Osteoarthritis (OA) is a common degenerative joint disease characterized by progressive cartilage degradation, subchondral bone remodeling, and low-grade inflammation (Yin et al., 2022). The prevalence of OA remains high in the world, associates with an age-associated increasing in incidence (Mandl, 2019). Obesity and joint injury are also well-known as important risk factors for OA (Allen et al., 2022). Clinically, joint pain, deformities, and dysfunction are the main manifestations of OA, which greatly affect people's quality of life (He et al., 2021). Self-management, exercises, and weight loss are the cornerstones of OA management (Hunter and Bierma-Zeinstra, 2019). The administration of anti-inflammatory and analgesic drugs, and injection of glucocorticoids or hyaluronic acid is also widely used in the treatment of OA (Chen et al., 2022). Although the outcomes of patients with OA are benefited from the advances in treatment to some degrees, exploring of novel therapeutic targets with high efficiency and safety is still needed.

Ubiquitination is a post-translational modification tagging proteins for proteasomal degradation, which plays an important role in all aspects of physiological and pathological processes (Popovic et al., 2014). Of note, ubiquitination modification also participates in maintaining skeletal homeostasis, and many components are considered as potential targets for the treatment of OA (Liu et al., 2021). To date, emerging evidence has determined that some ubiquitin ligases exert key regulatory roles in the progression of OA. For examples, FBXW7 is involved in the action mechanisms of mechanical overloading on the development of OA (Zhang et al., 2022). FBXO21 inhibits cartilage degeneration in a rat model of OA (Lin et al., 2021). FBXO6 is down-regulated in cartilage tissues from patients with OA, and its up-regulation inhibits OA-induced injury in a mouse model (Wang et al., 2020). HECTD1 is also down-regulated in human OA cartilage tissues, and its up-regulation contributes to the remission of OA in mice (Liao et al., 2023). STUB1, also names CHIP is a chaperone-dependent E3 ubiquitin ligase which is also involved in the progression of many human diseases, such as neurological diseases, cardiac diseases, bone metabolic diseases, cancers, etc. (Zhan et al., 2017). Of note, Gong et al. revealed that circZSWIM6 enhances the progression of OA in both a traumatic and non-traumatic model in mice through regulating STUB1 (Gong et al., 2023). However, the detail function of STUB1 in OA is rarely reported yet.

Ubiquitin ligases usually exerts their function through modifying target proteins. Nowadays, many targets of STUB1 have been determined, such as IFNγ-R1 (Apriamashvili et al., 2022), BMAL1 (Ullah et al., 2020), Dicer (Zhang et al., 2022), AHR (Wang et al., 2022), TGM2 (Liu et al., 2020), and SMYD2 (Pan et al., 2022). NRF2 is an important nuclear transcription factor that involved in the regulation of a variety of cellular processes, including oxidative stress, autophagy, metabolism, inflammation, stem cell quiescence, unfolded protein response, etc. (Kaspar et al., 2009, Tonelli et al., 2018, He et al., 2020). Of note, NRF also acts a potential therapeutic target in OA. Yan et al. found that the activation of NRF2 contributes to the alleviation of OA via inhibiting inflammasome activation (Yan et al., 2020). Marchev et al. revealed that NRF2 inhibits the oxidative stress and inflammation at the cellular level, contributing to the remission of OA (Marchev et al., 2017). Previous studies also reported that NRF2 is the target of many ubiquitin ligases in human diseases, including KEAP1 (Baird and Yamamoto, 2020), RBX1 (Niture et al., 2014), KEAP1 (Ichimura et al., 2013), TRIM22 (Liu et al., 2022), MIB1 (Wang et al., 2022), and so on. Therefore, we suspect that whether there is an interaction between STUB1 and NRF2 in OA.

Macrophage polarization is a multifactorial process that is important in regulating the inflammatory and immune responses (Kishore and Petrek, 2021). The polarization of M1 macrophages can enhance the pathological process of OA via driving inflammation (Lv et al., 2021). Accumulating evidence has determined that targeting macrophage polarization contributes to the remission of OA through inhibiting joint inflammation (Sun et al., 2020). In addition, STUB1 and NRF2 are also involved in the regulation of macrophage polarization. Wei et al. found that silencing of STUB1 activates M2 alveolar macrophages in mice with airway inflammation (Wei et al., 2014). Wang et al. revealed that the activation of NRF2 may relieve OA via inhibiting M1 polarization and promoting M2 polarization (Wang and He, 2022). Therefore, there may be a regulatory axis of STUB1-NRF2-macrophage polarization in OA. In this study, a rat model of OA was established to evaluate the function of STUB1 in OA. The action mechanisms of STUB1 involving NRF2 and macrophage polarization were further analyzed in vitro. Our findings may lay the foundation for the therapeutic potential of STUB1-NRF2 against OA.