Hypoxic-inflammatory responses under acute hypoxia: In Vitro experiments and prospective observational expedition trial.

Tobias Kammerer,Julia Kröpfl,Mareike Horstmann,Simon Schäfer,Nikolai Hulde,Manfred Stangl,Markus Rehm,Valentina Faihs,Christina Spengler,Simone Kreth,Florian Brettner

doi:10.3390/ijms21031034

Abstract

Induction of hypoxia-inducible-factor-1α (HIF-1α) pathway and HIF-target genes allow adaptation to hypoxia and are associated with reduced incidence of acute mountain sickness (AMS). Little is known about HIF-pathways in conjunction with inflammation or exercise stimuli under acute hypobaric hypoxia in non-acclimatized individuals. We therefore tested the hypotheses that (1) both hypoxic and inflammatory stimuli induce hypoxic-inflammatory signaling pathways in vitro, (2) similar results are seen in vivo under hypobaric hypoxia, and (3) induction of HIF-dependent genes is associated with AMS in 11 volunteers. In vitro, peripheral blood mononuclear cells (PBMCs) were incubated under hypoxic (10%/5% O2) or inflammatory (CD3/CD28) conditions. In vivo, Interleukin 1β (IL-1β), C-X-C Chemokine receptor type 4 (CXCR-4), and C-C Chemokine receptor type 2 (CCR-2) mRNA expression, cytokines and receptors were analyzed under normoxia (520 m above sea level (a.s.l.)), hypobaric hypoxia (3883 m a.s.l.) before/after exercise, and after 24 h under hypobaric hypoxia. In vitro, isolated hypoxic (p = 0.004) or inflammatory (p = 0.006) stimuli induced IL-1β mRNA expression. CCR-2 mRNA expression increased under hypoxia (p = 0.005); CXCR-4 mRNA expression remained unchanged. In vivo, cytokines, receptors, and IL-1β, CCR-2 and CXCR-4 mRNA expression increased under hypobaric hypoxia after 24 h (all p ≤ 0.05). Of note, proinflammatory IL-1β and CXCR-4 mRNA expression changes were associated with symptoms of AMS. Thus, hypoxic-inflammatory pathways are differentially regulated, as combined hypoxic and exercise stimulus was stronger in vivo than isolated hypoxic or inflammatory stimulation in vitro.

Highlights

Mammalian responses to hypoxia in combination with either inflammation or exercise stress, are highly related and induce the same hypoxia-inducible-factor-1α (HIF-1α) signaling pathway [1,2,3,4,5,6]
We analyzed the mRNA expression of Interleukin 1β (IL-1β), a master cytokine of the inflammatory response, linking hypoxic and inflammatory pathways
In vitro, compared to unstimulated controls (0.27 (0.14–0.47)), mRNA expression of IL-1β was increased both following isolated inflammatory stimulation with CD3/C28 (0.49 (0.31–0.84); p = 0.006) or profound hypoxia (5% O2: 0.39 (0.24–0.88); p = 0.004, Figure 2A for 24 h

Summary

Introduction

Mammalian responses to hypoxia in combination with either inflammation or exercise stress, are highly related and induce the same hypoxia-inducible-factor-1α (HIF-1α) signaling pathway [1,2,3,4,5,6]. When oxygen partial pressure decreases, activity of HIF-degrading prolinhydroxylases (PHD) drop and HIF is stabilized, dimerizes with HIF-1β [5], and induces the transcription of HIF-1 target genes to overcome hypoxia inducing erythropoiesis, vasodilatation, vascular growth, or release of various mediators [6,7] In this regard, previous research mainly focused on cell-cultures in vitro or experiments using animals [8,9,10,11]. A preceding and repeated inflammatory stimulus can induce cellular tolerance, a phenomenon known from inflammatory disorders as lipopolysaccharid tolerance [6,7,12,13] Tolerant cells lose their ability to induce HIF-1α mRNA expression and cellular protein content even when a strong second stimulus is given [9]. This inhibited HIF-pathway shows that immunosuppressive phenotypes are immediately initiated after the onset of sepsis [6,7,12,14]

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Conclusion