92:
316:
improve cancer's sensitivity to radiation and chemotherapy. Chk1 has important implications in drug targeting for cancer as its function acts in response to DNA damage. The cytotoxic effects of chemotherapy are currently being studied in the modulation of the G2/M transition, concerning both checkpoint abrogation or checkpoint arrest. Many therapies focus on inactivating the checkpoint in order to force cells with excess DNA damage to proceed through mitosis and induce cell death.
17:
178:
inactivated through tyrosine phosphorylation by Wee1 and Myt1. In the case of unreplicated DNA, the cyclin concentration threshold for Cdc2 activation is further increased. Through this mechanism, there exists two separate steady-state conditions separated by an unstable steady state. The bistable and hysteretic nature of CyclinB-Cdc2 ensures a highly regulated nature of the G2/M checkpoint.
243:(Ataxia Telangiectasia and Rad3 related) pathways which activate the Chk2 and Chk1 kinases, respectively. These kinases act upstream of Cdc25 and Wee1, the direct regulators of the CyclinB-Cdc2 complex. Chk1 and Chk2 phosphorylate Cdc25, inhibiting its phosphorylating activity and marking it for ubiquitinated degradation. These pathways also stimulate the tumor suppressor
33:
302:
Wee1 and Cdc25 abolishes the G2-M DNA damage checkpoint. Absence of Wee1 or removal of the tyrosine-15 site removes negative regulation of Cdc2 activity and causes cells to enter mitosis without completing repair, which effectively abolishes the G2-M checkpoint. Absence of Cdc25 arrests cells in G2,
272:
between the positive regulation of Wee1 and the negative regulation of Cdc25 by Chk1 in response to unreplicated or damaged DNA results in a strong G2 arrest. The increase in the amount of Wee1 and the decrease in the amount of Cdc25 contributes to the increase in the cyclin B concentration threshold
194:
Proteins that function in the G2-M checkpoint were originally identified in yeast screens that looked for mutants which show enhanced sensitivity to radiation, termed "rad" mutants. Inefficient repair of DNA damaged by ionizing radiation or chemical agents in these mutants revealed proteins essential
289:
The maintenance of such arrest in the G2 phase is further sustained by p53 and p21. In the absence of p53 or p21, it was demonstrated that radiated cells progressed into mitosis. The absence of p21 or 14-3-3 cannot sufficiently inhibit the CyclinB-Cdc2 complex, thus exhibiting the regulatory control
186:
Proteins that localize to sites of DNA damage in the G2 phase initiate a signaling cascade that regulates important components of the pathway, as described above, therefore controlling mitotic entry via CyclinB-Cdc2 activity. Negative regulation of CyclinB-Cdc2 activity results in a delay in mitotic
199:
in vertebrates, that are believed to localize to sites of DNA damage. Rad3 phosphorylates rad26 which is required to initiate, but not maintain the checkpoint. Rad3 also phosphorylates a number of other proteins whose absence abolishes checkpoint DNA repair, including rad1, rad9, hus1 and rad17. It
315:
Many cell cycle regulators like Cdks, cyclins, and p53 have been found to have abnormal expression in cancer. More specifically, they have been implicated in being involved in the G2/M transition by localizing to the centrosome, which thus leads to studies in manipulating such proteins in order to
285:
that is required for G2 arrest even when Chk1 is phosphorylated and active. Thus, rad18 is required for G2/M checkpoint maintenance while Chk1 is required for checkpoint initiation. This is further supported by its additional function in DNA repair, specifically in the maintenance of chromosomal
177:
cell-free egg extracts, such model was confirmed as the basis for entry into mitosis. Once cyclin concentration reaches a certain minimum activation threshold, Cdc2 is rapidly activated. It remains in this state until activity falls below a separate inactivation threshold at which it is abruptly
306:
Inactivation of Chk1 is sufficient to surpass the checkpoint and promote entry into mitosis, regardless if DNA damage is repaired. Yet, little is still known about the exact mechanism regarding checkpoint termination with possible mechanisms including protein phosphatases reversing activating
212:. In agreement with this idea, rad17 is similar to proteins involved in loading the clamp onto DNA. This supports a model where phosphorylation by rad3 causes recruitment of these proteins to sites of DNA damage where they mediate the activity of DNA polymerases involved in
223:, which is required for the G2-M arrest in response to DNA-damaging agents. Chk1 is an effector protein kinase that maintains mitotic cyclin in an inactive state and is phosphorylated by rad3 between S phase and mitosis, implicating its specific role in G2 arrest. Its
166:), and activates Cdc25 through phosphorylation with combined action activating Cdc2. The combined activity and complex of Cdc2, Cdc25, and Plk1 with the accumulation of cyclin B activates the CyclinB-Cdc2 complex, promoting entry into mitosis.
231:
can induce arrest independent of DNA damage. In addition, overexpression of Chk1 rescues the radiation sensitivity of rad mutants, presumably by allowing DNA repair to take place before entry into mitosis.
264:
The ATM/ATR pathway also results in the negative regulation of Plk1 that contributes to the stability of Wee1. The stabilization of Wee1 and Myt1 ensures the cells arrest in G2 and allows for DNA repair.
142:
and Myt1. Cdc25 activates the complex through the removal of phosphates from the active site while Wee1 inactivates the complex through the phosphorylation of tyrosine residues, specifically tyrosine-15.
260:
of Wee1 by Cdk1 allows for the binding of 14-3-3, sequestering Wee1 to the nucleus and enhancing its ability to phosphorylate Cdc2. The phosphorylation of both Wee1 and Cdc25 prevents Cdc2 activation.
255:
that phosphorylate (and thereby inactivate) and sequester Cdc25 in the cytoplasm, respectively. Recent studies have also suggested that Cdk1 and 14-3-3 positively regulate Wee1 in a similar manner. The
1095:
Bunz, F.; Dutriaux, A.; Lengauer, C.; Waldman, T.; Zhou, S.; Brown, J. P.; Sedivy, J. M.; Kinzler, K. W.; Volgestein, B. (1998). "Requirement for p53 and p21 to
Sustain G2 Arrest After DNA Damage".
281:
Rad3 is required for activation of Chk1 and initiation of G2 arrest, but different proteins are believed to maintain G2 arrest so that sufficient DNA repair can occur. One such protein is
290:
of p53 and p21 in the G2 checkpoint in response to DNA damage. p53 mutations can result in a significant checkpoint deficit, which has important implications in the treatment of cancer.
268:
Multiple pathways are involved in the checkpoint response and thus, the targeting of Cdc25 is not the sole mechanism underlying cell cycle delay, as some models have proposed. The
111:
regulatory proteins at different checkpoints of the cell cycle. Different phases of the cell cycle experience activation and/or deactivation of specific cyclin-CDK complexes.
303:
but still allows activation of the G2-M checkpoint, implicating that both the activation of Wee1 and deactivation of Cdc25 as important regulatory steps in the checkpoint.
169:
Many proteins involved in this positive feedback loop drive the activation of the CyclinB-Cdc2 complex because entry into mitosis requires an all-or-none response. The
1130:
Lundgren, K.; Walworth, N.; Booher, R.; Dembski, M.; Kirschner, M.; Beach, D. (1991). "Mik1 and wee1 cooperate in the inhibitory tyrosine phosphorylation of cdc2".
286:
structures. Its necessity is demonstrated by the fact that in the absence of rad18, DNA is unable to be repaired even when G2 arrest is prolonged by other means.
173:
is a mathematical model used to explain such regulatory loop that predicted the irreversible transition into mitosis driven by hysteresis. Through experiments in
68:
or death after cell division if they enter the M phase before repairing their DNA. The defining biochemical feature of this checkpoint is the activation of
307:
phosphorylations, targeted ubiquitin degradation of activating proteins, and checkpoint antagonists promoting mitosis through independent pathways.
195:
in this pathway. Early signaling proteins in the checkpoint pathway are members of a family of phosphatidylinositol 3-kinases, rad3 in yeast and
535:
Novak, B.; Tyson, J. J. (1993). "Numerical analysis of a comprehensive model of M-phase control in
Xenopus oocyte extracts and intact embryos".
427:
Gould, K. L.; Nurse, P. (1989). "Tyrosine phosphorylation of the fission yeast cdc2+ protein kinase regulates entry into mitosis".
240:
196:
403:
722:
Walworth, N.; Davey, S.; Beach, D. (1993). "Fission yeast chkl protein kinase links the rad checkpoint pathway to cdc2".
200:
has been hypothesized that rad9, hus1 and rad17 are similar to proteins involved in forming the clamp that increases the
362:
72:
154:, Bora accumulates and forms an activation complex with Aurora A. This complex then regulates the activation of
1202:
236:
104:
480:"Bora and the Kinase Aurora A Cooperatively Activate the Kinase Plk1 and Control Mitotic Entry"
60:
until damaged or incompletely replicated DNA is sufficiently repaired. Cells with a defective G
341:
Cuddihy, Andrew R.; O'Connell, Matthew J. (2003). "Cell-cycle responses to DNA damage in G2".
187:
entry, which is important for cells to repair any DNA damage that may have accumulated after
49:
162:
phosphorylates Wee1, targeting it for degradation through the SCF ubiquitin ligase complex (
731:
583:
491:
436:
257:
8:
1207:
1197:
735:
587:
495:
440:
1155:
930:
904:"Centrosome-associated regulators of the G2/M checkpoint as targets for cancer therapy"
903:
802:
777:
755:
649:
512:
479:
460:
155:
699:
682:
658:
633:
606:
571:
354:
1147:
1143:
1112:
1077:
1072:
1047:
1028:
984:
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954:
935:
855:
807:
747:
704:
663:
611:
552:
517:
452:
409:
399:
376:
368:
358:
170:
146:
This loop is further amplified indirectly through the coordinated interaction of the
131:
127:
76:
1159:
1139:
1104:
1067:
1059:
1046:
Verkade, H. M.; Bugg, S. J.; Lindsay, H. D.; Carr, A. M.; O'Connell, M. J. (1999).
1018:
974:
966:
925:
915:
845:
797:
789:
759:
739:
694:
653:
645:
601:
591:
544:
507:
499:
464:
444:
350:
1108:
1023:
1006:
793:
634:"DNA repair mutants defining G2 checkpoint pathways in Schizosaccharomyces pombe"
252:
209:
147:
123:
478:
Seki, A.; Coppinger, J. A.; Jang, C.-Y.; Yates, J. R.; Fang, G. (20 June 2008).
91:
228:
205:
1191:
683:"A sliding clamp model for the Rad1 family of cell cycle checkpoint proteins"
372:
269:
1048:"Rad18 is required for DNA repair and checkpoint responses in fission yeast"
850:
833:
596:
503:
413:
1081:
1032:
988:
939:
859:
811:
708:
615:
548:
521:
380:
224:
201:
1151:
1116:
1063:
920:
751:
667:
556:
456:
114:
CyclinB-CDK1 activity is specific to the G2/M checkpoint. Accumulation of
970:
572:"Hysteresis drives cell-cycle transitions in Xenopus laevis egg extracts"
163:
122:
as cells prepare to enter mitosis. Cdc2 activity is further regulated by
118:
increases the activity of the cyclin dependent kinase Cdk1 human homolog
902:
Wang, Y.; Ji, P.; Liu, J.; Broaddus, R. R.; Xue, F.; Zhang, W. (2009).
213:
100:
53:
1173:
DiPaola, R. S. (2002). "To Arrest or Not To G2-M Cell-Cycle Arrest".
743:
448:
80:
65:
151:
115:
188:
69:
57:
16:
108:
273:
in the hysteresis loop needed to drive the cell into mitosis.
1129:
282:
135:
32:
834:"The G(2) DNA damage checkpoint targets both Wee1 and Cdc25"
1094:
220:
159:
139:
119:
130:
of its corresponding activators and inhibitors. Through a
955:"Positive regulation of Wee1 by Chk1 and 14-3-3 proteins"
248:
244:
138:
which in turn deactivates the CyclinB-Cdc2 inhibitors,
1045:
680:
477:
831:
775:
340:
247:. p53 regulates the function of the Cdk2 inhibitor
721:
681:Thelen, M. P.; Venclovas, C.; Fidelis, K. (1999).
393:
901:
191:and necessary before cell division can continue.
181:
56:organisms that ensures that cells don't initiate
1189:
952:
1005:Harper, J. W.; Elledge, S. J. (December 2007).
631:
576:Proceedings of the National Academy of Sciences
1004:
276:
134:loop, CyclinB-Cdc2 activates the phosphatase
953:Lee, J.; Kumagai, A.; Dunphy, W. G. (2001).
75:, which phosphorylate proteins that promote
396:The cell cycle : principles of control
1007:"The DNA Damage Response: Ten Years After"
778:"Turning off the G2 DNA damage checkpoint"
534:
426:
293:
86:
1071:
1022:
978:
929:
919:
849:
832:Raleigh, J. M.; O'Connell, M. J. (2000).
801:
776:Calonge, T. M.; O'Connell, M. J. (2007).
698:
657:
605:
595:
511:
570:Sha, Wei; et al. (September 2002).
235:The presence of DNA damage triggers the
90:
31:
15:
1172:
1190:
884:. New Science Press. pp. 227–245.
879:
632:Al-Khodairy, F.; Carr, A. M. (1992).
219:The main rad3 effector is the kinase
1000:
998:
897:
895:
893:
891:
882:The Cell Cycle Principles of Control
875:
873:
871:
869:
827:
825:
823:
821:
771:
769:
627:
625:
569:
336:
334:
332:
330:
328:
239:(Ataxia telangiectasia mutated) or
13:
650:10.1002/j.1460-2075.1992.tb05179.x
150:and the Bora cofactor. During the
24:-M checkpoint occurs between the G
14:
1219:
995:
888:
866:
818:
766:
622:
343:International Review of Cytology
325:
1166:
1123:
1088:
1039:
946:
79:assembly and bring the cell to
715:
674:
563:
528:
471:
420:
387:
182:Pathway response to DNA damage
20:Steps of the cell cycle. The G
1:
1109:10.1126/science.282.5393.1497
1052:Molecular Biology of the Cell
959:Molecular Biology of the Cell
700:10.1016/s0092-8674(00)80587-5
398:. London: New Science Press.
355:10.1016/s0074-7696(02)22013-6
319:
103:is driven by proteins called
95:CyclinB-Cdk1 Hysteresis Graph
1144:10.1016/0092-8674(91)90266-2
1024:10.1016/j.molcel.2007.11.015
794:10.1016/j.dnarep.2007.07.017
7:
394:Morgan, David Owen (2007).
64:-M checkpoint will undergo
10:
1224:
277:Maintaining the checkpoint
310:
1175:Clinical Cancer Research
105:cyclin dependent kinases
46:-M DNA damage checkpoint
851:10.1242/jcs.113.10.1727
838:Journal of Cell Science
597:10.1073/pnas.0235349100
537:Journal of Cell Science
504:10.1126/science.1157425
294:Checkpoint inactivation
87:Cyclin B-CDK 1 activity
880:Morgan, David (2007).
549:10.1242/jcs.106.4.1153
96:
37:
29:
1064:10.1091/mbc.10.9.2905
921:10.1186/1476-4598-8-8
94:
50:cell cycle checkpoint
35:
19:
971:10.1091/mbc.12.3.551
258:hyperphosphorylation
107:that associate with
73:cyclin-CDK complexes
1103:(5393): 1497–1501.
736:1993Natur.363..368W
588:2003PNAS..100..975S
496:2008Sci...320.1655S
490:(5883): 1655–1658.
441:1989Natur.342...39G
1203:Molecular genetics
156:Polo-like kinase 1
97:
38:
30:
844:(10): 1727–1736.
782:DNA Repair (Amst)
730:(6427): 368–371.
405:978-0-19-920610-0
171:Novak-Tyson model
132:positive feedback
128:dephosphorylation
1215:
1183:
1182:
1181:(11): 3311–3314.
1170:
1164:
1163:
1138:(6): 1111–1122.
1127:
1121:
1120:
1092:
1086:
1085:
1075:
1058:(9): 2905–2918.
1043:
1037:
1036:
1026:
1002:
993:
992:
982:
950:
944:
943:
933:
923:
908:Molecular Cancer
899:
886:
885:
877:
864:
863:
853:
829:
816:
815:
805:
773:
764:
763:
744:10.1038/363368a0
719:
713:
712:
702:
678:
672:
671:
661:
644:(4): 1343–1350.
638:The EMBO Journal
629:
620:
619:
609:
599:
567:
561:
560:
543:(4): 1153–1168.
532:
526:
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515:
475:
469:
468:
449:10.1038/342039a0
424:
418:
417:
391:
385:
384:
338:
298:Inactivation of
48:is an important
1223:
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1218:
1217:
1216:
1214:
1213:
1212:
1188:
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819:
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767:
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679:
675:
630:
623:
568:
564:
533:
529:
476:
472:
435:(6245): 39–45.
425:
421:
406:
392:
388:
365:
339:
326:
322:
313:
296:
279:
253:14-3-3 proteins
210:DNA replication
184:
148:Aurora A kinase
124:phosphorylation
89:
63:
45:
27:
23:
12:
11:
5:
1221:
1211:
1210:
1205:
1200:
1185:
1184:
1165:
1122:
1087:
1038:
1017:(5): 739–745.
1011:Molecular Cell
994:
965:(3): 551–563.
945:
887:
865:
817:
788:(2): 136–140.
765:
714:
693:(6): 769–770.
673:
621:
582:(3): 975–980.
562:
527:
470:
419:
404:
386:
363:
323:
321:
318:
312:
309:
295:
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229:overexpression
206:DNA polymerase
183:
180:
175:Xenopus laevis
88:
85:
61:
43:
25:
21:
9:
6:
4:
3:
2:
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364:9780123646262
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270:cooperativity
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59:
55:
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34:
28:and M phases.
18:
1178:
1174:
1168:
1135:
1131:
1125:
1100:
1096:
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1055:
1051:
1041:
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225:upregulation
218:
202:processivity
193:
185:
174:
168:
145:
113:
98:
41:
39:
164:SCF complex
36:G2-M arrest
1208:DNA repair
1198:Cell cycle
1192:Categories
349:: 99–140.
320:References
214:DNA repair
101:cell cycle
54:eukaryotic
373:0074-7696
81:metaphase
66:apoptosis
1160:36864093
1082:10473635
1033:18082599
989:11251070
940:19216791
914:(1): 8.
860:10769204
812:17851138
709:10102265
616:12509509
522:18566290
414:70173205
381:12503848
251:and the
227:through
158:(Plk1).
152:G2 phase
116:cyclin B
1152:1706223
1117:9822382
1097:Science
931:2657106
803:2233850
760:4312997
752:8497322
732:Bibcode
668:1563350
584:Bibcode
557:8126097
513:2834883
492:Bibcode
484:Science
465:4287870
457:2682257
437:Bibcode
208:during
189:S phase
77:spindle
70:M-phase
58:mitosis
1158:
1150:
1115:
1080:
1070:
1031:
987:
977:
938:
928:
858:
810:
800:
758:
750:
724:Nature
707:
666:
659:556583
656:
614:
607:298711
604:
555:
520:
510:
463:
455:
429:Nature
412:
402:
379:
371:
361:
311:Cancer
109:cyclin
1156:S2CID
1073:25529
980:30963
756:S2CID
461:S2CID
283:rad18
136:Cdc25
1148:PMID
1132:Cell
1113:PMID
1078:PMID
1029:PMID
985:PMID
936:PMID
856:PMID
808:PMID
748:PMID
705:PMID
687:Cell
664:PMID
612:PMID
553:PMID
518:PMID
453:PMID
410:OCLC
400:ISBN
377:PMID
369:ISSN
359:ISBN
300:both
221:Chk1
160:Plk1
140:Wee1
120:Cdc2
99:The
40:The
1140:doi
1105:doi
1101:282
1068:PMC
1060:doi
1019:doi
975:PMC
967:doi
926:PMC
916:doi
846:doi
842:113
798:PMC
790:doi
740:doi
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