DNA methylation repels binding of hypoxia-inducible transcription factors to maintain tumor immunotolerance.

BACKGROUND: Hypoxia is pervasive in cancer and other diseases. Cells sense and adapt to hypoxia by activating hypoxia-inducible transcription factors (HIFs), but it is still an outstanding question why cell types differ in their transcriptional response to hypoxia. RESULTS: We report that HIFs fail to bind CpG dinucleotides that are methylated in their consensus binding sequence, both in in vitro biochemical binding assays and in vivo studies of differentially methylated isogenic cell lines. Based on in silico structural modeling, we show that 5-methylcytosine indeed causes steric hindrance in the HIF binding pocket. A model wherein cell-type-specific methylation landscapes, as laid down by the differential expression and binding of other transcription factors under normoxia, control cell-type-specific hypoxia responses is observed. We also discover ectopic HIF binding sites in repeat regions which are normally methylated. Genetic and pharmacological DNA demethylation, but also cancer-associated DNA hypomethylation, expose these binding sites, inducing HIF-dependent expression of cryptic transcripts. In line with such cryptic transcripts being more prone to cause double-stranded RNA and viral mimicry, we observe low DNA methylation and high cryptic transcript expression in tumors with high immune checkpoint expression, but not in tumors with low immune checkpoint expression, where they would compromise tumor immunotolerance. In a low-immunogenic tumor model, DNA demethylation upregulates cryptic transcript expression in a HIF-dependent manner, causing immune activation and reducing tumor growth. CONCLUSIONS: Our data elucidate the mechanism underlying cell-type-specific responses to hypoxia and suggest DNA methylation and hypoxia to underlie tumor immunotolerance.

Bidirectional modulation of HIF-2 activity through chemical ligands.

Hypoxia-inducible factor-2 (HIF-2) is a heterodimeric transcription factor formed through dimerization between an oxygen-sensitive HIF-2α subunit and its obligate partner subunit ARNT. Enhanced HIF-2 activity drives some cancers, whereas reduced activity causes anemia in chronic kidney disease. Therefore, modulation of HIF-2 activity via direct-binding ligands could provide many new therapeutic benefits. Here, we explored HIF-2α chemical ligands using combined crystallographic, biophysical, and cell-based functional studies. We found chemically unrelated antagonists to employ the same mechanism of action. Their binding displaced residue M252 from inside the HIF-2α PAS-B pocket toward the ARNT subunit to weaken heterodimerization. We also identified first-in-class HIF-2α agonists and found that they significantly displaced pocket residue Y281. Its dramatic side chain movement increases heterodimerization stability and transcriptional activity. Our findings show that despite binding to the same HIF-2α PAS-B pocket, ligands can manifest as inhibitors versus activators by mobilizing different pocket residues to allosterically alter HIF-2α-ARNT heterodimerization.

A nonalcoholic fatty liver disease model in human induced pluripotent stem cell-derived hepatocytes, created by endoplasmic reticulum stress-induced steatosis.

Hepatic steatosis, a reversible state of metabolic dysregulation, can promote the onset of nonalcoholic steatohepatitis (NASH), and its transition is thought to be critical in disease evolution. The association between endoplasmic reticulum (ER) stress response and hepatocyte metabolism disorders prompted us to characterize ER stress-induced hepatic metabolic dysfunction in human induced pluripotent stem cell-derived hepatocytes (hiPSC-Hep), to explore regulatory pathways and validate a phenotypic in vitro model for progression of liver steatosis. We treated hiPSC-Hep with a ratio of unsaturated and saturated fatty acids in the presence of an inducer of ER stress to synergistically promote triglyceride accumulation and dysregulate lipid metabolism. We monitored lipid accumulation by high-content imaging and measured gene regulation by RNA sequencing and reverse transcription quantitative PCR analyses. Our results show that ER stress potentiated intracellular lipid accumulation by 5-fold in hiPSC-Hep in the absence of apoptosis. Transcriptome pathway analysis identified ER stress pathways as the most significantly dysregulated of all pathways affected. Obeticholic acid dose dependently inhibited lipid accumulation and modulated gene expression downstream of the farnesoid X receptor. We were able to identify modulation of hepatic markers and gene pathways known to be involved in steatosis and nonalcoholic fatty liver disease (NAFLD), in support of a hiPSC-Hep disease model that is relevant to clinical data for human NASH. Our results show that the model can serve as a translational discovery platform for the understanding of molecular pathways involved in NAFLD, and can facilitate the identification of novel therapeutic molecules based on high-throughput screening strategies.

Visualizing the Architectures and Interactions of Nuclear Receptors.

Nuclear receptors (NRs) are master regulators of broad genetic programs in metazoans. These programs are regulated in part by the small-molecule ligands that bind NRs and modulate their interactions with transcriptional coregulatory factors. X-ray crystallography is now delivering more complete pictures of how the multidomain architectures of NR homo- and heterodimers are physically arranged on their DNA elements and how ligands and coactivator peptides act through these complexes. Complementary studies are also pointing to a variety of novel mechanisms by which NRs access their DNA-response elements within chromatin. Here, we review the new structural advances together with proteomic discoveries that shape our understanding of how NRs form a variety of functional interactions with collaborating factors in chromatin.

SETDB2 Links Glucocorticoid to Lipid Metabolism through Insig2a Regulation.

Transcriptional and chromatin regulations mediate the liver response to nutrient availability. The role of chromatin factors involved in hormonal regulation in response to fasting is not fully understood. We have identified SETDB2, a glucocorticoid-induced putative epigenetic modifier, as a positive regulator of GR-mediated gene activation in liver. Insig2a increases during fasting to limit lipid synthesis, but the mechanism of induction is unknown. We show Insig2a induction is GR-SETDB2 dependent. SETDB2 facilitates GR chromatin enrichment and is key to glucocorticoid-dependent enhancer-promoter interactions. INSIG2 is a negative regulator of SREBP, and acute glucocorticoid treatment decreased active SREBP during refeeding or in livers of Ob/Ob mice, both systems of elevated SREBP-1c-driven lipogenesis. Knockdown of SETDB2 or INSIG2 reversed the inhibition of SREBP processing. Overall, these studies identify a GR-SETDB2 regulatory axis of hepatic transcriptional reprogramming and identify SETDB2 as a potential target for metabolic disorders with aberrant glucocorticoid actions.

Towards understanding methyllysine readout.

BACKGROUND: Lysine methylation is the most versatile covalent posttranslational modification (PTM) found in histones and non-histone proteins. Over the past decade a number of methyllysine-specific readers have been discovered and their interactions with histone tails have been structurally and biochemically characterized. More recently innovative experimental approaches have emerged that allow for studying reader interactions in the context of the full nucleosome and nucleosomal arrays. SCOPE OF REVIEW: In this review we give a brief overview of the known mechanisms of histone lysine methylation readout, summarize progress recently made in exploring interactions with methylated nucleosomes, and discuss the latest advances in the development of small molecule inhibitors of the methyllysine-specific readers. MAJOR CONCLUSIONS: New studies reveal various reader-nucleosome contacts outside the methylated histone tail, thus offering a better model for association of histone readers to chromatin and broadening our understanding of the functional implications of these interactions. In addition, some progress has been made in the design of antagonists of these interactions. GENERAL SIGNIFICANCE: Specific lysine methylation patterns are commonly associated with certain chromatin states and genomic elements, and are linked to distinct biological outcomes such as transcription activation or repression. Disruption of patterns of histone modifications is associated with a number of diseases, and there is tremendous therapeutic potential in targeting histone modification pathways. Thus, investigating binding of readers of these modifications is not only important for elucidating fundamental mechanisms of chromatin regulation, but also necessary for the design of targeted therapeutics. This article is part of a Special Issue entitled: Molecular mechanisms of histone modification function.

Understanding nuclear receptor form and function using structural biology.

Nuclear receptors (NRs) are a major transcription factor family whose members selectively bind small-molecule lipophilic ligands and transduce those signals into specific changes in gene programs. For over two decades, structural biology efforts were focused exclusively on the individual ligand-binding domains (LBDs) or DNA-binding domains of NRs. These analyses revealed the basis for both ligand and DNA binding and also revealed receptor conformations representing both the activated and repressed states. Additionally, crystallographic studies explained how NR LBD surfaces recognize discrete portions of transcriptional coregulators. The many structural snapshots of LBDs have also guided the development of synthetic ligands with therapeutic potential. Yet, the exclusive structural focus on isolated NR domains has made it difficult to conceptualize how all the NR polypeptide segments are coordinated physically and functionally in the context of receptor quaternary architectures. Newly emerged crystal structures of the peroxisome proliferator-activated receptor-γ-retinoid X receptor α (PPARγ-RXRα) heterodimer and hepatocyte nuclear factor (HNF)-4α homodimer have recently revealed the higher order organizations of these receptor complexes on DNA, as well as the complexity and uniqueness of their domain-domain interfaces. These emerging structural advances promise to better explain how signals in one domain can be allosterically transmitted to distal receptor domains, also providing much better frameworks for guiding future drug discovery efforts.

Recognition of methylated histones: new twists and variations.

Histone tails undergo methylation at their lysines and arginines. These chemical marks act as traffic signals that direct activity of chromatin remodeling complexes to appropriate regions of the genome. A surprisingly diverse group of effector protein modules in chromatin remodeling complexes and their associated factors are involved in the recognition of histone methyllysines. Previous studies generally painted a picture of individual lysines recognized by these protein modules in a 1:1 fashion. However, recent structural studies show more complex interactions where the critical lysines are recognized in pairs, or in the context of nucleosomal DNA, or within the central pore of repeat motifs. These interactions extend our understanding of how histone tail recognition can be enhanced through coupled interactions within a single module or through the cooperation of two different molecules.

Epigenetic virtues of chromodomains.

The chromatin organization modifier domain (chromodomain) was first identified as a motif associated with chromatin silencing in Drosophila. There is growing evidence that chromodomains are evolutionary conserved across different eukaryotic species to control diverse aspects of epigenetic regulation. Although originally reported as histone H3 methyllysine readers, the chromodomain functions have now expanded to recognition of other histone and non-histone partners as well as interaction with nucleic acids. Chromodomain binding to a diverse group of targets is mediated by a conserved substructure called the chromobox homology region. This motif can be used to predict methyllysine binding and distinguish chromodomains from related Tudor "Royal" family members. In this review, we discuss and classify various chromodomains according to their context, structure and the mechanism of target recognition.

The HP1a disordered C terminus and chromo shadow domain cooperate to select target peptide partners.

Drosophila melanogaster heterochromatin protein 1a (HP1a) is essential for compacted heterochromatin structure and the associated gene silencing. Its chromo shadow domain (CSD) is well known for binding to peptides that contain a PXVXL motif. Heterochromatin protein 2 (HP2) is a non-histone chromosomal protein that associates with HP1a in the pericentric heterochromatin, telomeres, and the fourth chromosome. Using NMR spectroscopy, fluorescence polarization, and site-directed mutagenesis, we identified an LCVKI motif in HP2 that binds to the HP1a CSD. The binding affinity of the HP2 fragment is approximately two orders of magnitude higher than that of peptides from PIWI (with a PRVKV motif), AF10 (with a PLVVL motif), or CG15356 (with LYPLL and LSIVA motifs). To delineate differential interactions of the HP1a CSD, we characterized its structure, backbone dynamics, and dimerization constant. We found that the dimerization constant is bracketed by the affinities of HP2 and PIWI, which dock to the same HP1a homodimer surface. This suggests that HP2, but not PIWI, interaction can drive the homodimerization of HP1a. Interestingly, the integrity of the disordered C-terminal extension (CTE) of HP1a is essential for discriminatory binding, whereas swapping the PXVXL motifs does not confer specificity. Serine phosphorylation at the peptide binding surface of the CSD is thought to regulate heterochromatin assembly. Glutamic acid substitution at these sites destabilizes HP1a dimers, but improves the interaction with both binding partners. Our studies underscore the importance of CSD dimerization and cooperation with the CTE in forming distinct complexes of HP1a.

Corecognition of DNA and a methylated histone tail by the MSL3 chromodomain.

MSL3 resides in the MSL (male-specific lethal) complex, which upregulates transcription by spreading the histone H4 Lys16 (H4K16) acetyl mark. We discovered a DNA-dependent interaction of MSL3 chromodomain with the H4K20 monomethyl mark. The structure of a ternary complex shows that the DNA minor groove accommodates the histone H4 tail, and monomethyllysine inserts in a four-residue aromatic cage in MSL3. H4K16 acetylation antagonizes MSL3 binding, suggesting that MSL function is regulated by a combination of post-translational modifications.

Specificity of the chromodomain Y chromosome family of chromodomains for lysine-methylated ARK(S/T) motifs.

Previous studies have shown two homologous chromodomain modules in the HP1 and Polycomb proteins exhibit discriminatory binding to related methyllysine residues (embedded in ARKS motifs) of the histone H3 tail. Methylated ARK(S/T) motifs have recently been identified in other chromatin factors (e.g. linker histone H1.4 and lysine methyltransferase G9a). These are thought to function as peripheral docking sites for the HP1 chromodomain. In vertebrates, HP1-like chromodomains are also present in the chromodomain Y chromosome (CDY) family of proteins adjacent to a putative catalytic motif. The human genome encodes three CDY family proteins, CDY, CDYL, and CDYL2. These have putative functions ranging from establishment of histone H4 acetylation during spermiogenesis to regulation of transcription co-repressor complexes. To delineate the biochemical functions of the CDY family chromodomains, we analyzed their specificity of methyllysine recognition. We detected substantial differences among these factors. The CDY chromodomain exhibits discriminatory binding to lysine-methylated ARK(S/T) motifs, whereas the CDYL2 chromodomain binds with comparable strength to multiple ARK(S/T) motifs. Interestingly, subtle amino acid changes in the CDYL chromodomain prohibit such binding interactions in vitro and in vivo. However, point mutations can rescue binding. In support of the in vitro binding properties of the chromodomains, the full-length CDY family proteins exhibit substantial variability in chromatin localization. Our studies underscore the significance of subtle sequence differences in a conserved signaling module for diverse epigenetic regulatory pathways.

Centromeric Aurora-B activation requires TD-60, microtubules, and substrate priming phosphorylation.

The chromosome passenger complex (CPC) controls chromosome congression, kinetochore-microtubule attachments, and spindle checkpoint signaling during mitosis. Aurora-B kinase is the catalytic subunit of the CPC. To understand how a single kinase can regulate such diverse events, we have investigated the activation of Aurora-B and describe two distinct activation mechanisms. First, Aurora-B activation in vitro requires two cofactors, telophase disc-60kD (TD-60) and microtubules. TD-60 is critical to localize both the CPC and Haspin kinase activity to centromeres and thus regulates Aurora-B at several levels. Second, Aurora-B substrates can inhibit kinase activation, and this is relieved by phosphorylation of these substrates by the centromeric kinases Plk1 and Haspin. These regulatory mechanisms suggest models for phosphorylation by Aurora-B of centromeric substrates at unaligned chromosomes and merotelic attachments.

Identification of heme as the ligand for the orphan nuclear receptors REV-ERBalpha and REV-ERBbeta.

The nuclear receptors REV-ERBalpha (encoded by NR1D1) and REV-ERBbeta (NR1D2) have remained orphans owing to the lack of identified physiological ligands. Here we show that heme is a physiological ligand of both receptors. Heme associates with the ligand-binding domains of the REV-ERB receptors with a 1:1 stoichiometry and enhances the thermal stability of the proteins. Results from experiments of heme depletion in mammalian cells indicate that heme binding to REV-ERB causes the recruitment of the co-repressor NCoR, leading to repression of target genes including BMAL1 (official symbol ARNTL), an essential component of the circadian oscillator. Heme extends the known types of ligands used by the human nuclear receptor family beyond the endocrine hormones and dietary lipids described so far. Our results further indicate that heme regulation of REV-ERBs may link the control of metabolism and the mammalian clock.

Implications of a histone code mimic in epigenetic signaling.

In this issue of Molecular Cell, Sampath et al. show a lysine methylase exhibits substrate promiscuity and variability in degree of product methylation (Sampath et al., 2007). Two lysines are found to be automethylated in G9a, and one is H3K9-like and can establish a docking site for HP1 chromodomain.

The Arabidopsis LHP1 protein colocalizes with histone H3 Lys27 trimethylation.

Polycomb proteins are required for maintenance of silent chromatin states via histone H3 Lys27 trimethylation (H3K27me3) in animals, but homologs are not found in plant genomes. Using a DamID-chip method, we found that the Arabidopsis thaliana chromodomain-containing protein LHP1 colocalizes with H3K27me3 genome-wide. The LHP1 chromodomain also binds H3K27me3 with high affinity, suggesting that LHP1 has functions similar to those of Polycomb.

Recognition of trimethyllysine by a chromodomain is not driven by the hydrophobic effect.

Posttranslational modifications of histone proteins regulate gene expression via complex protein-protein and protein-DNA interactions with chromatin. One such modification, the methylation of lysine, has been shown to induce binding to chromodomains in an aromatic cage [Nielsen PR, et al. (2002) Nature 416:103-107]. The binding generally is attributed to the presence of cation-pi interactions between the methylated lysine and the aromatic pocket. However, whether the cationic component of the interaction is necessary for binding in the aromatic cage has not been addressed. In this article, the interaction of trimethyllysine with tryptophan is compared with that of its neutral analog, tert-butylnorleucine (2-amino-7,7-dimethyloctanoic acid), within the context of a beta-hairpin peptide model system. These two side chains have near-identical size, shape, and polarizabilities but differ in their charges. Comparison of the two peptides reveals that the neutral side chain has no preference for interacting with tryptophan, unlike trimethyllysine, which interacts strongly in a defined geometry. In vitro binding studies of the histone 3A peptide containing trimethyllysine or tert-butylnorleucine to HP1 chromodomain indicate that the cationic moiety is critical for binding in the aromatic cage. This difference in binding affinities demonstrates the necessity of the cation-pi interaction to binding with the chromodomain and its role in providing specificity. This article presents an excellent example of synergy between model systems and in vitro studies that allows for the investigation of the key forces that control biomolecular recognition.

Molecular implications of evolutionary differences in CHD double chromodomains.

Double chromodomains occur in CHD proteins, which are ATP-dependent chromatin remodeling factors implicated in RNA polymerase II transcription regulation. Biochemical studies suggest important differences in the histone H3 tail binding of different CHD chromodomains. In human and Drosophila, CHD1 double chromodomains bind lysine 4-methylated histone H3 tail, which is a hallmark of transcriptionally active chromatin in all eukaryotes. Here, we present the crystal structure of the yeast CHD1 double chromodomains, and pinpoint their differences with that of the human CHD1 double chromodomains. The most conserved residues in these double chromodomains are the two chromoboxes that orient adjacently. Only a subset of CHD chromoboxes can form an aromatic cage for methyllysine binding, and methyllysine binding requires correctly oriented inserts. These factors preclude yeast CHD1 double chromodomains from interacting with the histone H3 tail. Despite great sequence similarity between the human CHD1 and CHD2 chromodomains, variation within an insert likely prevents CHD2 double chromodomains from binding lysine 4-methylated histone H3 tail as efficiently as in CHD1. By using the available structural and biochemical data we highlight the evolutionary specialization of CHD double chromodomains, and provide insights about their targeting capacities.