c-Jun and hypoxia-inducible factor 1 functionally cooperate in hypoxia-induced gene transcription

ERKs activation and calcium signaling are both required for VEGF induction by vanadium in mouse epidermal Cl41 cells

The previous studies have demonstrated that vanadium exposure can cause a variety of biological effects. However, the mechanisms involved in the biological effects caused by vanadium are not well understood. Our previous studies have shown that exposure of mouse epidermal Cl 41 cells to vanadate stimulated the phosphorylation of both ERKs and p38K, and calcium signaling leading NFAT activation. In view of the evidence that ERKs and p38 kinase contribute to VEGF induction, we investigated in the present study the potential roles of ERKs, p38K, and calcium signaling in VEGF induction caused by vanadium exposure. Exposure of Cl 41 cells to vanadium led to VEGF induction in both time- and dose-dependent manners. Pre-treatment of Cl 41 cells with PD98059, an inhibitor of MEK1/2-ERKs pathway, but not SB202190, an inhibitor for p38K pathway, resulted in a dramatic inhibition of VEGF induction by vanadium. More interesting, pre-treatment of Cl 41 cells with intracellular calcium chelator, but not calcium channel blocker, resulted in a dramatic decrease in VEGF induction by vanadium. However, both PI-3K inhibitors and overexpression of Deltap85, a dominant negative PI-3K mutant, resulted in only a marginal decrease in VEGF induction by vanadium. Moreover, mTOR, as a downstream molecule of PI-3K, did not attribute to VEGF induction by vanadium because rapamycin pre-treatment did not show any inhibitory effect on VEGF induction. These results indicate that ERKs and intracellular stored calcium release play a critical role in VEGF induction by vanadium. PI-3K is partially involved in VEGF induction by vanadium, while p38K and mTOR are not involved. Those results will help us to understand the molecular mechanisms involved in vanadium-induced biological effects.

Critical role for NF-kappaB-induced JunB in VEGF regulation and tumor angiogenesis

Regulation of vascular endothelial growth factor (VEGF) expression is a complex process involving a plethora of transcriptional regulators. The AP-1 transcription factor is considered as facilitator of hypoxia-induced VEGF expression through interaction with hypoxia-inducible factor (HIF) which plays a major role in mediating the cellular hypoxia response. As yet, both the decisive AP-1 subunit leading to VEGF induction and the molecular mechanism by which this subunit is activated have not been deciphered. Here, we demonstrate that the AP-1 subunit junB is a target gene of hypoxia-induced signaling via NF-kappaB. Loss of JunB in various cell types results in severely impaired hypoxia-induced VEGF expression, although HIF is present and becomes stabilized. Thus, we identify JunB as a critical independent regulator of VEGF transcription and provide a mechanistic explanation for the inherent vascular phenotypes seen in JunB-deficient embryos, ex vivo allantois explants and in vitro differentiated embryoid bodies. In support of these findings, tumor angiogenesis was impaired in junB(-/-) teratocarcinomas because of severely impaired paracrine-acting VEGF and the subsequent inability to efficiently recruit host-derived vessels.

Regulation of angiogenesis by hypoxia and hypoxia-inducible factors

Maintenance of oxygen homeostasis is critical for the survival of multicellular organs. As a result, both invertebrates and vertebrates have developed highly specialized mechanisms to sense changes in oxygen levels and to mount adequate cellular and systemic responses to these changes. Hypoxia, or low oxygen tension, occurs in physiological situations such as during embryonic development, as well as in pathological conditions such as ischemia, wound healing, and cancer. A primary effector of the adaptive response to hypoxia in mammals is the hypoxia-inducible factor (HIF) family of transcription regulators. These proteins activate the expression of a broad range of genes that mediate many of the responses to decreased oxygen concentration, including enhanced glucose uptake, increased red blood cell production, and the formation of new blood vessels via angiogenesis. This latter process is dynamic and results in the establishment of a mature vascular system that is indispensable for proper delivery of oxygen and nutrients to all cells in both normal tissue and hypoxic regions. Angiogenesis is essential for normal development and neoplastic disease as tumors must develop mechanisms to stimulate vascularization to meet increasing metabolic demands. The link between hypoxia and the regulation of angiogenesis is an area of intense research and the molecular details of this connection are still being elaborated. This chapter will provide an overview of current knowledge and highlight new insights into the importance of HIF and hypoxia in angiogenesis in both physiological and pathophysiological conditions.

Induction of activating transcription factor 3 by anoxia is independent of p53 and the hypoxic HIF signalling pathway

Solid tumors often have an inadequate blood supply, which results in large regions that are subjected to hypoxic or anoxic stress. Hypoxia-inducible factor-1 (HIF-1) is a transcription factor that regulates much of the transcriptional response of cells to hypoxia. Activating transcription factor 3 (ATF3) is another transcription factor that responds to a variety of stresses and is often upregulated in cancer. We investigated the regulation of ATF3 by oxygen deprivation. ATF3 induction occurred most robustly under anoxia, is common, and it is not dependent on presence of HIF-1 or p53, but is sensitive to the inhibition of c-Jun NH2-terminal kinase activation and the antioxidant N-acetylcystein. ATF3 could also be induced by desferrioxamine but not by the mitochondrial poison cyanide or the nonspecific 2-oxoglutarate dioxygenase inhibitor dimethyloxalylglycine. We also show that anoxic ATF3 mRNA is more stable than normoxic mRNA providing a mechanism for this induction. Thus, this study demonstrates that the regulation of ATF3 under anoxia is independent of 2-oxoglutarate dioxygenase, HIF-1 and p53, presumably involving multiple regulatory pathways.

c-Jun kinase mediates expression of VEGF induced at transcriptional level by Rac1 and Cdc42Hs but not by RhoA

Tumour angiogenesis is mediated by increased levels of vascular endothelial growth factor (VEGF). We have studied the mechanism by which endogenous activation of Rho oncoproteins regulates VEGF expression in COS-7 and NIH3T3 cells. We carried out transient and stable transfection with constitutively activated rhoA, rac1, and cdc42 mutants in COS-7 and NIH3T3 cells, respectively in the absence of external stimuli. Western blot and inmunohistochemistry assays of those cells revealed increased VEGF protein expression. Cotransfection with constitutively activated rhoA, rac1, and cdc42 mutants and a VEGF promoter-reporter construct showed an increase in VEGF promoter transcriptional activity induced by Rho oncoproteins in COS-7 and NIH3T3. c-Jun kinase had been described as a MAPK involved in Rho oncoproteins pathways. Interestingly, we found that c-Jun kinase chemical inhibition as well as transient transactivation assays using dominant negative c-Jun kinase mutant abolished the VEGF promoter transcriptional induction by Rac1 and Cdc42 but not by RhoA. These findings indicate that Rho oncoprotein endogenously activated regulates VEGF expression through a transcriptional mechanism, and that the c-Jun kinase activity is a mediator in the expression of VEGF induced by Rac1 and Cdc42 oncoproteins, but not of that induced by RhoA.

Gene expression profiling reveals the profound upregulation of hypoxia-responsive genes in primary human astrocytes

Oxygen is vital for the development and survival of mammals. In response to hypoxia, the brain initiates numerous adaptive responses at the organ level as well as at the molecular and cellular levels, including the alteration of gene expression. Astrocytes play critical roles in the proper functioning of the brain; thus the manner in which astrocytes respond to hypoxia is likely important in determining the outcome of brain hypoxia. Here, we used microarray gene expression profiling and data-analysis algorithms to identify and analyze hypoxia-responsive genes in primary human astrocytes. We also compared gene expression patterns in astrocytes with those in human HeLa cells and pulmonary artery endothelial cells (ECs). Remarkably, in astrocytes, five times as many genes were induced as suppressed, whereas in HeLa and pulmonary ECs, as many as or more genes were suppressed than induced. More genes encoding hypoxia-inducible functions, such as glycolytic enzymes and angiogenic growth factors, were strongly induced in astrocytes compared with HeLa cells. Furthermore, gene ontology and computational algorithms revealed that many target genes of the EGF and insulin signaling pathways and the transcriptional regulators Myc, Jun, and p53 were selectively altered by hypoxia in astrocytes. Indeed, Western blot analysis confirmed that two major signal transducers mediating insulin and EGF action, Akt and MEK1/2, were activated by hypoxia in astrocytes. These results provide a global view of the signaling and regulatory network mediating oxygen regulation in human astrocytes.

Role for casein kinase 2 in the regulation of HIF-1 activity

Hypoxia-inducible factor-1 (HIF-1) is a heterodimeric transcription factor that plays a major role in cellular adaptation to hypoxia. The mechanisms regulating HIF-1 activity occurs at multiple levels in vivo. The HIF-1alpha subunit is highly sensible to oxygen and is rapidly degraded by the proteasome 26S in normoxia. Activation in hypoxia occurs through a multistep process including inhibition of HIF-1alpha degradation, but also increase in the transactivation activity of HIF-1. Several data indicate that phosphorylation could play a role in this regulation. In this report, we investigated the role of casein kinase 2 (CK2), an ubiquitous serine/threonine kinase, in the regulation of HIF-1 activity. Hypoxia was capable of increasing the expression of the beta subunit of CK2, of inducing a relocalization of this subunit at the plasma membrane, of inducing nuclear translocation of the alpha subunit and of increasing CK2 activity. Three inhibitors of this kinase, DRB (5,6-dichloro-1-beta-D-ribofuranosyl-benzimidazole), TBB (4,5,6,7-tetrabromotriazole) and apigenin, as well as overexpression of a partial dominant negative mutant of CK2alpha, were shown to inhibit HIF-1 activity as measured by a reporter assay and through hypoxia-induced VEGF and aldolase expression. This does not occur at the stabilization process since they did not affect HIF-1alpha protein level. DNA-binding activity was also not inhibited. We conclude that CK2 is an important regulator of HIF-1 transcriptional activity but the mechanism of this regulation remains to be determined. Since HIF-1 plays a major role in tumor angiogenesis and since CK2 has been described to be overexpressed in tumor cells, this new pathway of regulation can be one more way for tumor cells to survive.

Hypoxia-induced IL-6 production is associated with activation of MAP kinase, HIF-1, and NF-kappaB on HEI-OC1 cells

In the present study, we investigated the signal transduction pathways of expression of IL-6 in the desferrioxamine (DFX)-stimulated cochlear auditory cell line, HEI-OC1 cells. DFX increased the expression of HIF-1alpha and NF-kappaB in HEI-OC1 cells. DFX significantly increased the production of IL-6 (P<0.05) and expression of IL-6 mRNA but did not affect TNF-alpha production. DFX also induced the activation of mitogen-activated protein kinase (MAPK) including p38, ERK, and JNK on HEI-OC1. Increased IL-6 by DFX was significantly inhibited by p38 inhibitor, SB203580 (about 72% inhibition, P=0.027) but not ERK inhibitor, PD98059 or JNK inhibitor, SP600125. SB203580 inhibited the expression of IL-6 mRNA. Increased IL-6 production was partially inhibited by treatment of iron (HIF-1 inhibitor) or pyrriolidine-dithiocarbamate (PDTC, NF-kappaB inhibitor). DFX also induced IL-6 production and HIF-1alpha expression in the inner ear. We demonstrated the regulatory effects of MAPK, HIF-1alpha, and NF-kappaB on DFX-induced IL-6 production in a HEI-OC1 for the first time. In conclusion, these data indicate that regulation of inflammatory cytokine IL-6 by DFX, through mimicking hypoxic conditions, might explain its beneficial effect in the treatment of hypoxia-induced inner ear diseases.

MCP-1/CCL2 protects cardiac myocytes from hypoxia-induced apoptosis by a G(alphai)-independent pathway

Chemokines, in addition to their chemotactic properties, act upon resident cells within a tissue and mediate other cellular functions. In a previous study, we demonstrated that CCL2 protects cultured mouse neonatal cardiac myocytes from hypoxia-induced cell death. Leukocyte chemotaxis has been shown to contribute to ischemic injury. While the chemoattractant properties of CCL2 have been established, the protective effects of this chemokine suggest a novel role for CCL2 in myocardial ischemia/reperfusion injury. The present study examined the cellular signaling pathways that promote this protection. Treatment of cardiac myocyte cultures with CCL2 protected them from hypoxia-induced apoptosis. This protection was not mediated through the activation of G(alphai) signaling that mediates monocyte chemotaxis. Inhibition of the ERK1/2 signaling pathway abrogated CCL2 protection. Caspase 3 activation and JNK/SAPK phosphorylation were decreased in hypoxic myocytes co-treated with CCL2 as compared to hypoxia only-treated cultures. Expression of the Bcl-2 family proteins, Bcl-xL and Bag-1, was increased in CCL2-treated myocytes subjected to hypoxia. There was also downregulation of Bax protein levels as a result of CCL2 co-treatment. These data suggest that CCL2 cytoprotection and chemotaxis may occur through distinct signaling mechanisms.

Essential role of PI-3K, ERKs and calcium signal pathways in nickel-induced VEGF expression

Exposure to a highly nickel-polluted environment has the potential to cause a variety of adverse health effects, such as the respiratory tract cancers. Since numerous studies have demonstrated that nickel generally has weak mutagenic activity, research focus had turned to cell signalling activation leading to gene modulation and epigenetic changes as a plausible mechanism of carcinogenesis. Previous studies have revealed that nickel compounds can induce the expression of vascular endothelial growth factor (VEGF), which is a key mediator of angiogenesis both in physiological and pathologic conditions. In the present study, we investigated the potential roles of PI-3K, ERKs, p38 kinase and calcium signalling in VEGF induction by nickel in Cl 41 cells. Exposure of Cl 41 cells to nickel compounds led to VEGF induction in both time- and dose-dependent manners. Pre-treatment of Cl 41 cells with PI-3K inhibitor, wortmannin or Ly294002, resulted in a striking inhibition of VEGF induction by nickel compounds, implicating the role of PI-3K in the induction. However, mTOR, one of downstream molecules of PI-3K, may not contribute to the induction because pre-treatment of Cl 41 cells with its inhibitor, rapamycin, did not show obvious decrease in nickel-induced VEGF expression. Furthermore, pre-treatment of Cl 41 cells with MEK1/2-ERKs pathway inhibitor, PD98059, significantly inhibited VEGF induction by both NiCl2 and Ni3S2, whereas p38 kinase inhibitor, SB202190, did not impair the induction. Pre-treatment of Cl 41 cells with intracellular calcium chelator, but not calcium channel blocker, inhibited VEGF induction by nickel. Collectively these data demonstrate that PI-3K, ERKs and cytosolic calcium, but not p38 kinase, play essential roles in VEGF induction by nickel compounds.

Erythropoietin disrupts hypoxia-inducible factor signaling in ovarian cancer cells

OBJECTIVE

The objective of this study was to evaluate the effects of recombinant erythropoietin (EPO) on HIF-1alpha induced angiogenic pathways in ovarian cancer cells.

METHODS

Using Western blots and both quantitative and non-quantitative RT-PCR, HIF-1alpha protein and VEGF transcription levels were assessed. Cell growth was measured using flow cytometry.

RESULTS

EPO treatment decreased hypoxia-induced HIF-1alpha protein levels and VEGF transcription, with no effect on cell growth. Inhibition of HIF-1alpha signaling by EPO was also observed in MCF-7 breast cancer cells.

CONCLUSION

These novel findings suggest that EPO may exhibit anti-angiogenic properties, thus encouraging further exploration of signaling pathways between EPO and HIF-1alpha.

Cytokines and the regulation of hypoxia-inducible factor (HIF)-1alpha

Hypoxia-inducible factor (HIF)--an oxygen sensor? The HIF-oxygen sensing association type of dogma is, unequivocally, well anchored. But this is only one face of, at least, a double-sided coin. Current concepts charge HIF of taking sides with a yet not well-founded identity--an immunologic sensor and/or regulator. Or, is it really a sensor, put it more correctly, a key player in sensing mechanisms? The evolving association between HIF and immunity emanates from an established linkage that bonds oxidative stress and inflammation--notably the 'biologic response modifiers', or cytokines. HIF is a redox(y)-sensitive transcription factor, and so are cytokines. Recently, cytokines emerged as major regulators of HIF, under physiologic conditions extending the realm of hypoxia. Alternatively, can HIF, like the so infamous inflammatory transcription factor NF-(kappa)B, prove itself as a key player in the regulation of cytokines and, subsequently, the inflammatory process. The targeting of HIF would be, at least theoretically, of therapeutic value, but does it make sense given its intricate role in hypoxia signaling? It is the theme of HIF being an immunologic sensor that will be explored therein--with special emphasis on the regulatory role of cytokines.

Negative and positive regulation of HIF-1: a complex network

Hypoxia inducible factor-1 (HIF-1) is as a key transcriptional mediator of the hypoxic response in eukaryotic cells, regulating the expression of a myriad of genes involved in oxygen transport, glucose uptake and glycolysis and angiogenesis. Deregulation of HIF-1 activity occurs in many human cancers, usually at the level of the HIF-1alpha subunit. HIF-1 is regulated by a variety of mechanisms including transcription, translation post-translational modification, protein-protein interaction and degradation. Our understanding of the key signalling pathways that regulate HIF-1 has significantly progressed in recent years and has highlighted the potential for targeting the HIF-1 pathway as a basis for the development of new cancer therapies.

Mechanism of inhibition of ascites tumor growth in mice by curcumin is mediated by NF-kB and caspase activated DNase

One of the most clinically relevant biological activities of curcumin is its anti-cancer property, implicating multiple intracellular pathways in the process. In the present report, we investigated the effect of curcumin on the activation of apoptotic and anti-angiogenic pathways in Ehrlich Ascites Tumor (EAT) cells. Treatment with curcumin in vivo resulted in inhibition of proliferation of EAT cells and ascites formation. Further, we demonstrate that the induction of apoptosis in EAT cells showed nuclear condensation, DNA fragmentation and translocation of caspase-activated DNase (CAD) to nucleus upon curcumin treatment. Curcumin-induced apoptosis is mediated through activation of caspase-3, which is specifically inhibited by the caspase-3 inhibitor, Ac-DEVD-CHO. On the other hand, the decreased secretion of ascites by EAT cells is corroborated by reduction in VEGF secretion upon curcumin treatment. Further, CD31 immunohistological staining of peritoneum sections in curcumin-treated mice suggests its efficacy in acting as anti-angiogenic compound in EAT cells by inhibiting proliferation of endothelial cells in mouse peritoneum. However, immunoflurescence studies of NF-kB revealed that the inhibition of nuclear translocation of NF-kB p65, a transcription factor required for VEGF gene expression, in curcumin-treated EAT cells. These results suggest a further possible clinical application of this diet-derived compound curcumin, as both proapoptotic and anti-angiogenic compound in association with conventional chemotherapeutic agents.

Vascular endothelial growth factor is regulated by hypoxic stress via MAPK and HIF-1 alpha in the inner ear

Vascular endothelial growth factor (VEGF) is a key regulator of angiogenesis. The iron-chelator desferrioxamine (DFX) increased the expression of hypoxia-inducible factor (HIF)-1alpha in the hair cell line, HEI-OC1. The increased VEGF production by DFX was inhibited by iron. DFX also induced the activation of mitogen-activated protein kinase (MAPK) on HEI-OC1. The increased VEGF production by DFX was inhibited by a specific inhibitor of MAPK. In addition, DFX induced the VEGF production and HIF-1alpha stabilization in vivo. These results indicate that VEGF production is regulated via MAPK and HIF-1alpha under hypoxic condition in the inner ear.

The interaction between HIF-1 and AP-1 transcription factors in response to low oxygen

Hypoxia-inducible factor-1 (HIF-1) is a critical regulator of the transcriptional response to low oxygen conditions (hypoxia/anoxia) experienced by mammalian cells in both physiological and pathophysiological circumstances. As our understanding of the biology and biochemistry of HIF-1 has grown, it has become apparent that cells adapt to signals generated by low oxygen through a network of stress responsive transcription factors or complexes, which are influenced by HIF-1 activity. This review summarizes our current understanding of the interaction of HIF-1 with AP-1, a classic example of a family of pleiotropic transcription factors that impact on diverse cellular processes and phenotypes, including the adaptation to low oxygen stress. The review focuses on experimental studies involving cultured cells exposed to hypoxia/anoxia, and describes both established and possible interactions between HIF-1 and AP-1 at different levels of cellular organization.

Chronic hypoxia inhibits Kv channel gene expression in rat distal pulmonary artery

In pulmonary arterial smooth muscle cells (PASMCs), voltage-gated K+ (Kv) channels play an important role in regulating membrane potential, cytoplasmic free Ca2+ concentration, and pulmonary vasomotor tone. Previous studies demonstrated that exposure of rats to chronic hypoxia decreased Kv channel function in PASMCs from distal pulmonary arteries (dPA). To determine whether this decrease in function was due to decreased expression of Kv channel proteins and which Kv proteins might be involved, we analyzed Kv channel gene expression in intact, endothelium-denuded dPAs obtained from rats exposed to 10% O2 for 3 wk. Kv1.1, Kv1.2, Kv1.4, Kv1.5, Kv1.6, Kv2.1, Kv3.1, Kv4.3, and Kv9.3 channel alpha-subunits and Kv1, Kv2, and Kv3 beta-subunits were expressed in rat dPAs. Exposure to chronic hypoxia decreased mRNA and protein levels of Kv1.1, Kv1.5, Kv1.6, Kv2.1, and Kv4.3 alpha-subunits in dPAs but did not alter gene or protein expression of these channels in aorta. Furthermore, chronic hypoxia did not alter the mRNA levels of beta-subunits in dPAs. These results suggest that diminished transcription of Kv alpha-subunits may reduce the number of functional Kv channels in dPAs during prolonged hypoxia, causing the decreased Kv current previously observed in PASMCs and leading to pulmonary artery vasoconstriction.

Cellular reaction to hypoxia: sensing and responding to an adverse environment

Multicellular organisms have developed sophisticated physiologic mechanisms by which they maintain their tissues at the optimal oxygen concentration. This level is important so that the benefits of free oxygen can be realized, while limiting the potential harms. Despite these efforts, there exist physiologic and pathophysiologic conditions where oxygen delivery drops below what is necessary for the tissue. Under these circumstances, the cell then goes through a series of coordinated responses in a time and oxygen concentration-dependent manner. The gene expression changes are designed to maintain cellular and tissue viability, and are comprised of transcriptional as well as post-transcriptional events. As we understand more about the hypoxic response, we realize how it can impact normal development, wound healing, and the malignant progression of a solid tumor.

Stress kinase signaling in cancer: fact or fiction?

Cancer results from genetic alterations in intracellular signaling pathways, which normally orchestrate the execution of developmental programs and the organismic response to extrinsic factors. Mutations in upstream activators and components of the cytoplasmic (Ras-Raf MEK-ERK) cascade frequently occur in tumors. In vitro and in vivo studies have shown that isolated activation of this pathway is both, necessary and sufficient for transformation. During the last years two new groups of related kinases have joined the ranks of mitogen-activated protein kinases, stress-activated protein kinases/Jun N-terminal kinases and p38. Their activation not only occurs during cellular responses to unphysiological stimuli but also downstream of cytokine and pathogen receptors and has been observed in tumors. In this article we will review the role of stress kinases in cancer, and discuss the mechanisms through which they regulate the transformation process.

c-Jun NH2-Terminal Kinase Activation Contributes to Hypoxia-Inducible Factor 1α–Dependent P-Glycoprotein Expression in Hypoxia

We previously have shown that hypoxia increases the expression of P-glycoprotein, which in turn increases tumor cell capacity to actively extrude chemotherapeutic agents and may contribute to tumor drug resistance. This event is mediated through the hypoxia-inducible factor (HIF-1). Here, we investigated the role of the stress-activated protein kinase c-Jun NH(2)-terminal kinase (JNK) in the signaling mechanisms underlying these events. Hypoxia activates JNK activity in vitro and in vivo. Overexpression of mitogen-activated protein kinase (MAPK) kinase kinase (MEKK-1), which preferentially activates JNK, mimics, in a nonadditive way, hypoxia-induced activity of the MDR1 promoter and expression of MDR1 mRNA and P-glycoprotein. Furthermore, the JNK inhibitor SP600125 selectively and specifically inhibits hypoxia- and MEKK-1-induced MDR1 promoter activity in a dose-dependent manner. JNK inhibition also reversed hypoxia- and MEKK-1-induced activity of an HIF-1-dependent reporter gene. MEKK-1-induced MDR1 expression depends on a functional HIF-1 binding site (hypoxia-responsive element). Hypoxia- but not cobalt chloride-dependent HIF-1-DNA binding and transcriptional activation was inhibited by SP600125, indicating that hypoxia-induced signaling to HIF-1 depends on JNK activation. Because it has been reported that reactive oxygen species are increased in hypoxia and related to JNK activation, we investigated their role in signaling this response. Whereas exogenous addition of H(2)O(2) was sufficient to activate JNK, reactive oxygen species scavengers were without effect on hypoxia-induced JNK or HIF-1 activation. Thus, hypoxia-elicited MDR1 expression, which depends on HIF-1 activation, depends at least in part on signaling via activation of JNK. Furthermore, these events are independent of the generation of reactive oxygen intermediates. Thus, JNK may represent a therapeutic target in the prevention of tumor resistance to chemotherapeutic treatment.

Regulation of malignant progression by the hypoxia-sensitive transcription factors HIF-1alpha and MTF-1

Solid tumors are known to develop microenvironmental hypoxia or anoxia due to malfunction and malformation of blood vessels and the energy demands of the highly proliferative tumor cells. Oxygen deprivation can cause aberrant modifications of signaling pathways and their downstream transcription factors that are believed to contribute to malignancy. Here, we review the latest studies related to the involvement of hypoxia-inducible transcription factor-1alpha (HIF-1alpha), the first known mammalian intracellular hypoxia sensor, in tumor development. We propose that a second far less studied protein, metal transcription factor-1 (MTF-1), acts as a more general oxygen sensor, responding to both hypoxia and oxidative stress, and is also intimately involved in malignant progression. Existing evidence suggests that activation of these two ubiquitous proteins, by hypoxia and genetic modifications, modulate the expression patterns of a number of important proteins involved in tumorigenesis.

Glucose utilization is essential for hypoxia-inducible factor 1 alpha-dependent phosphorylation of c-Jun

Hypoxia and anoxia are important microenvironmental stresses that contribute to pathological events such as solid-tumor development. We have been investigating the effects of hypoxia and anoxia on expression of the proto-oncogene c-jun and the regulation of c-Jun/AP-1 transcription factors. In earlier work using genetically manipulated mouse embryo fibroblasts (mEFs), we found a functional relationship among c-jun expression, c-Jun N-terminal phosphorylation, and the presence of hypoxia-inducible factor 1 alpha (HIF-1 alpha), the oxygen-regulated subunit of the HIF-1 transcription factor. Both the induction of c-jun mRNA expression and c-Jun N-terminal phosphorylation in cells exposed to hypoxia or anoxia were found to be dependent on the presence of HIF-1 alpha, but this was not the case in cells exposed to less-severe hypoxia. Here we describe new findings concerning HIF-1-dependent c-Jun N-terminal phosphorylation in cells exposed to hypoxia or anoxia. Specifically, we report that hypoxia-inducible c-Jun N-terminal kinase (JNK) activity, which involves JNKs or stress-activated protein kinases (SAPKs), is dependent on enhanced glucose utilization mediated by HIF-1. These results suggest a model in which hypoxia-inducible JNK activity is connected to oxygen sensing through increased glucose absorption and/or glycolytic activity regulated by the HIF-1 system. We also found that basal threonine and tyrosine phosphorylation (within the TEY motif) of extracellular signal-regulated kinases 1 and 2 (ERK1/2) and the corresponding ERK1/2 activity were defective in hypoxic HIF-1 alpha-null mEFs but not in wild-type mEFs, independently of glucose uptake. Therefore, the activities of both JNKs/SAPKs and ERK1/2 are sensitive to HIF-1-dependent processes in cells exposed to hypoxia or anoxia.

HIFs and tumors--causes and consequences

For most organisms oxygen is essential fo life. When oxygen levels drop below those required to maintain the minimum physiological oxygen requirement of an organism or tissue it is termed hypoxia. To counter act possible deleterious effects of such a state, an immediate molecular response is initiated causing adaptation responses aimed at cell survival. This response is mediated by the hypoxia-inducible factor-1 (HIF-1), which is a heterodimer consisting of an alpha- and a beta-subunit. HIF-1 alpha protein is stabilized under hypoxic conditions and therefore confers selectivity to this response. Hypoxia is characteristic of tumors, mainly because of impaired blood supply resulting from abnormal growth. Over the past few years enormous progress has been made in the attempt to understand how the activation of the physiological response to hypoxia influences neoplastic growth. In this review some aspects of HIF-1 pathway activation in tumors and the consequences for pathophysiology and treatment of neoplasia are discussed.

Microarray analysis identifies Autotaxin, a tumour cell motility and angiogenic factor with lysophospholipase D activity, as a specific target of cell transformation by v-Jun

We have used chicken cDNA microarrays to investigate gene-expression changes induced during transformation of chick embryo fibroblasts (CEF) by the viral Jun oncoprotein encoded by ASV17. This analysis reveals that v-Jun induces increases and decreases of varying magnitude in the expression of genes involved in diverse cellular functions, most of which have not been detected in previous screens for putative v-Jun targets. In all, 27 individual genes were identified, whose expression is increased threefold or more in v-Jun-transformed cells, including genes involved in energy generation, protein synthesis, and gene transcription. Interestingly, this group includes the hypoxia-inducible factor-1 alpha (Hif-1alpha) transcription factor and the glycolytic enzyme enolase, suggesting that adaptation to hypoxia could play a role in tumorigenesis by v-Jun. We also identified 32 genes whose expression is decreased threefold or more, including chaperones, components of the cytoskeleton, and, unexpectedly, DNA replication factors. The gene whose expression is upregulated most dramatically (approximately 100-fold) encodes Autotaxin (ATX), a secreted tumor motility-promoting factor with lysophospholipase D activity. Strikingly, v-Jun-transformed CEF secrete catalytically active ATX and chemotactic activity, which can be detected in conditioned medium. ATX is not detectably expressed in normal CEF or CEF transformed by the v-Src or v-Myc oncoproteins, indicating that induction of this putative autocrine/paracrine factor is a specific consequence of cell transformation by v-Jun. ATX has been implicated in both angiogenesis and invasion, and could therefore play an important role in tumorigenesis by v-Jun in vivo.

Hypoxia-inducible factor-1-independent regulation of vascular endothelial growth factor by hypoxia in colon cancer

The induction of vascular endothelial growth factor (VEGF) is an essential feature of tumor angiogenesis, and the hypoxia-inducible factor-1 (HIF-1) transcription factor is known to be a key mediator of this process. In colon cancer, the frequently mutated K-ras oncogene also can regulate VEGF expression, but the role that K-ras may play in hypoxia is unknown. Hypoxia induced VEGF promoter activity, mRNA, and protein levels in colon cancer cells. Although HIF-1alpha was induced by hypoxia, VEGF reporter constructs with selectively mutated hypoxia-response elements remained responsive to hypoxia. In addition, "knockdown" of HIF-1alpha by RNA interference only minimally inhibited the hypoxic induction of VEGF. A region of the VEGF promoter between -420 and -90 bp mediated this HIF-independent induction by hypoxia. The introduction of K-ras(Val12) augmented the hypoxic induction of VEGF, and this was observed in wild-type and HIF-1alpha knockdown colon cancer cells. Thus, VEGF may be induced by hypoxia through HIF-dependent and HIF-independent pathways, and K-ras also can induce VEGF in hypoxia independent of HIF-1. These findings suggest the existence of multiple mechanisms regulating the hypoxic induction of VEGF in colon cancer.

Hypoxic up-regulation of triosephosphate isomerase expression in mouse brain capillary endothelial cells

A protein with a molecular mass of 27kDa was induced by hypoxia in a mouse brain capillary endothelial cell line and identified as triosephosphate isomerase (TPI) by amino-terminal sequencing. Hypoxia caused an elevation of the TPI protein level, concomitant with an increase of the TPI mRNA level. However, hypoxia resulted in an insufficient elevation of TPI activity level, compared to an increase of TPI protein level. When cells expressing the recombinant TPI protein with histidine tag were exposed to hypoxia and the TPI protein was affinity-purified, the catalytic activity (specific activity) of the TPI protein purified from hypoxic cells was substantially lower than that obtained from normoxic cells. In addition, three TPI isoforms with an electrophoretic multiplicity were found; two of the three isoforms were substantially increased in response to the hypoxia, but the level of the most acidic isoform was barely changed. The induction of TPI gene expression by hypoxia was suppressed by (1) a chelator of intracellular Ca(2+), (2) a blocker of non-selective cation channels, (3) a blocker of Na(+)/Ca(2+) exchangers, (4) an inhibitor of Ca(2+)/calmodulin-dependent protein kinases, and (5) an inhibitor of c-jun/AP-1 activation.

The von Hippel Lindau/Hypoxia-inducible Factor (HIF) Pathway Regulates the Transcription of the HIF-Proline Hydroxylase Genes in Response to Low Oxygen

Most of the genes induced by hypoxia are regulated by a family of transcription factors termed hypoxia-inducible factors (HIF). Under normoxic conditions, HIFalpha proteins are very unstable due to hydroxylation by a recently described family of proline hydroxylases termed EGL-Nine homologs (EGLN). Upon hydroxylation, HIFalpha is recognized by the product of the tumor suppressor vhl and targeted for proteosomal degradation. Since EGLNs require oxygen to catalyze HIF hydroxylation, this reaction does not efficiently occur under low oxygen tension. Thus, under hypoxia, HIFalpha escapes from degradation and transcribes target genes. The mRNA levels of two of the three EGLNs described to date are induced by hypoxia, suggesting that they might be novel HIF target genes; however, no proof for this hypothesis has been reported. Here we show that the induction of EGLN1 and -3 by hypoxia is found in a wide range of cell types. The basal levels of EGLN3 are always well below those of EGLN1 and EGLN2, and its induction by hypoxia is larger than that found for EGLN1. The inhibitor of transcription, actinomycin D, prevents the increase of EGLN3 mRNA induced by hypoxia, indicating that it is due to enhanced gene expression. Interestingly, EGLN1 and EGLN3 mRNAs were also triggered by EGLN inhibitors, suggesting the involvement of HIFalpha in the control of its transcription. In agreement with this possibility, pVHL-deficient cell lines, which present high HIF activity under normoxia, also showed dramatically increased normoxic levels of EGLN3. Moreover, the overexpression of an oxygen-insensitive mutant form of HIFalpha resulted in increased normoxic levels of EGLN3 mRNA. Finally, hypoxic induction of EGLNs was not observed in cells lacking functional HIFalpha.

Estrogen receptor inhibits c-Jun-dependent stress-induced cell death by binding and modifying c-Jun activity in human breast cancer cells

c-Jun, a major component of the AP-1 transcription factor, is either pro- or anti-apoptotic with cellular determinants unknown. Nuclear estrogen receptor (ER), on the other hand, regulates gene expression through both estrogen response elements and AP-1. Here we show that stress stimulates c-Jun phosphorylation and AP-1 activity in both ER+ and ER- human breast cancer cells and only induces cell death in ER- cells, indicating a determinant role of ER in c-Jun/AP-1 activity. The inhibitory effect of ER in stress-induced cell death is confirmed by ER transfection into ER- cells. Furthermore, inhibition of c-Jun activation by a dominant negative c-Jun blocks AP-1 activity in ER+ cells and attenuates stress-induced cell death but not AP-1 activity in ER- cells, suggesting that the c-Jun/AP-1 activity has distinct properties depending on ER status. ER was shown to inhibit stress-induced cell death through its physical interaction with c-Jun. This is because ER binds c-Jun in breast cancer cells, stress treatment further increases the ER-bound phosphorylated c-Jun, and the c-Jun binding-deficient ER mutant fails to protect stress-induced cell death. Together, our studies reveal a novel function of ER in stress response by modification of c-Jun activity.

Mechanisms of hypoxic gene regulation of angiogenesis factor Cyr61 in melanoma cells

Hypoxia has a profound influence on progression and metastasis of malignant tumors. In the present report, we used the oligonucleotide microarray technique to identify new hypoxia-inducible genes in malignant melanoma with a special emphasis on angiogenesis factors. A commercially available Affymetrix gene chip system was used to analyze five melanoma cell lines of different aggressiveness. A total of 160 hypoxia-inducible genes were identified, clustering in four different functional clusters. In search of putative angiogenesis and tumor progression factors within these clusters, Cyr61, a recently discovered angiogenesis factor, was identified. Cyr61 was hypoxia-inducible in low aggressive melanoma cells; however, it showed constitutive high expression in highly aggressive melanoma cells. Further analyses of transcriptional mechanisms underlying Cyr61 gene expression under hypoxia demonstrated that an AP-1 binding motif within the Cyr61 promoter plays a central role in the hypoxic regulation of Cyr61. It could be shown by use of in vitro luciferase assays, electrophoretic mobility shift assays, and immunoprecipitation that hypoxia-inducible factor-1alpha interacts with c-Jun/AP-1 and may thereby contribute to Cyr61 transcriptional regulation under hypoxia. Taken together, the presented data show that Cyr61 is a hypoxia-inducible angiogenesis factor in malignant melanoma with tumor stage-dependent expression. This may argue for a hypoxia-induced selection process during tumor progression toward melanoma cells with constitutive high Cyr61 expression.

Hepatocyte growth factor signaling regulates transactivation of genes belonging to the plasminogen activation system via hypoxia inducible factor-1

Hepatocyte growth factor (HGF) plays an important role in tumor growth and progression also by regulating invasive/metastatic phenotype and angiogenesis. Here we report that a molecular mechanism possibly contributing to these functions of HGF may be hypoxia inducible factor-1 (HIF-1)-dependent expression of genes of the plasminogen activation system. The following findings support this conclusion: (1) HGF enhanced the activity of a luciferase reporter construct under the control of multiple HIF-1 responsive elements (HRE) in HepG2 cells, and the cotransfection of the dominant negative for the beta-subunit (ARNT) prevented this increase; (2) HGF activated uPA and PAI-1 promoters through HIF-1 activity regulated by PI3K/JNK1 transducers, as demonstrated by cotransfection with the reporter gene promoters and the dominant negative for ARNT, p85 subunit of PI3K or JNK1; (3) hypoxia was additive to HGF in increasing reporter vector activities, but probably through different transduction pathways; (4) JNK1 wild-type expression vector increased HIF-1alpha protein expression probably in a phosphorylated state and, thus, functional for transactivating activity; and (5) c-Jun did not seem to be involved in the activation of the luciferase construct containing multiple HREs because it was not prevented by expression of TAM-67, which is the dominant negative mutant form for c-Jun.

Hypoxia induces differentiation of pulmonary artery adventitial fibroblasts into myofibroblasts

Activation of the alpha-smooth muscle actin (alpha-SMA) gene during the conversion of fibroblasts into myofibroblasts is an essential feature of various fibrotic conditions. Microvascular compromise and thus local environmental hypoxia are important components of the fibrotic response. The present study was thus undertaken to test the hypothesis that hypoxia can induce transdifferentiation of vascular fibroblasts into myofibroblasts and also to evaluate potential signaling mechanisms governing this process. We found that hypoxia significantly upregulates alpha-SMA protein levels in bovine pulmonary artery adventitial fibroblasts. Increased alpha-SMA expression is controlled at the transcriptional level because the alpha-SMA gene promoter activity, assayed via a luciferase reporter, was markedly increased in transfected fibroblasts exposed to hypoxia. Hypoxic induction of the alpha-SMA gene was mimicked by overexpression of constitutively active Galphai2 (alphai2Q205L) but not Galpha16 (alpha-16Q212L). Blockade of hypoxia-induced alpha-SMA expression with pertussis toxin, a Galphai antagonist, confirmed a role for Galphai in the hypoxia-induced transdifferentiation process. c-Jun NH2-terminal kinase (JNK) inhibitor II and SB202190, but not U0126, also attenuated alpha-SMA expression in hypoxic fibroblasts, suggesting the importance of JNK in the differentiation process. Hypoxia-induced increase in bromodeoxyuridine incorporation, which occurred concomitantly with hypoxia-induced differentiation, was blocked by U0126, suggesting that DNA synthesis and alpha-SMA expression take place through simultaneously activated parallel signaling pathways. Neutralizing antibody against transforming growth factor-beta1 blocked only 30% of the hypoxia-induced alpha-SMA promoter activity. Taken together, our results suggest that hypoxia induces differentiation of vascular fibroblasts into myofibroblasts by upregulating the expression of alpha-SMA, and this increase in alpha-SMA level occurs through Galphai- and JNK-dependent signaling pathways.

Role of the glucocorticoid receptor for regulation of hypoxia-dependent gene expression

Glucocorticoids are secreted from the adrenal glands and act as a peripheral effector of the hypothalamic-pituitary-adrenal axis, playing an essential role in stress response and homeostatic regulation. In target cells, however, it remains unknown how glucocorticoids fine-tune the cellular pathways mediating tissue and systemic adaptation. Recently, considerable evidence indicates that adaptation to hypoxic environments is influenced by glucocorticoids and there is cross-talk between hypoxia-dependent signals and glucocorticoid-mediated regulation of gene expression. We therefore investigated the interaction between these important stress-responsive pathways, focusing on the glucocorticoid receptor (GR) and hypoxia-inducible transcription factor HIF-1. Here we show that, under hypoxic conditions, HIF-1-dependent gene expression is further up-regulated by glucocorticoids via the GR. This up-regulation cannot be substituted by the other steroid receptors and is suggested to result from the interaction between the GR and the transactivation domain of HIF-1 alpha. Moreover, our results also indicate that the ligand binding domain of the GR is essential for this interaction, and the critical requirement for GR agonists suggests the importance of the ligand-mediated conformational change of the GR. Because these proteins are shown to colocalize in the distinct compartments of the nucleus, we suggest that these stress-responsive transcription factors have intimate communication in close proximity to each other, thereby enabling the fine-tuning of cellular responses for adaptation.

Expression of proinflammatory cytokines via HIF-1alpha and NF-kappaB activation on desferrioxamine-stimulated HMC-1 cells

We investigated the expression and the role of hypoxia-inducible factor 1alpha (HIF-1alpha) on the desferrioxamine (DFX)-induced cytokine production in human mast cells, HMC-1 cells. HIF-1alpha mRNA was constitutively expressed in mast cell lines including the P815, RBL-2H3, and HMC-1. DFX (100 microM) resulted in a great increase in protein levels of HIF-1alpha in HMC-1 cells, but it did not affect HIF-1alpha mRNA expression. Iron (HIF-1 inhibitor) inhibited increase of HIF-1alpha and NF-kappaB protein levels. Pyrriolidine-dithiocarbamate (PDTC, NF-kappaB inhibitor) inhibited increase of NF-kappaB protein levels, but it slightly increased HIF-1alpha protein levels. In addition, DFX significantly increased the production of IL-6, IL-8, and TNF-alpha in HMC-1 (P<0.05). These increased cytokine levels were significantly inhibited by treatment of iron or PDTC in a dose-dependent manner (P<0.05). We demonstrated the regulatory effects of HIF-1alpha on the DFX-induced proinflammatory cytokine production in human mast cells for the first time. These data indicate that inflammatory cytokines seem to be under HIF-1alpha or NF-kappaB transcriptional regulation in the hypoxic conditions on mast cells.

SSeCKS regulates angiogenesis and tight junction formation in blood-brain barrier

The blood-brain barrier (BBB) is essential for maintaining brain homeostasis and low permeability. BBB maintenance is important in the central nervous system (CNS) because disruption of the BBB may contribute to many brain disorders, including Alzheimer disease and ischemic stroke. The molecular mechanisms of BBB development remain ill-defined, however. Here we report that src-suppressed C-kinase substrate (SSeCKS) decreases the expression of vascular endothelial growth factor (VEGF) through AP-1 reduction and stimulates expression of angiopoietin-1 (Ang-1), an antipermeability factor in astrocytes. Conditioned media from SSeCKS-overexpressing astrocytes (SSeCKS-CM) blocked angiogenesis in vivo and in vitro. Moreover, SSeCKS-CM increased tight junction proteins in endothelial cells, consequently decreasing [3H]sucrose permeability. Furthermore, immunoreactivity to SSeCKS gradually increased during the BBB maturation period, and SSeCKS-expressing astrocytes closely interacted with zonula occludens (ZO)-1-expressing blood vessels in vivo. Collectively, our results suggest that SSeCKS regulates BBB differentiation by modulating both brain angiogenesis and tight junction formation.

Hepatocyte growth factor induces apoptosis through the extrinsic pathway in hepatoma cells: favouring role of hypoxia-inducible factor-1 deficiency

Two hepatocarcinoma cell lines, the Hepa-1 wild-type (c1c7) and the beta-subunit mutated (c4) lacking hypoxia-inducible factor-1 (HIF-1) activity, were differentially susceptible to apoptosis by hepatocyte growth factor (HGF). The c4 cells were 40% apoptotic 48 h after HGF treatment. On the contrary, the wild-type c1c7 cells showed modest signs of apoptosis only at 72 h. The revertant vT[2] cells, consisting of c4 cells stably transfected with HIF-1beta expression vector, behaved as the parental cells. To understand the mechanisms of this different sensitivity, we examined a panel of genes involved in apoptosis: ornithine decarboxylase, c-Myc and p53 protein levels progressively decreased while JNK1, caspase 8 and 3 activities persistently increased in c4 cells undergoing apoptosis. Distinct time-related events in c1c7 cells were the transient activations of JNK1 and caspase 8 followed by the accumulation of ODC and c-Myc proteins. The proapoptotic effect of HGF in c4 hepatocarcinoma cells seems to be related to HIF-1 deficiency with loss of cytoprotective and signalling functions. This may contribute to the triggering of the extrinsic pathway consisting in caspase 8 activation, which in turn causes BID cleavage and cytochrome c release. The effector caspase 3 is also activated.

Molecular basis of skeletal muscle plasticity--from gene to form and function

Skeletal muscle shows an enormous plasticity to adapt to stimuli such as contractile activity (endurance exercise, electrical stimulation, denervation), loading conditions (resistance training, microgravity), substrate supply (nutritional interventions) or environmental factors (hypoxia). The presented data show that adaptive structural events occur in both muscle fibres (myofibrils, mitochondria) and associated structures (motoneurons and capillaries). Functional adaptations appear to involve alterations in regulatory mechanisms (neuronal, endocrine and intracellular signalling), contractile properties and metabolic capacities. With the appropriate molecular techniques it has been demonstrated over the past 10 years that rapid changes in skeletal muscle mRNA expression occur with exercise in human and rodent species. Recently, gene expression profiling analysis has demonstrated that transcriptional adaptations in skeletal muscle due to changes in loading involve a broad range of genes and that mRNA changes often run parallel for genes in the same functional categories. These changes can be matched to the structural/functional adaptations known to occur with corresponding stimuli. Several signalling pathways involving cytoplasmic protein kinases and nuclear-encoded transcription factors are recognized as potential master regulators that transduce physiological stress into transcriptional adaptations of batteries of metabolic and contractile genes. Nuclear reprogramming is recognized as an important event in muscle plasticity and may be related to the adaptations in the myosin type, protein turnover, and the cytoplasma-to-myonucleus ratio. The accessibility of muscle tissue to biopsies in conjunction with the advent of high-throughput gene expression analysis technology points to skeletal muscle plasticity as a particularly useful paradigm for studying gene regulatory phenomena in humans.

Molecular responses to hypoxia in tumor cells

Highly aggressive, rapidly growing tumors are exposed to hypoxia or even anoxia which occurs as a consequence of inadequate blood supply. Both hypoxia and consecutive hypoxia/reoxygenation exert a variety of influences on tumor cell biology. Among these are activation of certain signal transduction pathways and gene regulatory mechanisms, induction of selection processes for gene mutations, tumor cell apoptosis and tumor angiogenesis. Most of these mechanisms contribute to tumor progression. Therefore, tissue hypoxia has been regarded as a central factor for tumor aggressiveness and metastasis. In this review, we summarize the current knowledge about the molecular mechanisms induced by tumor cell hypoxia with a special emphasis on intracellular signal transduction, gene regulation, angiogenesis and apoptosis. Interfering with these pathways might open perspectives for future innovative treatment of highly aggressive metastasizing tumors.

Evidence for a role of p38 kinase in hypoxia-inducible factor 1-independent induction of vascular endothelial growth factor expression by sodium arsenite

Recently we have demonstrated that sodium arsenite induces the expression of hypoxia-inducible factor 1alpha (HIF-1alpha) protein and vascular endothelial growth factor (VEGF) in OVCAR-3 human ovarian cancer cells. We now show that arsenic trioxide, an experimental anticancer drug, exerts the same effects. The involvement of phosphatidylinositol 3-kinase and mitogen-activated protein kinase (MAPK) pathways in the effects of sodium arsenite was investigated. By using kinase inhibitors in OVCAR-3 cells, both effects of sodium arsenite were found to be independent of phosphatidylinositol 3-kinase and p44/p42 MAPKS but were attenuated by inhibition of p38 MAPK. A role for p38 in the regulation of HIF-1alpha and VEGF expression was supported further by analysis of activation kinetics. Experiments in mouse fibroblast cell lines, lacking expression of c-Jun N-terminal kinases 1 and 2, suggested that these kinases are not required for induction of HIF-1alpha protein and VEGF mRNA. Unexpectedly, sodium arsenite did not activate a HIF-1-dependent reporter gene in OVCAR-3 cells, indicating that functional HIF-1 was not induced. In agreement with this hypothesis, up-regulation of VEGF mRNA was not reduced in HIF-1alpha(-/-) mouse fibroblast cell lines. Altogether, these data suggest that not HIF-1, but rather p38, mediates induction of VEGF mRNA expression by sodium arsenite.

Hypoxia and High Altitude

Increased erythropoietin plasma levels and the consequent augmented production of red blood cells is the best known systemic adaptation to reduced oxygen partial pressure (pO2). Intensive research during the last years revealed that the molecular mechanism behind the regulation of erythropoietin is ubiquitous and has far more implications than first thought. Erythropoietin regulation results from the activation of the hypoxia-inducible factor-1 (HIF-1) pathway under hypoxic conditions. HIF-1 is a heterodimer consisting of an oxygen sensitive--HIF-1--and an oxygen-independent subunit--HIF-1beta (also known as the aryl hydrocarbon receptor nuclear translocator--ARNT). In addition to erythropoietin, more than 30 genes are now known to be up-regulated by HIF-1. Recently, the critical involvement of HIF-1alpha post-translational modifications in the cellular oxygen sensing mechanism was discovered. In this review we will focus on the regulation of the HIF-1 pathway and the cellular oxygen sensor and discuss their implications in high altitude hypoxia.

Activation of Rac, Cdc42 and other downstream signalling molecules by Bartonella bacilliformis during entry into human endothelial cells

Bartonella bacilliformis is an intracellular bacterial pathogen of human endothelial cells. In vitro incubation of B. bacilliformis with human endothelial cells leads to the formation of filamentous actin extensions (filopodia) within 30 min, followed by formation of membrane rufflings or lamellipodia within 1 h of incubation. By immunofluorescence, F-actin phalloidin staining and anti-Rac antibodies were shown to co-localize in the membrane rufflings, indicating the recruitment of activated Rac at lamellipodia. Preincubation of endothelial cells with the Clostridial toxin, TcdB-10463, which inactivates the Rho-family GTPases, Rho, Rac and Cdc42, inhibited the entry of B. bacilliformis by 50-90%. Preincubation of endothelial cells with the Clostridial toxin, TcsL-1522, which specifically inactivates Rac and, to a lesser extent, Cdc42, but not Rho, inhibited entry by 30-40%. A 3.4-5.0-fold increase in activated (GTP-bound) -intracellular Rac and Cdc42 was observed in affinity precipitation assays. Increased kinase activity of p21-activated kinase (PAK), a specific downstream effector of activated Rac/Cdc42 was also observed during the time course of infection. Activation of SAPK/JNK-1 and 2, and p38 MAPKs in signalling pathways, was also detected during infection with Bartonella, as was increased binding activity of AP-1 transcription factor.

The response of c-jun/AP-1 to chronic hypoxia is hypoxia-inducible factor 1 alpha dependent

Hypoxia (low-oxygen tension) is an important physiological stress that influences responses to a wide range of pathologies, including stroke, infarction, and tumorigenesis. Prolonged or chronic hypoxia stimulates expression of the stress-inducible transcription factor gene c-jun and transient activation of protein kinase and phosphatase activities that regulate c-Jun/AP-1 activity. Here we describe evidence obtained by using wild-type and HIF-1 alpha nullizygous mouse embryonic fibroblasts (mEFs) that the induction of c-jun mRNA expression and c-Jun phosphorylation by prolonged hypoxia are completely dependent on the presence of the oxygen-regulated transcription factor hypoxia-inducible factor 1 alpha (HIF-1 alpha). In contrast, transient hypoxia induced c-jun expression in both types of mEFs, showing that the early or rapid induction of this gene is independent of HIF-1 alpha. These findings indicate that the c-jun gene has a biphasic response to hypoxia consisting of inductions that depend on the degree or duration of exposure. To more completely define the relationship between prolonged hypoxia and c-Jun phosphorylation, we used mEFs from mice containing inactivating mutations of critical phosphorylation sites in the c-Jun N-terminal region (serines 63 and 73 or threonines 91 and 93). Exposure of these mEFs to prolonged hypoxia demonstrated an absolute requirement for N-terminal sites for HIF-1 alpha-dependent phosphorylation of c-Jun. Taken together, these findings suggest that c-Jun/AP-1 and HIF-1 cooperate to regulate gene expression in pathophysiological microenvironments.