CST Guide:

Section I: Research Areas
Cell Cycle Control: G1/S Checkpoint
Ultra Violet
Stress Response
Growth
Factor
Withdrawal
Ubiquitination
Hormones
p19 INK4D
GSK-3β
SCF
Differentiation
p18 INK4C
Myc
G1-PHASE
Replicative
Senescence
p16 INK4A
Cyclin D
BMI1
CDK4/6
TGF-β
p15 INK4B
Myc
Smad3
Smad4
cdc25A
Growth Factor
Receptor Activation
R
Akt
FoxO1/3
p27 Kip1
CDK2
Cyclin E
Myc
S-PHASE
p21 Cip1
Myc
p53
ATM/
ATR
Replicative
Senescence
SCF
DNA
Damage
Chk1/2
Ubiquitination
Ubiquitination
Cell Cycle Control: G2/M DNA Damage Checkpoint
HIPK2
Nuclear Export,
Ubiquitination
Nucleolar
Sequestration
or
p53 Stabilization
p19 Arf cdc25A
IR UV
DNA Repair
DNA-PK
ATM/
ATR
Caffeine
WIP1
MDM4
MDM2 p53
Chk2
TopoII BRCA1 14-3-3σ Reprimo GADD45 p21 Cip1 CDK7
Nuclear
Exclusion
TRIP12
MDM2
p53
p300/
PCAF
WIP1
BRCA1
Chk1
cdc25
chapter 03: Cell Growth and Death
Critically Short
Telomeres
POT1 TRF2
PLK1
Myt1
Rad50
NBS1
Mre11
BRCA1
c-Abl
14-3-3
cdc25A/C
cdc25A/B
(MRN)
SCF
SCF
FANCD2
Rad51
p90RSK
DNA
Repair
Rad52
Nuclear Exclusion
Ubiquitination
AurA
Bora
FoxO1
DBE
Bim
FasL
TRAIL
Apoptosis
Suv39H1
Abl
Rb HDAC
E2F
DP-1
OFF
Rb
E2F
DP-1
ON
E2F/DP Target Genes:
Cyclin E/A, E2F-1/2/3,
cdc2, c-Myc, p107,
RanGAP, TK, DHFR,
PCNA, H2A, etc.
G2-PHASE
cdc2
Cyclin B
Wee1
M-PHASE
Ubiquitination
The primary G1/S cell cycle checkpoint controls the commitment of eukaryotic cells to transition through the G1 phase to enter into the DNA synthesis S phase. Two cell
cycle kinase complexes, CDK4/6-Cyclin D and CDK2-Cyclin E, work in concert to relieve inhibition of a dynamic transcription complex that contains the retinoblastoma protein
(Rb) and E2F. In G1-phase uncommitted cells, hypo-phosphorylated Rb binds to the E2F-DP1 transcription factors forming an inhibitory complex with HDAC to repress key
downstream transcription events. Commitment to enter S-phase occurs through sequential phosphorylation of Rb by Cyclin D-CDK4/6 and Cyclin E-CDK2 that dissociates
the HDAC-repressor complex, permitting transcription of genes required for DNA replication. In the presence of growth factors, Akt can phosphorylate FoxO1/3, which inhibits
their function by nuclear export, thereby allowing cell survival and proliferation. Importantly, a multitude of different stimuli exert checkpoint control, including TGF-β, DNA
damage, replicative senescence, and growth factor withdrawal. These stimuli act though transcription factors to induce specific members of the INK4 or Kip/Cip families of
cyclin dependent kinase inhibitors (CKIs). Notably, the oncogenic polycomb protein Bmi1 acts as a negative regulator of INK4A/B expression in stem cells and human cancer.
In addition to regulating CKIs, TGF-β also inhibits cdc25A transcription, a phosphatase directly required for CDK activation. At a critical convergence point with the DNAdamage
checkpoint, cdc25A is ubiquitinated and targeted for degradation via the SCF ubiquitin ligase complex downstream of the ATM/ATR/Chk-pathway. However, timely
degradation of cdc25A in mitosis (M-phase) via the APC ubiquitin ligase complex allows progression through mitosis. Furthermore, growth factor withdrawal activates GSK-3β
to phosphorylate Cyclin D, which leads to its rapid ubiquitination and proteasomal degradation. Collectively, ubiquitin/proteasome-dependent degradation and nuclear export
are mechanisms commonly used to effectively reduce the concentration of cell cycle control proteins. Importantly, Cyclin D1/CKD4/6 complexes are explored as therapeutic
targets for cancer treatment as researchers have found this checkpoint to be invariantly deregulated in human tumors.
Select Reviews:
Besson, A., Dowdy, S.F., and Roberts, J.M. (2008) Dev. Cell. 14, 159–169. • Gil, J. and Peters, G. (2006) Nat. Rev. Mol. Cell Biol. 7, 667–677. • Malumbres, M. and
Barbacid, M. (2009) Nat. Rev. Cancer 9, 153–166. • Musgrove, E.A., Caldon, C.E., Barraclough, J., Stone, A., and Sutherland, R.L. (2011) Nat. Rev. Cancer 11, 558–572.
• Skaar, J.R. and Pagano, M. (2009) Curr. Opin. Cell Biol. 21, 816–824. • Sparmann, A. and van Lohuizen, M. (2006) Nat. Rev. Cancer 6, 846–856. • Tzivion, G.,
Dobson, M., and Ramakrishnan, G. (2011) Biochim. Biophys. Acta. 1813, 1938–1945. • van den Heuvel, S. and Dyson, N.J. (2008) Nat. Rev. Mol. Cell Biol. 9, 713–724.
• Yang, J.Y. and Hung, M.C. (2009) Clin. Cancer Res. 15, 752–757.
The G2/M DNA damage checkpoint serves to prevent the cell from entering mitosis (M-phase) with genomic DNA damage. Specifically, the activity of the Cyclin B-cdc2 (CDK1)
complex is pivotal in regulating the G2-phase transition wherein cdc2 is maintained in an inactive state by the tyrosine kinases Wee1 and Myt1. It is thought that coordinated
action of the kinase Aurora A and the cofactor Bora activate PLK1 as cells approach the M-phase, which in turn activates the phosphatase cdc25 and downstream cdc2 activity,
hence establishing a feedback amplification loop that efficiently drives the cell into mitosis. Importantly, DNA damage cues activate the sensory DNA-PK/ATM/ATR kinases, which
relay two parallel cascades that ultimately serve to inactivate the Cyclin B-cdc2 complex. The first cascade rapidly inhibits progression into mitosis: the Chk kinases phosphorylate
and inactivate cdc25, which prevents activation of cdc2. The slower second parallel cascade involves phosphorylation of p53 and allows for its dissociation from MDM2 and
MDM4 (MdmX), which activates DNA binding and transcriptional regulatory activity, respectively. The transcriptional ability of p53 is further augmented through acetylation by the
co-activator complex p300/PCAF. The second cascade constitutes the p53 downstream-regulated genes including: 14-3-3, which binds to the phosphorylated Cyclin B-cdc2
complex and exports it from the nucleus; GADD45, which binds to and dissociates the Cyclin B-cdc2 complex; and p21 Cip1, an inhibitor of a subset of the cyclin-dependent
kinases including cdc2. Recent data suggest an important role for the p53-regulated WIP1 phosphatase that acts as a critical dampener of DNA damage signaling in cancer.
In human cancer, researchers have found p53 to be commonly mutated, indicating that this checkpoint is a critical barrier to tumor formation. In addition, sporadic and familial
mutations in the DNA-repair proteins such as the BRCA-family, ATM, and the Fanconi Anemia proteins further highlight this as a key tumor suppressor checkpoint.
Select Reviews:
Abbas, T. and Dutta, A. (2009) Nat. Rev. Cancer 9, 400–414. • Al-Ejeh, F., Kumar, R., Wiegmans, A., Lakhani, S.R., Brown, M.P., and Khanna, K.K. (2010) Oncogene 29,
6085–6098. • Boutros, R., Lobjois, V., and Ducommun, B. (2007) Nat. Rev. Cancer 7, 495–507. • Ciccia, A. and Elledge, S.J. (2010) Mol. Cell 40, 179–204. • Freed-
Pastor, W.A. and Prives, C. (2012) Genes Dev. 26, 1268–1286. • Huen, M.S., Sy, S.M., and Chen, J. (2010) Nat. Rev. Mol. Cell Biol. 11, 138–148. • Junttila, M.R. and
Evan, G.I. (2009) Nat. Rev. Cancer 9, 821–829. • Lens, S.M., Voest, E.E., and Medema, R.H. (2010) Nat. Rev. Cancer 10, 825–841. • Kee, Y. and D’Andrea, A.D. (2010)
Genes Dev. 24,1680–1694. • Nam, E.A. and Cortez, D. (2011) Biochem. J. 436, 527–536. • Reinhardt, H.C. and Yaffe, M.B. (2009) Curr. Opin. Cell Biol. 21, 245–255.
© 2002–2015 Cell Signaling Technology, Inc. • We would like to thank Dr. Hans Widlund, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA, for reviewing this diagram.
104 For Research Use Only. Not For Use in Diagnostic Procedures. See pages 302 & 303 for Pathway Diagrams, Application, and Reactivity keys.
© 2002–2015 Cell Signaling Technology, Inc. • We would like to thank Dr. Hans Widlund, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA, for reviewing this diagram.
www.cellsignal.com/cstpathways 105