Nucleoside diphosphate kinases, nm23, and tumor metastasis: Possible biochemical mechanisms

The Role of Calmodulin in Tumor Cell Migration, Invasiveness, and Metastasis

Calmodulin (CaM) is the principal Ca²⁺ sensor protein in all eukaryotic cells, that upon binding to target proteins transduces signals encoded by global or subcellular-specific changes of Ca²⁺ concentration within the cell. The Ca²⁺/CaM complex as well as Ca²⁺-free CaM modulate the activity of a vast number of enzymes, channels, signaling, adaptor and structural proteins, and hence the functionality of implicated signaling pathways, which control multiple cellular functions. A basic and important cellular function controlled by CaM in various ways is cell motility. Here we discuss the role of CaM-dependent systems involved in cell migration, tumor cell invasiveness, and metastasis development. Emphasis is given to phosphorylation/dephosphorylation events catalyzed by myosin light-chain kinase, CaM-dependent kinase-II, as well as other CaM-dependent kinases, and the CaM-dependent phosphatase calcineurin. In addition, the role of the CaM-regulated small GTPases Rac1 and Cdc42 (cell division cycle protein 42) as well as CaM-binding adaptor/scaffold proteins such as Grb7 (growth factor receptor bound protein 7), IQGAP (IQ motif containing GTPase activating protein) and AKAP12 (A kinase anchoring protein 12) will be reviewed. CaM-regulated mechanisms in cancer cells responsible for their greater migratory capacity compared to non-malignant cells, invasion of adjacent normal tissues and their systemic dissemination will be discussed, including closely linked processes such as the epithelial–mesenchymal transition and the activation of metalloproteases. This review covers as well the role of CaM in establishing metastatic foci in distant organs. Finally, the use of CaM antagonists and other blocking techniques to downregulate CaM-dependent systems aimed at preventing cancer cell invasiveness and metastasis development will be outlined.

Rotavirus Induces Formation of Remodeled Stress Granules and P Bodies and Their Sequestration in Viroplasms To Promote Progeny Virus Production

Rotavirus replicates in unique virus-induced cytoplasmic inclusion bodies called viroplasms (VMs), the composition and structure of which have yet to be understood. Based on the analysis of a few proteins, earlier studies reported that rotavirus infection inhibits stress granule (SG) formation and disrupts P bodies (PBs). However, the recent demonstration that rotavirus infection induces cytoplasmic relocalization and colocalization with VMs of several nuclear hnRNPs and AU-rich element-binding proteins (ARE-BPs), which are known components of SGs and PBs, suggested the possibility of rotavirus-induced remodeling of SGs and PBs, prompting us to analyze a large number of the SG and PB components to understand the status of SGs and PBs in rotavirus-infected cells. Here we demonstrate that rotavirus infection induces molecular triage by selective exclusion of a few proteins of SGs (G3BP1 and ZBP1) and PBs (DDX6, EDC4, and Pan3) and sequestration of the remodeled/atypical cellular organelles, containing the majority of their components, in the VM. The punctate SG and PB structures are seen at about 4 h postinfection (hpi), coinciding with the appearance of small VMs, many of which fuse to form mature large VMs with progression of infection. By use of small interfering RNA (siRNA)-mediated knockdown and/or ectopic overexpression, the majority of the SG and PB components, except for ADAR1, were observed to inhibit viral protein expression and virus growth. In conclusion, this study demonstrates that VMs are highly complex supramolecular structures and that rotavirus employs a novel strategy of sequestration in the VM and harnessing of the remodeled cellular RNA recycling bins to promote its growth.IMPORTANCE Rotavirus is known to replicate in specialized virus-induced cytoplasmic inclusion bodies called viroplasms (VMs), but the composition and structure of VMs are not yet understood. Here we demonstrate that rotavirus interferes with normal SG and PB assembly but promotes formation of atypical SG-PB structures by selective exclusion of a few components and employs a novel strategy of sequestration of the remodeled SG-PB granules in the VMs to promote virus growth by modulating their negative influence on virus infection. Rotavirus VMs appear to be complex supramolecular structures formed by the union of the triad of viral replication complexes and remodeled SGs and PBs, as well as other host factors, and designed to promote productive virus infection. These observations have implications for the planning of future research with the aim of understanding the structure of the VM, the mechanism of morphogenesis of the virus, and the detailed roles of host proteins in rotavirus biology.

Molecular biology of prostate cancer

Development of any cancer reflects a progressive accumulation of alterations in various genes. Oncogenes, tumour suppressor genes, DNA repair genes and metastasis suppressor genes have been investigated in prostate cancer. Here, we review current understanding of the molecular biology of prostate cancer. Detailed understanding of the molecular basis of prostate cancer will provide insights into the aetiology and prognosis of the disease, and suggest avenues for therapeutic intervention in the future.

Point mutations affecting the oligomeric structure of Nm23-H1 abrogates its inhibitory activity on colonization and invasion of prostate cancer cells

In order to identify Nm23-H1's structural motifs influencing its metastasis-inhibitory activity, we transfected DU 145 human prostate carcinoma cells with the expression vector encoding the Nm23-H1 protein with mutations at the following amino acids: serine-44, a phosphorylation site; proline-96, a site corresponding to the k-pn mutation that causes developmental defects in Drosophila; and serine-120, a site of mutation in human neuroblastoma and phosphorylation. Significant decrease in colonization in soft agar and invasiveness of DU 145 cells was observed in the wild type nm23-H1 transfectants, and also in the serine-44 and serine-120 to alanine mutant nm23-H1-transfected cell lines. However, the k-pn type proline-96 to serine (P96S) and neuroblastoma type serine-120 to glycine (S120G) mutations of Nm23-H1 abrogated its inhibitory activity on colonization and invasion. Meanwhile, all of the recombinant mutant Nm23-H1 proteins produced in Escherichia coli exhibited NDP kinase activity levels at the wild type protein, although the P96S and S120G mutant proteins exhibited decreased histidine protein kinase activity and autophosphorylation level, respectively. Interestingly, only two of the mutant recombinant Nm23-H1 proteins examined, P96S and S120G, exhibited reduced hexameric and increased dimeric oligomerization relative to the wild type. These correlative data suggest that the metastasis-suppressing activity of Nm23-H1 may depend on its oligomeric structure, but not on its NDP kinase activity.

Catalysis of DNA cleavage and nucleoside triphosphate synthesis by NM23-H2/NDP kinase share an active site that implies a DNA repair function

NM23/NDP kinases play an important role in development and cancer but their biological function is unknown, despite an intriguing collection of biochemical properties including nucleoside-diphosphate kinase (NDP kinase), DNA binding and transcription, a mutator function, and cleavage of unusually structured DNA by means of a covalent enzyme-DNA complex. To assess the role of the nuclease in human NM23-H2, we sought to identify the amino acid responsible for covalent catalysis. By sequencing a DNA-linked peptide and by site-directed mutagenesis, we identified lysine-12, a phylogenetically conserved residue, as the amino acid forming the covalent complex with DNA. In particular, the epsilon-amino group acts as the critical nucleophile, because substitution with glutamine but not arginine completely abrogated covalent adduct formation and DNA cleavage, whereas the DNA-binding properties remained intact. These findings and chemical modification data suggest that phosphodiester-bond cleavage occurs by a DNA glycosylase/lyase-like mechanism known as the signature of base excision DNA repair nucleases. Involvement of NM23/NDP kinase in a DNA repair pathway would be consistent with its role in normal and tumor cell development. Additionally, lysine-12, which is known in the x-ray crystallographic structure to lie in the catalytic pocket involved in the NDP kinase phosphorylation reaction, was found essential also for the NDP kinase activity of NM23-H2, suggesting that the two catalytic activities of NM23-H2 are fundamentally connected.

Decreased expression of nucleoside diphosphate kinase alpha isoform, an nm23-H2 gene homolog, is associated with metastatic potential of rat mammary-adenocarcinoma cells

The nm23 gene [encoding nucleoside diphosphate kinase (NDPK)] may act as a metastasis suppressor in certain tumor cells. We investigated the role of NDPK isoforms (alpha and beta) in the metastatic processes, using rat mammary-adenocarcinoma cell lines of poor (MTC) and high (MTLn3) spontaneous metastatic potential respectively. In these cell lines, as in most rat tissues, the alpha isoform (nm23-H2 homolog) was more highly expressed than the beta isoform (nm23-H1 homolog) at the mRNA and protein levels. When examined by Northern- and Western-blot analyses, expression of the 2 isoforms was reduced in highly metastatic MTLn3 cells compared with poorly metastatic MTC cells. The reduced expression was also associated with diminished NDPK-enzyme activity in the cell extracts. Southern-blot and RT-PCR-SSCP analyses suggested that the 2 genes were not grossly altered or mutated in their translation regions. MTLn3 cell clones transfected with NDPKalpha or NDPKbeta cDNA were all tumorigenic when implanted into the mammary fat pad of syngeneic rats. Among those, only clones transfected with the NDPKalpha gene exhibited reduced lung metastasis in a spontaneous metastasis assay.

A novel serine/threonine-specific protein phosphotransferase activity of Nm23/nucleoside-diphosphate kinase

Two human nm23 genes have been identified, designated nm23-H1 and nm23-H2, which encode the 88% identical nucleoside-diphosphate kinase (NDPK) A and NDPK B polypeptides, respectively. The nm23-H1 gene product has been shown to play a functional role in the suppression of tumor metastasis. The Nm23 proteins/NDPK are highly conserved throughout evolution and are implicated in controlling cellular differentiation and development in various species, while the underlying mechanisms remain undefined. Neither the NDPK activity nor the DNA-binding activity, identified recently for NDPK B, can satisfactory explain the regulatory functions of Nm23. The present study provides evidence that purified Nm23 proteins are capable of transferring a phosphate group to other proteins when non-denaturing amounts of urea are present. This novel Nm23/NDPK activity was found to be specific for serine and threonine residues, and the transphosphorylation of substrate proteins occurred stoichiometrically. Because of the absence of a substrate turn-over, the novel function was termed protein phosphotransferase activity instead of protein kinase activity. It is demonstrated that urea stimulates the interaction of NDPK with other proteins. Identical phosphoprotein patterns were obtained using purified NDPK preparations from human, Drosophila, yeast and Dictyostelium in the presence of urea. Partially purified NDPK from human erythrocytes produced a similar phosphorylation pattern independent of urea addition and also acted stoichiometrically. In this preparation, a protein phosphotransferase activity of Nm23 species may possibly be generated and/or stabilized by the interaction with copurified proteins. Using different mutants of Dictyostelium NDPK it was shown that the protein phosphotransferase activity depends on the same active site as the NDPK activity. A phosphotransfer mechanism analogous to that of protein-histidine kinases is proposed, involving a high-energy phosphohistidine intermediate. Furthermore, the novel Nm23 function is compared with an apparently similar protein phosphotransferase activity which was observed previously with partially purified NDPK from different plant species.

Theories on the metastatic process and possible therapeutic options

A sequence of steps are prerequisite for cancer cells before metastases are established. Metastasis has been shown to be an inefficient process limited by both random and selective events. By differentiating invasion from metastasis, sequential steps in the metastatic cascade have been defined and studied separately. This approach has yielded a variety of new potential therapeutic strategies. However, increasing knowledge of the mechanisms relating to metastasis has also revealed the complexity of each step. In spite of difficulties in translating results obtained in preclinical models into the clinical setting, continued development of such model systems and further research into the genetic control of metastatic dissemination will lead to improved strategies for prevention of metastasis formation and for treatment of metastatic tumor cells.

Nm23/nucleoside diphosphate kinase: toward a structural and biochemical understanding of its biological functions

The nm23 gene, a putative metastasis suppressor gene, was originally identified by its reduced expression in highly metastatic K-1735 murine melanoma cell lines, as compared to related, low metastatic melanoma cell lines. Transfection of nm23 cDNA has been reported to suppress malignant progression in Drosophila and mammalian cells. Highly conserved homologues of nm23 have been found in organisms ranging from the prokaryote Myxococcus xanthus to Drosophila, where the gene is involved in normal development and differentiation. The product of the nm23 gene exhibits a nucleoside diphosphate kinase activity, yet the nucleoside diphosphate kinase activity of Nm23 does not correlate with its apparent biological functions. We review recent cellular, genetic, biochemical and X-ray crystallographic data to formulate and evaluate hypotheses concerning the molecular mechanism of nm23 action.

High level of Nm23-H1 gene expression is associated with local colorectal cancer progression not with metastases

This study aimed to determine the expression of Nm23-H1 in colorectal cancer and liver metastases and to correlate Nm23-H1 expression with clinicopathological variables. Specimens from 59 primary colorectal cancers and five liver metastases were studied using Northern blot hybridisation. The mean +/- s.e. of tumour/normal (T/N) ratio of Nm23-H1 RNA expression was 4.3 +/- 0.4 (P < 0.001) and 5.1 +/- 0.90 (P < 0.01) for colorectal cancer and liver metastases respectively. No significant relationship was observed between the level of Nm23-H1 RNA and the patient's age, sex, tumour location, differentiation, presence of lymph node involvement or distant metastases. Nm23-H1 RNA level was 2.6 +/- 0.5 for tumour size less than 3.0 cm and 4.6 +/- 0.5 for those > or = 3.0 cm (P = 0.05). There appeared to be a trend between increasing relative Nm23-H1 RNA and bowel wall invasion, irrespective of metastatic status (T1 = 1.9 +/- 0.3, T2 = 4.1 +/- 0.6, T3 = 4.1 +/- 0.5 and T4 = 6.4 +/- 1.6). This difference was statistically significant when T1 was compared against > or = T2 lesions (P = 0.01). Western blot analysis reveals two Nm23H-1 bands (17.0 kDa and 18.5 kDa). In 16 colorectal patients, the T/N fold-increase in protein expression was 2.66 +/- 0.46 (P < 0.001) and 2.40 +/- 0.32 (P < 0.001) for the 17.0 and 18.5 kDa band respectively. Both Nm23-H1 RNA and protein levels in primary colorectal cancers do not appear to correlate with synchronous regional or distant metastases. Since Nm23-H1 RNA expression is associated with increasing tumour size and tumour local invasion, Nm23-H1 RNA expression may be associated with local disease progression.

NDP-K/nm23 expression in human breast cancer in relation to relapse, survival, and other prognostic factors: an immunohistochemical study

The nm23 gene was originally identified by Steeg et al. by screening of cDNA libraries from murine melanoma cell lines of varying metastatic potential. An inverse relationship between metastatic potential and nm23 RNA and/or protein was found in four different metastatic model systems. It was proposed that nm23 may function as a suppressor gene for tumour metastasis. It has recently been found that the sequence of nm23 and NDP-kinase (NDP-K) is identical. Using an immunohistochemical technique and employing a polyclonal antibody to purified NDP-K A, we have determined NDP-K expression in a series of 197 breast carcinomas. One hundred and sixty (81.2 per cent) of these tumours were scored positive for NDP-K and 37 (18.8 per cent) scored negative. No relationship was found between NDP-K/nm23 expression and patient relapse or survival. Furthermore, no relationship was found between NDP-K/nm23 expression and a number of other prognostic factors including tumour grade, oestrogen receptor, progesterone receptor, and p53 expression. Our results contradict the hypothesis concerning the possible role of NDP-K/nm23 as a metastatic suppressor gene in human breast cancer, but further studies using antibodies specific for NDP-K/nm23 subtypes are clearly indicated.

Nm23 and breast cancer metastasis

The majority of breast cancer patients succumb to metastatic disease. We summarize published and recent research concerning the nm23 gene in breast cancer metastasis. In a murine developmental study, nm23 expression increased with the functional differentiation of the mammary gland in nulliparous and pregnant animals. In human breast cancer, five studies have now demonstrated a significant association between reduced nm23 expression, at the RNA or protein levels, and aggressive tumor behavior. Nm23-negative tumor cells have been observed in comedo ductal carcinoma in situ lesions in two independent studies, indicating that decreases in nm23 expression begin prior to actual histologically identifiable invasion. Transfection studies, in which human nm23-H1 cDNA was expressed in the metastatic human MDA-MB-435 breast carcinoma cell line, indicate that nm23-H1 suppresses in vivo metastatic potential by 50-90%. Finally, our data in melanoma and breast carcinoma transfection systems suggest that the biochemical mechanism of nm23 suppressive activity is likely not due to its nucleoside diphosphate kinase activity, association with GAP proteins, or secretion from cells.