Apoptosis and Angiogenesis: Breast Cancer
Normally, angiogenesis and apoptosis function as the natural regulators of cellular activity, proliferation and programmed cell death. As defined by Harmey (2004), “angiogenesis is regulated by a network of inducing and inhibiting factors under physiologic conditions, whereas in pathologic conditions, such as regulation is altered or absent” (p.40). According to Gasparini (2001), angiogenesis normally permits growth and development, cellular repair, and wound healing by increasing the vascularization within the damaged region, which consequently increases blood circulation and cellular nutrient distribution.
Meanwhile, apoptosis or “programmed cellular death” acts as the natural regulator of cellular quantity directed by DNA transcriptase and cellular mediators controlling cellular proliferation and the allowed number of differentiated cells within the area (Coleman and Tsongalis, 2001 p.357). Angiogenesis and apoptosis have been the principal elements of focus in clinical cancer studies over the past decade acknowledging their pathological role in tumor growth, invasion of cancer cells, and metastasis in many cancer malignant cases (Harmey, 2004 p.40). Physiologically, apoptosis targets the molecular pathways of DNA replication during the normal cell cycle deregulating and interrupting the cellular repair through mitosis as well as DNA replication, which consequently inactivates the cellular regulating mechanism known as the “programmed cellular death (Coleman and Tsongalis, 2001 p.357). Basically, tumor development turns-off the natural regulator of cellular proliferation by genetically derailing apoptotic mechanism, while at the same time stimulating further cellular proliferation via angiogenesis.
a. Biological Role: Apoptosis and Angiogenesis
Angiogenesis and deactivation of apoptosis both contribute a major role in the process of tumor formation, cancer development and metastasis. Invasive breast cancer is usually caused by significant apoptotic-angiogenic reactions marked by abnormal stromal-epithelial interactions consisting of (a) fibroblast growth factor, (b) increased angiogenic factors, and (c) genetic expressions (Senaratne, Pirianov and Mansi et al., 2000). These angiogenic and apoptotic elements facilitate a tumor-stimulating microenvironment enhanced by growth factors, evasion of apoptosis, and rich vascularization (Atluri, Karakousis and Porrett et al., 2005 p.71).
In angiogenesis, the physiological highlight of tumor development is the formation of new vessels intricately especially during endothelial proliferation and metastasis (Harris, Lippman and Morrow et al., 2004 p.1163). A network of inducing and inhibiting angiogenic factors trigger the process of angiogenesis targeting the most potent vascularities for the tumor growth stimulation. According to Harris, Lippman and Morrow et al. (2004), tumor growth is generally impossible to facilitate without the tumor angiogenesis since angiogenic-formed vessels are responsible for tumoral nutrition and blood circulation (p.1163). Pro-angiogenic factors, specifically (a) vascular endothelial growth factor (VEGF) or vascular permeability factor (VPF), (b) acidic fibroblast growth factor (aFGF), (c) basic fibroblast growth factor (bFGF), and (d) epidermal growth factor (EGF), play major contributions in tumor angiogenesis marking the possible disease outcome and prognosis of breast cancer (Harmey, 2004 p.40). Pro-angiogenic factors enable further vascularization within the tumor microenvironment permitting the additional cellular supplementation that aids in the speeding of tumor growth. According to Harmey (2004), VEGF is known as the marker of on-going angiogenesis usually identifiable during the first stage of breast cancer development (p.42). VEGF stimulate neovascularization or tumor growth, while other pro-angiogenic factors (i.e. FGF and EGF) induce tumor growth and metastatic action (Atluri, Karakousis and Porrett et al., 2005 p.71). According to Presta, Dell-Era and Mitola et al. (2005), FGF – a heparin-binding growth factor – interact with various endothelial cell surface receptors (e.g. tyrosine kinase receptors, heparin-sulfate proteoglycans, etc.) together with EGF in order to induce neovascularization.
Meanwhile, the biological role of apoptosis in breast cancer development involves protein molecules capable of inhibiting apoptotic mechanism, namely (1) Bcl-2 family members, (2) Akt Pathway inhibitors, (3) inhibitor apoptosis protein (IAP), and (4) FLIP (apoptosis inhibitor) protein (Lipkowitz, 2003 p.75). Anti-apoptotic proteins inhibit normal regulation of cellular proliferation, especially when overexpressed, transforming these proteins into oncogenes capable of contributing to tumor growth (Ryan, Philips and Vousden, 2001). According to Lipkowtiz (2003), Bcl-2, Bcl-xL, Akt ErB-2 and PTEN inactivation contribute to the deregulation of apoptosis triggering the uncontrolled cellular proliferation, and eventually leading to tumor formations (p.75). Overexpressed protein oncogenes predominantly Bcl-2, Bcl-xL and Akt proteins suppress the cellular death by damaging p53 gene responsible for apoptotic action located within the DNA strands (Lowe and Lin, 2000). Apoptotic protein function mutated by several angiogenic elements (e.g. FGF, EGF, etc.) losses the suppressing function of p53 gene consequently deactivating apoptosis and deregulating cellular proliferation.
b. Multiple Genetic Pathways
Both angiogenesis and apoptosis play significant role in the alteration of genetic pathways responsible for the suppression of tumor development and breast cancer development. According to Harris, Lippman and Morrow et al. (2004), angiogenesis involves two critical phases, namely (1) activation and (2) resolution. During activation phase, cellular degradation of basement membrane, endothelial cell migration, and invasion of extracellular matrix occur allowing the proliferation of endothelial cells and formation of capillary lumen (p.1163). In this phase of angiogenesis, VEGF is able to increase vascular permeability by using itself as an endothelial cellular mitogen capable of binding to vascular receptors (Harmey, 2004 p.42). According to Skobe, Hawighorst and Jackson (2001), once VEGF and other pro-angiogenic proteins attached to vascular receptors and triggered neovascularization, DNA deregulation starts to occur brought by the increased vascular activity, permeability and capillary flow permitting the overexpression of anti-apoptotic proteins (e.g. Bcl-2, Bcl-xL, Akt ErB-2, etc.), which transform these components into cancer-inducing oncogenes. Normally during the resolution phase, maturation and stabilization of newly formed vascular networks via pericytes occur enabling various control mechanisms, such as (a) inhibition of endothelial proliferation, (b) reconstitution of basement membrane, and (c) junctional complex formation (Harris, Lippman and Morrow et al., 2004 p.1163). However, overexpressed oncogenes brought by anti-apoptotic mechanism and neovascularization by pro-angiogenic components start to cause generalized mutation of regulator genes, such as p53, Bcl-2, ced-9, etc., consequently triggering the deregulation of the body’s normal apoptotic action (Mirza, Mirza and Vlastos et al., 2002).
Clinical studies (Lowe and Lin, 2000; Mirza, Mirza and Vlastos et al., 2002) suggest the importance of genetic pathways traced through the apoptotic gene deactivation (Bcl-2 and p53). According to Skobe, Hawighorst and Jackson et al. (2001), VEGF-C overexpression is the responsible angiogenic factor inducing growth to the mammary tissues, which chronically trigger tumor formation and eventually mutation of breast apoptotic genes, such as BRCA1, BRCA2 and COX-2. According to Hedenfalk, Duggan and Chen et al. (2001), an analysis of variance between gene expression and genotype using gene samples (n=176) derived from breast cancer resulted to the overexpression of BRCA1 and BRCA2. Meanwhile, the study of Costa, Soares and Reis-Filho et al. (2002) suggests the variable of COX-2 expression – derived through immunochemistry – in detecting breast carcinomas since the density of capillary vessels are higher in patients (n=8 out of 46) with such biological expression. As proposed by the study, COX-2 expression has been found significantly associated with high apoptotic index (p=0.03) and tumoral angiogenesis (p=0.03) (Costa, Soares and Reis-Filho et al., 2002).
c. Prognostic Significance of Apoptosis and Angiogenesis
Protein expressions through immunohistochemistry (IHC) often provide significant prognosis of the disease. In angiogenesis, the peritumoral vascularization during the early the early phase of invasive breast cancer has been associated with the prognosis of the disease (Harmey, 2004 p.40). After studying three essential angiogenic peptides, namely VEGF, platelet derived-endothelial cell growth factor (PD-ECGF), and aFGF and bFGF, Gasparini (2001) has concluded VEGF as the most powerful prognostic indicator of angiogenesis and tumor development. According to the early review study of Gasparini (2000), VEGF has been found to possess significant correlation with relapse-free survival and/or overall survival among patients with low-angiogenic tumors (8 out of 9 published retrospective studies on VEGF). Meanwhile, according to Bronchud, Foote and Robinson (2000), anti-apoptotic overexpressions of the genes, BCL-1 and BCL-2, suggest good prognosis of breast cancer correlated with their decreased apoptotic and cellular proliferative scores, which indicate an anti-apoptotic action facilitating a cancer-enhancing environment (p.245). Current therapies of breast cancer utilize (a) the re-activation of apoptosis via chemical reagents (e.g. Biphosphonates, etc.) and (b) anti-angiogenic factors (e.g. anti-VEGF, angiostatin, endostatin, etc.) in order to inhibit further cellular proliferation and adhesion of cancer cells to the nearby areas of the breasts (Senaratne, Pirianov and Mansi et al., 2000). Indeed, therapies and prognosis currently being used involve the overexpressions of gene and protein substances stimulated by anti-apoptosis and pro-angiogenic factors.
In conclusion, angiogenesis and apoptosis play major roles in the micropathology of breast cancer by creating a tumor/cancer-conducive cellular environment. Normally, angiogenesis aids in growth and development by developing necessary vascularization to promote cellular proliferation, which is on the other hand controlled by apoptotic function. In breast cancer formation, the process of tumor angiogenesis with its pro-factors stimulates neovascularization capable of overnourishing cells and triggering mutation. Cellular mutation deregulates apoptotic genes overexpressing affected genes in the tissues of the breast, which theoretically trigger breast cancer. Hence, detection of gene and/or protein overexpressions provide good disease prognosis.
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