Trithorax requires Hsp90 for maintenance of active chromatin at sites of gene expression

Organ transformation by environmental disruption of protein integrity and epigenetic memory in Drosophila

Despite significant progress in understanding epigenetic reprogramming of cells, the mechanistic basis of "organ reprogramming" by (epi-)gene-environment interactions remained largely obscure. Here, we use the ether-induced haltere-to-wing transformations in Drosophila as a model for epigenetic "reprogramming" at the whole organism level. Our findings support a mechanistic chain of events explaining why and how brief embryonic exposure to ether leads to haltere-to-wing transformations manifested at the larval stage and on. We show that ether interferes with protein integrity in the egg, leading to altered deployment of Hsp90 and widespread repression of Trithorax-mediated establishment of active H3K4me3 chromatin marks throughout the genome. Despite this global reduction, Ubx targets and wing development genes preferentially retain higher levels of H3K4me3 that predispose these genes for later up-regulation in the larval haltere disc, hence the wing-like outcome. Consistent with compromised protein integrity during the exposure, the penetrance of bithorax transformations increases by genetic or chemical reduction of Hsp90 function. Moreover, joint reduction in Hsp90 and trx gene dosage can cause bithorax transformations without exposure to ether, supporting an underlying epistasis between Hsp90 and trx loss-of-functions. These findings implicate environmental disruption of protein integrity at the onset of histone methylation with altered epigenetic regulation of developmental patterning genes. The emerging picture provides a unique example wherein the alleviation of the Hsp90 "capacitor function" by the environment drives a morphogenetic shift towards an ancestral-like body plan. The morphogenetic impact of chaperone response during a major setup of epigenetic patterns may be a general scheme for organ transformation by environmental cues.

Potential Role of DNA Methylation as a Driver of Plastic Responses to the Environment Across Cells, Organisms, and Populations

There is great interest in exploring epigenetic modifications as drivers of adaptive organismal responses to environmental change. Extending this hypothesis to populations, epigenetically driven plasticity could influence phenotypic changes across environments. The canonical model posits that epigenetic modifications alter gene regulation and subsequently impact phenotypes. We first discuss origins of epigenetic variation in nature, which may arise from genetic variation, spontaneous epimutations, epigenetic drift, or variation in epigenetic capacitors. We then review and synthesize literature addressing three facets of the aforementioned model: (i) causal effects of epigenetic modifications on phenotypic plasticity at the organismal level, (ii) divergence of epigenetic patterns in natural populations distributed across environmental gradients, and (iii) the relationship between environmentally induced epigenetic changes and gene expression at the molecular level. We focus on DNA methylation, the most extensively studied epigenetic modification. We find support for environmentally associated epigenetic structure in populations and selection on stable epigenetic variants, and that inhibition of epigenetic enzymes frequently bears causal effects on plasticity. However, there are pervasive confounding issues in the literature. Effects of chromatin-modifying enzymes on phenotype may be independent of epigenetic marks, alternatively resulting from functions and protein interactions extrinsic of epigenetics. Associations between environmentally induced changes in DNA methylation and expression are strong in plants and mammals but notably absent in invertebrates and nonmammalian vertebrates. Given these challenges, we describe emerging approaches to better investigate how epigenetic modifications affect gene regulation, phenotypic plasticity, and divergence among populations.

Mask exhibits trxG-like behavior and associates with H3K27ac marked chromatin

The Trithorax group (trxG) proteins counteract the repressive effect of Polycomb group (PcG) complexes and maintain transcriptional memory of active states of key developmental genes. Although chromatin structure and modifications appear to play a fundamental role in this process, it is not clear how trxG prevents PcG-silencing and heritably maintains an active gene expression state. Here, we report a hitherto unknown role of Drosophila Multiple ankyrin repeats single KH domain (Mask), which emerged as one of the candidate trxG genes in our reverse genetic screen. The genome-wide binding profile of Mask correlates with known trxG binding sites across the Drosophila genome. In particular, the association of Mask at chromatin overlaps with CBP and H3K27ac, which are known hallmarks of actively transcribed genes by trxG. Importantly, Mask predominantly associates with actively transcribed genes in Drosophila. Depletion of Mask not only results in the downregulation of trxG targets but also correlates with diminished levels of H3K27ac. The fact that Mask positively regulates H3K27ac levels in flies was also found to be conserved in human cells. Strong suppression of Pc mutant phenotype by mutation in mask provides physiological relevance that Mask contributes to the anti-silencing effect of trxG, maintaining expression of key developmental genes. Since Mask is a downstream effector of multiple cell signaling pathways, we propose that Mask may connect cell signaling with chromatin mediated epigenetic cell memory governed by trxG.

Hsp70 and Hsp90 Elaborately Regulate RNAi Efficiency in Plutella xylostella

Heat-shock proteins (HSPs) serve as molecular chaperones in the RNA interference (RNAi) pathway of eukaryotic organisms. In model organisms, Hsp70 and Hsp90 facilitate the folding and remodeling of the client protein Argonaute (Ago). However, the specific function of HSPs in the RNAi pathway of Plutella xylostella (L.) (Lepidoptera: Plutellidae) remains unknown. In this study, we identified and analyzed the coding sequences of PxHsc70-4 and PxHsp83 (also known as PxHsp90). Both PxHsc70-4 and PxHsp83 exhibited three conserved domains that covered a massive portion of their respective regions. The knockdown or inhibition of PxHsc70-4 and PxHsp83 in vitro resulted in a significant increase in the gene expression of the dsRNA-silenced reporter gene PxmRPS18, leading to a decrease in its RNAi efficiency. Interestingly, the overexpression of PxHsc70-4 and PxHsp83 in DBM, Sf9, and S2 cells resulted in an increase in the bioluminescent activity of dsRNA-silenced luciferase, indicating a decrease in its RNAi efficiency via the overexpression of Hsp70/Hsp90. Furthermore, the inhibition of PxHsc70-4 and PxHsp83 in vivo resulted in a significant increase in the gene expression of PxmRPS18. These findings demonstrated the essential involvement of a specific quantity of Hsc70-4 and Hsp83 in the siRNA pathway in P. xylostella. Our study offers novel insights into the roles played by HSPs in the siRNA pathway in lepidopteran insects.

Impact of Heat Stress on Transposable Element Expression and Derived Small RNAs in Drosophila subobscura

Global warming is forcing insect populations to move and adapt, triggering adaptive genetic responses. Thermal stress is known to alter gene expression, repressing the transcription of active genes, and inducing others, such as those encoding heat shock proteins. It has also been related to the activation of some specific transposable element (TE) families. However, the actual magnitude of this stress on the whole genome and the factors involved in these genomic changes are still unclear. We studied mRNAs and small RNAs in gonads of two Drosophila subobscura populations, considered a good model to study adaptation to temperature changes. In control conditions, we found that a few genes and TE families were differentially expressed between populations, pointing out their putative involvement in the adaptation of populations to their different environments. Under heat stress, sex-specific changes in gene expression together with a trend toward overexpression, mainly of heat shock response-related genes, were observed. We did not observe large changes of TE expression nor small RNA production due to stress. Only population and sex-specific expression changes of some TE families (mainly retrotransposons), or the amounts of siRNAs and piRNAs, derived from specific TE families were observed, as well as the piRNA production from some piRNA clusters. Changes in small RNA amounts and TE expression could not be clearly correlated, indicating that other factors as chromatin modulation could also be involved. This work provides the first whole transcriptomic study including genes, TEs, and small RNAs after a heat stress in D. subobscura.

Phase separation promotes a highly active oligomeric scaffold of the MLL1 core complex for regulation of histone H3K4 methylation

Enzymes that regulate the degree of histone H3 lysine 4 (H3K4) methylation are crucial for proper cellular differentiation and are frequently mutated in cancer. The Mixed lineage leukemia (MLL) family of enzymes deposit H3K4 mono-, di-, or trimethylation at distinct genomic locations, requiring precise spatial and temporal control. Despite evidence that the degree of H3K4 methylation is controlled in part by a hierarchical assembly pathway with key subcomplex components, we previously found that the assembled state of the MLL1 core complex is not favored at physiological temperature. To better understand this paradox, we tested the hypothesis that increasing the concentration of subunits in a biomolecular condensate overcomes this thermodynamic barrier via mass action. Here, we demonstrate that MLL1 core complex phase separation stimulates enzymatic activity up to 60-fold but not primarily by concentrating subunits into droplets. Instead, we found that stimulated activity is largely due to the formation of an altered oligomeric scaffold that greatly reduces substrate Km. We posit that phase separation-induced scaffolding of the MLL1 core complex is a potential "switch-like" mechanism for spatiotemporal control of H3K4 methylation through the rapid formation or dissolution of biomolecular condensates within RNA Pol II transcription factories.

Chromatin Immunoprecipitation (ChIP) of Heat Shock Protein 90 (Hsp90)

Chromatin immunoprecipitation followed by sequencing (ChIP-seq) is a widely used technique for genome-wide mapping of protein-DNA interactions and epigenetic marks in vivo. Recent studies have suggested an important role of heat shock protein 90 (Hsp90) in chromatin. This molecular chaperone assists other proteins to acquire their mature and functional conformation and helps in the assembly of many complexes. In this chapter, we provide specific details on how to perform Hsp90 ChIP-seq from Drosophila Schneider (S2) cells. Briefly, cells are simultaneously lyzed and reversibly cross-linked to stabilize protein-DNA interactions. Chromatin is prepared from isolated nuclei and sheared by sonication. Hsp90-bound loci are immunoprecipitated and the corresponding DNA fragments are purified and sequenced. The described approach revealed that Hsp90 binds close to the transcriptional start site of around one-third of all Drosophila coding genes and characterized the role of the chaperone at chromatin.

Quantitative analysis of redox proteome reveals oxidation-sensitive protein thiols acting in fundamental processes of developmental hematopoiesis

Fetal and adult hematopoietic stem and progenitor cells (HSPCs) are characterized by distinct redox homeostasis that may influence their differential cellular behavior in normal and malignant hematopoiesis. In this work, we have applied a quantitative mass spectrometry-based redox proteomic approach to comprehensively describe reversible cysteine modifications in primary mouse fetal and adult HSPCs. We defined the redox state of 4,438 cysteines in fetal and adult HSPCs and demonstrated a higher susceptibility to oxidation of protein thiols in fetal HSPCs. Our data identified ontogenic changes to oxidation state of thiols in proteins with a pronounced role in metabolism and protein homeostasis. Additional redox proteomic analysis identified oxidation changes to thiols acting in mitochondrial respiration as well as protein homeostasis to be triggered during onset of MLL-ENL leukemogenesis in fetal HSPCs. Our data has demonstrated that redox signaling contributes to the regulation of fundamental processes of developmental hematopoiesis and has pinpointed potential targetable redox-sensitive proteins in in utero-initiated MLL-rearranged leukemia.

Direct targeted therapy for MLL-fusion-driven high-risk acute leukaemias

BACKGROUND

Improving the poor prognosis of infant leukaemias remains an unmet clinical need. This disease is a prototypical fusion oncoprotein-driven paediatric cancer, with MLL (KMT2A)-fusions present in most cases. Direct targeting of these driving oncoproteins represents a unique therapeutic opportunity. This rationale led us to initiate a drug screening with the aim of discovering drugs that can block MLL-fusion oncoproteins.

METHODS

A screen for inhibition of MLL-fusion proteins was developed that overcomes the traditional limitations of targeting transcription factors. This luciferase reporter-based screen, together with a secondary western blot screen, was used to prioritize compounds. We characterized the lead compound, disulfiram (DSF), based on its efficient ablation of MLL-fusion proteins. The consequences of drug-induced MLL-fusion inhibition were confirmed by cell proliferation, colony formation, apoptosis assays, RT-qPCR, in vivo assays, RNA-seq and ChIP-qPCR and ChIP-seq analysis. All statistical tests were two-sided.

RESULTS

Drug-induced inhibition of MLL-fusion proteins by DSF resulted in a specific block of colony formation in MLL-rearranged cells in vitro, induced differentiation and impeded leukaemia progression in vivo. Mechanistically, DSF abrogates MLL-fusion protein binding to DNA, resulting in epigenetic changes and down-regulation of leukaemic programmes setup by the MLL-fusion protein.

CONCLUSION

DSF can directly inhibit MLL-fusion proteins and demonstrate antitumour activity both in vitro and in vivo, providing, to our knowledge, the first evidence for a therapy that directly targets the initiating oncogenic MLL-fusion protein.

Is there a role for HSF1 in viral infections?

Cells undergo numerous processes to adapt to new challenging conditions and stressors. Heat stress is regulated by a family of heat shock factors (HSFs) that initiate a heat shock response by upregulating the expression of heat shock proteins (HSPs) intended to counteract cellular damage elicited by increased environmental temperature. Heat shock factor 1 (HSF1) is known as the master regulator of the heat shock response and upon its activation induces the transcription of genes that encode for molecular chaperones, such as HSP40, HSP70, and HSP90. Importantly, an accumulating body of studies relates HSF1 with viral infections; the induction of fever during viral infection may activate HSF1 and trigger a consequent heat shock response. Here, we review the role of HSF1 in different viral infections and its impact on the health outcome for the host. Studying the relationship between HSF1 and viruses could open new potential therapeutic strategies given the availability of drugs that regulate the activation of this transcription factor.

Heat Shock Proteins and Ferroptosis

Ferroptosis is a new form of regulatory cell death named by Dixon in 2012, which is characterized by the accumulation of lipid peroxides and iron ions. Molecular chaperones are a class of evolutionarily conserved proteins in the cytoplasm. They recognize and bind incompletely folded or assembled proteins to help them fold, transport or prevent their aggregation, but they themselves do not participate in the formation of final products. As the largest number of molecular chaperones, heat shock proteins can be divided into five families: HSP110 (HSPH), HSP90 (HSPC), HSP70 (HSPA), HSP40 (DNAJ) and small heat shock proteins (HSPB). Different heat shock proteins play different roles in promoting or inhibiting ferroptosis in different diseases. It is known that ferroptosis is participated in tumors, nervous system diseases, renal injury and ischemia-reperfusion injury. However, there are few reviews about the relationship of heat shock proteins and ferroptosis. In this study, we systematically summarize the roles of heat shock proteins in the occurrence of ferroptosis, and predict the possible mechanisms of different families of heat shock proteins in the development of ferroptosis.

Polycomb Requires Chaperonin Containing TCP-1 Subunit 7 for Maintaining Gene Silencing in Drosophila

In metazoans, heritable states of cell type-specific gene expression patterns linked with specialization of various cell types constitute transcriptional cellular memory. Evolutionarily conserved Polycomb group (PcG) and trithorax group (trxG) proteins contribute to the transcriptional cellular memory by maintaining heritable patterns of repressed and active expression states, respectively. Although chromatin structure and modifications appear to play a fundamental role in maintenance of repression by PcG, the precise targeting mechanism and the specificity factors that bind PcG complexes to defined regions in chromosomes remain elusive. Here, we report a serendipitous discovery that uncovers an interplay between Polycomb (Pc) and chaperonin containing T-complex protein 1 (TCP-1) subunit 7 (CCT7) of TCP-1 ring complex (TRiC) chaperonin in Drosophila. CCT7 interacts with Pc at chromatin to maintain repressed states of homeotic and non-homeotic targets of PcG, which supports a strong genetic interaction observed between Pc and CCT7 mutants. Depletion of CCT7 results in dissociation of Pc from chromatin and redistribution of an abundant amount of Pc in cytoplasm. We propose that CCT7 is an important modulator of Pc, which helps Pc recruitment at chromatin, and compromising CCT7 can directly influence an evolutionary conserved epigenetic network that supervises the appropriate cellular identities during development and homeostasis of an organism.

The Nutrient-Responsive Molecular Chaperone Hsp90 Supports Growth and Development in Drosophila

Animals can sense internal nutrients, such as amino acids/proteins, and are able to modify their developmental programs in accordance with their nutrient status. In the fruit fly, Drosophila melanogaster, amino acid/protein is sensed by the fat body, an insect adipose tissue, through a nutrient sensor, target of rapamycin (TOR) complex 1 (TORC1). TORC1 promotes the secretion of various peptide hormones from the fat body in an amino acid/protein-dependent manner. Fat-body-derived peptide hormones stimulate the release of insulin-like peptides, which are essential growth-promoting anabolic hormones, from neuroendocrine cells called insulin-producing cells (IPCs). Although the importance of TORC1 and the fat body-IPC axis has been elucidated, the mechanism by which TORC1 regulates the expression of insulinotropic signal peptides remains unclear. Here, we show that an evolutionarily conserved molecular chaperone, heat shock protein 90 (Hsp90), promotes the expression of insulinotropic signal peptides. Fat-body-selective Hsp90 knockdown caused the transcriptional downregulation of insulinotropic signal peptides. IPC activity and systemic growth were also impaired in fat-body-selective Hsp90 knockdown animals. Furthermore, Hsp90 expression depended on protein/amino acid availability and TORC1 signaling. These results strongly suggest that Hsp90 serves as a nutrient-responsive gene that upregulates the fat body-IPC axis and systemic growth. We propose that Hsp90 is induced in a nutrient-dependent manner to support anabolic metabolism during the juvenile growth period.

High-fat diet-induced obesity amplifies HSP70-2a and HSP90 expression in testicular tissue; correlation with proliferating cell nuclear antigen (PCNA)

AIMS

Current study was conducted to uncover the effect of high-fat diet (HFD)-induced obesity on heat shock proteins 70-2a and 90 expression levels and to investigate the network between these proteins with PCNA expression, endocrine status of testicular tissue and nucleotide backbone damages.

MAIN METHODS

For this purpose, 20 mature male Wistar rats were divided into two groups of control and HFD-received obese animals (n = 10/group). After 8 weeks from obesity approval, the animals were euthanized. The expression levels of Hsp70-2a, Hsp90 and PCNA were analyzed by qRT-PCR and immunohistochemical staining techniques. The Leydig cell distribution/mm² of interstitial tissue, serum level of testosterone, testicular total antioxidant capacity (TAC), and mRNA and DNA damage were investigated.

KEY FINDINGS

The obese (HFD-received) animals represented a remarkable (p < 0.05) increment in the mRNA levels of hsp70-2a and Hsp90, and the percentages of Hsp70-2a⁺ and Hsp90⁺ cells/seminiferous tubules with the same criteria. The PCNA mRNA level and the percentage of PCNA⁺ cells were decreased in the obese (HFD-received) group. The obesity, significantly decreased testicular TAC and with no effect on the Leydig cell distribution, but by reducing their steroidogenic activity resulted in a remarkable (p < 0.05) reduction in serum testosterone level. Finally, severe mRNA and DNA damage were revealed in the obese (HFD-received) group.

SIGNIFICANCE

Therefore, considering massive testicular DNA damage in the obese (HFD-received) animals, we can conclude that an increased expression of Hsp70-2a and Hsp90 with no harmony with PCNA could not properly maintain the cellular DNA integrity and/or appropriately finalize the DNA repair process.

Probing the evolutionary history of epigenetic mechanisms: what can we learn from marine diatoms

Recent progress made on epigenetic studies revealed the conservation of epigenetic features in deep diverse branching species including Stramenopiles, plants and animals. This suggests their fundamental role in shaping species genomes across different evolutionary time scales. Diatoms are a highly successful and diverse group of phytoplankton with a fossil record of about 190 million years ago. They are distantly related from other super-groups of Eukaryotes and have retained some of the epigenetic features found in mammals and plants suggesting their ancient origin. Phaeodactylum tricornutum and Thalassiosira pseudonana, pennate and centric diatoms, respectively, emerged as model species to address questions on the evolution of epigenetic phenomena such as what has been lost, retained or has evolved in contemporary species. In the present work, we will discuss how the study of non-model or emerging model organisms, such as diatoms, helps understand the evolutionary history of epigenetic mechanisms with a particular focus on DNA methylation and histone modifications.

The Role of HSP90 in Preserving the Integrity of Genomes Against Transposons Is Evolutionarily Conserved

The HSP90 protein is a molecular chaperone intensively studied for its role in numerous cellular processes both under physiological and stress conditions. This protein acts on a wide range of substrates with a well-established role in cancer and neurological disorders. In this review, we focused on the involvement of HSP90 in the silencing of transposable elements and in the genomic integrity maintenance. The common feature of transposable elements is the potential jumping in new genomic positions, causing chromosome structure rearrangements, gene mutations, and influencing gene expression levels. The role of HSP90 in the control of these elements is evolutionarily conserved and opens new perspectives in the HSP90-related mechanisms underlying human disorders. Here, we discuss the hypothesis that its role in the piRNA pathway regulating transposons may be implicated in the onset of neurological diseases.

The molecular mechanism behind insulin protective effects on testicular tissue of hyperglycemic rats

AIMS

The present study assessed the possible mechanisms by which the insulin regulates the heat shock (HSPs) and transitional proteins expression and consequently ameliorates the oxidative stress-induced damages in germ and sperm cells DNA contents.

MAIN METHODS

Mature male Wistar rats were distributed into control, Hyperglycemia-induced (HG) and insulin-treated HG-induced (HG-I) groups. Following 8 weeks from HG induction, testicular total antioxidant capacity (TAC), immunoreactivity of 8-oxodG, germ cells mRNA damage, Hsp70-2a, Hsp90, transitional proteins 1 and 2 (TP-1 and -2) mRNA and protein expressions were analyzed. Moreover, the sperm chromatin condensation was assessed by aniline-blue staining, and DNA integrity of germ and sperm cells were analyzed by TUNEL and acrdine-orange staining techniques.

KEY FINDINGS

The HG animals exhibited significant (p < 0.05) reduction in TAC, HSp70-2a, TP-1 and TP-2 expression levels, and increment in 8-oxodG immunoreactivity, mRNA damage, and Hsp90 expression. However, insulin treatment resulted in (p < 0.05) enhanced TAC level, Hsp70-2a, Hsp90, TP-1 and TP-2 expressions, besides reduced 8-oxodG immunoreactivity and mRNA damage compared to the HG group (p < 0.05). The chromatin condensation and the germ and sperm cells DNA fragmentation were decreased in HG-I group.

SIGNIFICANCE

Insulin treatment amplifies the testicular TAC level, improves the Hsp70-2a, TP-1, and TP-2 expressions, and boosts the Hsp90-mediated role in DNA repairment process. Consequently, altogether could maintain the HG-induced DNA integrity in the testicular and sperm cells.

Kinome-Wide RNAi Screen Uncovers Role of Ballchen in Maintenance of Gene Activation by Trithorax Group in Drosophila

Polycomb group (PcG) and trithorax group (trxG) proteins are evolutionary conserved factors that contribute to cell fate determination and maintenance of cellular identities during development of multicellular organisms. The PcG maintains heritable patterns of gene silencing while trxG acts as anti-silencing factors by conserving activation of cell type specific genes. Genetic and molecular analysis has revealed extensive details about how different PcG and trxG complexes antagonize each other to maintain cell fates, however, the cellular signaling components that contribute to the preservation of gene expression by PcG/trxG remain elusive. Here, we report an ex vivo kinome-wide RNAi screen in Drosophila aimed at identifying cell signaling genes that facilitate trxG in counteracting PcG mediated repression. From the list of trxG candidates, Ballchen (BALL), a histone kinase known to phosphorylate histone H2A at threonine 119 (H2AT119p), was characterized as a trxG regulator. The ball mutant exhibits strong genetic interactions with Polycomb (Pc) and trithorax (trx) mutants and loss of BALL affects expression of trxG target genes. BALL co-localizes with Trithorax on chromatin and depletion of BALL results in increased H2AK118 ubiquitination, a histone mark central to PcG mediated gene silencing. Moreover, BALL was found to substantially associate with known TRX binding sites across the genome. Genome wide distribution of BALL also overlaps with H3K4me3 and H3K27ac at actively transcribed genes. We propose that BALL mediated signaling positively contributes to the maintenance of gene activation by trxG in counteracting the repressive effect of PcG.

Febrile temperature causes transcriptional downregulation of Plasmodium falciparum Sirtuins through Hsp90-dependent epigenetic modification

Sirtuins (PfSIR2A and PfSIR2B) are implicated to play pivotal roles in the silencing of sub-telomeric genes and the maintenance of telomere length in P. falciparum 3D7 strain. Here, we identify the key factors that regulate the cellular abundance and activity of these two histone deacetylases. Our results demonstrate that PfSIR2A and PfSIR2B are transcriptionally downregulated at the mid-ring stage in response to febrile temperature. We found that the molecular chaperone PfHsp90 acts as a repressor of PfSIR2A & B transcription. By virtue of its presence in the PfSIR2A & B promoter proximal regions PfHsp90 helps recruiting H3K9me3, conferring heterochromatic state, and thereby leading to the downregulation of PfSIR2A & B transcription. Such transcriptional downregulation can be reversed by the addition of 17-(allylamino)-17-demethoxygeldanamycin or Radicicol, two potent inhibitors of PfHsp90. The reduced occupancy of PfSir2 at sub-telomeric var promoters leads to the de-repression of var genes. Thus, here we uncover how exposure to febrile temperature, a hallmark of malaria, enables the parasites to manipulate the expression of the two prominent epigenetic modifiers PfSir2A and PfSir2B.

Heat-Shock Protein 90 Controls the Expression of Cell-Cycle Genes by Stabilizing Metazoan-Specific Host-Cell Factor HCFC1

Molecular chaperones such as heat-shock proteins (HSPs) help in protein folding. Their function in the cytosol has been well studied. Notably, chaperones are also present in the nucleus, a compartment where proteins enter after completing de novo folding in the cytosol, and this raises an important question about chaperone function in the nucleus. We performed a systematic analysis of the nuclear pool of heat-shock protein 90. Three orthogonal and independent analyses led us to the core functional interactome of HSP90. Computational and biochemical analyses identify host cell factor C1 (HCFC1) as a transcriptional regulator that depends on HSP90 for its stability. HSP90 was required to maintain the expression of HCFC1-targeted cell-cycle genes. The regulatory nexus between HSP90 and the HCFC1 module identified in this study sheds light on the relevance of chaperones in the transcription of cell-cycle genes. Our study also suggests a therapeutic avenue of combining chaperone and transcription inhibitors for cancer treatment.

Testosterone amplifies HSP70-2a, HSP90 and PCNA expression in experimental varicocele condition: Implication for DNA fragmentation

The DNA fragmentation and failure in post-meiotic maturation of the spermatozoa because of testosterone withdrawal can affect the fertilization potential in varicocele (VCL) patients. To find out the exact mechanism of VCL-induced failure in histone-protamine replacement process and DNA fragmentation, the correlations between the levels of expression of HSP70-2a, HSP90, PCNA, TP1/2 and PCNA genes and the patterns of DNA methylation were investigated before and after testosterone administration in rats. In total, 40 mature male Wistar rats (10 in each group) were assigned between control (with no intervention), control-sham (undergone a simple laparotomy), VCL-induced (VCL-sole), and testosterone-treated VCL-induced (VCLT) groups. The HSP70-2a, HSP90, PCNA, TP1, and TP2 genes expressions and the patterns of global DNA methylation were determined in all groups. A statistically significant (p < 0.05) reduction were found in the HSP70-2a, HSP90, PCNA, TP1 and TP2 genes expressions in VCL-sole group. In VCLT group, testosterone was shown to significantly (p < 0.05) up-regulate the HSP70-2a, HSP90, PCNA, and TP2expression levels, but TP1 expression has not been changed. Furthermore, the VCLT group exhibited higher DNA methylation rates compared to VCL-sole animals. In conclusion, testosterone, by up-regulating the HSP70-2a and HSP90 expressions and maintaining the pre-existing HSP70-2a and HSP90 proteins levels, may be the reason for the significant increment in TP2 expression during post-meiotic stage and can boost the global methylation rates of DNA via up-regulating the PCNA expression, suggesting that administration of testosterone can mitigate the VCL-impaired histone-protamine replacement and DNA methylation rates and protect the cellular DNA content from VCL-induced oxidative stress.

Drosophila Morgana is an Hsp90-interacting protein with a direct role in microtubule polymerisation

Morgana (Mora, also known as CHORD in flies) and its mammalian homologue, called CHORDC1 or CHP1, is a highly conserved cysteine and histidine-rich domain (CHORD)-containing protein that has been proposed to function as an Hsp90 co-chaperone. Morgana deregulation promotes carcinogenesis in both mice and humans while, in Drosophila, loss of mora causes lethality and a complex mitotic phenotype that is rescued by a human morgana transgene. Here, we show that Drosophila Mora localises to mitotic spindles and co-purifies with the Hsp90-R2TP-TTT supercomplex and with additional well-known Hsp90 co-chaperones. Acute inhibition of Mora function in the early embryo results in a dramatic reduction in centrosomal microtubule stability, leading to small spindles nucleated from mitotic chromatin. Purified Mora binds to microtubules directly and promotes microtubule polymerisation in vitro, suggesting that Mora directly regulates spindle dynamics independently of its Hsp90 co-chaperone role.

Active BR signalling adjusts the subcellular localisation of BES1/HSP90 complex formation

Heat shock proteins 90 (HSP90) are essential and play critical roles in the adaptation of organisms to diverse stimuli. In plants, HSP90 are involved in auxin, jasmonate and brassinosteroid (BR) signalling pathways. The BR-promoted activation of the BES1 transcription factor regulates BR-responsive genes. Using genetic, physiological, fluorescence live cell imaging, molecular and biochemical approaches, such as phenotypic analysis, co-immunoprecipitation assay, yeast-two hybrid and Bimolecular fluorescence complementation (BiFC), we studied complex formation between BES1 and HSP90 under control conditions and active BR signalling. Further, we determined the effect of the pharmacological inhibition of HSP90 ATPase activity on hypocotyl elongation of bes1-D mutant. We determined that HSP90 interact with BES1 in the nucleus and in the cytoplasm. During active BR signalling, nuclear complexes were absent while cytoplasmic HSP90/BES1 complexes were prominent. Our results showed that the hypocotyl length of bes1-D mutants was highly reduced when HSP90 was challenged by the geldanamycin (GDA) inhibitor of the ATPase activity of HSP90. Active BR signalling could not rescue the GDA effect on the hypocotyl elongation of bes1-D. Our results reveal that the constitutively active BES1 in the bes1-D mutant is hypersensitive to GDA. The interaction of HSP90 with BES1 argues that HSP90 facilitate the nuclear metastable conformation of BES1 to regulate BR-dependent gene expression, and our data show that HSP90 assist in the compartmentalised cycle of BES1 during active BR signalling.

The Hsp90 Molecular Chaperone Regulates the Transcription Factor Network Controlling Chromatin Accessibility

Genomic events including gene regulation and chromatin status are controlled by transcription factors. Here we report that the Hsp90 molecular chaperone broadly regulates the transcription factor protein family. Our studies identified a biphasic use of Hsp90 in which early inactivation (15 min) of the chaperone triggered a wide reduction of DNA binding events along the genome with concurrent changes to chromatin structure. Long-term loss (6 h) of Hsp90 resulted in a decline of a divergent yet overlaying pool of transcription factors that produced a distinct chromatin pattern. Although both phases involve protein folding, the early point correlated with Hsp90 acting in a late folding step that is critical for DNA binding function, whereas prolonged Hsp90 inactivation led to a significant decrease in the steady-state transcription factor protein levels. Intriguingly, despite the broad chaperone impact on a variety of transcription factors, the operational influence of Hsp90 was at the level of chromatin with only a mild effect on gene regulation. Thus, Hsp90 selectively governs the transcription factor process overseeing local chromatin structure.

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.

Genome-wide RNAi screen in Drosophila reveals Enok as a novel trithorax group regulator

Background

Polycomb group (PcG) and trithorax group (trxG) proteins contribute to the specialization of cell types by maintaining differential gene expression patterns. Initially discovered as positive regulators of HOX genes in forward genetic screens, trxG counteracts PcG-mediated repression of cell type-specific genes. Despite decades of extensive analysis, molecular understanding of trxG action and regulation are still punctuated by many unknowns. This study aimed at discovering novel factors that elicit an anti-silencing effect to facilitate trxG-mediated gene activation.

Results

We have developed a cell-based reporter system and performed a genome-wide RNAi screen to discover novel factors involved in trxG-mediated gene regulation in Drosophila. We identified more than 200 genes affecting the reporter in a manner similar to trxG genes. From the list of top candidates, we have characterized Enoki mushroom (Enok), a known histone acetyltransferase, as an important regulator of trxG in Drosophila. Mutants of enok strongly suppressed extra sex comb phenotype of Pc mutants and enhanced homeotic transformations associated with trx mutations. Enok colocalizes with both TRX and PC at chromatin. Moreover, depletion of Enok specifically resulted in an increased enrichment of PC and consequently silencing of trxG targets. This downregulation of trxG targets was also accompanied by a decreased occupancy of RNA-Pol-II in the gene body, correlating with an increased stalling at the transcription start sites of these genes. We propose that Enok facilitates trxG-mediated maintenance of gene activation by specifically counteracting PcG-mediated repression.

Conclusion

Our ex vivo approach led to identification of new trxG candidate genes that warrant further investigation. Presence of chromatin modifiers as well as known members of trxG and their interactors in the genome-wide RNAi screen validated our reverse genetics approach. Genetic and molecular characterization of Enok revealed a hitherto unknown interplay between Enok and PcG/trxG system. We conclude that histone acetylation by Enok positively impacts the maintenance of trxG-regulated gene activation by inhibiting PRC1-mediated transcriptional repression.

A role for epigenetic adaption in evolution

The outcome of epigenetic responses to stress depends strictly on genetic background, suggesting that altered phenotypes, when induced, are created by a combination of induced epigenetic factors and pre-existing allelic ones. When individuals with altered phenotypes are selected and subjected to successive breeding, alleles that potentiate epigenetic responses could accumulate in offspring populations. It is reasonable to suppose that many, if not all, of these allelic genes could also be involved in creating new phenotypes under nonstressful conditions. In this review, I discuss the possibility that the accumulation of such alleles in selected individuals with an epigenetic phenotype could give rise to individuals that exhibit the same phenotype even in the absence of stress.

Over-expression of Hsp83 in grossly depleted hsrω lncRNA background causes synthetic lethality and l(2)gl phenocopy in Drosophila

We examined interactions between the 83 kDa heat-shock protein (Hsp83) and hsrω long noncoding RNAs (lncRNAs) in hsrω⁶⁶ Hsp90GFP homozygotes, which almost completely lack hsrω lncRNAs but over-express Hsp83. All +/+; hsrω⁶⁶ Hsp90GFP progeny died before the third instar. Rare Sp/CyO; hsrω⁶⁶ Hsp90GFP reached the third instar stage but phenocopied l(2)gl mutants, becoming progressively bulbous and transparent with enlarged brain and died after prolonged larval life. Additionally, ventral ganglia too were elongated. However, hsrω⁶⁶ Hsp90GFP/TM6B heterozygotes, carrying +/+ or Sp/CyO second chromosomes, developed normally. Total RNA sequencing (+/+, +/+; hsrω66/hsrω⁶⁶, Sp/CyO; hsrω⁶⁶/ hsrω⁶⁶, +/+; Hsp90GFP/Hsp90GFP and Sp/CyO; hsrω66 Hsp90GFP/hsrω66 Hsp90GFP late third instar larvae) revealed similar effects on many genes in hsrω66 and Hsp90GFP homozygotes. Besides additive effect on many of them, numerous additional genes were affected in Sp/CyO; hsrω66 Hsp90GFP larvae, with l(2)gl and several genes regulating the central nervous system being highly down-regulated in surviving Sp/CyO; hsrω66 Hsp90GFP larvae, but not in hsrω⁶⁶ or Hsp90GFP single mutants. Hsp83 and several omega speckle-associated hnRNPs were bioinformatically found to potentially bind with these gene promoters and transcripts. Since Hsp83 and hnRNPs are also known to interact, elevated Hsp83 in an altered background of hnRNP distribution and dynamics, due to near absence of hsrω lncRNAs and omega speckles, can severely perturb regulatory circuits with unexpected consequences, including down-regulation of tumoursuppressor genes such as l(2)gl.

HSP90: Enabler of Cancer Adaptation

The stability and function of many oncogenic mutant proteins depend on heat shock protein 90 (HSP90). This unique activity has inspired the exploration of HSP90 as an anticancer target for over two decades. Unfortunately, while clinical trials of highly optimized HSP90 inhibitors have demonstrated modest benefit for patients with advanced cancers, most commonly stabilization of disease, no HSP90 inhibitor has demonstrated sufficient efficacy to achieve FDA approval to date. This review discusses potential reasons for the limited success of these agents and how our increasingly sophisticated understanding of HSP90 suggests alternative, potentially more effective strategies for targeting it to treat cancers. First, we focus on insights gained from model organisms that suggest a fundamental role for HSP90 in supporting the adaptability and heterogeneity of cancers, key factors underlying their ability to evolve and acquire drug resistance. Second, we examine how HSP90’s role in promoting the stability of mutant proteins might be targeted in genetically unstable tumor cells to reveal their aberrant, foreign proteome to the immune system. Both of these emerging aspects of HSP90 biology suggest that the most effective use of HSP90 inhibitors may not be at high doses with the intent to kill cancer cells, but rather in combination with other molecularly targeted therapies at modest, non-heat shock-inducing exposures that limit the adaptive capacity of cancers.

Targeting the MYC Oncogene in Burkitt Lymphoma through HSP90 Inhibition

Overexpression of the MYC oncogene is a key feature of many human malignancies including Burkitt lymphoma. While MYC is widely regarded to be a promising therapeutic target, a clinically effective MYC inhibitor is still elusive. Here, we report an alternative strategy, targeting MYC indirectly through inhibition of the HSP90 machinery. We found that inhibition of HSP90 function reduces MYC expression in human Burkitt lymphoma through suppression of MYC transcription and destabilization of MYC protein, thereby diminishing the proliferation of tumor cells. Consistently, treatment of Burkitt lymphoma cell lines with HSP90 inhibitors (17-AAG or 17-DMAG) was accompanied by downregulation of canonical MYC target genes. Combination treatment with 17-DMAG and the proteasome inhibitor, MG-132, led to accumulation of MYC protein, indicating that upon HSP90 inhibition, MYC is degraded by the proteasome. Using co-immunoprecipitation, we furthermore demonstrated a direct interaction between MYC and HSP90, indicating that MYC is an HSP90 client protein in Burkitt lymphoma. Together, we report here the use of HSP90 inhibitors as an alternative approach to target the MYC oncogene and its network in Burkitt lymphoma.

Systemic signalling and local effectors in developmental stability, body symmetry, and size

Symmetric growth and the origins of fluctuating asymmetry are unresolved phenomena of biology. Small, and sometimes noticeable, deviations from perfect bilateral symmetry reflect the vulnerability of development to perturbations. The degree of asymmetry is related to the magnitude of the perturbations and the ability of an individual to cope with them. As the left and right sides of an individual were presumed to be genetically identical, deviations of symmetry were traditionally attributed to non-genetic effects such as environmental and developmental noise. In this review, we draw attention to other possible sources of variability, especially to somatic mutations and transposons. Mutations are a major source of phenotypic variability and recent genomic data have highlighted somatic mutations as ubiquitous, even in phenotypically normal individuals. We discuss the importance of factors that are responsible for buffering and stabilizing the genome and for maintaining size robustness and quality through elimination of less-fit or damaged cells. However, the important question that arises from these studies is whether this self-correcting capacity and intrinsic organ size controls are sufficient to explain how symmetric structures can reach an identical size and shape. Indeed, recent discoveries in the fruit fly have uncovered a conserved hormone of the insulin/IGF/relaxin family, Dilp8, that is responsible for stabilizing body size and symmetry in the face of growth perturbations. Dilp8 alarm signals periphery growth status to the brain, where it acts on its receptor Lgr3. Loss of Dilp8-Lgr3 signaling renders flies incapable of detecting growth perturbations and thus maintaining a stable size and symmetry. These findings help to understand how size and symmetry of somatic tissues remain undeterred in noisy environments, after injury or illnesses, and in the presence of accumulated somatic mutations.

Hsp83/Hsp90 Physically Associates with Insulin Receptor to Promote Neural Stem Cell Reactivation

Neural stem cells (NSCs) have the ability to exit quiescence and reactivate in response to physiological stimuli. In the Drosophila brain, insulin receptor (InR)/phosphatidylinositol 3-kinase (PI3K)/Akt pathway triggers NSC reactivation. However, intrinsic mechanisms that control the InR/PI3K/Akt pathway during reactivation remain unknown. Here, we have identified heat shock protein 83 (Hsp83/Hsp90), a molecular chaperone, as an intrinsic regulator of NSC reactivation. Hsp83 is both necessary and sufficient for NSC reactivation by promoting the activation of InR pathway in larval brains in the presence of dietary amino acids. Both Hsp83 and its co-chaperone Cdc37 physically associate with InR. Finally, reactivation defects observed in brains depleted of hsp83 were rescued by over-activation of the InR/PI3K/Akt pathway, suggesting that Hsp83 functions upstream of the InR/PI3K/Akt pathway during NSC reactivation. Given the conservation of Hsp83 and the InR pathway, our finding may provide insights into the molecular mechanisms underlying mammalian NSC reactivation.

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Hsp83/Hsp90 and its co-chaperone Cdc37 are required for NSC reactivation

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Hsp83 overexpression results in premature NSC reactivation on fed condition

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Hsp83 and Cdc37 physically associate with InR

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Hsp83 and Cdc37 are required for the activation of InR pathway in NSCs

Role of human oocyte-enriched factors in somatic cell reprograming

Cellular reprograming paves the way for creating functional patient-specific tissues to eliminate immune rejection responses by applying the same genetic profile. However, the epigenetic memory of a cell remains a challenge facing the current reprograming methods and does not allow transcription factors to bind properly. Because somatic cells can be reprogramed by transferring their nuclear contents into oocytes, introducing specific oocyte factors into differentiated cells is considered a promising approach for mimicking the reprograming process that occurs during fertilization. Mammalian metaphase II oocyte possesses a superior capacity to epigenetically reprogram somatic cell nuclei towards an embryonic stem cell-like state than the current factor-based reprograming approaches. This may be due to the presence of specific factors that are lacking in the current factor-based reprograming approaches. In this review, we focus on studies identifying human oocyte-enriched factors aiming to understand the molecular mechanisms mediating cellular reprograming. We describe the role of oocyte-enriched factors in metabolic switch, chromatin remodelling, and global epigenetic transformation. This is critical for improving the quality of resulting reprogramed cells, which is crucial for therapeutic applications.

It's not magic - Hsp90 and its effects on genetic and epigenetic variation

Canalization, or phenotypic robustness in the face of environmental and genetic perturbation, is an emergent property of living systems. Although this phenomenon has long been recognized, its molecular underpinnings have remained enigmatic until recently. Here, we review the contributions of the molecular chaperone Hsp90, a protein that facilitates the folding of many key regulators of growth and development, to canalization of phenotype - and de-canalization in times of stress - drawing on studies in eukaryotes as diverse as baker's yeast, mouse ear cress, and blind Mexican cavefish. Hsp90 is a hub of hubs that interacts with many so-called 'client proteins,' which affect virtually every aspect of cell signaling and physiology. As Hsp90 facilitates client folding and stability, it can epistatically suppress or enable the expression of genetic variants in its clients and other proteins that acquire client status through mutation. Hsp90's vast interaction network explains the breadth of its phenotypic reach, including Hsp90-dependent de novo mutations and epigenetic effects on gene regulation. Intrinsic links between environmental stress and Hsp90 function thus endow living systems with phenotypic plasticity in fluctuating environments. As environmental perturbations alter Hsp90 function, they also alter Hsp90's interaction with its client proteins, thereby re-wiring networks that determine the genotype-to-phenotype map. Ensuing de-canalization of phenotype creates phenotypic diversity that is not simply stochastic, but often has an underlying genetic basis. Thus, extreme phenotypes can be selected, and assimilated so that they no longer require environmental stress to manifest. In addition to acting on standing genetic variation, Hsp90 perturbation has also been linked to increased frequency of de novo variation and several epigenetic phenomena, all with the potential to generate heritable phenotypic change. Here, we aim to clarify and discuss the multiple means by which Hsp90 can affect phenotype and possibly evolutionary change, and identify their underlying common feature: at its core, Hsp90 interacts epistatically through its chaperone function with many other genes and their gene products. Its influence on phenotypic diversification is thus not magic but rather a fundamental property of genetics.

Implications of Epigenetic Mechanisms and their Targets in Cerebral Ischemia Models

BACKGROUND

Understanding the complexities associated with the ischemic condition and identifying therapeutic targets in ischemia is a continued challenge in stroke biology. Emerging evidence reveals the potential involvement of epigenetic mechanisms in the incident and outcome of stroke, suggesting novel therapeutic options of targeting different molecules related to epigenetic regulation.

OBJECTIVE

This review summarizes our current understanding of ischemic pathophysiology, describes various in vivo and in vitro models of ischemia, and examines epigenetic modifications associated with the ischemic condition.

METHOD

We focus on microRNAs, DNA methylation, and histone modifying enzymes, and present how epigenetic studies are revealing novel drug target candidates in stroke.

CONCLUSION

Finally, we discuss emerging approaches for the prevention and treatment of stroke and post-stroke effects using pharmacological interventions with a wide therapeutic window.

Gene expression regulation by heat-shock proteins: the cardinal roles of HSF1 and Hsp90

The ability to permit gene expression is managed by a set of relatively well known regulatory mechanisms. Nonetheless, this property can also be acquired during a life span as a consequence of environmental stimuli. Interestingly, some acquired information can be passed to the next generation of individuals without modifying gene information, but instead by the manner in which cells read and process such information. Molecular chaperones are classically related to the proper preservation of protein folding and anti-aggregation properties, but one of them, heat-shock protein 90 (Hsp90), is a refined sensor of protein function facilitating the biological activity of properly folded client proteins that already have a preserved tertiary structure. Interestingly, Hsp90 can also function as a critical switch able to regulate biological responses due to its association with key client proteins such as histone deacetylases or DNA methylases. Thus, a growing amount of evidence has connected the action of Hsp90 to post-translational modifications of soluble nuclear factors, DNA, and histones, which epigenetically affect gene expression upon the onset of an unfriendly environment. This response is commanded by the activation of the transcription factor heat-shock factor 1 (HSF1). Even though numerous stresses of diverse nature are known to trigger the stress response by activation of HSF1, it is still unknown whether there are different types of molecular sensors for each type of stimulus. In the present review, we will discuss various aspects of the regulatory action of HSF1 and Hsp90 on transcriptional regulation, and how this regulation may affect genetic assimilation mechanisms and the health of individuals.

Chromatin Immunoprecipitation (ChIP) of Heat Shock Protein 90 (Hsp90)

Chromatin immunoprecipitation followed by sequencing (ChIP-seq) is a widely used technique for genome-wide mapping of protein-DNA interactions and epigenetic marks in vivo. Recent studies have suggested an important role of heat shock protein 90 (Hsp90) at chromatin. This molecular chaperone assists other proteins to acquire their mature and functional conformation and helps in the assembly of many complexes. In this chapter, we provide specific details on how to perform Hsp90 ChIP-seq from Drosophila Schneider (S2) cells. Briefly, the cells are simultaneously lyzed and reversibly cross-linked to stabilize protein-DNA interactions. Chromatin is prepared from isolated nuclei and sheared by sonication. Hsp90-bound loci are immunoprecipitated and the corresponding DNA fragments are purified and sequenced. The described approach revealed that Hsp90 binds close to the transcriptional start site of around one-third of all Drosophila coding genes and characterized the role of the chaperone at chromatin.

Canalization by Selection of de Novo Induced Mutations

One of the most fascinating scientific problems, and a subject of intense debate, is that of the mechanisms of biological evolution. In this context, Waddington elaborated the concepts of "canalization and assimilation" to explain how an apparently somatic variant induced by stress could become heritable through the germline in Drosophila He resolved this seemingly Lamarckian phenomenon by positing the existence of cryptic mutations that can be expressed and selected under stress. To investigate the relevance of such mechanisms, we performed experiments following the Waddington procedure, then isolated and fixed three phenotypic variants along with another induced mutation that was not preceded by any phenocopy. All the fixed mutations we looked at were actually generated de novo by DNA deletions or transposon insertions, highlighting a novel mechanism for the assimilation process. Our study shows that heat-shock stress produces both phenotypic variants and germline mutations, and suggests an alternative explanation to that of Waddington for the apparent assimilation of an acquired character. The selection of the variants, under stress, for a number of generations allows for the coselection of newly induced corresponding germline mutations, making the phenotypic variants appear heritable.

Hsp90 induces increased genomic instability toward DNA-damaging agents by tuning down RAD53 transcription

The molecular mechanism behind hyperthermia coupled to radiation-induced DNA damage sensitivity is not known. The model organism Saccharomyces cerevisiae is used to establish that a transient heat shock and particularly the concomitant induction of Hsp90 lead to increased genomic instability via transcriptional regulation of the major checkpoint kinase Rad53.

It is well documented that elevated body temperature causes tumors to regress upon radiotherapy. However, how hyperthermia induces DNA damage sensitivity is not clear. We show that a transient heat shock and particularly the concomitant induction of Hsp90 lead to increased genomic instability under DNA-damaging conditions. Using Saccharomyces cerevisiae as a model eukaryote, we demonstrate that elevated levels of Hsp90 attenuate efficient DNA damage signaling and dictate preferential use of the potentially mutagenic double-strand break repair pathway. We show that under normal physiological conditions, Hsp90 negatively regulates RAD53 transcription to suppress DNA damage checkpoint activation. However, under DNA damaging conditions, RAD53 is derepressed, and the increased level of Rad53p triggers an efficient DNA damage response. A higher abundance of Hsp90 causes increased transcriptional repression on RAD53 in a dose-dependent manner, which could not be fully derepressed even in the presence of DNA damage. Accordingly, cells behave like a rad53 loss-of-function mutant and show reduced NHEJ efficiency, with a drastic failure to up-regulate RAD51 expression and manifestly faster accumulation of CLN1 and CLN2 in DNA-damaged G1, cells leading to premature release from checkpoint arrest. We further demonstrate that Rad53 overexpression is able to rescue all of the aforementioned deleterious effects caused by Hsp90 overproduction.

Hsp90: A Global Regulator of the Genotype-to-Phenotype Map in Cancers

Cancer cells have the unusual capacity to limit the cost of the mutation load that they harbor and simultaneously harness its evolutionary potential. This property fuels drug resistance, a key failure mode in oncogene-directed therapy. However, the factors that regulate this capacity might also provide an Achilles' heel that could be exploited therapeutically. Recently, insight has come from a seemingly distant field: protein folding. It is now clear that protein homeostasis broadly supports malignancy and fuels the rapid evolution of drug resistance. Among protein homeostatic mechanisms that influence cancer biology, the essential ATP-driven molecular chaperone heat-shock protein 90 (Hsp90) is especially important. Hsp90 catalyzes folding of many proteins that regulate growth and development. These "client" kinases, transcription factors, and ubiquitin ligases often play critical roles in human disease, especially cancer. Studies in a wide range of systems-from single-celled organisms to human tumor samples-suggest that Hsp90 can broadly reshape the map between genotype and phenotype, acting as a "capacitor" and "potentiator" of genetic variation. Indeed, it has likely done so to such a degree that it has left an impress on diverse genome sequences. Hsp90 can constitute as much as 5% of total protein in transformed cells and increased levels of heat-shock activation correlate with poor prognosis in breast cancer. These findings and others have motivated a flurry of interest in Hsp90 inhibitors as cancer therapeutics, which have met with rather limited success as single agents, but may eventually prove invaluable in limiting the emergence of resistance to other chemotherapeutics, both genotoxic and molecularly targeted. Here, we provide an overview of Hsp90 function, review its relationship to genetic variation and the evolution of new traits, and discuss the importance of these findings for cancer biology and future efforts to drug this pathway.

Hsp90 as a "Chaperone" of the Epigenome: Insights and Opportunities for Cancer Therapy

The cellular functions of Hsp90 have historically been attributed to its ability to chaperone client proteins involved in signal transduction. Although numerous stimuli and the signaling cascades they activate contribute to cancer progression, many of these pathways ultimately require transcriptional effectors to elicit tumor-promoting effects. Despite this obvious connection, the majority of studies evaluating Hsp90 function in malignancy have focused upon its regulation of cytosolic client proteins, and particularly members of receptor and/or kinase families. However, in recent years, Hsp90 has emerged as a pivotal orchestrator of nuclear events. Discovery of an expanding repertoire of Hsp90 clients has illuminated a vital role for Hsp90 in overseeing nuclear events and influencing gene transcription. Hence, this chapter will cast a spotlight upon several regulatory themes involving Hsp90-dependent nuclear functions. Highlighted topics include a summary of chaperone-dependent regulation of key transcription factors (TFs) and epigenetic effectors in malignancy, as well as a discussion of how the complex interplay among a subset of these TFs and epigenetic regulators may generate feed-forward loops that further support cancer progression. This chapter will also highlight less recognized indirect mechanisms whereby Hsp90-supported signaling may impinge upon epigenetic regulation. Finally, the relevance of these nuclear events is discussed within the framework of Hsp90's capacity to enable phenotypic variation and drug resistance. These newly acquired insights expanding our understanding of Hsp90 function support the collective notion that nuclear clients are major beneficiaries of Hsp90 action, and their impairment is likely responsible for many of the anticancer effects elicited by Hsp90-targeted approaches.

Contribution of the Type II Chaperonin, TRiC/CCT, to Oncogenesis

The folding of newly synthesized proteins and the maintenance of pre-existing proteins are essential in sustaining a living cell. A network of molecular chaperones tightly guides the folding, intracellular localization, and proteolytic turnover of proteins. Many of the key regulators of cell growth and differentiation have been identified as clients of molecular chaperones, which implies that chaperones are potential mediators of oncogenesis. In this review, we briefly provide an overview of the role of chaperones, including HSP70 and HSP90, in cancer. We further summarize and highlight the emerging the role of chaperonin TRiC (T-complex protein-1 ring complex, also known as CCT) in the development and progression of cancer mediated through its critical interactions with oncogenic clients that modulate growth deregulation, apoptosis, and genome instability in cancer cells. Elucidation of how TRiC modulates the folding and function of oncogenic clients will provide strategies for developing novel cancer therapies.

Hsp90 in Cancer: Transcriptional Roles in the Nucleus

Hsp90 plays a key role in fostering metabolic pathways essential in tumorigenesis through its functions as a molecular chaperone. Multiple oncogenic factors in the membrane and cytoplasm are thus protected from degradation and destruction. Here, we have considered Hsp90's role in transcription in the nucleus. Hsp90 functions both in regulating the activity of sequence-specific transcription factors such as nuclear receptors and HSF1, as well as impacting more globally acting factors that act on chromatin and RNA polymerase II. Hsp90 influences transcription by modulating histone modification mediated by its clients SMYD3 and trithorax/MLL, as well as by regulating the processivity of RNA polymerase II through negative elongation factor. It is not currently clear how the transcriptional role of Hsp90 may be influenced by the cancer milieu although recently discovered posttranslational modification of the chaperone may be involved. Dysregulation of Hsp90 may thus influence malignant processes both by modulating the function of specific transcription factors and effects on more globally acting general components of the transcriptional machinery.

Hsp90, the concertmaster: tuning transcription

In the last decade, Hsp90 has emerged as a major regulator of cancer cell growth and proliferation. In cancer cells, it assists in giving maturation to oncogenic proteins including several kinases and transcription factors (TF). Recent studies have shown that apart from its chaperone activity, it also imparts regulation of transcription machinery and thereby alters the cellular physiology. Hsp90 and its co-chaperones modulate transcription at least at three different levels. In the first place, they alter the steady-state levels of certain TFs in response to various physiological cues. Second, they modulate the activity of certain epigenetic modifiers, such as histone deacetylases or DNA methyl transferases, and thereby respond to the change in the environment. Third, they participate in the eviction of histones from the promoter region of certain genes and thereby turn on gene expression. In this review, we discuss the role of Hsp90 in all the three aforementioned mechanisms of transcriptional control, taking examples from various model organisms with a special emphasis on cancer progression.

Hsp90: a chaperone for HIV-1

HIV-1 replication has been intensively investigated over the past 30 years. Hsp90 is one of the most abundant proteins in human cells, important in the formation and function of several protein complexes that maintain cell homeostasis. Remarkably, the impact of Hsp90 on HIV-1 infection has started to be appreciated only recently. Hsp90 has been shown to (a) promote HIV-1 gene expression in acutely infected cells, (b) localize at the viral promoter DNA, (c) mediate enhanced replication in conditions of hyperthermia and (d) activate the P-TEFb complex, which is essential for efficient HIV-1 transcription. Hsp90 has been implicated in buffering deleterious mutations of the viral core and in the regulation of innate and acquired immune responses to HIV-1 infection. Therefore, Hsp90 is an important host factor promoting several steps of the HIV-1 life cycle. Several small Hsp90 inhibitors are in Phase II clinical trials for human cancers and might potentially be used to inhibit HIV-1 infection at multiple levels.

Epigenetics as an answer to Darwin's "special difficulty," Part 2: natural selection of metastable epialleles in honeybee castes

In a recent perspective in this journal, Herb (2014) discussed how epigenetics is a possible mechanism to circumvent Charles Darwin's "special difficulty" in using natural selection to explain the existence of the sterile-fertile dimorphism in eusocial insects. Darwin's classic book "On the Origin of Species by Means of Natural Selection" explains how natural selection of the fittest individuals in a population can allow a species to adapt to a novel or changing environment. However, in bees and other eusocial insects, such as ants and termites, there exist two or more castes of genetically similar females, from fertile queens to multiple sub-castes of sterile workers, with vastly different phenotypes, lifespans, and behaviors. This necessitates the selection of groups (or kin) rather than individuals in the evolution of honeybee hives, but group and kin selection theories of evolution are controversial and mechanistically uncertain. Also, group selection would seem to be prohibitively inefficient because the effective population size of a colony is reduced from thousands to a single breeding queen. In this follow-up perspective, we elaborate on possible mechanisms for how a combination of both epigenetics, specifically, the selection of metastable epialleles, and genetics, the selection of mutations generated by the selected metastable epialleles, allows for a combined means for selection amongst the fertile members of a species to increase colony fitness. This "intra-caste evolution" hypothesis is a variation of the epigenetic directed genetic error hypothesis, which proposes that selected metastable epialleles increase genetic variability by directing mutations specifically to the epialleles. Natural selection of random metastable epialleles followed by a second round of natural selection of random mutations generated by the metastable epialleles would allow a way around the small effective population size of eusocial insects.

Alcohol and HCV: implications for liver cancer

Liver cancers are one of the deadliest known malignancies which are increasingly becoming a major public health problem in both developed and developing countries. Overwhelming evidence suggests a strong role of infection with hepatitis B and C virus (HBV and HCV), alcohol abuse, as well as metabolic diseases such as obesity and diabetes either individually or synergistically to cause or exacerbate the development of liver cancers. Although numerous etiologic mechanisms for liver cancer development have been advanced and well characterized, the lack of definite curative treatments means that gaps in knowledge still exist in identifying key molecular mechanisms and pathways in the pathophysiology of liver cancers. Given the limited success with current therapies and preventive strategies against liver cancer, there is an urgent need to identify new therapeutic options for patients. Targeting HCV and or alcohol-induced signal transduction, or virus-host protein interactions may offer novel therapies for liver cancer. This review summarizes current knowledge on the mechanistic development of liver cancer associated with HCV infection and alcohol abuse as well as highlights potential novel therapeutic strategies.