Hypoxia-inducible factors and the response to hypoxic stress

Activation of Hif1α by the prolylhydroxylase inhibitor dimethyoxalyglycine decreases radiosensitivity

Hypoxia inducible factor 1α (Hif1α) is a stress responsive transcription factor, which regulates the expression of genes required for adaption to hypoxia. Hif1α is normally hydroxylated by an oxygen-dependent prolylhydroxylase, leading to degradation and clearance of Hif1α from the cell. Under hypoxic conditions, the activity of the prolylhydroxylase is reduced and Hif1α accumulates. Hif1α is also constitutively expressed in tumor cells, where it is associated with resistance to ionizing radiation. Activation of the Hif1α transcriptional regulatory pathway may therefore function to protect normal cells from DNA damage caused by ionizing radiation. Here, we utilized the prolylhydroxylase inhibitor dimethyloxalylglycine (DMOG) to elevate Hif1α levels in mouse embryonic fibroblasts (MEFs) to determine if DMOG could function as a radioprotector. The results demonstrate that DMOG increased Hif1α protein levels and decreased the sensitivity of MEFs to ionizing radiation. Further, the ability of DMOG to function as a radioprotector required Hif1α, indicating a key role for Hif1α's transcriptional activity. DMOG also induced the Hif1α -dependent accumulation of several DNA damage response proteins, including CHD4 and MTA3 (sub-units of the NuRD deacetylase complex) and the Suv39h1 histone H3 methyltransferase. Depletion of Suv39h1, but not CHD4 or MTA3, reduced the ability of DMOG to protect cells from radiation damage, implicating increased histone H3 methylation in the radioprotection of cells. Finally, treatment of mice with DMOG prior to total body irradiation resulted in significant radioprotection of the mice, demonstrating the utility of DMOG and related prolylhydroxylase inhibitors to protect whole organisms from ionizing radiation. Activation of Hif1α through prolylhydroxylase inhibition therefore identifies a new pathway for the development of novel radiation protectors.

Repairing critical-sized calvarial defects with BMSCs modified by a constitutively active form of hypoxia-inducible factor-1α and a phosphate cement scaffold

Tissue engineering combined with gene therapy represents a promising approach for bone regeneration. The Hypoxia-inducible factor-1α (HIF-1α) gene is a pivotal regulator of vascular reactivity and angiogenesis. Our recent study has showed that HIF-1α could promote osteogenesis of bone mesenchymal stem cells (BMSCs) using a gene point mutant technique. To optimize the function of HIF-1α on inducing stem cells, another constitutively active form of HIF-1α (CA5) was constructed with truncation mutant method and its therapeutic potential on critical-sized bone defects was evaluated with calcium-magnesium phosphate cement (CMPC) scaffold in a rat model. BMSCs were treated with Lenti (lentivirus) -CA5, Lenti-WT (wild-type HIF-1α), and Lenti-LacZ. These genetically modified BMSCs were then combined with CMPC scaffolds to repair critical-sized calvarial defects in rats. The results showed that the overexpression of HIF-1α obviously enhanced the mRNA and protein expression of osteogenic markers in vitro and robust new bone formation with the higher local bone mineral density (BMD) was found in vivo in the CA5 and WT groups. Furthermore, CA5 showed significantly greater stability and osteogenic activity in BMSCs compared with WT. These data suggest that BMSCs transduced with truncation mutanted HIF-1α gene can promote the overexpression of osteogenic markers. CMPC could serve as a potential substrate for HIF-1α gene modified tissue engineered bone to repair critical sized bony defects.

PGC-1 family coactivators and cell fate: roles in cancer, neurodegeneration, cardiovascular disease and retrograde mitochondria-nucleus signalling

Over the past two decades, a complex nuclear transcriptional machinery controlling mitochondrial biogenesis and function has been described. Central to this network are the PGC-1 family coactivators, characterised as master regulators of mitochondrial biogenesis. Recent literature has identified a broader role for PGC-1 coactivators in both cell death and cellular adaptation under conditions of stress, here reviewed in the context of the pathology associated with cancer, neurodegeneration and cardiovascular disease. Moreover, we propose that these studies also imply a novel conceptual framework on the general role of mitochondrial dysfunction in disease. It is now well established that the complex nuclear transcriptional control of mitochondrial biogenesis allows for adaptation of mitochondrial mass and function to environmental conditions. On the other hand, it has also been suggested that mitochondria alter their function according to prevailing cellular energetic requirements and thus function as sensors that generate signals to adjust fundamental cellular processes through a retrograde mitochondria-nucleus signalling pathway. Therefore, altered mitochondrial function can affect cell fate not only directly by modifying cellular energy levels or redox state, but also indirectly, by altering nuclear transcriptional patterns. The current literature on such retrograde signalling in both yeast and mammalian cells is thus reviewed, with an outlook on its potential contribution to disease through the regulation of PGC-1 family coactivators. We propose that further investigation of these pathways will lead to the identification of novel pharmacological targets and treatment strategies to combat disease.

Hypoxia inducible factor-2 α is translationally repressed in response to dietary iron deficiency in Sprague-Dawley rats

Iron regulatory proteins (IRP) regulate cellular iron metabolism by binding to iron-responsive elements (IRE) located in untranslated regions of mRNA-encoding proteins of iron metabolism. Recently, IRE have been identified in mRNA-encoding proteins with previously uncharacterized roles in iron metabolism, thus expanding the role of IRP beyond the regulation of cellular iron homeostasis. The mRNA for HIF 2-α contains an IRE and undergoes iron-dependent regulation in vitro, though the translational regulation of HIF-2α in vivo remains unknown. To examine HIF-2α translational regulation in vivo, we evaluated the effects of iron deficiency on the regulation of hepatic IRP activity and HIF-2α translation. Rats were fed either a control (C; 50 mg Fe/kg diet) or iron-deficient (ID; <5 mg Fe/kg diet) diet or were pair-fed (PF) the C diet for 21 d. In ID rats, there was a 2-fold increase in IRP activity compared to the PF group (P < 0.05), which was reflected by a 30-40% increase in HIF-2α repression (P < 0.05). In agreement with a decrease in translation, the levels of HIF-2α proteins were also decreased. The relative abundance of HIF-2α mRNA did not differ between treatment groups. Taken together, these results suggest that the translation of HIF-2α in the liver is regulated in part by the action of IRP in response to dietary iron deficiency and provide evidence that IRP may assist in coordinating the cellular response to alterations in iron and oxygen status associated with iron deficiency anemia.

Stereotyped fetal brain disorganization is induced by hypoxia and requires lysophosphatidic acid receptor 1 (LPA1) signaling

Fetal hypoxia is a common risk factor that has been associated with a range of CNS disorders including epilepsy, schizophrenia, and autism. Cellular and molecular mechanisms through which hypoxia may damage the developing brain are incompletely understood but are likely to involve disruption of the laminar organization of the cerebral cortex. Lysophosphatidic acid (LPA) is a bioactive lipid capable of cortical influences via one or more of six cognate G protein-coupled receptors, LPA(1-6), several of which are enriched in fetal neural progenitor cells (NPCs). Here we report that fetal hypoxia induces cortical disruption via increased LPA(1) signaling involving stereotyped effects on NPCs: N-cadherin disruption, displacement of mitotic NPCs, and impaired neuronal migration, as assessed both ex vivo and in vivo. Importantly, genetic removal or pharmacological inhibition of LPA(1) prevented the occurrence of these hypoxia-induced phenomena. Hypoxia resulted in overactivation of LPA(1) through selective inhibition of G protein-coupled receptor kinase 2 expression and activation of downstream pathways including G(αi) and Ras-related C3 botulinum toxin substrate 1. These data identify stereotyped and selective hypoxia-induced cerebral cortical disruption requiring LPA(1) signaling, inhibition of which can reduce or prevent disease-associated sequelae, and may take us closer to therapeutic treatment of fetal hypoxia-induced CNS disorders and possibly other forms of hypoxic injury.

Hypoxia-inducible factors in OSCC

Oral squamous cell carcinoma (OSCC) is a characteristic locally aggressive tumor in which hypoxia levels are very high, causing a low response to chemotherapy and providing basic resistance to anticancer drugs. Tumoral hypoxia directly depends on hypoxia-inducible factors (HIF). The goal of this paper is to describe HIF basic biology and tumor cells (HIF-1α, mainly), analyzing the effects of its expression in OSCC, study its relation with other molecules such as nitric oxide (NO), carbonic anhydrase (CA) or VEGF and assess the possibility of its manipulation as a therapeutic target.

Gene expression patterns in livers of Hispanic patients infected with hepatitis C virus

We report a gene expression study aimed at the identification of genes differentially expressed in the livers of Hispanic patients infected with hepatitis C virus (HCV). Six uninfected controls were compared with 14 HCV(+) patients in which the liver biopsies were obtained at the time of diagnosis. Among the latter, five patients were also analyzed 4 weeks after the onset of standard anti-HCV therapy (pegylated interferon-α + ribavirin). We identified many genes up- or down-regulated by the infection with HCV in the human livers. When these genes were subjected to pathway analysis, several prominent pathways were revealed including many interferon (IFN)-inducible pathways as well as immune cell trafficking, inflammation, anti-microbial responses, and even cancer. We detected expression of many genes that have previously been associated with HCV infection, as well as several novel genes including CD47. The genes induced by HCV infection showed large expression changes, whereas the genes induced by the IFN-α combination therapy were relatively few (including MX2, ORMDL3, GPAM, KOPX18, TMEM56, and HBP1) and they reflected relatively small expression changes. This is the first study to identify changes in gene expression in livers of HCV(+) Hispanic patients and the first to identify genes induced by anti-HCV combination therapy in the human liver.

The SirT3 divining rod points to oxidative stress

Sirtuins are NAD(+) dependent deacetylases that counter aging and diseases of aging. Sirtuin research has focused on SirT1, which deacetylates transcription factors and cofactors in the nucleus. More recent findings highlight SirT3 as a mitochondrial sirtuin that regulates metabolism and oxidative stress. This review focuses on new data linking SirT3 to management of reactive oxygen species from mitochondria, which may have profound implications for aging and late-onset diseases.

Roles for HIF-1α in neural stem cell function and the regenerative response to stroke

Stroke represents a leading cause of long-term disability worldwide, with few therapeutic options available for improving behavioral recovery. Identification of endogenous neural stem and progenitor cells (NSPCs) that are capable of promoting reparative responses following brain injury and stroke make these cells attractive therapeutic targets for stimulating cell replacement and neuronal plasticity. Interest in the mechanisms that support NSPC survival and replenishment of damaged cells within the ischemic brain has led to elucidation of new roles for hypoxia-inducible factor-1α (HIF-1α) in NSPC function. HIF-1α is a well-studied mediator of adaptive cellular responses to hypoxia through direct transcriptional regulation of cellular metabolism and angiogenesis. Recent evidence also indicates novel roles for HIF-1α in stem cell differentiation through modulation of Notch and Wnt/β-catenin signaling pathways. In this review, we will explore the hypothesis that HIF-1α represents an important mediator of NSPC function under both non-pathological conditions and stroke; and plays a central role in the regulation of NSPC response to hypoxia, metabolism and maintenance of the vascular environment of the neural stem cell niche.

Brain microvasculature and hypoxia-related proteins in Alzheimer's disease

Alzheimer's disease (AD) is a progressive, neurodegenerative disease of increasing incidence. The pathologic processes that underlie this disorder are incompletely understood, however, hypoperfusion/hypoxia is thought to contribute to disease pathogenesis. Hypoxia inducible factor 1-alpha (HIF-1α), a key regulator of cellular responses to hypoxia, is elevated in the microcirculation of AD patients. Cerebral hypoxia is a potent stimulus for vascular activation and angiogenesis. Microvessels isolated from the brains of AD patients express a large number of angiogenic proteins. Despite considerable data in human tissues regarding vascular expression of hypoxia-related angiogenic proteins, there is little information regarding these proteins in the brain vasculature of transgenic AD mice. The objectives of this study were to determine expression of HIF-1α, angiogenic proteins, angiopoietin-2 (Ang-2), and matrix metalloproteinase 2 (MMP2), and survival/apoptotic proteins (Bcl-xL, caspase 3) in the cerebromicrovasculature of AD transgenic mice and to determine the direct effect of hypoxia on cerebral endothelial expression of these proteins in vitro. Cultured brain endothelial cells were subjected to hypoxia for 4-6 h and analyzed by western blot and immunofluorescence. Our results demonstrated that HIF-1α is induced in cultured brain endothelial cells exposed to hypoxia and that expression of Ang-2, MMP2 and caspase 3 was elevated and the anti-apoptotic protein Bcl-xL decreased. Brain sections from AD and control mice showed that HIF-1α, Ang-2, MMP2 and caspase 3 are elevated and Bcl-xL decreased in the microvasculature of AD mice. These data suggest the cerebromicrovasculature is an important target for the effects of hypoxia in the AD brain.

The Loss of HIF1α Leads to Increased Susceptibility to Cadmium-Chloride-Induced Toxicity in Mouse Embryonic Fibroblasts

Wild-type and HIF1α −/− MEF cells were used to determine the role of HIF1α in cadmium-induced toxicity. Cadmium treatment did not affect HIF1-mediated transcription but led to caspase activation and apoptotic cell death in wild-type and HIF1α −/− cells. Cadmium-induced cell death, however, was significantly higher in HIF1α −/− cells as compared to their wild-type counterparts. Increased cell death in the HIF1α −/− cells was correlated with lower metallothionein protein, elevated levels of reactive oxygen species, and decreased superoxide dismutase enzyme activity. The total and oxidized glutathione levels, and, correspondingly, lipid peroxidation levels were elevated in the null cells compared to wild-type cells, indicating increased antioxidant demand and greater oxidative stress. Overall, the results suggest that basal levels of HIF1α play a protective role against cadmium-induced cytotoxicity in mouse embryonic fibroblasts by maintaining metallothionein and antioxidant activity levels.

Principles and mechanisms of vessel normalization for cancer and other angiogenic diseases

Despite having an abundant number of vessels, tumours are usually hypoxic and nutrient-deprived because their vessels malfunction. Such abnormal milieu can fuel disease progression and resistance to treatment. Traditional anti-angiogenesis strategies attempt to reduce the tumour vascular supply, but their success is restricted by insufficient efficacy or development of resistance. Preclinical and initial clinical evidence reveal that normalization of the vascular abnormalities is emerging as a complementary therapeutic paradigm for cancer and other vascular disorders, which affect more than half a billion people worldwide. Here, we discuss the mechanisms, benefits, limitations and possible clinical translation of vessel normalization for cancer and other angiogenic disorders.

Increasing vascularity to improve healing of a segmental defect of the rat femur

BACKGROUND

Segmental bone loss remains a challenging clinical problem. A frequent mitigating factor is inadequate blood supply. Small molecules that activate the hypoxia-inducible factor pathway can be used to stimulate angiogenesis. We investigated an approach to promote healing using angiogenic and osteogenic compounds in combination with a biodegradable, weightbearing scaffold.

METHODS

Adult rats underwent removal of a 5-mm segment of femur stabilized by a cylindrical biodegradable implant and intramedullary fixation. Treatment groups included 1) saline (negative control); 2) desferrioxamine (DFO, a hypoxia-inducible factor activator; 3) low-dose recombinant human bone morphogenetic protein-2 (rhBMP-2) (5 μg); 4) DFO and low-dose rhBMP-2 (5 μg); or 5) rh-BMP-2 (10 μg). Angiography was used to evaluate vascularity. Bone healing was assessed by radiographs, microcomputed tomography, histology, and biomechanical testing.

RESULTS

Increased vascularity was seen at 6 weeks in the DFO treatment group. There appeared to be increased bone bridging as assessed by radiographic scores and microcomputed tomography in the BMP groups, although the quantification of bone volume did not show statistically significant differences. Biomechanical testing revealed improved stiffness in the treatment groups.

CONCLUSIONS

DFO improved angiogenesis and stiffness of bone healing in segmental defects. BMP improved radiographic scores and stiffness. Use of angiogenic compounds in segmental bone loss is promising.

CLINICAL RELEVANCE

Activation of the hypoxia-inducible factor pathway may prove useful for bone defects, particularly where impaired blood supply exists.The low-cost approach could be useful in segmental bone defects clinically.

Nothing Iffy about HIF in the Hypothalamus

Two crucial biological processes are (1) the sensing and coordination of responses to low oxygen levels and (2) the control of food intake and energy expenditure. The hypoxia-inducible factor (HIF) family of proteins is known to regulate responses to low oxygen, whereas neuropeptides derived from proopiomelanocortin (POMC) are implicated in the control of food intake and energy expenditure. It is now becoming apparent that these two apparently disparate processes may be linked, with the exciting discovery that HIF proteins can act in the brain to regulate food intake and energy expenditure as reported in the current issue of PLoS Biology. This primer discusses the traditional role of HIF proteins in terms of responding to oxygen levels in the periphery and also their new role in coordinating responses to nutrients in the brain through regulation of POMC.

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.

Deletion of the Mint3/Apba3 gene in mice abrogates macrophage functions and increases resistance to lipopolysaccharide-induced septic shock

Two major metabolic systems are usually used to generate ATP: oxidative phosphorylation (OXPHOS) in the mitochondria and glycolysis. Most types of cells employ OXPHOS for ATP production during normoxia but then shift energy production from OXPHOS to glycolysis when exposed to hypoxia. Hypoxia-inducible factor-1 (HIF-1) is the master transcription factor regulating this metabolic shift. On the other hand, macrophages are unique in making use of glycolysis for ATP generation constitutively even during normoxia. We recently proposed that in macrophages, Mint3/APBA3 inhibits factor inhibiting HIF-1 (FIH-1) during normoxia, which in turn releases the suppression of HIF-1 activity by FIH-1. To demonstrate the physiological function of APBA3 in macrophages, we established Apba3(-/-) mice. The mutant mice presented no apparent gross phenotype but exhibited significant resistance against LPS-induced septic shock. The level of ATP in macrophages obtained from the mutant mice was reduced to 60% of the level observed in wild type cells, which in turn led to reduced ATP-dependent activities such as glycolysis, cytokine production, and motility. We also generated mutant mice with the Apba3 gene deleted specifically from cells of the myeloid lineage and confirmed that LPS-induced septic shock is mitigated significantly. Thus, we show cell type-specific regulation of energy production by APBA3 in macrophages using genetically manipulated mice. The specific function of APBA3 in macrophages might allow us to develop therapeutics to regulate aberrant macrophage function during infection and diseases.

Clinical significance of expression of HIF-1α and EMT-related factors in pancreatic ductal adenocarcinoma

AIM: To investigate the expression of hypoxia inducible factor-1α (HIF-1α) and epithelial-mesenchvmal transition (EMT)-related factors in pancreatic ductal adenocarcinoma (PDAC) and to analyze their clinical significance.

METHODS: Immunohistochemistry was used to examine the expression of HIF-1α, Snail, Slug, E-cadherin and N-cadherin in 92 cases of PDAC and 10 cases of normal pancreatic tissue.

RESULTS: The positive expression rates of HIF-1α, Snail, Slug, E-cadherin and N-cadherin were 69.57%, 69.57%, 58.70%, 36.96% and 73.91% in PDAC, and 10%, 0%, 0%, 90% and 0% in normal pancreas tissue. HIF-1α expression was significantly associated with lymph node metastasis. Snail expression was correlated with TNM stage. Slug expression was positively associated with TNM stage and lymph node metastasis. N-cadherin expression was significantly related to TNM stage, lymph node metastasis and histological grade. HIF-1α expression in PDAC was positively correlated with Snail and Slug expression. There was a negative correlation between Snail and E-cadherin expression in PDAC. Expression of E-cadherin was negatively related to that of N-cadherin. Multivariate Cox analysis demonstrated that HIF-1α, Snail and E-cadherin were independent factors for prognosis of PDAC.

CONCLUSION: HIF-1α, Snail, Slug, E-cadherin and N-cadherin play significant roles in the progression and prognosis of PDAC.

In vivo T cell activation in lymphoid tissues is inhibited in the oxygen-poor microenvironment

Activation of immune cells is under control of immunological and physiological regulatory mechanisms to ensure adequate destruction of pathogens with the minimum collateral damage to “innocent” bystander cells. The concept of physiological negative regulation of immune response has been advocated based on the finding of the critical immunoregulatory role of extracellular adenosine. Local tissue oxygen tension was proposed to function as one of such physiological regulatory mechanisms of immune responses. In the current study, we utilized in vivo marker of local tissue hypoxia pimonidazole hydrochloride (Hypoxyprobe-1) in the flowcytometric analysis of oxygen levels to which lymphocytes are exposed in vivo. The level of exposure to hypoxia in vivo was low in B cells and the levels increased in the following order: T cells < NKT cells < NK cells. The thymus was more hypoxic than the spleen and lymph nodes, suggesting the variation in the degree of oxygenation among lymphoid organs and cell types in normal mice. Based on in vitro studies, tissue hypoxia has been assumed to be suppressive to T cell activation in vivo, but there was no direct evidence demonstrating that T cells exposed to hypoxic environment in vivo are less activated. We tested whether the state of activation of T cells in vivo changes due to their exposure to hypoxic tissue microenvironments. The parallel analysis of more hypoxic and less hypoxic T cells in the same mouse revealed that the degree of T cell activation was significantly stronger in better-oxygenated T cells. These observations suggest that the extent of T cell activation in vivo is dependent on their localization and is decreased in environment with low oxygen tension.

Molecular mechanisms and clinical applications of angiogenesis

Blood vessels deliver oxygen and nutrients to every part of the body, but also nourish diseases such as cancer. Over the past decade, our understanding of the molecular mechanisms of angiogenesis (blood vessel growth) has increased at an explosive rate and has led to the approval of anti-angiogenic drugs for cancer and eye diseases. So far, hundreds of thousands of patients have benefited from blockers of the angiogenic protein vascular endothelial growth factor, but limited efficacy and resistance remain outstanding problems. Recent preclinical and clinical studies have shown new molecular targets and principles, which may provide avenues for improving the therapeutic benefit from anti-angiogenic strategies.

Emerging roles for Lys11-linked polyubiquitin in cellular regulation

Polyubiquitin chains are assembled via one of seven lysine (Lys) residues or the N terminus. The cellular roles of Lys48- and Lys63-linked polyubiquitin have been extensively studied; however, the cellular functions of Lys11-linked chains are less well understood. Recent insights into Lys11-linked ubiquitin chains have revealed their important function in cell cycle control. Additionally, Lys11 linkages have been identified in the context of mixed chains in many other cellular pathways. In this review, we introduce the specific enzymes that mediate Lys11-linked chain assembly and disassembly, and discuss the diverse cellular processes in which Lys11 linkages participate. Notably, mechanistic insights have revealed how the E2 ubiquitin-conjugating enzyme UBE2S achieves its Lys11 linkage specificity, and two structures of Lys11-linked polyubiquitin highlight the dynamic nature of this compact chain type.

The viral interferon regulatory factors of KSHV: immunosuppressors or oncogenes?

Kaposi’s sarcoma-associated herpesvirus (KSHV) is a large double-stranded DNA gammaherpesvirus, and the etiological agent for three human malignancies: Kaposi’s sarcoma, primary effusion lymphoma, and multicentric Castleman’s disease. To establish and maintain infection, KSHV has evolved unique mechanisms to evade the host immune response. Cellular interferon regulatory factors (IRFs) are a critical part of the host anti-viral immune response. KSHV encodes four homologs of IRFs, vIRF1–4, which inhibit the activity of their cellular counterparts. vIRF1, 2, and 3 have been shown to interact directly with cellular IRFs. Additionally, the vIRFs have other functions such as modulation of Myc, p53, Notch, transforming growth factor-β, and NF-κB signaling. These activities of vIRFs may contribute to KSHV tumorigenesis. KSHV vIRF1 and vIRF3 have been implicated as oncogenes, making the understanding of KSHV vIRF function vital to understanding KSHV pathogenesis.

Dynamic modelling of oestrogen signalling and cell fate in breast cancer cells

Cancers of the breast and other tissues arise from aberrant decision-making by cells regarding their survival or death, proliferation or quiescence, damage repair or bypass. These decisions are made by molecular signalling networks that process information from outside and from within the breast cancer cell and initiate responses that determine the cell's survival and reproduction. Because the molecular logic of these circuits is difficult to comprehend by intuitive reasoning alone, we present some preliminary mathematical models of the basic decision circuits in breast cancer cells that may aid our understanding of their susceptibility or resistance to endocrine therapy.

Hypoxia. Cross talk between oxygen sensing and the cell cycle machinery

A fundamental physiological property of mammalian cells is the regulation of proliferation according to O(2) availability. Progression through the cell cycle is inhibited under hypoxic conditions in many, but not all, cell types, and this G1 arrest is dependent on hypoxia-inducible factor (HIF) 1α. Components of the hexameric MCM helicase, which binds to replication origins before the onset of DNA synthesis, are present in large excess in mammalian cells relative to origins, suggesting that they may have additional functions. Screens for HIF-1α interacting proteins revealed that MCM7 binds to the amino-terminal PER-SIM-ARNT (PAS) domain of HIF-1α and stimulates prolyl hydroxylation-dependent ubiquitination and degradation of HIF-1α, whereas MCM3 binds to the carboxyl terminus of HIF-1α and enhances asparaginyl hydroxylation-dependent inhibition of HIF-1α transactivation domain function. Thus MCM proteins inhibit HIF activity via two distinct O(2)-dependent mechanisms. Under prolonged hypoxic conditions, MCM mRNA expression is inhibited in a HIF-1α-dependent manner. Thus HIF and MCM proteins act in a mutually antagonistic manner, providing a novel molecular mechanism for homeostatic regulation of cell proliferation based on the relative levels of these proteins.

The emerging role of hypoxia, HIF-1 and HIF-2 in multiple myeloma

Hypoxia is an imbalance between oxygen supply and demand, which deprives cells or tissues of sufficient oxygen. It is well-established that hypoxia triggers adaptive responses, which contribute to short- and long-term pathologies such as inflammation, cardiovascular disease and cancer. Induced by both microenvironmental hypoxia and genetic mutations, the elevated expression of the hypoxia-inducible transcription factor-1 (HIF-1) and HIF-2 is a key feature of many human cancers and has been shown to promote cellular processes, which facilitate tumor progression. In this review, we discuss the emerging role of hypoxia and the HIFs in the pathogenesis of multiple myeloma (MM), an incurable hematological malignancy of BM PCs, which reside within the hypoxic BM microenvironment. The need for current and future therapeutic interventions to target HIF-1 and HIF-2 in myeloma will also be discussed.

Ischaemia-induced retinal neovascularisation and diabetic retinopathy in mice with conditional knockout of hypoxia-inducible factor-1 in retinal Müller cells

AIMS/HYPOTHESIS

Retinal Müller cells are known to produce inflammatory and angiogenic cytokines, which play important roles in diabetic retinopathy. Hypoxia-inducible factor (HIF)-1 has been shown to play a crucial role in retinal inflammation and neovascularisation. We sought to determine the role of Müller cell-derived HIF-1 in oxygen-induced retinopathy (OIR) and diabetic retinopathy using conditional Hif-1α (also known as Hif1a) knockout (KO) mice.

METHODS

Conditional Hif-1α KO mice were generated by crossing mice expressing cyclisation recombinase (cre, also known as P1_gp003) in Müller cells with floxed Hif-1α mice and used for OIR and streptozotocin-induced diabetes to induce retinal neovascularisation and inflammation, respectively. Abundance of HIF-1α and pro-angiogenic and pro-inflammatory factors was measured by immunoblotting and immunohistochemistry. Retinal neovascularisation was visualised by angiography and quantified by counting pre-retinal nuclei. Retinal inflammation was evaluated by leucostasis and vascular leakage.

RESULTS

While the Hif-1α KO mice showed significantly decreased HIF-1α levels in the retina, they exhibited no apparent histological or visual functional abnormalities under normal conditions. Compared with wild-type counterparts, Hif-1α KO mice with OIR demonstrated attenuated overproduction of vascular endothelial growth factor (VEGF) and intercellular adhesion molecule (ICAM)-1, reduced vascular leakage and alleviated neovascularisation in the retina. Under diabetes conditions, disruption of Hif-1α in Müller cells attenuated the increases of retinal vascular leakage and adherent leucocytes, as well as the overproduction of VEGF and ICAM-1.

CONCLUSIONS/INTERPRETATION

Müller cell-derived HIF-1α is a key mediator of retinal neovascularisation, vascular leakage and inflammation, the major pathological changes in diabetic retinopathy. Müller cell-derived HIF-1α is therefore a promising therapeutic target for diabetic retinopathy.

Differential hypoxic regulation of hypoxia-inducible factors 1alpha and 2alpha

The hypoxia-inducible transcription factors (HIF) 1α and HIF-2α play a critical role in cellular response to hypoxia. Elevated HIF-α expression correlates with poor patient survival in a large number of cancers. Recent evidence suggests that HIF-2α appears to be preferentially expressed in neuronal tumor cells that exhibit cancer stem cell characteristics. These observations suggest that expression of HIF-1α and HIF-2α is differentially regulated in the hypoxic tumor microenvironment. However, the underlying mechanisms remain to be fully investigated. In this study, we investigated the transcriptional regulation of HIF-1α and HIF-2α under different physiologically relevant hypoxic conditions. We found that transcription of HIF-2α was consistently increased by hypoxia, whereas transcription of HIF-1α showed variable levels of repression. Mechanistically, differential regulation of HIF-α transcription involved hypoxia-induced changes in acetylation of core histones H3 and H4 associated with the proximal promoters of the HIF-1α or HIF-2α gene. We also found that, although highly stable under acute hypoxia, HIF-1α and HIF-2α proteins become destabilized under chronic hypoxia. Our results have thus provided new mechanistic insights into the differential regulation of HIF-1α and HIF-2α by the hypoxic tumor microenvironment. These findings also suggest an important role of HIF-2α in the regulation of tumor progression under chronic hypoxia.

Productive parvovirus B19 infection of primary human erythroid progenitor cells at hypoxia is regulated by STAT5A and MEK signaling but not HIFα

Human parvovirus B19 (B19V) causes a variety of human diseases. Disease outcomes of bone marrow failure in patients with high turnover of red blood cells and immunocompromised conditions, and fetal hydrops in pregnant women are resulted from the targeting and destruction of specifically erythroid progenitors of the human bone marrow by B19V. Although the ex vivo expanded erythroid progenitor cells recently used for studies of B19V infection are highly permissive, they produce progeny viruses inefficiently. In the current study, we aimed to identify the mechanism that underlies productive B19V infection of erythroid progenitor cells cultured in a physiologically relevant environment. Here, we demonstrate an effective reverse genetic system of B19V, and that B19V infection of ex vivo expanded erythroid progenitor cells at 1% O(2) (hypoxia) produces progeny viruses continuously and efficiently at a level of approximately 10 times higher than that seen in the context of normoxia. With regard to mechanism, we show that hypoxia promotes replication of the B19V genome within the nucleus, and that this is independent of the canonical PHD/HIFα pathway, but dependent on STAT5A and MEK/ERK signaling. We further show that simultaneous upregulation of STAT5A signaling and down-regulation of MEK/ERK signaling boosts the level of B19V infection in erythroid progenitor cells under normoxia to that in cells under hypoxia. We conclude that B19V infection of ex vivo expanded erythroid progenitor cells at hypoxia closely mimics native infection of erythroid progenitors in human bone marrow, maintains erythroid progenitors at a stage conducive to efficient production of progeny viruses, and is regulated by the STAT5A and MEK/ERK pathways.

Antiangiogenic therapy, hypoxia, and metastasis: risky liaisons, or not?

All human cells, including cancer cells, need oxygen and nutrients to survive. A widely used strategy to combat cancer is therefore the starvation of tumor cells by cutting off the blood supply of tumors. Clinical experience indeed shows that tumor progression can be delayed by anti-angiogenic agents. However, emerging evidence indicates that in certain experimental conditions, hypoxia as a result of pruning of the tumor microvasculature can promote tumor invasion and metastasis, although these findings are contextual and debated. Genetic studies in mice unveiled that vascular-targeting strategies that avoid aggravation of tumor hypoxia or even promote tumor oxygenation might prevent such an invasive metastatic switch. In this article, we will discuss the emerging link between hypoxia signaling and the various steps of metastasis.

Integrating physiological regulation with stem cell and tissue homeostasis

Stem cells are uniquely able to self-renew, to undergo multilineage differentiation, and to persist throughout life in a number of tissues. Stem cells are regulated by a combination of shared and tissue-specific mechanisms and are distinguished from restricted progenitors by differences in transcriptional and epigenetic regulation. Emerging evidence suggests that other aspects of cellular physiology, including mitosis, signal transduction, and metabolic regulation, also differ between stem cells and their progeny. These differences may allow stem cells to be regulated independently of differentiated cells in response to circadian rhythms, changes in metabolism, diet, exercise, mating, aging, infection, and disease. This allows stem cells to sustain homeostasis or to remodel relevant tissues in response to physiological change. Stem cells are therefore not only regulated by short-range signals that maintain homeostasis within their tissue of origin, but also by long-range signals that integrate stem cell function with systemic physiology.

Pharmacological activation of a novel p53-dependent S-phase checkpoint involving CHK-1

We have recently shown that induction of the p53 tumour suppressor protein by the small-molecule RITA (reactivation of p53 and induction of tumour cell apoptosis; 2,5-bis(5-hydroxymethyl-2-thienyl)furan) inhibits hypoxia-inducible factor-1α and vascular endothelial growth factor expression in vivo and induces p53-dependent tumour cell apoptosis in normoxia and hypoxia. Here, we demonstrate that RITA activates the canonical ataxia telangiectasia mutated/ataxia telangiectasia and Rad3-related DNA damage response pathway. Interestingly, phosphorylation of checkpoint kinase (CHK)-1 induced in response to RITA was influenced by p53 status. We found that induction of p53, phosphorylated CHK-1 and γH2AX proteins was significantly increased in S-phase. Furthermore, we found that RITA stalled replication fork elongation, prolonged S-phase progression and induced DNA damage in p53 positive cells. Although CHK-1 knockdown did not significantly affect p53-dependent DNA damage or apoptosis induced by RITA, it did block the ability for DNA integrity to be maintained during the immediate response to RITA. These data reveal the existence of a novel p53-dependent S-phase DNA maintenance checkpoint involving CHK-1.

Deletion of a subgroup of ribosome-related genes minimizes hypoxia-induced changes and confers hypoxia tolerance

Hypoxia is a widely occurring condition experienced by diverse organisms under numerous physiological and disease conditions. To probe the molecular mechanisms underlying hypoxia responses and tolerance, we performed a genome-wide screen to identify mutants with enhanced hypoxia tolerance in the model eukaryote, the yeast Saccharomyces cerevisiae. Yeast provides an excellent model for genomic and proteomic studies of hypoxia. We identified five genes whose deletion significantly enhanced hypoxia tolerance. They are RAI1, NSR1, BUD21, RPL20A, and RSM22, all of which encode functions involved in ribosome biogenesis. Further analysis of the deletion mutants showed that they minimized hypoxia-induced changes in polyribosome profiles and protein synthesis. Strikingly, proteomic analysis by using the iTRAQ profiling technology showed that a substantially fewer number of proteins were changed in response to hypoxia in the deletion mutants, compared with the parent strain. Computational analysis of the iTRAQ data indicated that the activities of a group of regulators were regulated by hypoxia in the wild-type parent cells, but such regulation appeared to be diminished in the deletion strains. These results show that the deletion of one of the genes involved in ribosome biogenesis leads to the reversal of hypoxia-induced changes in gene expression and related regulators. They suggest that modifying ribosomal function is an effective mechanism to minimize hypoxia-induced specific protein changes and to confer hypoxia tolerance. These results may have broad implications in understanding hypoxia responses and tolerance in diverse eukaryotes ranging from yeast to humans.

Iron regulatory protein 1 outcompetes iron regulatory protein 2 in regulating cellular iron homeostasis in response to nitric oxide

In mammals, iron regulatory proteins (IRPs) 1 and 2 posttranscriptionally regulate expression of genes involved in iron metabolism, including transferrin receptor 1, the ferritin (Ft) H and L subunits, and ferroportin by binding mRNA motifs called iron responsive elements (IREs). IRP1 is a bifunctional protein that mostly exists in a non-IRE-binding, [4Fe-4S] cluster aconitase form, whereas IRP2, which does not assemble an Fe-S cluster, spontaneously binds IREs. Although both IRPs fulfill a trans-regulatory function, only mice lacking IRP2 misregulate iron metabolism. NO stimulates the IRE-binding activity of IRP1 by targeting its Fe-S cluster. IRP2 has also been reported to sense NO, but the intrinsic function of IRP1 and IRP2 in NO-mediated regulation of cellular iron metabolism is controversial. In this study, we exposed bone marrow macrophages from Irp1(-/-) and Irp2(-/-) mice to NO and showed that the generated apo-IRP1 was entirely responsible for the posttranscriptional regulation of transferrin receptor 1, H-Ft, L-Ft, and ferroportin. The powerful action of NO on IRP1 also remedies the defects of iron storage found in IRP2-null bone marrow macrophages by efficiently reducing Ft overexpression. We also found that NO-dependent IRP1 activation, resulting in increased iron uptake and reduced iron sequestration and export, maintains enough intracellular iron to fuel the Fe-S cluster biosynthetic pathway for efficient restoration of the citric acid cycle aconitase in mitochondria. Thus, IRP1 is the dominant sensor and transducer of NO for posttranscriptional regulation of iron metabolism and participates in Fe-S cluster repair after exposure to NO.

Hypoxia-inducible factor plays a gut-injurious role in intestinal ischemia reperfusion injury

Gut injury and loss of normal intestinal barrier function are key elements in the paradigm of gut-origin systemic inflammatory response syndrome, acute lung injury, and multiple organ dysfunction syndrome (MODS). As hypoxia-inducible factor (HIF-1) is a critical determinant of the physiological and pathophysiological response to hypoxia and ischemia, we asked whether HIF-1 plays a proximal role in the induction of gut injury and subsequent lung injury. Using partially HIF-1α-deficient mice in an isolated superior mesenteric artery occlusion (SMAO) intestinal ischemia reperfusion (I/R) injury model (45 min SMAO followed by 3 h of reperfusion), we showed a direct relationship between HIF-1 activation and intestinal I/R injury. Specifically, partial HIF-1α deficiency attenuated SMAO-induced increases in intestinal permeability, lipid peroxidation, mucosal caspase-3 activity, and IL-1β mRNA levels. Furthermore, partial HIF-1α deficiency prevented the induction of ileal mucosal inducible nitric oxide synthase (iNOS) protein levels after SMAO and iNOS deficiency ameliorated SMAO-induced villus injury. Resistance to SMAO-induced gut injury was also associated with resistance to lung injury, as reflected by decreased levels of myeloperoxidase, IL-6 and IL-10 in the lungs of HIF-1α(+/-) mice. In contrast, a short duration of SMAO (15 min) followed by 3 h of reperfusion neither induced mucosal HIF-1α protein levels nor caused significant gut and lung injury in wild-type or HIF-1α(+/-) mice. This study indicates that intestinal HIF-1 activation is a proximal regulator of I/R-induced gut mucosal injury and gut-induced lung injury. However, the duration and severity of the gut I/R insult dictate whether HIF-1 plays a gut-protective or deleterious role.

Hypoxia-inducible factor regulates expression of surfactant protein in alveolar type II cells in vitro

Alveolar type II (ATII) cells cultured at an air-liquid (A/L) interface maintain differentiation, but they lose these properties when they are submerged. Others showed that an oxygen tension gradient develops in the culture medium as ATII cells consume oxygen. Therefore, we wondered whether hypoxia inducible factor (HIF) signaling could explain differences in the phenotypes of ATII cells cultured under A/L interface or submerged conditions. ATII cells were isolated from male Sprague-Dawley rats and cultured on inserts coated with a mixture of rat-tail collagen and Matrigel, in medium including 5% rat serum and 10 ng/ml keratinocyte growth factor, with their apical surfaces either exposed to air or submerged. The A/L interface condition maintained the expression of surfactant proteins, whereas that expression was down-regulated under the submerged condition, and the effect was rapid and reversible. Under submerged conditions, there was an increase in HIF1α and HIF2α in nuclear extracts, mRNA levels of HIF inducible genes, vascular endothelial growth factor, glucose transporter-1 (GLUT1), and the protein level of pyruvate dehydrogenase kinase isozyme-1. The expression of surfactant proteins was suppressed and GLUT1 mRNA levels were induced when cells were cultured with 1 mM dimethyloxalyl glycine. The expression of surfactant proteins was restored under submerged conditions with supplemented 60% oxygen. HIF signaling and oxygen tension at the surface of cells appears to be important in regulating the phenotype of rat ATII cells.

The role of iron in learning and memory

Iron deficiency (ID) is the most common nutrient deficiency, affecting 2 billion people and 30% of pregnant women and their offspring. Early life ID affects at least 3 major neurobehavioral domains, including speed of processing, affect, and learning and memory, the latter being particularly prominent. The learning and memory deficits occur while the infants are iron deficient and persist despite iron repletion. The neural mechanisms underlying the short- and long-term deficits are being elucidated. Early ID alters the transcriptome, metabolome, structure, intracellular signaling pathways, and electrophysiology of the developing hippocampus, the brain region responsible for recognition learning and memory. Until recently, it was unclear whether these effects are directly due to a lack of iron interacting with important transcriptional, translational, or post-translational processes or to indirect effects such as hypoxia due to anemia or stress. Nonanemic genetic mouse models generated by conditionally altering expression of iron transport proteins specifically in hippocampal neurons in late gestation have led to a greater understanding of iron's role in learning and memory. The learning deficits in adulthood likely result from interactions between direct and indirect effects that contribute to abnormal hippocampal structure and plasticity.

Review: Oxygen and trophoblast biology--a source of controversy

Oxygen is necessary for life yet too much or too little oxygen is toxic to cells. The oxygen tension in the maternal plasma bathing placental villi is <20 mm Hg until 10-12 weeks' gestation, rising to 40-80 mm Hg and remaining in this range throughout the second and third trimesters. Maldevelopment of the maternal spiral arteries in the first trimester predisposes to placental dysfunction and sub-optimal pregnancy outcomes in the second half of pregnancy. Although low oxygen at the site of early placental development is the norm, controversy is intense when investigators interpret how defective transformation of spiral arteries leads to placental dysfunction during the second and third trimesters. Moreover, debate rages as to what oxygen concentrations should be considered normal and abnormal for use in vitro to model villous responses in vivo. The placenta may be injured in the second half of pregnancy by hypoxia, but recent evidence shows that ischemia with reoxygenation and mechanical damage due to high flow contributes to the placental dysfunction of diverse pregnancy disorders. We overview normal and pathologic development of the placenta, consider variables that influence experiments in vitro, and discuss the hotly debated question of what in vitro oxygen percentage reflects the normal and abnormal oxygen concentrations that occur in vivo. We then describe our studies that show cultured villous trophoblasts undergo apoptosis and autophagy with phenotype-related differences in response to hypoxia.

Cellular metabolic stress: considering how cells respond to nutrient excess

Nutrient stress is generally considered from the standpoint of how cells detect and respond to an insufficient supply of nutrients to meet their bioenergetic needs. However, cells also experience stress as a result of nutrient excess, during which reactive oxygen species (ROS) production exceeds that required for normal physiological responses. This may occur as a result of oncogene activation or chronic exposure to growth factors combined with high levels of nutrients. As a result, multiple mechanisms have evolved to allow cells to detect and adapt to elevated levels of intracellular metabolites, including promotion of signaling and proliferation by ROS, amino acid-dependent mTOR activation, and regulation of signaling and transcription through metabolite-sensitive protein modifications. We discuss how each of these responses can contribute to the development and/or progression of cancer under conditions of cellular nutrient excess and their potential roles in linking chronic organismal over-nutrition (obesity) with cancer.

The aging stress response

Aging is the outcome of a balance between damage and repair. The rate of aging and the appearance of age-related pathology are modulated by stress response and repair pathways that gradually decline, including the proteostasis and DNA damage repair networks and mitochondrial respiratory metabolism. Highly conserved insulin/IGF-1, TOR, and sirtuin signaling pathways in turn control these critical cellular responses. The coordinated action of these signaling pathways maintains cellular and organismal homeostasis in the face of external perturbations, such as changes in nutrient availability, temperature, and oxygen level, as well as internal perturbations, such as protein misfolding and DNA damage. Studies in model organisms suggest that changes in signaling can augment these critical stress response systems, increasing life span and reducing age-related pathology. The systems biology of stress response signaling thus provides a new approach to the understanding and potential treatment of age-related diseases.

Autophagy and the integrated stress response

Autophagy is a tightly regulated pathway involving the lysosomal degradation of cytoplasmic organelles or cytosolic components. This pathway can be stimulated by multiple forms of cellular stress, including nutrient or growth factor deprivation, hypoxia, reactive oxygen species, DNA damage, protein aggregates, damaged organelles, or intracellular pathogens. Both specific, stimulus-dependent and more general, stimulus-independent signaling pathways are activated to coordinate different phases of autophagy. Autophagy can be integrated with other cellular stress responses through parallel stimulation of autophagy and other stress responses by specific stress stimuli, through dual regulation of autophagy and other stress responses by multifunctional stress signaling molecules, and/or through mutual control of autophagy and other stress responses. Thus, autophagy is a cell biological process that is a central component of the integrated stress response.

Regulation of the mTOR complex 1 pathway by nutrients, growth factors, and stress

The large serine/threonine protein kinase mTOR regulates cellular and organismal homeostasis by coordinating anabolic and catabolic processes with nutrient, energy, and oxygen availability and growth factor signaling. Cells and organisms experience a wide variety of insults that perturb the homeostatic systems governed by mTOR and therefore require appropriate stress responses to allow cells to continue to function. Stress can manifest from an excess or lack of upstream signals or as a result of genetic perturbations in upstream effectors of the pathway. mTOR nucleates two large protein complexes that are important nodes in the pathways that help buffer cells from stresses, and are implicated in the progression of stress-associated phenotypes and diseases, such as aging, tumorigenesis, and diabetes. This review focuses on the key components of the mTOR complex 1 pathway and on how various stresses impinge upon them.

Translational regulation of gene expression during conditions of cell stress

A number of stresses, including nutrient stress, temperature shock, DNA damage, and hypoxia, can lead to changes in gene expression patterns caused by a general shutdown and reprogramming of protein synthesis. Each of these stress conditions results in selective recruitment of ribosomes to mRNAs whose protein products are required for responding to stress. This recruitment is regulated by elements within the 5' and 3' untranslated regions of mRNAs, including internal ribosome entry segments, upstream open reading frames, and microRNA target sites. These elements can act singly or in combination and are themselves regulated by trans-acting factors. Translational reprogramming can result in increased life span, and conversely, deregulation of these translation pathways is associated with disease including cancer and diabetes.

Isolation and characterization of Pseudomonas aeruginosa PAO mutant that produces altered elastase

Triple-negative breast cancer (TNBC) is cancer that tested as negative for estrogen receptors (ER), progesterone receptors (PR), and excess human epidermal growth factor receptor 2 (HER2) protein which accounts for 15%–20% of all breast cancer cases. TNBC is considered to be a poorer prognosis than other types of breast cancer, mainly because it involves more aggressive phenotypes that are similar to stem cell–like cancer cells (cancer stem cell, CSC). Thus, targeted treatment of TNBC remains a major challenge in clinical practice. This review article surveys the latest evidence concerning the role of genomic alteration in current TNBC treatment responses, current clinical trials and potential targeting sites, CSC and drug resistance, and potential strategies targeting CSCs in TNBC. Furthermore, the role of insulin-like growth factor 1 receptor (IGF-1R) and nicotinic acetylcholine receptors (nAChR) in stemness expression, chemoresistance, and metastasis in TNBC and their relevance to potential treatments are also discussed and highlighted.