Aristolochic acid I (AA-I), one of the main active components in Aristolochaia herbs, may induce aristolochic acid nephropathy (AAN). Renal interstitial fibrosis is one of the most typical features of AAN. To investigate the mechanism of Aristolochic acid I (AA-I) -induced renal epithelial-mesenchymal transition (EMT) and determine the role of aquaporin-1 (AQP1) in this process, we established an AA-I-induced EMT model in human proximal tubular epithelial cells (HK-2 cells). Morphological examination, MTT assay, and Western blot analysis were performed. Aquaporin 1 (AQP1) and several EMT-related proteins were detected, thereby suggesting the occurrence of AA-I-induced EMT. Two main pathways of transforming growth factor-β (TGF-β) signaling, namely, Smad-dependent and Smad-independent signaling pathways, were also detected. The results showed that the TGF-β / Smad-independent signaling pathways (β-catenin, Ras-Raf-Erk1/2 signaling pathways) were activated, and AQP1 expression was decreased during the AA-I induced EMT on HK-2 cells. With the presence of TGF-β1 receptor inhibitor (LY364947) and Erk1/2 inhibitor (PD98059), AQP1 expression was altered by PD98059, suggested that AQP1 could be adjusted by Erk1/2 signaling. Moreover, the inhibitory effect of AA-I on AQP1 was stronger than that of TGF-β1, suggested that AQP1 may be an important target on AAN clinical therapy.

Rhein is one of the main active compounds in total rhubarb anthraquinones (TRAs) that were reported to cause nephrotoxicity. This paper explored the mechanism of how rhein induced apoptosis in human renal proximal tubular epithelial cells (HK-2 cells). In this study, rhein was found to induce apoptosis in HK-2 cells according to the results of annexin V/PI staining assay. The underlying mechanisms were investigated, and the mitochondria-mediated pathway was found to be critical. A series of related biological events were explored, including the disruption of mitochondrial membrane potential (MMP), the decrease of the ATP level, the release of cytochrome c (Cyt-c) from the mitochondrion to the cytosol, and down-regulation of Bcl-2 and up-regulation of Bax. Furthermore, rhein significantly increased the levels of ROS and inhibited the expression of mitochondrial uncoupling protein 2 (UCP2). UCP2 inhibition dramatically boosted oxidative stress and exacerbated rhein-induced apoptosis, whereas co-incubation with an ROS scavenger -acetylcysteine (NAC) could decrease rhein-induced apoptosis. In conclusion, our results have demonstrated that rhein induced apoptosis in HK-2 cells the UCP2-related mitochondrial pathway and rhein might be a weak inhibitor of UCP2. Our findings provide new evidence that UCP2 plays an important role in the mitochondrial apoptotic pathway.