Glycogen synthase kinase 3 phosphorylates hypoxia-inducible factor 1alpha and mediates its destabilization in a VHL-independent manner

Loss of Mel-18 induces tumor angiogenesis through enhancing the activity and expression of HIF-1α mediated by the PTEN/PI3K/Akt pathway

Mel-18 has been implicated in several processes in tumor progression, in which the Akt pathway is involved as an important key molecular event. However, the function of Mel-18 in human cancers has not been fully established yet. Here, we examined the effect of Mel-18 on tumor angiogenesis in human breast cancer, and found that Mel-18 was a novel regulator of HIF-1α. Mel-18 negatively regulated the HIF-1α expression and its target gene VEGF transcription during both normoxia and hypoxia. We demonstrated that Mel-18 regulated the HIF-1α expression and activity via the PI3K/Akt pathway. Loss of Mel-18 downregulated Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) expression, consequently activating the PI3K/Akt/MDM2 pathway, and leading to an increase of HIF-1α protein level. Mel-18 modulated the HIF-1α transcriptional activity via regulating the cytoplasmic retention of FOXO3a, a downstream effector of Akt, and recruitment of HIF-1α/CBP complex to the VEGF promoter. Furthermore, our data shows that Mel-18 blocked tumor angiogenesis both in vitro and in vivo. Mel-18 overexpression inhibited in vitro tube formation in human umbilical endothelial cells (HUVECs). Xenografts in NOD/SCID mice derived from stably Mel-18 knocked down MCF7 human breast cancer cells showed increased tumor volume, microvessel density, and phospho-Akt and HIF-1α expression levels. In conclusion, our findings provide that Mel-18 is a novel regulator of tumor angiogenesis through regulating HIF-1α and its target VEGF expressions mediated by the PTEN/PI3K/Akt pathway, suggesting a new tumor-suppressive role of Mel-18 in human breast cancer.

What Are the bona fide GSK3 Substrates?

Nearly 100 proteins are proposed to be substrates for GSK3, suggesting that this enzyme is a fundamental regulator of almost every process in the cell, in every tissue in the body. However, it is not certain how many of these proposed substrates are regulated by GSK3 in vivo. Clearly, the identification of the physiological functions of GSK3 will be greatly aided by the identification of its bona fide substrates, and the development of GSK3 as a therapeutic target will be highly influenced by this range of actions, hence the need to accurately establish true GSK3 substrates in cells. In this paper the evidence that proposed GSK3 substrates are likely to be physiological targets is assessed, highlighting the key cellular processes that could be modulated by GSK3 activity and inhibition.

Vhl is required for normal pancreatic β cell function and the maintenance of β cell mass with age in mice

Type 2 diabetes is hallmarked by insulin resistance and insufficient β-cell function. Islets of type 2 diabetes patients have been shown to have decreased hypoxia-inducible factor (HIF)-1α/β expression. Target genes of the HIF pathway are involved in angiogenesis, survival, proliferation, and energy metabolism, and von Hippel-Lindau protein (VHL) is a negative regulator of this pathway. We hypothesized that increased HIF-mediated gene transcription by VHL deletion in the β-cells would increase β-cell mass and function. We generated β-cell-specific VHL-knockout mice using the Cre-loxP recombination system driven by the rat insulin promoter to assess the role of VHL in glucose homeostasis and β-cell function. VHL deletion in the pancreatic β-cells led to impaired glucose tolerance due to defects in glucose-stimulated insulin secretion and β-cell mass with age. VHL-knockout islets had decreased GLUT2, but increased glucose transporter 1 and vascular endothelial growth factor expression. Furthermore, there were significant aberrations in islet morphology in the VHL-knockout mice, likely due to increased islet vasculature. Given that erythropoietin (EPO) is a target gene of the HIF pathway, which is not expressed in islets, we tested whether activating EPO signaling by systemic administration with recombinant human EPO (rHuEPO) can overcome the β-cell defects that occurred with VHL loss. We observed improved glucose tolerance and restoration of GLUT2 expression in VHL-deficient β-cells in response to rHuEPO. Contrary to our hypothesis, loss of VHL and increased transcription of HIF-target genes resulted in impaired β-cell function and mass, which can be overcome with exogenous EPO. Our results indicate a critical role for VHL in β-cell function and mass, and that EPO administration improved β-cell function making it a potential strategy for diabetes treatment.

Evolutionary conserved regulation of HIF-1β by NF-κB

Hypoxia Inducible Factor-1 (HIF-1) is essential for mammalian development and is the principal transcription factor activated by low oxygen tensions. HIF-α subunit quantities and their associated activity are regulated in a post-translational manner, through the concerted action of a class of enzymes called Prolyl Hydroxylases (PHDs) and Factor Inhibiting HIF (FIH) respectively. However, alternative modes of HIF-α regulation such as translation or transcription are under-investigated, and their importance has not been firmly established. Here, we demonstrate that NF-κB regulates the HIF pathway in a significant and evolutionary conserved manner. We demonstrate that NF-κB directly regulates HIF-1β mRNA and protein. In addition, we found that NF-κB–mediated changes in HIF-1β result in modulation of HIF-2α protein. HIF-1β overexpression can rescue HIF-2α protein levels following NF-κB depletion. Significantly, NF-κB regulates HIF-1β (tango) and HIF-α (sima) levels and activity (Hph/fatiga, ImpL3/ldha) in Drosophila, both in normoxia and hypoxia, indicating an evolutionary conserved mode of regulation. These results reveal a novel mechanism of HIF regulation, with impact in the development of novel therapeutic strategies for HIF–related pathologies including ageing, ischemia, and cancer.

The mechanisms by which cells and organisms respond to oxygen are of extreme importance for development and also for certain pathologies such as cancer, ageing, and ischemia. These are mediated by a family of transcription factors called hypoxia inducible factor (HIF), a factor that coordinates expression of a great number of genes. Significantly, these processes are evolutionary conserved from worms to humans. It is known that regulation of HIF occurs to a great extent through protein degradation. However, other important mechanisms of HIF control are currently being investigated. In this study, we have uncovered a novel mechanism of HIF regulation that relies on the action of another transcription factor family called NF-κB. We have found that NF-κB controls the levels of HIF-1α and HIF-1β genes by direct regulation. Furthermore, through its control of HIF-1β, NF-κB indirectly controls HIF-2α. Importantly, we find that this mechanism is conserved in Drosophila and mice. These results suggest an alternative avenue for therapeutic intervention in the HIF pathway, which has important implications for many human diseases.

GSK-3β: a signaling pathway node modulating neural stem cell and endothelial cell interactions

The neurogenic areas of the brain are highly organized structures in which there is dynamic reciprocal modulation of neural stem cells (NSC) and microvascular endothelial cells (BEC) resulting in control of neural stem cell and vascular proliferation, survival and differentiation throughout the life of the individual. Select molecules such as GSK-3β, functioning as signaling nodes, and their downstream signaling components including HIF-1α, HIF-2α and β-catenin participate in regulating and orchestrating the diverse responses involved in this complex process. In this report we demonstrate GSK-3β's role as a signaling node in two mouse strains (C57BL/6, which have been found to respond to and recover from a hypoxic insult from P3 to P11 poorly and CD-1, which have been found to respond to and recover from a hypoxic insult from P3 to P11 well both in vivo and in vitro) which mimic the wide range of responsiveness to hypoxic insult observed in the very low birth weight premature infant population. Differences in levels of neural stem cell and microvascular endothelial cell GSK-3β activation, β-catenin serine phosphorylation, HIF-1α and 2α, BDNF, SDF-1 and VEGF, β-III-tubulin and cleaved notch-1 expression in C57BL/6 and CD-1 subventricular zone tissues, and cultured NSC and BEC were noted. Specifically, CD1 pups, SVZ tissues and isolated NSC and BEC exhibit less GSK-3β and β-catenin serine phoslphorylation and greater HIF-1α and 2α, BDNF, SDF-1 and VEGF, β-III-tubulin and cleaved notch-1 expression compared to C57BL/6. Correlating with these changes were differences of several neural stem cell and microvascular endothelial cell behaviors including proliferation, apoptosis, migration and differentiation with CD1 NSC exhibiting greater proliferation and migration and decreased apoptosis and differentiation and CD1 BEC exhibiting greater angiogenesis. Further, upon treatment with nanomolar concentrations of a GSK-3β inhibitor (SB412682), C57 NSC and BEC behaviors could be brought to CD1 levels, consistent with the concept of GSK-3β functioning as a multifunctional signaling pathway node, modulating several behaviors in these cells. Lastly, the therapeutic potential of targeting GSK-3β is discussed.

Interactions between nitric oxide and hypoxia-inducible factor signaling pathways in inflammatory disease

Induction and activation of nitric oxide (NO) synthases (NOS) and excessive production of NO are common features of almost all diseases associated with infection and acute or chronic inflammation, although the contribution of NO to the pathophysiology of these diseases is highly multifactorial and often still a matter of controversy. Because of its direct impact on tissue oxygenation and cellular oxygen (O(2)) consumption and re-distribution, the ability of NO to regulate various aspects of hypoxia-induced signaling has received widespread attention. Conditions of tissue hypoxia and the activation of hypoxia-inducible factors (HIF) have been implicated in hypoxia or in cancer biology, but are also being increasingly recognized as important features of acute and chronic inflammation. Thus, the activation of HIF transcription factors has been increasingly implicated in inflammatory diseases, and recent studies have indicated its critical importance in regulating phagocyte function, inflammatory mediator production, and regulation of epithelial integrity and repair processes. Finally, HIF also appears to contribute to important features of tissue fibrosis and epithelial-to-mesenchymal transition, processes that are associated with tissue remodeling in various non-malignant chronic inflammatory disorders. In this review, we briefly summarize the current state of knowledge with respect to the general mechanisms involved in HIF regulation and the impact of NO on HIF activation. Secondly, we will summarize the major recent findings demonstrating a role for HIF signaling in infection, inflammation, and tissue repair and remodeling, and will address the involvement of NO. The growing interest in hypoxia-induced signaling and its relation with NO biology is expected to lead to further insights into the complex roles of NO in acute or chronic inflammatory diseases and may point to the importance of HIF signaling as key feature of NO-mediated events during these disorders.

Regulation of PTEN stability and activity by Plk3

By studying primary isogenic murine embryonic fibroblasts (MEFs), we have shown that PLK3 null MEFs contain a reduced level of phosphatase and tensin homolog (PTEN) and increased Akt1 activation coupled with decreased GSK3β activation under normoxia and hypoxia. Purified recombinant Plk3, but not a kinase-defective mutant, efficiently phosphorylates PTEN in vitro. Mass spectrometry identifies threonine 366 and serine 370 as two putative residues that are phosphorylated by Plk3. Immunoblotting using a phosphospecific antibody confirms these sites as Plk3 phosphorylation sites. Moreover, treatment of MEFs with LiCl, an inhibitor of GSK3β and CK2, only partially suppresses the phosphorylation, suggesting Plk3 as an additional kinase that phosphorylates these sites in vivo. Plk3-targeting mutants of PTEN are expressed at a reduced level in comparison with the wild-type counterpart, which is associated with an enhanced activity of PDK1, an upstream activator of Akt1. Furthermore, the reduced level of PTEN in PLK3 null MEFs is stabilized by treatment with MG132, a proteosome inhibitor. Combined, our study identifies Plk3 as a new player in the regulation of the PI3K/PDK1/Akt signaling axis by phosphorylation and stabilization of PTEN.

Plk3 functions as an essential component of the hypoxia regulatory pathway by direct phosphorylation of HIF-1alpha

Polo-like kinase 3 (Plk3) plays an important role in the regulation of cell cycle progression and stress responses. Plk3 also has a tumor-suppressing activity as aging PLK3-null mice develop tumors in multiple organs. The growth of highly vascularized tumors in PLK3-null mice suggests a role for Plk3 in angiogenesis and cellular responses to hypoxia. By studying primary isogenic murine embryonic fibroblasts, we tested the hypothesis that Plk3 functions as a component in the hypoxia signaling pathway. PLK3(-/-) murine embryonic fibroblasts contained an enhanced level of HIF-1α under hypoxic conditions. Immunoprecipitation and pulldown analyses revealed that Plk3 physically interacted with HIF-1α under hypoxia. Purified recombinant Plk3, but not a kinase-defective mutant, phosphorylated HIF-1α in vitro, resulting in a major mobility shift. Mass spectrometry identified two unique serine residues that were phosphorylated by Plk3. Moreover, ectopic expression followed by cycloheximide or pulse-chase treatment demonstrated that phospho-mutants exhibited a much longer half-life than the wild-type counterpart, strongly suggesting that Plk3 directly regulates HIF-1α stability in vivo. Combined, our study identifies Plk3 as a new and essential player in the regulation of the hypoxia signaling pathway.

Ras regulates interleukin-1β-induced HIF-1α transcriptional activity in glioblastoma

We observed elevated levels of pro-inflammatory cytokine IL-1β in glioblastoma multiforme tumor samples. Since hypoxia-inducible factor-1α (HIF-1α) plays a crucial role in linking inflammatory and oncogenic pathways, we investigated the effect of IL-1β on HIF-1α expression in glioma cells under normoxia. IL-1β-mediated elevation of HIF-1α transcriptional activity was dependent on Ras-induced NF-κB activation, as IL-1β failed to induce NF-κB and HIF-1α activity in cells transfected with dominant negative RasN17. Increased Ras expression was accompanied by increased phosphorylation of Ras effectors AKT, ERK, JNK, and p38MAPK. While inhibition of these effectors individually failed to block the IL-1β-mediated increase in HIF-1α induction, co-inhibition of both AKT and ERK resulted in a significant decrease in IL-1β-induced HIF-1α activation. Interestingly, IL-1β elevated Wnt-1 expression in a Ras-dependent manner, and small interfering RNA (siRNA)-mediated knockdown of Wnt-1 decreased HIF-1α activity. Although Wnt-1-mediated HIF-1α was independent of the canonical Wnt/β-catenin signaling pathway, it regulated HIF-1α through NF-κB. siRNA-mediated HIF-1α knockdown attenuated elevated IL-1β mRNA levels induced upon IL-1β treatment. This was accompanied by increased interaction of HIF-1α with HIF responsive element on the IL-1β promoter upon IL-1β treatment, under normoxia. Our studies highlights for first time that (1) Ras is a key mediator of IL-1β-induced NF-κB and HIF-1α activation, under normoxia; (2) Wnt-1 regulates IL-1β-mediated HIF-1α induction via NF-κB; (3) Ras and Wnt-1 are intermediaries in the canonical IL-1β-NF-κB signaling pathway downstream of MyD88; and (4) IL-1β-induced HIF-1α drives a HIF-1α-IL-1β autocrine loop to maintain persistently elevated IL-1β level.

The microtubule as a breast cancer target

Manifestations of non-equilibrium polarity, random transgressions, and catastrophes are not conditions usually associated with a sense of normalcy. Yet these disquieting features distinguish a utilitarian behavior known as dynamic instability, the signature characteristic of the microtubule. Long known to be a tumor target, disruption of this fragile attribute is associated with some of the most effective agents used to treat breast cancer today. Although the biology of the microtubule is under intense investigation much still remains unknown. As such, our understanding of regulatory molecules and resistance mechanisms are still rudimentary, further compromising our ability to develop novel therapeutic strategies to improve microtubule inhibitors. This review focuses on several classes of anti-microtubule agents and their effects on the functional dynamics of the targeted polymer. The primary objective is to critically examine the molecular mechanisms that contribute to tumor cell death, tumor-resistance, and incident neurotoxicity.

Neuroprotective multifunctional iron chelators: from redox-sensitive process to novel therapeutic opportunities

Accumulating evidence suggests that many cytotoxic signals occurring in the neurodegenerative brain can initiate neuronal death processes, including oxidative stress, inflammation, and accumulation of iron at the sites of the neuronal deterioration. Neuroprotection by iron chelators has been widely recognized with respect to their ability to prevent hydroxyl radical formation in the Fenton reaction by sequestering redox-active iron. An additional neuroprotective mechanism of iron chelators is associated with their ability to upregulate or stabilize the transcriptional activator, hypoxia-inducible factor-1alpha (HIF-1alpha). HIF-1alpha stability within the cells is under the control of a class of iron-dependent and oxygen-sensor enzymes, HIF prolyl-4-hydroxylases (PHDs) that target HIF-1alpha for degradation. Thus, an emerging novel target for neuroprotection is associated with the HIF system to promote stabilization of HIF-1alpha and increase transcription of HIF-1-related survival genes, which have been reported to be regulated in patient's brains afflicted with diverse neurodegenerative diseases. In accordance, a new potential therapeutic strategy for neurodegenerative diseases is explored, by which iron chelators would inhibit PHDs, target the HIF-1-signaling pathway and ultimately activate HIF-1-dependent neuroprotective genes. This review discusses two interrelated approaches concerning therapy targets in neurodegeneration, sharing in common the implementation of iron chelation activity: antioxidation and HIF-1-pathway activation.

Suppression of Her2/neu expression through ILK inhibition is regulated by a pathway involving TWIST and YB-1

In a previous study it was found that the therapeutic effects of QLT0267, a small molecule inhibitor of integrin-linked kinase (ILK), were influenced by Her2/neu expression. To understand how inhibition or silencing of ILK influences Her2/neu expression, Her2/neu signaling was evaluated in six Her2/neu-positive breast cancer cell lines (LCC6^Her2, MCF7^Her2, SKBR3, BT474, JIMT-1 and KPL-4). Treatment with QLT0267 engendered suppression (32–87%) of total Her2/neu protein in these cells. Suppression of Her2/neu was also observed following small interfering RNA-mediated silencing of ILK expression. Time course studies suggest that ILK inhibition or silencing caused transient decreases in P-AKT^ser473, which were not temporally related to Her2/neu downregulation. Attenuation of ILK activity or expression was, however, associated with decreases in YB-1 (Y-box binding protein-1) protein and transcript levels. YB-1 is a known transcriptional regulator of Her2/neu expression, and in this study it is demonstrated that inhibition of ILK activity using QLT0267 decreased YB-1 promoter activity by 50.6%. ILK inhibition was associated with changes in YB-1 localization, as reflected by localization of cytoplasmic YB-1 into stress granules. ILK inhibition also suppressed TWIST (a regulator of YB-1 expression) protein expression. To confirm the role of ILK on YB-1 and TWIST, cells were engineered to overexpress ILK. This was associated with a fourfold increase in the level of YB-1 in the nucleus, and a 2- and 1.5-fold increase in TWIST and Her2/neu protein levels, respectively. Taken together, these data indicate that ILK regulates the expression of Her2/neu through TWIST and YB-1, lending support to the use of ILK inhibitors in the treatment of aggressive Her2/neu-positive tumors.

Arsenite stabilizes HIF-1α protein through p85α-mediated up-regulation of inducible Hsp70 protein expression

Hypoxia-inducible factor-1α (HIF-1α) has been reported to regulate over 100 gene expressions in response to hypoxia and other stress conditions. In the present study, we found that arsenite could induce HIF-1α protein accumulation in both mouse epidermal Cl41 cells and mouse embryonic fibroblasts (MEFs). Knockout of p85α, a regulatory subunit of PI-3K, in MEFs (p85α(-/-)) dramatically decreased the arsenite-induced HIF-1α accumulation, indicating that p85α is crucial for arsenite effects on the stabilization of HIF-1α protein. Our further studies suggest that arsenite could induce inducible Hsp70 expression, and transfection of inducible Hsp70 into p85α(-/-) MEFs could restore HIF-1α protein accumulation. Moreover, the results using EMSA and Supershift assays indicate that p85α is crucial for arsenite-induced activation of the heat-shock transcription factor 1 (HSF-1), which is responsible for transcription of inducible Hsp70. Taken together, p85α-mediated HIF-1α stabilization upon arsenite exposure is specifically through HSF-1 activation and subsequent up-regulation of the inducible Hsp70 expression.

Aspartyl-(asparaginyl) beta-hydroxylase, hypoxia-inducible factor-alpha and Notch cross-talk in regulating neuronal motility

Aspartyl-(Asparaginyl)-β-Hydroxylase (AAH ) promotes cell motility by hydroxylating Notch. Insulin and insulin-like growth factor, type 1 (IGF-I) stimulate AAH through Erk MAP K and phosphoinositol-3-kinase-Akt (PI3K-Akt). However, hypoxia/oxidative stress may also regulate AAH . Hypoxia-inducible factor-1alpha (HIF-1α) regulates cell migration, signals through Notch, and is regulated by hypoxia/oxidative stress, insulin/IGF signaling and factor inhibiting HIF-1α (FIH) hydroxylation. To examine cross-talk between HIF-1α and AAH , we measured AAH , Notch-1, Jagged-1, FIH, HIF-1α, HIF-1β and the hairy and enhancer of split 1 (HE S-1) transcription factor expression and directional motility in primitive neuroectodermal tumor 2 (PNET2) human neuronal cells that were exposed to H₂O₂ or transfected with short interfering RNA duplexes (siRNA) targeting AAH , Notch-1 or HIF-1α. We found that: (1) AAH , HIF-1α and neuronal migration were stimulated by H2O2; (2) si-HIF-1α reduced AAH expression and cell motility; (3) si-AAH inhibited Notch and cell migration, but not HIF-1α and (4) si-Notch-1 increased FIH and inhibited HIF-1α. These findings suggest that AAH and HIF-1α crosstalk within a hydroxylation-regulated signaling pathway that may be transiently driven by oxidative stress and chronically regulated by insulin/IGF signaling.

The hypoxia-inducible factor-2alpha is stabilized by oxidative stress involving NOX4

The hypoxia-inducible factor-2alpha (HIF-2alpha) contributes to the vascular response to hypoxia. Hypoxia inhibits prolyl hydroxylation of the N-terminal transactivation domain (N-TAD), thus preventing binding of the von Hippel-Lindau protein (pVHL) and proteasomal degradation; additionally, hypoxia inhibits asparagyl hydroxylation of the C-TAD, thus diminishing cofactor recruitment. Reactive oxygen species (ROS) derived from NADPH oxidases (NOXs) have been shown to control vascular functions and to promote vascular remodeling. However, whether HIF-2alpha, ROS, and NOXs are linked under such nonhypoxic conditions is unclear. We found that activation of NOX4 by thrombin or H(2)O(2) increased HIF-2alpha protein because of decreased pVHL binding in pulmonary artery smooth muscle cells (PASMCs). Thrombin, H(2)O(2), and NOX4 overexpression increased HIF-2alpha N-TAD and C-TAD activity, which was prevented by ascorbate treatment or mutation of the hydroxylation sites in the TADs. HIF-2alpha also mediated induction of plasminogen activator inhibitor-1 and the proliferative response to thrombin, H(2)O(2), or NOX4 overexpression. Thus, ROS derived from NOX4 in response to thrombin stabilize HIF-2alpha by preventing hydroxylation of the N- and C-TAD, thus allowing formation of transcriptionally active HIF-2alpha, which promotes PASMC proliferation. Together, these findings present the first evidence that HIF-2alpha is critically involved in the ROS-regulated vascular remodeling processes.

Casein kinase 1 regulates human hypoxia-inducible factor HIF-1

Hypoxia-inducible factor 1 (HIF-1), a transcriptional activator that mediates cellular response to hypoxia and a promising target of anticancer therapy, is essential for adaptation to low oxygen conditions, embryogenesis and tumor progression. HIF-1 is a heterodimer of HIF-1alpha, expression of which is controlled by oxygen levels as well as by various oxygen-independent mechanisms, and HIF-1beta (or ARNT), which is constitutively expressed. In this work, we investigate the phosphorylation of the N-terminal heterodimerization (PAS) domain of HIF-1alpha and identify Ser247 as a major site of in vitro modification by casein kinase 1delta (CK1delta). Mutation of this site to alanine, surprisingly, enhanced the transcriptional activity of HIF-1alpha, a result phenocopied by inhibition or small interfering RNA (siRNA)-mediated silencing of CK1delta under hypoxic conditions. Conversely, overexpression of CK1delta or phosphomimetic mutation of Ser247 to aspartate inhibited HIF-1alpha activity without affecting its stability or nuclear accumulation. Immunoprecipitation and in vitro binding experiments suggest that CK1-dependent phosphorylation of HIF-1alpha at Ser247 impairs its association with ARNT, a notion also supported by modeling the structure of the complex between HIF-1alpha and ARNT PAS-B domains. We suggest that modification of HIF-1alpha by CK1 represents a novel mechanism that controls the activity of HIF-1 during hypoxia by regulating the interaction between its two subunits.

Inhibition of the PI3K-Akt pathway suppresses sFlt1 expression in human placental hypoxia models in vitro

OBJECTIVE

Although elevated expression of soluble fms-like tyrosine kinase 1 (sFlt1) plays a major role in the pathogenesis of pre-eclampsia, it is unclear how hypoxia regulates placental sFlt1 expression. Thus, we investigated sFlt1 expression in placentas from normal and preeclamptic pregnancies and in human placental hypoxia models in vitro to examine the role of the PI3K-Akt pathway in regulating the expression of this molecule.

METHODS

We examined the expression of VEGF, PlGF, sFlt1, PI3K, Akt, and HIF-1 in placental samples from ten women with pre-eclampsia and ten normotensive control patients and in human choriocarcinoma trophoblast cells treated with 600muM CoCl(2) by Western blotting. Using models of placental hypoxia, we also determined whether inhibition of the PI3K-Akt pathway plays a direct role in regulating the expression of sFlt1.

RESULTS

The VEGF, PlGF, sFlt1, PI3K, Akt, and HIF-1 levels were significantly higher in the preeclamptic placentas than the normal placentas. In the placental hypoxia models, the expression of VEGF and PlGF increased in a time-dependent manner, whereas the expression of sFlt1 plateaued after 3h of CoCl(2) treatment. The expression levels of p-Akt and PI3K were maximal after 6 and 12h of CoCl(2) treatment, respectively. The expression of HIF-1alpha increased in a time-dependent manner with CoCl(2) treatment. Inhibition of the PI3K-Akt pathway with the PI3K-specific inhibitor LY294002 leads to decreased sFlt1 levels and unchanged or increased VEGF and PlGF levels.

CONCLUSION

Inhibition of the PI3K-Akt pathway may be a useful therapeutic approach, if it were to decrease sFlt1 secretion without inhibiting VEGF or PlGF secretion. This pathway provides a potential target for a new treatment strategy in patients with pre-eclampsia.

Inhibition of hypoxia-inducible factor-1alpha (HIF-1alpha) protein synthesis by DNA damage inducing agents

Hypoxia-inducible factor (HIF) is a heterodimeric transcription factor that is composed of a hypoxia-inducible α subunit (HIF-1α and HIF-2α) and a constitutively expressed β subunit (HIF-1β). HIF mediates the adaptation of cells and tissues to low oxygen concentrations. It also plays an important role in tumorigenesis and constitutes an important therapeutic target in anti-tumor therapy. We have screened a number of reported HIF inhibitors for their effects on HIF-transcriptional activity and found that the DNA damage inducing agents camptothecin and mitomycin C produced the most robust effects. Camptothecin is a reported inhibitor of HIF-1α translation, while mitomycin C has been reported to induce p53-dependent HIF-1α degradation. In this study we demonstrate that the inhibitory effect of mitomycin C on HIF-1α protein expression is not dependent on p53 and protein degradation, but also involves HIF-1α translational regulation. Initiation of a DNA damage response with the small molecule p53 activator NSC-652287 (RITA) has been reported to inhibit HIF-1α protein synthesis by increasing the phosphorylation of eIF2α. However, we show here that even when eIF2α phosphorylation is prevented, the DNA damage inducing drugs mitomycin C, camptothecin and NSC-652287 still inhibit HIF-1α protein synthesis to the same extent. The inhibitory effects of camptothecin on HIF-1α expression but not that of mitomycin C and NSC-652287 were dependent on cyclin-dependent kinase activity. In conclusion, specific types of DNA damage can bring about selective inhibition of HIF-1α protein synthesis. Further characterization of the involved mechanisms may reveal important novel therapeutic targets.

Quantitative phosphoproteomic analysis of the STAT3/IL-6/HIF1alpha signaling network: an initial study in GSC11 glioblastoma stem cells

Initiation and maintenance of several cancers including glioblastoma (GBM) may be driven by a small subset of cells called cancer stem cells (CSCs). CSCs may provide a repository of cells in tumor cell populations that are refractory to chemotherapeutic agents developed for the treatment of tumors. STAT3 is a key transcription factor associated with regulation of multiple stem cell types. Recently, a novel autocrine loop (IL-6/STAT3/HIF1alpha) has been observed in multiple tumor types (pancreatic, prostate, lung, and colon). The objective of this study was to probe perturbations of this loop in a glioblastoma cancer stem cell line (GSC11) derived from a human tumor by use of a JAK2/STAT3 phosphorylation inhibitor (WP1193), IL-6 stimulation, and hypoxia. A quantitative phosphoproteomic approach that employed phosphoprotein enrichment, chemical tagging with isobaric tags, phosphopeptide enrichment, and tandem mass spectrometry in a high-resolution instrument was applied. A total of 3414 proteins were identified in this study. A rapid Western blotting technique (<1 h) was used to confirm alterations in key protein expression and phosphorylation levels observed in the mass spectrometric experiments. About 10% of the phosphoproteins were linked to the IL-6 pathway, and the majority of remaining proteins could be assigned to other interlinked networks. By multiple comparisons between the sample conditions, we observed expected changes and gained novel insights into the contribution of each factor to the IL6/STAT3/HIF1alpha autocrine loop and the CSC response to perturbations by hypoxia, inhibition of STAT3 phosphorylation, and IL-6 stimulation.

Hypoxia-inducible factors: post-translational crosstalk of signaling pathways

Hypoxia-inducible factor-1 (HIF-1) has a central role in the mammalian program by which cells respond to hypoxia in both physiological and pathological situations. HIF-1 transcriptional activity, protein stabilization, protein-protein interaction, and cellular localization are mainly modulated by Post-translational modifications such as hydroxylation, acetylation, phosphorylation, S-nitrosylation, and SUMOylation. Here, we summarize current knowledge about Post-translational HIF-1 regulation and give additional information about useful methods to determine some of these various modifications.

Regulation of protein stability by GSK3 mediated phosphorylation

Glycogen synthase kinase-3 (GSK3) plays important roles in numerous signaling pathways that regulate a variety of cellular processes including cell proliferation, differentiation, apoptosis and embryonic development. In the canonical Wnt signaling pathway, GSK3 phosphorylation mediates proteasomal targeting and degradation of beta-catenin via the destruction complex. We recently reported a biochemical screen that discovered multiple additional protein substrates whose stability is regulated by Wnt signaling and/or GSK3 and these have important implications for Wnt/GSK3 regulation of different cellular processes.(1) In this article, we also present a bio-informatics based screen for proteins whose stability may be controlled by GSK3 and beta-Trcp, the SCF E3 ubiquitin ligase that is responsible for beta-catenin degradation in the Wnt signaling pathway. Furthermore, we review various GSK3 regulated proteolysis substrates described in the literature. We propose that GSK3 phosphorylation dependent proteolysis is a widespread mechanism that the cell employs to regulate a variety of cell processes in response to signals.

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

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

HAF : the new player in oxygen-independent HIF-1alpha degradation

Adaptation to hypoxia is primarily mediated by the hypoxia-inducible transcription factor, HIF. The regulation of HIF activity by the oxygen-dependent degradation of the HIF-1alpha and HIF-2alpha subunits by the pVHL E3 ligase complex has been well characterized. We have recently described the hypoxia-associated factor, HAF, as an E3 ligase for HIF-1alpha that does not degrade HIF-2alpha. Here we summarize the mechanism of HAF-mediated HIF-1alpha degradation and the importance of oxygen-independent HIF-1alpha regulation in cancer. We also discuss the implications of the new HAF: HIF-1alpha degradation pathway with respect to other novel mediators of oxygen-independent HIF-alpha degradation. Finally, we review the significance of HAF as an isoform-specific E3 ligase in light of new information on the non-overlapping functions of HIF-1alpha and HIF-2alpha in cancer.

Sphingosine-1-phosphate: a novel nonhypoxic activator of hypoxia-inducible factor-1 in vascular cells

OBJECTIVE

Sphingosine-1-phosphate (S1P) is a potent bioactive phospholipid responsible for a variety of vascular cell responses. Hypoxia-inducible factor-1 (HIF-1) is a transcriptional activator of genes essential for adaptation to low oxygen. S1P and HIF-1 are both important mediators of vascular cell responses such as migation, proliferation, and survival. Studies have shown that nonhypoxic stimuli can activate HIF-1 in oxygenated conditions. Here, we attempt to determine whether S1P can modulate the vascular activation of HIF-1.

METHODS AND RESULTS

We show that in vascular endothelial and smooth muscle cells, activation of the S1P type-2 receptor by S1P strongly increases HIF-1 alpha protein levels, the active subunit of HIF-1. This is achieved through pVHL-independent stabilization of HIF-1 alpha. We demonstrate that the HIF-1 nuclear complex, formed on S1P stimulation, is transcriptionally active and specifically binds to a hypoxia-responsive elements. Moreover, S1P activates the expression of genes known to be closely regulated by HIF-1.

CONCLUSIONS

Our results identify S1P as a novel and potent nonhypoxic activator of HIF-1. We believe that understanding the role played by HIF-1 in S1P gene regulation will have a strong impact on different aspects of vascular biology.

Mechanism of hypoxia-induced GCM1 degradation: implications for the pathogenesis of preeclampsia

Preeclampsia is a major pregnancy-specific disorder affecting 5-7% of pregnancies worldwide. Although hypoxia caused by incomplete trophoblast invasion and impaired spiral arterial remodeling is thought to be a major cause of preeclampsia, how hypoxia affects placental development remains uncertain. GCM1 (glial cells missing homolog 1) is a transcription factor critical for placental development. In preeclampsia, GCM1 and its target genes syncytin 1 and placental growth factor, important for syncytiotrophoblast formation and placental vasculogenesis, are all decreased. Here we present evidence that GCM1 is a major target of hypoxia associated with preeclampsia. We show that hypoxia triggers GCM1 degradation by suppressing the phosphatidylinositol 3-kinase-Akt signaling pathway, leading to GSK-3beta activation. Activated GSK-3beta phosphorylates GCM1 on Ser322, which in turn recruits the F-box protein FBW2, leading to GCM1 ubiquitination and degradation. Importantly, the GSK-3beta inhibitor LiCl prevented hypoxia-induced GCM1 degradation. Our study identifies a molecular basis for the disrupted GCM1 transcription network in preeclampsia and provides a potential avenue for therapeutic intervention.

Metabolic transformation in cancer

In 2000, Douglas Hanahan and Robert Weinberg published a review detailing the six hallmarks of cancer. These are six phenotypes that a tumour requires in order to become a fully fledged malignancy: persistent growth signals, evasion of apoptosis, insensitivity to anti-growth signals, unlimited replicative potential, angiogenesis and invasion and metastasis. However, it is becoming increasingly clear that these phenotypes do not portray the whole story and that other hallmarks are necessary: one of which is a shift in cellular metabolism. The tumour environment creates a unique collection of stresses to which cells must adapt in order to survive. This environment is formed by the uncontrolled proliferation of cells, which ignore the cues that would create normal tissue architecture. As a result, the cells forming the tumour are exposed to low oxygen and nutrient levels, as well as high levels of toxic cellular waste products, which is thought to propel cells towards a more transformed phenotype, resistant to cell death and pro-metastatic.

Hypoxia confers protection against apoptosis via the PI3K/Akt pathway in endothelial progenitor cells

AIM

The recruitment of endothelial progenitor cells (EPC) to ischemia has recently been suggested as an important mechanism of tissue repair. Although tissue ischemia can facilitate EPC mobilization, recruitment, and retention at the hypoxic site, the effects of hypoxia on EPC survival are not well known. In the present study, we examined whether hypoxia (2% O2) would suppress apoptosis induced by serum withdrawal and whether survival signals, such as the phosphatidylinositol 3-kinase (PI3K)/Akt and extracellular signal-regulated protein kinase (ERK) pathways, were involved in this process.

METHODS

After being serum-starved for 24 h, EPC were cultured under normoxic or hypoxic conditions (2% O2) for 24 h. Cell survival was assessed by 3-(4,5- dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, annexin V-propidium iodide dual-color flow cytometry, and terminal deoxynucleotidyl transferase-mediated digoxigenin-dUTP nick-end labeling assay. The expressions of signaling proteins were evaluated by Western blot analysis.

RESULTS

Under hypoxic conditions, EPC were resistant to apoptosis induced by serum starvation. The inhibition of the PI3K/Akt pathway using the LY294002 inhibitor prevented hypoxia-inhibited apoptosis in EPC and altered the phosphorylation state of glycogen synthase kinase-3beta, an effector protein involved in regulation of EPC apoptosis. However, ERK inhibitor PD98059 had no significant effect on cell survival.

CONCLUSION

Our data demonstrated that hypoxia inhibited serum withdrawal-induced apoptosis in EPC, which might be associated with the activation of the PI3K/Akt pathway.

Sphingosine kinase 1: a new modulator of hypoxia inducible factor 1alpha during hypoxia in human cancer cells

Here, we provide the first evidence that sphingosine kinase 1 (SphK1), an oncogenic lipid kinase balancing the intracellular level of key signaling sphingolipids, modulates the transcription factor hypoxia inducible factor 1alpha (HIF-1alpha), master regulator of hypoxia. SphK1 activity is stimulated under low oxygen conditions and regulated by reactive oxygen species. The SphK1-dependent stabilization of HIF-1alpha levels is mediated by the Akt/glycogen synthase kinase-3beta signaling pathway that prevents its von Hippel-Lindau protein-mediated degradation by the proteasome. The pharmacologic and RNA silencing inhibition of SphK1 activity prevents the accumulation of HIF-1alpha and its transcriptional activity in several human cancer cell lineages (prostate, brain, breast, kidney, and lung), suggesting a canonical pathway. Therefore, we propose that SphK1 can act as a master regulator for hypoxia, giving support to its inhibition as a valid strategy to control tumor hypoxia and its molecular consequences.

Hypoxia-associated factor, a novel E3-ubiquitin ligase, binds and ubiquitinates hypoxia-inducible factor 1alpha, leading to its oxygen-independent degradation

The hypoxia-inducible factor 1alpha (HIF-1alpha) is the master regulator of the cellular response to hypoxia. A key regulator of HIF-1alpha is von Hippel-Lindau protein (pVHL), which mediates the oxygen-dependent, proteasomal degradation of HIF-1alpha in normoxia. Here, we describe a new regulator of HIF-1alpha, the hypoxia-associated factor (HAF), a novel E3-ubiquitin ligase that binds HIF-1alpha leading to its proteasome-dependent degradation irrespective of cellular oxygen tension. HAF, a protein expressed in proliferating cells, binds and ubiquitinates HIF-1alpha in vitro, and both binding and E3 ligase activity are mediated by HAF amino acids 654 to 800. Furthermore, HAF overexpression decreases HIF-1alpha levels in normoxia and hypoxia in both pVHL-competent and -deficient cells, whereas HAF knockdown increases HIF-1alpha levels in normoxia, hypoxia, and under epidermal growth factor stimulation. In contrast, HIF-2alpha is not regulated by HAF. In vivo, tumor xenografts from cells overexpressing HAF show decreased levels of HIF-1alpha accompanied by decreased tumor growth and angiogenesis. Therefore, HAF is the key mediator of a new HIF-1alpha-specific degradation pathway that degrades HIF-1alpha through a new, oxygen-independent mechanism.

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.

Inhibition of HIF-1 alpha and VEGF expression by the chemopreventive bioflavonoid apigenin is accompanied by Akt inhibition in human prostate carcinoma PC3-M cells

Progression of cancer leads to hypoxic solid tumors that mount specific cell signaling responses to low oxygen conditions. An important objective of anti-cancer therapy is the development of new drugs that suppress hypoxic responses in solid tumors. Apigenin is a natural flavone that has been shown to have chemopreventive and/or anti-cancer properties against a number of tumor types. However, the mechanisms underlying apigenin's chemopreventive properties are not yet completely understood. In this study, we have investigated the effects of apigenin on expression of hypoxia-inducible factor-1 (HIF-1) in human metastatic prostate PC3-M cancer cells. We found that hypoxia induced a time-dependent increase in the level of HIF-1alpha subunit protein in PC3-M cells, and treatment with apigenin markedly decreased HIF-1alpha expression under both normoxic and hypoxic conditions. Further, apigenin prevented the activation of the HIF-1 downstream target gene vascular endothelial growth factor (VEGF). We then showed that apigenin inhibited expression of HIF-1alpha by reducing stability of the protein as well as by reducing the level of HIF-1alpha mRNA. We also found that apigenin inhibited Akt and GSK-3beta phosphorylation in PC3-M cells. Further experiments demonstrated that constitutively active Akt blunted the effect of apigenin on HIF-1alpha expression. Taken together, our results identify apigenin as a bioflavonoid that inhibits hypoxia-activated pathways linked to cancer progression in human prostate cancer, in particular the PI3K/Akt/GSK-3 pathway. Further studies on the mechanism of action of apigenin will likely provide new insight into its applicability for pharmacologic targeting of HIF-1alpha for cancer therapeutic or chemopreventive purposes.

Polo-like kinase 3 functions as a tumor suppressor and is a negative regulator of hypoxia-inducible factor-1 alpha under hypoxic conditions

Polo-like kinase 3 (Plk3) is an important mediator of the cellular responses to genotoxic stresses. In this study, we examined the physiologic function of Plk3 by generating Plk3-deficient mice. Plk3(-/-) mice displayed an increase in weight and developed tumors in various organs at advanced age. Many tumors in Plk3(-/-) mice were large in size, exhibiting enhanced angiogenesis. Plk3(-/-) mouse embryonic fibroblasts were hypersensitive to the induction of hypoxia-inducible factor-1 alpha (HIF-1 alpha) under hypoxic conditions or by nickel and cobalt ion treatments. Ectopic expression of the Plk3-kinase domain (Plk3-KD), but not its Polo-box domain or a Plk3-KD mutant, suppressed the nuclear accumulation of HIF-1 alpha induced by nickel or cobalt ions. Moreover, hypoxia-induced HIF-1 alpha expression was tightly associated with a significant down-regulation of Plk3 expression in HeLa cells. Given the importance of HIF-1 alpha in mediating the activation of the "survival machinery" in cancer cells, these studies strongly suggest that enhanced tumorigenesis in Plk3-null mice is at least partially mediated by a deregulated HIF-1 pathway.

Transcriptional regulation of serine/threonine kinase-15 (STK15) expression by hypoxia and HIF-1

The serine/threonine kinase-15 (STK15) acts as a cell cycle regulator being overexpressed in various tumors. One mechanism that could contribute to overexpression of STK15 is tumor hypoxia where hypoxia-inducible factor-1 (HIF-1) is a major regulator of transcription. Therefore, we analyzed whether hypoxia and HIF-1 could contribute to overexpression of STK15. We found that hypoxia increased STK15 expression and STK15 promoter activity in HepG2 tumor cells. Overexpression of HIF-1 alpha induced STK15 gene transcription, whereas HIF-1 alpha siRNA and overexpression of prolyl hydroxylase 2 (PHD-2), a negative regulator of HIF-1 alpha, reversed this effect. In addition, site-directed mutagenesis experiments and chromatin immunoprecipitation revealed that from the three putative hypoxia responsive elements (HRE) within the STK15 promoter only HRE-2 was functional and bound HIF-1. Further, siRNA against STK15 inhibited proliferation of HepG2 cells induced by hypoxia. These results show that STK15 gene transcription can be regulated by hypoxia and HIF-1 via HRE-2 of the STK15 promoter. Thus, tumor hypoxia may trigger overexpression of STK15 observed in various tumors.

Induction of vascular endothelial growth factor by peptidyl-prolyl isomerase Pin1 in breast cancer cells

Pin1, a peptidyl-prolyl isomerase, is overexpressed in most types of cancer tissues and plays an important role in oncogenesis. We found that Pin1 increases the transcriptional activity and protein level of vascular endothelial growth factor (VEGF) in the human breast cancer cell line, MCF-7. Reporter gene analyses showed that Pin1 overexpression increased the reporter activities in cells transfected with reporters containing the VEGF gene promoter or with minimal reporters of activator protein-1 (AP-1) and hypoxia response element (HRE). VEGF reporter gene activity was significantly inhibited by either hypoxia-inducible factor-alpha (HIF-1alpha) siRNA or AP-1 decoy ODN. Moreover, the reporter activities of the VEGF promoter, AP-1 and HRE in Pin1-/- mouse embryonic fibroblast (MEF) cells were decreased compared with those in wild-type Pin1 MEF cells. These results imply that Pin1 stimulates VEGF expression by activating HIF-1alpha and AP-1, and suggest that Pin1 is a potential therapeutic target of angiogenesis during cancer development.

Hypoxia-inducible factors and cancer

Decreased oxygen availability is a common feature during embryonic development as well of malignant tumours. Hypoxia regulates many transcription factors, and one of the most studied is the hypoxia-inducible factor (HIF). As a consequence of HIF stabilisation, the cell constitutively upregulates the hypoxic programme resulting in the expression of genes responsible for global changes in cell proliferation, angiogenesis, metastasis, invasion, de-differentiation and energy metabolism. Of the three known alpha subunits of HIF transcription factors, HIF-1alpha and HIF-2alpha have been the most studied. Their differential expression and function have been widely discussed, however no clear picture has been drawn on how these two transcription factors differently regulate common and unique target genes. Their role as oncogenes has also been suggested in several studies. In this review we provide an overview of the current knowledge on some of the most important aspects of HIFalpha regulation, its role in tumour angiogenesis and energetic metabolism. We also give an overview of how the modulation of HIF regulating pathways is a potential therapeutic target that may have benefits in the treatment of cancer.

How lithium treatment generates neutrophilia by enhancing phosphorylation of GSK-3, increasing HIF-1 levels and how this path is important during engraftment

Lithium is commonly used in psychiatry for mood stabilization. Lithium treatment results in neutrophilia, increased platelets and increased circulating CD34+ haematopoietic stem cells, HSC. This paper outlines the newly discovered mechanism by which this occurs. Glycogen synthase kinase-3, GSK-3, phosphorylates and thereby inactivates hypoxia-induced factor-1, HIF-1. HIF-1 is a transcription factor triggering transcription of multiple genes related to adaptation to hypoxia, among which is CXCL12. CXCL12 forms the primary homing gradient for CD34+ HSCs towards the hypoxic, trophic bone marrow niche to which they must go to thrive. Lithium inhibits GSK-3 thereby increasing active HIF-1 that results in a stronger CXCL12 homing gradient. Trophic niche function is enhanced, ultimately resulting in increased production of neutrophils, platelets and CD34+ cells. Sitagliptin is a new drug to treat diabetes that coincidentally inhibits destruction of CXCL12. Thus, lithium and sitagliptin enhance CXCL12 by different paths, potentially increasing trophic niche function. Awareness of this path is important in HSC transplantation.

The hypoxia-inducible factor is stabilized in circulating hematopoietic stem cells under normoxic conditions

The hypoxia-inducible factor (HIF) transcriptional system enables cell adaptation to limited O(2) availability, transducing this signal into patho-physiological responses such as angiogenesis, erythropoiesis, vasomotor control, and altered energy metabolism, as well as cell survival decisions. However, other factors beyond hypoxia are known to activate this pleiotropic transcription factor. The aim of this study was to characterize HIF in human hematopoietic stem cells (HSCs) and evidence is provided that granulocyte colony stimulating factor-mobilized CD34+- and CD133+-HSCs express a stabilized cytoplasmic form of HIF-1alpha under normoxic conditions. It is shown that HIF-1alpha stabilization correlates with down-regulation of the tumour suppressor von Hippel-Lindau protein (pVHL) and is positively controlled by NADPH-oxidase-dependent production of reactive oxygen species, indicating a specific O(2)-independent post-transcriptional control of HIF in mobilized HSCs. This novel finding is discussed in the context of the proposed role of HIF as a mediator of progenitor cell recruitment to injured ischemic tissues and/or in the control of the maintenance of the undifferentiated state.