Hzf, a p53-responsive gene, regulates maintenance of the G2 phase checkpoint induced by DNA damage

Functional Investigations of p53 Acetylation Enabled by Heterobifunctional Molecules

Post-translational modifications (PTMs) dynamically regulate the critical stress response and tumor suppressive functions of p53. Among these, acetylation events mediated by multiple acetyltransferases lead to differential target gene activation and subsequent cell fate. However, our understanding of these events is incomplete due to, in part, the inability to selectively and dynamically control p53 acetylation. We recently developed a heterobifunctional small molecule system, AceTAG, to direct the acetyltransferase p300/CBP for targeted protein acetylation in cells. Here, we expand AceTAG to leverage the acetyltransferase PCAF/GCN5 and apply these tools to investigate the functional consequences of targeted p53 acetylation in human cancer cells. We demonstrate that the recruitment of p300/CBP or PCAF/GCN5 to p53 results in distinct acetylation events and differentiated transcriptional activities. Further, we show that chemically induced acetylation of multiple hotspot p53 mutants results in increased stabilization and enhancement of transcriptional activity. Collectively, these studies demonstrate the utility of AceTAG for functional investigations of protein acetylation.

Quantitative Proteomics Reveal the Mechanism of MiR-138-5p Suppressing Cervical Cancer via Targeting ZNF385A

MicroRNAs are short, noncoding RNA molecules that exert pivotal roles in cancer development and progression by modulating various target genes. There is growing evidence that miR-138-5p is significantly involved in cervical cancer (CC). However, its precise molecular mechanism has yet to be fully understood. In the current investigation, a quantitative proteomics approach was utilized to detect possible miR-138-5p targets in HeLa cells systematically. In total, 364 proteins were downregulated, and 150 were upregulated after miR-138-5p overexpression. Bioinformatic analysis of these differentially expressed proteins (DEPs) revealed significant enrichment in several cancer-related pathways. Zinc finger protein 385A (ZNF385A) was determined as a novel direct target of miR-138-5p and discovered to facilitate the proliferation, migration, and cell cycle progression of HeLa cells. SFN and Fas cell surface death receptor(FAS) were then identified as functional downstream effectors of ZNF385A and miR-138-5p. Moreover, a tumor xenograft experiment was conducted to validate the association of miR-138-5p-ZNF385A-SFN/FAS axis with the development of CC in vivo. Our findings have collectively established a catalog of proteins mediated by miR-138-5p and have provided an in-depth comprehension of the molecular mechanisms responsible for the inhibitory effect of miR-138-5p on CC. The miR-138-5p-ZNF385A-SFN/FAS axis could also be beneficial to the identification of new therapeutic targets.

ZNF385A and ZNF346 Serve as Prognostic Biomarkers Associated with an Inflamed Immunosuppressive Tumor Microenvironment in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) has a high mortality rate worldwide, and there are still many problems in the early diagnosis, molecular targeted therapy, and immunotherapy. It is necessary to explore valuable diagnostic markers and new therapeutic targets in HCC. Zinc finger protein 385A (ZNF385A) and zinc finger protein 346 (ZNF346) represent a unique class of RNA-binding Cys2 His2 (C2H2) zinc finger proteins that are involved in the regulation of cell cycle and apoptosis, but little is known of their roles in HCC. Based on multiple databases and analysis tools, we explored the expression, clinical relation, prognostic value, possible biological function, and pathways of ZNF385A and ZNF346, and their relationship with immune infiltration. Our results revealed that ZNF385A and ZNF346 were highly expressed and were associated with poor prognosis in HCC. Hepatitis B virus (HBV) infection may lead to the overexpression of ZNF385A and ZNF346, which was accompanied by elevated apoptosis and chronic inflammation. Moreover, ZNF385A and ZNF346 were positively correlated with immune-suppressive cells, inflammatory cytokines, immune checkpoint genes, and poor immunotherapy efficacy. Finally, the knockdown of ZNF385A and ZNF346 was observed to negatively affect the proliferation and migration of HepG2 cells in vitro. In conclusion, ZNF385A and ZNF346 are promising candidate biomarkers for the diagnosis, prognosis, and response to immunotherapy in HCC, and this study may help to understand the tumor microenvironment (TME) of liver cancer, and to develop new therapeutic targets.

Elimination of Age-Associated Hepatic Steatosis and Correction of Aging Phenotype by Inhibition of cdk4-C/EBPα-p300 Axis

The aging liver is affected by several disorders, including steatosis, that can lead to a decline of liver functions. Here, we present evidence that the cdk4-C/EBPα-p300 axis is a critical regulator of age-associated disorders, including steatosis. We found that patients with non-alcoholic fatty liver disease (NAFLD) have increased levels of cdk4 and that cdk4-resistant C/EBPα-S193A mice do not develop hepatic steatosis with advancing age. Underlying mechanisms include a block in C/EBPα activation and subsequent failure in activation of enzymes involved in the development of NAFLD. Inhibition of cdk4 in aged wild-type (WT) mice by a specific cdk4 inhibitor, PD-0332991, reduces C/EBPα-p300 complexes and eliminates hepatic steatosis. Moreover, the inhibition of cdk4 in aged mice reverses many age-related disorders. Mechanisms of correction include elimination of cellular senescence and alterations in the chromatin structure of hepatocytes. Thus, the inhibition of cdk4 might be considered as a therapeutic approach to correct age-associated liver disorders.

Nguyen et al. show that nuclear elevation of cdk4 leads to age-associated disorders, such as hepatic steatosis, and to age-dependent decline of liver functions and morphology. Elevation of cdk4 changes multiple molecular aspects of liver biology. Inhibition of cdk4 in old mice eliminates hepatic steatosis and corrects age-associated liver disorders.

Reversible regulation of ORC2 SUMOylation by PIAS4 and SENP2

The small ubiquitin-related modifier (SUMO) system is essential for smooth progression of cell cycle at the G2/M phase. Many centromeric proteins are reversibly SUMOylated to ensure proper chromosome segregation at the mitosis. SUMOylation of centromeric Origin Recognition Complex subunit 2 (ORC2) at the G2/M phase is essential in maintaining genome integrity. However, how ORC2 SUMOylation is regulated remains largely unclear. Here we show that ORC2 SUMOylation is reversibly controlled by SUMO E3 ligase PIAS4 and De-SUMOylase SENP2. Either depletion of PIAS4 or overexpression of SENP2 eliminated SUMOylation of ORC2 at the G/M phase and consequently resulted in abnormal centromeric histone H3 lysine 4 methylation. Cells stably expressing SENP2 protein or small interfering RNA for PIAS4 bypassed mitosis and endoreduplicated their genome to become polyploidy. Furthermore, percentage of polyploid cells is reduced after coexpression of ORC2-SUMO2 fusion protein. Thus, the proper regulation of ORC2 SUMOylation at the G2/M phase by PIAS4 and SENP2 is critical for smooth progression of the mitotic cycle of cells.

Wild type p53 transcriptionally represses the SALL2 transcription factor under genotoxic stress

SALL2- a member of the Spalt gene family- is a poorly characterized transcription factor found deregulated in various cancers, which suggests it plays a role in the disease. We previously identified SALL2 as a novel interacting protein of neurotrophin receptors and showed that it plays a role in neuronal function, which does not necessarily explain why or how SALL2 is deregulated in cancer. Previous evidences indicate that SALL2 gene is regulated by the WT1 and AP4 transcription factors. Here, we identified SALL2 as a novel downstream target of the p53 tumor suppressor protein. Bioinformatic analysis of the SALL2 gene revealed several putative p53 half sites along the promoter region. Either overexpression of wild-type p53 or induction of the endogenous p53 by the genotoxic agent doxorubicin repressed SALL2 promoter activity in various cell lines. However R175H, R249S, and R248W p53 mutants, frequently found in the tumors of cancer patients, were unable to repress SALL2 promoter activity, suggesting that p53 specific binding to DNA is important for the regulation of SALL2. Electrophoretic mobility shift assay demonstrated binding of p53 to one of the identified p53 half sites in the Sall2 promoter, and chromatin immunoprecipitation analysis confirmed in vivo interaction of p53 with the promoter region of Sall2 containing this half site. Importantly, by using a p53ER (TAM) knockin model expressing a variant of p53 that is completely dependent on 4-hydroxy-tamoxifen for its activity, we show that p53 activation diminished SALL2 RNA and protein levels during genotoxic cellular stress in primary mouse embryo fibroblasts (MEFs) and radiosensitive tissues in vivo. Thus, our finding indicates that p53 represses SALL2 expression in a context-specific manner, adding knowledge to the understanding of SALL2 gene regulation, and to a potential mechanism for its deregulation in cancer.

ZNF385B is characteristically expressed in germinal center B cells and involved in B-cell apoptosis

We previously identified zinc finger (ZF) protein ZNF385B as a molecule specifically expressed in Burkitt's lymphoma (BL) among hematologic malignancies. Here, we investigated ZNF385B expression in healthy B cells in a variety of hematological tissues by RT-PCR and immunohistochemistry. ZNF385B expression was found to be limited to a subset of GC B cells, the healthy counterpart to BL B cells. To elucidate the function of ZNF385B in healthy B cells, we established a tetracycline-controlled protein-inducible system in B-cell lines and observed that ectopic expression of the longest transcript variant of ZNF385B, possessing four ZF domains, induced upregulation of PERP and FAS/CD95, a downstream target of p53, and activation of caspase, resulting in apoptosis induction. However, a ZNF385B deletion mutant with three ZF domains corresponding to shorter isoforms, did not induce upregulation; rather it inhibited apoptosis induced by CD20 cross-linking and BCR stimulation. The direct binding of ZNF385B with p53 has suggested the involvement of ZNF385B in B-cell apoptosis via modulation of p53 transactivation; our data indicate that ZNF385B characteristically expressed in GC B cells has both proapoptotic and antiapoptotic activities depending on the type of isoform and should be a novel player in GC B-cell selection.

Cooperative role of the RNA-binding proteins Hzf and HuR in p53 activation

The RNA-binding protein Hzf (hematopoietic zinc finger) plays important roles in mRNA translation in cerebellar Purkinje cells and adipocytes. We along with others have reported that the expression of the Hzf gene is transcriptionally regulated by the p53 tumor suppressor protein. We show here that Hzf regulates p53 expression in cooperation with HuR. Hzf and HuR independently interact with the 3' untranslated region (UTR) of p53 mRNA, which facilitates the cytoplasmic localization of p53 mRNA in the presence of the ARF tumor suppressor protein. In the absence of Hzf and HuR, p53 induction by p19(ARF) is significantly attenuated, and the cells consequently acquire resistance to p19(ARF). Thus, these findings demonstrate that in addition to Mdm2 inhibition, p19(ARF) increases the concentration of p53 through posttranscriptional control of p53 mRNA and suggest critical roles for the RNA-binding proteins Hzf and HuR in p53 induction.

Life or death: p53-induced apoptosis requires DNA binding cooperativity

The tumor suppressor p53 provides exquisite protection from cancer by balancing cell survival and death in response to stress. Sustained stress or irreparable damage trigger p53's killer functions to permanently eliminate genetically-altered cells as a potential source of cancer. To prevent the unnecessary loss of cells that could cause premature aging as a result of stem cell attrition, the killer functions of p53 are tightly regulated and balanced against protector functions that promote damage repair and support survival in response to low stress or mild damage. In molecular terms these p53-based cell fate decisions involve protein interactions with cofactors and modifying enzymes, which modulate the activation of distinct sets of p53 target genes. In addition, we demonstrate that part of this regulation occurs at the level of DNA binding. We show that the killer function of p53 requires the four DNA binding domains within the p53 tetramer to interact with one another. These intermolecular interactions enable cooperative binding of p53 to less perfect response elements in the genome, which are present in many target genes essential for apoptosis. Modulating p53 interactions within the tetramer could therefore present a novel promising strategy to fine-tune p53-based cell fate decisions.

Production of rat monoclonal antibodies against RNA-binding protein Hzf

The hematopoietic zinc finger protein, Hzf, is induced in response to DNA damage or by Arf tumor suppressor in a p53-dependent manner. Recent studies have revealed that Hzf is an RNA-binding protein that regulates localization and translation of specific mRNA. The RNA-binding activity of Hzf is required for the functions of cerebellar purkinje cells and adipocytes, although their molecular mechanisms underlying the mRNA regulation largely remain unknown. To further investigate the molecular function of Hzf, we raised two rat monoclonal antibodies (MAb) against a peptide corresponding to the C-terminal region of the mouse/human Hzf protein. Both MAbs reacted with the native protein expressed in mammalian cells, and were highly efficient in detecting endogenous Hzf by immunoblotting, immunoprecipitation, and immunofluorescence. These MAbs should therefore be useful for further analysis of molecular functions of the Hzf protein and for identification of Hzf-binding proteins.

Activation of p53 by Nutlin-3a, an antagonist of MDM2, induces apoptosis and cellular senescence in adult T-cell leukemia cells

It has been reported that the induction of cellular senescence through p53 activation is an effective strategy in tumor regression. Unfortunately, however, tumors including adult T-cell leukemia/lymphoma (ATL) have disadvantages such as p53 mutations and a lack of p16(INK4a) and/or p14(ARF). In this study we characterized Nutlin-3a-induced cell death in 16 leukemia/lymphoma cell lines. Eight cell lines, including six ATL-related cell lines, had wild-type p53 and Nutlin-3a-activated p53, and the cell lines underwent apoptosis or cell-cycle arrest, whereas eight cell lines with mutated p53 were resistant. Interestingly, senescence-associated-beta-galactosidase (SA-beta-gal) staining revealed that only ATL-related cell lines with wild-type p53 showed cellular senescence, although they lack both p16(INK4a) and p14(ARF). These results indicate that cellular senescence is an important event in p53-dependent cell death in ATL cells and is inducible without p16(INK4a) and p14(ARF). Furthermore, knockdown of Tp53-induced glycolysis and apoptosis regulator (TIGAR), a novel target gene of p53, by small interfering RNA(siRNA) indicated its important role in the induction of cellular senescence. As many patients with ATL carry wild-type p53, our study suggests that p53 activation by Nutlin-3a is a promising strategy in ATL. We also found synergism with a combination of Nutlin-3a and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), suggesting the application of Nutlin-3a-based therapy to be broader than expected.

Stress-dependent Daxx-CHIP interaction suppresses the p53 apoptotic program

Our previous studies have implicated CHIP (carboxyl terminus of Hsp70-interacting protein) as a co-chaperone/ubiquitin ligase whose activities yield protection against stress-induced apoptotic events. In this report, we demonstrate a stress-dependent interaction between CHIP and Daxx (death domain-associated protein). This interaction interferes with the stress-dependent association of HIPK2 with Daxx, blocking phosphorylation of serine 46 in p53 and inhibiting the p53-dependent apoptotic program. Microarray analysis confirmed suppression of the p53-dependent transcriptional portrait in CHIP(+/+) but not in CHIP(-/-) heat shocked mouse embryonic fibroblasts. The interaction between CHIP and Daxx results in ubiquitination of Daxx, which is then partitioned to an insoluble compartment of the cell. In vitro ubiquitination of Daxx by CHIP revealed that ubiquitin chain formation utilizes non-canonical lysine linkages associated with resistance to proteasomal degradation. The ubiquitination of Daxx by CHIP utilizes lysines 630 and 631 and competes with the sumoylation machinery of the cell at these residues. These studies implicate CHIP as a stress-dependent regulator of Daxx that counters the pro-apoptotic influence of Daxx in the cell. By abrogating p53-dependent apoptotic pathways and by ubiquitination competitive with Daxx sumoylation, CHIP integrates the proteotoxic stress response of the cell with cell cycle pathways that influence cell survival.

Guardian of corpulence: a hypothesis on p53 signaling in the fat cell

Adipocytes provide an organism with fuel in times of caloric deficit, and are an important type of endocrine cell in the maintenance of metabolic homeostasis. In addition, as a lipid-sink, adipocytes serve an equally important role in the protection of organs from the damaging effects of ectopic lipid deposition. For the organism, it is of vital importance to maintain adipocyte viability, yet the fat depot is a demanding extracellular environment with high levels of interstitial free fatty acids and associated lipotoxic effects. These surroundings are less than beneficial for the overall health of any resident cell, adipocyte and preadipocyte alike. In this review, we discuss the process of adipogenesis and the potential involvement of the p53 tumor-suppressor protein in alleviating some of the cellular stress experienced by these cells. In particular, we discuss p53-mediated mechanisms that prevent damage caused by reactive oxygen species and the effects of lipotoxicity. We also suggest the potential for two p53 target genes, START domain-containing protein 4 (StARD4) and oxysterol-binding protein (OSBP), with the concomitant synthesis of the signaling molecule oxysterol, to participate in adipogenesis.

Analysis of gene expression in normal and cancer cells exposed to gamma-radiation

The expression of many genes is modulated after exposure to ionizing radiation. Identification of specific genes may allow the determination of pathways important in radiation responses. We previously identified modulation of the expression of several genes in response to ionizing radiation treatment. In the present study, we monitored the expression of RGS1, CC3, THBS1, vWF, MADH7, and a novel gene encoding a secreted protein in irradiated Jurkat, TK6, HeLa, and HFL1 cells. The RGS1 is involved in G-protein signaling pathway, CC3 belongs to the complement system, THBS1 is a component of the extracellular matrix, vWF takes part in blood coagulation, and MADH7 is a member of the TGF-β signal transduction pathway. Our objective was to find similarities and differences in the expression of these genes in ionizing radiation-exposed diverse cell types. RGS1 was downregulated in Jurkat cells but was upregulated in TK6 and HFL1 cells. The expression of CC3 was repressed in Jurkat and HFL1 cells but was induced in TK6 and HeLa cells. THBS1 was downregulated in irradiated TK6 and HFL1 cells. vWF was induced in radiation-exposed HeLa cells, but its expression was downregulated in Jurkat cells. The expression of MADH7 was induced in all the cell types examined. These results indicate cell specific modulation of gene expression and suggest the involvement of different pathways in cellular response to radiation treatment in different cells.

Hzf regulates adipogenesis through translational control of C/EBPalpha

Adipocyte differentiation requires a well-defined programme of gene expression in which the transcription factor C/EBPalpha (CCAAT/enhancer-binding protein) has a central function. Here, we show that Hzf (haematopoietic zinc-finger), a previously identified p53 transcriptional target, regulates C/EBPalpha expression. Hzf is induced during differentiation of preadipocyte cell lines, and its suppression by short hairpin RNA disrupts adipogenesis. In Hzf's absence, expression of C/EBPalpha is severely impaired because of reduced translation of its mRNA. Hzf physically interacts with the 3' untranslated region of C/EBPalpha mRNA to enhance its translation. Taken together, these findings underscore a critical role of Hzf in the adipogenesis regulatory cascade.

Inhibition of Chk1 kills tetraploid tumor cells through a p53-dependent pathway

Tetraploidy constitutes an adaptation to stress and an intermediate step between euploidy and aneuploidy in oncogenesis. Tetraploid cells are particularly resistant against genotoxic stress including radiotherapy and chemotherapy. Here, we designed a strategy to preferentially kill tetraploid tumor cells. Depletion of checkpoint kinase-1 (Chk1) by siRNAs, transfection with dominant-negative Chk1 mutants or pharmacological Chk1 inhibition killed tetraploid colon cancer cells yet had minor effects on their diploid counterparts. Chk1 inhibition abolished the spindle assembly checkpoint and caused premature and abnormal mitoses that led to p53 activation and cell death at a higher frequency in tetraploid than in diploid cells. Similarly, abolition of the spindle checkpoint by knockdown of Bub1, BubR1 or Mad2 induced p53-dependent apoptosis of tetraploid cells. Chk1 inhibition reversed the cisplatin resistance of tetraploid cells in vitro and in vivo, in xenografted human cancers. Chk1 inhibition activated p53-regulated transcripts including Puma/BBC3 in tetraploid but not in diploid tumor cells. Altogether, our results demonstrate that, in tetraploid tumor cells, the inhibition of Chk1 sequentially triggers aberrant mitosis, p53 activation and Puma/BBC3-dependent mitochondrial apoptosis.

Living with p53, dying of p53

The p53 tumor suppressor protein acts as a major defense against cancer. Among its most distinctive features is the ability to elicit both apoptotic death and cell cycle arrest. In this issue of Cell, Das et al. (2007) and Tanaka et al. (2007) provide new insights into the mechanisms that dictate the life and death decisions of p53.

Hzf Determines cell survival upon genotoxic stress by modulating p53 transactivation

A critical unresolved issue about the genotoxic stress response is how the resulting activation of the p53 tumor suppressor can lead either to cell-cycle arrest and DNA repair or to apoptosis. We show here that hematopoietic zinc finger (Hzf), a zinc-finger-containing p53 target gene, modulates p53 transactivation functions in an autoregulatory feedback loop. Hzf is induced by p53 and binds to its DNA-binding domain, resulting in preferential transactivation of proarrest p53 target genes over its proapoptotic target genes. Thus, p53 activation results in cell-cycle arrest in Hzf wild-type MEFs, while in Hzf(-/-) MEFs, apoptosis is induced. Exposure of Hzf null mice to ionizing radiation resulted in enhanced apoptosis in several organs, as compared to in wild-type mice. These findings provide novel insights into the regulation of p53 transactivation function and suggest that Hzf functions as a key player in regulating cell fate decisions in response to genotoxic stress.

APC/C(Cdc20) controls the ubiquitin-mediated degradation of p21 in prometaphase

During the G1/S transition, p21 proteolysis is mediated by Skp2; however, p21 reaccumulates in G2 and is degraded again in prometaphase. How p21 degradation is controlled in mitosis remains unexplored. We found that Cdc20 (an activator of the ubiquitin ligase APC/C) binds p21 in cultured cells and identified a D box motif in p21 necessary for APC/C(Cdc20)-mediated ubiquitylation of p21. Overexpression of Cdc20 or Skp2 destabilized wild-type p21; however, only Skp2, but not Cdc20, was able to destabilize a p21(D box) mutant. Silencing of Cdc20 induced an accumulation of p21, increased the fraction of p21 bound to Cdk1, and inhibited Cdk1 activity in p21(+/+) prometaphase cells, but not in p21(-/-) cells. Thus, in prometaphase Cdc20 positively regulates Cdk1 by mediating the degradation of p21. We propose that the APC/C(Cdc20)-mediated degradation of p21 contributes to the full activation of Cdk1 necessary for mitotic events and prevents mitotic slippage during spindle checkpoint activation.

p53 modulation of the DNA damage response

The tumor suppressor p53 plays a central role in the DNA damage response. After exposure to genotoxic stress, p53 can both positively and negatively regulate cell fate. Initially, p53 promotes cell survival by inducing cell cycle arrest, DNA repair, and other pro-survival pathways. However, when cells accumulate DNA damage or demonstrate aberrant growth, p53 can direct the elimination of damaged cells. In this review, we will discuss the transcriptional-dependent and -independent roles of p53 in regulating the DNA damage response.

The Wnt signaling pathway has tumor suppressor properties in retinoblastoma

Retinoblastoma is a pediatric retinal tumor caused by mutational inactivation of the tumor suppressor pRb. Additional genetic changes, as yet unidentified, are believed to be required for tumor initiation. Mutations in the Wnt signaling pathway have been implicated in the pathogenesis of many cancers. Multiple Wnt pathway genes are expressed in the retina and the pRb and Wnt pathways interact biochemically, raising the possibility that alterations in the Wnt pathway contribute to retinoblastoma. Our studies showed that Wnt signaling activation significantly decreased the viability of retinoblastoma cell lines by inducing cell cycle arrest, which was associated with upregulated p53. Furthermore, immunolocalization of the Wnt signaling mediator beta-catenin in human and mouse retinoblastoma tissue indicated that canonical Wnt signaling is suppressed in tumors in vivo. These studies are consistent with the Wnt pathway acting as a tumor suppressor in retinoblastoma and suggest that loss of Wnt signaling is tumorigenic in the retina.