ScholarWorks@UNO

The effect of short-term hypoxia on HIF expression levels in Fundulus grandis

Low dissolved oxygen, hypoxia, is a prevalent stressor in aquatic environments arising from natural and anthropogenic events. Included in the biological responses to hypoxia are changes in gene expression that are coordinated by the hypoxia-inducible factor (HIF) family of transcription factors. The paradigm for HIF signaling is that alpha subunits are degraded during normoxia, but degradation is suppressed at low oxygen, leading to the accumulation of HIF1alpha, dimerization with beta subunits, and regulation of target genes transcription. In fish, there is evidence that mRNA levels also increase during hypoxia, but these measurements are from experiments without corresponding protein levels. Therefore, in this study we addressed two questions. Do levels of HIF1alpha mRNA increase after short-term hypoxic exposure? Do levels of HIF1alpha mRNA correlate with protein levels measured in the same tissues? Fundulus grandis, a small, abundant, estuarine fish, was exposed to approximately 1 mg/l O2 for 6 or 24 h, after which RNA was extracted from liver, gills, ovary, and skeletal muscle. HIF1alpha mRNA was quantified by quantitative PCR. The results show that there were no changes in HIF1alpha mRNA under these conditions, even when HIF1alpha protein levels were significantly elevated. Our results support a role for protein stabilization, rather than new transcription in the initial response of fish to low oxygen, in accord with the paradigm developed in mammalian cells.