Hypoxia Activates HIF-1a and Affects Gene Expression and Transcriptional Regulation of PHD in Tegillarca granosa

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