Identification of alternative spliced variants of human hypoxia-inducible factor-1 alpha

Mammalian cells are able to sense oxygen and regulate a number of genes in response to hypoxia. The transcription factor Hypoxia Inducible Factor-1 (H IF-1) was identified as an important key component of the hypoxia signaling pathway. HIF-1 is a heterodimer composed of two members of the basic helix -loop-helix transcription factor superfamily containing a PAS (PER-ARNT-SIM ) domain: HIF-1 alpha and HIF-1 beta/ARNT, During the cloning by reverse tr anscriptase-polymerase chain reaction of the human HIF-1 alpha subunit, we isolated two cDNA clones which corresponded to alternative splicing of the HIF-1 alpha gene. Polymerase chain reaction analysis and sequencing reveale d that both clones possessed three additional base pairs between exons 1 an d 2, Also, one of them lacked 127 base pairs corresponding to exon 14, We d emonstrate that the mRNA of this truncated form is expressed in several hum an cells Lines and human skin but apparently not in rodents. When transfect ed in HEK 293 cells, the corresponding 736 amino acid protein (HIF-1 alpha( 736)) is regulated by hypoxia in a similar manner as the full-length HIF-1 alpha (HIF-1 alpha(FL)). In luciferase transfection assays, both recombinan t proteins HIF-1 alpha(736) and HIF-1 alpha(FL) dimerize with HIF-1 beta/ A RNT and activate the VEGF promoter upon hypoxia, However, the shorter HIF-1 alpha isoform is 3-fold less active than HIF-1 alpha(FL), a result consist ent with the lack of the C-terminal transactivation domain. As expected, th is small isoform can compete with the endogenous and transfected full-lengt h HIF-1 alpha. Altogether, these results suggest that the HIF-1 alpha(736) isoform modulates gene expression upon hypoxia.