High‐Altitude Exposure and Sustained Hypoxia Increase Expression of Inflammatory Genes and Enhance the Peripheral Blood Inflammatory Response to LPS

Abstract

Transcriptional responses to hypoxia and inflammatory stimuli are linked via hypoxia inducible factor (HIF) and NF-kB crosstalk. Tissues experience hypoxia during infection or inflammatory states due to increased metabolic demand of immune cells, or impaired oxygen delivery by occluded blood supply or fluid buildup. However, while the evolutionarily conserved inflammatory responses to oxygen limitation are necessary for modulating the physiological responses to hypoxic stress, during chronic and/or systemic hypoxia, the crosstalk between these two pathways can become maladaptive. For example, hypoxia-induced inflammation may contribute to the development of Acute and Chronic Mountain Sickness. We hypothesized that acute sustained hypoxia exposure induces a systemic inflammatory response and changes sensitivity to subsequent inflammatory stimuli. To test this hypothesis, we used a high-altitude exposure model (1-3 days). We compared the RNA profiles in whole blood collected in the morning during fasting at sea level, and after 1 night and 3 nights at high altitude (3800m elevation) in 14 healthy sojourners (4 women, 10 men). RNA sequencing was coupled with Nanostring direct digital detection of 255 inflammatory pathway genes. 127 genes were significantly upregulated, and 140 genes were significantly downregulated with a fold change threshold > 1 and adjusted p-value < 0.05 on the first day at high altitude. Of all significant genes, 13 were also detected as significantly upregulated via Nanostring: HMGB1, TGFBR1, HMGN1, DDIT3, TLR8, MAPK1, TLR1, IFIT1, IFI44, MAPK8, HMGB2, TLR5, IFIT2 (adj. p < 0.05 for all). Among the significant gene ontology terms for all differentially expressed genes with adj p < 0.05 were: toll-like receptor signaling pathway (p<0.0001), MyD88-dependent toll-like receptor signaling pathway (p=0.012), interleukin-1-mediated signaling pathway (p=0.003), and I-kappaB kinase/NF-kappaB signaling (p<<0.0001). To test if upregulation of toll-like receptor signaling pathways increased sensitivity of peripheral blood mononuclear cells to inflammatory stimuli, we cultured whole blood and isolated monocytes in the presence of normoxia (18% O2, 5% CO2 at sea level) or hypoxia (1% O2, 5% CO2 at sea level) and with or without a lipopolysaccharide (LPS) stimulus for 24 hours. Both LPS (p<<0.0001) and hypoxia (p=0.01) independently increased TNF-a expression in whole blood and there was a significant interaction between hypoxia and LPS (p=0.002) in which hypoxia augmented TNF-a expression in response to LPS. TNF-a expression was higher in monocyte cultures exposed to LPS (p<<0.0001) and showed a nonsignificant trend to be higher in the presence of hypoxia, but there was no interaction. In conclusion, our results indicate that acute sustained hypoxia exposure may exacerbate the response to subsequent inflammatory stimuli. This may be particularly important in the context of pulmonary diseases if hypoxemia exacerbates the inflammatory response to viral or bacterial infection.