In vitro study on the regulation of cellular mRNA levels by changes in transcription rate and transcript stability in fish red blood cells.

The interplay of transcriptional and post-transcriptional processes in the regulation of gene expression has been extensively studied in mammals but little is known in other vertebrates so far. Most non-mammalian vertebrates are faced with environmental cues and stressors distinct from those experienced by mammals and thus it is likely that the gene expression strategies differ from those of mammals. Here we performed experiments to study in vitro the various levels of gene expression regulation in nucleated fish red blood cells. Three critical environmental cues frequently experienced by fish were chosen: exposure to hypoxia (2.5%), ambient water temperature increase by 10°C (from 10°C to 20°C), and exposure to stress hormones (represented by the β-adrenergic agonist isoproterenol). We found that β-adrenergic stimulation increases the stability of the β3b-adrenergic receptor (β3b-ar) mRNA, suggesting that mRNA stability can play a role in the regulation of hormonal stress responses in fish. The β3b-ar gene encodes a unique β-adrenergic receptor subtype in fish red blood cells which controls the β-Na(+)/H(+) exchanger activity - an important component of responses to oxygen limitations. Our results furthermore show a yet undescribed link between the Hif1a signaling pathway and the β-adrenergic receptor response. After β-adrenergic stimulation, the transcription of hif1a was activated significantly after 4 hours of exposure. So far, such a response has only been described from mammalian species. This indicates that the β-AR is fundamental to the molecular and physiological responses to hypoxia and that Hif1a might have additional functions than those already known.