PRL-3 siRNA inhibits the metastasis of B16-BL6 mouse melanoma cells in vitro and in vivo

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.

Anti-Human CD9 Fab Fragment Antibody Blocks the Extracellular Vesicle-Mediated Increase in Malignancy of Colon Cancer Cells

Intercellular communication between cancer cells themselves or with healthy cells in the tumor microenvironment and/or pre-metastatic sites plays an important role in cancer progression and metastasis. In addition to ligand-receptor signaling complexes, extracellular vesicles (EVs) are emerging as novel mediators of intercellular communication both in tissue homeostasis and in diseases such as cancer. EV-mediated transfer of molecular activities impacting morphological features and cell motility from highly metastatic SW620 cells to non-metastatic SW480 cells is a good in vitro example to illustrate the increased malignancy of colorectal cancer leading to its transformation and aggressive behavior. In an attempt to intercept the intercellular communication promoted by EVs, we recently developed a monovalent Fab fragment antibody directed against human CD9 tetraspanin and showed its effectiveness in blocking the internalization of melanoma cell-derived EVs and the nuclear transfer of their cargo proteins into recipient cells. Here, we employed the SW480/SW620 model to investigate the anti-cancer potential of the anti-CD9 Fab antibody. We first demonstrated that most EVs derived from SW620 cells contain CD9, making them potential targets. We then found that the anti-CD9 Fab antibody, but not the corresponding divalent antibody, prevented internalization of EVs from SW620 cells into SW480 cells, thereby inhibiting their phenotypic transformation, i.e., the change from a mesenchymal-like morphology to a rounded amoeboid-like shape with membrane blebbing, and thus preventing increased cell migration. Intercepting EV-mediated intercellular communication in the tumor niche with an anti-CD9 Fab antibody, combined with direct targeting of cancer cells, could lead to the development of new anti-cancer therapeutic strategies.

The phosphatase PRL-3 affects intestinal homeostasis by altering the crypt cell composition

Expression of the phosphatase of regenerating liver-3 (PRL-3) is known to promote tumor growth in gastrointestinal adenocarcinomas, and the incidence of tumor formation upon inflammatory events correlates with PRL-3 levels in mouse models. These carcinomas and their onset are associated with the impairment of intestinal cell homeostasis, which is regulated by a balanced number of Paneth cells and Lgr5 expressing intestinal stem cells (Lgr5+ ISCs). Nevertheless, the consequences of PRL-3 overexpression on cellular homeostasis and ISC fitness in vivo are unexplored. Here, we employ a doxycycline-inducible PRL-3 mouse strain to show that aberrant PRL-3 expression within a non-cancerous background leads to the death of Lgr5+ ISCs and to Paneth cell expansion. A higher dose of PRL-3, resulting from homozygous expression, led to mice dying early. A primary 3D intestinal culture model obtained from these mice confirmed the loss of Lgr5+ ISCs upon PRL-3 expression. The impaired intestinal organoid formation was rescued by a PRL inhibitor, providing a functional link to the observed phenotypes. These results demonstrate that elevated PRL-3 phosphatase activity in healthy intestinal epithelium impairs intestinal cell homeostasis, which correlates this cellular mechanism of tumor onset with PRL-3-mediated higher susceptibility to tumor formation upon inflammatory or mutational events.

Key messages

• Transgenic mice homozygous for PRL-3 overexpression die early.

• PRL-3 heterozygous mice display disrupted intestinal self-renewal capacity.

• PRL-3 overexpression alone does not induce tumorigenesis in the mouse intestine.

• PRL-3 activity leads to the death of Lgr5+ ISCs and Paneth cell expansion.

• Impairment of cell homeostasis correlates PRL-3 action with tumor onset mechanisms.

The dual inhibition against the activity and expression of tyrosine phosphatase PRL-3 from a rhodanine derivative

Increasing evidences demonstrated that PRL-3 was associated with metastatic potential in a variety of cancers including CRC, gastric cancer, ovarian cancer and so on. PRL-3 knock down inhibited the development of metastasis by reducing the size of primary tumors and inhibiting the invasion and growth of cancer cells. Therefore, PRL-3 is a promising diagnostic marker and therapeutic target in tumors. So far, only several PRL-3 inhibitors have been reported. In this study, six rhodanine derivatives were synthesized and characterized. The compounds were evaluated against tyrosine phosphatase PRL-3. Among these compounds, 5-(5-chloro-2-(trifluoromethyl)benzylidene)-2-thioxothiazolidin-4-one (4) could effectively inhibit PRL-3 with IC₅₀ value of 15.22 μM. Fluorescent assays suggested compound 4 tightly bound to tyrosine phosphatase PRL-3 with the molar ratio of 1:1, and the binding constant of 1.74 × 10⁶ M^-1. Compound 4 entered into SW-480 cells, selectively inhibited the expression of PRL-3 and increased the phosphorylation of PRL-3 substrates, and decreased the survival rate of SW-480 cells with IC₅₀ of 6.64 μM and induced apoptosis. The results revealed that compound 4 is a dual functional inhibitor against the activity and expression of PRL-3 and a promising anti-cancer candidate targeting PRL-3.

A screen of FDA-approved drugs identifies inhibitors of protein tyrosine phosphatase 4A3 (PTP4A3 or PRL-3)

Protein tyrosine phosphatase 4A3 (PTP4A3 or PRL-3) is highly expressed in a variety of cancers, where it promotes tumor cell migration and metastasis leading to poor prognosis. Despite its clinical significance, small molecule inhibitors of PRL-3 are lacking. Here, we screened 1443 FDA-approved drugs for their ability to inhibit the activity of the PRL phosphatase family. We identified five specific inhibitors for PRL-3 as well as one selective inhibitor of PRL-2. Additionally, we found nine drugs that broadly and significantly suppressed PRL activity. Two of these broad-spectrum PRL inhibitors, Salirasib and Candesartan, blocked PRL-3-induced migration in human embryonic kidney cells with no impact on cell viability. Both drugs prevented migration of human colorectal cancer cells in a PRL-3 dependent manner and were selective towards PRLs over other phosphatases. In silico modeling revealed that Salirasib binds a putative allosteric site near the WPD loop of PRL-3, while Candesartan binds a potentially novel targetable site adjacent to the CX₅R motif. Inhibitor binding at either of these sites is predicted to trap PRL-3 in a closed conformation, preventing substrate binding and inhibiting function.

Oncogenic Tyrosine Phosphatases: Novel Therapeutic Targets for Melanoma Treatment

Despite a large number of therapeutic options available, malignant melanoma remains a highly fatal disease, especially in its metastatic forms. The oncogenic role of protein tyrosine phosphatases (PTPs) is becoming increasingly clear, paving the way for novel antitumor treatments based on their inhibition. In this review, we present the oncogenic PTPs contributing to melanoma progression and we provide, where available, a description of new inhibitory strategies designed against these enzymes and possibly useful in melanoma treatment. Considering the relevance of the immune infiltrate in supporting melanoma progression, we also focus on the role of PTPs in modulating immune cell activity, identifying interesting therapeutic options that may support the currently applied immunomodulating approaches. Collectively, this information highlights the value of going further in the development of new strategies targeting oncogenic PTPs to improve the efficacy of melanoma treatment.

PRL3 pseudophosphatase activity is necessary and sufficient to promote metastatic growth

Phosphatases of regenerating liver (PRLs) are markers of cancer and promote tumor growth. They have been implicated in a variety of biochemical pathways but the physiologically relevant target of phosphatase activity has eluded 20 years of investigation. Here, we show that PRL3 catalytic activity is not required in a mouse model of metastasis. PRL3 binds and inhibits CNNM4, a membrane protein associated with magnesium transport. Analysis of PRL3 mutants specifically defective in either CNNM-binding or phosphatase activity demonstrate that CNNM binding is necessary and sufficient to promote tumor metastasis. As PRLs do have phosphatase activity, they are in fact pseudo-pseudophosphatases. Phosphatase activity leads to formation of phosphocysteine, which blocks CNNM binding and may play a regulatory role. We show levels of PRL cysteine phosphorylation vary in response to culture conditions and in different tissues. Examination of related protein phosphatases shows the stability of phosphocysteine is a unique and evolutionarily conserved property of PRLs. The demonstration that PRL3 functions as a pseudophosphatase has important ramifications for the design of PRL inhibitors for cancer.

Protein tyrosine phosphatase 4A3 (PTP4A3/PRL-3) drives migration and progression of T-cell acute lymphoblastic leukemia in vitro and in vivo

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive blood cancer. There are no immunotherapies and few molecularly targeted therapeutics available for treatment of this malignancy. The identification and characterization of genes and pathways that drive T-ALL progression are critical for the development of new therapies for T-ALL. Here, we determined that the protein tyrosine phosphatase 4A3 (PTP4A3 or PRL-3) plays a critical role in T-ALL initiation and progression by promoting leukemia cell migration. PRL-3 is highly expressed in patient T-ALL samples at both the mRNA and protein levels compared to normal lymphocytes. Knock-down of PRL-3 expression using short-hairpin RNA (shRNA) in human T-ALL cell lines significantly impeded T-ALL cell migration capacity in vitro and reduced their ability to engraft and proliferate in vivo in xenograft mouse models. Additionally, PRL-3 overexpression in a Myc-induced zebrafish T-ALL model significantly accelerated disease onset and shortened the time needed for cells to enter blood circulation. Reverse-phase protein array (RPPA) and gene set enrichment analysis (GSEA) revealed that the SRC signaling pathway is affected by PRL-3. Immunoblot analyses validated that manipulation of PRL-3 expression in T-ALL cells affected the SRC signaling pathway, which is directly involved in cell migration, although Src was not a direct substrate of PRL-3. More importantly, T-ALL cell growth and migration were inhibited by small molecule inhibition of PRL-3, suggesting that PRL-3 has potential as a therapeutic target in T-ALL. Taken together, our study identifies PRL-3 as an oncogenic driver in T-ALL both in vitro and in vivo and provides a strong rationale for targeted therapies that interfere with PRL-3 function.

Targeting phosphatases of regenerating liver (PRLs) in cancer

The phosphatase of regenerating liver (PRL) family, also known as protein tyrosine phosphatase 4A (PTP4A), are dual-specificity phosphatases with largely unknown cellular functions. However, accumulating evidence indicates that PRLs are oncogenic across a broad variety of human cancers. PRLs are highly expressed in advanced tumors and metastases compared to early stage cancers or matched healthy tissue, and high expression of PRLs often correlates with poor patient prognosis. Consequentially, PRLs have been considered potential therapeutic targets in cancer. Persistent efforts have been made to define their role and mechanism in cancer progression and to create specific PRL inhibitors for basic research and drug development. However, targeting PRLs with small molecules remains challenging due to the highly conserved active site of protein tyrosine phosphatases and a high degree of sequence similarity between the PRL protein families. Here, we review the current PRL inhibitors, including the strategies used for their identification, their biological efficacy, potency, and selectivity, with a special focus on how PRL structure can inform future efforts to develop specific PRL inhibitors.

A chemical genetics approach identifies PTP4A3 as a regulator of colon cancer cell adhesion

Dysregulation of the tightly controlled protein phosphorylation networks that govern cellular behavior causes cancer. The membrane-associated, intracellular protein tyrosine phosphatase PTP4A3 is overexpressed in human colorectal cancer and contributes to cell migration and invasion. To interrogate further the role of PTP4A3 in colorectal cancer cell migration and invasion, we deleted the Ptp4a3 gene from murine colorectal tumor cells. The resulting PTP4A3^-/- cells exhibited impaired colony formation, spheroid formation, migration, and adherence compared with the paired PTP4A3^fl/fl cells. We replicated these phenotypic changes using the new small-molecule, allosteric PTP4A3 inhibitor JMS-053. A related structure, JMS-038, which lacked phosphatase inhibition, displayed no cellular activity. Reduction in cell viability and colony formation by JMS-053 occurred in both mouse and human colorectal cell lines and required PTP4A3 expression. Ptp4a3 deletion increased the expression of extracellular matrix (ECM) and adhesion genes, including the tumor suppressor Emilin 1. JMS-053 also increased Emilin 1 gene expression. Moreover, The Cancer Genome Atlas genomic database revealed human colorectal tumors with high Ptp4a3 expression had low Emilin 1 expression. These chemical and biologic reagents reveal a previously unknown communication between the intracellular PTP4A3 phosphatase and the ECM and support efforts to pharmacologically target PTP4A3.-McQueeney, K. E., Salamoun, J. M., Ahn J. G., Pekic, P., Blanco, I. K., Struckman, H. L., Sharlow, E. R., Wipf, P., Lazo, J. S. A chemical genetics approach identifies PTP4A3 as a regulator of colon cancer cell adhesion.

Expression of PRL-3 regulates proliferation and invasion of breast cancer cells in vitro

PURPOSE

The protein tyrosine phosphatase PRL-3 plays an important role in cancer cell migration, invasion and metastasis. In breast cancer, PRL-3 is overexpressed in 70-75% of tumors and even more frequently in lymph node metastases. Moreover, PRL-3 overexpression in breast cancer is associated with an adverse disease outcome. Aim of this study was to determine the role of PRL-3 in breast cancer cell proliferation, migration and invasion in vitro.

METHODS

PRL-3 mRNA expression was evaluated in 6 breast cancer cell lines by quantitative real-time PCR. To investigate the effect of PRL-3 expression in breast cancer cells in vitro we both up- and downregulated PRL-3 expression in breast cancer cells and performed in vitro wound repair cell motility assays and invasion assays. The influence of PRL-3 knockdown in MCF-7 cells on the expression of several gene products involved in cell invasion and cytoskeletal function was evaluated with real-time PCR.

RESULTS

PRL-3 mRNA expression was demonstrated in all breast cancer cell lines evaluated. Knockdown of PRL-3 in MCF-7 cells resulted in decreased proliferation, wound healing and invasion. PRL-3 knockdown in MCF-7 cells resulted in a significant reduction of heparanase, MMP-9, actin gamma-2 and Myosin 9 expression, and significant elevation of E-cadherin.

CONCLUSIONS

We conclude that PRL-3 is an important regulatory factor for breast cancer cell proliferation and invasion. Loss of PRL-3 function induces an antimetastatic gene expression profile in breast cancer cells. Due to its role in tumor growth and metastasis, PRL-3 emerges as a new therapeutic target in breast cancer therapy.

Novel Anticancer Agents Based on Targeting the Trimer Interface of the PRL Phosphatase

Phosphatase of regenerating liver (PRL) oncoproteins are phosphatases overexpressed in numerous types of human cancer. Elevated levels of PRL associate with metastasis and poor clinical outcomes. In principle, PRL phosphatases offer appealing therapeutic targets, but they remain underexplored due to the lack of specific chemical probes. In this study, we address this issue by exploiting a unique property of PRL phosphatases, namely, that they may function as homotrimers. Starting from a sequential structure-based virtual screening and medicinal chemistry strategy, we identified Cmpd-43 and several analogs that disrupt PRL1 trimerization. Biochemical and structural analyses demonstrate that Cmpd-43 and its close analogs directly bind the PRL1 trimer interface and obstruct PRL1 trimerization. Cmpd-43 also specifically blocks the PRL1-induced cell proliferation and migration through attenuation of both ERK1/2 and Akt activity. Importantly, Cmpd-43 exerted potent anticancer activity both in vitro and in vivo in a murine xenograft model of melanoma. Our results validate a trimerization-dependent signaling mechanism for PRL and offer proof of concept for trimerization inhibitors as candidate therapeutics to treat PRL-driven cancers. Cancer Res; 76(16); 4805-15. ©2016 AACR.

Procyanidins Negatively Affect the Activity of the Phosphatases of Regenerating Liver

Natural polyphenols like oligomeric catechins (procyanidins) derived from green tea and herbal medicines are interesting compounds for pharmaceutical research due to their ability to protect against carcinogenesis in animal models. It is nevertheless still unclear how intracellular pathways are modulated by polyphenols. Monomeric polyphenols were shown to affect the activity of some protein phosphatases (PPs). The three phosphatases of regenerating liver (PRLs) are close relatives and promising therapeutic targets in cancer. In the present study we show that several procyanidins inhibit the activity of all three members of the PRL family in the low micromolar range, whereas monomeric epicatechins show weak inhibitory activity. Increasing the number of catechin units in procyanidins to more than three does not further enhance the potency. Remarkably, the tested procyanidins showed selectivity in vitro when compared to other PPs, and over 10-fold selectivity toward PRL-1 over PRL-2 and PRL-3. As PRL overexpression induces cell migration compared to control cells, the effect of procyanidins on this phenotype was studied. Treatment with procyanidin C2 led to a decrease in cell migration of PRL-1- and PRL-3-overexpressing cells, suggesting the compound-dependent inhibition of PRL-promoted cell migration. Treatment with procyanidin B3 led to selective suppression of PRL-1 overexpressing cells, thereby corroborating the selectivity toward PRL-1- over PRL-3 in vitro. Together, our results show that procyanidins negatively affect PRL activity, suggesting that PRLs could be targets in the polypharmacology of natural polyphenols. Furthermore, they are interesting candidates for the development of PRL-1 inhibitors due to their low cellular toxicity and the selectivity within the PRL family.

PRL-3 Promotes the Malignant Progression of Melanoma via Triggering Dephosphorylation and Cytoplasmic Localization of NHERF1

Phosphatase of regenerating liver-3 (PRL-3) has been reported to have a critical role in metastatic progression of cancers. Here, we investigate how PRL-3 increases the malignant degree of melanoma cells. The expression of PRL-3 increased gradually during the malignant progression of melanoma. The phosphorylation of Akt was elevated in highly malignant melanoma cells, which was accompanied by a decrease in nuclear phosphatase and tensin homolog (PTEN). The phosphorylation of NHERF1 in the serine site was regulated by PRL-3 and showed cytoplasmic translocation upon dephosphorylation, which resulted in a decrease in nuclear PTEN. The co-translocation of NHERF1 and PTEN from the nucleus to the cytoplasm was observed during the malignant progression of melanoma cells. Tumor growth was inhibited significantly, and the survival was prolonged upon knockdown of cytoplasmic NHERF1 in B16BL6 cells prior to the inoculation into mice. Taken together, to our knowledge previously unreported, we have identified NHERF1 as a potential substrate of PRL-3. Its phosphorylation status as well as its change in cellular localization and association with PTEN correlated with the malignant progression of melanoma. Our data provide an explanation for how PRL-3 promotes the malignant progression of melanoma, as well as a diagnostic marker or therapeutic target for malignant melanoma.

Membrane protein CNNM4-dependent Mg2+ efflux suppresses tumor progression

Intracellular Mg(2+) levels are strictly regulated; however, the biological importance of intracellular Mg(2+) levels and the pathways that regulate them remain poorly understood. Here, we determined that intracellular Mg(2+) is important in regulating both energy metabolism and tumor progression. We determined that CNNM4, a membrane protein that stimulates Mg(2+) efflux, binds phosphatase of regenerating liver (PRL), which is frequently overexpressed in malignant human cancers. Biochemical analyses of cultured cells revealed that PRL prevents CNNM4-dependent Mg(2+) efflux and that regulation of intracellular Mg(2+) levels by PRL and CNNM4 is linked to energy metabolism and AMPK/mTOR signaling. Indeed, treatment with the clinically available mTOR inhibitor rapamycin suppressed the growth of cancer cells in which PRL was overexpressed. In ApcΔ(14/+) mice, which spontaneously form benign polyps in the intestine, deletion of Cnnm4 promoted malignant progression of intestinal polyps to adenocarcinomas. IHC analyses of tissues from patients with colon cancer demonstrated an inverse relationship between CNNM4 expression and colon cancer malignancy. Together, these results indicate that CNNM4-dependent Mg(2+) efflux suppresses tumor progression by regulating energy metabolism.

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.

Phosphatase of regenerating liver 2 (PRL2) deficiency impairs Kit signaling and spermatogenesis

The Phosphatase of Regenerating Liver (PRL) proteins promote cell signaling and are oncogenic when overexpressed. However, our understanding of PRL function came primarily from studies with cultured cell lines aberrantly or ectopically expressing PRLs. To define the physiological roles of the PRLs, we generated PRL2 knock-out mice to study the effects of PRL deletion in a genetically controlled, organismal model. PRL2-deficient male mice exhibit testicular hypotrophy and impaired spermatogenesis, leading to decreased reproductive capacity. Mechanistically, PRL2 deficiency results in elevated PTEN level in the testis, which attenuates the Kit-PI3K-Akt pathway, resulting in increased germ cell apoptosis. Conversely, increased PRL2 expression in GC-1 cells reduces PTEN level and promotes Akt activation. Our analyses of PRL2-deficient animals suggest that PRL2 is required for spermatogenesis during testis development. The study also reveals that PRL2 promotes Kit-mediated PI3K/Akt signaling by reducing the level of PTEN that normally antagonizes the pathway. Given the strong cancer susceptibility to subtle variations in PTEN level, the ability of PRL2 to repress PTEN expression qualifies it as an oncogene and a novel target for developing anti-cancer agents.

Src-mediated phosphorylation of the tyrosine phosphatase PRL-3 is required for PRL-3 promotion of Rho activation, motility and invasion

The metastasis-associated tyrosine phosphatase PRL-3/PTP4A is upregulated in numerous cancers, but the mechanisms modulating PRL-3 activity other than its expression levels have not been investigated. Here we report evidence for both Src-dependent tyrosine phosphorylation of PRL-3 and Src-mediated regulation of PRL-3 biological activities. We used structural mutants, pharmacological inhibitors and siRNA to demonstrate Src-dependent phosphorylation of endogenous PRL-3 in SW480 colon cancer cells. We also demonstrated that PRL-3 was not tyrosine phosphorylated in SYF mouse embryo fibroblasts deficient in Src, Yes and Fyn unless Src was re-expressed. Further, we show that platelet-derived growth factor (PDGF) can stimulate PRL-3 phosphorylation in a Src-dependent manner. Finally, we show that PRL-3-induced cell motility, Matrigel invasion and activation of the cytoskeleton-regulating small GTPase RhoC were abrogated in the presence of the phosphodeficient PRL-3 mutant Y53F, or by use of a Src inhibitor. Thus, PRL-3 requires the activity of a Src kinase, likely Src itself, to promote these cancer-associated phenotypes. Our data establish a model for the regulation of PRL-3 by Src that supports the possibility of their coordinate roles in signaling pathways promoting invasion and metastasis, and supports simultaneous use of novel molecularly targeted therapeutics directed at these proteins.

PRL-3 suppresses c-Fos and integrin α2 expression in ovarian cancer cells

Background

Phosphatase of regenerating liver-3 (PRL-3), a protein tyrosine phosphatase, is highly expressed in multiple human cancers and strongly implicated in tumor progression and cancer metastasis. However, the mechanisms by which PRL-3 promotes cancer cell migration, invasion, and metastasis are not very well understood. In this study, we investigated the contribution and molecular mechanisms of PRL-3 in ovarian cancer progression.

Methods

PRL-3 protein expression was detected on ovarian cancer tissue microarrays using immunohistochemistry. Stable PRL-3 depleted cell lines were generated using short hairpin RNA (shRNA) constructs. The migration and invasion potential of these cells were analyzed using Transwell and Matrigel assays, respectively. Immunoblotting and immunofluorescence were used to detect protein levels and distribution in PRL-3-ablated cells and the control cells. Cell morphology was observed with hematoxylin-eosin staining and transmission electron microscopy. Finally, PRL-3-ablated and control cells were injected into nude mice for xenograft tumorigenicity assays.

Results

Elevated PRL-3 expression was detected in 19% (26 out of 135) of human ovarian cancer patient samples, but not in normal ovary tissues (0 out of 14). Stable depletion of PRL-3 in A2780 ovarian cancer cells resulted in decreased migration ability and invasion activity compared with control parental A2780 cells. In addition, PRL-3-ablated cells also exhibited flattened morphology and extended lamellipodia. To address the possible molecular basis for the altered phenotypes associated with PRL-3 down-regulation, we assessed the expression profiles of various proteins involved in cell-matrix adhesion. Depletion of PRL-3 dramatically enhanced both RNA and protein levels of the cell surface receptor integrin α2, but not its heterologous binding partner integrin β1. Inhibition of PRL-3 also correlated with elevated expression and phosphorylation of paxillin. A pronounced increase in the expression and activation of c-fos, a transcriptional activator of integrin α2, was observed in these PRL-3 knock-down cells. Moreover, forced expression of EGFP-PRL-3 resulted in the suppression of both integrin α2 and c-fos expression in A2780 cells. Significantly, using a xenograft tumor model, we observed a greatly reduced tumorigenicity of A2780 PRL-3 knock-down cells in vivo.

Conclusions

These results suggest that PRL-3 plays a critical role in ovarian cancer tumorigenicity and maintaining the malignant phenotype. PRL-3 may inhibit c-fos transcriptional regulation of integrin α2 signaling. Our results strongly support a role for PRL-3 as a promising therapeutic target and potential early biomarker in ovarian cancer progression.

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.

Clinical importance of phosphatase of regenerating liver-3 expression in breast cancer

BACKGROUND

Several biomarkers have been previously studied for breast cancer to define risk of recurrence and metastasis. Phosphatase of regenerating liver-3 (PRL-3) is one of them. High PRL-3 expression has been found to be correlated with axillary lymph node metastasis and survival in breast cancer. Herein, we evaluated the prognostic significance of PRL-3 expression and the relationship between PRL-3 and other clinicopathological factors.

METHODS

PRL-3 expression was analyzed immunohistochemically in 122 invasive breast cancer tissues. We evaluated the correlation between PRL-3 and other clinicopathological factors by χ² test. Kaplan-Meier test and log rank method were used to define prognostic importance of PRL-3 expression.

RESULTS

Of 122 breast cancer tumor samples, 46 (37.7 %) were negative while 76 (62.3 %) were positive in respect to PRL-3 expression. There was significant correlation between PRL-3 expression and other clinicopathological factors, such as histology, lymphovascular invasion (LVI), necrosis, progesterone receptor (PR) status, and the presence of triple negative disease. Tumors with LVI and necrosis had more positive PRL-3 expression compared to tumors without LVI or necrosis (P = 0.05 and 0.03, respectively). Triple negative and cerb-B overexpressed breast cancers were found to be more positive PRL-3 expression than hormone receptor positive with cerb-B negative groups (luminal A) (P = 0.02).We could not find any relationship between PRL-3 expression and overall survival (OS) or disease-free survival (DFS) (P > 0.05).

CONCLUSION

Although PRL-3 expression was related to LVI or necrosis which is important for tumor invasiveness, we could not find that PRL-3 as an important prognostic factor in breast cancer patients. In addition, triple negative and cerb-B overexpressed tumors, which had worse prognosis compared to hormone receptor positive without cerb-B expressed group, associated with also PRL-3 positivity more than PRL-3 negative group.

Synthesis and biological evaluation of novel sinomenine derivatives as anti-inflammatory agents

Sinomenine (1) is clinically available for the treatment of rheumatoid arthritis (RA), however, its efficacy is quite weak. In the present study, a library of novel sinomenine-based homodimers and monomers through variable-length linkers were designed and synthesized, and their bioactivities were evaluated using RAW264.7 cells and mice. Among the compounds, 2f and 3b possessed much more potent inhibitory effects on the production of nitric oxide (NO), interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) than 1. Preliminary mechanism investigation revealed that 3b inhibited nuclear factor-κB (NF-κB) signaling pathway specifically, 2f suppressed both NF-κB and mitogen-activated protein kinase (MAPK) cascades. Moreover, 3b and 2f significantly alleviated the lipopolysaccharide (LPS)-induced mortality. These two compounds might serve as valuable candidates for anti-inflammatory drug discovery.

Tissue-specific alterations of PRL-1 and PRL-2 expression in cancer

The PRL-1 and PRL-2 phosphatases have been implicated as oncogenic, however the involvement of these molecules in human neoplasms is not well understood. To increase understanding of the role PRL-1 and PRL-2 play in the neoplastic process, in situ hybridization was used to examine PRL-1 and PRL-2 mRNA expression in 285 normal, benign, and malignant human tissues of diverse origin. Immunohistochemical analysis was performed on a subset of these. PRL-1 and PRL-2 mRNA expression was also assessed in a small set of samples from a variety of diseases other than cancer. Where possible, associations with clinicopathological characteristics were evaluated. Alterations in PRL-1 or -2 expression were a frequent event, but the nature of those alterations was highly tumor type specific. PRL-1 was significantly overexpressed in 100% of hepatocellular and gastric carcinomas, but significantly under-expressed in 100% of ovarian, 80% of breast, and 75% of lung tumors. PRL-2 expression was significantly increased in 100% of hepatocellular carcinomas, yet significantly downregulated in 54% of kidney carcinomas. PRL-1 expression was correlated to patient gender in the bladder and to patient age in the brain and skeletal muscle. PRL-1 expression was also associated with tumor grade in the prostate, ovary, and uterus. These results suggest a pleiotropic role for PRL-1 and PRL-2 in the neoplastic process. These molecules may associate with tumor progression and serve as clinical markers of tumor aggressiveness in some tissues, but be involved in inhibition of tumor formation or growth in others.

Knockdown of proteolipid protein 2 or focal adhesion kinase with an artificial microRNA reduces growth and metastasis of B16BL6 melanoma cells

Proteolipid protein 2 (PLP2) promotes the metastasis of B16F10 cells in an experimental metastasis model. However, the effect of PLP2 on spontaneous metastasis has yet to be demonstrated, and whether PLP2 may become a new therapeutic target for malignant tumors is as yet unknown. In this study, PLP2 or focal adhesion kinase (FAK) microRNA-based short hairpin RNAs (miRNAs) were used as target molecules to specifically reduce the expression of PLP2 or FAK in B16BL6 cells. In vitro, the knockdown of PLP2 or FAK significantly inhibited cell proliferation, adhesion, migration and invasion. In a spontaneous metastatic tumor model using a footpad injection, the knockdown of PLP2 or FAK markedly inhibited the proliferation of the primary tumor and prevented tumor cells from invading the popliteal lymph nodes. The results indicate that downregulation of PLP2 or FAK may improve outcomes of malignant tumor therapy.

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.

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.

Overexpression of stathmin1 in the diffuse type of gastric cancer and its roles in proliferation and migration of gastric cancer cells

BACKGROUND

Stathmin1 is a microtubule-regulating protein that has an important role in the assembly and disassembly of the mitotic spindle. The roles of stathmin1 in carcinogenesis of various cancers, including prostate and breast cancer, have been explored. However, its expression and roles in gastric cancer have not yet been described.

METHODS

Stathmin1 expression in paraffin-embedded tissue sections from 226 patients was analysed by immunohistochemistry. Roles of stathmin1 were studied using a specific small interfering RNA (siRNA).

RESULTS

The expression of stathmin1 was positively correlated with lymph node metastasis, TNM stages and vascular invasion, and negatively with recurrence-free survival, in the diffuse type of gastric cancer. The median recurrence-free survival in patients with a negative and positive expression of stathmin1 was 17.0 and 7.0 months, respectively (P=0.009). When the expression of stathmin1 was knocked down using siRNA, the proliferation, migration and invasion of poorly differentiated gastric cancer cells in vitro were significantly inhibited. Moreover, stathmin1 siRNA transfection significantly slowed the growth of xenografts in nude mice.

CONCLUSION

These results suggest that stathmin1 can be a good prognostic factor for recurrence-free survival rate and is a therapeutic target in diffuse-type gastric cancer.

Enzyme activity of phosphatase of regenerating liver is controlled by the redox environment and its C-terminal residues

Phosphatase of regenerating liver-1 (PRL-1) belongs to a unique subfamily of protein tyrosine phosphatases (PTPases) associated with oncogenic and metastatic phenotypes. While considerable evidence supports a role for PRL-1 in promoting proliferation, the biological regulators and effectors of PRL-1 activity remain unknown. PRL-1 activity is inhibited by disulfide bond formation at the active site in vitro, suggesting PRL-1 may be susceptible to redox regulation in vivo. Because PRL-1 has been observed to localize to several different subcellular locations and cellular redox conditions vary with tissue type, age, stage of cell cycle, and subcellular location, we determined the reduction potential of the active site disulfide bond that controls phosphatase activity to improve our understanding of the function of PRL-1 in various cellular environments. We used high-resolution solution NMR spectroscopy to measure the potential and found it to be -364.3 +/- 1.5 mV. Because normal cellular environments range from -170 to -320 mV, we concluded that nascent PRL-1 would be primarily oxidized inside cells. Our studies show that a significant conformational change accompanies activation, suggesting a post-translational modification may alter the reduction potential, conferring activity. We further demonstrate that alteration of the C-terminus renders the protein reduced and active in vitro, implying the C-terminus is an important regulator of PRL-1 function. These data provide a basis for understanding how subcellular localization regulates the activity of PRL-1 and, with further investigation, may help reveal how PRL-1 promotes unique outcomes in different cellular systems, including proliferation in both normal and diseased states.

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

Background

Phosphatase of regenerating liver-3 (PRL-3) is a member of the novel phosphatases of regenerating liver family, characterized by one protein tyrosine phosphatase active domain and a C-terminal prenylation (CCVM) motif. Though widely proposed to facilitate metastasis in many cancer types, PRL-3's cellular localization and the function of its CCVM motif in metastatic process remain unknown.

Methodology/Principal Findings

In the present study, a series of Myc tagged PRL-3 wild type or mutant plasmids were expressed in B16F1 melanoma cells to investigate the relationship between PRL-3's cellular localization and metastasis. With immuno-fluorescence microcopy and cell adhesion/migration assay in vitro, and an experimental passive metastasis model in vivo, we found that CCVM motif is critical for the localization of PRL-3 on cell plasma membrane and the lung metastasis of melanoma. In particular, Cystine170 is the key site for prenylation in this process.

Conclusions/Significance

These results suggest that cellular localization of PRL-3 is highly correlated with its function in tumor metastasis, and inhibition of PRL-3 prenylation might be a new approach to cancer therapy.

PRL-3 is essentially overexpressed in primary colorectal tumours and associates with tumour aggressiveness

Phosphatase PRL-3 has been involved in different types of cancer, especially in metastases from colorectal carcinoma (CRC). In this study, we explored both isoforms of PRL-3 as a biomarker to predict the recurrence of stage IIIB-C CRC. Overexpression of PRL-3 was investigated in primary human colorectal tumours (n=20) and hepatic metastases (n=36) xenografted in nude mice, samples characterised by absence of human non-tumoral cells, showing a high degree of expression in metastases (P=0.001). In 27 cases of matched normal colonic mucosa/primary tumour/hepatic metastases, PRL-3 overexpression occurs in primary tumours vs normal mucosa (P=0.001) and in hepatic metastases vs primary tumours (P=0.045). Besides, our results in a series of 80 stage IIIB-C CRC primary tumours showed that high levels of PRL-3 were an independent predictor of metastasis (P<0.0001; OR: 9.791) in multivariate analysis of a binary logistic regression and that PRL-3 expression tightly correlates with parameters of bad outcome. Moreover, PRL-3 expression associated with poor outcome in univariate (P<0.0001) and multivariate Cox models (hazard ratio: 3.322, 95%, confidence interval: 1.405–7.852, P=0.006). In conclusion, PRL-3 is a good marker of aggressiveness of locally advanced CRS and a promising predictor of distant metastases. Nevertheless, for prognosis purposes, it is imperative to validate the cutoff value of PRL-3 expression in a larger and consecutive series and adjuvant setting.

PRL PTPs: mediators and markers of cancer progression

Aberrant protein tyrosine phosphorylation resulting from the altered activity of protein tyrosine phosphatases (PTPs) is increasingly being implicated in the genesis and progression of human cancer. Accumulating evidence indicates that the dysregulated expression of members of the phosphatase of regenerating liver (PRL) subgroup of PTPs is linked to these processes. Enhanced expression of the PRLs, notably PRL-1 and PRL-3, promotes the acquisition of cellular properties that confer tumorigenic and metastatic abilities. Up-regulation of PRL-3 is associated with the progression and eventual metastasis of several types of human cancer. Indeed, PRL-3 shows promise as a biomarker and prognostic indicator in colorectal, breast, and gastric cancers. However, the substrates and molecular mechanisms of action of the PRLs have remained elusive. Recent findings indicate that PRLs may function in regulating cell adhesion structures to effect epithelial-mesenchymal transition. The identification of PRL substrates is key to understanding their roles in cancer progression and exploiting their potential as exciting new therapeutic targets for cancer treatment.

Translational control of C-terminal Src kinase (Csk) expression by PRL3 phosphatase

Phosphatase of regenerating liver 3 (PRL3) is up-regulated in cancer metastases. However, little is known of PRL3-mediated cellular signaling pathways. We previously reported that elevated PRL3 expression increases Src kinase activity, which likely contributes to the increased tumorigenesis and metastasis potential of PRL3. PRL3-induced Src activation is proposed to be indirect through down-regulation of Csk, a negative regulator of Src. Given the importance of PRL3 in tumor metastasis and the role of Csk in controlling Src activity, we addressed the mechanism by which PRL3 mediates Csk down-regulation. PRL3 is shown to exert a negative effect on Csk protein synthesis, rather than regulation of Csk mRNA levels or protein turnover. Interestingly, the preferential decrease in Csk protein synthesis is a consequence of increased eIF2 phosphorylation resulting from PRL3 expression. Reduced Csk synthesis also occurs in response to cellular stress that induces eIF2 phosphorylation, indicating that this regulatory mechanism may occur in response to a wider spectrum of cellular conditions known to direct translational control. Thus, we have uncovered a previously uncharacterized role for PRL3 in the gene-specific translational control of Csk expression.

Small interfering RNA-mediated knockdown of PRL phosphatases results in altered Akt phosphorylation and reduced clonogenicity of pancreatic cancer cells

The PRL phosphatases have been implicated in cancer cell growth and metastasis in a variety of tumor types. Using cDNA microarray, we previously identified and reported PRL-1 as being highly up-regulated in pancreatic cancer cell lines. In this study, we sought to further evaluate the expression of all three PRL phosphatases in pancreatic cancer cell lines and extend our findings to in situ analysis of primary pancreatic tumors taken directly from patients. Additionally, we determine if small interfering RNA-mediated knockdown of relevant PRLs confers antitumor effects in pancreatic cancer cells. Using oligonucleotide expression arrays, mRNA levels of PRL-1 and PRL-2 but not PRL-3 were identified as up-regulated in pancreatic cancer cell lines and tumor samples taken directly from patients compared with those of normal pancreas. Focusing on PRL-1 and PRL-2, high levels of both proteins were detected in a subset of pancreatic cancer cell lines and tumor samples using Western blotting and immunohistochemistry, respectively. Small interfering RNA-mediated knockdown of PRL-1 and PRL-2 in combination resulted in a moderate reduction of cellular growth and migration in MIA PaCa-2 and PANC-1 cells. More importantly, knockdown of both PRL-1 and PRL-2 significantly inhibited colony formation of these cells in soft agar as well as serum-induced Akt phosphorylation. These data support the hypothesis that PRL phosphatases regulate key pathways involved in tumorigenesis and metastasis and that knockdown of both PRL-1 and PRL-2 is required to disrupt PRL phosphatase function in pancreatic cancer cells.