The tumour microenvironment is closely correlated with the malignant degrees, metastasis, and recurrence of tumours. Besides, the acid environment, oxygen deficiency, and other inducible factors may severely affect the efficacies of routine therapies, radiotherapy and chemotherapy. Recent studies have also proved that many Chinese herbs could fight against tumour vascular angiogenesis, lower serum VEGF concentration, and inhibit expressions of VEGF. This may lead to the development of new potential antiangiogenic drugs.
Angiogenesis
Angiogenesis, the sprouting of new capillaries, is required for the development of the vascular system and, consequently, the growth of vertebrates. Angiogenic proteins, including several from the fibroblast growth factor family were found to be mitogenic not only for vascular endothelial cells but also for a wide variety of other types of cells and appeared to promote angiogenesis as part of coordinated tissue growth and repair. In the late 1980s the first selective angiogenic growth factor was purified on the basis of its ability to induce transient vascular leakage and endothelial cell mitogenesis called vascular endothelial growth factor (VEGF)/vascular permeability factor (VPF) (Neufeld et al 1994). The identification of VEGF (Ferrara 1993) set the stage for a rapid expansion in the understanding of what now appears to be one of the most important mediators of physiologic and pathologic angiogenesis yet discovered.
Transcription of VEGF mRNA is induced by a variety of factors. Serum-derived and paracrine growth factors and cytokines, including Platelet-Derived Growth Factor BB (PDGF-BB), basic fibroblast growth factor (bFGF) (Sipos et al 2002), epidermal growth factor, tumor necrosis factor α (Frank et al 1995), nitric oxide (Frank et al 1999), transforming growth factor-β1, and interleukin-1β (Li et al 1995; Jung et al 2001), can each induce expression of VEGF from 3- to 20-fold in a variety of cultured cells.
Hypoxia
Without an independent blood supply, tumours must rely on diffusion to obtain oxygen and other nutrients, and typically cannot grow more than 2-3 mm in size. Thus, a growing tumour without sufficient vasculature will have hypoxic areas.
In response to hypoxic conditions, tumours secrete vascular endothelial growth factor (VEGF) in order to recruit new vasculature, which then provides a supply of oxygen (Gimbrone et al., 1972). Hypoxia is known to induce angiogenesis, thereby providing a compensatory mechanism by which tissues can increase oxygenation. Therefore, diminished O2 is one of the most intriguing transcriptional inducers of VEGF (Shweiki et al 1992) and its receptors (Tuder, Flook & Voelkel 1995) in normal and transformed cells. Hypoxic induction of VEGF appears to be a general response since many types of cultured cells have been observed to increase VEGF mRNA levels by approximately 10-50-fold as a consequence of lowering the percentage of O2 from ambient 21% to the range of 0-3% (Sipos et al 2002).
Vascular permeability factor (VPF)
The microvasculature of tumours is hyperpermeable compared with that of most normal tissues and as a consequence, fluid and plasma accumulate in the interstitium of solid tumors (Heldin et al 2004) and this barrier is an obstacle in tumour treatment, as it results in inefficient uptake of therapeutic agents. Vascular permeability factor (VPF), also known as vascular endothelial growth factor (VEGF), is a multifunctional cytokine expressed and secreted at high levels by many tumor cells of animal and human origin. VPF/VEGF is likely to have a number of important roles in tumor biology related, but not limited to, the process of tumor angiogenesis. As a potent permeability factor, VPF/VEGF promotes extravasation of plasma fibrinogen, leading to fibrin deposition, which alters the tumor extracellular matrix. This matrix promotes the ingrowth of macrophages, fibroblasts, and endothelial cells. Moreover, VPF/VEGF is a selective endothelial cell (EC) growth factor in vitro, and it presumably stimulates EC proliferation in vivo. Furthermore, VPF/VEGF has been found in animal and human tumor effusions by immunoassay and by functional assays and very likely accounts for the induction of malignant ascites. In addition to its role in tumors, VPF/VEGF has recently been found to have a role in wound healing and its expression by activated macrophages suggests that it probably also participates in certain types of chronic inflammation (Senger et al 1993; Baban & Seymour 1998). Although VEGF is known to be a powerful growth factor for therapeutic angiogenesis/vascularization in the ischemic hind limb and myocardium, it has other activities that can increase the proliferation and permeability of capillary endothelial cells. These activities may produce unwanted side effects, such as tumor angiogenesis, vascular leakage, oedema, and inflammation (Chae et al, 2000).
Medicinal herbs and their phytochemicals are potential novel leads for developing antiangiogenic drugs. Jeong et al., (2011) conducted a review that aimed to assess the current status of research with medicinal herbs and their phytochemicals for the development of antiangiogenic agents for cancer and other angiogenesis-related diseases including inflammation, diabetic retinopathy, endometriosis and obesity. Most studies reviewed have focused on vascular endothelial growth factor (VEGF)/vascular endothelial growth factor receptor 2 (VEGFR-2) signaling for endothelial response processes and have led to the identification of many potential antiangiogenic agents.
Since human clinical trials with antiangiogenic modalities targeting VEGF/VEGFR-2 signaling have shown limited efficacy and occasional toxic side effects, screening strategies for herbal phytochemicals based on other signaling pathways important for cancer-endothelial and stromal crosstalks should be emphasized in the future.
Reference
Ferrara N. (1993) Trends Cardiovasc. Med. 3, 244–250
Neufeld G, Tessler S, Gitay-Goren H, Cohen T & Levi B-Z. (1994) Prog. Growth Factor Res. 5, 89–97
Tuder RM, Flook BE & Voelkel NF. (1995) J. Clin. Invest. 95, 1798–1807
Cordyceps sinensis
The aqueous extract of Cordyceps sinensis (Cs), one of the traditional Chinese medicines, has been used for the treatment of a wide range of disorders for centuries. It is generally accepted that its cultivated Cs fungi possess the same functions as Cs natural herbs. Although polysaccharide from Cs is one of its bioactive compositions, its antitumor ability has not been confirmed. In a study, Yang et al., (2005) investigated the effects of the exopolysaccharide fraction (EPSF) of a cultivated Cs fungus on c-Myc, c-Fos, and vascular endothelial growth factor (VEGF) expression of tumor-bearing mice. The mice (C57BL/6) were administered three different doses of EPSF peritoneally every 2 days, starting from the day of implantation of B16 melanoma cells through their tail veins for 27 days (14 times).
Sections from mouse paraffin-embedded liver and lung tissues were subjected to immunohistochemical analyses. The results of c-Myc, c-Fos, and VEGF expression were analyzed using SimplePCI image analysis software. The c-Myc, c-Fos, and VEGF levels in the lungs and livers of EPSF-treated mice were found to be significantly lower than those of untreated mice (p<0.05). This suggests that EPSF had inhibited tumor growth in the lungs and livers of mice, and that it might be a potential adjuvant in cancer therapy.
Reference
Pathol Res Pract. 2005;201(11):745-50. Epub 2005 Oct 19.
Ligustrazine
Ligustrazine is isolated from Ligustici Chuangxiong and can significantly inhibit the growth of vascular endothelial cell line (VEC-304), induce VEC-304 apoptosis and down-regulate the expression of VEGF (Peng, Jiang, & Wu, 2006).
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Ginsenoside Rg2
Ginseng saponins 20(S)-ginsenoside Rg2 extracted from cultured Panax notoginseng cells in a fermenter show a protection effect on human umbilical cord vein endothelial cells (VEC-304) from H2O2-induced cell apoptosis. When 50 mg/ml 20(S)-ginsenoside Rg2 was present in the culture medium for 8 h, the H2O2-damaged VEC-304 cells acquired about 11-fold ( p < 0.01) on the amount and about 2-fold ( p < 0.05) increase in PA activity compared with those untreated cells. And the Rg2 has a strong ability in scavenging intracellular ROS induced by H2O2 (Xin et al., 2005).
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