PTP4A3 is a subclass of a protein tyrosine phosphatase super family and is expressed in a range of epithelial neoplasms. We evaluated PTP4A3 expression and its association with clinicopathological parameters in different types of functioning pituitary adenomas.

METHODS

A total of 34 functioning pituitary adenomas samples were evaluated in this observational study. PTP4A3 expression was examined by immunohistochemical staining, and, possible correlations between PTP4A3 and some clinicopathological variables were investigated.

RESULTS

PTP4A3 was expressed in 19 out of 34 tumours (55%), at the cytoplasmic level of tumorous cells. Moreover, there was significant association (p=0.042) between PTP4A3 expression and tumorous size.

CONCLUSIONS

PTP4A3 was expressed in more than half of the tumours analysed, with there being a significant association with the tumorous size of functioning adenomas. This allows to speculate that PTP4A3 may regulate tumour growth, although further investigations are necessary to determine if this phosphatase can serve as a biomarker or used as a therapeutic target in pituitary macroadenomas.

Vitamin D Inhibition of TRPV5 Expression During Osteoclast Differentiation

BACKGROUND

Vitamin D is an important steroid that can regulate bone metabolism including osteoclast (OC) differentiation. Transient receptor potential cation channel subfamily V member 5 (TRPV5), is a calcium channel protein involved in OC differentiation. However, the impact of vitamin D on TRPV5 expression during OC differentiation is not clear.

OBJECTIVES

To determine if 1,25-dihydroxyvitamin D3 (1,25(OH)₂D₃) regulates the expression of TRPV5 during OC differentiation.

METHODS

Bone marrow mononuclear macrophage (BMMs) were induced to differentiate into OC with or without treatment with 10 nM 1,25(OH)₂D₃. The expression levels of vitamin D receptor (VDR) and TRPV5 were examined. The expression of several OC markers, including tartrate resistant acid phosphatase (TRAP), carbonic anhydrase II (Ca II), cathepsin K (CTSK), and vacuolar-type H⁺-ATPase (V-ATPase) were also detected.

RESULTS

We found that the VDR was expressed in murine bone marrow-derived macrophages at the early stage of OC differentiation. TRPV5 expression was increased during OC differentiation, which was down-regulated by 1,25(OH)₂D₃ after a prolonged exposure. The 1,25(OH)₂D₃ and TRPV5 inhibitors inhibited OC differentiation.

CONCLUSIONS

1,25(OH)₂D₃ can inhibit TRPV5 expression as well as TRPV5 inhibitors during OC differentiation. This suggests that 1,25(OH)₂D₃ may suppress OC differentiation by inhibiting TRPV5 expression.

Chemotherapy induced PRL3 expression promotes cancer growth via plasma membrane remodeling and specific alterations of caveolae-associated signaling

BACKGROUND

The outcome of cancer therapy is greatly defined by the ability of a tumor cell to evade treatment and re-establish its bulk mass after medical interventions. Consequently, there is an urgent need for the characterization of molecules affecting tumor reoccurrence. The phosphatase of regenerating liver 3 (PRL3) protein was recently emerged among the targets that could affect such a phenomenon.

METHODS

The expression induction of PRL3 in melanoma cells treated with chemotherapeutic agents was assessed by western blotting. The effect of PRL3 expression on cancer growth was investigated both in vitro and in vivo. The association of PRL3 with the caveolae structures of the plasma membrane was analyzed by detergent free raft purification. The effect of PRL3 expression on the membrane organization was assayed by electron microscopy and by membrane biophysical measurements. Purification of the plasma membrane fraction and co-immunoprecipitation were used to evaluate the altered protein composition of the plasma membrane upon PRL3 expression.

RESULTS

Here, we identified PRL3 as a genotoxic stress-induced oncogene whose expression is significantly increased by the presence of classical antitumor therapeutics. Furthermore, we successfully connected the presence of this oncogene with increased tumor growth, which implies that tumor cells can utilize PRL3 effects as a survival strategy. We further demonstrated the molecular mechanism that is connected with the pro-growth action of PRL3, which is closely associated with its localization to the caveolae-type lipid raft compartment of the plasma membrane. In our study, PRL3 was associated with distinct changes in the plasma membrane structure and in the caveolar proteome, such as the dephosphorylation of integrin β1 at Thr788/Thr789 and the increased partitioning of Rac1 to the plasma membrane. These alterations at the plasma membrane were further associated with the elevation of cyclin D1 in the nucleus.

CONCLUSIONS

This study identifies PRL3 as an oncogene upregulated in cancer cells upon exposure to anticancer therapeutics. Furthermore, this work contributes to the existing knowledge on PRL3 function by characterizing its association with the caveolae-like domains of the plasma membrane and their resident proteins.

Targeting ovarian cancer and endothelium with an allosteric PTP4A3 phosphatase inhibitor

Overexpression of protein tyrosine phosphatase PTP4A oncoproteins is common in many human cancers and is associated with poor patient prognosis and survival. We observed elevated levels of PTP4A3 phosphatase in 79% of human ovarian tumor samples, with significant overexpression in tumor endothelium and pericytes. Furthermore, PTP4A phosphatases appear to regulate several key malignant processes, such as invasion, migration, and angiogenesis, suggesting a pivotal regulatory role in cancer and endothelial signaling pathways. While phosphatases are attractive therapeutic targets, they have been poorly investigated because of a lack of potent and selective chemical probes. In this study, we disclose that a potent, selective, reversible, and noncompetitive PTP4A inhibitor, JMS-053, markedly enhanced microvascular barrier function after exposure of endothelial cells to vascular endothelial growth factor or lipopolysaccharide. JMS-053 also blocked the concomitant increase in RhoA activation and loss of Rac1. In human ovarian cancer cells, JMS-053 impeded migration, disrupted spheroid growth, and decreased RhoA activity. Importantly, JMS-053 displayed anticancer activity in a murine xenograft model of drug resistant human ovarian cancer. These data demonstrate that PTP4A phosphatases can be targeted in both endothelial and ovarian cancer cells, and confirm that RhoA signaling cascades are regulated by the PTP4A family.

Phosphatase of regenerating liver-3 inhibits invasiveness and proliferation in non-small cell lung cancer by regulating the epithelial-mesenchymal transition

Phosphatase of regenerating liver-3 (PRL-3) has been reported to be associated with colon and gastric cancer metastasis. However, the role and function of PRL-3 in human non-small cell lung cancer cells is unknown. Our studies showed that the expression of PRL-3mRNA and protein are higher in less invasive human lung adenocarcinoma cells than in highly invasive cell lines. Ectopic expression of PRL-3 reduced cell capacity for anchorage-dependent growth, anchorage-independent growth, migration, and invasion in vitro, as well as tumorigenesis in vivo. Conversely, catalytic (C104S) and prenylation-site (C170S) mutants enhanced cell invasion. Microarray profiling of PRL-3 transfectants revealed the pathways potentially involving PRL-3, including the epithelial-mesenchymal transition (EMT), extracellular matrix remodeling, and the WNT signaling pathway. Furthermore, we demonstrated that increased PRL-3 reduced Slug and enhanced E-cadherin gene expression through the AKT/GSK3β/β-catenin pathway. In conclusion, our data suggest that PRL-3 might play a tumor suppressor role in lung cancer, distinct from other cancers, by inhibiting EMT-related pathways.

Phosphatase of regenerating liver: a novel target for cancer therapy

INTRODUCTION

Phosphatases of regenerating livers (PRLs) are novel oncogenes that interact with many well-established cell signaling pathways that are misregulated in cancer, and are known to drive cancer metastasis when overexpressed.

AREAS COVERED

This review covers basic information of the discovery and characteristics of the PRL family. We also report findings on the role of PRL in cancer, cell functions and cell signaling. Furthermore, PRL's suitability as a novel drug target is discussed along with current methods being developed to facilitate PRL inhibition.

EXPERT OPINION

PRLs show great potential as novel drug targets for anticancer therapeutics. Studies indicate that PRL can perturb major cancer pathways such as Src/ERK1/2 and PTEN/PI3K/Akt. Upregulation of PRLs has also been shown to drive cancer metastasis. However, in order to fully realize its therapeutic potential, a deeper understanding of the function of PRL in normal tissue and in cancer must be obtained. Novel and integrated biochemical, chemical, biological, and genetic approaches will be needed to identify PRL substrate(s) and to provide proof-of-concept data on the druggability of the PRL phosphatases.

Expression of phosphatase of regenerating liver-3 (PRL-3) in endometrioid cancer and lymph nodes metastases

PURPOSE

We identify the expression of PRL-3 in primary endometrioid endometrial cancer and metastases in relation to the clinicopathological characteristics.

MATERIAL/METHODS

The study involved 30 patients with type I endometrial cancer. Twelve of them were diagnosed with metastases in various localization of abdomen. The PRL-3 expression was evaluated on the basis of immunohistochemistry results by the use of monoclonal antibody anti-PRL3 clone 3B6.

RESULTS

The intensity of PRL-3 expression in correlation with tumor stage was statistically significant (p = 0.024). The strongest reaction was noted in cases classified as a 1a and 1b stage defined by FIGO. The strength of PRL-3 expression is significantly associated with the degree of histological tumor grade (p = 0.035).

CONCLUSIONS

The strong expression of PRL-3 in the primary tumor that was significantly correlated with the grade and clinical stage suggest that PTP4A3 participates in the process of endometrial carcinogenesis.

Drosophila PRL-1 is a growth inhibitor that counteracts the function of the Src oncogene

Phosphatase of Regenerating Liver (PRL) family members have emerged as molecular markers that significantly correlate to the ability of many cancers to metastasize. However, contradictory cellular responses to PRL expression have been reported, including the inhibition of cell cycle progression. An obvious culprit for the discrepancy is the use of dozens of different cell lines, including many isolated from tumors or cultured cells selected for immortalization which may have missing or mutated modulators of PRL function. We created transgenic Drosophila to study the effects of PRL overexpression in a genetically controlled, organismal model. Our data support the paradigm that the normal cellular response to high levels of PRL is growth suppression and furthermore, that PRL can counter oncogenic activity of Src. The ability of PRL to inhibit growth under normal conditions is dependent on a CAAX motif that is required to localize PRL to the apical edge of the lateral membrane. However, PRL lacking the CAAX motif can still associate indiscriminately with the plasma membrane and retains its ability to inhibit Src function. We propose that PRL binds to other membrane-localized proteins that are effectors of Src or to Src itself. This first examination of PRL in a model organism demonstrates that PRL performs as a tumor suppressor and underscores the necessity of identifying the conditions that enable it to transform into an oncogene in cancer.

Can dehydroepiandrostenedione (DHEA) target PRL-3 to prevent colon cancer metastasis?

Colorectal cancer (CRC) is a frequently diagnosed cancer and causing significant mortality in the patients. Metastasis caused by CRC is mainly responsible for this cancer-related deaths. Despite recent advancements in the treatment methods, prognosis remains poor. Therefore, effective treatment strategies need to be designed for successful management of this disease. Dehydroepiandrostenedione (DHEA), a 17-ketosteroid hormone produced by adrenal glands, gonads and including gastrointestinal tract is required for several physiological processes. Deregulation of DHEA levels leads to various disease conditions including cancer. In fact, several experimental studies strongly suggest that DHEA could be used as a chemopreventive agent against colon cancer. Prenlyation of certain membrane proteins such as phosphatase of regenerating liver-3 (PRL-3) is crucial for metastatic progression of colon cancer cells. The ability of DHEA to target prenylation pathway could be utilized to inhibit PRL-3 prenylation for successful prevention of CRC metastases. As DHEA is a widely consumed drug for various ailments, incorporation of DHEA in the treatment regimen may be beneficial to prevent or delay the occurrence of metastasis resulting from CRC.

Suppression of breast tumor growth by DNA vaccination against phosphatase of regenerating liver 3

Phosphatase of regenerating liver (PRL)-3 is highly expressed in multiple cancers and has important roles in cancer development. Some small-molecule inhibitors and antibodies targeting PRL-3 have been recently reported to inhibit tumor growth effectively. To determine whether PRL-3-targeted DNA vaccination can induce immune response to prevent or inhibit the tumor growth, we established mouse D2F2 breast cancer cells expressing PRL-3 (D2F2/PRL-3) and control cells (D2F2/NC) with lentivirus, and constructed pVAX1-Igκ-PRL-3 plasmid (named as K-P3) as DNA vaccine to immunize BALB/c mice. We found that the K-P3 vaccine delivered by gene gun significantly prevented the growth of D2F2/PRL-3 compared with pVAX1-vector (P<0.01), but not of D2F2/NC, and improved the survival of D2F2/PRL-3-innoculated mice. Both PRL-3-targeted cytotoxic T lymphocytes (CTLs) and T-helper type 1 cell immune response (production of high levels of interferon-γ and tumor necrosis factor-α) were found to be involved in the preventive effect. Furthermore, PRL-3-targeted DNA immunization inhibited tumor growth of D2F2/PRL-3 cells in mice. We also evaluated the potential of immunization with PRL-3 protein, but no significant therapeutic or preventive effect was obtained on tumor growth. To enhance the immunity of PRL-3, we incorporated different molecular adjuvants, such as Mycobacterium tuberculosis heat-shock protein, CTL antigen 4 and M. tuberculosis T-cell stimulatory epitope (MT), into K-P3 vaccine for expressing the fusion proteins. We found that these adjuvant molecules did not significantly improve the antitumor activity of PRL-3 vaccine, but enhanced the production of PRL-3 antibodies in immunized mice. Summarily, our findings demonstrate that PRL-3-targeted DNA vaccine can generate significantly preventive and therapeutic effects on the growth of breast cancer expressing PRL-3 through the induction of cellular immune responses to PRL-3.

Plant cyclopeptide RA-V kills human breast cancer cells by inducing mitochondria-mediated apoptosis through blocking PDK1-AKT interaction

In the present paper, we examined the effects of a natural cyclopeptide RA-V on human breast cancer cells and the underlying mechanisms. RA-V significantly inhibited the growth of human breast cancer MCF-7, MDA-MB-231 cells and murine breast cancer 4T1 cells. In addition, RA-V triggered mitochondrial apoptotic pathway which was indicated by the loss of mitochondrial membrane potential, the release of cytochrome c, and the activation of caspase cascade. Further study showed that RA-V dramatically inhibited phosphorylation of AKT and 3-phosphoinositide dependent protein kinase 1 (PDK1) in MCF-7 cells. Moreover, RA-V disrupted the interaction between PDK1 and AKT in MCF-7 cells. Furthermore, RA-V-induced apoptosis could be enhanced by phosphatidylinositol 3-kinase inhibitor or attenuated by over-expression of AKT in all the three kinds of breast cancer cells. Taken together, this study shows that RA-V, which can induce mitochondria-mediated apoptosis, exerts strong anti-tumor activity against human breast cancer. The underlying anti-cancer mechanism of RA-V is related to the blockage of the interaction between PDK1 and AKT.

PRL-3 activates NF-κB signaling pathway by interacting with RAP1

Phosphatase of regenerating liver (PRL-3) promotes cancer metastasis through enhanced cell motility and invasiveness, however its role in tumorigenesis remains unclear. Herein, we reported that PRL-3 interacts with telomere-related protein RAP1. PRL-3 promotes the cytosolic localization of RAP1, which is counteracted by silencing of PRL-3. Immunohistochemical staining of colon cancer tissue array (n=170) revealed that high level of PRL-3 associates with cytosolic localization of RAP1 (p=0.01). Microarray analysis showed that PRL-3 regulates expression of diverse genes and enhances phosphorylation of p65 subunit of NF-κB in a RAP1-dependent manner. Furthermore, PRL-3 transcriptionally activates RAP1 expression, which is counteracted by ablating p65. Therefore, our results demonstrate PRL-3 as a novel regulator of NF-κB signaling pathway through RAP1.

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.

A novel chromone derivative with anti-inflammatory property via inhibition of ROS-dependent activation of TRAF6-ASK1-p38 pathway

The p38 MAPK signaling pathway plays a pivotal role in inflammation. Targeting p38 MAPK may be a potential strategy for the treatment of inflammatory diseases. In the present study, we show that a novel chromone derivative, DCO-6, significantly reduced lipopolysaccharide (LPS)-induced production of nitric oxide, IL-1β and IL-6, decreased the levels of iNOS, IL-1β and IL-6 mRNA expression in both RAW264.7 cells and mouse primary peritoneal macrophages, and inhibited LPS-induced activation of p38 MAPK but not of JNK, ERK. Moreover, DCO-6 specifically inhibited TLR4-dependent p38 activation without directly inhibiting its kinase activity. LPS-induced production of intracellular reactive oxygen species (ROS) was remarkably impaired by DCO-6, which disrupted the formation of the TRAF6-ASK1 complex. Administering DCO-6 significantly protected mice from LPS-induced septic shock in parallel with the inhibition of p38 activation and ROS production. Our results indicate that DCO-6 showed anti-inflammatory properties through inhibition of ROS-dependent activation of TRAF6-ASK1-p38 pathway. Blockade of the upstream events required for p38 MAPK action by DCO-6 may provide a new therapeutic option in the treatment of inflammatory diseases.

Therapeutic potential of PRL-3 targeting and clinical significance of PRL-3 genomic amplification in gastric cancer

Background

Phosphatase of regenerating liver-3 (PRL-3) has deserved attention as a crucial molecule in the multiple steps of metastasis. In the present study, we examined the mechanisms regulating PRL-3 expression, and assessed the clinical potential of PRL-3-targeted therapy in gastric cancer.

Methods

PRL-3 genomic amplification was analyzed using quantitative-polymerase chain reaction and/or fluorescence in situ hybridization in 77 primary gastric tumors. The anticancer activity of PRL-3 inhibitor (1-4-bromo-2-benzylidene rhodanine) treatment was evaluated against cancer cells with different genetic and expression status.

Results

PRL-3 genomic amplification was closely concordant with high level of its protein expression in cell lines, and was found in 20% (8/40) among human primary tumors with its expression, which were all stage III/IV disease (40%, 8/20), but in none (0/37) among those without expression. Additionally, PRL-3 genomic amplification was associated with metastatic lymph node status, leading to advanced stage and thereby poor outcomes in patients with lymph node metastasis (P = 0.021). PRL-3 small interfering RNA robustly repressed metastatic properties, including cell proliferation, invasion, and anchorage-independent colony formation. Although neither PRL-3 genomic amplification nor expression level was responsible for the sensitivity to PRL-3 inhibitor treatment, the inhibitor showed dose-dependent anticancer efficacy, and remarkably induced apoptosis on all the tested cell lines with PRL-3 expression.

Conclusions

We have for the first time, demonstrated that PRL-3 genomic amplification is one of the predominant mechanisms inducing its expression, especially in more advanced stage, and that PRL-3-targeted therapy may have a great potential against gastric cancer with its expression.

PRL-3 phosphatase and cancer metastasis

The deregulated expression of members of the phosphatase of regenerating liver (PRL) family has been implicated in the metastatic progression of multiple human cancers. Importantly, PRL-1 and PRL-3 both possess the capacity to drive key steps in metastatic progression. Yet, little is known about the regulation and oncogenic mechanisms of this emerging class of dual-specificity phosphatases. This prospect article details the involvement of PRLs in the metastatic cascade, the regulatory mechanisms controlling PRL expression, and recent efforts in the characterization of PRL-modulated pathways and substrates using biochemical and high-throughput approaches. Current advances and future prospects in anti-cancer therapy targeting this family are also discussed.

CDH22 expression is reduced in metastatic melanoma

Cadherin-like protein 22 (CDH22) is a transmembrane glycoprotein implicated in cell-cell adhesion and cancer metastasis. The expression of CDH22 has been shown to be increased in colorectal cancers. However, the role of CDH22 in melanomagenesis is not known. To investigate the role of CDH22 in melanoma progression, we examined the expression of CDH22 in melanocytic lesions at different stages and analysed the correlation between CDH22 expression and clinicopathlogic parameters and patient survival. Using tissue microarray and immunohisto-chemistry, we evaluated CDH22 staining in 76 dysplastic nevi, 247 primary melanomas, and 143 metastatic melanomas. We found that metastatic melanomas had a significantly higher percentage of negative CDH22 staining than dysplastic nevi (P = 0.012) and primary melanomas (P = 0.038). CDH22 expression was also reduced in thick (≥2 mm) and ulcerative melanomas (P = 0.003 and 0.022, respectively). Melanomas of AJCC stage II, III, and IV had a higher percentage of negative CDH22 staining than AJCC stage I melanomas (P = 0.004, P < 0.0001, and P = 0.009, respectively). Melanomas with negative CDH22 expression had significantly poorer disease-specific 5-year survival than those with positive CDH22 staining. Additionally, CDH22 expression depended on AJCC stage to predict patient survival. These data indicate that reduced CDH22 expression is associated with melanoma metastasis and poor patient prognosis.

The farnesyl transferase inhibitor lonafarnib inhibits mTOR signaling and enforces sorafenib-induced apoptosis in melanoma cells

Farnesyl transferase inhibitors (FTIs) inhibit the farnesylation of proteins, including RAS and RHEB (Ras homolog enriched in brain). RAS signals to the RAF-MEK-ERK (MAPK) and PI3K-AKT-mTOR (AKT) signaling pathways, which have a major role in melanoma progression. RHEB positively regulates mammalian target of rapamycin (mTOR). We investigated the effects of the FTI lonafarnib alone and in combination with MAPK (mitogen-activated protein kinase) or AKT (acutely transforming retrovirus AKT8 in rodent T-cell lymphoma) pathway inhibitors on proliferation, survival, and invasive tumor growth of melanoma cells. Lonafarnib alone did not sufficiently inhibit melanoma cell growth. Combinations of lonafarnib with AKT pathway inhibitors did not significantly increase melanoma cell growth inhibition. In contrast, combinations of lonafarnib with MAPK pathway inhibitors yielded additional growth-inhibiting effects. In particular, the combination of the FTI lonafarnib with the pan-RAF inhibitor sorafenib synergistically inhibited melanoma cell growth, significantly enhanced sorafenib-induced apoptosis, and completely suppressed invasive tumor growth in monolayer and organotypic cultures, respectively. Apoptosis induction was associated with upregulation of the endoplasmic reticulum stress-related transcription factors p8 and CHOP (CAAT/enhancer binding protein (C/EBP) homologous protein), and downregulation of the antiapoptotic Bcl-2 (B-cell lymphoma-2) family protein Mcl-1(myeloid cell leukemia 1). Lonafarnib did not affect MAPK and AKT but did affect mTOR signaling. Together, these findings suggest that the FTI lonafarnib inhibits mTOR signaling and enforces sorafenib-induced apoptosis in melanoma cells and may therefore represent an effective alternative for melanoma treatment.

Phosphatase of regenerating liver-3 as a convergent therapeutic target for lymph node metastasis in esophageal squamous cell carcinoma

Phosphatase of regenerating liver-3 (PRL-3) is a molecule associated with metastasis in a diverse of cancers, which, however, remains largely unknown in esophageal squamous cell carcinoma (ESCC). We examined both the clinical significance of PRL-3 expression and its biological roles, and assessed possibilities as a therapeutic target in ESCC. PRL-3 expression was found in 78% (69 of 88) of the primary ESCC on immunohistochemistry; it was the strong independent predictor for lymph node metastasis (LNM) on a multivariate logistic regression model (p = 0.0014, relative risk =15.20). Additionally, gene amplification was found in 3 (7.9%) of the 38 primary tumors with PRL-3 overexpression by fluorescence in situ hybridization, but in none of the 19 tumors without it. PRL-3 small interfering RNA robustly repressed cell proliferation, anchorage-independent colony formation and invasion and augmented 5-FU-induced apoptosis in all the tested ESCC cell lines with PRL-3 overexpression, irrespective of its gene amplification status. PRL-3 inhibitor (1-4-bromo-2-benzylidene rhodanine) also suppressed such metastatic properties in the cell lines with PRL-3 overexpression, but not with its low expression. Inverse effects were observed by PRL-3 forced expression. Collectively, PRL-3 overexpression is a frequent event associated with LNM and plays a causative role in promoting cancer progression. Moreover, the expression status may be a landmark to select patients with benefit from PRL-3-targeted therapy. Thus, PRL-3 could be a convergent therapeutic target against ESCC with LNM.