HIF-1α/Bnip3/VEGF axis regulates autophagy to mitigate intervertebral disc degeneration induced by inflammatory factor TNF-α
Authors: Sun Xiaoxian , bai Xue , Zhao Zitong , Wang Lining , Liu Mengmin , Tu Pengcheng , Lin Shun , Yan Zheng , Wang Zhiqiang , Zhou Qinfeng , Yuan Yongfeng , Li Xiaofeng , Liu Jintao , Ma Yong , Guo Yang 
Published: May 2024

Background
Intervertebral disc degeneration (IDD) is one of the main factors leading to low back pain. However, its potential pathogenesis targets are poorly understood. Therefore, the potential pathogenesis of IDD must be further explored to identify more appropriate treatment angles.
Objective
To investigate the effects of the hypoxia-inducing factor-1α (HIF-1α)/BCL-2 interacting protein 3 (Bnip3)/vascular endothelial growth factor (VEGF) signaling axis on IDD induced by the inflammatory factor tumor necrosis factor-alpha (TNF-α) and determine whether IDD progression can be delayed by regulating nucleus pulposus cell (NPC) autophagy.
Experimental methods:
Differences in TNF-α, VEGF, and HIF-1α expression between IDD model rats and normal rats were observed, and the effects of the HIF-1α inhibitor YC-1 on TNF-α and VEGF expression, HIF-1α/Bnip3 axis autophagy-related molecule levels, and IDD progression were verified in vivo. Finally, the effects of HIF-1α knockdown on HIF-1α and VEGF expression in TNF-α-induced NPCs, HIF-1α/Bnip3/VEGF signaling axis autophagy-related molecule levels, and IDD progression were explored in vitro.
Results
In vivo experimental results revealed obvious degeneration and significantly higher TNF-α, HIF-1α, and VEGF expression in the model group compared with the normal group. The YC-1 intervention downregulated TNF-α and VEGF in IDD, as verified by qPCR, WB, and immunofluorescence assays. In addition, YC-1 intrusion decreased autophagy flux in intervertebral discs (IVDs), as indicated by the decreased number of autophagosomes and lysosomes under transmission electron microscopy and decreased levels of HIF-1α/Bnip3 axis autophagy-associated molecules in immunohistochemical, WB, qPCR, and immunofluorescence assays. In addition, the YC-1 intervention led to a further decrease in the signal intensity of the intervertebral disc under T2-weighted magnetic resonance imaging (MRI) and an increase in the Pfirrmann score compared with the model group. Hematoxylin and eosin (HE), Masson’s, and safranin O-fast green staining also showed that in the YC-1 group, coagulation and shrinkage of the nucleus pulposus were more obvious, annulus fibrosus was more serious, loss of proteoglycan and collagen was increased, and IDD was further intensified compared with that in the model and dimethyl sulfoxide (DMSO) groups. In vitro experiments showed that the LV-HIF-1α intervention decreased HIF-1α and VEGF expression and autophagy-related molecule levels in the HIF-1α/Bnip3 axis of TNF-α-induced NPCs, as reflected in a decrease in the number of autophagosomes and lysosomes in the Lv-HIF-1α transfection group compared with the Lv-NC-H group. HIF-1α/Bnip3 axis-related mRNA and protein expression was downregulated, type II collagen and proteoglycan were significantly downregulated, and matrix catabolic markers matrix metalloproteinase 13 (MMP13) and A disintegrin and metalloproteinase with thrombospondin motifs 5 (ADAMTS-5) were significantly downregulated, indicating that IDD progression in NPCs was accelerated after HIF-1α knockdown.
Conclusion
In IDD, inflammatory factor TNF-α may activate autophagy by regulating the HIF-1α/Bnip3/VEGF signaling axis, which promotes NPC death, thereby protecting IVDs and delaying further IDD. However, intervention with HIF-1α blockers reduced autophagy and VEGF expression, which further aggravated IDD progression, thus supporting our hypothesis. These findings provide insights for developing targeted IDD therapies.