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

Oxygen controls most metazoan metabolism, yet in mammals, tissue O 2 levels vary widely. While extensive research has explored cellular responses to hypoxia, understanding how cells respond to physiologically high O 2 levels remains uncertain. To address this problem, we investigated respiratory epithelia as their contact with air exposes them to some of the highest O 2 levels in the body. We asked how the O 2 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 13 C tracing on primary cultures of human airway basal cells, we found that the O 2 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 O 2 levels that would be hyperoxic for most tissues. These findings identify mitochondrial citrate export as a cellular mechanism for responding to physiologically high O 2 levels.