Chemical inhibitors of cyclin-dependent kinases

Transient activation o f cyclin-dependent kinases (CDKs) is responsible for transition through the successive phases of the cell-division cycle. Major changes in the expression and regulation of CDKs have been described in human tumours. Enzymatic screening is starting to uncover chemical inhibitors o f CDKs that arrest the cell cycle at various steps. This review summarizes our knowledge of the first generation inhibitors, their molecular mechanisms of action and their effects on the cell cycle and apoptosis, and discusses their potential as synchronizing agents, as ligands for affinity chromatography and as therapeutic agents.     

2-Aminoquinazoline inhibitors of cyclin-dependent kinases

The inhibition of cyclin-dependent kinase 4 (Cdk4) causes cell cycle arrest and restores a checkpoint that is absent in the majority of tumor cells. Compounds that inhibit Cdk4 selectively are targeted for treating cancer. Appropriate substitution of 2-aminoquinazolines is demonstrated to produce high levels of selectivity for Cdk4 versus closely related serine-threonine kinases.     

Beta-carbolines as specific inhibitors of cyclin-dependent kinases

Harmine (3), 7-fluoro-1-methyl beta-carboline (35) and 1-(5-methyl-imidazol-4-yl) beta-carboline (41) were potent and specific inhibitors of cyclin-dependent kinases. The degree of aromaticity of the tricyclic ring and the positioning of substituents are important for inhibitory activity. While most beta-carbolines inhibited CDK2 and CDK5 to the same extent, selective inhibition against CDK2 was observed in 1-(2-chlorophenyl)- (12), 1-(2-fluorophenyl)- (15), and 1-(2-chloro-5-nitrophenyl)- (28) beta-carbolines.     

8-Azapurines as new inhibitors of cyclin-dependent kinases

Purine inhibitors of cyclin-dependent kinases (CDK) seem to be a potential anticancer drug candidate as one of the first representatives, roscovitine, is passing Phase II clinical trials for cancer and glomerulonephritis. In this article, we describe a novel modification of the purine scaffold influencing CDK2 inhibitory activities as well as anticancer properties in cell lines of different histopathological origin. The introduced N at position 8 of the purine ring generally lowered CDK2 inhibitory activity of new 8-azapurines (1,2,3-triazolo[4,5-d]pyrimidines) in comparison to the model trisubstituted purines, whereas the antiproliferative potential of some derivatives remained very high, reflecting their ability to activate p53 tumor suppressor.     

Genetic analysis of mammalian cyclin-dependent kinases and their inhibitors

Entry into the cell cycle, in particular the G1/S transition, is a tightly regulated process that involves a combination of mitogenic signaling pathways and cell cycle checkpoints. Some of the key regulators of this process are frequently altered in human cancer. Although the proteins that control the G1/S transition have been extensively studied at the biochemical level, little is known regarding their physiological role in vivo. During the last few years, a series of mouse strains carrying gene targeted mutations in key regulators of the G1/S transition have been generated. They include the Rb family of proteins and some of their downstream and upstream regulators. The latter include the regulatory (cyclin) and catalytic (Cdk) subunits of some of the kinases responsible for Rb inactivation as well as all the members of two families of cell cycle inhibitors, the INK4 and the Cip/Kip proteins. In this review, we summarize the most relevant information derived from the characterization of these strains of mice and attempt to integrate it within a functional framework of cell cycle regulation in vivo.     

Cdk pathway: Cyclin-dependent kinases and cyclin-dependent kinase inhibitors

Many mechanisms either activate or inhibit the cdks and thereby either promote or arrest progression through the mitotic cell cycle. Since the signal transduction pathways emanating from extracellular mitogens and the agents controlling these pathways are complicated there may yet be novel mechanisms of cell cycle regulation remaining to be elucidated. In this article we outline the different techniques used to study the cell cycle and its regulation. These include: establishing that the cell cycle is arrested by propidium iodide staining followed by FACS analysis or by measuring ³H-thymidine incorporation into DNA; measuring the amount of cyclin/cdk associated kinase activity; assessing the steady-state expression profiles of cyclins, cdks and ckis by immunoblotting; and investigating the formation of complexes between these proteins by coimmunoprecipitations. Caveats and advantages of each technique are discussed. Following this paradigm yielded the discovery of the cell cycle inhibitors p27^Kip1 and p21^Cip1 and could very well lead to the discovery or novel cell cycle regulatory mechanisms.     

Pyrazolo[3,4-b]quinoxalines. A new class of cyclin-dependent kinases inhibitors

Protein kinases are involved in most physiological processes and in numerous diseases. Therefore, inhibitors of protein kinases have therefore a wide therapeutic potential. While screening for inhibitors of cyclin-dependent kinases (CDK's) and glycogen synthase kinase-3 (GSK-3), we identified pyrazolo[3,4-b]quinoxalines as sub-micromolar inhibitors of CDK1/cyclin B. A preliminary structure-activity relationship study suggests that this family of compounds can be optimized to inhibit CDK's and GSK-3. Compounds were tested for their anti-proliferative activity and the results show that several of them displayed a significant inhibitory effect on CDK1/cyclin B. The most active compound (1) was also tested against the brain kinases CDK5/p25 and GSK-3, and proved to be a good inhibitor of both of them. On the contrary, none of the compounds showed any activity in the CDC25 phosphatase assay. As an additional approach, affinity chromatography on immobilized pyrazolo[3,4-b]quinoxalines will be used to identify the intracellular targets of this family of compounds.     

1H-Pyrazolo[3,4-b]pyridine inhibitors of cyclin-dependent kinases

1H-Pyrazolo[3,4-b]pyridine 3 (SQ-67563) has been shown to be a potent, selective inhibitor of CDK1/CDK2 in vitro. In cells 3 acts as a cytotoxic agent with the ability to block cell cycle progression and/or induce apoptosis. The solid state structure of 3 bound to CDK2 shows 3 resides coincident with the ATP purine binding site and forms important H-bonding interactions with Leu83 on the protein backbone.     

Activation of human cyclin-dependent kinases in vitro.

We have analyzed the activation of human cyclin-dependent kinases in a cell-free system. Human CDC2, cyclin-dependent kinase 2 (CDK2), cyclin A, and cyclin B1 were produced in insect cells by infection with recombinant baculoviruses. CDC2 or CDK2 monomers in lysates of infected cells could be activated by the addition of lysates containing cyclin A or B1. CDC2 activation by cyclin B1, as well as CDK2 activation by cyclins A and B1, was accompanied by the formation of high molecular weight complexes. In contrast, CDC2 did not bind effectively to cyclin A. CDC2 activation by cyclin B1 was studied in detail and was found to be accompanied by phosphorylation of CDC2 on Threonine 161. The binding of CDC2 to cyclin B1 also occurred under conditions where CDC2 phosphorylation was prevented, resulting in an inactive complex that could then be phosphorylated and activated on addition of cell extract. Highly purified CDC2 and cyclin B1 also formed inactive complexes that could be activated in an ATP-dependent fashion by unidentified components in crude cell extracts. These data suggest that the CDC2 activation process begins with cyclin binding, after which CDC2 phosphorylation, catalyzed by a separate enzyme, leads to activation.     

Characterization of isoenzyme-selective inhibitors of human sphingosine kinases

Sphingosine kinases (SKs) are promising new therapeutic targets for cancer because they regulate the balance between pro-apoptotic ceramides and mitogenic sphingosine-1-phosphate. The functions of the two SK isoenzymes, SK1 and SK2, are not redundant, with genetic ablation of SK2 having more pronounced anticancer effects than removal of SK1. Although several small molecule inhibitors of SKs have been described in the literature, detailed characterization of their molecular and cellular pharmacology, particularly their activities against human SK1 and SK2, have not been completed. Computational modeling of the putative active sites of SK1 and SK2 suggests structural differences that might allow isozyme-selective inhibitors. Therefore, we characterized several SK-inhibitory compounds which revealed differential inhibitory effects on SK1 and SK2 as follows: SKI-II and ABC294735 are SK1/2-dual inhibitors; CB5468139 is a SK1-selective inhibitor; and ABC294640 is a SK2-selective inhibitor. We examined the effects of the SK inhibitors on several biochemical and phenotypic processes in A498 kidney adenocarcinoma cells. The SK2-selective inhibitor ABC294640 demonstrated the most pronounced effects on SK1 and SK2 mRNA expression, decrease of S1P levels, elevation of ceramide levels, cell cycle arrest, and inhibition of proliferation, migration and invasion. ABC294640 also down-regulated the expression or activation of several signaling proteins, including STAT3, AKT, ERK, p21, p53 and FAK. These effects were equivalent or superior to responses to the SK1/2-dual inhibitors. Overall, these results suggest that inhibition of SK2 results in stronger anticancer effects than does inhibition of SK1 or both SK1 and SK2.     

Synthesis and biological activity of N-aryl-2-aminothiazoles: potent pan inhibitors of cyclin-dependent kinases

N-Aryl aminothiazoles 6-9 were prepared from 2-bromothiazole 5 and found to be CDK inhibitors. In cells they act as potent cytotoxic agents. Selectivity for CDK1, CDK2, and CDK4 was dependent of the nature of the N-aryl group and distinct from the CDK2 selective N-acyl analogues. The N-2-pyridyl analogues 7 and 19 showed pan CDK inhibitory activity. Elaborated analogues 19 and 23 exhibited anticancer activity in mice against P388 murine leukemia. The solid-state structure of 7 bound to CDK2 shows a similar binding mode to the N-acyl analogues.     

Cyclins,cyclin-dependent kinases and differentiation

Cyclin-dependent kinases and their regulatory subunits, the cyclins, are known to regulate progression through the cell cycle. Yet these same proteins are often expressed in non-cycling, differentiated cells. This review surveys the available information about cyclins and cyclin-dependent kinases in differentiated cells and explores the possibility that these proteins may have important functions that are independent of cell cycle regulation.     

NKIAMRE,a novel conserved CDC2-related kinase with features of both mitogen-activated protein kinases and cyclin-dependent kinases

We report the cloning of the NKIAMRE gene located on human chromosome 5q31.1. It encodes a novel 52kDa Cdc2-related kinase with a 1.5kb open reading frame. Like MAP kinases, NKIAMRE contains a Thr-X-Tyr (TXY) motif in the activation loop domain. Similar to cdks, NKIAMRE contains the putative negative regulatory Ser14 and Tyr15 residues and the cyclin-binding motif, NKIAMRE, from which it derives its name. Human NKIAMRE has significant amino acid identity to related kinases in rat, mouse, Caenorhabditis elegans, and Drosophila, and is widely expressed in human tissues and cell lines. Confocal microscopy demonstrates that NKIAMRE localizes to the cytoplasm. NKIAMRE is activated by treatment of cells with phorbol 12-myristate 13-acetate. Mutation of the ATP-binding Lys-33 to arginine and the Thr-Glu-Tyr motif to Ala-Glu-Phe abolished its ability to phosphorylate myelin basic protein. NKIAMRE is a member of a conserved family of kinases with homology to both MAP kinases and cyclin-dependent kinases.     

Regulation of cyclin-dependent kinases in Arabidopsis thaliana

In plants, different families of cyclin-dependent kinases (CDKs) and cyclins have been identified, indicating that also in plants the progression through the cell cycle is regulated by CDKs. In all eukaryotes, CDKs exert their activity through well-controlled phosphorylations of specific substrates on serine/threonine residues. Such post-translational modifications are universal mechanisms in signal transduction pathways. They allow the organism to differentiate, regulate growth and/or adapt to environmental changes, the latter being crucial for plants because of their sedentary life-style. This adaptation might explain the occurrence of a special CDK type with plant-specific features. This review focuses on the involvement of plant CDKs in different phases of the cell cycle in Arabidopsis thaliana and outlines their regulation by binding to other proteins, and by phosphorylation and dephosphorylation.