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asay in vitro, supporting a direct effect of c Abl on the E6 E6AP complex 94 . By analogy to Mdm2, where low ubiquitination leads Aurora Kinase Inhibitors to nuclear export and polyubiquitination results in degradation, it's tempting to propose that mono ubiquitination of p53 by E6 E6AP could also signal p53 for nuclear export, when poly ubiquitination could stimulate degradation within the nucleus along with the cytoplasm. The capability of c Abl to block degradation and nuclear export, suggests that it may manage both levels of p53 ubiquitination. However, this possibility is yet to be tested. It is important to note that p53 protected by c Abl is transcriptionally active. In non infected cells co expression of c Abl with E6 restores the capability of p53 to induce p53 target promoters. Likewise, in HPV infected cells c Abl was identified to trigger p53 activity 94 .
In addition, ectopic expression of c Abl in HPV infected Aurora Kinase Inhibitors cells induces efficient apoptosis, which can be blocked by down regulation of p53 making use of siRNA for p53 T.G. and Y.H unpublished data . These findings render c Abl a possible candidate for the development of drugs aimed at restoration BAY 11-7082 of p53 apoptotic activity Extispicy in HPV induced cancer, for instance cervical carcinoma. The mechanism by which c Abl protects p53 from E6 E6AP is just not clear yet. It can be intriguing, though, that c Abl protects p53 from both the normal cellular degradation machinery, too as machinery employed by the virus, pathways which are mutually exclusive 105 . This could suggest that c Abl could defend p53 by targeting a crucial step needed by Mdm2 too as the E6 E6AP complex.
Considering that the kinase activity of c Abl is needed for its protection from Mdm2, it's tempting to propose that E6 E6AP or other component within the degradation pathway is targeted by c Abl for phosphorylation. Other doable mechanisms cannot be ruled out at this stage. C Abl seems to be abundantly expressed in HPV infected BAY 11-7082 cells T.G. and Y.H unpublished results . This raises the question why endogenous c Abl is insufficient for triggering p53 by DNA damage to efficiently kill HPV infected cells. It can be doable that HPV viral protein s deregulate Aurora Kinase Inhibitors c Abl, by way of example by abrogating the kinase activity or by interfering with the sub cellular localization of c Abl. The finding that c Abl can trigger p53 in HPV infected cells supports this notion, on the other hand this really is yet to be explored.
Oncogenic c Abl and BAY 11-7082 p53 Constitutive activation of c Abl kinase activity can be oncogenic. This occurs when the N terminal portion of c Abl is replaced with fragments of genes, for instance bcr, tel or the viral gag, producing the fusion tyrosine kinases Bcr Abl, Tel Abl, and v Abl, respectively reviewed in 112,113 . In these fusion proteins, various portions from the N terminal inhibitory region of c Abl are removed, resulting inside a constitutive tyrosine kinase activity from the fusion proteins reviewed in 109 . This section focuses on the big oncogenic form, Bcr Abl, the product from the Philadelphia chromosome t 9;22 . The Bcr gene also contributes towards the transforming capability of Bcr Abl. The oncogenic effects of Bcr require the coiled coil domain, which is responsible for the tetramerization of Bcr Abl and hence its tyrosine kinase activation 114 .
Tyrosine 177 of Bcr is also significant considering that it recruits Grb 2 thereby linking Bcr towards the Ras pathway 115 . The roles of these websites had been demonstrated inside a mouse model for Bcr Abl inducing a myeloproliferative disorder Aurora Kinase Inhibitors 116 . Bcr Abl is present in over 95 of chronic myelogenous leukemia CML and in 25 30 of acute lymphoblastic leukemia ALL patients reviewed in 112,117 . The Bcr Abl is really a cytoplasmic protein that triggers mitogenic and anti apoptotic signals see beneath . These effects are opposite to those exerted by c Abl in response to pressure conditions. It has been argued that the ratio amongst these antagonistic effects of wt c Abl and Bcr Abl is important for the progression of CML 112 . In contrast towards the majority of cancer sorts, the frequency of p53 mutations in CML is comparatively low.
During the chronic phase of CML p53 mutations are observed in only 3 5 , but this figure increases to 25 30 as the disease progresses to blast crisis 118 120 . Oncogenic stimuli trigger p53, usually via the activation of ARF see beneath , resulting BAY 11-7082 either in robust selection for p53 mutations, or compensating alterations in p53 regulators, for instance the elevation of p53 inhibitors Mdm2 or Mdmx . The wt status of p53 in CML is especially puzzling simply because Bcr Abl elevates p53 levels in response to DNA damage 121,122 . Recent studies provide some insight into why p53 levels are elevated in CML. We have recently shown that in CML cells p53 is constitutively phosphorylated on serine 20 by the checkpoint kinase 1 Chk1 , which is constitutively activated inside a Bcr Abl dependent manner. Serine 20 phosphorylation partially relieves p53 from inhibition by Mdm2 123 , explaining the elevated levels of p53 in CML cells. In addition to serine 20 phosphorylation, it has re