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Tuesday, 11 August 2020

Cancer and Cell Cycle checkpoints


How Does Cancer Affect Normal Cell Functioning


Motivation: the term cell cycle is commonly depicted as uncontrolled multiple division of the cell in result proliferation of cells that change into a malignant tumor form. This cell  outgrowth produce metastasis .the past several years witnessed that cancer play a dramatic role in destroying the life of peoples so a lot of work done on ontogenesis clinically as well as in drug making industries. Cancer cells encompass a series of missteps that change a normal cell into proliferative tumor cells, which also affect surrounding cells. There are various cell cycle, regulators which control division of cell cycle but when they are disturbed or deregulate then a normal working cell transformed into tumor cells. Series of checkpoints are involved in controlling cycle cell but aberrations of these checkpoints provide an opportunity for Cancer cells present study is conducted to find out and summarize all work done on Cell cycle and oncogenesis.

Implications:  So Cancer  can be controlled by multiple steps and checkpoints .there are two main regulators which are involved in controlling cell cycle are retinoblastoma protein RB and P53 transcription factor which is also called guardian of the cell cycle. other  checkpoints involve in the cell cycle are Cyclin-dependent kinase  (CDK) Complex of protein which is either directly or indirectly involved in cell progression, deregulation of these the complex of protein notice in mutated cancer cell



Cell cycle phases and control: Cell division consist of series of steps that lead towards duplication of DNA for production of daughter cell, it’s an important process by which a single cell duplicate itself and fertilized egg develop into a complete adult individual. The division process is very necessary for development of various internal parts of the body such as hair, nail, blood cells, brain cell etc., but any sort of imbalance and deregulatory damage the whole division process. Cell during division enters in distinct four phases G1,,S ,G2,M phase. (Pardee AB, Dubrow R .1978) G1 ( Gap 1 ) S( Synthesis phase,G2 ( Gap 2 phase)are collectively called as interphase, M phase is called as Mitosis phase .the period between mitosis also term as interphase in which DNA Replication took place in S phase(Baserga R,1985). Mutated cancerous cells show various features in which cell damage. insensitive to  medicine, the proliferation of surrounding cells, decreased of  checkpoints activity(Hanahan D. and Weinberg ,R.A  2000).,In S phase DNA duplicate itself and exact copy of DNA form,G1 ,G2 both  gap phase in which cell prepare itself for a different type of metabolic activity, different enzymatic reaction also take place. various checkpoints are available which monitor the growth activities of cells as well as the length of phases. Cells have another temporary phase in which cells enter for a short time ,stop to divide itself called the Go phase, this phase also called the resting phase. Different events show that mutation and deregulatory in these checkpoints disturb the normal division of the cell. he prevention of human cancer   (Moon Taek  Park and Su-Jae Lee.2003).

cell cycle

Different cyclins and different Cdk  complexes proteins  are expressed in the whole-cell cycle at different points that check and maintain cell cycle (Nigg,1995).following diagram show the  stages and description of their activities (Kathleen       figure show the process of replication of DNA and  division of the cell in series,,these are coordinated steps ,Coordinated the mechanism is control by  Cascade of kinase protein which is step control next upcoming step. this is a high kinase activity. 2nd regulatory step is CDKs which is strictly monitor the cell cycle process, but if there is flaw occurs in regulatory checkpoints then it will be harmful for replication of DNA process as well in Segregation of Chromosomes.

Cell cycle machinery control the mutation process as well as proliferation of cells, because Cancer is a disease of proliferation in which uncontrolled division of cell take place cells not able to perform function well and insensitive to drugs and genes in mammals there are different CDKs bind to various types of CDKCs ,which are known as Cdk1 (first human Cyclin-dependent kinase protein identified),it is also associated with Cyclin A /B. There are also other mammalians Cdk 2,Cdk4 ,Cdk6. These are regulatory check points control the cell cycle at different phases of cell cycle .but the CDKs remain same  and specific in cell cycle but the level of Cyclin  vary, which lead towards progression in cell cycle.


cell cycle phases

                                                 Figure 2: expression of Cyclin A, B, D and E during G1, S, G2 phases of the cell cycle (waehrend_Zellzklu, 2011).

Family of protein that inhibits Cyclin-dependent kinase ( CDKs) are termed as  Cyclin-dependent kinase inhibitor CDKI play a vital role in progression of cell cycle. Subfamily (MTS1, MTS 2, P 18) of these complex CDKI proteins has homology with the tumor-suppressing gene. The subfamily proteins are structurally and functionally similar to tumor-suppressing genes.

Another factor which regulates the cell cycle and mutation in this checkpoint plays a vital role in the development of Cancer. Cellular Senescence is a phenomena in which permanent form of cell cycle cease, which can be triggered by DNA damaged as well as activation of oncogenes( Lowe SW , Cepero E, Evan G.,2004)  p53 also depicted as  TP53, also called as tumor protein  because it is the gene that regulates and maintain cell cycle hence function as tumor suppression,p53 has a great role  in conserving stability by preventing mutation in the genome ( Strachan and Read ,1999) , cellular senescence has been using in response to anti-tumor treatments.  If the p53 gene damaged then the tumor suppression is reduced, few people inherit one functional copy of p53 will most likely develop a tumor in adulthood, a disease called Li –Fraumeni Syndrome. In health p53 is regularly produced and then also degrade in Cell, but this is associated with MDM-2 (which is own regulatory gene).p53 has also play role in DNA repair and apoptosis (cell death). p53 activates the target genes which are Apaf1 and Bax .apoptosis  can be induced by binding with cascade protein  caspase 9,but this is done in a situation to avoid the proliferation of cell which containing damaged ,mutated DNA  otherwise Cell lead to Cancerous.

Related Work

 Cancer research in the last few decades has generated complex knowledge, revealing cancer a disease which involves a change in genomic sequence. These changes produce due to loss of suppressor genes, alternation of recessive genes loss of function. two classes of Cancer genes identified through alternation  in human as well as animal cancer  by depicting cancer phenotypes in experimental models ( Bishop and Weinberg,1996). Pathological analysis of various organ sites expose lesions that represent the intermediate steps in a process in which normal cells transformed into  premalignant sites into invasive cancer (Foulds 1954).Cancer cell genotypes  is a tangible  of six  alternations in cell physiology  which collectively Commands malignant growth of cell  and mutate the function of the cell cycle . There are at least four mechanisms by which the normal mutation rate could lead to mutations leading to tumor progression without assisting  an increased mutation rate in cancers. Firstly, cancer-causing genes could contain sequences that mutate at exceptionally high frequencies. Mutagenic hot spots have been found in a variety of genes (Coulondre, C., and Miller, J.H ,1977) though mutational activation of ras genes can occur at multiple sites, mutations in cancers are commonly observed at only a few sites (Barbacid,1987). It is not known whether the localization of mutations to specific nucleotides in ras genes results from an increased rate of DNA damage, or destroy DNA repair, or an elevated frequency of misleading by normal DNA polymerases opposite unrepaired lesions, or from the selective building up of cells refuge specific ras mutation mutations which lead to frank cancers could have multiple causal origins. Both spontaneous and chemically incorporate mutations could contribute to tumor progression. The 100-fold higher incidence of human cancer in the large intestine compared to that in the small intestine could reveal the part of carcinogens produced within the large intestine by the bacterial flora. Although the cell turnover rate is slightly greater in the small intestine, the track from stem to discriminating cells may be complex and different in the small and large intestines. Most vitally, premalignant changes in cells may not be manifested due to the constant shedding of cells into the intestinal lumen. a multifactorial scheme could present for the presence of large numbers of mutations in single cells. However, chemically induced DNA damage does not coinciding the sequential chromosomal changes that features the process of tumor progression. Unrepaired DNA damage could participate in quiescent cells and be amenable for a large number of mutations at the first cell division, but the DNA harm would not be present in tumor cell progeny .the evolutionary process show that CDK family in mammals divide into subfamilies (cdk1, cdk4, and cdk5), it also divides into five transcriptional subfamilies which are (Cdk7, Cdk8, Cdk9, Cdk11 and, Cdk 20), mostly these Cdks bind to one more than one cyclins. These proteins are an important feature of several diseases, which include Cancer and various drugs used in clinical trials. CDKs have to be activated and inactivated in a complex way at a specific point during the cell cycle .such mechanism of CDKs was first time analyzed on the maturation promoting factor also called as Cyclin B/Cdc2 complex. its activity is a vital process foe entering in M phase. Cyclin B accumulate throughout the G form a complex and undergoes to a process called phosphorylation by cdc2 activating kinase (CAK) , TFIIH is also responsible for phosphorylation(Feaver W,1994). (Roy R, 1994).mammals use a different type of Cdks and cyclins, among CDK D are associated with CDK 4 and CDK 6,but in hematopoietic cells CDK 6 are more prominent during G1 phase. Cyclin D 1 is predominant in the murine macrophage cell line .Cyclin E is also operating in the G1 phase Cyclin D also associate with CDK 2,the kinase activity is at peak during this stage, but if the level of  Cyclin D or Cyclin E  minimize the span of G1 phase in fibroblast .but if the Cyclin D inactivate then cell do not enter in S phase.  (OhtsuboM, 199).Kinase activity also necessary for cell cycle continuity, but when cell enter in S phase the level of Cyclin E decrease, with the decreasing level kinase activity also cease. Rendering of cyclin A activity in mammalian fibroblast during the G phase either by antibody or si RNA result in the inhibition of DNA synthesis (Pagano M, 1991).

Role of Rb protein in cell cycle and Cancer

Rb is another protein which shows its effects in the cell cycle and cancer .the phosphorylation of Rb protein is highly monitored during cell cycle .but during the G1 phase Rb occur as phosphorylated or UN phosphorylated form. The phosphorylation status of Rb is tightly monitored during the late G1 through the end of M phase, it is relevant phosphorylated .the RB protein is bind to cellular transcription factors which are depicted as E2F, these have five contiguous related proteins E2F-1,E2F-2,E2F-3,E2F-4 and   E2F-5 .its depicted as tumor suppressor protein that is dysfunctional in some major form of Cancer .( Murphee AL,1984)the  most important function of  Rb is to prevent excessive cell cycle growth by restrain  cell cycle progression .when this protein is absent cell progression start and cell undergoes  proliferation Rb gene also control chromatin remolding enzymes which are acetylases and methylases.( Shao Z, Robbins  PD,1995).Rb bind and inhibit the transcription factors of E2F Family  which are composed of a dimer of E2F protein and dimerization protein (Zukerberg LR,1995).when it is time of cell to wriggle into S phase,CDKs  and Cyclin phosphorylates Rb to pRb form, restrain its activity. The inceptive phosphorylation is performed by Cyclin D/CDK4/CDK6 and followed by additional phosphorylation by Cyclin E/CDK2. PRb remains phosphorylated all aspects to S, G2 and M phase and Inactivation of pRb during embryogenesis foster inappropriate cell cycle activity. This follows from the ingenue of pRb in negatively regulating entry into the cell cycle. In distinction to apprehension, however, the increased cell cycle activity in Rb null mice does not crop in a net increase in cell number. This is due to a commensurate increase in cell death that precisely eliminates the abnormally cycling cells. This cell death (apoptosis) often debased on the function of p53, as authenticate from the analysis of RB/p53 double-mutant embryos.


cell cycle and machinery

Figure4. Schematic representation of cell cycle and machinery (Toshiyasu, 1995)  

Recent research  prove clinical target in a prostate cancer as well as in breast cancer, the majority of AR contains unique functions , ER-negative tumor get advantage from the combined target of AR and the ErbB2 /HER2 also play a role in metastatic disease. AR is a protein receptor which binds to has C-N terminus it has functional domain AF-1 and AF-2. AF-1 is transactivation factor and AF-2 is highly conserved protein. androgen is very essential for normal growth in female but during the postmenstrual cycle level of estrogen fall down, the binding of  androgens to the LBD  cause conformational changes due to homodimerization AR  move to the nucleus, bind the AREs and regulate gene transcriptions factor (Mononen  N et al.2002). mutation occur  Mutations occurring in the growth signaling network  result in the loss of proteins which function to suppress oncogenes , which cause deregulation of cell cycle .this disturbance results in the proliferation of cell cause cancer.(Gareth H Williams and Kai Stoeber,2012).PTEEN is a tumor-suppressing gene present on the chromosome 10q23 ,which encode  a protein as well as phospholipid phosphatase .there are number of mutation which are somatic in human malignancies. The activity of ILK is coincidently increase in a serum- and the harbor-independent manner in PTEN-mutant cells, and transfection of wild-type (WT) PTEN into these cells cease ILK exertion. Transfection of a kinase-deficient, dominant-negative form of ILK or Manifesta to a small molecule ILK inhibitor suppresses the constitutive phosphorylation of PKB/Akt on Ser-473, but not on Thr-308, in the PTEN-mutant prostate carcinoma cell lines PC-3 and LNCaP. Transfection of dominant-negative ILK and WT PTEN into these cells also results in the inhibition of PKB/Akt kinase activity. Furthermore, dominant-negative ILK or WT PTEN indulge G1 phase cycle arrest and increase apoptosis. Together, these data authenticate a critical role for ILK in PTEN-dependent cell cycle regulation and survival and indicate that inhibition of ILK may be of significant value in PTEN-mutant tumor therapy

Proliferating cells undergo a metabolic reprogramming to “aerobic glycolysis” in which glycolytic fluctuation boost up oxidative phosphorylation is superseded, pyruvate is neophyte primarily to lactate, and intermediates of the TCA cycle are diverted into anabolic pathways to facilitate cell growth (Vandar Heiden et al,2009) A similar metabolic switch to aerobic glycolysis, termed the Warburg effect is also analyzed in tumor cells, and recent studies suggest that p53 normally inhibits this process by limiting glycolytic fluctuation through various mechanisms (  Feng and Levine, 2010;  Warburg, 1956).

; from the above-cited study it is clear that cancer cells behave in a different way as compared to cancer cells. Cancer cell multiply very rapidly and affect other cell in metastatic way. Cancer cells have their Replicative mortality” so they fail to undergo the programmed cell death processor cell might get mutated and give rise to a malignant tumor. And invade other tissue Development and Progression are due to disturbance of cell cycle regulator which are CDKs and Different kind of Cyclin protein, several proteins that transmit growth factor signals are coded by proto-oncogenes .if one of protein absent or become overproduce it may lead to transmitting signal even in a situation when no growth the factor is available .the Ras protein are also encoded by proto-oncogene. Cancer-causing mutation often changes the Ras protein which leads to an inactive form. there are another tumor suppressor proteins are present which monitor the cell cycle but inactivation of   pRB and p53 result in  dysfunctioning of protein which shift normal cell to cancer.p53 called the guardian of the cell,so its absent cause damaging od DNA replication .cells homozygous p53 mutation cause  Li-Fraumeni disease,(Yien et al,1992), but many discoveries and studies on cell cycle and cancer do not come only from human but research also stress on cell cycle regulation on Zebrafish, Yeast, Drosophila. Different regulators checkpoints which control the cell cycle process are androgens, cyclins, CDKs, CDIs. estrogen receptors and is very necessary to discover various drugs for control target genes which proliferate cancer .there is lot of discoveries and work done cell cycle for complete understanding as well for the discovery of drugs being used for treatment of is a time to elucidate and focus on the drug industry also for coping with such harmful disease.


 Cancer is not a one disease but is a complex form of many diseases. There is a lot of work done in this field. Researchers have very curious about the actual facts of Cancer .information are beings continually added in research of cell cycle which is also aspirant  for others in the research and medical field. The recent advancement in genetics, immunology is paving the way and explore a new method to control target affecting genes .and perhaps develop various cures for Cancer. Researchers have developed a breakthrough new treatment to get cancer cell and kill themselves. The new development allows treatments to specify target cells and genes. The study of how blocking of certain Proteins which might cause Cancer. There is a need to prepare more and more anticancer drugs and enhance research in novel therapies. It’s a need of time to develop more key genes which control cancer and become a hope for cancer patients.



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