Question:

Hypoxia of the alveolar epithelium decreases the expression and activity of ion transporters to maintain cellular ATP demand and cell integrity probably by mitochondrial ROS production. Here we asked whether HIF is involved in the regulation of ion transport activity in hypoxia.

Methods:

HIF-1α and HIF-2α were silenced in primary rat alveolar epithelial cells by adenoviral transfer of shRNAs. Cells were exposed to hypoxia (1.5%) for 24h. Ion transport was measured in Ussing chambers, oxygen consumption by respirometry, mRNA by RT qPCR.

Results:

ENaC activity and mRNA expression was decreased by hypoxia. Silencing HIFs decreased ENaC in normoxia and hypoxia. Silencing HIF prevented the decrease in the mRNA subunits of Na-K/ATPase but not the decrease in activity. Hypoxia decreased mitochondrial oxygen consumption (JO₂) which was prevented by HIF-1α silencing. HIF-2α silencing did not prevent the decrease in JO₂ but further decreased JO₂ both in normoxic and hypoxic cells. Hypoxia or HIF silencing did not change the expression of mitochondrial proteins. Hypoxia decreased mitochondrial membrane potential. In HIF-2α silenced cells HIF-1α mRNA and protein expression was increased in normoxia and hypoxia.

Conclusion:

These data show that HIFs are not required for inhibition of Na-transport activity and expression in hypoxia.

It can be speculated that the HIF dependent decrease in mitochondrial function prevents mitochondrial ROS formation when cells are hypoxic yet providing sufficient energy for maintaining basal, though decreased, ion transport activity. Both effects together protect the cells from excessive damage.