Complex regulation of the transactivation function of hypoxia-inducible factor-1 alpha by direct interaction with two distinct domains of the CREB-binding protein/p300

Discovery of niclosamide as a p300/transcription factor protein-protein interaction inhibitor

Protein-protein interactions (PPIs) are crucial in various biological processes and are attractive targets for drug discovery. In this study, we identified niclosamide (9) as a novel inhibitor of the hypoxia-inducible factor 1α (HIF-1α)/p300 PPI from the RIKEN NPDepo compound library using a fluorescence anisotropy-based screening method. We synthesized niclosamide azide (10) as a photoaffinity labelling probe to identify the p300 binding site of compound 9 and elucidated the binding mode using photoaffinity labelling experiments and molecular docking simulations. Furthermore, we demonstrated that compound 9 inhibited not only HIF-1α/p300 PPI but also p300-transcription factor PPIs, including interaction with p53 and STAT3, thereby suppressing the expression of BAX and c-MYC, respectively.

Synthetic transcription factors establish the function of nine amino acid transactivation domains of Komagataella phaffii Mxr1

The zinc finger (ZF) transcription factor Mxr1 (methanol expression regulator 1) of the methylotrophic yeast Komagataella phaffii (formerly Pichia pastoris) harbors a DNA-binding domain consisting of two C2H2 ZFs (Mxr1ZF) between amino acids 36 and 101 and a previously identified nine amino acid transactivation domain (9aaTAD) between residues 365 and 373 (TAD A, QELESSLNA). Beyond this, 21 putative 9aaTADs (designated TAD B-V) located between amino acids 401 and 1155 remain to be characterized. Here, we demonstrate that a compact synthetic transcription factor composed of Mxr1ZF and three tandem copies of TAD A can activate the transcription of Mxr1 target genes for ethanol and methanol metabolism with specificity and efficiency comparable to the full-length protein. Expression of individual synthetic transcription factors containing Mxr1ZF and each of the 20 putative 9aaTADs in K. phaffii Δmxr1 strain revealed that 10 of these putative TADs are functional, capable of reversing the growth defect of the mutant and activating transcription of target genes required for ethanol and methanol metabolism. Functional analysis indicates that Mxr1 9aaTADs rely on General Control Nondepressible 5 (Gcn5), a histone acetyltransferase, for transactivation. These findings suggest that recruitment of Gcn5-mediated histone acetylation at target promoters is a critical step in transcriptional activation by Mxr1 9aaTADs. This study represents the first comprehensive characterization of 9aaTADs in a K. phaffii ZF transcription factor, providing insights into their mechanism and potential applications in synthetic biology.

Epigenetic remodelling under hypoxia

Hypoxia is intrinsic to tumours and contributes to malignancy and metastasis while hindering the efficiency of existing treatments. Epigenetic mechanisms play a crucial role in the regulation of hypoxic cancer cell programs, both in the initial phases of sensing the decrease in oxygen levels and during adaptation to chronic lack of oxygen. During the latter, the epigenetic regulation of tumour biology intersects with hypoxia-sensitive transcription factors in a complex network of gene regulation that also involves metabolic reprogramming. Here, we review the current literature on the epigenetic control of gene programs in hypoxic cancer cells. We highlight common themes and features of such epigenetic remodelling and discuss their relevance for the development of therapeutic strategies.

Hypoxia-inducible factor affects hepatitis B virus transcripts and genome levels as well as the expression and subcellular location of the hepatitis B virus core protein

Globally, a chronic-hepatitis B virus (HBV) infection is the leading cause of hepatocellular carcinoma (HCC). The transcription factor hypoxia-inducible factor 1 (HIF1) is often elevated in HCC, including HBV-associated HCC. Previous studies have suggested that the expression of the HIF1 subunit, HIF1α, is elevated in HBV-infected hepatocytes; however, whether HIF1 activity affects the HBV lifecycle has not been fully explored. We used a liver-derived cell line and ex vivo cultured primary hepatocytes as models to determine how HIF1 affects the HBV lifecycle. We observed that HIF1 elevates HBV RNA transcript levels, core protein levels, core protein localization to the cytoplasm, and HBV genome replication. Attenuating the transcription activity of HIF1 blocked HIF1-mediated effects on the HBV lifecycle. Our studies show that HIF1 regulates various stages of the HBV lifecycle in hepatocytes and could be a therapeutic target for blocking HBV replication and the development of HBV-associated diseases.

Genome editing and cancer therapy: handling the hypoxia-responsive pathway as a promising strategy

The precise characterization of oxygen-sensing pathways and the identification of pO2-regulated gene expression are both issues of critical importance. The O2-sensing system plays crucial roles in almost all the pivotal human processes, including the stem cell specification, the growth and development of tissues (such as embryogenesis), the modulation of intermediate metabolism (including the shift of the glucose metabolism from oxidative to anaerobic ATP production and vice versa), and the control of blood pressure. The solid cancer microenvironment is characterized by low oxygen levels and by the consequent activation of the hypoxia response that, in turn, allows a complex adaptive response characterized mainly by neoangiogenesis and metabolic reprogramming. Recently, incredible advances in molecular genetic methodologies allowed the genome editing with high efficiency and, above all, the precise identification of target cells/tissues. These new possibilities and the knowledge of the mechanisms of adaptation to hypoxia suggest the effective development of new therapeutic approaches based on the manipulation, targeting, and exploitation of the oxygen-sensor system molecular mechanisms.

Mivebresib alleviates systemic lupus erythematosus-associated diffuse alveolar hemorrhage via inhibiting infiltration of monocytes and M1 polarization of macrophages

BACKGROUND

Diffuse alveolar hemorrhage (DAH) is a serious complication that can arise from systemic lupus erythematosus (SLE) and other autoimmune diseases. While current treatments for DAH have limitations and adverse side effects, recent evidence suggests that inflammatory macrophages play a crucial role in the development of DAH. In this study, we investigated Mivebresib, a BET protein-bromodomain-containing protein 4 (BRD4) inhibitor, as a potential treatment for DAH.

RESULTS

Our findings show that Mivebresib effectively protected C57BL/6J mice against pristane-induced DAH by inhibiting the migration and polarization of monocytes and macrophages, as well as pathogenic B and T cells. Specifically, Mivebresib modified the distribution of leukocytes, impeded the polarization of inflammatory macrophages, and reduced the frequency of CD19 + CD5 + B cells in the lungs of pristane-treated mice. Furthermore, in vitro experiments demonstrated that Mivebresib inhibited LPS-induced M1 polarization of macrophages and the expression of pro-inflammatory cytokines, M1 marker genes, and chemokines-chemokine receptors while thwarting the secretion of IL-6 and TNF-α. Transcriptomic analysis suggested and experiments comfimed that Mivebresib inhibits M1 polarization via interrupting the p300/BRD4/HIF1A axis.

CONCLUSIONS

Our study demonstrates that Mivebresib has therapeutic potential for the life-threatening complication of DAH caused by SLE. By inhibiting macrophage polarization and the infiltration of inflammatory cells, Mivebresib may offer a promising treatment option for patients suffering from this disease.

Transcriptional Response to Hypoxia: The Role of HIF-1-Associated Co-Regulators

The Hypoxia Inducible Factor 1 (HIF-1) plays a major role in the cellular response to hypoxia by regulating the expression of many genes involved in adaptive processes that allow cell survival under low oxygen conditions. Adaptation to the hypoxic tumor micro-environment is also critical for cancer cell proliferation and therefore HIF-1 is also considered a valid therapeutical target. Despite the huge progress in understanding regulation of HIF-1 expression and activity by oxygen levels or oncogenic pathways, the way HIF-1 interacts with chromatin and the transcriptional machinery in order to activate its target genes is still a matter of intense investigation. Recent studies have identified several different HIF-1- and chromatin-associated co-regulators that play important roles in the general transcriptional activity of HIF-1, independent of its expression levels, as well as in the selection of binding sites, promoters and target genes, which, however, often depends on cellular context. We review here these co-regulators and examine their effect on the expression of a compilation of well-characterized HIF-1 direct target genes in order to assess the range of their involvement in the transcriptional response to hypoxia. Delineating the mode and the significance of the interaction between HIF-1 and its associated co-regulators may offer new attractive and specific targets for anticancer therapy.

Gene regulation by histone-modifying enzymes under hypoxic conditions: a focus on histone methylation and acetylation

Oxygen, which is necessary for sustaining energy metabolism, is consumed in many biochemical reactions in eukaryotes. When the oxygen supply is insufficient for maintaining multiple homeostatic states at the cellular level, cells are subjected to hypoxic stress. Hypoxia induces adaptive cellular responses mainly through hypoxia-inducible factors (HIFs), which are stabilized and modulate the transcription of various hypoxia-related genes. In addition, many epigenetic regulators, such as DNA methylation, histone modification, histone variants, and adenosine triphosphate-dependent chromatin remodeling factors, play key roles in gene expression. In particular, hypoxic stress influences the activity and gene expression of histone-modifying enzymes, which controls the posttranslational modification of HIFs and histones. This review covers how histone methylation and histone acetylation enzymes modify histone and nonhistone proteins under hypoxic conditions and surveys the impact of epigenetic modifications on gene expression. In addition, future directions in this area are discussed.

New sequencing technologies are revealing how cells respond to hypoxia, insufficient oxygen, by managing gene activation. In multicellular organisms, gene activation is managed by how tightly a section of DNA is wound around proteins called histones; genes in tightly packed regions are inaccessible and inactive, whereas those in looser regions can be activated. Kyunghwan Kim, Sun-Ju Yi, and co-workers at Chungbuk National University in South Korea have reviewed recent data on how cells regulate gene activity under hypoxic conditions. Advances in sequencing technology have allowed genome-wide studies of how hypoxia affects DNA structure and gene activation, revealing that gene-specific modifications may be more important than genome-wide modifications. Hypoxia is implicated in several diseases, such as cancer and chronic metabolic diseases, and a better understanding of how it affects gene activation may help identify new treatments for hypoxia-related diseases.

Lactate metabolism coordinates macrophage response and regeneration in zebrafish

Rationale: Macrophages are multifunctional cells with a pivotal role on tissue development, homeostasis and regeneration. Indeed, in response to tissue injury and the ensuing regeneration process, macrophages are challenged and undergo massive metabolic adaptations and changes. However, the control of this metabolic reprogramming by macrophage microenvironment has never been deciphered in vivo. Methods: In this study, we used zebrafish model and caudal fin resection as a robust regeneration system. We explored specific changes in gene expression after tissue amputation via single-cell RNA sequencing analysis and whole-tissue transcriptomic analysis. Based on the identification of key modifications, we confirmed the role of the lactate pathway in macrophage response and fin regeneration, through the combination of chemical and genetic inhibitors of this pathway. Results: Single cell RNA sequencing revealed the upregulation of different genes associated with glycolysis and lactate metabolism in macrophages, upon fin regeneration. Hence, using chemical inhibitors of the LDH enzyme, we confirmed the role of lactate in macrophage recruitment and polarization, to promote a pro-inflammatory phenotype and enhance fin regeneration. The genetic modulation of monocarboxylate transporters illustrated a complex regulation of lactate levels, based on both intracellular and extracellular supplies. Commonly, the different sources of lactate resulted in macrophage activation with an increased expression level of inflammatory cytokines such as TNFa during the first 24 hours of regeneration. Transcriptomic analyses confirmed that lactate induced a global modification of gene expression in macrophages. Conclusion: Altogether, our findings highlight the crucial role of lactate at the onset of macrophage differentiation toward a pro-inflammatory phenotype. The deep modifications of macrophage phenotype mediated by lactate and downstream effectors play a key role to coordinate inflammatory response and tissue regeneration.

Co-condensation between transcription factor and coactivator p300 modulates transcriptional bursting kinetics

The coactivator p300/CREB-binding protein (CBP) regulates genes by facilitating the assembly of transcriptional machinery and by acetylating histones and other factors. However, it remains mostly unclear how both functions of p300 are dynamically coordinated during gene control. Here, we showed that p300 can orchestrate two functions through the formation of dynamic clusters with certain transcription factors (TFs), which is mediated by the interactions between a TF's transactivation domain (TAD) and the intrinsically disordered regions of p300. Co-condensation can enable spatially defined, all-or-none activation of p300's catalytic activity, priming the recruitment of coactivators, including Brd4. We showed that co-condensation can modulate transcriptional initiation rate and burst duration of target genes, underlying nonlinear gene regulatory functions. Such modulation is consistent with how p300 might shape gene bursting kinetics globally. Altogether, these results suggest an intriguing gene regulation mechanism, in which TF and p300 co-condensation contributes to transcriptional bursting regulation and cooperative gene control.

Role of HIF-1α in Cold Ischemia Injury of Rat Donor Heart Via the miR-21/PDCD4 Pathway

BACKGROUND

Hypoxia-inducible factor 1 alpha (HIF-1α) is a transcription factor that plays a major role under hypoxia conditions. Cold storage during heart transplantation causes the donor heart long-term hypoxia. There is some evidence indicating a conceivable HIF-1α/microRNA-21 (miR-21)/phosphatase and tensin homolog (PTEN)/programmed cell death 4 (PDCD4) pathway. We assessed the hypothesis that HIF-1α has a positive effect during donor heart cold storage by making the miR-21 upregulate to reduce the expression of PDCD4.

METHODS

We established the rat heart cold storage model and stratified it into 6-hour groups from 0 to 24 hours. Western blot and quantitative reverse transcription polymerase chain reaction were performed to detect the expression of HIF-1α, miR-21, PDCD4, and PTEN.

RESULTS

After cold storage the expression of HIF-1α increased from 0 to 6 hours and then gradually decreased, but the expression level was relatively higher compared with the control group. The miR-21 was upregulated from 0 to 12 hours then downregulated. The messenger RNA expression of PDCD4 was upregulated gradually, but the protein expression was significantly downregulated at 12th hour then continued to upregulate. Interestingly, the expression level of miR-21 was highest in the 12th hour, which indicated miR-21 could inhibit the PDCD4. We subsequently detected the messenger RNA of PTEN, which can inhibit HIF-1α and be inhibited by miR-21. The expression of PTEN was also significantly downregulated at the 12th hour.

CONCLUSION

In conclusion, there is possible interaction between HIF-1α and miR-21, and the conceivable HIF-1α/miR-21/PTEN/PDCD4 pathway plays a protective role in cold storage of the heart.

Molecular Understanding of the Cardiomodulation in Myocardial Infarction and the Mechanism of Vitamin E Protections

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Myocardial infarction is a major cause of deaths globally. Modulation of several molecular mechanisms occurs during the initial stages of myocardial ischemia prior to permanent cardiac tissue damage, which involves both pathogenic as well as survival pathways in the cardiomyocyte. Currently, there is increasing evidence regarding the cardioprotective role of vitamin E in alleviating the disease. This fat-soluble vitamin does not only act as a powerful antioxidant; but it also has the ability to regulate several intracellular signalling pathways including HIF-1, PPAR-γ, Nrf-2, and NF-κB that influence the expression of a number of genes and their protein products. Essentially, it inhibits the molecular progression of tissue damage and preserves myocardial tissue viability. This review aims to summarize the molecular understanding of the cardiomodulation in myocardial infarction as well as the mechanism of vitamin E protection.

Advances in inhibition of protein-protein interactions targeting hypoxia-inducible factor-1 for cancer therapy

Hypoxia is a common characteristic of many types of solid tumors and is associated with tumor propagation, malignant progression, and resistance to anti-cancer therapy. HIF-1 pathway is one of the survival pathways activated in tumor in response to hypoxia. In hypoxic condition, hypoxia-inducible factor-1α (HIF-1α) is stabilized and translocated into nucleus where it forms heterodimer with HIF-1β and regulates the expression of a plethora of genes involved in different processes, such as cell proliferation, differentiation, apoptosis, vascularization/angiogenesis, tumor invasion and metastasis. Recruitment of co-activator p300 or CBP to HIF-1α is critical to the transactivation activity of HIF-1 dimer, therefore, small molecules which can block the dimerization of HIF-1α and HIF-1β or inhibit the interaction between HIF-1α and p300 can function as inhibitors of HIF-1 and have the potential to be developed as novel therapies for the treatment of human cancers. In this review, recent progress of small molecular inhibitors of protein-protein interactions targeting HIF-1 is summarized, the mechanism of functions of these compounds and their potential usage as anti-cancer agents have also been discussed.

A Mesenchymal stem cell line (B10) increases angiogenesis in a rat MCAO model

A human mesenchymal stem cell line (B10) transplantation has been shown to improve ischemia-induced neurological deficits in animal stroke models. To understand the underlying mechanism, we have investigated the effects of B10 transplantation on cerebral angiogenesis in a rat middle cerebral artery occlusion (MCAO) model. B10 cells were transplanted intravenously 24 h after MCAO. Immunofluorescence staining results showed that compared to PBS-groups, vWF positive vessel and endoglin positive new vessels were increased in B10-transplanted MCAO groups in the lesion areas. The mRNA of angiogenesis factors including placental growth factor and hypoxia inducible factor (HIF)-1α were increased 3 days after MCAO in the core and IBZ areas of B10-transplanted group. Angiopoetin1 mRNA was increased only in the IBZ. Western blotting results showed that HIF-1α and vascular endothelial growth factor (VEGF) proteins were increased in B10-transplanted group. Both HIF-1α and VEGF were expressed in macrophage/microglia in the core area. In the IBZ, however, HIF-1α was expressed both in astrocytes and macrophage/microglia, while VEGF was expressed only in macrophage/microglia. Moreover, TGFβ protein levels were found to be increased in B10-transplanted group in the core and IBZ regions. Cell culture experiments using a human microglia cell line (HMO6) and B10 showed that IL-1β induced VEGF mRNA expression in both cell types. IL-1β was found to be highly expressed in B10 cells, and its co-culture with HMO6 further increased that in B10. Co-culture increased VEGF mRNA in both B10 and HMO6. In the rat brains, IL-1β was expressed in macrophage/microglia and transplanted-B10 cells in the core. IL-1β positive cell number was increased slightly, but significantly in B10-transplanted rats. To explore further, IL-1β expression was silenced in B10 cells by transfecting mRNA specific siRNA, and then transplanted in MCAO rats. Immunostaining result showed that endoglin positive area was decreased in IL-1β-silenced B10 transplanted groups compared to nonsilenced-B10 transplanted groups. Interestingly, vessel-like structure appeared as early as 3 days after MCAO in IL-1β-silenced B10-transplanted group. Thus our results demonstrated that B10 cells increased angiogenesis in MCAO rat model, through the regulation of HIF-1α and VEGF expression, where IL-1β might play a role.

HIF-dependent and reversible nucleosome disassembly in hypoxia-inducible gene promoters

Hypoxia causes dramatic changes in gene expression profiles, and the mechanism of hypoxia-inducible transcription has been analyzed for use as a model system of stress-inducible gene regulation. In this study, changes in chromatin organization in promoters of hypoxia-inducible genes were investigated during hypoxia-reoxygenation conditions. Most of the hypoxia-inducible gene promoters were hypersensitive to DNase I under both normal and hypoxic conditions, and our data indicate an immediate recruitment of transcription factors under hypoxic conditions. In some of the hypoxia-inducible promoters, nucleosome-free DNA regions (NFRs) were established in parallel with hypoxia-induced transcription. We also show that the hypoxia-inducible formation of NFRs requires that hypoxia-inducible transcription factors (HIFs) bind to the promoters together with the transcriptional coactivator CBP. Within 1 h after the hypoxia exposure was ended (reoxygenation), HIF complexes were dissociated from the promoter regions. Within 24 h of reoxygenation, the hypoxia-induced transcription returned to basal levels and the nucleosome structure was reassembled in the hypoxia-inducible NFRs. Nucleosome reassembly required the function of the transcriptional coregulator SIN3A. Thus, reversible changes in nucleosome organization mediated by transcription factors are notable features of stress-inducible gene regulation.

Combination of betulinic acid and chidamide inhibits acute myeloid leukemia by suppression of the HIF1α pathway and generation of reactive oxygen species

Acute myeloid leukemia (AML) is a heterogeneous disorder of the hematopoietic system with no common genetic “Achilles heel” that can be targeted. Most patients respond well to standard therapy, while a majority relapse, and development of an effective therapy for AML patients is still urgently needed. In this study, we demonstrated that betulinic acid (BA) significantly increased Aryl hydrocarbon receptor (AHR) expression through demethylation on the AHR promoter in AML cells, and the increased AHR expression interacts with and sequesters ARNT, subsequently suppressing hypoxia-inducible factor-1α (HIF1α) pathway. We also found that histone deacetylase inhibitor chidamide (CDM) treatment significantly increased p300 over-acetylation in AML cells with dissociation of p300 with HIF1α, and subsequently suppressed the HIF1α pathway. Further investigation showed that BA/CDM combination additively increased generation of reactive oxygen species (ROS) with DNA damage, apoptosis and mitochondrial dysfunction. Also, BA/CDM combination additively suppressed the HIF1α pathway with decreased VEGF expression. in vivo mice study showed that BA/CDM combination significantly suppressed AML tumor growth, and overexpression of SOD2 and a constitutive HIF1α (HIF1C) completely diminished this effect. We conclude that a BA/CDM combination inhibits AML tumors through ROS over-generation and HIF1α pathway suppression. This is the first time we have shown the potential effect and possible mechanism of BA and CDM on the inhibition of AML tumor growth.

Hypoxia inducible factor (HIF) as a model for studying inhibition of protein-protein interactions

The state of the art in identifying protein–protein interaction inhibitors of hypoxia inducible factor – a promising target for anticancer drug design – is described.

The modulation of protein–protein interactions (PPIs) represents a major challenge in modern chemical biology. Current approaches (e.g. high-throughput screening, computer aided ligand design) are recognised as having limitations in terms of identification of hit matter. Considerable success has been achieved in terms of developing new approaches to PPI modulator discovery using the p53/hDM2 and Bcl-2 family of PPIs. However these important targets in oncology might be considered as “low-hanging-fruit”. Hypoxia inducible factor (HIF) is an emerging, but not yet fully validated target for cancer chemotherapy. Its role is to regulate the hypoxic response and it does so through a plethora of protein–protein interactions of varying topology, topography and complexity: its modulation represents an attractive approach to prevent development of new vasculature by hypoxic tumours.

A compendium of proteins that interact with HIF-1α

Hypoxia-inducible factor 1 (HIF-1) is the founding member of a family of transcription factors that function as master regulators of oxygen homeostasis. HIF-1 is composed of an O₂-regulated HIF-1α subunit and a constitutively expressed HIF-1β subunit. This review provides a compendium of proteins that interact with the HIF-1α subunit, many of which regulate HIF-1 activity in either an O₂-dependent or O₂-independent manner.

Regulation of hypoxia-inducible gene expression after HIF activation

Hypoxia causes dramatic changes in the expression profiles of genes that encode glycolytic enzymes, vascular endothelial growth factors, erythropoietin, and other factors in a tissue-specific manner through activating hypoxia-inducible transcription factors (HIFs) such as HIF1α and HIF2α. It has been elucidated that the activity of HIFs is fundamentally regulated by their protein stability in an oxygen-dependent manner. However, little is known about how stabilized HIFs regulate transcription of their target genes in hypoxic cells. Additionally, the roles of HIF3α, the third member of the HIFs, are still enigma due to its various splicing variants and the complicated phenotypes of Hif3a-gene modified mouse lines. Here, we summarize how molecular systems fine-tune hypoxia-inducible transcription with the cooperation of HIFs and their negative regulators, including IPAS, one of the HIF3α splicing variants. Since epigenetic mechanisms contribute to stress-inducible and cell-type specific gene regulation, the HIF-dependent reorganization of nucleosome structures in hypoxia-inducible gene promoters is also discussed.

K63-polyubiquitinated HAUSP deubiquitinates HIF-1α and dictates H3K56 acetylation promoting hypoxia-induced tumour progression

Intratumoural hypoxia induces HIF-1α and promotes tumour progression, metastasis and treatment resistance. HIF-1α stability is regulated by VHL-E3 ligase-mediated ubiquitin-dependent degradation; however, the hypoxia-regulated deubiquitinase that stabilizes HIF-1α has not been identified. Here we report that HAUSP (USP7) deubiquitinase deubiquitinates HIF-1α to increase its stability, induce epithelial-mesenchymal transition and promote metastasis. Hypoxia induces K63-linked polyubiquitinated HAUSP at lysine 443 to enhance its functions. Knockdown of HAUSP decreases acetylation of histone 3 lysine 56 (H3K56Ac). K63-polyubiquitinated HAUSP interacts with a ubiquitin receptor CBP to specifically mediate H3K56 acetylation. ChIP-seq analysis of HAUSP and HIF-1α binding reveals two motifs responsive to hypoxia. HectH9 is the E3 ligase for HAUSP and a prognostic marker together with HIF-1α. This report demonstrates that hypoxia-induced K63-polyubiquitinated HAUSP deubiquitinates HIF-1α and causes CBP-mediated H3K56 acetylation on HIF-1α target gene promoters to promote EMT/metastasis, further defining HAUSP as a therapeutic target in hypoxia-induced tumour progression.

Tuning the Transcriptional Response to Hypoxia by Inhibiting Hypoxia-inducible Factor (HIF) Prolyl and Asparaginyl Hydroxylases

The hypoxia-inducible factor (HIF) system orchestrates cellular responses to hypoxia in animals. HIF is an α/β-heterodimeric transcription factor that regulates the expression of hundreds of genes in a tissue context-dependent manner. The major hypoxia-sensing component of the HIF system involves oxygen-dependent catalysis by the HIF hydroxylases; in humans there are three HIF prolyl hydroxylases (PHD1-3) and an asparaginyl hydroxylase (factor-inhibiting HIF (FIH)). PHD catalysis regulates HIFα levels, and FIH catalysis regulates HIF activity. How differences in HIFα hydroxylation status relate to variations in the induction of specific HIF target gene transcription is unknown. We report studies using small molecule HIF hydroxylase inhibitors that investigate the extent to which HIF target gene expression is induced by PHD or FIH inhibition. The results reveal substantial differences in the role of prolyl and asparaginyl hydroxylation in regulating hypoxia-responsive genes in cells. PHD inhibitors with different structural scaffolds behave similarly. Under the tested conditions, a broad-spectrum 2-oxoglutarate dioxygenase inhibitor is a better mimic of the overall transcriptional response to hypoxia than the selective PHD inhibitors, consistent with an important role for FIH in the hypoxic transcriptional response. Indeed, combined application of selective PHD and FIH inhibitors resulted in the transcriptional induction of a subset of genes not fully responsive to PHD inhibition alone. Thus, for the therapeutic regulation of HIF target genes, it is important to consider both PHD and FIH activity, and in the case of some sets of target genes, simultaneous inhibition of the PHDs and FIH catalysis may be preferable.

The TIP60 Complex Is a Conserved Coactivator of HIF1A

Hypoxia-inducible factors (HIFs) are critical regulators of the cellular response to hypoxia. Despite their established roles in normal physiology and numerous pathologies, the molecular mechanisms by which they control gene expression remain poorly understood. We report here a conserved role for the TIP60 complex as a HIF1 transcriptional cofactor in Drosophila and human cells. TIP60 (KAT5) is required for HIF1-dependent gene expression in fly cells and embryos and colorectal cancer cells. HIF1A interacts with and recruits TIP60 to chromatin. TIP60 is dispensable for HIF1A association with its target genes but is required for HIF1A-dependent chromatin modification and RNA polymerase II activation in hypoxia. In human cells, global analysis of HIF1A-dependent gene activity reveals that most HIF1A targets require either TIP60, the CDK8-Mediator complex, or both as coactivators for full expression in hypoxia. Thus, HIF1A employs functionally diverse cofactors to regulate different subsets of genes within its transcriptional program.

Class I and IIa HDACs Mediate HIF-1α Stability Through PHD2-Dependent Mechanism, While HDAC6, a Class IIb Member, Promotes HIF-1α Transcriptional Activity in Nucleus Pulposus Cells of the Intervertebral Disc

The objective of this study was to determine the role of histone deacetylases (HDACs) in regulating HIF-1α protein stability and activity in nucleus pulposus (NP) cells. Treatment of NP cells with pan-HDAC inhibitor TSA resulted in decreased HIF-1α levels under both normoxia and hypoxia in a dose-dependent fashion. TSA-mediated HIF-1α degradation was rescued by concomitant inhibition of not only the 26S proteasome but also PHD2 function. Moreover, TSA treatment of PHD2(-/-) cells had little effect on HIF-1α levels, supporting the notion that inhibition of PHD2 function by HDACs contributed to HIF-1α stabilization. Surprisingly, class-specific HDAC inhibitors did not affect HIF-1α protein stability, indicating that multiple HDACs controlled HIF-1α stability by regulating HIF-1α-PHD2 interaction in NP cells. Interestingly, lower-dose TSA that did not affect HIF-1α stability decreased its activity and target gene expression. Likewise, rescue of TSA-mediated HIF-1α protein degradation by blocking proteasomal or PHD activity did not restore HIF-1 activity, suggesting that HDACs independently regulate HIF-1α stability and activity. Noteworthy, selective inhibition of HDAC6 and not of class I and IIa HDACs decreased HIF-1-mediated transcription under hypoxia to a similar extent as lower-dose TSA, contrasting the reported role of HDAC6 as a transcriptional repressor in other cell types. Moreover, HDAC6 inhibition completely blocked TSA effects on HIF-1 activity. HDAC6 associated with and deacetylated HSP90, an important cofactor for HIF-1 function in NP cells, and HDAC6 inhibition decreased p300 transactivation in NP cells. Taken together, these results suggest that although multiple class I and class IIa HDACs control HIF-1 stability, HDAC6, a class IIb HDAC, is a novel mediator of HIF-1 activity in NP cells possibly through promoting action of critical HIF-1 cofactors. © 2016 American Society for Bone and Mineral Research.

Targeting Protein-Protein Interactions in the HIF System

Animals respond to chronic hypoxia by increasing the levels of a transcription factor known as the hypoxia-inducible factor (HIF). HIF upregulates multiple genes, the products of which work to ameliorate the effects of limited oxygen at cellular and systemic levels. Hypoxia sensing by the HIF system involves hydroxylase-catalysed post-translational modifications of the HIF α-subunits, which 1) signal for degradation of HIF-α and 2) limit binding of HIF to transcriptional coactivator proteins. Because the hypoxic response is relevant to multiple disease states, therapeutic manipulation of the HIF-mediated response has considerable medicinal potential. In addition to modulation of catalysis by the HIF hydroxylases, the HIF system manifests other possibilities for therapeutic intervention involving protein-protein and protein-nucleic acid interactions. Recent advances in our understanding of the structural biology and biochemistry of the HIF system are facilitating medicinal chemistry efforts. Herein we give an overview of the HIF system, focusing on structural knowledge of protein-protein interactions and how this might be used to modulate the hypoxic response for therapeutic benefit.

Expression levels of microRNA-199 and hypoxia-inducible factor-1 alpha in brain tissue of patients with intractable epilepsy

OBJECTIVES

During the last decade, experimental evidence has demonstrated an important role of hypoxia, which leads to neuronal cell death and angiogenesis, in the mechanisms of seizure precipitation and recurrence. MicroRNA-199 targets hypoxia-inducible factor-1alpha (HIF-1α), which has recently been implicated in the pathophysiology of the hypoxic state and brain injury. However, little is known about the roles of MicroRNA-199 and HIF-1α in the human epileptogenic process.

DESIGN AND METHODS

In this study, we investigated the expression of miR-199a-5p, miR-199b-5p and HIF-1α using real-time PCR, immunohistochemistry and western blots in the temporal neocortex of twenty four patients with intractable epilepsy and twelve control subjects.

RESULTS

Compared with the control group, the expression of miR-199a-5p and miR-199b-5p was significantly lower in epileptic brain tissues (p < 0.05). The levels of HIF-1α mRNA and protein were highly up-regulated in epileptic brain tissues compared with those of control subjects (p < 0.05).

CONCLUSION

These data suggest that the abnormal expression of miR-199 and HIF-1α in epileptic brain tissue may be involved in the pathophysiology of human epilepsy and that the expression of HIF-1α may be regulated by miR-199. These findings may provide new insights into the treatment of epilepsy.

Oxygen regulates molecular mechanisms of cancer progression and metastasis

Oxygen is the basic molecule which supports life and it truly is "god's gift to life." Despite its immense importance, research on "oxygen biology" has never received the light of the day and has been limited to physiological and biochemical studies. It seems that in modern day biology, oxygen research is summarized in one word "hypoxia." Scientists have focused on hypoxia-induced transcriptomics and molecular-cellular alterations exclusively in disease models. Interestingly, the potential of oxygen to control the basic principles of biology like homeostatic maintenance, transcription, replication, and protein folding among many others, at the molecular level, has been completely ignored. Here, we present a perspective on the crucial role played by oxygen in regulation of basic biological phenomena. Our conclusion highlights the importance of establishing novel research areas like oxygen biology, as there is great potential in this field for basic science discoveries and clinical benefits to the society.

Transcriptional regulation by hypoxia inducible factors

The cellular response to oxygen deprivation is governed largely by a family of transcription factors known as Hypoxia Inducible Factors (HIFs). This review focuses on the molecular mechanisms by which HIFs regulate the transcriptional apparatus to enable the cellular and organismal response to hypoxia. We discuss here how the various HIF polypeptides, their posttranslational modifications, binding partners and transcriptional cofactors affect RNA polymerase II activity to drive context-dependent transcriptional programs during hypoxia.

BRG1 and BRM chromatin-remodeling complexes regulate the hypoxia response by acting as coactivators for a subset of hypoxia-inducible transcription factor target genes

Chromatin remodeling is an active process, which represses or enables the access of transcription machinery to genes in response to external stimuli, including hypoxia. However, in hypoxia, the specific requirement, as well as the molecular mechanism by which the chromatin-remodeling complexes regulate gene expression, remains unclear. In this study, we report that the Brahma (BRM) and Brahma-related gene 1 (BRG1) ATPase-containing SWI/SNF chromatin-remodeling complexes promote the expression of the hypoxia-inducible transcription factor 1α (HIF1α) and HIF2α genes and also promote hypoxic induction of a subset of HIF1 and HIF2 target genes. We show that BRG1 or BRM knockdown in Hep3B and RCC4T cells reduces hypoxic induction of HIF target genes, while reexpression of BRG1 or BRM in BRG1/BRM-deficient SW13 cells increases HIF target gene activation. Mechanistically, HIF1 and HIF2 increase the hypoxic induction of HIF target genes by recruiting BRG1 complexes to HIF target gene promoters, which promotes nucleosome remodeling of HIF target gene promoters in a BRG1 ATPase-dependent manner. Importantly, we found that the function of BRG1 complexes in hypoxic SW13 and RCC4T cells is dictated by the HIF-mediated hypoxia response and could be opposite from their function in normoxic SW13 and RCC4T cells.

Epigenetic regulation of hypoxia-responsive gene expression: focusing on chromatin and DNA modifications

Mammalian cells constantly encounter hypoxia, which is a stress condition occurring during development and physiological processes. To adapt to this inevitable condition, cells develop various mechanisms to cope with this stress and survive. In addition to the activation/stabilization of transcriptional regulators (hypoxia-inducible factors), other epigenetic mechanisms of gene regulation are used. These mechanisms are mediated by various players including transcriptional coregulators, chromatin-modifying complexes, histone modification enzymes and changes in DNA methylation status. Recent progress in all the fields mentioned above has greatly improved the knowledge of how gene regulation contributes to the hypoxic response. This review should shed light on the molecular epigenetic mechanisms of hypoxia-induced gene regulation and help understand the processes adapted by cells to cope with hypoxia.

STAT3 and HIF1α cooperatively activate HIF1 target genes in MDA-MB-231 and RCC4 cells

Solid tumors often exhibit simultaneously inflammatory and hypoxic microenvironments. The 'signal transducer and activator of transcription-3' (STAT3)-mediated inflammatory response and the hypoxia-inducible factor (HIF)-mediated hypoxia response have been independently shown to promote tumorigenesis through the activation of HIF or STAT3 target genes and to be indicative of a poor prognosis in a variety of tumors. We report here for the first time that STAT3 is involved in the HIF1, but not HIF2-mediated hypoxic transcriptional response. We show that inhibiting STAT3 activity in MDA-MB-231 and RCC4 cells by a STAT3 inhibitor or STAT3 small interfering RNA significantly reduces the levels of HIF1, but not HIF2 target genes in spite of normal levels of hypoxia-inducible transcription factor 1α (HIF1α) and HIF2α protein. Mechanistically, STAT3 activates HIF1 target genes by binding to HIF1 target gene promoters, interacting with HIF1α protein and recruiting coactivators CREB binding protein (CBP) and p300, and RNA polymerase II (Pol II) to form enhanceosome complexes that contain HIF1α, STAT3, CBP, p300 and RNA Pol II on HIF1 target gene promoters. Functionally, the effect of STAT3 knockdown on proliferation, motility and clonogenic survival of tumor cells in vitro is phenocopied by HIF1α knockdown in hypoxic cells, whereas STAT3 knockdown in normoxic cells also reduces cell proliferation, motility and clonogenic survival. This indicates that STAT3 works with HIF1 to activate HIF1 target genes and to drive HIF1-depedent tumorigenesis under hypoxic conditions, but also has HIF-independent activity in normoxic and hypoxic cells. Identifying the role of STAT3 in the hypoxia response provides further data supporting the effectiveness of STAT3 inhibitors in solid tumor treatment owing to their usefulness in inhibiting both the STAT3 and HIF1 pro-tumorigenic signaling pathways in some cancer types.

A small molecule approach to engineering vascularized tissue

The repertoire of growth factors determines the biological engagement of human mesenchymal stromal cells (hMSCs) in processes such as immunomodulation and tissue repair. Hypoxia is a strong modulator of the secretome and well known stimuli to increase the secretion of pro-angiogenic molecules. In this manuscript, we employed a high throughput screening assay on an hMSCs cell line in order to identify small molecules that mimic hypoxia. Importantly, we show that the effect of these small molecules was cell type/species dependent, but we identified phenanthroline as a robust hit in several cell types. We show that phenanthroline induces high expression of hypoxia-target genes in hMSCs when compared with desferoxamine (DFO) (a known hypoxia mimic) and hypoxia incubator (2% O(2)). Interestingly, our microarray and proteomics analysis show that only phenanthroline induced high expression and secretion of another angiogenic cytokine, interleukin-8, suggesting that the mechanism of phenanthroline-induced hypoxia is distinct from DFO and hypoxia and involves the activation of other signaling pathways. We showed that phenanthroline alone was sufficient to induce blood vessel formation in a Matrigel plug assay in vivo paving the way to its application in ischeamic-related diseases.

Hypoxia-inducing factors as master regulators of stemness properties and altered metabolism of cancer- and metastasis-initiating cells

Accumulating lines of experimental evidence have revealed that hypoxia-inducible factors, HIF-1α and HIF-2α, are key regulators of the adaptation of cancer- and metastasis-initiating cells and their differentiated progenies to oxygen and nutrient deprivation during cancer progression under normoxic and hypoxic conditions. Particularly, the sustained stimulation of epidermal growth factor receptor (EGFR), insulin-like growth factor-1 receptor (IGF-1R), stem cell factor (SCF) receptor KIT, transforming growth factor-β receptors (TGF-βRs) and Notch and their downstream signalling elements such as phosphatidylinositol 3′-kinase (PI3K)/Akt/molecular target of rapamycin (mTOR) may lead to an enhanced activity of HIFs. Moreover, the up-regulation of HIFs in cancer cells may also occur in the hypoxic intratumoral regions formed within primary and secondary neoplasms as well as in leukaemic cells and metastatic prostate and breast cancer cells homing in the hypoxic endosteal niche of bone marrow. The activated HIFs may induce the expression of numerous gene products such as induced pluripotency-associated transcription factors (Oct-3/4, Nanog and Sox-2), glycolysis- and epithelial-mesenchymal transition (EMT) programme-associated molecules, including CXC chemokine receptor 4 (CXCR4), snail and twist, microRNAs and angiogenic factors such as vascular endothelial growth factor (VEGF). These gene products in turn can play critical roles for high self-renewal ability, survival, altered energy metabolism, invasion and metastases of cancer cells, angiogenic switch and treatment resistance. Consequently, the targeting of HIF signalling network and altered metabolic pathways represents new promising strategies to eradicate the total mass of cancer cells and improve the efficacy of current therapies against aggressive and metastatic cancers and prevent disease relapse.

Upstream stimulatory factor 2 and hypoxia-inducible factor 2α (HIF2α) cooperatively activate HIF2 target genes during hypoxia

While the functions of hypoxia-inducible factor 1α (HIF1α)/aryl hydrocarbon receptor nuclear translocator (ARNT) and HIF2α/ARNT (HIF2) proteins in activating hypoxia-inducible genes are well established, the role of other transcription factors in the hypoxic transcriptional response is less clear. We report here for the first time that the basic helix-loop-helix-leucine-zip transcription factor upstream stimulatory factor 2 (USF2) is required for the hypoxic transcriptional response, specifically, for hypoxic activation of HIF2 target genes. We show that inhibiting USF2 activity greatly reduces hypoxic induction of HIF2 target genes in cell lines that have USF2 activity, while inducing USF2 activity in cells lacking USF2 activity restores hypoxic induction of HIF2 target genes. Mechanistically, USF2 activates HIF2 target genes by binding to HIF2 target gene promoters, interacting with HIF2α protein, and recruiting coactivators CBP and p300 to form enhanceosome complexes that contain HIF2α, USF2, CBP, p300, and RNA polymerase II on HIF2 target gene promoters. Functionally, the effect of USF2 knockdown on proliferation, motility, and clonogenic survival of HIF2-dependent tumor cells in vitro is phenocopied by HIF2α knockdown, indicating that USF2 works with HIF2 to activate HIF2 target genes and to drive HIF2-depedent tumorigenesis.

Arylsulfonamide KCN1 inhibits in vivo glioma growth and interferes with HIF signaling by disrupting HIF-1α interaction with cofactors p300/CBP

PURPOSE

The hypoxia-inducible factor-1 (HIF-1) plays a critical role in tumor adaptation to hypoxia, and its elevated expression correlates with poor prognosis and treatment failure in patients with cancer. In this study, we determined whether 3,4-dimethoxy-N-[(2,2-dimethyl-2H-chromen-6-yl)methyl]-N-phenylbenzenesulfonamide, KCN1, the lead inhibitor in a novel class of arylsulfonamide inhibitors of the HIF-1 pathway, had antitumorigenic properties in vivo and further defined its mechanism of action.

EXPERIMENTAL DESIGN

We studied the inhibitory effect of systemic KCN1 delivery on the growth of human brain tumors in mice. To define mechanisms of KCN1 anti-HIF activities, we examined its influence on the assembly of a functional HIF-1α/HIF-1β/p300 transcription complex.

RESULTS

KCN1 specifically inhibited HIF reporter gene activity in several glioma cell lines at the nanomolar level. KCN1 also downregulated transcription of endogenous HIF-1 target genes, such as VEGF, Glut-1, and carbonic anhydrase 9, in a hypoxia-responsive element (HRE)-dependent manner. KCN1 potently inhibited the growth of subcutaneous malignant glioma tumor xenografts with minimal adverse effects on the host. It also induced a temporary survival benefit in an intracranial model of glioma but had no effect in a model of melanoma metastasis to the brain. Mechanistically, KCN1 did not downregulate the levels of HIF-1α or other components of the HIF transcriptional complex; rather, it antagonized hypoxia-inducible transcription by disrupting the interaction of HIF-1α with transcriptional coactivators p300/CBP.

CONCLUSIONS

Our results suggest that the new HIF pathway inhibitor KCN1 has antitumor activity in mouse models, supporting its further translation for the treatment of human tumors displaying hypoxia or HIF overexpression.

HIF2α-Sp1 interaction mediates a deacetylation-dependent FVII-gene activation under hypoxic conditions in ovarian cancer cells

Hypoxia-inducible factors (HIF)-1α and HIF2α are major transcription factors required for adaptive responses to hypoxia. HIFs form a complex with aryl hydrocarbon receptor nuclear translocator (ARNT) to bind to the regulatory regions of target genes. The acetylation of histones by histone acetyltransferases (HATs) is one of the epigenetic marks associated with active chromatin. Indeed, HIFs recruit p300 HAT to hypoxia response elements (HREs) within gene regulatory regions. Here, we report an unusual HIF-mediated transcriptional activation in ovarian clear cell carcinoma (CCC). While characterizing coagulation factor VII (FVII) gene induction during hypoxic conditions, we observed that the interaction of HIF2α with Sp1, but not with ARNT, could induce transcription of FVII in a HRE-independent manner. Unexpectedly, this gene activation is associated with histone deacetylation. We found that a class II HDAC, HDAC4, is recruited with HIF2α to the FVII promoter as a co-activator, while p300 HAT negatively regulated this process. Furthermore, this mechanism can be synergistically enhanced via a deacetylation-dependent pathway when cells are simultaneously exposed to hypoxic and serum-free conditions. These results suggest the presence of a stress-responsive transcription mediated by the HIF2α/Sp1/HDAC4 network and explain how CCC shed their procoagulant activity under hypoxia.

Synthetic transactivation screening reveals ETV4 as broad coactivator of hypoxia-inducible factor signaling

The human prolyl-4-hydroxylase domain (PHD) proteins 1–3 are known as cellular oxygen sensors, acting via the degradation of hypoxia-inducible factor (HIF) α-subunits. PHD2 and PHD3 genes are inducible by HIFs themselves, suggesting a negative feedback loop that involves PHD abundance. To identify novel regulators of the PHD2 gene, an expression array of 704 transcription factors was screened by a method that allows distinguishing between HIF-dependent and HIF-independent promoter regulation. Among others, the E-twenty six transcription factor ETS translocation variant 4 (ETV4) was found to contribute to PHD2 gene expression particularly under hypoxic conditions. Mechanistically, complex formation between ETV4 and HIF-1/2α was observed by mammalian two-hybrid and fluorescence resonance energy transfer analysis. HIF-1α domain mapping, CITED2 overexpression and factor inhibiting HIF depletion experiments provided evidence for cooperation between HIF-1α and p300/CBP in ETV4 binding. Chromatin immunoprecipitation confirmed ETV4 and HIF-1α corecruitment to the PHD2 promoter. Of 608 hypoxically induced transcripts found by genome-wide expression profiling, 7.7% required ETV4 for efficient hypoxic induction, suggesting a broad role of ETV4 in hypoxic gene regulation. Endogenous ETV4 highly correlated with PHD2, HIF-1/2α and several established markers of tissue hypoxia in 282 human breast cancer tissue samples, corroborating a functional interplay between the ETV4 and HIF pathways.

Deletion of the oxygen-dependent degradation domain results in impaired transcriptional activity of hypoxia-inducible factors

Hypoxia-inducible factors (HIF1α/HIF2α) are key transcription factors that promote angiogenesis. The overexpression of degradation-resistant HIF mutants is considered a promising pro-angiogenic therapeutic tool. We compared the transcriptional activity of HIF1α/HIF2α mutants that obtained their resistance to oxygen-dependent degradation either by deletion of their entire oxygen-dependent degradation (ODD) domain or by replacement of prolyl residues that are crucial for oxygen-dependent degradation. Although all HIF mutants translocated into the nucleus, HIF1α and HIF2α mutants inclosing the point mutations were significantly more effective in trans-activating the target gene VEGF and in inducing tube formation of endothelial cells than mutants lacking the complete ODD domain.

Hypoxia inducible factor-1 alpha expression is increased in infected positive HPV16 DNA oral squamous cell carcinoma and positively associated with HPV16 E7 oncoprotein

BACKGROUND

There is increasing evidence for the role of High Risk (HR) Human PapillomaVirus (HPV) in the pathogenesis of Oral Squamous Cell Carcinoma (OSCC). The E6 and E7 oncogenes from HR HPVs are responsible for the deregulation of p53 and pRB proteins involved in cell cycle and apoptotic pathways. In cell lines experiments, the HPV E7 protein seems to be able to enhance Hypoxia Inducible Factor-1 alpha (HIF-1α) activity, normally involved in the response to hypoxia and able to enhance angiogenesis.

RESULTS

We studied tumor specimens from 62 OSCC; a higher prevalence of tumors in TNM stage II and also in pT2 class between OSCC infected positive HPV16 DNA than non-infected ones was observed. HIF-1α positivity was detected throughout the analysed fields, not associated with areas of necrosis and also observed in cells immediately adjacent to blood vessels. A significant increase in mean values of the HIF-1α labeling indexes was observed for pT1-T2, as well for stage I-II, in the infected positive HPV16 DNA tumors than non-infected ones. HIF-1α and HPV16 E7 labeling indexes showed a significantly positive correlation which suggested a positive association between HPV16 E7 and HIF-1α expression.

CONCLUSIONS

In our specimens HIF-1α immunoreactivity hints for an O2-independent regulatory mechanism in infected positive HPV16 DNA tumors, especially for pT1-T2 and stage I-II tumors, suggesting a very early involvement in the development of HPV-induced OSCC. HIF-1α and HPV16 E7 labeling indexes suggest also a positive association between the two proteins in infected positive HPV16 DNA OSCC.

CITED2 controls the hypoxic signaling by snatching p300 from the two distinct activation domains of HIF-1α

HIF-1α plays a central role in cellular adaptation to hypoxia, and is closely related to the pathogeneses of life-threatening disorders. HIF-1α induces the expressions of numerous hypoxia-induced genes through two transactivation domains; N-terminal TAD (NAD) and C-terminal TAD (CAD). Furthermore, p300 is known to boost CAD-dependent transactivation, and CBP/p300-interacting transactivator with an ED-rich tail 2 (CITED2) inhibits HIF-1α-driven gene expression by interfering with the interaction between CAD and p300. However, few researches have focused on the role of CITED2 in the regulation of NAD activity, and thus, we addressed this point. CITED2 was found to attenuate the hypoxic activations of NAD-dependent and CAD-dependent genes, suggesting that CITED2 negatively regulates both CAD and NAD. Immunoprecipitation analyses showed that NAD interacts with the Cystein/Histidine region (CH) 1 and CH3 domains of p300. Moreover, CH1 and CH3 both were required for NAD-dependent transactivation. Furthermore, CITED2 was found to inactivate NAD by interfering with NAD binding to CH1, but not to CH3. These results indicate that CITED2 inactivates HIF-1α by blocking p300 recruitment by both NAD and CAD. We also found that pVHL inhibits NAD activity regardless of NAD degradation by blocking the interaction between p300 and NAD. Summarizing, NAD was activated by binding to p300, and this was blocked by either CITED2 or pVHL. We propose that pVHL controls NAD during normoxia and that CITED2 controls NAD during hypoxia. Our results provide a new strategy for controlling HIF-1α.

Epigenetics: new questions on the response to hypoxia

Reduction in oxygen levels below normal concentrations plays important roles in different normal and pathological conditions, such as development, tumorigenesis, chronic kidney disease and stroke. Organisms exposed to hypoxia trigger changes at both cellular and systemic levels to recover oxygen homeostasis. Most of these processes are mediated by Hypoxia Inducible Factors, HIFs, a family of transcription factors that directly induce the expression of several hundred genes in mammalian cells. Although different aspects of HIF regulation are well known, it is still unclear by which precise mechanism HIFs activate transcription of their target genes. Concomitantly, hypoxia provokes a dramatic decrease of general transcription that seems to rely in part on epigenetic changes through a poorly understood mechanism. In this review we discuss the current knowledge on chromatin changes involved in HIF dependent gene activation, as well as on other epigenetic changes, not necessarily linked to HIF that take place under hypoxic conditions.

Histone deacetylase inhibitors block nuclear factor-κB-dependent transcription by interfering with RNA polymerase II recruitment

Histone deacetylase inhibitors (HDACi) have been shown to exhibit anti-inflammatory activity, but their mechanism of action is poorly understood. Trichostatin A (TSA) and the cyclic tetrapeptide class inhibitor Ky-2 inhibit both lipopolysaccharide-induced tumor necrosis factor-α (TNF-α) production in rats and TNF-α-induced expression of inflammatory genes in HeLa cells. We assessed the molecular mechanism underlying TSA-induced anti-inflammatory activity by genetically dissecting activation of the nuclear factor-κB (NF-κB) pathway following stimulation with TNF-α. Trichostatin A did not inhibit degradation of IκBα, nuclear translocation and DNA binding of NF-κB; also, the drug did not affect transient expression from exogenous κB-reporter plasmids. However, endogenous expression of inflammatory cytokines such as interleukin-8 (IL-8) was greatly reduced, even in the absence of de novo protein synthesis, suggesting that HDACi directly inhibits NF-κB-induced transcription. Indeed, chromatin immunoprecipitation (ChIP) analysis showed that events related to transcriptional activation of the IL-8 gene region in response to TNF-α, including recruitment of RNA polymerase II (Pol II), were compromised in the presence of TSA. These data indicate that HDAC activity is required for the efficient initiation and/or elongation of inflammatory gene transcription mediated by NF-κB.

HIF-2alpha enhances beta-catenin/TCF-driven transcription by interacting with beta-catenin

The tumor-promoting factors β-catenin and hypoxia-inducible factor (HIF) are often found to be coactivated in rapidly growing tumors. Recently, it was shown that HIF-1α negatively regulates Wnt/β-catenin signaling by sequestering β-catenin from β-catenin/T-cell factor (TCF). However, no investigation has been undertaken on the involvement of HIF-2α in β-catenin regulation. In this study, it was found that, like HIF-1α, HIF-2α interacts with β-catenin, but at a different site. Furthermore, HIF-2α was found to assemble with β-catenin/TCF and facilitate gene transcription. Mutational analyses revealed that transactivation domains of HIF-2α promote p300 coactivator recruitment by β-catenin. Furthermore, HIF-2α and β-catenin were found to associate in the nuclei of 786-0 renal cell carcinoma cells, and HIF-2α was found to be required for β-catenin activation in these cells and for their proliferation. These results suggest that this interaction contributes to the unrestrained growth of tumor cells containing coactivated HIF-2α and β-catenin. Interestingly, these actions of HIF-2α oppose those of HIF-1α on β-catenin and cell growth, and this suggests that HIF-1α/HIF-2α balance may importantly determine cell growth when hypoxia and Wnt stimulation coexist.

Hypoxia-inducible factor-1alpha enhances haptoglobin gene expression by improving binding of STAT3 to the promoter

Haptoglobin (Hp) is known to play a role in angiogenesis as well as in anti-inflammation. STAT3 is a major transcription factor for expression of human Hp. We investigated whether hypoxia-inducible factor-1α (HIF-1α), a key mediator of angiogenesis, participates in Hp gene expression. HIF-1α overexpression by gene transfection or hypoxia augmented Hp transcription in HepG2 human hepatoma cells. Conversely, knockdown of HIF-1α by specific siRNA transfection diminished Hp expression, although the level of STAT3 phosphorylation remained unchanged. A luciferase reporter assay using mutant Hp promoters demonstrated that two adjacent DNA elements, a STAT3-binding element (SBE) and a cAMP-response element (CRE)-like site in human Hp promoter -120/-97, were required for HIF-1α-stimulated transactivation of the Hp gene. HIF-1α, STAT3, and p300/CBP were simultaneously bound to the SBE/CRE as a complex form. When HIF-1α was knocked down, STAT3 binding to the SBE in the Hp promoter was attenuated. Our findings suggest that HIF-1α assists STAT3 in strong binding to the proximal SBE in the Hp promoter. The CRE-like site located near the SBE may contribute to the formation of a stable complex of STAT3, HIF-1α, and p300/CBP, which leads to maximum transcription of the Hp gene.

Histone deacetylase inhibitors: the epigenetic therapeutics that repress hypoxia-inducible factors

Histone deacetylase inhibitors (HDACIs) have been actively explored as a new generation of chemotherapeutics for cancers, generally known as epigenetic therapeutics. Recent findings indicate that several types of HDACIs repress angiogenesis, a process essential for tumor metabolism and progression. Accumulating evidence supports that this repression is mediated by disrupting the function of hypoxia-inducible factors (HIF-1, HIF-2, and collectively, HIF), which are the master regulators of angiogenesis and cellular adaptation to hypoxia. Since HIF also regulate glucose metabolism, cell survival, microenvironment remodeling, and other alterations commonly required for tumor progression, they are considered as novel targets for cancer chemotherapy. Though the precise biochemical mechanism underlying the HDACI-triggered repression of HIF function remains unclear, potential cellular factors that may link the inhibition of deacetylase activity to the repression of HIF function have been proposed. Here we review published data that inhibitors of type I/II HDACs repress HIF function by either reducing functional HIF-1α levels, or repressing HIF-α transactivation activity. In addition, underlying mechanisms and potential proteins involved in the repression will be discussed. A thorough understanding of HDACI-induced repression of HIF function may facilitate the development of future therapies to either repress or promote angiogenesis for cancer or chronic ischemic disorders, respectively.