Nuclear cathepsin B-like protease cleaves transcription factor YY1 in differentiated cells

New Perspectives on the Role of Nuclear Proteases in Cell Death Pathways

Multiple factors can trigger cell death via various pathways, and nuclear proteases have emerged as essential regulators of these processes. While certain nuclear proteases have been extensively studied and their mechanisms of action are well understood, others remain poorly characterized. Regulation of nuclear protease activity is a promising therapeutic strategy that could selectively induce favorable cell death pathways in specific tissues or organs. Thus, by understanding the roles of newly discovered or predicted nuclear proteases in cell death processes, we can identify new pharmacological targets for improving therapeutic outcomes. In this article, we delved into the role of nuclear proteases in several types of cell death and explore potential avenues for future research and therapeutic development.

Involvement of Cathepsins in Innate and Adaptive Immune Responses in Periodontitis

Periodontitis is an infectious disease whereby the chronic inflammatory process of the periodontium stimulated by bacterial products induces specific host cell responses. The activation of the host cell immune system upregulates the production of inflammatory mediators, comprising cytokines and proteolytic enzymes, which contribute to inflammation and bone destruction. It has been well known that periodontitis is related to systemic inflammation which links to numerous systemic diseases, including diabetes and arteriosclerosis. Furthermore, periodontitis has been reported in association with neurodegenerative diseases such as Alzheimer's disease (AD) in the brain. Regarding immune responses and inflammation, cathepsin B (CatB) plays pivotal role for the induction of IL-1β, cathepsin K- (CatK-) dependent active toll-like receptor 9 (TLR9) signaling, and cathepsin S (CatS) which involves in regulating both TLR signaling and maturation of the MHC class II complex. Notably, both the production and proteolytic activities of cathepsins are upregulated in chronic inflammatory diseases, including periodontitis. In the present review, we focus on the roles of cathepsins in the innate and adaptive immune responses within periodontitis. We believe that understanding the roles of cathepsins in the immune responses in periodontitis would help to elucidate the therapeutic strategies of periodontitis, thus benefit for reduction of systemic diseases as well as neurodegenerative diseases in the global aging society.

Characterization of Novel Piperidine-Based Inhibitor of Cathepsin B-Dependent Bacterial Toxins and Viruses

Exploiting the host endocytic trafficking pathway is a common mechanism by which bacterial exotoxins gain entry to exert virulent effects upon the host cells. A previous study identified a small-molecule, 1-(2,6-dimethyl-1-piperidinyl)-3-[(2-isopropyl-5-methylcyclohexyl)oxy]-2-propanol, that blocks the process of anthrax lethal toxin (LT) cytotoxicity. Here, we report the characterization of the bioactivity of this compound, which we named RC1. We found that RC1 protected host cells independently of LT concentration and also blocked intoxication by other bacterial exotoxins, suggesting that the target of the compound is a host factor. Using the anthrax LT intoxication pathway as a reference, we show that while anthrax toxin is able to bind to cells and establish an endosomal pore in the presence of the drug, the toxin is unable to translocate into the cytosol. We demonstrate that RC1 does not inhibit the toxin directly but rather reduces the enzymatic activity of host cathepsin B that mediates the escape of toxins into the cytoplasm from late endosomes. We demonstrate that the pathogenicity of Human cytomegalovirus and Herpes simplex virus 1, which relies on cathepsin B protease activity, is reduced by RC1. This study reveals the potential of RC1 as a broad-spectrum host-oriented therapy against several aggressive and deadly pathogens.

Latency-Associated Expression of Human Cytomegalovirus US28 Attenuates Cell Signaling Pathways To Maintain Latent Infection

Reactivation of human cytomegalovirus (HCMV) latent infection from early myeloid lineage cells constitutes a threat to immunocompromised or immune-suppressed individuals. Consequently, understanding the control of latency and reactivation to allow targeting and killing of latently infected cells could have far-reaching clinical benefits. US28 is one of the few viral genes that is expressed during latency and encodes a cell surface G protein-coupled receptor (GPCR), which, during lytic infection, is a constitutive cell-signaling activator. Here we now show that in monocytes, which are recognized sites of HCMV latency in vivo, US28 attenuates multiple cell signaling pathways, including mitogen-activated protein (MAP) kinase and NF-κB, and that this is required to establish a latent infection; viruses deleted for US28 initiate a lytic infection in infected monocytes. We also show that these monocytes then become potent targets for the HCMV-specific host immune response and that latently infected cells treated with an inverse agonist of US28 also reactivate lytic infection and similarly become immune targets. Consequently, we suggest that the use of inhibitors of US28 could be a novel immunotherapeutic strategy to reactivate the latent viral reservoir, allowing it to be targeted by preexisting HCMV-specific T cells.

Human cytomegalovirus (HCMV) is a betaherpesvirus and a leading cause of morbidity and mortality among immunosuppressed individuals. HCMV can establish latent infection, where the viral genome is maintained in an infected cell, without production of infectious virus. A number of genes, including US28, are expressed by HCMV during latent infection. US28 has been shown to activate many cellular signaling pathways during lytic infection, promoting lytic gene expression and virus production. As such, the role of US28 remains unclear and seems at odds with latency. Here, we show that US28 has the opposite phenotype in cells that support latent infection—it attenuates cellular signaling, thereby maintaining latency. Inhibition of US28 with a small-molecule inhibitor causes HCMV latent infection to reactivate, allowing latently infected cells to be detected and killed by the immune system. This approach could be used to treat latent HCMV to clear it from human transplants.

NF-κB, CRE and YY1 elements are key functional regulators of CMV promoter-driven transient gene expression in CHO cells

Transient gene expression (TGE) in CHO cells is utilized to produce material for use in early stage drug development. These systems typically utilize the cytomegalovirus (CMV) promoter to drive recombinant gene transcription. In this study, we have mechanistically dissected CMV-mediated TGE in CHO cells in order to identify the key regulators of this process. An in silico analysis of the promoter composition of transcription factor regulatory elements (TFREs) and the CHO cell repertoire of transcription factors identified eight TFREs as likely effectors of CMV activity. We determined the regulatory function of these elements by preventing their cognate transcription factors from binding at the CMV promoter. This was achieved by both scrambling promoter binding site sequences and using decoy molecules to sequester intracellular transcription factors. We determined that the vast majority of CMV activity is mediated by just two discrete TFREs, showing that simultaneous inhibition of NF-κB and CRE-mediated transactivation reduced CMV-driven transient secreted alkaline phosphatase (SEAP) production by over 75%. Further, we identified a mechanism by which CMV-mediated TGE is negatively regulated in CHO cells, showing that inhibition of YY1-mediated transrepression increased SEAP production 1.5-fold. This work enables optimization and control of CMV-mediated TGE in CHO cells, in order to improve transient protein production yields.

Human cytomegalovirus latency and reactivation in and beyond the myeloid lineage

ABSTRACT: 

After primary infection with human cytomegalovirus (HCMV), which rarely causes any serious clinical problems in the immune competent, the virus persists subclinically for the lifetime of the host due, at least in part, to its ability to undergo latent infection. By contrast, HCMV can be a serious cause of morbidity, and in some cases mortality, upon primary infection of, or reactivation in, immune suppressed individuals. While current antivirals that target its lytic lifecycle have helped enormously in managing HCMV disease, to date, there are no available antivirals that target latent infection. In this review, we discuss research using natural and experimental models of latency that has led to some understanding of how HCMV latency is maintained, and reactivation controlled, in the myeloid lineage. Such analyses are now beginning to inform us of novel rationales that could allow the development of novel antivirals to target latency, itself.

A novel Entamoeba histolytica cysteine proteinase, EhCP4, is key for invasive amebiasis and a therapeutic target

Entamoeba histolytica cysteine proteinases (EhCPs) play a key role in disrupting the colonic epithelial barrier and the innate host immune response during invasion of E. histolytica, the protozoan cause of human amebiasis. EhCPs are encoded by 50 genes, of which ehcp4 (ehcp-a4) is the most up-regulated during invasion and colonization in a mouse cecal model of amebiasis. Up-regulation of ehcp4 in vivo correlated with our finding that co-culture of E. histolytica trophozoites with mucin-producing T84 cells increased ehcp4 expression up to 6-fold. We have expressed recombinant EhCP4, which was autocatalytically activated at acidic pH but had highest proteolytic activity at neutral pH. In contrast to the other amebic cysteine proteinases characterized so far, which have a preference for arginine in the P2 position, EhCP4 displayed a unique preference for valine and isoleucine at P2. This preference was confirmed by homology modeling, which revealed a shallow, hydrophobic S2 pocket. Endogenous EhCP4 localized to cytoplasmic vesicles, the nuclear region, and perinuclear endoplasmic reticulum (ER). Following co-culture with colonic cells, EhCP4 appeared in acidic vesicles and was released extracellularly. A specific vinyl sulfone inhibitor, WRR605, synthesized based on the substrate specificity of EhCP4, inhibited the recombinant enzyme in vitro and significantly reduced parasite burden and inflammation in the mouse cecal model. The unique expression pattern, localization, and biochemical properties of EhCP4 could be exploited as a potential target for drug design.

Cathepsin B is involved in the trafficking of TNF-alpha-containing vesicles to the plasma membrane in macrophages

TNF-alpha is a potent proinflammatory cytokine, essential for initiating innate immune responses against invading microbes and a key mediator involved in the pathogenesis of acute and chronic inflammatory diseases. To identify molecules involved in the production of TNF-alpha, we used a functional gene identification method using retroviral integration-mediated mutagenesis, followed by LPS-stimulated TNF-alpha production analysis in macrophages. We found that cathepsin B, a lysosomal cysteine proteinase, was required for optimal posttranslational processing of TNF-alpha in response to the bacterial cell wall component LPS. Mouse bone marrow-derived macrophages from cathepsin B-deficient mice and macrophages treated with the cathepsin B-specific chemical inhibitor CA074 methyl ester or small interfering RNA against cathepsin B secreted significantly less TNF-alpha than wild-type or nontreated macrophages. We further showed that the inhibition of cathepsin B caused accumulation of 26-kDa pro-TNF-containing vesicles. Ectopic expression of GFP-conjugated pro-TNF further suggests that pro-TNF failed to reach the plasma membrane without intracellular cathepsin B activity. Altogether, these data suggest that intracellular cathepsin B activity is involved in the TNF-alpha-containing vesicle trafficking to the plasma membrane.

Human cytomegalovirus: Latency and reactivation in the myeloid lineage

Human cytomegalovirus (HCMV) persists as a sub-clinical, lifelong infection in the human host which is maintained at least in part by its carriage in the absence of detectable infectious virus: a hallmark of latent infection. In contrast, reactivation from latency in immuno-compromised individuals can result in serious disease. Understanding virus latency and reactivation, therefore, is essential for a full understanding of the biology and pathogenesis of this persistent human herpesvirus. However, the precise cellular sites in which HCMV is carried and the mechanisms regulating its latency and reactivation, during natural infection, remain poorly understood. Recent work, however, has led to a consensus opinion that cells of the myeloid lineage are one site of carriage of HCMV in vivo and that in myeloid dendritic cell (DC) progenitors the viral genome is carried latently in the absence of virus lytic gene expression. In contrast, differentiation of these cells to a mature DC phenotype is linked with reactivation of infectious virus resulting from differentiation-dependent chromatin remodelling of the viral major immediate-early promoter. Thus there is a crucial link between the differentiation of myeloid cells and transcriptional reactivation of latent virus which is likely to play a key role in viral pathogenesis.

Cathepsin B mediates cleavage of herpes simplex virus type 1 origin binding protein (OBP) to yield OBPC-1, and cleavage is dependent upon viral DNA replication

Although the seven viral proteins required for herpes simplex virus type 1 (HSV-1) DNA replication have been identified, the mechanism by which viral DNA synthesis is regulated is unclear. HSV-1 DNA replication is thought to occur in two stages: origin-dependent DNA replication (stage I) mediated by the origin binding protein (OBP), followed by origin- and OBP-independent DNA replication (stage II). The mechanism that facilitates the switch from stage I to stage II is unknown; however, it must involve the loss of OBP function or OBP itself from the replication initiation complex. Previous studies from this laboratory identified a transcript (UL8.5) and protein (OBPC) that are in frame with and comprise the C terminus of the gene specifying OBP. Because of its DNA binding ability, OBPC has been hypothesized to mediate the switch from stage I to stage II. Here, we identify a second protein (OBPC-2) that is also in frame with the C terminus of OBP but comprises a smaller portion of the protein. We demonstrate that the protein originally identified (OBPC-1) is a cathepsin B-mediated cleavage product of OBP, while OBPC-2 may be the product of the UL8.5 transcript. We further demonstrate that the cleavage of OBP to yield OBPC-1 is dependent upon viral DNA replication. These results suggest that cleavage may be a mechanism by which OBP levels and/or activity are regulated during infection.

Multifunctional transcription factor YY1: a therapeutic target in human cancer?

The multifunctional transcription factor Yin Yang 1 (YY1) is a complex protein that has been shown to play pivotal roles in development, differentiation, cellular proliferation and apoptosis. It can act as a transcriptional repressor, an activator, or an initiator element binding protein that directs and initiates transcription of numerous cellular and viral genes. Because the expression and function of YY1 are known to be intimately associated with cell-cycle progression, the physiological significance of YY1 activity has recently been applied to models of cancer biology. Several lines of evidence imply that YY1 expression and/or activation is associated with tumourigenesis, in addition to its regulatory roles in normal biological processes. However, controversial results also raised and indicated that further studies are still needed to piece all of the seemingly contradictory data into a complete picture. On the basis of YY1 regulations and functions, novel drugs and specific treatment strategies may be developed with new therapeutic applications for tumour patients in the future.

Latency and reactivation of human cytomegalovirus

Human cytomegalovirus (HCMV) persists as a subclinical, lifelong infection in the normal human host, maintained at least in part by its carriage in the absence of detectable infectious virus – the hallmark of latent infection. Reactivation from latency in immunocompromised individuals, in contrast, often results in serious disease. Latency and reactivation are defining characteristics of the herpesviruses and key to understanding their biology. However, the precise cellular sites in which HCMV is carried and the mechanisms regulating its latency and reactivation during natural infection remain poorly understood. This review will detail our current knowledge of where HCMV is carried in healthy individuals, which viral genes are expressed upon carriage of the virus and what effect this has on cellular gene expression. It will also address the accumulating evidence suggesting that reactivation of HCMV from latency appears to be linked intrinsically to the differentiation status of the myeloid cell, and how the cellular mechanisms that normally control host gene expression play a critical role in the differential regulation of viral gene expression during latency and reactivation.

Oncogenes as novel targets for cancer therapy (part III): transcription factors

This is the third paper in a four-part serial review on potential therapeutic targeting of oncogenes. The previous parts described the involvement of oncogenes in different aspects of cancer growth and development, and considered the new technologies responsible for the advancement of oncogene identification, target validation, and drug design. Because of such advances, new specific and more efficient therapeutic agents can be developed for cancer. This part of the review continues the exploration of various oncogenes that we have grouped within seven categories: growth factors, tyrosine kinases, intermediate signaling molecules, transcription factors, cell cycle regulators, DNA damage repair genes, and genes involved in apoptosis. Part one discussed growth factors and tyrosine kinases and part two discussed intermediate signaling molecules. This portion of the review covers transcription factors and the various strategies being used to inhibit their expression or decrease their activities.

Targeted inhibition of the transcription factor YY1 in an embryonal carcinoma cell line results in retarded cell growth, elevated levels of p53 but no increase in apoptotic cell death

The ubiquitous cellular transcription factor Yin Yang-1 (YY1) is involved in the transcriptional regulation of many cellular and viral genes. It is known to bind to, and repress the activity of, the major immediate-early promoter of human cytomegalovirus (HCMV) in non-permissive T2 cells. Thus, YY1 is at least partly responsible for the lack of productive lytic infection of these cells. In this study, we have used short interfering RNA (siRNA) to specifically knock down YY1 expression in T2 cells. We wished to assess whether the removal of this negatively acting factor would render these ordinarily non-permissive cells permissive for infection. We show that we can potently inhibit YY1 expression but that this knock down has dramatic effects on the normal biology of the cells. In particular, we noted growth retardation, altered morphology and increased levels of p53. However, the cells do not undergo apoptosis, are not induced to differentiate, do not exhibit excessive levels of DNA damage, and synthesise DNA normally.

The C-terminal subunit of artificially truncated human cathepsin B mediates its nuclear targeting and contributes to cell viability

BACKGROUND

Splicing variants of human cathepsinB primary transcripts (CB(-2,3)) result in an expression product product which lacks the signal peptide and parts of the propeptide. This naturally truncated Delta51CB is thus unable to follow the regular CB processing and sorting pathway. It is addressed to the mitochondria through an activated N-terminal mitochondrial targeting signal instead. Although Delta51CB is supposed to be devoid of the typical CB enzymatic activity, it might play a role in malignancies and trigger cell death/apoptosis independent from the function of the regular enzyme. Cytoplasmic presence of the mature CB might occur as a result of lysosomal damage.

RESULTS

We investigated such "aberrant" proteins by artificial CB-GFP chimeras covering various sequence parts in respect to their enzymatic activity, their localization in different cell types, and the effects on the cell viability. Unlike the entire full length CB form, the artificial single chain form was not processed and did not reveal typical enzymatic CB activity during transient overexpression in large cell lung carcinoma cells. Delta51CB was found predominantly in mitochondria. In contrast, the shorter artificial CB constructs localized in the cytoplasm, inside the cell nucleus, and in the midbodies of dividing cells. Bleaching experiments revealed both mobile and immobile fractions of these constructs in the nucleus. Nuclear accumulation of artificially truncated CB variants led to disintegration of nuclei, followed by cell death.

CONCLUSION

We propose that cell death associated with CB is not necessarily triggered by its regular enzymatic activity but alternatively by a yet unknown activity profile of truncated CB. Cytoplasmic CB might be able to enter the cell nucleus. According to a mutational analysis, the part of CB that mediates its nuclear import is a signal patch within its heavy chain domain. The results suggest that besides the N-terminal signal peptide also other CB domains contain patterns which are responsible for a differentiated targeting of the molecule, e.g. to the mitochondria, to the nucleus, or to vesicles. We propose a hierarchy of targeting signals depending on their strength and availability. This implies other possible transport mechanisms besides the usual trafficking via the mannose-6-sound recording copyright sign pathway.

Ets-2 Repressor Factor (ERF) mediates repression of the human cytomegalovirus major immediate-early promoter in undifferentiated non-permissive cells

The repression of human cytomegalovirus immediate-early (IE) lytic gene expression is crucial for the maintenance of the latent viral state. By using conditionally permissive cell lines, which provide a good model for the differentiation state-dependent repression of IE gene expression, we have identified several cellular factors that bind to the major immediate-early promoter (MIEP) and whose expression is down-regulated after differentiation to a permissive phenotype. Here we show that the cellular protein Ets-2 Repressor Factor (ERF) physically interacts with the MIEP and represses MIEP activity in undifferentiated non-permissive T2 embryonal carcinoma cells. This factor binds to the dyad element and the 21 bp repeats within the MIEP - regions known to be important for the negative regulation of MIEP activity. Finally, we show that following differentiation to a permissive phenotype ERF's repressive effects are severely abrogated.