Abstract: BACKGROUND: Intervertebral disc degeneration is a condition caused by the combined effects of various factors, such as cellular apoptosis, oxidative stress, and inflammatory responses. Currently, it is believed that the core of this condition lies in the degeneration and apoptosis of nucleus pulposus cells (NPCs). However, the specific pathological mechanisms remain unclear.
OBJECTIVE: By investigating the expression of the phosphatidylinositol-3 kinase (PI3K)/protein kinase B (Akt)/hypoxia-inducible factor 1-alpha (HIF-1α) signaling pathway and the apoptosis of NPCs under different oxygen concentrations, to clarify the biological characteristics of NPCs under varying oxygen levels, and to explore the mechanisms of intervertebral disc degeneration.
METHODS: Normal and degenerated NPCs were collected. The second-generation cells were used for imaging identification. The third-generation cells were cultured in the following conditions: 37°C, air, 100% humidity. Leibovitz’s medium containing 100 μmol/L CoCl2 and 10% fetal bovine serum medium containing 100 μmol/L H2O2 were used to create distinct oxygen concentration environments for the third-generation cells, allowing for the investigation of cellular responses and behaviors under normal, low oxygen, and oxidative stress conditions. The third-generation cells were divided into six groups: normal NPCs+hypoxia, normal NPCs+normoxia, normal NPCs+oxidative stress, degenerated NPCs+hypoxia, degenerated NPCs+normoxia, and degenerated NPCs+oxidative stress groups. Cell viability and proliferation were assessed using the cell counting kit-8 method. Cell apoptosis was detected using flow cytometry. RT-PCR and western blot assay were utilized to examine the protein and mRNA expression levels of PI3K, AKT, and HIF-1α.
RESULTS AND CONCLUSION: At different oxygen concentrations, the cell proliferation rate of both normal and degenerated NPCs decreased as the oxygen concentration increased. Conversely, the apoptosis rate increased as the oxygen concentration rose (P < 0.05). With the normal NPCs+hypoxia group as the control group, the effect of degenerated NPCs on the apoptosis rate was highly significant (P < 0.001). Oxygen concentration had a highly significant impact on both NPC proliferation rate and apoptosis rate (P < 0.001). The interaction between cell degeneration and oxygen concentration significantly affected both NPC proliferation and apoptosis rates (P < 0.05). At different oxygen concentrations, the protein and mRNA expression levels of PI3K, AKT, and HIF-1α in normal NPCs were highest under low oxygen concentration, followed by oxidative stress environment, and lowest under normoxia. In degenerated NPCs, the protein and mRNA expression levels of PI3K, AKT, and HIF-1α decreasd as the oxygen concentration increased. The protein and mRNA expression levels of PI3K and Akt in normal NPCs were significantly higher than those in degenerated NPCs (P < 0.05). With the normal NPCs+hypoxia group as the control group, the effects of normal or degenerated NPCs, as well as oxygen concentration, on the protein and mRNA expression of PI3K, AKT, and HIF-1α were highly significant. The interaction between cell degeneration and oxygen concentration also had an extremely significant effect on the protein and mRNA expression of PI3K, AKT, and HIF-1α (P < 0.001). To conclude, there is a close correlation between the proliferation and apoptosis of NPCs and both oxygen concentration and the cellular functional state. The PI3K/Akt/HIF-1α signaling pathway exhibits antagonistic regulatory effects under various cellular functional states and different oxygen concentration environments. The mechanism may be associated with the activation of the PI3K/Akt signaling pathway, which consequently transcribes HIF-1α, thus maintaining the balance of reactive oxygen species metabolism. This pathway plays a significant role in inhibiting oxidative stress, antagonizing NPCs apoptosis, and consequently delaying intervertebral disc degeneration.

Key words: oxygen concentration, nucleus pulposus cell, cell apoptosis, oxidative stress, biological characteristics, signaling pathway