Hypoxia enhances the antiviral activity of interferons

Role of Hypoxia in the Interferon Response

In solid tumors, as the tumor grows and the disease progresses, hypoxic regions are often generated, but in contrast to most normal cells which cannot survive under these conditions, tumour cells adapt to hypoxia by HIF-driven mechanisms. Hypoxia can further promote cancer development by generating an immunosuppressive environment within the tumour mass, which allows tumour cells to escape the immune system recognition. This is achieved by recruiting immunosuppressive cells and by upregulating molecules which block immune cell activation. Hypoxia can also confer resistance to antitumor therapies by inducing the expression of membrane proteins that increase drug efflux or by inhibiting the apoptosis of treated cells. In addition, tumor cells require an active interferon (IFN) signalling pathway for the success of many anticancer therapies, such as radiotherapy or chemotherapy. Therefore, hypoxic effects on this pathway needs to be addressed for a successful treatment.

Contribution of hypoxia inducible factor-1 during viral infections

Hypoxia-inducible factor 1 (HIF-1) is a transcription factor that plays critical roles during the cellular response to hypoxia. Under normoxic conditions, its function is tightly regulated by the degradation of its alpha subunit (HIF-1α), which impairs the formation of an active heterodimer in the nucleus that otherwise regulates the expression of numerous genes. Importantly, HIF-1 participates in both cancer and infectious diseases unveiling new therapeutic targets for those ailments. Here, we discuss aspects related to the activation of HIF-1, the effects of this transcription factor over immune system components, as well as the involvement of HIF-1 activity in response to viral infections in humans. Although HIF-1 is currently being assessed in numerous clinical settings as a potential therapy for different diseases, up to date, there are no clinical studies evaluating the pharmacological modulation of this transcription factor as a possible new antiviral treatment. However, based on the available evidence, clinical trials targeting this molecule are likely to occur soon. In this review we discuss the role of HIF-1 in viral immunity, the modulation of HIF-1 by different types of viruses, as well as the effects of HIF-1 over their life cycle and the potential use of HIF-1 as a new target for the treatment of viral infections.

Hypoxia impairs anti-viral activity of natural killer (NK) cells but has little effect on anti-fibrotic NK cell functions in hepatitis C virus infection

BACKGROUND & AIMS

Natural killer (NK) cells have been shown to exert anti-viral as well as anti-fibrotic functions in hepatitis C virus (HCV) infection. Previous studies, however, analyzed NK cell functions exclusively under atmospheric oxygen conditions despite the fact that the liver microenvironment is hypoxic. Here, we analyzed the effects of low oxygen tension on anti-viral and anti-fibrotic NK cell activity.

METHODS

Peripheral (n=34) and intrahepatic (n=15) NK cells from HCV(+) patients as well as circulating NK cells from healthy donors (n=20) were studied with respect to anti-viral and anti-fibrotic activity via co-culture experiments with HuH7 replicon cells and hepatic stellate cells, respectively.

RESULTS

Anti-viral activity of resting NK cells from healthy controls was not affected by hypoxia. However, hypoxia significantly reduced the response of healthy NK cells to cytokine stimulation. In contrast to healthy controls, we observed resting and cytokine activated peripheral NK cells from HCV patients to display a significantly decreased anti-viral activity when cultured at 5% or 1% oxygen, suggesting HCV NK cells to be very sensitive to hypoxia. These findings could be confirmed when intrahepatic NK cells were tested. Finally, we show that anti-fibrotic NK cell activity was not affected by low oxygen tension.

CONCLUSIONS

Our results show that anti-viral function of NK cells from HCV(+) patients is critically affected by a hypoxic microenvironment and, therefore, indicate that in order to obtain an accurate understanding of intrahepatic NK cell anti-HCV activity, the laboratory modelling should take into account the liver specific levels of oxygen.

BC-box protein domain-related mechanism for VHL protein degradation

The tumor suppressor VHL (von Hippel-Lindau) protein is a substrate receptor for Ubiquitin Cullin Ring Ligase complexes (CRLs), containing a BC-box domain that associates to the adaptor Elongin B/C. VHL targets hypoxia-inducible factor 1α to proteasome-dependent degradation. Gam1 is an adenoviral protein, which also possesses a BC-box domain that interacts with the host Elongin B/C, thereby acting as a viral substrate receptor. Gam1 associates with both Cullin2 and Cullin5 to form CRL complexes targeting the host protein SUMO enzyme SAE1 for proteasomal degradation. We show that Gam1 protein expression induces VHL protein degradation leading to hypoxia-inducible factor 1α stabilization and induction of its downstream targets. We also characterize the CRL-dependent mechanism that drives VHL protein degradation via proteasome. Interestingly, expression of Suppressor of Cytokine Signaling (SOCS) domain-containing viral proteins and cellular BC-box proteins leads to VHL protein degradation, in a SOCS domain-containing manner. Our work underscores the exquisite ability of viral domains to uncover new regulatory mechanisms by hijacking key cellular proteins.

Effect of 4-hydroxynonenal on Antioxidant Capacity and Apoptosis Induction in Jurkat T Cells

4-Hydroxynonenal (HNE) is one of the major end products of lipid peroxidation and may have either physiological or pathological significance regulating cell proliferation. We studied some biochemical effects of HNE, at various concentrations (0.1-100 microM), on Jurkat T cells incubated thereafter for 24, 48 and 72 h. HNE at low concentrations significantly enhanced the proliferation index, whereas at higher concentrations progressively blocked cell proliferation. Caspase 3 activity increased significantly at HNE concentrations between 1 and 10 microM and decreased at higher concentrations. Superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and glutathione reductase (GSH-Rd) increased progressively with HNE concentrations, particularly GSH-Px. Glucose-6-phosphate dehydrogenase (G6PDH) showed a different pattern, increasing at low HNE (1-5 microM) concentrations and rapidly declined thereafter. These results show that HNE may induce growth inhibition of Jurkat T cells and regulate the activity of typical antioxidant enzymes. Furthermore, the protective effect of doubling the foetal calf serum still points out the risk that cultured cells undergo oxidative stress during incubation.

Hypoxia inhibits Moloney murine leukemia virus expression in activated macrophages

Hypoxia, a local decrease in oxygen tension, occurring in many pathological processes, modifies macrophage (Mphi) gene expression and function. Here, we provide the first evidence that hypoxia inhibits transgene expression driven by the Moloney murine leukemia virus-long terminal repeats (MoMLV-LTR) in IFN-gamma-activated Mphi. Hypoxia silenced the expression of several MoMLV-LTR-driven genes, including v-myc, enhanced green fluorescence protein, and env, and was effective in different mouse Mphi cell lines and on distinct MoMLV backbone-based viruses. Down-regulation of MoMLV mRNA occurred at the transcriptional level and was associated with decreased retrovirus production, as determined by titration experiments, suggesting that hypoxia may control MoMLV retroviral spread through the suppression of LTR activity. In contrast, genes driven by the CMV or the SV40 promoter were up-regulated or unchanged by hypoxia, indicating a selective inhibitory activity on the MoMLV promoter. It is interesting that hypoxia was ineffective in suppressing MoMLV-LTR-controlled gene expression in T or fibroblast cell lines, suggesting a Mphi lineage-selective action. Finally, we found that MoMLV-mediated gene expression in Mphi was also inhibited by picolinic acid, a tryptophan catabolite with hypoxia-like activity and Mphi-activating properties, suggesting a pathophysiological role of this molecule in viral resistance and its possible use as an antiviral agent.

Loss of VHL confers hypoxia-inducible factor (HIF)-dependent resistance to vesicular stomatitis virus: role of HIF in antiviral response

Hypoxia-inducible factor (HIF) is a central regulator of cellular responses to hypoxia, and under normal oxygen tension the catalytic alpha subunit of HIF is targeted for ubiquitin-mediated destruction via the VHL-containing E3 ubiquitin ligase complex. Principally known for its association with oncogenesis, HIF has been documented to have a role in the antibacterial response. Interferons, cytokines with antiviral functions, have been shown to upregulate the expression of HIF-1alpha, but the significance of HIF in the antiviral response has not been established. Here, using renal carcinoma cells devoid of VHL or reconstituted with functional wild-type VHL or VHL mutants with various abilities to negatively regulate HIF as an ideal model system of HIF activity, we show that elevated HIF activity confers dramatically enhanced resistance to vesicular stomatitis virus (VSV)-mediated cytotoxicity. Inhibition of HIF activity using a small-molecule inhibitor, chetomin, enhanced cellular sensitivity to VSV, while treatment with hypoxia mimetic CoCl2 promoted resistance. Similarly, targeting HIF-2alpha by RNA interference also enhanced susceptibility to VSV. Expression profiling studies show that upon VSV infection, the induction of genes with known antiviral activity, such as that encoding beta interferon (IFN-beta), is significantly enhanced by HIF. These results reveal a previously unrecognized role of HIF in the antiviral response by promoting the expression of the IFN-beta gene and other genes with antiviral activity upon viral infection.

Effects of ozone on isolated peripheral blood mononuclear cells

We have investigated the release of cytokines from isolated peripheral human blood mononuclear cells (PBMC) exposed to various ozone concentrations for 10 min and the release of both proinflammatory and immunosuppressive cytokine after 24, 48 and 76 h incubation. Ozonation was performed by exposing for 10 min equal cell numbers and volumes of cell suspension to equal volumes of a gas mixture (1:1 ratio) composed of oxygen-ozone with precise ozone concentrations ranging from 1.0 up to 80 microg/ml (0.02 up to 1.68 mM). Markers of oxidative stress showed a significant relationship between ozone doses and both lipid peroxidation and protein thiol groups content. With the exception of the lowest ozone concentration, the cytokine production of PBMC was depressed particularly at concentrations from 40 mug/ml upwards. There was no significant effect on IL-6 production between exposed or unexposed cells, up to 72 h of incubation. IL-4 production was markedly affected by ozone exposure, showing a marked decrease even at the lowest ozone concentration (2.5 microg/ml) already after 24 h incubation. On the other hand, production of IFN-gamma and TNF-alpha was slightly stimulated by the lowest ozone dose either at all times or only after 72 h incubation, respectively. Analysis of the proliferation index (PI) is consistent with these results showing that, while the lowest concentration stimulates it, progressively increasing O(3) concentrations inhibit the PI. These data show that there is a significant relationship between cytokine production and ozone concentrations and that PBMC are very sensitive to oxidation particularly in presence of serum with low antioxidant capacity.

Hypoxia induces the expression and release of interleukin 1 receptor antagonist in mitogen-activated mononuclear cells

Hypoxia modulates the expression of inflammatory mediators in a variety of cell types. Since interleukin (IL-)1 receptor antagonist (Ra) is a cytokine widely associated with an inflammatory state and is expressed by activated mononuclear cells, we investigated whether hypoxia induces IL-1Ra expression in human peripheral blood mononuclear cells (PBMC) activated by phytohaemagglutinin (PHA). RNase protection assay, conducted on PHA-activated PBMC cultured under hypoxic conditions (2% O(2)) for 16-40 h, revealed that hypoxia enhances IL-1Ra mRNA expression. Further, IL-1Ra release was significantly affected by hypoxia, as determined by ELISA. Concomitantly, hypoxia enhanced, even though at a lesser extent, both IL-1alpha and IL-1beta mRNA expression and release, as determined by RPA and ELISA. However, at 40 h of treatment, hypoxia did not affect cell viability and DNA fragmentation, but caused an inhibition of the proliferation index after PHA stimulation, obtained by MTT assay. These results suggest that activated mononuclear cells tend to respond to hypoxic stress by modulating the expression of IL-1Ra and IL-1-related molecules and their release in the surrounding microenvironment.

Hypoxia modulates cyclin and cytokine expression and inhibits peripheral mononuclear cell proliferation

Previously, we found that hypoxia can deeply affect the production of cytokines in human peripheral mononuclear cells (PBMC). Here, we demonstrated that the cycle progression of hypoxic PBMC, cultured in the presence or not of a specific T cell activator such as phytohaemagglutinin (PHA), was delayed when compared with aerobic cultures. This delay was accompanied by a decrease of the expression of specific cyclins associated to cell cycle progression phases. Ribonuclease Protection Assay (RPA) studies reveal a decrease in the expression of cyclin A and B in PHA-stimulated PBMC kept for 40 hr under hypoxic condition (2% O(2)), when compared with aerobic cultures (20% O(2)). In concomitance, a decrease of cyclin D2 expression was present after 16 hr of hypoxic treatment. However, the decrease was transient and disappeared after 40 hr of hypoxic treatment. Furthermore, cyclin C expression was not affected by hypoxia. Hypoxia-induced cyclin modulation was accompanied by an increased synthesis of interleukin (IL)-2 and IL-4, analyzed by ELISA. By evaluating these results, it appears that hypoxia induces a growth suppressive state in mitogen-activated PBMC by inhibiting the synthesis of mitotic cyclins A and B. However hypoxic PBMC maintain their viability and capability of producing stimulatory cytokines, after mitogen treatment. This should be important in local hypoxia, usually associated with necrotic areas, in inflammation, and infections, where T lymphocyte capability of producing stimulatory cytokines is desirable.

Autocrine regulation of epithelial permeability by hypoxia: role for polarized release of tumor necrosis factor alpha

BACKGROUND & AIMS

The intestinal mucosa is lined by a monolayer of protective epithelial cells. This barrier is regulated by immune-derived factors such as interferon gamma (IFN-gamma). Because of the high volume of blood flow, the intestine is a primary target for hypoxic damage. We hypothesize that epithelial cytokine responses are regulated by hypoxia.

METHODS

T84 intestinal epithelial cells were used to assess alterations in permeability, major histocompatibility complex class II induction, cytokine receptor expression, and cytokine release in response to combinations of IFN-gamma and cellular hypoxia.

RESULTS

Hypoxia potentiated the influence of IFN-gamma on epithelial barrier function. Such responses were conferrable in a >/=10-kilodalton conditioned media fraction from hypoxic epithelia. Subsequent experiments identified this factor as epithelium-derived tumor necrosis factor alpha (TNF-alpha). Add-back of recombinant TNF-alpha in combination with IFN-gamma to normoxic epithelia recapitulated hypoxia and identified basolaterally polarized TNF-alpha receptor types I and II on intestinal epithelia. A similar pattern of TNF-alpha-receptor expression was observed on native intestinal epithelia. Specific inhibition of TNF-alpha using neutralizing antibody or alpha-N-phthalimidoglutarimide (thalidomide) resulted in reversal of the hypoxia-evoked responses.

CONCLUSIONS

These studies indicate that during hypoxia, epithelium-derived mediators such as TNF-alpha have the potential to regulate permeability through autocrine pathways.

Hypoxia affects cytokine production and proliferative responses by human peripheral mononuclear cells

We have shown that hypoxia (2% O2 approximately pO2 14 mmHg) as opposed to O2 atmospheric pressure (20.9% O2 approximately pO2 140 mmHg) can deeply affect the production of cytokines in human peripheral mononuclear cells (PBMC) in the presence or absence of a specific T-cell activator such as phytohemagglutinin (PHA). In hypoxia, interleukin (IL)-2, IL-4, and interferon (IFN)-gamma production increased by 110, 70, and 50% over that of controls, respectively, in PHA-stimulated PBMC (P < 0.05). Moreover, in hypoxia, IL-6 production was significantly enhanced in both resting and PHA-stimulated PBMC by 36 and 37%, respectively (P < 0.05). However, in hypoxia, IL-10 production decreased in both resting and stimulated PBMC, being 80 and 67% of controls, respectively (P < 0.05). PBMC proliferation was not significantly affected by hypoxia, although PBMC susceptibility to PHA was about 80% of that of the control (P < 0.05) after 40 hr of treatment, whereas the cycle progression of hypoxic PBMC was delayed. From an evaluation of these results, hypoxia apparently modifies the production of cytokines by PBMC. These results have both theoretical and practical interest because local hypoxia is very common in several conditions, such as inflammation and local ischemia, and is a host-nonspecific defense against infection. Furthermore, these results suggest a differential pattern of cytokine production in vivo in hypoxic tissues.

An essential role for p300/CBP in the cellular response to hypoxia

p300 and CBP are homologous transcription adapters targeted by the E1A oncoprotein. They participate in numerous biological processes, including cell cycle arrest, differentiation, and transcription activation. p300 and/or CBP (p300/CBP) also coactivate CREB. How they participate in these processes is not yet known. In a search for specific p300 binding proteins, we have cloned the intact cDNA for HIF-1 alpha. This transcription factor mediates hypoxic induction of genes encoding certain glycolytic enzymes, erythropoietin (Epo), and vascular endothelial growth factor. Hypoxic conditions lead to the formation of a DNA binding complex containing both HIF-1 alpha and p300/CBP. Hypoxia-induced transcription from the Epo promoter was specifically enhanced by ectopic p300 and inhibited by E1A binding to p300/CBP. Hypoxia-induced VEGF and Epo mRNA synthesis were similarly inhibited by E1A. Hence, p300/CBP-HIF complexes participate in the induction of hypoxia-responsive genes, including one (vascular endothelial growth factor) that plays a major role in tumor angiogenesis. Paradoxically, these data, to our knowledge for the first time, suggest that p300/ CBP are active in both transformation suppression and tumor development.

Effects of hypoxia on the antiproliferative activity of human interferons

We studied the effects of pharmacologically attainable concentrations of interferon-alpha (IFN-alpha) and gamma (IFN-gamma) on the growth of cells incubated under hypoxic conditions (2% O2; approximately 14 mm Hg partial pressure) or exposed to oxygen at atmospheric pressure (21% O2; approximately 147 mm Hg). The cells were from four IFN-sensitive lines: A-549 lung carcinoma and G-361 human melanoma cells grow better under hypoxic conditions, but the growth of Hep-2 laryngeal carcinoma and WISH amnion cells is not affected by the environmental oxygen tension. The antiproliferative effects of the IFN were assessed in terms of cell cloning efficiency and also from the number of cells, relative to controls, measured 1, 2, and 3 days after seeding. Under hypoxic conditions, the cloning efficiency of A-549 and G-361 cells was increased, and they became significantly less responsive to the antiproliferative effect of IFN, and especially of IFN-gamma. No such effects were seen with WISH or Hep-2 cells. Hypoxic conditions are found in the necrotic areas present in most solid tumors, and our results suggest that these may decrease the antiproliferative effects of IFN. They may in part explain why IFNs have so little antitumor activity in such tumors, and they also suggest methods that may increase this activity.