Nm23-H1 homologs suppress tumor cell motility and anchorage independent growth

PRUNE1 and NME/NDPK family proteins influence energy metabolism and signaling in cancer metastases

We describe here the molecular basis of the complex formation of PRUNE1 with the tumor metastasis suppressors NME1 and NME2, two isoforms appertaining to the nucleoside diphosphate kinase (NDPK) enzyme family, and how this complex regulates signaling the immune system and energy metabolism, thereby shaping the tumor microenvironment (TME). Disrupting the interaction between NME1/2 and PRUNE1, as suggested, holds the potential to be an excellent therapeutic target for the treatment of cancer and the inhibition of metastasis dissemination. Furthermore, we postulate an interaction and regulation of the other Class I NME proteins, NME3 and NME4 proteins, with PRUNE1 and discuss potential functions. Class I NME1-4 proteins are NTP/NDP transphosphorylases required for balancing the intracellular pools of nucleotide diphosphates and triphosphates. They regulate different cellular functions by interacting with a large variety of other proteins, and in cancer and metastasis processes, they can exert pro- and anti-oncogenic properties depending on the cellular context. In this review, we therefore additionally discuss general aspects of class1 NME and PRUNE1 molecular structures as well as their posttranslational modifications and subcellular localization. The current knowledge on the contributions of PRUNE1 as well as NME proteins to signaling cascades is summarized with a special regard to cancer and metastasis.

Mechanisms of action of NME metastasis suppressors - a family affair

Metastatic progression is regulated by metastasis promoter and suppressor genes. NME1, the prototypic and first described metastasis suppressor gene, encodes a nucleoside diphosphate kinase (NDPK) involved in nucleotide metabolism; two related family members, NME2 and NME4, are also reported as metastasis suppressors. These proteins physically interact with members of the GTPase dynamin family, which have key functions in membrane fission and fusion reactions necessary for endocytosis and mitochondrial dynamics. Evidence supports a model in which NDPKs provide GTP to dynamins to maintain a high local GTP concentration for optimal dynamin function. NME1 and NME2 are cytosolic enzymes that provide GTP to dynamins at the plasma membrane, which drive endocytosis, suggesting that these NMEs are necessary to attenuate signaling by receptors on the cell surface. Disruption of NDPK activity in NME-deficient tumors may thus drive metastasis by prolonging signaling. NME4 is a mitochondrial enzyme that interacts with the dynamin OPA1 at the mitochondria inner membrane to drive inner membrane fusion and maintain a fused mitochondrial network. This function is consistent with the current view that mitochondrial fusion inhibits the metastatic potential of tumor cells whereas mitochondrial fission promotes metastasis progression. The roles of NME family members in dynamin-mediated endocytosis and mitochondrial dynamics and the intimate link between these processes and metastasis provide a new framework to understand the metastasis suppressor functions of NME proteins.

The metastasis suppressor protein NM23-H1 modulates the PI3K-AKT axis through interaction with the p110α catalytic subunit

The PI3K pathway is one of the most deregulated pathways in cancer, which is predominantly due to gain of function mutations or altered expression of the PI3KCA gene. This is codified by what is seen for the class I PI3K catalytic subunit p110α, a common feature of many cancers. The metastasis suppressor protein NM23-H1 (NME1), whose ability to suppress the metastasis activities of different tumors has been widely described and was previously reported to alter phosphatidylinositol signaling. Here, we show interaction of NM23-H1 with the p110α subunit and the functional consequence of this interaction. This interaction is predominantly localized at the plasma membrane with some signals seen in the cytoplasmic compartment. Analysis of NM23-H1 levels showed a negative correlation between NM23-H1 expression and Akt phosphorylation, the key marker of PI3K pathway activation. Investigating the functional consequence of this interaction using cell motility and clonogenicity assays showed that expression of NM23-H1 reversed the enhanced migration, invasion, adhesion, and filopodia structure formation in cells expressing the p110α catalytic subunit. A similar trend was seen in anchorage-independent assays. Notably, differential analyses using NM23-H1 mutants which lacked the enzymatic and metastasis suppressor activity, showed no detectable interaction between p110α and the NM23-H1 mutant proteins P96S, H118F, and S120G, as well as no dysregulation of the PI3K-AKT axis.

Loss of the Metastasis Suppressor NME1, But Not of Its Highly Related Isoform NME2, Induces a Hybrid Epithelial-Mesenchymal State in Cancer Cells

Epithelial-mesenchymal transition (EMT) is important for the initial steps of metastasis. Although it is well accepted that the nucleoside diphosphate kinase NME1 is a metastasis suppressor, its effect on EMT remains poorly documented, as does that of its closely related isoform, NME2. Here, by using gene silencing, inactivation and overexpression strategies in a variety of cellular models of cancer, we show that NME1 is a powerful inhibitor of EMT. Genetic manipulation of NME2, by contrast, had no effect on the EMT phenotype of cancer cells, indicating a specific function of NME1 in EMT regulation. Loss of NME1 in epithelial cancer cells resulted in a hybrid phenotype intermediate between epithelial and mesenchymal cells, which is known to be associated with cells with a highly metastatic character. Conversely, overexpression of NME1 in mesenchymal cancer cells resulted in a more epithelial phenotype. We found that NME1 expression was negatively associated with EMT markers in many human cancers and was reduced in human breast tumor cell lines with the aggressive 'triple-negative' phenotype when compared to human breast tumor cell lines positive for estrogen receptor. We show that NME1, but not NME2, is an inhibitor of essential concerted intracellular signaling pathways involved in inducing EMT, including the AKT and MAPK (ERK, p38, and JNK) pathways. Additionally, NME1 depletion considerably altered the distribution of E-cadherin, a gatekeeper of the epithelial phenotype, shifting it from the plasma membrane to the cytosol and resulting in less E-cadherin on the cell surface than in control cells. Functional aggregation and dispersion assays demonstrated that inactivation of NME1 decreases E-cadherin-mediated cell-cell adhesion. We conclude that NME1, but not NME2, acts specifically to inhibit EMT and prevent the earliest stages of metastasis.

Characterization of Nme5-Like Gene/Protein from the Red Alga Chondrus Crispus

The Nme gene/protein family of nucleoside diphosphate kinases (NDPK) was originally named after its member Nm23-H1/Nme1, the first identified metastasis suppressor. Human Nme proteins are divided in two groups. They all possess nucleoside diphosphate kinase domain (NDK). Group I (Nme1-Nme4) display a single type NDK domain, whereas Group II (Nme5-Nme9) display a single or several different NDK domains, associated or not associated with extra-domains. Data strongly suggest that, unlike Group I, none of the members of Group II display measurable NDPK activity, although some of them autophosphorylate. The multimeric form is required for the NDPK activity. Group I proteins are known to multimerize, while there are no data on the multimerization of Group II proteins. The Group II ancestral type protein was shown to be conserved in several species from three eukaryotic supergroups. Here, we analysed the Nme protein from an early branching eukaryotic lineage, the red alga Chondrus crispus. We show that the ancestral type protein, unlike its human homologue, was fully functional multimeric NDPK with high affinity to various types of DNA and dispersed localization throughout the eukaryotic cell. Its overexpression inhibits both cell proliferation and the anchorage-independent growth of cells in soft agar but fails to deregulate cell apoptosis. We conclude that the ancestral gene has changed during eukaryotic evolution, possibly in correlation with the protein function.

The mitochondrial nucleoside diphosphate kinase (NDPK-D/NME4), a moonlighting protein for cell homeostasis

Mitochondrial nucleoside diphosphate kinase (NDPK-D; synonyms: NME4, NM23-H4) represents the major mitochondrial NDP kinase. The homohexameric complex emerged as a protein with multiple functions in bioenergetics and phospholipid signaling. It occurs at different but precise mitochondrial locations and can affect among other mitochondrial shapes and dynamics, as well as the specific elimination of defective mitochondria or cells via mitophagy or apoptosis. With these various functions in cell homeostasis, NDPK-D/NME4 adds to the group of so-called moonlighting (or gene sharing) proteins.

Characterization of a group I Nme protein of Capsaspora owczarzaki-a close unicellular relative of animals

Nucleoside diphosphate kinases are enzymes present in all domains of life. In animals, they are called Nme or Nm23 proteins, and are divided into group I and II. Human Nme1 was the first protein identified as a metastasis suppressor. Because of its medical importance, it has been extensively studied. In spite of the large research effort, the exact mechanism of metastasis suppression remains unclear. It is unknown which of the biochemical properties or biological functions are responsible for the antimetastatic role of the mammalian Nme1. Furthermore, it is not clear at which point in the evolution of life group I Nme proteins acquired the potential to suppress metastasis, a process that is usually associated with complex animals. In this study we performed a series of tests and assays on a group I Nme protein from filasterean Capsaspora owczarzaki, a close unicellular relative of animals. The aim was to compare the protein to the well-known human Nme1 and Nme2 homologs, as well as with the homolog from a simple animal-sponge (Porifera), in order to see how the proteins changed with the transition to multicellularity, and subsequently in the evolution of complex animals. We found that premetazoan-type protein is highly similar to the homologs from sponge and human, in terms of biochemical characteristics and potential biological functions. Like the human Nme1 and Nme2, it is able to diminish the migratory potential of human cancer cells in culture.

RGS19 upregulates Nm23-H1/2 metastasis suppressors by transcriptional activation via the cAMP/PKA/CREB pathway

The Nm23 metastasis suppressor family is involved in physiological and pathological processes including tumorigenesis and metastasis. Although the inverse correlation of Nm23 level with tumor metastasis potential has been widely observed, the mechanisms that regulate the expression of Nm23 remain poorly understood. Our previous studies have revealed that Nm23-H1/2 isoforms are upregulated by RGS19, a regulator of G protein signaling (RGS) protein which accelerates the termination of G_i signals. Here, we examined the ability of RGS19 to stimulate transcriptional regulation of Nm23 by screening a panel of luciferase reporter genes. Transient and stable overexpression of RGS19 upregulated the Nm23-H1/2 protein levels and activated several transcription factors including CREB, AP-1 and SRE in HEK293 cells. Interestingly, agents that increase the intracellular cAMP level and the phosphorylation of CREB (e.g., adrenergic receptor agonist, forskolin, and cAMP analogues) upregulated the expression of Nm23-H1/2 in HEK293 cells and several cancer cell lines including A549, HeLa, MDA-MB-231, and MDA-MB-435s cells. Conversely, inhibition of protein kinase A (PKA) by H-89 suppressed the phosphorylation of CREB and reduced the expression of Nm23-H1/2. Furthermore, activation of PKA attenuated cancer cell migration in wound healing and transwell assays. Collectively, these results revealed a PKA-dependent mechanism for controlling Nm23-H1/2 expression.

Mutation of the nm23-H1 gene has a non-dominant role in colorectal adenocarcinoma

Nm23-H1 is a metastasis suppressor gene, which is has a reduced expression in patients with digestive system cancer. However, the mechanistic basis for the genetic instability remains unknown. To study the expression of the nm23-H1 gene in patients with colorectal cancer, polymerase chain reaction-single strand conformation polymorphism was used to analyze any point mutation, and immunohistochemistry was used to detect the expression of nm23-H1. Results revealed that all 63 specimens of Chinese human colorectal cancer tissues exhibit no point mutation. Among those 63 specimens, 19 (30%) exhibited positive immunostaining for the nm23-H1 protein and 44 (70%) exhibited negative immunostaining. These observations suggested that the protein and gene expression levels of nm23-H1 are reduced in colorectal cancer compared with the adjacent normal tissues, and the point mutation in the nm23-H1 gene is not the dominant cause of metastatic colorectal cancer.

Profilin-1 overexpression in MDA-MB-231 breast cancer cells is associated with alterations in proteomics biomarkers of cell proliferation, survival, and motility as revealed by global proteomics analyses

Despite early screening programs and new therapeutic strategies, metastatic breast cancer is still the leading cause of cancer death in women in industrialized countries and regions. There is a need for novel biomarkers of susceptibility, progression, and therapeutic response. Global analyses or systems science approaches with omics technologies offer concrete ways forward in biomarker discovery for breast cancer. Previous studies have shown that expression of profilin-1 (PFN1), a ubiquitously expressed actin-binding protein, is downregulated in invasive and metastatic breast cancer. It has also been reported that PFN1 overexpression can suppress tumorigenic ability and motility/invasiveness of breast cancer cells. To obtain insights into the underlying molecular mechanisms of how elevating PFN1 level induces these phenotypic changes in breast cancer cells, we investigated the alteration in global protein expression profiles of breast cancer cells upon stable overexpression of PFN1 by a combination of three different proteome analysis methods (2-DE, iTRAQ, label-free). Using MDA-MB-231 as a model breast cancer cell line, we provide evidence that PFN1 overexpression is associated with alterations in the expression of proteins that have been functionally linked to cell proliferation (FKPB1A, HDGF, MIF, PRDX1, TXNRD1, LGALS1, STMN1, LASP1, S100A11, S100A6), survival (HSPE1, HSPB1, HSPD1, HSPA5 and PPIA, YWHAZ, CFL1, NME1) and motility (CFL1, CORO1B, PFN2, PLS3, FLNA, FLNB, NME2, ARHGDIB). In view of the pleotropic effects of PFN1 overexpression in breast cancer cells as suggested by these new findings, we propose that PFN1-induced phenotypic changes in cancer cells involve multiple mechanisms. Our data reported here might also offer innovative strategies for identification and validation of novel therapeutic targets and companion diagnostics for persons with, or susceptibility to, breast cancer.

Inhibition of invasion and metastasis of human liver cancer HCCLM3 cells by portulacerebroside A

CONTEXT

Portulacerebroside A (PCA) is a novel cerebroside compound isolated from Portulaca oleracea L. (Portulacaceae), an edible and medicinal plant distributed in the temperate and tropical zones worldwide.

OBJECTIVE

This study investigates the effects of PCA in human liver cancer HCCLM3 cells on metastasis and invasion.

MATERIALS AND METHODS

After the cells were treated with PCA (2.5, 5, and 10 μg/ml) for 6, 12, 24, or 48 h, adhesion, transwell invasion, and scratch tests were conducted and cell functions were evaluated. Western blot and FQ-RT-PCR assays explored the mechanism of PCA-inhibited invasion and metastasis in the cells.

RESULTS

The adhesion rate of the cells was suppressed at 0.5 h (79.4 ± 1.0, 68.7 ± 1.3, and 58.1 ± 1.3%, versus 100 ± 1.5% in the control), 1 h (78.2 ± 1.2, 70.9 ± 1.6, and 55.4 ± 1.9%, versus 100 ± 1.2% in the control), and 1.5 h (71.6 ± 1.1, 62.3 ± 0.9, and 50.4 ± 0.9%, versus 100 ± 1.1% in the control). The 24 h invasion ability was decreased (356.6 ± 11.2, 204.0 ± 17.6, and 113.0 ± 9.5%, versus 443.6 ± 15.4% in the control). The migration capability was also restrained by PCA for 24 h (324.8 ± 25.4, 250.4 ± 21.0, and 126.3 ± 10.1, versus 381.6 ± 30.6 in the control) and 48 h (470.3 ± 34.3, 404.0 ± 19.7, and 201.0 ± 15.4, versus 752.0 ± 63.6 in the control). There was an increase in the mRNA and protein expression levels of TIMP-2 and nm23-H1, inhibition in the mRNA expression of MTA1, MMP-2, and MMP-9, and suppression in the protein expression of MTA1, RhoA, Rac1/Cdc42, MMP-2, but not RhoC and MMP-9.

CONCLUSION

PCA suppresses the invasion and metastasis of HCCLM3 cells possibly by modulation of the mRNA and protein expression of related parameters. This is the first study to reveal a new potential therapeutic application of PCA in antimetastatic therapy for liver cancer.

Blockade of extracellular NM23 or its endothelial target slows breast cancer growth and metastasis

BACKGROUND

Nucleoside Diphosphate Kinase (NDPK), described as NM23 a metastasis suppressor, is found in the culture medium of cancer cells lines suggesting that the kinase may have an extracellular role. We propose that extracellular NM23 released from breast cancers in vivo stimulates tumor cell migration, proliferation and endothelial cell angiogenesis in support of metastasis development.

METHODS

NM23 in the bloodstream of immunocompromised mice carrying human triple-negative breast cancers or in breast cancer patients was measured by ELISA. Primary and metastatic tumor development, the impact of blockade of NM23 and/or its stimulation of nucleotide receptors were measured using in vivo imaging. NM23 expression data in the Curtis breast dataset was examined to test our hypothesis that NM23 may play a mechanistic role in breast cancer development.

RESULTS

SCID mice carrying metastatic MDA-MB-231Luc+ triple-negative human breast tumor cells elaborate NM23 into the circulation correlated with primary tumor growth. Treatment of mice with the NM23 inhibitor ellagic acid (EA) or the purinergic receptor antagonist MRS2179 slowed primary tumor growth. At 16 weeks following implantation, lung metastases were reduced in mice treated with EA, MRS2179 or the combination. Expression of NM23 in the Curtis breast dataset confirmed a likely role for NM23 in tumor metastasis.

CONCLUSIONS

Extracellular NM23 may constitute both a biomarker and a therapeutic target in the management of breast cancer.

Plakoglobin represses SATB1 expression and decreases in vitro proliferation, migration and invasion

Plakoglobin (γ-catenin) is a homolog of β-catenin with dual adhesive and signaling functions. Plakoglobin participates in cell-cell adhesion as a component of the adherens junction and desmosomes whereas its signaling function is mediated by its interactions with various intracellular protein partners. To determine the role of plakoglobin during tumorigenesis and metastasis, we expressed plakoglobin in the human tongue squamous cell carcinoma (SCC9) cells and compared the mRNA profiles of parental SCC9 cells and their plakoglobin-expressing transfectants (SCC9-PG). We observed that the mRNA levels of SATB1, the oncogenic chromatin remodeling factor, were decreased approximately 3-fold in SCC9-PG cells compared to parental SCC9 cells. Here, we showed that plakoglobin decreased levels of SATB1 mRNA and protein in SCC9-PG cells and that plakoglobin and p53 associated with the SATB1 promoter. Plakoglobin expression also resulted in decreased SATB1 promoter activity. These results were confirmed following plakoglobin expression in the very low plakoglobin expressing and invasive mammary carcinoma cell line MDA-MB-231 cells (MDA-231-PG). In addition, knockdown of endogenous plakoglobin in the non-invasive mammary carcinoma MCF-7 cells (MCF-7-shPG) resulted in increased SATB1 mRNA and protein. Plakoglobin expression also resulted in increased mRNA and protein levels of the metastasis suppressor Nm23-H1, a SATB1 target gene. Furthermore, the levels of various SATB1 target genes involved in tumorigenesis and metastasis were altered in MCF-7-shPG cells relative to parental MCF-7 cells. Finally, plakoglobin expression resulted in decreased in vitro proliferation, migration and invasion in different carcinoma cell lines. Together with the results of our previous studies, the data suggests that plakoglobin suppresses tumorigenesis and metastasis through the regulation of genes involved in these processes.

Engineered reversal of drug resistance in cancer cells--metastases suppressor factors as change agents

Building molecular correlates of drug resistance in cancer and exploiting them for therapeutic intervention remains a pressing clinical need. To identify factors that impact drug resistance herein we built a model that couples inherent cell-based response toward drugs with transcriptomes of resistant/sensitive cells. To test this model, we focused on a group of genes called metastasis suppressor genes (MSGs) that influence aggressiveness and metastatic potential of cancers. Interestingly, modeling of 84 000 drug response transcriptome combinations predicted multiple MSGs to be associated with resistance of different cell types and drugs. As a case study, on inducing MSG levels in a drug resistant breast cancer line resistance to anticancer drugs caerulomycin, camptothecin and topotecan decreased by more than 50–60%, in both culture conditions and also in tumors generated in mice, in contrast to control un-induced cells. To our knowledge, this is the first demonstration of engineered reversal of drug resistance in cancer cells based on a model that exploits inherent cellular response profiles.

c-Abl and Arg induce cathepsin-mediated lysosomal degradation of the NM23-H1 metastasis suppressor in invasive cancer

Metastasis suppressors comprise a growing class of genes whose downregulation triggers metastatic progression. In contrast to tumor suppressors, metastasis suppressors are rarely mutated or deleted, and little is known regarding the mechanisms by which their expression is downregulated. Here, we demonstrate that the metastasis suppressor, NM23-H1, is degraded by lysosomal cysteine cathepsins (L,B), which directly cleave NM23-H1. In addition, activation of c-Abl and Arg oncoproteins induces NM23-H1 degradation in invasive cancer cells by increasing cysteine cathepsin transcription and activation. Moreover, c-Abl activates cathepsins by promoting endosome maturation, which facilitates trafficking of NM23-H1 to the lysosome where it is degraded. Importantly, the invasion- and metastasis-promoting activity of c-Abl/Arg is dependent on their ability to induce NM23-H1 degradation, and the pathway is clinically relevant as c-Abl/Arg activity and NM23-H1 expression are inversely correlated in primary breast cancers and melanomas. Thus, we demonstrate a novel mechanism by which cathepsin expression is upregulated in cancer cells (via Abl kinases). We also identify a novel role for intracellular cathepsins in invasion and metastasis (degradation of a metastasis suppressor). Finally, we identify novel crosstalk between oncogenic and metastasis suppressor pathways, thereby providing mechanistic insight into the process of NM23-H1 loss, which may pave the way for new strategies to restore NM23-H1 expression and block metastatic progression.

Targeting tumor metastasis by regulating Nm23 gene expression

The Nm23 gene is a metastatic suppressor identified in a melanoma cell line and expressed in different tumors where their levels of expression are associated with reduced or increased metastatic potential. Nm23 is one of the over 20 metastasis suppressor genes (MSGs) confirmed in vivo. It is highly conserved from yeast to human, implying a critical developmental function. Tumors with alteration of the p53 gene and reduced expression of the Nm23 gene are more prone to metastasis. Nm23-H1 has 3'-5' exonuclease activity. This review focuses on the role of Nm23 in cancer progression and also a potential novel target for cancer therapy.

Bcl3 selectively promotes metastasis of ERBB2-driven mammary tumors

Bcl3 is a putative proto-oncogene deregulated in hematopoietic and solid tumors. Studies in cell lines suggest that its oncogenic effects are mediated through the induction of proliferation and inhibition of cell death, yet its role in endogenous solid tumors has not been established. Here, we address the oncogenic effect of Bcl3 in vivo and describe how this Stat3-responsive oncogene promotes metastasis of ErbB2-positive mammary tumors without affecting primary tumor growth or normal mammary function. Deletion of the Bcl3 gene in ErbB2-positive (MMTV-Neu) mice resulted in a 75% reduction in metastatic tumor burden in the lungs with a 3.6-fold decrease in cell turnover index in these secondary lesions with no significant effect on primary mammary tumor growth, cyclin D1 levels, or caspase-3 activity. Direct inhibition of Bcl3 by siRNA in a transplantation model of an Erbb2-positive mammary tumor cell line confirmed the effect of Bcl3 in malignancy, suggesting that the effect of Bcl3 was intrinsic to the tumor cells. Bcl3 knockdown resulted in a 61% decrease in tumor cell motility and a concomitant increase in the cell migration inhibitors Nme1, Nme2, and Nme3, the GDP dissociation inhibitor Arhgdib, and the metalloprotease inhibitors Timp1 and Timp2. Independent knockdown of Nme1, Nme2, and Arhgdib partially rescued the Bcl3 motility phenotype. These results indicate for the first time a cell-autonomous disease-modifying role for Bcl3 in vivo, affecting metastatic disease progression rather than primary tumor growth.

Nucleoside diphosphate kinase Nm23-M1 involves in oligodendroglial versus neuronal cell fate decision in vitro

The adult glial progenitor cells were recently shown to be able to produce neurons in central nervous system (CNS) and to become multipotent in vitro. Although the fate decision of glial progenitors was studied extensively, the signals and factors which regulate the timing of neuronal differentiation still remain unknown. To elucidate the mechanisms underlying the neuronal differentiation from glial progenitors, we modified the gene expression profile in NG2(+) glial progenitor cells using enhanced retroviral mutagen (ERM) technique followed by phenotype screening to identify possible gene(s) responsible for glial-neuronal cell fate determination. Among the identified molecules, we found the gene named non-metastatic cell 1 which encodes a nucleoside diphosphate kinase protein A (Nm23-M1 or NME1). So far, the Nm23 members have been shown to be involved in various molecular processes including tumor metastasis, cell proliferation, differentiation and cell fate determination. In the present study, we provide evidence suggesting the role of NME1 in glial-neuronal cell fate determination in vitro. We showed that NME1 is widely expressed in neuronal structures throughout adult mouse CNS. Our immunohistochemical results revealed that NME1 is strongly colocalized with NF200 through white matter of spinal cord and brain. Interestingly, NME1 overexpression in oligodendrocyte progenitor OLN-93 cells potently induced the acquisition of neuronal fate, while its silencing was shown to promote oligodendrocyte differentiation. Furthermore, we demonstrated that dual-functional role of NME1 is achieved through cAMP-dependent protein kinase (PKA). Our data therefore suggested that NME1 acts as a switcher or reprogramming factor which involves in oligodentrocyte versus neuron cell fate specification in vitro.

Nm23-H1 regulates contact inhibition of locomotion, which is affected by ephrin-B1

Contact inhibition of locomotion (CIL) is the process by which cells stop the continual migration in the same direction after collision with another cell. Highly invasive malignant cells exhibit diminished CIL when they contact stromal cells, which allows invasion of the tissue by tumors. We show that Nm23-H1 is essential for the suppression of Rac1 through inactivation of Tiam1 at the sites of cell-cell contact, which plays a pivotal role in CIL. U87MG cells show CIL when they contact normal glia. In spheroid confrontation assays U87MG cells showed only limited invasion of the glial population, but reduction of Nm23-H1 expression in U87MG cells abrogated CIL resulting in invasion. In U87MG cells, Nm23-H1 is translocated to the sites of contact with glia through association with α-catenin and N-cadherin. Mutants of Nm23-H1, which lacked the binding ability with Tiam1, or α-catenin did not restore CIL. Moreover, the expression of ephrin-B1 in tumor cells disrupted CIL and promoted invasion. As one mechanism, ephrin-B1 inhibits the association of Nm23-H1 with Tiam1, which contributes for activation of Rac1. These results indicate a novel function of Nm23-H1 to control CIL, and its negative regulation by ephrin-B1.

[NM23, an example of a metastasis suppressor gene]

Metastasis suppressor genes - unlike tumor suppressor genes - are defined by their capacity to control metastatic dissemination in vivo without affecting growth of the primary tumor. The first of these metastasis suppressor genes, NM23, was identified in 1988. Since then, expression of NM23 has been studied widely in human tumor cohorts, often with contradictory results. Not only is NM23 overexpressed in most human solid tumors when compared to healthy tissues, but also low expression of NM23 correlates with metastasis and poor clinical prognosis in the advanced stages of a number of epithelial cancer types, including melanoma, breast, colon, and liver carcinoma. This does not hold true, however, for other cancer types such as neuroblastoma and hematological malignancies, in which high NM23 expression correlates with more aggressive disease. Genetic alterations in the NM23 gene - loss of heterozygosity, spontaneous mutations and polymorphisms - are rarely found in tumors; thus, the metastatic potential of tumor cells is probably affected by NM23 protein levels. Three lines of evidence demonstrate the anti-metastatic activity of NM23: first, overexpression of NM23 in metastatic cell lines reduces their metastatic potential in xenograft models; second, the incidence of lung metastases is elevated in NM23 knockout mice prone to develop hepatocellular carcinoma, and, third, silencing NM23 by RNA interference confers a "metastatic phenotype" on non-invasive human epithelial liver and colon cancer cell lines. It appears that NM23 is crucial for inhibiting invasive migration, so acting at early stages of metastatic dissemination. The mechanistic basis of the metastasis suppressor function of NM23 and its regulated expression still remains obscure, however. Reactivation of expression of the endogenous NM23 gene in tumor cells, or stimulation of the pathways it controls, constitutes a promising avenue for anti-metastatic therapy.

The nucleoside diphosphate kinase gene Nme3 acts as quantitative trait locus promoting non-Mendelian inheritance

The t-haplotype, a variant form of the t-complex region on mouse chromosome 17, acts as selfish genetic element and is transmitted at high frequencies (> 95%) from heterozygous (t/+) males to their offspring. This phenotype is termed transmission ratio distortion (TRD) and is caused by the interaction of the t-complex responder (Tcr) with several quantitative trait loci (QTL), the t-complex distorters (Tcd1 to Tcd4), all located within the t-haplotype region. Current data suggest that the distorters collectively impair motility of all sperm derived from t/+ males; t-sperm is rescued by the responder, whereas (+)-sperm remains partially dysfunctional. Recently we have identified two distorters as regulators of RHO small G proteins. Here we show that the nucleoside diphosphate kinase gene Nme3 acts as a QTL on TRD. Reduction of the Nme3 dosage by gene targeting of the wild-type allele enhanced the transmission rate of the t-haplotype and phenocopied distorter function. Genetic and biochemical analysis showed that the t-allele of Nme3 harbors a mutation (P89S) that compromises enzymatic activity of the protein and genetically acts as a hypomorph. Transgenic overexpression of the Nme3 t-allele reduced t-haplotype transmission, proving it to be a distorter. We propose that the NME3 protein interacts with RHO signaling cascades to impair sperm motility through hyperactivation of SMOK, the wild-type form of the responder. This deleterious effect of the distorters is counter-balanced by the responder, SMOK(Tcr), a dominant-negative protein kinase exclusively expressed in t-sperm, thus permitting selfish behaviour and preferential transmission of the t-haplotype. In addition, the previously reported association of NME family members with RHO signaling in somatic cell motility and metastasis, in conjunction with our data involving RHO signaling in sperm motility, suggests a functional conservation between mechanisms for motility control in somatic cells and spermatozoa.

Learning about the functions of NME/NM23: lessons from knockout mice to silencing strategies

The human NME gene family (also known as NM23) comprises ten genes that are involved in diverse physiological and pathological processes including proliferation, differentiation, development, ciliary functions, and metastasis. For the moment, only the NME1, NME2, and NME7 genes have been inactivated in transgenic knockout mice, as well as a double NME1-NME2 gene knockout. Mice lacking NME1 or NME2 grow to adulthood without health problems, although NME1 (-/-) mice have modest growth retardation. Double knockout NME1 (-/-)-NME2 (-/-) mice, by contrast, are highly hypotrophic and die at birth from profound anemia due to impaired erythroblast development. Evidence for a metastasis suppressor function of NME1 in vivo comes from crossing NME1 (-/-) mice with mice prone to develop hepatocellular carcinoma; the double transgenic mice present a higher incidence of lung metastases. Silencing of NME1 by siRNA interference has confirmed this function by conferring a "metastatic phenotype" on non-invasive human epithelial cancer cell lines. This function is specific to NME1 and is not observed when the NME2 is silenced. The data indicate that NME1 loss is causally involved at the early stages of the metastatic cascade. NME2 (-/-) mice and NME2 silencing experiments reveal a specific role of NME2 in activation of heterotrimeric G proteins and of KCa3.1 channel in T cells, pointing to a role of NME2 as a histidine phosphotransferase. Regarding NME7, consistent with its expression in axonemal structures, NME7 (-/-) mice present lesions similar to primary ciliary dyskinesia. This review summarizes the recent data obtained by knockout and silencing of NME/NM23 genes that provide mechanistic insights into their respective roles in physiology and pathology.

Mechanisms of non-metastatic 2 (NME2)-mediated control of metastasis across tumor types

Non-metastatic 23 [NM23/nucleoside diphosphate kinases (NDPK)] genes are the first discovered metastasis suppressor genes. More than two decades of research has demonstrated their roles in a variety of biological processes with NME1 and NME2 being most studied in the context of metastasis suppression. Although NME1 and NME2 share >85% homology at amino acid level, they show redundant as well as unique molecular functions. Phenotypic analyses of knockout (KO) mice for NM23 members (NDPK-A, B) and compound KO (A as well as B) showed requirement of both proteins in hematopoiesis suggesting shared functions in development disease. Several reviews have discussed NME1, however the role of NME2 appears to be relatively less understood in the context of metastasis suppression. Here, we focus on NME2 and by meta-analysis of gene expression from multiple tumor types, and survey of in vivo and vitro studies, suggest the possibility that NME2 may be one of the key factors in metastasis. This along with the relevance of normal physiological functions of NME2 in the context of metastasis is discussed. We further examined the genetic and epigenetic features of NME2 and NME1 gene promoters and found aspects of transcription control that could be unique to NME2/NME1. Findings on signaling pathways and small molecules which regulate the expression of NME2 that could be therapeutically important are also discussed.

A role for nucleotides in support of breast cancer angiogenesis: heterologous receptor signalling

Background:

Human breast carcinoma cells secrete an adenosine 5′-diphosphate transphosphorylase (sNDPK) known to induce endothelial cell tubulogenesis in a P2Y receptor-dependent manner. We examined sNDPK secretion and its effects on human endothelial cells.

Methods:

Nucleoside diphosphate kinase (NDPK) secretion was measured by western blot and enzyme-linked immunosorbent assay, while transphosphorylase activity was measured using the luciferin-luciferase ATP assay. Activation of MAPK was determined by western blot analysis, immunofluorescence and endothelial cell proliferation and migration.

Results:

A panel of breast cancer cell lines with origin as ductal carcinoma, adenocarcinoma or medullary carcinoma, secrete sNDPK-A/B. Addition of purified NDPK-B to endothelial cultures activated VEGFR-2 and Erk_1/2, both of which were blocked by inhibitors of NDPK and P2Y receptors. Activation of VEGFR-2 and ErK_1/2 by 2-methylthio-ATP (2MeS-ATP) was blocked by pretreatment with the P2Y₁-specific antagonist MRS2179, the proto-oncogene non-receptor tyrosine kinase (Src) inhibitor PP2 or the VEGFR-2 antagonist SU1498. Nucleoside diphosphate kinase-B stimulates cell growth and migration in a concentration-dependent manner comparable to the effect of vascular endothelial growth factor. Treatment of endothelial cells with either NDPK-B or 2MeS-ATP induced migration, blocked by P2Y₁, Src or VEGFR-2 antagonists.

Conclusion:

sNDPK supports angiogenesis. Understanding the mechanism of action of sNDPK and P2Y₁ nucleotide signalling in metastasis and angiogenesis represent new therapeutic targets for anti-angiogenic therapies to benefit patients.

MDM2 interacts with NME2 (non-metastatic cells 2, protein) and suppresses the ability of NME2 to negatively regulate cell motility

MDM2 expression, combined with increased p53 expression, is associated with reduced survival in several cancers, but is particularly of interest in renal cell carcinoma (RCC) where evidence suggests the presence of tissue-specific p53/MDM2 pathway defects. We set out to identify MDM2-interacting proteins in renal cells that could act as mediators/targets of MDM2 oncogenic effects in renal cancers. We identified the non-metastatic cells 2, protein; NME2 (NDPK-B, NM23-B/-H2), a nucleoside diphosphate kinase, as an MDM2-interacting protein using both a proteomic-based strategy [affinity chromatography and tandem mass spectrometry [MS/MS] from HEK293 cells] and a yeast two-hybrid screen of a renal carcinoma cell-derived complementary DNA library. The MDM2-NME2 interaction is highly specific, as NME1 (87.5% amino acid identity) does not interact with MDM2 in yeast. Specific NME proteins display well-documented cell motility and metastasis-suppressing activity. We show that NME2 contributes to motility suppression under conditions where MDM2 is expressed at normal physiological/low levels. However, up-regulation of MDM2 in RCC cells abolishes the ability of NME2 to suppress motility. Significantly, when MDM2 expression is down-regulated in these cells using small interfering RNA, the motility-suppressing activity of NME2 is rescued, confirming that MDM2 expression causes the loss of NME2 cell motility regulatory function. Thus MDM2 up-regulation in renal cancer cells can act in a dominant manner to abrogate the function of a potent suppressor of motility and metastasis. Our studies identify a novel protein-protein interaction between MDM2 and NME2, which suggests a mechanism that could explain the link between MDM2 expression and poor patient survival in RCC.

Mass spectrometry detection of histidine phosphorylation on NM23-H1

Phosphorylation is a ubiquitous protein post-translational modification that is intimately involved in most aspects of cellular regulation. Currently, most proteomic analyses are performed with phosphorylation searches for serine, threonine, and tyrosine modifications, as the phosphorylated residues of histidine and aspartic acid are acid labile and thus undetectable with most proteomic methodologies. Here, we present a novel buffer system to show histidine phosphorylation of NM23-H1, the product of the first identified putative human metastasis suppressor gene (NME1), which catalyzes the transfer of the γ-phosphate from nucleoside triphosphates to nucleoside diphosphates. On the basis of a pH titration of LC elution buffers and MS/MS identification, recombinant NM23-H1 subjected to autophosphorylation was shown to contain phosphorylated histidine at residue 118 at pH 5 and 6, with each level giving over 75% peptide coverage for identification. The solvent system presented permits the detection of all five possible phosphorylation moieties. Application of histidine and aspartic acid phosphorylation modifications to proteomic analyses will significantly advance the understanding of phosphorylation relay signaling in cellular regulation, including elucidation of the role of NM23-H1 in metastasis.

Purinergic mechanisms in breast cancer support intravasation, extravasation and angiogenesis

Several advances have recently expanded models of tumor growth and promoted the concept of tumor homeostasis, the hypothesis that primary tumors exert an anti-proliferative effect on both themselves and subclinical secondary metastases. Recent trials indicate that the characterization of tumor growth as uncontrolled is inconsistent with animal models, clinical models, and epidemiological models. There is a growing body of evidence which lends support to an updated concept of tumor growth: tumor homeostasis. In the case of breast cancer, if not all metastasizing tumors, these advances suggest an inconvenient truth. That is, if breast tumor cells metastasize to distant sites early in the tumorigenesis process, then removal of a breast tumor may hasten the development of its metastases. We explore the heretofore unappreciated notion that nucleotides generated by tumor cells following the secretion of an ADP-kinase can promote metastasis and support angiogenesis. Evidence is presented that blockade of the actions of nucleotides in the setting of newly diagnosed breast cancer may provide a useful adjunct to current anti-angiogenesis treatment.

Plakoglobin interacts with and increases the protein levels of metastasis suppressor Nm23-H2 and regulates the expression of Nm23-H1

Plakoglobin (gamma-catenin) is a homolog of beta-catenin with similar dual adhesive and signaling functions. The adhesive function of these proteins is mediated by their interactions with cadherins, whereas their signaling activity is regulated by association with various intracellular partners. In this respect, beta-catenin has a well-defined oncogenic activity through its role in the Wnt signaling pathway, whereas plakoglobin acts as a tumor/metastasis suppressor through mechanisms that remain unclear. We previously expressed plakoglobin in SCC9 squamous carcinoma cells (SCC9-P) and observed a mesenchymal-to-epidermoid transition. Comparison of the protein and RNA profiles of parental SCC9 cells and SCC9-P transfectants identified various differentially expressed proteins and transcripts, including the nonmetastatic protein 23 (Nm23). In this study, we show that Nm23-H1 mRNA and Nm23-H2 protein are increased after plakoglobin expression. Coimmunoprecipitation and confocal microscopy studies using SCC9-P and various epithelial cell lines with endogenous plakoglobin expression revealed that Nm23 interacts with plakoglobin, cadherins and alpha-catenin. Furthermore, Nm23-H2 is the primary isoform involved in these interactions, which occur prominently in the cytoskeleton-associated pool of cellular proteins. In addition, we show that plakoglobin-Nm23 interaction requires the N-terminal (alpha-catenin interacting) domain of plakoglobin. Our data suggest that by increasing the expression and stability of Nm23, plakoglobin has a role in regulating the metastasis suppressor activity of Nm23, which may further provide a potential mechanism for the tumor/metastasis suppressor function of plakoglobin itself.

Mitochondrial kinases and their molecular interaction with cardiolipin

Mitochondrial isoforms of creatine kinase (MtCK) and nucleoside diphosphate kinase (NDPK-D) are not phylogenetically related but share functionally important properties. They both use mitochondrially generated ATP with the ultimate goal of maintaining proper nucleotide pools, are located in the intermembrane/cristae space, have symmetrical oligomeric structures, and show high affinity binding to anionic phospholipids, in particular cardiolipin. The structural basis and functional consequences of the cardiolipin interaction have been studied and are discussed in detail in this review. They mainly result in a functional interaction of MtCK and NDPK-D with inner membrane adenylate translocator, probably by forming proteolipid complexes. These interactions allow for privileged exchange of metabolites (channeling) that ultimately regulate mitochondrial respiration. Further functions of the MtCK/membrane interaction include formation of cardiolipin membrane patches, stabilization of mitochondria and a role in apoptotic signaling, as well as in case of both kinases, a role in facilitating lipid transfer between two membranes. Finally, disturbed cardiolipin interactions of MtCK, NDPK-D and other proteins like cytochrome c and truncated Bid are discussed more generally in the context of apoptosis and necrosis.

Metastasis suppressors and the tumor microenvironment

The most dangerous attribute of cancer cells is their ability to metastasize. Throughout the process of metastasis, tumor cells interact with other tumor cells, host cells and extracellular molecules. This brief review explores how a new class of molecules – metastasis suppressors – regulate tumor cell–microenvironmental interactions. Data are presented which demonstrate that metastasis suppressors act at multiple steps of the metastatic cascade. A brief discussion for how metastasis suppressor regulation of cellular interactions might be exploited is presented.