Analysis of adenosine kinase mutants of baby hamster kidney cells using affinity-purified antibody

A Novel Adenosine Kinase from Bombyx mori: Enzymatic Activity, Structure, and Biological Function

Adenosine kinase (ADK) is the first enzyme in the adenosine remediation pathway that catalyzes adenosine phosphorylation into adenosine monophosphate, thus regulating adenosine homeostasis in cells. To obtain new insights into ADK from Bombyx mori (BmADK), we obtained recombinant BmADK, and analyzed its activity, structure, and function. Gel-filtration showed BmADK was a monomer with molecular weight of approximately 38 kDa. Circular dichroism spectra indicated BmADK had 36.8% α-helix and 29.9% β-strand structures, respectively. The structure of BmADK was stable in pH 5.0–11.0, and not affected under 30 °C. The melting temperature and the enthalpy and entropy changes in the thermal transition of BmADK were 46.51 ± 0.50 °C, 253.43 ± 0.20 KJ/mol, and 0.79 ± 0.01 KJ/(mol·K), respectively. Site-directed mutagenesis demonstrated G68, S201, E229, and D303 were key amino acids for BmADK structure and activity. In particular, S201A mutation significantly increased the α-helix content of BmADK and its activity. BmADK was located in the cytoplasm and highly expressed in the silk gland during the pre-pupal stage. RNA interference revealed the downregulation of BmADK decreased ATG-8, Caspase-9, Ec-R, E74A, and Br-C expression, indicating it was likely involved in 20E signaling, apoptosis, and autophagy to regulate silk gland degeneration and silkworm metamorphosis. Our study greatly expanded the knowledge on the activity, structure, and role of ADK.

A rapid and sensitive colorimetric assay for the determination of adenosine kinase activity

Adenosine kinase (ADK) plays an important role in the growth and development of organisms. A convenient, quick, reliable, sensitive and low-cost assay for ADK activity is of great significance. Here, we found the reaction system with bromothymol blue as the pH indicator had a maximum absorption peak at 614 nm. The absorbance change in 614 nm was positively correlated with the generated hydrogen ions in the reaction catalyzed by ADK. Then, we demonstrated this assay was feasible for ADK activity. Further, we analyzed the effects of buffer, bromothymol blue concentrations on the sensitivity of the assay, and investigated the sensitivity of ADK contents and adenosine concentration on the assay. Finally, we calculated the K_m and Vmax of ADK from Bombyx mori with this assay. Our results suggested this assay was quick, convenient, reliable, sensitive and economic for the activity of ADK. It is an excellent alternative for the conventional ADK assays.

Focused screening to identify new adenosine kinase inhibitors

Adenosine kinase (AdK) is a key player in controlling intra- and extracellular concentrations of the signaling molecule adenosine. Extensive evidence points to an important role of AdK in several diseases, and suggests that AdK inhibition might be a promising therapeutic strategy. The development of a new AdK assay and subsequent screening of part of our focused compound library led to the identification of 12 hit compounds (hit rate of 6%) representing six new classes of non-nucleoside human AdK inhibitors. The most potent inhibitor 1 displayed a K_i value of 184nM. Compound screening with a newly developed assay was useful and efficient for discovering novel AdK inhibitors which may serve as lead structures for developing drugs for adenosine augmentation therapy.

Adenosine kinase: exploitation for therapeutic gain

Adenosine kinase (ADK; EC 2.7.1.20) is an evolutionarily conserved phosphotransferase that converts the purine ribonucleoside adenosine into 5'-adenosine-monophosphate. This enzymatic reaction plays a fundamental role in determining the tone of adenosine, which fulfills essential functions as a homeostatic and metabolic regulator in all living systems. Adenosine not only activates specific signaling pathways by activation of four types of adenosine receptors but it is also a primordial metabolite and regulator of biochemical enzyme reactions that couple to bioenergetic and epigenetic functions. By regulating adenosine, ADK can thus be identified as an upstream regulator of complex homeostatic and metabolic networks. Not surprisingly, ADK dysfunction is involved in several pathologies, including diabetes, epilepsy, and cancer. Consequently, ADK emerges as a rational therapeutic target, and adenosine-regulating drugs have been tested extensively. In recent attempts to improve specificity of treatment, localized therapies have been developed to augment adenosine signaling at sites of injury or pathology; those approaches include transplantation of stem cells with deletions of ADK or the use of gene therapy vectors to downregulate ADK expression. More recently, the first human mutations in ADK have been described, and novel findings suggest an unexpected role of ADK in a wider range of pathologies. ADK-regulating strategies thus represent innovative therapeutic opportunities to reconstruct network homeostasis in a multitude of conditions. This review will provide a comprehensive overview of the genetics, biochemistry, and pharmacology of ADK and will then focus on pathologies and therapeutic interventions. Challenges to translate ADK-based therapies into clinical use will be discussed critically.

Molecular characterization of Chinese hamster cells mutants affected in adenosine kinase and showing novel genetic and biochemical characteristics

BACKGROUND

Two isoforms of the enzyme adenosine kinase (AdK), which differ at their N-terminal ends, are found in mammalian cells. However, there is no information available regarding the unique functional aspects or regulation of these isoforms.

RESULTS

We show that the two AdK isoforms differ only in their first exons and the promoter regions; hence they arise via differential splicing of their first exons with the other exons common to both isoforms. The expression of these isoforms also varied greatly in different rat tissues and cell lines with some tissues expressing both isoforms and others expressing only one of the isoforms. To gain insights into cellular functions of these isoforms, mutants resistant to toxic adenosine analogs formycin A and tubercidin were selected from Chinese hamster (CH) cell lines expressing either one or both isoforms. The AdK activity in most of these mutants was reduced to <5% of wild-type cells and they also showed large differences in the expression of the two isoforms. Thus, the genetic alterations in these mutants likely affected both regulatory and structural regions of AdK. We have characterized the molecular alterations in a number of these mutants. One of these mutants lacking AdK activity was affected in the conserved NxxE motif thereby providing evidence that this motif involved in the binding of Mg2+ and phosphate ions is essential for AdK function. Another mutant, FomR-4, exhibiting increased resistance to only C-adenosine analogs and whose resistance was expressed dominantly in cell-hybrids contained a single mutation leading to Ser191Phe alteration in AdK. We demonstrate that this mutation in AdK is sufficient to confer the novel genetic and biochemical characteristics of this mutant. The unusual genetic and biochemical characteristics of the FomR-4 mutant suggest that AdK in this mutant might be complexed with the enzyme AMP-kinase. Several other AdK mutants were altered in surface residues that likely affect its binding to the adenosine analogs and its interaction with other cellular proteins.

CONCLUSIONS

These AdK mutants provide important insights as well as novel tools for understanding the cellular functions of the two isoforms and their regulation in mammalian cells.

Subcellular localization of adenosine kinase in mammalian cells: The long isoform of AdK is localized in the nucleus

Two isoforms of adenosine kinase (AdK) have been identified in mammalian organisms with the long isoform (AdK-long) containing extra 20-21 amino acids at the N-terminus (NTS). The subcellular localizations of these isoforms are not known and they contain no identifiable targeting sequence. Immunofluorescence labeling of mammalian cells expressing either only AdK-long or both isoforms with AdK-specific antibody showed only nuclear labeling or both nucleus and cytoplasmic labeling, respectively. The AdK-long and -short isoforms fused at the C-terminus with c-myc epitope also localized in the nucleus and cytoplasm, respectively. Fusion of the AdK-long NTS to green fluorescent protein also resulted in its nuclear localization. AdK-long NTS contains a cluster of conserved amino acids (PKPKKLKVE). Replacement of KK in this sequence with either AA or AD abolished its nuclear localization capability, indicating that this cluster likely serves as a nuclear localization signal. AdK in nucleus is likely required for sustaining methylation reactions.

Development of off-line and on-line capillary electrophoresis methods for the screening and characterization of adenosine kinase inhibitors and substrates

Fast and convenient CE assays were developed for the screening of adenosine kinase (AK) inhibitors and substrates. In the first method, the enzymatic reaction was performed in a test tube and the samples were subsequently injected into the capillary by pressure and detected by their UV absorbance at 260 nm. An MEKC method using borate buffer (pH 9.5) containing 100 mM SDS (method A) was suitable for separating alternative substrates (nucleosides). For the CE determination of AMP formed as a product of the AK reaction, a phosphate buffer (pH 7.5 or 8.5) was used and a constant current (95 microA) was applied (method B). The methods employing a fused-silica capillary and normal polarity mode provided good resolution of substrates and products of the enzymatic reaction and a short analysis time of less than 10 min. To further optimize and miniaturize the AK assays, the enzymatic reaction was performed directly in the capillary, prior to separation and quantitation of the product employing electrophoretically mediated microanalysis (EMMA, method C). After hydrodynamic injection of a plug of reaction buffer (20 mM Tris-HCl, 0.2 mM MgCl2, pH 7.4), followed by a plug containing the enzyme, and subsequent injection of a plug of reaction buffer containing 1 mM ATP, 100 microM adenosine, and 20 microM UMP as an internal standard (I.S.), as well as various concentrations of an inhibitor, the reaction was initiated by the application of 5 kV separation voltage (negative polarity) for 0.20 min to let the plugs interpenetrate. The voltage was turned off for 5 min (zero-potential amplification) and again turned on at a constant current of -60 microA to elute the products within 7 min. The method employing a polyacrylamide-coated capillary of 20 cm effective length and reverse polarity mode provided good resolution of substrates and products. Dose-response curves and calculated K(i) values for standard antagonists obtained by CE were in excellent agreement with data obtained by the standard radioactive assay.

Cloning and expression of the adenosine kinase gene from rat and human tissues

Adenosine kinase is ubiquitous in eukaryotes and is a key enzyme in the regulation of the intracellular levels of adenosine, an important physiological effector of many cells and tissues. In this paper we report the cloning of cDNAs encoding adenosine kinase from both rat and human tissues. Two distinct forms of adenosine kinase mRNA were identified in human tissues. Sequence variation between the two forms is restricted to the extreme 5'-end of the adenosine kinase mRNA, including a portion of the coding region, and is consistent with differential splicing of a single transcriptional product. We have expressed both forms in E. coli and produced soluble active enzyme which catalyzes the phosphorylation of adenosine with high specific activity in vitro and is susceptible to known adenosine kinase inhibitors.

Adenosine kinase-deficient mutant of Saccharomyces cerevisiae

A cordycepin-resistant mutant strain of Saccharomyces cerevisiae (CD-R2) was found to be deficient in adenosine kinase. This mutant accumulated S-adenosylhomocysteine during growth in the presence of exogenous adenosine and it grew in a pseudohyphal manner in the presence of this nucleotide.

The bis(adenosin-N6-yl) alkanes, a family of potential dinucleoside polyphosphate analogue precursors. Mechanism of growth inhibition and suppression of adenosine toxicity in lymphoid cells

The potential diadenosine polyphosphate analogue precursor, bis(adenosin-N6-yl)dodecane (A[CH2]12A) (Chen, H. & McLennan, A. G. (1993) Eur. J. Biochem. 213, 935-944.) is equally toxic to both wild-type and adenosine-kinase-deficient BHK cells at concentrations up to 100 microM; at higher concentrations, wild-type cells are more sensitive, as are cells over-expressing adenosine kinase. Thus both the nucleoside and its nucleotide products are toxic. In contrast to adenosine toxicity, the toxicity of A[CH2]12A to S-49 T-lymphoma cells could not be reversed by uridine or by L-homocysteine thiolactone. A[CH2]12A and all its shorter chain bis(adenosin-N6-yl)alkane homologues could relieve the toxicity of low adenosine concentrations (< 20 microM) to S-49 cells, mainly through inhibition of adenosine kinase, while relief of the toxicity of high adenosine concentrations (> 20 microM) required the longer chain homologues. A[CH2]12A at 10 microM completely eliminated adenosine toxicity. Deoxyadenosine toxicity could also be relieved, but only that due to low concentrations (< 4 microM). A[CH2]12A had only a slight stimulatory effect on S-adenosylhomocysteine-hydrolase activity.

The bis(adenosin-N6-yl)alkanes, a family of potential dinucleoside-polyphosphate analogue precursors. Cytotoxicity, adenosine-receptor binding and metabolism

A series of bis(adenosin-N6-yl)alkanes, in which two adenosine residues are linked via their N6 positions by alkyl bridges comprising between 2 and 14 methylene units, were synthesized as potential precursors to dinucleoside-polyphosphate analogues. These compounds were moderately cytotoxic to mammalian cells, the toxicity increasing with the length of the alkyl chain. For example, the dose of bis(adenosin-N6-yl)dodecane, A[CH2]12A, leading to 50% inhibition of cell growth (ID50) for BHK fibroblasts, Walker 256 carcinoma cells and S-49 T-lymphoma cells were 90 +/- 8, 100 +/- 5 and 23 +/- 4 microM respectively. A significant amount of A[CH2]12A bound to serum albumin in the growth media; thus the ID50 for S-49 cells grown in serum-free medium was 9 +/- 2 microM. The corresponding bis-cytidine analogues were much less toxic; however the presence of a second adenosine moeity/molecule had little significant effect on cell growth when compared to N6-alkyladenosines. Toxicity to S-49 cells was unaffected by the nucleoside-transporter inhibitor nitrobenzylthioinosine and was even higher (ID50 = 5 +/- 0.5 microM) towards nucleoside-transport-deficient AE-1 cells, showing that the analogues could pass freely through the plasma membrane. Interaction with A1 adenosine receptors was shown by displacement of [3H]N6-R-phenylisopropyladenosine (Kd = 6 nM) from rat adipocyte membranes, with Ki values of 45, 65, 85 and 390 nM for the compounds containing 12, 8, 6 and 4 methylene units, respectively. Affinity for human platelet membrane A2 adenosine receptors was about 100-fold less, however the compounds were weak A2 agonists, producing up to a threefold increase in intracellular cyclic AMP in WI-38/VA-13 cells. Thus, these compounds behave, not surprisingly, as adenosine analogues. In addition, A[CH2]12A was metabolized in vitro and intracellularly by adenosine kinase (Ki = 70 nM) and adenylate kinase to yield a number of phosphorylated derivatives with the potential to act as diadenosine polyphosphate analogues. One of these, the bismonophosphate, was recognized by and inhibited adenylate kinase more effectively than adenosine(5')tetraphospho(5')adenosine (Ap4A, Ki = 3 microM).

Synthesis and applications of 8-azido photoaffinity analogs of P1,P3-bis(5'-adenosyl)triphosphate and P1,P4-bis(5'-adenosyl)tetraphosphate

32P-labeled photoaffinity analogs of bis(5'-adenosyl)-tetraphosphate and bis(5'-adenosyl)triphosphate which contain a single photoreactive 8-azidoadenosine group distal to the radiolabel have been synthesized from commercially available components using a combination of chemical and enzymatic procedures including a water-soluble carbodiimide. The method is simple, rapid, and produces yields of high specific activity products of around 60%. The analog of bis(5'-adenosyl)-tetraphosphate is very similar to the parent compound in its inhibition of rat liver adenosine kinase and its efficiency as a substrate for the bis(5'-nucleosidyl)tetraphosphate pyrophosphohydrolase from Artemia embryos. In the latter case, ATP and 8-azidoAMP are the preferred products. As would be expected, this analog is a much more effective photoprobe for both adenosine and adenylate kinases than the corresponding analog of bis(5'-adenosyl)triphosphate. Both compounds have been used to photoaffinity label crude extracts of Artemia, Vero cells, and Clostridium acetobutylicum and preferential specific labeling of different polypeptides by each analog has been shown. In extracts of C. acetobutylicum, the labeling of a polypeptide of Mr 48,500 by the bis(5'-adenosyl)tetraphosphate analog was totally dependent on the presence of Co2+ ions. These compounds should therefore prove valuable both for the active site labeling of purified binding proteins and for the detection and identification of new target proteins for these nucleotides.

Imbalance of purine nucleotides in alanosine-resistant baby hamster kidney cells

Human DNA was used to transform adenosine kinase (AK)-deficient BHK cells followed by selection of AK+ cells in medium containing alanosine, adenosine, and uridine (AAU medium). Twenty AAUr isolates were analyzed, and none of them contained AK activity. Several purine salvage enzymes were, however, found to be affected in these cells. The levels of hypoxanthine-guanine phosphoribosyltransferase and adenylosuccinate synthetase activities were elevated, while the adenylosuccinase activity was reduced. AAU-resistance may be explained by elevated activity of adenylosuccinate synthetase to overcome the alanosine block; thus AAUr cells were able to convert exogenous adenosine----inosine----hypoxanthine----IMP----AMPS----AMP. Moreover, these AAUr cells required exogenous purines for growth. HPLC analyses of endogenous nucleotide pools of AAUr cells showed that the levels of adenine nucleotides have diminished to less than 10% of the parental levels. These results suggest that the AAU-resistant mutation, which elicits pleiotropic phenotypes in BHK cells, affects an important component in the regulation of adenine nucleotide synthesis. By including erthyro-9-(2-hydroxy-3-nonyl)adenine in the AAU medium (renamed as AAUE medium) to block deamination of adenosine, AK+ BHK cells were isolated.