Adenosine A2areceptor-mediated, normoxic induction of HIF-1 through PKC and PI-3K-dependent pathways in macrophages

The supernatant of apoptotic cells causes transcriptional activation of hypoxia-inducible factor-1alpha in macrophages via sphingosine-1-phosphate and transforming growth factor-beta

Macrophages infiltrating solid tumors exhibit a tumor-supporting phenotype and are critical for tumor development. Little is known which tumor-derived signal provokes this phenotype shift and how these signals are interpreted in macrophages to support tumor growth. We used the supernatant of apoptotic cells and noticed transcriptional, nuclear factor of activated T cells-dependent up-regulation of hypoxia-inducible factor (HIF)-1alpha mRNA, subsequent protein expression, and HIF-1 activity. Blocking calcineurin with cyclosporine A attenuated nuclear factor of activated T cells binding during electrophoretic mobility shift assay analysis and circumvented the HIF-1alpha mRNA increase. Knockdown experiments, receptor analysis, and antibody neutralization pointed to sphingosine-1-phosphate and transforming growth factor-beta as the initiators of the HIF-1 response. The use of macrophages from conditional HIF-1alpha knockout mice revealed that macrophages, under the impact of apoptotic cell supernatants, use HIF-1 to produce factors that induce CD31 expression in murine embryonic stem cells. Our study supports the notion that soluble factors produced from apoptotic tumor cells activate the HIF-1 system under normoxia in macrophages to enhance their tumor-promoting capacity by, for example, releasing vascular endothelial growth factor. This shows the importance of HIF-1-elicited responses in regulatory macrophages under normoxia.

Differential regulation of HIF-1alpha isoforms in murine macrophages by TLR4 and adenosine A(2A) receptor agonists

Adenosine A(2A)R and TLR agonists synergize to induce an "angiogenic switch" in macrophages, down-regulating TNF-alpha and up-regulating VEGF expression. This switch involves transcriptional regulation of VEGF by HIF-1, transcriptional induction of HIF-1alpha by LPS (TLR4 agonist), and A(2A)R-dependent post-transcriptional regulation of HIF-1alpha stability. Murine HIF-1alpha is expressed as two mRNA isoforms: HIF-1alphaI.1 and -I.2, which contain alternative first exons and promoters. HIF-1alphaI.2 is expressed ubiquitously, and HIF-1alphaI.1 is tissue-specific. We investigated the regulation of these isoforms in macrophages by TLR4 and A(2A)R agonists. HIF-1alphaI.1 is induced strongly compared with HIF-1alphaI.2 upon costimulation with LPS and A(2A)R agonists (NECA or CGS21680). In unstimulated cells, the I.1 isoform constituted approximately 4% of HIF-1alpha transcripts; in LPS and NECA- or CGS21680-treated macrophages, this level was approximately 15%, indicating a substantial contribution of HIF-1alphaI.1 to total HIF-1alpha expression. The promoters of both isoforms were induced by LPS but not enhanced further by NECA, suggesting A(2A)R-mediated post-transcriptional regulation. LPS/NECA-induced expression of HIF-1alphaI.1 was down-regulated by Bay 11-7085 (NF-kappaB inhibitor) and ZM241385 (A(2A)R antagonist). Although VEGF and IL-10 expression by HIF-1alphaI.1-/- macrophages was equivalent to that of wild-type macrophages, TNF-alpha, MIP-1alpha, IL-6, IL-12p40, and IL-1beta expression was significantly greater, suggesting a role for HIF-1alphaI.1 in modulating expression of these cytokines. A(2A)R expression in unstimulated macrophages was low but was induced rapidly by LPS in a NF-kappaB-dependent manner. LPS-induced expression of A(2A)Rs and HIF-1alpha and A(2A)R-dependent HIF-1alpha mRNA and protein stabilization provide mechanisms for the synergistic effects of LPS and A(2A)R agonists on macrophage VEGF expression.

Variable activation of phosphoinositide 3-kinase influences the response of liver grafts to ischemic preconditioning

BACKGROUND/AIMS

The efficacy of ischemic preconditioning (IPC) in preventing reperfusion injury in human liver transplants is still questioned. Phosphoinositide-3-kinase (PI3K) is essential for IPC development in rodent livers. This work investigates whether PI3K-dependent signals might account for the inconsistent responses to IPC of transplanted human livers.

METHODS

Forty livers from deceased donors were randomized to receive or not IPC before recovery. PI3K activation was evaluated in biopsies obtained immediately before IPC and 2 h after reperfusion by measuring the phosphorylation of the PI3K downstream kinase PKB/Akt and the levels of the PI3K antagonist phosphatase tensin-homologue deleted from chromosome 10 (PTEN).

RESULTS

IPC increased PKB/Akt phosphorylation (p = 0.01) and decreased PTEN levels (p = 0.03) in grafts, but did not significantly ameliorate post-transplant reperfusion injury. By calculating T(2h)/T(0) PKB/Akt phosphorylation ratios, 10/19 (53%) of the preconditioned grafts had ratios above the control threshold (IPC-responsive), while the remaining nine grafts showed ratios comparable to controls (IPC-non-responsive). T(2h)/T(0) PTEN ratios were also decreased (p < or = 0.03) only in IPC-responsive grafts. The patients receiving IPC-responsive organs had ameliorated (p < or = 0.05) post-transplant aminotransferase and bilirubin levels, while prothrombin activity was unchanged.

CONCLUSIONS

Impaired PI3K signaling might account for the variability in the responses to IPC of human grafts from deceased donors.

T regulatory cells: hypoxia-adenosinergic suppression and re-direction of the immune response

T regulatory cells (Treg cells) suppress immune responses to maintain self tolerance, but they also protect cancerous tissues. I propose a model to potentially unify the diverse functions of Treg cells. This assumes that Treg cells provide a complementary immunological arm to a physiological tissue-protecting mechanism, driven by low oxygen tension (i.e. hypoxia) in inflamed or cancerous tissues. The cAMP-elevating A2A and A2B adenosine receptors, hypoxia inducible transcription factor 1alpha (HIF), the cAMP response element (CRE)- and hypoxia response element (HRE)-mediated transcription in Treg and effector cells have key roles in this model. Both the T cell receptor (TCR)-triggered- and HRE- and CRE-driven activities of Treg cells are required to achieve a maximal level of immune suppression.

Embryonic caffeine exposure induces adverse effects in adulthood

The purpose of this study was to determine both the short-term effects on cardiac development and embryo growth and the long-term effects on cardiac function and body composition of in utero caffeine exposure. Pregnant mice (C57BL/6) were exposed to hypoxia (10% O(2)) or room air from embryonic days (E) 8.5-10.5, and treated with caffeine (20 mg/kg, i.p.) or vehicle (normal saline, 0.9% NaCl). This caffeine dose results in a circulating level that is equivalent to 2 cups of coffee in humans. Hypoxic exposure acutely reduced embryonic growth by 30%. Exposure to a single dose of caffeine inhibited cardiac ventricular development by 53% in hypoxia and 37% in room air. Caffeine exposure resulted in inhibition of hypoxia-induced HIF1alpha protein expression in embryos by 40%. When offspring from dams treated with a single dose of caffeine were studied in adulthood, we observed that caffeine treatment alone resulted in a decrease in cardiac function of 38%, as assessed by echocardiography. We also observed a 20% increase in body fat with male mice exposed to caffeine. Caffeine was dissolved in normal saline, so it was used as a control. Room air controls were used to compare to the hypoxic mice. Exposure to a single dose of caffeine during embryogenesis results in both short-term effects on cardiac development and long-term effects on cardiac function.

Mechanisms of induction of adenosine receptor genes and its functional significance

Adenosine is a metabolite generated and released from cells, particularly under injury or stress. It elicits protective or damaging responses via signaling through the adenosine receptors, including the adenylyl cyclase inhibitory A(1) and A(3), and the adenylyl cyclase stimulatory A(2A) and A(2B). Multiple adenosine receptor types, including stimulatory and inhibitory, can be found in the same cell, suggesting that a careful balance of adenosine receptor expression in a particular cell is necessary for a specific adenosine-induced response. This balance could be controlled by differential expression of the adenosine receptor genes under different stimuli. Here, we have reviewed an array of studies that have characterized basal or induced expression of the adenosine receptors and common as well as distinct mechanisms of effect, in hopes that ongoing studies on this topic will further elucidate detailed mechanisms of adenosine receptor regulation, leading to potential therapeutic applications.

Hypoxia-adenosinergic immunosuppression: tumor protection by T regulatory cells and cancerous tissue hypoxia

Cancerous tissue protection from tumor-recognizing CD8(+) and CD4(+) T cells (antitumor T cells) limits the therapeutic potential of immunotherapies. We propose that tumor protection is to a large extent due to (a) inhibition of antitumor T cells by hypoxia-driven accumulation of extracellular adenosine in local tumor microenvironment and due to (b) T regulatory cell-produced extracellular adenosine. The adenosine triggers the immunosuppressive signaling via intracellular cyclic AMP-elevating A2A adenosine receptors (A2AR) on antitumor T cells. In addition, the activated antitumor T cells in hypoxic tumor microenvironment could be inhibited by elevated levels of immunosuppressive hypoxia-inducible factor-1alpha. Complete rejection or tumor growth retardation was observed when A2AR has been genetically eliminated or antagonized with synthetic drug or with natural A2AR antagonist 1,3,7-trimethylxanthine (caffeine). The promising strategy may be in combining the anti-hypoxia-adenosinergic treatment that prevents inhibition of antitumor T cells by tumor-produced and T regulatory cell-produced adenosine with targeting of other negative regulators, such as CTL antigen-4 blockade. Observations of tumor rejection in mice and massive prospective epidemiologic studies support the feasibility of anti-hypoxia-adenosinergic combined immunotherapy.

Treating lung inflammation with agonists of the adenosine A2A receptor: promises, problems and potential solutions

Adenosine A(2A) receptor agonists may be important regulators of inflammation. Such conclusions have come from studies demonstrating that, (i) adenosine A(2A) agonists exhibit anti-inflammatory properties in vitro and in vivo, (ii) selective A(2A) antagonists enhance inflammation in vivo and, (iii) knock outs of this receptor aggravate inflammation in a wide variety of in vivo models. Inflammation is a hallmark of asthma and COPD and adenosine has long been suggested to be involved in disease pathology. Two recent publications, however, suggested that an inhaled adenosine A(2A) receptor agonist (GW328267X) did not affect either the early and late asthmatic response or symptoms associated with allergic rhinitis suggesting that the rationale for treating inflammation with an adenosine A(2A) receptor agonist may be incorrect. A barrier to fully investigating the role of adenosine A(2A) receptor agonists as anti-inflammatory agents in the lung is the side effect profile due to systemic exposure, even with inhalation. Unless strategies can be evolved to limit the systemic exposure of inhaled adenosine A(2A) receptor agonists, the promise of treating lung inflammation with such agents may never be fully explored. Using strategies similar to that devised to improve the therapeutic index of inhaled corticosteroids, UK371,104 was identified as a selective agonist of the adenosine A(2A) receptor that has a lung focus of pharmacological activity following delivery to the lung in a pre clinical in vivo model of lung function. Lung-focussed agents such as UK371,104 may be suitable for assessing the anti-inflammatory potential of inhaled adenosine A(2A) receptor agonists.

Adenosine-dependent activation of hypoxia-inducible factor-1 induces late preconditioning in liver cells

The cellular mechanisms by which ischemic preconditioning increases liver tolerance to ischemia/reperfusion injury are still poorly understood. This study investigated the role of the hypoxia-inducible factor-1 (HIF-1) in the protection associated with the late phase of liver preconditioning. Late preconditioning was induced in primary cultured rat hepatocytes by a transient (10 minute) hypoxic stress or by 15 minutes incubation with the adenosine A(2A) receptors agonist CGS21680 24 hours before exposure to 90 minutes of hypoxia in a serum-free medium. Late preconditioning induced the nuclear translocation of HIF-1 and the expression of carbonic anhydrase IX (CAIX), a HIF-1-regulated transmembrane enzyme that catalyzes bicarbonate production. Such effects were associated with prevention of hepatocyte killing by hypoxia and the amelioration of intracellular acidosis and Na+ accumulation. The inhibition of PKC-mediated and PI3-kinase-mediated signals with, respectively, chelerythrine and wortmannin abolished HIF-1 activation and blocked both CAIX expression and the protective action of late preconditioning. CAIX expression was also prevented by interfering with the transcriptional activity of HIF-1 using a dominant negative HIF-1beta subunit. The inhibition of CAIX with acetazolamide or the block of bicarbonate influx with disodium-4-acetamido-4'-isothiocyanato-stilben-2,2'-disulfonate also reverted the protective effects of late preconditioning on intracellular acidosis and Na+ accumulation.

CONCLUSION

The stimulation of adenosine A(2A) receptors induced late preconditioning in liver cells through the activation of HIF-1. HIF-1-induced expression of CAIX increases hepatocyte tolerance to ischemia by maintaining intracellular Na+ homeostasis. These observations along with the importance of HIF-1 in regulating cell survival indicates HIF-1 activation as a possible key event in liver protection by late preconditioning.

Role of HIF-1 and NF-kappaB transcription factors in the modulation of transferrin receptor by inflammatory and anti-inflammatory signals

Inflammation generates various changes in body iron homeostasis, including iron sequestration in the reticuloendothelial system with ensuing hypoferremia and anemia of chronic disease. Increased iron accumulation is caused by hepcidin-mediated down-regulation of the iron export protein ferroportin and higher iron uptake. However, enhanced iron acquisition by macrophages cannot be accounted for by the previously reported transferrin receptor (TfR1) down-regulation in macrophages exposed to lipopolysaccharide (LPS)/interferon gamma (IFNgamma) because it impairs a major iron uptake mechanism. Because TfR1 is up-regulated by the hypoxia-inducible factor (HIF-1), we investigated the effect of inflammatory and anti-inflammatory signals on HIF-1-mediated TfR1 gene expression. Exposure of mouse macrophages (RAW 264.7 and J774A.1 cells or peritoneal macrophages) to LPS/IFNgamma up-regulated NF-kappaB, which in turn rapidly and transiently activated HIF-1-dependent TfR1 expression and iron uptake. Activation of an anti-inflammatory pathway by pre-exposure to the adenosine A(2A) receptor agonist CGS21680 prevented the inducing effect of LPS/IFNgamma on HIF-1 and TfR1 expression by inhibiting NF-kappaB activity, whereas treatment with CGS21680 alone increased HIF-1-mediated TfR1 expression by means of an NF-kappaB-independent signaling pathway. In conclusion, an interplay of the HIF-1 and NF-kappaB pathways controls TfR1 transcription in inflammation. The consequent changes in TfR1 expression may be involved in modulating iron retention in inflammatory macrophages, thus possibly contributing to the development of hypoferremia in the early phases preceding the down-regulation of macrophage ferroportin by hepcidin.

Novel PKC signaling is required for LPS-induced soluble Flt-1 expression in macrophages

In vitro activation of macrophages by LPS induces rapid release of vascular endothelial growth factor (VEGF) and soluble fms-like tyrosine kinase-1 receptor (sFlt-1), which are thought to be the effectors to cause sepsis. However, the signal pathway that controls the VEGF and sFlt-1 expressions in LPS-activated macrophages remains unclear. In this study, we demonstrated that phosphorylation of protein kinase C (PKC)delta played a key role in the VEGF and sFlt-1 signaling pathway of LPS-activated macrophages. PKC is a family of serine-threonine kinases, which are classified into three major groups based on homology and cofactor requirements: conventional PKCs, novel PKCs, and atypical PKCs. In the murine RAW264.7 cells, as well as in primary human monocytes/macrophages, pretreatment with a general PKC inhibitor GF109203X or with a novel PKCdelta inhibitor rottlerin or overexpression of a kinase-inactive form of PKCdelta (K376R) eliminated LPS-induced sFlt-1 expression and augmented LPS-induced VEGF expression at the protein and the transcription levels. In contrast, Gö6976, an inhibitor for the conventional PKCs, or myristoylated PKCzeta pseudosubstrate peptide, an inhibitor for the atypical PKCs, failed to exert the same effects. These data suggest that PKCdelta signaling is involved in LPS-induced sFlt-1 expression and serves as a negative mediator in LPS-induced VEGF expression in macrophages. A novel strategy controlling the LPS-induced PKC pathways, especially the PKCdelta isoform, may be developed based on this study.

HIF-1 alpha is an essential effector for purine nucleoside-mediated neuroprotection against hypoxia in PC12 cells and primary cerebellar granule neurons

Hypoxia-inducible factor-1 alpha (HIF-1alpha) and purine nucleosides adenosine and inosine are critical mediators of physiological responses to acute and chronic hypoxia. The specific aim of this paper was to evaluate the potential role of HIF-1alpha in purine-mediated neuroprotection. We show that adenosine and inosine efficiently rescued clonal rat pheochromocytoma (PC12) cells (up to 43.6%) as well as primary cerebellar granule neurons (up to 25.1%) from hypoxic insult, and furthermore, that HIF-1alpha is critical for purine-mediated neuroprotection. Next, we studied hypoxia or purine nucleoside increased nuclear accumulation of HIF-1alpha in PC12 cells. As a possible result of increased protein stabilization or synthesis an up to 2.5-fold induction of HIF-1alpha accumulation was detected. In cerebellar granule neurons, purine nucleosides induced an up to 3.1-fold HIF-1alpha accumulation in cell lysates. Concomitant with these results, small interfering RNA-mediated reduction of HIF-1alpha completely abolished adenosine- and inosine-mediated protection in PC12 cells and severely hampered purine nucleoside-mediated protection in primary neurons (up to 94.2%). Data presented in this paper thus clearly demonstrate that HIF-1alpha is a key regulator of purine nucleoside-mediated rescue of hypoxic neuronal cells.

A2A receptors in inflammation and injury: lessons learned from transgenic animals

Adenosine regulates the function of the innate and adaptive immune systems through targeting virtually every cell type that is involved in orchestrating an immune/inflammatory response. Of the four adenosine receptors (A(1), A(2A), A(2B), A(3)), A(2A) receptors have taken center stage as the primary anti-inflammatory effectors of extracellular adenosine. This broad, anti-inflammatory effect of A(2A) receptor activation is a result of the predominant expression of A(2A) receptors on monocytes/macrophages, dendritic cells, mast cells, neutrophils, endothelial cells, eosinophils, epithelial cells, as well as lymphocytes, NK cells, and NKT cells. A(2A) receptor activation inhibits early and late events occurring during an immune response, which include antigen presentation, costimulation, immune cell trafficking, immune cell proliferation, proinflammatory cytokine production, and cytotoxicity. In addition to limiting inflammation, A(2A) receptors participate in tissue remodeling and reparation. Consistent with their multifaceted, immunoregulatory action on immune cells, A(2A) receptors have been shown to impact the course of a wide spectrum of ischemic, autoimmune, infectious, and allergic diseases. Here, we review the regulatory roles of A(2A) receptors in immune/inflammatory diseases of various organs, including heart, lung, gut, liver, kidney, joints, and brain, as well as the role of A(2A) receptors in regulating multiple organ failure and sepsis.