EU formulas can be used in the treatment of OP by promoting osteogenesis and inhibiting bone resorption through multiple pathways, which in turn can improve bone mineral density (BMD). For example, Du-Zhong-Wan (DZW), a TCM formula made from a 1:1 weight ratio of Eucommiae Cortex and Radix Dipsaci has shown promising results. Animal studies in ovariectomized (OVX) rats have shown that DZW prevents estrogen deficiency-induced BMD decline by activating estrogen signaling through an estrogen receptor-dependent pathway. Through this pathway, DZW can increase the levels of osteocalcin (OCN) and estradiol (E2), as well as protect trabecular microarchitecture and biomechanical properties (Li et al., 2016). H-type vessels, a newly discovered subtype of skeletal blood vessels, provide essential nutrients to bone tissue and also effectively promote bone formation and bone repair. Research targeting H-vessels offers new directions in the treatment of skeletal diseases (Peng et al., 2020). DZW has been shown to promote H-type vessel angiogenesis at the fractured end by increasing the expression of the pro-angiogenic factor, SLIT3, thereby enhancing osteogenesis and supporting the repair of osteoporotic fractures (Tian et al., 2022). Inflammation plays a key role in many diseases, and OP is no exception (Iantomasi et al., 2023). Broadly targeted plant metabolomics technology, combined with animal experiments, revealed that the Yishen Gushu Formula can reduce the expression of pro-inflammatory factors by regulating the TNF-α and IL-17 signaling pathways in postmenopausal osteoporotic rats, thereby increasing trabecular thickness and number, decreasing trabecular separation, and exhibiting excellent osteoprotective effects (Liu et al., 2024). In addition, another formula composed of EU, Cuscuta, and Drynaria was found to inhibit osteoclast differentiation by suppressing the PI3K/Akt signaling pathway, resulting in improved BMD and bone histomorphology (Han et al., 2021). Interestingly, the mechanism underlying OP treatment by Zhuang-Gu-Fang, with EU as the chief herb, appears to be related to the regulation of gut hormones. Administration of Zhuang-Gu-Fang to OVX rats led to elevated levels of leptin, ghrelin, and PYY, as well as improved bone microarchitecture (Chen et al., 2020). Although Remmel et al. have also revealed an association between gut hormones and bone mineralization, the exact mechanisms require further investigation (Remmel et al., 2015).

Studies on the treatment of OA with EU formulas have mainly focused on their role in reducing inflammation and protecting cartilage tissue. A retrospective study has demonstrated that the combination of a compound EU bone tonic granules with meloxicam is more effective than meloxicam alone in treating the condition. This combination reduced serum concentrations of IL-17 and S100A12, suppressed inflammation, and significantly alleviated OA symptoms in patients (Hu et al., 2020). In a surgically-induced OA rabbit model, a formula composed of Eucommiae Cortex, Pomegranate, and Achyranthis Radix mixed in a 4:5:1 ratio also exhibited significant anti-inflammatory effects, effectively protecting cartilage tissue (Choi et al., 2020). COX-2, a target of NSAIDs is also a target of EU formulas. In a rat model of OA induced by anterior cruciate ligament transection followed by treadmill running, a combination of Eucommiae Cortex, Dipsaci Radix, Achyranthis Bidentatae Radix, and Psoraleae Fructus produced anti-inflammatory and symptom-relieving effects by inhibiting the expression of p-IKKαβ and COX-2 expression, and regulating the NF-κB pathway (Siu et al., 2019). In addition, the Ryupunghwan formula, with EU as the chief herb, was also able to decrease COX-2 expression. Surprisingly, the study showed that this herbal formula had no significant effect on COX-1, suggesting fewer gastrointestinal side effects compared to traditional NSAIDs (Hong et al., 2018). These studies indicate that EU formulas have great potential in the treatment of OA, with the advantage of causing fewer side effects than existing drugs. The results provide a strong scientific foundation for the further development of these herbal treatments.

Studies on the compatibility and synergistic effects of EU remain relatively limited. The Qing'E Formula, a TCM formula with EU and Psoraleae Fructus as core ingredients, complemented by Garlic Rhizoma and Juglandis Semen, is commonly used to treat lumbar and knee pain. The estrogen-like effects of Qing'E Formula have been confirmed in cellular and animal experiments. Individual studies on EU and Psoraleae Fructus have shown their estrogen-like effects, while Garlic Rhizoma and Juglandis Semen do not produce these effects independently but enhance the estrogen-like effects of EU and Psoraleae Fructus when included in the formulae (Xiong et al., 2022). The combination of EU and Achyranthis Radix also showed synergistic effects in a zebrafish model of glucocorticoid-induced OP. This formula protected osteoblast function by enhancing the expression of osteogenic genes, such as Runx2 and β-catenin, and exerted the greatest effect at a mass ratio of 1:1 between the two herbs (Lee et al., 2022). These findings further confirm the scientific basis for combining EU with other herbs into a formula for the treatment of aging-related bone diseases while also emphasizing the importance of the synergistic effects among the ingredients in the formula to enhance therapeutic efficacy. By increasing or decreasing the number of herbs or adjusting their proportions, TCM practitioners can change the therapeutic effect of formulas and optimize therapeutic efficacy by flexibly combining herbs for different diseases, thus demonstrating their unique treatment concepts. However, the synergistic mechanisms among herbs are highly complex and have not yet been fully elucidated. Therefore, in-depth research on the rationality of herbal compatibility is of great significance in promoting the modernization of TCM.

Studies have shown that the ethanol extract of EU cortex can treat chronic kidney disease mineral bone disorder (CKD-MBD) by activating the PPARG/AMPK signaling pathway. This activation not only attenuates secondary bone damage but also exerts therapeutic effects on the primary disease (Shen et al., 2023). Similarly, in diabetic OP mice, the aqueous extract of EU cortex reduced oxidative stress by activating the Nrf2/HO-1 signaling pathway, while also increasing renal and intestinal calcium uptake. This resulted in improved BMD, better bone microarchitecture, and alleviation of OP caused by metabolic disorders. Furthermore, the aqueous extract of EU cortex reduced the blood glucose level in the model mice in vivo (Shen et al., 2024). EU has also been shown to regulate blood glucose and protect renal function (Park et al., 2006; Li et al., 2021). However, it is unclear whether there is an intrinsic correlation between the efficacy of EU in different diseases. It is commonly accepted that the human body is an organic whole with mutual interactions among different physiological pathways. EU may indirectly affect bone metabolism by modulating kidney function and blood glucose levels. Interestingly, administration of the aqueous extract of EU leaves increased gut microbiota diversity and elevated fecal and serum short-chain fatty acids, which improved OP. This phenomenon indirectly demonstrates the speculation that EU cortex may have systemic modulatory effects (Zhao et al., 2020).

Another study on the aqueous extracts of EU cortex examined the effects of EU on growth hormone. The results showed that the aqueous extract of EU cortex induced the release of growth hormone, which in turn promoted the proliferation of osteoblasts and inhibited the proliferation of osteoclasts. This suggests that EU may play a crucial role in promoting bone metabolism by regulating growth hormone levels (Ha et al., 2003). A Korean study further confirms these findings: administering the ethanol extract of EU cortex to four-week-old rats led to an increase in BMP-2 and IGF-1 levels, as well as an increase in longitudinal bone growth rate and growth plate height (Kim et al., 2015). As IGF-1 mediates growth hormone action, it plays a critical role not only in growth and development but also in bone metabolism (Dixit et al., 2021).

In rats injected with collagen to induce OA, pannus formation and synovial hyperplasia were observed in the joints, which was reduced by ethanol extract of EU cortex. Serum assays showed a reduction in the expression of inflammatory factors after administration, suggesting that the ethanol extract can attenuate joint inflammation by decreasing the levels of inflammatory factors (Xing et al., 2020). Further studies suggest that this effect may be related to the PI3K/Akt signaling pathway. By inhibiting this pathway, the aqueous extract of EU cortex reduces the expression of inflammatory factors, while also decreasing the secretion of matrix metalloproteinases such as MMP-3, thereby protecting cartilage tissue (Xie et al., 2015). In addition, the aqueous extract of EU cortex can exert anti-inflammatory effects by inhibiting the IFN-β/STAT, NF-κB, and MAPK pathways. This not only delays OA progression but also offers the possibility of articular cartilage repair (Koh et al., 2017).

Limited evidence has elucidated the multi-target therapeutic mechanisms of EU in the treatment of rheumatoid arthritis (RA). Based on network pharmacology, the pharmacological mechanisms of EU in treating RA have been predicted, revealing that EU may exert its effects through pathways such as the TNF pathway and the IL-17 pathway (Ying et al., 2022). The findings from the cell and animal studies by Wang et al. provide further validation for the therapeutic potential of EU in treating RA. Specifically, the 70% ethanol extract of EU demonstrates a multifaceted therapeutic effect on RA through a series of interrelated mechanisms. It effectively inhibits synovial hyperplasia, thereby reducing the proliferation of inflamed synovial cells. This action is complemented by its ability to lower the population of Th17 cells and the corresponding levels of serum IL-17, while simultaneously enhancing the IL-10-mediated anti-inflammatory response. Additionally, the extract suppresses the production of TNF-α and IL-1β in both serum and tissues, ultimately mitigating cartilage and bone degradation. These mechanisms work in concert to alleviate RA symptoms comprehensively (Wang et al., 2016). Notably, the iridoid components of EU modulate and attenuate the invasion/migration of HFLS-RA cells through the JAK2/STAT3 pathway, which is evidenced by the decreased phosphorylation of p-JAK2/p-STAT3 and the downregulation of inflammatory genes (Tang et al., 2023). Furthermore, the ethanol extract of EU male flowers exhibits dose-dependent inhibitory effects on synovial proliferation by suppressing the NF-κB pathway, accompanied by pro-apoptotic effects. In collagen-induced arthritis (CIA) models, this intervention reduces osteoclast differentiation, joint inflammation, and the expression of angiogenic factors, while also delaying structural joint damage (Zhang et al., 2021). Collectively, these findings position EU as a pleiotropic therapeutic agent with the potential to address the inflammatory cascades and tissue remodeling issues in the pathogenesis of RA.

Quercetin is a flavonoid widely found in fruits, vegetables, and a variety of herbs. It is well known for its excellent antioxidant, antiviral, antibacterial, and anti-inflammatory properties, as well as its good regulatory effects on blood glucose, blood pressure, and lipids. These effects have been extensively reviewed from different perspectives (Di Petrillo et al., 2022; Qi et al., 2022; Hosseini et al., 2021). As one of the main active ingredients in EU, quercetin not only possesses these properties but has also shown to be effective in the treatment of OP. In an iron overload-induced OP mouse model, quercetin inhibited reactive oxygen species (ROS) production by activating the Nrf2/HO-1 signaling pathway, thereby significantly attenuating oxidative stress and reducing osteoblast apoptosis. Additionally, quercetin further protects bone tissue from oxidative damage by regulating the expression of anti-apoptotic (e.g., BCL-2) and pro-apoptotic factors (e.g., caspase-3 and BAX) (Xiao et al., 2023). Quercetin also activates the MAPK1/ERK2 signaling pathway, which promotes osteoblast proliferation and differentiation, highlighting its potential as a natural anti-OP agent (Zhou and Wu, 2022). Furthermore, by reducing PPARA levels, quercetin can delay intervertebral disc degeneration (Xu et al., 2023). However, the clinical application of quercetin is limited by poor bioavailability. Hence, further improvements are needed to increase its practical efficacy, and some progress has been achieved in the research on quercetin derivatives (Alizadeh and Ebrahimzadeh, 2022).

Aucubin is an iridoid glycoside that has garnered significant interest due to its wide range of pharmacological effects. Modern pharmacological studies have found that aucubin has antioxidant, anti-inflammatory, anti-tumor, neuroprotective, and osteoprotective properties (Zeng et al., 2020). In the treatment of OP, aucubin promotes angiogenesis and improves bone metabolism by modulating the VEGF/VEGFR and Ang/Tie signaling pathways (He et al., 2023). The AMPK pathway is another pathway of action for aucubin. By activating the AMPK pathway, aucubin enhances cellular autophagy, thereby inhibiting dexamethasone-induced apoptosis in osteoblasts (Yue et al., 2021). Using untargeted metabolomics techniques, Wang et al. found that arachidonic acid may play a key role in aucubin treatment of Glucocorticoid-induced OP. Further Western blot and RT-qPCR assays showed that aucubin promotes the metabolism of arachidonic acid to produce PGA2, which support bone synthesis (Wang et al., 2024). Aucubin also exerts a protective effect on articular cartilage. In mice with OA induced by meniscal ligament transection, aucubicin inhibited the expression of pro-apoptotic factors (e.g., BAX, caspase-9, and caspase-3), increased BCL-2 expression, and reduced ROS production (Wang et al., 2019b).

Geniposide, another iridoid glycoside, exhibits a range of pharmacological effects, including anti-inflammatory, antidiabetic, antioxidant, neuroprotective, hepatoprotective, and choleretic activities (Shan et al., 2017). In OP, geniposide acts via multiple pathways. It can activate the Nrf2 pathway and reduce oxidized low-density lipoprotein-induced osteoblast apoptosis by downregulating the NF-κB pathway (Xiao et al., 2023). The study revealed that geniposide exerts dual efficacy in the treatment of OP and OA by promoting autophagy, which not only increases BMD and trabecular number, but also protects chondrocytes (Huang et al., 2023; Huang et al., 2022). Given its ability to modulate multiple pathological processes, geniposide may represent a multifunctional therapeutic agent in managing aging-related bone diseases. The integration of autophagy enhancement and anti-inflammatory activity makes it especially valuable for complex conditions like OP coexisting with OA. Further investigations are needed to clarify its long-term efficacy, potential synergism with current treatments, and applicability in clinical settings.

Geniposidic acid, another iridoid compound, is known for its antioxidant and anti-inflammatory properties, and has been experimentally demonstrated to have an ameliorative effect on diseases such as renal fibrosis, cholestatic hepatitis, and colitis (Wang et al., 2024; Song et al., 2022; Jiang et al., 2023). Sun et al. examined the mechanism of geniposidic acid in the treatment of OA and found that it acts by activating the Nrf2 signaling pathway and inhibiting the activation of NF-κB (Sun et al., 2024). It shares similarities with the mechanism of geniposide in the treatment of OP, but further research is needed to see if both compounds treat OP and OA through the same mechanism. Two other studies demonstrated the ability of geniposidic acid to activate the FXR/Runx2 signaling pathway, which promotes osteogenesis. Importantly, in Fxr-knockout rats, geniposidic acid failed to upregulate Runx2 or promote osteogenesis, indicating that its osteogenic effects depend on FXR signaling (Liu et al., 2023; Liu et al., 2022). However, the potential link between the three signaling pathways (FXR, Nrf2, and NF-κB), and the specific mechanism of action for geniposidic acid, remain unclear. Notably, geniposidic acid enters Caco-2 cells by passive diffusion, but salt treatments enhance its cellular uptake, suggesting that preparation methods may influence EU efficacy (Lu et al., 2018). This processing-dependent alteration in bioactive compound bioavailability underscores the imperative for mechanistic investigations to optimize the therapeutic outcomes and safety profiles of botanical preparations.