Cell-type-specific regulation of degradation of hypoxia-inducible factor 1 alpha: role of subcellular compartmentalization

HIF-1α Promotes Luteinization via NDRG1 Induction in the Human Ovary

Background/Objectives: Hypoxia-inducible factor-1α (HIF-1α) is a transcription factor that plays a crucial role in various physiological and pathological processes of the ovary. However, the timing of HIF-1α expression and its specific biological function in the follicular development of the human ovary remain unclear. Therefore, in this study, we aimed to examine whether HIF-1α and its downstream gene, N-myc downstream-regulated gene 1 (NDRG1), exhibit stage-specific expression during the follicular development process in the human ovary. Methods: We used ovarian tissues from eight women with regular menstrual cycles who were not undergoing hormonal treatment. We investigated HIF-1α and NDRG1 expression and localization using immunohistochemistry. Further, we transfected human ovarian granulosa (KGN) cells with HIF-1α small interfering RNA (siRNA) to investigate the influence of HIF-1α on NDRG1 expression and progesterone synthesis. Results: The immunohistochemical analysis of human ovarian tissues revealed that HIF-1α was localized in the cytoplasm of granulosa cells (GCs) at both the primary and secondary follicular stages. Conversely, in tertiary and later developmental stages, HIF-1α was observed exclusively in the nucleus of GCs. Furthermore, while NDRG1 was not detected in primary follicles, it was present in all GCs beyond the tertiary stage. Notably, transfection of KGN cells with HIF-1α siRNA significantly decreased NDRG1 expression, at both the mRNA and protein levels, and in progesterone synthesis. Conclusion: Our results indicate that HIF-1α and NDRG1 are integral to follicular development and the early luteinization of pre-ovulatory follicles.

Repression of hypoxia-inducible factor-1 contributes to increased mitochondrial reactive oxygen species production in diabetes

Background:

Excessive production of mitochondrial reactive oxygen species (ROS) is a central mechanism for the development of diabetes complications. Recently, hypoxia has been identified to play an additional pathogenic role in diabetes. In this study, we hypothesized that ROS overproduction was secondary to the impaired responses to hypoxia due to the inhibition of hypoxia-inducible factor-1 (HIF-1) by hyperglycemia.

Methods:

The ROS levels were analyzed in the blood of healthy subjects and individuals with type 1 diabetes after exposure to hypoxia. The relation between HIF-1, glucose levels, ROS production and its functional consequences were analyzed in renal mIMCD-3 cells and in kidneys of mouse models of diabetes.

Results:

Exposure to hypoxia increased circulating ROS in subjects with diabetes, but not in subjects without diabetes. High glucose concentrations repressed HIF-1 both in hypoxic cells and in kidneys of animals with diabetes, through a HIF prolyl-hydroxylase (PHD)-dependent mechanism. The impaired HIF-1 signaling contributed to excess production of mitochondrial ROS through increased mitochondrial respiration that was mediated by Pyruvate dehydrogenase kinase 1 (PDK1). The restoration of HIF-1 function attenuated ROS overproduction despite persistent hyperglycemia, and conferred protection against apoptosis and renal injury in diabetes.

Conclusions:

We conclude that the repression of HIF-1 plays a central role in mitochondrial ROS overproduction in diabetes and is a potential therapeutic target for diabetic complications. These findings are timely since the first PHD inhibitor that can activate HIF-1 has been newly approved for clinical use.

Funding:

This work was supported by grants from the Swedish Research Council, Stockholm County Research Council, Stockholm Regional Research Foundation, Bert von Kantzows Foundation, Swedish Society of Medicine, Kung Gustaf V:s och Drottning Victorias Frimurarestifelse, Karolinska Institute’s Research Foundations, Strategic Research Programme in Diabetes, and Erling-Persson Family Foundation for S-B.C.; grants from the Swedish Research Council and Swedish Heart and Lung Foundation for T.A.S.; and ERC consolidator grant for M.M.

Intranuclear Delivery of HIF-1α-TMD Alleviates EAE via Functional Conversion of TH17 Cells

T helper 17 (TH17) cells are involved in several autoimmune diseases such as multiple sclerosis (MS) and rheumatoid arthritis (RA). In addition to retinoic acid receptor-related orphan nuclear receptor gamma t (ROR-γt), hypoxia-inducible factor-1α (HIF-1α) is essential for the differentiation and inflammatory function of TH17 cells. To investigate the roles of HIF-1α in the functional regulation of TH17 cells under the normal physiological condition without genetic modification, the nucleus-transducible form of transcription modulation domain (TMD) of HIF-1α (ntHIF-1α-TMD) was generated by conjugating HIF-1α-TMD to Hph-1 protein transduction domain (PTD). ntHIF-1α-TMD was effectively delivered into the nucleus of T cells without cellular cytotoxicity. ntHIF-1α-TMD significantly blocked the differentiation of naïve T cells into TH17 cells in a dose-dependent manner via IL-17A and ROR-γt expression inhibition. However, T-cell activation events such as induction of CD69, CD25, and IL-2 and the differentiation potential of naïve T cells into TH1, TH2, or Treg cells were not affected by ntHIF-1α-TMD. Interestingly, TH17 cells differentiated from naïve T cells in the presence of ntHIF-1α-TMD showed a substantial level of suppressive activity toward the activated T cells, and the increase of Foxp3 and IL-10 expression was detected in these TH17 cells. When mRNA expression pattern was compared between TH17 cells and ntHIF-1α-TMD-treated TH17 cells, the expression of the genes involved in the differentiation and functions of TH17 cells was downregulated, and that of the genes necessary for immune-suppressive functions of Treg cells was upregulated. When the mice with experimental autoimmune encephalomyelitis (EAE) were treated with ntHIF-1α-TMD with anti-IL-17A mAb as a positive control, the therapeutic efficacy of ntHIF-1α-TMD in vivo was comparable with that of anti-IL-17A mAb, and ntHIF-1α-TMD-mediated therapeutic effect was contributed by the functional conversion of TH17 cells into immune-suppressive T cells. The results in this study demonstrate that ntHIF-1α-TMD can be a new therapeutic reagent for the treatment of various autoimmune diseases in which TH17 cells are dominant and pathogenic T cells.

Subcellular Localization Signals of bHLH-PAS Proteins: Their Significance, Current State of Knowledge and Future Perspectives

The bHLH-PAS (basic helix-loop-helix/ Period-ARNT-Single minded) proteins are a family of transcriptional regulators commonly occurring in living organisms. bHLH-PAS members act as intracellular and extracellular "signals" sensors, initiating response to endo- and exogenous signals, including toxins, redox potential, and light. The activity of these proteins as transcription factors depends on nucleocytoplasmic shuttling: the signal received in the cytoplasm has to be transduced, via translocation, to the nucleus. It leads to the activation of transcription of particular genes and determines the cell response to different stimuli. In this review, we aim to present the current state of knowledge concerning signals that affect shuttling of bHLH-PAS transcription factors. We summarize experimentally verified and published nuclear localization signals/nuclear export signals (NLSs/NESs) in the context of performed in silico predictions. We have used most of the available NLS/NES predictors. Importantly, all our results confirm the existence of a complex system responsible for protein localization regulation that involves many localization signals, which activity has to be precisely controlled. We conclude that the current stage of knowledge in this area is still not complete and for most of bHLH-PAS proteins an experimental verification of the activity of further NLS/NES is needed.

Triggering of a Dll4-Notch1 loop impairs wound healing in diabetes

Diabetic foot ulcerations (DFUs) represent a major medical, social, and economic problem. Therapeutic options are restricted due to a poor understanding of the pathogenic mechanisms. The Notch pathway plays a pivotal role in cell differentiation, proliferation, and angiogenesis, processes that are profoundly disturbed in diabetic wounds. Notch signaling is activated upon interactions between membrane-bound Notch receptors (Notch 1-4) and ligands (Jagged 1-2 and Delta-like 1, 3, 4), resulting in cell-context-dependent outputs. Here, we report that Notch1 signaling is activated by hyperglycemia in diabetic skin and specifically impairs wound healing in diabetes. Local inhibition of Notch1 signaling in experimental wounds markedly improves healing exclusively in diabetic, but not in nondiabetic, animals. Mechanistically, high glucose levels activate a specific positive Delta-like 4 (Dll4)-Notch1 feedback loop. Using loss-of-function genetic approaches, we demonstrate that Notch1 inactivation in keratinocytes is sufficient to cancel the repressive effects of the Dll4-Notch1 loop on wound healing in diabetes, thus making Notch1 signaling an attractive locally therapeutic target for the treatment of DFUs.

The Expression of Adipophilin Is Frequently Found in Solid Subtype Adenocarcinoma and Is Associated with Adverse Outcomes in Lung Adenocarcinoma

BACKGROUND

The up-regulation of the lipogenic pathway has been reported in many types of malignant tumors. However, its pathogenic role or clinical significance is not fully understood. The objective of this study was to examine the expression levels of adipophilin and related hypoxic signaling proteins and to determine their prognostic impacts and associations with the pathologic characteristics of lung adenocarcinoma.

METHODS

Expression levels of adipophilin, heat shock protein 27 (HSP27), carbonic anhydrase IX, and hypoxia-inducible factor 1α were examined by immunohistochemical staining using tissue microarray blocks. Correlations between protein expression levels and various clinicopathologic features were analyzed.

RESULTS

A total of 230 cases of primary adenocarcinoma of the lung were enrolled in this study. Adipophilin expression was more frequent in males and with the solid histologic type. It was correlated with HSP27 expression. Patients with adipophilin-positive adenocarcinoma showed a shorter progression-free survival (PFS) (median PFS, 17.2 months vs 18.4 months) in a univariable survival analysis, whereas HSP27 positivity correlated with favorable overall survival (OS) and PFS. In a multivariable analysis, adipophilin and HSP27 were independent prognostic markers of both OS and PFS.

CONCLUSIONS

Activated lipid metabolism and the hypoxic signaling pathway might play a major role in the progression of lung adenocarcinoma, especially in the solid histologic type.

Co-immunoprecipitation Assay Using Endogenous Nuclear Proteins from Cells Cultured Under Hypoxic Conditions

Low oxygen levels (hypoxia) trigger a variety of adaptive responses with the Hypoxia-inducible factor 1 (HIF-1) complex acting as a master regulator. HIF-1 consists of a heterodimeric oxygen-regulated α subunit (HIF-1α) and constitutively expressed β subunit (HIF-1β) also known as aryl hydrocarbon receptor nuclear translocator (ARNT), regulating genes involved in diverse processes including angiogenesis, erythropoiesis and glycolysis. The identification of HIF-1 interacting proteins is key to the understanding of the hypoxia signaling pathway. Besides the regulation of HIF-1α stability, hypoxia also triggers the nuclear translocation of many transcription factors including HIF-1α and ARNT. Notably, most of the current methods used to study such protein-protein interactions (PPIs) are based on systems where protein levels are artificially increased through protein overexpression. Protein overexpression often leads to non-physiological results arising from temporal and spatial artifacts. Here we describe a modified co-immunoprecipitation protocol following hypoxia treatment using endogenous nuclear proteins, and as a proof of concept, to show the interaction between HIF-1α and ARNT. In this protocol, the hypoxic cells were harvested under hypoxic conditions and the Dulbecco's Phosphate-Buffered Saline (DPBS) wash buffer was also pre-equilibrated to hypoxic conditions before usage to mitigate protein degradation or protein complex dissociation during reoxygenation. In addition, the nuclear fractions were subsequently extracted to concentrate and stabilize endogenous nuclear proteins and avoid possible spurious results often seen during protein overexpression. This protocol can be used to demonstrate endogenous and native interactions between transcription factors and transcriptional co-regulators under hypoxic conditions.

Synthetic 10FN3-based mono- and bivalent inhibitors of MDM2/X function

Engineered non-antibody scaffold proteins constitute a rapidly growing technology for diagnostics and modulation/perturbation of protein function. Here, we describe the rapid and systematic development of high-affinity 10FN3 domain inhibitors of the MDM2 and MDMX proteins. These are often overexpressed in cancer and represent attractive drug targets. Using facile in vitro expression and pull-down assay methodology, numerous design iterations addressing insertion site(s) and spacer length were screened for optimal presentation of an MDM2/X dual peptide inhibitor in the 10FN3 scaffold. Lead inhibitors demonstrated robust, on-target cellular inhibition of MDM2/X leading to activation of the p53 tumor suppressor. Significant improvement to target engagement was observed by increasing valency within a single 10FN3 domain, which has not been demonstrated previously. We further established stable reporter cell lines with tunable expression of EGFP-fused 10FN3 domain inhibitors, and showed their intracellular location to be contingent on target engagement. Importantly, competitive inhibition of MDM2/X by small molecules and cell-penetrating peptides led to a readily observable phenotype, indicating significant potential of the developed platform as a robust tool for cell-based drug screening.

An Elongin-Cullin-SOCS Box Complex Regulates Stress-Induced Serotonergic Neuromodulation

Neuromodulatory cells transduce environmental information into long-lasting behavioral responses. However, the mechanisms governing how neuronal cells influence behavioral plasticity are difficult to characterize. Here, we adapted the translating ribosome affinity purification (TRAP) approach in C. elegans to profile ribosome-associated mRNAs from three major tissues and the neuromodulatory dopaminergic and serotonergic cells. We identified elc-2, an Elongin C ortholog, specifically expressed in stress-sensing amphid neuron dual ciliated sensory ending (ADF) serotonergic sensory neurons, and we found that it plays a role in mediating a long-lasting change in serotonin-dependent feeding behavior induced by heat stress. We demonstrate that ELC-2 and the von Hippel-Lindau protein VHL-1, components of an Elongin-Cullin-SOCS box (ECS) E3 ubiquitin ligase, modulate this behavior after experiencing stress. Also, heat stress induces a transient redistribution of ELC-2, becoming more nuclearly enriched. Together, our results demonstrate dynamic regulation of an E3 ligase and a role for an ECS complex in neuromodulation and control of lasting behavioral states.

GPER mediates the angiocrine actions induced by IGF1 through the HIF-1α/VEGF pathway in the breast tumor microenvironment

BACKGROUND

The G protein estrogen receptor GPER/GPR30 mediates estrogen action in breast cancer cells as well as in breast cancer-associated fibroblasts (CAFs), which are key components of microenvironment driving tumor progression. GPER is a transcriptional target of hypoxia inducible factor 1 alpha (HIF-1α) and activates VEGF expression and angiogenesis in hypoxic breast tumor microenvironment. Furthermore, IGF1/IGF1R signaling, which has angiogenic effects, has been shown to activate GPER in breast cancer cells.

METHODS

We analyzed gene expression data from published studies representing almost 5000 breast cancer patients to investigate whether GPER and IGF1 signaling establish an angiocrine gene signature in breast cancer patients. Next, we used GPER-positive but estrogen receptor (ER)-negative primary CAF cells derived from patient breast tumours and SKBR3 breast cancer cells to investigate the role of GPER in the regulation of VEGF expression and angiogenesis triggered by IGF1. We performed gene expression and promoter studies, western blotting and immunofluorescence analysis, gene silencing strategies and endothelial tube formation assays to evaluate the involvement of the HIF-1α/GPER/VEGF signaling in the biological responses to IGF1.

RESULTS

We first determined that GPER is co-expressed with IGF1R and with the vessel marker CD34 in human breast tumors (n = 4972). Next, we determined that IGF1/IGF1R signaling engages the ERK1/2 and AKT transduction pathways to induce the expression of HIF-1α and its targets GPER and VEGF. We found that a functional cooperation between HIF-1α and GPER is essential for the transcriptional activation of VEGF induced by IGF1. Finally, using conditioned medium from CAFs and SKBR3 cells stimulated with IGF1, we established that HIF-1α and GPER are both required for VEGF-induced human vascular endothelial cell tube formation.

CONCLUSIONS

These findings shed new light on the essential role played by GPER in IGF1/IGF1R signaling that induces breast tumor angiogenesis. Targeting the multifaceted interactions between cancer cells and tumor microenvironment involving both GPCRs and growth factor receptors has potential in future combination anticancer therapies.

A Notch-independent mechanism contributes to the induction of Hes1 gene expression in response to hypoxia in P19 cells

Hes1 is a Notch target gene that plays a major role during embryonic development. Previous studies have shown that HIF-1α can interact with the Notch intracellular domain and enhance Notch target gene expression. In this study, we have identified a Notch-independent mechanism that regulates the responsiveness of the Hes1 gene to hypoxia. Using P19 cells we show that silencing the Notch DNA binding partner CSL does not prevent hypoxia-dependent upregulation of Hes1 expression. In contrast to CSL, knockdown of HIF-1α or Arnt expression prevents Hes1 induction in hypoxia. Deletion analysis of the Hes1 promoter identified a minimal region near the transcription start site that is still responsive to hypoxia. In addition, we show that mutating the GA-binding protein (GABP) motif significantly reduced Hes1 promoter-responsiveness to hypoxia or to HIF-1 overexpression whereas mutation of the hypoxia-responsive element (HRE) present in this region had no effect. Chromatin immunoprecipitation assays demonstrated that HIF-1α binds to the proximal region of the Hes1 promoter in a Notch-independent manner. Using the same experimental approach, the presence of GABPα and GABPβ1 was also observed in the same region of the promoter. Loss- and gain-of-function studies demonstrated that Hes1 gene expression is upregulated by hypoxia in a GABP-dependent manner. Finally, co-immunoprecipitation assays demonstrated that HIF-1α but not HIF-2α is able to interact with either GABPα or GABPβ1. These results suggest a Notch-independent mechanism where HIF-1 and GABP contribute to the upregulation of Hes1 gene expression in response to hypoxia.

Endotoxin-induced acute lung injury in mice is protected by 5,7-dihydroxy-8-methoxyflavone via inhibition of oxidative stress and HIF-1α

Up to date, the morbidity and mortality rates of acute lung injury (ALI) still rank high among clinical illnesses. Endotoxin, also called lipopolysaccharide (LPS), induced sepsis is the major cause for ALI. Beneficial biological effects, such as antioxidation, anti-inflammation, and neuroprotection was found to express by 5,7-dihydroxy-8-methoxyflavone (DHMF). The purpose of present study was to investigate the potential protective effects of DHMF and the possibile mechanisms involved in LPS-induced ALI. In our experimental model, ALI was induced in mice by intratracheal injection of LPS, and DHMF at various concentrations was injected intraperitoneally for 30 min prior to LPS administration. Pretreatment with DHMF inhibited not only the histolopatholgical changes occurred in lungs but also leukocytes infiltration in LPS-induced ALI. Decreased activity of antioxidative enzymes (AOE) such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) caused by LPS was reversed by DHMF. LPS-induced lipid peroxidation HIF-1α accumulation, NF-κB phosphorylation, and IκBα degradation were all inhibited by DHMF. In addition, LPS-induced expression of proinflammatory mediators such as TNF-α and IL-1β were also inhibited by 5,7-dihydroxy-8-methoxyflavone. These results suggested that the protective mechanisms of DHMF on endotoxin-induced ALI might be via up-regulation of antioxidative enzymes, inhibition of NFκB phosphorylation, and HIF-1α accumulation. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 1700-1709, 2016.

CXCL16 induces angiogenesis in autocrine signaling pathway involving hypoxia-inducible factor 1α in human umbilical vein endothelial cells

Chemokine (C-X-C motif) ligand 16 (CXCL16) is a new angiogenic factor inducing angiogenesis via extracellular signal-regulated kinases pathway. To further understand the molecular mechanism underlying CXCL16‑induced angiogenesis, we explored involvement of other relevant pathways in CXCL16-induced angiogenesis. In the present study, we investigated the mechanisms underlying CXCL16-induced angiogenesis in human umbilical vein endothelial cells (HUVECs). CXCL16 promoted HUVEC proliferation, tube formation and migration. Enzyme-linked immunosorbent assay revealed that CXCL16 induced vascular endothelial growth factor secretion from HUVECs. Western blot analysis showed that CXCL16 increased the level of hypoxia‑inducible factor 1α, p-extracellular signal-regulated kinases (ERK), p-p38 and p-Akt dose- and time-dependently. ERK-, p38- and Akt-selective inhibitors significantly suppressed HUVEC proliferation, migration, tube formation and hypoxia-inducible factor 1α (HIF-1α) expression induced by CXCL16. Furthermore, CXCL16 peptides induced CXCL16 secretion via ERK, p38 and Akt pathways, which was suppressed by HIF-1α-selective inhibitor PX12. Our data suggest that CXCL16 induces angiogenesis in autocrine manner via ERK, Akt, p38 pathways and HIF-1α modulation.

The transition zone protein Rpgrip1l regulates proteasomal activity at the primary cilium

Rpgrip1l regulates proteasomal activity at the basal body via Psmd2 and thereby controls ciliary signaling.

Mutations in RPGRIP1L result in severe human diseases called ciliopathies. To unravel the molecular function of RPGRIP1L, we analyzed Rpgrip1l^−/− mouse embryos, which display a ciliopathy phenotype and die, at the latest, around birth. In these embryos, cilia-mediated signaling was severely disturbed. Defects in Shh signaling suggested that the Rpgrip1l deficiency causes an impairment of protein degradation and protein processing. Indeed, we detected a cilia-dependent decreased proteasomal activity in the absence of Rpgrip1l. We found different proteasomal components localized to cilia and identified Psmd2, a component of the regulatory proteasomal 19S subunit, as an interaction partner for Rpgrip1l. Quantifications of proteasomal substrates demonstrated that Rpgrip1l regulates proteasomal activity specifically at the basal body. Our study suggests that Rpgrip1l controls ciliary signaling by regulating the activity of the ciliary proteasome via Psmd2.

Hypoxia Up-Regulates Galectin-3 in Mammary Tumor Progression and Metastasis

The tumor microenvironment encompasses several stressful conditions for cancer cells such as hypoxia, oxidative stress and pH alterations. Galectin-3, a well-studied member of the beta-galactoside-binding animal family of lectins has been implicated in multiple steps of metastasis as cell-cell and cell-ECM adhesion, promotion of angiogenesis, cell proliferation and resistance to apoptosis. However, both its aberrantly up- and down-regulated expression was observed in several types of cancer. Thus, the mechanisms that regulate galectin-3 expression in neoplastic settings are not clear. In order to demonstrate the putative role of hypoxia in regulating galectin-3 expression in canine mammary tumors (CMT), in vitro and in vivo studies were performed. In malignant CMT cells, hypoxia was observed to induce expression of galectin-3, a phenomenon that was almost completely prevented by catalase treatment of CMT-U27 cells. Increased galectin-3 expression was confirmed at the mRNA level. Under hypoxic conditions the expression of galectin-3 shifts from a predominant nuclear location to cytoplasmic and membrane expressions. In in vivo studies, galectin-3 was overexpressed in hypoxic areas of primary tumors and well-established metastases. Tumor hypoxia thus up-regulates the expression of galectin-3, which may in turn increase tumor aggressiveness.

Hypoxia-inducible factor 1α mediates neuroprotection of hypoxic postconditioning against global cerebral ischemia

Hypoxia administered after transient global cerebral ischemia (tGCI) has been shown to induce neuroprotection in adult rats, but the underlying mechanisms for this protection are unclear. Here, we tested the hypothesis that hypoxic postconditioning (HPC) induces neuroprotection through upregulation of hypoxia-inducible factor 1α (HIF-1α) and vascular endothelial growth factor (VEGF), and that this involves phosphatidylinositol-3-kinase (PI3K), p38 mitogen-activated protein kinase (p38 MAPK), and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) pathways. The expression of HIF-1α, VEGF, and cleaved caspase-9 were determined by immunohistochemistry and Western blot. As pharmacologic interventions, the HIF-1α inhibitor 2-methoxyestradiol (2ME2), PI3K inhibitor LY294002, p38 MAPK inhibitor SB203580, and MEK inhibitor U0126 were administered before HPC or after tGCI. We found that HPC maintained the higher expression of HIF-1α and VEGF and decreased cleaved caspase-9 levels in CA1 after tGCI. These effects were reversed by 2ME2 administered before HPC, and the neuroprotection of HPC was abolished. LY294002 and SB203580 decreased the expression of HIF-1α and VEGF after HPC, whereas U0126 increased HIF-1α and VEGF after tGCI. These findings suggested that HIF-1α exerts neuroprotection induced by HPC against tGCI through VEGF upregulation and cleaved caspase-9 downregulation, and that the PI3K, p38 MAPK, and MEK pathways are involved in the regulation of HIF-1α and VEGF.

Role of compartmentalization on HiF-1α degradation dynamics during changing oxygen conditions: a computational approach

HiF-1α is the central protein driving the cellular response to hypoxia. Its accumulation in cancer cells is linked to the appearance of chemoresistant and aggressive tumor phenotypes. As a consequence, understanding the regulation of HiF-1α dynamics is a major issue to design new anti-cancer therapies. In this paper, we propose a model of the hypoxia pathway, involving HiF-1α and its inhibitor pVHL. Based on data from the literature, we made the hypothesis that the regulation of HiF-1α involves two compartments (nucleus and cytoplasm) and a constitutive shuttle of the pVHL protein between them. We first show that this model captures correctly the main features of HiF-1α dynamics, including the bi-exponential degradation profile in normoxia, the kinetics of induction in hypoxia, and the switch-like accumulation. Second, we simulated the effects of a hypoxia/reoxygenation event, and show that it generates a strong instability of HiF-1α. The protein concentration rapidly increases 3 hours after the reoxygenation, and exhibits an oscillating pattern. This effect vanishes if we do not consider compartmentalization of HiF-1α. This result can explain various counter-intuitive observations about the specific molecular and cellular response to the reoxygenation process. Third, we simulated the HiF-1α dynamics in the tumor case. We considered different types of mutations associated with tumorigenesis, and we compared their consequences on HiF-1α dynamics. Then, we tested different therapeutics strategies. We show that a therapeutic decrease of HiF-1α nuclear level is not always correlated with an attenuation of reoxygenation-induced instabilities. Thus, it appears that the design of anti-HiF-1α therapies have to take into account these two aspects to maximize their efficiency.

Mutations in APOPT1, encoding a mitochondrial protein, cause cavitating leukoencephalopathy with cytochrome c oxidase deficiency

Cytochrome c oxidase (COX) deficiency is a frequent biochemical abnormality in mitochondrial disorders, but a large fraction of cases remains genetically undetermined. Whole-exome sequencing led to the identification of APOPT1 mutations in two Italian sisters and in a third Turkish individual presenting severe COX deficiency. All three subjects presented a distinctive brain MRI pattern characterized by cavitating leukodystrophy, predominantly in the posterior region of the cerebral hemispheres. We then found APOPT1 mutations in three additional unrelated children, selected on the basis of these particular MRI features. All identified mutations predicted the synthesis of severely damaged protein variants. The clinical features of the six subjects varied widely from acute neurometabolic decompensation in late infancy to subtle neurological signs, which appeared in adolescence; all presented a chronic, long-surviving clinical course. We showed that APOPT1 is targeted to and localized within mitochondria by an N-terminal mitochondrial targeting sequence that is eventually cleaved off from the mature protein. We then showed that APOPT1 is virtually absent in fibroblasts cultured in standard conditions, but its levels increase by inhibiting the proteasome or after oxidative challenge. Mutant fibroblasts showed reduced amount of COX holocomplex and higher levels of reactive oxygen species, which both shifted toward control values by expressing a recombinant, wild-type APOPT1 cDNA. The shRNA-mediated knockdown of APOPT1 in myoblasts and fibroblasts caused dramatic decrease in cell viability. APOPT1 mutations are responsible for infantile or childhood-onset mitochondrial disease, hallmarked by the combination of profound COX deficiency with a distinctive neuroimaging presentation.

Ischemia induces different levels of hypoxia inducible factor-1α protein expression in interneurons and pyramidal neurons

Introduction

Pyramidal (glutamatergic) neurons and interneurons are morphologically and functionally well defined in the central nervous system. Although it is known that glutamatergic neurons undergo immediate cell death whereas interneurons are insensitive or survive longer during cerebral ischemia, the protection mechanisms responsible for this interneuronal survival are not well understood. Hypoxia inducible factor-1 (HIF-1) plays an important role in protecting neurons from hypoxic/ischemic insults. Here, we studied the expression of HIF-1α, the regulatable subunit of HIF-1, in the different neuronal phenotypes under in vitro and in vivo ischemia.

Results

In a primary cortical culture, HIF-1α expression was observed in neuronal somata after hypoxia (1% oxygen) in the presence of 5 or 25 mM glucose but not under normoxia (21% oxygen). Interestingly, only certain MAP2-positive neurons containing round somata (interneuron-like morphology) co-localized with HIF-1α staining. Other neurons such as pyramidal-like neurons showed no expression of HIF-1α under either normoxia or hypoxia. The HIF-1α positive neurons were GAD65/67 positive, confirming that they were interneuron-type cells. The HIF-1α expressing GAD65/67-positive neurons also possessed high levels of glutathione. We further demonstrated that ischemia induced significant HIF-1α expression in interneurons but not in pyramidal neurons in a rat model of middle cerebral artery occlusion.

Conclusion

These results suggest that HIF-1α protein expression induced by ischemia is neuron-type specific and that this specificity may be related to the intracellular level of glutathione (GSH).

Ηypoxia-inducible factor-1α, von Hippel-Lindau protein, and heat shock protein expression in ophthalmic pterygium and normal conjunctiva

PURPOSE

Εnhanced expression of transcription factor hypoxia inducible factor HIF-1α is known to play a critical role in the modulation of cell metabolism and survival pathways as well as having stem-cell-like properties. Furthermore, accumulated data reveal the existence of cross-regulation between the oxygen-sensing and heat shock pathways contributing to the adaptation of cells under stressful conditions. Pterygium, a stem cell disorder with premalignant features, has been reported to demonstrate hypoxia. The purpose of this study was to investigate the co-expression patterns of transcription factor HIF-1α and von Hippel Lindau protein (pVHL)--which normally acts to keep levels of HIF-1α activity low under normoxic conditions--in pterygium and normal conjunctival human samples. Additionally, expression of HIF-1α compared to the activation of heat shock proteins (Hsp90, Hsp70, and Hsp27) was studied. Emphasis was placed on the detection of HIF-1α and Hsp90, which associates with and stabilizes HIF-1α to promote its transcriptional activity.

METHODS

Semi-serial paraffin-embedded sections and tissue extracts from pterygium and normal conjunctival samples were studied by immunohistochemistry and western blot analysis, respectively, with the use of specific antibodies. Double labeling immunofluorescence studies on cryostat sections were also included.

RESULTS

Statistically significant increased expression of HIF-1α and Hsps (Hsp90, Hsp70, and Hsp27) in pterygia compared to normal conjunctiva was demonstrated (p<0.05). In contrast, no significant difference was detected for pVHL expression (p>0.05). Immunohistochemical findings revealed nuclear HIF-1α immunoreactivity in all the epithelial layers of 23/32 (71.8%) pterygium tissues. Furthermore, all epithelial layers of the majority (75%) of pterygium samples showed strong cytoplasmic immunoreactivity for Hsp27 while Hsp27 expression was detected in all pterygia (100%) examined. Hsp27 expression was not observed in the superficial layer of goblet cells. In some samples, focal basal epithelial cells exhibited weak Hsp27 expression or were Hsp27 immunonegative. Ιmmunoreactivity of phopsho-Hsp27 showed the same distribution pattern as Hsp27 did. Epithelium of all pterygia (100%) displayed moderate to strong Hsp90 cytoplasmic immunoreactivity. Furthermore, the majority of pterygia, specifically, 30/32 (93.7%) and 27/32 (84.3%) demonstrated, respectively, Hsp70 and pVHL cytoplasmic immunoreactivity. Hsp90, Hsp70, and pVHL immunoreactivity was mainly detected in basal and suprabasal epithelial layers even though strong immunoreactivity in all epithelial layers was also observed in some pterygia. Stroma vessels were immunopositive for Hsps (Hsp90, Hsp70, and Hsp27) and pVHL. A statistically significant correlation between the expression of HIF-1α and the activation status of Hsps (Hsp90, Hsp70, and Hsp27; p<0.05) was observed whereas HIF-1α expression did not correlate with pVHL expression (p>0.05). Double labeling immunofluorescence studies showed nuclear HIF-1α co-localization with cytoplasmic Hsp90 expression in cells distributed in the entire epithelium of pterygia, in contrast to, normal conjunctiva, which exhibited only a few scattered epithelial cells with cytoplasmic HIF-1α expression and basal epithelial cells with Hsp90 expression.

CONCLUSIONS

The upregulation of coordinated activation of HIF-1α and Hsps in pterygium may represent an adaptive process for the survival of cells under stressful conditions. The significance of the association of HIF-1α with Hsp90 with respect to the therapeutic approach of pterygium requires further evaluation.

Acute hypoxia induces apoptosis of pancreatic β-cell by activation of the unfolded protein response and upregulation of CHOP

The success of pancreatic β-cells transplantation to treat type 1 diabetes has been hindered by massive β-cell dysfunction and loss of β-cells that follows the procedure. Hypoxia-mediated cell death has been considered one of the main difficulties that must be overcome for transplantation to be regarded as a reliable therapy. Here we have investigated the mechanisms underlying β-cell death in response to hypoxia (1% O₂). Our studies show that mouse insulinoma cell line 6 (Min6) cells undergo apoptosis with caspase-3 activation occurring as early as 2 h following exposure to hypoxia. Hypoxia induces endoplasmic reticulum stress in Min6 cells leading to activation of the three branches of the unfolded protein response pathway. In response to hypoxia the pro-apoptotic transcription factor C/EBP homologous protein (CHOP) is upregulated. The important role of CHOP in the apoptotic process was highlighted by the rescue of Min6 cells from hypoxia-mediated apoptosis observed in CHOP-knockdown cells. Culturing isolated pancreatic mouse islets at normoxia showed intracellular hypoxia with accumulation of hypoxia-inducible factor-1α and upregulation of CHOP, the latter one occurring as early as 4 h after isolation. Finally, we observed that pancreatic islets of type 2 db/db diabetic mice were more hypoxic than their counterpart in normoglycemic animals. This finding indicates that hypoxia-mediated apoptosis may occur in type 2 diabetes.

HIF-1α signaling by airway epithelial cell K-α1-tubulin: role in fibrosis and chronic rejection of human lung allografts

Long term survival of the human lung allografts are hindered by chronic rejection, manifested clinically as bronchiolitis obliterans syndrome (BOS). We previously demonstrated significant correlation between the development of antibodies (Abs) to K-α1-tubulin (Kα1T) and BOS. In this study, we investigated the molecular basis for fibrinogenesis mediated by ligation of Kα1T expressed on airway epithelial cells by its specific Abs. Using RT-PCR we demonstrate that normal human bronchial epithelial (NHBE) cells upon ligation of Kα1T with specific Abs caused upregulation of pro-fibrotic growth factors. Western blot analysis of NHBE incubated with Kα1T Abs increased hypoxia inducible factor (HIF-1α). Kα1T Ab-mediated growth factor expression is dependent on HIF-1α as inhibition of HIF-1α returned fibrotic growth factor expression to basal levels. In conclusion, we propose that HIF-1α -mediated upregulation of fibrogenic growth factors induced by ligation of Kα1T Abs is critical for development of fibrosis leading to chronic rejection of lung allograft.

Hypoxia-inducible factor-1α protein expression is associated with poor survival in normal karyotype adult acute myeloid leukemia

We examined the predictive impact of HIF-1α protein expression on clinical outcome of 84 normal karyotype acute myeloid leukemia (NK-AML) patients (median age 66.5 years) at our institute. Thirty percent of NK-AML cells expressed cytoplasmic HIF-1α. In univariate analysis, low HIF-1α (≤ 5%, n = 66) was associated with improved event-free survival (p = 0.0453, HR = 0.22). Multivariate analysis incorporating age, complete remission, FLT3-ITD mutation, and marrow blast percentage demonstrated that HIF-1α was independently associated with poorer overall and event-free survival. HIF-1α expression correlated with VEGF-C but not VEGF-A, marrow angiogenesis, FLT3 ITD or NPM1 mutations. These results support HIF-1α as an outcome marker for NK-AML.

Selective inhibition of early--but not late--expressed HIF-1α is neuroprotective in rats after focal ischemic brain damage

The expression of hypoxia-inducible factor-1-alpha (HIF-1α) is upregulated in ischemic stroke, but its function is still unclear. In the present study, biphasic expression of HIF-1α was observed during 1-12 h and after 48 h in neurons exposed to ischemic stress in vitro and in vivo. Treating neurons with 2-methoxyestradiol (2ME2) 0.5 h after ischemic stress or pre-silencing HIF-1α with small interfering RNA (siRNA) decreased brain injury, brain edema and number of apoptotic cell, and downregulates Nip-like protein X (Nix) expression. Conversely, applying 2ME2 to neurons 8 h after ischemic stress or silencing the HIF-1α with siRNA 12 h after oxygen-glucose deprivation (OGD) increased neuron damage and decreased vascular endothelial growth factor (VEGF) expression. Taken together, these results demonstrate that HIF-1α induced by ischemia in early and late times leads cellular apoptosis and survival, respectively, and provides a new insight into the divergent roles of HIF-1α expression in neurons after ischemic stroke.

Continuous expression of HIF-1α in neural stem/progenitor cells

Hypoxia-inducible factor-1 alpha subunit (HIF-1α) is a transcriptional activator mediating adaptive cellular response to hypoxia. Normally degraded in most cell types and tissues, HIF-1α becomes stable and transcriptionally active under conditions of hypoxia. In contrast, we found that HIF-1α is continuously expressed in adult brain neurogenic zones, as well as in neural stem/progenitor cells (NSPCs) from the embryonic and post-natal mouse brain. Our in vitro studies suggest that HIF-1α does not undergo typical hydroxylation, ubiquitination, and degradation within NSPCs under normoxic conditions. Based on immunofluorescence and cell fractionation, HIF-1α is primarily sequestered in membranous cytoplasmic structures, identified by immuno-electron microscopy as HIF-1α-bearing vesicles (HBV), which may prevent HIF-1α from degradation within the cytoplasm. HIF-1α shRNAi-mediated knockdown reduced the resistance of NSPCs to hypoxia, and markedly altered the expression levels of Notch-1 and β-catenin, which influence NSPC differentiation. These findings indicate a unique regulation of HIF-1α protein stability in NSPCs, which may have importance in NSPCs properties and function.

Hypoxia-inducible factor-1α enhances the malignant phenotype of multicellular spheroid HeLa cells in vitro

The purpose of this study was to clarify the direct effect of hypoxia-inducible factor-1α (HIF-1α) on tumor growth, apoptosis and migration in vitro. To achieve this aim, a comparison was made of the differences in growth rates, apoptotic indices and cell invasive ability in the human cervical cancer cell line HeLa and the HIF-1α-blocked counterpart in a three-dimensional spheroid culture. A significant decrease in cell proliferation and invasion, and an increase in cell apoptosis were observed in HIF-1α-blocked cells in the three-dimensional culture. The data indicated that a multicellular spheroid culture is an ideal model of hypoxia in vitro and that HIF-1α is a significant regulator of adaptive processes that promote tumor cell malignant phenotypes, such as proliferation, anti-apoptosis and invasive ability.

Nanoscopy of the cellular response to hypoxia by means of fluorescence resonance energy transfer (FRET) and new FRET software

Background

Cellular oxygen sensing is fundamental to all mammalian cells to adequately respond to a shortage of oxygen by increasing the expression of genes that will ensure energy homeostasis. The transcription factor Hypoxia-Inducible-Factor-1 (HIF-1) is the key regulator of the response because it coordinates the expression of hypoxia inducible genes. The abundance and activity of HIF-1 are controlled through posttranslational modification by hydroxylases, the cellular oxygen sensors, of which the activity is oxygen dependent.

Methods

Fluorescence resonance energy transfer (FRET) was established to determine the assembly of the HIF-1 complex and to study the interaction of the α-subunit of HIF-1 with the O₂-sensing hydroxylase. New software was developed to improve the quality and reliability of FRET measurements.

Results

FRET revealed close proximity between the HIF-1 subunits in multiple cells. Data obtained by sensitized FRET in this study were fully compatible with previous work using acceptor bleaching FRET. Interaction between the O₂-sensing hydroxylase PHD1 and HIF-1α was demonstrated and revealed exclusive localization of O₂-sensing in the nucleus. The new software FRET significantly improved the quality and speed of FRET measurements.

Conclusion

FRET measurements do not only allow following the assembly of the HIF-1 complex under hypoxic conditions but can also provide important information about the process of O₂-sensing and its localisation within a cell.

MCS codes: 92C30, 92C05, 92C40

15-Deoxy-Delta(12,14)-prostaglandin-J(2) reveals a new pVHL-independent, lysosomal-dependent mechanism of HIF-1alpha degradation

Hypoxia-inducible factor-1alpha (HIF-1alpha) protein is degraded under normoxia by its association to von Hippel-Lindau protein (pVHL) and further proteasomal digestion. However, human renal cells HK-2 treated with 15-deoxy-Delta(12,14)-prostaglandin-J(2) (15d-PGJ(2)) accumulate HIF-1alpha in normoxic conditions. Thus, we aimed to investigate the mechanism involved in this accumulation. We found that 15d-PGJ(2) induced an over-accumulation of HIF-1alpha in RCC4 cells, which lack pVHL and in HK-2 cells treated with inhibitors of the pVHL-proteasome pathway. These results indicated that pVHL-proteasome-independent mechanisms are involved, and therefore we aimed to ascertain them. We have identified a new lysosomal-dependent mechanism of HIF-1alpha degradation as a target for 15d-PGJ(2) based on: (1) HIF-1alpha colocalized with the specific lysosomal marker Lamp-2a, (2) 15d-PGJ(2) inhibited the activity of cathepsin B, a lysosomal protease, and (3) inhibition of lysosomal activity did not result in over-accumulation of HIF-1alpha in 15d-PGJ(2)-treated cells. Therefore, expression of HIF-1alpha is also modulated by lysosomal degradation.

A mouse mammary gland involution mRNA signature identifies biological pathways potentially associated with breast cancer metastasis

Mouse mammary gland involution resembles a wound healing response with suppressed inflammation. Wound healing and inflammation are also associated with tumour development, and a 'wound-healing' gene expression signature can predict metastasis formation and survival. Recent studies have shown that an involuting mammary gland stroma can promote metastasis. It could therefore be hypothesised that gene expression signatures from an involuting mouse mammary gland may provide new insights into the physiological pathways that promote breast cancer progression. Indeed, using the HOPACH clustering method, the human orthologues of genes that were differentially regulated at day 3 of mammary gland involution and showed prolonged expression throughout the first 4 days of involution distinguished breast cancers in the NKI 295 breast cancer dataset with low and high metastatic activity. Most strikingly, genes associated with copper ion homeostasis and with HIF-1 promoter binding sites were the most over-represented, linking this signature to hypoxia. Further, six out of the ten mRNAs with strongest up-regulation in cancers with poor survival code for secreted factors, identifying potential candidates that may be involved in stromal/matrix-enhanced metastasis formation/breast cancer development. This method therefore identified biological processes that occur during mammary gland involution, which may be critical in promoting breast cancer metastasis that could form a basis for future investigation, and supports a role for copper in breast cancer development.

Real-time imaging of HIF-1alpha stabilization and degradation

HIF-1α is overexpressed in many human cancers compared to normal tissues due to the interaction of a multiplicity of factors and pathways that reflect specific genetic alterations and extracellular stimuli. We developed two HIF-1α chimeric reporter systems, HIF-1α/FLuc and HIF-1α(ΔODDD)/FLuc, to investigate the tightly controlled level of HIF-1α protein in normal (NIH3T3 and HEK293) and glioma (U87) cells. These reporter systems provided an opportunity to investigate the degradation of HIF-1α in different cell lines, both in culture and in xenografts. Using immunofluorescence microscopy, we observed different patterns of subcellular localization of HIF-1α/FLuc fusion protein between normal cells and cancer cells; similar differences were observed for HIF-1α in non-transduced, wild-type cells. A dynamic cytoplasmic-nuclear exchange of the fusion protein and HIF-1α was observed in NIH3T3 and HEK293 cells under different conditions (normoxia, CoCl₂ treatment and hypoxia). In contrast, U87 cells showed a more persistent nuclear localization pattern that was less affected by different growing conditions. Employing a kinetic model for protein degradation, we were able to distinguish two components of HIF-1α/FLuc protein degradation and quantify the half-life of HIF-1α fusion proteins. The rapid clearance component (t_1/2 ∼4–6 min) was abolished by the hypoxia-mimetic CoCl_2, MG132 treatment and deletion of ODD domain, and reflects the oxygen/VHL-dependent degradation pathway. The slow clearance component (t_1/2 ∼200 min) is consistent with other unidentified non-oxygen/VHL-dependent degradation pathways. Overall, the continuous bioluminescence readout of HIF-1α/FLuc stabilization in vitro and in vivo will facilitate the development and validation of therapeutics that affect the stability and accumulation of HIF-1α.

SEPT9_v1 up-regulates hypoxia-inducible factor 1 by preventing its RACK1-mediated degradation

A critical mediator of the cellular response to hypoxia is hypoxia-inducible factor 1 (HIF-1). Increased levels of HIF-1alpha are often associated with increased tumor metastasis, therapeutic resistance, and poorer prognosis. We recently identified a novel interaction between HIF-1alpha and the mammalian septin family member, SEPT9_v1. Septins are a highly conserved family of GTP-binding cytoskeletal proteins that are implicated in multiple cellular functions, including cell division and oncogenesis. SEPT9_v1 binds and stabilizes HIF-1alpha protein and stimulates HIF-1 transcriptional activity. SEPT9_v1-HIF-1 activation promotes tumor growth and angiogenesis. The structural and functional relationships between SEPT9_v1 and HIF-1alpha were analyzed. We found that SEPT9_v1 binds specifically with HIF-1alpha but not with HIF-2alpha. The GTPase domain of SEPT9_v1 was identified as essential for HIF-1alpha binding. A GTPase domain-derived polypeptide, corresponding to amino acids 252-379, was able to disrupt HIF-1alpha-SEPT9_v1 interaction and to inhibit HIF-1 transcriptional activity. SEPT9_v1 also protected HIF-1alpha from degradation induced by HSP90 inhibition by preventing the interaction of HIF-1alpha with the RACK1 protein, which promotes its oxygen-independent proteasomal degradation. In conclusion, a new mechanism of oxygen-independent activation of HIF-1 has been identified that is mediated by SEPT9_v1 blockade of RACK1 activity on HIF-1alpha degradation.

HIF-1 regulation: not so easy come, easy go

The hypoxia-inducible factor-1 (HIF-1) is the master regulator of the cellular response to hypoxia and its expression levels are tightly controlled through synthesis and degradation. It is widely accepted that HIF-1alpha protein accumulation during hypoxia results from inhibition of its oxygen-dependent degradation by the von Hippel Lindau protein (pVHL) pathway. However, recent data describe new pVHL- or oxygen-independent mechanisms for HIF-1alpha degradation. Furthermore, the hypoxia-induced increase in HIF-1alpha levels is facilitated by the continued translation of HIF-1alpha during hypoxia despite the global inhibition of protein translation. Recent work has contributed to an increased understanding of the mechanisms that control the translation and degradation of HIF-1alpha under both normoxic and hypoxic conditions.

Nuclear oxygen sensing: induction of endogenous prolyl-hydroxylase 2 activity by hypoxia and nitric oxide

The abundance of the transcription factor hypoxia-inducible factor is regulated through hydroxylation of its alpha-subunits by a family of prolyl-hydroxylases (PHD1-3). Enzymatic activity of these PHDs is O2-dependent, which enables PHDs to act as cellular O2 sensor enzymes. Herein we studied endogenous PHD activity that was induced in cells grown under hypoxia or in the presence of nitric oxide. Under such conditions nuclear extracts contained much higher PHD activity than the respective cytoplasmic extracts. Although PHD1-3 were abundant in both compartments, knockdown experiments for each isoenzyme revealed that nuclear PHD activity was only due to PHD2. Maximal PHD2 activity was found between 120 and 210 microm O2. PHD2 activity was strongly decreased below 100 microm O2 with a half-maximum activity at 53 +/- 13 microm O2 for the cytosolic and 54 +/- 10 microm O2 for nuclear PHD2 matching the physiological O2 concentration within most cells. Our data suggest a role for PHD2 as a decisive oxygen sensor of the hypoxia-inducible factor degradation pathway within the cell nucleus.

VDUP1 mediates nuclear export of HIF1alpha via CRM1-dependent pathway

Hypoxia-inducible factor 1alpha (HIF1alpha) is a critical transcriptional factor for inducing tumor metastasis, and stabilized under hypoxia but degraded by von Hippel-Lindau protein (pVHL) under normoxia. For the maximal degradation of HIF1alpha, it must be exported to the cytoplasm via an unidentified transporter. Here, we demonstrate that vitamin D3 up-regulated protein 1 (VDUP1) associates with the beta-domain of pVHL and enhances the interaction between pVHL and HIF1alpha to promote the nuclear export and degradation of HIF1alpha hypoxia-independently. Blocking of VDUP1 translocation either by leptomycin B or by nuclear export signal mutation inhibited the nuclear export of pVHL/HIF1alpha and relieved the destabilization of HIF1alpha. VDUP1 suppressed cell invasiveness and tumor metastasis, which were also recovered by blocking of nuclear export. Taken together, these findings indicate that VDUP1 is a novel tumor suppressor which mediates the nuclear export of pVHL/HIF1alpha complex to destabilize HIF1alpha.

Regulation of the Drosophila hypoxia-inducible factor alpha Sima by CRM1-dependent nuclear export

Hypoxia-inducible factor alpha (HIF-alpha) proteins are regulated by oxygen levels through several different mechanisms that include protein stability, transcriptional coactivator recruitment, and subcellular localization. It was previously reported that these transcription factors are mainly nuclear in hypoxia and cytoplasmic in normoxia, but so far the molecular basis of this regulation is unclear. We show here that the Drosophila melanogaster HIF-alpha protein Sima shuttles continuously between the nucleus and the cytoplasm. We identified the relevant nuclear localization signal and two functional nuclear export signals (NESs). These NESs are in the Sima basic helix-loop-helix (bHLH) domain and promote CRM1-dependent nuclear export. Site-directed mutagenesis of either NES provoked Sima nuclear retention and increased transcriptional activity, suggesting that nuclear export contributes to Sima regulation. The identified NESs are conserved and probably functional in the bHLH domains of several bHLH-PAS proteins. We propose that rapid nuclear export of Sima regulates the duration of cellular responses to hypoxia.

Kaposi's sarcoma-associated herpesvirus viral IFN regulatory factor 3 stabilizes hypoxia-inducible factor-1 alpha to induce vascular endothelial growth factor expression

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent associated with Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. Hypoxia-inducible factor-1 (HIF-1) is the master regulator of both developmental and pathologic angiogenesis, composed of an oxygen-sensitive alpha-subunit and a constitutively expressed beta-subunit. HIF-1 activity in tumors depends on the availability of the HIF-1 alpha subunit, the levels of which are increased under hypoxic conditions. Recent studies have shown that HIF-1 plays an important role in KSHV reactivation from latency and pathogenesis. Here, we report a novel mechanism by which KSHV activates HIF-1 activity. Specific interaction between KSHV viral IFN regulatory factor 3 (vIRF3) and the HIF-1 alpha subunit led to the HIF-1 alpha stabilization and transcriptional activation, which induced vascular endothelial growth factor expression and ultimately facilitated endothelial tube formation. Remarkably, the central domain of vIRF3, containing double alpha-helix motifs, was sufficient not only for binding to HIF-1 alpha but also for blocking its degradation in normoxic conditions. This indicates that KSHV has developed a unique mechanism to enhance HIF-1 alpha protein stability and transcriptional activity by incorporating a viral homologue of cellular IRF gene into its genome, which may contribute to viral pathogenesis.

Interaction with factor inhibiting HIF-1 defines an additional mode of cross-coupling between the Notch and hypoxia signaling pathways

Cells adapt to hypoxia by a cellular response, where hypoxia-inducible factor 1alpha (HIF-1alpha) becomes stabilized and directly activates transcription of downstream genes. In addition to this "canonical" response, certain aspects of the pathway require integration with Notch signaling, i.e., HIF-1alpha can interact with the Notch intracellular domain (ICD) to augment the Notch downstream response. In this work, we demonstrate an additional level of complexity in this cross-talk: factor-inhibiting HIF-1 (FIH-1) regulates not only HIF activity, but also the Notch signaling output and, in addition, plays a role in how Notch signaling modulates the hypoxic response. We show that FIH-1 hydroxylates Notch ICD at two residues (N(1945) and N(2012)) that are critical for the function of Notch ICD as a transactivator within cells and during neurogenesis and myogenesis in vivo. FIH-1 negatively regulates Notch activity and accelerates myogenic differentiation. In its modulation of the hypoxic response, Notch ICD enhances recruitment of HIF-1alpha to its target promoters and derepresses HIF-1alpha function. Addition of FIH-1, which has a higher affinity for Notch ICD than for HIF-1alpha, abrogates the derepression, suggesting that Notch ICD sequesters FIH-1 away from HIF-1alpha. In conclusion, the data reveal posttranslational modification of the activated form of the Notch receptor and an intricate mode of cross-coupling between the Notch and hypoxia signaling pathways.

Nuclear expression of hypoxia-inducible factor-1alpha in clear cell renal cell carcinoma is involved in tumor progression

OBJECTIVES

The most frequent genomic abnormality in clear cell renal cell carcinoma (cc-RCC) is inactivation of Von Hippel-Lindau gene (VHL). pVHL19 is a ligase promoting proteosomal degradation of hypoxia-inducible factor-1alpha (HIF-1alpha); pVHL30 is associated with microtubules. VHL exert its oncogenetic action both directly and through HIF-1alpha activation. TNM classification is unable to define a correct prognostic evaluation of intracapsular cc-RCC. The nucleo-cytoplasmic trafficking in VHL/HIF-1alpha pathway could be relevant in understanding the molecular pathogenesis of renal carcinogenesis. This study analyzes VHL/HIF-1alpha proteins in a large series of intracapsular cc-RCCs, correlating their expression and cellular localization with prognosis.

MATERIALS AND METHODS

Two anti-pVHL (clones Ig32 and Ig33) and 1 anti-HIF-1alpha were used on tissue microarrays from 136 intracapsular cc-RCCs (mean follow-up: 74 mo). Clone 32 recognizes both pVHLs, whereas clone 33 only pVHL30. Results were matched with clinicopathologic variables and tumor-specific survival (TSS).

RESULTS

A strong cytoplasmic positivity was found for all antibodies in the largest part of cases, associated to a strong nuclear localization in the case of HIF-1alpha. All pVHL-negative cases were associated with high HIF-1alpha expression. pVHL negativity and HIF-1alpha nuclear positivity significantly correlated with shorter TSS. In multivariate analysis both pVHL negativity and HIF-1alpha nuclear expression were independent predictors of TSS.

CONCLUSIONS

The localization of the proteins well matches with their role and with the supposed tumor molecular pathways. The correlation with prognosis of VHL/HIF-1alpha alterations confirms the relevance of their molecular pathway and of the cellular trafficking of their products in the pathogenesis of renal cancer.

Bortezomib inhibits tumor adaptation to hypoxia by stimulating the FIH-mediated repression of hypoxia-inducible factor-1

Bortezomib (PS-341), a proteasome inhibitor, has been examined clinically for the treatment of multiple myeloma and several solid tumors. Bortezomib directly induces tumor cell death and has also been reported to inhibit tumor adaptation to hypoxia by functionally inhibiting hypoxia-inducible factor-1alpha (HIF-1alpha). However, the mechanism underlying HIF-1 inhibition by bortezomib remains obscure. In the present study, we demonstrated that bortezomib attenuated the hypoxic induction of erythropoietin and vascular endothelial growth factor at subnanomolar concentrations in multiple myeloma and liver cancer cell lines, regardless of cytotoxic concentrations of bortezomib. Bortezomib repressed HIF-1alpha activity by inhibiting the recruitment of p300 coactivator. Specifically, bortezomib targeted HIF-1alpha C-terminal transactivation domain (CAD) but not the CAD lacking Asn803, which is a hydroxylation site by the factor inhibiting HIF-1 (FIH). Accordingly, this effect of bortezomib on CAD was augmented by FIH expression and abolished by FIH knock-down. Furthermore, bortezomib stimulated the interaction between CAD and FIH under hypoxic conditions, and FIH inhibition reversed the suppressions of erythropoietin and vascular endothelial growth factor by bortezomib. We propose that the mechanism underlying the inhibitory effects of bortezomib on tumor angiogenesis and hypoxic adaptation involves the repression of HIF-1alpha transcriptional activity by reinforcing the FIH-mediated inhibition of p300 recruitment.

Anti-apoptotic Bcl-2 enhancing requires FGF-2/FGF receptor 1 binding in mouse osteoblasts

In this study, we investigated the role of prostaglandin F2alpha (PGF2alpha) in mouse osteoblast survival and the function of fibroblast growth factor 2 (FGF-2) and fibroblast growth factor receptor 1 (FGFR1) in this process. In particular, for the first time, we demonstrated that PGF2alpha increased osteoblast survival in a dose-dependent manner and we showed that the effect is correlated with an increase in Bcl-2/Bax ratio. Furthermore, we demonstrated that PGF2alpha caused a decrement of the active caspases 9 and 3. By blocking FGF-2 with the specific neutralizing antibody and by depletion of FGFR1 gene with a specific siRNA, we showed that FGFR1 and FGF-2 are critical for the increment of Bcl-2/Bax ratio and the decrement of the active caspases 9 and 3, induced by PGF2alpha. Moreover, transmission electron microscopy studies showed that PGF2alpha increased binding of FGF-2 and FGFR1 and co-localization of reactive sites at plasma membrane level. In conclusion, we report a novel mechanism in which PGF2alpha induces FGF-2 binding to its specific cell surface receptor 1 leading to a cascade pathway that culminates with increased mouse osteoblast survival.

Abnormal heart development and lung remodeling in mice lacking the hypoxia-inducible factor-related basic helix-loop-helix PAS protein NEPAS

Hypoxia-inducible factors (HIFs) are crucial for oxygen homeostasis during both embryonic development and postnatal life. Here we show that a novel HIF family basic helix-loop-helix (bHLH) PAS (Per-Arnt-Sim) protein, which is expressed predominantly during embryonic and neonatal stages and thereby designated NEPAS (neonatal and embryonic PAS), acts as a negative regulator of HIF-mediated gene expression. NEPAS mRNA is derived from the HIF-3alpha gene by alternative splicing, replacing the first exon of HIF-3alpha with that of inhibitory PAS. NEPAS can dimerize with Arnt and exhibits only low levels of transcriptional activity, similar to that of HIF-3alpha. NEPAS suppressed reporter gene expression driven by HIF-1alpha and HIF-2alpha. By generating mice with a targeted disruption of the NEPAS/HIF-3alpha locus, we found that homozygous mutant mice (NEPAS/HIF-3alpha(-)(/)(-)) were viable but displayed enlargement of the right ventricle and impaired lung remodeling. The expression of endothelin 1 and platelet-derived growth factor beta was increased in the lung endothelial cells of NEPAS/HIF-3alpha-null mice. These results demonstrate a novel regulatory mechanism in which the activities of HIF-1alpha and HIF-2alpha are negatively regulated by NEPAS in endothelial cells, which is pertinent to lung and heart development during the embryonic and neonatal stages.

Modulation of vascular gene expression by hypoxia

PURPOSE OF REVIEW

Angiogenesis and inflammation are important features in atherosclerotic plaque destabilization. The transcription factors hypoxia-inducible factor-1alpha and Notch are key regulators of angiogenesis. In addition, hypoxia-inducible factor-1alpha has been linked to regulation of inflammatory processes and innate immunity. This review will document how hypoxia-inducible factor-mediated signaling pathways are initiated in hypoxic cells, and how the hypoxia-inducible factor and Notch-dependent signaling pathways are functionally integrated.

RECENT FINDINGS

Activation of the hypoxia-inducible factor-mediated signaling events by hypoxia is complex and regulated by a cascade of molecular events that will be reviewed in detail. The activated form of hypoxia-inducible factor enhances Notch-dependent activation of Notch target genes, thereby providing a mechanism by which hypoxia can regulate the differentiation status of a cell. Recent observations implicate the Notch signaling pathway in proper specification of cell identity, position and behavior in a developing blood vessel sprout, and the hypoxia-inducible factor-mediated signaling pathway is critical for induction of expression of vascular endothelial growth factor.

SUMMARY

Hypoxia-inducible factor and Notch transcription factors represent potentially attractive targets for regulation of angiogenesis and possibly inflammation. In view of the pleiotropic effects of these transcription factors, however, successful targeting of these signaling pathways will require the development of gene specific (and possibly tissue-specific) modulators and extensive validation in relevant model systems.

An in silico model for HIF-alpha regulation and hypoxia response in tumor cells

The dependency of the growth and metastasis of tumors on the new blood vessel formation, or angiogenesis, has opened up new potentials to tumor therapy, nevertheless understanding the molecular mechanisms involved in angiogenesis is crucial in the bioengineering of novel anti-angiogenic drugs. The key component in hypoxia sensing in tumor cells is the hypoxia-inducible factor, HIF-1alpha, which is inactivated through proteosome-mediated degradation under normoxic conditions. Two enzymes have been reported to hydroxylate HIF-1alpha, namely prolyl hydroxylase (PH), recruiting the proetasome complex and degrading cytoplasmic HIF-1alpha, and asparaginyl hydroxylase/factor inhibiting HIF-1alpha (FIH-1), downregulating the recruitment of p300 to the promoter, thereby reducing the transcriptional activity of HIF-1alpha. In this study, we have constructed an in silico model of a tumor cell using the GEPASI 3.30 biochemical simulation software (http://www.gepasi.org) and studied the performances of PH and FIH-1 on HIF-1alpha degradation and inactivation, respectively, as monitored by expression of the vascular endothelial growth factor, VEGF, during hypoxia. In our biochemical models, FIH-1 can successfully increase hypoxic transcription of VEGF, however FIH-1 on its own is not sufficient to inactivate HIF-1 completely, leading to background VEGF transcription under normoxic conditions. On the other hand, PH is necessary to increase the hypoxic transcriptional response, and can effectively shut off normoxic transcription. We therefore propose that regulating PH activity can be a primary target for anti-angiogenic bioengineering research.