Lack of evidence for the involvement of the phosphoinositide 3-kinase/Akt pathway in the activation of hypoxia-inducible factors by low oxygen tension.
Hypoxia-inducible factors (HIF) belong to an evolutionary conserved family of transcription factors, the activity of which is tightly regulated by oxygen levels. We have recently demonstrated that hypoxia activates the phosphoinositide 3-kinase (PI3K)/Akt pathway in some cell types, and other works have suggested that this pathway is involved in the activation of HIF. In the present work we studied the role of this pathway in the induction of HIF by hypoxia. Under hypoxic conditions the PI3K/Akt pathway was activated in some (PC12 and HeLa) but not all cell types (HepG2) tested, whereas the HIF protein was induced by hypoxia in all cases. Kinetics analysis showed that, when observed, the activation of PI3K/Akt occurred after HIF induction. In addition, the chemical inhibition of PI3K had no significant effect on the induction of the HIF protein or its transcriptional activity but prevented Akt activation. Accordingly, transient overexpression of a dominant negative form of the regulatory subunit of PI3K in HEK293T cells did not interfere with the induction of the HIF-alpha protein by hypoxia or affect HIF-mediated transcription in any of the cell types tested. Moreover, forced activation of the PI3K/Akt pathway did not affect the transcriptional activity of HIF under normoxic or hypoxic conditions. Thus, our data suggest that the activation of PI3K/Akt by hypoxia is cell type-specific and, when observed, lies downstream of HIF activation or in a parallel pathway. Furthermore, the activity of the PI3K/Akt is not sufficient for the activation of HIF nor is it essential for its induction by hypoxia.
Alvarez-Tejado
Miguel
M
Servicio de Inmunologia, Hospital de la Princesa, Universidad Autónoma de Madrid, Diego de León 62, 28006 Madrid, Spain.
Alfranca
Arántzazu
A
Aragonés
Julian
J
Vara
Alicia
A
Landázuri
Manuel O
MO
del Peso
Luis
L
D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D015153:Blotting, Western; D002460:Cell Line; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005799:Genes, Dominant; D017930:Genes, Reporter; D006367:HeLa Cells; D006801:Humans; D000860:Hypoxia; D051793:Hypoxia-Inducible Factor 1; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D007700:Kinetics; D009687:Nuclear Proteins / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism*; D016716:PC12 Cells; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D010957:Plasmids / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats; D015398:Signal Transduction; D014157:Transcription Factors; D014158:Transcription, Genetic; D014407:Tumor Cells, Cultured; D000077191:Wortmannin
D000730:Androstadienes; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; D051793:Hypoxia-Inducible Factor 1; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D009687:Nuclear Proteins; D010100:Oxygen; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D014157:Transcription Factors; D000077191:Wortmannin; C497442:HIF1A protein, human; C497444:Hif1a protein, rat; C494918:AKT1 protein, human; C494920:Akt1 protein, rat