Hypoxia is a feature of the microenvironment in a number of chronic inflammatory conditions due to increased metabolic activity and disrupted perfusion at the inflamed site. Hypoxia contributes to inflammation through the regulation of gene expression via key oxygen-sensitive transcriptional regulators including the hypoxia-inducible factor (HIF) and NF-κB. Recent studies have revealed a high degree of interdependence between HIF and NF-κB signaling; however, the relative contribution of each to hypoxia-induced inflammatory gene expression remains unclear. In this study, we use transgenic mice expressing luciferase under the control of NF-κB to demonstrate that hypoxia activates NF-κB in the heart and lungs of mice in vivo. Using small interfering RNA targeted to the p65 subunit of NF-κB, we confirm a unidirectional dependence of hypoxic HIF-1α accumulation upon an intact canonical NF-κB pathway in cultured cells. Cyclooxygenase-2 and other key proinflammatory genes are transcriptionally induced by hypoxia in a manner that is both HIF-1 and NF-κB dependent, and in mouse embryonic fibroblasts lacking an intact canonical NF-κB pathway, there is a loss of hypoxia-induced inflammatory gene expression. Finally, under conditions of hypoxia, HIF-1α and the p65 subunit of NF-κB directly bind to the cyclooxygenase-2 promoter. These results implicate an essential role for NF-κB signaling in inflammatory gene expression in response to hypoxia both through the regulation of HIF-1 and through direct effects upon target gene expression.

Antibiotic resistance is a significant and developing problem in general medical practice and a common clinical complication in cystic fibrosis patients infected with Pseudomonas aeruginosa. Such infections occur within hypoxic mucous deposits in the cystic fibrosis lung; however, little is known about how the hypoxic microenvironment influences pathogen behavior. Here we investigated the impact of hypoxia on antibiotic resistance in P. aeruginosa. The MICs of a selection of antibiotics were determined for P. aeruginosa grown under either normoxic or hypoxic conditions. The expression of mRNAs for resistancenodulation-cell division (RND) multidrug efflux pump linker proteins was determined by real-time PCR, and multidrug efflux pump activity was inhibited using Phe-Arg ␤-naphthylamide dihydrochloride. The MIC values of a subset of clinically important P. aeruginosa antibiotics were higher for bacteria incubated under hypoxia than under normoxia. Furthermore, hypoxia altered the stoichiometry of multidrug efflux pump linker protein subtype expression, and pharmacologic inhibition of these pumps reversed hypoxia-induced antibiotic resistance. We hypothesize that hypoxia increases multidrug resistance in P. aeruginosa by shifting multidrug efflux pump linker protein expression toward a dominance of MexEF-OprN. Thus, microenvironmental hypoxia may contribute significantly to the development of antibiotic resistance in P. aeruginosa infecting cystic fibrosis patients.

No abstract

Pseudomonas aeruginosa (P. aeruginosa) is an opportunistic pathogen commonly associated with lung and wound infections. Hypoxia is a frequent feature of the microenvironment of infected tissues which induces the expression of genes associated with innate immunity and inflammation in host cells primarily through the activation of the hypoxia-inducible factor (HIF) and Nuclear factor kappaB (NF-κB) pathways which are regulated by oxygen-dependent prolyl-hydroxylases. Hypoxia also affects virulence and antibiotic resistance in bacterial pathogens. However, less is known about the impact of hypoxia on host-pathogen interactions such as bacterial adhesion and infection. In the current study, we demonstrate that hypoxia decreases the internalization of P. aeruginosa into cultured epithelial cells resulting in decreased host cell death. This response can also be elicited by the hydroxylase inhibitor Dimethyloxallyl Glycine (DMOG). Reducing HIF-2α expression or Rho kinase activity diminished the effects of hypoxia on P. aeruginosa infection. Furthermore, in an in vivo pneumonia infection model, application of DMOG 48 h before infection with P. aeruginosa significantly reduced mortality. Thus, hypoxia reduces P. aeruginosa internalization into epithelial cells and pharmacologic manipulation of the host pathways involved may represent new therapeutic targets in the treatment of P. aeruginosa infection.

This study has demonstrated that Prevotella isolated from various sources harbour a common pool of resistance genes and possess RND-type efflux pumps, which may contribute to tetracycline resistance. The findings indicate that antibiotic resistance is common in Prevotella spp., but the genotypic traits investigated do not reflect phenotypic antibiotic resistance in every instance.

Hepatocyte death is an important contributing factor in a number of diseases of the liver. PHD1 confers hypoxic sensitivity upon transcription factors including the hypoxia inducible factor (HIF) and nuclear factor-kappaB (NF-κB). Reduced PHD1 activity is linked to decreased apoptosis. Here, we investigated the underlying mechanism(s) in hepatocytes. Basal NF-κB activity was elevated in PHD1(-/-) hepatocytes compared to wild type controls. ChIP-seq analysis confirmed enhanced binding of NF-κB to chromatin in regions proximal to the promoters of genes involved in the regulation of apoptosis. Inhibition of NF-κB (but not knock-out of HIF-1 or HIF-2) reversed the anti-apoptotic effects of pharmacologic hydroxylase inhibition. We hypothesize that PHD1 inhibition leads to altered expression of NF-κB-dependent genes resulting in reduced apoptosis. This study provides new information relating to the possible mechanism of therapeutic action of hydroxylase inhibitors that has been reported in pre-clinical models of intestinal and hepatic disease.

Our understanding of how the course of opportunistic bacterial infection is influenced by the microenvironment is limited. We demonstrate that the pathogenicity of Pseudomonas aeruginosa strains derived from acute clinical infections is higher than that of strains derived from chronic infections, where tissues are hypoxic. Exposure to hypoxia attenuated the pathogenicity of strains from acute (but not chronic) infections, implicating a role for hypoxia in regulating bacterial virulence. Mass spectrometric analysis of the secretome of P. aeruginosa derived from an acute infection revealed hypoxia-induced repression of multiple virulence factors independent of altered bacterial growth. Pseudomonas aeruginosa lacking the Pseudomonas prolyl-hydroxylase domain-containing protein, which has been implicated in bacterial oxygen sensing, displays reduced virulence factor expression. Furthermore, pharmacological hydroxylase inhibition reduces virulence factor expression and pathogenicity in a murine model of pneumonia. We hypothesize that hypoxia reduces P. aeruginosa virulence at least in part through the regulation of bacterial hydroxylases.

The synthesis of N-heterocyclic carbene (NHC) silver(I) acetate complexes with varying lipophilic benzyl-substituents at the 1 and 3 positions starting from 4,5-diphenylimidazole, opened a new class of antibiotic drug candidates. These NHC-silver(I) acetate derivatives exhibit interesting structural motifs in the solid state and proved to be soluble and stable in biological media. The leading candidate, SBC3, which was known to exhibit good antibacterial activity in preliminary Kirby-Bauer tests, was tested quantitatively using minimum inhibitory concentrations. NHC-silver(I) acetate complexes were found to have MIC values ranging from 20 to 3.13 μg/mL for a variety of Gram-positive, Gram-negative and mycobacteria tested. These values represent good antibiotic activities against potential pathogens when compared to clinically approved antibiotics. Most striking is the fact that SBC3 is active against methicillin-resistant Staphylococcus aureus with a MIC value of 12.5 μg/mL.