title: PI3K/Akt is required for heat shock proteins to protect hypoxia-inducible factor 1alpha from pVHL-independent degradation.
abstract: Hypoxia inducible factor 1 (HIF-1), a heterodimeric transcription factor composed of HIF-1alpha and HIF-1beta subunits, serves as a key regulator of metabolic adaptation to hypoxia. The amount of HIF-1alpha protein is regulated either by attenuating von Hippel-Lindau protein (pVHL)-dependent ubiquitination and subsequent 26 S proteasomal degradation or by enhancing cap-dependent mRNA translation, presumably involving a phosphatidyinositol 3-kinase (PI3K)/Akt-regulated pathway. In addition, it became apparent that Hsp90 protects HIF-1alpha from oxygen-independent degradation. Here we present evidence that PI3K/Akt is required for heat shock proteins to stabilize HIF-1alpha. In pVHL-deficient renal cell carcinoma cells, PI3K inhibition by LY294002 and wortmannin or transfection of either a dominant negative PI3K or a kinase-dead Akt mutant substantially lowered constitutively expressed HIF-1alpha without altering HIF-1alpha mRNA. Inhibitors of mitogen-activated protein kinase kinase (MAPKK) such as PD98059 or the p38 MAPK inhibitor SB203580 showed no interference. Considering that PI3K inhibitors down-regulated heat shock protein 90 (Hsp90) as well as Hsp70 expression and moreover attenuated heat- or hypoxia-induced Hsp70 as well as hypoxia-induced Hsp90 up-regulation we conclude that PI3K inhibition promoted degradation of HIF-1alpha indirectly by reducing steady state concentrations of Hsp90 and/or Hsp70. HIF-1alpha co-immunoprecipitated with Hsp90/Hsp70 and direct binding of Hsp70 to the oxygen-dependent degradation domain (ODD) of HIF-1alpha was proven by a pull-down assay and a peptide array. PI3K-mediated degradation of HIF-1alpha was confirmed in HEK 293 cells under hypoxia, suggesting that heat shock proteins constitute an integral component for HIF-1alpha accumulation. We conclude that PI3K/Akt contributes to HIF-1alpha stabilization by provoking expression of heat shock proteins.
authors.affiliation: Department of Cell Biology, Faculty of Biology, University of Kaiserslautern, 67663 Kaiserslautern, Germany.
authors.forename: Jie
authors.initials: J
authors.lastname: Zhou
authors.forename: Tobias
authors.initials: T
authors.lastname: Schmid
authors.forename: Ronald
authors.initials: R
authors.lastname: Frank
authors.forename: Bernhard
authors.initials: B
authors.lastname: Brüne
mesh_terms: D000595:Amino Acid Sequence; D015687:Cell Hypoxia / Q000502:physiology; D002460:Cell Line; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015870:Gene Expression; D018840:HSP70 Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism*; D018841:HSP90 Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D007668:Kidney / Q000166:cytology; D008969:Molecular Sequence Data; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D017346:Protein Serine-Threonine Kinases; D017434:Protein Structure, Tertiary; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt; D015398:Signal Transduction; D014157:Transcription Factors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D025521:Tumor Suppressor Proteins / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases / Q000378:metabolism; D051794:Von Hippel-Lindau Tumor Suppressor Protein
chemical_list: D004791:Enzyme Inhibitors; D018840:HSP70 Heat-Shock Proteins; D018841:HSP90 Heat-Shock Proteins; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D014157:Transcription Factors; D025521:Tumor Suppressor Proteins; D044767:Ubiquitin-Protein Ligases; D051794:Von Hippel-Lindau Tumor Suppressor Protein; C497442:HIF1A protein, human; C494918:AKT1 protein, human; C497446:VHL protein, human