Hypoxia and Innate Immunity: Keeping Up with the HIFsters.

Recent years have witnessed an emergence of interest in understanding metabolic changes associated with immune responses, termed immunometabolism. As oxygen is central to all aerobic metabolism, hypoxia is now recognized to contribute fundamentally to inflammatory and immune responses. Studies from a number of groups have implicated a prominent role for oxygen metabolism and hypoxia in innate immunity of healthy tissue (physiologic hypoxia) and during active inflammation (inflammatory hypoxia). This inflammatory hypoxia emanates from a combination of recruited inflammatory cells (e.g., neutrophils, eosinophils, and monocytes), high rates of oxidative metabolism, and the activation of multiple oxygen-consuming enzymes during inflammation. These localized shifts toward hypoxia have identified a prominent role for the transcription factor hypoxia-inducible factor (HIF) in the regulation of innate immunity. Such studies have provided new and enlightening insight into our basic understanding of immune mechanisms, and extensions of these findings have identified potential therapeutic targets. In this review, we summarize recent literature around the topic of innate immunity and mucosal hypoxia with a focus on transcriptional responses mediated by HIF.

Hypoxia research, where to now?

Investigating how cells and organisms sense and respond to O2 levels is essential to our understanding of physiology and pathology. This field has advanced considerably since the discovery of the major transcription factor family, hypoxia-inducible factor (HIF), and the enzymes that control its levels: prolyl hydroxylases (PHDs). However, with its expansion, new complexities have emerged. Herein we highlight three main areas where, in our opinion, the research community could direct some of their attention. These include non-transcriptional roles of HIFs, specificity and O2 sensitivity of 2-oxoglutarate-dependent dioxygenases (2-OGDDs), and new tools and methods to detect O2 concentrations in cells and organs. A greater understanding of these areas would answer big questions and help drive our knowledge of cellular responses to hypoxia forward.

Mechanisms and Consequences of Oxygen and Carbon Dioxide Sensing in Mammals.

Molecular oxygen (O2) and carbon dioxide (CO2) are the primary gaseous substrate and product of oxidative phosphorylation in respiring organisms, respectively. Variance in the levels of either of these gasses outside of the physiological range presents a serious threat to cell, tissue, and organism survival. Therefore, it is essential that endogenous levels are monitored and kept at appropriate concentrations to maintain a state of homeostasis. Higher organisms such as mammals have evolved mechanisms to sense O2 and CO2 both in the circulation and in individual cells and elicit appropriate corrective responses to promote adaptation to commonly encountered conditions such as hypoxia and hypercapnia. These can be acute and transient nontranscriptional responses, which typically occur at the level of whole animal physiology or more sustained transcriptional responses, which promote chronic adaptation. In this review, we discuss the mechanisms by which mammals sense changes in O2 and CO2 and elicit adaptive responses to maintain homeostasis. We also discuss crosstalk between these pathways and how they may represent targets for therapeutic intervention in a range of pathological states.

Hypoxia induces a glycolytic complex in intestinal epithelial cells independent of HIF-1-driven glycolytic gene expression.

The metabolic adaptation of eukaryotic cells to hypoxia involves increasing dependence upon glycolytic adenosine triphosphate (ATP) production, an event with consequences for cellular bioenergetics and cell fate. This response is regulated at the transcriptional level by the hypoxia-inducible factor-1(HIF-1)-dependent transcriptional upregulation of glycolytic enzymes (GEs) and glucose transporters. However, this transcriptional upregulation alone is unlikely to account fully for the levels of glycolytic ATP produced during hypoxia. Here, we investigated additional mechanisms regulating glycolysis in hypoxia. We observed that intestinal epithelial cells treated with inhibitors of transcription or translation and human platelets (which lack nuclei and the capacity for canonical transcriptional activity) maintained the capacity for hypoxia-induced glycolysis, a finding which suggests the involvement of a nontranscriptional component to the hypoxia-induced metabolic switch to a highly glycolytic phenotype. In our investigations into potential nontranscriptional mechanisms for glycolytic induction, we identified a hypoxia-sensitive formation of complexes comprising GEs and glucose transporters in intestinal epithelial cells. Surprisingly, the formation of such glycolytic complexes occurs independent of HIF-1-driven transcription. Finally, we provide evidence for the presence of HIF-1α in cytosolic fractions of hypoxic cells which physically interacts with the glucose transporter GLUT1 and the GEs in a hypoxia-sensitive manner. In conclusion, we provide insights into the nontranscriptional regulation of hypoxia-induced glycolysis in intestinal epithelial cells.

SUMOylation indirectly suppresses activity of the HIF-1α pathway in intestinal epithelial cells.

The hypoxia-inducible factor (HIF) is a master regulator of the cellular transcriptional response to hypoxia. While the oxygen-sensitive regulation of HIF-1α subunit stability via the ubiquitin-proteasome pathway has been well described, less is known about how other oxygen-independent post-translational modifications impact the HIF pathway. SUMOylation, the attachment of SUMO (small ubiquitin-like modifier) proteins to a target protein, regulates the HIF pathway, although the impact of SUMO on HIF activity remains controversial. Here, we examined the effects of SUMOylation on the expression pattern of HIF-1α in response to pan-hydroxylase inhibitor dimethyloxalylglycine (DMOG) in intestinal epithelial cells. We evaluated the effects of SUMO-1, SUMO-2, and SUMO-3 overexpression and inhibition of SUMOylation using a novel selective inhibitor of the SUMO pathway, TAK-981, on the sensitivity of HIF-1α in Caco-2 intestinal epithelial cells. Our findings demonstrate that treatment with TAK-981 decreases global SUMO-1 and SUMO-2/3 modification and enhances HIF-1α protein levels, whereas SUMO-1 and SUMO-2/3 overexpression results in decreased HIF-1α protein levels in response to DMOG. Reporter assay analysis demonstrates reduced HIF-1α transcriptional activity in cells overexpressing SUMO-1 and SUMO-2/3, whereas pretreatment with TAK-981 increased HIF-1α transcriptional activity in response to DMOG. In addition, HIF-1α nuclear accumulation was decreased in cells overexpressing SUMO-1. Importantly, we showed that HIF-1α is not directly SUMOylated, but that SUMOylation affects HIF-1α stability and activity indirectly. Taken together, our results indicate that SUMOylation indirectly suppresses HIF-1α protein stability, transcriptional activity, and nuclear accumulation in intestinal epithelial cells.

Intracellular energy production and distribution in hypoxia.

The hydrolysis of ATP is the primary source of metabolic energy for eukaryotic cells. Under physiological conditions, cells generally produce more than sufficient levels of ATP to fuel the active biological processes necessary to maintain homeostasis. However, mechanisms underpinning the distribution of ATP to subcellular microenvironments with high local demand remain poorly understood. Intracellular distribution of ATP in normal physiological conditions has been proposed to rely on passive diffusion across concentration gradients generated by ATP producing systems such as the mitochondria and the glycolytic pathway. However, subcellular microenvironments can develop with ATP deficiency due to increases in local ATP consumption. Alternatively, ATP production can be reduced during bioenergetic stress during hypoxia. Mammalian cells therefore need to have the capacity to alter their metabolism and energy distribution strategies to compensate for local ATP deficits while also controlling ATP production. It is highly likely that satisfying the bioenergetic requirements of the cell involves the regulated distribution of ATP producing systems to areas of high ATP demand within the cell. Recently, the distribution (both spatially and temporally) of ATP-producing systems has become an area of intense investigation. Here, we review what is known (and unknown) about intracellular energy production and distribution and explore potential mechanisms through which this targeted distribution can be altered in hypoxia, with the aim of stimulating investigation in this important, yet poorly understood field of research.

Hypoxia-inducible factor-driven glycolytic adaptations in host-microbe interactions.

Mammalian cells utilize glucose as a primary carbon source to produce energy for most cellular functions. However, the bioenergetic homeostasis of cells can be perturbed by environmental alterations, such as changes in oxygen levels which can be associated with bacterial infection. Reduction in oxygen availability leads to a state of hypoxia, inducing numerous cellular responses that aim to combat this stress. Importantly, hypoxia strongly augments cellular glycolysis in most cell types to compensate for the loss of aerobic respiration. Understanding how this host cell metabolic adaptation to hypoxia impacts the course of bacterial infection will identify new anti-microbial targets. This review will highlight developments in our understanding of glycolytic substrate channeling and spatiotemporal enzymatic organization in response to hypoxia, shedding light on the integral role of the hypoxia-inducible factor (HIF) during host-pathogen interactions. Furthermore, the ability of intracellular and extracellular bacteria (pathogens and commensals alike) to modulate host cellular glucose metabolism will be discussed.

Carbon Dioxide Sensing by Immune Cells Occurs through Carbonic Anhydrase 2-Dependent Changes in Intracellular pH.

CO2, the primary gaseous product of respiration, is a major physiologic gas, the biology of which is poorly understood. Elevated CO2 is a feature of the microenvironment in multiple inflammatory diseases that suppresses immune cell activity. However, little is known about the CO2-sensing mechanisms and downstream pathways involved. We found that elevated CO2 correlates with reduced monocyte and macrophage migration in patients undergoing gastrointestinal surgery and that elevated CO2 reduces migration in vitro. Mechanistically, CO2 reduces autocrine inflammatory gene expression, thereby inhibiting macrophage activation in a manner dependent on decreased intracellular pH. Pharmacologic or genetic inhibition of carbonic anhydrases (CAs) uncouples a CO2-elicited intracellular pH response and attenuates CO2 sensitivity in immune cells. Conversely, CRISPR-driven upregulation of the isoenzyme CA2 confers CO2 sensitivity in nonimmune cells. Of interest, we found that patients with chronic lung diseases associated with elevated systemic CO2 (hypercapnia) display a greater risk of developing anastomotic leakage following gastrointestinal surgery, indicating impaired wound healing. Furthermore, low intraoperative pH levels in these patients correlate with reduced intestinal macrophage infiltration. In conclusion, CO2 is an immunomodulatory gas sensed by immune cells through a CA2-coupled change in intracellular pH.

Transmigrating neutrophils shape the mucosal microenvironment through localized oxygen depletion to influence resolution of inflammation.

Acute intestinal inflammation involves early accumulation of neutrophils (PMNs) followed by either resolution or progression to chronic inflammation. Based on recent evidence that mucosal metabolism influences disease outcomes, we hypothesized that transmigrating PMNs influence the transcriptional profile of the surrounding mucosa. Microarray studies revealed a cohort of hypoxia-responsive genes regulated by PMN-epithelial crosstalk. Transmigrating PMNs rapidly depleted microenvironmental O2 sufficiently to stabilize intestinal epithelial cell hypoxia-inducible factor (HIF). By utilizing HIF reporter mice in an acute colitis model, we investigated the relative contribution of PMNs and the respiratory burst to "inflammatory hypoxia" in vivo. CGD mice, lacking a respiratory burst, developed accentuated colitis compared to control, with exaggerated PMN infiltration and diminished inflammatory hypoxia. Finally, pharmacological HIF stabilization within the mucosa protected CGD mice from severe colitis. In conclusion, transcriptional imprinting by infiltrating neutrophils modulates the host response to inflammation, via localized O2 depletion, resulting in microenvironmental hypoxia and effective inflammatory resolution.

Hypoxia-inducible factor as a bridge between healthy barrier function, wound healing, and fibrosis.

The healthy mammalian intestine is lined by a single layer of epithelial cells. These cells provide a selectively permeable barrier to luminal contents and normally do so in an efficient and effective manner. Barrier function in the healthy mucosa is provided via several mechanisms including epithelial junctional complexes, mucus production, as well as mucosal-derived antimicrobial proteins. As tissue metabolism is central to the maintenance of homeostasis in the mucosa, intestinal [Formula: see text] levels are uniquely low due to counter-current blood flow and the presence of the microbiota, resulting in the stabilization of the transcription factor hypoxia-inducible factor (HIF). Ongoing studies have revealed that HIF molds normal intestinal metabolism and is central to the coordination of barrier regulation during both homeostasis and active disease. During acute inflammation, HIF is central to controlling the rapid restitution of the epithelium consistent with normal wound healing responses. In contrast, HIF may also contribute to the fibrostenotic response associated with chronic, nonresolving inflammation. As such, HIF may function as a double-edged sword in the overall course of the inflammatory response. Here, we review recent literature on the contribution of HIF to mucosal barrier function, wound healing, and fibrosis.

Collagen release by human hepatic stellate cells requires vitamin C and is efficiently blocked by hydroxylase inhibition.

Liver fibrosis is characterized by the accumulation of extracellular matrix proteins, mainly composed of collagen. Hepatic stellate cells (HSCs) mediate liver fibrosis by secreting collagen. Vitamin C (ascorbic acid) is a cofactor of prolyl-hydroxylases that modify newly synthesized collagen on the route for secretion. Unlike most animals, humans cannot synthesize ascorbic acid and its role in liver fibrosis remains unclear. Here, we determined the effect of ascorbic acid and prolyl-hydroxylase inhibition on collagen production and secretion by human HSCs. Primary human HSCs (p-hHSCs) and the human HSCscell line LX-2 were treated with ascorbic acid, transforming growth factor-beta (TGFß) and/or the pan-hydroxylase inhibitor dimethyloxalylglycine (DMOG). Expression of collagen-I was analyzed by RT-qPCR (COL1A1), Western blotting, and immunofluorescence microscopy. Collagen secretion was determined in the medium by Western blotting for collagen-I and by HPLC for hydroxyproline concentrations. Expression of solute carrier family 23 members 1 and 2 (SLC23A1/SLC23A2), encoding sodium-dependent vitamin C transporters 1 and 2 (SVCT1/SVCT2) was quantified in healthy and cirrhotic human tissue. In the absence of ascorbic acid, collagen-I accumulated intracellularly in p-hHSCs and LX-2 cells, which was potentiated by TGFß. Ascorbic acid co-treatment strongly promoted collagen-I excretion and enhanced extracellular hydroxyproline concentrations, without affecting collagen-I (COL1A1) mRNA levels. DMOG inhibited collagen-I release even in the presence of ascorbic acid and suppressed COL1A1 and alpha-smooth muscle actin (αSMA/ACTA2) mRNA levels, also under hypoxic conditions. Hepatocytes express both ascorbic acid transporters, while p-hHSCs and LX-2 express the only SVCT2, which is selectively enhanced in cirrhotic livers. Human HSCs rely on ascorbic acid for the efficient secretion of collagen-I, which can be effectively blocked by hydroxylase antagonists, revealing new therapeutic targets to treat liver fibrosis.

The putative bacterial oxygen sensor Pseudomonas prolyl hydroxylase (PPHD) suppresses antibiotic resistance and pathogenicity in Pseudomonas aeruginosa.

Pseudomonas aeruginosa is an extracellular opportunistic bacterial pathogen commonly associated with infectious complications in susceptible individuals, such as those with underlying diseases including HIV/AIDS and cystic fibrosis. Antibiotic resistance in multiple strains of P. aeruginosa is a rapidly developing clinical problem. We have previously demonstrated that the oxygen levels at the site of P. aeruginosa infection can strongly influence virulence and antibiotic resistance in this pathogen, although the oxygen-sensing and -signaling mechanisms underpinning this response have remained unknown. In this study, we investigated the potential role of the putative oxygen sensor Pseudomonas prolyl hydroxylase (PPHD) in the control of virulence and antibiotic resistance in P. aeruginosa We found that a P. aeruginosa strain lacking PPHD (PAO310) exhibits increased virulence associated with increased bacterial motility. Furthermore, PPHD-deficient P. aeruginosa displayed enhanced antibiotic resistance against tetracycline through increased expression of the xenobiotic transporters mexEF-oprN and MexXY. Of note, the effect of the PPHD knockout on antibiotic resistance was phenocopied in bacteria exposed to atmospheric hypoxia. We conclude that PPHD is a putative bacterial oxygen sensor that may link microenvironmental oxygen levels to virulence and antibiotic resistance in P. aeruginosa.

Mucosal inflammation downregulates PHD1 expression promoting a barrier-protective HIF-1α response in ulcerative colitis patients.

The HIF hydroxylase enzymes (PHD1-3 and FIH) are cellular oxygen-sensors which confer hypoxic-sensitivity upon the hypoxia-inducible factors HIF-1α and HIF-2α. Microenvironmental hypoxia has a strong influence on the epithelial and immune cell function through HIF-dependent gene expression and consequently impacts upon the course of disease progression in ulcerative colitis (UC), with HIF-1α being protective while HIF-2α promotes disease. However, little is known about how inflammation regulates hypoxia-responsive pathways in UC patients. Here we demonstrate that hypoxia is a prominent microenvironmental feature of the mucosa in UC patients with active inflammatory disease. Furthermore, we found that inflammation drives transcriptional programming of the HIF pathway including downregulation of PHD1 thereby increasing the tissue responsiveness to hypoxia and skewing this response toward protective HIF-1 over detrimental HIF-2 activation. We identified CEBPα as a transcriptional regulator of PHD1 mRNA expression which is downregulated in both inflamed tissue derived from patients and in cultured intestinal epithelial cells treated with inflammatory cytokines. In summary, we propose that PHD1 downregulation skews the hypoxic response toward enhanced protective HIF-1α stabilization in the inflamed mucosa of UC patients.

Hydroxylase Inhibition Selectively Induces Cell Death in Monocytes.

Hypoxia is a common and prominent feature of the microenvironment at sites of bacteria-associated inflammation in inflammatory bowel disease. The prolyl-hydroxylases (PHD1/2/3) and the asparaginyl-hydroxylase factor-inhibiting HIF are oxygen-sensing enzymes that regulate adaptive responses to hypoxia through controlling the activity of HIF and NF-κB-dependent transcriptional pathways. Previous studies have demonstrated that the pan-hydroxylase inhibitor dimethyloxalylglycine (DMOG) is effective in the alleviation of inflammation in preclinical models of inflammatory bowel disease, at least in part, through suppression of IL-1ß-induced NF-κB activity. TLR-dependent signaling in immune cells, such as monocytes, which is important in bacteria-driven inflammation, shares a signaling pathway with IL-1ß. In studies into the effect of pharmacologic hydroxylase inhibition on TLR-induced inflammation in monocytes, we found that DMOG selectively triggers cell death in cultured THP-1 cells and primary human monocytes at concentrations well tolerated in other cell types. DMOG-induced apoptosis was independent of increased caspase-3/7 activity but was accompanied by reduced expression of the inhibitor of apoptosis protein 1 (cIAP1). Based on these data, we hypothesize that pharmacologic inhibition of the HIF-hydroxylases selectively targets monocytes for cell death and that this may contribute to the anti-inflammatory activity of HIF-hydroxylase inhibitors.

The Shc protein Rai enhances T-cell survival under hypoxia.

Hypoxia occurs in physiological and pathological conditions. T cells experience hypoxia in pathological and physiological conditions as well as in lymphoid organs. Indeed, hypoxia-inducible factor 1α (HIF-1α) affects T cell survival and functions. Rai, an Shc family protein member, exerts pro-survival effects in hypoxic neuroblastoma cells. Since Rai is also expressed in T cells, we here investigated its role in hypoxic T cells. In this work, hypoxia differently affected cell survival, proapoptotic, and metabolic programs in T cells, depending upon Rai expression. By using Jurkat cells stably expressing Rai and splenocytes from Rai-/- mice, we demonstrated that Rai promotes T cell survival and affects cell metabolism under hypoxia. Upon exposure to hypoxia, Jurkat T cells expressing Rai show (a) higher HIF-1α protein levels; (b) a decreased cell death and increased Akt/extracellular-signal-regulated kinase phosphorylation; (c) a decreased expression of proapoptotic markers, including caspase activities and poly(ADP-ribose) polymerase cleavage; (d) an increased glucose and lactate metabolism; (e) an increased activation of nuclear factor-kB pathway. The opposite effects were observed in hypoxic splenocytes from Rai-/- mice. Thus, Rai plays an important role in hypoxic signaling and may be relevant in the protection of T cells against hypoxia.

The hypoxia-inducible factor (HIF) couples immunity with metabolism.

Crosstalk between metabolic and immune pathways has recently become appreciated to be key to the regulation of host defence. The hypoxia-inducible factor (HIF) is a transcription factor which was initially described as a ubiquitous master regulator of the transcriptional response to hypoxia. In this role, HIF regulates genes promoting adaptation to hypoxia including a number which influence the cellular metabolic strategy of a cell. It has more recently been appreciated that the regulation of HIF is not restricted to oxygen-dependent pathways, and is now known to be mediated by a number of additional metabolic and immune cues including metabolites and cytokines respectively. Furthermore, our understanding of the functional role of HIF has expanded to it now being appreciated as a major regulator of host immunity. This places HIF in an ideal position to act as a regulatory hub which links metabolic activity with immunity. In this review we synthesise recent data which identifies HIF as both a target and effector for metabolic and immune processes. Developing our understanding of the role of HIF in this context will uncover new therapeutic targets for inflammatory and infectious disease.

Hypoxia-inducible factor hydroxylase inhibition enhances the protective effects of cyclosporine in colitis.

Inflammatory bowel disease (IBD) is characterized by epithelial barrier dysfunction with resultant inflammation as the mucosal immune system becomes exposed to luminal antigens. The hydroxylase inhibitor dimethyloxalylglycine (DMOG) reduces symptoms in experimental colitis through the upregulation of genes promoting barrier function and inhibition of epithelial cell apoptosis. The immunosuppressive drug cyclosporine reduces inflammation associated with IBD via suppression of immune cell activation. Given the distinct barrier protective effect of DMOG and the anti-inflammatory properties of cyclosporine, we hypothesized that combining these drugs may provide an enhanced protective effect by targeting both barrier dysfunction and inflammation simultaneously. We used the dextran sulfate sodium model of colitis in C57BL/6 mice to determine the combinatorial efficacy of cyclosporine and DMOG. While cyclosporine and DMOG ameliorated disease progression, in combination they had an additive protective effect that surpassed the level of protection afforded by either drug alone. The ability of DMOG to augment the anti-inflammatory effects of cyclosporine was largely due to preservation of barrier function and at least in part due to zonula occludens-1 regulation. We propose that combining the barrier protective effects of a hydroxylase inhibitor with the anti-inflammatory effects of cyclosporine provides added therapeutic benefit in colitis.NEW & NOTEWORTHY Inflammatory bowel disease is the result of decreased intestinal epithelial barrier function leading to exposure of the mucosal immune system to luminal antigens causing inflammation, which in turn further decreases epithelial barrier function. We demonstrate for the first time that strengthening the epithelial barrier with a hydroxylase inhibitor in combination with the administration of the immunosuppressive cyclosporine provides additive therapeutic advantage in a murine model of colitis.

Pharmacologic inhibition of hypoxia-inducible factor (HIF)-hydroxylases ameliorates allergic contact dermatitis.

BACKGROUND: When an immune cell migrates from the bloodstream to a site of chronic inflammation, it experiences a profound decrease in microenvironmental oxygen levels leading to a state of cellular hypoxia. The hypoxia-inducible factor-1α (HIF-1α) promotes an adaptive transcriptional response to hypoxia and as such is a major regulator of immune cell survival and function. HIF hydroxylases are the family of oxygen-sensing enzymes primarily responsible for conferring oxygen dependence upon the HIF pathway. METHODS: Using a mouse model of allergic contact dermatitis (ACD), we tested the effects of treatment with the pharmacologic hydroxylase inhibitor DMOG, which mimics hypoxia, on disease development. RESULTS: Re-exposure of sensitized mice to 2,4-dinitrofluorobenzene (DNFB) elicited inflammation, edema, chemokine synthesis (including CXCL1 and CCL5) and the recruitment of neutrophils and eosinophils. Intraperitoneal or topical application of the pharmacologic hydroxylase inhibitors dymethyloxalylglycine (DMOG) or JNJ1935 attenuated this inflammatory response. Reduced inflammation was associated with diminished recruitment of neutrophils and eosinophils but not lymphocytes. Finally, hydroxylase inhibition reduced cytokine-induced chemokine production in cultured primary keratinocytes through attenuation of the JNK pathway. CONCLUSION: These data demonstrate that hydroxylase inhibition attenuates the recruitment of neutrophils to inflamed skin through reduction of chemokine production and increased neutrophilic apoptosis. Thus, pharmacologic inhibition of HIF hydroxylases may be an effective new therapeutic approach in allergic skin inflammation.

FIH Regulates Cellular Metabolism through Hydroxylation of the Deubiquitinase OTUB1.

The asparagine hydroxylase, factor inhibiting HIF (FIH), confers oxygen-dependence upon the hypoxia-inducible factor (HIF), a master regulator of the cellular adaptive response to hypoxia. Studies investigating whether asparagine hydroxylation is a general regulatory oxygen-dependent modification have identified multiple non-HIF targets for FIH. However, the functional consequences of this outside of the HIF pathway remain unclear. Here, we demonstrate that the deubiquitinase ovarian tumor domain containing ubiquitin aldehyde binding protein 1 (OTUB1) is a substrate for hydroxylation by FIH on N22. Mutation of N22 leads to a profound change in the interaction of OTUB1 with proteins important in cellular metabolism. Furthermore, in cultured cells, overexpression of N22A mutant OTUB1 impairs cellular metabolic processes when compared to wild type. Based on these data, we hypothesize that OTUB1 is a target for functional hydroxylation by FIH. Additionally, we propose that our results provide new insight into the regulation of cellular energy metabolism during hypoxic stress and the potential for targeting hydroxylases for therapeutic benefit.

Trypanosoma brucei metabolite indolepyruvate decreases HIF-1α and glycolysis in macrophages as a mechanism of innate immune evasion.

The parasite Trypanasoma brucei causes African trypanosomiasis, known as sleeping sickness in humans and nagana in domestic animals. These diseases are a major burden in the 36 sub-Saharan African countries where the tsetse fly vector is endemic. Untreated trypanosomiasis is fatal and the current treatments are stage-dependent and can be problematic during the meningoencephalitic stage, where no new therapies have been developed in recent years and the current drugs have a low therapeutic index. There is a need for more effective treatments and a better understanding of how these parasites evade the host immune response will help in this regard. The bloodstream form of T. brucei excretes significant amounts of aromatic ketoacids, including indolepyruvate, a transamination product of tryptophan. This study demonstrates that this process is essential in bloodstream forms, is mediated by a specialized isoform of cytoplasmic aminotransferase and, importantly, reveals an immunomodulatory role for indolepyruvate. Indolepyruvate prevents the LPS-induced glycolytic shift in macrophages. This effect is the result of an increase in the hydroxylation and degradation of the transcription factor hypoxia-inducible factor-1α (HIF-1α). The reduction in HIF-1α levels by indolepyruvate, following LPS or trypanosome activation, results in a decrease in production of the proinflammatory cytokine IL-1ß. These data demonstrate an important role for indolepyruvate in immune evasion by T. brucei.