Characterisation of pomegranate juice polyphenols identifies oenothein B as an endothelium-dependent vasodilator and regulator of endothelial function that enhances actions of oligomeric procyanidins

Abstract Background Cardiovascular disease mortality is reduced in individuals with high daily consumption of dietary polyphenols. Optimal daily intakes, choice of polyphenol, and mechanism of action have yet to be fully defined. Pomegranate juice (PJ) reduces blood pressure and improves endothelial function. These effects are attributed to the high levels of polyphenols, primarily ellagitannins. Purpose The aims of these investigations were (1) to identify PJ polyphenol(s) that altered endothelial function and could potentially lower blood pressure, and (2) to compare these endothelial actions to oligomeric procyanidins (OPC) purified from apple polyphenol extracts. Methods PJ polyphenols were purified by Sephadex LH-20 chromatography and RP-HPLC, and identified by LC-MS/MS and NMR. Fractions were screened for biological activity using cultured bovine aortic endothelial cells by measuring the degree of inhibition of endothelin-1 synthesis. Endothelial actions were further assessed by measuring changes in intracellular Ca2+ using FLIPR Ca2+-6 assay, qRT-PCR (EDN1, eNOS and KLF2 mRNAs), and NO synthesis (nitrite). Results Six PJ polyphenols were isolated. These were identified as punicalin, four punicalagin isomers, and oenothein B (OTB, a cyclic dimeric ellagitannin; LC-MS [M-H]-= 1567.1). At concentrations ≤5 µg/ml only OTB inhibited ET-1 synthesis (IC50 1.5 µg/ml). The identity of OTB was confirmed by NMR, and by comparison with OTB purified from Epilobium species. Further characterisation of the actions of OTB on endothelial cells showed pre-incubation with 1.25, 2.5 and 5 µg/ml OTB increased binding of biotinylated-OPC by 1.7, 2.3 and 3.8 fold. Subsequent studies of the interactions between OTB and OPC showed synergistic effects for decreases in ET-1 synthesis, EDN1 mRNA, and increases in KLF2 mRNA, eNOS mRNA and NO release. The effect of combining OTB (1 µg/ml) and OPC (1 µg/ml) lasted for over 24 h (relative to control: ET-1 -57%, EDN1 mRNA -63%, eNOS mRNA +69%, KLF2 mRNA +356%), when individually these polyphenols no longer had any significant effect. Intracellular Ca2+ responses also showed synergistic interactions, however the profile of [Ca2+] changes for OTB and OPC alone differed, suggesting separate signalling pathways initiating these endothelial responses. Endothelium-dependent vasodilator effects of OTB were confirmed on isolated mouse aorta (EC50 0.27 µg/ml). Conclusions These effects of OTB may underlie previously reported decreases in blood pressure and improvements in endothelial function after pomegranate juice consumption. Detailed understanding of the mechanism of action of OTB, and its interactions with OPC could lead to new therapeutic modalities for treating endothelial dysfunction in a broad spectrum of people at risk of cardiovascular disease.