Protein kinase B/AKT phosphorylates hypoxia-inducible factor-3α1 in response to insulin, promoting cell growth and migration

Hypoxia-inducible factors (HIFs) are best known for their roles in the adaptation to low oxygen environments. Besides hypoxia, HIF-1/2 α-subunits are also regulated by various non-hypoxic stimuli including insulin which can act via the PI3K/protein kinase B (PKB) signaling pathway. However, with respect to insulin little is known about HIF-3α. We aimed to investigate this relationship and found that insulin stimulates HIF-3α expression under both normal and low oxygen conditions. Blocking PKB activity reversed the effects of insulin, indicating that HIF-3α is a direct target of PKB. We identified serine 524, located in the oxygen-dependent degradation domain of HIF-3α, as a phosphorylation site of PKB. Mutating serine 524 impaired binding of PKB to HIF-3α and its ubiquitination, suggesting that PKB regulates HIF-3α stability through phosphorylation, thereby affecting important cellular processes such as cell viability and cell adhesion. Importantly, we discovered that this phosphorylation site also influenced insulin-dependent cell migration. These findings shed light on a novel mechanism by which insulin affects PKB-dependent HIF-3α expression and activity, with potential implications in metabolic diseases and cancer.

ER-stress promotes VHL-independent degradation of hypoxia-inducible factors via FBXW1A/βTrCP

Metabolic adaptation and signal integration in response to hypoxic conditions is mainly regulated by hypoxia-inducible factors (HIFs). At the same time, hypoxia induces ROS formation and activates the unfolded protein response (UPR), indicative of endoplasmic reticulum (ER) stress. However, whether ER stress would affect the hypoxia response remains ill-defined. Here we report that feeding mice a high fat diet causes ER stress and attenuates the response to hypoxia. Mechanistically, ER stress promotes HIF-1α and HIF-2α degradation independent of ROS, Ca²⁺, and the von Hippel-Lindau (VHL) pathway, involving GSK3β and the ubiquitin ligase FBXW1A/βTrCP. Thereby, we reveal a previously unknown function of the GSK3β/HIFα/βTrCP1 axis in ER homeostasis and demonstrate that inhibition of the HIF-1 and HIF-2 response and genetic deficiency of GSK3β affects proliferation, migration, and sensitizes cells for ER stress promoted apoptosis. Vice versa, we show that hypoxia affects the ER stress response mainly through the PERK-arm of the UPR. Overall, we discovered previously unrecognized links between the HIF pathway and the ER stress response and uncovered an essential survival pathway for cells under ER stress.

The Role of Hypoxia-Inducible Factor Post-Translational Modifications in Regulating Its Localisation, Stability, and Activity

The hypoxia signalling pathway enables adaptation of cells to decreased oxygen availability. When oxygen becomes limiting, the central transcription factors of the pathway, hypoxia-inducible factors (HIFs), are stabilised and activated to induce the expression of hypoxia-regulated genes, thereby maintaining cellular homeostasis. Whilst hydroxylation has been thoroughly described as the major and canonical modification of the HIF-α subunits, regulating both HIF stability and activity, a range of other post-translational modifications decorating the entire protein play also a crucial role in altering HIF localisation, stability, and activity. These modifications, their conservation throughout evolution, and their effects on HIF-dependent signalling are discussed in this review.

Loss of USF2 promotes proliferation, migration and mitophagy in a redox-dependent manner

The upstream stimulatory factor 2 (USF2) is a transcription factor implicated in several cellular processes and among them, tumor development seems to stand out. However, the data with respect to the role of USF2 in tumor development are conflicting suggesting that it acts either as tumor promoter or suppressor. Here we show that absence of USF2 promotes proliferation and migration. Thereby, we reveal a previously unknown function of USF2 in mitochondrial homeostasis. Mechanistically, we demonstrate that deficiency of USF2 promotes survival by inducing mitophagy in a ROS-sensitive manner by activating both ERK1/2 and AKT.

Altogether, this study supports USF2′s function as tumor suppressor and highlights its novel role for mitochondrial function and energy homeostasis thereby linking USF2 to conditions such as insulin resistance, type-2 diabetes mellitus, and the metabolic syndrome.

Transcriptomic and Proteomic Analysis of Clear Cell Foci (CCF) in the Human Non-Cirrhotic Liver Identifies Several Differentially Expressed Genes and Proteins with Functions in Cancer Cell Biology and Glycogen Metabolism

Clear cell foci (CCF) of the liver are considered to be pre-neoplastic lesions of hepatocellular adenomas and carcinomas. They are hallmarked by glycogen overload and activation of AKT (v-akt murine thymoma viral oncogene homolog)/mTOR (mammalian target of rapamycin)-signaling. Here, we report the transcriptome and proteome of CCF extracted from human liver biopsies by laser capture microdissection. We found 14 genes and 22 proteins differentially expressed in CCF and the majority of these were expressed at lower levels in CCF. Using immunohistochemistry, the reduced expressions of STBD1 (starch-binding domain-containing protein 1), USP28 (ubiquitin-specific peptidase 28), monad/WDR92 (WD repeat domain 92), CYB5B (Cytochrome b5 type B), and HSPE1 (10 kDa heat shock protein, mitochondrial) were validated in CCF in independent specimens. Knockout of Stbd1, the gene coding for Starch-binding domain-containing protein 1, in mice did not have a significant effect on liver glycogen levels, indicating that additional factors are required for glycogen overload in CCF. Usp28 knockout mice did not show changes in glycogen storage in diethylnitrosamine-induced liver carcinoma, demonstrating that CCF are distinct from this type of cancer model, despite the decreased USP28 expression. Moreover, our data indicates that decreased USP28 expression is a novel factor contributing to the pre-neoplastic character of CCF. In summary, our work identifies several novel and unexpected candidates that are differentially expressed in CCF and that have functions in glycogen metabolism and tumorigenesis.

DUBs, Hypoxia, and Cancer

Alterations in protein ubiquitylation and hypoxia are commonly associated with cancer. Ubiquitylation is carried out by three sequentially acting ubiquitylating enzymes and can be opposed by deubiquitinases (DUBs), which have emerged as promising drug targets. Apart from protein localization and activity, ubiquitylation regulates degradation of proteins, among them hypoxia-inducible factors (HIFs). Thereby, various E3 ubiquitin ligases and DUBs regulate HIF abundance. Conversely, several E3s and DUBs are regulated by hypoxia. While hypoxia is a powerful HIF regulator, less is known about hypoxia-regulated DUBs and their impact on HIFs. Here, we review current knowledge about the relationship of E3s, DUBs, and hypoxia signaling. We also discuss the reciprocal regulation of DUBs by hypoxia and use of DUB-specific drugs in cancer.

The Pro-Oncogenic Adaptor CIN85 Acts as an Inhibitory Binding Partner of Hypoxia-Inducible Factor Prolyl Hydroxylase 2

The EGFR adaptor protein, CIN85, has been shown to promote breast cancer malignancy and hypoxia-inducible factor (HIF) stability. However, the mechanisms underlying cancer promotion remain ill defined. Here we show that CIN85 is a novel binding partner of the main HIF-prolyl hydroxylase, PHD2, but not of PHD1 or PHD3. Mechanistically, the N-terminal SRC homology 3 domains of CIN85 interacted with the proline-arginine-rich region within the N-terminus of PHD2, thereby inhibiting PHD2 activity and HIF degradation. This activity is essential in vivo, as specific loss of the CIN85-PHD2 interaction in CRISPR/Cas9-edited cells affected growth and migration properties, as well as tumor growth in mice. Overall, we discovered a previously unrecognized tumor growth checkpoint that is regulated by CIN85-PHD2 and uncovered an essential survival function in tumor cells by linking growth factor adaptors with hypoxia signaling. SIGNIFICANCE: This study provides unprecedented evidence for an oxygen-independent mechanism of PHD2 regulation that has important implications in cancer cell survival. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/79/16/4042/F1.large.jpg.

A Golgi-associated redox switch regulates catalytic activation and cooperative functioning of ST6Gal-I with B4GalT-I

Glycosylation, a common modification of cellular proteins and lipids, is often altered in diseases and pathophysiological states such as hypoxia, yet the underlying molecular causes remain poorly understood. By utilizing lectin microarray glycan profiling, Golgi pH and redox screens, we show here that hypoxia inhibits terminal sialylation of N- and O-linked glycans in a HIF- independent manner by lowering Golgi oxidative potential. This redox state change was accompanied by loss of two surface-exposed disulfide bonds in the catalytic domain of the α-2,6-sialyltransferase (ST6Gal-I) and its ability to functionally interact with B4GalT-I, an enzyme adding the preceding galactose to complex N-glycans. Mutagenesis of selected cysteine residues in ST6Gal-I mimicked these effects, and also rendered the enzyme inactive. Cells expressing the inactive mutant, but not those expressing the wild type ST6Gal-I, were able to proliferate and migrate normally, supporting the view that inactivation of the ST6Gal-I help cells to adapt to hypoxic environment. Structure comparisons revealed similar disulfide bonds also in ST3Gal-I, suggesting that this O-glycan and glycolipid modifying sialyltransferase is also sensitive to hypoxia and thereby contribute to attenuated sialylation of O-linked glycans in hypoxic cells. Collectively, these findings unveil a previously unknown redox switch in the Golgi apparatus that is responsible for the catalytic activation and cooperative functioning of ST6Gal-I with B4GalT-I.

Systemic inactivation of hypoxia-inducible factor prolyl 4-hydroxylase 2 in mice protects from alcohol-induced fatty liver disease

Alcoholic fatty liver disease (AFLD) is a growing health problem for which no targeted therapy is available. We set out to study whether systemic inactivation of the main hypoxia-inducible factor prolyl 4-hydroxylase, HIF-P4H-2 (PHD2/EglN1), whose inactivation has been associated with protection against metabolic dysfunction, could ameliorate it. HIF-P4H-2-deficient and wild-type (WT) mice or HIF-P4H inhibitor-treated WT mice were subjected to an ethanol diet for 3-4 weeks and their metabolic health, liver and white adipose tissue (WAT) were analyzed. Primary hepatocytes from the mice were used to study cellular ethanol metabolism. The HIF-P4H-2-deficient mice retained a healthier metabolic profile, including less adiposity, better lipoprotein profile and restored insulin sensitivity, while on the ethanol diet than the WT. They also demonstrated protection from alcohol-induced steatosis and liver damage and had less WAT inflammation. In liver and WAT the expression of the key lipogenic and adipocytokine mRNAs, such as Fas and Ccl2, were downregulated, respectively. The upregulation of metabolic and antioxidant hypoxia-inducible factor (HIF) target genes, such as Slcs 16a1 and 16a3 and Gclc, respectively, and a higher catalytic activity of ALDH2 in the HIF-P4H-2-deficient hepatocytes improved handling of the toxic ethanol metabolites and oxidative stress. Pharmacological HIF-P4H inhibition in the WT mice phenocopied the protection against AFLD. Our data show that global genetic inactivation of HIF-P4H-2 and pharmacological HIF-P4H inhibition can protect mice from alcohol-induced steatosis and liver injury, suggesting that HIF-P4H inhibitors, now in clinical trials for renal anemia, could also be studied in randomized clinical trials for treatment of AFLD.

The Circadian Clock Protein CRY1 Is a Negative Regulator of HIF-1α

The circadian clock and the hypoxia-signaling pathway are regulated by an integrated interplay of positive and negative feedback limbs that incorporate energy homeostasis and carcinogenesis. We show that the negative circadian regulator CRY1 is also a negative regulator of hypoxia-inducible factor (HIF). Mechanistically, CRY1 interacts with the basic-helix-loop-helix domain of HIF-1α via its tail region. Subsequently, CRY1 reduces HIF-1α half-life and binding of HIFs to target gene promoters. This appeared to be CRY1 specific because genetic disruption of CRY1, but not CRY2, affected the hypoxia response. Furthermore, CRY1 deficiency could induce cellular HIF levels, proliferation, and migration, which could be reversed by CRISPR/Cas9- or short hairpin RNA-mediated HIF knockout. Altogether, our study provides a mechanistic explanation for genetic association studies linking a disruption of the circadian clock with hypoxia-associated processes such as carcinogenesis.

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Hypoxia and HIFs affect the circadian rhythm

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CRY1 directly interacts with both HIF-1α and HIF-2α

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CRY1 inhibits binding of HIFs to its target gene promoters

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The CRY1-HIFα interaction has opposite roles on cellular growth and migration

Hypoxia and Reactive Oxygen Species as Modulators of Endoplasmic Reticulum and Golgi Homeostasis

SIGNIFICANCE

Eukaryotic cells execute various functions in subcellular compartments or organelles for which cellular redox homeostasis is of importance. Apart from mitochondria, hypoxia and stress-mediated formation of reactive oxygen species (ROS) were shown to modulate endoplasmic reticulum (ER) and Golgi apparatus (GA) functions. Recent Advances: Research during the last decade has improved our understanding of disulfide bond formation, protein glycosylation and secretion, as well as pH and redox homeostasis in the ER and GA. Thus, oxygen (O₂) itself, NADPH oxidase (NOX) formed ROS, and pH changes appear to be of importance and indicate the intricate balance of intercompartmental communication.

CRITICAL ISSUES

Although the interplay between hypoxia, ER stress, and Golgi function is evident, the existence of more than 20 protein disulfide isomerase family members and the relative mild phenotypes of, for example, endoplasmic reticulum oxidoreductin 1 (ERO1)- and NOX4-knockout mice clearly suggest the existence of redundant and alternative pathways, which remain largely elusive.

FUTURE DIRECTIONS

The identification of these pathways and the key players involved in intercompartmental communication needs suitable animal models, genome-wide association, as well as proteomic studies in humans. The results of those studies will be beneficial for the understanding of the etiology of diseases such as type 2 diabetes, Alzheimer's disease, and cancer, which are associated with ROS, protein aggregation, and glycosylation defects.

USP28 Deficiency Promotes Breast and Liver Carcinogenesis as well as Tumor Angiogenesis in a HIF-independent Manner

Recent studies suggest that the ubiquitin-specific protease USP28 plays an important role in cellular repair and tissue remodeling, which implies that it has a direct role in carcinogenesis. The carcinogenic potential of USP28 was investigated in a comprehensive manner using patients, animal models, and cell culture. The findings demonstrate that overexpression of USP28 correlates with a better survival in patients with invasive ductal breast carcinoma. Mouse xenograft experiments with USP28-deficient breast cancer cells also support this view. Furthermore, lack of USP28 promotes a more malignant state of breast cancer cells, indicated by an epithelial-to-mesenchymal (EMT) transition, elevated proliferation, migration, and angiogenesis as well as a decreased adhesion. In addition to breast cancer, lack of USP28 in mice promoted an earlier onset and a more severe tumor formation in a chemical-induced liver cancer model. Mechanistically, the angio- and carcinogenic processes driven by the lack of USP28 appeared to be independent of HIF-1α, p53, and 53BP1.Implications: The findings of this study are not limited to one particular type of cancer but are rather applicable for carcinogenesis in a more general manner. The obtained data support the view that USP28 is involved in tumor suppression and has the potential to be a prognostic marker. Mol Cancer Res; 16(6); 1000-12. ©2018 AACR.

The mTOR and PP2A Pathways Regulate PHD2 Phosphorylation to Fine-Tune HIF1α Levels and Colorectal Cancer Cell Survival under Hypoxia

Oxygen-dependent HIF1α hydroxylation and degradation are strictly controlled by PHD2. In hypoxia, HIF1α partly escapes degradation because of low oxygen availability. Here, we show that PHD2 is phosphorylated on serine 125 (S125) by the mechanistic target of rapamycin (mTOR) downstream kinase P70S6K and that this phosphorylation increases its ability to degrade HIF1α. mTOR blockade in hypoxia by REDD1 restrains P70S6K and unleashes PP2A phosphatase activity. Through its regulatory subunit B55α, PP2A directly dephosphorylates PHD2 on S125, resulting in a further reduction of PHD2 activity that ultimately boosts HIF1α accumulation. These events promote autophagy-mediated cell survival in colorectal cancer (CRC) cells. B55α knockdown blocks neoplastic growth of CRC cells in vitro and in vivo in a PHD2-dependent manner. In patients, CRC tissue expresses higher levels of REDD1, B55α, and HIF1α but has lower phospho-S125 PHD2 compared with a healthy colon. Our data disclose a mechanism of PHD2 regulation that involves the mTOR and PP2A pathways and controls tumor growth.

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PHD2 is phosphorylated at Ser125 by P70S6K and dephosphorylated by PP2A/B55α

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PHD2 dephosphorylation impairs its function, resulting in increased HIF1α accumulation

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HIF1α promotes CRC survival in hypoxia via autophagy in a PHD2/B55α-dependent fashion

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B55α silencing blocks CRC tumor growth in vitro and in vivo; this is PHD2 dependent

Non-electron transfer chain mitochondrial defects differently regulate HIF-1α degradation and transcription

Mitochondria are the main consumers of molecular O2 in a cell as well as an abundant source of reactive oxygen species (ROS). Both, molecular oxygen and ROS are powerful regulators of the hypoxia-inducible factor-1α-subunit (HIF-α). While a number of mechanisms in the oxygen-dependent HIF-α regulation are quite well known, the view with respect to mitochondria is less clear. Several approaches using pharmacological or genetic tools targeting the mitochondrial electron transport chain (ETC) indicated that ROS, mainly formed at the Rieske cluster of complex III of the ETC, are drivers of HIF-1α activation. However, studies investigating non-ETC located mitochondrial defects and their effects on HIF-1α regulation are scarce, if at all existing. Thus, in the present study we examined three cell lines with non-ETC mitochondrial defects and focused on HIF-1α degradation and transcription, target gene expression, as well as ROS levels. We found that cells lacking the key enzyme 2-enoyl thioester reductase/mitochondrial enoyl-CoA reductase (MECR), and cells lacking manganese superoxide dismutase (MnSOD) showed a reduced induction of HIF-1α under long-term (20h) hypoxia. By contrast, cells lacking the mitochondrial DNA depletion syndrome channel protein Mpv17 displayed enhanced levels of HIF-1α already under normoxic conditions. Further, we show that ROS do not exert a uniform pattern when mediating their effects on HIF-1α, although all mitochondrial defects in the used cell types increased ROS formation. Moreover, all defects caused a different HIF-1α regulation via promoting HIF-1α degradation as well as via changes in HIF-1α transcription. Thereby, MECR- and MnSOD-deficient cells showed a reduction in HIF-1α mRNA levels whereas the Mpv17 lacking cells displayed enhanced HIF-1α mRNA levels under normoxia and hypoxia. Altogether, our study shows for the first time that mitochondrial defects which are not related to the ETC and Krebs cycle contribute differently to HIF-1α regulation by affecting HIF-1α degradation and HIF-1α transcription where ROS play not a major role.

Differential transcriptional regulation of hypoxia-inducible factor-1α by arsenite under normoxia and hypoxia: involvement of Nrf2

Abstract

Arsenite (As(III)) is widely distributed in nature and can be found in water, food, and air. There is significant evidence that exposure to As(III) is associated with human cancers originated from liver, lung, skin, bladder, kidney, and prostate. Hypoxia plays a role in tumor growth and aggressiveness; adaptation to it is, at least to a large extent, mediated by hypoxia-inducible factor-1α (HIF-1α). In the current study, we investigated As(III) effects on HIF-1α under normoxia and hypoxia in the hepatoma cell line HepG2. We found that As(III) increased HIF-1α protein levels under normoxia while the hypoxia-mediated induction of HIF1α was reduced. Thereby, the As(III) effects on HIF-1α were dependent on both, transcriptional regulation via the transcription factor Nrf2 mediated by NOX4, PI3K/Akt, and ERK1/2 as well as by modulation of HIF-1α protein stability. In line, the different effects of As(III) via participation of HIF-1α and Nrf2 were also seen in tube formation assays with endothelial cells where knockdown of Nrf2 and HIF-1α abolished As(III) effects. Overall, the present study shows that As(III) is a potent inducer of HIF-1α under normoxia but not under hypoxia which may explain, in part, its carcinogenic as well as anti-carcinogenic actions.

Key message

As(III) increased HIF-1α under normoxia but reduced its hypoxia-dependent induction.

The As(III) effects on HIF-1α were dependent on ROS, NOX4, PI3K/Akt, and ERK1/2.

The As(III) effects under normoxia involved transcriptional regulation via Nrf2.

Knockdown of Nrf2 and HIF-1α abolished As(III) effects in tube formation assays.

The data may partially explain As(III)’s carcinogenic and anti-carcinogenic actions.

Hypoxia-Inducible Factors (HIFs) and Phosphorylation: Impact on Stability, Localization, and Transactivity

The hypoxia-inducible factor α-subunits (HIFα) are key transcription factors in the mammalian response to oxygen deficiency. The HIFα regulation in response to hypoxia occurs primarily on the level of protein stability due to posttranslational hydroxylation and proteasomal degradation. However, HIF α-subunits also respond to various growth factors, hormones, or cytokines under normoxia indicating involvement of different kinase pathways in their regulation. Because these proteins participate in angiogenesis, glycolysis, programmed cell death, cancer, and ischemia, HIFα regulating kinases are attractive therapeutic targets. Although numerous kinases were reported to regulate HIFα indirectly, direct phosphorylation of HIFα affects HIFα stability, nuclear localization, and transactivity. Herein, we review the role of phosphorylation-dependent HIFα regulation with emphasis on protein stability, subcellular localization, and transactivation.

PHD1 regulates p53-mediated colorectal cancer chemoresistance

Overcoming resistance to chemotherapy is a major challenge in colorectal cancer (CRC) treatment, especially since the underlying molecular mechanisms remain unclear. We show that silencing of the prolyl hydroxylase domain protein PHD1, but not PHD2 or PHD3, prevents p53 activation upon chemotherapy in different CRC cell lines, thereby inhibiting DNA repair and favoring cell death. Mechanistically, PHD1 activity reinforces p53 binding to p38α kinase in a hydroxylation-dependent manner. Following p53-p38α interaction and chemotherapeutic damage, p53 can be phosphorylated at serine 15 and thus activated. Active p53 allows nucleotide excision repair by interacting with the DNA helicase XPB, thereby protecting from chemotherapy-induced apoptosis. In accord with this observation, PHD1 knockdown greatly sensitizes CRC to 5-FU in mice. We propose that PHD1 is part of the resistance machinery in CRC, supporting rational drug design of PHD1-specific inhibitors and their use in combination with chemotherapy.

Antioxidant responses and cellular adjustments to oxidative stress

Redox biological reactions are now accepted to bear the Janus faceted feature of promoting both physiological signaling responses and pathophysiological cues. Endogenous antioxidant molecules participate in both scenarios. This review focuses on the role of crucial cellular nucleophiles, such as glutathione, and their capacity to interact with oxidants and to establish networks with other critical enzymes such as peroxiredoxins. We discuss the importance of the Nrf2-Keap1 pathway as an example of a transcriptional antioxidant response and we summarize transcriptional routes related to redox activation. As examples of pathophysiological cellular and tissular settings where antioxidant responses are major players we highlight endoplasmic reticulum stress and ischemia reperfusion. Topologically confined redox-mediated post-translational modifications of thiols are considered important molecular mechanisms mediating many antioxidant responses, whereas redox-sensitive microRNAs have emerged as key players in the posttranscriptional regulation of redox-mediated gene expression. Understanding such mechanisms may provide the basis for antioxidant-based therapeutic interventions in redox-related diseases.

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Antioxidant responses are crucial for both redox signaling and redox damage.

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Glutathione-mediated reactions and Nrf2-Keap1 pathway are key antioxidant responses.

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Redox-related post-translational modifications activate specific signaling pathways.

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Redox-sensitive microRNAs contribute to redox-mediated gene expression regulation.

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ER stress and ischemia-reperfusion are antioxidant-related pathophysiological events.

The Human Mitochondrial DNA Depletion Syndrome Gene MPV17 Encodes a Non-selective Channel That Modulates Membrane Potential

The human MPV17-related mitochondrial DNA depletion syndrome is an inherited autosomal recessive disease caused by mutations in the inner mitochondrial membrane protein MPV17. Although more than 30 MPV17 gene mutations were shown to be associated with mitochondrial DNA depletion syndrome, the function of MPV17 is still unknown. Mice deficient in Mpv17 show signs of premature aging. In the present study, we used electrophysiological measurements with recombinant MPV17 to reveal that this protein forms a non-selective channel with a pore diameter of 1.8 nm and located the channel's selectivity filter. The channel was weakly cation-selective and showed several subconductance states. Voltage-dependent gating of the channel was regulated by redox conditions and pH and was affected also in mutants mimicking a phosphorylated state. Likewise, the mitochondrial membrane potential (Δψm) and the cellular production of reactive oxygen species were higher in embryonic fibroblasts from Mpv17(-/-) mice. However, despite the elevated Δψm, the Mpv17-deficient mitochondria showed signs of accelerated fission. Together, these observations uncover the role of MPV17 as a Δψm-modulating channel that apparently contributes to mitochondrial homeostasis under different conditions.

Direct phosphorylation events involved in HIF-α regulation: the role of GSK-3β

Hypoxia-inducible factors (HIFs), consisting of α- and β-subunits, are critical regulators of the transcriptional response to hypoxia under both physiological and pathological conditions. To a large extent, the protein stability and the recruitment of coactivators to the C-terminal transactivation domain of the HIF α-subunits determine overall HIF activity. The regulation of HIF α-subunit protein stability and coactivator recruitment is mainly achieved by oxygen-dependent posttranslational hydroxylation of conserved proline and asparagine residues, respectively. Under hypoxia, the hydroxylation events are inhibited and HIF α-subunits stabilize, translocate to the nucleus, dimerize with the β-subunits, and trigger a transcriptional response. However, under normal oxygen conditions, HIF α-subunits can be activated by various growth and coagulation factors, hormones, cytokines, or stress factors implicating the involvement of different kinase pathways in their regulation, thereby making HIF-α-regulating kinases attractive therapeutic targets. From the kinases known to regulate HIF α-subunits, only a few phosphorylate HIF-α directly. Here, we review the direct phosphorylation of HIF-α with an emphasis on the role of glycogen synthase kinase-3β and the consequences for HIF-1α function.

Nutritional countermeasures targeting reactive oxygen species in cancer: from mechanisms to biomarkers and clinical evidence

Reactive oxygen species (ROS) exert various biological effects and contribute to signaling events during physiological and pathological processes. Enhanced levels of ROS are highly associated with different tumors, a Western lifestyle, and a nutritional regime. The supplementation of food with traditional antioxidants was shown to be protective against cancer in a number of studies both in vitro and in vivo. However, recent large-scale human trials in well-nourished populations did not confirm the beneficial role of antioxidants in cancer, whereas there is a well-established connection between longevity of several human populations and increased amount of antioxidants in their diets. Although our knowledge about ROS generators, ROS scavengers, and ROS signaling has improved, the knowledge about the direct link between nutrition, ROS levels, and cancer is limited. These limitations are partly due to lack of standardized reliable ROS measurement methods, easily usable biomarkers, knowledge of ROS action in cellular compartments, and individual genetic predispositions. The current review summarizes ROS formation due to nutrition with respect to macronutrients and antioxidant micronutrients in the context of cancer and discusses signaling mechanisms, used biomarkers, and its limitations along with large-scale human trials.

Shift of syndecan-1 expression from epithelial to stromal cells during progression of solid tumours

Syndecan-1 (SDC-1), a protein found on cells and in the extracellular matrix, participates in cell proliferation, cell migration and cell-matrix interactions. SDC-1 expression correlates with the maintenance of epithelial morphology and inhibition of invasiveness. In the present study, a second SDC-1 mRNA isoform was identified and the expression of both transcripts was investigated in various normal and malignant tissues. Both transcripts were coexpressed at equal levels in all tissues and organs analysed. Cancer-profiling array (CPA) analysis of 241 non-enriched tumour and normal cDNAs revealed stronger upregulation of SDC-1 in tumour tissues as compared with oligonucleotide array-based expression analysis of SDC-1 in microdissected breast, prostate, lung, and colon carcinoma cells. With in situ hybridisation and immunohistochemistry it was demonstrated that this difference in SDC-1 expression originates from stromal cells present in tumour connective tissue. But only the cells in connective tissue surrounding breast, lung, colon and bladder carcinoma showed upregulation of SDC-1. These stromal cells were characterised as spindle cells with myofibroblastic differentiation and they may contribute to the dedifferentiation of tumour cells and the development of metastasis.

Activation of myogenesis by the homeobox gene Lbx1 requires cell proliferation

Myogenic differentiation can be initiated by a limited number of molecules. In this work, we analyzed the function of the homeobox gene Lbx1 in chicken embryos and explant cultures. We demonstrate that overexpression of Lbx1 in vivo and in vitro leads to a strong activation of various muscle markers. We show that cell proliferation, which is strongly stimulated by Lbx1 and Pax3, is required for Lbx1- or Pax3-dependent myogenic activation. Inhibition of cell proliferation prevents expression of muscle differentiation markers, while the activation of other putative downstream targets of Pax3 and Lbx1 is not affected. Our findings imply that a critical function of Pax3 and Lbx1 during muscle cell formation is the enlargement of muscle cell populations. The growth of the muscle precursor cell population may increase the bias for myogenic differentiation and thus enable myogenic cells to respond to environmental cues.

Sonic hedgehog is a survival factor for hypaxial muscles during mouse development

Sonic hedgehog (Shh) has been proposed to function as an inductive and trophic signal that controls development of epaxial musculature in vertebrate embryos. In contrast, development of hypaxial muscles was assumed to occur independently of Shh. We here show that formation of limb muscles was severely affected in two different mouse strains with inactivating mutations of the Shh gene. The limb muscle defect became apparent relatively late and initial stages of hypaxial muscle development were unaffected or only slightly delayed. Micromass cultures and cultures of tissue fragments derived from limbs under different conditions with or without the overlaying ectoderm indicated that Shh is required for the maintenance of the expression of myogenic regulatory factors (MRFs) and, consecutively, for the formation of differentiated limb muscle myotubes. We propose that Shh acts as a survival and proliferation factor for myogenic precursor cells during hypaxial muscle development. Detection of a reduced but significant level of Myf5 expression in the epaxial compartment of somites of Shh homozygous mutant embryos at E9.5 indicated that Shh might be dispensable for the initiation of myogenesis both in hypaxial and epaxial muscles. Our data suggest that Shh acts similarly in both somitic compartments as a survival and proliferation factor and not as a primary inducer of myogenesis.

Heterozygous myogenic factor 6 mutation associated with myopathy and severe course of Becker muscular dystrophy

Myogenic factors (MYF) belong to the basic helix-loop-helix (bHLH) transcription factor family and regulate myogenesis and muscle regeneration. The physiological importance of both functions was demonstrated in homozygous Myf knockout mice and mdx mice. Myf5 and Myod are predominantly expressed in proliferating myoblasts while Myf4 and Myf6 are involved in differentiation of myotubes. In a boy with myopathy and an increase of muscle fibres with central nuclei we detected a heterozygous 387G-->T nucleotide transversion in the MYF6 gene (MIM*159991). Protein-protein interaction of mutant MYF6 was reduced, and DNA-binding potential and transactivation capacity were abolished, thus demonstrating MYF6 haploinsufficiency. The boy's father carried the identical mutation and, in addition, an in-frame deletion of exons 45-47 in his dystrophin gene. This mutation is normally associated with a mild to moderate course of Becker muscular dystrophy but the father suffered from a severe course of Becker muscular dystrophy suggesting MYF6 as a modifier.

Two highly related homeodomain proteins, Nkx5-1 and Nkx5-2, display different DNA binding specificities

The mouse Nkx5-1 and Nkx5-2 genes are related to NK genes in Drosophila and encode proteins with very similar homeodomains. In higher vertebrates Nkx5 genes are specifically expressed in the inner ear. Inactivation of the mouse Nkx5-1 gene by homologous recombination revealed a critical role for the formation of vestibular inner ear structures. Here, we investigated biochemical properties of the proteins encoded by the Nkx5 genes. A similar consensus binding sequence was isolated for both Nkx5 proteins using binding site selection. This sequence is related to target sequences previously identified for other Nkx proteins and contains the conserved homeodomain binding core TAAT. An additional, novel and unrelated high affinity binding sequence could be identified for the Nkx5-2 protein.

Pax-3 is necessary but not sufficient for lbx1 expression in myogenic precursor cells of the limb

In vertebrates all skeletal muscles of trunk and limbs are derived from condensations of the paraxial mesoderm, the somites. Limb muscle precursor cells migrate during embryogenesis from somites to limb buds where migration stops and differentiation occurs. We have characterized lbx1 homeobox genes in chicken and mice and found them to be expressed in migrating limb muscle precursor cells in both species. Analysis of splotch mutant mice showed that lbx1 and c-met are differently affected by the lack of Pax-3. Limb buds of splotch (Pax-3 mutant) mice were devoid of lbx1 transcripts, while expression of c-met was still detectable at a low level. The presence of c-met-positive cells in splotch mice entering the limbs indicates that migration of cells from somites to limbs is not entirely dependent on Pax-3. We show that induction of epithelial to mesenchymal transition of Pax-3-positive cells by SF/HGF was not sufficient to induce ectopic lbx-1 expression at the inter-limb level, while ectopic limb formation was able to activate lbx1 expression. We postulate that Pax-3 is necessary for lbx1 expression in the lateral tips of somites but additional, yet unknown signals derived from limb buds are needed to initiate lbx1 expression. The role of limb bud-derived signals involved in targeted muscle precursor cell migration, and lbx1 activation was further confirmed by analysis of explanted somite/limb bud co-cultures in collagen gels.