Ecto-5'-nucleotidase (CD73) regulation by hypoxia-inducible factor-1 mediates permeability changes in intestinal epithelia

Hypoxia-inducible factor in cancer: from pathway regulation to therapeutic opportunity

Cancer remains one of the most formidable challenges in modern medicine, due to its complex and dynamic nature, which demands innovative therapeutic approaches. One major challenge to cancer treatment is the tumour microenvironment and in particular tumour hypoxia (low oxygen levels), which contributes to tumour progression and immune evasion. At the cellular level, this is primarily governed by hypoxia-inducible factor (HIF). HIF is a transcription factor that orchestrates cellular responses to low oxygen levels, driving angiogenesis, metabolic adaptation and immune regulation. HIF's dysregulation is frequently observed in various cancer types and correlates with increased aggressiveness, metastasis, resistance to therapy and poor patient prognosis. Consequently, understanding the cellular mechanisms underlying HIF activation and its downstream effects has become crucial to developing targeted cancer therapies for improving cancer patient outcomes and represents a key step towards precision medicine. Recent advancements in drug development have led to the emergence of HIF inhibitors, which aim to disrupt HIF-driven processes in cancer providing therapeutic benefit. Here, we provide a review of the molecular mechanisms through which HIF promotes tumour growth and resistance, emphasising the potential clinical benefits of HIF-targeted therapies. This review will discuss the challenges and opportunities associated with translating HIF inhibition into clinical practice, including ongoing clinical trials and future directions in the development of HIF-based cancer treatments.

MicroRNA-223 attenuates hepatocarcinogenesis by blocking hypoxia-driven angiogenesis and immunosuppression

OBJECTIVE

The current treatment for hepatocellular carcinoma (HCC) to block angiogenesis and immunosuppression provides some benefits only for a subset of patients with HCC, thus optimised therapeutic regimens are unmet needs, which require a thorough understanding of the underlying mechanisms by which tumour cells orchestrate an inflamed tumour microenvironment with significant myeloid cell infiltration. MicroRNA-223 (miR-223) is highly expressed in myeloid cells but its role in regulating tumour microenvironment remains unknown.

DESIGN

Wild-type and miR-223 knockout mice were subjected to two mouse models of inflammation-associated HCC induced by injection of diethylnitrosamine (DEN) or orthotopic HCC cell implantation in chronic carbon tetrachloride (CCl4)-treated mice.

RESULTS

Genetic deletion of miR-223 markedly exacerbated tumourigenesis in inflammation-associated HCC. Compared with wild-type mice, miR-223 knockout mice had more infiltrated programmed cell death 1 (PD-1+) T cells and programmed cell death ligand 1 (PD-L1+) macrophages after DEN+CCl4 administration. Bioinformatic analyses of RNA sequencing data revealed a strong correlation between miR-223 levels and tumour hypoxia, a condition that is well-documented to regulate PD-1/PD-L1. In vivo and in vitro mechanistic studies demonstrated that miR-223 did not directly target PD-1 and PD-L1 in immune cells rather than indirectly downregulated them by modulating tumour microenvironment via the suppression of hypoxia-inducible factor 1α-driven CD39/CD73-adenosine pathway in HCC. Moreover, gene delivery of miR-223 via adenovirus inhibited angiogenesis and hypoxia-mediated PD-1/PD-L1 activation in both HCC models, thereby hindering HCC progression.

CONCLUSION

The miR-223 plays a critical role in modulating hypoxia-induced tumour immunosuppression and angiogenesis, which may serve as a novel therapeutic target for HCC.

Defining the spatial distribution of extracellular adenosine revealed a myeloid-dependent immunosuppressive microenvironment in pancreatic ductal adenocarcinoma

BACKGROUND

The prognosis for patients with pancreatic ductal adenocarcinoma (PDAC) remains extremely poor. It has been suggested that the adenosine pathway contributes to the ability of PDAC to evade the immune system and hence, its resistance to immuno-oncology therapies (IOT), by generating extracellular adenosine (eAdo).

METHODS

Using genetically engineered allograft models of PDAC in syngeneic mice with defined and different immune infiltration and response to IOT and autochthonous tumors in KPC mice we investigated the impact of the adenosine pathway on the PDAC tumor microenvironment (TME). Flow cytometry and imaging mass cytometry (IMC) were used to characterize the subpopulation frequency and spatial distribution of tumor-infiltrating immune cells. Mass spectrometry imaging (MSI) was used to visualize adenosine compartmentalization in the PDAC tumors. RNA sequencing was used to evaluate the influence of the adenosine pathway on the shaping of the immune milieu and correlate our findings to published data sets in human PDAC.

RESULTS

We demonstrated high expression of adenosine pathway components in tumor-infiltrating immune cells (particularly myeloid populations) in the murine models. MSI demonstrated that extracellular adenosine distribution is heterogeneous in tumors, with high concentrations in peri-necrotic, hypoxic regions, associated with rich myeloid infiltration, demonstrated using IMC. Protumorigenic M2 macrophages express high levels of the Adora2a receptor; particularly in the IOT resistant model. Blocking the in vivo formation and function of eAdo (Adoi), using a combination of anti-CD73 antibody and an Adora2a inhibitor slowed tumor growth and reduced metastatic burden. Additionally, blocking the adenosine pathway improved the efficacy of combinations of cytotoxic agents or immunotherapy. Adoi remodeled the TME, by reducing the infiltration of M2 macrophages and regulatory T cells. RNA sequencing analysis showed that genes related to immune modulation, hypoxia and tumor stroma were downregulated following Adoi and a specific adenosine signature derived from this is associated with a poorer prognosis in patients with PDAC.

CONCLUSIONS

The formation of eAdo promotes the development of the immunosuppressive TME in PDAC, contributing to its resistance to conventional and novel therapies. Therefore, inhibition of the adenosine pathway may represent a strategy to modulate the PDAC immune milieu and improve therapy response in patients with PDAC.

Epithelial HIF2α expression induces intestinal barrier dysfunction and exacerbation of arthritis

OBJECTIVE

To investigate how the mucosal barrier in the intestine influences the development of arthritis, considering that metabolic changes in the intestinal epithelium influence its barrier function.

METHODS

Intestinal hypoxia inducible factor (HIF)-2α expression was assessed before, at onset and during experimental arthritis and human rheumatoid arthritis (RA). Intestinal epithelial cell-specific HIF2α conditional knock-out mice were generated (HIF2α∆IEC) and subjected to collagen-induced arthritis. Clinical and histological courses of arthritis were recorded; T-cell and B-cell subsets were analysed in the gut and secondary lymphatic organs; and intestinal epithelial cells were subjected to molecular mRNA sequencing in HIF2α∆IEC and littermate control mice. The gut intestinal HIF2α target genes were delineated by chromatin immunoprecipitation and luciferase experiments. Furthermore, pharmacological HIF2α inhibitor PT2977 was used for inhibition of arthritis.

RESULTS

Intestinal HIF2α expression peaked at onset of experimental arthritis and RA. Conditionally, deletion of HIF2α in gut epithelial cells inhibited arthritis and was associated with improved intestinal barrier function and less intestinal and lymphatic Th1 and Th17 activation. Mechanistically, HIF2α induced the transcription of the pore-forming claudin (CLDN)-15, which inhibits intestinal barrier integrity. Furthermore, treatment with HIF2α inhibitor decreased claudin-15 expression in epithelial cells and inhibited arthritis.

CONCLUSION

These findings show that the HIF2α-CLDN15 axis is critical for the breakdown of intestinal barrier function at onset of arthritis, highlighting the functional link between intestinal homeostasis and arthritis.

ATP and Adenosine Metabolism in Cancer: Exploitation for Therapeutic Gain

Adenosine is an evolutionary ancient metabolic regulator linking energy state to physiologic processes, including immunomodulation and cell proliferation. Tumors create an adenosine-rich immunosuppressive microenvironment through the increased release of ATP from dying and stressed cells and its ectoenzymatic conversion into adenosine. Therefore, the adenosine pathway becomes an important therapeutic target to improve the effectiveness of immune therapies. Prior research has focused largely on the two major ectonucleotidases, ectonucleoside triphosphate diphosphohydrolase 1/cluster of differentiation (CD)39 and ecto-5'-nucleotidase/CD73, which catalyze the breakdown of extracellular ATP into adenosine, and on the subsequent activation of different subtypes of adenosine receptors with mixed findings of antitumor and protumor effects. New findings, needed for more effective therapeutic approaches, require consideration of redundant pathways controlling intratumoral adenosine levels, including the alternative NAD-inactivating pathway through the CD38-ectonucleotide pyrophosphatase phosphodiesterase (ENPP)1-CD73 axis, the counteracting ATP-regenerating ectoenzymatic pathway, and cellular adenosine uptake and its phosphorylation by adenosine kinase. This review provides a holistic view of extracellular and intracellular adenosine metabolism as an integrated complex network and summarizes recent data on the underlying mechanisms through which adenosine and its precursors ATP and ADP control cancer immunosurveillance, tumor angiogenesis, lymphangiogenesis, cancer-associated thrombosis, blood flow, and tumor perfusion. Special attention is given to differences and commonalities in the purinome of different cancers, heterogeneity of the tumor microenvironment, subcellular compartmentalization of the adenosine system, and novel roles of purine-converting enzymes as targets for cancer therapy. SIGNIFICANCE STATEMENT: The discovery of the role of adenosine as immune checkpoint regulator in cancer has led to the development of novel therapeutic strategies targeting extracellular adenosine metabolism and signaling in multiple clinical trials and preclinical models. Here we identify major gaps in knowledge that need to be filled to improve the therapeutic gain from agents targeting key components of the adenosine metabolic network and, on this basis, provide a holistic view of the cancer purinome as a complex and integrated network.

Prognostic significance and immune correlates of CD73 expression in renal cell carcinoma

Background

CD73–adenosine signaling in the tumor microenvironment is immunosuppressive and may be associated with aggressive renal cell carcinoma (RCC). We investigated the prognostic significance of CD73 protein expression in RCC leveraging nephrectomy samples. We also performed a complementary analysis using The Cancer Genome Atlas (TCGA) dataset to evaluate the correlation of CD73 (ecto-5′-nucleotidase (NT5E), CD39 (ectonucleoside triphosphate diphosphohydrolase 1 (ENTPD1)) and A2 adenosine receptor (A2AR; ADORA2A) transcript levels with markers of angiogenesis and antitumor immune response.

Methods

Patients with RCC with available archived nephrectomy samples were eligible for inclusion. Tumor CD73 protein expression was assessed by immunohistochemistry and quantified using a combined score (CS: % positive cells×intensity). Samples were categorized as CD73_negative (CS=0), CD73_low or CD73_high (< and ≥median CS, respectively). Multivariable Cox regression analysis compared disease-free survival (DFS) and overall survival (OS) between CD73 expression groups. In the TCGA dataset, samples were categorized as low, intermediate and high NT5E, ENTPD1 and ADORA2A gene expression groups. Gene expression signatures for infiltrating immune cells, angiogenesis, myeloid inflammation, and effector T-cell response were compared between NT5E, ENTPD1 and ADORA2A expression groups.

Results

Among the 138 patients eligible for inclusion, ‘any’ CD73 expression was observed in 30% of primary tumor samples. High CD73 expression was more frequent in patients with M1 RCC (29% vs 12% M0), grade 4 tumors (27% vs 13% grade 3 vs 15% grades 1 and 2), advanced T-stage (≥T3: 22% vs T2: 19% vs T1: 12%) and tumors with sarcomatoid histology (50% vs 12%). In the M0 cohort (n=107), patients with CD73_high tumor expression had significantly worse 5-year DFS (42%) and 10-year OS (22%) compared with those in the CD73_negative group (DFS: 75%, adjusted HR: 2.7, 95% CI 1.3 to 5.9, p=0.01; OS: 64%, adjusted HR: 2.6, 95% CI 1.2 to 5.8, p=0.02) independent of tumor stage and grade. In the TCGA analysis, high NT5E expression was associated with significantly worse 5-year OS (p=0.008). NT5E and ENTPD1 expression correlated with higher regulatory T cell (Treg) signature, while ADORA2A expression was associated with increased Treg and angiogenesis signatures.

Conclusions

High CD73 expression portends significantly worse survival outcomes independent of stage and grade. Our findings provide compelling support for targeting the immunosuppressive and proangiogenic CD73–adenosine pathway in RCC.

Conversion of ATP to adenosine by CD39 and CD73 in multiple myeloma can be successfully targeted together with adenosine receptor A2A blockade

Background

PD1/PDL1-directed therapies have been unsuccessful for multiple myeloma (MM), an incurable cancer of plasma cells in the bone marrow (BM). Therefore, other immune checkpoints such as extracellular adenosine and its immunosuppressive receptor should be considered. CD39 and CD73 convert extracellular ATP to adenosine, which inhibits T-cell effector functions via the adenosine receptor A2A (A2AR). We set out to investigate whether blocking the adenosine pathway could be a therapy for MM.

Methods

Expression of CD39 and CD73 on BM cells from patients and T-cell proliferation were determined by flow cytometry and adenosine production by Liquid chromatograpy-mass spectrometry (HPCL/MS). ENTPD1 (CD39) mRNA expression was determined on myeloma cells from patients enrolled in the publicly available CoMMpass study. Transplantable 5T33MM myeloma cells were used to determine the effect of inhibiting CD39, CD73 and A2AR in mice in vivo.

Results

Elevated level of adenosine was found in BM plasma of MM patients. Myeloma cells from patients expressed CD39, and high gene expression indicated reduced survival. CD73 was found on leukocytes and stromal cells in the BM. A CD39 inhibitor, POM-1, and an anti-CD73 antibody inhibited adenosine production and reduced T-cell suppression in vitro in coculture of myeloma and stromal cells. Blocking the adenosine pathway in vivo with a combination of Sodium polyoxotungstate (POM-1), anti-CD73, and the A2AR antagonist AZD4635 activated immune cells, increased interferon gamma production, and reduced the tumor load in a murine model of MM.

Conclusions

Our data suggest that the adenosine pathway can be successfully targeted in MM and blocking this pathway could be an alternative to PD1/PDL1 inhibition for MM and other hematological cancers. Inhibitors of the adenosine pathway are available. Some are in clinical trials and they could thus reach MM patients fairly rapidly.

DMOG, a Prolyl Hydroxylase Inhibitor, Increases Hemoglobin Levels without Exacerbating Hypertension and Renal Injury in Salt-Sensitive Hypertensive Rats

Prolyl hydroxylase (PHD) inhibitors are being developed as alternatives to recombinant human erythropoietin (rHuEPO) for the treatment of anemia in patients with chronic kidney disease (CKD). However, the effects of PHD inhibitors and rHuEPO on blood pressure and CKD in animal models susceptible to hypertension and nephropathy have not been studied. The present study compared the effects of dimethyloxaloylglycine (DMOG), a PHD inhibitor, and rHuEPO on the development of hypertension and renal injury in Dahl salt-sensitive rats fed an 8% salt diet for 3 weeks. DMOG and rHuEPO were equally effective at raising hemoglobin levels. Systolic blood pressure rose to a greater extent in rHuEPO-treated rats (267 ± 10 vs. 226 ± 4 mm Hg) than in rats given DMOG (189 ± 8 mm Hg). Urinary protein excretion increased to 568 ± 54 versus 353 ± 25 mg/day in rats treated with rHuEPO and vehicle; however, it only rose to 207 ± 21 mg/day in rats receiving DMOG. DMOG significantly attenuated the degree of glomerulosclerosis and renal interstitial fibrosis as compared with that in vehicle and rHuEPO-treated rats. This was associated with lower renal levels of monocyte chemoattractant protein-1 and interleukin-1β and increased vascular endothelial growth factor expression in cortex and medulla. These results indicate that DMOG and rHuEPO are equally effective in increasing hemoglobin levels in Dahl S rats; however, rHuEPO aggravates hypertension and renal injury, whereas DMOG has marked renoprotective effects. These results suggest that PHD inhibitors may have a therapeutic advantage for the treatment of anemia in CKD. SIGNIFICANCE STATEMENT: Prolyl hydroxylase (PHD) inhibitors are in phase 3 clinical trials as alternatives to recombinant human erythropoietin (rHuEPO) for the treatment of anemia in chronic kidney disease (CKD). The present study reveals that dimethyloxaloylglycine (DMOG), a PHD inhibitor, and rHuEPO are equally effective in increasing hemoglobin levels in Dahl S rats; however, rHuEPO aggravated hypertension and renal injury, whereas DMOG attenuated the development of hypertension and prevented renal injury. PHD inhibitors may provide a safer therapeutic option for the treatment of anemia in CKD.

The Purinergic System as a Pharmacological Target for the Treatment of Immune-Mediated Inflammatory Diseases

Immune-mediated inflammatory diseases (IMIDs) encompass a wide range of seemingly unrelated conditions, such as multiple sclerosis, rheumatoid arthritis, psoriasis, inflammatory bowel diseases, asthma, chronic obstructive pulmonary disease, and systemic lupus erythematosus. Despite differing etiologies, these diseases share common inflammatory pathways, which lead to damage in primary target organs and frequently to a plethora of systemic effects as well. The purinergic signaling complex comprising extracellular nucleotides and nucleosides and their receptors, the P2 and P1 purinergic receptors, respectively, as well as catabolic enzymes and nucleoside transporters is a major regulatory system in the body. The purinergic signaling complex can regulate the development and course of IMIDs. Here we provide a comprehensive review on the role of purinergic signaling in controlling immunity, inflammation, and organ function in IMIDs. In addition, we discuss the possible therapeutic applications of drugs acting on purinergic pathways, which have been entering clinical development, to manage patients suffering from IMIDs.

The Adaptive Complexity of Cancer

Cancer treatment options are expanding to the benefit of significant segments of patients. However, their therapeutic power is not equally realized for all cancer patients due to drug toxicity and disease resistance. Overcoming these therapeutic challenges would require a better understanding of the adaptive survival mechanisms of cancer. In this respect, an integrated view of the disease as a complex adaptive system is proposed as a framework to explain the dynamic coupling between the various drivers underlying tumor growth and cancer resistance to therapy. In light of this system view of cancer, the immune system is in principal the most appropriate and naturally available therapeutic instrument that can thwart the adaptive survival mechanisms of cancer. In this respect, new cancer therapies should aim at restoring immunosurveillance by priming the induction of an effective immune response through a judicious targeting of immunosuppression, inflammation, and the tumor nutritional lifeline extended by the tumor microenvironment.

The Hypoxia-Adenosine Link during Intestinal Inflammation

Intestinal inflammation is a key element in inflammatory bowel disease and is related to a combination of factors, including genetics, mucosal barrier dysfunction, bacteria translocation, deleterious host-microbe interactions, and dysregulated immune responses. Over the past decade, it has been appreciated that these inflammatory lesions are associated with profound tissue hypoxia. Interestingly, an endogenous adaptive response under the control of hypoxia signaling is enhancement in adenosine signaling, which impacts these different endpoints, including promoting barrier function and encouraging anti-inflammatory activity. In this review, we discuss the hypoxia-adenosine link in inflammatory bowel disease, intestinal ischemia/reperfusion injury, and colon cancer. In addition, we provide a summary of clinical implications of hypoxia and adenosine signaling in intestinal inflammation and disease.

CD73 Predicts Favorable Prognosis in Patients with Nonmuscle-Invasive Urothelial Bladder Cancer

Aims. CD73 is a membrane associated 5′-ectonucleotidase that has been proposed as prognostic biomarker in various solid tumors. The aim of this study is to evaluate CD73 expression in a cohort of patients with primary bladder cancer in regard to its association with clinicopathological features and disease course. Methods. Tissue samples from 174 patients with a primary urothelial carcinoma were immunohistochemically assessed on a tissue microarray. Associations between CD73 expression and retrospectively obtained clinicopathological data were evaluated by contingency analysis. Survival analysis was performed to investigate the predictive value of CD73 within the subgroup of pTa and pT1 tumors in regard to progression-free survival (PFS). Results. High CD73 expression was found in 46 (26.4%) patients and was significantly associated with lower stage, lower grade, less adjacent carcinoma in situ and with lower Ki-67 proliferation index. High CD73 immunoreactivity in the subgroup of pTa and pT1 tumors (n = 158) was significantly associated with longer PFS (HR: 0.228; p = 0.047) in univariable Cox regression analysis. Conclusion. High CD73 immunoreactivity was associated with favorable clinicopathological features. Furthermore, it predicts better outcome in the subgroup of pTa and pT1 tumors and may thus serve as additional tool for the selection of patients with favorable prognosis.

Hypoxia-Inducible Factor-2α Limits Natural Killer T Cell Cytotoxicity in Renal Ischemia/Reperfusion Injury

Natural killer T (NKT) cells are the major early-acting immune cell type and fundamental immune modulators in ischemia-reperfusion injury (IRI). Because lymphocytes are exposed to various oxygen tensions under pathophysiologic conditions, we hypothesize that hypoxia-inducible factors (HIFs) have roles in NKT cell activation, and thus determine the final outcome of renal IRI. In this study, we used Lck-Cre transgenic mice to specifically disrupt HIF-2α in T/NKT cells and found that HIF-2α knockout led to upregulated Fas ligand expression on peripheral NKT cells, but not on conventional T cells. HIF-2α knockout promoted infiltration of NKT cells into ischemic kidneys and exacerbated IRI, which could be mitigated by in vivo NK1.1(+) cell depletion or Fas ligand blockade. Compared with wild-type NKT cells, HIF-2α(-/-) NKT cells adoptively transferred to Rag1-knockout mice elicited more severe renal injury, and these mice were not protected by CGS21680, an adenosine A2A receptor agonist. Mechanistically, hypoxia-induced expression of adenosine A2A receptor in NKT cells and CGS21680-induced cAMP production in thymocytes were HIF-2α-dependent. Hydrogen peroxide-induced Fas ligand expression on thymic wild-type NKT cells was significantly attenuated by CGS21680 treatment, but this effect was lost in HIF-2α(-/-) NKT cells. Finally, CGS21680 and LPS, an inducer of HIF-2α in endothelium, synergistically reduced renal IRI substantially, but this effect was absent in Mx1-Cre-induced global HIF-2α-knockout mice. Taken together, our results reveal a hypoxia/HIF-2α/adenosine A2A receptor axis that restricts NKT cell activation when confronted with oxidative stress and thus protects against renal IRI.

Pathophysiological role of extracellular purinergic mediators in the control of intestinal inflammation

Purinergic mediators such as adenosine 5′-triphosphate (ATP) are released into the extracellular compartment from damaged tissues and activated immune cells. They are then recognized by multiple purinergic P2X and P2Y receptors. Release and recognition of extracellular ATP are associated with both the development and the resolution of inflammation and infection. Accumulating evidence has recently suggested the potential of purinergic receptors as novel targets for drugs for treating intestinal disorders, including intestinal inflammation and irritable bowel syndrome. In this review, we highlight recent findings regarding the pathophysiological role of purinergic mediators in the development of intestinal inflammation.

IFN-γ-mediated induction of an apical IL-10 receptor on polarized intestinal epithelia

Cytokines secreted at sites of inflammation impact the onset, progression, and resolution of inflammation. In this article, we investigated potential proresolving mechanisms of IFN-γ in models of inflammatory bowel disease. Guided by initial microarray analysis, in vitro studies revealed that IFN-γ selectively induced the expression of IL-10R1 on intestinal epithelia. Further analysis revealed that IL-10R1 was expressed predominantly on the apical membrane of polarized epithelial cells. Receptor activation functionally induced canonical IL-10 target gene expression in epithelia, concomitant with enhanced barrier restitution. Furthermore, knockdown of IL-10R1 in intestinal epithelial cells results in impaired barrier function in vitro. Colonic tissue isolated from murine colitis revealed that levels of IL-10R1 and suppressor of cytokine signaling 3 were increased in the epithelium and coincided with increased tissue IFN-γ and IL-10 cytokines. In parallel, studies showed that treatment of mice with rIFN-γ was sufficient to drive expression of IL-10R1 in the colonic epithelium. Studies of dextran sodium sulfate colitis in intestinal epithelial-specific IL-10R1-null mice revealed a remarkable increase in disease susceptibility associated with increased intestinal permeability. Together, these results provide novel insight into the crucial and underappreciated role of epithelial IL-10 signaling in the maintenance and restitution of epithelial barrier and of the temporal regulation of these pathways by IFN-γ.

CD73-dependent generation of adenosine and endothelial Adora2b signaling attenuate diabetic nephropathy

Nucleotide phosphohydrolysis by the ecto-5'-nucleotidase (CD73) is the main source for extracellular generation of adenosine. Extracellular adenosine subsequently signals through four distinct adenosine A receptors (Adora1, Adora2a, Adora2b, or Adora3). Here, we hypothesized a functional role for CD73-dependent generation and concomitant signaling of extracellular adenosine during diabetic nephropathy. CD73 transcript and protein levels were elevated in the kidneys of diabetic mice. Genetic deletion of CD73 was associated with more severe diabetic nephropathy, whereas treatment with soluble nucleotidase was therapeutic. Transcript levels of renal adenosine receptors showed a selective induction of Adora2b during diabetic nephropathy. In a transgenic reporter mouse, Adora2b expression localized to the vasculature and increased after treatment with streptozotocin. Adora2b(-/-) mice experienced more severe diabetic nephropathy, and studies in mice with tissue-specific deletion of Adora2b in tubular epithelia or vascular endothelia implicated endothelial Adora2b signaling in protection from diabetic nephropathy. Finally, treatment with a selective Adora2b agonist (BAY 60-6583) conveyed potent protection from diabetes-associated kidney disease. Taken together, these findings implicate CD73-dependent production of extracellular adenosine and endothelial Adora2b signaling in kidney protection during diabetic nephropathy.

TLR9 deficiency promotes CD73 expression in T cells and diabetes protection in nonobese diabetic mice

TLR9-deficient (TLR9⁻/⁻) NOD mice develop a significantly reduced incidence of diabetes. This study was to investigate the molecular mechanisms of the protective role of TLR9 deficiency. Through gene screening and confirmation by both mRNA and protein expression, we found a significant increase in CD73-expressing immune cells from peripheral lymphoid tissues in TLR9⁻/⁻ NOD mice. The elevated frequency of CD73-expressing immune cells seemed to be specific for TLR9 deficiency and was MyD88 independent. Moreover, the increased frequency of CD73 expression was limited to the NOD background. Increased frequency of CD73 expression was also associated with lower levels of proinflammatory cytokines and more anti-inflammatory cytokine production in CD4⁺ T cells in TLR9⁻/⁻ NOD mice. Purified CD73⁺CD4⁺ T cells showed stronger immunosuppressive function in vitro and delayed diabetes development in vivo. The immunosuppression appeared to be mediated by TGF-β. In addition, elevated frequency of CD73-expressing cells was associated with improved β cell function. Our observations were further confirmed by protection from diabetes with similar alterations in CD73 in the NY8.3 TCR NOD mouse model crossed with TLR9⁻/⁻ mice and by the use of a TLR9 inhibitor in NOD mice. Our novel findings suggest an important immune-regulatory role of CD73 in regulation of diabetes development and may offer a new therapeutic strategy for specific intervention to prevent type 1 diabetes.

Expression and clinical significance of CD73 and hypoxia-inducible factor-1α in gastric carcinoma

AIM: To investigate the expression of CD73 and hypoxia-inducible factor-1α (HIF-1α) in human gastric carcinoma, and explore their clinical significance and prognostic value.

METHODS: CD73 and HIF-1α expressions were detected by immunohistochemistry in consecutive sections of tissue samples from 68 gastric carcinoma patients. The peritumor tissues 2 cm away from the tumor were obtained and served as controls. The presence of CD73 and HIF-1α was analyzed by immunohistochemistry using the Envision technique.

RESULTS: CD73 and HIF-1α expressions in gastric carcinoma were significantly higher than those in gastric mucosal tissues as control (P < 0.001) and showed a close correlation (Spearman r = 0.390, P = 0.001). Overexpression of CD73 was positively correlated with differentiation of tumor (P = 0.000), histopathology (P = 0.041), depth of invasion (P < 0.001), nodal status (P = 0.003), metastasis (P = 0.013), and the American Joint Committee on Cancer (AJCC) stage (P < 0.001). High expression of HIF-1α was positively correlated with tumor diameter (P = 0.031), depth of invasion (P = 0.022), and AJCC stage (P = 0.035). The overall survival rate was low in the patients with high expression of CD73 (P < 0.001). Moreover, CD73+/HIF-1α+ patients had the worst prognosis (P < 0.001). CD73 expression was proven to be an independent predictor for patients with gastric carcinoma by both multivariate Cox regression analysis (P = 0.021) and receiver operating characteristic curves (P = 0.001).

CONCLUSION: CD73 expression correlates closely with HIF-1α expression in gastric carcinoma. CD73 could be an independent prognostic indicator for gastric carcinoma.

Ecto-nucleoside triphosphate diphosphohydrolase 7 controls Th17 cell responses through regulation of luminal ATP in the small intestine

Extracellular ATP is released from live cells in controlled conditions, as well as dying cells in inflammatory conditions, and, thereby, regulates T cell responses, including Th17 cell induction. The level of extracellular ATP is closely regulated by ATP hydrolyzing enzymes, such as ecto-nucleoside triphosphate diphosphohydrolases (ENTPDases). ENTPDase1/CD39, which is expressed in immune cells, was shown to regulate immune responses by downregulating the ATP level. In this study, we analyzed the immunomodulatory function of ENTPDase7, which is preferentially expressed in epithelial cells in the small intestine. The targeted deletion of Entpd7 encoding ENTPDase7 in mice resulted in increased ATP levels in the small intestinal lumen. The number of Th17 cells was selectively increased in the small intestinal lamina propria in Entpd7(-/-) mice. Th17 cells were decreased by oral administration of antibiotics or the ATP antagonist in Entpd7(-/-) mice, indicating that commensal microbiota-dependent ATP release mediates the enhanced Th17 cell development in the small intestinal lamina propria of Entpd7(-/-) mice. In accordance with the increased number of small intestinal Th17 cells, Entpd7(-/-) mice were resistant to oral infection with Citrobacter rodentium. Entpd7(-/-) mice suffered from severe experimental autoimmune encephalomyelitis, which was associated with increased numbers of CD4(+) T cells producing both IL-17 and IFN-γ. Taken together, these findings demonstrate that ENTPDase7 controls the luminal ATP level and, thereby, regulates Th17 cell development in the small intestine.

Central role for endothelial human deneddylase-1/SENP8 in fine-tuning the vascular inflammatory response

A deeper understanding of the mechanisms that control responses to inflammation is critical to the development of effective therapies. We sought to define the most proximal regulators of the Cullin (Cul)-RING ligases, which play a central role in the stabilization of NF-κB and hypoxia-inducible factor (HIF). In these studies, we identify the human deneddylase-1 (SENP8) as a key regulator of Cul neddylation response in vitro and in vivo. Using human microvascular endothelial cells (HMECs), we examined inflammatory responses to LPS or TNF-α by assessing Cul neddylation status, NF-κB and HIF-1α stabilization, and inflammatory cytokine secretion. HMECs with an intact neddylation pathway showed a time-dependent induction of Cul-1 neddylation, nuclear translocation of NF-κB, stabilization of HIF-1α, and increased NF-κB/HIF-α promoter activity in response to LPS. HMECs lacking SENP8 were unable to neddylate Cul-1 and subsequently were unable to activate NF-κB or HIF-1α. Pharmacological targeting of neddylation (MLN4924) significantly abrogated NF-κB responses, induced HIF-1α promoter activity, and reduced secretion of TNF-α-elicited proinflammatory cytokines. MLN4924 stabilized HIF and abrogated proinflammatory responses while maintaining anti-inflammatory IL-10 responses in vivo following LPS administration. These studies identify SENP8 as a proximal regulator of Cul neddylation and provide an important role for SENP8 in fine-tuning the inflammatory response. Moreover, our findings provide feasibility for therapeutic targeting of the Culs during inflammation.

CD73-generated adenosine: orchestrating the tumor-stroma interplay to promote cancer growth

Despite the coming of age of cancer immunotherapy, clinical benefits are still modest. An important barrier to successful cancer immunotherapy is that tumors employ a number of mechanisms to facilitate immune escape, including the production of anti-inflammatory cytokines, the recruitment of regulatory immune subsets, and the production of immunosuppressive metabolites. Significant therapeutic opportunity exists in targeting these immunosuppressive pathways. One such immunosuppressive pathway is the production of extracellular adenosine by CD73, an ectonucleotidase overexpressed in various types of cancer. We hereafter review the biology of CD73 and its role in cancer progression and metastasis. We describe the role of extracellular adenosine in promoting tumor growth through paracrine and autocrine action on tumor cells, endothelial cells, and immune cells.

CD73 is critical for the resolution of murine colonic inflammation

CD73 is a glycosyl-phosphatidylinositol-(GPI-) linked membrane protein that catalyzes the extracellular dephosphorylation of adenosine monophosphate (AMP) to adenosine. Adenosine is a negative regulator of inflammation and prevents excessive cellular damage. We investigated the role of extracellular adenosine in the intestinal mucosa during the development of Dextran-Sulfate-Sodium-(DSS-)salt-induced colitis in mice that lack CD73 (CD73^−/−) and are unable to synthesize extracellular adenosine. We have found that, compared to wild-type (WT) mice, CD73^−/− mice are highly susceptible to DSS-induced colitis. CD73^−/− mice exhibit pronounced weight loss, slower weight recovery, an increase in gut permeability, a decrease in expression of tight junctional adhesion molecules, as well as unresolved inflammation following the removal of DSS. Moreover, colonic epithelia in CD73^−/− mice exhibited increased TLR9 expression, high levels of IL-1β and TNF-α, and constitutive activation of NF-κB. We conclude that CD73 expression in the colon is critical for regulating the magnitude and the resolution of colonic immune responses.

The CD39-adenosinergic axis in the pathogenesis of immune and nonimmune diabetes

Diabetes mellitus encompasses two distinct disease processes: autoimmune Type 1 (T1D) and nonimmune Type 2 (T2D) diabetes. Despite the disparate aetiologies, the disease phenotype of hyperglycemia and the associated complications are similar. In this paper, we discuss the role of the CD39-adenosinergic axis in the pathogenesis of both T1D and T2D, with particular emphasis on the role of CD39 and CD73.

Production of adenosine by ectonucleotidases: a key factor in tumor immunoescape

It is now well known that tumor immunosurveillance contributes to the control of cancer growth. Many mechanisms can be used by cancer cells to avoid the antitumor immune response. One such mechanism relies on the capacity of cancer cells or more generally of the tumor microenvironment to generate adenosine, a major molecule involved in antitumor T cell response suppression. Adenosine is generated by the dephosphorylation of extracellular ATP released by dying tumor cells. The conversion of ATP into adenosine is mediated by ectonucleotidase molecules, namely, CD73 and CD39. These molecules are frequently expressed in the tumor bed by a wide range of cells including tumor cells, regulatory T cells, Th17 cells, myeloid cells, and stromal cells. Recent evidence suggests that targeting adenosine by inhibiting ectonucleotidases may restore the resident antitumor immune response or enhance the efficacy of antitumor therapies. This paper will underline the impact of adenosine and ectonucleotidases on the antitumor response.

Tissue-resident ecto-5' nucleotidase (CD73) regulates leukocyte trafficking in the ischemic brain

Ectoenzymes expressed on the surface of vascular cells and leukocytes modulate the ambient nucleotide milieu. CD73 is an ecto-5' nucleotidase that catalyzes the terminal phosphohydrolysis of AMP and resides in the brain on glial cells, cells of the choroid plexus, and leukocytes. Though CD73 tightens epithelial barriers, its role in the ischemic brain remains undefined. When subjected to photothrombotic arterial occlusion, CD73(-/-) mice exhibited significantly larger (49%) cerebral infarct volumes than wild-type mice, with concordant increases in local accumulation of leukocyte subsets (neutrophils, T lymphocytes, macrophages, and microglia). CD73(-/-) mice were rescued from ischemic neurologic injury by soluble 5'-nucleotidase. In situ, CD73(-/-) macrophages upregulated expression of costimulatory molecules far more than wild-type macrophages, with a sharp increase of the CD80/CD86 ratio. To define the CD73-bearing cells responsible for ischemic cerebroprotection, mice were subjected to irradiative myeloablation, marrow reconstitution, and then stroke following engraftment. Chimeric mice lacking CD73 in tissue had larger cerebral infarct volumes and more tissue leukosequestration than did mice lacking CD73 on circulating cells. These data show a cardinal role for CD73 in suppressing ischemic tissue leukosequestration. This underscores a critical role for CD73 as a modulator of brain inflammation and immune function.

Ecto-5'-nucleotidase (CD73) decreases mortality and organ injury in sepsis

The extracellular concentrations of adenosine are increased during sepsis, and adenosine receptors regulate the host's response to sepsis. In this study, we investigated the role of the adenosine-generating ectoenzyme, ecto-5'-nucleotidase (CD73), in regulating immune and organ function during sepsis. Polymicrobial sepsis was induced by subjecting CD73 knockout (KO) and wild type (WT) mice to cecal ligation and puncture. CD73 KO mice showed increased mortality in comparison with WT mice, which was associated with increased bacterial counts and elevated inflammatory cytokine and chemokine concentrations in the blood and peritoneum. CD73 deficiency promoted lung injury, as indicated by increased myeloperoxidase activity and neutrophil infiltration, and elevated pulmonary cytokine levels. CD73 KO mice had increased apoptosis in the thymus, as evidenced by increased cleavage of caspase-3 and poly(ADP-ribose) polymerase and increased activation of NF-κB. Septic CD73 KO mice had higher blood urea nitrogen levels and increased cytokine levels in the kidney, indicating increased renal dysfunction. The increased kidney injury of CD73 KO mice was associated with augmented activation of p38 MAPK and decreased phosphorylation of Akt. Pharmacological inactivation of CD73 in WT mice using α, β-methylene ADP augmented cytokine levels in the blood and peritoneal lavage fluid. These findings suggest that CD73-derived adenosine may be beneficial in sepsis.

An endogenously anti-inflammatory role for methylation in mucosal inflammation identified through metabolite profiling

Tissues of the mucosa are lined by an epithelium that provides barrier and transport functions. It is now appreciated that inflammatory responses in inflammatory bowel diseases are accompanied by striking shifts in tissue metabolism. In this paper, we examined global metabolic consequences of mucosal inflammation using both in vitro and in vivo models of disease. Initial analysis of the metabolic signature elicited by inflammation in epithelial models and in colonic tissue isolated from murine colitis demonstrated that levels of specific metabolites associated with cellular methylation reactions are significantly altered by model inflammatory systems. Furthermore, expression of enzymes central to all cellular methylation, S-adenosylmethionine synthetase and S-adenosylhomocysteine hydrolase, are increased in response to inflammation. Subsequent studies showed that DNA methylation is substantially increased during inflammation and that epithelial NF-κB activity is significantly inhibited following treatment with a reversible S-adenosylhomocysteine hydrolase inhibitor, DZ2002. Finally, these studies demonstrated that inhibition of cellular methylation in a murine model of colitis results in disease exacerbation while folate supplementation to promote methylation partially ameliorates the severity of murine colitis. Taken together, these results identify a global change in methylation, which during inflammation, translates to an overall protective role in mucosal epithelia.

Hypoxia-inducible factor-1α-dependent protection from intestinal ischemia/reperfusion injury involves ecto-5'-nucleotidase (CD73) and the A2B adenosine receptor

Intestinal ischemia/reperfusion injury (IR) is characterized by intermittent loss of perfusion to the gut, resulting in dramatic increases in morbidity and mortality. Based on previous studies indicating an anti-inflammatory role for hypoxia-inducible factor (HIF)-1-elicited enhancement of extracellular adenosine production via ecto-5'-nucleotidase (CD73) and signaling through the A2B adenosine receptor (A2BAR), we targeted HIF-1 during IR using pharmacological or genetic approaches. Initial studies with pharmacological HIF activation indicated attenuation of intestinal injury with dimethyloxallyl glycine (DMOG) treatment during murine IR. Although DMOG treatment was associated with induction of CD73 transcript and protein, DMOG protection was abolished in cd73(-/-) mice. Similarly, DMOG treatment enhanced A2BAR transcript and protein levels, whereas DMOG protection was abolished in A2BAR(-/-) mice. Finally, studies of mice with conditional HIF-1α deletion in intestinal epithelia or pharmacological inhibition of HIF-1 with 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin revealed enhanced tissue injury during IR. These studies indicated a tissue-protective role of HIF-dependent enhancement of intestinal adenosine generation and signaling during intestinal IR.

Adenosine generation and signaling during acute kidney injury

Acute kidney injury (AKI) is among the leading causes of morbidity and mortality in hospitalized patients. Particularly in the perioperative period, the most common cause of AKI is renal ischemia. At present, therapeutic modalities to prevent or treat AKI are extremely limited and the search for novel therapeutic interventions for ischemic AKI is an area of intense investigation. Recent studies implicate the endogenous signaling molecule, adenosine, in kidney protection from ischemia. As such, enzymatic production of adenosine from its precursor molecules ATP and AMP, and signaling events through adenosine receptors, play a critical role in attenuating renal inflammation and preserving kidney function during episodes of renal ischemia. Utilizing genetic mouse models with defects in adenosine generation or signaling provide strong evidence for the key role of extracellular adenosine in adapting renal tissues to limited oxygen availability and attenuating hypoxia-driven inflammation of the kidneys. Moreover, experimental therapeutics targeting individual adenosine receptors demonstrate strong prophylactic or therapeutic effects during murine AKI. If these experimental strategies can be translated into a clinical setting, adenosine receptor therapeutics may become an integral part in the prevention or treatment of AKI from renal ischemia.

IFN-γ attenuates hypoxia-inducible factor (HIF) activity in intestinal epithelial cells through transcriptional repression of HIF-1β

Numerous studies have revealed that hypoxia and inflammation occur coincidentally in mucosal disorders, such as inflammatory bowel disease. During inflammation, epithelial-expressed hypoxia-inducible factor (HIF) serves an endogenously protective function. In this study, we sought to explore how mucosal immune responses influence HIF-dependent end points. Guided by a screen of relevant inflammatory mediators, we identified IFN-γ as a potent repressor of HIF-dependent transcription in human intestinal epithelial cells. Analysis of HIF levels revealed that HIF-1β, but not HIF-1α, is selectively repressed by IFN-γ in a JAK-dependent manner. Cloning and functional analysis of the HIF-1β promoter identified a prominent region for IFN-γ-dependent repression. Further studies revealed that colonic IFN-γ and HIF-1β levels were inversely correlated in a murine colitis model. Taken together, these studies demonstrated that intestinal epithelial HIF is attenuated by IFN-γ through transcriptional repression of HIF-1β. These observations are relevant to the pathophysiology of colitis (i.e., that loss of HIF signaling during active inflammation may exacerbate disease pathogenesis).

SP1-dependent induction of CD39 facilitates hepatic ischemic preconditioning

Ischemia/reperfusion injury (IRI) of the liver is an important cause of hepatic dysfunction. Ischemic preconditioning (IP) is associated with adenosine-mediated tissue protection from subsequent IRI. Extracellular nucleotides (e.g., ATP) represent the main source for extracellular adenosine. Therefore, we hypothesized that phosphohydrolysis of ATP/ADP via the ectonucleoside triphosphate diphosphohydrolase-1 (CD39), conversion of ATP/ADP to AMP, mediates IP-dependent liver protection. We found that hepatic IP was associated with significant induction of CD39 transcript, heightened protein expression, and improved outcomes after IRI. Targeted gene deletion or pharmacological inhibition of CD39 abolished hepatoprotection by IP as measured by serum markers of liver injury or histology. Therapeutic studies to mimic IP with i.p. apyrase (a soluble ectonucleoside triphosphate diphosphohydrolase, NTPDase) in the absence of IP attenuated hepatic injury after IRI. In additional in vivo studies, small interfering RNA treatment was used to achieve repression of the transcription factor Sp1, known to be implicated in CD39 transcriptional regulation. In fact, Sp1 small interfering RNA treatment was associated with attenuated CD39 induction and increased hepatic injury in vivo. Our data suggest a Sp1-dependent regulatory pathway for CD39 during hepatic IP. These studies reveal a novel role of CD39 in hepatic protection and suggest soluble apyrase for the treatment of liver ischemia.

Feedback effects of host-derived adenosine on enteropathogenic Escherichia coli

Enteropathogenic E. coli (EPEC) is a common cause of diarrhea in children in developing countries. After adhering to intestinal cells, EPEC secretes effector proteins into host cells, causing cell damage and eventually death. We previously showed that EPEC infection triggers the release of ATP from host cells and that ATP is broken down to ADP, AMP, and adenosine. Adenosine produced from the breakdown of extracellular ATP triggers fluid secretion in intestinal monolayers and may be an important mediator of EPEC-induced diarrhea. Here we examined whether adenosine has any effects on EPEC bacteria. Adenosine stimulated EPEC growth in several types of media in vitro. Adenosine also altered the pattern of EPEC adherence to cultured cells from a localized adherence pattern to a more diffuse pattern. Adenosine changed the expression of virulence factors in EPEC, inhibiting the expression of the bundle-forming pilus (BFP) and enhancing expression of the EPEC secreted proteins (Esps). In vivo, experimental manipulations of adenosine levels had strong effects on the outcome of EPEC infection in rabbit intestinal loops. In addition to its previously reported effects on host tissues, adenosine has strong effects on EPEC bacteria, stimulating EPEC growth, altering its adherence pattern, and changing the expression of several important virulence genes. Adenosine, like noradrenaline, is a small, host-derived molecule that is utilized as a signal by EPEC.

Contribution of adenosine A2B receptors to inflammatory parameters of experimental colitis

Inflammatory diseases influence tissue metabolism, significantly altering the profile of extracellular adenine nucleotides. A number of studies have suggested that adenosine (Ado) may function as an endogenously generated anti-inflammatory molecule. Given the central role of intestinal epithelial cells to the development of colitis, we hypothesized that specific Ado receptors would contribute to disease resolution in mucosal inflammation as modeled by dextran sodium sulfate (DSS) colitis. Initial profiling studies revealed that murine intestinal epithelial cells express predominantly the Ado A2B receptor (AA2BR) and to a lesser extent AA2AR. Guided by these results, we examined the contribution of AA2BR to colitis. Initial studies indicated that the severity of colitis was increased in Aa2br(-/-) mice relative to Aa2br(+/+) controls, as reflected by increased weight loss, colonic shortening, and disease activity indices. Likewise, enteral administration of the selective AA2BR inhibitor PSB1115 to Aa2br(+/+) mice resulted in a similar increase in severity of DSS colitis. Cytokine profiling of colonic tissue revealed specific deficiencies in IL-10 in Aa2br(-/-) mice relative to controls. Extensions of these findings in cultured human intestinal epithelial cells revealed that stable Ado analogs induce IL-10 mRNA and protein and that such increases can be blocked with PSB1115. Taken together, these studies indicate a central regulatory role for AA2BR-modulated IL-10 in the acute inflammatory phase of DSS colitis, thereby implicating AA2BR as an endogenously protective molecule expressed on intestinal epithelial cells.

Control of IFN-alphaA by CD73: implications for mucosal inflammation

Inflammatory diseases influence tissue metabolism, altering regulation of extracellular adenine nucleotides, with a resultant protective influence of adenosine. Ecto-5'-nucleotidase (CD73) is a central surface enzyme generating extracellular adenosine. Thus, we hypothesized that CD73 is protective in mucosal inflammation as modeled by trinitrobenzene sulfonate (TNBS) colitis. Initial studies revealed a >3-fold induction of CD73 mRNA levels after TNBS colitis. Additionally, the severity of colitis was increased, as determined by weight loss and colonic shortening, in cd73(-/-) mice relative to cd73(+/+) controls. Likewise, enteral administration of the selective CD73 inhibitor alpha,beta-methylene ADP to cd73(+/+) mice resulted in a similar increase in severity of TNBS colitis. Gene array profiling of cytokine mRNA expression, verified by real-time PCR, revealed a >90% down-regulation of IFN-alphaA in cd73(-/-) mice and alpha,beta-methylene ADP-treated cd73(+/+) mice, compared with cd73(+/+) mice. Exogenous administration of recombinant IFN-alphaA partially protected TNBS-treated cd73(-/-) mice. Cytokine profiling revealed similar increases in both IFN-gamma and TNF-alpha mRNA in colitic animals, independent of genotype. However, IL-10 mRNA increased in wild-type mice on day 3 after TNBS administration, whereas cd73(-/-) mice mounted no IL-10 response. This IL-10 response was restored in the cd73(-/-) mice by exogenous IFN-alphaA. Further cytokine profiling revealed that this IL-10 induction is preceded by a transient IFN-alphaA induction on day 2 after TNBS exposure. Together, these studies indicate a critical regulatory role for CD73-modulated IFNalphaA in the acute inflammatory phase of TNBS colitis, thereby implicating IFN-alphaA as a protective element of adenosine signaling during mucosal inflammation.

Identification of Pur alpha as a new hypoxia response factor responsible for coordinated induction of the beta 2 integrin family

Central to the process of inflammation are hypoxic conditions that lead to the binding of circulating leukocytes to the endothelium. We have previously shown that such binding is mediated by monocytes being able to directly sense hypoxic conditions and respond by inducing their surface expression of the beta(2) integrin family of adhesion molecules. In this study, we show that coordinated induction of the beta(2) integrins during direct hypoxia-sensing occurs through transcriptional activation of each of the genes by which they are encoded. Certain of the molecular mechanisms that mediate this activation in transcription are dependent upon hypoxia-inducible factor-1 (HIF-1), whereas others are HIF-1 independent. In search of these HIF-1-independent mechanisms, we identified Pur alpha as a new hypoxia-response factor. Binding of Pur alpha to the HIF-1-independent beta(2) integrin promoters is induced by hypoxia and mutagenesis of these Pur alpha-binding sites almost completely abolishes the ability of the promoters to respond to hypoxic conditions. Additional studies using siRNA directed against Pur alpha also revealed a loss in the hypoxic response of the beta(2) integrin promoters. Taken together, our findings demonstrate that hypoxia induces a coordinated up-regulation in beta(2) integrin expression that is dependent upon transcriptional mechanisms mediated by HIF-1 and Pur alpha.

Identification of ectonucleotidases CD39 and CD73 in innate protection during acute lung injury

Acute lung injury (ALI), such as that which occurs with mechanical ventilation, contributes to morbidity and mortality of critical illness. Nonetheless, in many instances, ALI resolves spontaneously through unknown mechanisms. Therefore, we hypothesized the presence of innate adaptive pathways to protect the lungs during mechanical ventilation. In this study, we used ventilator-induced lung injury as a model to identify endogenous mechanisms of lung protection. Initial in vitro studies revealed that supernatants from stretch-induced injury contained a stable factor which diminished endothelial leakage. This factor was subsequently identified as adenosine. Additional studies in vivo revealed prominent increases in pulmonary adenosine levels with mechanical ventilation. Because ectoapyrase (CD39) and ecto-5'-nucleotidase (CD73) are rate limiting for extracellular adenosine generation, we examined their contribution to ALI. In fact, both pulmonary CD39 and CD73 are induced by mechanical ventilation. Moreover, we observed pressure- and time-dependent increases in pulmonary edema and inflammation in ventilated cd39(-/-) mice. Similarly, pharmacological inhibition or targeted gene deletion of cd73 was associated with increased symptom severity of ventilator-induced ALI. Reconstitution of cd39(-/-) or cd73(-/-) mice with soluble apyrase or 5'-nucleotidase, respectively, reversed such increases. In addition, ALI was significantly attenuated and survival improved after i.p. treatment of wild-type mice with soluble apyrase or 5'-nucleotidase. Taken together, these data reveal a previously unrecognized role for CD39 and CD73 in lung protection and suggest treatment with their soluble compounds as a therapeutic strategy for noninfectious ALI.

Ecto-5'-nucleotidase and intestinal ion secretion by enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) triggers a large release of adenosine triphosphate (ATP) from host intestinal cells and the extracellular ATP is broken down to adenosine diphosphate (ADP), AMP, and adenosine. Adenosine is a potent secretagogue in the small and large intestine. We suspected that ecto-5'-nucleotidase (CD73, an intestinal enzyme) was a critical enzyme involved in the conversion of AMP to adenosine and in the pathogenesis of EPEC diarrhea. We developed a nonradioactive method for measuring ecto-5'-nucleotidase in cultured T84 cell monolayers based on the detection of phosphate release from 5'-AMP. EPEC infection triggered a release of ecto-5'-nucleotidase from the cell surface into the supernatant medium. EPEC-induced 5'-nucleotidase release was not correlated with host cell death but instead with activation of phosphatidylinositol-specific phospholipase C (PI-PLC). Ecto-5'-nucleotidase was susceptible to inhibition by zinc acetate and by alpha,beta-methylene-adenosine diphosphate (alpha,beta-methylene-ADP). In the Ussing chamber, these inhibitors could reverse the chloride secretory responses triggered by 5'-AMP. In addition, alpha,beta-methylene-ADP and zinc blocked the ability of 5'-AMP to stimulate EPEC growth under nutrient-limited conditions in vitro. Ecto-5'-nucleotidase appears to be the major enzyme responsible for generation of adenosine from adenine nucleotides in the T84 cell line, and inhibitors of ecto-5'-nucleotidase, such as alpha,beta-methylene-ADP and zinc, might be useful for treatment of the watery diarrhea produced by EPEC infection.

Antiinflammatory adaptation to hypoxia through adenosine-mediated cullin-1 deneddylation

A major adaptive pathway for hypoxia is hypoxic preconditioning (HPC), a form of endogenous protection that renders cells tolerant to severe challenges of hypoxia. We sought to define the antiinflammatory properties of HPC. cDNA microarray analysis of lung tissue from mice subjected to hypoxia or HPC identified a cluster of NF-kappaB-regulated genes whose expression is attenuated by HPC. Studies using an NF-kappaB luciferase reporter assay confirmed a significant suppression of NF-kappaB activation during HPC. HPC-elicited activity was conferrable, as a soluble supernatant from HPC-treated cells, and the active fraction was purified and identified as adenosine (Ado). Guided by recent studies demonstrating bacterial inhibition of NF-kappaB through cullin-1 (Cul-1) deneddylation, we found a dose-dependent deneddylation of Cul-1 by Ado receptor stimulation predominantly mediated by the Ado A2B receptor subtype. Further, siRNA-mediated repression of CSN5, a subunit of the COP9 signalosome responsible for deneddylation of Cul-1, partially reversed HPC-mediated inhibition of NF-kappaB. Cul-1 deneddylation was evident in a murine model of HPC and lost in animals lacking extracellular Ado (Cd73-/- mice). Taken together, these results demonstrate that HPC induces extracellular accumulation of Ado and suppresses NF-kappaB activity through deneddylation of Cul-1. These results define a molecular regulatory pathway by which Ado provides potent antiinflammatory properties.

Protective role of ecto-5'-nucleotidase (CD73) in renal ischemia

Acute renal failure from ischemia significantly contributes to cardiovascular morbidity and mortality. Extracellular adenosine has been implicated as an anti-inflammatory metabolite particularly during conditions of limited oxygen availability (e.g., ischemia). Because ecto-5'-nucleotidase (CD73) is rate limiting for extracellular adenosine generation, this study examined the contribution of CD73-dependent adenosine production to ischemic preconditioning (IP) of the kidneys. After the initial observation that murine CD73 transcript, protein, and function are induced by renal IP, its role in IP-mediated kidney protection was studied. In fact, increases in renal adenosine concentration with IP are attenuated in cd73(-/-) mice. Moreover, pharmacologic inhibition of CD73 or its targeted gene deletion abolished renal protection by IP as measured by clearance studies, plasma electrolytes, and renal tubular destruction, and reconstitution of cd73(-/-) mice with soluble 5'-nucleotidase resulted in complete restoration of renal protection by IP. Finally, renal injury after ischemia was attenuated by intraperitoneal treatment of wild-type mice with soluble 5'-nucleotidase to a similar degree as by IP. Taken together, these data reveal what is believed to be a previously unrecognized role of CD73 in renal protection from ischemia and suggest treatment with soluble 5'-nucleotidase as a novel therapeutic approach in the treatment of renal diseases that are precipitated by limited oxygen availability.

From "Hellstrom Paradox" to anti-adenosinergic cancer immunotherapy

Cancer therapy by endogenous or adoptively transferred anti-tumor T cells is considered complementary to conventional cancer treatment by surgery, radiotherapy or chemotherapy. However, the scope of promising immunotherapeutic protocols is currently limited because tumors can create a ‘hostile–immunosuppressive microenvironment that prevents their destruction by anti-tumor T cells. There is a possibility to develop better and more effective immunotherapies by inactivating mechanisms that inhibit anti-tumor T cells in the tumor microenvironment and thereby protect cancerous tissues from immune damage. This may be now possible because of the recent demonstration that genetic deletion of immunosuppressive A2A and A2B adenosine receptors (A2AR and A2BR) or their pharmacological inactivation can prevent the inhibition of anti-tumor T cells by the hypoxic tumor microenvironment and as a result facilitate full tumor rejection [Ohta A, Gorelik E, Prasad SJ et al (2006) Proc Natl Acad Sci USA 103(35):13132–3137]. This approach is based on in vivo genetic evidence that A2AR play a critical role in the protection of normal tissues from overactive immune cells in acutely inflamed and hypoxic areas. The observations of much improved T-cell-mediated rejection of tumors in mice with inactivated A2AR strongly suggest that A2AR also protects hypoxic cancerous tissues and that A2AR should be inactivated in order to improve tumor rejection by anti-tumor T cells.

T regulatory and primed uncommitted CD4 T cells express CD73, which suppresses effector CD4 T cells by converting 5'-adenosine monophosphate to adenosine

CD73 (5'-ectonucleotidase) is expressed by two distinct mouse CD4 T cell populations: CD25+ (FoxP3+) T regulatory (Treg) cells that suppress T cell proliferation but do not secrete IL-2, and CD25- uncommitted primed precursor Th (Thpp) cells that secrete IL-2 but do not suppress in standard Treg suppressor assays. CD73 on both Treg and Thpp cells converted extracellular 5'-AMP to adenosine. Adenosine suppressed proliferation and cytokine secretion of Th1 and Th2 effector cells, even when target cells were activated by anti-CD3 and anti-CD28. This represents an additional suppressive mechanism of Treg cells and a previously unrecognized suppressive activity of Thpp cells. Infiltration of either Treg or Thpp cells at inflammatory sites could potentially convert 5'-AMP generated by neutrophils or dying cells into the anti-inflammatory mediator adenosine, thus dampening excessive immune reactions.

Ecto-5'-nucleotidase (CD73)-mediated adenosine production is tissue protective in a model of bleomycin-induced lung injury

Adenosine signaling has diverse actions on inflammation and tissue injury. Levels of adenosine are rapidly elevated in response to tissue injury; however, the mechanisms responsible for adenosine production in response to injury are not well understood. In this study, we found that adenosine levels are elevated in the lungs of mice injured by the drug bleomycin. In addition, increased activity of ecto-5'-nucleotidase (CD73) was found in the lungs in conjunction with adenosine elevations. To determine the contribution of CD73 to the generation of adenosine in the lung, CD73(-/-) mice were subjected to bleomycin challenges. Results demonstrated that CD73(-/-) mice challenged with bleomycin no longer accumulated adenosine in their lungs, suggesting that the primary means of adenosine production following bleomycin injury resulted from the release and subsequent dephosphorylation of adenine nucleotides. CD73(-/-) mice challenged with bleomycin exhibited enhanced pulmonary inflammation and fibrosis as well as exaggerated expression of proinflammatory and profibrotic mediators in the lung. Intranasal instillations of exogenous nucleotidase restored the ability of lungs of CD73(-/-) mice to accumulate adenosine following bleomycin challenge. Furthermore, these treatments were associated with a decrease in pulmonary inflammation and fibrosis. CD73(+/+) animals challenged with bleomycin and supplemented with exogenous nucleotidase also exhibited reduced inflammation. Together, these findings suggest that CD73-dependent adenosine production contributes to anti-inflammatory pathways in bleomycin-induced lung injury.

Integration of oxygen signaling at the consensus HRE

The hypoxia-inducible factor 1 (HIF-1) was initially identified as a transcription factor that regulated erythropoietin gene expression in response to a decrease in oxygen availability in kidney tissue. Subsequently, a family of oxygen-dependent protein hydroxylases was found to regulate the abundance and activity of three oxygen-sensitive HIFalpha subunits, which, as part of the HIF heterodimer, regulated the transcription of at least 70 different effector genes. In addition to responding to a decrease in tissue oxygenation, HIF is proactively induced, even under normoxic conditions, in response to stimuli that lead to cell growth, ultimately leading to higher oxygen consumption. The growing cell thus profits from an anticipatory increase in HIF-dependent target gene expression. Growth stimuli-activated signaling pathways that influence the abundance and activity of HIFs include pathways in which kinases are activated and pathways in which reactive oxygen species are liberated. These pathways signal to the HIF protein hydroxylases, as well as to HIF itself, by means of covalent or redox modifications and protein-protein interactions. The final point of integration of all of these pathways is the hypoxia-response element (HRE) of effector genes. Here, we provide comprehensive compilations of the known growth stimuli that promote increases in HIF abundance, of protein-protein interactions involving HIF, and of the known HIF effector genes. The consensus HRE derived from a comparison of the HREs of these HIF effectors will be useful for identification of novel HIF target genes, design of oxygen-regulated gene therapy, and prediction of effects of future drugs targeting the HIF system.

Dynamic purine signaling and metabolism during neutrophil-endothelial interactions

During episodes of hypoxia and inflammation, polymorphonuclear leukocytes (PMN) move into underlying tissues by initially passing between endothelial cells that line the inner surface of blood vessels (transendothelial migration, TEM). TEM creates the potential for disturbances in vascular barrier and concomitant loss of extravascular fluid and resultant edema. Recent studies have demonstrated a crucial role for nucleotide metabolism and nucleoside signaling during inflammation. These studies have implicated multiple adenine nucleotides as endogenous tissue protective mechanisms invivo. Here, we review the functional components of vascular barrier, identify strategies for increasing nucleotide generation and nucleoside signaling, and discuss potential therapeutic targets to regulate the vascular barrier during inflammation.