Promotion of mammalian angiogenesis by plant-derived secondary metabolites

Abstract

Historically, plant natural products and their derivatives have been invaluable sources of therapeutic agents to enhance human health and to treat disease. Angiogenesis is a complex and highly regulated process of generating new capillary networks from mature blood vessels. Failure to grow blood vessels has been reported to be associated with diseases including stroke, cardiovascular disorders and wound healing. However, pro-angiogenic drugs are less researched than anti-angiogenic drugs. Soybean is a source of nutrition and it contains a number of small metabolites that confer many beneficial health effects. Some of these metabolites have been found to modulate angiogenesis in animal tissue. Therefore, soybean-derived secondary metabolites and their potential therapeutic applications were the focus of this thesis. In chapter 3, a bioassay-directed discovery approach utilising size exclusion and liquid chromatography enabled the isolation and purification of a number of bioactive fractions from soybean xylem sap. Using high resolution accurate mass spectrometry and NMR, the structure of two pro-angiogenic molecules were elucidated and shown to be erythro-guaiacylglycerol-8-O-4'-coniferyl alcohol (FK1), and threo-guaiacylglycerol-8-O-4'-coniferyl alcohol (FK2). Independently sourced samples of FK1 and FK2 exhibited comparable pro-angiogenic activity to the soybean molecules. The mode of action of these molecules was investigated in chapter 4 by studying their effect on endothelial cell proliferation, migration, tube formation and adhesion to the extracellular matrix (ECM) components, fibronectin and vitronectin. These compounds enhanced endothelial cell proliferation and endothelial cell tube formation on an artificial ECM, Matrigel, but did not affect endothelial cell migration or adhesion to fibronectin and vitronectin. It is proposed that FK1 and FK2 might enhance angiogenesis via potentiating the potent mitogen, bFGF, and/or its downstream signaling in endothelial cells. Flavonoids are implicated in positively influencing a number of human conditions and disease by their anti-tumour, anti-inflammatory and anti-oxidant activities. However, the bioactivities of these compounds have been mostly studied at high concentrations (more than 1 micromolar) in in vitro and in vivo experiments and at these concentrations are physiologically irrelevant. Chapter 5 examined the potential pro-angiogenic activity of a series of eighteen flavonoids at serum concentrations resulting from a normal diet. Seven flavonoids including genistein and naringenin showed significant promotion of angiogenesis at sub-micromolar concentrations while genistein inhibited angiogenesis at high micromolar concentrations. By examining structure-activity relationships we found that pro-angiogenic molecules lack a C2-C3 double bond. In conclusion, we have explored the potential of plant-derived secondary metabolites to promote angiogenesis in mammals. With novel bioactivity and a possible novel mechanism of action, FK1 and FK2 may have the potential to be developed for the therapeutic treatment of aberrant angiogenesis-related conditions such as cardiovascular disease, ischemia, stroke, chronic wounds and hypertension. Moreover, pro-angiogenic activity of dietary flavonoids, genistein and naringenin, at physiological concentrations has given some insight into the importance of measuring the bioactivity of natural-derived compounds at their physiological concentrations. Show more