Functional requirement of p23 and Hsp90 in telomerase complexes

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.

Clinical and laboratory results in vaginal wall restoration using a fractional-pixel-CO2 laser: histological findings and changes in the Ki67 protein and telomere length

Thermal deposition of laser energy in the vaginal epithelium in genitourinary syndrome of menopause (GSM) results in clinical and biological effects, but many cellular and molecular changes indicating cell proliferation or senescence inhibition are unknown. The aim of this study is to evaluate the clinical efficacy of the fractional-pixel-CO2 laser in the possible improvement of GMS signs and symptoms that can be correlated with histological changes or with cellular or molecular indicators of restoration. A detailed prospective study was designed to assess 17 women diagnosed with GSM who were treated intravaginally with two laser sessions. Seven non-treated women diagnosed with GSM were used as controls. Three validated outcome questionnaires for assessment of quality of sexual life and urinary incontinence were performed. Vaginal biopsies were collected before the first laser treatment and 4 months following the second session. Histological status, elastin, collagen, and hyaluronic acid content of the biopsies were also evaluated. Cell proliferation was assessed by Ki67 staining. Telomere length (TL) was measured by qPCR. The results show an improvement of the clinical symptoms of GSM (p < 0.05), vaginal epithelium recovery and enhancement of collagen (p < 0.05), elastic fibers (p < 0.005), and hyaluronic acid (p < 0.0005) content in the lamina propria after fractional-pixel-CO2 laser treatment. The laser treatment induced a significant rise on the TL of vaginal epithelial cells (VECs), and a positive correlation was found between the improvements of the collagen and hyaluronic acid content and TL changes (r = 0.82, p < 0.05; r = 0.38, p < 0.05). The percentage of proliferative Ki67-positive VECs was increased in patients whose vaginal TL lengthened after laser treatment (p < 0.05). In conclusion, the results indicate that laser treatment may induce restoration of the vaginal epithelium which is associated to increased TL and proliferation in the VECs. Performing a TL assay could be a suitable tool to evaluate the efficacy of vaginal laser treatment.

The p23 co-chaperone is a succinate-activated COX-2 transcription factor in lung adenocarcinoma tumorigenesis

P23, historically known as a heat shock protein 90 (HSP90) co-chaperone, exerts some of its critical functions in an HSP90-independent manner, particularly when it translocates into the nucleus. The molecular nature underlying how this HSP90-independent p23 function is achieved remains as a biological mystery. Here, we found that p23 is a previously unidentified transcription factor of COX-2, and its nuclear localization predicts the poor clinical outcomes. Intratumor succinate promotes p23 succinylation at K7, K33, and K79, which drives its nuclear translocation for COX-2 transcription and consequently fascinates tumor growth. We then identified M16 as a potent p23 succinylation inhibitor from 1.6 million compounds through a combined virtual and biological screening. M16 inhibited p23 succinylation and nuclear translocation, attenuated COX-2 transcription in a p23-dependent manner, and markedly suppressed tumor growth. Therefore, our study defines p23 as a succinate-activated transcription factor in tumor progression and provides a rationale for inhibiting p23 succinylation as an anticancer chemotherapy.

Telomere loss is accompanied by decreased pool of shelterin proteins TRF2 and RAP1, elevated levels of TERRA and enhanced glycolysis in imatinib-resistant CML cells

Telomere length may be maintained by telomerase nucleoprotein complex and shelterin complex, namely TRF1, TRF2, TIN2, TPP1, POT1 and RAP1 proteins and modulated by TERRA expression. Telomere loss is observed during progression of chronic myeloid leukemia (CML) from the chronic phase (CML-CP) to the blastic phase (CML-BP). The introduction of tyrosine kinase inhibitors (TKIs), such as imatinib (IM), has changed outcome for majority of patients, however, a number of patients treated with TKIs may develop drug resistance. The molecular mechanisms underlying this phenomenon are not fully understood and require further investigation. In the present study, we demonstrate that IM-resistant BCR::ABL1 gene-positive CML K-562 and MEG-A2 cells are characterized by decreased telomere length, lowered protein levels of TRF2 and RAP1 and increased expression of TERRA in comparison to corresponding IM-sensitive CML cells and BCR::ABL1 gene-negative HL-60 cells. Furthermore, enhanced activity of glycolytic pathway was observed in IM-resistant CML cells. A negative correlation between a telomere length and advanced glycation end products (AGE) was also revealed in CD34⁺ cells isolated from CML patients. In conclusion, we suggest that affected expression of shelterin complex proteins, namely TRF2 and RAP1, TERRA levels, and glucose consumption rate may promote telomere dysfunction in IM-resistant CML cells.

Proteomic analysis defines the interactome of telomerase in the protozoan parasite, Trypanosoma brucei

Telomerase is a ribonucleoprotein enzyme responsible for maintaining the telomeric end of the chromosome. The telomerase enzyme requires two main components to function: the telomerase reverse transcriptase (TERT) and the telomerase RNA (TR), which provides the template for telomeric DNA synthesis. TR is a long non-coding RNA, which forms the basis of a large structural scaffold upon which many accessory proteins can bind and form the complete telomerase holoenzyme. These accessory protein interactions are required for telomerase activity and regulation inside cells. The interacting partners of TERT have been well studied in yeast, human, and Tetrahymena models, but not in parasitic protozoa, including clinically relevant human parasites. Here, using the protozoan parasite, Trypanosoma brucei (T. brucei) as a model, we have identified the interactome of T. brucei TERT (TbTERT) using a mass spectrometry-based approach. We identified previously known and unknown interacting factors of TbTERT, highlighting unique features of T. brucei telomerase biology. These unique interactions with TbTERT, suggest mechanistic differences in telomere maintenance between T. brucei and other eukaryotes.

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.

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.

Activation of telomerase activity and telomere elongation of host cells by Theileria annulata infection

Theileria annulata-transformed cells share many phenotypes with cancer cells, including uncontrolled proliferation, immortalization, and dissemination. Telomeres are DNA-protein complex at the end of eukaryotic chromosomes that function to maintain genome stability and cell replicative capacity. Telomere length maintenance is primarily dependent on telomerase activity. In up to 90% of human cancer cells, telomerase is reactivated through expression of its catalytic subunit TERT. However, the effect of T. annulata infection on telomere and telomerase activity in bovine cells has not yet been described. In the present study, we confirmed that telomere length and telomerase activity are upregulated after T. annulata infection in three types of cell lines. This change depends on the presence of parasites. After eliminating Theileria from cells with antitheilerial drug buparvaquone, telomerase activity and the expression level of bTERT were decreased. In addition, inhibition of bHSP90 by novobiocin led to decreased AKT phosphorylation levels and telomerase activity, indicating that the bHSP90-AKT complex is a potent factor modulates telomerase activity in T. annulata-infected cells.

Targeting HSP90 as a Novel Therapy for Cancer: Mechanistic Insights and Translational Relevance

Heat shock protein (HSP90), a highly conserved molecular chaperon, is indispensable for the maturation of newly synthesized poly-peptides and provides a shelter for the turnover of misfolded or denatured proteins. In cancers, the client proteins of HSP90 extend to the entire process of oncogenesis that are associated with all hallmarks of cancer. Accumulating evidence has demonstrated that the client proteins are guided for proteasomal degradation when their complexes with HSP90 are disrupted. Accordingly, HSP90 and its co-chaperones have emerged as viable targets for the development of cancer therapeutics. Consequently, a number of natural products and their analogs targeting HSP90 have been identified. They have shown a strong inhibitory effect on various cancer types through different mechanisms. The inhibitors act by directly binding to either HSP90 or its co-chaperones/client proteins. Several HSP90 inhibitors—such as geldanamycin and its derivatives, gamitrinib and shepherdin—are under clinical evaluation with promising results. Here, we review the subcellular localization of HSP90, its corresponding mechanism of action in the malignant phenotypes, and the recent progress on the development of HSP90 inhibitors. Hopefully, this comprehensive review will shed light on the translational potential of HSP90 inhibitors as novel cancer therapeutics.

The role of cycloastragenol at the intersection of NRF2/ARE, telomerase, and proteasome activity

Aging is well-characterized by the gradual decline of cellular functionality. As redox balance, proteostasis, and telomerase systems have been found to be associated with aging and age-related diseases, targeting these systems with small compounds has been considered a promising therapeutic approach. Cycloastragenol (CA), a small molecule telomerase activator obtained from Astragalus species, has been reported to positively affect several age-related pathophysiologies, but the mechanisms underlying CA activity have yet to be reported. Here, we presented that CA increased NRF2 nuclear localization and activity leading to upregulation of cytoprotective enzymes and attenuation of oxidative stress-induced ROS levels. Furthermore, CA-mediated induction of telomerase activity was found to be regulated by NRF2. CA not only increased the expression of hTERT but also its nuclear localization via upregulating the Hsp90-chaperon complex. In addition to modulating nuclear hTERT levels at unstressed conditions, CA alleviated oxidative stress-induced mitochondrial hTERT levels while increasing nuclear hTERT levels. Concomitantly, H₂O₂-induced mitochondrial ROS level was found to be significantly decreased by CA administration. Our data also revealed that CA strongly enhanced proteasome activity and assembly. More importantly, the proteasome activator effect of CA is dependent on the induction of telomerase activity, which is mediated by NRF2 system. In conclusion, our results not only revealed the cross-talk among NRF2, telomerase, and proteasome systems but also that CA functions at the intersection of these three major aging-related cellular pathways.

Direct Interaction between N-Acetylcysteine and Cytotoxic Electrophile—An Overlooked In Vitro Mechanism of Protection

In laboratory experiments, many electrophilic cytotoxic agents induce cell death accompanied by reactive oxygen species (ROS) production and/or by glutathione (GSH) depletion. Not surprisingly, millimolar concentrations of N-acetylcysteine (NAC), which is used as a universal ROS scavenger and precursor of GSH biosynthesis, inhibit ROS production, restore GSH levels, and prevent cell death. The protective effect of NAC is generally used as corroborative evidence that cell death induced by a studied cytotoxic agent is mediated by an oxidative stress-related mechanism. However, any simple interpretation of the results of the protective effects of NAC may be misleading because it is unable to interact with superoxide (O₂•⁻), the most important biologically relevant ROS, and is a very weak scavenger of H₂O₂. In addition, NAC is used in concentrations that are unnecessarily high to stimulate GSH synthesis. Unfortunately, the possibility that NAC as a nucleophile can directly interact with cytotoxic electrophiles to form non-cytotoxic NAC–electrophile adduct is rarely considered, although it is a well-known protective mechanism that is much more common than expected. Overall, apropos the possible mechanism of the cytoprotective effect of NAC in vitro, it is appropriate to investigate whether there is a direct interaction between NAC and the cytotoxic electrophile to form a non-cytotoxic NAC–electrophilic adduct(s).

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.

Lactate Dehydrogenase B Is Required for Pancreatic Cancer Cell Immortalization Through Activation of Telomerase Activity

Telomerase activity is elevated in most cancer cells and is required for telomere length maintenance and immortalization of cancer cells. Glucose metabolic reprogramming is a hallmark of cancer and accompanied with increased expression of key metabolic enzymes. Whether these enzymes influence telomerase activity and cell immortalization remains unclear. In the current study, we screened metabolic enzymes using telomerase activity assay and identified lactate dehydrogenase B (LDHB) as a regulator of telomerase activity. Sodium lactate and sodium pyruvate did not influence telomerase activity, indicating LDHB regulates telomerase activity independent of its metabolism regulating function. Further studies revealed that LDHB directly interacted with TERT and regulated the interaction between TERT and TERC. Additionally, long-term knockdown of LDHB inhibited cancer cell growth and induced cell senescence in vitro and in vivo. Higher LDHB expression was detected in pancreatic cancer tissues compared with that in adjacent normal tissues and expression of LDHB correlated negatively with prognosis. Thus, we identified LDHB as the first glucose metabolic enzyme contributing to telomerase activity and pancreatic cancer cell immortalization.

Four-protein model for predicting prognostic risk of lung cancer

Patients with lung cancer at the same stage may have markedly different overall outcome and a lack of specific biomarker to predict lung cancer outcome. Heat-shock protein 90 β (HSP90β) is overexpressed in various tumor cells. In this study, the ELISA results of HSP90β combined with CEA, CA125, and CYFRA21-1 were used to construct a recursive partitioning decision tree model to establish a four-protein diagnostic model and predict the survival of patients with lung cancer. Survival analysis showed that the recursive partitioning decision tree could distinguish the prognosis between high- and low-risk groups. Results suggested that the joint detection of HSP90β, CEA, CA125, and CYFRA21-1 in the peripheral blood of patients with lung cancer is plausible for early diagnosis and prognosis prediction of lung cancer.

Telomerase-based therapies in haematological malignancies

Telomeres are specialized genetic structures present at the end of all eukaryotic linear chromosomes. They progressively get shortened after each cell division due to end replication problems. Telomere shortening (TS) and chromosomal instability cause apoptosis and massive cell death. Following oncogene activation and inactivation of tumour suppressor genes, cells acquire mechanisms such as telomerase expression and alternative lengthening of telomeres to maintain telomere length (TL) and prevent initiation of cellular senescence or apoptosis. Significant TS, telomerase activation and alteration in expression of telomere-associated proteins are frequent features of different haematological malignancies that reflect on the progression, response to therapy and recurrence of these diseases. Telomerase is a ribonucleoprotein enzyme that has a pivotal role in maintaining the TL. However, telomerase activity in most somatic cells is insufficient to prevent TS. In 85-90% of tumour cells, the critically short telomeric length is maintained by telomerase activation. Thus, overexpression of telomerase in most tumour cells is a potential target for cancer therapy. In this review, alteration of telomeres, telomerase and telomere-associated proteins in different haematological malignancies and related telomerase-based therapies are discussed.

Telomeres and Cancer

Telomeres cap the ends of eukaryotic chromosomes and are indispensable chromatin structures for genome protection and replication. Telomere length maintenance has been attributed to several functional modulators, including telomerase, the shelterin complex, and the CST complex, synergizing with DNA replication, repair, and the RNA metabolism pathway components. As dysfunctional telomere maintenance and telomerase activation are associated with several human diseases, including cancer, the molecular mechanisms behind telomere length regulation and protection need particular emphasis. Cancer cells exhibit telomerase activation, enabling replicative immortality. Telomerase reverse transcriptase (TERT) activation is involved in cancer development through diverse activities other than mediating telomere elongation. This review describes the telomere functions, the role of functional modulators, the implications in cancer development, and the future therapeutic opportunities.

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.

Ovarian Telomerase and Female Fertility

Women's fertility is characterized both quantitatively and qualitatively mainly by the pool of ovarian follicles. Monthly, gonadotropins cause an intense multiplication of granulosa cells surrounding the oocyte. This step of follicular development requires a high proliferation ability for these cells. Telomere length plays a crucial role in the mitotic index of human cells. Hence, disrupting telomere homeostasis could directly affect women's fertility. Strongly expressed in ovaries, telomerase is the most effective factor to limit telomeric attrition and preserve ovarian reserve. Considering these facts, two situations of infertility could be correlated with the length of telomeres and ovarian telomerase activity: PolyCystic Ovary Syndrome (PCOS), which is associated with a high density of small antral follicles, and Premature Ovarian Failure (POF), which is associated with a premature decrease in ovarian reserve. Several authors have studied this topic, expecting to find long telomeres and strong telomerase activity in PCOS and short telomeres and low telomerase activity in POF patients. Although the results of these studies are contradictory, telomere length and the ovarian telomerase impact in women's fertility disorders appear obvious. In this context, our research perspectives aimed to explore the stimulation of ovarian telomerase to limit the decrease in the follicular pool while avoiding an increase in cancer risk.

P23 Acts as Functional RBP in the Macrophage Inflammation Response

Macrophages exert the primary cellular immune response. Pathogen components like bacterial lipopolysaccharides (LPS) stimulate macrophage migration, phagocytotic activity and cytokine expression. Previously, we identified the poly(A)⁺ RNA interactome of RAW 264.7 macrophages. Of the 402 RNA-binding proteins (RBPs), 32 were classified as unique in macrophages, including nineteen not reported to interact with nucleic acids before. Remarkably, P23 a HSP90 co-chaperone, also known as cytosolic prostaglandin E2 synthase (PTGES3), exhibited differential poly(A)⁺ RNA binding in untreated and LPS-induced macrophages. To identify mRNAs bound by P23 and to elucidate potential regulatory RBP functions in macrophages, we immunoprecipitated P23 from cytoplasmic extracts of cross-linked untreated and LPS-induced cells. RNAseq revealed that enrichment of 44 mRNAs was reduced in response to LPS. Kif15 mRNA, which encodes kinesin family member 15 (KIF15), a motor protein implicated in cytoskeletal reorganization and cell mobility was selected for further analysis. Noteworthy, phagocytic activity of LPS-induced macrophages was enhanced by P23 depletion. Specifically, in untreated RAW 264.7 macrophages, decreased P23 results in Kif15 mRNA destabilization, diminished KIF15 expression and accelerated macrophage migration. We show that the unexpected RBP function of P23 contributes to the regulation of macrophage phagocytotic activity and migration.

Molecular Chaperones and Thyroid Cancer

Thyroid cancers are the most common of the endocrine system malignancies and progress must be made in the areas of differential diagnosis and treatment to improve patient management. Advances in the understanding of carcinogenic mechanisms have occurred in various fronts, including studies of the chaperone system (CS). Components of the CS are found to be quantitatively increased or decreased, and some correlations have been established between the quantitative changes and tumor type, prognosis, and response to treatment. These correlations provide the basis for identifying distinctive patterns useful in differential diagnosis and for planning experiments aiming at elucidating the role of the CS in tumorigenesis. Here, we discuss studies of the CS components in various thyroid cancers (TC). The chaperones belonging to the families of the small heat-shock proteins Hsp70 and Hsp90 and the chaperonin of Group I, Hsp60, have been quantified mostly by immunohistochemistry and Western blot in tumor and normal control tissues and in extracellular vesicles. Distinctive differences were revealed between the various thyroid tumor types. The most frequent finding was an increase in the chaperones, which can be attributed to the augmented need for chaperones the tumor cells have because of their accelerated metabolism, growth, and division rate. Thus, chaperones help the tumor cell rather than protect the patient, exemplifying chaperonopathies by mistake or collaborationism. This highlights the need for research on chaperonotherapy, namely the development of means to eliminate/inhibit pathogenic chaperones.

Telomerase Regulation: A Role for Epigenetics

Simple Summary

Maintenance of telomeres is a fundamental step in human carcinogenesis and is primarily regulated by telomerase and the human telomerase reverse transcriptase gene (TERT). Improved understanding of the transcriptional control of this gene may provide potential therapeutic targets. Epigenetic modifications are a prominent mechanism to control telomerase activity and regulation of the TERT gene. TERT-targeting miRNAs have been widely studied and their function explained through pre-clinical in vivo model-based validation studies. Further, histone deacetylase inhibitors are now in pre and early clinical trials with significant clinical success. Importantly, TERT downregulation through epigenetic modifications including TERT promoter methylation, histone deacetylase inhibitors, and miRNA activity might contribute to clinical study design. This review provides an overview of the epigenetic mechanisms involved in the regulation of TERT expression and telomerase activity.

Abstract

Telomerase was first described by Greider and Blackburn in 1984, a discovery ultimately recognized by the Nobel Prize committee in 2009. The three decades following on from its discovery have been accompanied by an increased understanding of the fundamental mechanisms of telomerase activity, and its role in telomere biology. Telomerase has a clearly defined role in telomere length maintenance and an established influence on DNA replication, differentiation, survival, development, apoptosis, tumorigenesis, and a further role in therapeutic resistance in human stem and cancer cells including those of breast and cervical origin. TERT encodes the catalytic subunit and rate-limiting factor for telomerase enzyme activity. The mechanisms of activation or silencing of TERT remain open to debate across somatic, cancer, and stem cells. Promoter mutations upstream of TERT may promote dysregulated telomerase activation in tumour cells but additional factors including epigenetic, transcriptional and posttranscriptional modifications also have a role to play. Previous systematic analysis indicated methylation and mutation of the TERT promoter in 53% and 31%, respectively, of TERT expressing cancer cell lines supporting the concept of a key role for epigenetic alteration associated with TERT dysregulation and cellular transformation. Epigenetic regulators including DNA methylation, histone modification, and non-coding RNAs are now emerging as drivers in the regulation of telomeres and telomerase activity. Epigenetic regulation may be responsible for reversible silencing of TERT in several biological processes including development and differentiation, and increased TERT expression in cancers. Understanding the epigenetic mechanisms behind telomerase regulation holds important prospects for cancer treatment, diagnosis and prognosis. This review will focus on the role of epigenetics in telomerase regulation.

The Connection Between Cell Fate and Telomere

Abolition of telomerase activity results in telomere shortening, a process that eventually destabilizes the ends of chromosomes, leading to genomic instability and cell growth arrest or death. Telomere shortening leads to the attainment of the "Hayflick limit", and the transition of cells to state of senescence. If senescence is bypassed, cells undergo crisis through loss of checkpoints. This process causes massive cell death concomitant with further telomere shortening and spontaneous telomere fusions. In functional telomere of mammalian cells, DNA contains double-stranded tandem repeats of TTAGGG. The Shelterin complex, which is composed of six different proteins, is required for the regulation of telomere length and stability in cells. Telomere protection by telomeric repeat binding protein 2 (TRF2) is dependent on DNA damage response (DDR) inhibition via formation of T-loop structures. Many protein kinases contribute to the DDR activated cell cycle checkpoint pathways, and prevent DNA replication until damaged DNA is repaired. Thereby, the connection between cell fate and telomere length-associated telomerase activity is regulated by multiple protein kinase activities. Contrarily, inactivation of DNA damage checkpoint protein kinases in senescent cells can restore cell-cycle progression into S phase. Therefore, telomere-initiated senescence is a DNA damage checkpoint response that is activated with a direct contribution from dysfunctional telomeres. In this review, in addition to the above mentioned, the choice of main repair pathways, which comprise non-homologous end joining and homologous recombination in telomere uncapping telomere dysfunctions, are discussed.

Role of Telomeres and Telomeric Proteins in Human Malignancies and Their Therapeutic Potential

Telomeres are the ends of linear chromosomes comprised of repetitive nucleotide sequences in humans. Telomeres preserve chromosomal stability and genomic integrity. Telomere length shortens with every cell division in somatic cells, eventually resulting in replicative senescence once telomere length becomes critically short. Telomere shortening can be overcome by telomerase enzyme activity that is undetectable in somatic cells, while being active in germline cells, stem cells, and immune cells. Telomeres are bound by a shelterin complex that regulates telomere lengthening as well as protects them from being identified as DNA damage sites. Telomeres are transcribed by RNA polymerase II, and generate a long noncoding RNA called telomeric repeat-containing RNA (TERRA), which plays a key role in regulating subtelomeric gene expression. Replicative immortality and genome instability are hallmarks of cancer and to attain them cancer cells exploit telomere maintenance and telomere protection mechanisms. Thus, understanding the role of telomeres and their associated proteins in cancer initiation, progression and treatment is very important. The present review highlights the critical role of various telomeric components with recently established functions in cancer. Further, current strategies to target various telomeric components including human telomerase reverse transcriptase (hTERT) as a therapeutic approach in human malignancies are discussed.

Telomerase Biogenesis and Activities from the Perspective of Its Direct Interacting Partners

Telomerase reverse transcriptase (TERT)—the catalytic subunit of telomerase—is reactivated in up to 90% of all human cancers. TERT is observed in heterogenous populations of protein complexes, which are dynamically regulated in a cell type- and cell cycle-specific manner. Over the past two decades, in vitro protein–protein interaction detection methods have discovered a number of endogenous TERT binding partners in human cells that are responsible for the biogenesis and functionalization of the telomerase holoenzyme, including the processes of TERT trafficking between subcellular compartments, assembly into telomerase, and catalytic action at telomeres. Additionally, TERT have been found to interact with protein species with no known telomeric functions, suggesting that these complexes may contribute to non-canonical activities of TERT. Here, we survey TERT direct binding partners and discuss their contributions to TERT biogenesis and functions. The goal is to review the comprehensive spectrum of TERT pro-malignant activities, both telomeric and non-telomeric, which may explain the prevalence of its upregulation in cancer.

Human telomerase reverse transcriptase positively regulates mitophagy by inhibiting the processing and cytoplasmic release of mitochondrial PINK1

Mutations in the phosphatase and tensin homologue-induced putative kinase 1 (PINK1) gene have been linked to an early-onset autosomal recessive form of familial Parkinson′s disease (PD). PINK1, a mitochondrial serine/threonine-protein kinase, plays an important role in clearing defective mitochondria by mitophagy – the selective removal of mitochondria through autophagy. Evidence suggests that alteration of the PINK1 pathway contributes to the pathogenesis of PD, but the mechanisms by which the PINK1 pathway regulates mitochondrial quality control through mitophagy remain unclear. Human telomerase reverse transcriptase (hTERT) is a catalytic subunit of telomerase that functions in telomere maintenance as well as several non-telomeric activities. For example, hTERT has been associated with cellular immortalization, cell growth control, and mitochondrial regulation. We determined that hTERT negatively regulates the cleavage and cytosolic processing of PINK1 and enhances its mitochondrial localization by inhibiting mitochondrial processing peptidase β (MPPβ). Consequently, hTERT promotes mitophagy following carbonyl cyanide m-chlorophenylhydrazone (CCCP)-induced mitochondrial dysfunction and improves the function of damaged mitochondria by modulating PINK1. These findings suggest that hTERT positively regulates PINK1 function, leading to increased mitophagy following mitochondrial damage.

Selective Autophagy Maintains the Aryl Hydrocarbon Receptor Levels in HeLa Cells: A Mechanism That Is Dependent on the p23 Co-Chaperone

The aryl hydrocarbon receptor (AHR) is an environmental sensing molecule which impacts diverse cellular functions such as immune responses, cell growth, respiratory function, and hematopoietic stem cell differentiation. It is widely accepted that the degradation of AHR by 26S proteasome occurs after ligand activation. Recently, we discovered that HeLa cells can modulate the AHR levels via protein degradation without exogenous treatment of a ligand, and this degradation is particularly apparent when the p23 content is down-regulated. Inhibition of autophagy by a chemical agent (such as chloroquine, bafilomycin A1, or 3-methyladenine) increases the AHR protein levels in HeLa cells whereas activation of autophagy by short-term nutrition deprivation reduces its levels. Treatment of chloroquine retards the degradation of AHR and triggers physical interaction between AHR and LC3B. Knockdown of LC3B suppresses the chloroquine-mediated increase of AHR. Down-regulation of p23 promotes AHR degradation via autophagy with no change of the autophagy-related gene expression. Although most data in this study were derived from HeLa cells, human lung (A549), liver (Hep3B), and breast (T-47D and MDA-MB-468) cells also exhibit AHR levels sensitive to chloroquine treatment and AHR-p62/LC3 interactions. Here we provide evidence supporting that AHR undergoes the p62/LC3-mediated selective autophagy in HeLa cells.

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.

HnRNP F/H associate with hTERC and telomerase holoenzyme to modulate telomerase function and promote cell proliferation

Human telomerase RNA component hTERC comprises multiple motifs that contribute to hTERC biogenesis, holoenzyme activity, and enzyme recruitment to telomeres. hTERC contains several guanine tracts (G-tracts) at its 5′-end, but its associated proteins and potential roles in telomerase function are still poorly understood. The heterogeneous nuclear ribonucleoproteins F, H1, and H2 (hnRNP F/H) are splicing factors that preferentially bind to poly(G)-rich sequences RNA. Here, we demonstrate that hnRNP F/H associate with both hTERC and telomerase holoenzyme to regulate telomerase activity. We reveal hnRNP F/H bind to the 5′-end region of hTERC in vitro and in vivo, and identify the first three G-tracts of hTERC and qRRM1 domain of hnRNP F/H are required for their interaction. Furthermore, hnRNP F/H also directly interact with telomerase holoenzyme. Functionally, we show that hnRNP F/H plays important roles in modulating telomerase activity and telomere length. Moreover, hnRNP F/H deletion greatly impair cancer and stem cell proliferation, and induce stem cell senescence, while hnRNP F/H overexpression delay stem cell senescence. Collectively, our findings unveil a novel role of hnRNP F/H as the binding partners of hTERC and telomerase holoenzyme to regulate telomerase function.

Impact of Carcinogenic Chromium on the Cellular Response to Proteotoxic Stress

Worldwide, several million workers are employed in the various chromium (Cr) industries. These workers may suffer from a variety of adverse health effects produced by dusts, mists and fumes containing Cr in the hexavalent oxidation state, Cr(VI). Of major importance, occupational exposure to Cr(VI) compounds has been firmly associated with the development of lung cancer. Counterintuitively, Cr(VI) is mostly unreactive towards most biomolecules, including nucleic acids. However, its intracellular reduction produces several species that react extensively with biomolecules. The diversity and chemical versatility of these species add great complexity to the study of the molecular mechanisms underlying Cr(VI) toxicity and carcinogenicity. As a consequence, these mechanisms are still poorly understood, in spite of intensive research efforts. Here, we discuss the impact of Cr(VI) on the stress response—an intricate cellular system against proteotoxic stress which is increasingly viewed as playing a critical role in carcinogenesis. This discussion is preceded by information regarding applications, chemical properties and adverse health effects of Cr(VI). A summary of our current understanding of cancer initiation, promotion and progression is also provided, followed by a brief description of the stress response and its links to cancer and by an overview of potential molecular mechanisms of Cr(VI) carcinogenicity.

The Nuclear and DNA-Associated Molecular Chaperone Network

Maintenance of a healthy and functional proteome in all cellular compartments is critical to cell and organismal homeostasis. Yet, our understanding of the proteostasis process within the nucleus is limited. Here, we discuss the identified roles of the major molecular chaperones Hsp90, Hsp70, and Hsp60 with client proteins working in diverse DNA-associated pathways. The unique challenges facing proteins in the nucleus are considered as well as the conserved features of the molecular chaperone system in facilitating DNA-linked processes. As nuclear protein inclusions are a common feature of protein-aggregation diseases (e.g., neurodegeneration), a better understanding of nuclear proteostasis is warranted.

Therapeutic strategies for targeting telomerase in cancer

Telomere and telomerase play important roles in abnormal cell proliferation, metastasis, stem cell maintenance, and immortalization in various cancers. Therefore, designing of drugs targeting telomerase and telomere is of great significance. Over the past two decades, considerable knowledge regarding telomere and telomerase has been accumulated, which provides theoretical support for the design of therapeutic strategies such as telomere elongation. Therefore, the development of telomere-based therapies such as nucleoside analogs, non-nucleoside small molecules, antisense technology, ribozymes, and dominant negative human telomerase reverse transcriptase are being prioritized for eradicating a majority of tumors. While the benefits of telomere-based therapies are obvious, there is a need to address the limitations of various therapeutic strategies to improve the possibility of clinical applications. In this study, current knowledge of telomere and telomerase is discussed, and therapeutic strategies based on recent research are reviewed.

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.

PES1 is a critical component of telomerase assembly and regulates cellular senescence

Telomerase defers the onset of telomere shortening and cellular senescence by adding telomeric repeat DNA to chromosome ends, and its activation contributes to carcinogenesis. Telomerase minimally consists of the telomerase reverse transcriptase (TERT) and the telomerase RNA (TR). However, how telomerase assembles is largely unknown. Here, we demonstrate that PES1 (Pescadillo), a protein overexpressed in many cancers, forms a complex with TERT and TR through direct interaction with TERT, regulating telomerase activity, telomere length maintenance, and senescence. PES1 does not interact with the previously reported telomerase components Reptin, Pontin, p23, and Hsp90. PES1 facilitates telomerase assembly by promoting direct interaction between TERT and TR without affecting TERT and TR levels. PES1 expression correlates positively with telomerase activity and negatively with senescence in patients with breast cancer. Thus, we identify a previously unknown telomerase complex, and targeting PES1 may open a new avenue for cancer therapy.

Targeting hsp90 family members: A strategy to improve cancer cell death

A crucial process in biology is the conversion of the genetic information into functional proteins that carry out the genetic program. However, a supplementary step is required to obtain functional proteins: the folding of the newly translated polypeptides into well-defined, three-dimensional conformations. Proteins chaperones are crucial for this final step in the readout of genetic information, which results in the formation of functional proteins. In this review, a special attention will be given to the strategies targeting hsp90 family members in order to increase cancer cell death. We argue that disruption of hsp90 machinery and the further client protein degradation is the main consequence of hsp90 oxidative cleavage taking place at the N-terminal nucleotide-binding site. Moreover, modulation of Grp94 expression will be discussed as a potential therapeutic goal looking for a decrease in cancer relapses.

Telomeropathies: Etiology, diagnosis, treatment and follow-up. Ethical and legal considerations

Telomeropathies involve a wide variety of infrequent genetic diseases caused by mutations in the telomerase maintenance mechanism or the DNA damage response (DDR) system. They are considered a family of rare diseases that often share causes, molecular mechanisms and symptoms. Generally, these diseases are not diagnosed until the symptoms are advanced, diminishing the survival time of patients. Although several related syndromes may still be unrecognized this work describes those that are known, highlighting that because they are rare diseases, physicians should be trained in their early diagnosis. The etiology and diagnosis are discussed for each telomeropathy and the treatments when available, along with a new classification of this group of diseases. Ethical and legal issues related to this group of diseases are also considered.

Biological Actions of the Hsp90-binding Immunophilins FKBP51 and FKBP52

Immunophilins are a family of proteins whose signature domain is the peptidylprolyl-isomerase domain. High molecular weight immunophilins are characterized by the additional presence of tetratricopeptide-repeats (TPR) through which they bind to the 90-kDa heat-shock protein (Hsp90), and via this chaperone, immunophilins contribute to the regulation of the biological functions of several client-proteins. Among these Hsp90-binding immunophilins, there are two highly homologous members named FKBP51 and FKBP52 (FK506-binding protein of 51-kDa and 52-kDa, respectively) that were first characterized as components of the Hsp90-based heterocomplex associated to steroid receptors. Afterwards, they emerged as likely contributors to a variety of other hormone-dependent diseases, stress-related pathologies, psychiatric disorders, cancer, and other syndromes characterized by misfolded proteins. The differential biological actions of these immunophilins have been assigned to the structurally similar, but functionally divergent enzymatic domain. Nonetheless, they also require the complementary input of the TPR domain, most likely due to their dependence with the association to Hsp90 as a functional unit. FKBP51 and FKBP52 regulate a variety of biological processes such as steroid receptor action, transcriptional activity, protein conformation, protein trafficking, cell differentiation, apoptosis, cancer progression, telomerase activity, cytoskeleton architecture, etc. In this article we discuss the biology of these events and some mechanistic aspects.

Telomeres in Plants and Humans: Not So Different, Not So Similar

Parallel research on multiple model organisms shows that while some principles of telomere biology are conserved among all eukaryotic kingdoms, we also find some deviations that reflect different evolutionary paths and life strategies, which may have diversified after the establishment of telomerase as a primary mechanism for telomere maintenance. Much more than animals, plants have to cope with environmental stressors, including genotoxic factors, due to their sessile lifestyle. This is, in principle, made possible by an increased capacity and efficiency of the molecular systems ensuring maintenance of genome stability, as well as a higher tolerance to genome instability. Furthermore, plant ontogenesis differs from that of animals in which tissue differentiation and telomerase silencing occur during early embryonic development, and the “telomere clock” in somatic cells may act as a preventive measure against carcinogenesis. This does not happen in plants, where growth and ontogenesis occur through the serial division of apical meristems consisting of a small group of stem cells that generate a linear series of cells, which differentiate into an array of cell types that make a shoot and root. Flowers, as generative plant organs, initiate from the shoot apical meristem in mature plants which is incompatible with the human-like developmental telomere shortening. In this review, we discuss differences between human and plant telomere biology and the implications for aging, genome stability, and cell and organism survival. In particular, we provide a comprehensive comparative overview of telomere proteins acting in humans and in Arabidopsis thaliana model plant, and discuss distinct epigenetic features of telomeric chromatin in these species.

A suboptimal maternal diet combined with accelerated postnatal growth results in an altered aging profile in the thymus of male rats

Reduced fetal nutrition and rapid postnatal growth accelerates the aging phenotype in many organ systems; however, effects on the immune system are unclear. We addressed this by studying the thymus from a rat model of developmental programming. The recuperated group was generated by in utero protein restriction, followed by cross-fostering to control-fed mothers, and were then compared with controls. Fat infiltration and adipocyte size increased with age ( P < 0.001) and in recuperated thymi ( P < 0.05). Cortex/medulla ratio decreased with age ( P < 0.001) and decreased ( P < 0.05) in 12-mo recuperated thymi. Age-associated decreases in thymic-epithelial cell ( P < 0.01) and thymocyte markers ( P < 0.01) were observed in both groups and was decreased ( P < 0.05) in recuperated thymi. These data demonstrate effects of developmental programming upon thymic involution. The recuperated group had longer thymic telomeres than controls ( P < 0.001) at 22 d and at 3 mo, which was associated with increased expression of telomere-length maintenance molecules [telomerase RNA component ( Terc; P < 0.01), P23 ( P = 0.02), and Ku70 and Ku80 ( P < 0.01)]. By 12 mo, recuperated offspring had shorter thymic telomeres than controls had ( P < 0.001) and reduced DNA damage-response markers [( DNA-PKcs, Mre11 ( P < 0.01), Xrcc4 ( P = 0.02), and γ-H2ax ( P < 0.001], suggesting failure of earlier compensatory responses. Our results suggest that low birth weight with rapid postnatal growth results in premature thymic maturation, resulting in accelerated thymic aging. This could lead to increased age-associated vulnerability to infection.-Tarry-Adkins, J. L., Aiken, C. E., Ashmore, T. J., Fernandez-Twinn, D. S., Chen, J.-H., Ozanne, S. E. A suboptimal maternal diet combined with accelerated postnatal growth results in an altered aging profile in the thymus of male rats.

Bioflavonoids protect cells against halogenated boroxine-induced genotoxic damage by upregulation of hTERT expression

Plant bioflavonoids are widely present in the human diet and have various protective properties. In this study, we have demonstrated the capacity of delphinidin and luteolin to increase human telomerase reverse transcriptase (hTERT) expression level and act as protective agents against halogenated boroxine-induced genotoxic damage. Halogenated boroxine K2(B3O3F4OH) (HB), is a novel compound with potential for the treatment of both benign and malignant skin changes. In vivo and in vitro studies have confirmed the inhibitory effects of HB on carcinoma cell proliferation and cell cycle progression as well as enzyme inhibition. However, minor genotoxic effects of HB are registered in higher applied concentrations, but those can be suppressed by in vitro addition of delphinidin and luteolin in appropriate concentrations. Fresh peripheral blood samples were cultivated for 72 h followed by independent and concomitant treatments of HB with luteolin or delphinidin. We analyzed the differences in relative hTERT expression between series of treatments compared with controls, which were based on normalized ratios with housekeeping genes. The obtained results have shown that selected bioflavonoids induce upregulation of hTERT that may contribute to the repair of genotoxic damage in vitro.

Age-related response to an acute innate immune challenge in mice: proteomics reveals a telomere maintenance-related cost

Ageing is characterized by the impairment of the acute innate immune response and the upregulation of low-grade inflammation, i.e. inflammaging. At the cellular level, telomeres are considered as a marker of biological ageing as their length is progressively eroded in the absence of repair mechanisms. However, the link between telomeres and inflammaging remains underexplored. We aimed to identify proteins that are differentially expressed between age classes in response to an acute inflammatory challenge. We challenged young (two months) and old (12 months) C57BL/6 mice using bacterial lipopolysaccharide (LPS) and measured telomere length and proteomic profiles in splenocytes. In total, 233 out of the 1966 proteins we quantified differed among experimental groups. A hierarchical clustering analysis revealed that nine of those 233 proteins were differently expressed among the experimental groups. Young mice responded to LPS by increasing the expression of proteins involved in the innate immune response, and interestingly, in telomere length maintenance. However, this regulation was impaired at older ages. These results are in agreement with the assumption that the strength of selection declines with age, potentially explaining the maintenance of costly, dysregulated, immune responses at old age. We suggest that the immune response is competing with the telomere maintenance process, highlighting how telomeres reflect the ageing trade-off even in a species where telomere length is not related to lifespan.

Involvement of telomerase activity inhibition and telomere dysfunction in silver nanoparticles anticancer effects

AIM

To investigate the possible mechanisms of telomerase and telomere underlying the anticancer effects of silver nanoparticles (AgNPs).

MATERIALS & METHODS

25nm polyvinylpyrrolidone-coated AgNPs were used. The telomerase activity and telomere function were evaluated. The anticancer effects of AgNPs were gauged with cell viability assay under different statement of telomerase and telomere.

RESULTS & CONCLUSION

AgNPs could inhibit telomerase activity and lead to telomere shortening and dysfunction. Overexpression of telomerase attenuated the anticancer activity of AgNPs, whereas downregulation of telomerase activity or dysfunction of the telomere enhanced the cytotoxicity of AgNPs in HeLa cells. Our findings provided strong evidence that the anticancer effects of AgNPs were mediated via interference with the telomerase/telomere.

p23 protects the human aryl hydrocarbon receptor from degradation via a heat shock protein 90-independent mechanism

The aryl hydrocarbon receptor (AHR) is a ligand-activated signaling molecule which is involved in diverse biological functions ranging from cancer metastasis to immune regulation. This receptor forms a cytoplasmic complex with Hsp90, p23, and XAP2. We have previously reported that down-regulation of p23 triggers degradation of the AHR protein, uncovering a potentially dynamic event which controls the cellular AHR levels without ligand treatment. Here we investigate the underlying mechanisms for this p23 effect using wild-type HeLa and the p23 knockdown HeLa cells. Reduction of the Hsp90 and XAP2 contents, however, did not affect the AHR protein levels, implying that this p23 effect on AHR is more than just alteration of the cytoplasmic complex dynamics. Association of p23 with Hsp90 is not important for the modulation of the AHR levels since exogenous expression of p23 mutants with modest Hsp90-binding affinity effectively restored the AHR message and protein levels. The protein folding property of p23 which resides at the terminal 50-amino acid region is not involved for this p23 effect. Results from our interaction study using the affinity purified thioredoxin fusion proteins and GST fusion proteins showed that p23 directly interacts with AHR and the interaction surface lies within AHR amino acid 1-216 and p23 amino acid 1-110. Down-regulation of the p23 protein content promotes the ubiquitination of AHR, indicating that p23 protects AHR from the ubiquitin-meditated protein degradation.