Inhibition of high mobility group box 1 (HMGB1) attenuates podocyte apoptosis and epithelial-mesenchymal transition by regulating autophagy flux.

BACKGROUND

Podocyte injury, characterized by podocyte hypertrophy, apoptosis, and epithelial-mesenchymal transition (EMT), is the major causative factor of diabetic nephropathy (DN). Autophagy dysfunction is regarded as the major risk factor for podocyte injury including EMT and apoptosis. High mobility group box 1 (HMGB1) is involved in the progression of DN through the induction of autophagy. However, the underlying mechanism remains unknown.

METHODS

Plasma HMGB1 concentrations were determined in DN patients using ELISA. Apoptosis of DN serum-treated podocytes was evaluated by flow cytometry. Podocyte autophagy flux was measured using immunofluorescence. Western blotting analysis was used to investigate HMGB1 expression, EMT, and autophagy-related signaling pathways.

RESULTS

Upregulation of HMGB1 was found in DN patients and DN serum-treated podocytes. Removal of HMGB1 inhibited DN serum-mediated podocyte apoptosis by inhibiting autophagy and activating AKT/mammalian target of rapamycin (mTOR) signaling. In addition, HMGB1 depletion repressed the progression of podocyte EMT by inhibiting transforming growth factor (TGF)-β/smad1 signaling in vitro and in vivo. The combination of HMGB1 short interference (si) RNA and the autophagy activator rapamycin protected against podocyte apoptosis and EMT progression by inhibiting the AKT/mTOR and TGF-β/smad signaling pathway, respectively.

CONCLUSIONS

Although HMGB1 siRNA and rapamycin treatment had opposite effects on autophagy and AKT/mTOR signaling, there was no contradiction about the role of HMGB1 siRNA and rapamycin on AKT/mTOR pathway because autophagy and AKT/mTOR signaling play dual roles in intracellular biological processes. Based on the findings of this study, we may assume that HMGB1-initiated autophagy is harmful, whereas rapamycin is beneficial to podocyte survival. Possibly combined treatment with HMGB1 siRNA and rapamycin improved podocyte damage and EMT by regulating autophagy homeostasis.