Hypoxia-induced energy stress regulates mRNA translation and cell growth

C. elegans are protected from lethal hypoxia by an embryonic diapause

At least 100 mammalian species exhibit embryonic diapause, where fertilized embryos arrest development in utero until suitable seasonal or nutritional environments are encountered. Delaying maternal investments in producing offspring allows these animals to utilize limited resources to survive while searching for better conditions and ensures that progeny are not produced when they are unlikely to survive. In addition, embryos may be protected from external environmental vicissitudes while in utero. Here we demonstrate embryonic diapause in C. elegans, and show that this diapause protects embryos from otherwise lethal hypoxia. Diapausing embryos in utero require san-1 to survive, indicating that hypoxia-induced embryonic diapause may be mechanistically related to suspended animation. Furthermore, we show that neuronal HIF-1 activity in the adult dictates the O(2) tension at which embryonic diapause is engaged. We suggest that the maternal perception of hypoxia stimulates a response to protect embryos in utero by inducing diapause, a natural form of suspended animation. This response is likely to be an important strategy to improve offspring survival in harsh conditions and allow adults to find environments more suitable for reproductive success.

Hepatocyte growth factor induces hypoxia-related interleukin-8 expression in lung adenocarcinoma cells

Rapid growth of cancer cells often creates insufficient supply of oxygen and nutrients in the tumour nest. The frequent detection of hypoxia-inducible factor (HIF) and interleukin-8 (IL-8) in afflicted tissues suggests that IL-8 expression could be associated with elevated levels of HIF. Recently, we found that hypoxia also upregulated the expression of hepatocyte growth factor (HGF) in lung adenocarcinoma (LAD) cells. However, the relationship between HGF and IL-8 has not been investigated in LAD cells. In this study, we found that HGF induced IL-8 expression in LAD. Interestingly, hypoxia also increased the level of prostaglandin F(2alpha) (PGF(2alpha)), a product of dihydrodiol dehydrogenase (DDH). When expression of DDH was suppressed by siRNA, the levels of PGF(2alpha), HGF and IL-8 were reduced; however, their levels returned to normal after DDH was reintroduced. These data suggest that hypoxia induces biosynthesis of PGF(2alpha), which then activates HGF and IL-8 expression. The results provide a reasonable explanation of how PGF(2alpha), HGF and IL-8 exert their effects on cancer cell metastasis.

A major effect of simulated microgravity on several stages of preimplantation mouse development is lethality associated with elevated phosphorylated SAPK/JNK

We tested whether microgravity affects mouse development during a period when gravity cues chick and frog embryo development. A rotating vessel developed approximately 0.1% simulated microgravity (MGS) for embryos. Microgravity simulation resulted in blocked cell accumulation in E2.5 embryos. E1.5 and E3.5 embryos showed lesser effects. For E1.5/2.5 embryos, cell accumulation block was followed by lethality at 48 hours after MGS. For E3.5 embryos, MGS blocked development without lethality but with apoptosis. E1.5-3.5 embryos from the rotational control developed lesser effects than MGS embryos. Embryonic stress-activated protein kinase (SAPK) was phosphorylated during MGS and mediated apoptosis. Increased pSAPK suggested that lethality is due to cellular stress induced by MGS, unlike the dysfunctional development after gravitational disorientation in frog and chick embryos. Thus, MGS causes lethality, a novel phenotype not often observed in microgravity or MGS. Embryonic lethality at E2.5 and apoptosis at E3.5 are associated with SAPK function, suggesting that MGS causes a general stress response that immediately affects many aspects of development. In addition, MGS and many aspects of In vitro fertilization/assisted reproductive technologies (IVF/ART) produce nonphysiological, nonevolutionary stresses that are mediated by SAPK, suggesting the primacy of this protein kinase in a wide range of mechanisms mediating negative reproductive outcomes in IVF/ART and potentially in spaceflight.

The role of protein synthesis in cell cycling and cancer

Cell cycling and protein synthesis are both key physiological tasks for cancer cells. Here we present a model for how the elongation phase of protein synthesis, governed by elongation factor 2 and elongation factor 2 kinase, both modulates and responds to cell cycling. Within this framework we also discuss survivin, a protein with both pro-mitotic and anti-apoptotic roles whose persistence in the cell is tied to protein synthesis due to its short half-life. Finally, we provide a brief overview of efforts of cancer researchers to target EF2 and EF2 kinase.

Taking advantage of tumor cell adaptations to hypoxia for developing new tumor markers and treatment strategies

Cancer cells in hypoxic areas of solid tumors are to a large extent protected against the action of radiation as well as many chemotherapeutic drugs. There are, however, two different aspects of the problem caused by tumor hypoxia when cancer therapy is concerned: One is due to the chemical reactions that molecular oxygen enters into therapeutically targeted cells. This results in a direct chemical protection against therapy by the hypoxic microenvironment, which has little to do with cellular biological regulatory processes. This part of the protective effect of hypoxia has been known for more than half a century and has been studied extensively. However, in recent years there has been more focus on the other aspect of hypoxia, namely the effect of this microenvironmental condition on selecting cells with certain genetic prerequisites that are negative with respect to patient prognosis. There are adaptive mechanisms, where hypoxia induces regulatory cascades in cells resulting in a changed metabolism or changes in extracellular signaling. These processes may lead to changes in cellular intrinsic sensitivity to treatment irrespective of oxygenation and, furthermore, may also have consequences for tissue organization. Thus, the adaptive mechanisms induced by hypoxia itself may have a selective effect on cells, with a fine-tuned protection against damage and stress of many kinds. It therefore could be that the adaptive mechanisms may take advantage of for new tumor labeling/imaging and treatment strategies. One of the Achilles' heels of hypoxia research has always been the exact measurements of tissue oxygenation as well as the control of oxygenation in biological tumor models. Thus, development of technology that can ease this control is vital in order to study mechanisms and perform drug development under relevant conditions. An integrated EU Framework project 2004-2009, termed EUROXY, demonstrates several pathways involved in transcription and translation control of the hypoxic cell phenotype and evidence of cross-talk with responses to pH and redox changes. The carbonic anhydrase isoenzyme CA IX was selected for further studies due to its expression on the surface of many types of hypoxic tumors. The effort has led to marketable culture flasks with sensors and incubation equipment, and the synthesis of new drug candidates against new molecular targets. New labeling/imaging methods for cancer diagnosing and imaging of hypoxic cancer tissue are now being tested in xenograft models and are also in early clinical testing, while new potential anti-cancer drugs are undergoing tests using xenografted tumor cancers. The present article describes the above results in individual consortium partner presentations.

Hypoxic activation of AMPK is dependent on mitochondrial ROS but independent of an increase in AMP/ATP ratio

AMP-activated protein kinase (AMPK) is a sensor of cellular energy status found in metazoans that is known to be activated by stimuli that increase the cellular AMP/ATP ratio. Full activation of AMPK requires specific phosphorylation within the activation loop of the catalytic domain of the alpha-subunit by upstream kinases such as the serine/threonine protein kinase LKB1. Here we show that hypoxia activates AMPK through LKB1 without an increase in the AMP/ATP ratio. Hypoxia increased reactive oxygen species (ROS) levels and the antioxidant EUK-134 abolished the hypoxic activation of AMPK. Cells deficient in mitochondrial DNA (rho(0) cells) failed to activate AMPK during hypoxia but are able to in the presence of exogenous H(2)O(2). Furthermore, we provide genetic evidence that ROS generated within the mitochondrial electron transport chain and not oxidative phosphorylation is required for hypoxic activation of AMPK. Collectively, these data indicate that oxidative stress and not an increase in the AMP/ATP ratio is required for hypoxic activation of AMPK.

LKB1 and AMP-activated protein kinase control of mTOR signalling and growth

The AMP-activated serine/threonine protein kinase (AMPK) is a sensor of cellular energy status found in all eukaryotes that is activated under conditions of low intracellular ATP following stresses such as nutrient deprivation or hypoxia. In the past 5 years, work from a large number of laboratories has revealed that one of the major downstream signalling pathways regulated by AMPK is the mammalian target-of-rapamycin [mammalian target of rapamycin (mTOR) pathway]. Interestingly, like AMPK, the mTOR serine/threonine kinase plays key roles not only in growth control and cell proliferation but also in metabolism. Recent work has revealed that across eukaryotes mTOR orthologues are found in two biochemically distinct complexes and only one of those complexes (mTORC1 in mammals) is acutely sensitive to rapamycin and regulated by nutrients and AMPK. Many details of the molecular mechanism by which AMPK inhibits mTORC1 signalling have also been decoded in the past 5 years. AMPK directly phosphorylates at least two proteins to induce rapid suppression of mTORC1 activity, the TSC2 tumour suppressor and the critical mTORC1 binding subunit raptor. Here we explore the molecular connections between AMPK and mTOR signalling pathways and examine the physiological processes in which AMPK regulation of mTOR is critical for growth or metabolic control. The functional conservation of AMPK and TOR in all eukaryotes, and the sequence conservation around the AMPK phosphorylation sites in raptor across all eukaryotes examined suggest that this represents a fundamental cell growth module connecting nutrient status to the cell growth machinery. These findings have broad implications for the control of cell growth by nutrients in a number of cellular and organismal contexts.

Alpha1-AMP-activated protein kinase regulates hypoxia-induced Na,K-ATPase endocytosis via direct phosphorylation of protein kinase C zeta

Hypoxia promotes Na,K-ATPase endocytosis via protein kinase C zeta (PKC zeta)-mediated phosphorylation of the Na,K-ATPase alpha subunit. Here, we report that hypoxia leads to the phosphorylation of 5'-AMP-activated protein kinase (AMPK) at Thr172 in rat alveolar epithelial cells. The overexpression of a dominant-negative AMPK alpha subunit (AMPK-DN) construct prevented the hypoxia-induced endocytosis of Na,K-ATPase. The overexpression of the reactive oxygen species (ROS) scavenger catalase prevented hypoxia-induced AMPK activation. Moreover, hypoxia failed to activate AMPK in mitochondrion-deficient rho(0)-A549 cells, suggesting that mitochondrial ROS play an essential role in hypoxia-induced AMPK activation. Hypoxia-induced PKC zeta translocation to the plasma membrane and phosphorylation at Thr410 were prevented by the pharmacological inhibition of AMPK or by the overexpression of the AMPK-DN construct. We found that AMPK alpha phosphorylates PKC zeta on residue Thr410 within the PKC zeta activation loop. Importantly, the activation of AMPK alpha was necessary for hypoxia-induced AMPK-PKC zeta binding in alveolar epithelial cells. The overexpression of T410A mutant PKC zeta prevented hypoxia-induced Na,K-ATPase endocytosis, confirming that PKC zeta Thr410 phosphorylation is essential for this process. PKC zeta activation by AMPK is isoform specific, as small interfering RNA targeting the alpha1 but not the alpha2 catalytic subunit prevented PKC zeta activation. Accordingly, we provide the first evidence that hypoxia-generated mitochondrial ROS lead to the activation of the AMPK alpha1 isoform, which binds and directly phosphorylates PKC zeta at Thr410, thereby promoting Na,K-ATPase endocytosis.

Activating transcription factor 4 and CCAAT/enhancer-binding protein-beta negatively regulate the mammalian target of rapamycin via Redd1 expression in response to oxidative and endoplasmic reticulum stress

Regulation of mRNA translation in mammalian cells involves the coordinated control of mammalian target of rapamycin (mTOR) signaling. At present, limited information is available on the potential relevance of mTOR regulation, although translation inhibition during oxidative and endoplasmic reticulum (ER) stress is clearly important. In this study, we show that activating transcription factor 4 (ATF4) and CCAAT/enhancer-binding protein-beta (C/EBP-beta) negatively regulate mTOR via Redd1 expression in response to oxidative and ER stress. Oxidative and ER stress conditions induce rapid and significant activation of ATF4 downstream of eIF2alpha phosphorylation, which is responsible for Redd1 expression. In our experiment, overexpression of ATF4 was associated with reduced mTOR activity via Redd1 expression, whereas suppression of ATF4 levels with small interfering RNA led to the recovery of decreased mTOR activity mediated by downregulation of Redd1 during oxidative and ER stress. We additionally identified Redd1 as a downstream effector of C/EBP-beta stimulated by ATF4 activated under the stress conditions examined. RNA interference studies provided further evidence of the requirement of C/EBP-beta for Redd1 expression. We conclude that the Redd1 gene is transactivated by the ATF4 and C/EBP family of transcription factors, leading to mTOR inhibition in response to oxidative and ER stress.

The Rheb-mTOR pathway is upregulated in reactive astrocytes of the injured spinal cord

Astrocytes in the CNS respond to tissue damage by becoming reactive. They migrate, undergo hypertrophy, and form a glial scar that inhibits axon regeneration. Therefore, limiting astrocytic responses represents a potential therapeutic strategy to improve functional recovery. It was recently shown that the epidermal growth factor (EGF) receptor is upregulated in astrocytes after injury and promotes their transformation into reactive astrocytes. Furthermore, EGF receptor inhibitors were shown to enhance axon regeneration in the injured optic nerve and promote recovery after spinal cord injury. However, the signaling pathways involved were not elucidated. Here we show that in cultures of adult spinal cord astrocytes EGF activates the mTOR pathway, a key regulator of astrocyte physiology. This occurs through Akt-mediated phosphorylation of the GTPase-activating protein Tuberin, which inhibits Tuberin's ability to inactivate the small GTPase Rheb. Indeed, we found that Rheb is required for EGF-dependent mTOR activation in spinal cord astrocytes, whereas the Ras-MAP kinase pathway does not appear to be involved. Moreover, astrocyte growth and EGF-dependent chemoattraction were inhibited by the mTOR-selective drug rapamycin. We also detected elevated levels of activated EGF receptor and mTOR signaling in reactive astrocytes in vivo in an ischemic model of spinal cord injury. Furthermore, increased Rheb expression likely contributes to mTOR activation in the injured spinal cord. Interestingly, injured rats treated with rapamycin showed reduced signs of reactive gliosis, suggesting that rapamycin could be used to harness astrocytic responses in the damaged nervous system to promote an environment more permissive to axon regeneration.

Comparative proteome analysis using amine-reactive isobaric tagging reagents coupled with 2D LC/MS/MS in 3T3-L1 adipocytes following hypoxia or normoxia

Hypoxia during the expansion of adipocytes is known to contribute both to the secretion of multiple inflammation-related adipokines as well as to obesity. We therefore investigated the nature of protein changes occurring in adipocytes during hypoxia by observation of the intracellular proteins that are expressed in 3T3-L1 adipocytes. Lysates were utilized for quantitative proteome analysis using isobaric tags for relative and absolute quantitation (iTRAQ) combined with peptide separation by multi-dimensional liquid chromatography. Antioxidants and elongation factors, as well as glycolytic enzymes were increased in hypoxic adipocytes. These changes were supported by similar changes suggested by real-time PCR. The proteins showing changes are all potential targets for revering the mechanism behind the phenomenon of induction of obese adipocytes by hypoxia. This study can therefore aid in defining the proteomic changes that occur in adipocytes in response to oxygen stress, and can further characterize adipocyte metabolism and adaptation to low oxygen conditions.

Inhibition of 4E-BP1 sensitizes U87 glioblastoma xenograft tumors to irradiation by decreasing hypoxia tolerance

PURPOSE

Eukaryotic initiation factor 4E (eIF4E) is an essential rate-limiting factor for cap-dependent translation in eukaryotic cells. Elevated eIF4E activity is common in many human tumors and is associated with disease progression. The growth-promoting effects of eIF4E are in turn negatively regulated by 4E-BP1. However, although 4E-BP1 harbors anti-growth activity, its expression is paradoxically elevated in some tumors. The aim of this study was to investigate the functional role of 4E-BP1 in the context of solid tumors.

METHODS AND MATERIALS

In vitro and in vivo growth properties, hypoxia tolerance, and response to radiation were assessed for HeLa and U87 cells, after stable expression of shRNA specific for 4E-BP1.

RESULTS

We found that loss of 4E-BP1 expression did not significantly alter in vitro growth but did accelerate the growth of U87 tumor xenografts, consistent with the growth-promoting function of deregulated eIF4E. However, cells lacking 4E-BP1 were significantly more sensitive to hypoxia-induced cell death in vitro. Furthermore, 4E-BP1 knockdown cells produced tumors more sensitive to radiation because of a reduction in the viable fraction of radioresistant hypoxic cells. Decreased hypoxia tolerance in the 4E-BP1 knockdown tumors was evident by increased cleaved caspase-3 levels and was associated with a reduction in adenosine triphosphate (ATP).

CONCLUSIONS

Our results suggest that although tumors often demonstrate increases in cap-dependent translation, regulation of this activity is required to facilitate energy conservation, hypoxia tolerance, and tumor radioresistance. Furthermore, we suggest that targeting translational control may be an effective way to target hypoxic cells and radioresistance in metabolically hyperactive tumors.

Tumor suppressors and cell metabolism: a recipe for cancer growth

Growing tumors face two major metabolic challenges-how to meet the bioenergetic and biosynthetic demands of increased cell proliferation, and how to survive environmental fluctuations in external nutrient and oxygen availability when tumor growth outpaces the delivery capabilities of the existing vasculature. Cancer cells display dramatically altered metabolic circuitry that appears to directly result from the oncogenic mutations selected during the tumorigenic process. An emerging theme in cancer biology is that many of the genes that can initiate tumorigenesis are intricately linked to metabolic regulation. In turn, it appears that a number of well-established tumor suppressors play critical roles in suppressing growth and/or proliferation when intracellular supplies of essential metabolites become reduced. In this review, we consider the potential role of tumor suppressors as metabolic regulators.

Cellular oxygen sensing, signalling and how to survive translational arrest in hypoxia

Hypoxia is a consequence of inadequate oxygen availability. At the cellular level, lowered oxygen concentration activates signal cascades including numerous receptors, ion channels, second messengers, as well as several protein kinases and phosphatases. This, in turn, activates trans-factors like transcription factors, RNA-binding proteins and miRNAs, mediating an alteration in gene expression control. Each cell type has its unique constellation of oxygen sensors, couplers and effectors that determine the activation and predominance of several independent hypoxia-sensitive pathways. Hence, altered gene expression patterns in hypoxia result from a complex regulatory network with multiple divergences and convergences. Although hundreds of genes are activated by transcriptional control in hypoxia, metabolic rate depression, as a consequence of reduced ATP level, causes inhibition of mRNA translation. In a multi-phase response to hypoxia, global protein synthesis is suppressed, mainly by phosphorylation of eIF2-alpha by PERK and inhibition of mTOR, causing suppression of 5'-cap-dependent mRNA translation. Growing evidence suggests that mRNAs undergo sorting at stress granules, which determines the fate of mRNA as to whether being translated, stored, or degraded. Data indicate that translation is suppressed only at 'free' polysomes, but is active at subsets of membrane-bound ribosomes. The recruitment of specific mRNAs into subcellular compartments seems to be crucial for local mRNA translation in prolonged hypoxia. Furthermore, ribosomes themselves may play a significant role in targeting mRNAs for translation. This review summarizes the multiple facets of the cellular adaptation to hypoxia observed in mammals.

Inhibition of oxygen sensors as a therapeutic strategy for ischaemic and inflammatory disease

Cells in the human body need oxygen to function and survive, and severe deprivation of oxygen, as occurs in ischaemic heart disease and stroke, is a major cause of mortality. Nevertheless, other organisms, such as the fossorial mole rat or diving seals, have acquired the ability to survive in conditions of limited oxygen supply. Hypoxia tolerance also allows the heart to survive chronic oxygen shortage, and ischaemic preconditioning protects tissues against lethal hypoxia. The recent discovery of a new family of oxygen sensors--including prolyl hydroxylase domain-containing proteins 1-3 (PHD1-3)--has yielded exciting novel insights into how cells sense oxygen and keep oxygen supply and consumption in balance. Advances in understanding of the role of these oxygen sensors in hypoxia tolerance, ischaemic preconditioning and inflammation are creating new opportunities for pharmacological interventions for ischaemic and inflammatory diseases.

Small-molecule inhibitors of HIF-2a translation link its 5'UTR iron-responsive element to oxygen sensing

Cells transiently adapt to hypoxia by globally decreasing protein translation. However, specific proteins needed to respond to hypoxia evade this translational repression. The mechanisms of this phenomenon remain unclear. We screened for and identified small molecules that selectively decrease HIF-2a translation in an mTOR-independent manner, by enhancing the binding of Iron-Regulatory Protein 1 (IRP1) to a recently reported iron-responsive element (IRE) within the 5'-untranslated region (UTR) of the HIF-2a message. Knocking down the expression of IRP1 by shRNA abolished the effect of the compounds. Hypoxia derepresses HIF-2a translation by disrupting the IRP1-HIF-2a IRE interaction. Thus, this chemical genetic analysis describes a molecular mechanism by which translation of the HIF-2a message is maintained during conditions of cellular hypoxia through inhibition of IRP-1-dependent repression. It also provides the chemical tools for studying this phenomenon.

Cytoplasmic polyadenylation-element-binding protein (CPEB)1 and 2 bind to the HIF-1alpha mRNA 3'-UTR and modulate HIF-1alpha protein expression

The heterodimeric HIF (hypoxia-inducible factor)-1 is a transcriptional master regulator of several genes involved in mammalian oxygen homoeostasis. Besides the well described regulation of the HIF-1alpha subunit via hydroxylation-mediated protein stability in hypoxia, there are several indications of an additional translational control of the HIF-1alpha mRNA, especially after growth factor stimulation. We identified an interaction of CPEB (cytoplasmic polyadenylation-element-binding protein) 1 and CPEB2 with the 3'-UTR (untranslated region) of HIF-1alpha mRNA. Overexpression of CPEB1 and CPEB2 affected HIF-1alpha protein levels mediated by the 3'-UTR of HIF-1alpha mRNA. Stimulation of neuroblastoma SK-N-MC cells with insulin and thus activation of endogenous CPEBs increased the expression of a luciferase reporter gene fused to the 3'-UTR of HIF-1alpha as well as endogenous HIF-1alpha protein levels. This could be abrogated by treating the cells with CPEB1 or CPEB2 siRNAs (short interfering RNAs). Injection of HIF-1alpha cRNA into Xenopus oocytes verified the elongation of the poly(A)+ (polyadenylated) tail by cytoplasmic polyadenylation. Thus CPEB1 and CPEB2 are involved in the regulation of HIF-1alpha following insulin stimulation.

Wound healing essentials: let there be oxygen

The state of wound oxygenation is a key determinant of healing outcomes. From a diagnostic standpoint, measurements of wound oxygenation are commonly used to guide treatment planning such as amputation decision. In preventive applications, optimizing wound perfusion and providing supplemental O(2) in the perioperative period reduces the incidence of postoperative infections. Correction of wound pO(2) may, by itself, trigger some healing responses. Importantly, approaches to correct wound pO(2) favorably influence outcomes of other therapies such as responsiveness to growth factors and acceptance of grafts. Chronic ischemic wounds are essentially hypoxic. Primarily based on the tumor literature, hypoxia is generally viewed as being angiogenic. This is true with the condition that hypoxia be acute and mild to modest in magnitude. Extreme near-anoxic hypoxia, as commonly noted in problem wounds, is not compatible with tissue repair. Adequate wound tissue oxygenation is required but may not be sufficient to favorably influence healing outcomes. Success in wound care may be improved by a personalized health care approach. The key lies in our ability to specifically identify the key limitations of a given wound and in developing a multifaceted strategy to specifically address those limitations. In considering approaches to oxygenate the wound tissue it is important to recognize that both too little as well as too much may impede the healing process. Oxygen dosing based on the specific need of a wound therefore seems prudent. Therapeutic approaches targeting the oxygen sensing and redox signaling pathways are promising.

A stress-responsive RNA switch regulates VEGFA expression

Ligand binding to structural elements in noncoding regions of mRNA modulates gene expression1,2. Ligands such as free metabolites or other small molecules directly bind and induce conformational changes in regulatory RNA elements known as riboswitches1-4. Other types of RNA switches are activated by complexed metabolites, e.g., RNA-ligated metabolites such as aminoacyl-charged tRNA in the T-box system5, or protein-bound metabolites in the glucose- or amino acid-stimulated terminator-antiterminator systems6,7. All of these switch types are found in bacteria, fungi, and plants8-10. Here, we report an RNA switch in human vascular endothelial growth factor-A (VEGF) mRNA 3’UTR that integrates signals from interferon (IFN)-γ and hypoxia to regulate VEGF translation in myeloid cells. Analogous to riboswitches, the VEGF 3’UTR undergoes a binary conformational change in response to environmental signals. However, the VEGF 3’UTR switch is metabolite-independent, and the conformational change is dictated by mutually exclusive, stimulus-dependent binding of proteins, namely, the IFN-γ-activated inhibitor of translation (GAIT) complex11,12 and heterogenous nuclear ribonucleoprotein (hnRNP) L. We speculate the VEGF switch represents the founding member of a family of signal-mediated, protein-dependent RNA switches that evolved to regulate gene expression in multicellular animals where precise integration of disparate inputs may be more important than rapidity of response.

Hypoxia-induced energy stress inhibits the mTOR pathway by activating an AMPK/REDD1 signaling axis in head and neck squamous cell carcinoma

The mammalian target of rapamycin (mTOR) signaling network is frequently hyperactivated in patients with head and neck squamous cell carcinoma (HNSCC). Recent studies suggest that hypoxia, a common microenvironmental stress found in tumors, blocks this mitogenic pathway. Here, we demonstrate that in HNSCC cell lines, the expression of the phosphorylated forms of the mTOR downstream targets S6 kinase and S6 (pS6) decreased after hypoxia. These events were associated with a marked up-regulation of the regulated in development and DNA damage 1 (REDD1), a recently characterized hypoxia-induced protein that negatively controls mTOR activity. Conversely, pS6 levels were retained under hypoxia in REDD1 knock-down cells and in HNSCC cells lacking endogenous REDD1 expression. Furthermore, we observed that prolonged hypoxia induced an energy-depleting response as evidenced by decreased cellular ATP levels and AMP-activated protein kinase (AMPK) activation. Interestingly, AMPK inhibition before prolonged hypoxia prevented REDD1 expression, thereby sustaining mTOR activity. These results suggest a novel mechanism by which AMPK activation after hypoxia-induced energy stress may be crucial in regulating REDD1 expression to control the mTOR pathway in HNSCC. Furthermore, we found that, in some HNSCC cells, the reduced mTOR activity in response to hypoxia through AMPK/REDD1 was deregulated, which hence might contribute to the persistent activation of the mTOR pathway in this cancer type.

Preferential cytotoxicity of bortezomib toward hypoxic tumor cells via overactivation of endoplasmic reticulum stress pathways

Hypoxia is a dynamic feature of the tumor microenvironment that contributes to drug resistance and cancer progression. We previously showed that components of the unfolded protein response (UPR), elicited by endoplasmic reticulum (ER) stress, are also activated by hypoxia in vitro and in vivo animal and human patient tumors. Here, we report that ER stressors, such as thapsigargin or the clinically used proteasome inhibitor bortezomib, exhibit significantly higher cytotoxicity toward hypoxic compared with normoxic tumor cells, which is accompanied by enhanced activation of UPR effectors in vitro and UPR reporter activity in vivo. Treatment of cells with the translation inhibitor cycloheximide, which relieves ER load, ameliorated this enhanced cytotoxicity, indicating that the increased cytotoxicity is ER stress-dependent. The mode of cell death was cell type-dependent, because DLD1 colorectal carcinoma cells exhibited enhanced apoptosis, whereas HeLa cervical carcinoma cells activated autophagy, blocked apoptosis, and eventually led to necrosis. Pharmacologic or genetic ablation of autophagy increased the levels of apoptosis. These results show that hypoxic tumor cells, which are generally more resistant to genotoxic agents, are hypersensitive to proteasome inhibitors and suggest that combining bortezomib with therapies that target the normoxic fraction of human tumors can lead to more effective tumor control.

Emerging role of adipose tissue hypoxia in obesity and insulin resistance

Recent studies consistently support a hypoxia response in the adipose tissue in obese animals. The observations have led to the formation of an exciting concept, adipose tissue hypoxia (ATH), in the understanding of major disorders associated with obesity. ATH may provide cellular mechanisms for chronic inflammation, macrophage infiltration, adiponectin reduction, leptin elevation, adipocyte death, endoplasmic reticulum stress and mitochondrial dysfunction in white adipose tissue in obesity. The concept suggests that inhibition of adipogenesis and triglyceride synthesis by hypoxia may be a new mechanism for elevated free fatty acids in the circulation in obesity. ATH may represent a unified cellular mechanism for a variety of metabolic disorders and insulin resistance in patients with metabolic syndrome. It suggests a new mechanism of pathogenesis of insulin resistance and inflammation in obstructive sleep apnea. In addition, it may help us to understand the beneficial effects of caloric restriction, physical exercise and angiotensin II inhibitors in the improvement of insulin sensitivity. In this review article, literatures are reviewed to summarize the evidence and possible cellular mechanisms of ATH. The directions and road blocks in the future studies are analyzed.

Selective mRNA translation coordinates energetic and metabolic adjustments to cellular oxygen deprivation and reoxygenation in Arabidopsis thaliana

Cellular oxygen deprivation (hypoxia/anoxia) requires an acclimation response that enables survival during an energy crisis. To gain new insights into the processes that facilitate the endurance of transient oxygen deprivation, the dynamics of the mRNA translation state and metabolites were quantitatively monitored in Arabidopsis thaliana seedlings exposed to a short (2 h) or prolonged (9 h) period of oxygen and carbon dioxide deprivation and following 1 h of re-aeration. Hypoxia stress and reoxygenation promoted adjustments in the levels of polyribosomes (polysomes) that were highly coordinated with cellular ATP content. A quantitative comparison of steady-state and polysomal mRNA populations revealed that over half of the cellular mRNAs were restricted from polysome complexes during the stress, with little or no change in abundance. This selective repression of translation was rapidly reversed upon reoxygenation. Comparison of the adjustment in gene transcripts and metabolites demonstrated that profiling of polysomal mRNAs strongly augments the prediction of cellular processes that are altered during cellular oxygen deprivation. The selective translation of a subset of mRNAs promotes the conservation of ATP and facilitates the transition to anaerobic metabolism during low-oxygen stress.

The role of mTOR in the adaptation and failure of beta-cells in type 2 diabetes

Mammalian target of rapamycin (mTOR) is an important nutrient sensor that plays a critical role in cellular metabolism, growth, proliferation and apoptosis and in the cellular response to oxidative stress. In addition, mTOR-raptor complex, also called mammalian target of rapamycin complex 1 (mTORC1), generates an inhibitory feedback loop on insulin receptor substrate proteins. It was suggested that nutrient overload leads to insulin/insulin-like growth factor 1 resistance in peripheral insulin-responsive tissues and in the beta-cells through sustained activation of mTORC1. In this review, we summarize the literature on the regulation and function of mTOR, its role in the organism's response to nutrients and its potential impact on lifespan, insulin resistance and the metabolic adaptation to hyperglycaemia in type 2 diabetes. We also propose a hypothesis based on data in the literature as well as data generated in our laboratory, which assigns a central positive role to mTOR in the maintenance of beta-cell function and mass in the diabetic environment.

Hypoxia signalling through mTOR and the unfolded protein response in cancer

Hypoxia occurs in the majority of tumours, promoting angiogenesis, metastasis and resistance to therapy. Responses to hypoxia are orchestrated in part through activation of the hypoxia-inducible factor family of transcription factors (HIFs). Recently, two additional O(2)-sensitive signalling pathways have also been implicated: signalling through the mammalian target of rapamycin (mTOR) kinase and signalling through activation of the unfolded protein response (UPR). Although they are activated independently, growing evidence suggests that HIF-, mTOR- and UPR-dependent responses to hypoxia act in an integrated way, influencing each other and common downstream pathways that affect gene expression, metabolism, cell survival, tumorigenesis and tumour growth.

Hypoxia-activated autophagy accelerates degradation of SQSTM1/p62

Sequestosome 1 (SQSTM1/p62) is a multifunctional protein involved in signal transduction, protein degradation and cell transformation. Hypoxia is a common feature of solid tumours that promotes cancer progression. Here, we report that p62 is downregulated in hypoxia in carcinoma cells and that the expression is rapidly restored in response to reoxygenation. The hypoxic p62 downregulation did not occur at the mRNA level and was independent of the hypoxic signal mediators hypoxia-inducible factor (HIF) and von Hippel-Lindau tumour suppressor protein as well as the activity of HIF-prolyl hydroxylases and was not mediated by proteosomal destruction. Autophagy was activated in hypoxia and was required for p62 degradation. The hypoxic degradation of p62 was blocked by autophagy inhibitors as well as by the attenuation of Atg8/LC3 expression. Downregulation of p62 was required for hypoxic extracellular regulated kinase (ERK)-1/2 phosphorylation. Attenuation of p62 in normoxia activated and forced expression of p62 in hypoxia blocked the activation of ERK-1/2. The results demonstrate that hypoxic activation of autophagy induces clearance of p62 protein and implies a role for p62 in the regulation of hypoxic cancer cell survival responses.

Discovery, identification, and characterization of candidate pharmacodynamic markers of methionine aminopeptidase-2 inhibition

The catalytic activity of methionine aminopeptidase-2 (MetAP2) has been pharmacologically linked to cell growth, angiogenesis, and tumor progression, making this an attractive target for cancer therapy. An assay for monitoring specific protein changes in response to MetAP2 inhibition, allowing pharmacokinetic (PK)/pharmacodynamic (PD) models to be established, could dramatically improve clinical decision-making. Candidate MetAP2-specific protein substrates were discovered from undigested cell culture-derived proteomes by MALDI-/SELDI-MS profiling and a biochemical method using (35)S-Met labeled protein lysates. Substrates were identified either as intact proteins by FT-ICR-MS or applying in-gel protease digestions followed by LC-MS/MS. The combination of these approaches led to the discovery of novel MetAP2-specific substrates including thioredoxin-1 (Trx-1), SH3 binding glutamic acid rich-like protein (SH3BGRL), and eukaryotic elongation factor-2 (eEF2). These studies also confirmed glyceraldehye 3-phosphate dehydrogenase (GAPDH) and cyclophillin A (CypA) as MetAP2 substrates. Additional data in support of these proteins as MetAP2-specific substrates were provided by in vitro MetAP1/MetAP2 enzyme assays with the corresponding N-terminal derived peptides and 1D/2D Western analyses of cellular and tissue lysates. FT-ICR-MS characterization of all intact species of the 18 kDa substrate, CypA, enabled a SELDI-MS cell-based assay to be developed for correlating N-terminal processing and inhibition of proliferation. The MetAP2-specific protein substrates discovered in this study have diverse properties that should facilitate the development of reagents for testing in preclinical and clinical environments.

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.

Activation of Hsp90-eNOS and increased NO generation attenuate respiration of hypoxia-treated endothelial cells

Hypoxia induces various adoptive signaling in cells that can cause several physiological changes. In the present work, we have observed that exposure of bovine aortic endothelial cells (BAECs) to extreme hypoxia (1-5% O(2)) attenuates cellular respiration by a mechanism involving heat shock protein 90 (Hsp90) and endothelial nitric oxide (NO) synthase (eNOS), so that the cells are conditioned to consume less oxygen and survive in prolonged hypoxic conditions. BAECs, exposed to 1% O(2), showed a reduced respiration compared with 21% O(2)-maintained cells. Western blot analysis showed an increase in the association of Hsp90-eNOS and enhanced NO generation on hypoxia exposure, whereas there was no significant accumulation of hypoxia-inducible factor-1alpha (HIF-1alpha). The addition of inhibitors of Hsp90, phosphatidylinositol 3-kinase, and NOS significantly alleviated this hypoxia-induced attenuation of respiration. Thus we conclude that hypoxia-induced excess NO and its derivatives such as ONOO(-) cause inhibition of the electron transport chain and attenuate O(2) demand, leading to cell survival at extreme hypoxia. More importantly, such an attenuation is found to be independent of HIF-1alpha, which is otherwise thought to be the key regulator of respiration in hypoxia-exposed cells, through a nonphosphorylative glycolytic pathway. The present mechanistic insight will be helpful to understand the difference in the magnitude of endothelial dysfunction.

Effects of 4E-BP1 expression on hypoxic cell cycle inhibition and tumor cell proliferation and survival

Elevated activity of the eIF4F complex, which controls initiation of cap-dependent mRNA translation, has been linked to cancer progression. eIF4E recruitment to eIF4F is the rate limiting step of complex assembly and is regulated by eIF4E-Binding Proteins (4E-BPs). When stimulated, the mammalian Target of Rapamycin complex 1 (mTORC1) phosphorylates 4E-BP1, which then releases eIF4E. Hypoxia inhibits mTORC1 activity and therefore cap-dependent protein synthesis. To establish a novel genetic test of the role of eIF4F activity in regulating cell division and viability within hypoxic tumor microenvironments, we generated shRNA mediated 4E-BP1 knock-down in Rh30 rhabdomyosarcoma cells. 4E-BP1 knock-down relieved hypoxia-mediated inhibition of cycle progression in vitro and was correlated with increased expression of cyclin D1 and c-Myc. Xenograft tumors derived from these cells also displayed enhanced expression of cyclin D1 and c-Myc along with antiapoptotic genes encoding Bcl-x(L), and XIAP, and failed to develop the extensive necrotic zones and edema observed in control tumors. Surprisingly, 4E-BP1 knock-down also leads to a dramatic increase in aberrant mitoses in vivo and enhanced expression of Mad2 and securin. Thus, reduced expression of the negative regulator of eIF4E has significant effects on tumor development, and is associated with enhanced cell proliferation and survival.

Hypoxic reactive oxygen species regulate the integrated stress response and cell survival

Under hypoxic conditions, cells suppress energy-intensive mRNA translation by modulating the mammalian target of rapamycin (mTOR) and pancreatic eIF2α kinase (PERK) pathways. Much is known about hypoxic inhibition of mTOR activity; however, the cellular processes activating PERK remain unclear. Since hypoxia is known to increase intracellular reactive oxygen species (ROS), we hypothesized that hypoxic ROS regulate mTOR and PERK to control mRNA translation and cell survival. Our data indicate that although exogenous ROS inhibit mTOR, eIF2α, and eEF2, mTOR and eEF2 were largely refractory to ROS generated under moderate hypoxia (0.5% O₂). In direct contrast, the PERK/eIF2α/ATF4 integrated stress response (ISR) was activated by hypoxic ROS and contributed to global protein synthesis inhibition and adaptive ATF4-mediated gene expression. The ISR as well as exogenous growth factors were critical for cell viability during extended hypoxia, since ISR inhibition decreased the viability of cells deprived of O₂ and growth factors. Collectively, our data support an important role for ROS in hypoxic cell survival. Under conditions of moderate hypoxia, ROS induce the ISR, thereby promoting energy and redox homeostasis and enhancing cellular survival.

Lack of hypoxic response in uterine leiomyomas despite severe tissue hypoxia

Hypoxia is now established as a key factor influencing the pathophysiology of malignant growth. Among other effects, hypoxia modulates the expression of a multitude of genes through the induction of hypoxia-inducible transcription factors. This differential gene expression favors angiogenesis, cell survival, an invasive/metastatic phenotype, and resistance to anticancer therapies. Because benign tumors do not exhibit these traits, one might expect these entities to be neither hypoxic nor to induce the genetic hypoxia response program. To test this hypothesis, an investigation of the oxygenation status of 17 leiomyomas and 1 leiomyosarcoma of the uterus using polarographic needle electrodes (Eppendorf pO(2) sensor) and the expression of hypoxia-related markers in biopsy specimens of the same tumors was carried out. Marker expression in eight additional archival leiomyosarcomas was also assessed. Leiomyoma tissue was generally found to be severely hypoxic, with median oxygen (O(2)) partial pressure values ranging from 1 to 5 mm Hg. In contrast, none of the hypoxia-related markers hypoxia-inducible factor (HIF)-1alpha, HIF-2alpha, glucose transporter-1, or carbonic anhydrase IX were expressed in any leiomyoma. Larger intercapillary distances were correlated with a poorer oxygenation status. Conversely, the expression of hypoxia-related markers was abundant in the leiomyosarcomas and they also exhibited a high-turnover phenotype (significantly increased proliferation and apoptosis). Uterine leiomyoma might therefore represent a state of oxygen-limited proliferation. Malignancy in the same organ system is associated with growth and metabolism beyond tissue-inherent limitations leading to the induction of hypoxia-related markers, thereby contributing to a self-perpetuating aggressive phenotype.

Regulation of gene expression by hypoxia

Hypoxia induces profound changes in the cellular gene expression profile. The discovery of a major transcription factor family activated by hypoxia, HIF (hypoxia-inducible factor), and the factors that contribute to HIF regulation have greatly enhanced our knowledge of the molecular aspects of the hypoxic response. However, in addition to HIF, other transcription factors and cellular pathways are activated by exposure to reduced oxygen. In the present review, we summarize the current knowledge of how additional hypoxia-responsive transcription factors integrate with HIF and how other cellular pathways such as chromatin remodelling, translation regulation and microRNA induction, contribute to the co-ordinated cellular response observed following hypoxic stress.

Hypoxia induces resistance to 5-fluorouracil in oral cancer cells via G(1) phase cell cycle arrest

Malignant tumors are exposed to various levels of hypoxic condition in vivo. It has been known that tumor cells under hypoxia are resistant to chemotherapies. To clarify the mechanism of the hypoxia-induced chemoresistance, we evaluated the effects of hypoxia on the resistance of oral squamous cell carcinoma (OSCC) cell lines to 5-fluorouracil (5-FU). OSCC cells were divided to two groups by the proliferation activity under hypoxic condition; hypoxia-resistant (HR) and hypoxia-sensitive (HS) cells. Growth of HS cells were inhibited by hypoxia and introduced to G(1) arrest in cell cycle. 5-FU effect on HS cell viability was markedly reduced in hypoxic condition without an induction of chemoresistant related protein, P-glycoprotein. However, proliferation, cell cycle, and 5-FU sensitivity of HR cells were not affected by hypoxia. Hypoxia-inducible factor (HIF)-1alpha was induced by hypoxia in all OSCC cell lines, but diminished in HS cells within 48h. Expression of p21 and p27 was strongly augmented and CyclinD expression was reduced by hypoxia in HS cells. However, the expression of these proteins was constitutive in HR cells during 48h hypoxic culture. Phosphorylation of mammalian target of rapamycin (mTOR) was reduced by hypoxia in HS cells. From these findings, we concluded that HS OSCC cells acquire 5-FU resistance under hypoxia by G(1)/S transition through an upregulation of cell cycle inhibitors.

An RNA interference screen identifies a novel regulator of target of rapamycin that mediates hypoxia suppression of translation in Drosophila S2 cells

In addition to its central role in energy production, oxygen has pervasive regulatory actions. Hypoxia (oxygen limitation) triggers the shutdown of major cellular processes, including gene expression. We carried out a genome-wide RNA interference (RNAi) screen in Drosophila S2 cells for functions required to down-regulate translation during hypoxia. RNAi knockdown of specific genes allowed induction of a green fluorescent protein (GFP) reporter gene and continued protein synthesis during hypoxia. Among the identified genes, Tsc1 and Tsc2, which together form the tuberose sclerosis complex that negatively regulates target of rapamycin (TOR) kinase, gave an especially strong effect. This finding is consistent with the involvement of TOR in promoting translation. Another gene required for efficient inhibition of protein translation during hypoxia, the protein tyrosine phosphatase 61F (Ptp61F), down-regulates TOR activity under hypoxia. Lack of Ptp61F or Tsc2 improves cell survival under prolonged hypoxia in a TOR-dependent manner. Our results identify Ptp61F as a novel modulator of TOR activity and suggest that its function during hypoxia contributes to the down-regulation of protein synthesis.

Transformation, translation and TRAIL: an unexpected intersection

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a cytokine with roles in tumor surveillance and tolerance. TRAIL selectively induces apoptosis in many malignant but not normal cells but the underlying cause for spontaneous TRAIL sensitivity remains elusive. We propose a novel hypothesis that links TRAIL sensitivity to translational arrest following stresses that inactivate eukaryotic elongation factor 2 (EF2). Affected cells experience a reduction in apoptotic threshold because, due to their short half-lives, levels of anti-apoptotic proteins quickly drop off once translation elongation is inhibited leaving pro-apoptotic proteins unchallenged. This change in protein profile renders affected cells sensitive to TRAIL-mediated apoptosis and places EF2 into the role of a sensor for cellular damage.

The TSC1-TSC2 complex: a molecular switchboard controlling cell growth

TSC1 and TSC2 are the tumour-suppressor genes mutated in the tumour syndrome TSC (tuberous sclerosis complex). Their gene products form a complex that has become the focus of many signal transduction researchers. The TSC1-TSC2 (hamartin-tuberin) complex, through its GAP (GTPase-activating protein) activity towards the small G-protein Rheb (Ras homologue enriched in brain), is a critical negative regulator of mTORC1 (mammalian target of rapamycin complex 1). As mTORC1 activity controls anabolic processes to promote cell growth, it is exquisitely sensitive to alterations in cell growth conditions. Through numerous phosphorylation events, the TSC1-TSC2 complex has emerged as the sensor and integrator of these growth conditions, relaying signals from diverse cellular pathways to properly modulate mTORC1 activity. In the present review we focus on the molecular details of TSC1-TSC2 complex regulation and function as it relates to the control of Rheb and mTORC1.

Downregulation of osteopontin contributes to metastasis suppression by breast cancer metastasis suppressor 1

Breast cancer metastasis suppressor 1 (BRMS1) inhibits the ability of multiple human and murine cancer cell lines to metastasize to lymph nodes, bones and lungs. Comparison of mRNA expression in metastatic MDA-MB-435 human carcinoma cells (435) and metastasis-suppressed BRMS1 transfectants (435/BRMS1) showed a marked (>90%) reduction of osteopontin (OPN) mRNA and protein expression in BRMS1-overexpressing cells. OPN expression is associated with disease progression in patients, with higher levels of OPN produced by cancer cells associated with poorer patient survival. Furthermore, OPN has been suggested to promote survival of cancer cells in response to stress, although the mechanisms by which this may occur remain poorly understood. This study tested the hypothesis that re-expression of OPN in metastasis-suppressed 435/BRMS1 cells would reverse metastasis suppression and confer protection from stress-induced apoptosis. A stable pooled population of OPN overexpressing 435/BRMS1 cells was created (435/BRMS1/OPN). OPN re-expression did not affect in vitro cell growth rates; however, increased anchorage independent growth/survival and protection from hypoxia-induced apoptosis was observed (p < 0.05). In vivo, OPN re-expression in BRMS1 transfected cells did not affect in vivo primary tumor growth but did increase the incidence of spontaneous metastasis to lymph nodes and lungs in mice. These novel findings suggest that OPN downregulation by BRMS1 may be responsible, at least in part, for BRMS1-mediated metastasis suppression by sensitizing cancer cells to stress induced apoptosis. These studies clarify one mechanism by which BRMS1 can suppress metastasis.

Hypoxia induces microRNA miR-210 in vitro and in vivo ephrin-A3 and neuronal pentraxin 1 are potentially regulated by miR-210

Shortage of oxygen is one of the prime stress conditions in tissues. In this study, we looked for microRNAs expressed during hypoxia and showed that miR-210 expression was upregulated in response to hypoxia in vitro and in vivo. An active form of the HIF-1alpha induced the expression of miR-210, showing the involvement of the HIF-1 signaling pathway in miR-210 gene transcription. Furthermore, miR-210 was shown to bind to the predicted target sites of ephrin-A3 or neuronal pentraxin 1, causing repression in luciferase reporter activity. Contrary to the microRNA-mediated repression hypothesis, ephrin-A3 was expressed at very high levels in post-ischemic mouse hippocampus in vivo. Thus, the regulatory effects of miR-210 on its targets in vivo need to be further characterized.

AMPK phosphorylation of raptor mediates a metabolic checkpoint

AMPK is a highly conserved sensor of cellular energy status that is activated under conditions of low intracellular ATP. AMPK responds to energy stress by suppressing cell growth and biosynthetic processes, in part through its inhibition of the rapamycin-sensitive mTOR (mTORC1) pathway. AMPK phosphorylation of the TSC2 tumor suppressor contributes to suppression of mTORC1; however, TSC2-deficient cells remain responsive to energy stress. Using a proteomic and bioinformatics approach, we sought to identify additional substrates of AMPK that mediate its effects on growth control. We report here that AMPK directly phosphorylates the mTOR binding partner raptor on two well-conserved serine residues, and this phosphorylation induces 14-3-3 binding to raptor. The phosphorylation of raptor by AMPK is required for the inhibition of mTORC1 and cell-cycle arrest induced by energy stress. These findings uncover a conserved effector of AMPK that mediates its role as a metabolic checkpoint coordinating cell growth with energy status.

Hypoxia-induced signaling in the cardiovascular system

Low oxygen (O2) levels are a naturally occurring feature of embryonic development, adult physiology, and diseases such as those of the cardiovascular system. Although many responses to O2 deprivation are mediated by hypoxia-inducible factors (HIFs), researchers are finding a growing number of HIF-independent pathways that promote O2 conformance and hypoxia tolerance. Here, we describe HIF-independent responses and how they impact cardiovascular tissue homeostasis.

IGFBPs contribute to survival of pancreatic cancer cells under severely hypoxic conditions

Protein synthesis in general is suppressed under hypoxic conditions in both cancer cells and normal cells. In human pancreatic cancer AsPC-1 cells, which are resistant to low oxygen tension, protein synthesis at day 4 under hypoxia (1% O(2)) was about 20% of that under normoxia. Secretion of some proteins actually increased in spite of a general reduction in protein synthesis; two of these proteins were insulin-like growth factor binding protein-1 (IGFBP-1) and IGFBP-3. Hypoxia also induced transcription of multiple IGFBPs. The IGFBPs contributed to reduced IGF signaling and to the survival of cancer cells under conditions of low oxygen tension.

Hypoxia-mediated selective mRNA translation by an internal ribosome entry site-independent mechanism

Although it is advantageous for hypoxic cells to inhibit protein synthesis and conserve energy, it is also important to translate mRNAs critical for adaptive responses to hypoxic stress. Because internal ribosome entry sites (IRES) have been postulated to mediate this preferential synthesis, we analyzed the 5 '-untranslated regions from a panel of stress-regulated mRNAs for m(7)GTP cap-independent translation and identified putative IRES elements in encephalomyocarditis virus, vascular endothelial growth factor, hypoxia-inducible factors (HIFs) 1alpha and 2alpha, glucose transporter-like protein 1, p57(Kip2), La, BiP, and triose phosphate isomerase transcripts. However, when capped and polyadenylated dicistronic RNAs were synthesized in vitro and transfected into cells, cellular IRES-mediated translation accounted for less than 1% that of the level of cap-dependent translation. Moreover, hypoxic stress failed to activate cap-independent synthesis, indicating that it is unlikely that this is the primary mechanism for the maintenance of the translation of these mRNAs under low O(2). Furthermore, although HIF-1alpha is frequently cited as an example of an mRNA that is preferentially translated, we demonstrate that under different levels and durations of hypoxic stress, changes in newly synthesized HIF-1alpha and beta-actin protein levels mirror alterations in corresponding mRNA abundance. In addition, our data suggest that cyclin-dependent kinase inhibitor p57(Kip2) and vascular endothelial growth factor mRNAs are selectively translated by an IRES-independent mechanism under hypoxic stress.