Cell cycle arrest in response to DNA damage is an important mechanism for maintaining genomic integrity. This cell cycle arrest provides time for DNA repair to prevent replication or segregation of damaged DNA. Induction of growth arrest by DNA damage occurs mainly through the activation of checkpoint pathways that delay cell cycle progression at G1, S, and G2 (Ref.1).
In vertebrate cells, the commitment to complete a round of mitotic division takes place during the initial phase of the cell cycle (G1), at a stage called the restriction (R) point, preceding the onset of DNA synthesis (S-phase) (Ref.2). Two cell cycle
,and the transcription complex that includes Rb
(Retinoblastoma)-family proteins (also called "pocket proteins") and E2F
are pivotal in controlling this checkpoint. pRb-family proteins are negative growth-regulators in their active form. During
G1-phase, the Rb
(Histone Deacetylases) repressor complex binds to the
transcription factors, inhibiting the downstream transcription. Five E2F
proteins (E2F-1 to 5) and three DP proteins (DP-1
, DP-2 and DP-3) have been cloned. Heterodimerization between E2F
is required for efficient DNA binding,
transcriptional activity, and binding to pocket proteins.E2F1
bind predominantly pRb
binds all known pocket proteins and E2F5
binds predominantly p130
(Ref.3). Phosphorylation of Rb
dissociates the Rb
-repressor complex, permitting transcription of S-phase genes encoding for proteins that amplify the G1 to S-phase switch that is required for DNA replication. There are several proteins that can inhibit the cell cycle in G1.
If DNA damage has occurred, p53
accumulates in the cell and induces the p21(CIP1)
-mediated inhibition of Cyclin-D
(Ref.4). If the Cyclin-D
complex is inhibited, Rb
protein is in a state of low phosphorylation and is tightly bound to the transcription factor E2F
, inhibiting its activity.
accumulates during late G-phase and triggers the passage into S-phase. The entire genome is replicated during S-phase. The synthesis and accumulation of Cyclin-B
also begins during S-phase, but the complex is phosphorylated at Thr14 -Tyr15 and is inactive. Many different stimuli also exert checkpoint control including TGF-Beta
, DNA damage, contact inhibition, replicative senescence, and growth factor withdrawal. The first four act by inducing members of the INK4 ( p16INK4A
) or KIP/CIP (p21(CIP1)
and p57KIP2) families of cell cycle kinase inhibitors (Ref.5). Activation of TGF-Beta
receptors also induces the inhibition of Cyclin-D
by p15, while cAMP (cyclic Adenosine 3, 5’-Monophosphate) inhibits
complex via p27
additionally inhibits the transcription of CDC25A
, a phosphatase that activates the cell cycle kinases. Growth Factor
withdrawal activates GSK3Beta
(Glycogen Synthase Kinase-3), which phosphorylates Cyclin-D
, leading to its rapid ubiquitination and proteosomal degradation. Ubiquitination, nuclear export, and degradation are mechanisms
commonly used to rapidly reduce the concentration of cell cycle control proteins.
The balance between cell differentiation and proliferation is regulated at the transcriptional level and in the cell cycle the transition from G1 to S-phase (G1/S transition) is of paramount importance in this regard. It is only before this point that cells can be oriented toward the differentiation pathway beyond which the cells progress into the cycle in an autonomous manner.