The circadian factor Period 2 modulates p53 stability and transcriptional activity in unstressed cells.
Bottom Line: We found that hPer2 binds the C-terminal half of human p53 (hp53) and forms a stable trimeric complex with hp53's negative regulator, Mdm2.Down-regulation of hPer2 expression directly affects hp53 levels, whereas its overexpression influences both hp53 protein stability and transcription of targeted genes.Overall our findings place hPer2 directly at the heart of the hp53-mediated response by ensuring that basal levels of hp53 are available to precondition the cell when a rapid, hp53-mediated, transcriptional response is needed.
Affiliation: Integrated Cellular Responses Laboratory, Department of Biological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.Show MeSH
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Mentions: Binding of the E3 ligase Mdm2 to the N-terminus domain of hp53 allows for the formation of a trimeric complex in which hPer2 binding to p53’s C-terminus prevents its ubiquitination and promotes hp53 stabilization (Figures 2, C and D, 3, and 5). The stability of p53 has been the subject of numerous studies and is known to be largely influenced by posttranslational modifications, intracellular distribution, and binding to other interacting proteins (for review, see Lavin and Gueven, 2006). As a result, different scenarios should be considered when evaluating the mechanism by which hPer2 leads to hp53 stabilization, including 1) intracellular localization of the hPer2/hp53 complex (unpublished data), 2) inhibition of Mdm2 E3 ligase activity when in contact with hPer2 in the trimeric complex (Supplemental Figure S2C, lane 6), and, alternatively, 3) blockage of Mdm2 access to its substrate region within hp53. In support of these scenarios is the existence of multiple proteins that influence the stabilization of p53, although the major control mechanism remains its interaction with and ubiquitination by Mdm2 (Harris and Levine, 2005). However, proteins that reverse this modification and others that either enhance translation of p53 mRNA or alter its subcellular localization influence the total level of p53 present in the cell at a given time. The herpes virus–associated ubiquitin-specific protease (HAUSP) is a p53-interacting protein that stabilizes p53 by deubiquitination even in the presence of an excess of Mdm2 (Li et al., 2002). More recently, the ovarian tumor domain-containing ubiquitin aldehyde-binding protein 1 (Otub1) was found to directly suppress Mdm2-mediated p53 ubiquitination and drastically stabilize p53 in response to DNA damage (Sun et al., 2012). An additional ubiquitin-independent but proteasome-dependent p53 degradation process is linked to its association to NAD(P)H quinone oxidoreductase (NQO1; Asher and Shaul, 2005). Both p53 and p73 interact with NQO1, which is largely associated with the 20S proteasome, unless an excess of NADH is present and competes off their interaction, preventing p53 degradation. Unlike in unstressed cells, in which Mdm2 prevents p300 from associating with p53, both CBP/p300 transcriptional coactivators and Strap, a partner protein of p300, associate with p53 in response to DNA damage, increasing its level and half-life through a mechanism that involves p53 acetylation (Lambert et al., 1998; Barlev et al., 2001). Methylation of p53 by overexpression of various methyltransferases has also been linked to its hyperstabilization (Chuikov et al., 2004). Finally, protein–protein interaction has been proven effective in increasing the stability of various forms of mutant p53. For example, heat shock protein 90 (hsp90) binds to the C-terminus of mutant p53 when in complex with Mdm2; however, in this case, hsp90 acts by inhibiting the ubiquitin ligase activity of Mdm2, blocking the ubiquitination of both Mdm2 itself and mutant p53 (Peng et al., 2001). An additional mode of regulation results from association of the central and C-terminus regions of p53 to poly(ADP-ribose) polymerase-1 (PARP-1; Wesierska-Gadek et al., 2003). In this case, inactivation of PARP-1 resulted in decreased levels of p53, whereas inhibition of nuclear export by leptomycin B prevented accelerated degradation of p53 in PARP-1–knockout cells, favoring p53 accumulation. As a result, p53 stability results from its nuclear accumulation, as PARP-1 blocks p53’s nuclear export signal located in its carboxy-terminal area (Wesierska-Gadek et al., 2003). Overall the mechanisms for controlling p53 stabilization are many and varied, but they all serve the purpose of monitoring different aspects of cellular homeostasis.
Affiliation: Integrated Cellular Responses Laboratory, Department of Biological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.