The role of the hypoxia-inducible factor (HIF) subunits 1α and 2α in response to hypoxia is well established in lung epithelial cells, whereas little is known about HIF-3α with respect to transcriptional and translational regulation by hypoxia. HIF-3α and HIF-1α are two similar but distinct basic helix-loop-helix-PAS proteins, which have been postulated to activate hypoxia responsive genes in response to hypoxia. Here, we used quantitative real time RT-PCR and immunoblotting to determine the activation of HIF-3α vs. HIF-1α by hypoxia. HIF-3α was strongly induced by hypoxia (1% O 2 ) both at the level of protein and mRNA due to an increase in protein stability and transcriptional activation, whereas HIF-1α protein and mRNA levels enhanced transiently and then decreased because of a reduction in its mRNA stability in A549 cells, as measured on mRNA and protein levels. Interestingly, HIF-3α and HIF-1α exhibited strikingly similar responses to a variety of activating or inhibitory pharmacological agents. These results demonstrate that HIF-3α is expressed abundantly in lung epithelial cells, and that the transcriptional induction of HIF-3α plays an important role in the response to hypoxia in vitro. Our findings suggest that HIF-3α, as a member of the HIF system, is complementary rather than redundant to HIF-1α induction in protection against hypoxic damage in alveolar epithelial cells.

Isoflurane can up-regulate HIF-1alpha and enhance HIF-1-responsive genes heme oxygenase 1, inducible nitric oxide synthase, and VEGF mRNA expression in Hep3B cells. The induction of HIF-1alpha by isoflurane does not involve protein degradation but depends on translation pathway.

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