Efficient translation of mouse hypoxia-inducible factor-1α under normoxic and hypoxic conditions

The heterodimeric hypoxia-inducible factor-1 (HIF-1), consisting of the subunits HIF-1 alpha and HIF-1 beta/ARNT, is a master transcriptional regulator of oxygen homeostasis. Under hypoxic conditions, HIF-1 alpha levels very rapidly increase, mostly due to protein stabilization. However, translational regulation of HIF-1 alpha has not been directly analyzed so far. Mouse HIF-1 alpha exists as two mRNA isoforms (termed mHIF-1 alpha I.1 and mHIF-1 alpha I.2) containing structurally different 5'-termini which might modulate translation initiation. Whereas the in vitro translation efficiency of these two mRNA isoforms was about equal, the mHIF-1 alpha I.2 5'-untranslated region (5'-UTR) conferred significantly higher in vivo luciferase reporter gene activity than the mHIF-1 alpha I.1 5'-UTR. Similar corresponding luciferase mRNA levels indicate translational rather than transcriptional alterations. Reporter gene expression was not affected upon exposure of transiently transfected cells to hypoxia (1% oxygen). Direct assessment of translational regulation by polysomal profile analysis of HeLaS3 cells showed that HIF-1 alpha (and to a lower extent ARNT) mRNA was found mainly in the translationally active polyribosomal fractions under both normoxic and hypoxic conditions. In contrast, the association of mRNAs for beta-actin and ribosomal protein L28 with the polyribosomal fractions was substantially reduced under hypoxic conditions, suggesting decreased overall protein synthesis. Thus, efficient translation of mouse HIF-1 alpha in a situation where the general translation efficiency is reduced represents a prerequisite for the very rapid accumulation of HIF-1 alpha protein upon exposure to hypoxia, (C) 2000 Elsevier Science B.V. All rights reserved.