BAG-1M, an isoform of Bcl-2-interacting protein BAG-1, enhances gene expression driven by CMV promoter

Role of Heat Shock Proteins (HSP70 and HSP90) in Viral Infection

Heat shock proteins (HSPs) are a large group of chaperones found in most eukaryotes and bacteria. They are responsible for the correct protein folding, protection of the cell against stressors, presenting immune and inflammatory cytokines; furthermore, they are important factors in regulating cell differentiation, survival and death. Although the biological function of HSPs is to maintain cell homeostasis, some of them can be used by viruses both to fold their proteins and increase the chances of survival in unfavorable host conditions. Folding viral proteins as well as replicating many different viruses are carried out by, among others, proteins from the HSP70 and HSP90 families. In some cases, the HSP70 family proteins directly interact with viral polymerase to enhance viral replication or they can facilitate the formation of a viral replication complex and/or maintain the stability of complex proteins. It is known that HSP90 is important for the expression of viral genes at both the transcriptional and the translational levels. Both of these HSPs can form a complex with HSP90 and, consequently, facilitate the entry of the virus into the cell. Current studies have shown the biological significance of HSPs in the course of infection SARS-CoV-2. A comprehensive understanding of chaperone use during viral infection will provide new insight into viral replication mechanisms and therapeutic potential. The aim of this study is to describe the molecular basis of HSP70 and HSP90 participation in some viral infections and the potential use of these proteins in antiviral therapy.

Stress proteins: the biological functions in virus infection, present and challenges for target-based antiviral drug development

Stress proteins (SPs) including heat-shock proteins (HSPs), RNA chaperones, and ER associated stress proteins are molecular chaperones essential for cellular homeostasis. The major functions of HSPs include chaperoning misfolded or unfolded polypeptides, protecting cells from toxic stress, and presenting immune and inflammatory cytokines. Regarded as a double-edged sword, HSPs also cooperate with numerous viruses and cancer cells to promote their survival. RNA chaperones are a group of heterogeneous nuclear ribonucleoproteins (hnRNPs), which are essential factors for manipulating both the functions and metabolisms of pre-mRNAs/hnRNAs transcribed by RNA polymerase II. hnRNPs involve in a large number of cellular processes, including chromatin remodelling, transcription regulation, RNP assembly and stabilization, RNA export, virus replication, histone-like nucleoid structuring, and even intracellular immunity. Dysregulation of stress proteins is associated with many human diseases including human cancer, cardiovascular diseases, neurodegenerative diseases (e.g., Parkinson’s diseases, Alzheimer disease), stroke and infectious diseases. In this review, we summarized the biologic function of stress proteins, and current progress on their mechanisms related to virus reproduction and diseases caused by virus infections. As SPs also attract a great interest as potential antiviral targets (e.g., COVID-19), we also discuss the present progress and challenges in this area of HSP-based drug development, as well as with compounds already under clinical evaluation.

Human cytomegalovirus inhibits apoptosis involving upregulation of the antiapoptotic protein Bag-1

Human cytomegalovirus (HCMV) is an important opportunistic pathogen in immunocompromised individuals and is recognized as a major viral cause of birth defects. HCMV has the ability to establish lifelong persistence and latent infection following primary exposure. Apoptosis is an innate cellular defense response to viral infection. HCMV can block apoptosis in various cell types. Here we show that HCMV promotes survival of human embryonic lung fibroblasts by activating of MAPK/ERK signaling pathway. Bag-1 is up-regulated in a MAPK/ERK-dependent fashion in infected cells. Depletion of Bag-1 suppresses the antiapoptotic effect of HCMV. Taken together, these data indicate that Bag-1 up-regulation is required to maintain apoptosis resistance in HCMV infected cells.

Coregulator control of androgen receptor action by a novel nuclear receptor-binding motif

The androgen receptor (AR) is a ligand-activated transcription factor that is essential for prostate cancer development. It is activated by androgens through its ligand-binding domain (LBD), which consists predominantly of 11 α-helices. Upon ligand binding, the last helix is reorganized to an agonist conformation termed activator function-2 (AF-2) for coactivator binding. Several coactivators bind to the AF-2 pocket through conserved LXXLL or FXXLF sequences to enhance the activity of the receptor. Recently, a small compound-binding surface adjacent to AF-2 has been identified as an allosteric modulator of the AF-2 activity and is termed binding function-3 (BF-3). However, the role of BF-3 in vivo is currently unknown, and little is understood about what proteins can bind to it. Here we demonstrate that a duplicated GARRPR motif at the N terminus of the cochaperone Bag-1L functions through the BF-3 pocket. These findings are supported by the fact that a selective BF-3 inhibitor or mutations within the BF-3 pocket abolish the interaction between the GARRPR motif(s) and the BF-3. Conversely, amino acid exchanges in the two GARRPR motifs of Bag-1L can impair the interaction between Bag-1L and AR without altering the ability of Bag-1L to bind to chromatin. Furthermore, the mutant Bag-1L increases androgen-dependent activation of a subset of AR targets in a genome-wide transcriptome analysis, demonstrating a repressive function of the GARRPR/BF-3 interaction. We have therefore identified GARRPR as a novel BF-3 regulatory sequence important for fine-tuning the activity of the AR.

Heat shock enhances CMV-IE promoter-driven metabotropic glutamate receptor expression and toxicity in transfected cells

In CHO-K1 cells, heat shock strongly activated reporter-gene expression driven by the cytomegalovirus immediate-early (CMV-IE) promoter from adenoviral and plasmid vectors. Heat shock treatment (2h at 42.5 °C) significantly enhanced the promoter DNA-binding activity in nuclear extracts. In CHO cells expressing mGluR1a and mGluR5a receptors under the control of the CMV promoter, heat shock increased receptor protein expression, mRNA levels and receptor function estimated by measurement of PI hydrolysis, intracellular Ca²+ and cAMP. Hyperthermia increased average amplitudes of Ca²+ responses, the number of responding cells, and revealed the toxic properties of mGluR1a receptor. Heat shock also effectively increased the expression of EGFP. Hence, heat shock effects on mGluR expression and function in CHO cells may be attributed to the activation of the CMV promoter. Moreover, this effect was not limited to CHO cells as heat shock also increased EGFP expression in PC-12 and HEK293 cells. Heat shock treatment may be a useful tool to study the function of proteins expressed in heterologous systems under control of the CMV promoter. It may be especially valuable for increasing protein expression in transient transfections, for enhancing receptor expression in drug screening applications and to control the expression of proteins endowed with toxic properties. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Role of Heat Shock Proteins in Viral Infection

One of the most intriguing and less known aspects of the interaction between viruses and their host is the impact of the viral infection on the heat shock response (HSR). While both a positive and a negative role of different heat shock proteins (HSP) in the control of virus replication has been hypothesized, HSP function during the virus replication cycle is still not well understood. This chapter describes different aspects of the interactions between viruses and heat shock proteins during infection of mammalian cells: the first part focuses on the modulation of the heat shock response by human viral pathogens; the second describes the interactions of HSP and other chaperones with viral components, and their function during different steps of the virus replication cycle; the last part summarizes our knowledge on the effect of hyperthermia and HSR modulators on virus replication.

Multiple, but concerted cellular activities of the human protein Hap46/BAG-1M and isoforms

The closely related human and murine proteins Hap46/BAG-1M and BAG-1, respectively, were discovered more than a decade ago by molecular cloning techniques. These and the larger isoform Hap50/BAG-1L, as well as shorter isoforms, have the ability to interact with a seemingly unlimited array of proteins of completely unrelated structures. This problem was partially resolved when it was realized that molecular chaperones of the hsp70 heat shock protein family are major primary association partners, binding being mediated by the carboxy terminal BAG-domain and the ATP-binding domain of hsp70 chaperones. The latter, in turn, can associate with an almost unlimited variety of proteins through their substrate-binding domains, so that ternary complexes may result. The protein folding activity of hsp70 chaperones is affected by interactions with Hap46/BAG-1M or isoforms. However, there also exist several proteins which bind to Hap46/BAG-1M and isoforms independent of hsp70 mediation. Moreover, Hap46/BAG-1M and Hap50/BAG-1L, but not the shorter isoforms, can bind to DNA in a sequence-independent manner by making use of positively charged regions close to their amino terminal ends. This is the molecular basis for their effects on transcription which are of major physiological relevance, as discussed here in terms of a model. The related proteins Hap50/BAG-1L and Hap46/BAG-1M may thus serve as molecular links between such diverse bioactivities as regulation of gene expression and protein quality control. These activities are coordinated and synergize in helping cells to cope with conditions of external stress. Moreover, they recently became markers for the aggressiveness of several cancer types.

Different BAG-1 isoforms have distinct functions in modulating chemotherapeutic-induced apoptosis in breast cancer cells

AIM

BAG-1 is a multifunctional anti-apoptotic gene with four isoforms, and different BAG-1 isoforms have different anti-apoptotic functions. In this study, we transfected BAG-1 isoforms into the human breast cancer cell lines Hs578T (ER negative) and MCF-7 (ER positive) to study their effect on apoptosis with or without estrogens.

METHODS

The constructed recombinant expression vectors carrying individual BAG-1 isoforms was used to transfect human breast cancer cell lines Hs578T (ER negative) and MCF-7 (ER positive). After stable cell lines were made, a variety of apoptosis-inducing agents, including doxorubicin, docetaxel, and 5-FU, was used to treat these cell lines with or without estrogen to test the role of BAG-1. The mechanism by which BAG-1 affected the function of Bcl-2 was exploredby using the cycloheximide chase assay.

RESULTS

The BAG-1 p50 and p46 isoforms significantly enhanced the resistance to apoptosis in both cell lines according to flow cytometry analysis. BAG-1 p33 and p29 failed to protect the transfected cells from apoptosis. The cell viability assay showed that only BAG-1 p50, but not p46, p33, or p29, increased estrogen-dependent function in ER-positive cell line MCF-7. Only BAG-1 p50 dramatically increased its anti-apoptotic ability in the presence of estrogen, while estrogen has very little effect on the anti-apoptotic ability of other BAG-1 isoforms. In the detection of the expression of K-ras, Hsp70, cytochrome c, Raf-1, ER-alpha, and Bcl-2 in MCF-7 cells by Western blot, only Bcl-2 protein expression was significantly increased in MCF-7 cells transfected with BAG-1 p50 and p46, respectively. Furthermore, the cycloheximide chase assay indicated that the degradation of Bcl-2 protein was extended in the BAG-1 p50 and p46 transfected MCF-7 cells.

CONCLUSION

Distinct isoforms of BAG-1 have different anti-apoptotic functions in breast cancer cells, and that the BAG-1 p50 isoform can potentiate the role of estrogen in ER-positive breast cancer.

Reporter gene stimulation by MIDA1 through its DnaJ homology region

MIDA1 was reported as a protein that can associate with Id1. Its N-terminus has homology to Z-DNA binding protein, Zuotin, that contains DnaJ motif, considered to interact with Hsp70s, and Id binding domain. In the present study, we found that MIDA1 stimulates the transcription of the co-transfected genes. This stimulation was independent of promoter specificity because it was observed in various transfected genes. MIDA1 enhanced formation of DNA-protein complexes with E-box or TATA box without its direct binding to DNA. Analysis with deletion mutants of MIDA1 showed that the short protein fragment containing DnaJ motif within Zuotin homology region is sufficient for the stimulation of transcription and we demonstrated that MIDA1 associates with Hsp70. These data suggest involvement of MIDA1 in the stimulation of transcription in concert with Hsp70/Hsc70 molecular chaperones, thus providing a link between Hsp70/Hsc70 molecular chaperones and components of the transcriptional machinery.

BAG-1--a nucleotide exchange factor of Hsc70 with multiple cellular functions

BAG-1 (Bcl-2-associated athanogene) is a multifaceted protein implicated in the modulation of a large variety of cellular processes. Elucidating the molecular mechanisms that underlie the cellular functions of BAG-1 becomes an increasingly important task, particularly in light of the growing evidence connecting aberrant BAG-1 expression to certain human cancers. A common element of the remarkable functional diversity of BAG-1 appears to be the interaction with molecular chaperones of the Hsp70 family. In fact, BAG-1 functions as a nucleotide exchange factor of mammalian cytosolic Hsc70, thereby triggering substrate unloading from the chaperone. In addition, recent findings reveal an association of BAG-1 with the proteasome, which suggests a role in coordinating chaperone and degradation pathways.

Recruitment of Hsp70 chaperones: a crucial part of viral survival strategies

Virus proliferation depends on the successful recruitment of host cellular components for their own replication, protein synthesis, and virion assembly. In the course of virus particle production a large number of proteins are synthesized in a relatively short time, whereby protein folding can become a limiting step. Most viruses therefore need cellular chaperones during their life cycle. In addition to their own protein folding problems viruses need to interfere with cellular processes such as signal transduction, cell cycle regulation and induction of apoptosis in order to create a favorable environment for their proliferation and to avoid premature cell death. Chaperones are involved in the control of these cellular processes and some viruses reprogram their host cell by interacting with them. Hsp70 chaperones, as central components of the cellular chaperone network, are frequently recruited by viruses. This review focuses on the function of Hsp70 chaperones at the different stages of the viral life cycle emphasizing mechanistic aspects.

Transcriptional stimulation by the DNA binding protein Hap46/BAG-1M involves hsp70/hsc70 molecular chaperones

The hsp70/hsc70-associating protein Hap46 of human origin, also called BAG-1M (Bcl-2-associated athanogene 1), has been characterized previously as a DNA binding protein, which is able to stimulate transcription. By use of in vitro assays we now show that Hap46-mediated transcriptional activation can occur from linearized as well as from supercoiled circular DNA and does not require the presence of a transcription promoter. Accordingly, we observed no preferential binding of Hap46 to overlapping DNA fragments covering the sequence of the cytomegalovirus (CMV) early promoter, thus suggesting non-specific binding. The C-terminal deletion variant Hap46DeltaC47, which is unable to associate with hsp70/hsc70 molecular chaperones, produced greatly diminished effects on transcription, indicating a significant involvement of hsp70/hsc70 chaperones but not an absolute requirement. In contrast, deletion of the acidic hexarepeat region, as in variant Hap46Delta12-62, did not disturb transcriptional stimulation. While full-length Hap46 readily formed complexes with a series of structurally unrelated transcription factors, variant Hap46DeltaC47 proved incapable of doing so. Together these data suggest that transcriptional stimulation is a major biological activity of Hap46 and point to involvement of hsp70/hsc70 molecular chaperones in transcription in concert with Hap46, thus providing a link between hsp70/hsc70 molecular chaperones and components of the transcription machinery.

BAG-1: a multifunctional regulator of cell growth and survival

BAG-1 is multifunctional protein which interacts with a wide range of cellular targets to regulate growth control pathways important for normal and malignant cells, including apoptosis, signaling, proliferation, transcription and cell motility. Of particular relevance to tumour cells, BAG-1 interacts with the anti-apoptotic BCL-2 protein, various nuclear hormone receptors and the 70 kDa heat shock proteins, Hsc70 and Hsp70. Interaction with chaperones may account for many of the pleiotropic effects associated with BAG-1 overexpression. Recent studies have shown that BAG-1 expression is frequently altered in malignant cells, and BAG-1 expression may have clinical value as a prognostic/predictive marker. This review summarises current understanding of molecular mechanisms of BAG-1 expression and function.

Down-regulation of Bcl-2-interacting protein BAG-1 confers resistance to anti-cancer drugs

BAG-1 was originally identified as a binding partner of anti-apoptotic factor Bcl-2 [Takayama et al., Cell 80 (1995) 279-284]. Exogenous expression of BAG-1 was reported to confer cells resistance to several stresses [Chen et al., Oncogene 21 (2002) 7050]. We have obtained human cervical cancer HeLa cells with down-regulated BAG-1 levels by using a highly specific and efficient RNA interference approach. Surprisingly, cells with down-regulated BAG-1 exhibited significantly lower sensitivity against several anti-cancer drugs than parental cells expressing normal levels of the protein. Furthermore, growth rate of the cells was reduced when BAG-1 was down-regulated. Activity of ERK pathway appeared to be decreased in BAG-1 down-regulated cells, as shown by the reduced phosphorylation of ERK1/2 proteins. Taken together resistance against anti-cancer drugs acquired by BAG-1 down-regulated cells may well be accounted for by the retardation of cell cycle progression, implicating the importance of BAG-1 in cell growth regulation.

BAG-1—a nucleotide exchange factor of Hsc70 with multiple cellular functions

BAG-1 (Bcl-2-associated athanogene) is a multifaceted protein implicated in the modulation of a large variety of cellular processes. Elucidating the molecular mechanisms that underlie the cellular functions of BAG-1 becomes an increasingly important task, particularly in light of the growing evidence connecting aberrant BAG-1 expression to certain human cancers. A common element of the remarkable functional diversity of BAG-1 appears to be the interaction with molecular chaperones of the Hsp70 family. In fact, BAG-1 functions as a nucleotide exchange factor of mammalian cytosolic Hsc70, thereby triggering substrate unloading from the chaperone. In addition, recent findings reveal an association of BAG-1 with the proteasome, which suggests a role in coordinating chaperone and degradation pathways.

Distinct BAG-1 isoforms have different anti-apoptotic functions in BAG-1-transfected C33A human cervical carcinoma cell line

BAG-1 protein can be expressed as four isoforms of 50, 46, 33 and 29 kDa with different subcellular localizations, which may have different functions in anti-apoptosis, but the exact mechanism remains unclear. We constructed BAG-1 full length and deletion mutated plasmids in a pCR3.1 vector and established stable transfections of BAG-1 isoforms in low BAG-1 expressing C33A cells. Treatment of the transfected cells with cisplatin, staurosporine, paclitaxel and doxorubicine showed that BAG-1 p50, p46 and p33 isoforms enhanced the resistance to apoptosis. BAG-1 p50, p46 and p33 exhibited different degrees of apoptosis inhibition in the transfected cells and BAG-1 p46 isoform had the most pronounced effect on anti-apoptosis. BAG-1 p29 failed to protect the transfected cells from apoptosis. Resistance to apoptosis by BAG-1 isoforms was correlated with decreased caspase-3 activation. We also detected the expression of Bax, Bak, p53, Bcl-2, Bcl-X(L), AIF and MRP1 by Western blots. Bcl-2 protein expression was significantly increased in p50, p46 and p33 transfected cells, while the expression of Bax, Bak, p53, Bcl-X(L) and MRP1 was essentially unchanged. These in vitro results suggest that distinct isoforms of BAG-1 have different anti-apoptotic functions and their functions may be correlated to increased Bcl-2 expression.