Trimeric Structure of PRL-1 Phosphatase Reveals an Active Enzyme Conformation and Regulation Mechanisms

Drosophila Phosphatase of Regenerating Liver Is Critical for Photoreceptor Cell Polarity and Survival during Retinal Development

Establishing apicobasal polarity, involving intricate interactions among polarity regulators, is key for epithelial cell function. Though phosphatase of regenerating liver (PRL) proteins are implicated in diverse biological processes, including cancer, their developmental role remains unclear. In this study, we explore the role of Drosophila PRL (dPRL) in photoreceptor cell development. We reveal that dPRL, requiring a C-terminal prenylation motif, is highly enriched in the apical membrane of developing photoreceptor cells. Moreover, dPRL knockdown during retinal development results in adult Drosophila retinal degeneration, caused by hid-induced apoptosis. dPRL depletion also mislocalizes cell adhesion and polarity proteins like Armadillo, Crumbs, and DaPKC and relocates the basolateral protein, alpha subunit of Na⁺/K⁺-ATPase, to the presumed apical membrane. Importantly, this polarity disruption is not secondary to apoptosis, as suppressing hid expression does not rescue the polarity defect in dPRL-depleted photoreceptor cells. These findings underscore dPRL's crucial role in photoreceptor cell polarity and emphasize PRL's importance in establishing epithelial polarity and maintaining cell survival during retinal development, offering new insights into PRL's role in normal epithelium.

Burst kinetics and CNNM binding are evolutionarily conserved properties of phosphatases of regenerating liver

Phosphatases of regenerating liver (PRL or PTP4A) are a family of enigmatic protein phosphatases implicated in cell growth and metabolism. Despite their relevance in metastatic cancer, much remains unknown about the PRL family. They act as pseudophosphatases to regulate the CNNM family of magnesium transporters yet also have enzymatic activity on unknown substrates. In mammals, PRLs are mostly found trapped in an intermediate state that regulates their pseudophosphatase activity. Phosphocysteine, which is formed as an intermediate in the phosphatase catalytic cycle, is inefficiently hydrolyzed leading to burst enzyme kinetics and turnover numbers of less than one per hour. In flies, PRLs have recently been shown to have neuroprotective and neurodevelopmental roles raising the question whether they act as phosphatases, pseudophosphatases, or both. Here, we characterize the evolutionary development of PRLs and ask whether their unique structural and functional properties are conserved. We purified recombinant PRL proteins from 15 phylogenetically diverse organisms and characterized their catalytic activities and ability to bind CNNM proteins. We observed PRLs from humans to amoebae form a stable phosphocysteine intermediate and exhibit burst kinetics. Isothermal titration calorimetry experiments confirmed that the PRL-CNNM interaction is broadly conserved with nanomolar affinity in vertebrates. Lastly, we determined the crystal structure of the Drosophila melanogaster PRL-CNNM complex and identified mutants that specifically impair either phosphatase activity or CNNM binding. Our results reveal the unique properties of PRLs are conserved throughout the animal kingdom and open the door to using model organisms to dissect PRL function in cell signaling.

Increased PRL-1 in BM-derived MSCs triggers anaerobic metabolism via mitochondria in a cholestatic rat model

Mesenchymal stem cell (MSC) therapy in chronic liver disease is associated with mitochondrial anaerobic metabolism. Phosphatase of regenerating liver-1 (PRL-1), known as protein tyrosine phosphatase type 4A, member 1 (PTP4A1), plays a critical role in liver regeneration. However, its therapeutic mechanism remains obscure. The aim of this study was to establish genetically modified bone marrow (BM)-MSCs overexpressing PRL-1 (BM-MSCs^PRL-1) and to investigate their therapeutic effects on mitochondrial anaerobic metabolism in a bile duct ligation (BDL)-injured cholestatic rat model. BM-MSCs^PRL-1 were generated with lentiviral and nonviral gene delivery systems and characterized. Compared with naive cells, BM-MSCs^PRL-1 showed an improved antioxidant capacity and mitochondrial dynamics and decreased cellular senescence. In particular, mitochondrial respiration in BM-MSCs^PRL-1 generated using the nonviral system was significantly increased as well as mtDNA copy number and total ATP production. Moreover, transplantation of BM-MSCs^PRL-1 generated using the nonviral system had predominantly antifibrotic effects and restored hepatic function in a BDL rat model. Decreased cytoplasmic lactate and increased mitochondrial lactate upon the administration of BM-MSCs^PRL-1 indicated significant alterations in mtDNA copy number and ATP production, activating anaerobic metabolism. In conclusion, BM-MSCs^PRL-1 generated by a nonviral gene delivery system enhanced anaerobic mitochondrial metabolism in a cholestatic rat model, improving hepatic function.

Anticancer Profile of Rhodanines: Structure-Activity Relationship (SAR) and Molecular Targets-A Review

The rhodanine core is a well-known privileged heterocycle in medicinal chemistry. The rhodanines, as subtypes of thiazolidin-4-ones, show a broad spectrum of biological activity, including anticancer properties. This review aims to analyze the anticancer features of the rhodanines described over the last decade in the scientific literature. The structure–activity relationship of rhodanine derivatives, as well as some of the molecular targets, were discussed. The information contained in this review could be of benefit to the design of new, effective small molecules with anticancer potential among rhodanine derivatives or their related heterocycles.

The double lives of phosphatases of regenerating liver: A structural view of their catalytic and noncatalytic activities

Phosphatases of regenerating liver (PRLs) are protein phosphatases involved in the control of cell growth and migration. They are known to promote cancer metastasis but, despite over 20 years of study, there is still no consensus about their mechanism of action. Recent work has revealed that PRLs lead double lives, acting both as catalytically active enzymes and as pseudophosphatases. The three known PRLs belong to the large family of cysteine phosphatases that form a phosphocysteine intermediate during catalysis. Uniquely to PRLs, this intermediate is stable, with a lifetime measured in hours. As a consequence, PRLs have very little phosphatase activity. Independently, PRLs also act as pseudophosphatases by binding CNNM membrane proteins to regulate magnesium homeostasis. In this function, an aspartic acid from CNNM inserts into the phosphatase catalytic site of PRLs, mimicking a substrate–enzyme interaction. The delineation of PRL pseudophosphatase and phosphatase activities in vivo was impossible until the recent identification of PRL mutants defective in one activity or the other. These mutants showed that CNNM binding was sufficient for PRL oncogenicity in one model of metastasis, but left unresolved its role in other contexts. As the presence of phosphocysteine prevents CNNM binding and CNNM-binding blocks catalytic activity, these two activities are inherently linked. Additional studies are needed to untangle the intertwined catalytic and noncatalytic functions of PRLs. Here, we review the current understanding of the structure and biophysical properties of PRL phosphatases.

Fast Diffusion Sustains Plasma Membrane Accumulation of Phosphatase of Regenerating Liver-1

It has been proposed that the accumulation of farnesylated phosphatase of regenerating liver-1 (PRL-1) at the plasma membrane is mediated by static electrostatic interactions of a polybasic region with acidic membrane lipids and assisted by oligomerization. Nonetheless, localization at early and recycling endosomes suggests that the recycling compartment might also contribute to its plasma membrane accumulation. Here, we investigated in live cells the dynamics of PRL-1 fused to the green fluorescent protein (GFP-PRL-1). Blocking the secretory pathway and photobleaching techniques suggested that plasma membrane accumulation of PRL-1 was not sustained by recycling endosomes but by a dynamic exchange of diffusible protein pools. Consistent with this idea, fluorescence correlation spectroscopy in cells overexpressing wild type or monomeric mutants of GFP-PRL-1 measured cytosolic and membrane-diffusing pools of protein that were not dependent on oligomerization. Endogenous expression of GFP-PRL-1 by CRISPR/Cas9 genome edition confirmed the existence of fast diffusing cytosolic and membrane pools of protein. We propose that plasma membrane PRL-1 replenishment is independent of the recycling compartment and the oligomerization state and mainly driven by fast diffusion of the cytosolic pool.

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 phosphatases: promising targets in pancreatic ductal adenocarcinoma

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

Tapping the therapeutic potential of protein tyrosine phosphatase 4A with small molecule inhibitors

Protein tyrosine phosphatases (PTPs) are emerging new targets for drug discovery. PTPs and protein tyrosine kinases (PTKs) maintain cellular homeostasis through opposing roles: tyrosine O-dephosphorylation and -phosphorylation, respectively. An imbalance in the phosphorylation equilibrium results in aberrant protein signaling and pathophysiological conditions. PTPs have historically been considered 'undruggable', in part due to a lack of evidence defining their relationship to disease causality and a focus on purely competitive inhibitors. However, a better understanding of protein-protein interfaces and shallow active sites has recently renewed interest in the pursuit of allosteric and orthosteric modulators of targets outside the major druggable protein families. While their biological mechanism of action still remains to be clarified, PTP4A1-3 (also referred to as PRL1-3) are validated oncology targets and play an important role in cell proliferation, metastasis, and tumor angiogenesis. In this Digest, recent syntheses and structure-activity relationships (SAR) of small molecule inhibitors (SMIs) of PTP4A1-3 are summarized, and enzyme docking studies of the most potent chemotype are highlighted. In particular, the thienopyridone scaffold has emerged as a potent lead structure to interrogate the function and druggability of this dual-specificity PTP.

Mechanism of thienopyridone and iminothienopyridinedione inhibition of protein phosphatases

Thienopyridone (TP) has been proposed as a selective inhibitor of phosphatases of regenerating liver (PRL or PTP4A). PRLs are dual specificity phosphatases that promote cancer progression and are attractive anticancer targets. TP and iminothienopyridinedione (ITP), a more potent derivative, were shown to be effective inhibitors but the mechanism of inhibition was not established. Here, we perform NMR experiments and in vitro phosphatase assays to show that TP and ITP inhibit protein phosphatases non-specifically through oxidation of the phosphatase catalytic cysteine. We demonstrate that TP and ITP are redox active compounds, inhibiting PRL-3 and multiple other PTPs through oxidation. They also catalyze the oxidation of thioredoxin-1 as well as small molecules, like TCEP, DTT, and glutathione. The reported selectivity of TP and ITP is likely due to the higher susceptibility of PRLs to oxidation. Thus, while TP and ITP effectively inhibit PRLs, their use for studying the cellular function of PRLs is problematic due to the likelihood of off-target effects.

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.

Regulatory Mechanisms and Novel Therapeutic Targeting Strategies for Protein Tyrosine Phosphatases

An appropriate level of protein phosphorylation on tyrosine is essential for cells to react to extracellular stimuli and maintain cellular homeostasis. Faulty operation of signal pathways mediated by protein tyrosine phosphorylation causes numerous human diseases, which presents enormous opportunities for therapeutic intervention. While the importance of protein tyrosine kinases in orchestrating the tyrosine phosphorylation networks and in target-based drug discovery has long been recognized, the significance of protein tyrosine phosphatases (PTPs) in cellular signaling and disease biology has historically been underappreciated, due to a large extent to an erroneous assumption that they are largely constitutive and housekeeping enzymes. Here, we provide a comprehensive examination of a number of regulatory mechanisms, including redox modulation, allosteric regulation, and protein oligomerization, that control PTP activity. These regulatory mechanisms are integral to the myriad PTP-mediated biochemical events and reinforce the concept that PTPs are indispensable and specific modulators of cellular signaling. We also discuss how disruption of these PTP regulatory mechanisms can cause human diseases and how these diverse regulatory mechanisms can be exploited for novel therapeutic development.

Critical Roles of Dual-Specificity Phosphatases in Neuronal Proteostasis and Neurological Diseases

Protein homeostasis or proteostasis is a fundamental cellular property that encompasses the dynamic balancing of processes in the proteostasis network (PN). Such processes include protein synthesis, folding, and degradation in both non-stressed and stressful conditions. The role of the PN in neurodegenerative disease is well-documented, where it is known to respond to changes in protein folding states or toxic gain-of-function protein aggregation. Dual-specificity phosphatases have recently emerged as important participants in maintaining balance within the PN, acting through modulation of cellular signaling pathways that are involved in neurodegeneration. In this review, we will summarize recent findings describing the roles of dual-specificity phosphatases in neurodegeneration and offer perspectives on future therapeutic directions.

Characterization of the Protein Tyrosine Phosphatase LmPRL-1 Secreted by Leishmania major via the Exosome Pathway

Similar to other intracellular pathogens, Leishmania parasites are known to evade the antimicrobial effector functions of host immune cells. To date, however, only a few virulence factors have been described for Leishmania major, one of the causative agents of cutaneous leishmaniasis. Here, we have characterized the expression and function of an L. major phosphatase, which we termed LmPRL-1. This enzyme shows a strong structural similarity to the human phosphatases of regenerating liver (PRL-1, -2, and -3) that regulate the proliferation, differentiation, and motility of cells. The biochemical characterization of the L. major phosphatase revealed that the enzyme is redox sensitive. When analyzing the subcellular localization of LmPRL-1 in promastigotes, amastigotes, and infected macrophages, we found that the phosphatase was predominantly expressed and secreted by promastigotes via the exosome route. Finally, we observed that ectopic expression of LmPRL-1 in L. major led to an increased number of parasites in macrophages. From these data, we conclude that the L. major phosphatase LmPRL-1 contributes to the intracellular survival of the parasites in macrophages.

TALEN based HPV-E7 editing triggers necrotic cell death in cervical cancer cells

Human Papillomavirus E7 and E6 oncoproteins have been considered as suitable candidate anti-viral targets since they cause malignant conversion in cervical cancers. Transcription Activator-Like Effector Nucleases (TALENs) are recent editing tools to knockout genes by inducing double stranded breaks at specific sites in the genome. In here, we have designed specific TALENs to target E7 and analyzed their efficiency in inducing cell death in cervical cancer cells. We found that designed TALENs could yield about 10–12% editing activity as observed from T7E1 and nuclease resistance assays. Down-regulation of E7 and E6 was further evident at the transcript as well as proteins levels indicating that the selected TALENs were effective. TALEN-mediated E7 editing led to cell death as ascertained by cell cycle and Annexin V assays. Annexin profiling suggested that cell death could be due to necrosis as observed by upregulation of necrotic markers such as LDH A, Rip-1, and Cyclophilin A. Necrosis appears to be a better therapeutic response as it could further activate pro-inflammatory cytokines to attract immune cells to eliminate HPV-integrated cells and therefore TALEN editing strategy has the potential to be a promising tool as an adjuvant therapy in cervical cancer along with surgery.

Regulatory mechanisms of phosphatase of regenerating liver (PRL)-3

The phosphatase of regenerating liver (PRL)-3 is overexpressed in many human cancer types and tumor metastases when compared with healthy tissues. Different pathways and mechanisms have been suggested to modulate PRL-3 expression levels and activity, giving some valuable insights but still leaving an incomplete picture. Investigating these mechanisms could provide new targets for therapeutic drug development. Here, we present an updated overview and summarize recent findings concerning the different PRL-3 expression regulatory mechanisms and posttranslational modifications suggested to modulate the activity, localization, or stability of this phosphatase.

PRL3 phosphatase active site is required for binding the putative magnesium transporter CNNM3

The phosphatases of regenerating liver (PRLs) are involved in tumorigenesis and metastatic cancer yet their cellular function remains unclear. Recent reports have shown PRL phosphatases bind tightly to the CNNM family of membrane proteins to regulate magnesium efflux. Here, we characterize the interactions between the CBS-pair (Bateman) domain of CNNM3 and either PRL2 or PRL3 using X-ray crystallography, isothermal titration calorimetry, and activity assays. We report four new crystal structures of PRL proteins bound to the CNNM3 CBS-pair domain that reveal the effects of cysteine disulphide formation and nucleotide binding on complex formation. We use comprehensive mutagenesis of the PRL3 catalytic site to quantify the importance of different PRL amino acids, including cysteine 104, leucine 108, and arginine 110, for CNNM binding and phosphatase activity. We show the PRL3 R138E mutant is selectively deficient in CNNM3 binding with the potential to distinguish between the downstream effects of phosphatase and CNNM-binding activities in vivo. Through a novel activity assay, we show that PRL3 has magnesium-sensitive phosphatase activity with ATP and other nucleotides. Our results identify a strong correlation between phosphatase activity and CNNM binding and support the contention that PRL function as pseudophosphatases regulated by chemical modifications of their catalytic cysteine.

Structural Basis of the Oncogenic Interaction of Phosphatase PRL-1 with the Magnesium Transporter CNNM2

Phosphatases of regenerating liver (PRLs), the most oncogenic of all protein-tyrosine phosphatases (PTPs), play a critical role in metastatic progression of cancers. Recent findings established a new paradigm by uncovering that their association with magnesium transporters of the cyclin M (CNNM) family causes a rise in intracellular magnesium levels that promote oncogenic transformation. Recently, however, essential roles for regulation of the circadian rhythm and reproduction of the CNNM family have been highlighted. Here, we describe the crystal structure of PRL-1 in complex with the Bateman module of CNNM2 (CNNM2_BAT), which consists of two cystathionine β-synthase (CBS) domains (IPR000664) and represents an intracellular regulatory module of the transporter. The structure reveals a heterotetrameric association, consisting of a disc-like homodimer of CNNM2_BAT bound to two independent PRL-1 molecules, each one located at opposite tips of the disc. The structure highlights the key role played by Asp-558 at the extended loop of the CBS2 motif of CNNM2 in maintaining the association between the two proteins and proves that the interaction between CNNM2 and PRL-1 occurs via the catalytic domain of the phosphatase. Our data shed new light on the structural basis underlying the interaction between PRL phosphatases and CNNM transporters and provides a hypothesis about the molecular mechanism by which PRL-1, upon binding to CNNM2, might increase the intracellular concentration of Mg²⁺ thereby contributing to tumor progression and metastasis. The availability of this structure sets the basis for the rational design of compounds modulating PRL-1 and CNNM2 activities.

Phosphocysteine in the PRL-CNNM pathway mediates magnesium homeostasis

PRLs (phosphatases of regenerating liver) are frequently overexpressed in human cancers and are prognostic markers of poor survival. Despite their potential as therapeutic targets, their mechanism of action is not understood in part due to their weak enzymatic activity. Previous studies revealed that PRLs interact with CNNM ion transporters and prevent CNNM4-dependent Mg²⁺ transport, which is important for energy metabolism and tumor progression. Here, we report that PRL-CNNM complex formation is regulated by the formation of phosphocysteine. We show that cysteine in the PRL catalytic site is endogenously phosphorylated as part of the catalytic cycle and that phosphocysteine levels change in response to Mg²⁺ levels. Phosphorylation blocks PRL binding to CNNM Mg²⁺ transporters, and mutations that block the PRL-CNNM interaction prevent regulation of Mg²⁺ efflux in cultured cells. The crystal structure of the complex of PRL2 and the CBS-pair domain of the Mg²⁺ transporter CNNM3 reveals the molecular basis for the interaction. The identification of phosphocysteine as a regulatory modification opens new perspectives for signaling by protein phosphatases.

Identification of proteins suppressing the functions of oncogenic phosphatase of regenerating liver 1 and 3

The phosphatase of regenerating liver (PRL) family, including PRL-1, PRL-2, and PRL-3, comprises protein tyrosine phosphatases whose deregulation is associated with the tumorigenesis and metastasis of many types of cancer. However, the underlying mechanism is poorly understood. In this study, aiming to increase understanding of the molecular mechanisms underlying the functions of PRL-1 and PRL-3, a yeast two-hybrid system was employed to screen for their interacting proteins. Alignment with the NCBI BLAST database revealed 12 interactive proteins: Synaptic nuclear envelope protein 2, emerin, mannose 6-phosphate receptor-binding protein 1, low-density lipoprotein receptor-related protein 10, Rab acceptor 1, tumor protein D52-like 2, selectin P ligand (SELPLG), guanylate binding protein 1, transmembrane and ubiquitin-like domain-containing 2, NADH:ubiquinone oxidoreductase subunit B8, syndecan 4 and FK506-binding protein 8 (FKBP8). These proteins are associated with cell proliferation, apoptosis, immune response, cell fate specification and metabolic process in biological process categories, and involved in various signaling pathways, including Alzheimer's disease, Parkinson's disease, Huntington's disease, hypertrophic cardiomyopathy and cell adhesion molecules. Interactions of PRL-1 with the prey proteins SELPLG and FKBP8 were confirmed by immunoprecipitation or immunostaining. Furthermore, SELPLG and FKBP8 suppressed PRL-1- or PRL-3-mediated p53 activity. Identification of the proteins interacting with PRL family proteins may provide valuable information to better understand the mechanism of PRL-mediated signal transduction in cancer and other diverse diseases.

Discovery and Evaluation of PRL Trimer Disruptors for Novel Anticancer Agents

Overexpression of PRL phosphatases (PRL1, PRL2, and PRL3) has been found in a variety of late-stage tumors and their distant metastatic sites. Therefore, the oncogenic PRL phosphatases represent intriguing targets for cancer therapy. There is considerable interest in identifying small molecule inhibitors targeting PRLs as novel anticancer agents. However, it has been difficult to acquire phosphatase activity-based PRL inhibitors due to the unusual wide and shallow catalytic pockets of PRLs revealed by crystal structure studies. Here, we present a novel method to identify PRL1 inhibitors by targeting the PRL1 trimer interface and the procedure to characterize their biochemical and cellular activity.

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.

Inhibition of PRL-2·CNNM3 Protein Complex Formation Decreases Breast Cancer Proliferation and Tumor Growth

The oncogenic phosphatase of regenerating liver 2 (PRL-2) has been shown to regulate intracellular magnesium levels by forming a complex through an extended amino acid loop present in the Bateman module of the CNNM3 magnesium transporter. Here we identified highly conserved residues located on this amino acid loop critical for the binding with PRL-2. A single point mutation (D426A) of one of those critical amino acids was found to completely disrupt PRL-2·human Cyclin M 3 (CNNM3) complex formation. Whole-cell voltage clamping revealed that expression of CNNM3 influenced the surface current, whereas overexpression of the binding mutant had no effect, indicating that the binding of PRL-2 to CNNM3 is important for the activity of the complex. Interestingly, overexpression of the CNNM3 D426A-binding mutant in cancer cells decreased their ability to proliferate under magnesium-deprived situations and under anchorage-independent growth conditions, demonstrating a PRL-2·CNNM3 complex-dependent oncogenic advantage in a more stringent environment. We further confirmed the importance of this complex in vivo using an orthotopic xenograft breast cancer model. Finally, because molecular modeling showed that the Asp-426 side chain in CNNM3 buries into the catalytic cavity of PRL-2, we showed that a PRL inhibitor could abrogate complex formation, resulting in a decrease in proliferation of human breast cancer cells. In summary, we provide evidence that this fundamental regulatory aspect of PRL-2 in cancer cells could potentially lead to broadly applicable and innovative therapeutic avenues.

Characterization of the protein tyrosine phosphatase PRL from Entamoeba histolytica

Protein tyrosine phosphatase of regenerating liver (PRL) is a group of phosphatases that has not been broadly studied in protozoan parasites. In humans, PRLs are involved in metastatic cancer, the promotion of cell migration and invasion. PTPs have been increasingly recognized as important effectors of host-pathogen interactions. We characterized the only putative protein tyrosine phosphatase PRL (PTP EhPRL) in the eukaryotic human intestinal parasite Entamoeba histolytica. Here, we reported that the EhPRL protein possessed the classical HCX5R catalytic motif of PTPs and the CAAX box characteristic of the PRL family and exhibited 31-32% homology with the three human PRL isoforms. In amebae, the protein was expressed at low but detectable levels. The recombinant protein (rEhPRL) had enzymatic activity with the 3-o-methyl fluorescein phosphate (OMFP) substrate; this enzymatic activity was inhibited by the PTP inhibitor o-vanadate. Using immunofluorescence we showed that native EhPRL was localized to the cytoplasm and plasma membrane. When the trophozoites interacted with collagen, EhPRL relocalized over time to vesicle-like structures. Interaction with fibronectin increased the presence of the enzyme in the cytoplasm. Using RT-PCR, we demonstrated that EhPRL mRNA expression was upregulated when the trophozoites interacted with collagen but not with fibronectin. Trophozoites recovered from amoebic liver abscesses showed higher EhPRL mRNA expression levels than normal trophozoites. These results strongly suggest that EhPRL may play an important role in the biology and adaptive response of the parasite to the host environment during amoebic liver abscess development, thereby participating in the pathogenic mechanism.

Phosphatase of regenerating liver in hematopoietic stem cells and hematological malignancies

The phosphatases of regenerating liver (PRLs), consisting PRL1, PRL2 and PRL3, are dual-specificity protein phosphatases that have been implicated as biomarkers and therapeutic targets in several solid tumors. However, their roles in hematological malignancies are largely unknown. Recent findings demonstrate that PRL2 is important for hematopoietic stem cell self-renewal and proliferation. In addition, both PRL2 and PRL3 are highly expressed in some hematological malignancies, including acute myeloid leukemia (AML), chronic myeloid leukemia (CML), multiple myeloma (MM) and acute lymphoblastic leukemia (ALL). Moreover, PRL deficiency impairs the proliferation and survival of leukemia cells through regulating oncogenic signaling pathways. While PRLs are potential novel therapeutic targets in hematological malignancies, their exact biological function and cellular substrates remain unclear. This review will discuss how PRLs regulate hematopoietic stem cell behavior, what signaling pathways are regulated by PRLs, and how to target PRLs in hematological malignancies. An improved understanding of how PRLs function and how they are regulated may facilitate the development of PRL inhibitors that are effective in cancer treatment.

Structure and backbone dynamics of vanadate-bound PRL-3: comparison of 15N nuclear magnetic resonance relaxation profiles of free and vanadate-bound PRL-3

Phosphatases of regenerating liver (PRLs) constitute a novel class of small, prenylated phosphatases with oncogenic activity. PRL-3 is particularly important in cancer metastasis and represents a potential therapeutic target. The flexibility of the WPD loop as well as the P-loop of protein tyrosine phosphatases is closely related to their catalytic activity. Using nuclear magnetic resonance spectroscopy, we studied the structure of vanadate-bound PRL-3, which was generated by addition of sodium orthovanadate to PRL-3. The WPD loop of free PRL-3 extended outside of the active site, forming an open conformation, whereas that of vanadate-bound PRL-3 was directed into the active site by a large movement, resulting in a closed conformation. We suggest that vanadate binding induced structural changes in the WPD loop, P-loop, helices α4-α6, and the polybasic region. Compared to free PRL-3, vanadate-bound PRL-3 has a longer α4 helix, where the catalytic R110 residue coordinates with vanadate in the active site. In addition, the hydrophobic cavity formed by helices α4-α6 with a depth of 14-15 Å can accommodate a farnesyl chain at the truncated prenylation motif of PRL-3, i.e., from R169 to M173. Conformational exchange data suggested that the WPD loop moves between open and closed conformations with a closing rate constant k(close) of 7 s(-1). This intrinsic loop flexibility of PRL-3 may be related to their catalytic rate and may play a role in substrate recognition.

Dual-specificity phosphatases as molecular targets for inhibition in human disease

SIGNIFICANCE

The dual-specificity phosphatases (DUSPs) constitute a heterogeneous group of cysteine-based protein tyrosine phosphatases, whose members exert a pivotal role in cell physiology by dephosphorylation of phosphoserine, phosphothreonine, and phosphotyrosine residues from proteins, as well as other non-proteinaceous substrates.

RECENT ADVANCES

A picture is emerging in which a selected group of DUSP enzymes display overexpression or hyperactivity that is associated with human disease, especially human cancer, making feasible targeted therapy approaches based on their inhibition. A panoply of molecular and functional studies on DUSPs have been performed in the previous years, and drug-discovery efforts are ongoing to develop specific and efficient DUSP enzyme inhibitors. This review summarizes the current status on inhibitory compounds targeting DUSPs that belong to the MAP kinase phosphatases-, small-sized atypical-, and phosphatases of regenerating liver subfamilies, whose inhibition could be beneficial for the prevention or mitigation of human disease.

CRITICAL ISSUES

Achieving specificity, potency, and bioavailability are the major challenges in the discovery of DUSP inhibitors for the clinics. Clinical validation of compounds or alternative inhibitory strategies of DUSP inhibition has yet to come.

FUTURE DIRECTIONS

Further work is required to understand the dual role of many DUSPs in human cancer, their function-structure properties, and to identify their physiologic substrates. This will help in the implementation of therapies based on DUSPs inhibition.

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.

The family-wide structure and function of human dual-specificity protein phosphatases

Dual-specificity protein phosphatases (DUSPs), which dephosphorylate both phosphoserine/threonine and phosphotyrosine, play vital roles in immune activation, brain function and cell-growth signalling. A family-wide structural library of human DUSPs was constructed based on experimental structure determination supplemented with homology modelling. The catalytic domain of each individual DUSP has characteristic features in the active site and in surface-charge distribution, indicating substrate-interaction specificity. The active-site loop-to-strand switch occurs in a subtype-specific manner, indicating that the switch process is necessary for characteristic substrate interactions in the corresponding DUSPs. A comprehensive analysis of the activity-inhibition profile and active-site geometry of DUSPs revealed a novel role of the active-pocket structure in the substrate specificity of DUSPs. A structure-based analysis of redox responses indicated that the additional cysteine residues are important for the protection of enzyme activity. The family-wide structures of DUSPs form a basis for the understanding of phosphorylation-mediated signal transduction and the development of therapeutics.

Structure and catalytic mechanism of human protein tyrosine phosphatome

Together with protein tyrosine kinases (PTKs), protein tyrosine phosphatases (PTPs) serve as hallmarks in cellular signal transduction by controlling the reversible phosphorylation of their substrates. The human genome is estimated to encode more than 100 PTPs, which can be divided into eleven sub-groups according to their structural and functional characteristics. All the crystal structures of catalytic domains of sub-groups have been elucidated, enabling us to understand their precise catalytic mechanism and to compare their structures across all sub-groups. In this review, I describe the structure and mechanism of catalytic domains of PTPs in the structural context. [BMB Reports 2012; 45(12): 693-699]

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.

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.

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.

The metastasis-promoting phosphatase PRL-3 shows activity toward phosphoinositides

Phosphatase of regenerating liver 3 (PRL-3) is suggested as a biomarker and therapeutic target in several cancers. It has a well-established causative role in cancer metastasis. However, little is known about its natural substrates, pathways, and biological functions, and only a few protein substrates have been suggested so far. To improve our understanding of the substrate specificity and molecular determinants of PRL-3 activity, the wild-type (WT) protein, two supposedly catalytically inactive mutants D72A and C104S, and the reported hyperactive mutant A111S were tested in vitro for substrate specificity and activity toward phosphopeptides and phosphoinositides (PIPs), their structural stability, and their ability to promote cell migration using stable HEK293 cell lines. We discovered that WT PRL-3 does not dephosphorylate the tested phosphopeptides in vitro. However, as shown by two complementary biochemical assays, PRL-3 is active toward the phosphoinositide PI(4,5)P(2). Our experimental results substantiated by molecular docking studies suggest that PRL-3 is a phosphatidylinositol 5-phosphatase. The C104S variant was shown to be not only catalytically inactive but also structurally destabilized and unable to promote cell migration, whereas WT PRL-3 promotes cell migration. The D72A mutant is structurally stable and does not dephosphorylate the unnatural substrate 3-O-methylfluorescein phosphate (OMFP). However, we observed residual in vitro activity of D72A against PI(4,5)P(2), and in accordance with this, it exhibits the same cellular phenotype as WT PRL-3. Our analysis of the A111S variant shows that the hyperactivity toward the unnatural OMFP substrate is not apparent in dephosphorylation assays with phosphoinositides: the mutant is completely inactive against PIPs. We observed significant structural destabilization of this variant. The cellular phenotype of this mutant equals that of the catalytically inactive C104S mutant. These results provide a possible explanation for the absence of the conserved Ser of the PTP catalytic motif in the PRL family. The correlation of the phosphatase activity toward PI(4,5)P(2) with the observed phenotypes for WT PRL-3 and the mutants suggests a link between the PI(4,5)P(2) dephosphorylation by PRL-3 and its role in cell migration.

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.

Retinoblastoma-independent antiproliferative activity of novel intracellular antibodies against the E7 oncoprotein in HPV 16-positive cells

Background

"High risk" Human Papillomavirus strains are the causative agents of the vast majority of carcinomas of the uterine cervix. In these tumors, the physical integration of the HPV genome is a frequent, though not invariable occurrence, but the constitutive expression of the E6 and E7 viral genes is always observed, suggesting key roles for the E6 and E7 oncoproteins in the process of malignant transformation. The "intracellular antibody" technology using recombinant antibodies in single-chain format offers the possibility of targeting a protein in its intracellular environment even at the level of definite domains thus representing a valuable strategy to "knock out" the function of specific proteins.

Methods

In this study, we investigate the in vitro activity of two single-chain antibody fragments directed against the "high-risk" HPV 16 E7 oncoprotein, scFv 43M2 and scFv 51. These scFvs were expressed by retroviral system in different cell compartments of the HPV16-positive SiHa cells, and cell proliferation was analyzed by Colony Formation Assay and EZ4U assay. The binding of these scFvs to E7, and their possible interference with the interaction between E7 and its main target, the tumor suppressor pRb protein, were then investigated by immunoassays, PepSet™technology and Surface Plasmon Resonance.

Results

The expression of the two scFvs in the nucleus and the endoplasmic reticulum of SiHa cells resulted in the selective growth inhibition of these cells. Analysis of binding showed that both scFvs bind E7 via distinct but overlapping epitopes not corresponding to the pRb binding site. Nevertheless, the binding of scFv 43M2 to E7 was inhibited by pRb in a non-competitive manner.

Conclusions

Based on the overall results, the observed inhibition of HPV-positive SiHa cells proliferation could be ascribed to an interaction between scFv and E7, involving non-pRb targets. The study paves the way for the employment of specific scFvs in immunotherapeutic approaches against the HPV-associated lesions.

Probing residue-specific interactions in the stabilization of proteins using high-resolution NMR: a study of disulfide bond compensation

It is well established that the oxidation state of cysteine residues in proteins is critical to overall physical stability. Disulfide bonds most often impart thermodynamic stability, but in some cases, diminish it. Predicting the circumstances that lead to each outcome is difficult because mechanistic information is lacking. Because the techniques typically used to study protein stability do not provide sufficient detail, high-resolution NMR was used in combination with low-resolution analysis to obtain mechanistic information regarding disulfide bond formation in a model protein. T(m) (CD) and T(onset) (SLS) for the reduced and oxidized wild type and C104S and C49S mutants were measured. The mutant proteins have altered T(m)s and T(onset)s compared to the reduced wild type, indicating that differences in local interactions of the Cys side chains are important for stability. The NMR spectra clearly show distinct differences in the chemical environment surrounding these Cys residues and the overall tertiary structure. The C49S protein, which is less stable and more aggregation prone than reduced wild type, lacks a hydrogen bond between Y53 and H103. Increased flexibility of the Y53-containing loop is correlated with increased dynamics and unraveling of alpha2, which likely leads to edge strand initiated aggregation of the central beta-sheet.

Phosphatase of regenerating liver-1 promotes cell migration and invasion and regulates filamentous actin dynamics

The phosphatases of regenerating liver (PRLs) are a unique family of plasma membrane-associated protein tyrosine phosphatases that have been hypothesized to be involved in metastatic cancer. How PRLs control cancer cell migration, invasion, and proliferation remains largely unknown. In the current study, we demonstrate a role for PRL-1 in the regulation of filamentous actin dynamics, which could promote cell metastatic processes. Human A549 non-small-cell lung cancer cells stably expressing wild-type PRL-1 exhibited a 60% increase in migration and a 3-fold increase in invasion. Cells expressing catalytic mutants of PRL-1 (C104S and D72A) lacked increased cell migration and invasion, indicating that these phenotypic changes required PRL-1 phosphatase activity. In contrast, PRL-1 small interfering RNA decreased in vitro lung cancer cell migration and invasion. The cadherin-catenin complex and dynamic filamentous actin are believed to control cellular invasiveness. Expression of wild-type PRL-1, but not phosphatase-inactive PRL-1 (C104S or D72A), decreased E-cadherin, vinculin, and paxillin expression. Ectopic expression of wild-type PRL-1 increased RhoA levels, which have an important role in actin filament assembly and stabilization of focal adhesion, and decreased activated Cdc42 and Rac. The Rho-associated protein kinase inhibitor, (R)-(+)-trans-4-(1-aminoethyl)-N-(4-pyridyl) cyclohexanecarboxamide dihydrochloride (Y-27632), decreased RhoA activity, actin filament levels, and cellular migration and invasion in PRL-1-expressing cells. These results suggest that PRL-1 could be a productive cancer therapeutic target and support further efforts to identify its substrates.

Crystal structure of the yeast Sac1: implications for its phosphoinositide phosphatase function

Sac family phosphoinositide (PI) phosphatases are an essential family of CX(5)R(T/S)-based enzymes, involved in numerous aspects of cellular function such as PI homeostasis, cellular signalling, and membrane trafficking. Genetic deletions of several Sac family members result in lethality in animal models and mutations of the Sac3 gene have been found in human hereditary diseases. In this study, we report the crystal structure of a founding member of this family, the Sac phosphatase domain of yeast Sac1. The 2.0 A resolution structure shows that the Sac domain comprises of two closely packed sub-domains, a novel N-terminal sub-domain and the PI phosphatase catalytic sub-domain. The structure further shows a striking conformation of the catalytic P-loop and a large positively charged groove at the catalytic site. These findings suggest an unusual mechanism for its dephosphorylation function. Homology structural modeling of human Fig4/Sac3 allows the mapping of several disease-related mutations and provides a framework for the understanding of the molecular mechanisms of human diseases.

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.

1H, 15N, 13C resonance assignments of the reduced and active form of human Protein Tyrosine Phosphatase, PRL-1

Phosphatase of regenerating liver-1 (PRL-1) is a novel target for potentially treating cancer metastases. Although its specific biochemical role in these processes has yet to be delineated, considerable evidence suggests the phosphatase activity of PRL-1 is required for promoting cancer and metastasis. PRL-1 belongs to the protein tyrosine phosphatase (PTPase) family and functions using the CX(5)R consensus active site motif. Like other PTPases, PRL-1 is inhibited by oxidation at its active site Cys, however, disulfide bond formation occurs unusually readily in wild-type PRL-1. Chemical shift assignments are available for oxidized wild type, but numerous, substantial changes are observed in the spectra upon reduction. Because the reduced form is active, we sought to identify a stable mutant that would resist oxidation and be useful for facilitating drug screening and development using NMR-based assays. We present here NMR assignments for a full-length, reduced and active form of PRL-1, PRL-1-C170S-C171S, that is well suited for this purpose.

INTERACTIONS BETWEEN HUMAN SLINGSHOT PHOSPHATASE 2 AND PHOSPHO-COFILIN: A MOLECULAR DYNAMICS STUDY

Human slingshot phosphatase 2 (SSH2) is one of the dual specificity protein tyrosine phosphatases, which can activate cofilin substrate by binding its phosphorylation state. Because the interaction model of SSH2 and phospho-cofilin (P-cofilin) was unknown, we obtained the complex through macromolecular docking method. The molecular dynamics studies were used to investigate the complex dynamics in an aqueous solution. To understand the binding specificity, the free energy was calculated with the molecular mechanics Poisson–Boltzmann surface area (MM/PBSA) approach and the interaction mode in active site was analyzed. The results indicated that the interaction of the P-loop of SSH2 with phosphoserine of human P-cofilin was stabilized by molecular mechanics energy and nonpolar solvation energy components, while polar solvation energy and the entropic contributions were unfavorable for the combination of the two proteins. In addition, the electrostatic contributions were negative for the formation of the complex on the whole, but seen from the active local, the Coulomb interaction between the phosphoserine and the P-loop residues could play an important role in determining substrate specificity.

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.

Targeting PTPs with small molecule inhibitors in cancer treatment

Protein tyrosine phosphorylation plays a major role in cellular signaling. The level of tyrosine phosphorylation is controlled by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Disturbance of the normal balance between PTK and PTP activity results in aberrant tyrosine phosphorylation, which has been linked to the etiology of several human diseases, including cancer. A number of PTPs have been implicated in oncogenesis and tumor progression and therefore are potential drug targets for cancer chemotherapy. These include PTP1B, which may augment signaling downstream of HER2/Neu; SHP2, which is the first oncogene in the PTP superfamily and is essential for growth factor-mediated signaling; the Cdc25 phosphatases, which are positive regulators of cell cycle progression; and the phosphatase of regenerating liver (PRL) phosphatases, which promote tumor metastases. As PTPs have emerged as drug targets for cancer, a number of strategies are currently been explored for the identification of various classes of PTP inhibitors. These efforts have resulted many potent, and in some cases selective, inhibitors for PTP1B, SHP2, Cdc25 and PRL phosphatases. Structural information derived from these compounds serves as a solid foundation upon which novel anti-cancer agents targeted to these PTPs can be developed.

Analysis of molecular determinants of PRL-3

In order to analyse whether a C-terminal polybasic sequence represents a nuclear localization signal (NLS) we obtained several truncated and mutant forms of protein of regerating liver (PRL)-3 and evaluated their subcellular localization as compared to the wild-type form. Our results invalidate the hypothesis that this is an NLS. We also analysed the influence of the C- and N-terminal residues on the phosphatase activity of PRL-3. Our results provide in vitro evidence that the C-terminal CAAX motif, besides directing the protein farnesylation, plays an additional regulatory role by inhibiting the catalytic efficiency of PRL-3. Taking into account the results we obtained, as well as reported data, we propose a hypothetical molecular mechanism for the nucleocytoplasmic localization and transfer of PRL-3.