Lower back pain (LBP) affects people of all cultures and ages. It reportedly affects 700 million patients worldwide physically, psychologically, and economically and reduces the productivity of society by causing massive economic losses (Francisco et al., 2022). Intervertebral disc degeneration (IDD) is thought to be the most important component of LBP (Zhao et al., 2021). Intervertebral discs (IVDs) are composed of fibrous tissue and cartilage and have an avascular structure that contains three layers: the annulus fibrosus with its outer and inner parts, the central nucleus pulposus, and the endplate. The normal function of an IVD depends on the cells and stroma (Kos et al., 2019). Multiple studies have indicated that various pathological factors, such as extracellular matrix (ECM) degradation, oxidative stress, inflammatory responses, mitochondrial dysfunction, nutritional deficiency, abnormal mechanical loading, and epigenetic changes, are involved in IDD progression (Zhang G. Z. et al., 2020; Zhang et al., 2021; Liang et al., 2022). However, the regulatory mechanisms involved in IDD are complex; thus, many of them remain to be elucidated.

Autophagy is a process in which cells self-digest their own components. It nourishes cells during fasting, clearing them of excess or damaged organelles, misfolded proteins, and invading microorganisms (Levine and Kroemer 2008). In recent years, numerous studies have indicated that disordered autophagy may lead to IDD (Gong and Zhang 2021; Madhu et al., 2021). Studies investigating autophagy in IDD are still at a very early stage, and the physiological role of autophagy in normal IVDs and specific mechanisms involved in autophagy regulation in IVD cells remain unclear (Kritschil et al., 2022). More studies on relevant functions and mechanisms are needed to clarify the role of autophagy in IDD.

The term ferroptosis, coined in 2012 (Dixon et al., 2012), describes a unique form of cell death. It is driven by iron-dependent phospholipid peroxidation and regulated by multiple cellular metabolic events, such as redox homeostasis, mitochondrial activity, and metabolism of amino acids, lipids, and sugars, as well as many disease-related signaling pathways. Ferroptosis may be involved in many organ damage and degenerative pathologies (Jiang et al., 2021). Previous studies have suggested that ferroptosis might play a role in IDD (Zhang GZ. et al., 2020; Lu et al., 2021).

The current study used microarray data analysis to investigate IDD pathogenesis and potential biomarkers related to autophagy or ferroptosis in IDD.

IDD is an important cause of LBP and involves complex pathophysiological changes and molecular mechanisms. Research on the occurrence and developmental mechanisms involved in IDD has been limited, particularly regarding mechanisms relating to autophagy and ferroptosis. In the current study, microarray and bioinformatics analyses were used to investigate possible molecular mechanisms involved in IDD occurrence and development, and potential biomarkers were predicted.

Both GO and KEGG enrichment analyses of DEGs and hub genes highlighted the importance of oxidative stress, ECM, and PI3K-AKT signaling pathway, and all three of these were related to autophagy and ferroptosis. Activation of the PI3K-AKT signaling pathway can increase ECM content, prevent apoptosis, promote cell proliferation, induce or prevent autophagy, mitigate oxidative damage, and contribute to adaptation to a hypoxic microenvironment, thereby preventing IDD (Ouyang et al., 2017). Exosomes of the cartilage endplate activate the PI3K/AKT/autophagy pathway, inhibit NPC apoptosis, and slow the IDD process (Luo et al., 2021). Islet amyloid polypeptide regulates gene expression responsible for ECM anabolism and catabolism and controls the interaction between apoptosis and NPC autophagy via the PI3K/AKT/mTOR and p38/JNK MAPK signaling pathways (Wu et al., 2017). XIST may act as a competitive endogenous RNA of MIRI-19, and XIST/MIRI-19/PTEN crosstalk reduces NPC viability, promotes NPC degeneration, and induces NPC autophagy in IVD tissues via the PI3K/Akt signaling pathway inactivation, slowing IDD development (Chen W et al., 2021). Bone marrow mesenchymal stem cell-derived extracellular vesicles harboring circ_0072464 inhibit NPC ferroptosis by inhibiting miR-431 and upregulating NRF2, which promotes NPC matrix synthesis and proliferation, alleviating IDD (Yu et al., 2022). Under oxidative stress, ferroprotein imbalance leads to NP intercellular iron overload, resulting in NPC ferroptosis and accelerating IDD progression in vivo (Lu et al., 2021). Homocysteine promotes IDD development by upregulating oxidative stress and ferroptosis in the nucleus pulposus by enhancing GPX4 methylation (Zhang G. Z. et al., 2020). Previous studies have indicated that oxidative stress, ECM, and PI3K-Akt signaling pathway might be clinical biomarkers or therapeutic indicators in IDD, which is consistent with the current study. Based on the present analysis and previous studies, we suggest that the roles of autophagy and ferroptosis in IDD warrant attention.

TP53 encodes a tumor suppressor protein containing transcriptional activation, DNA binding, and oligomerization domains. The p53-mediated transcriptional suppression of SLC7A11 promotes ferroptosis in cancer cells. p53 R273H and R175H inhibit the expression of SLC7A11 by not binding to DNA and by inhibiting the activity of other transcription factors, suggesting that the expression of this core ferroptosis regulator is regulated by an integrated network of transcription factors (Chen X et al., 2021). p53 can enhance ferroptosis by enhancing the expression of SAT1 (spermidine/spermine N1-acetyltransferase 1) and GLS2 (glutaminase 2) or by inhibiting that of SLC7A11 (solute carrier family seven member 11). On the other hand, p53 suppresses ferroptosis through the direct inhibition of DPP4 (dipeptidyl peptidase 4) activity or by the promotion of CDKN1A/p21 (cyclin-dependent kinase inhibitor 1A) expression (Kang et al., 2019). The loss of TP53 prevents nuclear accumulation of DPP4, which promotes plasma membrane-associated DPP4-dependent lipid peroxidation, leading to ferroptosis (Xie et al., 2017). Previous studies have demonstrated the central role of TP53 in the ferroptosis regulatory network, controlling ferroptosis by directly regulating the expression of downstream ferroptosis-related target genes or regulating the expression of other transcription factors to control the expression of death-related genes. Our results indicated that TP53 might regulate downstream ferroptosis-related target gene SESN2 to regulate ferroptosis, while TP53 might be regulated by other transcription factors (such as DAZAP2, HLA-B, MSN, UBLCP1, and PPT1) to form a complex and integrated regulatory network of transcription factors, which was consistent with previous studies.

SESN2 encodes a member of the sestrin family of PA26-related proteins. SESN2 can play an antioxidant role in sepsis, downregulating the ATF4-CHOP-CHAC1 signaling pathway and inhibiting ferroptosis in dendritic cells (Li et al., 2021). Ferroptosis-mediated SESN2 induction protects against iron overload and ferroptosis-induced liver injury, relying on NRF2 (Park et al., 2019). Based on existing studies, we believe that TP53 may be a central gene in the regulatory network of ferroptosis in IDD, and it may play an important role in IDD by regulating SESN2 or a complex transcription factor regulatory network to control ferroptosis.

SLC38A1 functions as a sodium-dependent amino acid transporter. As a competitive endogenous RNA, long non-coding RNA ZFAS1 promotes the conversion of lung fibroblasts to myofibroblasts and ferroptosis via the MIR-150-5P/SLC38A1 axis (Yang et al., 2020). OGFRP1 regulates lung cancer cell proliferation and ferroptosis by inhibiting miR-299-3p to enhance SLC38A1 expression (Liu et al., 2022). SLC7A11 has been confirmed to be regulated by TP53. SLC38A1 and SLC7A11 belong to different family members of solute carrier (SLC) and participate in the regulation of ferroptosis. Whether TP53 and SLC38A1 have a regulatory relationship deserves further attention and research.

TP53 is reported to be involved in autophagy. DNA damage-regulated autophagy modulator and TP53-targeted autophagy-related genes, including unc-51-like autophagy activating kinase one and autophagy-related 7, are also essential for TP53-mediated apoptosis. Cytoplasmic TP53 inhibits AMP-dependent kinase, which, in turn, activates MTOR, leading to the inhibition of autophagy, whereas nuclear TP53 has the opposite effect. Mild stressors activate TP53, which, in turn, upregulates the level of basal autophagy without activation of MTOR-dependent induced autophagy. TP53/p53-FBXO22-TFEB controls basal autophagy, regulating hormesis (Suzuki et al., 2021). Increased levels of p53 acetylation suppress renal tubular epithelial cells (RTEC) autophagy after sepsis. Autophagy activation induced by p53 following deacetylation by Sirt1 reduces sepsis-associated acute kidney injury (SAKI) (Sun et al., 2021). UBE2T upregulates autophagy in NSCLC cells by activating the p53/AMPK/mTOR signaling pathway (Zhu et al., 2021).

CANX encodes a member of the calnexin family of molecular chaperones. Blocked mitosis in hypoxic cells may be due to the fact that after CANX knockout, the number of elongated mitochondria is increased, and the colocalization of autophagosomes and mitochondria is reduced (Wu et al., 2016). The endoplasmic reticulum (ER)-resident lectin chaperone Calnexin (CANX) and ER-phagy receptor FAM134B are required for autophagy-mediated quality control of endogenous procollagens (Forrester et al., 2019). VAMP3 is a member of the vesicle-associated membrane protein (VAMP)/synaptobrevin family. VAMP3 is required for the fusion between autophagosomes and multivesicular bodies (MVBs) in K562 cells (Fader et al., 2009). The STX6-VTI1B-VAMP3 complex facilitates xenophagy by regulating the fusion between recycling endosomes and autophagosomes (Nozawa et al., 2017). TP53 is not only an autophagy-related gene but also an important and potent ferroptosis-related gene. Revealing its role in IDD autophagy and regulatory axis of TP53-CANX/TP53-VAMP3 may inspire new ideas for the research on the regulatory network of IDD autophagy.

GNAI3 belongs to the G-alpha family. GNAI3 reduces autophagy flux and inhibits the autophagic degradation of hepatitis B virus antigen by enhancing the insulin-induced AKT-MTOR pathway (Wang et al., 2022). The activator of G-protein signaling 3 (AGS3), which is directly bound to light chain 3 (LC3), recruits Gαi3 to the LC3-positive membrane upon starvation and enhances autophagy by inhibiting G protein. Gα-interacting vesicle-associated protein (GIV) destroys the Gαi3-AGS three complex under the stimulation of growth factors, releases Gαi3 from the LC3-positive membrane, and inhibits autophagy by activating G protein (Garcia-Marcos et al., 2011). At present, there is no research involving GNAI3 and its encoded Gαi3 in the field of IDD. Elucidation of the role of GNAI3 in IDD may contribute to a further understanding of autophagy in IDD.

The present study had some limitations. First, the data used were downloaded from public databases, and it was not possible to evaluate whether some of those data contained errors. Second, only IDD tissue samples were analyzed, but a comprehensive analysis of venous blood samples and IDD tissue may be more helpful with respect to providing a general understanding of the potential mechanisms involved in IDD, which would be an improvement for future research. Lastly, a large amount of evidence is still needed to support our hypothesis. In subsequent studies, we will use various experiments, such as western blotting, immunohistochemistry assays, and dual luciferase reporter gene assays, to verify our conclusions and further clarify the mechanisms of IDD autophagy and ferroptosis.

In conclusion, the purpose of this study was to investigate the molecular mechanisms involved in IDD progression via comprehensive bioinformatics analysis to complement the understanding of autophagy and ferroptosis in IDD. This study might lead to the discovery of new IDD biomarkers, and more research is required to validate our results.