The biochemical role of the heat shock protein 90 chaperone complex in establishing human telomerase activity

Heat shock proteins as hallmarks of cancer: insights from molecular mechanisms to therapeutic strategies

Heat shock proteins are essential molecular chaperones that play crucial roles in stabilizing protein structures, facilitating the repair or degradation of damaged proteins, and maintaining proteostasis and cellular functions. Extensive research has demonstrated that heat shock proteins are highly expressed in cancers and closely associated with tumorigenesis and progression. The "Hallmarks of Cancer" are the core features of cancer biology that collectively define a series of functional characteristics acquired by cells as they transition from a normal state to a state of tumor growth, including sustained proliferative signaling, evasion of growth suppressors, resistance to cell death, enabled replicative immortality, the induction of angiogenesis, and the activation of invasion and metastasis. The pivotal roles of heat shock proteins in modulating the hallmarks of cancer through the activation or inhibition of various signaling pathways has been well documented. Therefore, this review provides an overview of the roles of heat shock proteins in vital biological processes from the perspective of the hallmarks of cancer and summarizes the small-molecule inhibitors that target heat shock proteins to regulate various cancer hallmarks. Moreover, we further discuss combination therapy strategies involving heat shock proteins and promising dual-target inhibitors to highlight the potential of targeting heat shock proteins for cancer treatment. In summary, this review highlights how targeting heat shock proteins could regulate the hallmarks of cancer, which will provide valuable information to better elucidate and understand the roles of heat shock proteins in oncology and the mechanisms of cancer occurrence and development and aid in the development of more efficacious and less toxic novel anticancer agents.

HSP90 multi-functionality in cancer

The 90-kDa heat shock proteins (HSP90s) are molecular chaperones essential for folding, unfolding, degradation and activity of a wide range of client proteins. HSP90s and their cognate co-chaperones are subject to various post-translational modifications, functional consequences of which are not fully understood in cancer. Intracellular and extracellular HSP90 family members (HSP90α, HSP90β, GRP94 and TRAP1) promote cancer by sustaining various hallmarks of cancer, including cell death resistance, replicative immortality, tumor immunity, angiogenesis, invasion and metastasis. Given the importance of HSP90 in tumor progression, various inhibitors and HSP90-based vaccines were developed for the treatment of cancer. Further understanding of HSP90 functions in cancer may provide new opportunities and novel therapeutic strategies for the treatment of cancer.

Post-Transcriptional and Post-Translational Modifications in Telomerase Biogenesis and Recruitment to Telomeres

Telomere length is associated with the proliferative potential of cells. Telomerase is an enzyme that elongates telomeres throughout the entire lifespan of an organism in stem cells, germ cells, and cells of constantly renewed tissues. It is activated during cellular division, including regeneration and immune responses. The biogenesis of telomerase components and their assembly and functional localization to the telomere is a complex system regulated at multiple levels, where each step must be tuned to the cellular requirements. Any defect in the function or localization of the components of the telomerase biogenesis and functional system will affect the maintenance of telomere length, which is critical to the processes of regeneration, immune response, embryonic development, and cancer progression. An understanding of the regulatory mechanisms of telomerase biogenesis and activity is necessary for the development of approaches toward manipulating telomerase to influence these processes. The present review focuses on the molecular mechanisms involved in the major steps of telomerase regulation and the role of post-transcriptional and post-translational modifications in telomerase biogenesis and function in yeast and vertebrates.

The Telomere-Telomerase System Is Detrimental to Health at High-Altitude

The hypobaric-hypoxia environment at high-altitude (HA, >2500 m) may influence DNA damage due to the production of reactive molecular species and high UV radiation. The telomere system, vital to chromosomal integrity and cellular viability, is prone to oxidative damages contributing to the severity of high-altitude disorders such as high-altitude pulmonary edema (HAPE). However, at the same time, it is suggested to sustain physical performance. This case-control study, comprising 210 HAPE-free (HAPE-f) sojourners, 183 HAPE-patients (HAPE-p) and 200 healthy highland natives (HLs) residing at ~3500 m, investigated telomere length, telomerase activity, and oxidative stress biomarkers. Fluidigm SNP genotyping screened 65 single nucleotide polymorphisms (SNPs) in 11 telomere-maintaining genes. Significance was attained at p ≤ 0.05 after adjusting for confounders and correction for multiple comparisons. Shorter telomere length, decreased telomerase activity and increased oxidative stress were observed in HAPE patients; contrarily, longer telomere length and elevated telomerase activity were observed in healthy HA natives compared to HAPE-f. Four SNPs and three haplotypes are associated with HAPE, whereas eight SNPs and nine haplotypes are associated with HA adaptation. Various gene-gene interactions and correlations between/among clinical parameters and biomarkers suggested the presence of a complex interplay underlining HAPE and HA adaptation physiology. A distinctive contribution of the telomere-telomerase system contributing to HA physiology is evident in this study. A normal telomere system may be advantageous in endurance training.

p23 and Aha1: Distinct Functions Promote Client Maturation

Hsp90 is a conserved molecular chaperone regulating the folding and activation of a diverse array of several hundreds of client proteins. The function of Hsp90 in client processing is fine-tuned by a cohort of co-chaperones that modulate client activation in a client-specific manner. They affect the Hsp90 ATPase activity and the recruitment of client proteins and can in addition affect chaperoning in an Hsp90-independent way. p23 and Aha1 are central Hsp90 co-chaperones that regulate Hsp90 in opposing ways. While p23 inhibits the Hsp90 ATPase and stabilizes a client-bound Hsp90 state, Aha1 accelerates ATP hydrolysis and competes with client binding to Hsp90. Even though both proteins have been intensively studied for decades, research of the last few years has revealed intriguing new aspects of these co-chaperones that expanded our perception of how they regulate client activation. Here, we review the progress in understanding p23 and Aha1 as promoters of client processing. We highlight the structures of Aha1 and p23, their interaction with Hsp90, and how their association with Hsp90 affects the conformational cycle of Hsp90 in the context of client maturation.

The Role of Hsp90-R2TP in Macromolecular Complex Assembly and Stabilization

Hsp90 is a ubiquitous molecular chaperone involved in many cell signaling pathways, and its interactions with specific chaperones and cochaperones determines which client proteins to fold. Hsp90 has been shown to be involved in the promotion and maintenance of proper protein complex assembly either alone or in association with other chaperones such as the R2TP chaperone complex. Hsp90-R2TP acts through several mechanisms, such as by controlling the transcription of protein complex subunits, stabilizing protein subcomplexes before their incorporation into the entire complex, and by recruiting adaptors that facilitate complex assembly. Despite its many roles in protein complex assembly, detailed mechanisms of how Hsp90-R2TP assembles protein complexes have yet to be determined, with most findings restricted to proteomic analyses and in vitro interactions. This review will discuss our current understanding of the function of Hsp90-R2TP in the assembly, stabilization, and activity of the following seven classes of protein complexes: L7Ae snoRNPs, spliceosome snRNPs, RNA polymerases, PIKKs, MRN, TSC, and axonemal dynein arms.

Gastric cancer exosomes contribute to the field cancerization of gastric epithelial cells surrounding gastric cancer

BACKGROUND

A dynamic molecular interaction between cancer and the surrounding normal cells is mediated through exosomes. We investigated whether exosomes derived from gastric cancer cells affected the fate of the surrounding gastric epithelial cells.

METHODS

We analyzed the cell viability and immortalization of primary normal stomach epithelial cells (PNSECs) after treatment with exosomes derived from AGS gastric cancer cells and/or H. pylori CagA. Cell proliferation and apoptosis were analyzed by BrdU incorporation, flow-cytometry, and colony formation assays. We examined telomere length, expression and activity of telomerase, and expression of telomere-related genes in PNSECs treated with cancer exosomes, and in 60 gastric cancer and corresponding mucosal tissues. The differentially expressed genes and transcriptional regulation of telomere-related genes were verified using real-time qPCR and ChIP analyses, respectively.

RESULTS

Gastric cancer exosomes increased cell viability and the population-doubling levels but inhibited the cellular senescence and apoptosis of PNSECs. The internalization of cancer exosomes in PNSECs dramatically increased the number of surviving colonies and induced a multilayer growth and invasion into the scaffold. Treatment of PNSECs with cancer exosomes markedly increased the expression and activity of telomerase and the T/S ratio and regulated the expression of the telomere-associated genes, heat-shock genes, and hedgehog genes. Compared to gastric mucosae, gastric cancer showed increased hTERT expression, which was positively correlated with telomere length. Interestingly, seven (46.7%) of 15 non-cancerous gastric mucosae demonstrated strong telomerase activity.

CONCLUSION

These results suggest that gastric cancer exosomes induced the transformation and field cancerization of the surrounding non-cancerous gastric epithelial cells.

Telomerase ribonucleoprotein and genome integrity-An emerging connection in protozoan parasites

Telomerase has an established role in telomere maintenance in eukaryotes. However, recent studies have begun to implicate telomerase in cellular roles beyond telomere maintenance. Specifically, evidence is emerging of cross-talks between telomerase mediated telomere homeostasis and DNA repair pathways. Telomere shortening due to the end replication problem is a constant threat to genome integrity in eukaryotic cells. This poses a particular problem in unicellular parasitic protists because their major virulence genes are located at the subtelomeric loci. Although telomerase is the major regulator of telomere lengthening in eukaryotes, it is less studied in the ancient eukaryotes, including clinically important human pathogens. Recent research is highlighting interplay between telomerase and the DNA damage response in human parasites. The importance of this interplay in pathogen virulence is only beginning to be illuminated, including the potential to highlight novel developmental regulation of telomerase in parasites who transition between multiple developmental stages throughout their life cycle. In this review, we will discuss the telomerase ribonucleoprotein enzyme and DNA repair pathways with emerging views in human parasites to give a broader perspective of the possible connection of telomere, telomerase, and DNA repair pathways across eukaryotic lineages and highlight their potential role in pathogen virulence. This article is categorized under: RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems RNA Evolution and Genomics > RNA and Ribonucleoprotein Evolution RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Possible Involvement of Hsp90 in the Regulation of Telomere Length and Telomerase Activity During the Leishmania amazonensis Developmental Cycle and Population Proliferation

The Leishmania developmental cycle comprises three main life forms in two hosts, indicating that the parasite is continually challenged due to drastic environmental changes. The disruption of this cycle is critical for discovering new therapies to eradicate leishmaniasis, a neglected disease that affects millions worldwide. Telomeres, the physical ends of chromosomes, maintain genome stability and cell proliferation and are potential antiparasitic drug targets. Therefore, understanding how telomere length is regulated during parasite development is vital. Here, we show that telomeres form clusters spread in the nucleoplasm of the three parasite life forms. We also observed that amastigotes telomeres are shorter than metacyclic and procyclic promastigotes and that in parasites with continuous in vitro passages, telomere length increases over time. These observed differences in telomere length among parasite’s life stages were not due to lack/inhibition of telomerase since enzyme activity was detected in all parasite life stages, although the catalysis was temperature-dependent. These data led us to test if, similar to other eukaryotes, parasite telomere length maintenance could be regulated by Hsp83, the ortholog of Hsp90 in trypanosomatids, and Leishmania (LHsp90). Parasites were then treated with the Hsp90 inhibitor 17AAG. The results showed that 17AAG disturbed parasite growth, induced accumulation into G2/M phases, and telomere shortening in a time-dependent manner. It has also inhibited procyclic promastigote’s telomerase activity. Besides, LHsp90 interacts with the telomerase TERT component as shown by immunoprecipitation, strongly suggesting a new role for LHsp90 as a parasite telomerase component involved in controlling telomere length maintenance and parasite life span.

Targeting telomerase for its advent in cancer therapeutics

Telomerase has emerged as an important primary target in anticancer therapy. It is a distinctive reverse transcriptase enzyme, which extends the length of telomere at the 3' chromosomal end, and uses telomerase reverse transcriptase (TERT) and telomerase RNA template-containing domains. Telomerase has a vital role and is a contributing factor in human health, mainly affecting cell aging and cell proliferation. Due to its unique feature, it ensures unrestricted cell proliferation in malignancy and plays a major role in cancer disease. The development of telomerase inhibitors with increased specificity and better pharmacokinetics is being considered to design and develop newer potent anticancer agents. Use of natural and synthetic compounds for the inhibition of telomerase activity can lead to an opening of new vistas in cancer treatment. This review details about the telomerase biochemistry, use of natural and synthetic compounds; vaccines and oncolytic virus in therapy that suppress the telomerase activity. We have discussed structure-activity relationships of various natural and synthetic telomerase inhibitors to help medicinal chemists and chemical biology researchers with a ready reference and updated status of their clinical trials. Suppression of human TERT (hTERT) activity through inhibition of hTERT promoter is an important approach for telomerase inhibition.

Regulation of human telomerase in homeostasis and disease

Telomerase is a ribonucleoprotein complex, the catalytic core of which includes the telomerase reverse transcriptase (TERT) and the non-coding human telomerase RNA (hTR), which serves as a template for the addition of telomeric repeats to chromosome ends. Telomerase expression is restricted in humans to certain cell types, and telomerase levels are tightly controlled in normal conditions. Increased levels of telomerase are found in the vast majority of human cancers, and we have recently begun to understand the mechanisms by which cancer cells increase telomerase activity. Conversely, germline mutations in telomerase-relevant genes that decrease telomerase function cause a range of genetic disorders, including dyskeratosis congenita, idiopathic pulmonary fibrosis and bone marrow failure. In this Review, we discuss the transcriptional regulation of human TERT, hTR processing, assembly of the telomerase complex, the cellular localization of telomerase and its recruitment to telomeres, and the regulation of telomerase activity. We also discuss the disease relevance of each of these steps of telomerase biogenesis.

Stilbene Compounds Inhibit Tumor Growth by the Induction of Cellular Senescence and the Inhibition of Telomerase Activity

Cellular senescence is a state of cell cycle arrest characterized by a distinct morphology, gene expression pattern, and secretory phenotype. It can be triggered by multiple mechanisms, including those involved in telomere shortening, the accumulation of DNA damage, epigenetic pathways, and the senescence-associated secretory phenotype (SASP), and so on. In current cancer therapy, cellular senescence has emerged as a potent tumor suppression mechanism that restrains proliferation in cells at risk for malignant transformation. Therefore, compounds that stimulate the growth inhibition effects of senescence while limiting its detrimental effects are believed to have great clinical potential. In this review article, we first review the current knowledge of the pro- and antitumorigeneic functions of senescence and summarize the key roles of telomerase in the regulation of senescence in tumors. Second, we review the current literature regarding the anticancer effects of stilbene compounds that are mediated by the targeting of telomerase and cell senescence. Finally, we provide future perspectives on the clinical utilization of stilbene compounds, especially resveratrol and pterostilbene, as novel cancer therapeutic remedies. We conclude and propose that stilbene compounds may induce senescence and may potentially be used as the therapeutic or adjuvant agents for cancers with high telomerase activity.

Proteomic identification of proteins differentially expressed following overexpression of hTERT (human telomerase reverse transcriptase) in cancer cells

Reverse transcriptase activity of telomerase adds telomeric repeat sequences at extreme ends of the newly replicated chromosome in actively dividing cells. Telomerase expression is not detected in terminally differentiated cells but is noticeable in 90% of the cancer cells. hTERT (human telomerase reverse transcriptase) expression seems to promote invasiveness of cancer cells. We here present proteomic profiles of cells overexpressing or knocked down for hTERT. This study also attempts to find out the potential interacting partners of hTERT in cancer cell lines. Two-dimensional gel electrophoresis (2-DE) of two different cell lines U2OS (a naturally hTERT negative cell line) and HeLa revealed differential expression of proteins in hTERT over-expressing cells. In U2OS cell line 28 spots were picked among which 23 spots represented upregulated and 5 represented down regulated proteins. In HeLa cells 21 were upregulated and 2 were down regulated out of 23 selected spots under otherwise identical experimental conditions. Some heat shock proteins viz. Hsp60 and Hsp70 and GAPDH, which is a housekeeping gene, were found similarly upregulated in both the cell lines. The upregulation of these proteins were further confirmed at RNA and protein level by real-time PCR and western blotting respectively.

Too much to handle - how gaining chromosomes destabilizes the genome

Most eukaryotic organisms are diploid, with 2 chromosome sets in their nuclei. Whole chromosomal aneuploidy, a deviation from multiples of the haploid chromosome number, arises from chromosome segregation errors and often has detrimental consequences for cells. In humans, numerical aneuploidy severely impairs embryonic development and the rare survivors develop disorders characterized by multiple pathologies. Moreover, as many as 75 % of malignant tumors display aneuploidy. Although the exact contribution of aneuploidy to tumorigenesis remains unclear, previous studies have suggested that aneuploidy may affect the maintenance of genome integrity. We found that human cells with extra chromosomes showed phenotypes suggestive of replication defects, a phenomenon which we went on to characterize as being due to the aneuploidy-driven downregulation of replication factors, in particular of the replicative helicase MCM2-7. Thus, missegregation of even a single chromosome can further promote genomic instability and thereby contribute to tumor development. In this review we will examine the possible causes of downregulation of replicative factors and discuss the consequences of genomic instability in aneuploid cells.

Telomerase Regulation from Beginning to the End

The vast body of literature regarding human telomere maintenance is a true testament to the importance of understanding telomere regulation in both normal and diseased states. In this review, our goal was simple: tell the telomerase story from the biogenesis of its parts to its maturity as a complex and function at its site of action, emphasizing new developments and how they contribute to the foundational knowledge of telomerase and telomere biology.

Telomerase repeat amplification protocol (TRAP) activity upon recombinant expression and purification of human telomerase in a bacterial system

Telomerase biogenesis is a highly regulated process that solves the DNA end-replication problem. Recombinant expression has so far been accomplished only within a eukaryotic background. Towards structural and functional analyses, we developed bacterial expression of human telomerase. Positive activity by the telomerase repeat amplification protocol (TRAP) was identified in cell extracts of Escherichia coli expressing a sequence-optimized hTERT gene, the full-length hTR RNA with a self-splicing hepatitis delta virus ribozyme, and the human heat shock complex of Hsp90, Hsp70, p60/Hop, Hsp40, and p23. The Hsp90 inhibitor geldanamycin did not affect post-assembly TRAP activity. By various purification methods, TRAP activity was also obtained upon expression of only hTERT and hTR. hTERT was confirmed by tandem mass spectrometry in a ∼120 kDa SDS-PAGE fragment from a TRAP-positive purification fraction. TRAP activity was also supported by hTR constructs lacking the box H/ACA small nucleolar RNA domain. End-point TRAP indicated expression levels within 3-fold of that from HeLa carcinoma cells, which is several orders of magnitude below detection by the direct assay. These results represent the first report of TRAP activity from a bacterium and provide a facile system for the investigation of assembly factors and anti-cancer therapeutics independently of a eukaryotic setting.

The HSP90 inhibitor alvespimycin enhances the potency of telomerase inhibition by imetelstat in human osteosarcoma

The unsatisfactory outcomes for osteosarcoma necessitate novel therapeutic strategies. This study evaluated the effect of the telomerase inhibitor imetelstat in pre-clinical models of human osteosarcoma. Because the chaperone molecule HSP90 facilitates the assembly of telomerase protein, the ability of the HSP90 inhibitor alvespimycin to potentiate the effect of the telomerase inhibitor was assessed. The effect of single or combined treatment with imetelstat and alvespimycin on long-term growth was assessed in osteosarcoma cell lines (143B, HOS and MG-63) and xenografts derived from 143B cells. Results indicated that imetelstat as a single agent inhibited telomerase activity, induced telomere shortening, and inhibited growth in all 3 osteosarcoma cell lines, though the bulk cell cultures did not undergo growth arrest. Combined treatment with imetelstat and alvespimycin resulted in diminished telomerase activity and shorter telomeres compared to either agent alone as well as higher levels of γH2AX and cleaved caspase-3, indicative of increased DNA damage and apoptosis. With dual telomerase and HSP90 inhibition, complete growth arrest of bulk cell cultures was achieved. In xenograft models, all 3 treatment groups significantly inhibited tumor growth compared with the placebo-treated control group, with the greatest effect seen in the combined treatment group (imetelstat, p = 0.045, alvespimycin, p = 0.034; combined treatment, p = 0.004). In conclusion, HSP90 inhibition enhanced the effect of telomerase inhibition in pre-clinical models of osteosarcoma. Dual targeting of telomerase and HSP90 warrants further investigation as a therapeutic strategy.

Targeting heat-shock-protein 90 (Hsp90) by natural products: geldanamycin, a show case in cancer therapy

Covering 2005 to 2013. In this review recent progress in the development of heat shock proteins (Hsp90) in oncogenesis is illuminated. Particular emphasis is put on inhibitors such as geldanamycin and analogues that serve as a natural product show case. Hsp90 has emerged as an important target in cancer therapy and/or against pathogenic cells which elicit abnormal Hsp patterns. Competition for ATP by geldanamycin and related compounds abrogate the chaperone function of Hsp90. In this context, this account pursues three topics in detail: a) Hsp90 and its biochemistry, b) Hsp90 and its role in oncogenesis and c) strategies to create compound libraries of structurally complex inhibitors like geldanamycin on which SAR studies and the development of drugs that are currently in different stages of clinical testing rely.

Tanshinones: sources, pharmacokinetics and anti-cancer activities

Tanshinones are a class of abietane diterpene compound isolated from Salvia miltiorrhiza (Danshen or Tanshen in Chinese), a well-known herb in Traditional Chinese Medicine (TCM). Since they were first identified in the 1930s, more than 40 lipophilic tanshinones and structurally related compounds have been isolated from Danshen. In recent decades, numerous studies have been conducted to investigate the isolation, identification, synthesis and pharmacology of tanshinones. In addition to the well-studied cardiovascular activities, tanshinones have been investigated more recently for their anti-cancer activities in vitro and in vivo. In this review, we update the herbal and alternative sources of tanshinones, and the pharmacokinetics of selected tanshinones. We discuss anti-cancer properties and identify critical issues for future research. Whereas previous studies have suggested anti-cancer potential of tanshinones affecting multiple cellular processes and molecular targets in cell culture models, data from in vivo potency assessment experiments in preclinical models vary greatly due to lack of uniformity of solvent vehicles and routes of administration. Chemical modifications and novel formulations had been made to address the poor oral bioavailability of tanshinones. So far, human clinical trials have been far from ideal in their design and execution for the purpose of supporting an anti-cancer indication of tanshinones.

Expanding the cellular molecular chaperone network through the ubiquitous cochaperones

Cellular environments are highly complex and contain a copious variety of proteins that must operate in unison to achieve homeostasis. To guide and preserve order, multifaceted molecular chaperone networks are present within each cell type. To handle the vast client diversity and regulatory demands, a wide assortment of chaperones are needed. In addition to the classic heat shock proteins, cochaperones with inherent chaperoning abilities (e.g., p23, Hsp40, Cdc37, etc.) are likely used to complete a system. In this review, we focus on the HSP90-associated cochaperones and provide evidence supporting a model in which select cochaperones are used to differentially modulate target proteins, contribute to combinatorial client regulation, and increase the overall reach of a cellular molecular chaperone network. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

ortho-Quinone tanshinones directly inhibit telomerase through an oxidative mechanism mediated by hydrogen peroxide

The tanshinone natural products possess a variety of pharmacological properties including anti-bacterial, anti-inflammatory, anti-oxidant, and anti-neoplastic activity. The molecular basis of these effects, however, remains largely unknown. In the present study, we explored the direct effect of tanshinones on the enzyme telomerase. Telomerase is up-regulated in the majority of cancer cells and is essential for their survival, making it a potential anti-cancer drug target. We found that the ortho-quinone tanshinone II-A inhibits telomerase in a time- and DTT-dependent fashion, and the hydrogen peroxide scavenger catalase protected telomerase from inactivation. These findings demonstrate that ortho-quinone containing tanshinones can inhibit telomerase owing to their ability to generate reactive oxygen species. The results also provide evidence that telomerase is directly and negatively regulated by reactive oxygen species.

Targeting pancreatic cancer with a G-quadruplex ligand

The integrity of telomeres in most cancer cells is maintained by the action of the telomerase enzyme complex, which catalyzes the synthesis of telomeric DNA repeats in order to replace those lost during replication. Telomerase is especially up-regulated in metastatic cancer and is thus emerging as a major therapeutic target. One approach to telomerase inhibition involves the sequestration of the single-stranded 3' ends of telomeric DNA into higher-order quadruplex structures. We have recently shown that tetra-substituted naphthalene diimide compounds are potent quadruplex-stabilizing molecules with telomerase inhibitory activity in cells. We show here that one such compound, BMSG-SH-3, which has been optimized by computer modeling, has significant in vivo antitumor activity against a model for pancreatic cancer, a cancer that is especially resistant to current therapies. A large reduction in telomerase activity in treated tumors was observed and the naphthalene diimide compound was found to be selectively localized in the treated tumors. We find that the expression of the therapeutically important chaperone protein HSP90, a regulator of telomerase is also reduced in vivo by BMSG-SH-3 treatment. The compound is a potent stabilizer of two G-quadruplex sequences found in the promoter region of the HSP90 gene, as well as a G-quadruplex from human telomeric DNA. It is proposed that the simultaneous targeting of these quadruplexes may be an effective anti-tumor strategy.

The role of Hsp90 in protein complex assembly

Hsp90 is a ubiquitous and essential molecular chaperone that plays central roles in many signaling and other cellular pathways. The in vivo and in vitro activity of Hsp90 depends on its association with a wide variety of cochaperones and cofactors, which form large multi-protein complexes involved in folding client proteins. Based on our proteomic work mapping the molecular chaperone interaction networks in yeast, especially that of Hsp90, as well as, on experiments and results presented in the published literature, one major role of Hsp90 appears to be the promotion and maintenance of proper assembly of protein complexes. To highlight this role of Hsp90, the effect of the chaperone on the assembly of the following seven complexes is discussed in this review: snoRNP, RNA polymerase II, phosphatidylinositol-3 kinase-related protein kinase (PIKK), telomere complex, kinetochore, RNA induced silencing complexes (RISC), and 26S proteasome. For some complexes, it is observed that Hsp90 mediates complex assembly by stabilizing an unstable protein subunit and facilitating its incorporation into the complex; for other complexes, Hsp90 promotes change in the composition of that complex. In all cases, Hsp90 does not appear to be part of the final assembled complex. This article is part of a Special Issue entitled:Heat Shock Protein 90 (HSP90).

Human telomerase domain interactions capture DNA for TEN domain-dependent processive elongation

Eukaryotic chromosome maintenance requires telomeric repeat synthesis by telomerase. It remains uncertain how telomerase domains interact to organize the active RNP and how this architecture establishes the specificity of the catalytic cycle. We combine human telomerase reconstitutions in vivo, affinity purifications, and discriminating activity assays to uncover a network of protein-protein and protein-RNA domain interactions. Notably, we find that complete single-repeat synthesis requires only a telomerase reverse transcriptase (TERT) core. Single-repeat synthesis does not require the TERT N-terminal (TEN) domain, but RNA-dependent positioning of the TEN domain captures substrate and allows repeat synthesis processivity. A TEN domain physically separate from the TERT core can capture even a minimal template-paired DNA substrate, with substrate association enhanced by the presence of a 5' single-stranded extension. Our results provide insights into active enzyme architecture, explain biological variations of the catalytic cycle, and predict altered activities for TERT proteins of some eukaryotes.

CHIP promotes human telomerase reverse transcriptase degradation and negatively regulates telomerase activity

The maintenance of eukaryotic telomeres requires telomerase, which is minimally composed of a telomerase reverse transcriptase (TERT) and an associated RNA component. Telomerase activity is tightly regulated by expression of human (h) TERT at both the transcriptional and post-translational levels. The Hsp90 and p23 molecular chaperones have been shown to associate with hTERT for the assembly of active telomerase. Here, we show that CHIP (C terminus of Hsc70-interacting protein) physically associates with hTERT in the cytoplasm and regulates the cellular abundance of hTERT through a ubiquitin-mediated degradation. Overexpression of CHIP prevents nuclear translocation of hTERT and promotes hTERT degradation in the cytoplasm, thereby inhibiting telomerase activity. In contrast, knockdown of endogenous CHIP results in the stabilization of cytoplasmic hTERT. However, it does not affect the level of nuclear hTERT and has no effect on telomerase activity and telomere length. We further show that the binding of CHIP and Hsp70 to hTERT inhibits nuclear translocation of hTERT by dissociating p23. However, Hsp90 binding to hTERT was not affected by CHIP overexpression. These results suggest that CHIP can remodel the hTERT-chaperone complexes. Finally, the amount of hTERT associated with CHIP peaks in G(2)/M phases but decreases during S phase, suggesting a cell cycle-dependent regulation of hTERT. Our data suggest that CHIP represents a new pathway for modulating telomerase activity in cancer.

HSP90 manages the ends

The telomere environment requires an efficient means to assemble and disassemble a multitude of structures to operate correctly and to help achieve cellular homeostasis. Telomeres are challenged by a common binding specificity displayed by many of the protein components for telomeric DNA, which could result in competitive DNA interactions, and by a cell cycle-restricted timing of events, which enforces a narrow working period in which to perform numerous tasks. In this review, we discuss how the HSP90 molecular chaperone network avoids these obstacles and facilitates an effective operation of the telomere system.

Induction of premature senescence by hsp90 inhibition in small cell lung cancer

Background

The molecular chaperone Hsp90 is a promising new target in cancer therapy and selective Hsp90 inhibitors are currently in clinical trials. Previously these inhibitors have been reported to induce either cell cycle arrest or cell death in cancer cells. Whether the cell cycle arrest is reversible or irreversible has not generally been assessed. Here we have examined in detail the cell cycle arrest and cell death responses of human small cell lung cancer cell lines to Hsp90 inhibition.

Methodology/Principal Findings

In MTT assays, small cell lung cancer cells showed a biphasic response to the Hsp90 inhibitors geldanamycin and radicicol, with low concentrations causing proliferation arrest and high concentrations causing cell death. Assessment of Hsp90 intracellular activity using loss of client protein expression showed that geldanamycin concentrations that inhibited Hsp90 correlated closely with those causing proliferation arrest but not cell death. The proliferation arrest induced by low concentrations of geldanamycin was not reversed for a period of over thirty days following drug removal and showed features of senescence. Rare populations of variant small cell lung cancer cells could be isolated that had additional genetic alterations and no longer underwent irreversible proliferation arrest in response to Hsp90 inhibitors.

Conclusions/Significance

We conclude that: (1) Hsp90 inhibition primarily induces premature senescence, rather than cell death, in small cell lung cancer cells; (2) small cell lung cancer cells can bypass this senescence through further genetic alterations; (3) Hsp90 inhibitor-induced cell death in small cell lung cancer cells is due to inhibition of a target other than cytosolic Hsp90. These results have implications with regard to how these inhibitors will behave in clinical trials and for the design of future inhibitors in this class.

Characterization of the octamer, a cis-regulatory element that modulates excretory cell gene-expression in Caenorhabditis elegans

BACKGROUND

We have previously demonstrated that the POU transcription factor CEH-6 is required for driving aqp-8 expression in the C. elegans excretory (canal) cell, an osmotic regulatory organ that is functionally analogous to the kidney. This transcriptional regulation occurs through a CEH-6 binding to a cis-regulatory element called the octamer (ATTTGCAT), which is located in the aqp-8 promoter.

RESULTS

Here, we further characterize octamer driven transcription in C. elegans. First, we analyzed the positional requirements of the octamer. To do so, we assayed the effects on excretory cell expression by placing the octamer within the well-characterized promoter of vit-2. Second, using phylogenetic footprinting between three Caenorhabditis species, we identified a set of 165 genes that contain conserved upstream octamers in their promoters. Third, we used promoter::GFP fusions to examine the expression patterns of 107 of the 165 genes. This analysis demonstrated that conservation of octamers in promoters increases the likelihood that the gene is expressed in the excretory cell. Furthermore, we found that the sequences flanking the octamers may have functional importance. Finally, we altered the octamer using site-directed mutagenesis. Thus, we demonstrated that some nucleotide substitutions within the octamer do not affect the expression pattern of nearby genes, but change their overall expression was changed. Therefore, we have expanded the core octamer to include flanking regions and variants of the motif.

CONCLUSIONS

Taken together, we have demonstrated that octamer-containing regions are associated with excretory cell expression of several genes that have putative roles in osmoregulation. Moreover, our analysis of the octamer sequence and its sequence variants could aid in the identification of additional genes that are expressed in the excretory cell and that may also be regulated by CEH-6.

A truncated form of p23 down-regulates telomerase activity via disruption of Hsp90 function

The Hsp90-associated protein p23 modulates Hsp90 activity during the final stages of the chaperone pathway to facilitate maturation of client proteins. Previous reports indicate that p23 cleavage induced by caspases during cell death triggers destabilization of client proteins. However, the specific role of truncated p23 (Delta p23) in this process and the underlying mechanisms remain to be determined. One such client protein, hTERT, is a telomerase catalytic subunit regulated by several chaperone proteins, including Hsp90 and p23. In the present study, we examined the effects of p23 cleavage on hTERT stability and telomerase activity. Our data showed that overexpression of Delta p23 resulted in a decrease in hTERT levels, and a down-regulation in telomerase activity. Serine phosphorylation of Hsp90 was significantly reduced in cells expressing high levels of Delta p23 compared with those expressing full-length p23. Mutation analyses revealed that two serine residues (Ser-231 and Ser-263) in Hsp90 are important for activation of telomerase, and down-regulation of telomerase activity by Delta p23 was associated with inhibition of cell growth and sensitization of cells to cisplatin. Our data aid in determining the mechanism underlying the regulation of telomerase activity by the chaperone complex during caspase-dependent cell death.

Structures of telomerase subunits provide functional insights

BACKGROUND

Telomerase continues to generate substantial attention both because of its pivotal roles in cellular proliferation and aging and because of its unusual structure and mechanism. By replenishing telomeric DNA lost during the cell cycle, telomerase overcomes one of the many hurdles facing cellular immortalization. Functionally, telomerase is a reverse transcriptase, and it shares structural and mechanistic features with this class of nucleotide polymerases. Telomerase is a very unusual reverse transcriptase because it remains stably associated with its template and because it reverse transcribes multiple copies of its template onto a single primer in one reaction cycle.

SCOPE OF REVIEW

Here, we review recent findings that illuminate our understanding of telomerase. Even though the specific emphasis is on structure and mechanism, we also highlight new insights into the roles of telomerase in human biology.

GENERAL SIGNIFICANCE

Recent advances in the structural biology of telomerase, including high resolution structures of the catalytic subunit of a beetle telomerase and two domains of a ciliate telomerase catalytic subunit, provide new perspectives into telomerase biochemistry and reveal new puzzles.

Recovery from stress is a function of age and telomere length

Cells are constantly exposed to a wide variety of stimuli and must be able to mount appropriate physiological responses in order to maintain proper form and function. Cells from every organism have evolved highly conserved mechanisms to cope with environmental changes, including the widely studied heat shock response (HSR), which is induced by a variety of cellular stresses such as heavy metal ion exposure. It has long been known that as organisms and individual cells age, their ability to appropriately cope with environmental stress is attenuated. Here, we examine the ability of two heavy metal ions (ZnCl(2), SnCl(2)) to induce the HSR in human fibroblasts by assessing the expression of heat shock proteins (Hsp90, Hsp70, and p23) and the ability of the cells to recover over time. We demonstrate that the induction and recovery of chaperone levels is attenuated with age and that cells immortalized with the human telomerase reverse transcriptase component of the telomerase enzyme do not attenuate their HSR as their replicative age increases. Our data suggest that the recovery of normal human cells from an HSR is related in part to age and the cell's overall telomere length.

Association of hsp90 to the hTERT promoter is necessary for hTERT expression in human oral cancer cells

Enhanced expression of human telomerase reverse transcriptase (hTERT) occurs frequently during cellular immortalization. The current study was undertaken to determine the mechanism regulating the hTERT promoter activity during cellular immortalization of human oral keratinocytes. Normal human oral keratinocytes (NHOKs) were immortalized with Bmi-1 and the E6 oncoprotein of human papillomavirus type 16 to establish the telomerase-positive HOK-Bmi-1/E6 cell line. Using DNA-protein-binding assay, we found that heat shock protein 90 (hsp90) physically interacts with the hTERT promoter in vitro. The hsp90 interaction with the promoter was detected more strongly in the telomerase-positive HOK-Bmi-1/E6 cells compared with that in senescing NHOK. Chromatin immunoprecipitation confirmed the in vivo interaction between hsp90 and the hTERT promoter in SCC4 cells, a telomerase-positive oral cancer cell line, but not in the NHOK. To determine the physiological significance of this interaction, SCC4 cells were exposed to geldanamycin (GA), a competitive inhibitor of hsp90. GA exposure led to decrease in telomerase activity, hTERT promoter activity and hTERT messenger RNA expression in SCC4 cells, even in the absence of de novo protein synthesis. Also, it abolished the in vivo interaction of the hTERT promoter region with hsp90 but not with Sp1 or c-Myc. These results indicate that physical interaction between hsp90 and the hTERT promoter occurs in telomerase-positive cells but not in normal human cells and is necessary for the enhanced hTERT expression and telomerase activity in cancer cells.

Upregulation of Telomerase Function During Tissue Regeneration

Telomerase plays a primary role in the maintenance of telomeres in immortal, germ, and tumor cells in humans but is lacking in most somatic cells and tissues. However, many species, including fish and inbred mice, express telomerase in most cells and tissues. Little is known about the expression of telomerase in aquatic species, although the importance of telomerase for longevity has been suggested. We compared telomerase activity and telomere lengths among a broad range of tissues from aquatic species and found telomerase at significant levels in both long- and short-lived aquatic species, suggesting constitutive telomerase expression has an alternative function. Telomere lengths in these aquatic species were comparable to those observed in normal human tissues and cell strains. Given that a host of aquatic species with short life spans have telomerase and a tremendous capacity to regenerate, we tested the hypothesis that telomerase upregulation is important for tissue regeneration. During regeneration, telomerase activity was upregulated and telomere lengths are maintained with the shortest telomeres being elongated, indicating the importance for maintaining telomere length and integrity during tissue regeneration. Thus, the expression of telomerase in aquatic animals is likely not related to longevity but to their ability to regenerate injured tissue.

Proteome alteration induced by hTERT transfection of human fibroblast cells

Background

Telomerase confers cellular immortality by elongating telomeres, thereby circumventing the Hayflick limit. Extended-life-span cells have been generated by transfection with the human telomerase reverse transcriptase (hTERT) gene. hTERT transfected cell lines may be of outstanding interest to monitor the effect of drugs targeting the telomerase activity. The incidence of hTERT gene transfection at the proteome level is a prerequisite to that purpose. The effect of the transfection has been studied on the proteome of human fibroblast (WI38). Cytosolic and nuclear fractions of WI38 cells, empty vector transfected WI38 (WI38-HPV) and hTERT WI38 cells were submitted to a 2D-DIGE (Two-Dimensional Differential In-Gel Electrophoresis) analysis. Only spots that had a similar abundance in WI38 and WI38-HPV, but were differentially expressed in WI38 hTERT were selected for MS identification. This method directly points to the proteins linked with the hTERT expression. Number of false positive differentially expressed proteins has been excluded by using control WI38-HPV cells. The proteome alteration induced by hTERT WI38 transfection should be taken into account in subsequent use of the cell line for anti-telomerase drugs evaluation.

Results

2D-DIGE experiment shows that 57 spots out of 2246 are significantly differentially expressed in the cytosolic fraction due to hTERT transfection, and 38 were confidently identified. In the nuclear fraction, 44 spots out of 2172 were selected in the differential proteome analysis, and 14 were identified. The results show that, in addition to elongating telomeres, hTERT gene transfection has other physiological roles, among which an enhanced ER capacity and a potent cell protection against apoptosis.

Conclusion

We show that the methodology reduces the complexity of the proteome analysis and highlights proteins implicated in other processes than telomere elongation. hTERT induced proteome changes suggest that telomerase expression enhances natural cell repair mechanisms and stress resistance probably required for long term resistance of immortalized cells. Thus, hTERT transfected cells can not be only consider as an immortal equivalent to parental cells but also as cells which are over-resistant to stresses. These findings are the prerequisite for any larger proteomics aiming to evaluate anti-telomerase drugs proteome alteration and thus therapeutics induced cell reactions.

The Hsp90 molecular chaperone: an open and shut case for treatment

The molecular chaperone Hsp90 (90 kDa heat-shock protein) is a remarkably versatile protein involved in the stress response and in normal homoeostatic control mechanisms. It interacts with 'client proteins', including protein kinases, transcription factors and others, and either facilitates their stabilization and activation or directs them for proteasomal degradation. By this means, Hsp90 displays a multifaceted ability to influence signal transduction, chromatin remodelling and epigenetic regulation, development and morphological evolution. Hsp90 operates as a dimer in a conformational cycle driven by ATP binding and hydrolysis at the N-terminus. The cycle is also regulated by a group of co-chaperones and accessory proteins. Here we review the biology of the Hsp90 molecular chaperone, emphasizing recent progress in our understanding of structure-function relationships and the identification of new client proteins. In addition we describe the exciting progress that has been made in the development of Hsp90 inhibitors, which are now showing promise in the clinic for cancer treatment. We also identify the gaps in our current understanding and highlight important topics for future research.

Transcriptional regulation of telomerase activity: roles of the the Ets transcription factor family

Telomerase maintains telomeres to preclude cell senescence. It remains elusive how telomerase activity is repressed in differentiated cells, but retained at high levels in stem cells and cancer. Recent studies suggest that the Ets transcription factor family, downstream of the mitogen signaling pathways of MAP kinase, regulates telomerase activity at the gene transcription level of human telomerase reverse transcriptase (hTERT). Several Ets transcription factors are involved in regulating hTERT gene expression, both directly and indirectly through the proto-oncogene c-myc. ER81 may mediate telomerase activation in telomerase-negative fibroblasts stimulated by oncogenes Her2/Neu, Ras, and Raf. Ets2 may also play an important role in regulating the hTERT gene; but further studies are required to decipher the mechanisms in the regulation of telomerase activity.

Physiological assembly and activity of human telomerase complexes

Telomerase is a specialized reverse transcriptase conserved throughout almost all eukaryotic life. It plays a fundamental role in genome maintenance, adding back the telomeric DNA repeats lost from chromosome ends due to incomplete replication or damage. The protein and RNA subunits of telomerase fold and function in a co-dependent manner to establish a high fidelity of telomeric repeat synthesis. Over the past two decades, studies of telomerase have uncovered previously unanticipated levels of complexity in its assembly, activity and regulation. This review describes the current understanding of telomerase in humans, with particular focus on telomerase biogenesis and regulation in its cellular context.

The hsp90 molecular chaperone modulates multiple telomerase activities

The Hsp90 molecular chaperone is a highly abundant eukaryotic molecular chaperone. While it is understood that Hsp90 modulates a significant number of proteins, the mechanistic contributions made by Hsp90 to a client protein typically are not well understood. Here we investigate the yeast Hsp90 regulatory roles with telomerase. Telomerase lengthens chromosome termini by specifically associating with single-stranded telomeric DNA and appending nucleotides by reverse transcription. We have found that the yeast Hsp90 homolog Hsp82p promotes both telomerase DNA binding and nucleotide addition properties. By isolating telomerase from different allelic backgrounds we observed distinct defects. For example, in an hsp82 T101I strain telomerase displayed decreased nucleotide processivity, whereas both DNA binding and extension activities were lowered in a G170D background. The decline in telomerase DNA binding correlated with a loss of Hsp82p association. No matter the defect, telomerase activity was recovered upon Hsp82p addition. Importantly, telomere length and telomerase telomere occupancy was yeast Hsp90 dependent. Taken together, our results indicate that Hsp82p promotes telomerase DNA association and facilitates DNA extension once telomerase is engaged with the DNA.

Chaperone activation of the hepadnaviral reverse transcriptase for template RNA binding is established by the Hsp70 and stimulated by the Hsp90 system

Hepadnaviruses are DNA viruses that replicate by protein-primed reverse transcription, employing a specialized reverse transcriptase (RT), P protein. DNA synthesis from the pregenomic RNA is initiated by binding of P to the ε signal. Using ε as template and a Tyr-residue for initiation, the RT synthesizes a DNA oligo (priming) as primer for full-length DNA. Priming strictly requires prior RT activation by chaperones. Active P–ε complexes have been reconstituted in vitro, but whether in addition to the heat-shock protein 70 (Hsp70) system the Hsp90 system is essential has been controversial. Here we quantitatively compared Hsp70 versus Hsp70 plus Hsp90 RT activation, and corroborated that the Hsp70 system alone is sufficient; however, Hsp90 as well the Hsp70 nucleotide exchange factor Bag-1 markedly stimulated activation by increasing the steady-state concentration of the activated metastable RT form P*, though by different mechanisms. Hsp90 inhibition in intact cells by geldanamycin analogs blocked hepadnavirus replication, however not completely and only at severely cytotoxic inhibitor concentrations. While compatible with a basal level of Hsp90 independent in vivo replication, unambiguous statements are precluded by the simultaneous massive upregulation of Hsp70 and Hsp90.

The p23 molecular chaperone promotes functional telomerase complexes through DNA dissociation

Telomeres are the composite of short DNA element tandem arrays and heterotypic protein components that protect and maintain chromosomal termini. As proper telomere maintenance requires a multitude of DNA extension events, it is important to understand the factors that modulate telomerase DNA association. Here, we show that the endogenous levels of the yeast p23 molecular chaperone Sba1p are required for telomere length maintenance and that Sba1p can modulate telomerase DNA binding and extension activities in vitro. Notably, telomere occupancy by telomerase and the extension rate of a shortened telomere fluctuated with changing Sba1 protein levels in vivo. In addition, we found that Sba1p displayed a cell cycle-dependent telomere interaction that paralleled telomerase binding; telomere association by Sba1p depended on its inherent chaperone activity. Taken together, our results support a model in which Sba1p modulates telomerase DNA binding activity for optimal function in vitro and in vivo.