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The tumor suppressor protein p53 is stabilized and activated to induce cell cycle arrest or apoptosis in response to diverse stress, including DNA damage and oncogenic stress, thus preventing cell from genomic instability and malignant transformation (1-3). Studies over the past decade have revealed that p53 also plays a crucial role in mediating the cellular response to a newly defined stress called ribosomal stress (4), which is induced by perturbation of ribosomal biogenesis. By doing so, p53 coordinates ribosomal biogenesis (cell growth) with cell cycle progression (cell division). Ribosomal biogenesis includes coordinated synthesis of ribosomal RNA (rRNA) and ribosomal proteins (RPs), 2 rRNA processing, and the assembly of the mature ribosome subunits in the nucleolus and their transport into the cytoplasm (4, 5). Disturbing any one of the steps perturbs ribosomal biogenesis and triggers ribosomal stress (4). Because ribosomal stress is often accompanied by the disruption of nucleolar integrity, it is also referred as to nucleolar stress (6). Examples of ribosomal stress include inhibition of rRNA synthesis by treatment with a low dose of actinomycin D (Act D) (7-9) or mycophenolic acid (MPA) (10), inhibition of rRNA processing by treatment with 5-fluorouracil (5-FU) (11), genetic disruption of the RNA polymerase I transcription initiation factor TIF-IA (12), and knockdown of individual RPs (13-15) or nucleolar factors such as nucleostemin (16).Emerging evidence has established a critical role for a group of RPs, including L5, L11, L23, L26, S7, and S27 (8, 9, 17-24), in mediating p53 signaling in response to ribosomal stress. These RPs, when overexpressed or in response to ribosomal stress, bind to MDM2 and inhibit MDM2-mediated p53 ubiquitination and degradation, leading to stabilization and activation of p53. It is well known that MDM2 forms an elegant feedback regulatory loop with p53, as MDM2 is transcriptionally induced by p53 (25-27), whereas it in turn directly inhibits p53 transcriptional activity (28, 29) and mediates p53 ubiquitination and proteasomal degradation (30, 31). Interestingly, knockdown of certain RPs including L5, L11, and S7 attenuates p53 signaling after ribosomal stress (7-11, 18, 24), indicating that these RPs play a non-redundant role in ribosomal stress-induced p53 activation. Furthermore, mice with a knockin of the cancer-associated MDM2 mutant, C305F (MDM2 C305F ), which fails to bind to L5 and L11 (32), displayed a specific defect in p53 signaling in response to ribosomal stress but not DNA damage, compellingly validating the ribosomal stress-induced RPs-MDM2-p53 signaling pathway in vivo (33).Multiple RPs are required for p53 activation in response to ribosomal stress, suggesting that they may form a multi-RP-MDM2 protein complex to optimally suppress MDM2 function. Supporting this notion, L5 and L11 synergistically inhibit MDM2, leading to robust activation of p53 compared with overexpression of L5 or L11 individually (34). Additionally, many of these RPs appear to bind to ...

The interstitial extracellular matrix (ECM) and epithelial-cell associated basement membrane (BM) play critical roles in the morphogenesis and differentiation of developing salivary glands. Early studies used ex vivo organ culture and tissue recombination methods to identify the importance of the ECM in organ development. Incorporation of transgenic mice and molecular tools has facilitated progress in our understanding of the mechanisms by which ECM proteins influence SMG development. Recent work has identified alterations in the ECM, BM, and associated proteins in salivary gland diseases, including Sjögren's syndrome and salivary gland cancers, but the significance of such changes is not known. Understanding the basic mechanisms controlling morphogenesis and differentiation in mammalian organ development is the first step towards understanding pathogenesis. Molecular characterization of the function of the ECM and BM in cellular processes is critical for future development of therapeutic approaches in regenerative medicine and tissue engineering. Here we provide a historical overview of experiments defining the functions of the ECM, ECM receptors, and associated molecules in salivary gland development. We include a discussion of the function of ECM-associated proteases and major growth factor signaling components that are in some way regulated by the ECM or associated molecules. We conclude with a discussion of defects in ECM and BM occurring in salivary gland pathologies and speculation on future areas of research pertaining to further understanding of the function of the ECM in the salivary gland.

Within the past decade, there has been a revolution in the types of drugs developed to treat cancer. Therapies that selectively target cancer-specific aberrations, such as kinase inhibitors, have made a dramatic impact on a subset of patients. In spite of these successes, there is still a dearth of treatment options for the vast majority of patients. Therefore, there is a need to design therapies with broader efficacy. The p53 tumor suppressor pathway is one of the most frequently altered in human cancers. However, about half of all cancers retain wild-type p53, yet through various mechanisms, the p53 pathway is otherwise inactivated. Targeting this pathway for reactivation truly represents the “holy grail” in cancer treatment. Most commonly, destabilization of p53 by various components of ubiquitin-proteasome system, notably the ubiquitin ligase MDM2 and its partner MDMX as well as various deubiquitinating enzymes (DUBs), render p53 inert and unresponsive to stress signals. Reinstating its function in cancer has been a long sought-after goal. Towards this end, a great deal of work has been devoted to the development of compounds that either interfere with the p53-MDM2 and p53-MDMX interactions, inhibit MDM2 E3 activity, or target individual DUBs. Here we review the current progress that has been made in the field, with a special emphasis on both MDM2 and DUB inhibitors. Developing inhibitors targeting the upstream of the p53 ubiquitination pathway will likely also be a valuable option.

Histone H2B monoubiquitination plays a critical role in the regulation of gene transcription. Deregulation of H2B monoubiquitination contributes to human pathologies, such as cancer. Here we report that human USP36 is a novel H2Bub1 deubiquitinase. We show that USP36 interacts with H2B and deubiquitinates H2Bub1 in cells and in vitro. Overexpression of USP36 markedly reduced the levels of H2Bub1 in cells. Using the p21 gene as a model, we demonstrate that depletion of USP36 increases H2Bub1 at the p21 locus, primarily within its gene body. Consistently, knockdown of USP36 induced the expression of p21 and inhibits cell proliferation. Together, our results reveal USP36 as a novel H2B deubiquitinase and shed light on its additional functions in regulating gene expression.

No abstract

Ribosomal proteins (RPs) play a critical role in tightly coordinating p53-dependent cell cycle checkpoint with ribosomal biogenesis. A number of RPs, including L5, L11, L23, L26, and S7, specifically bind to MDM2 and inhibit MDM2-mediated p53 ubiquitination and degradation, leading to p53 activation, in response to nucleolar stress triggered by the perturbation of ribosomal biogenesis. Here, we report S27a, a ribosomal protein from small subunit of ribosome and synthesized as an ubiquitin C-terminal extension protein (CEP80), as a novel MDM2 regulator. S27a interacts with MDM2 at the central acidic domain of MDM2 and suppresses MDM2-mediated p53 ubiquitination, leading to p53 activation and cell cycle arrest. Knockdown of endogenous S27a significantly attenuated p53 activation by treatment with a low dose (5 nM) of actinomycin D (Act D), which specifically inhibits RNA polymerase I activity leading to inhibition of rRNA synthesis and perturbation of ribosomal biogenesis, suggesting that S27a is a non-redundant MDM2 regulator in response to nucleolar stress. Interestingly, we also show that MDM2 ubiquitinates S27a, leading to protesomal degradation of S27a, in response to Act D treatment, suggesting that MDM2, in turn, controls S27a levels in response to nucleolar stress. Taken together, our results reveal that S27a regulates p53 activity in response to nucleolar stress via a novel and dynamic interplay between S27a and MDM2. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3087. doi:10.1158/1538-7445.AM2011-3087