Tissue-specific increases in hypoxia-inducible factor 1 alpha protein are independent of mRNA levels during acute hypoxic exposure of the Gulf killifish, Fundulus grandis

The hypoxia inducible factor 1 (HIF1) is a central regulator of the molecular responses to low oxygen in animals. It has been extensively studied in mammals, where its tissue levels are regulated by stabilization of the alpha subunit (HIF1) when oxygen levels decrease. Consistent with this, the initial characterization of HIF1 in fish cells in culture indicated that protein abundance increased during hypoxia even when transcription was blocked. Subsequent studies, however, have reported an increase in HIF1 mRNA levels during hypoxia in certain tissues of selected species, raising the question whether hypoxic exposure brings about coordinated changes in HIF1 mRNA and protein in tissues when measured in the same fish. We have directly addressed this question by determining levels of HIF1 protein and mRNA in the tissues of Gulf killifish, Fundulus grandis, exposed to short-term hypoxia (24 h at 1 mg O2 l-1). HIF1 protein was higher in brain, ovary, and skeletal muscle from fish exposed to hypoxia compared with normoxic controls by 6 h, and it remained elevated in brain and ovary at 24 h. In contrast, HIF1 mRNA levels were unaffected by hypoxia in any tissue. Moreover, levels of HIF1 protein and mRNA in the same tissues were not correlated with one another, during either normoxia or hypoxia. These results suggest that, during the initial response to low oxygen, HIF1 protein levels increase as the result of post-translational protein stabilization, rather than new transcription, as predicted from studies in mammalian and fish cells in culture. Summary StatementParallel measurements of protein and mRNA of the hypoxia inducible factor support post-translational protein stabilization rather than new transcription in the initial response of fish to low oxygen