N 6 -( 2 -isopentenyl)adenosine metabolism in man

Phytohormone metabolism in human cells: Cytokinins are taken up and interconverted in HeLa cell culture

Cytokinins (CKs) encompass a group of phytohormones, known to orchestrate many critical processes in plant development. Excluding Archaea, CKs are pervasive among all kingdoms, but much less is reported about their metabolism beyond plants. Recent evidence from mammalian tissues indicates the presence of six additional CK forms beyond the previously identified, single mammalian CK, N6-isopentenyladenosine (i6A). There is limited understanding of CK biosynthesis pathways in mammalian systems; therefore, human cervical cancer (HeLa) cells were used to further characterize CK processing by tracking the interconversion of CKs into their various structural derivatives in mammalian cells in a time-course study. Through high-performance liquid chromatography-positive electrospray ionization-tandem mass spectrometry (HPLC-(+ESI)-MS/MS), we document changes in the functional profiles of endogenous CKs in a human cell line following metabolism by HeLa cell cultures. The nucleotide CK fraction (iPRP) was found exclusively within the cell pellet (0.34 pmol/106 cells), and the active free base (FB) form (iP) and riboside fraction (iPR) were found in greater abundance extracellularly (1.67 and 0.10 nmol/L respectively). For further confirmation, we demonstrate that HeLa cells metabolize an exogenously supplied CK, N6-benzyladenosine (BAR). In the HeLa culture supernatant, a 12-fold decrease in BAR concentration was observed within the first 24 hours of incubation accompanied by a fivefold increase in the FB form, N6-benzyladenine (BA). These findings support the hypothesis that HeLa cells have the enzymatic pathways required for the metabolism of both endogenous and exogenous CKs.

The natural cytokinin 2OH3MeOBAR induces cell death by a mechanism that is different from that of the "classical" cytokinin ribosides

Cytokinin ribosides (N⁶-substituted adenosines) have demonstrated anticancer activity in various cultured cell lines, several xenografts and even a small clinical trial. Effects of kinetin riboside, N⁶-benzyladenosine (BAR) and N⁶-isopentenyladenosine on various parameters related to apoptosis have also been reported, but not directly compared with those of the highly active naturally occurring aromatic cytokinins oTR (ortho-topolin riboside) and 2OH3MeOBAR (N⁶-(2-hydroxy-3-methoxybenzyl)adenosine). Here we show that 2OH3MeOBAR is the most active cytokinin riboside studied to date (median, 1st quartile, 3rd quartile and range of GI50 in tests with the NCI60 cell panel: 0.19, 0.10, 0.43 and 0.02 to 15.7 μM, respectively) and it differs from other cytokinins by inducing cell death without causing pronounced ATP depletion. Analysis of NCI60 test data suggests that its activity is independent of p53 status. Further we demonstrate that its 5'-monophosphate, the dominant cancer cell metabolite, inhibits the candidate oncogene DNPH1. Synthesis, purification, HPLC-MS identification and HPLC-UV quantification of 2OH3MeOBAR metabolites are also reported.

N(6)-isopentenyladenosine and analogs activate the NRF2-mediated antioxidant response

N⁶-isopentenyladenosine (i⁶A), a naturally occurring modified nucleoside, inhibits the proliferation of human tumor cell lines in vitro, but its mechanism of action remains unclear. Treatment of MCF7 human breast adenocarcinoma cells with i⁶A or with three synthetic analogs (allyl⁶A, benzyl⁶A, and butyl⁶A) inhibited growth and altered gene expression. About 60% of the genes that were differentially expressed in response to i⁶A treatment were also modulated by the analogs, and pathway enrichment analysis identified the NRF2-mediated oxidative stress response as being significantly modulated by all four compounds. Luciferase reporter gene assays in transfected MCF7 cells confirmed that i⁶A activates the transcription factor NRF2. Assays for cellular production of reactive oxygen species indicated that i⁶A and analogs had antioxidant effects, reducing basal levels and inhibiting the H₂O₂- or 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced production in MCF7 or dHL-60 (HL-60 cells induced to differentiate along the neutrophilic lineage) cell lines, respectively. In vivo, topical application of i⁶A or benzyl⁶A to mouse ears prior to TPA stimulation lessened the inflammatory response and significantly reduced the number of infiltrating neutrophils. These results suggest that i⁶A and analogs trigger a cellular response against oxidative stress and open the possibility of i⁶A and benzyl⁶A being used as topical anti-inflammatory drugs.

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i⁶A and analogs (allyl⁶A, benzyl⁶A and butyl⁶A) inhibit growth of MCF7 cells.

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They activate NRF2-mediated oxidative stress response.

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They inhibit ROS production in MCF7 or dHL-60 cells treated with H₂O₂ or TPA.

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In vivo topical application of i⁶A or benzyl⁶A reduces TPA-induced inflammation.

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i⁶A and benzyl⁶A have potential as topical antioxidant and anti-inflammatory drugs.

Putative role of the adenosine A(3) receptor in the antiproliferative action of N (6)-(2-isopentenyl)adenosine

We tested a panel of naturally occurring nucleosides for their affinity towards adenosine receptors. Both N⁶-(2-isopentenyl)adenosine (IPA) and racemic zeatin riboside were shown to be selective human adenosine A₃ receptor (hA₃R) ligands with affinities in the high nanomolar range (K_i values of 159 and 649 nM, respectively). These values were comparable to the observed K_i value of adenosine on hA₃R, which was 847 nM in the same radioligand binding assay. IPA also bound with micromolar affinity to the rat A₃R. In a functional assay in Chinese hamster ovary cells transfected with hA₃R, IPA and zeatin riboside inhibited forskolin-induced cAMP formation at micromolar potencies. The effect of IPA could be blocked by the A₃R antagonist VUF5574. Both IPA and reference A₃R agonist 2-chloro-N⁶-(3-iodobenzyl)adenosine-5′-N-methylcarboxamide (Cl-IB-MECA) have known antitumor effects. We demonstrated strong and highly similar antiproliferative effects of IPA and Cl-IB-MECA on human and rat tumor cell lines LNCaP and N1S1. Importantly, the antiproliferative effect of low concentrations of IPA on LNCaP cells could be fully blocked by the selective A₃R antagonist MRS1523. At higher concentrations, IPA appeared to inhibit cell growth by an A₃R-independent mechanism, as was previously reported for other A₃R agonists. We used HPLC to investigate the presence of endogenous IPA in rat muscle tissue, but we could not detect the compound. In conclusion, the antiproliferative effects of the naturally occurring nucleoside IPA are at least in part mediated by the A₃R.

Anticancer activity of natural cytokinins: a structure-activity relationship study

Cytokinin ribosides (N(6)-substituted adenosine derivatives) have been shown to have anticancer activity both in vitro and in vivo. This study presents the first systematic analysis of the relationship between the chemical structure of cytokinins and their cytotoxic effects against a panel of human cancer cell lines with diverse histopathological origins. The results confirm the cytotoxic activity of N(6)-isopentenyladenosine, kinetin riboside, and N(6)-benzyladenosine and show that the spectrum of cell lines that are sensitive to these compounds and their tissues of origin are wider than previously reported. The first evidence that the hydroxylated aromatic cytokinins (ortho-, meta-, para-topolin riboside) and the isoprenoid cytokinin cis-zeatin riboside have cytotoxic activities is presented. Most cell lines in the panel showed greatest sensitivity to ortho-topolin riboside (IC(50)=0.5-11.6 microM). Cytokinin nucleotides, some synthesized for the first time in this study, were usually active in a similar concentration range to the corresponding ribosides. However, cytokinin free bases, 2-methylthio derivatives and both O- and N-glucosides showed little or no toxicity. Overall the study shows that structural requirements for cytotoxic activity of cytokinins against human cancer cell lines differ from the requirements for their activity in plant bioassays. The potent anticancer activity of ortho-topolin riboside (GI(50)=0.07-84.60 microM, 1st quartile=0.33 microM, median=0.65 microM, 3rd quartile=1.94 microM) was confirmed using NCI(60), a standard panel of 59 cell lines, originating from nine different tissues. Further, the activity pattern of oTR was distinctly different from those of standard anticancer drugs, suggesting that it has a unique mechanism of activity. In comparison with standard drugs, oTR showed exceptional cytotoxic activity against NCI(60) cell lines with a mutated p53 tumour suppressor gene. oTR also exhibited significant anticancer activity against several tumour models in in vivo hollow fibre assays.

Urinary nucleosides

The methods of analysis, origins, and clinical significance of urinary nucleosides are reviewed through 1997. Structures, chromatographic and mass spectral data and references to the clinical literature are presented for each of the 57 nucleosides currently identified in normal and pathogenic human urine samples. Data from the HPLC separation and GC/MS analysis of 37 individual HPLC fractions are presented and discussed. Methods, including sample preparation techniques, used for the analysis of urinary nucleosides including GC, HPLC, GC/MS, HPLC/MS and immunoassays are compared and the advantages and limitations of each method described. The conclusion is drawn that the urinary nucleosides do serve as biomarkers of cancer and other diseases, but analytical methods need further improvement if clinical decisions are to be made based on the levels of nucleosides in human urine.

Metabolism of 1-methyladenosine

1-[methyl-8-14C] Adenosine was synthesized and its metabolic fate was determined in intact rat. It was found that approximately 57% of 1-[methyl-8-14C] adenosine administered iv was excreted unchanged in the urine and 33% of the excreted radioactivity in the urine was associated with the major metabolite 1-methyl-hypoxanthine and about 4.5% was associated with 1-methylinosine. Very little adenosine or N6-methyladenosine was formed. It is concluded that 1-methyladenosine is initially deaminated by adenosine deaminase to 1-methylinosine which is then cleaved by nucleoside phosphorylase to 1-methylhypoxanthine.

Cytotoxic effects of Eleutherococcus senticosus aqueous extracts in combination with N6-(delta 2-isopentenyl)-adenosine and 1-beta-D-arabinofuranosylcytosine against L1210 leukemia cells

The use of the aqueous extracts of Eleutherococcus senticosus in combination with either cytarabine or N6-(delta 2-isopentenyl)-adenosine gave additive antiproliferative effects against L1210 murine leukemia. The ED50 for E. senticosus root extracts against L1210 cells was approximately 75 micrograms/mL. E. senticosus appears to be potentially useful for reducing the concentration of conventional antimetabolites used for their antiproliferative effects on tumor cells.

Enhancement of the antitumor activity of N6-(delta 2-isopentenyl)adenosine against cultured L-1210 leukemia cells by pentostatin using a polymeric delivery system

The adenosine deaminase inhibitor pentostatin (I), recently shown to be effective in the treatment of several types of acute and chronic human leukemias, was impregnated in a silicone polymer monolithic disk device for release in vitro in the presence of the antitumor nucleoside N6-(delta 2-isopentenyl)adenosine (II) against mouse L-1210 lymphocytic leukemia cells. Although I is ineffective alone against L-1210 cells, controlled release from the polymeric delivery matrix potentiates the antiproliferative effects of II during the midlog phase of growth (48 hr). Cytotoxicity is prolonged, leading to total cell death during the stationary phase of growth (96 hr). The present study suggests that polymeric delivery systems be used for controlled release of oncologic agents, alone or in combination with inhibitors, especially where liability is a concern.

Antineoplastic effects of N6-(delta 2-isopentenyl)adenosine against L-1210 mouse lymphocytic leukemic cells using a polymeric delivery system

N6-(delta 2-Isopentenyl)adenosine (I), a nucleoside previously shown to be cytotoxic against several types of tumor cells, was impregnated in silicone polymer monolithic disc devices for release in vitro against lymphocytic mouse leukemia cells. Plotting the cumulative amount of N6-(delta 2-isopentenyl)adenosine released per unit area of the device versus the square root of time revealed a linear relationship. However, the higher loading dose tended to rapidly release any drug deposited on the polymer surface. The optimum loading dose of the device for the most effective antileukemic activity in 24 hr was calculated based on a plot of the release rate versus the square root of an initial loading dose. The silicone polymer--I delivery system enabled a sustained and controllable release of additional agent. It was thus possible to achieve virtually total inhibition of leukemia cell replication using the polymeric delivery system. Increased concentrations of I, without the use of the polymeric system, resulted in maximum 24 hr inhibition of only approximately 81%, followed by a decline in overall antileukemic activity. It is possible to achieve a more predictable release rate of N6-(delta 2-isopentenyl)adenosine and corresponding antileukemic activity using a polymeric delivery system against L-1210 mouse leukemic cells in vitro. The relative data indicate the ED50 concentrations to be considerably less using the polymeric delivery system.

Metabolic fate of N6-benzyladenosine and N6-benzyladenosine-5'-phosphate in rats

The radiolabeled antitumor nucleoside (14C-8)-N6-benzyladenosine and its (14C-8)-5'-phosphate were administered to rats intravenously, and their metabolic fate was studied. Twenty-nine percent of the radioactivity was recovered in the 48-hr urine collection after (14C-8)-N6-benzyladenosine administration. The following metabolites were isolated: unchanged N6-benzyladenosine (20%), adenine (12%), uric acid (5%), and N6-benzyladenine (0.3%). In the case of (14C-8)-N6-benzyladenosine-5'-phosphate, a total of 28% of the radioactivity was recovered in the 48-hr urine collection and the following metabolites were isolated: N6-benzyladenosine (40%), uric acid (12%), adenine (trace), and unidentified urea derivatives (30%). Metabolism of N6-benzyladenosine appears to involve N-debenzylation to some extent, followed by conversion to adenine and uric acid. N6-Benzyladenosine and its 5'-phosphate differ from other adenosine analogs in being retained in significant amounts by the animals.

Adenosine phosphyorylase activity as distinct from inosine-guanosine phosphorylase activity in Sarcoma 180 cells and rat liver

Adenosine phosphorylase (EC 2.4.2.-) activity present in Sarcoma 180 cells grown in culture and in rat liver, is shown to be distinct from inosine-guanosine phosphorylase by several criteria: (a) treatment of Sarcoma 180 cell extract with p-chloromercuribenzoate inhibited the two activities to a different extent, (b) adenine selectively protected the adenosine phosphorylase activity of Sarcoma 180 and rat liver extract against heat inactivation, while hypoxanthine selectively protected inosine-guanosine phosphorylase activity, (c) at nearly saturating substrate concentrations and using Sarcoma 180 extract, the rates of ribosylation of a mixture of adenine + hypoxanthine or adenine + guanine, but not of hypoxanthine + guanine, were found to be almost equal to the sum of their individual rates as measured separately, (d) inosine selectively inhibited the ribosylation of hypoxanthine and guanine catalysed by Sarcoma 180 and rat liver extract while 2-chloroadenosine selectively inhibited the ribosylation of adenine and N6-furfuryladenine, (e) pH vs. activity curves were similar with hypoxanthine or guanine as the substrate but they were markedly different from the curve with adenine as the substrate. The potential role of adenosine phosphorylase activity in vivo is discussed.

Biosynthesis and cytokinin activity of 8-hydroxy and 2,8-dihydroxy derivatives of zeatin and N-6(increment-2-isopentenyl)adenine

8-Hydroxy and 2,8-dihydroxy derivatives of the cytokinins, 6-(4-hydroxy-3-methyl-trans-2-butenylamino)purine and N-6-(increment -2-isopentenyl)adenine, have been biosynthesized by xanthine oxidase oxidation. 8-Hydroxy derivatives have been shown to be the major intermdeiates. These compounds were tested for cytokinin activity in the tobacco bioassay. The results suggest that substitution of the 8 position with a hydroxyl group causes less decrease of cytokinin activity than substitution of both the 2 and 8 positions with hydroxyl groups.

Studies on the coronary dilator actions of some adenosine analogues

1 The cardiovascular actions of 23 adenosine analogues have been examined in anaesthetized open thorax dogs; the analogues were substituted in the 2-position of the purine ring, or in the exocyclic amino group, or were modified in the imidazole or sugar rings.2 The effects of these compounds on coronary blood flow, peripheral blood pressure, and heart rate were compared with those of adenosine.3 9-beta-D-Arabinofuranosyladenine had no cardiovascular action; the other analogues on intra-atrial administration caused an immediate increase in coronary blood flow, the magnitude and duration of which varied with the structures of the analogues.4 2-Fluoro-, 2-bromo-, 2-isobutylthio-, 2-ethylamino-, and 5'-deoxy-5'-chloro- adenosines had coronary dilator potencies equal to or greater than that of adenosine. No relationship was found between the dilator potency of the adenosine analogues and their duration of coronary dilator action.5 The coronary dilator action of adenosine was potentiated by inosine, 9-beta-D-arabinofurano-syladenine, tubercidin, N(6)-methyladenosine and 2-trifluoromethyl-N(6)-methyladenosine.6 There was no correlation between the substrate specificities of the shorter-acting analogues for adenosine deaminase or adenosine kinase and their duration of coronary dilator action.7 It is proposed that in the anaesthetized dog, uptake into tissues is a more important mode of removal of adenosine and adenosine analogues from the vascular system than inactivation by adenosine deaminase, that the duration of coronary dilator action of the analogues is related primarily to their specificity for the carrier which mediates adenosine uptake, and that the adenosine carrier is not associated with kinase action.