Stress on the Endoplasmic reticulum (ER) can severely disrupt cellular function by impairing protein folding and post-translational modifications, thereby leading to the accumulation of poor-quality proteins. However, research on its impact on photosynthesis remains limited. In this study, we investigated the impact of ER stress on the photosynthetic efficiency of Chlamydomonas reinhardtii using pharmacological inducers, tunicamycin (TM) and brefeldin A (BFA), which specifically target the ER. Our measurements of photosynthetic parameters showed that these ER stress-inducing compounds caused a significant decline in photosynthetic efficiency. A proteomic analysis confirmed that TM and BFA effectively induce ER stress, as evidenced by the upregulation of ER stress-related proteins. Furthermore, we observed a widespread downregulation of photosynthesis-related proteins, which is consistent with the results obtained from our measurements of photosynthetic parameters. These findings suggest that the stress on ER has a profound impact on chloroplast function, disrupting photosynthetic processes. This study highlights the critical interdependence between the ER and chloroplasts, and it underscores the broader implications of ER stress on the cellular metabolism and energy efficiency of photosynthetic organisms.

Diabetic kidney disease (DKD) is a leading cause of end-stage renal disease. The Shen-Kang Recipe (SKR) is a traditional Chinese medicine formula used clinically to slow DKD progression, but its bioactive constituents and molecular targets remain unclear. Solute carrier family 15 member 2 (SLC15A2/PEPT2), a high-affinity peptide transporter expressed in renal proximal tubules, has been implicated in kidney pathophysiology, yet its potential role in mediating the therapeutic effects of the SKR has not been explored. Here, we evaluated the effects of the SKR in db/db mice and found that SKR treatment significantly improved renal function, attenuated glomerulosclerosis, and reduced interstitial collagen deposition. Wide-target metabolomics and quantitative proteomics revealed that the SKR broadly reversed DKD-associated metabolic and proteomic disturbances, particularly in pathways related to energy and amino acid metabolism. Proteomic analysis identified SLC15A2 as a key proximal tubule protein downregulated in DKD and selectively restored by the SKR. UPLC-Q-TOF/MS-based serum pharmacochemistry and network pharmacology highlighted quercetin as a principal bioactive component of the SKR. Molecular docking, molecular dynamics simulations, and surface plasmon resonance (SPR) confirmed direct, high-affinity binding between quercetin and SLC15A2 (KD = 7.5 µM). In TGF-β1-stimulated HK-2 cells, quercetin suppressed epithelial-mesenchymal transition (EMT), as evidenced by restored E-cadherin and reduced N-cadherin, vimentin, and α-SMA expression; this effect was abrogated by siRNA-mediated SLC15A2 knockdown, demonstrating the functional necessity of this axis. Collectively, these findings identify a quercetin-SLC15A2 axis through which the SKR inhibits EMT and alleviates renal fibrosis in DKD, providing a mechanistic basis for its clinical application and nominating SLC15A2 as a potential therapeutic target.