4-Hydroxy-2-hexenal-induced apoptosis in human renal proximal tubular epithelial cells

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.

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.

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.

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.