E

Lavin and Khanna, 1999; Shiloh, 2001; Shiloh, 2003). This site is putatively conserved, but in zebrafish p53 the position is Ser6 followed by S(6)QE (Cheng et al., 1997). In humans, Ser15 phosphorylation interferes with Mdm2 binding to p53 (Shieh et al., 1997). Moreover, DNA damage induced by IR also results in ATM-dependent activation of Chk2 (Chehab et al., 2000; Hirao et al., 2000; Matsuoka et al., 2000; Shieh et al., 2000). Activated Chk2 phosphorylates Ser20 of human p53, which is also within the Mdm2-binding region and the phosphor-ylation of Ser 20, reduces the binding of Mdm2 (Chehab et al., 1999; Unger et al., 1999). However, zebrafish p53 does not contain a phosphorylation site equivalent to Ser20. Phosphorylation sites equivalent to Ser33 and Ser37 of human p53 are likewise lacking in zebrafish p53. Intriguingly, it has been shown that a Ser20 to Ala mutation generates a less stable form of human p53 (Chehab et al., 1999; Unger et al., 1999), whereas wildtype zebrafish p53 contains Ala10 followed by FA(10)E at the site equivalent to FS(20)D in human (Cheng et al., 1997) (Figure 28.5), suggesting permanent instability or basic lack of function through the Mdm2 pathway. More importantly, the zebrafish genome contains homologues of those checkpoint regulators and kinases, but thus far, to our knowledge, apparently lacks an ARF homologue. ARF is a small protein generated from the CDKN4 locus, and the same region also encodes the CDK4 inhibitor p16. ARF is a tumor suppressor and an important inhibitor of the ubiquitin ligase Mdm2, thus antagonizing p53 degradation. In addition, nucleolar relocalization of Mdm2 by ARF connotes a novel mechanism for preventing p53 turnover and provides a framework for understanding how stress signals cooperate to regulate p53 function. In mammals, ARF also connects pathways regulated by the pRB and p53. ARF inactivation reduces p53-dependent apoptosis induced by oncogenic signals. If is also in zebrafish, the consequences may reveal that zebrafish don't utilize the protein turnover machinery to regulate p53 activity through the ARF-Mdm2-p53 pathway. Furthermore, since zebrafish p53 lacks the potentially targeted Chk1 and Chk2 phosphorylation sites Ser33 and Ser37, the Chk1/Chk2-p53 pathway might differentially function through an alternative mechanism in zebrafish. This is worthwhile investigating from the comparatively biological and evolutionary point of view among vertebrates because p53, as a critical tumor suppressor, is among the most commonly mutated genes in human cancers (Vogelstein, 1990; Vogelstein and Kinzler, 1992; Vogelstein and Kinzler, 2004; Vogelstein et al., 2000). In humans, the Chk2 kinase is also a known tumor suppressor, and a wide variety of cancer-associated mutations and defects have been identified (Bartek and Lukas, 2003; Motoyama and Naka, 2004). Importantly, consistent with the tumor suppressor function of Chk2, recent studies demonstrated that Chk2 activation, caused by telomere attrition, triggers replicative senescence in human cells (Gire, 2004; Gire et al., 2004; Oh et al., 2003). In contrast, cancer-related defects in Chk1 are rare, and so far seem limited to some gastrointestinal carcinomas (Bartek and Lukas, 2003). Therefore, the zebrafish Chk1/Chk2 pathways need required to be characterized in terms of their role in cancer biology as well as for DNA damage and senescence response in zebrafish.

Collectively, p53 is a crucial molecule for regulation of the responses to DNA damage, cell-cycle arrest, apopto-sis, and senescence. Critically short or dysfunctional telomeres trigger a p53-dependent DNA damage response possibly through ATM activation (Bakkenist et al., 2004; d'Adda di Fagagna et al., 2003; Herbig et al., 2004). Moreover, telomere dysfunction enhances sensitivity to IR in mice. Loss of p53 function delays or abrogates the replicative senescence of human cells irrespective of the ATM states. However, both p53-independent ATM pathways and ATM-independent p53 pathways must be involved in the diversified regulations of senescence response (Figure 28.6).

Importantly, p53-dependent senescent growth arrest is reversible at least in some human cells, whereas the other senescent arrest, which is induced by the p16/pRB pathway, is irreversible even with subsequent inactivation of p53, p16, or pRB (Beausejour et al., 2003; Dirac and Bernards, 2003). So far, the involvement of the p53/p21 pathway in cell-cycle regulation and apoptosis has been demonstrated in several experiments using zebrafish, and we have already confirmed p53-dependent premature senescence induction during development and adult fin regeneration of zebrafish. The importance of the pRB pathway in zebrafish aging and senescence response remains to be characterized, and further elucidation of the signaling pathways and their network may reveal evolutionary conservation of mechanisms among vertebrates.

Telomeres

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Telomeres

Telomeres

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Telomeres

Senescence

Figure 28.6. Possible schematic presentation of a model for DNA damage-mediated senescence response in the ATM-p53 axis. In the presence or absence of ATM, a different balance of ATM-dependent and ATM-independent senescence response probably occurs through p53. ATM can maintain genome and telomere integrity. In the absence of ATM, genomic instability and telomere attrition are aberrantly induced by DNA damage and p53 is still activated through an ATM-independent pathway. This ATM-independent p53 activation leads to much more prominent induction of senescence, in other words accelerated premature senescence, in an A—T patient. The ATM-independent p53-dependent pathway may involve ATR, DNA-PK, and Chk1/2. ATRIP and Ku80 seem to function as sensor proteins of DNA damage to ATR and DNA-PK, respectively (Falck et a/., 2005). NBS1 could be another sensor protein responsible for ATM (Falck et a/., 2005).

Senescence

Senescence

Figure 28.6. Possible schematic presentation of a model for DNA damage-mediated senescence response in the ATM-p53 axis. In the presence or absence of ATM, a different balance of ATM-dependent and ATM-independent senescence response probably occurs through p53. ATM can maintain genome and telomere integrity. In the absence of ATM, genomic instability and telomere attrition are aberrantly induced by DNA damage and p53 is still activated through an ATM-independent pathway. This ATM-independent p53 activation leads to much more prominent induction of senescence, in other words accelerated premature senescence, in an A—T patient. The ATM-independent p53-dependent pathway may involve ATR, DNA-PK, and Chk1/2. ATRIP and Ku80 seem to function as sensor proteins of DNA damage to ATR and DNA-PK, respectively (Falck et a/., 2005). NBS1 could be another sensor protein responsible for ATM (Falck et a/., 2005).

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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