The Molecular Basis of G2 Arrest
To understand how the cell cycle is regulated in response to DNA damage, mutants of budding and fission yeast were identified in which the cell cycle was no longer delayed in response to DNA damage 58 . This approach led to the concept of the cell cycle checkpoint, and uncovered many genes that form an important part of what we know about the workings of the DNA-damage response. Cell cycle checkpoints are regulatory mechanisms that ensure that cell cycle processes occur at the right time and in...
G2 Checkpoint
G2 is the second restriction point in the cell cycle. This checkpoint assures that the replication machinery has completed DNA duplication. Simultaneously, it also makes sure that, in the replicated DNA, if any deletions or duplications of bases have occurred, they should be repaired before cells enter the prophase stage. Cdc25A is stable after phosphorylation by cyclin B-Cdc2 at G2 M in normal cells 140,141 . This phosphorylation allows this protein to remain functional during mitosis and...
Factors Regulating G1 Phase of the Cell Cycle
2.1.1. G1 Phase CDK Cyclin Complexes The progression through the cell cycle is governed by the periodic activation and inactivation of cyclin-dependent kinase CDK complexes. The CDK proteins are Ser Thr protein kinases, and their kinase activities are controlled by their association partners, which are called cyclins 22 . The protein levels of the CDKs remain constant through the cell cycle, whereas the levels of the cyclins vary during the cell cycle, owing to periodic expression and...
Bipin C Dash and Wafik S ElDeiry
Maintenance of genomic integrity is essential to avoid cellular transformation, neoplasia, or cell death. DNA synthesis, mitosis, and cytokinesis are important cellular processes required for cell division and the maintenance of cellular homeostasis they are governed by many extra- and intra-cellular stimuli. Progression of normal cell division depends on cyclin interaction with cyclin-dependent kinases Cdk and the degradation of cyclins before chromosomal segregation through ubiquitination....
Cdk Activation and Inhibition by Phosphorylation
Cdks are closely related in size 35-40 Kd , their peptide sequence contains about 40 identity, and all are activated by a cyclin regulatory subunit 3942 . The Cdk catalytic subunit contains 300 amino acids, representing a catalytic core. It is completely inactive when monomeric and unphosphorylated. Besides cyclin binding, complete Cdk activation requires phosphorylation at the T loop Thr 172 in Cdk4, Thr 160 in Cdk2, and Thr 161 in Cdc2, by the cyclin H-CAK Cdk7 complex 43-45 . Cdk 7, like its...
G2
Fig. 2. Regulation of G1 and G1 S transition. In quiesc t, GO cells, E2F-DP transcription factors are bound to p130 and become inactive. In G1, the E2F-DP-Rb complexes predominate. Mitogenic signals increase cyclin D synthesis, formation of active cyclin D-Cdk4 Cdk6 complexes, and initial phosphorylation of RB, and result in cyclin E biosynthesis through E2F-DP-Rb complexes. Cyclin E binds to Cdk2 and activates the kinase, resulting in the hyperphosphorylation of Rb. Rb becomes completely...
SPhase Checkpoint in Response to DNA Replication Stress
Several types of agents are known to interfere with the function of replication forks and to elicit the S-phase checkpoint. These include agents that Fig. 4. Molecular mechanism of S-phase progression and the S-phase checkpoints. Upon initiation of DNA replication, DNA replication initiation factors such as MCM10, CDC45-Sld3 budding yeast and RPA and checkpoint complexes bind to the pre-replicative complex pre-RC on chromatin and trigger the unwinding of DNA. The hexameric MCM2-7 complex is...
LargeScale Reprogramming of Transcription in Response to DNA Damage
With the knowledge that pl30 and pl07 are required to downregulate Cdc2 in response to DNA damage, we used Affymetrix microarrays to compare gene expression profiles in pl30 pl07-null and wild-type mouse embryo fibroblasts MEFs Taylor and Stark, unpublished observations . Untreated cells were compared with cells treated with adriamycin for either l2 or 24 h. Although the expression of a large number of genes changed, we focused on genes that were repressed in the wild-type but not in the pl30...
The ATMATRp53Mediated Pathway
The ATM ATR p53 pathway plays a pivotal role in one of the checkpoint mechanisms that arrest the cell cycle at G1 phase following DNA damage G1 checkpoint Fig. 3 . As described in Subheading 3, ATM is activated in response to IR, whereas ATR is activated in response to replication inhibition or UV-induced damage. The activated ATM or ATR then phosphorylates p53 on Ser-15 , and this phosphorylation causes MDM2 to dissociate from p53, which stabilizes p53 and leads to its accumulation 128,129 ....
The Function of p53
The p53 tumor suppressor was originally identified as a protein bound to the large T antigen of the SV40 tumor virus 104 . Early studies on p53 function suggested that it was an oncogene, because the first clones of p53 could immortalize primary cells and cooperate with other oncogenes such as Ha-ras to induce neoplastic transformation 105-108 . The true function of p53 as a tumor suppressor was established in a number of landmark studies showing that 1 transforming p53 alleles contained...
J I Oo
Fig. 1. The lNK4a ARF locus encodes p16INK4a and p19ARF, two degenerated tumor suppressor proteins. Absence of p16INK4a composed of ANK activates CDK4 6 cyclin D kinase, which phosphorylates pRB, thereby activating the E2F DP1 transcription factor. Since the transcription of the ARF gene is regulated by E2F, expression of ARF is induced. ARF separates MDM2 from p53 by recruiting MDM2 into the nucleolus, possibly in collaboration with cyclin G1 and cyclin G2. This stabilizes p53 in the nucleus...
Defects in G1S Checkpoint and Cancer
Defects in the genome maintenance mechanisms, including DNA repair and cell cycle checkpoint pathways, are believed to enhance genetic instability and cause the accumulation of mutations and chromosomal aberrations that is a hallmark of cancer cells 155 . Most of the G1 S checkpoint transducers and effectors are classified as either tumor suppressors or proto-oncogenes, and their loss-of-function mutations or overexpression appear to play pivotal roles in many types of human tumors. Mouse...
Tension Occupancy and the Source of the Signal
What event is measured at the kinetochore There are two models that have been proposed. The first is that tension across the kinetochores, the result of microtubule binding to opposite spindle poles, is assessed, and cells enter anaphase only when all chromosomes are under tension. The second is kineto-chore-microtubule occupancy. This model predicts that there are microtubule binding sites in sister kinetochores that must be stably bound by microtubules, and cells enter anaphase only when all...
The Role of Transcriptional Repression in G2 Arrest
To understand how the overexpression of p53 could induce G2 arrest, we focused on the regulation of Cdc2, a likely candidate to mediate this effect. In cells arrested in G2 by p53, Cdc2 activity was low, but CAK activity was unaffected 131 . Thus, our results suggest that the reduction in threonine 161 phosphorylation of Cdc2 in response to p21 waf1 is not owing to direct inactivation of CAK, and other mechanisms must be at play. Immunoblotting also showed that when cells were just reaching the...
Our Current Understanding of Cdc2 Regulation
Many of the regulatory steps that control Cdc2 activity have been elucidated and have been conserved during evolution, although some details are different reviewed in 32,33 . Cdc2 is active only at the G2 M border and is turned off as cells enter the anaphase stage of mitosis. The first step in generating active Cdc2 is its association with a cyclin Fig. 1 . In animal cells, Cdc2 associates with an A-type or a B-type cyclin, in fission yeast with Cdc13, and in budding yeast with the CLB...
Regulation of p53 Stability by ARF and MDM2
The lNK4a locus that generates p16INK4a also encodes a degenerated gene product called ARF after alternative reading frame 131 . Thus, the locus encodes two tumor suppressor proteins, p16INK4a and p19ARF p14ARF in humans , which activate the growth suppressive functions of pRB and p53, respectively 67 . ARF is a highly basic pI gt 12 , arginine-rich nucleolar protein 132 . Deletion of the ARF gene can inactivate p53 function in tumors where p53 itself remains intact. Transcription of the ARF...
The p53pRB Pathway Controls the G1S Transition
The p53 tumor suppressor gene TP53 is the most frequently mutated gene about 50 in human tumors, and encodes a 53 kDa transcription factor p53 that directly induces the expression of a substantial number of genes that are important for cell cycle regulation, DNA damage repair, and apoptosis 108,109 . Of the genes that are induced by p53, p21WAF1 plays a pivotal role in G1 arrest by inhibiting CDK4 6 cyclin D1 activity, thereby reducing the phosphorylation of pRB and promoting G1 arrest of the...