Regulation of heat shock transcription factor 1 by stress-induced SUMO-1 modification

Unveiling the impact of SUMOylation at K298 site of heat shock factor 1 on glioblastoma malignant progression

BACKGROUND

Glioblastoma (GBM) poses a significant medical challenge due to its aggressive nature and poor prognosis. Mitochondrial unfolded protein response (UPRmt) and the heat shock factor 1 (HSF1) pathway play crucial roles in GBM pathogenesis. Post-translational modifications, such as SUMOylation, regulate the mechanism of action of HSF1 and may influence the progression of GBM. Understanding the interplay between SUMOylation-modified HSF1 and GBM pathophysiology is essential for developing targeted therapies.

METHODS

We conducted a comprehensive investigation using cellular, molecular, and in vivo techniques. Cell culture experiments involved establishing stable cell lines, protein extraction, Western blotting, co-immunoprecipitation, and immunofluorescence analysis. Mass spectrometry was utilized for protein interaction studies. Computational modeling techniques were employed for protein structure analysis. Plasmid construction and lentiviral transfection facilitated the manipulation of HSF1 SUMOylation. In vivo studies employed xenograft models for tumor growth assessment.

RESULTS

Our research findings indicate that HSF1 primarily undergoes SUMOylation at the lysine residue K298, enhancing its nuclear translocation, stability, and downstream heat shock protein expression, while having no effect on its trimer conformation. SUMOylated HSF1 promoted the UPRmt pathway, leading to increased GBM cell proliferation, migration, invasion, and reduced apoptosis. In vivo studies have confirmed that SUMOylation of HSF1 enhances its oncogenic effect in promoting tumor growth in GBM xenograft models.

CONCLUSION

This study elucidates the significance of SUMOylation modification of HSF1 in driving GBM progression. Targeting SUMOylated HSF1 may offer a novel therapeutic approach for GBM treatment. Further investigation into the specific molecular mechanisms influenced by SUMOylated HSF1 is warranted for the development of effective targeted therapies to improve outcomes for GBM patients.

Protein modification in neurodegenerative diseases

Posttranslational modifications play a crucial role in governing cellular functions and protein behavior. Researchers have implicated dysregulated posttranslational modifications in protein misfolding, which results in cytotoxicity, particularly in neurodegenerative diseases such as Alzheimer disease, Parkinson disease, and Huntington disease. These aberrant posttranslational modifications cause proteins to gather in certain parts of the brain that are linked to the development of the diseases. This leads to neuronal dysfunction and the start of neurodegenerative disease symptoms. Cognitive decline and neurological impairments commonly manifest in neurodegenerative disease patients, underscoring the urgency of comprehending the posttranslational modifications' impact on protein function for targeted therapeutic interventions. This review elucidates the critical link between neurodegenerative diseases and specific posttranslational modifications, focusing on Tau, APP, α-synuclein, Huntingtin protein, Parkin, DJ-1, and Drp1. By delineating the prominent aberrant posttranslational modifications within Alzheimer disease, Parkinson disease, and Huntington disease, the review underscores the significance of understanding the interplay among these modifications. Emphasizing 10 key abnormal posttranslational modifications, this study aims to provide a comprehensive framework for investigating neurodegenerative diseases holistically. The insights presented herein shed light on potential therapeutic avenues aimed at modulating posttranslational modifications to mitigate protein aggregation and retard neurodegenerative disease progression.

Screening and Structural Characterization of Heat Shock Response Elements (HSEs) in Entamoeba histolytica Promoters

Entamoeba histolytica (E. histolytica) exhibits a remarkable capacity to respond to thermal shock stress through a sophisticated genetic regulation mechanism. This process is carried out via Heat Shock Response Elements (HSEs), which are recognized by Heat Shock Transcription Factors (EhHSTFs), enabling fine and precise control of gene expression. Our study focused on screening for HSEs in the promoters of the E. histolytica genome, specifically analyzing six HSEs, including Ehpgp5, EhrabB1, EhrabB4, EhrabB5, Ehmlbp, and Ehhsp100. We discovered 2578 HSEs, with 1412 in promoters of hypothetical genes and 1166 in coding genes. We observed that a single promoter could contain anywhere from one to five HSEs. Gene ontology analysis revealed the presence of HSEs in essential genes for the amoeba, including cysteine proteinases, ribosomal genes, Myb family DNA-binding proteins, and Rab GTPases, among others. Complementarily, our molecular docking analyses indicate that these HSEs are potentially recognized by EhHSTF5, EhHSTF6, and EhHSTF7 factors in their trimeric conformation. These findings suggest that E. histolytica has the capability to regulate a wide range of critical genes via HSE-EhHSTFs, not only for thermal stress response but also for vital functions of the parasite. This is the first comprehensive study of HSEs in the genome of E. histolytica, significantly contributing to the understanding of its genetic regulation and highlighting the complexity and precision of this mechanism in the parasite's survival.

Paralogue-Specific Roles of SUMO1 and SUMO2/3 in Protein Quality Control and Associated Diseases

Small ubiquitin-related modifiers (SUMOs) function as post-translational protein modifications and regulate nearly every aspect of cellular function. While a single ubiquitin protein is expressed across eukaryotic organisms, multiple SUMO paralogues with distinct biomolecular properties have been identified in plants and vertebrates. Five SUMO paralogues have been characterized in humans, with SUMO1, SUMO2 and SUMO3 being the best studied. SUMO2 and SUMO3 share 97% protein sequence homology (and are thus referred to as SUMO2/3) but only 47% homology with SUMO1. To date, thousands of putative sumoylation substrates have been identified thanks to advanced proteomic techniques, but the identification of SUMO1- and SUMO2/3-specific modifications and their unique functions in physiology and pathology are not well understood. The SUMO2/3 paralogues play an important role in proteostasis, converging with ubiquitylation to mediate protein degradation. This function is achieved primarily through SUMO-targeted ubiquitin ligases (STUbLs), which preferentially bind and ubiquitylate poly-SUMO2/3 modified proteins. Effects of the SUMO1 paralogue on protein solubility and aggregation independent of STUbLs and proteasomal degradation have also been reported. Consistent with these functions, sumoylation is implicated in multiple human diseases associated with disturbed proteostasis, and a broad range of pathogenic proteins have been identified as SUMO1 and SUMO2/3 substrates. A better understanding of paralogue-specific functions of SUMO1 and SUMO2/3 in cellular protein quality control may therefore provide novel insights into disease pathogenesis and therapeutic innovation. This review summarizes current understandings of the roles of sumoylation in protein quality control and associated diseases, with a focus on the specific effects of SUMO1 and SUMO2/3 paralogues.

On the Prevalence and Roles of Proteins Undergoing Liquid-Liquid Phase Separation in the Biogenesis of PML-Bodies

The formation and function of membrane-less organelles (MLOs) is one of the main driving forces in the molecular life of the cell. These processes are based on the separation of biopolymers into phases regulated by multiple specific and nonspecific inter- and intramolecular interactions. Among the realm of MLOs, a special place is taken by the promyelocytic leukemia nuclear bodies (PML-NBs or PML bodies), which are the intranuclear compartments involved in the regulation of cellular metabolism, transcription, the maintenance of genome stability, responses to viral infection, apoptosis, and tumor suppression. According to the accepted models, specific interactions, such as SUMO/SIM, the formation of disulfide bonds, etc., play a decisive role in the biogenesis of PML bodies. In this work, a number of bioinformatics approaches were used to study proteins found in the proteome of PML bodies for their tendency for spontaneous liquid-liquid phase separation (LLPS), which is usually caused by weak nonspecific interactions. A total of 205 proteins found in PML bodies have been identified. It has been suggested that UBC9, P53, HIPK2, and SUMO1 can be considered as the scaffold proteins of PML bodies. It was shown that more than half of the proteins in the analyzed proteome are capable of spontaneous LLPS, with 85% of the analyzed proteins being intrinsically disordered proteins (IDPs) and the remaining 15% being proteins with intrinsically disordered protein regions (IDPRs). About 44% of all proteins analyzed in this study contain SUMO binding sites and can potentially be SUMOylated. These data suggest that weak nonspecific interactions play a significantly larger role in the formation and biogenesis of PML bodies than previously expected.

A novel cell-permeable peptide prevents protein SUMOylation and supports the mislocalization and aggregation of TDP-43

SUMOylation is a post-translational modification (PTM) that exerts a regulatory role in different cellular processes, including protein localization, aggregation, and biological activities. It consists of the dynamic formation of covalent isopeptide bonds between a family member of the Small Ubiquitin Like Modifiers (SUMOs) and the target proteins. Interestingly, it is a cellular mechanism implicated in several neurodegenerative pathologies and potentially it could become a new therapeutic target; however, there are very few pharmacological tools to modulate the SUMOylation process. In this study, we have designed and tested the activity of a novel small cell-permeable peptide, COV-1, in a neuroblastoma cell line that specifically prevents protein SUMOylation. COV-1 inhibits UBC9-protein target interaction and efficiently decreases global SUMO-1ylation. Moreover, it can perturb RanGAP-1 perinuclear localization by inducing the downregulation of UBC9. In parallel, we found that COV-1 causes an increase in the ubiquitin degradation system up to its engulfment while enhancing the autophagic flux. Surprisingly, COV-1 modifies protein aggregation, and specifically it mislocalizes TDP-43 within cells, inducing its aggregation and co-localization with SUMO-1. These data suggest that COV-1 could be taken into future consideration as an interesting pharmacological tool to study the cellular cascade effects of SUMOylation prevention.

Repression motif in HSF1 regulated by phosphorylation

The heat shock factor 1 (HSF1) is a transcription factor that itself is a sensor for stress and integrates various intrinsic or environmental stress sensing pathways. Thus HSF1 orchestrates the heat shock response (HSR) by translating these pathways into a distinct transcriptional program that aids the cells to cope with and adapt to proteotoxic stress. Although heavily researched the regulation of HSF1 activation is still not completely understood. A conserved reaction to stress is the hyperphosphorylation of the otherwise confined constitutive phosphorylated HSF1. Therefore, this stress specific phosphorylation is believed to be involved in the regulatory mechanism and hence, was and is focus of many studies, ascribing various effects to single phosphorylation sites. To gain additional insight into effects of phosphorylation, HSF1 carrying amino acid substitutions on up to 18 amino acids were tested for their transactivation potential on an HSR reporter plasmid. A pattern of eleven phosphor-mimicking and diminishing amino acid substitutions on well-known phosphorylation sites of HSF1 were introduced to produce transcriptional active [11 M(+)] or repressed [11 M(-)] phenotypes. It could be confirmed that heat activates HSF1 regardless of phosphorylation. Distinct cellular stress, obtained by chemical HSR inducers or mimicked by a constitutively active HSF1, showed clear differences in the activation potential of HSF1-11 M(+) and 11 M(-). Further refinement to the single amino acid level identified the S303/307 double-phosphorylation motif, wherein phosphorylation of S303 was sole responsible for the repressing effect. The effect could be reproduced in different cell lines and is not entirely based on degradation. A small repression motif could be dissociated from the HSF1 context, which is still capable of repressing the background transcription of a specifically designed reporter plasmid. Taken together these results indicate, that besides already described mechanisms of pS303/307 mediated repression of HSF1 activation, an additional mechanism repressing the transcriptional output of the entire HSE containing promoter is mediated by this small repressive motif.

Effects of 5G-modulated 3.5 GHz radiofrequency field exposures on HSF1, RAS, ERK, and PML activation in live fibroblasts and keratinocytes cells

The potential health risks of exposure to radiofrequency electromagnetic fields from mobile communications technologies have raised societal concerns. Guidelines have been set to protect the population (e.g. non-specific heating above 1 °C under exposure to radiofrequency fields), but questions remain regarding the potential biological effects of non-thermal exposures. With the advent of the fifth generation (5G) of mobile communication, assessing whether exposure to this new signal induces a cellular stress response is one of the mandatory steps on the roadmap for a safe deployment and health risk evaluation. Using the BRET (Bioluminescence Resonance Energy-Transfer) technique, we assessed whether continuous or intermittent (5 min ON/ 10 min OFF) exposure of live human keratinocytes and fibroblasts cells to 5G 3.5 GHz signals at specific absorption rate (SAR) up to 4 W/kg for 24 h impact basal or chemically-induced activity of Heat Shock Factor (HSF), RAt Sarcoma virus (RAS) and Extracellular signal-Regulated Kinases (ERK) kinases, and Promyelocytic Leukemia Protein (PML), that are all molecular pathways involved in environmental cell-stress responses. The main results are (i), a decrease of the HSF1 basal BRET signal when fibroblasts cells were exposed at the lower SARs tested (0.25 and 1 W/kg), but not at the highest one (4 W/kg), and (ii) a slight decrease of As₂O₃ maximal efficacy to trigger PML SUMOylation when fibroblasts cells, but not keratinocytes, were continuously exposed to the 5G RF-EMF signal. Nevertheless, given the inconsistency of these effects in terms of impacted cell type, effective SAR, exposure mode, and molecular cell stress response, we concluded that our study show no conclusive evidence that molecular effects can arise when skin cells are exposed to the 5G RF-EMF alone or with a chemical stressor.

SUMOylation regulates pre-mRNA splicing to overcome DNA damage in fungi

Pathogenic fungi are subject to DNA damage stress derived from host immune responses during infection. Small ubiquitin-like modifier (SUMO) modification and precursor (pre)-mRNA splicing are both involved in DNA damage response (DDR). However, the mechanisms of how SUMOylation and splicing coordinated in DDR remain largely unknown. Combining with biochemical analysis, RNA-Seq method, and biological analysis, we report that SUMO pathway participates in DDR and virulence in Fusarium graminearum, a causal agent of Fusarium head blight of cereal crops world-wide. Interestingly, a key transcription factor FgSR is SUMOylated upon DNA damage stress. SUMOylation regulates FgSR nuclear-cytoplasmic partitioning and its phosphorylation by FgMec1, and promotes its interaction with chromatin remodeling complex SWI/SNF for activating the expression of DDR-related genes. Moreover, the SWI/SNF complex was found to further recruit splicing-related NineTeen Complex, subsequently modulates pre-mRNA splicing during DDR. Our findings reveal a novel function of SUMOylation in DDR by regulating a transcription factor to orchestrate gene expression and pre-mRNA splicing to overcome DNA damage during the infection of F. graminearum, which advances the understanding of the delicate regulation of DDR by SUMOylation in pathogenic fungi, and extends the knowledge of cooperation of SUMOylation and pre-mRNA splicing in DDR in eukaryotes.

Regulation of SUMOylation on RNA metabolism in cancers

Post-translational modifications of proteins play very important roles in regulating RNA metabolism and affect many biological pathways. Here we mainly summarize the crucial functions of small ubiquitin-like modifier (SUMO) modification in RNA metabolism including transcription, splicing, tailing, stability and modification, as well as its impact on the biogenesis and function of microRNA (miRNA) in particular. This review also highlights the current knowledge about SUMOylation regulation in RNA metabolism involved in many cellular processes such as cell proliferation and apoptosis, which is closely related to tumorigenesis and cancer progression.

Interplay between mammalian heat shock factors 1 and 2 in physiology and pathology

The heat-shock factors (HSFs) belong to an evolutionary conserved family of transcription factors that were discovered already over 30 years ago. The HSFs have been shown to a have a broad repertoire of target genes, and they also have crucial functions during normal development. Importantly, HSFs have been linked to several disease states, such as neurodegenerative disorders and cancer, highlighting their importance in physiology and pathology. However, it is still unclear how HSFs are regulated and how they choose their specific target genes under different conditions. Posttranslational modifications and interplay among the HSF family members have been shown to be key regulatory mechanisms for these transcription factors. In this review, we focus on the mammalian HSF1 and HSF2, including their interplay, and provide an updated overview of the advances in understanding how HSFs are regulated and how they function in multiple processes of development, aging, and disease. We also discuss HSFs as therapeutic targets, especially the recently reported HSF1 inhibitors.

SUMOylation in Skeletal Development, Homeostasis, and Disease

The modification of proteins by small ubiquitin-related modifier (SUMO) molecules, SUMOylation, is a key post-translational modification involved in a variety of biological processes, such as chromosome organization, DNA replication and repair, transcription, nuclear transport, and cell signaling transduction. In recent years, emerging evidence has shown that SUMOylation regulates the development and homeostasis of the skeletal system, with its dysregulation causing skeletal diseases, suggesting that SUMOylation pathways may serve as a promising therapeutic target. In this review, we summarize the current understanding of the molecular mechanisms by which SUMOylation pathways regulate skeletal cells in physiological and disease contexts.

HSF-1: Guardian of the Proteome Through Integration of Longevity Signals to the Proteostatic Network

Discoveries made in the nematode Caenorhabditis elegans revealed that aging is under genetic control. Since these transformative initial studies, C. elegans has become a premier model system for aging research. Critically, the genes, pathways, and processes that have fundamental roles in organismal aging are deeply conserved throughout evolution. This conservation has led to a wealth of knowledge regarding both the processes that influence aging and the identification of molecular and cellular hallmarks that play a causative role in the physiological decline of organisms. One key feature of age-associated decline is the failure of mechanisms that maintain proper function of the proteome (proteostasis). Here we highlight components of the proteostatic network that act to maintain the proteome and how this network integrates into major longevity signaling pathways. We focus in depth on the heat shock transcription factor 1 (HSF1), the central regulator of gene expression for proteins that maintain the cytosolic and nuclear proteomes, and a key effector of longevity signals.

Small-molecule inhibitors targeting small ubiquitin-like modifier pathway for the treatment of cancers and other diseases

SUMOylation is a key post-translational modification that involves the covalent attachment of small ubiquitin-like modifier (SUMO) to the lysine residues of target proteins. The well-balanced SUMOylation is essential for normal cellular behaviors, while disturbance of SUMOylation is associated with various cancers and other diseases. Herein, we summarize the structures and biological functions of proteins involved in the SUMOylation process, their dysregulation in human diseases, and the discovery of small-molecular inhibitors targeting this pathway. In addition, we highlight the emerging trends in this field.

The protein modifier SUMO is critical for integrity of the Arabidopsis shoot apex at warm ambient temperatures

SUMO is a protein modification whose conjugate levels peak during acute heat stress. We find that SUMO is also critical for plant longevity when Arabidopsis experiences a prolonged non-damaging period of only 28 degrees Celsius. Remarkably, this thermo-lethality at 28 degrees was not seen with any other mutant of the SUMO pathway tested. Autoimmunity due to low SUMO1/2 expression levels was not causal for this thermo-lethality. The role of SUMO for thermo-resilience was also distinct from its requirement for thermomorphogenesis - a growth response triggered by the same warm temperature, as only the latter response was dependent on the SUMO ligase SIZ1 as well. Thermo-resilience at 28 degrees Celsius and (acquired) thermotolerance (a response that allows plants to recover and acclimate to brief extreme temperatures) both depend on the HEAT SHOCK TRANSCRIPTION FACTOR A1 (HSFA1). Acquired thermotolerance was, however, normal in the sumo1/2 knockdown mutant. Thus, SUMO-dependent thermo-resilience is potentially controlled in a different way than the protein damage pathway that underpins thermotolerance. Close inspection of shoot apices revealed that the cell patterning and tissue integrity of the shoot apex of the SUMO1/2 knockdown mutant was lost at 28, but not 22 degrees Celsius. We thus describe a novel SUMO-dependent phenotype.

SUMO and Transcriptional Regulation: The Lessons of Large-Scale Proteomic, Modifomic and Genomic Studies

One major role of the eukaryotic peptidic post-translational modifier SUMO in the cell is transcriptional control. This occurs via modification of virtually all classes of transcriptional actors, which include transcription factors, transcriptional coregulators, diverse chromatin components, as well as Pol I-, Pol II- and Pol III transcriptional machineries and their regulators. For many years, the role of SUMOylation has essentially been studied on individual proteins, or small groups of proteins, principally dealing with Pol II-mediated transcription. This provided only a fragmentary view of how SUMOylation controls transcription. The recent advent of large-scale proteomic, modifomic and genomic studies has however considerably refined our perception of the part played by SUMO in gene expression control. We review here these developments and the new concepts they are at the origin of, together with the limitations of our knowledge. How they illuminate the SUMO-dependent transcriptional mechanisms that have been characterized thus far and how they impact our view of SUMO-dependent chromatin organization are also considered.

Heat shock transcription factor 1 is SUMOylated in the activated trimeric state

The heat shock response is a transcriptional program of organisms to counteract an imbalance in protein homeostasis. It is orchestrated in all eukaryotic cells by heat shock transcription factor 1 (Hsf1). Despite very intensive research, the intricacies of the Hsf1 activation-attenuation cycle remain elusive at a molecular level. Post-translational modifications belong to one of the key mechanisms proposed to adapt the Hsf1 activity to the needs of individual cells, and phosphorylation of Hsf1 at multiple sites has attracted much attention. According to cell biological and proteomics data, Hsf1 is also modified by small ubiquitin-like modifier (SUMO) at several sites. How SUMOylation affects Hsf1 activity at a molecular level is still unclear. Here, we analyzed Hsf1 SUMOylation in vitro with purified components to address questions that could not be answered in cell culture models. In vitro Hsf1 is primarily conjugated at lysine 298 with a single SUMO, though we did detect low-level SUMOylation at other sites. Different SUMO E3 ligases such as protein inhibitor of activated STAT 4 enhanced the efficiency of in vitro modification but did not alter SUMO site preferences. We provide evidence that Hsf1 trimerization and phosphorylation at serines 303 and 307 increases SUMOylation efficiency, suggesting that Hsf1 is SUMOylated in its activated state. Hsf1 can be SUMOylated when DNA bound, and SUMOylation of Hsf1 does neither alter DNA-binding affinity nor affects heat shock cognate 71kDa protein (HSPA8)+DnaJ homolog subfamily B member 1-mediated monomerization of Hsf1 trimers and concomitant dislocation from DNA. We propose that SUMOylation acts at the transcription level of the heat shock response.

SUMO and cellular adaptive mechanisms

The ubiquitin family member SUMO is a covalent regulator of proteins that functions in response to various stresses, and defects in SUMO-protein conjugation or deconjugation have been implicated in multiple diseases. The loss of the Ulp2 SUMO protease, which reverses SUMO-protein modifications, in the model eukaryote Saccharomyces cerevisiae is severely detrimental to cell fitness and has emerged as a useful model for studying how cells adapt to SUMO system dysfunction. Both short-term and long-term adaptive mechanisms are triggered depending on the length of time cells spend without this SUMO chain-cleaving enzyme. Such short-term adaptations include a highly specific multichromosome aneuploidy and large changes in ribosomal gene transcription. While aneuploid ulp2Δ cells survive, they suffer severe defects in growth and stress resistance. Over many generations, euploidy is restored, transcriptional programs are adjusted, and specific genetic changes that compensate for the loss of the SUMO protease are observed. These long-term adapted cells grow at normal rates with no detectable defects in stress resistance. In this review, we examine the connections between SUMO and cellular adaptive mechanisms more broadly.

Cellular stress caused by disrupting attachment of the ubiquitous small ubiquitin-like modifier (SUMO) proteins, which are present in most organisms and regulate numerous DNA processes and stress responses by attaching to key proteins, results in some remarkable adaptations. Mark Hochstrasser at Yale University, New Haven, USA, and co-workers review how this “sumoylation” is reversed by protease enzymes, and how imbalances between sumoylation and desumoylation may be linked to diseases including cancer. When certain SUMO proteases are deliberately disrupted, the cells quickly become aneuploid, i.e., carry an abnormal number of chromosomes. These cells show severe growth defects, but over many generations they regain the normal number of chromosomes. They also undergo genetic changes that promote alternative mechanisms that compensate for losing the SUMO protease and facilitate the same efficient stress responses as the original cells.

SUMOylation in α-Synuclein Homeostasis and Pathology

The accumulation and aggregation of α-synuclein are central to Parkinson’s disease (PD), yet the molecular mechanisms responsible for these events are not fully understood. Post-translational modifications of α-synuclein regulate several of its properties, including degradation, interaction with proteins and membranes, aggregation and toxicity. SUMOylation is a post-translational modification involved in various nuclear and extranuclear processes, such as subcellular protein targeting, mitochondrial fission and synaptic plasticity. Protein SUMOylation increases in response to several stressful situations, from viral infections to trauma. In this framework, an increasing amount of evidence has implicated SUMOylation in several neurodegenerative diseases, including PD. This review will discuss recent findings in the role of SUMOylation as a regulator of α-synuclein accumulation, aggregation and toxicity, and its possible implication in neurodegeneration that underlies PD.

Dietary flavonoid fisetin binds human SUMO1 and blocks sumoylation of p53

Flavonoids are plant-derived compounds that occur abundantly in fruits and vegetables and have been shown to possess potent anti-cancer, antioxidant, and anti-inflammatory properties. However, their direct targets and molecular mechanism of action are not well characterized, hampering exploitation of the beneficial properties of flavonoids for drug development. Small ubiquitin-related modifier 1 (SUMO1) is attached to target proteins as part of a post-translational modification system implicated in a myriad of cellular processes from nuclear trafficking to transcriptional regulation. Using a combination of surface plasmon resonance, differential scanning fluorimetry and fluorescence quenching studies, we provide evidence for direct binding of the dietary flavonoid fisetin to human SUMO1. Our NMR chemical shift perturbation analyses reveal that binding to fisetin involves four conserved amino acid residues (L65, F66, E67, M82) previously shown to be important for conjugation of SUMO1 to target proteins. In vitro sumoylation experiments indicate that fisetin blocks sumoylation of tumor suppressor p53, consistent with fisetin negatively affecting post-translational modification and thus the biological activity of p53. A series of differential scanning fluorimetry experiments suggest that high concentrations of fisetin result in destabilization and unfolding of SUMO1, presenting a molecular mechanism by which flavonoid binding affects its activity. Overall, our data establish a novel direct interaction between fisetin and SUMO1, providing a mechanistic explanation for the ability of fisetin to modulate multiple key signaling pathways inside cells.

Regulation of Hsf1 and the Heat Shock Response

The heat shock response (HSR) is characterized by the induction of molecular chaperones following a sudden increase in temperature. In eukaryotes, the HSR comprises the set of genes controlled by the transcription factor Hsf1. The HSR is induced by defects in co-translational protein folding, ribosome biogenesis, organellar targeting of nascent proteins, and protein degradation by the ubiquitin proteasome system. Upon heat shock, these processes may be endogenous sources of polypeptide ligands that activate the HSR. Mechanistically, these ligands are thought to titrate the chaperone Hsp70 away from Hsf1, releasing Hsf1 to induce the full arsenal of cellular chaperones to restore protein homeostasis. In metazoans, this cell-autonomous feedback loop is modulated by the microenvironment and neuronal cues to enable tissue-level and organism-wide coordination.

SUMO conjugation regulates immune signalling

Post-translational modifications (PTMs) are critical drivers and attenuators for proteins that regulate immune signalling cascades in host defence. In this review, we explore functional roles for one such PTM, the small ubiquitin-like modifier (SUMO). Very few of the SUMO conjugation targets identified by proteomic studies have been validated in terms of their roles in host defence. Here, we compare and contrast potential SUMO substrate proteins in immune signalling for flies and mammals, with an emphasis on NFκB pathways. We discuss, using the few mechanistic studies that exist for validated targets, the effect of SUMO conjugation on signalling and also explore current molecular models that explain regulation by SUMO. We also discuss in detail roles of evolutionary conservation of mechanisms, SUMO interaction motifs, crosstalk of SUMO with other PTMs, emerging concepts such as group SUMOylation and finally, the potentially transforming roles for genome-editing technologies in studying the effect of PTMs.

Inhibiting ubiquitination causes an accumulation of SUMOylated newly synthesized nuclear proteins at PML bodies

Protein ubiquitination and SUMOylation are required for the maintenance of cellular protein homeostasis, and both increase in proteotoxic conditions (e.g. heat shock or proteasome inhibition). However, we found that when ubiquitination was blocked in several human cell lines by inhibiting the ubiquitin-activating enzyme with TAK243, there was an unexpected, large accumulation of proteins modified by SUMO2/3 chains or SUMO1, but not by several other ubiquitin-like proteins. This buildup of SUMOylated proteins was evident within 3–4 h. It required the small ubiquitin-like modifier (SUMO)-conjugating enzyme, UBC9, and the promyelocytic leukemia protein (PML) and thus was not due to nonspecific SUMO conjugation by ubiquitination enzymes. The SUMOylated proteins accumulated predominantly bound to chromatin and were localized to PML nuclear bodies. Because blocking protein synthesis with cycloheximide prevented the buildup of SUMOylated proteins, they appeared to be newly-synthesized proteins. The proteins SUMOylated after inhibition of ubiquitination were purified and analyzed by MS. In HeLa and U2OS cells, there was a cycloheximide-sensitive increase in a similar set of SUMOylated proteins (including transcription factors and proteins involved in DNA damage repair). Surprisingly, the inhibition of ubiquitination also caused a cycloheximide-sensitive decrease in a distinct set of SUMOylated proteins (including proteins for chromosome modification and mRNA splicing). More than 80% of the SUMOylated proteins whose levels rose or fell upon inhibiting ubiquitination inhibition underwent similar cycloheximide-sensitive increases or decreases upon proteasome inhibition. Thus, when nuclear substrates of the ubiquitin–proteasome pathway are not efficiently degraded, many become SUMO-modified and accumulate in PML bodies.

SUMOylation Evoked by Oxidative Stress Reduced Lens Epithelial Cell Antioxidant Functions by Increasing the Stability and Transcription of TP53INP1 in Age-Related Cataracts

Oxidative stress plays an important role in the pathogenesis of cataracts. Small ubiquitin-like modifier (SUMO) proteins have great effects on cell stress response. Previous studies have shown that TP53INP1 can arrest cell growth and induce apoptosis by modulating p53 transcriptional activity and that both TP53INP1 and p53 are substrates of SUMOylation. However, no previous research has studied the effect of SUMOylation on the oxidative stress response in cataracts. This is the first study to investigate the effect of SUMOylation of TP53INP1 in oxidative stress-induced lens epithelial cell injury and age-related cataract formation. We found that the oxidative stress-induced endogenous SUMOylation of TP53INP1 promoted human lens epithelial cell (holed) apoptosis and regulated hLEC antioxidant effects by increasing the stability and transcription of TP53INP1 in age-related cataracts. SUMO-1, SUMOylation, and TP53INP1 were upregulated in lens tissues affected by age-related cataracts. A SUMO-1-specific protease, SENP1, acted as an oxidative stress-sensitive target gene in hLECs. This study identified for the first time that TP53INP1 can be SUMOylated in vivo, that the SUMOylation of TP53INP1 is induced by oxidative stress, and that SUMOylation/deSUMOylation can affect the stability and transcription of TP53INP1 in hLECs.

Tailoring of Proteostasis Networks with Heat Shock Factors

Heat shock factors (HSFs) are the main transcriptional regulators of the heat shock response and indispensable for maintaining cellular proteostasis. HSFs mediate their protective functions through diverse genetic programs, which are composed of genes encoding molecular chaperones and other genes crucial for cell survival. The mechanisms that are used to tailor HSF-driven proteostasis networks are not yet completely understood, but they likely comprise from distinct combinations of both genetic and proteomic determinants. In this review, we highlight the versatile HSF-mediated cellular functions that extend from cellular stress responses to various physiological and pathological processes, and we underline the key advancements that have been achieved in the field of HSF research during the last decade.

Modulation of Heat Shock Factor 1 Activity through Silencing of Ser303/Ser307 Phosphorylation Supports a Metabolic Program Leading to Age-Related Obesity and Insulin Resistance

Activation of the adaptive response to cellular stress orchestrated by heat shock factor 1 (HSF1), which is an evolutionarily conserved transcriptional regulator of chaperone response and cellular bioenergetics in diverse model systems, is a central feature of organismal defense from environmental and cellular stress. HSF1 activity, induced by proteostatic, metabolic, and growth factor signals, is regulated by posttranscriptional modifications, yet the mechanisms that regulate HSF1 and particularly the functional significance of these modifications in modulating its biological activity in vivo remain unknown. HSF1 phosphorylation at both Ser303 (S303) and Ser307 (S307) has been shown to repress HSF1 transcriptional activity under normal physiological growth conditions. To determine the biological relevance of these HSF1 phosphorylation events, we generated a knock-in mouse model in which S303 and S307 were replaced with alanine (HSF1^303A/307A). Our results confirmed that loss of phosphorylation in HSF1^303A/307A cells and tissues increases protein stability but also markedly sensitizes HSF1 activation under normal and heat- or nutrient-induced stress conditions. Interestingly, the enhanced HSF1 activation in HSF1^303A/307A mice activates a supportive metabolic program that aggravates the development of age-dependent obesity, fatty liver diseases, and insulin resistance. Thus, these findings highlight the importance of a posttranslational mechanism (through phosphorylation at S303 and S307 sites) of regulation of the HSF1-mediated transcriptional program that moderates the severity of nutrient-induced metabolic diseases.

Age Alters Chromatin Structure and Expression of SUMO Proteins under Stress Conditions in Human Adipose-Derived Stem Cells

Adult stem cells play a critical role in tissue homeostasis and repair. Aging leads to a decline in stem cells’ regenerative capacity that contributes significantly to the maintenance of organ and tissue functions. Age-dependent genomic and epigenetic modifications together play a role in the disruption of critical cellular pathways. However, the epigenetic mechanisms responsible for the decline of adult stem cell functions remain to be well established. Here, we investigated age-dependent, genome-wide alterations in the chromatin accessibility of primary human adipose-derived stem cells (ASCs) in comparison to age-matched fibroblasts via ATAC-seq technology. Our results demonstrate that aging ASCs possess globally more stable chromatin accessibility profiles as compared to aging fibroblasts, suggesting that robust regulatory mechanisms maintain adult stem cell chromatin structure against aging. Furthermore, we observed age-dependent subtle changes in promoter nucleosome positioning in selective pathways during aging, concurrent with altered small ubiquitin-related modifier (SUMO) protein expression under stress conditions. Together, our data suggest a significant role for nucleosome positioning in sumoylation pathway regulation in stress response during adult stem cell aging. The differences described here between the chromatin structure of human ASCs and fibroblasts will further elucidate the mechanisms regulating gene expression during aging in both stem cells and differentiated cells.

Gas-Phase Enrichment of Multiply Charged Peptide Ions by Differential Ion Mobility Extend the Comprehensiveness of SUMO Proteome Analyses

The small ubiquitin-like modifier (SUMO) is a member of the family of ubiquitin-like modifiers (UBLs) and is involved in important cellular processes, including DNA damage response, meiosis and cellular trafficking. The large-scale identification of SUMO peptides in a site-specific manner is challenging not only because of the low abundance and dynamic nature of this modification, but also due to the branched structure of the corresponding peptides that further complicate their identification using conventional search engines. Here, we exploited the unusual structure of SUMO peptides to facilitate their separation by high-field asymmetric waveform ion mobility spectrometry (FAIMS) and increase the coverage of SUMO proteome analysis. Upon trypsin digestion, branched peptides contain a SUMO remnant side chain and predominantly form triply protonated ions that facilitate their gas-phase separation using FAIMS. We evaluated the mobility characteristics of synthetic SUMO peptides and further demonstrated the application of FAIMS to profile the changes in protein SUMOylation of HEK293 cells following heat shock, a condition known to affect this modification. FAIMS typically provided a 10-fold improvement of detection limit of SUMO peptides, and enabled a 36% increase in SUMO proteome coverage compared to the same LC-MS/MS analyses performed without FAIMS. Graphical Abstract ᅟ.

Chemical Biology of H2S Signaling through Persulfidation

Signaling by H2S is proposed to occur via persulfidation, a posttranslational modification of cysteine residues (RSH) to persulfides (RSSH). Persulfidation provides a framework for understanding the physiological and pharmacological effects of H2S. Due to the inherent instability of persulfides, their chemistry is understudied. In this review, we discuss the biologically relevant chemistry of H2S and the enzymatic routes for its production and oxidation. We cover the chemical biology of persulfides and the chemical probes for detecting them. We conclude by discussing the roles ascribed to protein persulfidation in cell signaling pathways.

The Regulation of Chromatin by Dynamic SUMO Modifications

Protein modification by the small ubiquitin-related modifier (SUMO) protein regulates numerous cellular pathways and mounting evidence reveals a critical role for SUMO in modulating gene expression. Dynamic sumoylation of transcription factors, chromatin-modifying enzymes, histones, and other chromatin-associated factors significantly affects the transcriptional status of the eukaryotic genome. Recent studies have employed high-throughput ChIP-Seq analyses to gain clues regarding the role of the SUMO pathway in regulating chromatin-based transactions. Indeed, the global distribution of SUMO across chromatin reveals an important function for SUMO in controlling transcription, particularly of genes involved in protein synthesis. These newly appreciated patterns of genome-wide sumoylation will inform more directed studies aimed at analyzing how the dynamics of gene expression are controlled by posttranslational SUMO modification.

SUMO E3 Ligase SlSIZ1 Facilitates Heat Tolerance in Tomato

High temperature has become a major abiotic stress that limits crop productivity. Heat shock transcription factors (HSFs) and heat shock proteins (HSPs) play important roles in enhancing thermotolerance of plants. SUMOylation is an important post-translational modification in regulating cellular functions in eukaryotes. SIZ1, a well-characterized SUMO E3 ligase, mediates the process of SUMOylation. In this study, SUMO conjugations were clearly induced by high temperature. Overexpression of SIZ1 SUMO E3 ligase (SlSIZ1) in tomato could enhance the tolerance to heat stress in tomato. The RNA interference (RNAi) plants were more wilted than the wild type with heat treatment. Under heat stress, SlSIZ1 could decrease the accumulation of reactive oxygen species (ROS) and induce some genes of HSF and HSP transcription. Furthermore, overexpression of SlSIZ1 could increase the level of Hsp70 under high temperature. Yeast two-hybrid and bimolecular fluorescence complementation (BiFC) assays showed that SlSIZ1 could interact with SlHsfA1 to mediate the SUMOylation of SlHsfA1 and consequently enhance thermotolerance of tomato. In conclusion, overexpression of SlSIZ1 enhanced heat tolerance by regulating the activities of HsfA1 and increasing the content Hsp70.

SUMO1/UBC9‑decreased Nox1 activity inhibits reactive oxygen species generation and apoptosis in diabetic retinopathy

Diabetic retinopathy (DR) is an increasing global health concern that causes vision loss and blindness. Reactive oxygen species (ROS) are considered to be a principal cause of DR. An important source of ROS is the oxidization of NADPH. In the present study, NADPH oxidase 1 (Nox1)-expressing human retinal epithelial cell (HREC) lines were generated and infected with small ubiquitin-like modifier 1 (SUMO1) and/or ubiquitin conjugating enzyme E2 I (UBC9) lentiviral pGMLV constructs. The viabilities, apoptotic capacities and ROS production levels of the HREC lines were quantified using Hoechst 33258, annexin V/propidium iodide and dichlorodihydrofluorescein diacetate assays, respectively. Additionally, rat DR models were established. From these models, the apoptotic capacities of retinal tissues were visualized using terminal deoxynucleotidyl transferase dUTP nick end labeling assays, and the pathologies were evaluated. The mRNA and protein expression levels of SUMO1, UBC9 and Nox1 were analyzed using reverse transcription-quantitative polymerase chain reaction and western blot analyses, respectively. Compared with controls, the relative mRNA levels of SUMO1 and UBC9 were significantly upregulated, and the Nox1 levels significantly downregulated, in cells infected with SUMO1 or UBC9 alone or in combination. The ROS production and apoptosis rates of cells and retinal tissues were decreased. In addition, pathological symptoms in DR tissues improved when they were simultaneously transfected with SUMO1 and UBC9 via intraocular injection. In conclusion, the SUMO1/UBC9 axis may regulate Nox1-mediated DR by inhibiting ROS generation and apoptosis in rat and cellular model systems.

SUMO-Dependent Synergism Involving Heat Shock Transcription Factors with Functions Linked to Seed Longevity and Desiccation Tolerance

A transcriptional synergism between HaHSFA9 (A9) and HaHSFA4a (A4a) contributes to determining longevity and desiccation tolerance of sunflower (Helianthus annuus, L.) seeds. Potential lysine SUMOylation sites were identified in A9 and A4a and mutated to arginine. We show that A9 is SUMOylated in planta at K38. Although we did not directly detect SUMOylated A4a in planta, we provide indirect evidence from transient expression experiments indicating that A4a is SUMOylated at K172. Different combinations of wild type and SUMOylation site mutants of A9 and A4a were analyzed by transient expression in sunflower embryos and leaves. Although most of the precedents in literature link SUMOylation with repression, the A9 and A4a synergism was fully abolished when the mutant forms for both factors were combined. However, the combination of mutant forms of A9 and A4a did not affect the nuclear retention of A4a by A9; therefore, the analyzed mutations would affect the synergism after the mutual interaction and nuclear co-localization of A9 and A4a. Our results suggest a role for HSF SUMOylation during late, zygotic, embryogenesis. The SUMOylation of A9 (or A4a) would allow a crucial, synergic, transcriptional effect that occurs in maturing sunflower seeds.

Stress induced nuclear granules form in response to accumulation of misfolded proteins in Caenorhabditis elegans

Background

Environmental stress can affect the viability or fecundity of an organism. Environmental stressors may affect the genome or the proteome and can cause cellular distress by contributing to protein damage or misfolding. This study examines the cellular response to environmental stress in the germline of the nematode, C. elegans.

Results

Salt stress, oxidative stress, and starvation, but not heat shock, induce the relocalization of ubiquitin, proteasome, and the TIAR-2 protein into distinct subnuclear regions referred to as stress induced nuclear granules (SINGs). The SINGs form within 1 h of stress initiation and do not require intertissue signaling. K48-linked polyubiquitin chains but not K63 chains are enriched in SINGs. Worms with a mutation in the conjugating enzyme, ubc-18, do not form SINGs. Additionally, knockdown of ubc-20 and ubc-22 reduces the level of SING formation as does knockdown of the ubiquitin ligase chn-1, a CHIP homolog. The nuclear import machinery is required for SING formation. Stressed embryos containing SINGs fail to hatch and cell division in these embryos is halted. The formation of SINGs can be prevented by pre-exposure to a brief period of heat shock before stress exposure. Heat shock inhibition of SINGs is dependent upon the HSF-1 transcription factor.

Conclusions

The heat shock results suggest that chaperone expression can prevent SING formation and that the accumulation of damaged or misfolded proteins is a necessary precursor to SING formation. Thus, SINGs may be part of a novel protein quality control system. The data suggest an interesting model where SINGs represent sites of localized protein degradation for nuclear or cytosolic proteins. Thus, the physiological impacts of environmental stress may begin at the cellular level with the formation of stress induced nuclear granules.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-017-0136-x) contains supplementary material, which is available to authorized users.

Regulation of transcription factors by sumoylation

Transcription factors (TFs) are among the most frequently detected targets of sumoylation, and effects of the modification have been studied for about 200 individual TFs to date. TF sumoylation is most often associated with reduced target gene expression, which can be mediated by enhanced interactions with corepressors or by interference with protein modifications that promote transcription. However, recent studies show that sumoylation also regulates gene expression by controlling the levels of TFs that are associated with chromatin. SUMO can mediate this by modulating TF DNA-binding activity, promoting clearance of TFs from chromatin, or indirectly, by influencing TF abundance or localization.

Arabidopsis B-cell lymphoma2 (Bcl-2)-associated athanogene 7 (BAG7)-mediated heat tolerance requires translocation, sumoylation and binding to WRKY29

To cope with stress and increased accumulation of misfolded proteins, plants and animals use a survival pathway known as the unfolded protein response (UPR) that signals between the endoplasmic reticulum (ER) and the nucleus to maintain cell homeostasis via proper folding of proteins. B-cell lymphoma2 (Bcl-2)-associated athanogene (BAG) proteins are an evolutionarily conserved family of co-chaperones that are linked to disease states in mammals and responses to environmental stimuli (biotic and abiotic) in plants. Molecular and physiological techniques were used to functionally characterize a newly identified branch of the UPR initiated by the ER-localized co-chaperone from Arabidopsis thaliana, AtBAG7. AtBAG7 has functional roles in both the ER and the nucleus. Upon heat stress, AtBAG7 is sumoylated, proteolytically processed and translocated from the ER to the nucleus, where interaction with the WRKY29 transcription factor occurs. Sumoylation and translocation are required for the AtBAG7-WRKY29 interaction and subsequent stress tolerance. In the ER, AtBAG7 interacts with the ER-localized transcription factor, AtbZIP28, and established UPR regulator, the AtBiP2 chaperone. The results indicate that AtBAG7 plays a central regulatory role in the heat-induced UPR pathway.

Introduction to Sumoylation

Reversible post-translational modification is a rapid and efficient system to control the activity of pre-existing proteins. Modifiers range from small chemical moieties, such as phosphate groups, to proteins themselves as the modifier. The patriarch of the protein modifiers is ubiquitin which plays a central role in protein degradation and protein targeting. Over the last 20 years, the ubiquitin family has expanded to include a variety of ubiquitin-related small modifier proteins that are all covalently attached to a lysine residue on target proteins via series of enzymatic reactions. Of these more recently discovered ubiquitin-like proteins, the SUMO family has gained prominence as a major regulatory component that impacts numerous aspects of cell growth, differentiation, and response to stress. Unlike ubiquitinylation which often leads to proteins turn over, sumoylation performs a variety of function such as altering protein stability, modulating protein trafficking, directing protein-protein interactions, and regulating protein activity. This chapter will introduce the basic properties of SUMO proteins and the general tenets of sumoylation.

SUMO-interacting motifs (SIMs) in Polo-like kinase 1-interacting checkpoint helicase (PICH) ensure proper chromosome segregation during mitosis

Polo-like kinase 1 (Plk1)-interacting checkpoint helicase (PICH) localizes at the centromere and is critical for proper chromosome segregation during mitosis. However, the precise molecular mechanism of PICH's centromeric localization and function at the centromere is not yet fully understood. Recently, using Xenopus egg extract assays, we showed that PICH is a promiscuous SUMO binding protein. To further determine the molecular consequence of PICH/SUMO interaction on PICH function, we identified 3 SUMO-interacting motifs (SIMs) on PICH and generated a SIM-deficient PICH mutant. Using the conditional expression of PICH in cells, we found distinct roles of PICH SIMs during mitosis. Although all SIMs are dispensable for PICH's localization on ultrafine anaphase DNA bridges, only SIM3 (third SIM, close to the C-terminus end of PICH) is critical for its centromeric localization. Intriguingly, the other 2 SIMs function in chromatin bridge prevention. With these results, we propose a novel SUMO-dependent regulation of PICH's function on mitotic centromeres.

HSF1: Guardian of Proteostasis in Cancer

Proteomic instability is causally related to human diseases. In guarding proteome stability, the heat shock factor 1 (HSF1)-mediated proteotoxic stress response plays a pivotal role. Contrasting with its beneficial role of enhancing cell survival, recent findings have revealed a compelling pro-oncogenic role for HSF1. However, the mechanisms underlying the persistent activation and function of HSF1 within malignancy remain poorly understood. Emerging evidence reveals that oncogenic signaling mobilizes HSF1 and that cancer cells rely on HSF1 to avert proteomic instability and repress tumor-suppressive amyloidogenesis. In aggregate, these new developments suggest that cancer cells endure chronic proteotoxic stress and that proteomic instability is intrinsically associated with the malignant state, a characteristic that could be exploited to combat cancer.

Roles for SUMO in pre-mRNA processing

When the small ubiquitin-like modifier (SUMO)-1 protein is localized on the genome, it is found on proteins bound to the promoters of the most highly active genes and on proteins bound to the DNA-encoding exons. Inhibition of the SUMO-1 modification leads to reductions in initiation of messenger RNA (mRNA) synthesis and splicing. In this review, we discuss what is known about the SUMOylation of factors involved in transcription initiation, pre-mRNA processing, and polyadenylation. We suggest a mechanism by which SUMO modifications of factors at the promoters of high-activity genes trigger the formation of an RNA polymerase II complex that coordinates and integrates the stimulatory signals for each process to catalyze an extremely high level of gene expression. WIREs RNA 2016, 7:105-112. doi: 10.1002/wrna.1318 For further resources related to this article, please visit the WIREs website.

SUMO-regulated transcription: challenging the dogma

The small ubiquitin-like modifier SUMO regulates many aspects of cellular physiology to maintain cell homeostasis, both under normal conditions and during cell stress. Components of the transcriptional apparatus and chromatin are among the most prominent SUMO substrates. The prevailing view is that SUMO serves to repress transcription. However, as we will discuss in this review, this model needs to be refined, because recent studies have revealed that SUMO can also have profound positive effects on transcription.

Sumo and the cellular stress response

The ubiquitin family member Sumo has important functions in many cellular processes including DNA repair, transcription and cell division. Numerous studies have shown that Sumo is essential for maintaining cell homeostasis when the cell encounters endogenous or environmental stress, such as osmotic stress, hypoxia, heat shock, genotoxic stress, and nutrient stress. Regulation of transcription is a key component of the Sumo stress response, and multiple mechanisms have been described by which Sumo can regulate transcription. Although many individual substrates have been described that are sumoylated during the Sumo stress response, an emerging concept is modification of entire complexes or pathways by Sumo. This review focuses on the function and regulation of Sumo during the stress response.

When Cancer Fights Back: Multiple Myeloma, Proteasome Inhibition, and the Heat-Shock Response

Multiple myeloma is a plasma cell malignancy with an estimated 26,850 new cases and 11,240 deaths in 2015 in the United States. Two main classes of agents are the mainstays of therapy-proteasome inhibitors (PI) and immunomodulatory drugs (IMiD). Other new targets are emerging rapidly, including monoclonal antibodies and histone deacetylase (HDAC) inhibitors. These therapeutic options have greatly improved overall survival, but currently only 15% to 20% of patients experience long-term progression-free survival or are cured. Therefore, improvement in treatment options is needed. One potential means of improving clinical options is to target resistance mechanisms for current agents. For example, eliminating the cytoprotective heat-shock response that protects myeloma cells from proteasome inhibition may enhance PI-based therapies. The transcription factor heat-shock factor 1 (HSF1) is the master regulator of the heat-shock response. HSF1 is vital in the proteotoxic stress response, and its activation is controlled by posttranslational modifications (PTM). This review details the mechanisms of HSF1 regulation and discusses leveraging that regulation to enhance PI activity.

SUMOylation by a stress-specific small ubiquitin-like modifier E2 conjugase is essential for survival of Chlamydomonas reinhardtii under stress conditions

Posttranslational modification of proteins by small ubiquitin-like modifier (SUMO) is required for survival of virtually all eukaryotic organisms. Attachment of SUMO to target proteins is catalyzed by SUMO E2 conjugase. All haploid or diploid eukaryotes studied to date possess a single indispensable SUMO conjugase. We report here the unanticipated isolation of a Chlamydomonas reinhardtii (mutant5 [mut5]). in which the previously identified SUMO conjugase gene C. reinhardtii ubiquitin-conjugating enzyme9 (CrUBC9) is deleted. This surprising mutant is viable and unexpectedly, displays a pattern of protein SUMOylation at 25°C that is essentially identical to wild-type cells. However, unlike wild-type cells, mut5 fails to SUMOylate a large set of proteins in response to multiple stress conditions, a failure that results in a markedly reduced tolerance or complete lack of tolerance to these stresses. Restoration of expected stress-induced protein SUMOylation patterns as well as normal stress tolerance phenotypes in mut5 cells complemented with a CrUBC9 gene shows that CrUBC9 is an authentic SUMO conjugase and, more importantly, that SUMOylation is essential for cell survival under stress conditions. The presence of bona fide SUMOylated proteins in the mut5 mutant at 25°C can only be explained by the presence of at least one additional SUMO conjugase in C. reinhardtii, a conjugase tentatively identified as CrUBC3. Together, these results suggest that, unlike all other nonpolyploid eukaryotes, there are at least two distinct and functional SUMO E2 conjugases in C. reinhardtii, with a clear division of labor between the two sets: One (CrUBC9) is involved in essential stress-induced SUMOylations, and one (CrUBC3) is involved in housekeeping SUMOylations.

Post-transcriptional and post-translational regulations of drought and heat response in plants: a spider's web of mechanisms

Drought and heat tolerance are complex quantitative traits. Moreover, the adaptive significance of some stress-related traits is more related to plant survival than to agronomic performance. A web of regulatory mechanisms fine-tunes the expression of stress-related traits and integrates both environmental and developmental signals. Both post-transcriptional and post-translational modifications contribute substantially to this network with a pivotal regulatory function of the transcriptional changes related to cellular and plant stress response. Alternative splicing and RNA-mediated silencing control the amount of specific transcripts, while ubiquitin and SUMO modify activity, sub-cellular localization and half-life of proteins. Interactions across these modification mechanisms ensure temporally and spatially appropriate patterns of downstream-gene expression. For key molecular components of these regulatory mechanisms, natural genetic diversity exists among genotypes with different behavior in terms of stress tolerance, with effects upon the expression of adaptive morphological and/or physiological target traits.

SUMOylation regulates polo-like kinase 1-interacting checkpoint helicase (PICH) during mitosis

Mitotic SUMOylation has an essential role in faithful chromosome segregation in eukaryotes, although its molecular consequences are not yet fully understood. In Xenopus egg extract assays, we showed that poly(ADP-ribose) polymerase 1 (PARP1) is modified by SUMO2/3 at mitotic centromeres and that its enzymatic activity could be regulated by SUMOylation. To determine the molecular consequence of mitotic SUMOylation, we analyzed SUMOylated PARP1-specific binding proteins. We identified Polo-like kinase 1-interacting checkpoint helicase (PICH) as an interaction partner of SUMOylated PARP1 in Xenopus egg extract. Interestingly, PICH also bound to SUMOylated topoisomerase IIα (TopoIIα), a major centromeric small ubiquitin-like modifier (SUMO) substrate. Purified recombinant human PICH interacted with SUMOylated substrates, indicating that PICH directly interacts with SUMO, and this interaction is conserved among species. Further analysis of mitotic chromosomes revealed that PICH localized to the centromere independent of mitotic SUMOylation. Additionally, we found that PICH is modified by SUMO2/3 on mitotic chromosomes and in vitro. PICH SUMOylation is highly dependent on protein inhibitor of activated STAT, PIASy, consistent with other mitotic chromosomal SUMO substrates. Finally, the SUMOylation of PICH significantly reduced its DNA binding capability, indicating that SUMOylation might regulate its DNA-dependent ATPase activity. Collectively, our findings suggest a novel SUMO-mediated regulation of the function of PICH at mitotic centromeres.

Fever, immunity, and molecular adaptations

The heat shock response (HSR) is an ancient and highly conserved process that is essential for coping with environmental stresses, including extremes of temperature. Fever is a more recently evolved response, during which organisms temporarily subject themselves to thermal stress in the face of infections. We review the phylogenetically conserved mechanisms that regulate fever and discuss the effects that febrile-range temperatures have on multiple biological processes involved in host defense and cell death and survival, including the HSR and its implications for patients with severe sepsis, trauma, and other acute systemic inflammatory states. Heat shock factor-1, a heat-induced transcriptional enhancer is not only the central regulator of the HSR but also regulates expression of pivotal cytokines and early response genes. Febrile-range temperatures exert additional immunomodulatory effects by activating mitogen-activated protein kinase cascades and accelerating apoptosis in some cell types. This results in accelerated pathogen clearance, but increased collateral tissue injury, thus the net effect of exposure to febrile range temperature depends in part on the site and nature of the pathologic process and the specific treatment provided.

Expression of HSF2 decreases in mitosis to enable stress-inducible transcription and cell survival

In spite of global transcriptional inhibition, a decrease in HSF2 expression during mitosis allows for heat shock protein expression and protects cells against proteotoxicity.

Unless mitigated, external and physiological stresses are detrimental for cells, especially in mitosis, resulting in chromosomal missegregation, aneuploidy, or apoptosis. Heat shock proteins (Hsps) maintain protein homeostasis and promote cell survival. Hsps are transcriptionally regulated by heat shock factors (HSFs). Of these, HSF1 is the master regulator and HSF2 modulates Hsp expression by interacting with HSF1. Due to global inhibition of transcription in mitosis, including HSF1-mediated expression of Hsps, mitotic cells are highly vulnerable to stress. Here, we show that cells can counteract transcriptional silencing and protect themselves against proteotoxicity in mitosis. We found that the condensed chromatin of HSF2-deficient cells is accessible for HSF1 and RNA polymerase II, allowing stress-inducible Hsp expression. Consequently, HSF2-deficient cells exposed to acute stress display diminished mitotic errors and have a survival advantage. We also show that HSF2 expression declines during mitosis in several but not all human cell lines, which corresponds to the Hsp70 induction and protection against stress-induced mitotic abnormalities and apoptosis.

SUMOylation regulates AKT1 activity

Serine threonine kinase AKT has a central role in the cell, controlling survival, proliferation, metabolism and angiogenesis. Deregulation of its activity underlies a wide range of pathological situations, including cancer. Here we show that AKT is post-translationally modified by the small ubiquitin-like modifier (SUMO) protein. Interestingly, neither SUMO conjugation nor activation of SUMOylated AKT is regulated by the classical AKT targeting to the cell membrane or by the phosphoinositide 3-kinase pathway. We demonstrate that SUMO induces the activation of AKT, whereas, conversely, down-modulation of the SUMO machinery diminishes AKT activation and cell proliferation. Furthermore, an AKT SUMOylation mutant shows reduced activation, and decreased anti-apoptotic and pro-tumoral activities in comparison with the wild-type protein. These results identify SUMO as a novel key regulator of AKT phosphorylation and activity.