Krana,a Novel Negative Regulator Of P53

The p53 tumor suppressor is a transcriptional factor which lies at the center of a protein network that controls cell cycle progression and commitment to apoptosis and exerts anti-proliferative effects in response to various types of stress including DNA damage and abnormal proliferative signals. Tight regulation of p53 is essential for maintaining normal cell growth and preventing tumorigenesis. It is inactive in proliferating cells, due to the regulation by its negative regulators, including HDM2 oncoprotein, COP1, Sir2α, hCtBP2. In response to multiple stimuli, p53 is release from this negative regulation and activated through diverse pathways. The zinc finger proteins containing the Kruppel-associated box domain (KRAB) are the single largest class of transcription factors in human genome. Here, we have identified and characterized a novel KRAB zinc finger gene (KZNF), named Krana (Krab domain and nineteen zinc fingers-containing ATM-associated protein), which encodes a protein with an N-terminal KRAB domain, nineteen zinc fingers and three potential phosphorylation sites of ATM. Krana is mainly localized in the nucleus and partially accumulates in the nucleolus. Krana serves as a selective transcriptional repressor, and can repress the transcriptional activity of many kinds of transcription factors including p53. It physically interacts with p53 via its C-terminal zinc finger domains, and inhibits p53-dependent transcription and p53-mediated apoptosis. Importantly, knock down of endogenous Krana by RNAi enhances p53-mediated apoptosis. Krana is colocalized with p53 in the nucleoplasm and p53 overexpression decreases the distribution of Krana in nucleolus. Cellular stress including heat shock and DNA damage (MMS treatment) trigger the translocation of Krana from nucleoplasm to nucleolus and abolish the Krana-p53 interaction, and then the repression of p53-dependent transcription by Krana is downregulated. Meanwhile, Krana accumulates in the nucleolus and the colocalization with p53 disappeared. Notably, the negative regulation of Krana co-repressor function in response to heat shock and MMS treatment is dependent on the function of DNA damage kinase ATM. Kranaalso physically interacts with ATM via its N-terminal KRAB domain. Thus, Krana bridges ATM to p53 and stabilize the interaction between p53 and ATM in vivo which is also abolished by heat shock or MMS treatment. In addition, overexpression of pl4ARF , an important nucleolar regulator of p53 which is activated mainly by oncogenic activation, can also inhibit the interaction between Krana and p53, promotes the nucleolus location of Krana, and then downregulates its repression activity on p53. Taken together, these results demonstrate that Krana acts as an important negative regulator of p53, and represent a novel pathway in which the cellular stress signals can transduce from ATM to p53 through the negative regulation cascade between ATM, Krana and p53.