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Cellular senescence is, essentially, a permanent proliferation arrest induced by various cellular stresses or inappropriate stimuli. This arrest, which is associated with dramatic changes in cell morphology, metabolism and gene expression, involves a complex signalling network aiming at stable inactivation of CDKs, major cell cycle regulators. Notably, several tumour suppressors, such as p53, pRb or p16(Ink4a), play key roles both in the initiation of the senescence program and in its maintenance, which often involves epigenetic changes. While having widely recognized roles in tumour suppression and wound healing, senescence, like the roman god Janus, recently revealed another darker face. Mostly due to altered secretion phenotype favouring inflammation, senescent cells strongly influence surrounding tissue contributing to the development of age-related pathologies, including cancer.

Cell cycle progression corresponds to a series of events, which succeed one another to end in the division of a mother cell to give two daughter cells. The processes that allow the cell to divide are very precisely controlled by a multitude of enzymatic reactions among which protein phosphorylation, carried out by protein kinases, plays a key role. Serine/threonine kinases are enzymes that catalyse the transfer of a phosphate from ATP to a protein substrate, more precisely on a serine or threonine amino acid residue. Three important families of serine/threonine kinases are involved in the regulation of cell cycle progression, the cyclin dependent kinase (CDK) the polo-like kinase (PLK) and those of the Aurora family. The cancer is described as an uncontrolled cell division process. Cancer cells proliferate indeed in an anarchic way, and carry out cycles of cellular division by being unaware of the signals of alarm. A simple idea thus appeared soon: to stop or to slow down cell cycle progression would result in inhibiting cell proliferation and thus fighting against cancer. Cell cycle progression being controlled in particular by protein kinases of the CDK, PLK and Aurora families, it was rapidly decided to look for inhibitors of those protein kinases. We will first make a general recall on cell cycle progression and the mechanisms that control it. The functions of protein kinases of the CDK, PLK and Aurora families will then be described by concentrating on the sensitive phase of the cell cycle progression, i.e. mitosis. Finally, we will approach the consequences of the inhibition of these protein kinases within the framework of the fight against cancer.

Protein phosphorylation is a fundamental post-translational modification. It regulates a large number of critical cellular processes (differentiation, division, proliferation, apoptosis). Cell division is a process including a series of phases by which a parent cell divides into two daughter cells. The cells enter these stages then progress within the cell division under an accurate control by many proteins. These proteins are activated by phosphorylation. Cyclin-dependent kinases are responsible for this phosphorylation and therefore represent potential therapeutic targets especially in oncology.

Cellular rhythms represent a field of choice for studies in system biology. The examples of circadian rhythms and of the cell cycle show how the experimental and modeling approaches contribute to clarify the conditions in which periodic behavior spontaneously arises in regulatory networks at the cellular level. Circadian rhythms originate from intertwined positive and negative feedback loops controlling the expression of several clock genes. Models can be used to address the dynamical bases of physiological disorders related to dysfunctions of the mammalian circadian clock. The cell cycle is driven by a network of cyclin-dependent kinases (Cdks). Modeled in the form of four modules coupled through multiple regulatory interactions, the Cdk network operates in an oscillatory manner in the presence of sufficient amounts of growth factor. For circadian rhythms and the cell cycle, as for other recently observed cellular rhythms, periodic behavior represents an emergent property of biological systems related to their regulatory structure.

Liposarcoma of the vulva is a rare entity. This unusual localization with atypical clinical and histological appearance may induce diagnostic and treatment delay. We report the 13th case shown in the literature in a 31-year-old woman initially treated for a vulvar lipoma. Arguments based on clinical short term recurrence, histological infiltrating adipocytes, and cytogenentical findings evoked well-differentiated liposarcoma. Even though cytogenetic abnormalities, involving MDM2 and CDK4 genes, have been found, a certainty in malignity diagnosis could be difficult. In these cases, treatment decision may be uneasy. This case report recalls difficulties encountered in uterine hypercellular leiomyomas.

Dramatic changes of cell organisation occur at onset of mitosis. Genetic analysis of fission yeast and physiological studies of vertebrate and invertebrate oocytes showed that activation of cyclin B-cdc2 kinase triggers mitosis. Nevertheless, upstream mechanisms responsible for this activation remain largely unknown in somatic cells of higher eukaryotes. This review discusses possible pathways and mechanisms involved in triggering onset of mitosis in such cells, including inhibitory checkpoint mechanisms that detect defects in structural organisation of the cell.

During the past two years, new molecular targets have been discovered which link cell cycle, cell proliferation and cellular growth. It has become more and more evident that whereas gain-of-function mutations in specific genes can lead to cancer, genomic instability plays also an important role in tumour progression. With examples taken from the recent literature, we describe in this short review crucial findings on the molecular mechanisms controlling cell cycle and proliferation. We illustrate how specific combinations of proto-oncogenes alterations can result in tissue-specific tumours. Finally, impairment of the interactions of a cancer cell with its surrounding neighbours is also shown to participate in the progression toward aggressive phenotypes.

Replicative senescence is defined for human diploid fibroblasts in culture as a cell growth arrest appearing beyond 50 +/- 10 population doublings and associated with telomeres' shortening. This phenomenon shows an increased expression of growth cell inhibitors: p21Waf1 described as an universal CDK inhibitor and p16INK4a as a specific inhibitor for both G1 phase kinases CDK4 and CDK6. The cell proliferation inhibitor p14ARF, product of INK4a/ARF locus is involved in replicative senescence too. Overexpression or homozygotic deletion of these inhibitors demonstrated their role in senescence induction. These proteins are involved in two different metabolic pathways, the first including p53, represented by E2F, ARF, MDM2, p53, p21Waf1, and the second concerning pRb and p16INK4a. These two pathways present numerous interactions and the polymerase (PARP) in relation with p53 and activated by telomere shortening might represent via p21Waf1 a link between this shortening and cell cycle control. An another metabolic pathway involving PTEN and p27KIP1 is discussed in senescent-like phenotype induction, but its activity in replicative senescent is uncertain.

Cyclin-dependent kinases (CDKs) are widely involved in regulating the cell division cycle. The discovery of numerous alterations of CDKs, cyclins, their regulators and their substrates in many human tumours has strongly stimulated the search for chemical inhibitors of CDKs. The potential use of these potent and selective kinase inhibitors in cancer therapy is presently under investigation. Recently a group from Glaxo Wellcome unexpectedly discovered a potential new application of these CDK inhibitors to prevent chemotherapy-induced alopecia. These cyto-protective properties might also be extended to other tissues damaged during chemotherapy.

Comprehension of cell cycle regulation mechanisms has progressed very quickly these past few years and regulators of the cell cycle have gained widespread importance in cancer. This review first summarizes major advances in the understanding of the control of cell cycle mechanisms. Examples of how this control is altered in tumoral cells are then described.The typical mammalian cell cycle consists of four distinct phases occurring in a well-defined order, each of which should be completed successfully before the next begins. Progression of eukaryotic cells through major cell cycle transitions is mediated by sequential assembly and activation of a family of serine-threonine protein kinases, the cyclin dependent kinases (CDK). The timing of their activation is determined by their post-translational modifications (phosphorylations/dephosphorylations), and by the association of a protein called cyclin, which is the regulatory subunit of the kinase complex. The cyclin family is divided into two main classes. The 'G1 cyclins' include cyclins C, D1-3, and E, and their accumulation is rate-limiting for progression from the G1 to S phase. The 'mitotic or G2 cyclins', which include cyclin A and cyclin B, are involved in the control of G2/M transition and mitosis. The cyclins bind to and activate the CDK, which leads to phosphorylation (and then inhibition) of the tumor suppressor protein, pRb. pRb controls commitment to progress from the G1 to S phase, at least in part by repressing the activity of the E2F transcription factors known to promote cell proliferation. Both the D-type cyclins and their partner kinases CDK4/6 have proto-oncogenic properties, and their activity is carefully regulated at multiple levels including negative control by two families of CDK inhibitors. While members of the INK4 family (p16INK4A, p15INK4B, p18INK4C, p19INK4D) interact specifically with CDK4 and CDK6, the CIP/KIP inhibitors p21CIP1/WAF1, p27KIP1 and p57KIP2 inhibit a broader spectrum of CDK. The interplay between p16INK4A, cyclin D/CDK, and pRb/E2F together constitute a functional unit collectively known as the 'pRb pathway'. Each of the major components of this mechanism may become deregulated in cancer, and accumulating evidence points to the 'pRb pathway' as a candidate obligatory target in multistep oncogenesis of possibly all human tumor types.Major advances in the understanding of cell cycle regulation mechanisms provided a better knowledge of the molecular interactions involved in human cancer. This progress has led to the promotion of new therapeutic agents presently in clinical trials or under development. Moreover, the components of the cell cycle are probably involved in other non-cancerous diseases and their role must be defined.

In the past decade, the use of a large variety of cellular models and cell biology, biochemistry and molecular biology techniques has led to the discovery of key proteins that are intimately involved in the regulation of tumor growth. In particular, it has been shown that cyclin-dependent kinases (CDKs) are key regulators of the cell-division cycle. Their frequent deregulation in human tumors make them attractive targets for the identification of new antineoplasic agents. Intensive screening has led in the past few years to the identification of a series of selective and potent chemical inhibitors of CDKs. Drugs representing new lead structures like flavopiridol, indirubin and staurosporine++ derivatives have already been used in clinical evaluation for cancer treatment (clinical trials, phase I and II). Anticancer drug development is being pursued to reduce their toxic side effects, to improve their pharmacokinetic properties and to increase their anti-tumor activity. In this context, traditional drug screening methods in biological test systems have led to the discovery of new compounds such as purine derivatives and paullones, which display remarkable selectivity and efficiency. These novels drugs may result in substantial progress in cancer treatment in the near future.

International audience; Progression in cell cycle is controlled by CDKs (cyclin dependent kinases) and their cyclins regulatory subunits. In mammalian cells, three dual specificity phosphatases called CDC25 activate CDKs/cyclin complexes. The activity of CDC25 is regulated by phosphorylation and dephosphorylation events. CDC25 phosphatases also participate in cell cycle checkpoints activated in response to DNA damage. Two members of this family, CDC25 A and CDC25 B, have oncogenic properties, and their overexpression has been detected in various types of tumors.

When cells are exposed to ionizing radiation, they initiate a complex response that includes the arrest of cell cycle progression in G1 and G2, apoptosis and DNA repair. DNA is an important subcellular target of ionizing radiation, but oxydative damage to plasma membrane lipids initiates signal transduction pathways that activate apoptosis and that may play a role in cell cycle regulation. How is DNA damage converted into intracellular signals for cell cycle arrest? The ataxia telangectasia mutant (ATM) protein and/or the DNA-dependent protein kinase (DNA-PK), that are both activated by DNA damage, may initiate cell cycle arrest by activating the p53 tumor suppressor protein. The p53 protein acts as a transcription factor and regulates expression of several components implicated in pathways that regulate cell cycle progression. The best known, p21WAF1/CIP1 protein, is an inhibitor of cyclin-dependent kinases (CDK), a family of protein kinases known as key regulators of cell cycle progression. p21WAF1/CIP1 was shown to be able to inhibit several CDK, but is most effective toward G1/S cyclins. Other CDK inhibitors, p27KIP1 and p15INK4b are activated by irradiation and contribute to the G1 arrest. Moreover, radiation-induced G2 arrest was shown to require inhibitory phosphorylation of the kinase cdc2 via an ATM-dependent pathway. Mutations in cell cycle regulatory genes are common in human cancer and cell cycle regulatory deficiency can lead to increase resistance to ionizing radiation in cancer cells. The major function of p53-dependent G1 arrest may be elimination of cells containing DNA damage whereas G2 arrest following radiation has been shown to be important in protecting cells from death. Cell cycle checkpoints offer a new set of potential targets for chemotherapeutic compounds, especially the G2 checkpoint. Thus, abrogation of the G2 checkpoint with methylxanthines such as caffeine or protein kinase inhibitors such as staurosporine and UCN-01 (7-hydroxystaurosporine) was found to sensitize cells to ionizing radiation. These data did not lead to clinical applications, but confirm targeting of the G2 checkpoint may be an important strategy for cancer therapy.

Recent studies have demonstrated the close link between oncogenesis and cell cycle machinery. Cyclin dependent kinase inhibitory proteins (Ckis) have been shown to be implicated in cancer progression. We investigated the levels of the different regulatory inhibitory proteins involved in the G1 progression and G1/S in choroidal melanomas.Immunoblotting, immunoprecipitation and immunohistochemistry were performed on human choroidal cell lines and human choroïdal tumors.Our findings suggested a lack of expression of Cdk inhibitor p21 in two of three melanoma cell lines and a striking underexpression of p27 in the three transformed cell lines. The p16 level was found to be almost the same in both normal and transformed cells, a loss of p16-Cdk4 interaction was observed in two of the three melanoma cell lines. In immunohistochemistry, nuclear positivity for p16 was observed in six tumors. Nuclear positivity for p21 was observed in five tumors. All of theses tumors had scleral invasion (p = 0.003). Nuclear positivity for p27 was observed in only two tumors.Our results demonstrate that immunoreactivity for p16, p21 and p27 could be implicated in progression of melanoma tumors. More cases are required to further clarify this issue.

9p21 chromosomal region contains p15INK4b and p16INK4a genes which regulate G1 phase of the cell cycle by inhibition of cyclin-cyclin dependent kinases. The p19ARF protein is translated from an alternative transcript of the p16INK4a gene and regulates G1 and G2 phase of the cell cycle by degradation of the MDM2 protein. p16INK4a and p15INK4b gene homozygous deletions occur mostly in acute lymphoblastic leukemia, ATL secondary to HTLV1 infection, and some lymphoma. Point mutations of p16INK4a or p15INK4b gene seem to be extremely rare, however selective methylations of the p15INK4b or p16INK4a promoters are frequently found in myelodysplastic syndromes and acute leukemias, or lymphomas and myelomas respectively. These data demonstrate that inactivation of gene of the 9p21 region is currently the main molecular event found in hematological malignancies.

Since its recent discovery on chromosome 9p21 band, the p16INK4a/MTS1/CDKN2 gene has been reported as one of the most frequently impaired tumor suppressor genes (ranking second after p53) in a variety of malignancies, including acute lymphoblastic leukemias. In fact, the situation is likely to be more complex than expected: the gene has a very unusual status in that sense that it encodes two structurally unrelated but functionally similar proteins, p16INK4a and p19ARF. In this minireview, the present status of the gene is examined.

The size of a tissue such as the epidermis, and its potential growth, results from the balance between cell production and loss. The mechanisms controlling these two parameters are still poorly understood. Major advances have however been recently achieved in our knowledge of cell cycle regulation and programmed cell death. Dysregulation could explain the development of neoplastic tumors. Basal cell carcinoma basically results from a fall in programmed cell death. Spindle cell carcinoma results from an increased cell production. Both mechanisms are involved in melanoma. One must however keep in mind the fact that we are dealing with hypotheses which remain to be verified.

Aging is classically accompanied by a dysregulation of the immunologic machinery. As a consequence, the immune response developed in senescent organisms is usually inappropriate, often inefficient, sometimes aberrant, and potentially detrimental. The age-associated immune dysfunction may be implicated to some degree in the extreme susceptibility of the elderly to infection and neoplasia and may even participate in various aspects of senescence. The current understanding of the molecular mechanisms underlying immunosenescence is still fragmentary. The most extensively studied phenomenon is the progressive decline in the proliferative capacities of T lymphocytes with aging. The loss of proliferative potential in response to antigenic challenge is a characteristic feature of immune senescence. It is directly implicated in the emergence of the age-related immune deficiency. The purpose of this review is to show how the accumulation of various biochemical lesions with advancing age leads to the failure of a critical cell function, namely the activation-induced lymphocyte proliferation. The biochemical modifications responsible for the defect in transduction and execution of the proliferative signal are analyzed as a function of age. The multiple alterations observed on the various biochemical pathways may appear as a consequence of a unique deleterious mechanism more fundamentally related to the process of senescence such as the inability to cope with oxidative stress.

The use of in vitro cell transformation, as a complement to the direct study of tumors and established tumor cell lines, has enabled the analysis of the contribution of diverse genes to the tumor phenotype. The most recent results have underlined the importance of fundamental mechanisms regulating cell proliferation. The role in the long term survival of mammalian cells in culture and in tumor progression of the telomerase enzyme, which permits the maintenance of chromosome ends integrity, has now been demonstrated. Cell cycle progression and its regulation are ensured by a positive control exerted by cyclin-kinase complexes and by a negative one exerted by kinase inhibitors. Cyclin and kinase genes are frequently altered in transformed and tumor cells, as well as the genes coding for membrane proteins responsible for cell-cell and cell-matrix contacts.

Eukaryotic cell cycle progression is governed by multiple cyclin and cyclin-dependent kinases interactions that are in turn regulated by multiple factors. Recently, inhibitors of cyclin/cyclin-dependent kinases activities have been characterized and their genes have been cloned. Alterations of some these genes may play a major in tumor initiation or progression.

Mitotic cyclins constitute a regulatory subunit of the histone H1 kinase complex. On the basis of primary structure differences, they are divided into two classes, A and B. Both classes are necessary for mitosis to occur. Cyclins A and B differ in the timing of their cellular expression and in their affinity with the various members of the cdk (cyclin-dependent kinases) family. They also have specific functions: cyclin A plays a role in DNA replication, whereas cyclin B are involved in the inhibition of the fusion of early endosomes and in the activation of cdc25 phosphatase. Cyclins A and B can contribute to the development of neoplastic disorders, either directly (inappropriate expression of the cyclin A gene caused by the hepatitis B virus in hepatocellular carcinoma, interactions between cyclin A and factors involved in the regulation of cell division), or indirectly by causing phosphorylation of oncogene products by a cdk.