Metabolic control of airway basal stem cell fate through regulating ER homeostasis

The airway epithelium provides an important barrier function and maintains airway homeostasis. It is exposed to various of exogenous and endogenous stressors, including respiratory viruses and type 2 (T2) inflammation. Using air-liquid interface cultures, we assessed the overlap in gene expression from human epithelial cells (EpCs) stimulated with IL-4/13 and infected with rhinovirus. We identified a single transcript, CH25H, that was highly and significantly upregulated in both conditions. Cholesterol 25-Hydroxylase (Ch25h) is an endoplasmic reticulum- (ER-)resident enzyme that converts cholesterol to 25-hydroxycholesterol. Expression of CH25H is upregulated in the epithelium of T2 diseases including severe asthma, eosinophilic esophagitis, atopic dermatitis. Deletion of Ch25h protected mice from developing tracheal and lung inflammation and fibrosis elicited by repetitive administration of the mold aeroallergen Alternaria alternata (Aa). Ch25h was expressed in tracheal basal cells (BCs) and upregulated after Aa. As compared to WT BCs, Ch25h-/- BCs showed increased expression of genes in the ubiquitination pathway and reduced expression of genes in the unfolded protein response, suggesting an altered capacity to buffer cell stress. Moreover, after a single challenge with Aa, WT mice had evidence of airway injury with increased XBP1s, γH2AX and TUNEL staining in the tracheal epithelium that was reduced in Ch25h-/- mice. Stimulation of Ch25h-deficient BCs with the ER-stressor tunicamycin upregulated AMPK-dependent autophagy pathway proteins, including pAMPK, ULK1, Rab7 and LC3B that were reduced by an inhibitor of SREBP2. Furthermore, tunicamycin-elicited apoptosis was reduced in Ch25h-deficient BCs and restored by the SREBP2 inhibitor. These results suggest that Ch25h inhibits SREBP2-dependent autophagocytic function thereby promoting cell stress. Finally, 25-hydroxycholesterol, the Ch25h-dependent metabolite, inhibits expression of ULK1 in a dose-dependent fashion. Taken together, our data demonstrate the metabolic control of airway epithelial stem cell fate through regulating the AMPK-autophagy-UPR signaling pathway.