SUMOylation modulates the transcription repressor function of RIP140

Mitochondrial Imbalance in Down Syndrome: A Driver of Accelerated Brain Aging?

Down syndrome (DS), caused by trisomy of chromosome 21 (HSA21), is a complex condition associated with neurodevelopmental impairments and accelerated brain aging, often culminating in early-onset Alzheimer's disease (AD). Central to this accelerated aging is mitochondrial imbalance, characterized by disrupted energy metabolism, increased oxidative stress, impaired dynamics, and defective quality control mechanisms like mitophagy. These abnormalities exacerbate neuronal vulnerability, driving cognitive decline and neurodegeneration. This review examines the genetic and biochemical underpinnings of mitochondrial dysfunction in DS, with a focus on the role of HSA21-encoded genes. We also highlight how mitochondrial dysfunction, amplified by oxidative stress and HSA21 gene dosage effects, converges with cellular senescence and neuroinflammation to accelerate Alzheimer-like pathology and brain aging in DS. Finally, we discuss emerging therapeutic strategies targeting mitochondrial pathways, which hold promise for mitigating neurodegenerative phenotypes and improving outcomes in DS.

Depolymerization of SUMO chains induces slender to stumpy differentiation in T. brucei bloodstream parasites

Trypanosoma brucei are protozoan parasites that cause sleeping sickness in humans and nagana in cattle. Inside the mammalian host, a quorum sensing-like mechanism coordinates its differentiation from a slender replicative form into a quiescent stumpy form, limiting growth and activating metabolic pathways that are beneficial to the parasite in the insect host. The post-translational modification of proteins with the Small Ubiquitin-like MOdifier (SUMO) enables dynamic regulation of cellular metabolism. SUMO can be conjugated to its targets as a monomer but can also form oligomeric chains. Here, we have investigated the role of SUMO chains in T. brucei by abolishing the ability of SUMO to polymerize. We have found that parasites able to conjugate only SUMO monomers are primed for differentiation. This was demonstrated for monomorphic lines that are normally unable to produce stumpy forms in response to quorum sensing signaling in mice, and also for pleomorphic cell lines in which stumpy cells were observed at unusually low parasitemia levels. SUMO chain mutants showed a stumpy compatible transcriptional profile and better competence to differentiate into procyclics. Our study indicates that SUMO depolymerization may represent a coordinated signal triggered during stumpy activation program.

New insights into the genetic basis of premature ovarian insufficiency: Novel causative variants and candidate genes revealed by genomic sequencing

Ovarian deficiency, including premature ovarian insufficiency (POI) and diminished ovarian reserve (DOR), represents one of the main causes of female infertility. POI is a genetically heterogeneous condition but current understanding of its genetic basis is far from complete, with the cause remaining unknown in the majority of patients. The genes that regulate DOR have been reported but the genetic basis of DOR has not been explored in depth. Both conditions are likely to lie along a continuum of degrees of decrease in ovarian reserve. We performed genomic analysis via whole exome sequencing (WES) followed by in silico analyses and functional experiments to investigate the genetic cause of ovarian deficiency in ten affected women. We achieved diagnoses for three of them, including the identification of novel variants in STAG3, GDF9, and FANCM. We identified potentially causative FSHR variants in another patient. This is the second report of biallelic GDF9 and FANCM variants, and, combined with functional support, validates these genes as bone fide autosomal recessive "POI genes". We also identified new candidate genes, NRIP1, XPO1, and MACF1. These genes have been linked to ovarian function in mouse, pig, and zebrafish respectively, but never in humans. In the case of NRIP1, we provide functional support for the deleterious nature of the variant via SUMOylation and luciferase/β-galactosidase reporter assays. Our study provides multiple insights into the genetic basis of POI/DOR. We have further elucidated the involvement of GDF9, FANCM, STAG3 and FSHR in POI pathogenesis, and propose new candidate genes, NRIP1, XPO1, and MACF1, which should be the focus of future studies.

Steroidogenic Factor 1 (NR5A1) Activates ATF3 Transcriptional Activity

Steroidogenic Factor 1 (SF-1/NR5A1), an orphan nuclear receptor, is important for sexual differentiation and the development of multiple endocrine organs, as well as cell proliferation in cancer cells. Activating transcription factor 3 (ATF3) is a transcriptional repressor, and its expression is rapidly induced by DNA damage and oncogenic stimuli. Since both NR5A1 and ATF3 can regulate and cooperate with several transcription factors, we hypothesized that NR5A1 may interact with ATF3 and plays a functional role in cancer development. First, we found that NR5A1 physically interacts with ATF3. We further demonstrated that ATF3 expression is up-regulated by NR5A1. Moreover, the promoter activity of the ATF3 is activated by NR5A1 in a dose-dependent manner in several cell lines. By mapping the ATF3 promoter as well as the site-directed mutagenesis analysis, we provide evidence that NR5A1 response elements (-695 bp and -665 bp) are required for ATF3 expression by NR5A1. It is well known that the transcriptional activities of NR5A1 are modulated by post-translational modifications, such as small ubiquitin-related modifier (SUMO) modification and phosphorylation. Notably, we found that both SUMOylation and phosphorylation of NR5A1 play roles, at least in part, for NR5A1-mediated ATF3 expression. Overall, our results provide the first evidence of a novel relationship between NR5A1 and ATF3.

Immune and Neuroprotective Effects of Physical Activity on the Brain in Depression

Physical activity-a lifestyle factor that is associated with immune function, neuroprotection, and energy metabolism-modulates the cellular and molecular processes in the brain that are vital for emotional and cognitive health, collective mechanisms that can go awry in depression. Physical activity optimizes the stress response, neurotransmitter level and function (e.g., serotonergic, noradrenergic, dopaminergic, and glutamatergic), myokine production (e.g., interleukin-6), transcription factor levels and correlates [e.g., peroxisome proliferator-activated receptor C coactivator-1α [PGC-1α], mitochondrial density, nitric oxide pathway activity, Ca2+ signaling, reactive oxygen specie production, and AMP-activated protein kinase [AMPK] activity], kynurenine metabolites, glucose regulation, astrocytic health, and growth factors (e.g., brain-derived neurotrophic factor). Dysregulation of these interrelated processes can effectuate depression, a chronic mental illness that affects millions of individuals worldwide. Although the biogenic amine model has provided some clinical utility in understanding chronic depression, a need remains to better understand the interrelated mechanisms that contribute to immune dysfunction and the means by which various therapeutics mitigate them. Fortunately, convergent evidence suggests that physical activity improves emotional and cognitive function in persons with depression, particularly in those with comorbid inflammation. Accordingly, the aims of this review are to (1) underscore the link between inflammatory correlates and depression, (2) explicate immuno-neuroendocrine foundations, (3) elucidate evidence of neurotransmitter and cytokine crosstalk in depressive pathobiology, (4) determine the immunomodulatory effects of physical activity in depression, (5) examine protocols used to effectuate the positive effects of physical activity in depression, and (6) highlight implications for clinicians and scientists. It is our contention that a deeper understanding of the mechanisms by which inflammation contributes to the pathobiology of depression will translate to novel and more effective treatments, particularly by identifying relevant patient populations that can benefit from immune-based therapies within the context of personalized medicine.

Mitochondrial dysfunction in down syndrome: molecular mechanisms and therapeutic targets

Trisomy of chromosome 21 (TS21) is the most common autosomal aneuploidy compatible with postnatal survival with a prevalence of 1 in 700 newborns. Its phenotype is highly complex with constant features, such as mental retardation, dysmorphic traits and hypotonia, and variable features including heart defects, susceptibility to Alzheimer’s disease (AD), type 2 diabetes, obesity and immune disorders. Overexpression of genes on chromosome-21 (Hsa21) is responsible for the pathogenesis of Down syndrome (DS) phenotypic features either in a direct or in an indirect manner since many Hsa21 genes can affect the expression of other genes mapping to different chromosomes. Many of these genes are involved in mitochondrial function and energy conversion, and play a central role in the mitochondrial dysfunction and chronic oxidative stress, consistently observed in DS subjects.

Recent studies highlight the deep interconnections between mitochondrial dysfunction and DS phenotype. In this short review we first provide a basic overview of mitochondrial phenotype in DS cells and tissues. We then discuss how specific Hsa21 genes may be involved in determining the disruption of mitochondrial DS phenotype and biogenesis. Finally we briefly focus on drugs that affect mitochondrial function and mitochondrial network suggesting possible therapeutic approaches to improve and/or prevent some aspects of the DS phenotype.

The Emerging Role of Vitamin B6 in Inflammation and Carcinogenesis

Vitamin B6 serves as a coenzyme catalyzing more than 150 enzymes regulating metabolism and synthesis of proteins, carbohydrates, lipids, heme, and important bioactive metabolites. For several years vitamin B6 and its vitamers (B6) were recognized as antioxidant and antiinflammatory and in modulating immunity and gene expression. During the last 10 years, there were growing reports implicating B6 in inflammation and inflammation-related chronic illnesses including cancer. It is unclear if the deficiency of B6 or additional intake of B6, above the current requirement, should be the focus. Whether the current recommended daily intake for B6 is adequate should be revisited, since B6 is important to human health beyond its role as a coenzyme and its status is affected by many factors including but not limited to age, obesity, and inflammation associated with chronic illnesses. A link between inflammation B6 status and carcinogenesis is not yet completely understood. B6-mediated synthesis of H₂S, a gasotransmitter, and taurine in health and disease, especially in maintaining mitochondrial integrity and biogenesis and inflammation, remains an important area to be explored. Recent developments in the molecular role of B6 and its direct interaction with inflammasomes, and nuclear receptor corepressor and coactivator, receptor-interacting protein 140, provide a strong impetus to further explore the multifaceted role of B6 in carcinogenesis and human health.

Roles of FoxM1 in cell regulation and breast cancer targeting therapy

Forkhead box M1 (FoxM1) is an oncogenic transcription factor involved in a wide variety of cellular processes, such as cell cycle progression, proliferation, differentiation, migration, metabolism and DNA damage response. It is overexpressed in many human cancers, especially in breast cancers. Posttranslational modifications are known to play an important role in regulating the expression and transcriptional activity of FoxM1. In this review, we characterize the posttranslational modifications of FoxM1, summarize modifications of FoxM1 by different kinases, explore the relationship between the different sites of modifications and comprehensively describe how posttranslational modifications to regulate the function of FoxM1 by changing protein stability, nucleus localization and transcriptional activity. Additionally, we systematically summarize the roles of FoxM1 in breast cancer occurrence, therapy and drug resistance. The purpose of this paper tries to give a better understanding of the regulatory mechanisms of FoxM1 in cell regulation and highlights potential of a new method for breast cancer therapy by targeting FoxM1.

PIAS1 binds p300 and behaves as a coactivator or corepressor of the transcription factor c-Myb dependent on SUMO-status

The PIAS proteins (Protein Inhibitor of Activated STATs) constitute a family of multifunctional nuclear proteins operating as SUMO E3 ligases and being involved in a multitude of interactions. They participate in a range of biological processes, also beyond their well-established role in the immune system and cytokine signalling. They act both as transcriptional corepressors and coactivators depending on the context. In the present work, we investigated mechanisms by which PIAS1 causes activation or repression of c-Myb dependent target genes. Analysis of global expression data shows that c-Myb and PIAS1 knockdowns affect a subset of common targets, but with a dual outcome consistent with a role of PIAS1 as either a corepressor or coactivator. Our mechanistic studies show that PIAS1 engages in a novel interaction with the acetyltransferase and coactivator p300. Interaction and ChIP analysis suggest a bridging function where PIAS1 enhances p300 recruitment to c-Myb-bound sites through interaction with both proteins. In addition, the E3 activity of PIAS1 enhances further its coactivation. Remarkably, the SUMO status of c-Myb had a decisive role, indicating a SUMO-dependent switch in the way PIAS1 affects c-Myb, either as a coactivator or corepressor. Removal of the two major SUMO-conjugation sites in c-Myb (2KR mutant), which enhances its activity significantly, turned PIAS1 into a corepressor. Also, p300 was less efficiently recruited to chromatin by c-Myb-2KR. We propose that PIAS1 acts as a "protein inhibitor of activated c-Myb" in the absence of SUMOylation while, in its presence, PIAS behaves as a "protein activator of repressed c-Myb".

Downregulation of RIP140 in hepatocellular carcinoma promoted the growth and migration of the cancer cells

Hepatocellular carcinoma (HCC) is one of the most common malignancies with a poor response to chemotherapy. It is very important to identify novel diagnosis biomarkers and therapeutic targets. RIP140, a regulator of estrogen receptor, recently has been found to be involved in the tumorigenesis. However, its function in the progression of HCC remains poorly understood. Here, we found that the expression of RIP140 was downregulated in the HCC tissues. Moreover, overexpression of RIP140 in HCC cells inhibited cell proliferation and migration, while downregulation of RIP140 promoted the tumorigenicity of HCC cells in vitro and in vivo. Mechanistically, RIP140 interacted with beta-catenin and negatively regulated beta-catenin/TCF signaling. Taken together, our study suggests the suppressive roles of RIP140 in the pathogenesis of HCC.

Discriminating between lysine sumoylation and lysine acetylation using mRMR feature selection and analysis

Post-translational modifications (PTMs) are crucial steps in protein synthesis and are important factors contributing to protein diversity. PTMs play important roles in the regulation of gene expression, protein stability and metabolism. Lysine residues in protein sequences have been found to be targeted for both types of PTMs: sumoylations and acetylations; however, each PTM has a different cellular role. As experimental approaches are often laborious and time consuming, it is challenging to distinguish the two types of PTMs on lysine residues using computational methods. In this study, we developed a method to discriminate between sumoylated lysine residues and acetylated residues. The method incorporated several features: PSSM conservation scores, amino acid factors, secondary structures, solvent accessibilities and disorder scores. By using the mRMR (Maximum Relevance Minimum Redundancy) method and the IFS (Incremental Feature Selection) method, an optimal feature set was selected from all of the incorporated features, with which the classifier achieved 92.14% accuracy with an MCC value of 0.7322. Analysis of the optimal feature set revealed some differences between acetylation and sumoylation. The results from our study also supported the previous finding that there exist different consensus motifs for the two types of PTMs. The results could suggest possible dominant factors governing the acetylation and sumoylation of lysine residues, shedding some light on the modification dynamics and molecular mechanisms of the two types of PTMs, and provide guidelines for experimental validations.

Receptor-interacting protein 140 attenuates endoplasmic reticulum stress in neurons and protects against cell death

Inositol 1, 4, 5-trisphosphate receptor (IP3R)-mediated Ca(2+) release from the endoplasmic reticulum (ER) triggers many physiological responses in neurons, and when uncontrolled can cause ER stress that contributes to neurological disease. Here we show that the unfolded protein response (UPR) in neurons induces rapid translocation of nuclear receptor-interacting protein 140 (RIP140) to the cytoplasm. In the cytoplasm, RIP140 localizes to the ER by binding to the IP3R. The carboxyl-terminal RD4 domain of RIP140 interacts with the carboxyl-terminal gate-keeping domain of the IP3R. This molecular interaction disrupts the IP3R's 'head-tail' interaction, thereby suppressing channel opening and attenuating IP3R-mediated Ca(2+) release. This contributes to a rapid suppression of the ER stress response and provides protection from apoptosis in both hippocampal neurons in vitro and in an animal model of ER stress. Thus, RIP140 translocation to the cytoplasm is an early response to ER stress and provides protection against neuronal death.

SUMOylation of FOXM1B alters its transcriptional activity on regulation of MiR-200 family and JNK1 in MCF7 human breast cancer cells

Transcription factor Forkhead Box Protein M1 (FOXM1) is a well-known master regulator in controlling cell-cycle pathways essential for DNA replication and mitosis, as well as cell proliferation. Among the three major isoforms of FOXM1, FOXM1B is highly associated with tumor growth and metastasis. The activities of FOXM1B are modulated by post-translational modifications (PTMs), such as phosphorylation, but whether it is modified by small ubiquitin-related modifier (SUMO) remains unknown. The aim of the current study was to determine whether FOXM1B is post-translationally modified by SUMO proteins and also to identify SUMOylation of FOXM1B on its target gene transcription activity. Here we report that FOXM1B is clearly defined as a SUMO target protein at the cellular levels. Moreover, a SUMOylation protease, SENP2, significantly decreased SUMOylation of FOXM1B. Notably, FOXM1B is selectively SUMOylated at lysine residue 463. While SUMOylation of FOXM1B is required for full repression of its target genes MiR-200b/c and p21, SUMOylation of FOXM1B is essential for full activation of JNK1 gene. Overall, we provide evidence that FOXM1B is post-translationally modified by SUMO and SUMOylation of FOXM1B plays a functional role in regulation of its target gene activities.

NRIP1/RIP140 siRNA-mediated attenuation counteracts mitochondrial dysfunction in Down syndrome

Mitochondrial dysfunction, which is consistently observed in Down syndrome (DS) cells and tissues, might contribute to the severity of the DS phenotype. Our recent studies on DS fetal hearts and fibroblasts have suggested that one of the possible causes of mitochondrial dysfunction is the downregulation of peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PGC-1α or PPARGC1A)--a key modulator of mitochondrial function--and of several nuclear-encoded mitochondrial genes (NEMGs). Re-analysis of publicly available expression data related to manipulation of chromosome 21 (Hsa21) genes suggested the nuclear receptor interacting protein 1 (NRIP1 or RIP140) as a good candidate Hsa21 gene for NEMG downregulation. Indeed, NRIP1 is known to affect oxidative metabolism and mitochondrial biogenesis by negatively controlling mitochondrial pathways regulated by PGC-1α. To establish whether NRIP1 overexpression in DS downregulates both PGC-1α and NEMGs, thereby causing mitochondrial dysfunction, we used siRNAs to decrease NRIP1 expression in trisomic human fetal fibroblasts. Levels of PGC-1α and NEMGs were increased and mitochondrial function was restored, as shown by reactive oxygen species decrease, adenosine 5'-triphosphate (ATP) production and mitochondrial activity increase. These findings indicate that the Hsa21 gene NRIP1 contributes to the mitochondrial dysfunction observed in DS. Furthermore, they suggest that the NRIP1-PGC-1α axe might represent a potential therapeutic target for restoring altered mitochondrial function in DS.

Acidic residue Glu199 increases SUMOylation level of nuclear hormone receptor NR5A1

Steroidogenic factor 1 (NR5A1/SF1) is a well-known master regulator in controlling adrenal and sexual development, as well as regulating numerous genes involved in adrenal and gonadal steroidogenesis. Several studies including ours have demonstrated that NR5A1 can be SUMOylated on lysine 194 (K194, the major site) and lysine 119 (K119, the minor site), and the cycle of SUMOylation regulates NR5A1’s transcriptional activity. An extended consensus negatively charged amino acid-dependent SUMOylation motif (NDSM) enhances the specificity of substrate modification by SUMO has been reported; however, the mechanism of NDSM for NR5A1 remains to be clarified. In this study, we investigated the functional significance of the acidic residue located downstream from the core consensus SUMO site of NR5A1. Here we report that E199A (glutamic acid was replaced with alanine) of NR5A1 reduced, but not completely abolished, its SUMOylation level. We next characterized the functional role of NR5A1 E199A on target gene expression and protein levels. We found that E199A alone, as well as combination with K194R, increased Mc2r and Cyp19a1 reporter activities. Moreover, E199A alone as well as combination with K194R enhanced NR5A1-mediated STAR protein levels in mouse adrenocortical cancer Y1 cells. We also observed that E199A increased interaction of NR5A1 with CDK7 and SRC1. Overall, we provide the evidence that the acidic residue (E199) located downstream from the core consensus SUMO site of NR5A1 is, at least in part, required for SUMOylation of NR5A1 and for its mediated target gene and protein expression.

Identification of a non-covalent ternary complex formed by PIAS1, SUMO1, and UBC9 proteins involved in transcriptional regulation

Post-translational modifications with ubiquitin-like proteins require three sequentially acting enzymes (E1, E2, and E3) that must unambiguously recognize each other in a coordinated fashion to achieve their functions. Although a single E2 (UBC9) and few RING-type E3s (PIAS) operate in the SUMOylation system, the molecular determinants regulating the interactions between UBC9 and the RING-type E3 enzymes are still not well defined. In this study we use biochemical and functional experiments to characterize the interactions between PIAS1 and UBC9. Our results reveal that UBC9 and PIAS1 are engaged both in a canonical E2·E3 interaction as well as assembled into a previously unidentified non-covalent ternary complex with SUMO as evidenced by bioluminescence resonance energy transfer, nuclear magnetic resonance spectroscopy, and isothermal titration calorimetry studies. In this ternary complex, SUMO functions as a bridge by forming non-overlapping interfaces with UBC9 and PIAS1. Moreover, our data suggest that phosphorylation of serine residues adjacent to the PIAS1 SUMO-interacting motif favors formation of the non covalent PIAS1·SUMO·UBC9 ternary complex. Finally, our results also indicate that the non-covalent ternary complex is required for the known transcriptional repression activities mediated by UBC9 and SUMO1. Taken together, the data enhance our knowledge concerning the mode of interaction of enzymes of the SUMOylation machinery as well as their role in transcriptional regulation and establishes a framework for investigations of other ubiquitin-like protein systems.

SUMOylation of ATF3 alters its transcriptional activity on regulation of TP53 gene

Cyclic AMP-dependent transcription factor-3 (ATF3), a stress sensor, plays an essential role in cells to maintain homeostasis and has diverse functions in cellular survival and death signal pathways. ATF3 is a novel regulator of p53 protein stability and function. The activities of ATF3 are modulated by post-translational modifications (PTMs), such as ubiquitination, but whether it is modified by small ubiquitin-related modifier (SUMO) remains unknown. The aim of this study was to investigate whether ATF3 is post-translationally modified by SUMO proteins and also to elucidate SUMOylation of ATF3 on TP53 gene activity. Here we report that ATF3 is clearly defined as a SUMO target protein both in vitro SUMOylation assay using recombinant proteins and at the cellular levels. Furthermore, ATF3 interacted with UBE2I, the only SUMO E2 enzyme found so far. In addition, PIAS3β (a SUMO E3 ligase) enhanced and SENP2 and SENP7 (two SUMOylation proteases) decreased SUMOylation of ATF3, respectively. Finally, we found that ATF3 is selectively SUMOylated at lysine residue 42 but the SUMOylation does not alter subcellular localization of ATF3. We then characterized the functional role of ATF3 SUMOylation on TP53 gene expression. We found that SUMOylation of ATF3 is required for full repression of TP53 gene. Overall, we provide the first evidence that ATF3 is post-translationally modified by SUMO and SUMOylation of ATF3 plays a functional role in regulation of TP53 gene activity.

Loss of SUMOylation on ATF3 inhibits proliferation of prostate cancer cells by modulating CCND1/2 activity

SUMOylation plays an important role in regulating a wide range of cellular processes. Previously, we showed that ATF3, a stress response mediator, can be SUMOylated and lysine 42 is the major SUMO site. However, the significance of ATF3 SUMOylation in biological processes is still poorly understood. In the present study, we investigated the role of ATF3 SUMOylation on CCND activity and cellular proliferation in human prostate cancer cells. First, we showed that ATF3 can be SUMOylated endogenously in the overexpression system, and lysine 42 is the major SUMO site. Unlike normal prostate tissue and androgen-responsive LNCaP cancer cells, androgen-independent PC3 and DU145 cancer cells did not express ATF3 endogenously. Overexpression of ATF3 increased CCND1/2 expression in PC3 and DU145 cancer cells. Interestingly, we observed that SUMOylation is essential for ATF3-mediated CCND1/2 activation. Finally, we observed that SUMOylation plays a functional role in ATF3-mediated cellular proliferation in PC3 and DU145 cells. Taken together, our results demonstrate that SUMO modification of ATF3 influences CCND1/2 activity and cellular proliferation of prostate cancer PC3 and DU145 cells and explains at least in part how ATF3 functions to regulate cancer development.

AMPK regulation of fatty acid metabolism and mitochondrial biogenesis: implications for obesity

Skeletal muscle plays an important role in regulating whole-body energy expenditure given it is a major site for glucose and lipid oxidation. Obesity and type 2 diabetes are causally linked through their association with skeletal muscle insulin resistance, while conversely exercise is known to improve whole body glucose homeostasis simultaneously with muscle insulin sensitivity. Exercise activates skeletal muscle AMP-activated protein kinase (AMPK). AMPK plays a role in regulating exercise capacity, skeletal muscle mitochondrial content and contraction-stimulated glucose uptake. Skeletal muscle AMPK is also thought to be important for regulating fatty acid metabolism; however, direct genetic evidence in this area is currently lacking. This review will discuss the current paradigms regarding the influence of AMPK in regulating skeletal muscle fatty acid metabolism and mitochondrial biogenesis at rest and during exercise, and highlight the potential implications in the development of insulin resistance.

Thyroid hormone receptor isoform-specific modification by small ubiquitin-like modifier (SUMO) modulates thyroid hormone-dependent gene regulation

Thyroid hormone receptor (TR) α and β mediate thyroid hormone action at target tissues. TR isoforms have specific roles in development and in adult tissues. The mechanisms underlying TR isoform-specific action, however, are not well understood. We demonstrate that posttranslational modification of TR by conjugation of small SUMO to TRα and TRβ plays an important role in triiodothyronine (T3) action and TR isoform specificity. TRα was sumoylated at lysines 283 and 389, and TRβ at lysines 50, 146, and 443. Sumoylation of TRβ was ligand-dependent, and sumoylation of TRα was ligand-independent. TRα-SUMO conjugation utilized the E3 ligase PIASxβ and TRβ-SUMO conjugation utilized predominantly PIAS1. SUMO1 and SUMO3 conjugation to TR was important for T3-dependent gene regulation, as demonstrated in transient transfection assay and studies of endogenous gene regulation. The functional role of SUMO1 and SUMO3 in T3 induction in transient expression assays was closely matched to the pattern of TR and cofactor recruitment to thyroid hormone response elements (TREs) as determined by ChIP assays. SUMO1 was required for the T3-induced recruitment of the co-activator CREB-binding protein (CBP) and release of nuclear receptor co-repressor (NCoR) on a TRE but had no significant effect on TR DNA binding. SUMO1 was required for T3-mediated recruitment of NCoR and release of CBP from the TSHβ-negative TRE. SUMO3 was required for T3-stimulated TR binding to the TSHβ-negative TRE and recruitment of NCoR. These findings demonstrate that conjugation of SUMO to TR has a TR-isoform preference and is important for T3-dependent gene induction and repression.

Dynamic SUMOylation is linked to the activity cycles of androgen receptor in the cell nucleus

Despite of the progress in the molecular etiology of prostate cancer, the androgen receptor (AR) remains the major druggable target for the advanced disease. In addition to hormonal ligands, AR activity is regulated by posttranslational modifications. Here, we show that androgen induces SUMO-2 and SUMO-3 (SUMO-2/3) modification (SUMOylation) of the endogenous AR in prostate cancer cells, which is also reflected in the chromatin-bound receptor. Although only a small percentage of AR is SUMOylated at the steady state, AR SUMOylation sites have an impact on the receptor's stability, intranuclear mobility, and chromatin interactions and on expression of its target genes. Interestingly, short-term proteotoxic and cell stress, such as hyperthermia, that detaches the AR from the chromatin triggers accumulation of the SUMO-2/3-modified AR pool which concentrates into the nuclear matrix compartment. Alleviation of the stress allows rapid reversal of the SUMO-2/3 modifications and the AR to return to the chromatin. In sum, these results suggest that the androgen-induced SUMOylation is linked to the activity cycles of the holo-AR in the nucleus and chromatin binding, whereas the stress-induced SUMO-2/3 modifications sustain the solubility of the AR and protect it from proteotoxic insults in the nucleus.

Role of SUMOylation in full antiestrogenicity

The selective estrogen receptor downregulator (SERD) fulvestrant can be used as second-line treatment for patients relapsing after treatment with tamoxifen, a selective estrogen receptor modulator (SERM). Unlike tamoxifen, SERDs are devoid of partial agonist activity. While the full antiestrogenicity of SERDs may result in part from their capacity to downregulate levels of estrogen receptor alpha (ERα) through proteasome-mediated degradation, SERDs are also fully antiestrogenic in the absence of increased receptor turnover in HepG2 cells. Here we report that SERDs induce the rapid and strong SUMOylation of ERα in ERα-positive and -negative cell lines, including HepG2 cells. Four sites of SUMOylation were identified by mass spectrometry analysis. In derivatives of the SERD ICI164,384, SUMOylation was dependent on the length of the side chain and correlated with full antiestrogenicity. Preventing SUMOylation by the overexpression of a SUMO-specific protease (SENP) deSUMOylase partially derepressed transcription in the presence of full antiestrogens in HepG2 cells without a corresponding increase in activity in the presence of agonists or of the SERM tamoxifen. Mutations increasing transcriptional activity in the presence of full antiestrogens reduced SUMOylation levels and suppressed stimulation by SENP1. Our results indicate that ERα SUMOylation contributes to full antiestrogenicity in the absence of accelerated receptor turnover.

Absence of RIP140 reveals a pathway regulating glut4-dependent glucose uptake in oxidative skeletal muscle through UCP1-mediated activation of AMPK

Skeletal muscle constitutes the major site of glucose uptake leading to increased removal of glucose from the circulation in response to insulin. Type 2 diabetes and obesity are often associated with insulin resistance that can be counteracted by exercise or the use of drugs increasing the relative proportion of oxidative fibers. RIP140 is a transcriptional coregulator with a central role in metabolic tissues and we tested the effect of modulating its level of expression on muscle glucose and lipid metabolism in two mice models. Here, we show that although RIP140 protein is expressed at the same level in both oxidative and glycolytic muscles, it inhibits both fatty acid and glucose utilization in a fiber-type dependent manner. In RIP140-null mice, fatty acid utilization increases in the extensor digitorum longus and this is associated with elevated expression of genes implicated in fatty acid binding and transport. In the RIP140-null soleus, depletion of RIP140 leads to increased GLUT4 trafficking and glucose uptake with no change in Akt activity. AMPK phosphorylation/activity is inhibited in the soleus of RIP140 transgenic mice and increased in RIP140-null soleus. This is associated with increased UCP1 expression and mitochondrial uncoupling revealing the existence of a signaling pathway controlling insulin-independent glucose uptake in the soleus of RIP140-null mice. In conclusion, our findings reinforce the participation of RIP140 in the maintenance of energy homeostasis by acting as an inhibitor of energy production and particularly point to RIP140 as a promising therapeutic target in the treatment of insulin resistance.

Aryl hydrocarbon receptor modulation of estrogen receptor α-mediated gene regulation by a multimeric chromatin complex involving the two receptors and the coregulator RIP140

Although crosstalk between aryl hydrocarbon receptor (AhR) and estrogen receptor α (ERα) is well established, the mechanistic basis and involvement of other proteins in this process are not known. Because we observed an enrichment of AhR-binding motifs in ERα-binding sites of many estradiol (E2)-regulated genes, we investigated how AhR might modulate ERα-mediated gene transcription in breast cancer cells. Gene regulations were categorized based on their pattern of stimulation by E2 and/or dioxin and were denoted E2-responsive, dioxin-responsive, or responsive to either ligand. ERα, AhR, aryl hydrocarbon receptor translocator, and receptor interacting protein 140 (RIP140) were recruited to gene regulatory regions in a gene-specific and E2/dioxin ligand-specific manner. Knockdown of AhR markedly increased the expression of ERα-mediated genes upon E2 treatment. This was not attributable to a change in ERα level, or recruitment of ERα, phosphoSer5-RNA Pol II, or several coregulators but rather was associated with greatly diminished recruitment of the coregulator RIP140 to gene regulatory sites. Changing the cellular level of RIP140 revealed coactivator or corepressor roles for this coregulator in E2- and dioxin-mediated gene regulation, the choice of which was determined by the presence or absence of ERα at gene regulatory sites. Coimmunoprecipitation and chromatin immunoprecipitation (ChIP)-reChIP studies documented that E2- or dioxin-promoted formation of a multimeric complex of ERα, AhR, and RIP140 at ERα-binding sites of genes regulated by either E2 or dioxin. Our findings highlight the importance of cross-regulation between AhR and ERα and a novel mechanism by which AhR controls, through modulating the recruitment of RIP140 to ERα-binding sites, the kinetics and magnitude of ERα-mediated gene stimulation.

Gene repressive activity of RIP140 through direct interaction with CDK8

Receptor interacting protein 140 (RIP140) is a coregulator for numerous nuclear receptors and transcription factors and primarily exerts gene-repressive activities on various target genes. We previously identified a spectrum of posttranslational modifications on RIP140 that augment its property and biological activity. In T(3)-triggered biphasic regulation of cellular retinoic acid binding protein 1 (Crabp1) gene along the course of fibroblast-adipocyte differentiation, we found TRAP220(MED1) critical for T(3)-activated chromatin remodeling whereas RIP140 essential for T(3)-repressive chromatin remodeling of this gene promoter. In this current study, we aim to examine whether and how RIP140 replaces TRAP220(MED1) on the CrabpI promoter in differentiating adipocyte cultures. We find increasing recruitment of RIP140 to this promoter, with corresponding reduction in TRAP220(MED1) recruitment during the T(3)-repressive phase. We also uncover direct interaction of RIP140 with cyclin-dependent kinase (CDK)8 through the amino terminus of RIP140, which is stimulated by lysine acetylation on RIP140. We further validate the biological activity of lysine acetylation-mimetic RIP140, which elicits a stronger repressive effect and more efficiently recruits CDK8 and confirm CDK8's function in recruiting repressive components, such as G9a, to the RIP140 complex on this promoter. This underlies the T(3)-triggered repression of CrabpI gene. This study illustrates a new gene-repressive mechanism of RIP140 that can affect the transcription machinery by directly interacting with CDK8.

Estrogen-responsive genes in macrophages of the bony fish gilthead seabream: a transcriptomic approach

The role of sex steroids in the modulation of fish immune responses has received little attention. Previous studies have demonstrated that 17β-estradiol (E(2)) is able to alter the response of gilthead seabream leukocytes to infectious agents. We have used suppression subtractive hybridization to identify genes upregulated by E(2) (50 ng/ml) in macrophage cultures from gilthead seabream. We isolated 393 up-regulated cDNA fragments that led to the identification of 162 candidate estrogen-responsive genes. Functional analyses revealed the presence of several enriched immune processes and molecular pathways. The E(2) up-regulation of some immune-relevant genes was further confirmed by real time RT-PCR. Bioinformatics analysis revealed the ability of E(2) to orchestrate profound alterations in the macrophage expression profile, especially immune-related processes and pathways. This is the first report on E(2)-dependent modifications of fish macrophage transcriptome and lends weight to a suggested role for estrogen in the immune system, the possible significance of which is discussed.

Role of nuclear receptor corepressor RIP140 in metabolic syndrome

Obesity and its associated complications, which can lead to the development of metabolic syndrome, are a worldwide major public health concern especially in developed countries where they have a very high prevalence. RIP140 is a nuclear coregulator with a pivotal role in controlling lipid and glucose metabolism. Genetically manipulated mice devoid of RIP140 are lean with increased oxygen consumption and are resistant to high-fat diet-induced obesity and hepatic steatosis with improved insulin sensitivity. Moreover, white adipocytes with targeted disruption of RIP140 express genes characteristic of brown fat including CIDEA and UCP1 while skeletal muscles show a shift in fibre type composition enriched in more oxidative fibres. Thus, RIP140 is a potential therapeutic target in metabolic disorders. In this article we will review the role of RIP140 in tissues relevant to the appearance and progression of the metabolic syndrome and discuss how the manipulation of RIP140 levels or activity might represent a therapeutic approach to combat obesity and associated metabolic disorders. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

The metabolic coregulator RIP140: an update

RIP140 is a transcriptional coregulator highly expressed in metabolic tissues where it has important and diverse actions. RIP140-null mice show that it plays a crucial role in the control of lipid metabolism in adipose tissue, skeletal muscle, and the liver and is essential for female fertility. RIP140 has been shown to act as a ligand-dependent transcriptional corepressor for metabolic nuclear receptors such as estrogen-related receptors and peroxisome proliferator-activated receptors. The role of RIP140 as a corepressor has been strengthened by the characterization of RIP140-overexpressing mice, although it emerges through several studies that RIP140 can also behave as a coactivator. Nuclear localization of RIP140 is important for controlling transcription of target genes and is subject to regulation by posttranslational modifications. However, cytoplasmic RIP140 has been shown to play a role in the control of metabolism through direct regulation of glucose transport in adipocytes. In this review, we focus on recent advances highlighting the growing importance of RIP140 as a regulator of energy homeostasis.

PGC-1alpha regulation by exercise training and its influences on muscle function and insulin sensitivity

The peroxisome proliferator-activated receptor-gamma (PPARgamma) coactivator-1alpha (PGC-1alpha) is a major regulator of exercise-induced phenotypic adaptation and substrate utilization. We provide an overview of 1) the role of PGC-1alpha in exercise-mediated muscle adaptation and 2) the possible insulin-sensitizing role of PGC-1alpha. To these ends, the following questions are addressed. 1) How is PGC-1alpha regulated, 2) what adaptations are indeed dependent on PGC-1alpha action, 3) is PGC-1alpha altered in insulin resistance, and 4) are PGC-1alpha-knockout and -transgenic mice suitable models for examining therapeutic potential of this coactivator? In skeletal muscle, an orchestrated signaling network, including Ca(2+)-dependent pathways, reactive oxygen species (ROS), nitric oxide (NO), AMP-dependent protein kinase (AMPK), and p38 MAPK, is involved in the control of contractile protein expression, angiogenesis, mitochondrial biogenesis, and other adaptations. However, the p38gamma MAPK/PGC-1alpha regulatory axis has been confirmed to be required for exercise-induced angiogenesis and mitochondrial biogenesis but not for fiber type transformation. With respect to a potential insulin-sensitizing role of PGC-1alpha, human studies on type 2 diabetes suggest that PGC-1alpha and its target genes are only modestly downregulated (< or =34%). However, studies in PGC-1alpha-knockout or PGC-1alpha-transgenic mice have provided unexpected anomalies, which appear to suggest that PGC-1alpha does not have an insulin-sensitizing role. In contrast, a modest ( approximately 25%) upregulation of PGC-1alpha, within physiological limits, does improve mitochondrial biogenesis, fatty acid oxidation, and insulin sensitivity in healthy and insulin-resistant skeletal muscle. Taken altogether, there is substantial evidence that the p38gamma MAPK-PGC-1alpha regulatory axis is critical for exercise-induced metabolic adaptations in skeletal muscle, and strategies that upregulate PGC-1alpha, within physiological limits, have revealed its insulin-sensitizing effects.

SUMOylation can regulate the activity of ETS-like transcription factor 4

ETS-like transcription factor 4 (ELK4) (a.k.a. serum response factor accessory protein 1) belongs to the ternary complex factor (TCF) subfamily of E twenty-six (ETS) domain transcription factors. Compared to the other TCF subfamily members, ELK1 and ELK3 (NET), there is limited information of the mechanisms regulating the ELK4 activity. Here, we show that the ELK4 can be covalently modified (SUMOylated) by small ubiquitin-related modifier (SUMO) 1 protein, an important regulator of signaling and transcription. SUMOylation of ELK4 was reversed by SUMO-specific proteases (SENP) 1 and 2 and stimulated by SUMO E3 ligase PIAS3. Conserved lysine residue 167 that is located in the NET inhibitory domain of ELK4 was identified as the main site of SUMO-1 conjugation. Interestingly, mutation of the K167 disrupting the SUMOylation markedly enhanced the transcriptional activity of the ELK4, but weakened its repressive function on c-fos promoter. In conclusion, our results suggest that covalent modification by SUMO-1 can regulate the activity of ELK4, contributing to the transcriptional repression by the ELK4.

Emerging roles of the ubiquitin proteasome system in nuclear hormone receptor signaling

Nuclear receptor (NR)-mediated transcription is intimately tied to the ubiquitin proteasome system (UPS). The UPS targets numerous NR and coregulator proteins, regulating their stability and altering their transcriptional activities through the posttranslational placement of ubiquitin marks on them. Differences in the manner in which ubiquitin is attached to target proteins or itself have distinct regulatory consequences. Protein monoubiquitination, polyubiquitination, the site of ubiquitin attachment to a target protein, and the type of polyubiquitin chain linkage all lead to different biological outcomes and have an important regulatory function in NR-mediated transcription. Consistent with its role in protein degradation, the UPS is able to limit the biological actions of both NRs and coregulators by reducing their protein concentrations in the cell. However, in spite of its destructive capabilities, the UPS can play a positive role in facilitating NR-mediated transcription as well. In addition, ubiquitin-like modifications such as SUMOylation also modify and regulate NRs and coregulators. The UPS forms a key biological system that underlies a sophisticated postranslational regulatory scheme from which complex and dynamic regulation of NR-mediated transcription can occur.

Transcriptional control of mitochondrial biogenesis and function

Mitochondria play central roles in energy homeostasis, metabolism, signaling, and apoptosis. Accordingly, the abundance, morphology, and functional properties of mitochondria are finely tuned to meet cell-specific energetic, metabolic, and signaling demands. This tuning is largely achieved at the level of transcriptional regulation. A highly interconnected network of transcription factors regulates a broad set of nuclear genes encoding mitochondrial proteins, including those that control replication and transcription of the mitochondrial genome. The same transcriptional network senses cues relaying cellular energy status, nutrient availability, and the physiological state of the organism and enables short- and long-term adaptive responses, resulting in adjustments to mitochondrial function and mitochondrial biogenesis. Mitochondrial dysfunction is associated with many human diseases. Characterization of the transcriptional mechanisms that regulate mitochondrial biogenesis and function can offer insights into possible therapeutic interventions aimed at modulating mitochondrial function.

SUMOylation attenuates the function of PGC-1alpha

Peroxisome proliferator-activated receptor gamma-coactivator-1alpha (PGC-1alpha) is a key coordinator of gene programs in metabolism and energy homeostasis in mammals. It is highly responsive to changes in the cellular environment and physiological status of mammals and regulated by post-translational modifications: acetylation, phosphorylation, and methylation. Here, we show that PGC-1alpha is covalently modified by small ubiquitin-like modifier (SUMO) 1 protein, an important regulator of signaling and transcription. Conserved lysine residue 183 located in the activation domain of PGC-1alpha was identified as the major site of SUMO conjugation. Interestingly, the same Lys residue is also a target for acetylation. Therefore, the E185A mutation disrupting the SUMOylation consensus sequence was utilized to show that SUMOylation plays a role in the regulation of PGC-1alpha function. Our results show that SUMOylation does not have an apparent effect on the subcellular localization or the stability of PGC-1alpha, but it attenuates the transcriptional activity of the coactivator, probably by enhancing the interaction of PGC-1alpha with corepressor RIP140. Mutation that abolished the SUMOylation augments the activity of PGC-1alpha also in the context of PPARgamma-dependent transcription. Thus, our findings showing that reversible SUMOylation can adjust the activity of PGC-1alpha add a novel layer to the regulation of the coactivator.

Sumoylation of forkhead L2 by Ubc9 is required for its activity as a transcriptional repressor of the Steroidogenic Acute Regulatory gene

Forkhead L2 (FOXL2) is a member of the forkhead/hepatocyte nuclear factor 3 (FKH/HNF3) gene family of transcription factors and acts as a transcriptional repressor of the Steroidogenic Acute Regulatory (StAR) gene, a marker of granulosa cell differentiation. FOXL2 may play a role in ovarian follicle maturation and prevent premature follicle depletion leading to premature ovarian failure. In this study, we found that FOXL2 interacts with Ubc9, an E2-conjugating enzyme that mediates sumoylation, a key mechanism in transcriptional regulation. FOXL2 and Ubc9 are co-expressed in granulosa cells of small and medium ovarian follicles. FOXL2 is sumoylated by Ubc9, and this Ubc9-mediated sumoylation is essential to the transcriptional activity of FOXL2 on the StAR promoter. As FOXL2 is endogenous to granulosa cells, we generated a stable cell line expressing FOXL2 and found that activity of the StAR promoter in this cell line is greatly decreased in the presence of Ubc9. The sumoylation site was identified at lysine 25 of FOXL2. Mutation of lysine 25 to arginine leads to loss of transcriptional repressor activity of FOXL2. Taken together, we propose that Ubc9-mediated sumoylation at lysine 25 of FOXL2 is required for transcriptional repression of the StAR gene and may be responsible for controlling the development of ovarian follicles.

RIP140 gene and protein expression levels are downregulated in visceral adipose tissue in human morbid obesity

BACKGROUND

Receptor-interacting protein 140 (RIP140) is a corepressor for nuclear receptors with an important role in the inhibition of energy expenditure. Rip140-knockout mice are lean and resistant to diet-induced obesity due to an increase in mitochondrial biogenesis, fatty acid oxidation, and oxidative phosphorylation. The aim of the present work was to evaluate the effect of morbid obesity on gene and protein expression levels of RIP140 in visceral adipose tissue (VAT).

METHODS

VAT biopsies obtained from 17 subjects were used in the study. Patients were classified as lean (body mass index [BMI]=21.8+/-1.3 kg/m2) or obese (BMI=48.2+/-2.6 kg/m2). Reverse transcription polymerase chain reaction and Western blot analyses were performed to quantify the expression levels of RIP140 in VAT. We also analyzed glucose and lipid profile as well as some inflammatory factors.

RESULTS

Obese patients exhibited significantly lower RIP140 mRNA expression levels compared to lean subjects (lean=1.00+/-0.17 arbitrary units, obese=0.65+/-0.18 arbitrary units; P<0.05). Protein expression of RIP140 followed the same trend, being significantly higher in lean volunteers (lean=1.00+/-0.18 arbitrary units, obese=0.45+/-0.11 arbitrary units; P<0.05). Furthermore, a significant negative correlation was found between RIP140 protein levels and both BMI (rho=-0.85; P<0.001) and body fat percentage (rho=-0.88; P<0.001).

CONCLUSIONS

The lower gene and protein expression levels of RIP140 in obese subjects may suggest a compensatory mechanism in order to favor energy expenditure and reduce fat accumulation in obesity states.

SUMOylation inhibits SF-1 activity by reducing CDK7-mediated serine 203 phosphorylation

Steroidogenic factor 1 (SF-1) is an orphan nuclear receptor selectively expressed in the adrenal cortex and gonads, where it mediates the hormonal stimulation of multiple genes involved in steroid hormone biosynthesis. SF-1 is the target of both phosphorylation and SUMOylation, but how these modifications interact or contribute to SF-1 regulation of endogenous genes remains poorly defined. We found that SF-1 is selectively SUMOylated at K194 in Y1 adrenocarcinoma cells and that although SUMOylation does not alter the subcellular localization of SF-1, the modification inhibits the ability of SF-1 to activate target genes. Notably, whereas SF-1 SUMOylation is independent of S203 phosphorylation and is unaffected by adrenocorticotropin (ACTH) treatment, loss of SUMOylation leads to enhanced SF-1 phosphorylation at serine 203. Furthermore, preventing SF-1 SUMOylation increases the mRNA and protein levels of multiple steroidogenic enzyme genes. Analysis of the StAR promoter indicates that blockade of SF-1 SUMOylation leads to an increase in overall promoter occupancy but does not alter the oscillatory recruitment dynamics in response to ACTH. Notably, we find that CDK7 binds preferentially to the SUMOylation-deficient form of SF-1 and that CDK7 inhibition reduces phosphorylation of SF-1. Based on these observations, we propose a coordinated modification model in which inhibition of SF-1-mediated transcription by SUMOylation in adrenocortical cancer cells is mediated through reduced CDK7-induced phosphorylation of SF-1.

Lysine acetylation: codified crosstalk with other posttranslational modifications

Lysine acetylation has emerged as a major posttranslational modification for histones. Crossregulation between this and other modifications is crucial in modulating chromatin-based transcriptional control and shaping inheritable epigenetic programs. In addition to histones, many other nuclear proteins and various cytoplasmic regulators are subject to lysine acetylation. This review focuses on recent findings pertinent to acetylation of nonhistone proteins and emphasizes how this modification might crosstalk with phosphorylation, methylation, ubiquitination, sumoylation, and others to form code-like multisite modification programs for dynamic control of cellular signaling under diverse conditions.