Schisandrol A in Fufang Muniziqi Granules suppresses the abnormal expression of MHC-II in renal tubular epithelial cells to treat lupus nephritis through disrupting the interaction between STUB1 and PPARγ.

OBJECTIVE: In lupus nephritis (LN), renal tubular epithelial cells (RTECs) can acquire an abnormal MHC-II antigen-presenting cell-like phenotype and thereby amplify local immune responses. This study aimed to evaluate the therapeutic effects of the Uyghur medicine Fufang Muniziqi Granules (FMG) in LN, determine whether FMG acts by suppressing aberrant MHC-II expression in RTECs, and identify its potential targets and molecular mechanisms. METHODS: We treated spontaneous LN MRL/lpr mice with different doses of FMG. We assessed systemic immune activation by measuring the spleen index, autoantibodies, and inflammatory cytokines. We evaluated renal function and histopathology using renal function indicators, H&E staining, PAS staining, and Masson staining. We performed scRNA-seq and untargeted metabolomics analyses to examine changes in renal cell populations and metabolic status. In vitro, we used IFN-γ to induce an immune phenotype in HK-2 cells and then assessed CD74 expression, mitochondrial function, and PPARγ activity. Candidate active components of FMG were identified, and CETSA, molecular docking, and DARTS assays were subsequently used to validate their targets. Finally, we examined the effect of RTEC immune phenotype modulation on T cell activation using an HK-2 and Jurkat co-culture system. RESULTS: FMG markedly alleviated systemic inflammation and renal injury in MRL/lpr mice. It reduced spleen index, autoantibody levels, and inflammatory cytokines, and decreased Scr, BUN, and albumin-to-creatinine ratio (ACR), while improving renal histopathology. scRNA-seq analysis showed that FMG primarily targeted RTECs, where it downregulated MHC-II and IFN-γ-related gene sets and restored PPARγ-mediated antioxidative capacity. Non-targeted metabolomics analysis suggested that FMG could promote fatty acid oxidation in renal tissue. In vitro, Schisandrol A, identified as a candidate active component of FMG, suppressed IFN-γ-induced CD74 expression in HK-2 cells, improved mitochondrial function, and enhanced PPARγ transcriptional activity; these effects were attenuated by PPARγ inhibition. STUB1 was identified as a potential target through which Schisandrol A regulates PPARγ. Schisandrol A blocked the interaction between STUB1 and PPARγ, and inhibited STUB1-induced PPARγ ubiquitination. STUB1 silencing increased PPARγ activity and reduced MHC-II expression, mimicking the effect of Schisandrol A. Co-culture of HK-2 and Jurkat cells showed that Schisandrol A indirectly suppressed T cell activation (IL-2 level) via RTECs, with minimal direct effects on T cells. CONCLUSION: FMG exerts therapeutic effects in LN by modulating the MHC-II antigen-presenting-like phenotype of RTECs. Mechanistically, Schisandrol A was identified as a candidate active component of FMG that may contribute to this effect by disrupting the interaction between STUB1 and PPARγ and suppressing MHC-II-associated immune activation in RTECs.