Tribbles 1 (TRB1) is one of the mammalian orthologs of Drosophila Tribbles, which regulates development and cell proliferation. TRB1 is suggested to act as a scaffold protein in signaling pathways for important cellular processes. TRB1 has also been identified as a myeloid oncogenic driver and mediates leukemogenesis through the mitogen-activated protein extracellular kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway and CCAAT/enhancer binding protein (C/EBP) transcriptional factors. However, the physiological roles of TRB1 in solid tumors have not been clarified. Here, we show that TRB1 interacts with p53 and suppresses its tumor suppressor activity. TRB1 knockdown enhances transcriptional activity of p53 and decreases cell viability. Interestingly, TRB1 enhances histone deacety lase 1 (HDAC1)-mediated p53 deacetylation and decreases DNA binding of p53. These results suggest that TRB1 is involved in the proliferation of tumor cells by inhibiting the activities of tumor suppressor p53 in solid tumors.Key words Tribbles 1 (TRB1); p53; histone deacetylase 1 (HDAC1); deacetylationThe pseudokinase Tribbles 1 (TRB1; also termed Trib1, C8FW, SKIP1) is one of the mammalian orthologs of Tribbles, a cell cycle regulator during development in Drosophila. 1,2) TRB1 has a conserved motif, which is similar to the catalytic domain of a serine/threonine kinase, but lacks an ATP binding site or one of the conserved catalytic motifs essential for kinase activity.3,4) Therefore, TRB1 is considered as a scaffold protein or an adaptor protein to facilitate the degradation of their target proteins and to regulate several key signaling pathways for important cellular processes.5) In cancer, TRB1 has been identified as a myeloid oncogenic driver and induces acute myeloid leukemia (AML) in mice through the mitogenactivated protein extracellular kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway and CCAAT/enhancer binding protein (C/EBP) transcriptional factors.6,7) Importantly, TRB1 is often overexpressed in AML and myelodysplastic syndrome with gene amplification.4,5) TRB1 is also found to be overexpressed in solid tumors, including prostate cancer, 8,9) thyroid cancer, 10) ovarian cancer, 11) and colorectal cancer. 12)Moreover, Oncomine data analyses show that TRB1 is overexpressed in other types of cancer (e.g. breast, esophageal, head and neck, and melanoma). 12) However, whether and how TRB1 plays the physiological roles in solid tumors has not been fully investigated.The tumor suppressor p53 plays an important role in regulating cell proliferation during various stimuli, including genotoxic stress and oncogenic activation, and nearly all cancers show defects in p53 pathway. [13][14][15][16] p53 primarily functions as a transcriptional factor that activates various genes responsible for cell-cycle arrest, senescence or apoptosis to prevent tumor cell growth.17) The expression of p53 is tightly regulated by a mechanism involving the ubiquitin-proteasome-mediated degradation pathway. When cells experienc...

Kurarinone, a flavonoid isolated from the roots of Sophora flavescens, was suggested to exert potent antioxidant and immunosuppressive effects. However, the underlying mechanisms remain unclear. Nuclear factor erythroid 2-related factor 2 (Nrf2) is a key transcription factor that regulates the antioxidant defense system with anti-inflammatory activity. In the present study, we demonstrated that kurarinone activated Nrf2 and increased the expression of antioxidant enzymes, including heme oxygenase-1 (HO-1). Mechanistically, kurarinone downregulated the expression of kelch-like ECH-associated protein 1 (KEAP1), subsequently leading to the activation of Nrf2. Kurarinone also inhibited the expression of the inflammatory cytokine, interleukin (IL)-1β, and inducible nitric oxide synthase (iNos) in lipopolysaccharide (LPS)-stimulated RAW264.7 macrophages. The overexpression of HO-1 suppressed the LPS-induced production of inflammatory mediators in RAW264.7 cells, and the immunosuppressive effects of kurarinone were partially inhibited by a treatment with Tin Protomorphyrin IX (TinPPIX), an inhibitor of HO-1. These results indicate that kurarinone activates the KEAP1/Nrf2 pathway to induce HO-1 expression, thereby exerting immunosuppressive effects.

Edited by Xiao-Fan Wang Su(var)3-9, Enhancer-of-zeste, and Trithorax (SET) domaincontaining protein 8 (SET8) is the sole enzyme that monomethylates Lys-20 of histone H4 (H4K20). SET8 has been implicated in the regulation of multiple biological processes, such as gene transcription, the cell cycle, and senescence. SET8 quickly undergoes ubiquitination and degradation by several E3 ubiquitin ligases; however, the enzyme that deubiquitinates SET8 has not yet been identified. Here we demonstrated that ubiquitinspecific peptidase 17-like family member (USP17) deubiquitinates and therefore stabilizes the SET8 protein. We observed that USP17 interacts with SET8 and removes polyubiquitin chains from SET8. USP17 knockdown not only decreased SET8 protein levels and H4K20 monomethylation but also increased the levels of the cyclin-dependent kinase inhibitor p21. As a consequence, USP17 knockdown suppressed cell proliferation. We noted that USP17 was down-regulated in replicative senescence and that USP17 inhibition alone was sufficient to trigger cellular senescence. These results reveal a regulatory mechanism whereby USP17 prevents cellular senescence by removing ubiquitin marks from and stabilizing SET8 and transcriptionally repressing p21. Su(var)3-9, Enhancer-of-zeste, and Trithorax (SET) 5 domaincontaining protein 8 (SET8) (also known as SETD8, PR-Set7, and KMT5A) is the sole enzyme required to catalyze monomethylation of histone H4 lysine 20 (H4K20me1) (1-3). Besides H4K20me1, SET8 can methylate nonhistone proteins, including p53, proliferating cell nuclear antigen, Numb, and androgen receptor and estrogen receptor ␣ (4). SET8 is involved in vital cellular processes such as DNA replication, mitosis, DNA repair, and gene transcriptional regulation (5, 6). Therefore, precise modulation of SET8 levels is important for proper cell cycle regulation, and deregulation of SET8 expression has been suggested to cause cellular transformation and contribute to cancer progression (7). Aberrant expression of SET8 has been detected in many types of tumors (8-11). High levels of SET8 are also associated with poor survival in cancer patients (9-11). Moreover, recent studies have shown that SET8 prevents cellular senescence through epigenetic regulation (12, 13). Loss of SET8 is sufficient to establish cellular senescence, and H4K20me1 modification of the cyclin-dependent kinase inhibitor p21 gene during cellular senescence is regulated by SET8 (12, 13). Because knockdown of p21 alleviates the senescence state of SET8 knockdown cells, SET8 suppresses induction of cellular senescence by repressing p21 transcription (13). SET8 is regulated at several levels, including the transcriptional level (14), posttranscriptional level (15), and posttranslational level (7). Some E3 ubiquitin ligases have been shown to induce SET8 ubiquitination and degradation, which regulate cell cycle progression (7). The anaphase-promoting complex APC/C Cdh1 induces ubiquitination and degradation of SET8 during G 1 phase (16). In addition, Cullin-RING ubiquitin ...

Transcriptional coactivator with a PDZ-binding motif (TAZ) is one of the mammalian orthologs of Drosophila Yorkie, a transcriptional coactivator of the Hippo pathway. TAZ has been suggested to function as a regulator that modulates the expression of cell proliferation and anti-apoptotic genes in order to stimulate cell proliferation. TAZ has also been associated with a poor prognosis in several cancers, including breast cancer. However, the physiological role of TAZ in tumorigenesis remains unclear. We herein demonstrated that TAZ negatively regulated the activity of the tumor suppressor p53. The overexpression of TAZ down-regulated p53 transcriptional activity and its downstream gene expression. In contrast, TAZ knockdown up-regulated p21 expression induced by p53 activation. Regarding the underlying mechanism, TAZ inhibited the interaction between p53 and p300 and suppressed the p300-mediated acetylation of p53. Furthermore, TAZ knockdown induced cellular senescence in a p53-dependent manner. These results suggest that TAZ negatively regulates the tumor suppressor functions of p53 and attenuates p53-mediated cellular senescence.

Tribbles-related protein (TRB) family members are the mammalian orthologs of Drosophila tribbles. Tribbles was originally identified as a cell cycle regulator during Drosophila development. Tribbles genes are evolutionary conserved, and three TRB genes (TRB1, TRB2 and TRB3) have been identified in mammals. TRBs are considered pseudokinases because they lack an ATP binding site or one of the conserved catalytic motifs essential for kinase activity. Instead, TRBs play important roles in various cellular processes as scaffolds or adaptors to promote the degradation of target proteins and to regulate several key signaling pathways. Recent research has focused on the role of TRBs in tumorigenesis and neoplastic progression. In this review, we focus on the physiological roles of TRB family members in tumorigenesis through the regulation of the ubiquitin-proteasome system and discuss TRBs as biomarkers or potential therapeutic targets in cancer.