Hypoxia Inducible Factor Signaling Modulates Susceptibility to Mycobacterial Infection via a Nitric Oxide Dependent Mechanism

Abstract

Tuberculosis is a current major world-health problem, exacerbated by the causative pathogen, Mycobacterium tuberculosis (Mtb), becoming increasingly resistant to conventional antibiotic treatment. Mtb is able to counteract the bactericidal mechanisms of leukocytes to survive intracellularly and develop a niche permissive for proliferation and dissemination. Understanding of the pathogenesis of mycobacterial infections such as tuberculosis (TB) remains limited, especially for early infection and for reactivation of latent infection. Signaling via hypoxia inducible factor α (HIF-α) transcription factors has previously been implicated in leukocyte activation and host defence. We have previously shown that hypoxic signaling via stabilization of Hif-1α prolongs the functionality of leukocytes in the innate immune response to injury. We sought to manipulate Hif-α signaling in a well-established Mycobacterium marinum (Mm) zebrafish model of TB to investigate effects on the host's ability to combat mycobacterial infection. Stabilization of host Hif-1α, both pharmacologically and genetically, at early stages of Mm infection was able to reduce the bacterial burden of infected larvae. Increasing Hif-1α signaling enhanced levels of reactive nitrogen species (RNS) in neutrophils prior to infection and was able to reduce larval mycobacterial burden. Conversely, decreasing Hif-2α signaling enhanced RNS levels and reduced bacterial burden, demonstrating that Hif-1α and Hif-2α have opposing effects on host susceptibility to mycobacterial infection. The antimicrobial effect of Hif-1α stabilization, and Hif-2α reduction, were demonstrated to be dependent on inducible nitric oxide synthase (iNOS) signaling at early stages of infection. Our findings indicate that induction of leukocyte iNOS by stabilizing Hif-1α, or reducing Hif-2α, aids the host during early stages of Mm infection. Stabilization of Hif-1α therefore represents a potential target for therapeutic intervention against tuberculosis.

Hypoxia and tissue destruction in pulmonary TB

“…34 Our findings are consistent with the demonstration of activation of HIF-1α signalling pathways following infection in a zebrafish model of TB infection. 35 We showed expression of HIF-1α within TB granulomas, localising the activity to epithelioid macrophages and multinucleate giant cells, which we have previously shown are key in secretion of several MMPs. 15 We demonstrate that HIF-1α regulates MMP-1 expression in TB.…”

Hypoxia-dependent regulation of inflammatory pathways in immune cells

“…Consistent with these findings, loss of HIF-1α in mice increases susceptibility to P. aeruginosa-mediated keratitis (110). In other models of infection including Clostridium, Streptococcus, Staphylococcus, and Acinetobacter species, HIF has also been largely found to be protective against pathogens (111)(112)(113)(114)(115). Given the evolving issue of antimicrobial resistance in multiple pathogens, future studies will undoubtedly expand on the fascinating potential of targeting host oxygen-sensing pathways for the purpose of regulating host immunity in infectious disease.…”

Hypoxic regulation of neutrophil function and consequences for Staphylococcus aureus infection

“…In addition to ROS, neutrophils produce antimicrobial reactive nitrogen species, a process which does appear to be under HIF-1a control. In a zebrafish mycobacterial infection model, stabilisation of HIF-1a increased production of reactive nitrogen species via inducible nitric oxide synthase (iNOS), decreasing mycobacterial burden [70].…”