INHIBITION OF VASCULAR ENDOTHELIAL GROWTH FACTORS BY CHALCONES
While rationally developing some potential angiogenic inhibitors, either from nature or (semi-)synthetic, VEGF-1 and VEGF-2 remained the prime target for the pharmacotherapeutics (Figure 11.1) (Ferrara et al., 2003). These are solitarily formed in the cells and promote neovascularization. During the embryonic stages, these are vastly expressed and play a critical role in fetal vasculogenesis (Ferrara, 2002). In adults, these play a central role in the formation of blood vessels under injury conditions and cardiac blockade. Under tumor conditions, these are largely overexpressed and lead to swift growth of tumor and its spreading across the body due to the enormous availability of nutrients and oxygen in the metabolizing cells (McColl et al., 2004).
FIGURE 11.1 Illustration of the VEGF pathway.
The family of VEGF genes comprises of seven members, which include VEGF-E, a viral genome-derived component. VEGFs play an essential role in the cardiovascular system, atherosclerosis, cardiac myofibroblasts, central nervous system, nonendothelial cells, bones, hematopoietic cells, hematological malignancies, autocrine signaling, tumor cells, etc. (Przybylski, 2009). VEGF-A, VEGF-B, VEGF-C, and VEGF-D have been glimpsed to regulate the process of angiogenesis at early embryogenesis, show vascular permeability activity, have proangiogenic activity, stimulate cell migration in macrophage, and are considered as significant regulators of lymphangiogenesis (Nagy et ah, 2007). VEGFs of diverse origin have proangiogenic characteristics for the development and maintenance of blood vessel physiological levels to overcome ischemic diseases. Based on the conditions, inhibitors of angiogenesis have been developed and preclinically evaluated to determine, but none of them proved very promising for a complete cure (Baka et ah, 2006). Multikinase inhibitors and anti-VEGF-A neutralizing antibody have been produced as an alternative way of treatment; keeping possible side effects aside, however, the clinical efficacy is under question (Ferrara and Kerbel, 2005). Chalcone-based compounds have been reported veiy recently as promising inhibitors of VEGF and may be applied in the near future as angiogenesis inhibitors with several advantageous pharmacodynamics and pharmacokinetics attributes.
2-Hydroxy-4-methoxychalcone (1) has been found to exhibit in vivo anti- angiogenic activity by complete inhibition of COX-2 enzyme. The inhibition of COX-2 enzyme directly or indirectly produced a marked suppression of the proliferative nature of the cells (Lee et al., 2006).
A natural chalcone obtained from the dried flower of Cleistocalyx oper- culatus, 2,4-dihydroxy-6-methoxy-3,5-dimethylchalcone (2) (Figure 11.2), produced reversible inhibition of kinase domain receptor (KDR) tyrosine kinase phosphorylation (VEGF). VEGF is a key factor responsible for the growth of human vascular endothelial human dermal microvascular endothelial cells (HDMECs) and ultimately results in angiogenesis. The inhibition of VEGF leads to an acute reduction in the tumor vessel density and eventually causes in vivo antiangiogenic activity (Zhu et al., 2005).
Two novel methoxylated chalcones, 2,2',4'-trimethoxychalcone (3) and 3'-bromo-2,4-dimethoxychalcone (4), demonstrated significant antiendothe- lial characteristics in immortalized human microvascular endothelial cells (HMEC-1) and thwarted the angiogenic growth by preventing the tumor- induced neovascularization (Bertl et al., 2004).
Chalcone (5) has been observed as a potential antiangiogenic candidate by producing considerable inhibition of VEGF-induced migration and invasion of НерЗВ and human umbilical vein endothelial cells (HUVECs) (under nontoxic concentrations) as well as the inhibition of HIF-1 by downregulating the expression of HIF-la (under hypoxic conditions) (Wang et al., 2015).
The potential of 4'-hydroxy chalcone (6) in suppressing multiple-step angiogenesis was studied by Varinska and co-workers. The natural product showed no cytotoxic effects and effectively inhibited VEGF and fibroblast growth factor (FGF)-induced phosphorylation of extracellular signal-regulated kinase (ERK)-l/-2 and Akt kinase, which noticeably restricted the endothelial cell proliferation, their migration, and fonnation of tubules. The suppression of growth factor pathways translated into in vitro angiogenic attributes of endothelial cells (Varinska et al., 2012).
With antiangiogenic prospective, hydrophilic chalcones (7 and 8) have been screened against human mammary adenocarcinoma (MCF-7) cell line
FIGURE 11.2 Chalcones as VEGF inhibitors.
and multiple myeloma (RPMI-8226) cell line with an analogous motif. The study expressed tremendous antiangiogenic activity through the regulation of various pathways at low micromolar concentrations (Syam et al., 2012).
Quite similarly, chalcone molecules (9-12) demonstrated significant activities (in micromolar concentrations) against human adenocarcinoma cell line HT-29, human breast adenocarcinoma cell line MCF-7, human prostate cancer cell line PC3, human normal liver cell line WRL-68, and human lung adenocarcinoma cell line A549 (Nelson et al., 2013).
From the ethyl acetate extract of bark of Broussonetia papyrifera, brous- sochalcone A (13) was isolated and in vitro investigated for antiangiogenic ability against ER-positive breast cancer MCF-7 cells, where excellent results were perceived due to target modulatory effects (Guo et al., 2013).
Parasiticin-A, -B, and -C, three novel chalcone derivatives along with other four chalcone compounds, have been isolated from Cyclosorus parasiticus leaves and screened against six human cancer cell lines (A549, MDA-MB-231, ALL-SIL, HepG2, and MCF-7). The in vitro exploration demonstrated extensive cytotoxicity (micromolar IC50 values) along with the significant antiangiogenic activity. Parasiticin-C (14) and 2,4-dihydroxy-
6-methoxy-3,5-dimethylchalcone (2) showed the highest antiangiogenic activity (Wei et al., 2013).
In an interesting study, a natural chalcone, butein (15) has been scrutinized as potent in vivo suppressor of microvessel formation. The chalcone inhibited the major factor VEGF, which prevents sprouting of the minor vessels from the aortic rings (Chung et al., 2013).
Chalcone-based pigment, hydroxysafflor yellow A (16), obtained from the flower petals of Carthamus tinctorius, presented antiangiogenic activity by downregulating the (3FGF mRNA expression and VEGF in transplanted gastric adenocarcinoma (Xi et al., 2009).
