Regulatory functions of Nm23-H2 in tumorigenesis: insights from biochemical to clinical perspectives

Mitochondrial NME6 Influences Basic Cellular Processes in Tumor Cells In Vitro

NME6 belongs to the family of nucleoside diphosphate kinase enzymes, whose major role is to transfer the terminal phosphate from NTPs, mostly ATP, to other (d)NDPs via a high-energy intermediate. Beside this basic enzymatic activity, the family, comprising 10 genes/proteins in humans, executes a number of diverse biochemical/biological functions in the cell. A few previous studies have reported that NME6 resides in the mitochondria and influences oxidative phosphorylation while interacting with RCC1L, a GTPase involved in mitochondrial ribosome assembly and translation. Considering the multifunctional role of NME family members, the goal of the present study was to assess the influence of the overexpression or silencing of NME6 on fundamental cellular events of MDA-MB-231T metastatic breast cancer cells. Using flow cytometry, Western blotting, and a wound-healing assay, we demonstrated that the overexpression of NME6 reduces cell migration and alters the expression of EMT (epithelial-mesenchymal transition) markers. In addition, NME6 overexpression influences cell cycle distribution exclusively upon DNA damage and impacts the MAPK/ERK signaling pathway, while it has no effect on apoptosis. To conclude, our results demonstrate that NME6 is involved in different cellular processes, providing a solid basis for future, more precise investigations of its role.

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.

ATP and Adenosine Metabolism in Cancer: Exploitation for Therapeutic Gain

Adenosine is an evolutionary ancient metabolic regulator linking energy state to physiologic processes, including immunomodulation and cell proliferation. Tumors create an adenosine-rich immunosuppressive microenvironment through the increased release of ATP from dying and stressed cells and its ectoenzymatic conversion into adenosine. Therefore, the adenosine pathway becomes an important therapeutic target to improve the effectiveness of immune therapies. Prior research has focused largely on the two major ectonucleotidases, ectonucleoside triphosphate diphosphohydrolase 1/cluster of differentiation (CD)39 and ecto-5'-nucleotidase/CD73, which catalyze the breakdown of extracellular ATP into adenosine, and on the subsequent activation of different subtypes of adenosine receptors with mixed findings of antitumor and protumor effects. New findings, needed for more effective therapeutic approaches, require consideration of redundant pathways controlling intratumoral adenosine levels, including the alternative NAD-inactivating pathway through the CD38-ectonucleotide pyrophosphatase phosphodiesterase (ENPP)1-CD73 axis, the counteracting ATP-regenerating ectoenzymatic pathway, and cellular adenosine uptake and its phosphorylation by adenosine kinase. This review provides a holistic view of extracellular and intracellular adenosine metabolism as an integrated complex network and summarizes recent data on the underlying mechanisms through which adenosine and its precursors ATP and ADP control cancer immunosurveillance, tumor angiogenesis, lymphangiogenesis, cancer-associated thrombosis, blood flow, and tumor perfusion. Special attention is given to differences and commonalities in the purinome of different cancers, heterogeneity of the tumor microenvironment, subcellular compartmentalization of the adenosine system, and novel roles of purine-converting enzymes as targets for cancer therapy. SIGNIFICANCE STATEMENT: The discovery of the role of adenosine as immune checkpoint regulator in cancer has led to the development of novel therapeutic strategies targeting extracellular adenosine metabolism and signaling in multiple clinical trials and preclinical models. Here we identify major gaps in knowledge that need to be filled to improve the therapeutic gain from agents targeting key components of the adenosine metabolic network and, on this basis, provide a holistic view of the cancer purinome as a complex and integrated network.

The mitochondrially-localized nucleoside diphosphate kinase D (NME4) is a novel metastasis suppressor

Background

Mitochondrial nucleoside diphosphate kinase (NDPK-D, NME4, NM23-H4) is a multifunctional enzyme mainly localized in the intermembrane space, bound to the inner membrane.

Results

We constructed loss-of-function mutants of NDPK-D, lacking either NDP kinase activity or membrane interaction and expressed mutants or wild-type protein in cancer cells. In a complementary approach, we performed depletion of NDPK-D by RNA interference. Both loss-of-function mutations and NDPK-D depletion promoted epithelial-mesenchymal transition and increased migratory and invasive potential. Immunocompromised mice developed more metastases when injected with cells expressing mutant NDPK-D as compared to wild-type. This metastatic reprogramming is a consequence of mitochondrial alterations, including fragmentation and loss of mitochondria, a metabolic switch from respiration to glycolysis, increased ROS generation, and further metabolic changes in mitochondria, all of which can trigger pro-metastatic protein expression and signaling cascades. In human cancer, NME4 expression is negatively associated with markers of epithelial-mesenchymal transition and tumor aggressiveness and a good prognosis factor for beneficial clinical outcome.

Conclusions

These data demonstrate NME4 as a novel metastasis suppressor gene, the first localizing to mitochondria, pointing to a role of mitochondria in metastatic dissemination.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01155-5.

CTCF and EGR1 suppress breast cancer cell migration through transcriptional control of Nm23-H1

Tumor metastasis remains an obstacle in cancer treatment and is responsible for most cancer-related deaths. Nm23-H1 is one of the first metastasis suppressor proteins discovered with the ability to inhibit metastasis of many cancers including breast, colon, and liver cancer. Although loss of Nm23-H1 is observed in aggressive cancers and correlated with metastatic potential, little is known regarding the mechanisms that regulate its cellular level. Here, we examined the mechanisms that control Nm23-H1 expression in breast cancer cells. Initial studies in aggressive MDA-MB-231 cells (expressing low Nm23-H1) and less invasive MCF-7 cells (expressing high Nm23-H1) revealed that mRNA levels correlated with protein expression, suggesting that transcriptional mechanisms may control Nm23-H1 expression. Truncational analysis of the Nm23-H1 promoter revealed a proximal and minimal promoter that harbor putative binding sites for transcription factors including CTCF and EGR1. CTCF and EGR1 induced Nm23-H1 expression and reduced cell migration of MDA-MB-231 cells. Moreover, CTCF and EGR1 were recruited to the Nm23-H1 promoter in MCF-7 cells and their expression correlated with Nm23-H1 levels. This study indicates that loss of Nm23-H1 in aggressive breast cancer is apparently caused by downregulation of CTCF and EGR1, which potentially drive Nm23-H1 expression to promote a less invasive phenotype.

The multiple regulation of metastasis suppressor NM23-H1 in cancer

Metastasis is one of the leading causes of mortality in cancer patients. As the firstly identified metastasis suppressor, NM23-H1 has been endowed with expectation as a potent target in metastatic cancer therapy during the past decades. However, many challenges impede its clinical use. Accumulating evidence shows that NM23-H1 has a dichotomous role in tumor metastasis as a suppressor and promoter. It has potentially attributed to its versatile biochemical characteristics such as nucleoside diphosphate kinase (NDPK) activity, histidine kinase activity (HPK), exonuclease activity, and protein scaffold, which further augment the complexity and uncertainty of its physiological function. Simultaneously, tumor cells have evolved multiple ways to regulate the expression and function of NM23-H1 during tumorigenesis and metastasis. This review summarized and discussed the regulatory mechanisms of NM23-H1 in cancer including transcriptional activation, subcellular location, enzymatic activity, and protein degradation, which significantly modulate its anti-metastatic function.

Mutations at the dimer interface and surface residues of Nm23-H1 metastasis suppressor affect its expression and function

The Nm23 metastasis suppressor family is involved in a variety of physiological and pathological processes including cell proliferation, differentiation, tumorigenesis, and metastasis. Given that Nm23 proteins may function as hexamers composed of different members of the family, especially Nm23-H1 and H2 isoforms, it is pertinent to assess the importance of interface and surface residues in defining the functional characteristics of Nm23 proteins. Using molecular modeling to identify clusters of residues that may affect dimer formation and isoform specificity, mutants of Nm23-H1 were constructed and assayed for their ability to modulate cell migration. Mutations of dimer interface residues Gly²² and Lys³⁹ affected the expression level of Nm23-H1, without altering the transcript level. The reduced protein expression was not due to increased protein degradation or altered subcellular distribution. Substitution of the surface residues of Nm23-H1 with Nm23-H2-specific Ser¹³¹ and/or Lys^124/135 affected the electrophoretic mobility of the protein. Moreover, in cell migration assays, several mutants with altered surface residues exhibited impaired ability to suppress the mobility of MDA-MB-231 cells. Collectively, the study suggests that disrupting the dimer interface may affect the expression of Nm23-H1, while the residues at α-helix and β-sheet on the surface of Nm23-H1 may contribute to its metastasis suppressive function.

The Subcellular Localization and Oligomerization Preferences of NME1/NME2 upon Radiation-Induced DNA Damage

Nucleoside diphosphate kinases (NDPK/NME/Nm23) are enzymes composed of subunits NME1/NDPK A and NME2/NDPK B, responsible for the maintenance of the cellular (d)NTP pool and involved in other cellular processes, such as metastasis suppression and DNA damage repair. Although eukaryotic NDPKs are active only as hexamers, it is unclear whether other NME functions require the hexameric form, and how the isoenzyme composition varies in different cellular compartments. To examine the effect of DNA damage on intracellular localization of NME1 and NME2 and the composition of NME oligomers in the nucleus and the cytoplasm, we used live-cell imaging and the FRET/FLIM technique. We showed that exogenous NME1 and NME2 proteins co-localize in the cytoplasm of non-irradiated cells, and move simultaneously to the nucleus after gamma irradiation. The FRET/FLIM experiments imply that, after DNA damage, there is a slight shift in the homomer/heteromer balance between the nucleus and the cytoplasm. Collectively, our results indicate that, after irradiation, NME1 and NME2 engage in mutual functions in the nucleus, possibly performing specific functions in their homomeric states. Finally, we demonstrated that fluorophores fused to the N-termini of NME polypeptides produce the largest FRET effect and thus recommend this orientation for use in similar studies.

The NDPK/NME superfamily: state of the art

Nucleoside diphosphate kinases (NDPK) are nucleotide metabolism enzymes encoded by NME genes (also called NM23). Given the fact that not all NME-encoded proteins are catalytically active NDPKs and that NM23 generally refers to clinical studies on metastasis, we use here NME/NDPK to denote the proteins. Since their discovery in the 1950's, NMEs/NDPKs have been shown to be involved in multiple physiological and pathological cellular processes, but the molecular mechanisms have not been fully determined. Recent progress in elucidating these underlying mechanisms has been presented by experts in the field at the 10th International Congress on the NDPK/NME/AWD protein family in October 2016 in Dubrovnik, Croatia, and is summarized in review articles or original research in this and an upcoming issue of Laboratory Investigation. Within this editorial, we discuss three major cellular processes that involve members of the multi-functional NME/NDPK family: (i) cancer and metastasis dissemination, (ii) membrane remodeling and nucleotide channeling, and iii) protein histidine phosphorylation.

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.

[Effect of Nm23-H1 Nuclear Localization on Proliferation of Human Lung Adenocarcinoma Cell Line A549]

背景与目的

现有研究发现Nm23-H1还存在胞核表达，而既往的研究都是以过表达或抑制胞浆Nm23-H1为研究手段，由于Nm23-H1本身缺乏核引导序列，其研究结果并不能真实反映或重复临床中Nm23-H1以胞核定位为主的实际生物学效应。因此，本研究通过构建带有核引导序列的Nm23-H1载体并转染A549细胞以探讨Nm23-H1从胞浆向胞核转位对肺癌细胞增殖的影响。

方法

采用基因重组技术构建带核定位信号序列的pLentis-CMV-NME1-IRES2-PURO慢病毒载体，酶切和测序鉴定正确后，稳定转染A549细胞后用Western blot和激光共聚焦检测Nm23-H1蛋白的定位和表达，用CCK-8法检测细胞的增殖，流式细胞术检测细胞周期变化。

结果

成功构建了核内定向表达Nm23-H1的慢病毒载体。转染组在72 h、96 h和120 h时增殖率与空载体组相比均显著升高（P < 0.000, 1）。空载体组A549细胞在G₀期/G₁期所占比例为35.69%，高于转染组的28.28%（t=1.461, P=0.217）；而转染组细胞在G₂期/M期所占比例为58.7%，空载体组为31.30%（t=4.560, P=0.010）。

结论

Nm23-H1在人肺腺癌A549细胞的核内过表达使细胞主要分布在G₂期/M期并促进了细胞的体外增殖。

Two-Step Membrane Binding of NDPK-B Induces Membrane Fluidity Decrease and Changes in Lipid Lateral Organization and Protein Cluster Formation

Nucleoside diphosphate kinases (NDPKs) are crucial elements in a wide array of cellular physiological or pathophysiological processes such as apoptosis, proliferation, or metastasis formation. Among the NDPK isoenzymes, NDPK-B, a cytoplasmic protein, was reported to be associated with several biological membranes such as plasma or endoplasmic reticulum membranes. Using several membrane models (liposomes, lipid monolayers, and supported lipid bilayers) associated with biophysical approaches, we show that lipid membrane binding occurs in a two-step process: first, initiation by a strong electrostatic adsorption process and followed by shallow penetration of the protein within the membrane. The NDPK-B binding leads to a decrease in membrane fluidity and formation of protein patches. The ability of NDPK-B to form microdomains at the membrane level may be related to protein-protein interactions triggered by its association with anionic phospholipids. Such accumulation of NDPK-B would amplify its effects in functional platform formation and protein recruitment at the membrane.

Metastasis suppressors Nm23H1 and Nm23H2 differentially regulate neoplastic transformation and tumorigenesis

Nm23H1 and H2 are prototypical metastasis suppressors with diverse functions, but recent studies suggest that they may also regulate tumorigenesis. Here, we employed both cellular and in vivo assays to examine the effect of Nm23H1 and H2 on tumorigenesis induced by oncogenic Ras and/or p53 deficiency. Co-expression of Nm23H1 but not H2 in NIH3T3 cells effectively suppressed neoplastic transformation and tumorigenesis induced by the oncogenic H-Ras G12V mutant. Overexpression of Nm23H1 but not H2 also inhibited tumorigenesis by human cervical cancer HeLa cells with p53 deficiency. However, in human non-small-cell lung carcinoma H1299 cells harboring N-Ras Q61K oncogenic mutation and p53 deletion, overexpression of Nm23H1 did not affect tumorigenesis in nude mice assays, while overexpression of Nm23H2 enhanced tumor growth with elevated expression of the c-Myc proto-oncogene. Collectively, these results suggest that Nm23H1 and H2 have differential abilities to modulate tumorigenesis.