Search

Activating transcription factor 5 (ATF5) is a member of the ATF/cAMP response element-binding protein family of transcription factors. ATF5 regulates stress responses and cell survival, proliferation, and differentiation and also plays a role in viral infections, cancer, diabetes, schizophrenia, and the olfactory system. Moreover, it was found to also have a critical cell cycle-dependent structural function at the centrosome. However, the mechanism that controls the localization of ATF5 at the centrosome is unclear. Here we report that ATF5 is small ubiquitin-like modifier (SUMO) 2/3-modified at a conserved SUMO-targeting consensus site in various types of mammalian cells. We found that SUMOylation of ATF5 is elevated in the G 1 phase of the cell cycle and diminished in the G 2 /M phase. ATF5 SUMOylation disrupted the interaction of ATF5 with several centrosomal proteins and dislodged ATF5 from the centrosome at the end of the M phase. Of note, blockade of ATF5 SUMOylation deregulated the centrosome cycle, impeded ATF5 translocation from the centrosome, and caused genomic instability and G 2 /M arrest in HeLa cells. Our results indicate that ATF5 SUMOylation is an essential mechanism that regulates ATF5 localization and function at the centrosome., (© 2018 by The American Society for Biochemistry and Molecular Biology, Inc.)

Human cytomegalovirus (HCMV), a widespread beta-herpes virus, infects a high percentage of gliomas. HCMV is specifically detected in human gliomas at a low level of expression raises the possibility that it may regulate the malignant phenotype in a chronic manner. Although HCMV is not recognized as an oncogenic virus, it might dysregulate signaling pathways involved in initiation and promotion of malignancy.Here, our immunohistochemical staining reveals that nucleus staining of the HCMV 86-kDa immediate-early protein (IE86) is markedly increased in GBM (58.56%) compared with that in nontumorous samples (4.20%) and low-grade glioma(19.56%). IE86 staining positively correlates with the staining of activating transcription factor 5 (ATF5) which is essential for glioma cell viability and proliferation suggesting that HCMV IE86 could have important implications in glioma biology. Moreover, we find that the IE86 overexpression enhances glioma cell's growth in vitro and in vivo. We demonstrate that IE86 protein physically interacts with, and acetylates ATF5 thereby promoting glioma cell survival. Therefore, our findings illustrate the biological significance of HCMV infection in accelerating glioma progression, and provide novel evidence that HCMV infection may serve as a therapeutic target in human glioma.

Although it is known that the centrioles play instructive roles in pericentriolar material (PCM) assembly and that the PCM is essential for proper centriole formation, the mechanism that governs centriole-PCM interaction is poorly understood. Here, we show that ATF5 forms a characteristic 9-fold symmetrical ring structure in the inner layer of the PCM outfitting the proximal end of the mother centriole. ATF5 controls the centriole-PCM interaction in a cell-cycle- and centriole-age-dependent manner. Interaction of ATF5 with polyglutamylated tubulin (PGT) on the mother centriole and with PCNT in the PCM renders ATF5 as a required molecule in mother centriole-directed PCM accumulation and in PCM-dependent centriole formation. ATF5 depletion blocks PCM accumulation at the centrosome and causes fragmentation of centrioles, leading to the formation of multi-polar mitotic spindles and genomic instability. These data show that ATF5 is an essential structural protein that is required for the interaction between the mother centriole and the PCM., (Copyright © 2015 Elsevier Inc. All rights reserved.)

The activating transcription factor 5 (ATF5) is highly expressed in many kinds of tumors including glioblastoma and breast cancers, but its expression in epithelial ovarian neoplasms has not been investigated. Here, we show that ATF5 is highly expressed in the majority of epithelial ovarian cancer samples (43/60) as compared with benign ovarian tumor tissues (4/13) and normal ovarian tissues (1/10). Furthermore, we found that ATF5 expression significantly correlated with advanced clinical stage (P<0.05) and poor differentiation of epithelial ovarian carcinomas (P<0.05). Previous studies suggested that ATF5 is required for the survival of cancer cells, but the mechanisms by which ATF5 regulates genes and promotes cell survival are not clear. Our data additionally demonstrated that interference with the function of ATF5 could markedly increase the apoptosis of ovarian cancer cells and identified B-cell leukemia lymphoma-2 as an ATF5-targeted apoptosis-related gene. These findings may provide potential therapeutic application in epithelial ovarian cancer.

Background: Past work has established that human glioblastomas and breast cancer cells invariably express the activating transcription factor 5 (ATF5) and that loss of function of ATF5 caused massive apoptotic death of all cancer cell lines tested. ATF5 expression and function in pancreatic cancer cells have not been investigated., Materials and Methods: Quantitative real-time/reverse transcription-polymerase chain reaction (QRT/RT-PCR), western blotting (WB), immunohistochemistry (IHC) and promoter reporter assay were used for gene expression analysis. MTT [3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay and FACS (fluorescence-activated cell sorting) analysis were used to monitor cell viability/apoptosis., Results: ATF5 is highly expressed in pancreatic cancer cells as compared with non-tumor tissues. Both paclitaxel treatment and loss of function of ATF5 elicited apoptosis of SW1990 cells. Interference with ATF5 function in SW1990 cells resulted in down-regulation of BCL-2 and up-regulation of BAX, resulting in enhanced sensitivity to apoptosis induced by paclitaxel treatment., Conclusion: ATF5 is highly expressed in pancreatic cancer cells. Targeting ATF5 significantly enhances paclitaxel-induced apoptosis in human pancreatic cancer cells. ATF5 could be an important therapeutic target for pancreatic cancer treatment.

Nucleophosmin (NPM1/B23) and the activating transcription factor 5 (ATF5) are both known to subject to cell type-dependent regulation. NPM1 is expressed weakly in hepatocytes and highly expressed in hepatocellular carcinomas (HCC) with a clear correlation between enhanced NPM1 expression and increased tumor grading and poor prognosis, whereas in contrast, ATF5 is expressed abundantly in hepatocytes and down-regulated in HCC. Re-expression of ATF5 in HCC inhibits cell proliferation. We report here that using an unbiased approach, tandem affinity purification (TAP) followed with mass spectrometry (MS), we identified NPM1 as a novel ATF5-interacting protein. Unlike many other NPM1-interacting proteins that interact with the N-terminal oligomerization domain of NPM1, ATF5 binds via its basic leucine zipper to the C-terminal region of NPM1 where its nucleolar localization signal is located. NPM1 association with ATF5, whose staining patterns partially overlap in the nucleoli, promotes ATF5 protein degradation through proteasome-dependent and caspase-dependent pathways. NPM1-c, a mutant NPM1 that is defective in nucleolar localization, failed to stimulate ATF5 polyubiquitination and was unable to down-regulate ATF5. NPM1 interaction with ATF5 displaces HSP70, a known ATF5-interacting protein, from ATF5 protein complexes and antagonizes its role in stabilization of ATF5 protein. NPM1-promoted ATF5 down-regulation diminished ATF5-mediated repression of cAMP-responsive element-dependent gene transcription and abrogates ATF5-induced G(2)/M cell cycle blockade and inhibition of cell proliferation in HCC cells. Our study establishes a mechanistic link between elevated NPM1 expression and depressed ATF5 in HCC and suggests that regulation of ATF5 by NPM1 plays an important role in the proliferation and survival of HCC.

ATF5 has been shown to be a critical regulator of cell proliferation and survival; however, the underlying mechanism remains largely unknown. We demonstrate here that ATF5 interacts with the transcriptional coactivator p300, which acetylates ATF5 at lysine-29 (K29), which in turn enhances the interaction between ATF5 and p300 and binding of the ATF5/p300 complex to the ATF5 response element (ARE) region of the Egr-1 promoter. ARE-bound ATF5/p300 acetylates lysine-14 (K14) of nucleosomal histone H3 at both the ARE and serum response element (SRE) of the Egr-1 promoter, which facilitates binding of extracellular signal-regulated kinase (ERK)-phosphorylated Elk-1 to the SRE, activating the Egr-1 promoter. Interference of p300-dependent acetylation of ATF5 or nucleosomal histone H3 or blockade of ERK-dependent Elk-1 phosphorylation abrogates ATF5-dependent Egr-1 activation and cell proliferation and survival. These findings assign a central role for the ATF5/p300 complex in ATF5 function and suggest that coordinated actions by ATF5, p300, Elk-1, and ERK/mitogen-activated protein kinase (MAPK) are essential for ATF5-dependent Egr-1 activation and cell proliferation and survival.

Although both the heat shock protein 70 (HSP70) and the activating transcription factor 5 (ATF5) have been shown to promote cell survival of transformed cells but not survival of non-transformed cells, the relationship of the two molecules is unknown. Here we show that HSP70 and ATF5 are concomitantly up-regulated upon transient but down-regulated over prolonged cellular stress and apoptotic stimulation in the rat C6 glioma and human U87 glioma cells. HSP70 interacts strongly with the N-terminal activation domain of ATF5, which is expected to be rigid and uniquely structured under physiological conditions because of extraordinary high concentration (over 25%) of proline residues. Binding of HSP70 to ATF5 is an ATP-driven process and requires functional ATPase on the nucleotide binding domain of the HSP70 molecule. Overexpression of HSP70 dramatically stabilizes the ATF5 protein, which is otherwise subject to rapid degradation, facilitated by both proteasome-dependent and caspase-dependent processes, whereas HSP70 depletion leads to acceleration of ATF5 degradation and transcription repression of Bcl-2 and Egr-1, which are downstream targets of ATF5 in C6 and U87 glioma cells. Our data reveal an essential role for HSP70 in maintaining high levels of ATF5 expression in glioma cells and support the conclusion that ATF5 is an important substrate protein of HSP70 that mediates HSP70-promoted cell survival in glioma cells.

ATF5 loss of function has been shown previously to cause apoptotic cell death in glioblastoma and breast cancer cells but not in non-transformed astrocytes and human breast epithelial cells. The mechanism for the cell type-dependent survival function of ATF5 is unknown. We report here that the anti-apoptotic factor BCL-2 is a downstream target of ATF5 that mediates the prosurvival function of ATF5 in C6 glioma cells and MCF-7 breast cancer cells. ATF5 binds to an ATF5-specific regulatory element that is downstream of and adjacent to the negative regulatory element in the BCL-2 P2 promoter, stimulating BCL-2 expression. Highlighting the critical role of BCL-2 in ATF5-dependent cancer cell survival, expression of BCL-2 blocks death of C6 and MCF-7 cells induced by dominant-negative ATF5, and depletion of BCL-2 impairs ATF5-promoted cell survival. Moreover, we found that BCL-2 expression is not regulated by ATF5 in non-transformed rat astrocytes, mouse embryonic fibroblasts, and human breast epithelial cells, where expression of BCL-2 but not ATF5 is required for cell survival. These findings identify BCL-2 as an essential mediator for the cancer-specific cell survival function of ATF5 in glioblastoma and breast cancer cells and provide direct evidence that the cell type-specific function of ATF5 derives from differential regulation of downstream targets by ATF5 in different types of cells.

Recent reports indicate that the activating transcription factor 5 (ATF5) is required for the survival of cancer cells but not for noncancer cells. However, the mechanisms by which ATF5 regulates genes and promotes cell survival are not clear. Using a cyclic amplification and selection of targets (CASTing) approach, we identified a novel ATF5 consensus DNA binding sequence. We show in C6 glioma and MCF-7 breast cancer cells that ATF5 occupies this sequence and that ATF5 activates reporter gene expression driven by this site. Conversely, reporter activity is diminished when ATF5 activity is blocked or when ATF5 expression is down-regulated by serum withdrawal. We further show that early growth response factor 1 (Egr-1), whose promoter contains two adjacent ATF5 consensus binding sites at a conserved promoter position in rat, mouse, and human, is targeted and regulated by ATF5 in C6 and MCF-7 cells. These data provide new insight on the mechanisms by which ATF5 promotes gene regulation and cancer-specific cell survival.