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4-Hydroxy-2-hexenal (HHE), the aldehyde product of lipid peroxidation, may be responsible for the pathogenesis of progressive renal disease. Recently, paricalcitol (19-nor-1,25-dihydroxyvitamin D2) was shown to be renoprotective through its anti-inflammatory and antifibrotic effects in various experimental nephropathy models. In this study, we investigated the effects of paricalcitol on inflammation and epithelial-mesenchymal transition (EMT) after HHE-induced renal tubular epithelial cell injury. To investigate the molecular mechanisms underlying HHE-induced renal tubular cell injury, the human proximal tubular epithelial (HK-2) cells cultured with 10 µM HHE in the presence or absence of paricalcitol. In HK-2 cells, paricalcitol attenuated the HHE-induced expression of extracellular signal-regulated kinase, c-Jun N-terminal kinase, and p38 mitogen-activated protein kinase, and prevented nuclear factor-κB (NF-κB) activation. The expression of the inflammatory proteins inducible nitric oxide synthase and cyclooxygenase-2 was attenuated by paricalcitol pretreatment. In addition, HHE increased the expression of the transforming growth factor (TGF)-β/Smad signaling proteins and fibrotic proteins, such as α-smooth muscle actin and connective tissue growth factor; this inducible expression was suppressed by pretreatment with paricalcitol. Treatment with HHE resulted in the activation of the β-catenin signaling pathway, and paricalcitol pretreatment reduced the expression of β-catenin in HHE-treated HK-2 cells. Coimmunoprecipitation shows that paricalcitol induced vitamin D receptor (VDR)/β-catenin complex formation in HK-2 cells. Also immunofluorescence staining revealed that co-localization of VDR and β-catenin in the nuclei. ICG-001, an inhibitor of β-catenin, decreased the expression of TGF-β1 and attenuated HHE-induced tubular EMT. These results show that paricalcitol attenuated HHE-induced renal tubular cell injury by suppressing inflammation and EMT process through inhibition of the NF-κB, TGF-β/Smad, and β-catenin signaling pathways.

Background: The aldehyde products of lipid peroxidation such as 4-hydroxy-2-hexenal (HHE) might be responsible for the pathogenesis of kidney injury. The present study was aimed to investigate the effects of HHE on renal tubular epithelial cells and its signaling mechanisms., Methods: Human proximal tubular epithelial (HK-2) cells were treated with 10 μM of HHE. Cell viability was examined using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The fluorescent probe 2',7'-dichlorofluorescein diacetate was used to measure intracellular levels of reactive oxygen species (ROS). The protein expression of NF-κB, mitogen-activated protein kinase (MAPK), pro-apoptoic Bax and anti-apoptotic protein Bcl-2 was determined by semiquantitative immunoblotting. Apoptosis was assessed by flow cytometry analysis after the cells were stained by fluorescein isothiocyanate-conjugated annexin V protein and propidium iodine., Results: Treatment with various doses of HHE resulted in dose-dependent decreases of cell viability and increases of ROS. HHE increased the expression of p38 MAPK, extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK). HHE induced NF-κB activation and IκB-α degradation. Increased nuclear NF-κB activation was blocked by inhibitors of ERK (PD98059) or JNK (SP600125), but not affected by p38 MAPK inhibitor (SB203580). Flow cytometry analysis revealed HHE-induced apoptosis. HHE decreased the expression of Bcl-2, while it increased that of Bax, which was attenuated by the treatment of NF-κB inhibitor (Bay 11-7082) or N-acetyl-L-cysteine (NAC). An inhibition of NF-κB prevented HHE-induced apoptosis., Conclusions: HHE-induced tubular cell apoptosis is mediated by modulation of Bax and Bcl-2 via ROS generation. HHE-mediated accumulation of ROS may induce redox-sensitive transcription factor, NF-κB, through activation of ERK and JNK, resulting in cellular apoptosis in HK-2 cells.