Jing Nie

KRAB-type zinc-finger protein Apak specifically regulates p53-dependent apoptosis

Only a few p53 regulators have been shown to participate in the selective control of p53-mediated cell cycle arrest or apoptosis. How p53-mediated apoptosis is negatively regulated remains largely unclear. Here we report that Apak (ATM and p53-associated KZNF protein), a Krüppel-associated box (KRAB)-type zinc-finger protein, binds directly to p53 in unstressed cells, specifically downregulates pro-apoptotic genes, and suppresses p53-mediated apoptosis by recruiting KRAB-box-associated protein (KAP)-1 and histone deacetylase 1 (HDAC1) to attenuate the acetylation of p53. Apak inhibits p53 activity by interacting with ATM, a previously identified p53 activator. In response to stress, Apak is phosphorylated by ATM and dissociates from p53, resulting in activation of p53 and induction of apoptosis. These findings revealed Apak to be a negative regulator of p53-mediated apoptosis and showed the dual role of ATM in p53 regulation.

Smad Ubiquitylation Regulatory Factor 1/2 (Smurf1/2) Promotes p53 Degradation by Stabilizing the E3 Ligase MDM2

The tumor suppressor p53 protein is tightly regulated by a ubiquitin-proteasomal degradation mechanism. Several E3 ubiquitin ligases, including MDM2 (mouse double minute 2), have been reported to play an essential role in the regulation of p53 stability. However, it remains unclear how the activity of these E3 ligases is regulated. Here, we show that the HECT-type E3 ligase Smurf1/2 (Smad ubiquitylation regulatory factor 1/2) promotes p53 degradation by enhancing the activity of the E3 ligase MDM2. We provide evidence that the role of Smurf1/2 on the p53 stability is not dependent on the E3 activity of Smurf1/2 but rather is dependent on the activity of MDM2. We find that Smurf1/2 stabilizes MDM2 by enhancing the heterodimerization of MDM2 with MDMX, during which Smurf1/2 interacts with MDM2 and MDMX. We finally provide evidence that Smurf1/2 regulates apoptosis through p53. To our knowledge, this is the first report to demonstrate that Smurf1/2 functions as a factor to stabilize MDM2 protein rather than as a direct E3 ligase in regulation of p53 degradation.

Histone methyltransferase protein SETD2 interacts with p53 and selectively regulates its downstream genes.

SETD2 (SET domain containing protein 2) is a histone H3K36 trimethyltransferase protein that associates with hyperphosphorylated RNA polymerase II and involves in transcriptional elongation. However, whether and how SETD2 is implicated in the specific regulation of gene transcription remains unknown. Here we show that SETD2 could interact with p53 and selectively regulate the transcription factor activity of p53. The interaction was dependent of C-terminal region of SETD2, which contains the SET and WW domains, and the N-terminal transactivation domain (residues 1-45) of p53. Overexpression of SETD2 upregulated the expression levels of a subset of p53 targets including puma, noxa, p53AIP1, fas, p21, tsp1, huntingtin, but downregulated that of hdm2. In contrast, it had no significant effect on those of 14-3-3sigma, gadd45 and pig3. Consistently, knockdown of endogenous SETD2 expression by RNA interference resulted in converse effects as expected. In p53-deficient H1299 cells, SETD2 lost the ability to regulate these gene expression except hdm2, indicating the dependence of p53. Furthermore, we demonstrated that SETD2 downregulated hdm2 expression by targeting its P2 promoter and then enhanced p53 protein stability. Collectively, these findings suggest that the histone methyltransferase SETD2 could selectively regulate the transcription of subset genes via cooperation with the transcription factor p53.

HECT ubiquitin ligase Smurf1 targets the tumor suppressor ING2 for ubiquitination and degradation

MINT‐7894249: ING2 (uniprotkb:Q9H160) physically interacts (MI:0915) with ubiquitin (uniprotkb:P62988) by anti tag co‐immunoprecipitation (MI:0007)

REGγ proteasome mediates degradation of the ubiquitin ligase Smurf1

MINT‐7894558: Smurf1 (uniprotkb:Q9HCE7) physically interacts (MI:0915) with REG gamma (uniprotkb:P61289) by two hybrid (MI:0018)

CKIP-1: a scaffold protein and potential therapeutic target integrating multiple signaling pathways and physiological functions.

The PH domain-containing casein kinase 2 interacting protein-1 (CKIP-1, also known as PLEKHO1) acts as a scaffold protein mediating interactions with multiple proteins, including CK2α, CPα, AP-1/c-Jun, Akt, ATM, IFP35/Nmi and Smurf1. CKIP-1 functions through different ways, such as plasma membrane recruitment, transcriptional activity modulation and posttranscriptional modification regulation. Moreover, the subcellular localization of CKIP-1 is determined by several key amino acids in a cell type dependent style, and the nucleus/plasma membrane shuttle of CKIP-1 is regulated by different cell stresses. As an adaptor protein, CKIP-1 is involved in various important signaling pathways, controlling cell growth, apoptosis, differentiation, cytoskeleton and bone formation. Strikingly, CKIP-1 has been recently demonstrated to be a promising target for treatment of osteoporosis in rat models. In addition, more evidences suggest that CKIP-1 might also function as a potential tumor suppressor.

Ceap/BLOS2 interacts with BRD7 and selectively inhibits its transcription-suppressing effect on cellular proliferation-associated genes

The centrosome associated protein Ceap-16 (also termed BLOS2) can accelerate the proliferation of mouse fibroblast NIH3T3 cells, which mechanism remains unclear. Here we identified tumor suppressor candidate BRD7 (bromodomain containing protein 7), which could negatively regulate cell proliferation and growth, as a novel Ceap-16-interacting protein. Ceap-16 and BRD7 interacted with each other both in vitro and in vivo. The C-terminus of BRD7 and the central region of Ceap-16 mediated the interaction. Through this binding, Ceap-16 could translocate from cytoplasm to the nucleus where it selectively inhibited the transcriptional suppression activity of BRD7 towards certain target genes including E2F3 and cyclin A. Moreover, Ceap-16, BRD7 and histone H3/H4 could form a complex and Ceap-16 did not compete with BRD7 binding to histones. These findings suggest a novel function for Ceap-16 in the transcriptional regulation through associating with BRD7.

CKIP-1 acts as a colonic tumor suppressor by repressing oncogenic Smurf1 synthesis and promoting Smurf1 autodegradation

Dysregulation of cellular signaling pathways can lead to colon cancer. However, research on the key signaling effectors or regulators in colon carcinogenesis is limited. Casein kinase-2 interacting protein-1 (CKIP-1; also known as PLEKHO1) is crucial during adult bone formation and is a promising drug target for osteoporosis therapy. In this study, we observed that CKIP-1 was downregulated in human colon cancer tissues and colon cancer cell lines, and this result was correlated with colon cancer progression. CKIP-1 silencing in colon cancers involved promoter methylation. In colon cancer HCT116 and SW480 cells, CKIP-1 overexpression inhibited cell growth and migration. CKIP-1 also suppressed in-vivo tumor formation. Notably, the growth-suppressive role of CKIP-1 was dependent on the downregulation of the cell cycle-regulated oncogene Smad ubiquitylation regulatory factor-1 (Smurf1). During cell cycle progression, phosphatidylinositol-3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signaling increased Smurf1 production by an mTOR-dependent translational control mechanism. Rapamycin, the mTOR inhibitor, significantly reduced Smurf1 protein levels, and Smurf1 was degraded in mitosis. In colon cancer, CKIP-1 controlled Smurf1 expression by suppressing PI3K/Akt/mTOR signaling and enhancing Smurf1 autodegradation, and CKIP-1 downregulation was correlated with Smurf1 upregulation in colon carcinogenesis. These findings provide novel insight into the mechanisms of the candidate tumor suppressor CKIP-1.

CKIP-1 couples Smurf1 ubiquitin ligase with Rpt6 subunit of proteasome to promote substrate degradation

CKIP‐1 is an activator of the Smurf1 ubiquitin ligase acting to promote the ubiquitylation of Smad5 and MEKK2. The mechanisms involved in the recognition and degradation of these substrates by the proteasome remain unclear. Here, we show that CKIP‐1, through its leucine zipper, interacts directly with the Rpt6 ATPase of the 19S regulatory particle of the proteasome. CKIP‐1 mediates the Smurf1–Rpt6 interaction and delivers the ubiquitylated substrates to the proteasome. Depletion of CKIP‐1 reduces the degradation of Smurf1 and its substrates by Rpt6. These findings reveal an unexpected adaptor role of CKIP‐1 in coupling the ubiquitin ligase and the proteasome.

Progress in regulation of activity and stability of ubiquitin protein ligase MDM2: Progress in regulation of activity and stability of ubiquitin protein ligase MDM2

The ubiquitin protein ligase (E3) MDM2 (Murine double minute 2) possesses oncogenic activities. Overexpression of this protein enhances degradation and inactivation of the tumor suppressor p53. At least 7% of all human tumors exhibit inappropriate amplification of mdm2, whereas p53 gene remains in its wild-type configuration. This indicates that MDM2 may function in the p53-independent manner to promote tumorigenesis. Considering the critical role of MDM2, this review summarizes the current mechanisms and progress on MDM2 regulation in levels of gene control, mRNA transcription, post-translational modification, and interaction proteins.