Biology Reference
In-Depth Information
p21CIP1/WAF1.
29
p21 inhibits the G1/S-promoting cyclin E/Cdk2 kinase and,
as a consequence, the ability for DNA synthesis, that is, the duplication of
damaged DNA. Alternatively, if the DNA damage is extensive, p53 triggers
apoptotic cell death through intrinsic and extrinsic pathways, by activating
target genes such as
Puma
and
Noxa
30,31
(
Fig. 1
A). In addition to p53, check-
points are controlled by negatively regulating the Cdc25 family of phospha-
tases, direct substrates of the effector kinases Chk1 and Chk2, as well as
MK2.
24
Cdc25 proteins regulate G1/S and G2/M transitions through the
removal of inhibitory phosphorylations on Cyclin/Cdk complexes (
Fig. 1
B).
During the intra-S checkpoint, phosphorylation of Cdc25A by Chk1 leads to
rapid degradation of the phosphatase by the 26S proteasome.
21,32-34
Cdc25B
and Cdc25C are inhibited to enforce the G2/M checkpoint, where phosphor-
ylation by DDR effector kinases leads to 14-3-3 binding in the cytosol, resulting
in reduced catalytic activity and sequestration away from their substrates, the
Cyclin/Cdk complexes.
32,35,36
The increased selectivity of cancer cells to DNA-
damaging agents indicates that they have rewired their signaling networks and
established new dependencies between cell cycle checkpoint and survival
pathways that do not exist, or are less pronounced, in normal cells. The G1/S
boundary, for example, is the predominant checkpoint upon DNA damage in
normal mammalian cells. Most tumor cells, however, show a loss of function in
the tumor suppressor gene
p53
or its regulatory pathways,
29
leading to a
selective loss of this G1 checkpoint. Therefore, cancer cells are often complete-
ly dependent on the S and G2/M checkpoints to arrest cell cycle after genotoxic
target genes
Puma
and
Noxa
following DNA damage is responsible for cell death. The ATR/Chk1
module is activated by DNA single-strand breaks (SSBs) and bulky lesions. The p38MAPK/MK2
pathway is an additional checkpoint regulator that is activated downstream of both ATM and ATR
upon DNA damage and is especially important in the context of p53 deficiency. Targets of the
effector kinases Chk1, Chk2, and MK2 include Cdc25 phosphatases and Wee1 kinase, which are
regulators of cell cycle arrest. BRCA1/2 are components of the homologous recombination (HR)
repair pathway. DNA-PKcs is a third PI(3)K-like kinase mainly involved in the activation of DNA
damage repair pathways including nonhomologous end-joining (NHEJ). (B) Cell cycle checkpoint
mechanisms. Genotoxic and replicative stress activate checkpoints to delay progression from one
cell cycle phase to the next. Activation of p53 by DNA damage response kinases mediates arrest at
the G1 (Gap1)/S (synthesis) and intra-S transitions, mainly through the upregulation of p21. p21 in
turn inhibits the cell cycle promoting cyclin/Cdk kinase complexes. In addition, checkpoints are
regulated by Cdc25 phosphatases, which are inhibited by the effector kinases Chk1, Chk2, and
MK2. Cdc25 proteins drive S-phase and G2 (Gap2)/M (Mitosis) entry through removal of inhib-
itory phosphorylations on Cdk/Cyclin complexes. The Chk1 target Wee1 is responsible for gener-
ating these inhibitory phosphorylations on Cdk kinases. Many tumor cells are defective in p53
function, leading to a selective loss of the G1/S checkpoint. Therefore, cancer cells are often
dependent on the intra-S and G2/M checkpoints to arrest the cell cycle after genotoxic stress.
For this reason, interference with these two checkpoints has become an interesting and promising
strategy to sensitize G1-deficient cancer cells to DNA-damaging therapy.
