Hypoxia Activates <i>HIF-1α</i> and Affects Gene Expression and Transcriptional Regulation of <i>PHD</i> in <i>Tegillarca granosa</i>

Hypoxia is a common challenge faced by mollusks, and the role of hypoxia-inducible factor-1α (<i>HIF-1α</i>) in regulating related target genes under hypoxia in <i>Tegillarca granosa (Tg</i>) remains unclear. In this study, we identified gene <i>HIF-1α</i> and further explored its function. qRT-PCR was performed to determine the mRNA expression of <i>HIF-1α</i>, prolyl 4-hydroxylases (<i>PHD</i>), hemoglobin <i>(Hb)</i>, and myoglobin <i>(Mb)</i> in response to hypoxia. Dual-luciferase reporter analysis was used to assess the transcriptional activity of <i>HIF-1α</i> on the <i>PHD</i> and <i>Hb</i> genes during hypoxia. Results showed that the expression levels of <i>HIF-1α</i> and <i>PHD</i> mRNAs were highest in the gill and lowest in the adductor muscle. Under hypoxic conditions, <i>HIF-1α</i> and <i>PHD</i> mRNAs were both induced, and their expression levels increased significantly, peaking at 8 h and gradually decreasing thereafter. The transcription of <i>Tg-PHD</i> was induced by hypoxia and was <i>Tg-HIF-1α</i> dependent. Notably, the expression of <i>Hb</i> decreased in hemocytes (<i>p</i> < 0.01) after 8 h at 0.5 mg/L, while the expression of <i>MbI</i> and <i>MbII</i> increased (<i>p</i> < 0.01) in the hepatopancreas after 24 h and 120 h, respectively. Moreover, <i>Tg-HIF-1α</i> could transactivate the <i>PHD</i> promoter but not that of <i>Hb</i>. These findings provide valuable insights into the regulatory role of <i>Tg-HIF-1α</i> on <i>Tg-PHD</i>, safeguarding it from degradation by <i>PHD</i> and offering significant contributions to the understanding of <i>HIF-1α’s</i> function. Nonetheless, the expression of Tg-HIF-1α protein was not detected under hypoxic conditions, and additional investigations are required to quantify Tg-HIF-1α protein dynamics and establish the correlation between mRNA expression and protein levels.