Ethanolic extract of propolis from the Malaysian stingless bee Geniotrigona thoracica inhibits formation of THP-1 derived macrophage foam cells

Abstract

Propolis is resinous materials with various therapeutic potentials used by bees to protect hive. Propolis extracted from honey bees may inhibit atherosclerosis, but no study has been performed using propolis extracted from Indo-Malayan stingless bees. The study aims to investigate the effect of the ethanolic extract of propolis (EEP) from Geniotrigona thoracica (G. thoracica) on the formation of macrophage-derived foam cells. Propolis from G. thoracica was extracted using 80% ethanol. The optimal concentrations of EEP to treat THP-1 derived macrophages were determined. THP-1 cells were differentiated into macrophages using 100 ng/mL of Phorbol 12-myristate-13-acetate for 4 days. THP-1 derived macrophages were then treated with 80 µg/mL of oxidized low-density lipoprotein (oxLDL) and 20 µg/mL of EEP at 6, 24, and 48 hours. The lipid droplets were determined by Oil Red O staining, and the amount of cholesterol ester in the cells was quantified. Concentrations of interleukin 1 beta (IL-1β) and tumor necrosis factor alpha (TNF-α) secreted by treated cells were measured. Twenty µg/mL of EEP was selected for cell treatment. Microscopic observation showed that EEP reduced lipid droplet aggregation in the cytosol of THP-1 derived macrophages-treated oxLDL. Similarly, cholesterol ester contents in oxLDL-treated THP-1 derived macrophages were also significantly reduced with the addition of EEP. EEP treatment also significantly reduced TNF-α and IL-1β secretion in the supernatant of oxLDL-treated THP-1 derived macrophages. In conclusion, EEP inhibited the formation of THP-1 derived macrophage foam cells by suppressing TNF-α and IL-1β secretion, which normally is upregulated in atherosclerotic plaques.