Tumour Angiogenesis in Breast Cancer

Abstract

Since the last comprehensive assessment of antiangiogenic therapy was published in Breast Cancer Research 3 years ago, clinical trials in a variety of tumour types, including breast cancer, have underscored the key relevance of tumour neovascularization. Bevacizumab, a drug designed to target vascular endothelial cell growth factor, was utilised in many of these studies (VEGF). Clinical trials using antiangiogenic treatment in breast cancer have highlighted the critical role of tumour neovascularization. Personalised medicine will become increasingly important to generate maximum therapeutic benefit to the patient but also to realise the optimal economic advantage from the finite resources available, according to a report by the US Department of Health and Human Services (HHS) and the National Institute for Occupational and Environmental Health (NIH). This overview covers the history of breast tumour neovascularization in both in situ and invasive breast cancer, the processes by which it occurs, and the impact of the microenvironment, with a focus on hypoxia. The regulation of angiogenesis, as well as the antivascular drugs employed in antiangiogenic dosing schedules, both innovative and traditional, are discussed.

1. Introduction

Cancer has the potential to spread to nearby or distant organs, posing a life-threatening threat. For the metastatic spread of cancer tissue, the growth of the vascular network is crucial. Angiogenesis and lymphangiogenesis are the processes by which new blood and lymphatic vessels originate.

Cancer has the potential to spread to nearby or distant organs, posing a life-threatening threat. Tumour cells can enter blood or lymphatic vessels, circulate through the intravascular stream, and then spread to a new location (metastasis) [1]. The growth of the vascular network is crucial for cancer tissue metastatic dissemination. Angiogenesis and lymphangiogenesis are the processes that result in the formation of new blood and lymphatic vessels. Both are necessary for the formation of a new vascular network that will provide nutrients, oxygen, and immune cells while also removing waste. In tumour vascularization studies, angiogenic and lymphangiogenic factors are gaining popularity.

1.1 Angiogenesis in cancer

Endothelial cells, epithelial cells, mesothelial cells, and leucocytes, as well as cancer cells and host cells, all release chemicals that aid in angiogenesis. Plateletderived endothelial cell growth factor (I’D-ECGF), plateletderived growth factor (PDGF).

Angiogenesis is a series of events that are triggered by microvascular endothelial cells. Angiogenesis and lymphangiogenesis are essential for tumour growth and metastasis, and are triggered by chemical signals from tumour cells in the early stages of development. In a prior study, Muthukkaruppan and colleagues [2] looked at how cancer cells behaved when they were placed in different parts of the same organ. Blood circulation was present in the iris, but not in the anterior chamber [2]. Cancer cells without blood circulation grew to a diameter of 1–2 mm³ and then stopped growing when placed in an area where angiogenesis was possible, but they grew to a diameter of more than 2 mm³ when placed in an area where angiogenesis was possible.

If there is insufficient blood flow, tumours can become necrotic or even apoptotic [3, 4]. Angiogenesis thus aids cancer progression. The neovascularization stage of tumour angiogenesis is one of four steps in the process. Local injury to the basement membrane occurs first in tissues. Destruction and hypoxia take place almost immediately. Angiogenic chemicals cause endothelial cells to become activated and move. Endothelial cells multiply and settle in the third step of the process. Angiogenesis is still influenced by angiogenic stimuli, according to the fourth point.

Every 1000 days on average, vascular endothelial cells divide [5]. When tumour tissues need nutrition and oxygen, angiogenesis is induced. Activators and inhibitors of angiogenesis regulate the process. On the other hand, increasing angiogenic factor activity is insufficient to enhance neoplasm angiogenesis. Negative regulators or vascular growth inhibitors must also be inhibited [6].

1.2 Breast cancer: tumour angiogenesis

Clinical studies in a range of tumour types, including breast cancer, have proven the vital role of tumour neovascularization in the 3 years after the last comprehensive review of antiangiogenic therapy was published in Breast Cancer Research [7]. Bevacizumab (AvastinTM; Genentech, South San Francisco, CA, USA) was utilised in many of these trials since it was particularly intended to target vascular endothelial cell growth factor (VEGF). Bevacizumab is a recombinant VEGF antibody that binds to all known isoforms of VEGF-A and blocks receptor interaction, inhibiting angiogenesis and tumour growth. It was made from a mouse monoclonal antibody that had been humanised. One of the successes of antiangiogenic treatment, which was first suggested by Judah Folkman more than 35 years ago, is the critical contribution of this angiogenic factor in controlling many of the processes involved in angiogenesis, as well as its importance as a paradigm for the rational design of an anticancer agent.

Because all tumours (including liquid tumours like leukaemias) are angiogenesis-dependent, angiogenesis is highly restricted in adults, the endothelium of the vessels is accessible, and any treatment would be amplified through subsequent tumour infarction, the antiangiogenic approach has always appealed to researchers. Furthermore, because endothelial cells are non-neoplastic and should have a stable genome, cancer resistance should no longer be an issue [8].

To grow larger than a few centimetres in diameter, breast cancer, like other solid tumours, requires the formation of new blood vessels (neovascularization). The extra veins not only supply more nutrients to the tumour, but they also provide possible pathways for tumour dispersal and metastasis [9].

Tumour-induced angiogenesis first develops in pre-invasive high-grade ductal carcinoma in situ. In this case, a distinctive ring of microvessels emerges around the ducts, which are packed with proliferating epithelial cells. As the tumour grows, the amount of neovascularization increases [10]. Increased microvascular density or development, as well as variables that encourage new vessel growth, have been associated to poor breast cancer prognosis.

As a result, a significant amount of study has been focused on identifying the factors in the tumour microenvironment that promote and maintain angiogenesis in the hopes of limiting neovascularization and, as a result, tumour development and dissemination. Furthermore, unlike tumour cells, which are genetically unstable and can develop resistance to many therapeutic medications fast, normal vascular endothelium lacks mutations that would allow drug resistance [11, 12]. Both research lines are investigated in this paper.

Although the presence of axillary lymph nodes is the most important prognostic marker in operable breast cancer, it does not entirely explain for the wide range of disease outcomes. More precise prognostic indications would aid in the identification of patients at high risk of illness recurrence and mortality who would benefit from systemic adjuvant therapy [13]. Microvessel density (count or grade) in invasive breast cancer (a measure of tumour angiogenesis) is associated with metastasis and so may be a prognostic sign, according to recent research.

Breast tumour growth requires angiogenesis, or the rapid formation of new blood vessels, in order to acquire enough oxygen and nutrients [14]. Breast cancer cells, like all other biological tissues, rely on a vascular network of capillaries to provide food and oxygen on a regular basis. Endothelial cells (ECs), which line the interior surface of blood vessels, do not reproduce, hence capillaries do not proliferate. Hypoxia (low oxygen) triggers a variety of transcriptional responses that are mediated by transcription factors called hypoxia-inducible factors (HIFs) [15, 16, 17, 18]. HIFs are transcription factors that regulate the expression of genes involved in physiological processes like metabolism, angiogenesis, and cell division. Local angiogenesis is one of the tumour microenvironment’s long-term major responses to low O2 levels [19, 20].

It is the fusion of EC precursors that leads to the creation of capillary plexus, which thereafter evolves into blood vessels. Angiogenesis is required for a variety of normal processes, including embryonic development, growth, and wound healing [21].

As a result, the tumour activates an angiogenic switch and enters an irreversible active angiogenic state. Because of the tumour’s newly acquired status, it can recruit new capillaries, restoring oxygen and nutrients to both angiogenic and non-angiogenic cells, resulting in rapid tumour growth [9, 22, 23, 24]. Despite the fact that surgical excision of tumours is the current standard of care for breast cancer, adjuvant therapy, such as anti-angiogenic therapy, has been used after surgery in advanced disease stages when surgery is no longer an option [25].

Angiogenic growth factors, such as vascular endothelial growth factor (VEGF) and fibroblast growth factors, are primarily involved in the initiation and progression of tumour angiogenesis (FGF). Angiogenic factor levels, as well as the number of vascular networks created as a result, have been shown to predict breast cancer survival in many studies. To put it another way, high levels imply that the tumour cells are aggressive and are linked to a poor prognosis. The rate and degree to which blood vessels permeate are controlled by these variables in connection with beginning angiogenesis [26, 27, 28, 29]. Angiogenesis-targeting compounds have recently received a lot of attention in breast cancer research.

Bevacizumab, a humanised anti-VEGF monoclonal antibody, has been the most extensively investigated molecule [30, 31, 32, 33]. After promising results in preclinical trials targeting VEGF, the FDA authorised bevacizumab in 2008 for the treatment of metastatic HER2-negative breast cancer [34, 35].

Following that, multiple anti-angiogenic medicines targeting VEGF or blocking its receptor’s action were licenced, and they are now routinely utilised in the treatment of various malignancies [36, 37, 38, 39, 40]. The FDA, however, revoked its certification in 2011 due to conflicting results from earlier trials and allegations of increased toxicity as a result [41, 42, 43, 44].

While the discovery of these anti-angiogenic drugs and small molecules was heralded as a potential victory in one aspect of the cancer fight, the agents’ modest activities, such as their inability to arrest recurrent tumours in a latent state and the moderate improvement in overall patient survival, dampened the celebration.

1.3 The angiogenic cycle

Endothelial cells in normal, quiescent capillaries are in contact with a laminin-rich basement membrane and a layer of supportive pericytes that is 1- to 2-cell thick. Angiogenesis necessitates the weakening of connections between nearby pericytes as well as the degradation of the basement membrane [45]. The integrin adhesion receptors help endothelial cells re-enter the cell cycle and infiltrate the surrounding stromal matrix. Endothelial cells begin to resynthesize a basement membrane, which aids in cell cycle exit and promotes the creation of a capillary-like morphology [46]. Pericytes are then recruited to newly formed capillaries to help mature arteries stabilise [47]. Chronic exposure to angiogenic factors in the tumour microenvironment that promote basement membrane proteolysis or antagonise endothelial–pericyte interactions leads to the formation of a relatively unstable, highly permeable network of vessels that does not fully mature but can supply nutrients to meet the tumour’s growing metabolic demands. Increased arterial permeability is thought to encourage tumour cell extravasation and, eventually, spread [48, 49].

2. Factors that promote angiogenesis

2.1 Hypoxia

Hypoxia has long been suspected as a significant angiogenic stimulator within the tumour microenvironment. Densely packed, quickly proliferating cells with limited nutritional inputs are the source of low tissue oxygen tension [50]. In recent years, researchers have made tremendous progress in understanding the biochemical and molecular reactions to hypoxia, as well as how the tissue senses low oxygen tension [51]. It was discovered that the hypoxia-inducible factor (HIF), a heterodimeric transcription factor made up of the hypoxic response factor (HIF-1) and the constitutively expressed aryl hydrocarbon receptor nuclear translocator (ARNT or HIF-1), is particularly significant [52, 53].

HIF-1 binds to the von Hippel-Lindau (VHL) protein in oxygenated circumstances, causing ubiquitination and fast destruction [54]. In hypoxic settings, on the other hand, this factor is stabilised: it is unable to associate with VHL protein because prolyl hydroxylase, an enzyme that typically alters HIF-1 to facilitate its interactions with VHL protein, is inactive. As a result, the oxygen sensor has been proposed as prolyl hydroxylase [55, 56, 57, 58, 59].

Animals lacking HIF-1 had markedly reduced angiogenic responses, indicating that it plays a vital role in experimental tumour growth and tumour-associated angiogenesis. Human ductal carcinomas overexpress HIF-1, whereas benign tumours with little angiogenesis do not. In the hypoxic tumour microenvironment, stabilised HIF-1 induces the expression of a variety of proangiogenic mediators, including vascular endothelial growth factor (VEGF) and one of its receptors, VEGF receptor 1 (VEGFR1) [60, 61, 62].

2.2 Vascular endothelial growth factor

VEGF is a powerful and selective endothelium mitogen that can produce a rapid and full angiogenic response, as its name suggests. VEGF (VEGF-A), the most investigated and implicated in tumour-induced angiogenesis, is a family of glycoproteins (VEGF-A, -B, -C, and -D) that are linked to VEGF (VEGF-A). The lymphatic endothelium responds to VEGF-C and -D in a big way [63].

VEGF is produced and released by a range of normal cell types, but its expression is dramatically increased in tumour cells, including a variety of breast malignancies, as well as reactive breast tumour stromal cells [64]. In contrast to other cytokines produced by tumour cells, VEGF functions almost exclusively on endothelial cells because expression of the major VEGF receptor, VEGFR2, is confined to such cells. Interfering with VEGF or VEGFR2 allows for the specific targeting of tumour endothelium [65]. VEGFR1, on the other hand, is expressed by endothelial cells, monocytes, and macrophages, and its role was unknown until recently.

When VEGF binds to its receptor, it activates an intracellular signalling cascade that causes gene expression modifications that promote endothelial cell migration and proliferation [66]. Furthermore, because VEGF not only functions as an endothelium mitogen but also increases capillary permeability, it’s not surprising that the leakiness of tumour arteries is a fundamental distinguishing feature.

2.3 VEGF and breast tumour angiogenesis

An increase in VEGF synthesis by tumour cells and cells in the tumour stroma has been connected to angiogenesis induced by breast tumours, as previously mentioned. VEGFR2 expression was also shown to be greater in the endothelial cells of the adjacent breast tumour. Indeed, higher VEGF expression correlates with the first detectable breast-tumour driven angiogenesis in pre-invasive high grade ductal carcinoma in situ [67].

The elevated expression of VEGF in the breast tumour environment is thought to be due to a number of causes. Hypoxia and HIF-1 are clearly important factors. The fact that premenopausal women had higher levels of VEGF expression than postmenopausal women suggests that steroid hormones may also boost VEGF expression [68, 69]. Estradiol has long been known to be angiogenic, and evidence suggests that oestrogen effects may be mediated through VEGF induction. In certain breast cancer cell lines, estrogens increase VEGF expression whereas progestins lower it. Tamoxifen, an oestrogen receptor inhibitor, has recently been found to reduce VEGF transcription. However, whether oestrogen receptor expression is linked to VEGF expression and vascular density has to be determined.

VEGF production is also influenced by changes in the tumour environment. Matrix metalloproteinases, for example, are frequently secreted by numerous tumour cells, including human breast cancers [70]. Matrix metalloproteinase (MMP)-9, which is produced by tumour cells and expressed at high levels in human breast cancers, is one member of this family that has attracted a lot of attention. MMP-9 has been found to proteolyze the surrounding extracellular matrix, releasing trapped VEGF and thereby enhancing its bioavailability.

The expression of HER2 is another significant alteration in breast cancers. HER2 is a tyrosine kinase receptor that belongs to the epidermal growth factor receptor family and is expressed by the ERB2 gene [71, 72]. It signals in the lack of a known ligand. Furthermore, HER2 overexpression or heregulin stimulation causes an increase in VEGF mRNA, whereas treatment of breast tumours with an anti-HER2 neutralising antibody inhibits VEGF synthesis in a dose-dependent manner. Furthermore, HER2 was found to boost the rate of HIF-1 protein production in a new, rapamycin-dependent mechanism, rather than by blocking degradation as seen during hypoxia [73, 74].

VEGF production can also be boosted by changes in epithelial gene expression linked to tumorigenicity. The 644 integrin, which typically facilitates connections between breast epithelium and basement membrane, is upregulated and mislocalized in breast carcinoma cells, promoting tumour cell invasiveness. According to recent research, 644 signalling causes the inactivation of eIF-4E, a translational repressor, which enhances VEGF translation and, in turn, tumour cell survival [75, 76, 77]. The 644 signalling pathway, which enhances VEGF translation, converges on a rapamycin-sensitive route, similar to the HER2-mediated increases in HIF-1 and VEGF. Importantly, the tumour cells’ increased VEGF production has been shown to act in an autocrine manner, promoting epithelial cell survival directly.

2.4 Mechanisms of angiogenesis

Tumour development and metastasis are dependent on angiogenesis. Necrosis occurs when a tumour’s blood supply is cut off, preventing it from growing. After a while, any further metastatic spread into the systemic circulation is stopped. Scientists have been studying angiogenesis and the different variables that regulate it in order to better understand how it affects breast cancer and develop a strategy to limit tumour progression [25, 29]. Because of the dual nature of this process, it’s critical to understand and distinguish between normal angiogenesis processes, such as wound healing, normal growth, and embryo nutrition, and tumour-related angiogenesis mechanisms.

Angiogenesis, which involves communicating between tumour cells and a variety of other cell types within the tumour microenvironment, is initiated by some compounds known as angiogenic activators because of their capacity to stimulate cell proliferation in vitro. The generation of pro-angiogenic growth factors by tumour cells, which impact the existing vasculature, has been shown to be necessary for the induction of this process [21]. To generate and stabilise newly created blood vessels, a delicate signal balance between pro- and anti-angiogenic factors is vigorously maintained in the microenvironment during these closely regulated processes [78]. As a result, numerous investigations have demonstrated that these angiogenic activators are critical in the growth of malignancies.

Certain tumour cells express both pro- and anti-angiogenic proteins, which encourage and inhibit angiogenesis, according to previous research. Tumours are thought to turn on the angiogenic switch by reversing the balance of angiogenesis inducers and inhibitors [29, 37]. This switch can be made by altering gene transcription, as seen in various cancers where VEGF and/or FGF levels are higher than in healthy tissue. The levels of endogenous inhibitors are lowered in some cancers, on the other hand. The intricate mechanism that drives these alterations in the regulators’ balances, on the other hand, remains a fascinating subject of research (Figure 1).

Figure 1.

Angiogenesis is a physiological process that results in the formation of new blood vessels from existing ones. From pre-existing capillaries, new blood vessels emerge. The tumour receives crucial nutrients for growth from the new blood vessels that have sprouted near and within the tumour. Angiogenesis in healthy tissues is regulated by a balance of anti- and pro-angiogenic factors (bottom), but the presence of angiogenic factors in tumours disrupts this balance, resulting in abnormal blood vessel structure and function, as well as hypoxia. The vasculature is normalised and the balance is restored.

The tumours ability to switch on angiogenesis is determined by the balance of this switch. Further research revealed that a decrease in anti-angiogenic protein production activates the tumour angiogenic switch, promoting tumour growth and metastasis [79, 80, 81]. Stimulating angiogenesis in a tumour and forming the endothelial tubes that result is a multistep process governed by hypoxia at each stage. This pathway is heavily reliant on ECs expressing HIF-1, a heterodimeric transcription factor. Under hypoxic conditions, the HIF-1 protein is stabilised and forms a heterodimer with HIF-1, and this pair promotes the transcription of multiple target genes to adapt to the hypoxic environment in human cancer cells.

HIF-1, in conjunction with other members of the HIF family, has been demonstrated in certain studies to govern practically every element of angiogenesis, making the HIF pathway a master regulator of angiogenesis [82]. In various malignancies, HIF-1 and HIF-2 expression has also been linked to a poor prognosis and metastatic illness. As a result, it’s regarded as a promising therapeutic target for a variety of medical conditions (Figure 2).

Figure 2.

This figure depicts the balance hypothesis of the angiogenic switch. Angiogenesis switch mechanism is assumed to be in charge of normal angiogenesis (formation of new capillaries). By utilising angiogenesis inducers and inhibitors, which flip the switch, this balance can be tilted in favour of enhanced blood vessel formation. Reduced inhibitor levels (thrombospondin-1, 16 kD prolactin, interferon, platelet factor-4, Angiostatin, and others) or increased activator levels (aFGF, bFGF, VEGF, and others) can tip the balance and activate the switch, resulting in the formation of new blood vessels.

Hypoxia and activation of the HIF pathway in cancer cells are required for the sprouting and formation of new blood vessels because they control the expression of several pro-angiogenic genes [83]. Some of the most powerful cytokines are VEGF, an endothelial mitogen and pro-angiogenic factor, angiopoietin-1, angiopoietin-2, platelet-derived growth factor (PDGF), and basic fibroblast growth factor (bFGF) [84, 85, 86, 87, 88].

The FGF and VEGF families of angiogenetic growth factors have gotten more attention than the others. In 1983, the protein VEGF-A (vascular endothelial growth factor) was identified and sequenced. It was the first cytokine to be identified as a key contributor to tumour angiogenesis, was purified from tumour cell ascites as vascular permeability factor (VPF), and was also revealed to have pharmacological effects on EC mitogenesis; consequently, VPF is referred to as VEGF (Figure 3) [89, 90].

Figure 3.

This diagram depicts the receptor binding selectivity and signalling pathways of members of the vascular endothelial growth factor (VEGF) family. VEGF family members bind to VEGFR-1, VEGFR-2, and VEGFR-3 receptor tyrosine kinases, which activate a variety of signalling pathways and allow them to exert their physiological effects.

In vivo and in vitro, VEGF is now known to be a multifunctional peptide capable of triggering receptor-mediated endothelial cell proliferation and angiogenesis. The VEGF family contains at least five members, each of which has three VEGF receptors (VEGFR) [91, 92, 93]. These receptors use transmembrane receptor tyrosine kinases to communicate with the cell’s interior (RTKs). The VEGF gene is subject to complex transcriptional control, and four distinct RNA isoforms are produced with varying biological features as a result of alternative splicing of its pre-mRNAs. VEGF-B, VEGF-C, VEGF-D, VEGF-E, and platelet-derived growth factor are all produced as a result of this process (PDGF).

By attaching to VEGF receptors and ligands, VEGF, for example, can trigger angiogenesis. The effects of vascular endothelial growth factor (VEGF), as well as acidic and basic fibroblast growth factors (FGF1/2), can be employed to investigate the induction and progression of angiogenesis at various phases of tumour development. VEGF binds to its receptor (VEGFR) and ligands on the surface of ECs. It causes dimerization, autophosphorylation, and activation of the downstream signalling cascade after binding to and activating the transmembrane tyrosine kinase receptors on the cell’s surface [94, 95, 96]. Tube development and sprouting follow EC survival, proliferation, migration, and apoptosis avoidance through several cascade phases. Over time, this process results in the development of a complex network of new blood vessels. Vasodilation and vascular permeability, a key feature of tissue inflammation and the tumour microenvironment, are also induced by VEGF [97, 98, 99, 100, 101, 102, 103].

The activity of the ECs outlined above is caused by an increase in pro-angiogenic factors such as VEGF and proteolytic enzymes, as well as a decrease in anti-angiogenic factors. Finally, a capillary network is successfully established, supplying enough nutrition and oxygen to the growing tumour. Taking advantage of this new vascular bed, the tumour cell may reach the systemic circulation and induce distant metastases. As a result, the number of metastasis sites is proportional to the amount of cancer cells that enter the circulation at the outset [104, 105, 106, 107, 108, 109, 110, 111].

Angiogenic inducers have been implicated in the regulating process of angiogenesis in malignancies since their discovery a decade ago. Anti-angiogenic treatment decreases tumour vascular growth by interfering with VEGF and VEGFR intracellular signalling [112, 113, 114, 115, 116].

Angiogenesis was originally linked to cancer, arthritis, and psoriasis. However, the impact it has on a variety of other disorders has been documented. Tumours are innately primed for successful angiogenic development due to their nature and composition. An active vascular system is made up of adipose tissue that is encased in stromal cells and serves as a scaffold for the tumour’s vascular system to emerge [117, 118, 119].

Brown adipose tissue (made up of cells with numerous mitochondria) promotes tumour growth by supplying a steady supply of oxygen and nutrients, whereas white adipose tissue promotes the formation and progression of breast cancer in a mouse model. Both types of adipose tissues, which have been associated to breast cancer, produce angiogenic factors such as VEGF A, B, and C, basic fibroblast growth factor (bFGF)/FGF-2, matrix metalloproteinases (MMPs), and IL-8. This aberrant blood vessel creation has been linked to cardiovascular illness, cancer, blindness, and diabetic ulcers [120, 121, 122].

2.5 Non-angiogenic functions of VEGF in breast cancer

VEGF increases the formation of new blood vessels and lymphatics, as well as increasing vascular permeability, and has a variety of tumour-related effects. The importance of VEGF in vascular and lymphangiogenesis has dominated research in breast and other cancers [123]. The importance of VEGF in cancer behaviour cannot be overstated. The presence of hypoxic patches in most malignancies, on the other hand, implies that VEGF-induced angiogenesis is insufficient to alleviate hypoxia [124]. Hypoxia works as a strong selection pressure, allowing only the most aggressive and metastatic cells to thrive. Understanding the mechanisms that allow tumour cells to survive under hypoxia is therefore critical for interpreting cancer biology and developing therapeutic approaches [125, 126].

VEGF produced by tumour or stromal cells interacts to VEGF receptors on tumour cells, producing a signalling response that supports survival in the face of hypoxia and other apoptotic triggers, according to our and other labs’ research [127]. This process, which most likely operates in tandem with p53 inactivation, provides self-sufficiency to tumour cells, making it simpler for them to form tumours and increasing the possibility that they will spread to other parts of the body [128, 129]. To put it another way, we believe that hypoxia favours cells that can signal VEGF, and that the most aggressive tumour cells (metastatic cells) are determined by their dependency on VEGF.

A side effect of VEGF signalling in breast cancer cells is that it can help them move and invade more easily.

3. Breast carcinoma cells and VEGF signalling

3.1 Survival signalling by autocrine VEGF

Tumour cells receive signals from various sources as a result of the complex microenvironment of solid tumours, and these signals alter the activity of other cells. However, it is becoming obvious that cancer cells can attain a certain level of self-sufficiency by creating autocrine signalling pathways that aid critical tasks such as growth, survival, and invasion [130] within this web of paracrine signalling. As tumours develop towards invasive and metastatic illness, autocrine pathways become more critical as the tumour’s environment becomes increasingly hostile. As a result, autocrine signalling pathways are a major target for anti-tumour therapy. Our study on invasive breast carcinoma cell lines provided one of the first indications that VEGF may have autocrine functions in cancer [73, 131].

We discovered that a 50% reduction in VEGF expression resulted in a considerable increase in apoptosis, even in the presence of 10% serum, when we utilised an antisense oligonucleotide approach to limit VEGF expression. This evidence backs with the theory that these cells were selected in vivo because they rely on VEGF to survive [132]. The importance of VEGF in carcinoma and other cancer cell survival has now been validated by research from our lab and others.

Because it increased VEGF expression in invasive breast cancer cell lines, hypoxia inhibited apoptosis caused by serum deprivation. The mechanism by which autocrine VEGF maintains the survival of breast carcinoma cells appears to involve constitutive activation of the PI3-kinase pathway, as evidenced by the findings that reducing VEGF expression results in a significant decrease in PI3-kinase basal activity, hypoxia stimulates Akt activity, and inhibition of PI3-kinase induces apoptosis [133]. According to previous studies, VEGF inhibits apoptosis in breast cancer cells via upregulating the anti-apoptotic protein Bcl-2.

3.2 The role of VEGF in breast carcinoma migration and invasion

Carcinoma cells acquire the ability to migrate and infiltrate tissues as a result of malignant transformation and development. Although chemoattractant gradients may enhance carcinoma migration and invasion, it has been established that cells’ ability to form autocrine signalling pathways might boost their sensitivity to external stimuli [134]. Depleting VEGF expression in the presence of caspase inhibitors, which prevent apoptosis caused by VEGF expression loss, allowed us to find a role for autocrine VEGF in the migration and invasion of breast cancer cells towards chemokines. The capacity of breast cancer cells to migrate and invade in response to chemotactic stimuli is considerably diminished in such circumstances.

One mechanism for VEGF’s involvement in these events is its ability to alter the expression of the chemokine receptor CXCR4 [135]. This finding is significant for breast cancer growth since stromal-derived factor-1, the receptor’s ligand, is abundant in tumour stroma as well as organs such as the lymph and lung, which are the primary targets of invasive breast carcinoma cells, and CXCR4 inhibitors impede metastasis [136].

In addition to its survival benefits, VEGF autocrine signalling may contribute to tumour growth by boosting chemokine receptor expression and allowing tumour cells to migrate towards chemokine gradients [137].

3.3 Perspective

The revelation that breast cancer cells produce VEGF receptors is significant, but further research is needed to understand how these receptors are expressed as a result of transformation and progression, including EMT, and the mechanisms through which these receptors regulate tumour cell behaviour. Despite having inherent signalling capabilities, little is known about how NP-1 enhances VEGF165 signalling on breast cancer cells. In endothelial cells, it appears to work with either VEGFR1 or VEGFR2, although this has yet to be validated in breast cancer cells.

Another hypothesis is that NP-1 transmits NP-1 signals in neurons via interacting with non-VEGF receptors in cancer cells, such as plexins. Our findings reveal that plexin A1 is expressed in breast cancer cells and can affect cell motility. The study of plexin involvement in NP-1 signalling will require a much more in-depth understanding of plexin expression and function in breast and other cancers. In addition, more exact data on the location and relative expression of NP-1 in the mammary gland and human breast malignancies is needed.

VEGF-C and VEGF-D, for example, have been linked to angiogenesis and lymphangiogenesis in breast tumours. It’s critical to figure out whether these VEGFs have a paracrine or autocrine effect on breast cancer cells. Some data suggests that breast cancer cells can respond to VEGF-D autocrinely, although additional research is needed to confirm this.