Hypoxia-induced energy stress regulates mRNA translation and cell growth

Hypoxia modulates HLA-G gene expression in tumor cells

Human leukocyte antigen G (HLA-G) molecules are expressed in cytotrophoblasts and play a key role in maintaining immune tolerance at the maternal-fetal interface. HLA-G expression was also reported in inflammatory diseases, organ transplantation, and malignant tumors. The regulatory mechanisms of HLA-G gene expression differ from those of classical HLA class I genes and are still only partially elucidated. Focusing on tumor cells, we previously demonstrated a tight control of HLA-G gene expression by cis-acting epigenetic mechanisms. In the present study, we hypothesized that these processes are dependent of microenvironment conditions, and more particularly, stress conditions like hypoxia. Cellular response to hypoxia is mainly driven by a key transcription factor, hypoxia-inducible factor 1 (HIF-1), and other factors, such as NF-kappaB, involved in angiogenesis and cell survival. Here we confirmed the influence of hypoxia on HLA-G gene induction in the HLA-G-negative M8 melanoma cell line. Moreover, upon treatment with the hypoxia-mimicking desferrioxamine, we demonstrated a decrease in HLA-G gene expression in melanoma FON and choriocarcinoma JEG-3 cell lines, both expressing constitutively HLA-G. Finally, we demonstrated for the first time that the modulation of HLA-G gene expression is dependent of HIF-1 stabilization and thus might be relevant for the control of HLA-G gene expression in hypoxic tumors.

The mTOR pathway in the control of protein synthesis

Signaling through mammalian target of rapamycin (mTOR) is activated by amino acids, insulin, and growth factors, and impaired by nutrient or energy deficiency. mTOR plays key roles in cell physiology. mTOR regulates numerous components involved in protein synthesis, including initiation and elongation factors, and the biogenesis of ribosomes themselves.

mTOR and cancer therapy

Proteins regulating the mammalian target of rapamycin (mTOR), as well as some of the targets of the mTOR kinase, are overexpressed or mutated in cancer. Rapamycin, the naturally occurring inhibitor of mTOR, along with a number of recently developed rapamycin analogs (rapalogs) consisting of synthetically derived compounds containing minor chemical modifications to the parent structure, inhibit the growth of cell lines derived from multiple tumor types in vitro, and tumor models in vivo. Results from clinical trials indicate that the rapalogs may be useful for the treatment of subsets of certain types of cancer. The sporadic responses from the initial clinical trials, based on the hypothesis of general translation inhibition of cancer cells are now beginning to be understood owing to a more complete understanding of the dynamics of mTOR regulation and the function of mTOR in the tumor microenvironment. This review will summarize the preclinical and clinical data and recent discoveries of the function of mTOR in cancer and growth regulation.

Stress and mTORture signaling

The TOR (target of rapamycin) pathway is an evolutionarily conserved signaling module regulating cell growth (accumulation of mass) in response to a variety of environmental cues such as nutrient availability, hypoxia, DNA damage and osmotic stress. Its pivotal role in cellular and organismal homeostasis is reflected in the fact that unrestrained signaling activity in mammals is associated with the occurrence of disease states including inflammation, cancer and diabetes. The existence of TOR homologs in unicellular organisms whose growth is affected by environmental factors, such as temperature, nutrients and osmolarity, suggests an ancient role for the TOR signaling network in the surveillance of stress conditions. Here, we will summarize recent advances in the TOR signaling field with special emphasis on how stress conditions impinge on insulin/insulin-like growth factor signaling/TOR signaling.

Glucose metabolism and cancer

The first identified biochemical hallmark of tumor cells was a shift in glucose metabolism from oxidative phosphorylation to aerobic glycolysis. We now know that much of this metabolic conversion is controlled by specific transcriptional programs. Recent studies suggest that activation of the hypoxia-inducible factor (HIF) is a common consequence of a wide variety of mutations underlying human cancer. HIF stimulates expression of glycolytic enzymes and decreases reliance on mitochondrial oxidative phosphorylation in tumor cells, which occurs even under aerobic conditions. In addition, recent efforts have also connected the master metabolic regulator AMP-activated protein kinase (AMPK) to several human tumor suppressors. Several promising therapeutic strategies based on modulation of AMPK, HIF and other metabolic targets have been proposed to exploit the addiction of tumor cells to increased glucose uptake and glycolysis.

Perk-dependent translational regulation promotes tumor cell adaptation and angiogenesis in response to hypoxic stress

It has been well established that the tumor microenvironment can promote tumor cell adaptation and survival. However, the mechanisms that influence malignant progression have not been clearly elucidated. We have previously demonstrated that cells cultured under hypoxic/anoxic conditions and transformed cells in hypoxic areas of tumors activate a translational control program known as the integrated stress response (ISR). Here, we show that tumors derived from K-Ras-transformed Perk(-/-) mouse embryonic fibroblasts (MEFs) are smaller and exhibit less angiogenesis than tumors with an intact ISR. Furthermore, Perk promotes a tumor microenvironment that favors the formation of functional microvessels. These observations were corroborated by a microarray analysis of polysome-bound RNA in aerobic and hypoxic Perk(+/+) and Perk(-/-) MEFs. This analysis revealed that a subset of proangiogenic transcripts is preferentially translated in a Perk-dependent manner; these transcripts include VCIP, an adhesion molecule that promotes cellular adhesion, integrin binding, and capillary morphogenesis. Taken with the concomitant Perk-dependent translational induction of additional proangiogenic genes identified by our microarray analysis, this study suggests that Perk plays a role in tumor cell adaptation to hypoxic stress by regulating the translation of angiogenic factors necessary for the development of functional microvessels and further supports the contention that the Perk pathway could be an attractive target for novel antitumor modalities.

PML inhibits HIF-1alpha translation and neoangiogenesis through repression of mTOR

Loss of the promyelocytic leukaemia (PML) tumour suppressor has been observed in several human cancers. The tumour-suppressive function of PML has been attributed to its ability to induce growth arrest, cellular senescence and apoptosis. Here we identify PML as a critical inhibitor of neoangiogenesis (the formation of new blood vessels) in vivo, in both ischaemic and neoplastic conditions, through the control of protein translation. We demonstrate that in hypoxic conditions PML acts as a negative regulator of the synthesis rate of hypoxia-inducible factor 1alpha (HIF-1alpha) by repressing mammalian target of rapamycin (mTOR). PML physically interacts with mTOR and negatively regulates its association with the small GTPase Rheb by favouring mTOR nuclear accumulation. Notably, Pml-/- cells and tumours display higher sensitivity both in vitro and in vivo to growth inhibition by rapamycin, and lack of PML inversely correlates with phosphorylation of ribosomal protein S6 and tumour angiogenesis in mouse and human tumours. Thus, our findings identify PML as a novel suppressor of mTOR and neoangiogenesis.

Short-cycle hypoxia in the intact heart: hypoxia-inducible factor 1alpha signaling and the relationship to injury threshold

Hypoxia-inducible factor 1alpha (HIF-1alpha) transcriptionally activates multiple genes, which regulate metabolic cardioprotective and cross-adaptive mechanisms. Hypoxia and several other stimuli induce the HIF-1alpha signaling cascade, although little data exist regarding the stress threshold for activation in heart. We tested the hypothesis that relatively mild short-cycle hypoxia, which produces minimal cardiac dysfunction and no sustained or major disruption in energy state, can induce HIF-1alpha activation. We developed a short-cycle hypoxia protocol in isolated perfused rabbit heart to test this hypothesis. By altering cycling conditions, we identified a specific cycle with O(2) content and duration that operated near a threshold for causing functional injury in these rabbit hearts. Mild short-cycle hypoxia for 46 min elevated HIF-1alpha mRNA and protein within 45 min after reoxygenation. Expression also increased for multiple HIF-1alpha target genes, such as VEGF and heme oxygenase 1. After mild hypoxia, VEGF protein accumulation occurred, although HIF-1alpha and VEGF protein accumulation were suppressed after more severe hypoxia, which also caused depletion of ATP and nondiffusible nucleotides. In summary, these results indicate that mild near-threshold hypoxia induces HIF-1alpha cascade, but more severe hypoxia suppresses protein accumulation for this transcription factor and the target genes. Posttranscriptional suppression of these proteins occurs under conditions of altered energy state, exemplified by ATP depletion.

"Translating" tumor hypoxia: unfolded protein response (UPR)-dependent and UPR-independent pathways

Poor oxygenation (hypoxia) is present in the majority of human tumors and is associated with poor prognosis due to the protection it affords to radiotherapy and chemotherapy. Hypoxia also elicits multiple cellular response pathways that alter gene expression and affect tumor progression, including two recently identified separate pathways that strongly suppress the rates of mRNA translation during hypoxia. The first pathway is activated extremely rapidly and is mediated by phosphorylation and inhibition of the eukaryotic initiation factor 2alpha. Phosphorylation of this factor occurs as part of a coordinated endoplasmic reticulum stress response program known as the unfolded protein response and activation of this program is required for hypoxic cell survival and tumor growth. Translation during hypoxia is also inhibited through the inactivation of a second eukaryotic initiation complex, eukaryotic initiation factor 4F. At least part of this inhibition is mediated through a Redd1 and tuberous sclerosis complex 1/2-dependent inhibition of the mammalian target of rapamycin kinase. Inhibition of mRNA translation is hypothesized to affect the cellular tolerance to hypoxia in part by promoting energy homeostasis. However, regulation of translation also results in a specific increase in the synthesis of a subset of hypoxia-induced proteins. Consequently, both arms of translational control during hypoxia influence gene expression and phenotype. These hypoxic response pathways show differential activation requirements that are dependent on the level of oxygenation and duration of hypoxia and are themselves highly dynamic. Thus, the severity and duration of hypoxia can lead to different biological and therapeutic consequences.

Patterns of tumor oxygenation and their influence on the cellular hypoxic response and hypoxia-directed therapies

Deficiencies in the oxygenation of solid tumors are associated with poor patient prognosis due to changes in cell metabolism, angiogenesis, invasiveness and resistance to therapy. Work over the past 10 years has defined several distinct oxygen sensing pathways that together determine the cellular response to hypoxia. These include both a transcriptional response initiated by oxygen-dependent stabilisation of the HIF-1 transcription factor and an mRNA translational response characterized by activation of the unfolded protein response (UPR) and inhibition of mTOR signalling. Laboratory experiments have established the importance of these hypoxic response pathways for tumor growth and resistance to treatment. This has led to the development of agents aimed at targeting hypoxic response pathways in tumors, several of which are in clinical trials. However, several important features of the tumor microenvironment that may affect the success of these new therapies have not been thoroughly evaluated. Oxygenation patterns in human tumors have proven to be highly complex, leading to a large degree of heterogeneity with respect to the severity and duration of hypoxic exposure. Because both of these properties strongly influence the known cellular responses to hypoxia, this heterogeneity is expected to be a strong determinant of the fate of hypoxic cells and the success of new hypoxia-directed therapies. Here we summarize the important oxygen response pathways that currently serve as targets for therapy and their dependence on the specific oxygenation patterns that are expected in human tumors.

Oxygen-sensing in tumors

PURPOSE OF REVIEW

Tumor hypoxia induces cancer cell treatment resistance, angiogenesis, invasiveness, and overall poor clinical outcome. Cellular adaptations to hypoxia are largely driven by hypoxia-induced alterations in gene transcription, mRNA translation, and protein stability. This review will summarize recent advances in the understanding of mammalian oxygen-sensing mechanisms in normal and cancerous cells.

RECENT FINDINGS

Specific molecular candidates have been identified that are involved in the primary sensing of hypoxia or its secondary consequences. Chief amongst these are the iron and 2-oxoglutarate-dependent dioxygenases that hydroxylate the alpha subunits of hypoxia-inducible transcription factors. This oxygen-dependent reaction, which prevents the transcription of many genes, is relieved under hypoxia. Evidence for the regulated expression and decay of the hypoxia-inducible transcription factor hydroxylating enzymes suggests that the sensitivity of transcriptional responses to hypoxia can be dynamically adjusted. Recent results also argue that these hydroxylating enzymes may be able to sense not only oxygen availability, but also the accumulation of bioenergetic intermediates and reactive oxygen species. In cancer cells, changes in these metabolites may accompany hypoxia or may occur independently. Several organellar compartments including plasma membrane, mitochondria and endoplasmic reticulum also appear to contribute to oxygen sensing through the generation of metabolites or through regulation of protein translation.

SUMMARY

Oxygen-sensing mechanisms induce prominent clinically relevant changes in cancer cells and tumor biology through the control of gene expression. Significant overlap exists between oxygen-sensing mechanisms and other metabolic and cell stress sensing pathways, which allows nonhypoxic cell stresses to activate hypoxia-inducible responses.

Ras, PI(3)K and mTOR signalling controls tumour cell growth

All eukaryotic cells coordinate cell growth with the availability of nutrients in their environment. The mTOR protein kinase has emerged as a critical growth-control node, receiving stimulatory signals from Ras and phosphatidylinositol-3-OH kinase (PI(3)K) downstream from growth factors, as well as nutrient inputs in the form of amino-acid, glucose and oxygen availability. Notably, components of the Ras and PI(3)K signalling pathways are mutated in most human cancers. The preponderance of mutations in these interconnected pathways suggests that the loss of growth-control checkpoints and promotion of cell survival in nutrient-limited conditions may be an obligate event in tumorigenesis.

Hypoxia inhibits protein synthesis through a 4E-BP1 and elongation factor 2 kinase pathway controlled by mTOR and uncoupled in breast cancer cells

Hypoxia is a state of low oxygen availability that limits tumor growth. The mechanism of protein synthesis inhibition by hypoxia and its circumvention by transformation are not well understood. Hypoxic breast epithelial cells are shown to downregulate protein synthesis by inhibition of the kinase mTOR, which suppresses mRNA translation through a novel mechanism mitigated in transformed cells: disruption of proteasome-targeted degradation of eukaryotic elongation factor 2 (eEF2) kinase and activation of the regulatory protein 4E-BP1. In transformed breast epithelial cells under hypoxia, the mTOR and S6 kinases are constitutively activated and the mTOR negative regulator tuberous sclerosis complex 2 (TSC2) protein fails to function. Gene silencing of 4E-BP1 and eEF2 kinase or TSC2 confers resistance to hypoxia inhibition of protein synthesis in immortalized breast epithelial cells. Breast cancer cells therefore acquire resistance to hypoxia by uncoupling oxygen-responsive signaling pathways from mTOR function, eliminating inhibition of protein synthesis mediated by 4E-BP1 and eEF2.

Activation of AMP-activated protein kinase in the liver: a new strategy for the management of metabolic hepatic disorders

It is now becoming evident that the liver has an important role in the control of whole body metabolism of energy nutrients. In this review, we focus on recent findings showing that AMP-activated protein kinase (AMPK) plays a major role in the control of hepatic metabolism. AMPK integrates nutritional and hormonal signals to promote energy balance by switching on catabolic pathways and switching off ATP-consuming pathways, both by short-term effects on phosphorylation of regulatory proteins and by long-term effects on gene expression. Activation of AMPK in the liver leads to the stimulation of fatty acid oxidation and inhibition of lipogenesis, glucose production and protein synthesis. Medical interest in the AMPK system has recently increased with the demonstration that AMPK could mediate some of the effects of the fat cell-derived adiponectin and the antidiabetic drugs metformin and thiazolidinediones. These findings reinforce the idea that pharmacological activation of AMPK may provide, through signalling and metabolic and gene expression effects, a new strategy for the management of metabolic hepatic disorders linked to type 2 diabetes and obesity.

Targeting translation in hypoxic tumors

Recent insight into how mammalian cells adapt their translational machinery to hypoxic conditions raises the possibility of targeting components of the regulatory networks involved to selectively inhibit metabolically compromised tumor cells and possibly manipulate a broad range of other physiological processes.

Escherichia coli succinic thiolinase. Stoichiometry of phosphorylation and coenzyme A binding

Equilibrium and covalent binding studies of succinic thiokinase from Escherichia coli indicates that there can be a stoichiometric relationship between coenzyme A binding and the phosphoyrlation capacity of the enzyme. A comparison of homogeneous enzyme preparations has revealed that enzyme of high specific activity exhibits greater binding capacity and that this property is proportional to enzyme activity. Phosphorylation capacity was related to specific activity in highly active enzyme preparations, but leveled off at 1 mol of phosphorus/mol of thiokinase. These studies show that the "dimer of dimers" structure of succinic thiokinase contains the expected two active sites and that this enzyme does not demonstrate "half-the-sites" reactivity. A coenzyme A binding site of lower affinity can be detected in some enzyme samples of lower specific activity. Binding of coenzyme A to the higher affinity sites may involve positive cooperativity. ATP, unlike ADP, does not bind to phosphorylated enzyme.