MgcRacGAP interacts with HIF-1alpha and regulates its transcriptional activity

Genome editing and cancer therapy: handling the hypoxia-responsive pathway as a promising strategy

The precise characterization of oxygen-sensing pathways and the identification of pO2-regulated gene expression are both issues of critical importance. The O2-sensing system plays crucial roles in almost all the pivotal human processes, including the stem cell specification, the growth and development of tissues (such as embryogenesis), the modulation of intermediate metabolism (including the shift of the glucose metabolism from oxidative to anaerobic ATP production and vice versa), and the control of blood pressure. The solid cancer microenvironment is characterized by low oxygen levels and by the consequent activation of the hypoxia response that, in turn, allows a complex adaptive response characterized mainly by neoangiogenesis and metabolic reprogramming. Recently, incredible advances in molecular genetic methodologies allowed the genome editing with high efficiency and, above all, the precise identification of target cells/tissues. These new possibilities and the knowledge of the mechanisms of adaptation to hypoxia suggest the effective development of new therapeutic approaches based on the manipulation, targeting, and exploitation of the oxygen-sensor system molecular mechanisms.

Pyruvate dehydrogenase phosphatase 1 (PDP1) stimulates HIF activity by supporting histone acetylation under hypoxia

Cancer cells, when exposed to the hypoxic tumour microenvironment, respond by activating hypoxia-inducible factors (HIFs). HIF-1 mediates extensive metabolic re-programming, and expression of HIF-1α, its oxygen-regulated subunit, is associated with poor prognosis in cancer. Here we analyse the role of pyruvate dehydrogenase phosphatase 1 (PDP1) in the regulation of HIF-1 activity. PDP1 is a key hormone-regulated metabolic enzyme that dephosphorylates and activates pyruvate dehydrogenase (PDH), thereby stimulating the conversion of pyruvate into acetyl-CoA. Silencing of PDP1 down-regulated HIF transcriptional activity and the expression of HIF-dependent genes, including that of PDK1, the kinase that phosphorylates and inactivates PDH, opposing the effects of PDP1. Inversely, PDP1 stimulation enhanced HIF activity under hypoxia. Alteration of PDP1 levels or activity did not have an effect on HIF-1α protein levels, nuclear accumulation or interaction with its partners ARNT and NPM1. However, depletion of PDP-1 decreased histone H3 acetylation of HIF-1 target gene promoters and inhibited binding of HIF-1 to the respective hypoxia-response elements (HREs) under hypoxia. Furthermore, the decrease of HIF transcriptional activity upon PDP1 depletion could be reversed by treating the cells with acetate, as an exogenous source of acetyl-CoA, or the histone deacetylase (HDAC) inhibitor trichostatin A. These data suggest that the PDP1/PDH/HIF-1/PDK1 axis is part of a homeostatic loop which, under hypoxia, preserves cellular acetyl-CoA production to a level sufficient to sustain chromatin acetylation and transcription of hypoxia-inducible genes.

Short-term hypoxia triggers ROS and SAFB mediated nuclear matrix and mRNA splicing remodeling

The cellular response to hypoxia, in addition to HIF-dependent transcriptional reprogramming, also involves less characterized transcription-independent processes, such as alternative splicing of the VEGFA transcript leading to the production of the proangiogenic VEGF form. We now show that this event depends on reorganization of the splicing machinery, triggered after short-term hypoxia by ROS production and intranuclear redistribution of the nucleoskeletal proteins SAFB1/2. Exposure to low oxygen causes fast dissociation of SAFB1/2 from the nuclear matrix, which is reversible, inhibited by antioxidant treatment, and also observed under normoxia when the mitochondrial electron transport chain is blocked. This is accompanied by altered interactions between SAFB1/2 and the splicing machinery, translocation of kinase SRPK1 to the cytoplasm, and dephosphorylation of RS-splicing factors. Depletion of SAFB1/2 under normoxia phenocopies the hypoxic and ROS-mediated switch in VEGF mRNA splicing. These data suggest that ROS-dependent remodeling of the nuclear architecture can promote production of splicing variants that facilitate adaptation to hypoxia.

Cancer Cell Metabolism in Hypoxia: Role of HIF-1 as Key Regulator and Therapeutic Target

In order to meet the high energy demand, a metabolic reprogramming occurs in cancer cells. Its role is crucial in promoting tumor survival. Among the substrates in demand, oxygen is fundamental for bioenergetics. Nevertheless, tumor microenvironment is frequently characterized by low-oxygen conditions. Hypoxia-inducible factor 1 (HIF-1) is a pivotal modulator of the metabolic reprogramming which takes place in hypoxic cancer cells. In the hub of cellular bioenergetics, mitochondria are key players in regulating cellular energy. Therefore, a close crosstalk between mitochondria and HIF-1 underlies the metabolic and functional changes of cancer cells. Noteworthy, HIF-1 represents a promising target for novel cancer therapeutics. In this review, we summarize the molecular mechanisms underlying the interplay between HIF-1 and energetic metabolism, with a focus on mitochondria, of hypoxic cancer cells.

Specific Inhibition of HIF Activity: Can Peptides Lead the Way?

Simple Summary

Cancer cells in solid tumors often experience lack of oxygen (hypoxia), which they overcome with the help of hypoxia inducible transcription factors (HIFs). When HIFs are activated, they stimulate the expression of many genes and cause the production of proteins that help cancer cells grow and migrate even in the presence of very little oxygen. Many experiments have shown that agents that block the activity of HIFs (HIF inhibitors) can prevent growth of cancer cells under hypoxia and, subsequently, hinder formation of malignant tumors or metastases. Most small chemical HIF inhibitors lack the selectivity required for development of safe anticancer drugs. On the other hand, peptides derived from HIFs themselves can be very selective HIF inhibitors by disrupting specific associations of HIFs with cellular components that are essential for HIF activation. This review discusses the nature of available peptide HIF inhibitors and their prospects as effective pharmaceuticals against cancer.

Abstract

Reduced oxygen availability (hypoxia) is a characteristic of many disorders including cancer. Central components of the systemic and cellular response to hypoxia are the Hypoxia Inducible Factors (HIFs), a small family of heterodimeric transcription factors that directly or indirectly regulate the expression of hundreds of genes, the products of which mediate adaptive changes in processes that include metabolism, erythropoiesis, and angiogenesis. The overexpression of HIFs has been linked to the pathogenesis and progression of cancer. Moreover, evidence from cellular and animal models have convincingly shown that targeting HIFs represents a valid approach to treat hypoxia-related disorders. However, targeting transcription factors with small molecules is a very demanding task and development of HIF inhibitors with specificity and therapeutic potential has largely remained an unattainable challenge. Another promising approach to inhibit HIFs is to use peptides modelled after HIF subunit domains known to be involved in protein–protein interactions that are critical for HIF function. Introduction of these peptides into cells can inhibit, through competition, the activity of endogenous HIFs in a sequence and, therefore also isoform, specific manner. This review summarizes the involvement of HIFs in cancer and the approaches for targeting them, with a special focus on the development of peptide HIF inhibitors and their prospects as highly-specific pharmacological agents.

Calcitriol Suppresses HIF-1 and HIF-2 Transcriptional Activity by Reducing HIF-1/2α Protein Levels via a VDR-Independent Mechanism

Hypoxia-inducible transcription factors 1 and 2 (HIFs) are major mediators of cancer development and progression and validated targets for cancer therapy. Although calcitriol, the biologically active metabolite of vitamin D, was attributed with anticancer properties, there is little information on the effect of calcitriol on HIFs and the mechanism underling this activity. Here, we demonstrate the negative effect of calcitriol on HIF-1/2α protein levels and HIF-1/2 transcriptional activity and elucidate the molecular mechanism of calcitriol action. We also reveal that the suppression of vitamin D receptor (VDR) expression by siRNA does not abrogate the negative regulation of HIF-1α and HIF-2α protein levels and HIF-1/2 transcriptional activity by calcitriol, thus testifying that the mechanism of these actions is VDR independent. At the same time, calcitriol significantly reduces the phosphorylation of Akt protein kinase and its downstream targets and suppresses HIF-1/2α protein synthesis by inhibiting HIF1A and EPAS1 (Endothelial PAS domain-containing protein 1) mRNA translation, without affecting their mRNA levels. On the basis of the acquired data, it can be proposed that calcitriol reduces HIF-1α and HIF-2α protein levels and inhibits HIF-1 and HIF-2 transcriptional activity by a VDR-independent, nongenomic mechanism that involves inhibition of PI3K/Akt signaling pathway and suppression of HIF1A and EPAS1 mRNA translation.

ERK1/2 phosphorylates HIF-2α and regulates its activity by controlling its CRM1-dependent nuclear shuttling

Hypoxia-inducible factor 2 (HIF-2) is a principal component of the cellular response to oxygen deprivation (hypoxia). Its inducible subunit, HIF-2α (also known as EPAS1), is controlled by oxygen-dependent as well as oxygen-independent mechanisms, such as phosphorylation. We show here that HIF-2α is phosphorylated under hypoxia (1% O2) by extracellular signal-regulated protein kinases 1 and 2 (ERK1/2; also known as MAPK3 and MAPK1, respectively) at serine residue 672, as identified by in vitro phosphorylation assays. Mutation of this site to an alanine residue or inhibition of the ERK1/2 pathway decreases HIF-2 transcriptional activity and causes HIF-2α to mislocalize to the cytoplasm without changing its protein expression levels. Localization, reporter gene and immunoprecipitation experiments further show that HIF-2α associates with the exportin chromosomal maintenance 1 (CRM1, also known as XPO1) in a phosphorylation-sensitive manner and identify two critical leucine residues as part of an atypical CRM1-dependent nuclear export signal (NES) neighboring serine 672. Inhibition of CRM1 or mutation of these residues restores nuclear accumulation and activity of HIF-2α lacking the ERK1/2-mediated modification. In summary, we reveal a novel regulatory mechanism of HIF-2, involving ERK1/2-dependent phosphorylation of HIF-2α, which controls its nucleocytoplasmic shuttling and the HIF-2 transcriptional activity.This article has an associated First Person interview with the first author of the paper.

Hypoxia-Inducible Factors and the Regulation of Lipid Metabolism

Oxygen deprivation or hypoxia characterizes a number of serious pathological conditions and elicits a number of adaptive changes that are mainly mediated at the transcriptional level by the family of hypoxia-inducible factors (HIFs). The HIF target gene repertoire includes genes responsible for the regulation of metabolism, oxygen delivery and cell survival. Although the involvement of HIFs in the regulation of carbohydrate metabolism and the switch to anaerobic glycolysis under hypoxia is well established, their role in the control of lipid anabolism and catabolism remains still relatively obscure. Recent evidence indicates that many aspects of lipid metabolism are modified during hypoxia or in tumor cells in a HIF-dependent manner, contributing significantly to the pathogenesis and/or progression of cancer and metabolic disorders. However, direct transcriptional regulation by HIFs has been only demonstrated in relatively few cases, leaving open the exact and isoform-specific mechanisms that underlie HIF-dependency. This review summarizes the evidence for both direct and indirect roles of HIFs in the regulation of genes involved in lipid metabolism as well as the involvement of HIFs in various diseases as demonstrated by studies with transgenic animal models.

Protein phosphatase PPP3CA (calcineurin A) down-regulates hypoxia-inducible factor transcriptional activity

Hypoxia-inducible factors (HIF) are master regulators of the response to hypoxia. Although several kinases are known to modify their oxygen sensitive HIF-α subunits or affect indirectly their function, little is known about the role of phosphatases in HIF control. To address this issue, a library containing siRNAs for the 25 known catalytic subunits of human phosphatases was used to screen for their effect on HIF transcriptional activity in HeLa cells. Serine-threonine phosphatase PPP3CA (calcineurin A, isoform a) was identified as the strongest candidate for a negative regulator of HIF activity. Indeed, independent silencing of PPP3CA expression stimulated HIF transcriptional activity under hypoxia, without increasing the protein levels of HIF-1α or HIF-2α. Overexpression of a constitutively active PPP3CA form, but not its catalytically inactive counterpart, inhibited HIF activity and expression of HIF target genes but did not affect HIF-1α or HIF-2α expression. These results were phenocopied by treatment with the ionophore ionomycin, that activates endogenous PPP3CA. The effect of ionomycin was mediated by PPP3CA as it was largely abolished by PPP3CA silencing. Furthermore, ionomycin enhanced the down-regulation of HIF activity by wild-type PPP3CA overexpression. Overall, our results suggest the involvement of PPP3CA in fine-tuning the HIF-dependent transcriptional response to hypoxia.

HIF-1α-derived cell-penetrating peptides inhibit ERK-dependent activation of HIF-1 and trigger apoptosis of cancer cells under hypoxia

Hypoxia is frequently encountered in the microenvironment of solid tumors. Hypoxia-inducible factors (HIFs), the main effectors of cell response to hypoxia, promote cancer cell survival and progression. HIF-1α, the oxygen-regulated subunit of HIF-1, is often correlated with oncogenesis and represents an attractive therapeutic target. We have previously reported that activation HIF-1α by ERK involves modification of two serine residues and masking of a nuclear export signal (NES), all inside a 43-amino acid domain termed ERK Targeted Domain (ETD). Overexpression of ETD variants including wild-type, phospho-mimetic (SE) or NES-less (IA) mutant forms caused HIF-1 inactivation in two hepatocarcinoma cell lines, while a phospho-deficient (SA) form was ineffective and acted as a sequence-specific negative control. To deliver these ETD forms directly into cancer cells, they were fused to the HIV TAT-sequence and produced as cell-permeable peptides. When the TAT-ETD peptides were added to the culture medium of Huh7 cells, they entered the cells and, with the exception of ETD-SA, accumulated inside the nucleus, caused mislocalization of endogenous HIF-1α to the cytoplasm, significant reduction of HIF-1 activity and inhibition of expression of specific HIF-1, but not HIF-2, gene targets under hypoxia. More importantly, transduced nuclear TAT-ETD peptides restricted migration, impaired colony formation and triggered apoptotic cell death of cancer cells grown under hypoxia, while they produced no effects in normoxic cells. These data demonstrate the importance of ERK-mediated activation of HIF-1 for low oxygen adaptation and the applicability of ETD peptide derivatives as sequence-specific HIF-1 and cancer cell growth inhibitors under hypoxia.

Expression of AGPAT2, an enzyme involved in the glycerophospholipid/triacylglycerol biosynthesis pathway, is directly regulated by HIF-1 and promotes survival and etoposide resistance of cancer cells under hypoxia

Hypoxia inducible factor-1 (HIF-1) supports survival of normal cells under low oxygen concentration and cancer cells in the hypoxic tumor microenvironment. This involves metabolic reprogramming via upregulation of glycolysis, downregulation of oxidative phosphorylation and, less well documented, effects on lipid metabolism. To investigate the latter, we examined expression of relevant enzymes in cancer cells grown under hypoxia. We show that expression of acylglycerol-3-phosphate acyltransferase 2 (AGPAT2), also known as lysophosphatidic acid acyltransferase β (LPAATβ), was upregulated under hypoxia and this was impaired by siRNA-mediated knockdown of HIF-1α. Moreover, a sequence of the AGPAT2 gene promoter region, containing 6 putative Hypoxia Response Elements (HREs), activated transcription of a reporter gene under hypoxic conditions or in normoxic cells over-expressing HIF-1α. Chromatin immunoprecipitation experiments confirmed binding of HIF-1α to one of these HREs, mutation of which abolished hypoxic activation of the AGPAT2 promoter. Knockdown of AGPAT2 by siRNA reduced lipid droplet accumulation and cell viability under hypoxia and increased cancer cell sensitivity to the chemotherapeutic etoposide. In conclusion, our findings demonstrate that AGPAT2, which is mutated in patients with congenital generalized lipodystrophy and over-expressed in different types of cancer, is a direct transcriptional target of HIF-1, suggesting that upregulation of lipid storage by HIF-1 plays an important role in adaptation and survival of cancer cells under low oxygen conditions.

Hypoxia upregulates integrin gene expression in microvascular endothelial cells and promotes their migration and capillary-like tube formation

Tissue hypoxia affects gene expression through the hypoxia-inducible transcription factors, HIF-1 and HIF-2, in both physiological and pathological angiogenesis. Angiogenesis is a complex response of endothelial cells integrating cell proliferation, migration, tube formation, and their interaction with the extracellular matrix through integrin receptors. In this report, we studied the effect of hypoxia on the angiogenic functions of human microvascular endothelial cells (HMEC-1) as well as on expression of the angiogenic integrins α_ν β₃ , α_ν β₅ , and α₅ β₁ . Exposure of HMEC-1 to hypoxia (1% O₂ ) or to DMOG, a prolyl-4-hydroxylase inhibitor, caused significant reduction to their proliferation rate, whereas their migration ability toward laminin-1 or collagen IV and capillary-like tube formation were significantly enhanced. In addition, α_v , β₁ , β₃ , and β₅ integrins expression was increased under hypoxia in HMEC-1, while α₅ integrin was not affected. Both HIF-1 and HIF-2 protein expression and transcriptional activity were induced under hypoxia in HMEC-1. The knockdown of either HIF-1α or HIF-2α inhibited integrin β₃ hypoxic stimulation, suggesting a HIF-dependent induction of β₃ integrin in HMEC-1. Taken together, our results indicate that hypoxia transcriptionally up-regulates angiogenic integrins in microvascular endothelial cells along with promoting migration and tube formation of HMEC-1.

Mortalin-mediated and ERK-controlled targeting of HIF-1α to mitochondria confers resistance to apoptosis under hypoxia

Hypoxia inducible factor-1 (HIF-1) is the main transcriptional activator of the cellular response to hypoxia and an important target of anticancer therapy. Phosphorylation by ERK1 and/or ERK2 (MAPK3 and MAPK1, respectively; hereafter ERK) stimulates the transcriptional activity of HIF-1α by inhibiting its CRM1 (XPO1)-dependent nuclear export. Here, we demonstrate that phosphorylation by ERK also regulates the association of HIF-1α with a so-far-unknown interaction partner identified as mortalin (also known as GRP75 and HSPA9), which mediates non-genomic involvement of HIF-1α in apoptosis. Mortalin binds specifically to HIF-1α that lacks modification by ERK, and the HIF-1α-mortalin complex is localized outside the nucleus. Under hypoxia, mortalin mediates targeting of unmodified HIF-1α to the outer mitochondrial membrane, as well as association with VDAC1 and hexokinase II, which promotes production of a C-terminally truncated active form of VDAC1, denoted VDAC1-ΔC, and protection from apoptosis when ERK is inactivated. Under normoxia, transcriptionally inactive forms of unmodified HIF-1α or its C-terminal domain alone are also targeted to mitochondria, stimulate production of VDAC1-ΔC and increase resistance to etoposide- or doxorubicin-induced apoptosis. These findings reveal an ERK-controlled, unconventional and anti-apoptotic function of HIF-1α that might serve as an early protective mechanism upon oxygen limitation and promote cancer cell resistance to chemotherapy.

HIF-2α phosphorylation by CK1δ promotes erythropoietin secretion in liver cancer cells under hypoxia

Hypoxia inducible factor 2 (HIF-2) is a transcriptional activator implicated in the cellular response to hypoxia. Regulation of its inducible subunit, HIF-2α (also known as EPAS1), involves post-translational modifications. Here, we demonstrate that casein kinase 1δ (CK1δ; also known as CSNK1D) phosphorylates HIF-2α at Ser383 and Thr528 in vitro We found that disruption of these phosphorylation sites, and silencing or chemical inhibition of CK1δ, reduced the expression of HIF-2 target genes and the secretion of erythropoietin (EPO) in two hepatic cancer cell lines, Huh7 and HepG2, without affecting the levels of HIF-2α protein expression. Furthermore, when CK1δ-dependent phosphorylation of HIF-2α was inhibited, we observed substantial cytoplasmic mislocalization of HIF-2α, which was reversed upon the addition of the nuclear protein export inhibitor leptomycin B. Taken together, these data suggest that CK1δ enhances EPO secretion from liver cancer cells under hypoxia by modifying HIF-2α and promoting its nuclear accumulation. This modification represents a new mechanism of HIF-2 regulation that might allow HIF isoforms to undertake differing functions.

A tumoural angiogenic gateway blocker, Benzophenone-1B represses the HIF-1α nuclear translocation and its target gene activation against neoplastic progression

Hypoxia is an important module in all solid tumours to promote angiogenesis, invasion and metastasis. Stabilization and subsequent nuclear localization of HIF-1α subunits result in the activation of tumour promoting target genes such as VEGF, MMPs, Flt-1, Ang-1 etc. which plays a pivotal role in adaptation of tumour cells to hypoxia. Increased HIF-α and its nuclear translocation have been correlated with pronounced angiogenesis, aggressive tumour growth and poor patient prognosis leading to current interest in HIF-1α as an anticancer drug target. Benzophenone-1B ([4-(1H-benzimidazol-2-ylmethoxy)-3,5-dimethylphenyl]-(4-methoxyphenyl) methanone, or BP-1B) is a new antineoplastic agent with potential angiopreventive effects. Current investigation reports the cellular biochemical modulation underlying BP-1B cytotoxic/antiangiogenic effects. Experimental evidences postulate that BP-1B exhibits the tumour specific cytotoxic actions against various cancer types with prolonged action. Moreover BP-1B efficiently counteracts endothelial cell capillary formation in in-vitro, in-vivo non-tumour and tumour angiogenic systems. Molecular signaling studies reveal that BP-1B arrests nuclear translocation of HIF-1α devoid of p42/44 pathway under CoCl₂ induced hypoxic conditions in various cancer cells thereby leading to abrogated HIF-1α dependent activation of VEGF-A, Flt-1, MMP-2, MMP -9 and Ang-1 angiogenic factors resulting in retarded cell migration and invasions. The in-vitro results were reproducible in the reliable in-vivo solid tumour model. Taken together, we conclude that BP-1B impairs angiogenesis by blocking nuclear localization of HIF-1α which can be translated into a potent HIF-1α inhibitor.

CK1δ restrains lipin-1 induction, lipid droplet formation and cell proliferation under hypoxia by reducing HIF-1α/ARNT complex formation

Proliferation of cells under hypoxia is facilitated by metabolic adaptation, mediated by the transcriptional activator Hypoxia Inducible Factor-1 (HIF-1). HIF-1α, the inducible subunit of HIF-1 is regulated by oxygen as well as by oxygen-independent mechanisms involving phosphorylation. We have previously shown that CK1δ phosphorylates HIF-1α in its N-terminus and reduces its affinity for its heterodimerization partner ARNT. To investigate the importance of this mechanism for cell proliferation under hypoxia, we visually monitored HIF-1α interactions within the cell nucleus using the in situ proximity ligation assay (PLA) and fluorescence recovery after photobleaching (FRAP). Both methods show that CK1δ-dependent modification of HIF-1α impairs the formation of a chromatin binding HIF-1 complex. This is confirmed by analyzing expression of lipin-1, a direct target of HIF-1 that mediates hypoxic neutral lipid accumulation. Inhibition of CK1δ increases lipid droplet formation and proliferation of both cancer and normal cells specifically under hypoxia and in an HIF-1α- and lipin-1-dependent manner. These data reveal a novel role for CK1δ in regulating lipid metabolism and, through it, cell adaptation to low oxygen conditions.

An association study between hypoxia inducible factor-1alpha (HIF-1α) polymorphisms and osteonecrosis

Bone hypoxia resulting from impaired blood flow is the final pathway for the development of osteonecrosis (ON). The aim of this study was to evaluate if HIF-1α, the major transcription factor triggered by hypoxia, is genetically implicated in susceptibility to ON. For this we analyzed frequencies of three known HIF-1α polymorphisms: one in exon 2 (C111A) and two in exon 12 (C1772T and G1790A) and their association with ON in a Greek population. Genotype analysis was performed using PCR-RFLP and rare alleles were further confirmed with sequencing. We found that genotype and allele frequency of C1772T and G1790A SNP of HIF-1α (SNPs found in our cohort) were not significantly different in ON patients compared to control patients. Furthermore these SNPs could not be associated with the different subgroups of ON. At the protein level we observed that the corresponding mutations (P582S and A588T, respectively) are not significant for protein function since the activity, expression and localization of the mutant proteins is practically indistinguishable from wt in HEK293 and Saos-2 cells. These results suggest that these missense mutations in the HIF-1α gene are not important for the risk of developing ON.

Cobalt stimulates HIF-1-dependent but inhibits HIF-2-dependent gene expression in liver cancer cells

Hypoxia-inducible factors (HIFs) are transcriptional regulators that mediate the cellular response to low oxygen. Although HIF-1 is usually considered as the principal mediator of hypoxic adaptation, several tissues and different cell types express both HIF-1 and HIF-2 isoforms under hypoxia or when treated with hypoxia mimetic chemicals such as cobalt. However, the similarities or differences between HIF-1 and HIF-2, in terms of their tissue- and inducer-specific activation and function, are not adequately characterized. To address this issue, we investigated the effects of true hypoxia and hypoxia mimetics on HIF-1 and HIF-2 induction and specific gene transcriptional activity in two hepatic cancer cell lines, Huh7 and HepG2. Both hypoxia and cobalt caused rapid induction of both HIF-1α and HIF-2α proteins. Hypoxia induced erythropoietin (EPO) expression and secretion in a HIF-2-dependent way. Surprisingly, however, EPO expression was not induced when cells were treated with cobalt. In agreement, both HIF-1- and HIF-2-dependent promoters (of PGK and SOD2 genes, respectively) were activated by hypoxia while cobalt only activated the HIF-1-dependent PGK promoter. Unlike cobalt, other hypoxia mimetics such as DFO and DMOG activated both types of promoters. Furthermore, cobalt impaired the hypoxic stimulation of HIF-2, but not HIF-1, activity and cobalt-induced HIF-2α interacted poorly with USF-2, a HIF-2-specific co-activator. These data show that, despite similar induction of HIF-1α and HIF-2α protein expression, HIF-1 and HIF-2 specific gene activating functions respond differently to different stimuli and suggest the operation of oxygen-independent and gene- or tissue-specific regulatory mechanisms involving additional transcription factors or co-activators.

TNFα induces expression of HIF-1α mRNA and protein but inhibits hypoxic stimulation of HIF-1 transcriptional activity in airway smooth muscle cells

Airway smooth muscle cells (ASMCs) participate in tissue remodeling characteristic of airway inflammatory diseases like asthma. Inflammation and hypoxia pathways are often interconnected and the regulatory subunit of the hypoxia inducible factor, HIF-1α, has been recently shown to be induced by cytokines. Here we investigate the effect of individual or combined treatment of ASMCs with the inflammatory mediator TNFα and/or hypoxia on the expression of HIF-1α, HIF-1 targets and inflammation markers. TNFα enhances HIF-1α protein and mRNA levels, under both normoxia and hypoxia. TNFα-mediated induction of HIF-1α gene transcription is repressed by inhibition of the NF-κB pathway. Despite the up-regulation of HIF-1α protein, the transcription of HIF-1 target genes remains low in the presence of TNFα at normoxia and is even reduced at hypoxia. We show that the reduction in HIF-1 transcriptional activity by TNFα is due to inhibition of the interaction of HIF-1α with ARNT and subsequent blocking of its binding to HREs. Comparison between hypoxia and TNFα for their effects on the expression of inflammatory markers shows significant differences: hypoxia up-regulates the expression of IL-6, but not RANTES or ICAM, and reduces the induction of VCAM by TNFα. Finally, ex vivo treatment of rabbit trachea strips with TNFα increases HIF-1α protein levels, but reduces the expression of HIF-1 targets under hypoxia. Overall, TNFα induces HIF-1α mRNA synthesis via an NF-κB dependent pathway but inhibits binding of HIF-1α to ARNT and DNA, while hypoxia and TNFα have distinct effects on ASMC inflammatory gene expression.

MgcRacGAP, a cytoskeleton regulator, inhibits HIF-1 transcriptional activity by blocking its dimerization

Hypoxia inducible factor-1 (HIF-1), a dimeric transcription factor of the bHLH-PAS family, is comprised of HIF-1α, which is inducible by hypoxia and ARNT or HIF-1β, which is constitutively expressed. HIF-1 is involved in cellular homeostasis under hypoxia, in development and in several diseases affected by oxygen availability, particularly cancer. Since its expression is positively correlated with poor outcome prognosis for cancer patients, HIF-1 is a target for pharmaceutical therapy. We have previously shown that male germ cell Rac GTPase activating protein (MgcRacGAP), a regulator of Rho proteins which are principally involved in cytoskeletal organization, binds to HIF-1α and inhibits its transcriptional activity. In this work, we have explored the mechanism of the MgcRacGAP-mediated HIF-1 inactivation. We show that the Myo domain of MgcRacGAP, which is both necessary and sufficient for HIF-1 repression, binds to the PAS-B domain of HIF-1α. Furthermore MgcRacGAP competes with ARNT for binding to the HIF-1α PAS-B domain, as shown by in vitro binding pull down assays. In mammalian cells, ARNT overexpression can overcome the MgcRacGAP-mediated inhibition and MgcRacGAP binding to HIF-1α in vivo inhibits its dimerization with ARNT. We additionally present results indicating that MgcRacGAP binding to HIF-1α is specific, since it does not affect the transcriptional activity of HIF-2, a close evolutionary relative of HIF-1 also involved in hypoxia regulation and cancer. Our results reveal a new mechanism for HIF-1 transcriptional activity regulation, suggest a novel hypoxia-cytoskeleton link and provide new tools for selective HIF-1 inhibition.

Centralspindlin: at the heart of cytokinesis

The final step in the cell cycle is the formation of two genetically identical daughter cells by cytokinesis. At the heart of cytokinesis in animal cells is the centralspindlin complex which is composed of two proteins, a kinesin-like protein, Mitotic kinesin-like protein 1, and a Rho GTPase activating protein (RhoGAP), CYK-4. Through its targeted localization to a narrow region of antiparallel microtubule overlap immediately following chromosome segregation, centralspindlin initiates central spindle assembly. Centralspindlin has several critical functions during cell division including positioning of the division plane, regulation of Rho family GTPases, as well as midbody assembly and abscission. In this review, we will examine the biochemistry of centralspindlin and its multiple functions during cell division. Remarkably, several of its critical functions are somewhat unexpected. Although endowed with motor domains, centralspindlin has an important role in generating stable, antiparallel microtubule bundles. Although it contains a Rho family GAP domain, it has a central role in the activation of RhoA during cytokinesis. Finally, centralspindlin functions as a motor protein complex, as a scaffold protein for key regulators of abscission and as a conventional RhoGAP. Because of these diverse functions, centralspindlin lies at the heart of the cytokinetic mechanism.

Hypoxia causes triglyceride accumulation by HIF-1-mediated stimulation of lipin 1 expression

Adaptation to hypoxia involves hypoxia-inducible transcription factors (HIFs) and requires reprogramming of cellular metabolism that is essential during both physiological and pathological processes. In contrast to the established role of HIF-1 in glucose metabolism, the involvement of HIFs and the molecular mechanisms concerning the effects of hypoxia on lipid metabolism are poorly characterized. Here, we report that exposure of human cells to hypoxia causes accumulation of triglycerides and lipid droplets. This is accompanied by induction of lipin 1, a phosphatidate phosphatase isoform that catalyzes the penultimate step in triglyceride biosynthesis, whereas lipin 2 remains unaffected. Hypoxic upregulation of lipin 1 expression involves predominantly HIF-1, which binds to a single distal hypoxia-responsive element in the lipin 1 gene promoter and causes its activation under low oxygen conditions. Accumulation of hypoxic triglycerides or lipid droplets can be blocked by siRNA-mediated silencing of lipin 1 expression or kaempferol-mediated inhibition of HIF-1. We conclude that direct control of lipin 1 transcription by HIF-1 is an important regulatory feature of lipid metabolism and its adaptation to hypoxia.

Cytokinesis proteins Tum and Pav have a nuclear role in Wnt regulation

Wg/Wnt signals specify cell fates in both invertebrate and vertebrate embryos and maintain stem-cell populations in many adult tissues. Deregulation of the Wnt pathway can transform cells to a proliferative fate, leading to cancer. We have discovered that two Drosophila proteins that are crucial for cytokinesis have a second, largely independent, role in restricting activity of the Wnt pathway. The fly homolog of RacGAP1, Tumbleweed (Tum)/RacGAP50C, and its binding partner, the kinesin-like protein Pavarotti (Pav), negatively regulate Wnt activity in fly embryos and in cultured mammalian cells. Unlike many known regulators of the Wnt pathway, these molecules do not affect stabilization of Arm/beta-catenin (betacat), the principal effector molecule in Wnt signal transduction. Rather, they appear to act downstream of betacat stabilization to control target-gene transcription. Both Tum and Pav accumulate in the nuclei of interphase cells, a location that is spatially distinct from their cleavage-furrow localization during cytokinesis. We show that this nuclear localization is essential for their role in Wnt regulation. Thus, we have identified two modulators of the Wnt pathway that have shared functions in cell division, which hints at a possible link between cytokinesis and Wnt activity during tumorigenesis.

HIF-1 and HIF-2 transcription factors--similar but not identical

Hypoxia inducible factor (HIF)-1 and HIF-2 are heterodimeric transcription factors mediating the cellular response to hypoxia. Recent data indicate that not only ubiquitous HIF-1 alpha, but also more cell-specific HIF-2 alpha, is an important regulator of the hypoxia response. Although both alpha subunits are highly conservative at protein level, share similar domain structure, heterodimerize with HIF-1 beta, and bind to the same DNA sequence called hypoxia responsive element (HRE), their effect on the expression of some genes may vary. In this review we stressed the differences between the isoforms, their structure and expression pattern. Moreover, we described diversity of coactivators and proteins which interact with HIFs, and which are responsible for the specificity of their action. Finally, recent data showing link between HIFs and specific microRNA have been presented.

Isolation of hypoxia-inducible factor 1 (HIF-1) inhibitors from frankincense using a molecularly imprinted polymer

Hypoxia-Inducible Factor 1 (HIF-1), a transcriptional activator, is highly involved in the pathology of cancer. Inhibition of HIF-1 retards tumor growth and enhances treatment efficiency when used in combination with chemo- or radiation therapy. The recent validation of HIF-1 as an important drug target in cancer treatment has stimulated efforts to identify and isolate natural or synthetic HIF-1 inhibitors. In the present study, quercetin, a known inhibitor of HIF-1, was imprinted in a polymer matrix in order to prepare a Molecularly Imprinted Polymer (MIP), which was subsequently used for the selective isolation of new inhibitors from frankincense, a gum resin used as anticancer remedy in traditional medicine. The frankincense components isolated by Solid Phase Extraction on MIP (MIP-SPE), efficiently inhibited the transcriptional activity of HIF-1 and decreased the protein levels of HIF-1α, the regulated subunit of HIF-1. The selective retention of acetyl 11-ketoboswellic acid (AKBA, one of the main bioactive components of frankincense) by MIP led to the revealing of its inhibitory activity on the HIF-1 signaling pathway. AKBA was selectively retained by SPE on the quercetin imprinted polymer, with an imprinting effect of 8.1 ± 4.6. Overall, this study demonstrates the potential of MIP application in the screening, recognition and isolation of new bioactive compounds that aim selected molecular targets, a potential that has been poorly appreciated until.

Hypoxia inducible factor-1 alpha and vascular endothelial growth factor in biopsies of small cell lung carcinoma

Neoangiogenesis has been documented in small cell lung carcinoma (SCLC). In addition, antiangiogenic therapies are being tested in clinical trials that involve SCLC. However, study of the underlying mechanisms has been performed almost exclusively in cell lines. In the current study, we immunostained 30 biopsy samples of SCLC with antibodies to hypoxia inducible factor-1 alpha (HIF-1 alpha), vascular endothelial growth factor (VEGF), vascular endothelial growth factor-receptor 1 (VEGF-R1/flt-1) and vascular endothelial growth factor-receptor 2 (VEGF-R1/flk-1). The immunoreactivity was analyzed using a bivariate Spearman correlation test and linear regression analysis. We found significant correlation between HIF-1 alpha nuclear staining and VEGF staining. Moreover HIF-1 alpha+/VEGF+ cases were associated with poor survival. We also found a positive correlation between VEGF and VEGF-R2 expression. We suggest that a HIF-1 alpha/VEGF angiogenic pathway may exist in vivo in SCLC, similar to that in non-SCLC. Our data also suggest a potential VEGF/VEGFR-2 autocrine pathway in SCLC. The inclusion of novel inhibitors to HIF-1 alpha and other factors may optimize antiangiogenic interventions in SCLC.

Prognostic significance of Hypoxia-Inducible Factor 1 alpha(HIF-1 alpha) expression in serous ovarian cancer: an immunohistochemical study

Background

The hypoxia-inducible factor (HIF) has emerged as an attractive target for cancer therapy. The few publications addressing the prognostic significance of Hypoxia-Inducible Factor 1α (HIF-1α) cellular expression in ovarian cancer produced contradictory findings which are not permissible to widely acceptable conclusions and clinical applications. Our study was designed to investigate this by including a comparatively large number of cases and by using a combination of antibodies to analyze immunohistochemically the expression of HIF-1α.

Methods

One hundred (n = 100) neoplastic and 20 benign (controls) pathological samples from paraffin-embedded tissue were included. They were classified after surgery as stage I (n = 23) and stage III G3 (n = 55). Also 22 borderline serous adenocarcinoma patients and 20 benign controls were stained. The mean follow up was 3 years. Only patients with the diagnosis of serous carcinoma of stage III, G3 who received 6 cycles of postoperative TC (175–180 mg/m2 paclitaxel and carboplatin after calculating the area under the concentration curve) with complete medical records (n = 55) were selected for survival analysis. The survival analysis of the samples compared two groups after the patients were dichotomized by HIF-1α final score to positive and negative.

Results

The frequency of the nuclear expression of HIF-1α in benign tumours was significantly lower (median: no expression) than in borderline and ovarian cancer tumours combined (p < 0.001). HIF-1α expression in serous ovarian carcinoma was not stage dependent. The overall survival of patients with tumours that stained strongly for HIF-1α was significantly shorter than that of patients with tumours that stained weakly or were negative for HIF-1α (p = 0.01). Kaplan-Meier survival curves confirmed that HIF-1α "positive" had decreased overall survival compared to HIF-1α "negative" patients (p = 0.003) and this was an independent adverse prognostic factor (multivariable analysis p = 0.006). HIF-1α "positive" patients displayed a shorter median progress free interval (PFI) (not statistically significant p > 0.05). Interestingly the overall PFI of the subgroup of patients that have undergone suboptimal cytoreduction at primary surgery (n = 21) with tumours that stained strongly for HIF-1α was significantly worse than that of patients with tumours that stained weakly or were negative for HIF-1α (p = 0.03).

Conclusion

Our report confirms the prognostic value of HIF-1α when restricted to poorly differentiated serous ovarian carcinoma. In addition it shows that this association is elusive, since it is not only methodology-related but it can be antibody-depended. There is adequate evidence to speculate that targeting HIF-1α could improve the long-term prognosis of these patients In order to increase the overall sensitivity of the immunoassay, maintaining acceptable levels of specificity, a panel of antibodies should be used.

Atypical CRM1-dependent nuclear export signal mediates regulation of hypoxia-inducible factor-1alpha by MAPK

Hypoxia-inducible factor 1 (HIF-1) is the key transcriptional activator of hypoxia-inducible genes and an important anti-cancer target. Its regulated subunit, HIF-1alpha, is controlled by oxygen levels and major signaling pathways. We reported previously that phosphorylation of Ser(641/643) by p42/44 MAPK is essential for HIF-1alpha nuclear accumulation and activity. We now show that a fragment of HIF-1alpha (amino acids 616-658), termed MAPK target domain, contains a nuclear export signal (NES), which has atypical hydrophobic residue spacing. Localization, reporter gene, and co-immunoprecipitation assays demonstrate that the identified NES interacts with CRM1 in a phosphorylation-sensitive manner. Furthermore, disruption of the NES (I637A/L638A/I639A) restores nuclear localization and activity of nonphosphorylated HIF-1alpha and renders it largely resistant to inhibition of MAPK, an effect reproduced by a phosphomimetic mutation (S641E). As these data predict, overexpression of wild-type or mutant (S641A/S643A) MAPK target domain in HeLa cells modulates the activity and subcellular distribution of endogenous HIF-1alpha. We suggest that control of HIF-1alpha nuclear transport represents an important MAPK-dependent regulatory mechanism.

2-Oxoglutarate-dependent oxygenases control hepcidin gene expression

BACKGROUND/AIMS

Hepcidin is a liver-produced hormone that regulates systemic iron homeostasis. Hepcidin expression is stimulated upon iron overload or inflammation while iron deficiency, anemia and tissue hypoxia are negative regulators. We investigated the involvement of 2-oxoglutarate-dependent oxygenases, HIF-1 and other transcription factors in the hypoxic suppression of hepcidin.

METHODS

Northern blotting analysis and real time PCR were used to determine hepcidin mRNA levels in hepatoma cells and hepcidin promoter activity was measured using Huh7 cells transfected with suitable reporter constructs under various conditions.

RESULTS

Treatment of human cultured hepatoma cells with hypoxia or known inhibitors of 2-oxoglutarate-dependent oxygenases, such as the iron chelator desferrioxamine, cobalt or the 2-oxoglutarate analogue dimethyl-oxalylglycine significantly reduced hepcidin mRNA levels and down-regulated its gene promoter activity. This effect was not dependent on the HREs or other known putative response elements in the hepcidin promoter and was observed even under interleukin-6 treatment.

CONCLUSIONS

2-Oxoglutarate-dependent oxygenases are important to maintain high hepcidin mRNA expression in a HIF-1-independent manner. We suggest that modulation of oxygenase activity may be of therapeutic value in iron-related disorders.

Flavonoids induce HIF-1alpha but impair its nuclear accumulation and activity

Hypoxia-inducible factor-1alpha (HIF-1alpha) is the regulatory subunit of the transcription factor HIF-1, which is highly involved in the pathology of diseases associated with tissue hypoxia. In this study we investigated the ability of plant flavonoids to induce HIF-1alpha and regulate HIF-1 transcriptional activity in HeLa cells. We demonstrate for the first time that the flavonoids baicalein, luteolin and fisetin, as well as the previously investigated quercetin, induce HIF-1alpha under normal oxygen pressure, whereas kaempferol, taxifolin, and rutin are inactive. We further reveal that the capability of flavonoids to bind efficiently intracellular iron and their lipophilicity are essential for HIF-1alpha induction. Despite the ability of flavonoids to stabilize HIF-1alpha, the transcriptional activity of HIF-1 induced by flavonoids was significantly lower than that observed with the iron chelator and known HIF-1 inducer, desferrioxamine (DFO). Furthermore, when cells in which HIF-1 had been induced by DFO were also treated with flavonoids, the transcriptional activity of HIF-1 was strongly impaired without simultaneous reduction in HIF-1alpha protein levels. Localization of HIF-1alpha by immuno- and direct fluorescence microscopy and in vitro phosphorylation assays suggest that flavonoids inhibit HIF-1 activity by impairing the MAPK-dependent phosphorylation of HIF-1alpha, thereby decreasing its nuclear accumulation.