Cytokinins: development of a potent antagonist

An appraisal on synthetic and pharmaceutical perspectives of pyrazolo[4,3-d]pyrimidine scaffold

Pyrazolo[4,3-d]pyrimidine, a fused heterocycle bearing pyrazole and pyrimidine portions has gained a significant attention in the field of bioorganic and medicinal chemistry. Pyrazolo[4,3-d]pyrimidine derivatives have demonstrated numerous pharmacological activities particularly, anti-cancer, anti-infectious, phosphodiesterase inhibitors, adenosine antagonists and cytokinin antagonists etc. This review extensively unveils the synthetic and pharmacological diversity with special emphasis on structural variations around pyrazolo[4,3-d]pyrimidine scaffold. This endeavour has thus uncovered the medicinal worthiness of pyrazolo[4,3-d]pyrimidine framework. To the best of our knowledge this review is the first compilation on synthetic, medicinal and structure activity relationship (SAR) aspects of pyrazolo[4,3-d]pyrimidines since 1956.

Convenient and efficient syntheses of N(6)- and N(4)- substituted adenines and cytosines and their 2'-deoxyribosides

Convenient and efficient methods of the synthesis of N(6)- and N(4)-substituted derivatives of adenine and cytosine and their 2'-deoxyribosides were developed. The reactions of either unprotected nucleobases (adenine, cytosine) or unprotected 2'-deoxyribosides with aryl or alkyl aldehydes give corresponding Schiff bases that can be reduced to the target title compounds with high overall yields. In the case of aryl aldehydes the imine derivatives are obtained in the presence of methoxides in methanol and reduced with sodium borohydride. The corresponding reactions with alkyl aldehydes require the use of acetic acid and borane dimethyl sulfide complex instead.

Unraveling plant hormone signaling through the use of small molecules

Plants have acquired the capacity to grow continuously and adjust their morphology in response to endogenous and external signals, leading to a high architectural plasticity. The dynamic and differential distribution of phytohormones is an essential factor in these developmental changes. Phytohormone perception is a fast but complex process modulating specific developmental reprogramming. In recent years, chemical genomics or the use of small molecules to modulate target protein function has emerged as a powerful strategy to study complex biological processes in plants such as hormone signaling. Small molecules can be applied in a conditional, dose-dependent and reversible manner, with the advantage of circumventing the limitations of lethality and functional redundancy inherent to traditional mutant screens. High-throughput screening of diverse chemical libraries has led to the identification of bioactive molecules able to induce plant hormone-related phenotypes. Characterization of the cognate targets and pathways of those molecules has allowed the identification of novel regulatory components, providing new insights into the molecular mechanisms of plant hormone signaling. An extensive structure-activity relationship (SAR) analysis of the natural phytohormones, their designed synthetic analogs and newly identified bioactive molecules has led to the determination of the structural requirements essential for their bioactivity. In this review, we will summarize the so far identified small molecules and their structural variants targeting specific phytohormone signaling pathways. We will highlight how the SAR analyses have enabled better interrogation of the molecular mechanisms of phytohormone responses. Finally, we will discuss how labeled/tagged hormone analogs can be exploited, as compelling tools to better understand hormone signaling and transport mechanisms.

Properties, functions and evolution of cytokinin receptors

The discovery of cytokinin receptors of Arabidopsis thaliana ten years ago was a milestone in plant hormone research. Since then, research has yielded insights into the biochemical properties and functions of these sensor histidine kinases. Their affinities to both trans-zeatin and isopentenyladenine are in the low nM range. Cytokinin ribosides, cis-zeatin and thidiazuron were established as compounds with genuine cytokinin activity and the first cytokinin antagonists were identified. Numerous functions of cytokinin receptors in plant development, as well as in the plant's responses to the environment, have been elucidated and are summarized. Finally, we address the question how the receptors have evolved during plant evolution.

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.

Anticytokinin activity of substituted pyrrolo[2,3-d]pyrimidines

Ten substituted pyrrolo[2,3-d]pyrimidines were tested as cytokinins and anticytokinins in the tobacco bioassay. Eight new anticytokinins were identified and two were found to be highly active. The most potent species were 4-cyclohexylamino- and 4-cyclopentylamino-2-methylthiopyrrolo[2,3-d]pyrimidine, of which 0.05 and 0.009 muM concentrations, respectively, were required to produce detectable inhibition of the growth of tobacco callus cultured on a medium containing 0.003 muM 6-(3-methyl-2-butenylamino)purine. The inhibition of growth by moderate (</=6.6 muM) concentrations of these compounds was reversible by equal or higher concentrations of 6-(3-methyl-2-butenylamino)purine, but not by indole-3-acetic acid or gibberellic acid. These substituted pyrrolo[2,3-d]pyrimidines were also found to enhance bud formation at high cytokinin concentrations, suggesting that a cytokinin may act at more than one cellular site in exerting its growth-promoting and morphogenetic effects.

Cytokinin receptor antagonists derived from 6-benzylaminopurine

Recently we reported 6-(2-hydroxy-3-methylbenzylamino)purine (PI-55) as the first molecule to antagonize cytokinin activity at the receptor level. Here we report the synthesis and in vitro biological testing of eleven BAP derivatives substituted in the benzyl ring and in the C2, N7 and N9 positions of the purine moiety. The ability of the compounds to interact with Arabidopsis cytokinin receptors AHK3 and CRE1/AHK4 was tested in bacterial receptor and in live-cell binding assays, and in an Arabidopsis ARR5:GUS (Arabidopsis response regulator 5) reporter gene assay. Cytokinin activity of the compounds was determined in classical cytokinin biotests (tobacco callus, wheat leaf senescence and Amaranthus bioassays). 6-(2,5-Dihydroxybenzylamino)purine (LGR-991) was identified as a cytokinin receptor antagonist. At the molecular level LGR-991 blocks the cytokinin receptor CRE1/AHK4 with the same potency as PI-55. Moreover, LGR-991 acts as a competitive inhibitor of AHK3, and importantly shows reduced agonistic effects in comparison to PI-55 in the ARR5:GUS reporter gene assay and in cytokinin bioassays. LGR-991 causes more rapid germination of Arabidopsis seeds and increases hypocotyl length of dark-grown seedlings, which are characteristics of plants with a reduced cytokinin status. LGR-991 exhibits a structural motive that might lead to preparation of cytokinin antagonists with a broader specificity and reduced agonistic properties.

The purine derivative PI-55 blocks cytokinin action via receptor inhibition

One of several potential approaches to study mechanisms of action of biologically active compounds is to develop their agonists and antagonists. In the present study, we report the identification of the first known molecule antagonizing the activity of the plant hormone cytokinin at the receptor level. This compound, 6-(2-hydroxy-3-methylbenzylamino)purine, designated PI-55 in the present study, is structurally closely related to cytokinin 6-benzylaminopurine, but substitutions at specific positions of the aromatic side chain strongly diminished its cytokinin activity and conferred antagonistic properties. PI-55 competitively inhibited the binding of the natural ligand trans-zeatin to the Arabidopsis cytokinin receptors cytokinin response 1 (CRE1)/Arabidopsis histidine kinase (AHK) 4 and AHK3 and repressed induction of the cytokinin response gene ARR5:GUS. Genetic analysis revealed that CRE1/AHK4 is the primary target of PI-55. Cytokinin bioassays also demonstrated the anticytokinin effect of PI-55 in several other species. Furthermore, we show that PI-55 accelerated the germination of Arabidopsis seeds and promoted the root growth and formation of lateral roots, thus phenocopying the known consequences of a lowered cytokinin status and demonstrating its potential to inhibit cytokinin perception in planta. PI-55 is the first example for the targeted development of a cytokinin antagonist and represents an initial step for the preparation of cytokinin antagonists with broad activity and reduced agonistic properties.

Classical Anticytokinins Do Not Interact with Cytokinin Receptors but Inhibit Cyclin-dependent Kinases

Cytokinins are a class of plant hormones that regulate the cell cycle and diverse developmental and physiological processes. Several compounds have been identified that antagonize the effects of cytokinins. Based on structural similarities and competitive inhibition, it has been assumed that these anticytokinins act through a common cellular target, namely the cytokinin receptor. Here, we examined directly the possibility that various representative classical anticytokinins inhibit the Arabidopsis cytokinin receptors CRE1/AHK4 (cytokinin response 1/Arabidopsis histidine kinase 4) and AHK3 (Arabidopsis histidine kinase 3). We show that pyrrolo[2,3-d]pyrimidine and pyrazolo[4,3-d]pyrimidine anticytokinins do not act as competitors of cytokinins at the receptor level. Flow cytometry and microscopic analyses revealed that anticytokinins inhibit the cell cycle and cause disorganization of the microtubular cytoskeleton and apoptosis. This is consistent with the hypothesis that they inhibit regulatory cyclin-dependent kinase (CDK) enzymes. Biochemical studies demonstrated inhibition by selected anti-cytokinins of both Arabidopsis and human CDKs. X-ray determination of the crystal structure of a human CDK2-anticytokinin complex demonstrated that the antagonist occupies the ATP-binding site of CDK2. Finally, treatment of human cancer cell lines with anticytokinins demonstrated their ability to kill human cells with similar effectiveness as known CDK inhibitors.

Pyrazolo[4,3-d]pyrimidines as new generation of cyclin-dependent kinase inhibitors

A search among analogues of anti-CDK purines led to the identification of substituted pyrazolo[4,3-d]pyrimidines as novel inhibitors of CDK1/cyclin B. Some of these derivatives also show antiproliferative activity on cancer cell line K-562, thus may find an application as anticancer agents.

Fluorescent anticytokinins as a probe for binding. Isolation of cytokinin-binding proteins from the soluble fraction and identification of a cytokinin-binding site on ribosomes of tobacco callus cells

4-Substituted 2-methylthiopyrido[2,3-d]pyrimidines, a series of recently developed anticytokinins, have been found to fluoresce strongly in water and to be useful as probes for binding studies. The binding activity of the soluble proteins and particulate fraction of tobacco callus cells to the biologically most active member of the family, 4-n-butylamino-2-methylthiopyrido[2,3-d]pyrimidine (BAMPP), was studied fluorimetrically. We found that the binding activity is better monitored in terms of saturable binding rather than in terms of the amount of bound ligand, a conventional method used in isolation studies of hormone receptor proteins. Using this technique we isolated two kinds of high-affinity cytokinin-binding proteins from the soluble fraction and identified a high-affinity binding site on ribosomes.

Biological Effects of Cytokinin Antagonists 7-(Pentylamino) and 7-(Benzylamino)-3-Methylpyrazolo(4,3-d)Pyrimidines on Suspension-cultured Tobacco Cells

We have studied the biological effects of two structural analogs of cytokinins, 3-methyl-7-(pentylamino)pyrazolo(4,3-d)pyrimidine and 3-methyl-7-(benzylamino)pyrazolo(4,3-d)pyrimidine, on tobacco cell suspension cultures. These two cytokinin analogs are highly inhibitory to cytokinin-autonomous and cytokinin-requiring tobacco cells. The growth inhibitory effect is markedly antagonized by benzyladenine, but not by adenine. Cell suspensions of tobacco cells of a cytokinin-autonomous strain become cytokinin-dependent in the presence of 0.1 micromolar 3-methyl-7-(benzylamino)pyrazolo(4,3-d)pyrimidine. It is also demonstrated that growth inhibition of cell suspension cultures is accompanied by cell death and that only dividing cells are sensitive to this cytotoxic effect.

The nature of spontaneous changes in growth rate in isolated coleoptile segments

About 4 hours after they are cut from the seedling, corn (Zea mays L.) coleoptile segments mounted vertically show a strong increase in growth rate. This increase occurs in water or various buffers near pH 7 and is not accompanied by the accumulation of a growth promoter in the medium. The increase in growth rate is prevented by 1 mmp-fluorophenylalanine and is strongly inhibited by 0.1 mmp-chlorophenoxyisobutyric acid.The increased growth rate is accompanied by a 95% increase in the ability of tissue extracts to catalyze the conversion of (14)C-tryptophan to (14)C-indole-3-acetic acid and by a nearly 3-fold increase in indole-3-acetic acid oxidase activity. The increase in growth rate is also observed in segments from coleoptiles grown aseptically.The spontaneous increase in growth rate is completely but reversibly inhibited by 1 mum indole-3-acetic acid. Cytokinins have little effect on the spontaneous growth response, whereas gibberellic acid is observed to extend the latent period and reduce the magnitude of the response. It is tentatively concluded that the increase in endogenous growth rate may result from increased auxin production upon derepression of the auxin biosynthesis pathway after isolating the tissue from the normal supply of auxin from the tip.

Competitive inhibition of beef heart cyclic AMP phosphodiesterase by cytokinins and related compounds

Two cytokinins and four related analogs, none of which is a cyclic ribonucleotide, have been shown to act as competitive inhibitors of the high K(m) cyclic-AMP phosphodiesterase (3':5'-cyclic-AMP 5'-nucleotidohydrolase, EC 3.1.4.17) activity from beef heart. Weak inhibition of the low K(m) cyclic AMP phosphodiesterase activity was also observed, suggesting a possible mechanism for regulation of intracellular cyclic AMP levels by the exogenously added compounds. In addition to the kinetic data, obtained on the six inhibitors in four different heterocyclic series, 15 other cytokinins and related compounds have been shown to inhibit the high K(m) cyclic AMP phosphodiesterase activity at single concentrations of substrate and inhibitor. Heterocycles such as adenosine and 7-amino-3-methylpyrazolo[4,3-d]pyrimidine, which lack the N-substituent, were inactive as cyclic AMP phosphodiesterase inhibitors. The observed inhibition of cyclic AMP phophodiesterase supports prior observations which implicate exogenously added cytokinins in cyclic AMP metabolism.