Uncontrolled proliferation is a hallmark of cancer cells. Molecular analysis of human tumors and animal models have provided a clear basis for the understanding of the cellular processes that govern cell cycle progression in normal and tumor cells. Many cell cycle regulators controlling the correct entry and progression through the cell cycle are altered in tumors. In fact, most, if not all, human cancers show a deregulated control of G1 progression, a period when cells decide whether to start proliferation or to stay quiescent. In addition, tumor cells have impaired cell cycle checkpoints, resulting in the accumulation of genetic aberrations. Manipulation of these control mechanisms provides new avenues for the design of advanced therapeutic strategies against tumor development.
Cyclin-dependent kinases are involved in diverse cellular processes that include cell cycle control, apoptosis, neuronal physiology, differentiation, and transcription. Intensive screening and drug design based on CDK/inhibitor co-crystal structures and on SAR studies have led to the identification and characterization of a large variety of chemical inhibitors of CDKs. Although they all act by competing with ATP for binding at the catalytic site of the kinase, their kinase selectivity varies greatly and remains to be studied in most cases. The requirement for CDKs in many physiological processes justifies their evaluation as potential therapeutic targets against a much larger scope of diseases than initially anticipated.
Cardiovascular diseases are the leading cause of morbidity and mortality in industrialized countries. Most cardiovascular diseases result from complications of atherosclerosis, which is a chronic and progression inflammatory condition characterized by excessive cellular proliferation of vascular smooth muscle cells, endothelial cells and inflammatory cells leading to occlusive vascular disease, myocardial infarction and stroke. Recent studies have revealed the important role of the cyclins, the cyclin-dependent kinases (CDKs), and the cyclin-dependent kinase inhibitors (CKIs) in vascular and cardiac tissue injury, inflammation and wound repair. Tissue remodeling in the cardiovascular system is a regulated balance between pro- and anti-proliferative molecules, and this balance becomes derailed in cardiovascular pathology. Understanding the circuitry of the cyclin-CDK-CKI interactions in normal physiology and disease pathology allows a better understanding of the molecular mechanisms of cardiovascular diseases and permits the rationale design of new classes of therapeutic agents for these diseases.
Studies from eukaryotic model systems, ranging from yeast to human, indicate that Polo and Polo-like kinases (Plks) are essential for the activity of the microtubule organization center. Polo/Plks localize to centrosomes or spindle pole bodies and undergo dramatic subcellular relocation during the cell cycle. Deregulated activities of Plks often result in abnormalities in centrosome duplication, maturation, and/or microtubule dynamics. Genetic and biochemical approaches have identified several candidate genes that either lie in the same pathway as POLO/PLKs or whose products are direct targets of Polo/Plks during the centrosome cycle. Recent studies have demonstrated that mammalian Plks also regulate the function of the Golgi complex, a cellular organelle closely associated with the centrosome and also having microtubule organization activity. Furthermore, deregulated expression of human PLK1 and PLK3 is strongly correlated with the development of many types of malignancies, and ectopic expression of kinase-active Plk3 or Plk1 dominant negative protein leads to rapid cell death. Given that several effective anti-tumor drugs directly interfere with microtubule dynamics, mammalian Plks are excellent targets for the development of anticancer drugs.
The sundry cellular processes required to successfully replicate and divide cells are driven by the sequential activation and inactivation of a family of cyclin dependent kinases. Activation is driven predominately by the periodic expression of the cyclin subunit and requires activating phosphorylation of the kinase subunit. Inactivation is controlled by inhibitory phosphorylation of the kinase subunit, by ubiquitin-mediated degradation of the cyclin subunit and by interaction of the complex with small inhibitory proteins. The mechanisms that coordinate cell cycle progression under favorable and adverse conditions are reviewed.
Potent and selective small-molecule mediated inhibition of the cell's replication machinery remains a principal aim in the development of novel therapeutics and biological probes. Recent efforts have identified small molecules capable of arresting the cell cycle via specific interaction with a variety of intracellular protein targets. Advances in combinatorial and diversity oriented synthetic methods, coupled with a continued effort to identify sources of bioactive natural products, promise to contribute to the growing library of small-molecule inhibitors of the cell cycle.
Dynamic processes such as cell migration and division depend on the actin cytoskeleton, a dense meshwork of protein polymers capable of undergoing rapid cycles of assembly and disassembly, under the control of a large number of actin-associated proteins. In cancer cells, structural and functional perturbations of the actin cytoskeleton correlate with higher proliferation rates and uncontrolled movement. Therefore, small molecules that act on the actin cytoskeleton of tumour cells and thus inhibit cell division and movement, may be of high therapeutic value. The dynamic properties of the actin cytoskeleton and the mechanism of action of actin-targeting drugs will be described.
There is an urgent need to develop new drugs against eukaryotic parasitic protozoa such as Plasmodium, Trypanosoma and Leishmania, which cause the diseases malaria, trypanosomiasis and the leishmaniases respectively. The biology of these organisms has many unusual facets that might be exploited for drug design, and the recent availability of parasite genome sequence data has facilitated the search for novel drug targets. Here we review current understanding of the cell cycle in these parasites and show that important structural and functional differences exist between parasite and mammalian cell cycle control machineries and signal transduction pathways, which might be utilised for rational drug design. Potential targets include protein kinases from the cyclin-dependent kinase, cAMP-dependent kinase and mitogen activated protein kinase families.
The phospho-Ser/Thr-Pro specific prolyl-isomerase Pin1 has been implicated in multiple aspects of cell cycle regulation. It has been suggested that Pin1 function is required for both normal mitotic progression and reentry into the cell cycle from quiescence. In support of this hypothesis, numerous key regulators of G1 and mitosis have been identified as Pin1 interacting proteins. However, the cellular consequence of Pin1 binding to these proteins has rarely been rigorously characterized. In this review we focus on the role of Pin1 and its binding proteins in cell cycle regulation and the potential value of Pin1 as a therapeutic target.
The nucleolus is the ribosome factory and also a multifunctional domain that plays an important role in nuclear organization and function. Interestingly, ribosome biogenesis appears directly related to cell growth and proliferation. The nucleolus corresponds to a very dynamic nuclear domain resulting from an equilibrium between the level of ribosomal RNA synthesis and the efficiency of ribosomal RNA processing. Ribosome biogenesis is regulated throughout interphase and stops during mitosis. The mitotic silencing of transcription, the distribution of the processing machinery, as well as the link between transcription and processing were found to be dependent on cyclin-dependent kinase(s).
GSK-3 is a multifunctional protein kinase known to play a pivotal role in the regulation of metabolism, the cytoskeleton and gene expression. It also interacts with the cell cycle in a number of ways. GSK-3 forms part of both Wnt and Hh signalling pathways and hence controls expression of a number of cell cycle regulatory genes. Prominent among these is cyclin D1. GSK-3 also phosphorylates cyclin D1 to promote its nuclear export and subsequent degradation. In this chapter we examine how GSK-3 mediates these effects and consider how therapeutic strategies may be developed to specifically target these pathways.
The control of gene expression at the translational level has emerged in the past decade as an important aspect of cell growth, proliferation and malignant transformation. Translation is primarily regulated at the initiation step, and mitogen-dependent signaling pathways converge to modulate the activity of translation initiation factors. In most tumors tested, at least one translation initiation factor is overexpressed and overexpression of translation initiation factors often provokes transformation. Malignant transformation could be caused by the increased translation of a subset of mRNAs encoding important proteins which are required for cell growth and proliferation. These mRNAs usually possess regulatory sequences that render their translation more sensitive to changes in the activity of translation initiation factors. In this chapter, we describe recent advances illustrating the importance of translation in cell cycle progression and cell transformation. Control of translation initiation may represent an excellent target for antitumor drugs.
Rho family GTPases (Rho, Rac and CDC42) share around 30% sequence identity with RAS family GTPases, and are essential for RAS-induced malignant transformation, i.e., aberrant serum/anchorage-independent growth and actin cytoskeleton-linked morphological changes. Oncogenic RAS mutants such as v-Ha-RAS trigger cell cycle entry (G0-G1 transition) mainly by up-regulating cyclin D1, an activator of cyclin-dependent kinases (CDK), and down-regulating p27, a CDK inhibitor. Although both Rac and CDC42 are clearly activated by RAS, there is so far no evidence that RAS activates Rho. In this chapter, we will discuss the role of these Rho family GTPases and their effectors, in particular the Ser/Thr kinases PAK1 and Rock, in RAS-induced serum/anchorage-independent cell cycling, and discuss several potential therapeutics, peptides or chemical compounds, that could block this oncogenic cell cycle signalling pathway.
The presence of mutated Ras in more that 30% of human cancers has spurred interest in the identification of molecules that can block its uncontrolled signaling function. A particular focus in recent years has been a key posttranslational modification of Ras that places a farnesyl group on a cysteine residue near the C-terminus of the protein. In this chapter we describe recent progress in the design of inhibitors for the enzyme that catalyzes this step, protein farnesyltransferase, and show their potential for blocking oncogenic cell growth.
The precise duplication of chromosomal DNA during each cell cycle is essential for the maintenance of genetic stability. Failure to correctly regulate chromosomal DNA replication could lead to losses or duplication of chromosome segments. The precise duplication of chromosomes is normally achieved by correct regulation of the replication licensing system. Here we review our current knowledge of the licensing system and how this might be defective in cancer cells. We also review how detection of licensing components can be used for the diagnosis and prognosis of cancer. Finally we discuss the potential of the replication licensing system as a novel anti-cancer target.
Successful cell division requires that daughter cells inherit not only a complete set of chromosomes, but also only one centrosome, and similar amounts of organelles and cytoplasmic components. The different mitotic processes are driven by cell cycle-regulated protein kinases and phosphatases and their fidelity is closely monitored by a number of checkpoint mechanisms. Histone H3 is phosphorylated during mitosis, but the kinases involved were not known until recently. Recent work has revealed that Aurora kinases are required for mitotic phosphorylation of histone H3 and of its centromeric variant CENP-A. This finding has stimulated functional studies of the role(s) of Aurora kinases and H3 phosphorylation during mitosis, which are reviewed in this chapter.
Cells normally respond to DNA damage by activating checkpoints that delay the transition from G1 to S and from G2 to M while DNA is repaired. The checkpoints thus protect cells by blocking replication of damaged DNA and segregation of damaged chromosomes. Most cancer cells have an inoperative G1 checkpoint due to p53 inactivation, and a functioning but impaired G2 checkpoint. Inhibitors of the G2 checkpoint can selectively sensitize cells with inactive p53 to killing by DNA-damaging drugs or ionizing radiation and might be useful in cancer therapy. Cell-based and target-directed screens for checkpoint inhibitors have been developed and several checkpoint inhibitors have been identified. This review describes their chemical structures, biochemical targets and cellular effects and discusses their therapeutic potential.
The mechanism by which neurons die in human neurodegenerative diseases remains an enigma till today. Terminally differentiated neurons of normal brain are incapable of cell division. However, accumulating evidence has suggested that aberrant activation of the cell cycle in certain degenerative diseases leads to their demise. In Alzheimer's disease, regulators spanning every phase of the cell cycle are upregulated in affected neurons, leading to successful DNA replication, but unsuccessful mitosis. The end point of this nonproductive cycle of division is death. Elucidating the details of this cell cycle-mediated degenerative cascade may lead to novel strategies for curbing the onset and progression of degenerative diseases.
Ataxia-Telangiectasia mutated (ATM) and ATM- and Rad3-related (ATR) are members of the phosphatidyl inositol 3-kinase-like family of serine/threonine protein kinases (PIKKs), and play important roles in the cellular response to DNA damage. Activation of ATM by ionizing radiation results in the activation of signal transduction pathways that induce cell cycle arrest at G1/S, S and G2/M. ATR is required for cell cycle arrest in response to DNA-damaging agents such as ultraviolet radiation that cause bulky lesions. This review focuses on the role of ATM and ATR in various DNA damage response pathways, and discusses the potential for targeting these pathways for the development of novel therapeutics.
The ERK signaling pathway, also known as the p42/p44 MAP kinase pathway, is a major determinant in the control of cell growth, cell differentiation and cell survival. This pathway, which operates downstream of Ras, is often up-regulated in human tumors and as such represents an attractive target for anticancer therapy. In this chapter we review the rationale for targeting the components of the ERK pathway, either alone or in association with cytotoxic anticancer agents. We present the most advanced inhibitors of this pathway and discuss their specificity and mechanism of action.
