The oxygen level in air directs airway epithelial cell differentiation by controlling mitochondrial citrate export

Oxygen controls most metazoan metabolism, yet in mammals, tissue O₂ levels vary widely. While extensive research has explored cellular responses to hypoxia, understanding how cells respond to physiologically high O₂ levels remains uncertain. To address this problem, we investigated respiratory epithelia as their contact with air exposes them to some of the highest O₂ levels in the body. We asked how the O₂ level in air controls differentiation of airway basal stem cells into the ciliated epithelial cells essential for clearing airborne pathogens from the lung. Through a metabolomics screen and ¹³C tracing on primary cultures of human airway basal cells, we found that the O₂ level in air directs ciliated cell differentiation by increasing mitochondrial citrate export. Unexpectedly, disrupting mitochondrial citrate export elicited hypoxia transcriptional responses independently of HIF1α stabilization and at O₂ levels that would be hyperoxic for most tissues. These findings identify mitochondrial citrate export as a cellular mechanism for responding to physiologically high O₂ levels.