Chemical structure of flavonols in relation to modulation of angiogenesis and immune-endothelial cell adhesion

Abstract

The antioxidant activity of flavonoids has been suggested to contribute to several health benefits associated with the consumption of fruits and vegetables. Four flavonols — myricetin (M), quercetin (Q), kaempferol (K) and galangin (G), all with different numbers of hydroxyl moieties (–OH) — were examined for their antioxidant activity and cytotoxicity on human umbilical vein endothelial cells (HUVECs) and for their potential antiangiogenic and cell adhesion effects. The relative antioxidant capacity of these flavonols in cell culture medium (cell-free system) and their intracellular antioxidant activity were M=Q>K=G, which correlated respectively with the presence of 3, 2, 1 and 0 moieties of –OH on their B-ring. The higher the numbers of –OH moieties on the B-ring the less toxic the flavonol was to HUVEC, and the LD50 was determined as: M (100 μM)>Q (50 μM)>K (20 μM)>G (10 μM). These flavonols at ≈0.5 LD50 doses suppressed the vascular endothelial growth factor (VEGF)-stimulated HUVEC tubular structure formation by: M (47%)>Q (37%)>K (15%)>G (14%), which was not linearly associated with their numbers of –OH moieties. However, the magnitude of flavonols' suppression of activated U937 monocytic cells adhesion to HUVEC was associated with the number of –OH moieties on the B-ring. This was prominent when U937 cells were pretreated with these flavonols. In contrast, the numbers of –OH moiety had no apparent influence on the adhesion or expression of adhesion molecules when activated HUVECs were pretreated with these flavonols. The presence of different numbers of –OH moieties on the B-ring of the flavonols may contribute to their antioxidant activity as well as their toxicity and may play an important role in their potency for biological action such as angiogenesis and immune-endothelial cell adhesion, which, respectively, are important processes in the development of cancer and atherosclerosis.

Introduction

Flavonols are polyphenol compounds possessing two benzene rings joined by a linear three carbon chain (C2, C3, C4), represented as the C6–C3–C6 system. Flavonoids have been suggested to have several potential health benefits due to their antioxidant activities, which are attributed to the presence of phenolic hydroxyl (–OH) moieties on the structure [1], [2]. In molar bases, the antioxidant capacity of some of these flavonoids is much higher than those of vitamins C and E [3]. In addition to –OH moieties in the structural arrangements of flavonols, the resonance of electrons between A- and B-rings is very important for their antioxidant and biological activities. The B-ring –OH moiety is the most significant determinant factor in the scavenging of reactive oxygen species (ROS) [4], [5].

Flavonoids are the important phytonutrient components present in a wide range of fruits, vegetables, nuts and beverages, including wine and tea [6]. Myricetin with three –OH moieties on the B-ring (Fig. 1A) is one of the flavonols present in a large number of plants [7], including tea, berries, fruits, vegetables and medicinal herbs [8]. It is an effective scavenger of free radicals generated by both enzymatic and nonenzymatic systems [9]. It inhibits cancer development induced by polycyclic aromatic hydrocarbons [10] in SENCAR mice. In addition, it has been shown to have antiviral activity by inhibiting the reverse transcriptase [11], antiaggregatory effects on blood platelets [12] and antiatherosclerotic effect via inhibition of oxidative modification of low-density lipoprotein by macrophages [13]. Quercetin (Fig. 1B) is another important flavonol with two –OH moieties on the B-ring and is found in many fruits and vegetables, as well as in olive oil, red wine and tea [14], [15], [16], [17]. Quercetin, in addition to having antioxidant properties, has been suggested to prevent atherosclerosis and chronic inflammation [14], to modulate cell-cycle regulation, to interact with type II estrogen binding sites and to induce tumor cell apoptosis [15], [16]. It may also possess antiangiogenic potential [17]. Kaempferol (Fig. 1C), with one –OH moiety on the B-ring, is widely present in broccoli [18], Ginkgo biloba [19], fruits and vegetables [20], [21], [22]. Several biological activities have been attributed to kaempferol, including inhibition of lipoxygenase and cyclooxygenase [23], [24]. It also possesses antiaggregatory [25], antibacterial [26] and anticancer [27] activities. Galangin (Fig. 1D), with no –OH moiety on the B-ring, is another member of the flavonols and is present in high concentrations of honey and Alpinia officinarum, a spice plant that has also been used as an herbal medicine for a variety of ailments in Asia. Galangin is also present in propolis, which is a resinous material made by bees and is used in many Asian countries for the management of numerous diseases, including respiratory, subcutaneo-mucosal and viral infections [28], [29]. Galangin has been demonstrated to possess several biological actions such as antioxidative and radical scavenging activities [30], [31], antimutagenic [32], [33], anticlastogenic effect [34], [35], anti-inflammatory activity, [36], [37] as well as an inhibitory effect on cytochrome P450 hydroxylase in human liver microsomes [38], [39]. Galangin has been recently proposed as a candidate agent for cancer chemoprevention [40].

We used these flavonols containing different –OH moieties on the B-ring to examine their potential antioxidant activities in in vitro cell culture systems in relation to modulation of angiogenesis, which is necessary for solid tumor growth and immune-endothelial cell adhesion, a process important in the development of atherosclerosis.