Heat and SDS insensitive NDK dimers are largely stabilised by hydrophobic interaction to form functional hexamer in Mycobacterium smegmatis

The primary structure and function of nucleoside diphosphate kinase (NDK), a substrate non-specific enzyme involved in the maintenance of nucleotide pools is also implicated to play pivotal roles in many other cellular processes. NDK is conserved from bacteria to human and forms a homotetramer or hexamer to exhibit its biological activity. However, the nature of the functional oligomeric form of the enzyme differs among different organisms. The functional form of NDKs from many bacterial systems, including that of the human pathogen, Mycobacterium tuberculosis (MtuNDK), is a hexamer, although some bacterial NDKs are tetrameric in nature. The present study addresses the oligomeric property of MsmNDK and how a dimer, the basic subunit of a functional hexamer, is stabilized by hydrogen bonds and hydrophobic interactions. Homology modeling was generated using the three-dimensional structure of MtuNDK as a template; the residues interacting at the monomer-monomer interface of MsmNDK were mapped. Using recombinant enzymes of wild type, catalytically inactive mutant, and monomer-monomer interactive mutants of MsmNDK, the stability of the dimer was verified under heat, SDS, low pH, and methanol. The predicted residues (Gln17, Ser24 and Glu27) were engaged in dimer formation, however the mutated proteins retained the ATPase and GTPase activity even after introducing single (MsmNDK- Q17A, MsmNDK-E27A, and MsmNDK-E27Q) and double (MsmNDK-E27A/Q17A) mutation. However, the monomer-monomer interaction could be abolished using methanol, indicating the stabilization of the monomer-monomer interaction by hydrophobic interaction.

Phase II study of the CPT-11, mitoxantrone and dexamethasone regimen in combination with rituximab in elderly patients with relapsed diffuse large B-cell lymphoma

Standard treatment for elderly patients with relapsed or refractory DLBCL has not been established. CPT-11 has a broad spectrum of anticancer activities including a cytotoxic effect in a variety of malignant tumors. The results of combined treatment with CPT-11 and rituximab have not been reported. The R-CMD regimen was given to elderly patients with relapsed or refractory DLBCL. The safety and efficacy of this regimen were studied. In addition, the serum nm23-H1 level was determined to study whether or not it can serve as a prognostic factor. Thirty elderly patients with DLBCL were studied. The main non-hematological toxicities were infusion-related adverse events. Grade 3/4 hematological toxicity was seen in 19 patients. Following R-CMD treatment, the BNP and troponin T levels did not increase. The CR rate was 57%, PR rate was 17%, 2-year survival rate was 45.2%, and PFS rate was 37.2%. Patients with serum nm23-H1 levels of higher than 80 ng/mL before the treatment showed significantly poorer prognosis. The serum nm23-H1 level of the 30 subjects before the treatment was elevated at 39.4 +/- 41.3 ng/mL, but it significantly decreased only in the subset of patients who achieved CR. The R-CMD regimen was safe in elderly patients with DLBCL. No new signs of cardiotoxicity were observed with this regimen. It was also effective in patients with relapsed or refractory DLBCL who had previously used DXR.

Medroxyprogesterone acetate elevation of Nm23-H1 metastasis suppressor expression in hormone receptor-negative breast cancer

BACKGROUND

Reestablishment of metastasis suppressor gene expression may constitute a therapeutic strategy for high-risk breast cancer patients. We previously showed that medroxyprogesterone acetate (MPA), a progestin that has been tested as treatment for advanced breast cancer, elevates expression of the Nm23-H1 metastasis suppressor gene in hormone receptor-negative metastatic human breast carcinoma cell lines in vitro via a glucocorticoid receptor-based mechanism. Here, we tested whether MPA treatment inhibits metastatic colonization of a hormone receptor-negative breast cancer cell line in vivo.

METHODS

We tested the soft-agar colony-forming efficiency of untransfected MDA-MB-231T human breast carcinoma cells and MDA-MB-231T cells transfected with antisense Nm23-H1 in the presence and absence of MPA. Pharmacokinetic studies were used to establish dose and injection schedules that led to MPA serum levels in mice similar to those achievable in humans. For in vivo studies, nude mice were injected intravenously with MDA-MB-231T cells. After 4 weeks, mice were randomized to control or MPA arms. Endpoints included incidence, number, and size of gross pulmonary metastases; Nm23-H1 protein expression in gross metastases; and side effects. All statistical tests were two-sided.

RESULTS

MPA reduced colony formation of MDA-MB-231T cells by 40%-50% but had no effect on colony formation of Nm23-H1 antisense transfectants. Metastases developed in 100% (95% confidence interval [CI] = 78% to 100% and 77% to 100%, respectively) of control mice injected with MDA-MB-231T cells. In two independent experiments, only 73% (95% CI = 45% to 92%) and 64% (95% CI = 35% to 87%) of mice injected with 2 mg of MPA developed metastases. Mice injected with 2 mg of MPA showed reductions in the mean numbers, per mouse, of all metastases and of large (>3 mm) metastases (P = .04 and .013, respectively). Nm23-H1 was expressed at high levels in 43% of pulmonary metastases in MPA-treated mice but only 13% of metastases in untreated mice. Mice receiving at least 1-mg doses of MPA gained more weight than control-treated mice but exhibited no bone density alterations or abnormal mammary fat pad histology.

CONCLUSION

Our preclinical results show that MPA appears to elevate Nm23-H1 metastasis suppressor gene expression, thereby reducing metastatic colonization. The data suggest a new use for an old agent in a molecularly defined subset of breast cancer patients.

Nm23H2 facilitates coat protein complex II assembly and endoplasmic reticulum export in mammalian cells

The cytosolic coat protein complex II (COPII) mediates vesicle formation from the endoplasmic reticulum (ER) and is essential for ER-to-Golgi trafficking. The minimal machinery for COPII assembly is well established. However, additional factors may regulate the process in mammalian cells. Here, a morphological COPII assembly assay using purified COPII proteins and digitonin-permeabilized cells has been applied to demonstrate a role for a novel component of the COPII assembly pathway. The factor was purified and identified by mass spectrometry as Nm23H2, one of eight isoforms of nucleoside diphosphate kinase in mammalian cells. Importantly, recombinant Nm23H2, as well as a catalytically inactive version, promoted COPII assembly in vitro, suggesting a noncatalytic role for Nm23H2. Consistent with a function for Nm23H2 in ER export, Nm23H2 localized to a reticular network that also stained for the ER marker calnexin. Finally, an in vivo role for Nm23H2 in COPII assembly was confirmed by isoform-specific knockdown of Nm23H2 by using short interfering RNA. Knockdown of Nm23H2, but not its most closely related isoform Nm23H1, resulted in diminished COPII assembly at steady state and reduced kinetics of ER export. These results strongly suggest a previously unappreciated role for Nm23H2 in mammalian ER export.

Higher intracellular levels of uridinemonophosphate under nitrogen-limited conditions enhance metabolic flux of curdlan synthesis in Agrobacterium species

Changes of intracellular nucleotide levels and their stimulatory effects on curdlan synthesis in Agrobacterium species were investigated under different culture conditions. Under nitrogen-limited conditions where curdlan synthesis was stimulated, intracellular levels of UMP were as high as 87 and those of AMP were 78 nmol/mg of cellular protein, while those under nitrogen-sufficient conditions were lower than 45 nmol/mg-protein. The levels of other nucleotides such as UDP, UTP, UDP-glucose, ADP, ATP, and ADP-glucose were lower than 30 nmol/mg-protein under both nitrogen-limited and sufficient conditions. The time profiles of curdlan synthesis and cellular nucleotide levels showed that curdlan synthesis had a positive relationship with intracellular levels of UMP and AMP. After the ammonium concentration in the medium fell below 0.1 g/L, intracellular levels of UMP and AMP increased, followed by curdlan synthesis. However, no significant changes in the specific activities of UMP kinase, UDP kinase, and UDP-glucose pyrophosphorylase were observed during cultivation. In vitro enzyme reactions for the synthesis of UDP-glucose, which serve as a precursor for curdlan synthesis, demonstrated that the synthesis of UDP-glucose increased with the increase of UMP concentration. In contrast, AMP had no effect on UDP-glucose synthesis at all. Addition of UMP in the medium increased the curdlan synthesis, whereas curdlan synthesis was inhibited in the presence of AMP. From these results, we concluded that only the higher intracellular UMP levels caused by nitrogen limitation in the medium enhance the metabolic flux of curdlan synthesis by promoting cellular UDP-glucose synthesis.

Interaction of recombinant rat nucleoside diphosphate kinase alpha with bleached bovine retinal rod outer segment membranes: a possible mode of pH and salt effects

An attempt was made to reveal the mode of action of protons and salts on the recently discovered GTP gamma S-dependent interaction of bovine retinal rod outer segments (ROS)1 nucleoside diphosphate kinase (NDP kinase) with the complex between bleached visual receptor rhodopsin and retinal G-protein transducin in bovine ROS membranes. The properties of recombinant rat NDP kinase alpha, that is immunologically similar to the soluble NDP kinase from bovine ROS preparation, have been studied in solution by means of protein fluorescence at different pH and salt concentrations and results were compared with pH and salt effects on the binding of NDP kinase alpha to bleached bovine ROS membranes. The results suggest that NDP kinase alpha itself may serve as a target for protons and salts and mediates their effects on the interaction between the enzyme and ROS membranes.

Thermal stability of hexameric and tetrameric nucleoside diphosphate kinases. Effect of subunit interaction

The eukaryotic nucleoside diphosphate (NDP) kinases are hexamers, while the bacterial NDP kinases are tetramers made of small, single domain subunits. These enzymes represent an ideal model for studying the effect of subunit interaction on protein stability. The thermostability of NDP kinases of each class was studied by differential scanning calorimetry and biochemical methods. The hexameric NDP kinase from Dictyostelium discoideum displays one single, irreversible differential scanning calorimetry peak (Tm 62 degrees C) over a broad protein concentration, indicating a single step denaturation. The thermal stability of the protein was increased by ADP. The P105G substitution, which affects a loop implicated in subunit contacts, yields a protein that reversibly dissociates to folded monomers at 38 degrees C before the irreversible denaturation occurs (Tm 47 degrees C). ADP delays the dissociation, but does not change the Tm. These data indicate a "coupling" of the quaternary structure with the tertiary structure in the wild-type, but not in the mutated protein. We describe the x-ray structure of the P105G mutant at 2.2-A resolution. It is very similar to that of the wild-type protein. Therefore, a minimal change in the structure leads to a dramatic change of protein thermostability. The NDP kinase from Escherichia coli behaves like the P105G mutant of the Dictyostelium NDP kinase. The detailed study of their thermostability is important, since biological effects of thermolabile NDP kinases have been described in several organisms.

Inhibition by cromoglycate and some flavonoids of nucleoside diphosphate kinase and of exocytosis from permeabilized mast cells

1. The anti-allergic compound, cromoglycate, is reported to possess affinity for, and to suppress the autophosphorylation of a 72kDa protein having the sequence of nucleoside diphosphate kinase (NDPK). 2. We have tested the ability of cromoglycate, and a panel of ten structurally related flavonoids of plant origin, to inhibit the NDPK reaction and the exocytotic process of permeabilized mast cells. The conditions of permeabilization (use of an isotonic medium based on sodium glutamate) were selected to ensure that NDPK activity would be an essential component in the induction of Ca(2+)-induced exocytosis in which ATP is required for generation of GTP. For comparison, we also measured the inhibition of exocytosis induced by GTP-gamma-S; this proceeds in the absence of ATP and bypasses the need for NDPK activity. 3. We found that cromoglycate does not discriminate between Ca2+ and GTP-gamma-S-induced exocytosis and is a poor inhibitor of NDPK activity. Concentrations in the millimolar range are required for inhibition of all these functions. By comparison, many of the flavonoids are effective at concentrations in the micromolar range. 4. While we were unable to discern any systematic relationships between their ability to inhibit the three functions, two compounds, quercetin and genistein, inhibit Ca(2+)-induced, but not GTP-gamma-S-induced exocytosis. Inhibition of the late stages of the stimulus-response pathway in mast cells by these compounds is therefore likely to be due to inhibition of NDPK and the consequent failure to generate GTP.

Activation of nucleoside diphosphate kinase by mastoparan, a peptide isolated from wasp venom

We have previously reported that GDP-bound alpha beta gamma-trimeric GTP-binding (G) proteins can be converted into the active GTP-bound form with nucleoside diphosphate (NDP) kinase and ATP, although its exact activation mechanism still remains to be resolved. In the present study, we investigated whether NDP kinase activity was modified by mastoparan, a wasp venom peptide that is known to activate G proteins as an agonist-receptor complex. The activity of NDP kinase measured by the formation of GTP from ATP and GDP was markedly stimulated, when the kinase was incubated with mastoparan. The concentration of mastoparan required for the activation was much lower than that observed for the peptide-induced activation of G proteins under similar assay conditions. There was also an increase in the phosphorylated intermediate of NDP kinase as well as the catalytic activity upon its incubation with mastoparan. These results suggest that mastoparan not only activates G proteins directly via guanine nucleotide exchange reaction but also stimulates NDP kinase activity.