HIF1A-AS2 Regulates Hypoxia-Driven HIF Isoform Switching in Human Endothelial Cells

Hypoxia-inducible factors (HIFs) are central regulators of cellular adaptation to low oxygen, with a well-documented transition from HIF-1 to HIF-2 signaling during prolonged hypoxia. Here, we investigate the role of a natural antisense transcript of HIF1A, HIF1A-AS2, in modulating this transition in human endothelial cells (ECs). Genome-wide expression profiling revealed a strong hypoxia-induced upregulation of HIF1A-AS2 that coincided with a marked reduction in HIF1A mRNA levels, whereas EPAS1 remained stable. Interestingly, this negative correlation was consistent across multiple EC types and supported by silencing experiments which demonstrated that both HIF-1 and HIF-2 contribute to HIF1A-AS2 induction. Knockdown of HIF1A-AS2 partially restored HIF1A expression and increased HIF-1 protein and functional activity, as evidenced by elevated VEGFA and GLUT1 expression. Genomic analysis identified HIF-1 and HIF-2 binding motifs upstream of HIF1A-AS2, supporting the dual regulation. Furthermore, cancer tissue analysis revealed frequent downregulation of HIF1A-AS2, correlating with elevated HIF1A and EPAS1 levels. These findings suggest that HIF1A-AS2 acts as a hypoxia-responsive regulator that limits HIF-1 signaling and facilitates the HIF isoform switch, and suggests implications for endothelial adaptation, angiogenic signaling, and vascular remodeling under chronic hypoxia.