Downregulation in the mRNA expression of nuclear hormone receptor liver-X-receptor alpha (LXR-α) by TNF-α is abolished by the antioxidant kaempferol, but not ascorbic acid, in human hepatocarcinoma HepG2 cells

Background: Liver-X-Receptor alpha (LXR-α) plays an essential role in cholesterol metabolism, bile acids, and lipid synthesis. The proinflammatory cytokine TNF-α has been shown to downregulate the expression of LXR-α.

Objectives: We investigated the effects of antioxidants kaempferol and ascorbic acid on LXR-α mRNA expression in TNF-α stimulated HepG2 cells.

Methods: After the cells have reached 60% to 70% confluence, they were treated with 20 ng/ml of TNF-α for 24 hours prior to the stimulation with different concentrations of kaempferol (1 μM, 5 μM, 10 μM, and 20 μM) or ascorbic acid (15 μM, 150 μM, and 1500 μM) for another 24 hours. Total RNA was isolated using TRI Reagent LS (Molecular Research Centre). Reverse transcription quantitative real-time PCR was then conducted to determine the mRNA expression of LXR- α after their respective treatments.

Results: Kaempferol was able to relieve the downregulatory action of TNF-α on LXR-α in a dose-dependent manner. The highest dose of kaempferol (20 μM) almost abolished the action of TNF-α by restoring the levels of LXR-α to its basal levels in HepG2 cells. Twenty μM of kaempferol alone did not produce any significant effect on LXR-α mRNA expression. In contrast, ascorbic acid was unable to counteract the action of TNF-α. In contrast to kaempferol, the highest dose of ascorbic acid (1500 μM) suppressed the expression of LXR-α mRNA in TNF-α stimulated HepG2 cells to 0.571-fold compared with no treatment. In the presence of 1500 μM of ascorbic acid alone, LXR-α mRNA expression was also significantly suppressed to 0.320-fold.

Conclusion: This study demonstrated that kaempferol, but not ascorbic acid, was able to alleviate the TNF-α downregulatory effects on LXR-α mRNA in HepG2 cells. Kaempferol can be a better anti-inflammatory agent than ascorbic acid.