Natural Flavonoid Isoquercitrin Blocks STAT3 to Protect Kidneys in Diabetes

Diabetic nephropathy is the leading cause of renal failure globally, driven not only by metabolic and hemodynamic changes but increasingly recognized as fueled by persistent inflammation and progressive fibrosis. STAT3, a convergence node for multiple cytokine signals, is aberrantly activated by high glucose, advanced glycation end products, and inflammatory cytokines in the diabetic kidney, driving pro-inflammatory and profibrotic gene transcription. This study investigated whether isoquercitrin—a natural 3-O-glycoside derivative of quercetin with known anti-inflammatory and antioxidant properties—could inhibit STAT3 and protect against diabetic nephropathy.

Using BKS-db/db (db/db) mice, a well-established model of type 2 diabetic nephropathy, the team administered isoquercitrin at 40, 80, and 120 mg/kg for 12 weeks. Isoquercitrin dose-dependently reduced serum creatinine, blood urea nitrogen, and urinary albumin-to-creatinine ratio without altering blood glucose or body weight, suggesting direct renal action. Histological analyses (PAS, HE, Sirius Red staining) showed marked reduction in mesangial expansion, basement membrane thickening, tubular epithelial injury, and interstitial ECM accumulation. Collagen IV deposition and α-SMA expression—markers of fibrosis and epithelial-to-mesenchymal transition—were significantly reduced. Electron microscopy confirmed preservation of podocyte foot processes and basement membrane integrity.

To identify the molecular target, D-biotin-labeled isoquercitrin was used in protein microarray binding assays, pointing to the JAK-STAT pathway. Subsequent experiments confirmed STAT3 as the direct target. Molecular docking and binding studies revealed that isoquercitrin occupies the pY+1 binding pocket within STAT3's SH2 domain, forming stable noncovalent hydrogen bonds with Ser668, Gln635, and Gln633. This interaction physically obstructs STAT3 phosphorylation at Tyr705 and prevents the SH2-domain-mediated dimerization required for nuclear translocation and transcriptional activation. Cell-based assays in endothelial and renal tubular epithelial cells confirmed suppression of STAT3 phosphorylation, dimerization, and downstream pro-inflammatory and profibrotic gene expression.

Recognizing isoquercitrin's limited oral bioavailability and poor renal targeting, the researchers developed Iso@PEG-GK, a kidney-targeted nanocarrier system. This nanoparticle formulation significantly improved isoquercitrin absorption and renal tissue distribution compared to free compound, enhancing therapeutic precision. Toxicity assessments at all doses showed no significant pathological changes in heart, liver, or spleen over 12 weeks, supporting a favorable safety profile.

This is the first study to systematically demonstrate isoquercitrin's protective mechanism in diabetic nephropathy and to characterize its direct binding to STAT3 at atomic resolution. The work establishes isoquercitrin as a novel STAT3 inhibitor acting through a distinct pY+1 pocket mechanism and introduces a viable nanocarrier strategy to overcome pharmacokinetic limitations. Translational validation in human models and clinical trials will be needed before therapeutic application.