N6-isopentenyladenosine inhibits aerobic glycolysis in glioblastoma cells by targeting PKM2 expression and activity

Glioblastoma (GBM) is a primary tumor in the central nervous system with poor prognosis. It exhibits elevated glucose uptake and lactate production. This metabolic state of aerobic glycolysis is known as the Warburg effect. N6-isopentenyladenosine (iPA), a natural cytokine modified with an isopentenyl moiety derived from the mevalonate pathway, has well-established anti-tumor activity. It inhibits cell proliferation in glioma cells, inducing cell death by apoptosis and/or necroptosis. In the present study, we found that iPA inhibits aerobic glycolysis in unmodified U87MG cells and in the same cell line engineered to over-express wild-type epidermal growth factor receptor (EGFR) or EGFR variant III (vIII), as well as in a primary GBM4 patient-derived cell line. The detection of glycolysis showed that iPA treatment suppressed ATP and lactate production. We also evaluated the response of iPA treatment in normal human astrocyte primary cells, healthy counterpart cells of the brain. Aerobic glycolysis in treated normal human astrocyte cells did not show significant changes compared to GBM cells. To determine the mechanism of iPA action on aerobic glycolysis, we investigated the expression of certain enzymes involved in this metabolic pathway. We observed that iPA reduced the expression of pyruvate kinase M2 (PKM2), which plays a key role in the regulation of aerobic glycolysis, promoting tumor cell proliferation. The reduction of PKM2 expression is a result of the inhibition of the inhibitor of nuclear factor kappa-B kinase subunit, beta/nuclear factor-kappa B pathway upon iPA treatment. In conclusion, these experimental results show that iPA may inhibit aerobic glycolysis of GBM in stabilized cell lines and primary GBM cells by targeting the expression and activity of PKM2.

N6-Isopentenyladenosine Hinders the Vasculogenic Mimicry in Human Glioblastoma Cells through Src-120 Catenin Pathway Modulation and RhoA Activity Inhibition

BACKGROUND

Vasculogenic mimicry (VM) is a functional microcirculation pattern formed by aggressive tumor cells. Thus far, no effective drugs have been developed to target VM. Glioblastoma (GBM) is the most malignant form of brain cancer and is a highly vascularized tumor. Vasculogenic mimicry represents a means whereby GBM can escape anti-angiogenic therapies.

METHODS

Here, using an in vitro tube formation assay on Matrigel, we evaluated the ability of N6-isopentenyladenosine (iPA) to interfere with vasculogenic mimicry (VM). RhoA activity was assessed using a pull-down assay, while the modulation of the adherens junctions proteins was analyzed by Western blot analysis.

RESULTS

We found that iPA at sublethal doses inhibited the formation of capillary-like structures suppressing cell migration and invasion of U87MG, U343MG, and U251MG cells, of patient-derived human GBM cells and GBM stem cells. iPA reduces the vascular endothelial cadherin (VE-cadherin) expression levels in a dose-dependent manner, impairs the vasculogenic mimicry network by modulation of the Src/p120-catenin pathway and inhibition of RhoA-GTPase activity.

CONCLUSIONS

Taken together, our results revealed iPA as a promising novel anti-VM drug in GBM clinical therapeutics.

A highly efficient organogenesis protocol based on zeatin riboside for in vitro regeneration of eggplant

BACKGROUND

Efficient organogenesis induction in eggplant (Solanum melongena L.) is required for multiple in vitro culture applications. In this work, we aimed at developing a universal protocol for efficient in vitro regeneration of eggplant mainly based on the use of zeatin riboside (ZR). We evaluated the effect of seven combinations of ZR with indoleacetic acid (IAA) for organogenic regeneration in five genetically diverse S. melongena and one S. insanum L. accessions using two photoperiod conditions. In addition, the effect of six different concentrations of indolebutyric acid (IBA) in order to promote rooting was assessed to facilitate subsequent acclimatization of plants. The ploidy level of regenerated plants was studied.

RESULTS

In a first experiment with accessions MEL1 and MEL3, significant (p < 0.05) differences were observed for the four factors evaluated for organogenesis from cotyledon, hypocotyl and leaf explants, with the best results obtained (9 and 11 shoots for MEL1 and MEL3, respectively) using cotyledon tissue, 16 h light / 8 h dark photoperiod conditions, and medium E6 (2 mg/L of ZR and 0 mg/L of IAA). The best combination of conditions was tested in the other four accessions and confirmed its high regeneration efficiency per explant when using both cotyledon and hypocotyl tissues. The best rooting media was R2 (1 mg/L IBA). The analysis of ploidy level revealed that between 25 and 50% of the regenerated plantlets were tetraploid.

CONCLUSIONS

An efficient protocol for organogenesis of both cultivated and wild accessions of eggplant, based on the use of ZR, is proposed. The universal protocol developed may be useful for fostering in vitro culture applications in eggplant requiring regeneration of plants and, in addition, allows developing tetraploid plants without the need of antimitotic chemicals.

Regulation of maize kernel weight and carbohydrate metabolism by abscisic acid applied at the early and middle post-pollination stages in vitro

Abscisic acid (ABA) accumulates in plants under drought stress, but views on the role of ABA in kernel formation and abortion are not unified. The response of the developing maize kernel to exogenous ABA was investigated by excising kernels from cob sections at four days after pollination and culturing in vitro with different concentrations of ABA (0, 5, 10, 100μM). When ABA was applied at the early post-pollination stage (EPPS), significant weight loss was observed at high ABA concentration (100μM), which could be attributed to jointly affected sink capacity and activity. Endosperm cells and starch granules were decreased significantly with high concentration, and ABA inhibited the activities of soluble acid invertase and acid cell wall invertase, together with earlier attainment of peak values. When ABA was applied at the middle post-pollination stage (MPPS), kernel weight was observably reduced with high concentration and mildly increased with low concentration, which was regulated due to sink activity. The inhibitory effect of high concentration and the mild stimulatory effect of low concentration on sucrose synthase and starch synthase activities were noted, but a peak level of ADP-glucose pyrophosphorylase (AGPase) was stimulated in all ABA treatments. Interestingly, AGPase peak values were advanced by low concentration and postponed by high concentration. In addition, compared with the control, the weight of low ABA concentration treatments were not statistically significant at the two stages, whereas weight loss from high concentration applied at EPPS was considerably obvious compared with that of the MPPS, but neither led to kernel abortion. The temporal- and dose-dependent impacts of ABA reveal a complex process of maize kernel growth and development.

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.

Biological Activity of Vegetal Extracts Containing Phenols on Plant Metabolism

The influence of vegetal extracts derived from red grape, blueberry fruits and hawthorn leaves on Zea mays L. plant growth and the activity of phenylalanine ammonia-lyase (PAL), a key enzyme of the phenylpropanoid pathway, was investigated in laboratory experiments. The extracts were characterized using FT-IR and Raman spectroscopies in order to obtain a pattern of the main functional groups. In addition, phenols content was determined by HPLC, whereas the content of indoleacetic acid and isopentenyladenosine hormones was determined by ELISA test and the auxin and gibberellin-like activities by plant-bioassays. The treated maize revealed increased root and leaf biomass, chlorophyll and sugars content with respect to untreated plants. Hawthorn, red grape skin and blueberry at 1.0 mL/L induced high p-coumaric content values, whilst hawthorn also showed high amounts of gallic and p-hydroxybenzoic acids. PAL activity induced by hawthorn at 1.0 mL/L had the highest values (11.1-fold UNT) and was strongly and linearly related with the sum of leaf phenols. Our results suggest that these vegetal extracts contain more than one group of plant-promoting substances.

Plant regeneration and biochemical accumulation of hydroxybenzoic and hydroxycinnamic acid derivatives in Hypoxis hemerocallidea organ and callus cultures

Micropropagation of Hypoxis hemerocallidea Fisch. and C.A. Mey was used as a model system to study the influence of cytokinins (CKs) on plant regeneration and biochemical accumulation of hydroxybenzoic and hydroxycinnamic acid derivatives in organ and callus cultures and their antioxidant activity. Fourteen free phenolic acids were detected using ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) while antioxidant activity was evaluated using oxygen radical absorbance capacity (ORAC) and 2,2-diphenyl-1-picryl hydrazyl (DPPH) radical scavenging activity. Cytokinins had a significant effect on the biochemical accumulation of hydroxybenzoic and hydroxycinnamic acid derivatives in H. hemerocallidea organ cultures. In particular, meta-topolin-treated organ cultures produced high concentrations of gallic, protocatechuic, gentisic, p-hydroxybenzoic, m-hydroxybenzoic, salicylic, chlorogenic and trans-cinnamic acids. The isoprenoid CK, N(6)-(2-isopentenyl)-adenine significantly increased the accumulation of hydroxycinnamic acid derivatives, namely, caffeic, p-coumaric, sinapic and ferulic acids. Cytokinin-treated organ cultures exhibited a significant increase in antioxidant activity, particularly in the ORAC model. In callus cultures, CKs decreased the concentrations of hydroxycinnamic acid derivatives and antioxidant activity when compared to the control. Overall, both CK type and concentration had a significant effect on plant regeneration, callus proliferation, biochemical accumulation of free phenolic acids and antioxidant activity of the resultant extracts.

N6-isopentenyladenosine affects cytotoxic activity and cytokines production by IL-2 activated NK cells and exerts topical anti-inflammatory activity in mice

N6-isopentenyladenosine (iPA) is a modified adenosine with an isopentenyl moiety derived from the mevalonate pathway which displays pleiotropic biological effects, including anti-tumor and anti-angiogenic activity. Previous evidence revealed a biphasic effect of iPA on phytohemagglutinin-stimulated lymphocytes, being pro-proliferative at low doses and anti-proliferative at high doses. Analogously, we have recently shown that low iPA concentrations (<1μM) increased the immune response of natural killer (NK) cells against cancer targets. In the present study, we evaluated the effect of iPA at high concentration (10μM) on IL-2-activated NK cells. iPA, inhibited NK cell proliferation and cytotoxicity against their conventional tumor target, human K562 cells. This inhibition was associated with decreased expression and functionality of NK cell activating receptors NKp44 and NKG2D as well as impaired cyto/chemokines secretion (RANTES, MIP-1α, TNF-α and IFN-γ). ERK/MAPK and STAT5 activation in IL-2-activated NK cells were inhibited by iPA. The results obtained in vitro were validated in vivo in the inflammatory murine model of croton oil-induced ear dermatitis. The topical application of iPA significantly reduced mouse ear oedema, thus suggesting anti-inflammatory properties of this molecule. These results show the ability of iPA to exert anti-inflammatory effects both in vitro and in vivo directly targeting NK cells, providing a novel pharmacological tool in those diseases characterized by a deregulated immune-response, such as cancer or inflammatory conditions.

Endogenous cytokinin profiles of tissue-cultured and acclimatized 'Williams' bananas subjected to different aromatic cytokinin treatments

Endogenous cytokinin (CK) levels of in vitro-cultured and greenhouse-acclimatized 'Williams' bananas treated with six aromatic CKs were quantified using UPLC-MS/MS. The underground parts had higher endogenous CK levels than the aerial parts. Control plantlets had more isoprenoid CKs while the aromatic-type CKs were predominant in all other regenerants. Following acclimatization of the control and 10 μM CK regenerants, there was a rapid decline in both isoprenoid and aromatic CK in the greenhouse-grown plants. Apart from the control and 6-(3-Methoxybenzylamino)-9-tetrahydropyran-2-ylpurine (MemTTHP) treatment with higher level of isoprenoid CK, aromatic CK remain the predominant CK-type across all CK treatments. The most abundant CK forms were meta-topolin (mT) and benzyladenine (BA) in the micropropagated and acclimatized plants, respectively. Micropropagated plantlets had cis-Zeatin (cZ) as the major isoprenoid CK-type which was in turn replaced by isopentenyladenine (iP) upon acclimatization. On a structural and functional basis, 9-glucoside, a deactivation/detoxicification product was the most abundant and mainly located in the underground parts (micropropagation and acclimatization). The results establish the wide variation in metabolic products of the tested aromatic CKs during micropropagation and acclimatization. The findings are discussed with the possible physiological roles of the various CK constituents on the growth and development of banana plants.

Effect of shooting medium and source of material on grapevine (Vitis vinifera L.) shoot tip recovery after cryopreservation

BACKGROUND

Selecting experimental material at the optimal physiological stage is of paramount importance for successful cryopreservation.

OBJECTIVE

The study was to investigate the effect of the physiological state of grapevine buds on their regrowth after liquid nitrogen exposure.

METHODS

In a first set of experiments, we tested the regrowth of cryopreserved buds sampled from microcuttings cultured on shooting medium containing benzylaminopurine or zeatin riboside for various durations. In a second set of experiments, we studied the regrowth after liquid nitrogen exposure of buds sampled from different positions on the stem of in vitro plantlets.

RESULTS

Regrowth of cryopreserved buds sampled from microcuttings was higher (30%), compared to buds sampled directly from in vitro plantlets (23%), for all culture durations of microcuttings on shooting medium tested (2-6 weeks). Addition of cytokinin in the shooting medium improved regrowth of cryopreserved buds compared to buds sampled from microcuttings cultured on medium devoid of growth regulators; however similar results were obtained with the two cytokinins tested. Buds sampled on nodes 3-4 and 6-7 (from the top of the stem) displayed higher regrowth compared to shoot tips. No significant differences were noted in regrowth after cryopreservation between buds sampled from microcuttings produced from the terminal node, or nodes 3-4 and 6-7.

CONCLUSION

The physiological state of the plant material is important for cryopreservation success. Actively growing buds sampled from microcuttings displayed higher regrowth compared to buds sampled directly on in vitro plantlets.

Antioxidant and phenolic acid profiles of tissue cultured and acclimatized Merwilla plumbea plantlets in relation to the applied cytokinins

Merwilla plumbea (Lindl.) Speta is an important medicinal plant widely used in traditional medicine. We evaluated the effect of five cytokinins [benzyladenine (BA), 2-isopentenyladenine (2iP), meta-topolin (mT), meta-topolin riboside (mTR), and meta-methoxy-9-tetrahydropyran-2-yl-topolin (MemTTHP)] on the level of phenolic acids and antioxidant activity of M. plumbea during the tissue culture and acclimatization stages. Two cytokinins (mT and mTR) significantly improved the antioxidant activity of tissue culture plantlets while the control plantlets were better after acclimatization. Using UPLC-MS/MS, the levels of hydroxybenzoic and hydroxycinnamic acid derivatives (phenolic acids) varied significantly during tissue culture and acclimatization, depending on the cytokinin and plant part analyzed. Vanillic acid (24.9 μg g⁻¹) detected in underground parts of tissue culture plants supplemented with BA was the most abundant phenolic acid detected. The current findings indicate that the phytochemicals together with the bioactivity during in vitro propagation of M. plumbea is influenced by the cytokinin type used and the stage of plant material collection.

Functional identification of OsHk6 as a homotypic cytokinin receptor in rice with preferential affinity for iP

Cytokinins are involved in key developmental processes in rice (Oryza sativa), including the regulation of cell proliferation and grain yield. However, the in vivo action of histidine kinases (OsHks), putative cytokinin receptors, in rice cytokinin signaling remains elusive. This study examined the function and characteristics of OsHk3, 4 and 6 in rice. OsHk6 was highly sensitive to isopentenyladenine (iP) and was capable of restoring cytokinin-dependent ARR6 reporter expression in the ahk2 ahk3 Arabidopsis mutant upon treatment with 1 nM iP. OsHk4 recognized trans-zeatin (tZ) and iP, while OsHk3 scarcely induced cytokinin signaling activity. OsHk4 and OsHk6 mediated the canonical two-component signaling cascade of Arabidopsis to induce phosphorylation of ARR2. OsHk4 and OsHk6 were highly expressed in spikelets, suggesting that tZ and iP might play key roles in grain development. OsHk6 formed a self-interacting homomer in rice protoplasts, although the trans-phosphorylation activity between subunits was much lower than the intra-molecular trans-phosphorylation activity. This indicates that the action mechanism of OsHks is evolutionarily diverged from bacterial histidine kinases. Ectopic expression of OsHk6 in rice calli promoted green pigmentation and subsequent shoot induction, further supporting an OsHk6 in planta function as a cytokinin receptor. From the results of this study, OsHks are homomeric cytokinin receptors with distinctive cytokinin preferences in rice.

Functional identification of OsHk6 as a homotypic cytokinin receptor in rice with preferential affinity for iP

Cytokinins are involved in key developmental processes in rice (Oryza sativa), including the regulation of cell proliferation and grain yield. However, the in vivo action of histidine kinases (OsHks), putative cytokinin receptors, in rice cytokinin signaling remains elusive. This study examined the function and characteristics of OsHk3, 4 and 6 in rice. OsHk6 was highly sensitive to isopentenyladenine (iP) and was capable of restoring cytokinin-dependent ARR6 reporter expression in the ahk2 ahk3 Arabidopsis mutant upon treatment with 1 nM iP. OsHk4 recognized trans-zeatin (tZ) and iP, while OsHk3 scarcely induced cytokinin signaling activity. OsHk4 and OsHk6 mediated the canonical two-component signaling cascade of Arabidopsis to induce phosphorylation of ARR2. OsHk4 and OsHk6 were highly expressed in spikelets, suggesting that tZ and iP might play key roles in grain development. OsHk6 formed a self-interacting homomer in rice protoplasts, although the trans-phosphorylation activity between subunits was much lower than the intra-molecular trans-phosphorylation activity. This indicates that the action mechanism of OsHks is evolutionarily diverged from bacterial histidine kinases. Ectopic expression of OsHk6 in rice calli promoted green pigmentation and subsequent shoot induction, further supporting an OsHk6 in planta function as a cytokinin receptor. From the results of this study, OsHks are homomeric cytokinin receptors with distinctive cytokinin preferences in rice.

Effects of cytokinins, cytokinin ribosides and their analogs on the viability of normal and neoplastic human cells

We examined the effects of some cytokinins and cytokinin ribosides including a series of adenosine analogs differently substituted in the N(6) position, along with some hypoxanthine derivatives on the viability of normal and neoplastic human cells. Cytokinins such as trans-zeatin, isopentenyladenine and benzyladenine do not show any effect, while cytokinin ribosides such as trans-zeatin riboside, isopentenyladenosine, and benzylaminopurine riboside impair the viability of normal and neoplastic cells, apart from colon carcinoma LoVo cells.

The phenylquinazoline compound S-4893 is a non-competitive cytokinin antagonist that targets Arabidopsis cytokinin receptor CRE1 and promotes root growth in Arabidopsis and rice

We identified two phenylquinazoline compounds in a large-scale screening for cytokinin antagonists in yeast expressing the Arabidopsis cytokinin receptor cytokinin response 1/histidine kinase 4 (CRE1). After chemical modifications, we obtained compound S-4893, which non-competitively inhibited binding of the natural ligand 2-isopentenyladenine to CRE1. S-4893 antagonized cytokinin-induced activation of the Arabidopsis response regulator 5 promoter in Arabidopsis. Importantly, S-4893 had no detectable intrinsic cytokinin agonist activity in Arabidopsis or in the transformed yeast system. Cytokinin bioassay further demonstrated that S-4893 antagonized cytokinin-induced stimulation of callus formation and inhibition of root elongation. S-4893 also promoted seminal, crown and lateral root growth in rice, suggesting that S-4893 could potentially promote root growth in a variety of agronomically important plants. We believe S-4893 will be a useful tool in functional studies of cytokinin action in a wide range of plants and a lead compound for the development of useful root growth promoters in agriculture.

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.

Induction of in vitro flowering in Dendrobium Madame Thong-In (Orchidaceae) seedlings is associated with increase in endogenous N(6)-(Delta (2)-isopentenyl)-adenine (iP) and N (6)-(Delta (2)-isopentenyl)-adenosine (iPA) levels

We analysed the endogenous cytokinin levels of Dendrobium Madame Thong-In seedlings grown in vitro during vegetative and flowering-inductive periods. HPLC was used to fractionate the extracts and radioimmunoassay (RIA) was used for assay of zeatin (Z), dihydrozeatin (DZ), N(6)-(Delta(2)-isopentenyl)-adenine (iP) and their derivatives. Coconut water used in experiments was found to contain high level (>136 pmol ml(-1)) of zeatin riboside (ZR). Protocorms and seedlings cultured in medium with coconut water were found to contain 0.5-3.9 pmol g(-1) FW of the cytokinins analysed. Seedlings (1.0-1.5 cm) cultured in flowering-inductive liquid medium containing 6-benzyladenine (BA, 4.4 muM) and coconut water (CW, 15%) contained up to 200 and 133 pmol g(-1) FW of iP and iPA, respectively. These levels were significantly higher than all other cytokinins analysed in seedlings of the same stage and were about 80- to 150-folds higher than seedlings cultured in non-inductive medium. During the transitional (vegetative to reproductive) stage, the endogenous levels of iP (178 pmol g(-1) FW) and iPA (63 pmol g(-1) FW) were also significantly higher than cytokinins in the zeatine (Z) and dihydrozeatin (DZ) families in the same seedlings. Seedlings that grew on inductive medium but remained vegetative contained lower levels of iPA. The importance of the profiles of iP and its derivatives in induction of in vitro flowering of D. Madame Thong-In is discussed.

N6-isopentenyladenosine: A potential therapeutic agent for a variety of epithelial cancers

Isopentenyladenosine (i(6)A) is a product of isopentenyltransferases and, in mammals, occurs either bound to tRNA or as a free nucleoside. Sporadic reports have suggested an anticancer effect of i(6)A, mostly on leukemia cells. The present analysis of 9 human epithelial cancer cell lines derived from different types of malignant tissue revealed complete suppression of clonogenic activity in 8 of the lines after exposure to i(6)A at a concentration of 10 muM. Mechanistic studies showed that i(6)A tumor suppressor activity is associated with inhibition of cell proliferation, a block in DNA synthesis and morphological changes. These results point to i(6)A and to its possible derivatives as a new potential class of wide-spectrum anticancer agents.

N6‐isopentenyladenosine arrests tumor cell proliferation by inhibiting farnesyl diphosphate synthase and protein prenylation

The physiological effects of a variety of N6-substituted adenine and adenosine derivatives called cytokinins have been documented in plants, but information on their occurrence and function in other biological system is limited. Here we investigated the anti-proliferative effect of N6-isopentenyladenosine (i6A), an adenosine and isoprenoid derivative, in a thyroid cell system, FRTL-5 wild-type, and K-ras transformed KiMol cells. Addition of i6A to FRTL-5 cells caused a dose-dependent arrest of the G0-G1 cell phase transition associated with a reduction of cells in the S phase that was much more evident in KiMol cells. I6A arrested tumor cell proliferation by inhibiting farnesyl diphosphate synthase (FPPS) and protein prenylation. Indeed the addition of farnesol reversed these effects and i6A affected protein prenylation, in particular lamin B processing. I6A effect was not mediated by the adenosine receptor but was due to a direct modulation of FPPS enzyme activity as a result of its uptake inside the cells. I6A inhibited FPPS activity more efficaciously in KiMol cells than in normal FRTL-5. Moreover, the i6A anti-proliferative effect was evaluated in vivo in a nude mouse xenograft model, where KiMol cells were implanted subcutaneously. Mice treated with i6A showed a drastic reduction in tumor volume. Our findings indicate that this isoprenoid end product might be used for antineoplastic therapy, an application emulating that of the lovastatin and/or farnesyl-transferase inhibitors

Squash xylem sap has activities that inhibit proliferation and promote the elongation of tobacco BY-2 cell protoplasts

To elucidate the physiological functions of the substances in xylem sap, we analyzed the biological activities of xylem sap from squash (Cucurbita maxima Duch.) root using tobacco BY-2 (Nicotiana tabacum L. cv. Bright Yellow 2) cell protoplasts. When BY-2 cell protoplasts were cultivated with the total substance of squash xylem sap, the protoplasts elongated remarkably, and cell division was inhibited. Although trans-zeatin riboside (ZR), the most abundant cytokinin in squash xylem sap, had a concentration-dependent effect similar to that of total squash xylem sap, ZR concentrations several orders of magnitude greater than those found endogenously in squash xylem sap (i.e. 2 x 10(-8) M) were required to affect the growth of BY-2 cell protoplasts. The ability to stimulate cell elongation and inhibit cell division in BY-2 cell protoplasts was observed for the ethyl acetate phase fraction (pH 2) of squash xylem sap and an acetonitrile-eluate fraction from reverse-phase chromatography. The xylem sap also showed inhibitory activity for auxin-induced elongation of excised cucumber hypocotyls. These results suggest that an organic substance other than ZR is produced in the root and transported to above-ground organs through the xylem via the transpiration stream, where it is involved in regulating cell proliferation and elongation in the shoot, possibly as an auxin antagonist.

Regulation of sulfur-responsive gene expression by exogenously applied cytokinins in Arabidopsis thaliana

Effects of plant hormones on a sulfur-deficiency responsive element (betaSR) from the promoter region of the beta subunit gene of beta-conglycinin, a major seed storage protein of soybean, were investigated using transgenic Arabidopsis thaliana. Among the hormones tested, the cytokinins, trans-zeatin (Z) and trans-zeatin riboside, upregulated gene expression directed by the betaSR element both in the presence and in the absence of sulfate in the medium. Z also increased transcript accumulation of two endogenous sulfur-responsive genes, the adenosine 5'-phosphosulfate reductase (APR1) and the Sultr2;2, a sulfate transporter. Concentrations of cytokinins were unaltered during early stages of sulfur starvation when expression of these genes was upregulated. Z did not alter concentrations of O-acetyl-L-serine, a positive regulator of gene expression in sulfur starvation response. Concentrations of sucrose, which is known to upregulate expression of APR1, were increased in rosette leaves by Z. Sucrose application to the medium also increased expression directed by the betaSR element, although sucrose concentrations in tissues were not significantly altered by sulfur availability. These results suggest that exogenously applied cytokinins positively regulate expression of these sulfur responsive genes through a pathway independent of that from sulfur starvation, possibly through increasing sucrose concentrations in tissues.

A trans-zeatin riboside in root xylem sap negatively regulates adventitious root formation on cucumber hypocotyls

Shoot cultures of cucumber were used to analyse the roles of root-derived substances in adventitious root formation on hypocotyl tissues. Xylem sap collected from the roots of squash had a strong inhibitory effect on the formation of hypocotyl adventitious roots. Double-solvent extraction followed by fractionation with both normal and reverse phase column chromatographies and analysis by liquid chromatography/tandem mass spectrometry identified trans-zeatin riboside (ZR) as the primary suppressor of adventitious root formation. ZR was the predominant cytokinin present in the xylem sap, occurring at a concentration of 2x10(-8 )M. Application of ZR at concentrations from 3.16x10(-9) M effected inhibition of adventitious root formation. These results suggest that ZR transported from roots via xylem sap may act as an endogenous suppressor of hypocotyl adventitious root formation in planta.

Activation of caspase-like proteases and induction of apoptosis by isopentenyladenosine in tobacco BY-2 cells

The effects of isopentenyladenosine (iPA) on tobacco (Nicotiana tabacum L.) BY-2 cells were examined. The number of BY-2 cells decreased in a time- and concentration-dependent manner after being exposed to micromolar concentrations of iPA. This decrease was mainly due to a loss of cell viability, since no substantial changes in cell cycle progression were revealed by flow-cytometric analysis. Dying cells exhibited the typical morphological and biochemical hallmarks of apoptosis, including cell shrinkage, chromatin condensation, and degradation of nuclear DNA to nucleosomal size fragments. Caspase-1-like and caspase-3-like proteases also became activated, the former being dominant. Inhibitor-sensitivity studies revealed that although synthetic caspase inhibitors failed to prevent cell death they markedly reduced cell death in tobacco BY-2 cells, Nu-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone, a specific inhibitor for caspase-1, being the most effective. Our results indicate that caspase-like proteases, and particularly caspase-1-like protease, might be critically implicated in iPA-induced apoptosis of BY-2 cells. Finally, the outcome of inhibiting adenosine kinase by 4-amino-3-iodo-1(beta- D-ribofuranosyl)pyrazolo[3,4-d]-pyrimidine revealed that intracellular phosphorylation of iPA is required for its cytotoxicity to develop.

Induction of early bolting in Arabidopsis thaliana by triacontanol, cerium and lanthanum is correlated with increased endogenous concentration of isopentenyl adenosine (iPAdos)

The effects of triacontanol (TRIA), applied singly or in combination with cerium nitrate and lanthanum nitrate, on bolting of Arabidopsis thaliana were studied. Triacontanol (0.1 to 0.6 microM) added to the culture medium induced early bolting. TRIA (0.3 microM) applied with low concentrations of cerium and lanthanum caused a synergistic stimulation of bolting. In medium containing 0.3 microM TRIA, 0.1 microM cerium nitrate and 0.1 mM lanthanum nitrate, 82% of the plants bolted 20 d after seed sowing compared to only 8.6% in basal medium and 47.8% in medium with TRIA only. The changes in the endogenous concentrations of total cytokinins of the isopentenyl adenine (IP) subfamily in the leaf and root tissues were correlated with TRIA-induced early bolting. The combined treatment of TRIA (0.3 microM), cerium nitrate (0.1 microM) and lanthanum nitrate (0.1 mM) resulted in a significant increase in the endogenous concentrations of total cytokinins of the IP subfamily in the root and leaf tissues compared to plants growing in the basal medium and medium with TRIA. The exogenous application of six natural cytokinins to the plants revealed that only isopentenyl adenosine (iPAdos) was as effective as TRIA on floral bud formation. iPAdos was also found to have similar effects as TRIA on root growth and reproductive growth. These results suggest a correlation between the early bolting induced by TRIA, cerium and lanthanum and the production of higher concentrations of endogenous iPAdos.

Apoptosis induced by modified ribonucleosides in human cell culture systems

The in vitro modulation of apoptosis and cell proliferation by modified in comparison with non-modified ribonucleosides was investigated for the first time using peripheral blood lymphocytes, HL-60 cells and Caco-2 cells as human cell culture models. Modulating effects of several ribonucleosides were found in the range of 10(-7)-10(-3) mol/l. The following ribonucleosides induced significant apoptosis of HL-60 cells: adenosine, N6-dimethyladenosine, N6-(2-isopentenyl)-adenosine, N2-dimethylguanosine. A significant apoptotic effect on PBL was found with N6-dimethyladenosine and N6-(2-isopentenyl)-adenosine. N6-Dimethyladenosine, N6-(2-isopentenyl)-adenosine and guanosine had a pronounced inhibitory effect on Caco-2 cell apoptosis. Regarding the known function of ribonucleosides as pathobiochemical marker molecules for cancer, the possibility of a selective apoptotic effect against malignant cells is discussed.

Effects of norepinephrine and isopentenyladenosine on Na+/Ca2+ exchange currents in isolated guinea pig ventricular myocytes

AIM

To study the effects of norepinephrine (NE) and isopentenyladenosine (Iso) on Na+/Ca2+ exchange currents and the receptor mechanism.

METHODS

The quasi-steady state current-voltage relationship from the isolated guinea pig ventricular myocytes was measured using whole-cell voltage-clamp techniques with a ramp pulse protocol.

RESULTS

At potential of +50 mV, NE 0.005, 0.05, and 5 mumol.L-1 increased the Ni(2+)-sensitive current by 29% +/- 9%, 72% +/- 11%, and 124% +/- 31.4%, respectively; Iso 1.5, 150, and 1500 nmol.L-1 caused increases in the Ni(2+)-sensitive current by 2.8% +/- 2.8%, 56% +/- 13%, and 102% +/- 12%, respectively. Propranolol 10 mumol.L-1 completely inhibited the current changes induced by NE and Iso while phentolamine 50 mumol.L-1 showed no effects.

CONCLUSION

NE and Iso increased the Na+/Ca2+ exchange currents via stimulation of cardiac beta-adrenoceptor.

N6-isopentenyladenosine affects cAMP-dependent microfilament organization in FRTL-5 thyroid cells

N6-Isopentenyladenosine (i6A), an adenosine and mevalonate derivative, inhibits, like adenosine, TSH-induced cAMP increase and its related events (I- uptake and DNA synthesis) in FRTL-5 cells. This inhibition is dose-dependent and is measurable at 10(-8) M. However, unlike adenosine, i6A prevents TSH-promoted microfilament disassembly. The effect of i6A on cytoskeletal structure is antagonized by pertussis toxin and could be assigned to its N6 substitution since it can be mimicked by other synthetic N6-adenosine derivatives. It is suggested that a step beyond cAMP is involved, since i6A prevents also microfilament disassembly induced by 8-bromo-cAMP. This is the first demonstration that an adenosine derivative, which is also an end-product of the isoprenoid pathway, affects cAMP-dependent microfilament organization.

Accumulation of reactive oxygen species and oxidation of cytokinin in germinating soybean seeds

Seed germination is an important developmental switch when quiescent seed cells initiate oxidative phosphorylation for further development and differentiation. During early imbibition of soybean seeds (Glycine max L. cv. Weber), a superoxide dismutase (SOD) activity peak was observed, in embryonic axes, after 6 h imbibition. Peroxidase activities, including catalase, were significantly increased after 12 h inhibition and during germination phase III. Catalase was the most efficient enzyme in catabolizing H2O2 in embryonic axes. When stored at 42 degrees C and 100% relative humidity, seeds were stressed and lost their viability in a time-dependent manner. A significant increase in the Cu, Zn-superoxide-dismutase activity, and to a lesser extent, Mn superoxide dismutase activity was observed during germination in low-viability (stressed) seeds as compared to high-viability (unstressed) seeds. Northern blot analysis confirmed that superoxide dismutase induction resulted from an accumulation of its transcripts in response to the production of O2-. The induction of catalase did not occur in low-viability seeds, resulting in dramatic accumulation of H2O2. Using capillary electrophoresis, HPLC and NMR we found that the endogenous cytokinin, zeatin riboside, was present in large quantities in the high-viability seeds, but it was oxidized into adenine in the low-viability seeds. In vitro superoxide anion could also oxidize the cytokinin. Zeatin riboside, but not adenine, was found to act as a scavenger of superoxide anions and may help to maintain seed viability by detoxifying reactive oxygen species. Germination of stressed seeds was partially restored by the addition of exogenous cytokinin (zeatin riboside). Protection against oxidative stress by cytokinin seemed to be a general phenomenon, as Escherichia coli cells were also protected against superoxide stress in the presence of cytokinin.

Purines. LXII. Both enantiomers of N6-(1,3-dimethyl-2-butenyl)adenine and their 9-beta-D-ribofuranosides: synthesis and cytokinin activity

Both enantiomers [(1'R)-6 and (1'S)-6] of N6-(1,3-dimethyl-2-butenyl)adenine and their 9-beta-D-ribofuranosides [(1"R)-16 and (1"S)-16] have been synthesized for the first time from both enantiomers of alanine (15) in nine steps. These aglycones and nucleosides, together with N6-(3-methyl-2-butenyl)adenine (5) and its 9-beta-D-ribofuranoside (18) as well as 9-beta-D-ribofuranosyl-cis-zeatin (20) and 9-(2-deoxy-beta-D-ribofuranosyl)-cis-zeatin (19), were tested for cytokinin activity in the tobacco callus bioassay. The order of their activity was 5 > (1'R)-6 > (1"R)-16 approximately 18 > (1'S)-6 > (1"S)-16 > 20 > 19. The bioassay results are compared with those obtained previously for the derivatives modified analogously in the N6-substituent in the cis- and trans-zeatin series.

Increased cytotoxicity of N6-(delta 2-isopentenyl)adenosine in combination with pentostatin against L-1210 leukemia cells

Pentostatin (I), a tight-binding inhibitor of adenosine deaminase, was evaluated in combination with the partially effective antitumor nucleoside N6-(delta 2-isopentenyl)adenosine (II) for cytotoxic activity against cultured L-1210 lymphocytic mouse leukemia cells. Although I alone (less than or equal to 10 micrograms/ml) was ineffective, it significantly potentiated and prolonged the cytotoxic and cytostatic activities of II. The combination of I (2-10 micrograms/ml) with II (25 micrograms/ml) resulted in inhibition of cellular proliferation (80-96%) within 24 hr with maintenance at that level for an extended period of time due to the continued ability of I to prevent the facile deamination of the allylic side chain of II. This type of adjuvant chemoprotection has potential use for other labile oncologic agents.

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.

Inhibition of hepatoma cell growth by analogs of adenosine and cyclic AMP and the influence of enzymes in mammalian sera

The following evidence suggests that inhibition of hepatoma cell (HTC) growth by cyclic nucleotides is an adenosine-like effect that is greatly modified by the type and treatment of serum used in the culture medium and is probably not mediated by cyclic AMP-dependent protein kinase: 1) Heating serum reduces its phosphodiesterase content, thereby slowing metabolism of cyclic AMP and reducing the inhibition of HTC cell growth by cyclic AMP; 2) Using medium that contains phosphodiesterase but lacks adenosine deaminase causes adenosine to accumulate from cyclic AMP and increases the toxicity of cyclic AMP; 3) Uridine or cytidine reverses the growth inhibition caused by adenosine, 5'-AMP or cyclic AMP; 4) adenosine, 5'-AMP and N6-(delta 2-isopentenyl) adenosine are more toxic for HTC cells than is cyclic AMP, and N6,O2-dibutyryl cyclic AMP is not toxic; and 5) N6,O2'-dibutyryl cyclic AMP inhibits growth of Reuber H35 cells, but uridine prevents this inhibition of growth. We conclude that most, if not all, of the inhibitory effects of cyclic AMP and N6,O2'-dibutyryl cyclic AMP on HTc and Reuber H35 hepatoma cell growth are due to the generation of toxic metabolites.

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.

The effect of phytohormones produced by Arthrobacter sp. on the phosphatase activity in plant roots

Phytohormonal activity (auxins, gibberellins, cytokinins) was tested in the supernatant of a culture of Arthrobacter sp. Crude extract of the phytohormonal fraction was used as substrate for the growth of Lactuca sativa seedlings. Treating with bacterial hormones resulted in an increased plant develop- ment. Furthermore, a sharp increase of the acid phosphatase activity was observed in the roots.

Cyclic-AMP content in Escherichia coli B/b as affected by N1-(delta 2-isopentyl)adenine

N6-(delta 2-isopentenyl)adenine, like other cytokinins, does not detectably modify Escherichia coli growth, but strongly affects cellular levels of cAMP. A substantial delay of the highest level of intracellular cAMP, a reduction to about one half of such maximum level, and a slight increase of cAMP secreted into the medium are reported.

Potentiation by guanine nucleosides of the growth-inhibitory effects of adenosine analogs on L1210 and sarcoma 180 cells in culture

The growth-inhibitory effect of 6-methylmercaptopurine riboside (MMPR) against leukemia L1210 cells in culture was dramatically potentiated by the addition of guanine nucleosides to the medium. In the presence of either deoxyguanosine or guanosine, the concentration of MMPR that caused 50% inhibition of growth was 35 times lower than in the absence of these nucleosides. Similar potentiation was also observed against Sarcoma 180 cells in culture by guanosine. The metabolic basis of this synergism was approached in a study of the incorporation of [14C]glycine into 5'-phosphoribosyl-N-formylglycinamide in Sarcoma 180 cells. The results show that the site of inhibition resulting in synergism is an early step in purine biosynthesis, probably phosphoribosyl pyrophosphate amidotransferase (EC 2.4.2.14). In the L1210 cell system, the addition of hypoxanthine to the medium prevented the potentiation of MMPR by guanine nucleosides supporting the conclusion that the site of the synergistic interaction involves purine biosynthesis de novo. While hypoxanthine partially reversed the growth-inhibitory effects of MMPR, an even higher degree of protection was observed in the presence of both uridine and hypoxanthine, suggesting that MMPR may have additional sites of action concerned with pyrimidine metabolism.

N6-(delta2-isopentenyl)adenosine, an inhibitor of cellular transport of uridine and cytidine

N6-(delta2-Isopentenyl)adenosine (IPAR) inhibited severely the incorporation of uridine and cytidine into S-180 cells in culture. When IPAR and the nucleosides were simultaneously present in the medium the inhibition was competitive (Ki 3.4 muM) and indicated inhibition of transport. However, the inhibition occurred even in the absence of extracellular IPAR if the cells had been preincubated with IPAR. Since 5'-IPAMP was the product which accumulated in large quantities in S-180 cells when incubated with IPAR, the effects of this AMP analog of the intracellular metabolism of uridine had to be considered. No direct correlation between the amount of intracellular IPAMP and the degree of inhibition of uridine utilization was observed and the relative distribution of uridine nucleotides in the acid soluble pool of the cells was unaltered in cells treated with IPAR. Also, IPAMP was not an inhibitor of uridine kinase in a cell free system nor was the activity of this enzyme affected by treatment of cells with IPAR. In addition, a profound inhibition of uridine utilization was also observed in a resistant subline of S-180 cells, which is unable to form IPAMP. These data suggest that IPAMP was not the inhibitory agent. Furthermore, the observation that the inhibition in both sensitive and resistant cells was caused even by a 15-second exposure to 100 muM IPAR, followed by rinsing, suggests that IPAR itself is the effective agent. It is concluded that IPAR exerts its inhibitory effect on uridine and cytidine utilization by becoming lodged in the cell membrane and thereby preventing the passage of these nucleosides into the cells. It is also shown that the inhibition of uridine and cytidine utilization by IPAR and by other potent nucleoside uptake inhibitors is unrelated to inhibition of growth or RNA-synthesis when the cells do not depend on an extracellular source of a nucleoside for growth.