Inconsistent effects of acidosis on HIF-alpha protein and its target genes.
The transcription factor HIF-1alpha has been identified as a key regulator in the cellular and systemic response to hypoxia. Because hypoxia is frequently associated with acidosis, like in ischemia or tumour growth, we studied the impact of acidosis on the expression of the HIF-1alpha and HIF-2alpha proteins and that of the three HIF target genes carbonic anhydrase-9 (CA-9), glucose transporter-1 (Glut-1) and erythropoietin (EPO). Since the HIF-prolyl hydroxylases (PHD) modulate cellular HIF-alpha protein levels we also investigated changes in PHD mRNA expression under hypoxia and acidosis. HIF-1alpha immunoblots revealed, depending on the cell line investigated, a moderate induction of HIF-alpha protein levels by acidosis in normoxia (Hep3B cells) or hypoxia (HeLa cells). Concordantly, the activity of HIF-driven luciferase reporters was slightly enhanced at pH 7.0. In contrast, HIF target genes exhibited divergent responses to acidosis: basal and hypoxia-induced CA-9 mRNA levels were further increased, whereas hypoxic EPO mRNA induction was attenuated, and Glut-1 mRNA levels were not altered by acidosis. Except from a small increase of hypoxia-induced PHD3 mRNA levels in HeLa cells, there was also no significant effect of acidosis on PHD expression. In conclusion, albeit HIF protein levels slightly induced by acidosis and the inconsistent regulation of HIF target genes under acidosis suggest additional, yet unidentified pH-sensitive factors to be involved in the regulation of these genes.
Willam
Carsten
C
Department of Nephrology and Hypertension, Medizinische Klinik IV, University of Erlangen-Nuremberg, Germany. carsten.willam@med4.med.uni-erlangen.de
Warnecke
Christina
C
Schefold
Jörg C
JC
Kügler
Jan
J
Koehne
Petra
P
Frei
Ulrich
U
Wiesener
Michael
M
Eckardt
Kai-Uwe
KU
D000138:Acidosis / Q000235:genetics* / Q000378:metabolism*; D002245:Carbon Dioxide / Q000378:metabolism; D002460:Cell Line; D005786:Gene Expression Regulation; D006801:Humans; D000860:Hypoxia / Q000378:metabolism; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D014158:Transcription, Genetic; D015533:Transcriptional Activation
C497442:HIF1A protein, human; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D012333:RNA, Messenger; D002245:Carbon Dioxide; D006899:Mixed Function Oxygenases