By Mark Comer
Sustained angiogenesis is one of the six hallmarks of cancer, presented in the seminal publication by Hanahan & Weingberg in 2000. The ‘switch’ refers to a distinct event during cancer progression where the tumour microenvironment is altered to support growth of new vasculature. The exact steps and pattern of vascularisation are diverse and varies between different types of cancers. Vascularisation is typically tightly regulated by a balance between a variety of pro and anti-angiogenic factors and under normal conditions activation of angiogenesis is temporary, for example in wound healing and as part of the menstrual cycle. However, in cancer this process is permanently ‘switched on’ in order to support the rapid and uncontrolled growth of cells with an adequate supply of oxygen and nutrients (Hanahan & Weinberg, 2000). The growth of new blood vessels also facilitates the establishment of distant metastases (Bielenberh & Zetter, 1994). The importance of angiogenesis places further emphasis on the role of surrounding non-cancerous cells in the development of metastatic disease. The process of angiogenesis has also been tied to well characterised oncogenes such as Ras and Myc, as well as tumour suppressors such as p53 ((Hanahan & Weinberg, 2011). As a result, the process of angiogenesis in the development of cancer has garnered much attention as a target for therapeutics, especially due to its universal role amongst all cancers.
The majority of cancers are epithelial-derived, in the early stages of tumorigenesis the cancer remains in situ and vascularization is absent. As growth continues, cells secrete pro-angiogenic factors to facilitate further expansion. The primary pro-angiogenic molecule implicated in the process of angiogenesis is vascular endothelial growth factor (VEGF). Secretion of VEGF is stimulated in response to cellular stress as a result of hypoxia and low nutrient levels. Hypoxic conditions are ‘sensed’ by the cell through the hypoxia-inducible factor-1α (HIF-1α): ambient oxygen hydroxylates HIF-1α and prevents it from inducing transcription of pro-angiogenic factors such as VEGF. In the absence of oxygen, HIF-1α is free to drive secretion of VEGF and expression of VEGF’s cognate receptor VEGFR (Nishida et al, 2006). HIF-1α drives expression of approximately 100 genes, not all of which are related to angiogenesis, many genes regulated by HIF-1α aid tumour survival and growth (Masoud & Li, 2015). Expression of glucose transporters and glycolytic enzymes to allow for “aerobic glycolysis” by tumour cells is regulated by HIF-1α for example. The process of vascularisation is carried out by endothelial cells in response to the production of VEGF. Once the response is triggered, they produce matrix metalloproteinases (MMPs) which digest the local extracellular matrix (ECM) (Bielenberg & Zetter, 2015). Growth and invasion of the endothelial cells results in the formation of a plexus of new vasculature that supplies the growing tumour with oxygen and nutrients. The newly produced blood vessels are abnormal, their most distinguishing feature being the ‘leakiness’ of the vessels that boost the metastatic capacity of a tumour. It is also important to note that secretion of pro-angiogenic factors can occur from other tumour associated cells such as fibroblasts or immune cells (Bielenberg & Zetter, 2015). Associated cells can indirectly boost VEGF levels through proteolytic cleavage of the ECM, this is seen with secretion of MMP-9 by immune cells resulting in release of VEGF (Deryugina & Quigley, 2015). The recruitment of normal cells to the tumour microenvironment can be considered a key driver of tumour cell growth. The involvement of non-cancerous cells in this key process points to the importance of the cells associated with an in situ cancer as they aid both expansion and metastasis of a tumour.
Likewise, cancer cells will actively suppress mechanisms designed to inhibit angiogenesis. Given that angiogenesis is regulated by the balance of pro- and anti-angiogenic signals, simply upregulating expression of one pro-angiogenic factor may be insufficient to trip the angiogenic switch (Nishida et al, 2006). Consequently, inhibition of anti-angiogenic factors may be necessary for new vasculature to form. Downregulation of thrombospondin glycoproteins is common in cancer, as the thrombospondin family of proteins are potent antagonists to VEGF but also perform a variety of other anti-tumour functions such as regulation of growth factors (Yee et al, 2009). Anti-angiogenic pathways are often regulated by tumour suppressors, p53 has been shown to regulate the expression of thrombospondin. This suggests that other key ‘hallmarks’ of cancer are coregulated by the same set of genes. Integrin expression is another key regulator of angiogenesis; interference with integrin signalling from the ECM has been shown to inhibit angiogenesis (Mahabeleshwar et al, 2006). Similar to the release of VEGF from the ECM, proteolytic cleavage of type XVIII collagen releases endostatin which inhibits angiogenesis by blocking endothelial adhesion (Baeriswyl & Christofori, 2009). Fragments of plasminogen protein also inhibit angiogenesis with angiostatin inducing apoptosis in endothelial cells to prevent formation of new vasculature.
Angiogenesis is an attractive therapeutic target, conceptually this could greatly limit the expansion of an in situ cancer as well as prevent metastasis and invasion. The current strategy involves the use of monoclonal antibodies against pro-angiogenic signalling molecules such as VEGF and their corresponding receptors such as VEGFR to block angiogenic signalling (Teleanu et al, 2020), this strategy has proved effective in colorectal cancer and glioblastoma. As is common with a variety of other treatments, monoclonal antibody treatment against VEGF/VEGFR causes side effects including hypertension, blood clots, strokes, and proteinuria. Combination with traditional chemotherapy agents have shown promise with increases in progression-free survival (Teleanu et al, 2020) and this approach is the subject of ongoing research. Resistance to anti-angiogenic therapies follows the common themes of amplification of the target and signalling redundancy.
The tumour microenvironment has become an area of intense research and study over the last few decades. The role of non-cancerous cells and the ECM in driving expansion and growth of the tumour has been brought into sharp focus by the importance of the angiogenic switch. The process of angiogenesis differs between cancers, and is a complex multi-step process regulated by tissue-specific signalling that requires further study in order to develop effective therapies.
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