The NEDD8 inhibitor MLN4924 increases the size of the nucleolus and activates p53 through the ribosomal-Mdm2 pathway.

The ubiquitin-like molecule NEDD8 is essential for viability, growth and development, and is a potential target for therapeutic intervention. We found that the small molecule inhibitor of NEDDylation, MLN4924, alters the morphology and increases the surface size of the nucleolus in human and germline cells of Caenorhabditis elegans in the absence of nucleolar fragmentation. SILAC proteomics and monitoring of rRNA production, processing and ribosome profiling shows that MLN4924 changes the composition of the nucleolar proteome but does not inhibit RNA Pol I transcription. Further analysis demonstrates that MLN4924 activates the p53 tumour suppressor through the RPL11/RPL5-Mdm2 pathway, with characteristics of nucleolar stress. The study identifies the nucleolus as a target of inhibitors of NEDDylation and provides a mechanism for p53 activation upon NEDD8 inhibition. It also indicates that targeting the nucleolar proteome without affecting nucleolar transcription initiates the required signalling events for the control of cell cycle regulators.

Recruitment of RPL11 at promoter sites of p53-regulated genes upon nucleolar stress through NEDD8 and in an Mdm2-dependent manner.

Ribosomal proteins (RPs) activate the p53 tumour-suppressor protein upon disruption of the nucleolus. However, the exact mechanisms for p53 transcriptional activation through RPs are not well understood. We show that the RPL11 is rapidly but transiently recruited at promoter sites of p53-regulated genes upon nucleolar stress induced by actinomycin D (ActD). Characterisation of molecular events at p53 promoter sites shows that L11 is required for the recruitment of p53 transcriptional co-activators p300/CBP and p53 K382 acetylation. We found that direct binding to Mdm2 E3 ligase and NEDDylation of L11 are critical regulators for L11 promoter recruitment. Our data suggest that binding of L11 to Mdm2 at the promoter results in relief from Mdm2-mediated transcriptional repression of p53. Analysis of chromatin and RNA polymerase II markers suggests that L11 is involved in the initiation step of transcriptional activation. Furthermore, analysis of 36 ActD-induced genes shows that L11 and NEDD8 are global regulators of the p53 activation response. The studies provide insights on how nucleolar stress through L11 and NEDD8 can activate the transcriptional activity of p53.

NUB1 promotes cytoplasmic localization of p53 through cooperation of the NEDD8 and ubiquitin pathways.

Non-covalent recognition of ubiquitin (Ub) and ubiquitin-like molecules (Ubls) by interacting proteins has an important role in the regulation of protein function and initiation of signalling events. In addition, growing evidence suggests that regulation of p53 subcellular localization contributes to the biological outcome of the p53 response. Cytoplasmic p53 is shown to promote apoptosis and inhibit the induction of autophagy. In this study we show that NEDD8 ultimate buster 1 (NUB1), a non-covalent interactor of the Ubl NEDD8 (neural precursor cell expressed, developmentally downregulated 8), controls the localization of p53. Expression of NUB1 leads to decreased modification of p53 with NEDD8 and stimulation of p53 ubiquitination. The biological outcome is the cytoplasmic localization and inhibition of the transcriptional activity of p53. Although the effects of NUB1 on p53 depend on NEDDylation and the murine double minute 2 (Mdm2) E3-ligase, the cooperation of NEDD8 with ubiquitin is required. The data identify a role for NEDD8 in controlling p53 localization and suggest that NEDD8 can control protein function through its non-covalent recognition by interacting proteins.

Molecular evolution of the thermosensitive PAb1620 epitope of human p53 by DNA shuffling.

Conformational stability of the p53 protein is an absolute necessity for its physiological function as a tumor suppressor. Recent in vitro studies have shown that wild-type p53 is a highly temperature-sensitive protein at the structural and functional levels. Upon heat treatment at 37 degrees C, p53 loses its wild-type (PAb1620(+)) conformation and its ability to bind DNA, but can be stabilized by different classes of ligands. To further investigate the thermal instability of p53, we isolated p53 mutants resistant to heat denaturation. For this purpose, we applied a recently developed random mutagenesis technique called DNA shuffling and screened for p53 variants that could retain reactivity to the native conformation-specific anti-p53 antibody PAb1620 upon thermal treatment. After three rounds of mutagenesis and screening, mutants were isolated with the desired phenotype. The isolated mutants were translated in vitro in either Escherichia coli or rabbit reticulocyte lysate and characterized biochemically. Mutational analysis identified 20 amino acid residues in the core domain of p53 (amino acids 101-120) responsible for the thermostable phenotype. Furthermore, the thermostable mutants could partially protect the PAb1620(+) conformation of tumor-derived p53 mutants from thermal unfolding, providing a novel approach for restoration of wild-type structure and possibly function to a subset of p53 mutants in tumor cells.

Cocompartmentalization of p53 and Mdm2 is a major determinant for Mdm2-mediated degradation of p53.

The product of the Mdm2 oncogene directly interacts with p53 and promotes its ubiquitination and proteasomal degradation. Initial biological studies identified nuclear export sequences (NES), similar to that of the Rev protein from the human immunodeficiency virus, both in Mdm2 and p53. The reported phenotypes resulting from mutation of these NESs, together with results obtained using the nuclear export inhibitor leptomycin B (LMB), have led to a model according to which nuclear export of p53 (via either the NES of Mdm2 or its own NES) is required for efficient p53 degradation. In this study we demonstrate that Mdm2 can promote degradation of p53 in the nucleus or in the cytoplasm, provided both proteins are colocalized. We also investigated if nuclear export is an obligate step on the p53 degradation pathway. We find that (1) when proteasome activity is inhibited, ubiquitinated p53 accumulates in the nucleus and not in the cytoplasm; (2) Mdm2 with a mutated NES can efficiently mediate degradation of wild type p53 or p53 with a mutated NES; (3) the nuclear export inhibitor LMB can increase the steady-state level of p53 by inhibiting Mdm2-mediated ubiquitination of p53; and (4) LMB fails to inhibit Mdm2-mediated degradation of the p53NES mutant, demonstrating that Mdm2-dependent proteolysis of p53 is feasible in the nucleus in the absence of any nuclear export. Therefore, given cocompartmentalization, Mdm2 can promote ubiquitination and proteasomal degradation of p53 with no absolute requirement for nuclear to cytoplasmic transport.

Mdmx stabilizes p53 and Mdm2 via two distinct mechanisms.

The p53 protein maintains genomic integrity through its ability to induce cell cycle arrest or apoptosis in response to various forms of stress. Substantial regulation of p53 activity occurs at the level of protein stability, largely determined by the activity of the Mdm2 protein. Mdm2 targets both p53 and itself for ubiquitylation and subsequent proteasomal degradation by acting as an ubiquitin ligase, a function that needs an intact Mdm2 RING finger. For efficient degradation of p53 nuclear export appears to be required. The Mdmx protein, structurally homologous to Mdm2, does not target p53 for degradation, but even stabilizes both p53 and Mdm2, an activity most likely mediated by heterodimerization of the RING fingers of Mdm2 and Mdmx. Here we show that Mdmx expression leads to accumulation of ubiquitylated, nuclear p53 but does not significantly affect the Mdm2-mediated ubiquitylation of p53. In contrast, Mdmx stabilizes Mdm2 by inhibiting its self-ubiquitylation.