Telomere-induced senescence has been proved to be as effective as apoptosis in reducing cancer incidence and is mediated by the tumor suppressor gene, p53 26. Mutations in the p53 gene frequently appear in human tumors conferring aggressive oncogenic properties such as exacerbated malignant transformation and metastatic phenotype when over-expressed in p53-null cells. P53 gets activated upon genotoxic and non genotoxic stresses like oxidative damage and activates p21 and ultimately culminates the cell to senescence. Mice with a point mutation (p53(R172H)) in their endogenous p53 loci act as a model for the human Li-Fraumeni syndrome. Genetic alterations at chromosomes 3p, 6p, and 1lq were frequently found early in tumor development and showed additional allelic losses at chromosome arms 6q, 17p and 18q. Genes for telomerase suppression are presumably located on chromosomes 3, 4 and 6 27.

P53 over expression has been directly associated with unfavorable clinico-pathologic factors such as advanced stage, histologic subtype, advanced patient age and nodal metastasis in endometrial carcinomas while bcl-2 expression was related with younger age, favorable grade and PR expression by tumor cells. Patient survival is however not related to the tested biomarkers 28. In humans, TP53 codon 72 Arginine to Proline polymorphism was found to affect both cancer incidence and longevity as well 29. The senescence-associated signature of p53 isoform expression (that is, elevated p53beta and reduced Delta133p53) was observed in vivo in colon adenomas with senescent phenotypes. The increased Delta133p53 and decreased p53beta isoform expression found in colon carcinoma may signal an escape from the senescence barrier during the progression from adenoma to carcinoma 30.

P107 is required for the initiation of accelerated cellular senescence in the absence of Rb and p130 may be required to prevent the onset of this phenomenon in un-stimulated prostate cancer cells lacking a functional Rb allele 31. Cell cycle regulatory proteins are more sensitive to exogenous hormone treatment in postm-HBT (postmenopausal human breast tissue) than in pre-HBT (premenopausal human breast tissue) 32. Olsson et al advocates that bfl-1 (tumor suppressor bcl-2 family member) contributes to chemo resistance and might be a therapeutic target in B-cell chronic lymphocytic leukaemia 33. The activation of PI3K/Akt pathway is involved in the late-stage progression and metastasis of gastric cancer and attenuation of p-Akt by 2-ME suppresses metastasis 34. Yet another tumor suppressor Promyelolytic leukemia (PML) regulates p53 acetylation in both RS as well as Ras induced accelerated senescence 35.

A large number of in vitro studies have been reported where a wide range of chemotherapeutical antidotes induce senescence like morphological changes and SA-β-gal expression in cancer cells activating the pathway of senescence. Research into the induction of cellular senescence as cancer therapy has however, been hindered by a lack of compounds that efficiently induce this response. To overcome this, Ewald et al (2009) by using dual Hoechst 33342 and SA-β gal staining identified library compounds that induce senescence in prostate cancer cells 36. It is well acknowledged that telomerase and the maintenance of telomeres are key players in the ability of stem and cancer cells to bypass senescence and be immortal. Proliferation of telomerase (-) pre-malignant cells leads to telomere dysfunction and increased genomic instability suggesting one possible sequence of events leading to immortalization of breast epithelial cells during cancer progression 37. The increased h-TERT expression may be a cellular response to genomic insults by various metal toxicants like arsenic that may also act as a tumor promoter in mammalian carcinogenesis as studied in blood cells by Mumford et al hTERT-specific T cells could contribute to the immunosurveillance of breast cancer suggests novel opportunities for both therapeutic and prophylactic vaccine strategies for cancer 38.

In one of the studies, using non-small lung adenocarcinoma A549 cells, it was shown that after treatment with DNA damaging anti-tumor drugs like caffeine, cells become permanently growth-arrested as a result of so-called drug-induced premature senescence (pseudo-senescence) or SIPS. Similarly, lowered efficacy of anti-cancer doxorubicin (due to dose dependent toxicity) against breast cancer cells can be increased when used in conjunction with siRNA inhibitor of telomerase 39. Yet another study advocated the use ofGRN163L (novel telomerase template antagonist) in the treatment of breast cancer by augmenting the effects of paclitaxel 40. Hence clearly proposing that inhibition of telomerase is a potential treatment strategy for inducing senescence. It has also been shown that caveolin-1 targets Mdm2/p53-mediated pathway and causes senescence in breast cancer cells 41. Another study reported that bleomycin, a widely used anti-tumor agent, causes senescence of lung cancer cells by modulating the roles of caveolin-1, a protein abundant in lung fibroblasts and smooth muscle and endothelial cells 42.

A recent study showed that the activation of the p53-p21(Cip1/WAF1) pathway acts as a major mediator of cellular senescence induced by CKII inhibition in HCT116 colon carcinoma cells 43. A senescence-inducing effect of doxorubicin on the same cells, in another study, had a dual effect-it stopped the proliferation of the majority of the cells and led to the appearance of proliferating aneuploid cells 44. Likewise, while characterizing ashwagandha and its molecular mechanisms Wadhwa et al provided the first example that phytochemical(s) have both anti-cancer and anti senescent activities and pointed towards the molecular link between aging and cancer using normal human fibroblasts through decreased accumulation of molecular damage, down-regulation of the SA-β gal activity and the senescence marker protein, p21(WAF-1), protection against oxidative damage, and induction of proteasomal activity 45. In one of the studies, by silencing BRCA1 expression at different levels through RNA interference technology in a series of partially transformed (HBL100) and tumorigenic (MCF7 and T47D) breast cancer cell lines, cell models were probed by clonogenic assay for their response to several DNA-damaging agents (mitomycin C, cisplatin, doxorubicin, and etoposide) commonly used in cancer therapy 46. The increased sensitivity to these compounds displayed by BRCA1-defective cells was correlated to an increased fraction of growth-arrested, enlarged, multinucleated SA-β-galactosidase-positive senescent cells 46. Melanocytic nevi frequently harbor oncogenic BRAF mutations and recently it was found that a subpopulation of melanocytes possess the ability to survive BRAF induced senescence, and suggest that p53 inactivation may promote malignant transformation of these cells 47 and thus have implications in skin cancer treatment. In vitro experiments with therapeutic nucleic acids successfully inhibited E6/E7 oncogene expression and caused induction of apoptosis and/or senescence in cervical carcinoma cells. A useful assay was described by Lau et al 48 to predict the response of the patient to a set of medicines without administering them by testing the susceptibility of a sample of cancer cells in vitro and comparing it to the standard regimen.

Apart from these, it has been observed that cells' passage number controls the appearance of senescence. Normal human diploid fibroblasts approach senescence near passage 64 through RNaseT2 expression, which however fails to induce senescence in SV40 immortalized cell lines 49. Rat chondrocytes show the onset of senescence in the 4^th passage 50 while human rheumatoid arthritis fibroblast-like synoviocytes exhibit ageing at 10^th passage 51. Stable clones derived from hTERT-expressing normal and G6PD-deficient fibroblasts have normal karyotypes, and display no sign of senescence beyond 145 and 105 passages, respectively, suggesting that ectopic expression of hTERT, in addition to telomere length maintenance by activating telomerase, also functions in regulating senescence induction 52. Recently, a study explored the self-renewal potential of human breast stem cells and found that it gets exhausted within five in vitro passages of mammospheres, suggesting the need for further improvisation in culture conditions for their long-term maintenance 53.

Induction of senescence upon drug administration has been proposed as a possible anti-cancer treatment in various animal models. The finite proliferative potential of normal human cells leads to RS, which is a critical barrier to tumor progression in vivo. By studying embryonic fibroblast-derived cells with loss-of-senescence or H-RasV12/E1A-transformed phenotypes at different stages of oncogenic progression in nude mice, it was postulated that they may escape therapies aimed at metabolic inhibition of tumors with a fully developed Warburg phenotype 54. β-carotene provides protection against O₃-induced skin oxidative stress in female SKH-1 mice skin, which is consistent with a protective role for beta-carotene in the skin hence has implications in skin cancer and aging or senescence of skin 55. A novel target of NESH-SH3 (TARSH), cellular senescence related gene in mouse embryonic fibroblasts may suppress tumor development in pulmonary tumorigenesis mouse model by causing an increase in SA-β-gal activity and this was attributed to p53-dependent p21^(Cip1) accumulation 56. Pituitary tumor transforming gene deletion results in pituitary p21 induction and abrogates tumor development in Rb(+/-)Pttg(-/-) mice. Senescence was evidenced by increased p21 and SA-β-galactosidase. Aneuploid pituitary cell p21 may constrain pituitary tumor growth, thus accounting for the very low incidence of pituitary carcinomas 57. Work by Efimova et al using p38-null mice skin carcinogenesis model strongly suggests a role for p38delta (key regulator in senescence, tumorigenesis, survival, inflammation etc) in promoting cell proliferation and tumor development in epidermis and may have therapeutic implication for skin cancer 58. Three xenograft breast cancer mouse models, 2 of them with a TP53 mutation and one without it, were studied for their immediate response to high doses of epirubicin-cyclophosphamide. TP53 wild type stained positive for SA-β-galactosidase staining and also over expressed P21 but TP53 mutant did not succumb to senescence suggesting that treatment induced senescence is mediated via functional p53 in breast cancer 59. More in vivo studies are however, needed to elucidate the role of senescence in cancer. Although these concepts are well supported in these models, translating them to clinical oncology remains a challenge.

The physiological phenomenon of senescence serves as a lucrative pathway to annihilate deleterious cancer cells and tissues. This program of senescence is activated upon the administration of various anti-cancer regimens. Even though this is not a universal mechanism of curbing tumor cell growth, yet a considerable number of instances of in vitro as well as in vivo studies have been cited to decipher the metabolic pathway it targets and these studies have produced useful results that have enhanced and refined our knowledge about these pathways (Figure 1) and will be helpful in delineating new treatment strategies for curtailing cancer. Several studies 6061, however provide compelling evidence that some cancer cells which are mitotically non-viable escape cell death, due to the accumulation of some genetic and epigenetic mutations and p53/pRB/p53^Ink4a-dependent senescence checkpoint malfunctioning resulting in telomerase dysfunction 6162 and finally evade cell death via continued progression through neosis. Such cells acquire so called' immortality' and to eliminate them, different strategies need to be designed. These cells multiply by a unique route called 'neosis' that facilitates in its progression and existence thereby evading the program of senescence. It has been described as a parasexual, somatic, reduction division in cancer 62. Although neosis-like events have been reported in the literature sporadically for more than a century 63 under different names, they have been neglected due to the lack of appreciation of the significance of this process in cancer biology. Neosis may be a fundamental step in current concept of multi step carcinogenesis. Studying the behavior of individual neotic clones has revealed the significance of their central role in cancer 64. Non-synchronous occurrence of secondary/tertiary-neosis (Figure 2) creates the illusion of the existence of cancer stem cells and the 'mirage' of immortality of cancer cells.

Some of the genetic and epigenetic alterations become the achilles heel of the mutated tumors that bypass the effect of certain classes of anti-cancer agents. That is, patients whose tumors carry such defects can be stratified for respective therapy rendering some classic DNA damaging agents called neosicides into "targeted therapies." Development of novel strategies to improve current status of cancer therapy will require identification and exploitation of yet unrecognized differences between normal and tumor cells with respect to propagation, evolution and development of resistance to conventional treatments 65. The discovery of neosis has identified novel cellular targets, against which one can identify novel neosis-specific molecular targets in order to design anti-neotic agents or neosicides that will be more specific in their action and do less harm to non diseased cells. A judicial combination of senescent drugs with efficient neosicides could further improve the status of cancer control.

The focus of this section is on the different MKs families. These kinases are modulated by de-novo synthesis, stability factors, phosphorylation, and ubiquitin-dependent proteolysis. They, in turn, phosphorylate innumerable centrosomal/mitotic protein substrates, and have the ability to behave as oncogenes (i.e. Aurora-A, Plk-1), providing a compelling link between errors in mitosis and oncogenic processes 73. Additionally, dysregulation of MKs have been linked with improper cell cycle progression both in vitro and in vivo. Without getting into the basics of MKs, the main pre-clinical and clinical studies concerning MK inhibitors currently under investigation are reported and important considerations for their future development are discussed. Here is given a representation of kinases in different phases of cell cycle (Additional File 1: Table S1).

Cdk1 is vital participant in the mitotic cell cycle. Mitosis begins and ends with the activity of cdk1 with binding partner cyclin B1. First studied in fission yeast (Saccharomyces cerevesiae), Nurse 74 identified a gene that controlled mitosis and named it cdk1 or cdc2. Studies have revealed that functional p53 protein may enhance the anti-cancer activity of roscovitine (known cdk1 inhibitor) that could be beneficial for anti-cancer therapy 75. Tumorigenecity mediated by p53 loss does not require either Cdk2 or Cdk4, which necessitates consideration of the use of broad spectrum cell cycle inhibitors as a means of effective anti-Cdk cancer therapy 76. Gartner et al have reported for the first time reported an association of cyclins and Cdks with the microtubule network by immunoelectron microscopy and immuno-biochemical methods. Cyclins D, E, A and B as well as Cdks 1, 2 and 4 were also found to be associated and exhibit kinase activity towards the microtubule-associated protein tau 77. Bailet et al 78 have highlighted a new role for spleen tyrosine kinase (Syk) in regulating cellular senescence and identify Syk-mediated senescence as a novel tumor suppressor pathway, the inactivation of which may contribute to melanoma tumorigenicity. Study by Buchanan et al 79 on murine adenocarcinoma mammary cells provided new clues regarding the mechanism involved in the modulation of mammary tumor cell growth and survival induced by glypican-3. Gene expression profiling has generated hypotheses that led to an increase in our knowledge of the cellular effects of seliciclib (cdk inhibitor) and could provide potential pharmacodynamic or response biomarkers for use in animal models and clinical trials 80. Another Cdk inhibitor SU9516 is over expressed in HCT116 cells by the knockout of the p21^WAF1/CIP1 gene which suppresses thymidylate synthase and enhances chemosensitivity to 5-Flurouracil 81.

Chk1 and Chk2 are effector kinases in the cellular DNA damage response and impairment of their function is closely related to tumorigenesis. If DNA damage is detected after S and before G2/M transition, ATM/ATR is activated and phosphorylation of Chk1 and Chk2 occurs 82 leading to cell death during mitosis or MC. Experiments have demonstrated that there are alternate mechanisms for activating ATM that are both stress-specific and independent of the presence of DNA breaks 83. The activation of the ATR-Chk1 pathway in response to bifunctional DNA alkylator 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) treatment and the dependency of this response on the DNA mismatch repair capacity were investigated. Chk1 was found to be phosphorylated at serine 345 and exhibited increased kinase activity. Si-RNA knockdown of ATR also reduced Chk1 phosphorylation following exposure to BCNU. However, knockdown of ATM had no effect on the observed Chk1 phosphorylation, suggesting that ATR was primarily responsible for Chk1 activation 84.

A family of serine/threonine kinases also designated as tubulin-associated proteins actively participate during mitosis and comprises four distinct members: Plk1 (Plk), Plk2 (Snk), Plk3 (Prk or Fnk) and Plk4 (Sak) 85 each carrying out a multitude of distinct roles. Plk1 is the most extensively characterized among the family members, suggesting that the polo box domain of it can provide an additional structural basis for discovery of new anti-cancer drugs. It was also found out that Plk1 is required for chromosomal DNA replication under stressful conditions 86 and Plk3 is more potent in inhibiting cell proliferation and inducing apoptosis 87.

Plk1 gene expression is tightly regulated with mRNA increase beginning in S phase and peak mRNA levels detected at G2-M transitions and through mitosis 88. RNA-interference -mediated depletion of Plk1 to determine its potential for sensitizing pancreatic tumor cells to gemcitabine showed that small interfering RNA-mediated knockdown of Plk1 caused cell cycle arrest at G2/M and the reduction of cellular proliferation and decreased cell viability and increased cellular apoptosis 89. Transcription of Plk1 is inhibited along with other G2/M specific genes like cyclin B1, cyclin B2 and cdc25B by inhibition of Nuclear Factor kappa B at G2-M phases 90. Studies define and illuminate a late mitotic function of Plk1 that it is obligatory in the positioning and recruitment of Rho guanine nucleotide exchange factor (RhoGEF) Ect2 to the central spindle and abolishing RhoA GTPase localization to the equatorial cortex, and suppressing cleavage furrow formation and cell division 91. Increased plk-1 gene and protein perhaps play a key role in abnormal proliferation of acute leukemia cells and correlate with the malignancy of leukemia 92 prostate carcinoma, 93 and gastric carcinoma 94. Snk/Plk2 is transcriptionally down-regulated in B-cell neoplasms 95 and consequently provides a potential mechanistic basis underlying the strong selective pressure for abrogation of Plk2 function in B-cell neoplasia. Plk3 has been shown to catalyse the priming of Cdc25A by phosphorylated glycogen synthase kinase-3β (GSK-3β) and observations indicate that GSK-3β inactivation may account for Cdc25A overproduction in a subset of human tumors 96. LFM-A13 (alpha-cyano-beta-hydroxy-beta-methyl-N-(2,5-dibromophenyl)propenamide) has recently been identified as an inhibitor of Plks and markedly enhances the anti-cancer activity of paclitaxel 97 with anti-proliferative activity against human breast cancer 98.

Aurora kinases namely, Aurora A(Aurora 2), Aurora B(Aurora 1) and Aurora C(Aurora 3) are serine/threonine kinases also known as tubulin-associated proteins 99 which are expressed only in actively dividing cells and their increase is a factor of bad prognosis in cancer. Side effects, dosing and tolerability of inhibitors have been discussed in great length by Pinel et al 100 and enzymatic characterization of GSK1070916, a potent and selective Aurora-B/Aurora-C inhibitor was done and compared with other Aurora inhibitors AZD1152 and VX-680 101, GSK1070916 was found to exert a more prevailing inhibitory effect due to a slow rate of dissociation from the Aurora-B & C enzymes. Detailed kinetic analyses of two isogenic cell lines differing in p53 function and have been compared with the effects of ZM447439 and VE-465 to describe several mechanisms explaining how cells may evade killing by Aurora kinase inhibitors 102. It has been proposed that perikinetochoric rings of MCAK and Aurora-B define a novel transient centromere domain at least in mouse chromosomes during meiosis and also its functions have been illustrated by Parra et al 103.

The Bub family of kinases constitutes members that are concerned with spindle assembly functioning and APC/C regulation. In one of the studies p53 was sustained to express in K562 leukemic cells after being infected by recombinant adenoviruses carrying the wt-p53 gene and it was shown that wt-p53 can suppress excessive replication of centrosomes and may contribute to the upregulation of Gadd45a and BubR1 protein expression as well as the downregulation of Aurora A protein expression 104. A novel study reports that Ajuba, a microtubule-associated protein collaborates with Aurora B and BubR1 at the metaphase-anaphase transition and ensures proper chromosome segregation105.

The Nek or Nrk related kinase family are essential MKs first described in the filamentous fungus Aspergillus nidulans 106 containing 11 members (Nek1, 2, 3, 6, 7, 8, 9, and 11 are prominent) 107. Nek1 is involved early in the DNA damage sensing/repair pathway after IR and G(1)-S-phase checkpoint control can be rescued by ectopically over-expressing wild-type Nek1. Moreover, in cells without functional Nek1, DNA is not repaired properly, double-stranded DNA breaks persist long after low dose IR, and excessive numbers of chromosome breaks are observed 108. Recently, studies have explicated that ciliary localization of Nek8 in a subset of ureteric-bud-derived kidney tubules is essential for maintaining the integrity of those tubules in the mammalian kidney 109.

As current cancer therapies are still in their infancy and are not able to fulfill the expectations of cancer control, strategies targeting mitotic regulators could be a potentially pragmatic option, which may improve the therapeutic index when used either alone or in combination with current anti-cancer antidotes. The uniqueness of MKs lies in the fact that they are expressed in actively duplicating cells and not in differentiated cells further make it important targets against cancer cells. Targeting MKs would aid us in understanding the mechanism of chemo-resistance. The research efforts to examine the role of MKs and mitotic signaling pathways are, however, in its beginning. By presenting an overview of regulation of MKs in this review, we open promising avenues in designing novel therapeutic approaches in curbing cancer. Simultaneously, we also present the rationale for these kinases as an anti-cancer target. Hence, more concern needs to be laid on in vivo work to understand the role of MKs and their utility as targets before we can actually embark on translational studies in human.