Induction of apolipoprotein A-I gene expression by black seed (Nigella sativa) extracts

Context: Black seed [Nigella sativa L. (Ranunculaceae)] has been shown in animal models to lower serum cholesterol levels. Objectives: In order to determine if extracts from black seed have any effects on high-density lipoprotein (HDL), we characterized the effects of black seed extract on apolipoprotein A-I (apo A-I) gene expression, the primary protein component of HDL. Materials and methods: Hepatocytes (HepG2) and intestinal cells (Caco-2) were treated with black seed extracts, and Apo A-I, peroxisome proliferator-activated receptor alpha (PPAR alpha), and retinoid-x-receptor a (RXR alpha) were measured by Western blot analysis. Apo A-I mRNA levels were measured by quantitative real-time polymerase chain reaction and apo A-I gene transcription was measured by transient transfection of apo A-I reporter plasmids. Results: Extracts from black seeds significantly increased hepatic and intestinal apo A-I secretion, as well as apo A-I mRNA and gene promoter activity. This effect required a PPAR alpha binding site in the apo A-I gene promoter. Treatment of the extract with either heat or trypsin had no effect on its ability to induce apo A-I secretion. Treatment with black seed extract induced PPAR alpha expression 9-fold and RXR alpha expression 2.5-fold. Furthermore, the addition of PPAR alpha siRNA but not a control siRNA prevented some but not all the positive effects of black seed on apo A-I secretion. Discussion: Black seed extract is a potent inducer of apo A-I gene expression, presumably by enhancing PPAR alpha/RXR alpha expression. Conclusions: We conclude that black seed may have beneficial effects in treating dyslipidemia and coronary heart disease.