The essential role of FKBP38 in regulating phosphatase of regenerating liver 3 (PRL-3) protein stability

Signal peptide peptidase promotes tumor progression via facilitating FKBP8 degradation

Signal peptide peptidase (SPP) is an endoplasmic reticulum (ER)-resident aspartyl protease mediating intramembrane cleavage of type II transmembrane proteins. Increasing evidence has supported the role of SPP in ER-associated protein degradation. In the present study, we show that SPP expression is highly induced in human lung and breast cancers and correlated with disease outcome. Stable depletion of SPP expression in lung and breast cancer cell lines significantly reduced cell growth and migration/invasion abilities. Quantitative analysis of the proteomic changes of microsomal proteins in lung cancer cells by the stable isotope labeling with amino acids in cell culture (SILAC) approach revealed that the level of FKBP8, an endogenous inhibitor of mTOR, was significantly increased following SPP depletion. Co-immunoprecipitation assay and confocal immunofluorescence demonstrated that SPP interacted and colocalized with FKBP8 in ER, supporting that FKBP8 is a protein substrate of SPP. Cycloheximide chase and proteasome inhibition experiments revealed that SPP-mediated proteolysis facilitated FKBP8 protein degradation in cytosol. Further experiment demonstrated that the levels of phosphorylation in mTOR and its downstream effectors, S6K and 4E-BP1, were significantly lower in SPP-depleted cells. The reduced mTOR signaling and decreases of growth and migration/invasion abilities induced by SPP depletion in cancer cells could be reversed by FKBP8 downregulation. The implication of FKBP8 in SPP-mediated tumorigenicity was also observed in the xenograft model. Together, these findings disclose that SPP promotes tumor progression, at least in part, via facilitating the degradation of FKBP8 to enhance mTOR signaling.

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.

PRL-3 disrupts epithelial architecture by altering the post-mitotic midbody position

Disruption of epithelial architecture is a fundamental event during epithelial tumorigenesis. We show that the expression of the cancer-promoting phosphatase PRL-3 (PTP4A3), which is overexpressed in several epithelial cancers, in polarized epithelial MDCK and Caco2 cells leads to invasion and the formation of multiple ectopic, fully polarized lumens in cysts. Both processes disrupt epithelial architecture and are hallmarks of cancer. The pathological relevance of these findings is supported by the knockdown of endogenous PRL-3 in MCF-7 breast cancer cells grown in three-dimensional branched structures, showing the rescue from multiple-lumen- to single-lumen-containing branch ends. Mechanistically, it has been previously shown that ectopic lumens can arise from midbodies that have been mislocalized through the loss of mitotic spindle orientation or through the loss of asymmetric abscission. Here, we show that PRL-3 triggers ectopic lumen formation through midbody mispositioning without altering the spindle orientation or asymmetric abscission, instead, PRL-3 accelerates cytokinesis, suggesting that this process is an alternative new mechanism for ectopic lumen formation in MDCK cysts. The disruption of epithelial architecture by PRL-3 revealed here is a newly recognized mechanism for PRL-3-promoted cancer progression.

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.

Biological relevance of Hsp90-binding immunophilins in cancer development and treatment

Immunophilins are a family of intracellular receptors for immunosuppressive drugs. Those immunophilins that are related to immunosuppression are the smallest proteins of the family, i.e., FKBP12 and CyPA, whereas the other members of the family have higher molecular weight because the show additional domains to the drug-binding site. Among these extra domains, the TPR-domain is perhaps the most relevant because it permits the interaction of high molecular weight immunophilins with the 90-kDa heat-shock protein, Hsp90. This essential molecular chaperone regulates the biological function of several protein-kinases, oncogenes, protein phosphatases, transcription factors and cofactors . Hsp90-binding immunophilins where first characterized due to their association with steroid receptors. They regulate the cytoplasmic transport and the subcellular localization of these and other Hsp90 client proteins, as well as transcriptional activity, cell proliferation, cell differentiation and apoptosis. Hsp90-binding immunophilins are frequently overexpressed in several types of cancers and play a key role in cell survival. In this article we analyze the most important biological actions of the best characterized Hsp90-binding immunophilins in both steroid receptor function and cancer development and discuss the potential use of these immunophilins for therapeutic purposes as potential targets of specific small molecules.

Pleiotropic roles in cancer biology for multifaceted proteins FKBPs

BACKGROUND

FK506 binding proteins (FKBP) are multifunctional proteins highly conserved across the species and abundantly expressed in the cell. In addition to a well-established role in immunosuppression, FKBPs modulate several signal transduction pathways in the cell, due to their isomerase activity and the capability to interact with other proteins, inducing changes in conformation and function of protein partners. Increasing literature data support the concept that FKBPs control cancer related pathways.

SCOPE OF THE REVIEW

The aim of the present article is to review current knowledge on FKBPs roles in regulation of key signaling pathways associated with cancer.

MAJOR CONCLUSIONS

Some family members appear to promote disease while others are protective against tumorigenesis.

GENERAL SIGNIFICANCE

FKBPs family proteins are expected to provide new biomarkers and small molecular targets, in the near future, increasing diagnostic and therapeutic opportunities in the cancer field. This article is part of a Special Issue entitled Proline-Directed Foldases: Cell Signaling Catalysts and Drug Targets.

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.

Independent oncogenic and therapeutic significance of phosphatase PRL-3 in FLT3-ITD-negative acute myeloid leukemia

BACKGROUND

Internal tandem duplication of FMS-like tyrosine kinase (FLT3-ITD) is well known to be involved in acute myeloid leukemia (AML) progression, but FLT3-ITD-negative AML cases account for 70% to 80% of AML, and the mechanisms underlying their pathology remain unclear. This study identifies protein tyrosine phophatase PRL-3 as a key mediator of FLT3-ITD-negative AML.

METHODS

A total of 112 FLT3-ITD-negative AML patients were sampled between 2010 and 2013, and the occurrence of PRL-3 hyperexpression in FLT3-ITD-negative AML was evaluated by multivariate probit regression analysis. Overexpression or depletion of endogenous PRL-3 expression with the specific small interfering RNAs was performed to investigate the role of PRL-3 in AML progression. Xenograft models were also used to confirm the oncogenic role of PRL-3.

RESULTS

Compared to healthy donors, PRL-3 is upregulated more than 3-fold in 40.2% of FLT3-ITD-negative AML patients. PRL-3 expression level is adversely correlated to the overall survival of the AML patients, and the AML relapses accompany with re-upregulation of PRL-3. Mechanistically, aberrant PRL-3 expression promoted cell cycle progression and enhanced the antiapoptotic machinery of AML cells to drug cytotoxicity through downregulation of p21 and upregulation of Cyclin D1 and CDK2 and activation of STAT5 and AKT. Depletion of endogenous PRL-3 sensitizes AML cells to therapeutic drugs, concomitant with apoptosis by upregulation of cleaved PARP (poly ADP ribose polymerase) and apoptosis-related caspases. Xenograft assays further confirmed PRL-3's oncogenic role in leukemogenesis.

CONCLUSIONS

Our results demonstrated that PRL-3 is a novel independent crucial player in both FLT3-ITD-positive and FLT3-ITD-negative AML and could be a potential therapeutic target.

Integrated analysis of global mRNA and protein expression data in HEK293 cells overexpressing PRL-1

BACKGROUND

The protein tyrosine phosphatase PRL-1 represents a putative oncogene with wide-ranging cellular effects. Overexpression of PRL-1 can promote cell proliferation, survival, migration, invasion, and metastasis, but the underlying mechanisms by which it influences these processes remain poorly understood.

METHODOLOGY

To increase our comprehension of PRL-1 mediated signaling events, we employed transcriptional profiling (DNA microarray) and proteomics (mass spectrometry) to perform a thorough characterization of the global molecular changes in gene expression that occur in response to stable PRL-1 overexpression in a relevant model system (HEK293).

PRINCIPAL FINDINGS

Overexpression of PRL-1 led to several significant changes in the mRNA and protein expression profiles of HEK293 cells. The differentially expressed gene set was highly enriched in genes involved in cytoskeletal remodeling, integrin-mediated cell-matrix adhesion, and RNA recognition and splicing. In particular, members of the Rho signaling pathway and molecules that converge on this pathway were heavily influenced by PRL-1 overexpression, supporting observations from previous studies that link PRL-1 to the Rho GTPase signaling network. In addition, several genes not previously associated with PRL-1 were found to be significantly altered by its expression. Most notable among these were Filamin A, RhoGDIα, SPARC, hnRNPH2, and PRDX2.

CONCLUSIONS AND SIGNIFICANCE

This systems-level approach sheds new light on the molecular networks underlying PRL-1 action and presents several novel directions for future, hypothesis-based studies.

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.

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.

VEGF promotes the transcription of the human PRL-3 gene in HUVEC through transcription factor MEF2C

Phosphatase of regenerating liver 3 (PRL-3) is known to be overexpressed in many tumors, and its transcript level is high in the vasculature and endothelial cells of malignant tumor tissue. However, the mechanism(s) underlying its enhanced expression and its function in endothelial cells remain unknown. Here, we report that vascular endothelial growth factor (VEGF) can induce PRL-3 transcription in human umbilical vein endothelial cells (HUVEC). An analysis of its 5′UTR revealed that PRL-3 transcription is initiated from two distinct sites, which results in the formation of the two transcripts, PRL-3-iso1 and PRL-3-iso2, but only the latter is up-regulated in HUVEC by VEGF. The PRL-3-iso2 promoter region includes two functional MEF2 (myocyte enhancer factor2) binding sites. The over-expression of the constitutively active form of MEF2C promotes the abundance of the PRL-3-iso2 transcript in a number of human cell lines. The siRNA-induced knockdown of MEF2C abolished the stimulative effect of VEGF on PRL-3 transcript in HUVEC, indicating that the VEGF-induced promotion of PRL-3 expression requires the presence of MEF2C. Finally, blocking PRL-3 activity or expression suppresses tube formation by HUVEC. We suggest that PRL-3 functions downstream of the VEGF/MEF2C pathway in endothelial cells and may play an important role in tumor angiogenesis.