The IVD is a key structure in the human spinal joint, connecting the upper and lower cones to perform movements and carry the load of the spine. The IVD is made up of three components: NP, anulus fibrosus (AF) and the cartilage endplate (CEP) and cone endplate on the upper and lower surfaces (40) (Fig. 1). The NP is composed of chondrocyte-like cells and has a large number of randomly distributed collagen fibres and radially arranged elastic fibers, the most important component of which is aggregated proteoglycan (aggrecan) (41,42), which provides the osmotic properties needed to resist compression and maintain the height of the IVD (43). The AF is composed primarily of water, collagen, aggregated and non-aggregated proteoglycan and non-collagenous protein (44). The AF is structurally composed of 15-25 concentric rings or lamellae composed of type I collagen fibres that are reversed in their alignment between consecutive cones (45). The AF is used to accommodate the NP and maintain NP pressurization under compressive loading (43,46). The CEP mostly is composed of type II collagen, glycosaminoglycan (GAG), and water (47) in similar proportions to those found in articular cartilage (48). The CEP exists in a highly hydrated proteoglycan matrix reinforced by collagen fibres, which fibers may contribute to the connection between the CEP and AF (49), while elastic fibers are key for the connection between the NP and adjacent CEP (50). CEP can separate the IVD from the adjacent cone while accommodating the NP (51). CEP can act as a semipermeable membrane that facilitates material exchange between cone layers (52), which provides nutrients to the cone and eliminates metabolites. CEP can act as a cushion and evenly distribute the load (53).

Aberrant cell-mediated response to increasing collapse of structure is a prevalent route of IVDD (Fig. 1). Degenerative IVD is characterized by structural failure accompanied by accelerated cellular senescence (45). Although mechanical overload is hypothesized to be the direct cause of IVDD, it is only an aggravating factor. The premature weakening of IVD is one of the key factors leading to disc-related issues. This degeneration can be caused by various factors, including genetic predisposition, age, insufficient transport of metabolites, and loading history (45).

A key stage in the development of the IVDD is the degradation of the extracellular matrix (ECM) by NP and AF cells. In NP cells, the matrix exhibits a decrease in GAG, aggrecan and elastin, an alteration in collagen and an increase in fragmented aggrecan, fibronectin and advanced glycosylation end products (AGES) (54). The decrease in type II collagen expression in NP cells is accompanied by an increase in type I and III collagen, ensuring that the absolute amount of collagen remains constant. However, there is an association between changes in type X collagen and calcification of the CEP. High expression of type X collagen may stimulate preosteoblasts to undergo osteogenesis in chondrocytes, leading to calcification of CEP (55,56). In addition, fragments of aggrecan are filtered out of the IVD (57), and the decrease in aggrecan is hypothesized to be closely related to a decrease in the hydration capacity and ability to withstand load of NP cells (58). Additionally, the formation of AGES can increase stiffness and fragility of collagen fibres (54). This may lead to a decrease in the elasticity of the IVD, making it more susceptible to injury and degeneration, which in turn affects the stability and function of the spine (54). In addition, syndecan-4, a transmembrane acetyl heparan sulfate proteoglycan, may mediate degradation of ECM and aggregated proteins via thrombospondin motif 5 and MMPs (59,60). The small leucine-rich proteoglycans are also impaired by degradation (61). This may lead to a loss of elasticity and stability of the ECM, affecting cell-matrix interactions and, consequently, the physiological function and structural stability of the tissue (62). Decrease expression of islet amyloid polypeptide, calcitonin receptor and receptor activity-modifying proteins in AF cells disrupts ECM homeostasis, thereby promoting IVDD (61). Furthermore, the reduction of type I and type II collagen and lectins in AF cells leads to a transition of their load-bearing form to an inner annular elastic cushioning. Prolonged transition may result in compressed pain and AF damage (51). Alterations in the activity of certain enzymes, such as histoproteinases, matrix proteases and polymerase, may indirectly trigger degeneration of the disc (55). For example, MMPs may cause apoptosis in NP cells and calcification of the CEP (63,64).

Numerous signalling pathways are thought to be closely associated with IVDD, such as the NF-κB and MAPK pathways, which advance the progression of IVDD by regulating proinflammatory mediators (65) such as TNF-α, IL-1β and IL-6. In addition, initiation and development of disc disease may be dependent on the Wnt and the Notch pathways. Activation of the Wnt pathway accelerates NP cell senescence (66), while the hypoxia-activated Notch pathway can influence the course of IVDD via proinflammatory factors (66). DNA damage also plays a role in IVDD, leading to the loss of proteoglycans, decreased disc height and increased cellular senescence (67,68). DNA damage and telomere shortening induce replicative senescence in IVD cells primarily through activation of the p53/p21-retinoblastoma (Rb)/p16-Rb signalling pathway (69,70). In addition, oxidative stress serves an important role in IVDD and ROS damage the structure and function of IVD by damaging lipids, DNA and protein (71). Hydrogen peroxide promotes senescence by activating the senescence pathway, which stops NP cells at the G0/G1 phase of the cell cycle (72). The effects of oxidative stress also indicate activation of the p53/p21-Rb/p16-Rb signalling pathway (73). Inflammation promotes senescence in IVD cells and the inflammatory factors TNF-α and IL-1α, -1β, -6 and -17 enhance catabolism of the ECM (73).

Sirtuin (SIRT)1 can protect against IVD by inhibiting the p53-dependent and p16-dependent cellular senescence pathways and it can activate the Akt signalling pathway to protect NP cells from apoptosis (73,74). Moderate autophagy can mitigate apoptosis and delay IVDD in response to oxidative stress (75). Most autophagy is activated via AMPK/mTOR-dependent pathways (76). Overactivated autophagy may adversely affect IVD and hypoxic conditions can inactivate the AMPK/mTOR signalling pathway by limiting ROS production and decreasing activation of autophagy in NP cells through a pathway involving hypoxia-inducible factor 1α (75). In addition, autophagy safeguards CEP cells against calcification brought on by the intermittent, cyclic mechanical strain (77).

Apoptosis is associated with IVDD in NP cells, which are associated with activation of caspase-3 activity, oxidative stress and SIRT1 (37). Extrinsically, TNF-α regulates Fas/Fas ligand levels and thus activates caspase-3/9 activity (66), and may cause a significant increase in a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) family of disintegrins and ADAMTS-7 in degenerating NP cells (78). In addition, in degenerating IVD, microRNAs (miRs) are differentially expressed, including downregulation of miR-155, production of miR-10b and miR-27a, and elevation of miR-21, and downregulation of miR-155 also activates caspase-3 activity to promote apoptosis (37). Additionally, p38 MAPK, JNK1/2 and ERK1/2 in the MAPK family may damage the IVD (78). It has been shown that the MAPK family exerts proapoptotic effects on senescence via direct or indirect activation of the p53/p21-Rb/p16-Rb pathway (73). In addition, apoptosis in AF and CEP cells is associated with the presence of the MAPK family and caspase-3, -8 and -9 (37).

The structural formula of metformin (N, N-dimethylbiguanide) is C₄H₁₁N₅ (molecular weight, 129.16 g/mol); it is composed of a single protonated planar molecule in a non-polar methyl group and two imino groups. Metformin hydrochloride (molecular weight, 165.6 g/mol), which is widely used in the clinic, is constructed of dimethylamine hydrochloride and dicyandiamide (79). It is a white crystalline powder at room temperature. The density is 1.28 g/cm³, the melting point is 223-226°C and the boiling point is 224.1°C (79). This substance is almost insoluble in chloroform and ether. It can be dissolved in water at a ratio of 1:2, and in ethanol at a ratio of 1:100 (80,81). Metformin has low lipophilicity, so it cannot quickly and passively diffuse through the cell membrane (24,82,83). As a hydrophilic base, metformin has an acid-base constant of 2.8 and 11.5. It has numerous tautomeric structures, is monoprotonated at neutral pH (84) and exists as a cation at physiological pH. The concentration of metformin in physiological fluids is represented by the free base (molecular weight, 129.16 g/mol) (83) (Fig. 2).

Following oral administration, metformin is dispersed in the gastrointestinal tract, liver and kidney (85). The mean oral bioavailability of metformin is 55±16%. Absorption is mainly limited to the proximal small intestine, and it accumulates in the liver after absorption (83). The peak plasma concentration is reached ~3 h after oral administration, and the peak in the liver appears within 1 h of administration (83,86). The plasma concentrations of metformin are 1-4 mg/l in patients who received 1-2 g/day metformin oral treatment (12). At 4 h after drug intake, the liver absorbs 80% of metformin and the body eliminates 98% within 24 h (87). Metformin is primarily excreted through the kidneys. Elimination half-life and apparent total clearance in individuals with normal renal function are 5 h and 1,140±330 ml/min, respectively (83). The absorption, liver utilization and renal excretion of metformin are primarily mediated by organic cation transporters (OCTs). Studies have shown that proton inhibitors decrease metformin transport by inhibiting OCTs (24,83).

To lessen gastrointestinal adverse reactions such as diarrhoea, malignant vomiting and so on, metformin should be taken with meals. Eating can affect drug absorption, but the decreased rate of absorption is not clinically significant for most patients (24,83,88). The faecal recovery of metformin is 20-30% of the oral dose, and metformin is not readily detectable in faeces after intravenous administration, suggesting that faecal metformin is not absorbed or utilized (83).

Degenerative skeletal diseases include OP, OA and IVDD, as well as diseases such as ankylosing spondylitis (AS) and periodontitis (24) (Fig. 2). The use of metformin in the treatment of degenerative bone diseases has been verified to be beneficial (24,89). The incidence of OP is lower among patients who have diabetes with carcinoma in situ who are treated with metformin than those not receiving metformin (25). Metformin improves the long-term outcome for people who are significantly overweight and have knee OA (90). To the best of our knowledge however, there have been no clinical studies showing the effectiveness of metformin in the treatment of IVDD. Since the NP is similar to articular cartilage, which consists of water, collagen and proteoglycans, (91-93) and NP and bone tissue have common components and origins (94,95), the mechanisms by which metformin affects them are similar.

Many studies have shown that activation of AMPK has a notable effect on musculoskeletal disorder (96-98). AMPK activity is significantly downregulated in human and mouse chondrocytes with OA and significantly elevated following metformin administration (82). Metformin can lead to osteoblast differentiation in vitro by activating AMPK, promoting bone matrix synthesis and osteoblast proliferation (99). Metformin also stimulates the regenerative process in bone lesions in diabetic and non-diabetic rats (99). Metformin serves an effective role in inhibiting ossification and inflammation in AS fibroblasts by activating the AMPK pathway (100). The aforementioned studies suggest an important role of the AMPK signalling pathway in treatment of degenerative skeletal diseases with metformin. However, the direct targets of metformin that act via the AMPK pathway in these diseases remain unclear. Recent studies have shown that metformin binds presenilin Enhancer 2 (PEN2) and initiates a signalling pathway that intersects with the AMPK/activated lysosomal glucose sensing pathway via ATP6AP1 (101-103). Whether metformin acts through this signalling pathway in skeletal degenerative disease remains unclear.

Phosphoinositide 3-kinase (PI3K)/Akt is one of the key pathways that regulates cell behaviour (including apoptosis, proliferation and differentiation); previous studies have shown that this pathway is a key regulator of osteoblasts and is associated with fracture healing (104-107). Metformin ability to regulate the PI3K/Akt pathway could contribute to its efficacy in the treatment of AS (100). Metformin reduces insulin and insulin-like growth factor-1 (IGF-1) levels, thereby inhibiting phosphorylation of insulin receptor substrate 1/2 (IRS-1/2) and PI3K/Akt/mTOR signalling (24), which is key for the control of skeletal degenerative lesions.

SIRT family is a group of antiaging genes (108) that are closely associated with the aetiology of degenerative bone disease (109). Studies have demonstrated that metformin inhibits IL-1β-induced oxidative and osteoarthritic inflammatory changes by enhancing the SIRT3/PINK1/Parkin signalling pathway. In addition, metformin attenuates IL-1β-induced expression of catabolic genes, such as MMP3 and 13 and enhances the anabolic factor collagen II, which is beneficial for treatment of OA (110-113). In addition, metformin-induced activation of SIRT1 decreases inflammatory mediators and matrix degradation substances, inhibits accumulation of receptor for AGE (RAGE) and protects articular cartilage, thus preventing the development of OA (114). Another study confirmed that metformin inhibits lipopolysaccharide (LPS)-mediated enhancement of IL-6, IL-1β and TNF-α production in human gingival fibroblasts (HGFs) by increasing activating transcription factor-3 (ATF3) expression. Decreases in TNF-α and IL-1β protein expression suggest that metformin mitigates LPS-induced inflammatory responses via the AMPK/NF-κB pathway (115). However, whether this mechanism of metformin is involved in IVDD requires further experimental verification.

Metformin is an efficient and typically well-tolerated drug with notable hypoglycaemic effects but there are some adverse events (Fig. 2); the most prevalent negative reactions include gastrointestinal discomfort, including abdominal distension and pain, nausea and vomiting. These symptoms are typically mild, but medication may need to be suspended and the dose adjusted (24,88,116,117). In rare cases, metformin is related to lactic acidosis. Studies have shown that cardiovascular disease and liver and kidney damage following treatment are likely to be the main causes of lactic acidosis and there is no causal relationship with metformin (24,118,119). It has been reported that 1.4% of patients treated with metformin have insomnia, which may be associated with changes in blood glucose levels (120). DM with depression often suffer from insomnia (121,122). In addition, long-term metformin treatment may lead to vitamin B12 deficiency and require regular monitoring to decrease the incidence of side effects (123-125). Researchers determined the milk-to-plasma ratio of metformin in lactating patients and showed that metformin was safe during lactation and did not cause side effects (10,24). However, maternal and infant risks and individual differences should be considered and metformin is not recommended for pregnant patients (24).

As a highly evolutionarily conserved serine/threonine protein kinase, AMPK is commonly expressed in eukaryotic cells (126). The role of AMPK in the development of IVDD is becoming more obvious (Table I). Activation of the AMPK signalling pathway has been observed in human IVDD (127). AMPK phosphorylation is increased in rat IVDD. IVDD is a complex process involving both apoptosis and senescence (29). In degenerating rat NP cells, AMPK activation further dephosphorylates mTOR and inhibition of AMPK-mediated mTOR facilitates apoptosis and senescence while suppressing cell proliferation and cell cycle progression (34,128). Meanwhile, human AF cell apoptosis is influenced by AMPK (129).

Several in vivo studies have demonstrated a significant increase in the concentration of oxidative products in aging and degenerating IVD (130-132). Oxidative stress has been shown to induce apoptosis via the mitochondrial pathway and to induce premature cellular senescence (72,133). Chen et al (29) modelled apoptosis and senescence in medullary cells by inducing oxidative stress using tert-butyl hydroperoxide (TBHP) (29). The aforementioned study investigated the effects of metformin on apoptosis and senescence of myeloid cells under oxidative stress and examined expression of anabolic and catabolic genes in myeloid cells following metformin treatment; metformin had a significant protective effect against TBHP-induced apoptosis. Metformin pretreatment inhibits the TBHP-induced increase in cleaved-caspase3 and Cyclin-dependent kinase inhibitor 2A (p16INK4a) protein content in a dose-dependent manner. In addition, TBHP treatment decreases mRNA levels of the key ECM synthesizing gene collagen type II alpha 1 chain (Col2a1), whereas metformin significantly inhibits this downregulation. The effects of metformin are realized through the activation of autophagy by the AMPK pathway. Metformin significantly decreases Bax and caspase expression and increases BCL-2 under oxidative stress conditions via the mitochondrial pathway. Metformin also significantly decreases the senescence marker senescence-associated β-galactosidase (SA-β-gal) activity and expression of the senescence mediators p16INK4a, p21WAF1 and phosphorylated-p53, which results in the inhibition of oxidative stress-induced senescence in NP cells. Following metformin treatment, expression of catabolic enzymes MMP3 and ADAMTS-5 is inhibited and synthesis of type II collagen and aggrecan is increased, maintaining NP cell homeostasis from an ECM perspective (29). However, more investigation is necessary to show how metformin triggers autophagy via the AMPK pathway. Metformin binds to PEN2 and starts a signalling cascade that interacts with the AMPK-activated lysosomal glucose sensing pathway via ATP6AP1 (101). Low doses of metformin activate the PEN2/ATP6AP1/AMPK pathway, while high concentrations of metformin directly block mitochondrial respiratory chain complex I, thereby lowering ATP and increasing AMP to enable AMPK activation (102). To the best of our knowledge, however, there have been no studies that directly show that metformin improves IVDD by activating AMPK by binding to PEN2 (Fig. 3).

EVs are vesicular secretions produced by MSCs that mediate the biological effects of MSCs (134-136). Compared with MSC-based cell treatment, EV therapy provides a number of potential benefits, including decreased manufacturing costs, greater stability and ease of sterilization, storage and infusion therapy (134). EV therapy has been considered a promising alternative strategy for treatment with MSCs. Autophagic activity is associated with EV biogenesis and secretion and the regulation of autophagy may influence EV production (137,138). Metformin, which is an AMPK activator, is associated with autophagy activation (139). In a recent study, Liao et al (30) showed that metformin treatment effectively promotes the secretion of EVs in vitro and that this process is associated with autophagy activation (30). Metformin activates autophagy in MSCs primarily via AMPK signalling and promotes formation of multivesicular bodies (MVBs) or amphipathic bodies, which is followed by membrane fusion and EV release. In addition, metformin treatment induces phosphorylation of synaptosome associated protein (SNAP29), a key SNARE protein that mediates fusion of MVBs with the plasma membrane, thereby facilitating the release of EVs (30). Liao et al (30) also hypothesized that metformin facilitates release of EVs by altering the expression of inter-α-trypsin inhibitor heavy chain 4, which serves a key role in the regulation of cell proliferation and inflammatory responses. The secretome of MSCs (including EVs) exerts antioxidant and anti-inflammatory effects to rescue resident senescent cells in the IVD; EV treatment reduces the rate of senescent NP cells and delays the progression of IVDD (140).

One of the key indicators of the innate immune response is cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) signalling and its primary function is to recognize and respond to cytoplasmic DNA. After cGAS recognizes and binds DNA in the cytoplasm, STING initiates downstream responses (31). Senescent cells secrete a variety of cytokines called the senescence-associated secretory phenotype (SASP). SASP factors affect the intracellular environment and induce local or systemic dysfunction. However, it has been demonstrated that cGAS/STING signalling can stimulate production of inflammatory factors that lead to SASP factor secretion by senescent cells and accelerate ageing (141). Ren et al (31) demonstrated that metformin inhibits the activation of the cGAS/STING pathway by activating autophagy, thereby inhibiting the inflammatory response and NP cell ageing and ultimately delaying IVDD (31). During IVDD, release of inflammatory factors is the primary cause of excessive senescence in myeloid cells (31). Several studies have proven that oxidative stress is present in degenerative discs and ROS production disrupts intracellular homeostasis and leads to inflammation and accelerated senescence in NP cells (142-144). The cGAS/STING signalling axis consists of the second messenger loop cGAS of the interferon gene and circulating GMP-AMP receptor-stimulating factor. The inflammatory response and cellular senescence both rely on this pathway, which is part of the innate immune system. cGAS sequesters damaged DNA fragments in the cytoplasm in senescent myeloid cells, which then recruits downstream STING to activate the NF-κB signalling pathway, promoting the release of inflammatory cytokines (145). In addition, the cGAS/STING signalling pathway is a pathway that is responsive to cytoplasmic DNA. Under normal conditions, intracellular DNA is stable and present in the nucleus and mitochondria (146-148). When DNA is damaged, histone γ-H2AX (DNA damage marker) is phosphorylated and damaged DNA fragments enter the cytoplasm and are recognized by cGAS, thus activating STING and its downstream molecules (149). NF-κB has been identified as a downstream molecule of cGAS-STING. Damaged DNA fragments induce cGAS/STING activation, which phosphorylates P65 and promotes its translocation to the nucleus (150,151). This promotes the production of inflammatory cytokines, which then induce oxidative stress, leading to degradation and destruction of ECM and accelerated cellular senescence (152). The study has also shown that silencing STING gene inhibits senescence and that overexpression of STING promotes senescence in myeloid cells (152), which suggests that cGAS/STING signalling may serve as a novel therapeutic target to inhibit senescence and mitigate the progression of IVDD (Fig. 3).

NF-κB is a key mediator of cellular responses to inflammation; the downstream factor prostaglandin E2 (PGE2) disrupts IVD cell matrix homeostasis, thereby destroying the disc (153,154). There is substantial evidence that inhibiting NF-κB inhibits the inflammatory response and the production of catabolic factors in IVD cells (153,155), thus providing a potential strategy for inhibiting disc degeneration. Unlike conventional non-steroidal anti-inflammatory drugs, metformin does not inhibit COX-2 (PGE2 synthase) through an enzymatic response but rather regulates COX-2 expression via NF-κB (33). Metformin results in a large decrease in nuclear ectopic NF-κB and a significant decrease in PGE2 levels following dual stimulation of mechanical and inflammatory stress (33). The effect of metformin on proinflammatory gene expression is likely due to blockade of NF-κB translocation to the nucleus via the SIRT1/FOXO/Peroxisome proliferator-activated receptor gamma coactivator 1 (PGC1) pathway induced by activated AMPK (33,156).

High mobility group box 1 (HMGB1) is a nuclear protein that binds to DNA and is a cofactor for gene transcription (157). Under normal conditions, HMGB1 is in the nucleus and serves a role in regulating DNA stability and transcription, but it is released into ECM in response to certain stimulatory conditions (such as LPS) (158), which promotes expression of PGE2 and some inflammatory factors (TNF-α, IL-1β and IL-6) (159,160). Metformin exerts antiaging and anti-inflammatory effects by inhibiting HMGB1 translocation and catabolic products of AFSCs. HMGB1 also significantly decreases the levels of SA-β-gal activity, inflammatory factors (TNF-, IL-1 and IL-6), and certain catabolic enzymes (MMP3 and 13) in AFSCs (32) (Fig. 4).

IL-1, which is an inflammatory factor, is key to the development of IVDD (161). AGEs amplify the inflammatory response via the NOD-like receptor thermal protein domain associated protein 3 (NLRP3) inflammasome/IL-1β inflammatory response axis, which can lead to IVDD (162,163). In addition, IL-1β leads to a significant increase in ADAMTS and MMP expression, which promotes ECM degradation and leads to IVDD (164,165). Additionally, IL-1β can induce senescence and apoptosis in myeloid cells and inhibit their proliferation (161). It has also been shown that IL-1β increases intracellular ROS levels in murine vertebral MSCs, thereby exacerbating oxidative stress in IVD cells (166). Metformin inhibits IL-1β production by the SIRT3/PINK1/Parkin and the AMPK/SIRT1 pathway (110,114) and inhibits LPS-mediated enhancement of IL-1β in HGFs by increasing ATF3 expression (115). In addition to decreasing IL-1β production, metformin also reduces the effects of existing IL-1β by inhibiting activation of the NLRP3 inflammasome (167). Therefore, metformin may inhibit the proinflammatory effect of IL-1β via several pathways, thus exerting a protective effect against IVDD. In addition, resveratrol inhibits IL-1β-mediated apoptosis in the NP by regulating the PI3K/Akt pathway (168); metformin regulates the PI3K/Akt pathway (169), suggesting that metformin may also inhibit IL-1β-mediated apoptosis in myeloid cells by modulating the PI3K/Akt pathway; nonetheless, additional experiments are required to confirm this hypothesis (Fig. 4).

SIRT family is composed of NAD⁺-dependent histone deacetylases, which participate in biological processes, such as cell survival, senescence, proliferation, apoptosis, DNA repair, metabolism and caloric restriction (170). SIRT1-7 are considered attractive therapeutic targets for ageing-associated diseases such as T2DM and inflammatory and neurodegenerative disease (171). SIRT1 inhibits senescence and apoptosis in myeloid cells via autophagy. SIRT1 blocks the expression of MMP1 and 3 at the mRNA and protein levels in human dermal fibroblasts (172) and enhances the expression of cartilage-specific ECM genes, such as type II collagen and aggregated proteoglycan (173). SIRT1 inhibits matrix metalloproteinase (MMP) expression and activity, and SIRT1 also promotes the synthesis of matrix components, delaying IVDD through these pathways (174,175). SIRT1 acts on the disc via the PI3K/Akt pathway (74,176). There has been much evidence that metformin has a strong activating effect on SIRT1 (177,178). To the best of our knowledge, however, few studies have shown that metformin affects IVDD through SIRT1 signalling and further investigations still need to be conducted (179,180).

SIRT3 is a key factor for IVDD. A key mechanism of AGE-induced oxidative stress and secondary human NP apoptosis is impairment of SIRT3 activity and the mitochondrial antioxidant network (181). Nicotinamide mononucleotide (NMN) activates SIRT3 via the AMPK/PGC-1α pathways (181,182). By preventing compression-induced mitochondrial division, SIRT3 expression improves mitochondrial function, decreases the formation of ROS (183) and activates the SIRT3/FOXO3/SOD2 signalling pathway; SIRT3 activates transcription factor FOXO3a, which in turn promotes superoxide dismutase 2 (SOD2) synthesis and ROS catabolism (184), which reduces NP cell stress and the degradation of ECM. The activating effect of metformin on SIRT3 is evident, and metformin significantly increases the expression of Histone H3 Lysine 79 trimethylation (H3K79me3) in the SIRT3 promoter region by stimulating AMPK production, thereby increasing levels of SIRT3 (185). In addition, metformin inhibits the inflammatory response in myeloid cells via the SIRT3/PINK1/Parkin signalling pathway (110). When NAD is used to inhibit the effect of SIRT3, a significant decrease in the effect of metformin also occurs (186). This finding suggests that metformin may act via SIRT3 to improve IVDD.

In senescent NP cells, expression of SIRT6 is decreased, and SIRT6 decreases levels of the replicative senescence marker SA-β-gal, which inhibits IL-1β-induced apoptosis and activates autophagy in NP cells by inhibiting phosphorylation of mTOR; activation of autophagy inhibits stress-induced apoptosis in NP cells and regulates the expression and activity of certain mechanistic catabolic enzymes (187). For example, MMP-3 (Matrix Metalloproteinases) and MMP-13 are enzymes that are frequently regulated during autophagy, and they play important roles in extracellular matrix degradation and interstitial cell remodelling (188). ADAMTS-4 (A Disintegrin and Metalloproteinase with Thrombospondin Motifs) and ADAMTS-5 are typical examples of autophagy-regulated enzymes, and their activity and expression change significantly during articular cartilage and intervertebral disc degeneration (189). Autophagy may affect cell metabolism and survival by regulating the number and function of lysosomes to influence the activity of endogenous mechanolytic enzymes (190). Metformin augments the expression of SIRT6 in vitro (191). It has also been shown that SIRT2 can decrease the expression of P21 and P53 and thus inhibit ageing and oxidative stress in NP cells (192), and there is a relationship between metformin and SIRT2 (193). However, whether metformin ameliorates IVDD directly by acting on SIRT-related signalling pathways requires further study.