| The p53tumour suppressor acts as a major barrier against cancer. Inactivation ofp53’s tumour suppressive functions appears to be almost a must for a tumour to develop;such inactivation can occur directly through mutations in the TP53gene encodinghuman p53, as is the case in about half of all human cancers, or through alterations inother components of the intricate p53pathway.The tumour suppressor activities of p53rely on its ability to modulate a variety ofbiological processes in stressed as well as in unperturbed cells. Most notable in thatregard is the capacity of p53to enforce growth arrest or apoptosis in response to varyingdegrees and types of stress, as well as its role in ensuring genome stability. Wecharacterize a new mechanism of repression of a p53proapoptotic target in normal cellsthat is relieved upon DNA damage.Unlike growth arrest, cell death is a one-way road. Premature triggering of theapoptotic programme is therefore highly inadvisable. To avoid such undesirable‘accidents’, the cell uses sophisticated mechanisms for selective inhibition ofproapoptotic p53target genes, to assure that they stay ‘locked’ unless an apoptoticprogramme needs to be triggered. Some of those mechanisms are ‘class-specific’, as isthe case for deacetylation that prevents p53from efficiently activating a substantialsubset of proapoptotic genes. Other mechanisms, however, are tailored intricately torestrict the expression of individual genes. An elegant example of such gene-specificsafeguard mechanisms was provided by Gomes and Espinosa, who found that theproapoptotic p53target gene Puma is constitutively repressed through the action of theinsulator protein CTCF in a complex with cohesin. Remarkably, in the absence of a proper signal to trigger p53-mediated apoptosis, this mechanism results in production of an abortive transcript rather than of mRNA encoding functional PUMA protein.We now describe another interesting mechanism for gene-specific repression of a p53proapoptotic target in non-stressed cells. In earlier work, we identified Apak (ATM andp53-associated KZNF protein, also known as ZNF420) as a negative regulator of p53,which effectively inhibits p53-mediated apoptosis. Apak belongs to the family ofKRAB-type zinc finger proteins (KZNFs), typically involved in transcriptionalrepression. Tian and co-workers showed that Apak binds directly to p53, andconcomitantly—through its KRAB domain—also recruits the co-repressor KAP1(KRAB box-associated protein1). KAP-1, in turn, recruits the histone deacetylaseHDAC1, which quenches p53acetylation and thereby hampers the ability of p53totransactivate proapoptotic target genes. That study positioned Apak as a ‘class-specific’p53modulator. In their present study, we now report that Apak also doubles as agene-specific repressor, selectively targeting the proapoptotic p53target gene p53AIP1.We identified a particular DNA sequence—TCTTN(2–30)TTGT—as the binding motiffavoured by Apak. Remarkably, p53AIP1harbours such a sequence within its firstintron, and the authors confirmed the binding of Apak to this region. What makes thisobservation of particular interest is the fact that the Apak binding motif overlaps withthe p53RE located within this region, which is responsible for the ability of p53totransactivate p53AIP1. Indeed, we went on to show that Apak could compete with p53,preventing it from binding to its cognate p53RE and thereby strongly repressingp53AIP1transcription. Thus, Apak can dampen p53-mediated apoptosis by at least twocomplementary molecular mechanisms, class-specific as well as gene-specific.This ‘off’ state predominates in non-stressed cells. By contrast, when cells areexposed to genotoxic stress, Apak and its partner KAP1are phosphorylated through anATM-dependent mechanism. Consequently, Apak dissociates from p53and KAP1isprevented from driving HDAC1-mediated p53deacetylation; the resultant acetylated p53is now free to bind to and transactivate a variety of proapoptotic genes includingCD95and Noxa. Moreover, Apak phosphorylation also results in its dissociation fromthe p53AIP1intron1, enabling p53to bind to its p53RE and trigger expression ofp53AIP1mRNA. Together, reversal of the two complementary repressive activities ofApak enables the orchestration of a robust proapoptotic transcription programme,eventually leading to cell death. At later stages of the process, Apak is sequestered to thenucleolus, further ensuring that it stays out of p53’s way as long as the genotoxic signalpersists.This study found that Apak is the first protein which competes with p53to bind to thep53response element and riched p53selective regulation mechanisms. Meanwhile, weidentified that Apak protein is the first direct transcriptional repressor of p53AIP1gene,which provide a basis for the p53AIP1gene studies on regulation of expression. Apakprotein belongs to the largest transcription factor family in mammals-KRAB zinc fingerprotein (also known as KZNF) family, the number of this family are more than400members which are very conservative, but their functions were not clear, this study haslaid a solid foundation for other family members. |
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