Oxygen and dexamethasone modulate αENaC translation in fetal lung epithelia via availability of eIF4F

Abstract

At birth, fetal distal lung epithelia (FDLE) must rapidly upregulate Na+ transport, the rate limiting step of which is the amiloride-sensitive epithelial Na+ channel (ENaC). Although glucocorticoids (GC) mature the fetal lung it is unknown how GC interact with pO2 at birth to induce lung fluid absorption. We found that dexamethasone (DEX)-induced αENaC mRNA was efficiently translated into protein in primary cultures of FDLE only under postnatal (21%) O2 (Otulakowski et al., AJRCMB;in press). Sucrose density gradient analyses of polysomes showed changes in αENaC mRNA distribution consistent with specific modulation of αENaC translation initiation; ie. DEX treatment of cells under fetal (3%) O2 decreased association of αENaC mRNA with large polysomes, while shifting DEX-treated cells to 21% O2 restored association with large polysomes. αENaC mRNA possesses a long 5′UTR with a high GC content near the 5′-cap, typical of mRNAs sensitive to limiting availability of eIF4F, the mRNA 5′-cap binding complex composed of eIF4E and eIF4G. Western blots and m7GTP-Sepharose pull-down experiments showed DEX decreased formation of eIF4F and increased association of eIF4E with its inhibitor 4E-BP in FDLE cultured at 3% O2, mediated by changes in 4E-BP phosphorylation. Conversely, FDLE cultured at 21% O2 expressed lower levels of 4E-BP and maintained eIF4E-eIF4G association in the presence of DEX. This suggests that the 7-fold increase in pO2 acts synergistically with GC to prepare the newborn lung to actively absorb fetal lung liquid. Supported by CIHR.