HYPOXIA-INDUCIBLE FACTOR-1-ALPHA IS REGULATED AT THE POST-MESSENGER-RNA LEVEL

The hypoxia-inducible factor-1 (HIF-1) is involved in the induction of oxygen regulated genes such as erythropoietin and vascular endothelia l growth factor (VEGF). HIF-1 is a heterodimeric transcription factor consisting of an alpha and a beta subunit. The question of how HIF-1 i tself is regulated remains to be elucidated. Studies performed in huma n Hep3B hepatoma cells suggested that the prevalent mode of HIF-1 acti on is an increase in HIF-1 alpha steady-state mRNA and protein levels after hypoxic exposure. In contrast to the reported very low basal HIF -1 alpha mRNA levels, however, we detected HIF-1 alpha mRNA in several cell lines cultured under normoxic conditions, although no HIF-1 DNA binding activity was observed. Following hypoxic induction, VEGF mRNA levels and HIF-1 DNA binding activity increased, but HIF-1 alpha mRNA levels remained largely unchanged. One possible explanation for this d iscrepancy might be that HIF-1 DNA binding activity does not follow HI F-1 alpha mRNA kinetics. We therefore incubated HeLaS3 cells in tonome ters for 7.5 minutes up to four hours at either 20% O-2 or 0.5% O-2. A lthough there was some variation in HIF-1 alpha mRNA levels, we did no t find significant changes over this time frame, suggesting that HIF-1 alpha is not transcriptionally regulated.