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

Protein tyrosine phosphatases as potential therapeutic targets

Protein tyrosine phosphorylation is a key regulatory process in virtually all aspects of cellular functions. Dysregulation of protein tyrosine phosphorylation is a major cause of human diseases, such as cancers, diabetes, autoimmune disorders, and neurological diseases. Indeed, protein tyrosine phosphorylation-mediated signaling events offer ample therapeutic targets, and drug discovery efforts to date have brought over two dozen kinase inhibitors to the clinic. Accordingly, protein tyrosine phosphatases (PTPs) are considered next-generation drug targets. For instance, PTP1B is a well-known targets of type 2 diabetes and obesity, and recent studies indicate that it is also a promising target for breast cancer. SHP2 is a bona-fide oncoprotein, mutations of which cause juvenile myelomonocytic leukemia, acute myeloid leukemia, and solid tumors. In addition, LYP is strongly associated with type 1 diabetes and many other autoimmune diseases. This review summarizes recent findings on several highly recognized PTP family drug targets, including PTP1B, Src homology phosphotyrosyl phosphatase 2(SHP2), lymphoid-specific tyrosine phosphatase (LYP), CD45, Fas associated phosphatase-1 (FAP-1), striatal enriched tyrosine phosphatases (STEP), mitogen-activated protein kinase/dual-specificity phosphatase 1 (MKP-1), phosphatases of regenerating liver-1 (PRL), low molecular weight PTPs (LMWPTP), and CDC25. Given that there are over 100 family members, we hope this review will serve as a road map for innovative drug discovery targeting PTPs

THE ONCOGENIC ROLE OF THE PROTEIN TYROSINE PHOSPHATASE 4A3 (PTP4A3 OR PRL-3) IN T-CELL ACUTE LYMPHOBLASTIC LEUKEMIA

T-cell Acute Lymphoblastic Leukemia (T-ALL) is an aggressive blood cancer. There are no immunotherapies and few molecularly targeted therapeutics available for the treatment of this malignancy. The identification and characterization of genes and pathways that drive T-ALL progression is critical for the development of new therapies for T-ALL. The protein tyrosine phosphatase PTP4A3, also known as PRL-3, has been extensively reported to play a causative role in numerous cancers, including several types of blood malignancies. However, its role in T-ALL is not well defined. Here, we determined that the 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 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 xenograft mouse models. Additionally, PRL-3 over-expression 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 was inhibited by small molecule inhibition of PRL-3, suggesting that PRL-3 has potential as a therapeutic target in T-ALL. Finally, we identified a pool of PRL-3 interacting proteins in T-ALL using BioID-based proximal labeling approach. Co-immunoprecipitation assays were used to validate a number of direct interacting partners of PRL-3, including LCK and CD3. 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. We also identified PRL-3 interactomes in T-ALL, which lays foundation for future PRL-3 substrates study

UNCOVERING THE BIOLOGICAL FUNCTIONS OF PHOSPHATASE OF REGENERATING LIVER -2

The Phosphatase of Regenerating Liver (PRL) family, consisting of PRL-1, PRL-2, and PRL-3, is a group of prenylated phosphatases that are candidate cancer biomarkers and therapeutic targets. Individual PRLs are over-expressed in a variety of cancer cell lines and tissues, and elevated PRL expression has been associated with tumorigenesis and metastasis. Although several studies have documented that altered expression of PRL-1 or PRL-3 can influence cell proliferation, migration and invasion, there is an absence of knowledge about the biological functions of PRL-2. Thus, the current study was designed to evaluate the role of PRL-2 in cell migration and invasion in human cancer cells. I found that four lung cancer cells, including A549, over-expressed PRL-2 when compared with normal lung cells. PRL-2 suppression by siRNA or shRNA markedly inhibited cell migration and invasion. PRL-2 suppression by siRNA decreased p130Cas and vinculin expression, increased phosphorylation of Ezrin on tyrosine 146, and decreased ERK phosphorylation upon serum stimulation. There were no significant changes in total p53, Akt and c-Src expression levels or their phosphorylation status, suggesting PRL-2 suppression could inhibit tumor cell migration and invasion through a Src-independent p130Cas signaling pathway. Ectopic expression of wild type PRL-2, a catalytic inactive C101S mutant, and a C-terminal CAAX deletion revealed a requirement for both the PRL-2 catalytic functionality and prenylation site. Expression of wild type but not the mutant forms of PRL-2 caused ERK phosphorylation and nuclear translocation, and promoted tumor cell migration and invasion. These results support a model in which PRL-2 promotes cell migration and invasion through an ERK-dependent signaling pathway. In addition, thienopyridone, a previously reported PRL inhibitor, showed antiproliferative activity in a concentration-dependent manner, and decreased cell migration and invasion. In summary, these studies demonstrate for the first time that PRL-2 regulates cell migration and invasion in non-small cell lung cancer, and I propose that PRL-2 stimulates cell migration and invasion through an ERK signaling pathway

Novel anticancer agents based on targeting the trimer interface of the PRL phosphatase

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 which 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 cancer

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

Investigation of the action of phosphatase of regenerating liver on PTEN using murine models

Indiana University-Purdue University Indianapolis (IUPUI)The addition and removal of phosphate groups is a key regulatory mechanism for many cellular processes. The balance between phosphorylation and dephosphorylation is delicate and must be maintained in order for proper cell functions to be carried out. Protein kinases and phosphatases are the keepers of this balance with kinases adding phosphate groups and phosphatases removing them. As such, mutation and/or altered regulation of these proteins can be the driving factor in disease. Phosphatase of Regenerating Liver (PRL) is a family novel of three dual specificity phosphatases (DSPs) first discovered in the regenerating liver tissue of rats. PRLs have also been shown to act as oncogenes in cell culture and in animal models. However, the physiological substrate and mechanisms of the PRLs are not yet known. Recently, our lab has developed a PRL 2 knockout mouse and found several striking phenotypes all of which correspond to a significant increase in PTEN. We also found that PRL 2 is targetable by small molecular inhibitors that can potentially be used to disrupt tumor growth and spermatogenesis. Furthermore, a PTEN heterozygous mouse model crossed into our PRL 2 knockout line was generated to investigate the relevance of PRL interaction with PTEN in cancer

Protein Tyrosine Phosphatase PRL2 Mediates Notch and Kit Signals in Early T Cell Progenitors

The molecular pathways regulating lymphoid priming, fate, and development of multipotent bone marrow hematopoietic stem and progenitor cells (HSPCs) that continuously feed thymic progenitors remain largely unknown. While Notch signal is indispensable for T cell specification and differentiation, the downstream effectors are not well understood. PRL2, a protein tyrosine phosphatase that regulates hematopoietic stem cell proliferation and self-renewal, is highly expressed in murine thymocyte progenitors. Here we demonstrate that protein tyrosine phosphatase PRL2 and receptor tyrosine kinase c-Kit are critical downstream targets and effectors of the canonical Notch/RBPJ pathway in early T cell progenitors. While PRL2 deficiency resulted in moderate defects of thymopoiesis in the steady state, de novo generation of T cells from Prl2 null hematopoietic stem cells was significantly reduced following transplantation. Prl2 null HSPCs also showed impaired T cell differentiation in vitro. We found that Notch/RBPJ signaling upregulated PRL2 as well as c-Kit expression in T cell progenitors. Further, PRL2 sustains Notch-mediated c-Kit expression and enhances stem cell factor/c-Kit signaling in T cell progenitors, promoting effective DN1-DN2 transition. Thus, we have identified a critical role for PRL2 phosphatase in mediating Notch and c-Kit signals in early T cell progenitors