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

The heterodimeric hypoxia-inducible factor-1 (HIF-1), consisting of the sub units HIF-1 alpha and HIF-1 beta/ARNT, is a master transcriptional regulato r of oxygen homeostasis. Under hypoxic conditions, HIF-1 alpha levels very rapidly increase, mostly due to protein stabilization. However, translation al regulation of HIF-1 alpha has not been directly analyzed so far. Mouse H IF-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 modulat e translation initiation. Whereas the in vitro translation efficiency of th ese 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 luci ferase mRNA levels indicate translational rather than transcriptional alter ations. Reporter gene expression was not affected upon exposure of transien tly transfected cells to hypoxia (1% oxygen). Direct assessment of translat ional 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 trans lationally active polyribosomal fractions under both normoxic and hypoxic c onditions. In contrast, the association of mRNAs for beta-actin and ribosom al protein L28 with the polyribosomal fractions was substantially reduced u nder hypoxic conditions, suggesting decreased overall protein synthesis. Th us, efficient translation of mouse HIF-1 alpha in a situation where the gen eral translation efficiency is reduced represents a prerequisite for the ve ry rapid accumulation of HIF-1 alpha protein upon exposure to hypoxia, (C) 2000 Elsevier Science B.V. All rights reserved.