Characterization of a subset of the basic-helix-loop-helix-PAS superfamily that interacts with components of the dioxin signaling pathway

Now a Nobel gas: oxygen

The recent bestowal of the Nobel Prize 2019 in Physiology or Medicine to Gregg L. Semenza, Sir Peter J. Ratcliffe, and William G. Kaelin Jr. celebrates a series of remarkable discoveries that span from the physiological research question on how oxygen deficiency (hypoxia) induces the red blood cell forming hormone erythropoietin (Epo) to the first clinical application of a novel family of Epo-inducing drugs to treat patients suffering from renal anemia. This review looks back at the most important findings made by the three Nobel laureates, highlights current research trends, and sheds an eye on future perspectives of hypoxia research, including emerging and potential clinical applications.

NAD+ cellular redox and SIRT1 regulate the diurnal rhythms of tyrosine hydroxylase and conditioned cocaine reward

The diurnal regulation of dopamine is important for normal physiology and diseases such as addiction. Here we find a novel role for the CLOCK protein to antagonize CREB-mediated transcriptional activity at the tyrosine hydroxylase (TH) promoter, which is mediated by the interaction with the metabolic sensing protein, Sirtuin 1 (SIRT1). Additionally, we demonstrate that the transcriptional activity of TH is modulated by the cellular redox state, and daily rhythms of redox balance in the ventral tegmental area (VTA), along with TH transcription, are highly disrupted following chronic cocaine administration. Furthermore, CLOCK and SIRT1 are important for regulating cocaine reward and dopaminergic (DAergic) activity, with interesting differences depending on whether DAergic activity is in a heightened state and if there is a functional CLOCK protein. Taken together, we find that rhythms in cellular metabolism and circadian proteins work together to regulate dopamine synthesis and the reward value for drugs of abuse.

Tumor Hypoxia As an Enhancer of Inflammation-Mediated Metastasis: Emerging Therapeutic Strategies

Metastasis is the leading cause of cancer-related deaths. Recent research has implicated tumor inflammation as a promoter of metastasis. Myeloid, lymphoid, and mesenchymal cells in the tumor microenvironment promote inflammatory signaling amongst each other and together with cancer cells to modulate sustained inflammation, which may enhance cancer invasiveness. Tumor hypoxia, a state of reduced available oxygen present in the majority of solid tumors, acts as a prognostic factor for a worse outcome and is known to have a role in tumor inflammation through the regulation of inflammatory mediator signals in both cancer and neighboring cells in the microenvironment. Multiple methods to target tumor hypoxia have been developed and tested in clinical trials, and still more are emerging as the impacts of hypoxia become better understood. These strategies include mechanistic inhibition of the hypoxia inducible factor signaling pathway and hypoxia activated pro-drugs, leading to both anti-tumor and anti-inflammatory effects. This prompts a need for further research on the prevention of hypoxia-mediated inflammation in cancer. Hypoxia-targeting strategies seem to have the most potential for therapeutic benefit when combined with traditional chemotherapy agents. This paper will serve to summarize the role of the inflammatory response in metastasis, to discuss how hypoxia can enable or enhance inflammatory signaling, and to review established and emerging strategies to target the hypoxia-inflammation-metastasis axis.

Cell-Type-Specific Regulation of Nucleus Accumbens Synaptic Plasticity and Cocaine Reward Sensitivity by the Circadian Protein, NPAS2

The circadian transcription factor neuronal PAS domain 2 (NPAS2) is linked to psychiatric disorders associated with altered reward sensitivity. The expression of Npas2 is preferentially enriched in the mammalian forebrain, including the nucleus accumbens (NAc), a major neural substrate of motivated and reward behavior. Previously, we demonstrated that downregulation of NPAS2 in the NAc reduces the conditioned behavioral response to cocaine in mice. We also showed that Npas2 is preferentially enriched in dopamine receptor 1 containing medium spiny neurons (D1R-MSNs) of the striatum. To extend these studies, we investigated the impact of NPAS2 disruption on accumbal excitatory synaptic transmission and strength, along with the behavioral sensitivity to cocaine reward in a cell-type-specific manner. Viral-mediated knockdown of Npas2 in the NAc of male and female C57BL/6J mice increased the excitatory drive onto MSNs. Using Drd1a-tdTomato mice in combination with viral knockdown, we determined these synaptic adaptations were specific to D1R-MSNs relative to non-D1R-MSNs. Interestingly, NAc-specific knockdown of Npas2 blocked cocaine-induced enhancement of synaptic strength and glutamatergic transmission specifically onto D1R-MSNs. Last, we designed, validated, and used a novel Cre-inducible short-hairpin RNA virus for MSN-subtype-specific knockdown of Npas2 Cell-type-specific Npas2 knockdown in D1R-MSNs, but not D2R-MSNs, in the NAc reduced cocaine conditioned place preference. Together, our results demonstrate that NPAS2 regulates excitatory synapses of D1R-MSNs in the NAc and cocaine reward-related behavior.SIGNIFICANCE STATEMENT Drug addiction is a widespread public health concern often comorbid with other psychiatric disorders. Disruptions of the circadian clock can predispose or exacerbate substance abuse in vulnerable individuals. We demonstrate a role for the core circadian protein, NPAS2, in mediating glutamatergic neurotransmission at medium spiny neurons (MSNs) in the nucleus accumbens (NAc), a region critical for reward processing. We find that NPAS2 negatively regulates functional excitatory synaptic plasticity in the NAc and is necessary for cocaine-induced plastic changes in MSNs expressing the dopamine 1 receptor (D1R). We further demonstrate disruption of NPAS2 in D1R-MSNs produces augmented cocaine preference. These findings highlight the significance of cell-type-specificity in mechanisms underlying reward regulation by NPAS2 and extend our knowledge of its function.

The transcriptional factors HIF-1 and HIF-2 and their novel inhibitors in cancer therapy

Introduction: Hypoxia is one of the intrinsic features of solid tumors, and it is always associated with aggressive phenotypes, including resistance to radiation and chemotherapy, metastasis, and poor patient prognosis. Hypoxia manifests these unfavorable effects through activation of a family of transcription factors, Hypoxia-inducible factors (HIFs) play a pivotal role in the adaptation of tumor cells to hypoxic and nutrient-deprived conditions by upregulating the transcription of several pro-oncogenic genes. Several advanced human cancers share HIFs activation as a final common pathway. Areas covered: This review highlights the role and regulation of the HIF-1/2 in cancers and alludes on the biological complexity and redundancy of HIF-1/2 regulation. Moreover, this review summarizes recent insights into the therapeutic approaches targeting the HIF-1/2 pathway. Expert opinion: More studies are needed to unravel the extensive complexity of HIFs regulation and to develop more precise anticancer treatments. Inclusion of HIF-1/2 inhibitors to the current chemotherapy regimens has been proven advantageous in numerous reported preclinical studies. The combination therapy ideally should be personalized based on the type of mutations involved in the specific cancers, and it might be better to include two drugs that inhibit HIF-1/2 activity by synergistic molecular mechanisms.

Micro-CT observation of in vivo temporal change in mandibular condyle morphology in BMAL1 knockout mice

Brain and muscle Arnt-like protein-1 (BMAL1) knockout mice exhibit accelerated aging, abnormal glucose metabolism, and impaired adipocyte differentiation, among other phenotypes, which are effects associated with the BMAL1 gene. No study has investigated temporal changes in the deformation of the mandibular condyle and the presence of calcification in areas surrounding the mandibular condyle. In a study of 12 C57/BL strain mice under inhalation anesthesia, we collected images of the mandibular condyle at 6 weeks after birth and then every 5 weeks from 10 to 25 weeks after birth. At 25 weeks, deformation of the mandibular condyle was seen in 8 of 12 joints in BMAL1 knockout mice and in 2 of 12 joints in wild-type mice. At 20 and 25 weeks, deformation in areas surrounding the mandibular condyle, which are known to undergo calcification, was seen in 2 of 12 joints in BMAL1 knockout mice and in 0 of 12 joints in wild-type mice. BMAL1 knockout mice exhibited premature aging of the mandibular condyle, which suggests that circadian rhythms affect mandibular condyle morphology.

Periodicity, repression, and the molecular architecture of the mammalian circadian clock

Large molecular machines regulate daily cycles of transcriptional activity and help generate rhythmic behavior. In recent years, structural and biochemical analyses have elucidated a number of principles guiding the interactions of proteins that form the basis of circadian timing. In its simplest form, the circadian clock is composed of a transcription/translation feedback loop. However, this description elides a complicated process of activator recruitment, chromatin decompaction, recruitment of coactivators, expression of repressors, formation of a repressive complex, repression of the activators, and ultimately degradation of the repressors and reinitiation of the cycle. Understanding the core principles underlying the clock requires careful examination of molecular and even atomic level details of these processes. Here, we review major structural and biochemical findings in circadian biology and make the argument that shared protein interfaces within the clockwork are critical for both the generation of rhythmicity and timing of the clock.

Non-transcriptional processes in circadian rhythm generation

'Biological clocks' orchestrate mammalian biology to a daily rhythm. Whilst 'clock gene' transcriptional circuits impart rhythmic regulation to myriad cellular systems, our picture of the biochemical mechanisms that determine their circadian (∼24 hour) period is incomplete. Here we consider the evidence supporting different models for circadian rhythm generation in mammalian cells in light of evolutionary factors. We find it plausible that the circadian timekeeping mechanism in mammalian cells is primarily protein-based, signalling biological timing information to the nucleus by the post-translational regulation of transcription factor activity, with transcriptional feedback imparting robustness to the oscillation via hysteresis. We conclude by suggesting experiments that might distinguish this model from competing paradigms.

Involvement of the luteinizing hormone surge in the regulation of ovary and oviduct clock gene expression in mice

Circadian dysfunction perturbs the female reproductive cycle. In particular, mice lacking the clock gene Bmal1 show severe infertility, implying that BMAL1 plays roles in ovulation and luteinization. Here, we examined temporal changes in clock gene expression in the ovary and oviduct before and during gonadotropin-induced follicular growth, ovulation, and luteinization in sexually immature mice. While the oviduct did not show a drastic change in clock gene expression, Bmal1 expression in the ovary was higher than that in control mice during the period from 4 to 16 hr after human chorionic gonadotropin (hCG) administration. Bmal1 expression reached a maximum at 16 hr after hCG administration, when follicle luteinization occurred. In an interesting manner, administration of hCG to ex vivo-cultured oviduct triggered a shorter circadian period and inevitably resulted in phase advance. Together, our present data suggest that LH surge induces continuous expression of BMAL1 in the mouse ovary and modulates circadian phase in the mouse oviduct.

Physiology and pathophysiology of renal erythropoietin-producing cells

Anemia is a common complication and contributes to increased morbidity and mortality in chronic kidney disease (CKD) patients. Whereas there has been a significant improvement of understanding the underlying mechanism of erythropoiesis, the treatment of renal anemia is still restricted to erythropoietin (EPO)-stimulating agents. The purpose of this article is to review the physiology of erythropoiesis, functional role of EPO and underlying molecular and cellular basis that regulate EPO production. Regulation of EPO production is at mRNA level. When anemia or hypoxia occurs, transcriptional factor, hypoxia-inducible factor (HIF), binds to EPO 5' hypoxic response element and EPO gene transcription increases. The renal EPO is mainly produced by pericytes. In CKD, pericytes transdifferentiate to myofibroblasts, and subsequently the ability of EPO production decreases, leading to renal anemia. Recent experimental and clinical studies show the promising efficacy of prolyl hydroxylase inhibitors in renal anemia through increasing EPO production by stabilizing HIF. Recent advances on epigenetics create a new field to study EPO gene expression at chromatin level. We will discuss the role of demethylating agent on restoring EPO expression, providing a novel approach to the treatment of renal anemia.

Dendritic cell nuclear protein-1 regulates melatonin biosynthesis by binding to BMAL1 and inhibiting the transcription of N-acetyltransferase in C6 cells

Dendritic cell nuclear protein-1 (DCNP1) is a protein associated with major depression. In the brains of depression patients, DCNP1 is up-regulated. However, how DCNP1 participates in the pathogenesis of major depression remains unknown. In this study, we first transfected HEK293 cells with EGFP-DCNP1 and demonstrated that the full-length DCNP1 protein was localized in the nucleus, and RRK (the residues 117-119) composed its nuclear localization signal (NLS). An RRK-deletion form of DCNP1 (DCNP1ΔRRK) and truncated form (DCNP11-116), each lacking the RRK residues, did not show the specific nuclear localization like full-length DCNP1 in the cells. A rat glioma cell line C6 can synthesize melatonin, a hormone that plays important roles in both sleep and depression. We then revealed that transfection of C6 cells with full-length DCNP1 but not DCNP1ΔRRK or DCNP11-116 significantly decreased the levels of melatonin. Furthermore, overexpression of full-length DCNP1, but not DCNP1ΔRRK or DCNP11-116, in C6 cells significantly decreased both the mRNA and protein levels of N-acetyltransferase (NAT), a key enzyme in melatonin synthesis. Full-length DCNP1 but not DCNP1ΔRRK or DCNP11-116 was detected to interact with the Nat promoter and inhibited its activity through its E-box motif. Furthermore, full-length DCNP1 but not the mutants interacted with and repressed the transcriptional activity of BMAL1, a transcription factor that transactivates Nat through the E-box motif. In conclusion, we have shown that RRK (the residues 117-119) are the NLS responsible for DCNP1 nuclear localization. Nuclear DCNP1 represses NAT expression and melatonin biosynthesis by interacting with BMAL1 and repressing its transcriptional activity. Our study reveals a connection between the major depression candidate protein DCNP1, circadian system and melatonin biosynthesis, which may contribute to the pathogenesis of depression.

The role of CLOCK gene in psychiatric disorders: Evidence from human and animal research

The circadian clock system drives daily rhythms in physiology, metabolism, and behavior in mammals. Molecular mechanisms of this system consist of multiple clock genes, with Circadian Locomotor Output Cycles Kaput (CLOCK) as a core member that plays an important role in a wide range of behaviors. Alterations in the CLOCK gene are associated with common psychiatric disorders as well as with circadian disturbances comorbidities. This review addresses animal, molecular, and genetic studies evaluating the role of the CLOCK gene on many psychiatric conditions, namely autism spectrum disorder, schizophrenia, attention-deficit/hyperactivity disorder, major depressive disorder, bipolar disorder, anxiety disorder, and substance use disorder. Many animal experiments focusing on the effects of the Clock gene in behavior related to psychiatric conditions have shown consistent biological plausibility and promising findings. In humans, genetic and gene expression studies regarding disorder susceptibility, sleep disturbances related comorbidities, and response to pharmacological treatment, in general, are in agreement with animal studies. However, the number of controversial results is high. Literature suggests that the CLOCK gene exerts important influence on these conditions, and influences the susceptibility to phenotypes of psychiatric disorders.

Hypoxia-inducible Factor-1α directs renal regeneration induced by decellularized scaffolds

Although mammalian kidney regeneration has been reported to occur throughout life, mature kidneys in mammals are not thought to regenerate sufficiently, particularly glomeruli. In our previous work, we found that renal regeneration could be enhanced by decellularized renal scaffolds after partial nephrectomy. In this study, we verified that the enhanced renal regeneration mediated by decellularized scaffolds could be attributed to regenerated glomeruli, which were counted both indirectly and directly under a microscope. Using the isobaric tag for relative and absolute quantitation, we performed proteomics analysis and found that hypoxia-inducible factor (HIF)-1α may be a key factor involved in induced renal regeneration. Dimethyloxyallyl glycine (DMOG), a propyl hydroxylase inhibitor, was applied to stabilize constitutive expression of HIF-1α protein, and small interfering RNA was used to inhibit gene expression. Administration of DMOG to decellularized scaffold-grafted rats improved the induced renal regeneration, whereas siHif1α transfection decreased the regeneration capacity. These findings revealed the critical role of HIF-1α in renal regeneration and provided important insights into our understanding of kidney development and the treatment of various kidney diseases.

Bmal1 knockdown suppresses wake and increases immobility without altering orexin A, corticotrophin-releasing hormone, or glutamate decarboxylase

OBJECTIVE

To determine the effect of Bmal1 knockdown (KD) on sleep, activity, immobility, hypothalamic levels of orexin, corticotrophin-releasing hormone (CRH), and GABAergic glutamate decarboxylase (GAD).

METHODS

We used Bmal1 siRNA, or control siRNA intracerebroventricular (ICV) injection to knock down Bmal1 in C57BL/6 mice. Sleep polysomnography, wheel-running activity, and tail suspension test were performed. Polysomnographic (PSG) recordings in both groups were preceded by ICV injection made during both the light phase and the dark phase. We also measured brain orexin A and CRH using an ELISA and measured GAD using immunoblotting.

RESULTS

Compared with control group, Bmal1 KD group had reduced wheel activity and increased immobility. Compared with control, the Bmal1 KD group had reduced wheel activity and increased immobility. During the first 24 hours after treatment, we observed that control siRNA induced a much greater increase in sleep during the dark phase, which was associated with lower orexin levels. However, beginning 24 hours after treatment, we observed an increase in sleep and a decrease in time spent awake during the dark phase in the Bmal1 KD group. These changes were not associated with changes in brain levels of orexin A, CRH, or GAD.

CONCLUSION

Bmal1 KD led to reduced activity, increased immobility, and dramatic reduction in time spent awake as well as an increase in sleep during the dark phase. Early after injection, there was a slight change in sleep but brain levels of orexin, CRH, and GAD remain unchanged. Control siRNA also affected sleep associated with changes in orexin levels.

The SIN3A histone deacetylase complex is required for a complete transcriptional response to hypoxia

Cells adapt to environmental changes, including fluctuations in oxygen levels, through the induction of specific gene expression programs. To identify genes regulated by hypoxia at the transcriptional level, we pulse-labeled HUVEC cells with 4-thiouridine and sequenced nascent transcripts. Then, we searched genome-wide binding profiles from the ENCODE project for factors that correlated with changes in transcription and identified binding of several components of the Sin3A co-repressor complex, including SIN3A, SAP30 and HDAC1/2, proximal to genes repressed by hypoxia. SIN3A interference revealed that it participates in the downregulation of 75% of the hypoxia-repressed genes in endothelial cells. Unexpectedly, it also blunted the induction of 47% of the upregulated genes, suggesting a role for this corepressor in gene induction. In agreement, ChIP-seq experiments showed that SIN3A preferentially localizes to the promoter region of actively transcribed genes and that SIN3A signal was enriched in hypoxia-repressed genes, prior exposure to the stimulus. Importantly, SINA3 occupancy was not altered by hypoxia in spite of changes in H3K27ac signal. In summary, our results reveal a prominent role for SIN3A in the transcriptional response to hypoxia and suggest a model where modulation of the associated histone deacetylase activity, rather than its recruitment, determines the transcriptional output.

HIF-1α promotes autophagic proteolysis of Dicer and enhances tumor metastasis

HIF-1α, one of the most extensively studied oncogenes, is activated by a variety of microenvironmental factors. The resulting biological effects are thought to depend on its transcriptional activity. The RNAse enzyme Dicer is frequently downregulated in human cancers, which has been functionally linked to enhanced metastatic properties; however, current knowledge of the upstream mechanisms regulating Dicer is limited. In the present study, we identified Dicer as a HIF-1α-interacting protein in multiple types of cancer cell lines and different human tumors. HIF-1α downregulated Dicer expression by facilitating its ubiquitination by the E3 ligase Parkin, thereby enhancing autophagy-mediated degradation of Dicer, which further suppressed the maturation of known tumor suppressors, such as the microRNA let-7 and microRNA-200b. Consequently, expression of HIF-1α facilitated epithelial-mesenchymal transition (EMT) and metastasis in tumor-bearing mice. Thus, this study uncovered a connection between oncogenic HIF-1α and the tumor-suppressive Dicer. This function of HIF-1α is transcription independent and occurs through previously unrecognized protein interaction-mediated ubiquitination and autophagic proteolysis.

Epigenetic Regulation of Biological Rhythms: An Evolutionary Ancient Molecular Timer

Biological rhythms are pervasive in nature, yet our understanding of the molecular mechanisms that govern timing is far from complete. The rapidly emerging research focus on epigenetic plasticity has revealed a system that is highly dynamic and reversible. In this Opinion, I propose an epigenetic clock model that outlines how molecular modifications, such as DNA methylation, are integral components for timing endogenous biological rhythms. The hypothesis proposed is that an epigenetic clock serves to maintain the period of molecular rhythms via control over the phase of gene transcription and this timing mechanism resides in all cells, from unicellular to complex organisms. The model also provides a novel framework for the timing of epigenetic modifications during the lifespan and transgenerational inheritance of an organism.

Aryl Hydrocarbon Receptor Deficiency Alters Circadian and Metabolic Rhythmicity

PAS domain-containing proteins can act as environmental sensors that capture external stimuli to allow coordination of organismal physiology with the outside world. These proteins permit diverse ligand binding and heterodimeric partnership, allowing for varied combinations of PAS-dependent protein-protein interactions and promoting crosstalk among signaling pathways. Previous studies report crosstalk between circadian clock proteins and the aryl hydrocarbon receptor (AhR). Activated AhR forms a heterodimer with the circadian clock protein Bmal1 and thereby functionally inhibits CLOCK/Bmal1 activity. If physiological activation of AhR through naturally occurring, endogenous ligands inhibits clock function, it seems plausible to hypothesize that decreased AhR expression releases AhR-induced inhibition of circadian rhythms. Because both AhR and the clock are important regulators of glucose metabolism, it follows that decreased AhR will also alter metabolic function. To test this hypothesis, rhythms of behavior, metabolic outputs, and circadian and metabolic gene expression were measured in AhR-deficient mice. Genetic depletion of AhR enhanced behavioral responses to changes in the light-dark cycle, increased rhythmic amplitude of circadian clock genes in the liver, and altered rhythms of glucose and insulin. This study provides evidence of AhR-induced inhibition that influences circadian rhythm amplitude.

Systems Biology-Derived Discoveries of Intrinsic Clocks

A systems approach to studying biology uses a variety of mathematical, computational, and engineering tools to holistically understand and model properties of cells, tissues, and organisms. Building from early biochemical, genetic, and physiological studies, systems biology became established through the development of genome-wide methods, high-throughput procedures, modern computational processing power, and bioinformatics. Here, we highlight a variety of systems approaches to the study of biological rhythms that occur with a 24-h period-circadian rhythms. We review how systems methods have helped to elucidate complex behaviors of the circadian clock including temperature compensation, rhythmicity, and robustness. Finally, we explain the contribution of systems biology to the transcription-translation feedback loop and posttranslational oscillator models of circadian rhythms and describe new technologies and "-omics" approaches to understand circadian timekeeping and neurophysiology.

2,3,7,8-tetrachlorodibenzo-p-dioxin and the viral infection

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a widespread highly toxic environmental contaminant, suppresses immune response and leads to an increased susceptibility to infectious agents. In particular, several studies have provided evidence that TCDD decreases resistance to numerous viruses. Indeed, in vivo and in vitro investigations showed that the presence of TCDD is able to interfere with the replication of both human and animal viruses, such as influenza A viruses, coxsackie virus B3, immunodeficiency virus type-1 (HIV-1), cytomegalovirus (CMV), herpes simplex II, and bovine herpesvirus 1. Moreover, TCDD could induce an exacerbation of latent infection produced by HIV-1, CMV or Epstein-Barr virus. In this review, we first describe the general effects of TCDD exposure on mammalian cells, then we focus on its influence on the viral infections. Overall, the available data support the concept that TCDD exposure may act as an additional risk factor in promoting of viral diseases.

Molecular targeting of hypoxia in radiotherapy

Hypoxia (low O₂) is an essential microenvironmental driver of phenotypic diversity in human solid cancers. Hypoxic cancer cells hijack evolutionarily conserved, O₂- sensitive pathways eliciting molecular adaptations that impact responses to radiotherapy, tumor recurrence and patient survival. In this review, we summarize the radiobiological, genetic, epigenetic and metabolic mechanisms orchestrating oncogenic responses to hypoxia. In addition, we outline emerging hypoxia- targeting strategies that hold promise for individualized cancer therapy in the context of radiotherapy and drug delivery.

The Aryl Hydrocarbon Receptor and Its Ligands Inhibit Myofibroblast Formation and Activation: Implications for Thyroid Eye Disease

Thyroid eye disease (TED) is a degenerative disease that manifests with detrimental tissue remodeling, myofibroblast accumulation, and scarring in the orbit of affected individuals. Currently, there are no effective therapies for TED that target or prevent the excessive tissue remodeling caused by myofibroblast formation and activation. The canonical cytokine that induces myofibroblast formation is transforming growth factor (TGF)-β. The TGF-β signaling pathway is influenced by aryl hydrocarbon receptor (AHR) signaling pathways. We hypothesized that AHR agonists can prevent myofibroblast formation in fibroblasts from patients with TED, and thus AHR ligands are potential therapeutics for the disease. Orbital fibroblasts explanted from patients with TED were treated with TGF-β to induce myofibroblast formation, contraction, and proliferation. We found that AHR ligands prevent TGF-β-dependent myofibroblast formation, and this ability is dependent on AHR expression. The AHR and AHR ligands block profibrotic Wnt signaling by inhibiting the phosphorylation of GSK3β to prevent myofibroblast formation. These results provide new insight into the molecular pathways underlying orbital scarring in TED. These novel studies highlight the potential of the AHR and AHR ligands as future therapeutic options for eye diseases and possibly also for other scarring conditions.

Houttuynia cordata Thunb Promotes Activation of HIF-1A-FOXO3 and MEF2A Pathways to Induce Apoptosis in Human HepG2 Hepatocellular Carcinoma Cells

Houttuynia cordata Thunb (H cordata), a medicinal plant, has anticancer activity, as it inhibits cell growth and induces cell apoptosis in cancer. However, the potential anti-cancer activity and mechanism of H cordata for human liver cancer cells is not well understood. Recently, we identified hypoxia-inducible factor (HIF)-1A, Forkhead box (FOX)O3, and MEF2A as proapoptotic factors induced by H cordata, suggesting that HIF-1A, FOXO3, and MEF2A contribute to the apoptosis of HepG2 hepatocellular carcinoma cells. FOXO3 transcription factors regulate target genes involved in apoptosis. H cordata significantly increased the mRNA and protein expression of HIF-1A and FOXO3 and stimulated MEF2A expression in addition to increased apoptosis in HepG2 cells within 24 hours. Therefore, we determined the potential role of FOXO3 on apoptosis and on H cordata–induced MEF2A in HepG2 cells. HIF-1A silencing by siRNA attenuated MEF2A and H cordata–mediated FOXO3 upregulation in HepG2 cells. Furthermore, H cordata–mediated MEF2A expression enhanced caspase-3 and caspase-7, which were abolished on silencing FOXO3 with siRNA. In addition, H cordata inhibited growth of human hepatocellular carcinoma xenografts in nude mice. Taken together, our results demonstrate that H cordata enhances HIF-1A/FOXO3 signaling, leading to MEF2A upregulation in HepG2 cells, and in parallel, it disturbs the expression of Bcl-2 family proteins (Bax, Bcl-2, and Bcl-xL), which results in apoptosis. Taken together, these findings demonstrate that H cordata promotes the activation of HIF-1A–FOXO3 and MEF2A pathways to induce apoptosis in human HepG2 hepatocellular carcinoma cells and is, therefore, a promising candidate for antitumor drug development.

Differential sub-nuclear distribution of hypoxia-inducible factors (HIF)-1 and -2 alpha impacts on their stability and mobility

Cellular adaptation to hypoxia occurs via a complex programme of gene expression mediated by the hypoxia-inducible factor (HIF). The oxygen labile alpha subunits, HIF-1α/-2α, form a heterodimeric transcription factor with HIF-1β and modulate gene expression. HIF-1α and HIF-2α possess similar domain structure and bind to the same consensus sequence. However, they have different oxygen-dependent stability and activate distinct genes. To better understand these differences, we used fluorescent microscopy to determine precise localization and dynamics. We observed a homogeneous distribution of HIF-1α in the nucleus, while HIF-2α localized into speckles. We demonstrated that the number, size and mobility of HIF-2α speckles were independent of cellular oxygenation and that HIF-2α molecules were capable of exchanging between the speckles and nucleoplasm in an oxygen-independent manner. The concentration of HIF-2α into speckles may explain its increased stability compared with HIF-1α and its slower mobility may offer a mechanism for gene specificity.

Obstructive Sleep Apnea and Circulating Potassium Channel Levels

Background

Cardiac arrhythmias and sudden cardiac death are more frequent in patients with obstructive sleep apnea (OSA). OSA is associated with QT prolongation, and QT prolongation is an independent risk factor for sudden cardiac death. Because QT prolongation can be mediated by potassium channel loss of function, we tested whether OSA or continuous positive airway pressure therapy altered mRNA expression of circulating white blood cell potassium channels.

Methods and Results

In total, 28 patients with OSA newly diagnosed by polysomnogram and 6 participants without OSA were enrolled. Potassium channel levels in white blood cells at baseline and at a 4‐week follow‐up visit were compared. There was a significant inverse correlation between the severity of the OSA stratified by apnea–hypopnea index and mRNA expression of the main potassium channels assessed: KCNQ1 (r=−0.486, P=0.007), KCNH2 (r=−0.437, P=0.016), KCNE1 (r=−0.567, P=0.001), KCNJ2 (r=−0.442, P=0.015), and KCNA5 (r=−0.468, P=0.009). In addition, KCNQ1, KCNH2, and KCNE1 inversely correlated with the oxygen desaturation index 4. After 4 weeks of continuous positive airway pressure therapy, circulating KCNQ1 and KCNJ2 were increased 1.4±0.4‐fold (P=0.040) and 2.1±1.4‐fold (P=0.046) in the moderate OSA group. Compared with patients with mild or moderate OSA, patients with severe OSA had a persistently higher apnea–hypopnea index (mild 2.0±1.8, moderate 1.0±0.9, severe 5.8±5.6; P=0.015), perhaps explaining why the potassium channel changes were not seen in the severe OSA group.

Conclusions

The mRNA expression of most potassium channels inversely correlates with the severity of OSA and hypoxemia. Continuous positive airway pressure therapy improves circulating KCNQ1 and KCNJ2 in patients with moderate OSA.

Molecular mechanisms of OLIG2 transcription factor in brain cancer

Oligodendrocyte lineage transcription factor 2 (OLIG2) plays a pivotal role in glioma development. Here we conducted a comprehensive study of the critical gene regulatory networks involving OLIG2. These include the networks responsible for OLIG2 expression, its translocation to nucleus, cell cycle, epigenetic regulation, and Rho-pathway interactions. We described positive feedback loops including OLIG2: loops of epigenetic regulation and loops involving receptor tyrosine kinases. These loops may be responsible for the prolonged oncogenic activity of OLIG2. The proposed schemes for epigenetic regulation of the gene networks involving OLIG2 are confirmed by patient survival (Kaplan-Meier) curves based on the cancer genome atlas (TCGA) datasets. Finally, we elucidate the Coherent-Gene Modules (CGMs) networks-framework of OLIG2 involvement in cancer. We showed that genes interacting with OLIG2 formed eight CGMs having a set of intermodular connections. We showed also that among the genes involved in these modules the most connected hub is EGFR, then, on lower level, HSP90 and CALM1, followed by three lower levels including epigenetic genes KDM1A and NCOR1. The genes on the six upper levels of the hierarchy are involved in interconnections of all eight CGMs and organize functionally defined gene-signaling subnetworks having specific functions. For example, CGM1 is involved in epigenetic control. CGM2 is significantly related to cell proliferation and differentiation. CGM3 includes a number of interconnected helix-loop-helix transcription factors (bHLH) including OLIG2. Many of these TFs are partially controlled by OLIG2. The CGM4 is involved in PDGF-related: angiogenesis, tumor cell proliferation and differentiation. These analyses provide testable hypotheses and approaches to inhibit OLIG2 pathway and relevant feed-forward and feedback loops to be interrogated. This broad approach can be applied to other TFs.

Melatonin Signal Transduction Pathways Require E-Box-Mediated Transcription of Per1 and Per2 to Reset the SCN Clock at Dusk

Melatonin is released from the pineal gland into the circulatory system at night in the absence of light, acting as “hormone of darkness” to the brain and body. Melatonin also can regulate circadian phasing of the suprachiasmatic nucleus (SCN). During the day-to-night transition, melatonin exposure advances intrinsic SCN neural activity rhythms via the melatonin type-2 (MT₂) receptor and downstream activation of protein kinase C (PKC). The effects of melatonin on SCN phasing have not been linked to daily changes in the expression of core genes that constitute the molecular framework of the circadian clock. Using real-time RT-PCR, we found that melatonin induces an increase in the expression of two clock genes, Period 1 (Per1) and Period 2 (Per2). This effect occurs at CT 10, when melatonin advances SCN phase, but not at CT 6, when it does not. Using anti-sense oligodeoxynucleotides (α ODNs) to Per 1 and Per 2, as well as to E-box enhancer sequences in the promoters of these genes, we show that their specific induction is necessary for the phase-altering effects of melatonin on SCN neural activity rhythms in the rat. These effects of melatonin on Per1 and Per2 were mediated by PKC. This is unlike day-active non-photic signals that reset the SCN clock by non-PCK signal transduction mechanisms and by decreasing Per1 expression. Rather, this finding extends roles for Per1 and Per2, which are critical to photic phase-resetting, to a nonphotic zeitgeber, melatonin, and suggest that the regulation of these clock gene transcripts is required for clock resetting by diverse regulatory cues.

PRDX2 and PRDX4 are negative regulators of hypoxia-inducible factors under conditions of prolonged hypoxia

Hypoxia-inducible factors (HIFs) control the transcription of genes that are crucial for the pathogenesis of cancer and other human diseases. The transcriptional activity of HIFs is rapidly increased upon exposure to hypoxia, but expression of some HIF target genes decreases during prolonged hypoxia. However, the underlying mechanism for feedback inhibition is not completely understood. Here, we report that peroxiredoxin 2 (PRDX2) and PRDX4 interact with HIF-1α and HIF-2α in vitro and in hypoxic HeLa cells. Prolonged hypoxia increases the nuclear translocation of PRDX2 and PRDX4. As a result, PRDX2 and PRDX4 impair HIF-1 and HIF-2 binding to the hypoxia response elements of a subset of HIF target genes, thereby inhibiting gene transcription in cells exposed to prolonged hypoxia. PRDX2 and PRDX4 have no effect on the recruitment of p300 and RNA polymerase II to HIF target genes and the enzymatic activity of PRDX2 and PRDX4 is not required for inhibition of HIF-1 and HIF-2. We also demonstrate that PRDX2 is a direct HIF target gene and that PRDX2 expression is induced by prolonged hypoxia. These findings uncover a novel feedback mechanism for inhibition of HIF transcriptional activity under conditions of prolonged hypoxia.

Circadian metabolism in the light of evolution

Circadian rhythm, or daily oscillation, of behaviors and biological processes is a fundamental feature of mammalian physiology that has developed over hundreds of thousands of years under the continuous evolutionary pressure of energy conservation and efficiency. Evolution has fine-tuned the body's clock to anticipate and respond to numerous environmental cues in order to maintain homeostatic balance and promote survival. However, we now live in a society in which these classic circadian entrainment stimuli have been dramatically altered from the conditions under which the clock machinery was originally set. A bombardment of artificial lighting, heating, and cooling systems that maintain constant ambient temperature; sedentary lifestyle; and the availability of inexpensive, high-calorie foods has threatened even the most powerful and ancient circadian programming mechanisms. Such environmental changes have contributed to the recent staggering elevation in lifestyle-influenced pathologies, including cancer, cardiovascular disease, depression, obesity, and diabetes. This review scrutinizes the role of the body's internal clocks in the hard-wiring of circadian networks that have evolved to achieve energetic balance and adaptability, and it discusses potential therapeutic strategies to reset clock metabolic control to modern time for the benefit of human health.

The emerging role of hypoxia-inducible factor-2 involved in chemo/radioresistance in solid tumors

The hypoxic condition is a common feature that negatively impacts the efficacy of radio- and chemotherapy in solid tumors. Hypoxia-inducible factors (HIF-1, 2, 3) predominantly regulate the adaptation to hypoxia at the cellular or organismal level. HIF-2 is one of the three known alpha subunits of HIF transcription factors. Previous studies have shown that HIF-1 is associated with chemotherapy failure. Accumulating evidence in recent years suggests that HIF-2 also contributes to chemo/radioresistance in solid tumors. Despite sharing similar structures, HIF-1α and HIF-2α had highly divergent and even opposing roles in solid tumors under hypoxic conditions. Recent studies have also implied that HIF-2α had a role in chemo/radioresistance through different mechanisms, at least partly, compared to HIF-1α. The present paper summarizes the function of HIF-2 in chemo/radioresistance in solid tumors as well as some of its novel mechanisms that contributed to this pathological process.

A mammalian circadian clock model incorporating daytime expression elements

Models of the mammalian clock have traditionally been based around two feedback loops-the self-repression of Per/Cry by interfering with activation by BMAL/CLOCK, and the repression of Bmal/Clock by the REV-ERB proteins. Recent experimental evidence suggests that the D-box, a transcription factor binding site associated with daytime expression, plays a larger role in clock function than has previously been understood. We present a simplified clock model that highlights the role of the D-box and illustrate an approach for finding maximum-entropy ensembles of model parameters, given experimentally imposed constraints. Parameter variability can be mitigated using prior probability distributions derived from genome-wide studies of cellular kinetics. Our model reproduces predictions concerning the dual regulation of Cry1 by the D-box and Rev-ErbA/ROR response element (RRE) promoter elements and allows for ensemble-based predictions of phase response curves (PRCs). Nonphotic signals such as Neuropeptide Y (NPY) may act by promoting Cry1 expression, whereas photic signals likely act by stimulating expression from the E/E' box. Ensemble generation with parameter probability restraints reveals more about a model's behavior than a single optimal parameter set.

The role of hypoxia-inducible factor-2 in digestive system cancers

Hypoxia is an all but ubiquitous phenomenon in cancers. Two known hypoxia-inducible factors (HIFs), HIF-1α and HIF-2α, primarily mediate the transcriptional response to hypoxia. Despite the high homology between HIF-1α and HIF-2α, emerging evidence suggests differences between both molecules in terms of transcriptional targets as well as impact on multiple physiological pathways and tumorigenesis. To date, much progress has been made toward understanding the roles of HIF-2α in digestive system cancers. Indeed, HIF-2α has been shown to regulate multiple aspects of digestive system cancers, including cell proliferation, angiogenesis and apoptosis, metabolism, metastasis and resistance to chemotherapy. These findings make HIF-2α a critical regulator of this malignant phenotype. Here we summarize the function of HIF-2 during cancer development as well as its contribution to tumorigenesis in digestive system malignancies.

Signaling hypoxia by hypoxia-inducible factor protein hydroxylases: a historical overview and future perspectives

By the early 1900s, the close matching of oxygen supply with demand was recognized to be a fundamental requirement for physiological function, and multiple adaptive responses to environment hypoxia had been described. Nevertheless, the widespread operation of mechanisms that directly sense and respond to levels of oxygen in animal cells was not appreciated for most of the twentieth century with investigators generally stressing the regulatory importance of metabolic products. Work over the last 25 years has overturned that paradigm. It has revealed the existence of a set of “oxygen-sensing” 2-oxoglutarate dependent dioxygenases that catalyze the hydroxylation of specific amino acid residues and thereby control the stability and activity of hypoxia-inducible factor. The hypoxia-inducible factor hydroxylase pathway regulates a massive transcriptional cascade that is operative in essentially all animal cells. It transduces a wide range of responses to hypoxia, extending well beyond the classical boundaries of hypoxia physiology. Here we review the discovery and elucidation of these pathways, and consider the opportunities and challenges that have been brought into focus by the findings, including new implications for the integrated physiology of hypoxia and therapeutic approaches to ischemic/hypoxic disease.

NPAS1 represses the generation of specific subtypes of cortical interneurons

Little is known about genetic mechanisms that regulate the ratio of cortical excitatory and inhibitory neurons. We show that NPAS1 and NPAS3 transcription factors (TFs) are expressed in progenitor domains of the mouse basal ganglia (subpallium, MGE, and CGE). NPAS1(-/-) mutants had increased proliferation, ERK signaling, and expression of Arx in the MGE and CGE. NPAS1(-/-) mutants also had increased neocortical inhibition (sIPSC and mIPSC) and generated an excess of somatostatin(+) (SST) (MGE-derived) and vasoactive intestinal polypeptide(+) (VIP) (CGE-derived) neocortical interneurons, but had a normal density of parvalbumin(+) (PV) (MGE-derived) interneurons. In contrast, NPAS3(-/-) mutants showed decreased proliferation and ERK signaling in progenitors of the ganglionic eminences and had fewer SST(+) and VIP(+) interneurons. NPAS1 repressed activity of an Arx enhancer, and Arx overexpression resulted in increased proliferation of CGE progenitors. These results provide insights into genetic regulation of cortical interneuron numbers and cortical inhibitory tone.

The activation mechanism of the aryl hydrocarbon receptor (AhR) by molecular chaperone HSP90

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We showed the direct interaction of AhR and HSP90 using purified protein.

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The ligand 17-DMAG induces a dissociation of HSP90 from AhR.

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The AhR–HSP90 complex is not affected by the timing of β-naphthoflavone binding to AhR.

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The AhR–HSP90 complex was translocated to the nucleus after treatment with β-naphthoflavone.

The aryl hydrocarbon receptor is a member of the nuclear receptor superfamily that associates with the molecular chaperone HSP90 in the cytoplasm. The activation mechanism of the AhR is not yet fully understood. It has been proposed that after binding of ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3methylcholanthrene (3-MC), or β-naphthoflavone (β-NF), the AhR dissociates from HSP90 and translocates to the nucleus. It has also been hypothesized that the AhR translocates to the nucleus and forms a complex with HSP90 and other co-chaperones. There are a few reports about the direct association or dissociation of AhR and HSP90 due to difficulties in purifying AhR. We constructed and purified the PAS domain from AhR. Binding of the AhR-PAS domain to β-NF affinity resin suggested that it possesses ligand-binding affinity. We demonstrated that the AhR-PAS domain binds to HSP90 and the association is not affected by ligand binding. The ligand 17-DMAG inhibited binding of HSP90 to GST-PAS. In an immunoprecipitation assay, HSP90 was co-immunoprecipitated with AhR both in the presence or absence of ligand. Endogenous AhR decreased in the cytoplasm and increased in the nucleus of HeLa cells 15 min after treatment with ligand. These results suggested that the ligand-bound AhR is translocated to nucleus while in complex with HSP90.

We used an in situ proximity ligation assay to confirm whether AhR was translocated to the nucleus alone or together with HSP90. HSP90 was co-localized with AhR after the nuclear translocation. It has been suggested that the ligand-bound AhR was translocated to the nucleus with HSP90. Activated AhR acts as a transcription factor, as shown by the transcription induction of the gene CYP1A1 8 h after treatment with β-NF.

Interplay between sirtuins, MYC and hypoxia-inducible factor in cancer-associated metabolic reprogramming

In the early twentieth century, Otto Heinrich Warburg described an elevated rate of glycolysis occurring in cancer cells, even in the presence of atmospheric oxygen (the Warburg effect). Despite the inefficiency of ATP generation through glycolysis, the breakdown of glucose into lactate provides cancer cells with a number of advantages, including the ability to withstand fluctuations in oxygen levels, and the production of intermediates that serve as building blocks to support rapid proliferation. Recent evidence from many cancer types supports the notion that pervasive metabolic reprogramming in cancer and stromal cells is a crucial feature of neoplastic transformation. Two key transcription factors that play major roles in this metabolic reprogramming are hypoxia inducible factor-1 (HIF1) and MYC. Sirtuin-family deacetylases regulate diverse biological processes, including many aspects of tumor biology. Recently, the sirtuin SIRT6 has been shown to inhibit the transcriptional output of both HIF1 and MYC, and to function as a tumor suppressor. In this Review, we highlight the importance of HIF1 and MYC in regulating tumor metabolism and their regulation by sirtuins, with a main focus on SIRT6.

The role of HIFs in ischemia-reperfusion injury

The reduction or cessation of the blood supply to an organ results in tissue ischemia. Ischemia can cause significant tissue damage, and is observed as a result of a thrombosis, as part of a disease process, and during surgery. However, the restoration of the blood supply often causes more damage to the tissue than the ischemic episode itself. Research is therefore focused on identifying the cellular pathways involved in the protection of organs from the damage incurred by this process of ischemia reperfusion (I/R). The hypoxia-inducible factors (HIFs) are a family of heterodimeric transcription factors that are stabilized during ischemia. The genes that are expressed downstream of HIF activity enhance oxygen-independent ATP generation, cell survival, and angiogenesis, amongst other phenotypes. They are, therefore, important factors in the protection of tissues from I/R injury. Interestingly, a number of the mechanisms already known to induce organ protection against I/R injury, including preconditioning, postconditioning, and activation of signaling pathways such as adenosine receptor signaling, converge on the HIF system. This review describes the evidence for HIFs playing a role in I/R protection mediated by these factors, highlights areas that require further study, and discuss whether HIFs themselves are good therapeutic targets for protecting tissues from I/R injury.

Hypoxia-inducible regulation of placental BOK expression

Pre-eclampsia, a disorder of pregnancy, is characterized by placental hypoxia and cell death. Hypoxia shifts the intricate balance between death-inducing BOK and survival-promoting MCL1 towards BOK, thereby triggering placental cell death. Here, we show that BOK is a direct target of HIF.

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.

HSP90 affects the stability of BMAL1 and circadian gene expression

The mammalian circadian clock comprises a system of interconnected transcriptional and translational feedback loops. Proper oscillator function requires the precisely timed synthesis and degradation of core clock proteins. Heat shock protein 90 (HSP90), an adenosine triphosphate (ATP)-dependent molecular chaperone, has important functions in many cellular regulatory pathways by controlling the activity and stability of its various client proteins. Despite accumulating evidence for interplay between the heat shock response and the circadian system, the role of HSP90 in the mammalian core clock is not known. The results of this study suggest that inhibition of the ATP-dependent chaperone activity of HSP90 impairs circadian rhythmicity of cultured mouse fibroblasts whereby amplitude and phase of the oscillations are predominantly affected. Inhibition of HSP90 shortened the half-life of BMAL1, which resulted in reduced cellular protein levels and blunted expression of rhythmic BMAL1-CLOCK target genes. Furthermore, the HSP90 isoforms HSP90AA1 and HSP90AB1, and not HSP90B1-GRP94 or TRAP1, are responsible for maintaining proper cellular levels of BMAL1 protein. In summary, these findings provide evidence for a model in which cytoplasmic HSP90 is required for transcriptional activation processes by the positive arm of the mammalian circadian clock.

Overexpression of HIF-2α in pancreatic β cells does not alter glucose homeostasis

Both type 1 and type 2 diabetes are associated with insufficient functional β-cell mass. Understanding intracellular signaling pathways associated with this decline is important in broadening our understanding of the disease and potential therapeutic strategies. The hypoxia inducible factor pathway (HIF) plays a critical role in cellular adaptation to hypoxic conditions. Activation of this pathway increases expression of numerous genes involved in multiple cellular processes and has been shown to impact the regulation of β-cell function. Previously, deletion of HIF-1α or HIF-1β in pancreatic β-cells, as well as constitutive activation of the HIF pathway in β-cells, was shown to result in glucose intolerance and impaired insulin secretion. The objective of this study was to delineate roles of HIF-2α overexpression in pancreatic β-cells in vivo. We overexpressed HIF-2α in pancreatic β-cells by employing the Cre-loxP system driven by the Pdx1 promoter to delete a stop codon. Our study revealed that pancreatic HIF-2α overexpression does not result in significant differences in glucose tolerance, insulin sensitivity or β-cell area compared to wild-type littermates under basal conditions or after high fat diet. Together, our study shows excess HIF-2α in the pancreatic β-cells does not play a significant role in β-cell function and glucose homeostasis.

Circadian rhythms in liver physiology and liver diseases

In mammals, circadian rhythms function to coordinate a diverse panel of physiological processes with environmental conditions such as food and light. As the driving force for circadian rhythmicity, the molecular clock is a self-sustained transcription-translational feedback loop system consisting of transcription factors, epigenetic modulators, kinases/phosphatases, and ubiquitin E3 ligases. The molecular clock exists not only in the suprachiasmatic nuclei of the hypothalamus but also in the peripheral tissues to regulate cellular and physiological function in a tissue-specific manner. The circadian clock system in the liver plays important roles in regulating metabolism and energy homeostasis. Clock gene mutant animals display impaired glucose and lipid metabolism and are susceptible to diet-induced obesity and metabolic dysfunction, providing strong evidence for the connection between the circadian clock and metabolic homeostasis. Circadian-controlled hepatic metabolism is partially achieved by controlling the expression and/or activity of key metabolic enzymes, transcription factors, signaling molecules, and transporters. Reciprocally, intracellular metabolites modulate the molecular clock activity in response to the energy status. Although still at the early stage, circadian clock dysfunction has been implicated in common chronic liver diseases. Circadian dysregulation of lipid metabolism, detoxification, reactive oxygen species (ROS) production, and cell-cycle control might contribute to the onset and progression of liver steatosis, fibrosis, and even carcinogenesis. In summary, these findings call for a comprehensive study of the function and mechanisms of hepatic circadian clock to gain better understanding of liver physiology and diseases.

Oxygen sensing, hypoxia-inducible factors, and disease pathophysiology

Hypoxia-inducible factors (HIFs) are transcriptional activators that function as master regulators of oxygen homeostasis, which is disrupted in disorders affecting the circulatory system and in cancer. The role of HIFs in these diseases has been elucidated by clinical studies and by analyses of mouse models. HIFs play a protective role in the pathophysiology of myocardial ischemia due to coronary artery disease, limb ischemia due to peripheral arterial disease, pressure-overload heart failure, wound healing, and chronic rejection of organ transplants. In contrast, HIFs contribute to the pathogenesis of pulmonary arterial hypertension, systemic hypertension associated with sleep apnea, ocular neovascularization, hereditary erythrocytosis, and cancer.

Hypoxia, therapeutic resistance, and sphingosine 1-phosphate

Hypoxia, defined as a poor oxygenation, has been long recognized as a hallmark of solid tumors and a negative prognostic factor for response to therapeutics and survival of patients. Cancer cells have evolved biochemical mechanisms that allow them to react and adapt to hypoxia. At the cellular level, this adaptation is under the control of two related transcription factors, HIF-1 and HIF-2 (hypoxia-inducible factor), that respond rapidly to decreased oxygen levels to activate the expression of a broad range of genes promoting neoangiogenesis, glycolysis, metastasis, increased tumor growth, and resistance to treatments. Recent studies have identified the sphingosine kinase 1/sphingosine 1-phosphate (SphK1/S1P) signaling pathway-which elicits various cellular processes including cell proliferation, cell survival, or angiogenesis-as a new regulator of HIF-1 or HIF-2 activity. In this review, we will focus on how the inhibition/neutralization of the SphK1/S1P signaling could be exploited for cancer therapy.

Brain and muscle Arnt-like 1 is a key regulator of myogenesis

The circadian clock network is an evolutionarily conserved mechanism that imparts temporal regulation to diverse biological processes. Brain and muscle Arnt-like 1 (Bmal1), an essential transcriptional activator of the clock, is highly expressed in skeletal muscle. However, whether this key clock component impacts myogenesis, a temporally regulated event that requires the sequential activation of myogenic regulatory factors, is not known. Here we report a novel function of Bmal1 in controlling myogenic differentiation through direct transcriptional activation of components of the canonical Wnt signaling cascade, a major inductive signal for embryonic and postnatal muscle growth. Genetic loss of Bmal1 in mice leads to reduced total muscle mass and Bmal1-deficient primary myoblasts exhibit significantly impaired myogenic differentiation accompanied by markedly blunted expression of key myogenic regulatory factors. Conversely, forced expression of Bmal1 enhances differentiation of C2C12 myoblasts. This cell-autonomous effect of Bmal1 is mediated by Wnt signaling as both expression and activity of Wnt components are markedly attenuated by inhibition of Bmal1, and activation of the Wnt pathway partially rescues the myogenic defect in Bmal1-deficient myoblasts. We further reveal direct association of Bmal1 with promoters of canonical Wnt pathway genes, and as a result of this transcriptional regulation, Wnt signaling components exhibit intrinsic circadian oscillation. Collectively, our study demonstrates that the core clock gene, Bmal1, is a positive regulator of myogenesis, which may represent a temporal regulatory mechanism to fine-tune myocyte differentiation.