Exploring VEGF and VEGFR: key Regulatory Mechanisms of Angiogenesis
What is the Angiogenic Factor Family？
The angiogenic factor family is a group of proteins with key roles in cell biology, mainly regulating the growth and formation of new blood vessels. This family includes a variety of cytokines, the most famous of which is vascular endothelial growth factor (VEGF), but also basic fibroblast growth factor (bFGF). These factors play crucial roles in normal physiological processes and disease states.
The angiogenic factor family plays an important role in embryonic development, wound healing, tissue repair, and disease development. These factors maintain normal vascular structure and function in vivo by promoting the proliferation, migration and survival of endothelial cells, and supporting the formation of new blood vessels. Under disease conditions, overactive or abnormal expression of angiogenic factors may lead to abnormal angiogenesis, such as tumor angiogenesis, thereby affecting tumor growth and metastasis.
The angiogenic factor family has important application prospects in medicine. Inhibition of these factors can inhibit tumor growth and metastasis, thus becoming a potential strategy for antitumor therapy.
Understanding Vascular Endothelial Growth Factor (VEGF)
VEGF, which stands for Vascular Endothelial Growth Factor, is a vital class of protein molecules that holds a pivotal role in regulating angiogenesis, the process of forming new blood vessels within organisms. Angiogenesis is a complex and essential biological phenomenon responsible for creating new blood vessels to meet the diverse nutritional requirements of various tissues and organs.
Operating within the cytokine family, VEGF encompasses various subtypes, including VEGF-A, VEGF-B, VEGF-C, VEGF-D, and more. Its primary function centers around binding to its receptor, known as VEGFR (vascular endothelial growth factor receptor). This interaction triggers an intricate cascade of cell signaling pathways, profoundly influencing endothelial cells' activities, such as proliferation, migration, survival, and most notably, angiogenesis.
Initially, VEGF was linked closely to endothelial cell growth and migration during angiogenesis. In healthy physiological conditions, maintaining a balanced level of VEGF is paramount for the proper development of organs, tissue regeneration, and efficient wound healing processes. However, instances of abnormal VEGF expression can disturb this equilibrium, leading to irregular angiogenesis. Such anomalies have been associated with the onset and progression of various conditions, including tumors, diabetes, and more.
In essence, comprehending the intricate functions of VEGF provides a deeper insight into the mechanisms governing blood vessel formation and its far-reaching impacts on health and disease.
Decoding the VEGFR Structure
In the early 1990s, a breakthrough occurred with the isolation of genes responsible for novel tyrosine kinase receptors. These receptors were identified as crucial players in both promoting and inhibiting the development of blood and lymph vessels, and they were aptly named VEGFRs[1-2].
The VEGFR family comprises three genes encoding three full-length receptors—VEGFR-1, -2, and -3—along with one soluble molecule called sVEGFR-1. Remarkably, these receptors share similar overall structures, each characterized by three key domains. These domains encompass an extracellular ligand-binding domain (ECD), boasting seven immunoglobulin (Ig)-like domains, followed by a transmembrane domain, and finally, a tyrosine kinase domain demarcated by a kinase insert and a carboxy terminus[3-4]. It's within these kinase domains that the most striking conservation is observed, with a remarkable sequence identity of 78–80%.
At the heart of these intricate structures lies the VEGF-VEGFR system, a critical conductor in orchestrating tumor angiogenesis. Understanding the unique architecture of VEGFRs is paramount as it offers insights into potential vulnerabilities that could be targeted for therapeutic interventions aimed at disrupting tumor-driven angiogenesis. This burgeoning knowledge not only enhances our comprehension of the complex interplay within cellular processes but also holds the promise of innovative anti-angiogenic therapies with the potential to reshape the landscape of cancer treatment.
Fig.1 Structure of VEGFR[5]
Functions of VEGF
VEGF, or vascular endothelial growth factor, stands as a pivotal class of protein molecules orchestrating the intricate symphony of angiogenesis—vital blood vessel formation—in living organisms. This biological dance involves the creation of new blood vessels, nourishing diverse tissues and organs.
Nestled within the cytokine family, VEGF boasts diverse subtypes including VEGF-A, VEGF-B, VEGF-C, VEGF-D, and more. The linchpin of its activity is its interaction with VEGFR, the vascular endothelial growth factor receptor. This dynamic binding sets off an intricate chain reaction of cell signaling, deeply influencing endothelial cell proliferation, migration, survival, and, crucially, angiogenesis.
At its core, VEGF's paramount function is to usher in angiogenesis. By docking onto VEGF receptors dotting cell surfaces, it kickstarts a remarkable transformation within endothelial cells. These cells spring into action, proliferating, migrating, and weaving intricate vascular networks. This process is the lifeblood, quite literally, of delivering nutrients and oxygen to nascent tissues. It's a choreographer of development, a conductor during wound healing, and even an enabler of tumor expansion.
Beyond nurturing fresh blood highways, VEGF is no one-trick pony. It also demonstrates its prowess by enhancing blood vessel wall permeability. This strategic move eases the passage of cells and molecules from the bloodstream to the interstitial space, where they're needed the most. This maneuver grants immune cells and antibodies easy access to infection sites or injured areas, all while providing essential sustenance to cells.
But the story doesn't end there. In mature vasculature, VEGF dons the cape of a homeostatic guardian. It fine-tunes blood flow, ensuring oxygen supply marches hand in hand. It guards the health of vascular endothelial cells, standing as a stalwart shield against damage and inflammation. And let's not forget the nervous system, where VEGF moonlights as a conductor shaping neuron growth, migration, and connections.
In the realm of wound healing and tissue restoration, VEGF steps up once again. It's a staunch advocate for the formation of fresh blood vessels, a key pillar supporting the recovery of damaged tissues.
In essence, VEGF is an architect of vitality—enabling life-sustaining blood vessel networks, enhancing immune responses, nurturing neuronal growth, and championing tissue revival. It's a versatile maestro, conducting a symphony of biological functions that underpin the miracle of life itself.
Angiogenesis and Anti-angiogenic Therapy in Cancer
The proliferation and metastasis of solid tumors hinge on the growth of blood vessels, making angiogenesis a pivotal focus of tumor therapy investigation[6]. The VEGF-VEGFR system, a primary regulator of tumor angiogenesis, plays a central role in various human diseases like tumor-driven angiogenesis, metastasis, and inflammatory conditions such as rheumatoid arthritis, psoriasis, hyperthyroidism, and atherosclerosis[7-8]. Pathological angiogenesis might be stimulated by VEGFR-1, activating endothelial cells and recruiting bone marrow progenitor cells[9-10]. Furthermore, the trophoblast layer's sVEGFR-1 splice variant contributes to creating a regulatory barrier against abnormal vascular permeability and angiogenesis. Remarkably, elevated sVEGFR-1 levels correlate with preeclampsia symptoms like hypertension and proteinuria[11]. VEGFR-2 plays a direct role in tumor angiogenesis and blood vessel-related metastasis, especially during hypoxic stress induced by rapid tumor growth. Both hyperactivity and dysfunction of VEGFR-3 are linked to various human pathologies. Inactivation of VEGFR-3 exacerbates congenital lymphedema resulting from impaired lymphatic vessel transport capacity, causing chronic tissue swelling[13-14]. Conversely, VEGF-C shows promise in alleviating lymphedema caused by filariasis, trauma, or infection by reducing VEGFR-3 activation.
The VEGF-VEGFR system emerges as a prime target for novel drug development, particularly in managing malignancies. Diverse anti-angiogenic agents, including VEGF-neutralizing antibody (bevacizumab) and tyrosine kinase inhibitors (sunitinib and sorafenib), exhibit the potential to hinder tumor growth and metastasis. In cases where tumors develop resistance to standard cytotoxic therapies, anti-angiogenic agents present themselves as promising alternatives for cancer treatment.
Bevacizumab, a humanized monoclonal antibody targeting VEGF-A, effectively neutralizes VEGF-A but not other family members. Although FDA-approved for cancer treatment in 2004, its efficacy remains under scrutiny due to inconclusive data on overall survival from large-scale phase III clinical trials such as E2100, AVADO, and RIBBON-1[14]. Notably, Bevacizumab carries life-threatening adverse effects like hypertension, proteinuria, rhinorrhagia, thrombosis, and bleeding[15]. Moreover, certain cancers develop resistance to Bevacizumab through mechanisms such as enhancing alternative pro-angiogenic pathways, attracting bone marrow-derived pro-angiogenic cells, and increasing pericytes within tumors[16]. Sunitinib malate and sorafenib tosylate selectively target protein receptors, including VEGFRs, effectively inhibiting their kinase activity.
VEGF Signaling Pathway
The intricate Vascular Endothelial Growth Factor (VEGF) signaling pathway orchestrates cell growth, angiogenesis, and disease progression. As VEGF binds to its primary receptors, chiefly VEGFR-1 and VEGFR-2, it sets in motion a complex cascade of events.
Upon receptor binding, VEGF prompts receptor dimerization and autophosphorylation, initiating diverse downstream signaling pathways. The PI3K/Akt pathway and the Ras/Raf/ERK (MAPK) pathway stand out as pivotal players.
The PI3K/Akt pathway unfolds with autophosphorylated receptors activating Akt protein kinase via PI3K. Activated Akt facilitates cell survival, proliferation, migration, and impedes apoptosis. This pivotal role sustains endothelial cell health and spurs fresh blood vessel generation.
Activation of the Ras/Raf/ERK (MAPK) pathway sets off a series of reactions involving the Ras protein. This culminates in mitogen-activated protein kinase (ERK) phosphorylation, dictating gene expression and fueling cell proliferation, survival, and angiogenesis.
Beyond these core pathways, VEGF signaling stimulates the transcription factor HIF-1, activated in hypoxic environments. This activation leads to the expression of genes adapted to low-oxygen conditions and fuels neovascularization.
Collectively, VEGF signaling pathways govern angiogenesis and cellular activities at the micro level, providing crucial support for both normal physiological processes and disease states. An in-depth comprehension of the molecular intricacies of the VEGF signaling pathway holds immense significance in devising novel therapeutic strategies, particularly for cancer and vascular-related disorders.
Fig.2 VEGF-angiogenesis signaling and cell cycle regulation pathways.[17]
The Significance of Clinical Detection of VEGF
The clinical detection of VEGF (Vascular Endothelial Growth Factor) holds paramount medical importance, particularly in diagnosing, treating, and prognosticating tumors and vascular-related disorders. The significance of VEGF clinical detection is multifaceted:
- Tumor Diagnosis and Prognosis: VEGF is a pivotal player in tumor growth and metastasis. Elevated VEGF levels are often observed in tumor tissues, driving the creation of new blood vessels to supply oxygen and nutrients to tumors. Clinically, quantifying VEGF levels in blood or tissues serves as an indicator of tumor aggressiveness. Elevated VEGF levels correlate with a poorer prognosis, suggesting heightened tumor aggression and metastatic potential.
- Therapeutic Response Monitoring: Anti-angiogenic therapies, designed to impede tumor blood vessel formation and nutrient supply, are prominent strategies. Monitoring VEGF level fluctuations aids in evaluating treatment effectiveness. Reduced VEGF levels post-treatment could signify therapeutic success, whereas sustained high levels might suggest inadequate response.
- Drug Development and Assessment: VEGF inhibitors, pivotal in treating tumors and vascular conditions, necessitate rigorous drug development and evaluation. Clinical VEGF level detection becomes a yardstick for gauging new drug efficacy and safety, steering drug trials and regulatory approvals.
- Tailored Treatment Approach: Clinical trials often explore combining VEGF inhibitors with complementary treatments like radiotherapy or chemotherapy. By gauging VEGF levels in patients, physicians can adeptly tailor treatment strategies, optimizing patient outcomes by selecting the most fitting approach.
VEGF and VEGFR Protein
Recombinant Human VEGF Protein
Synonym:Vascular endothelial growth factor A, VEGF-A, Vascular permeability factor, VPF, VEGF, MGC70609.
Recombinant Human VEGFR2 Protein
Synonym:KDR D1-7, sKDR D1-7, Kinase insert domain receptor, Protein-tyrosine kinase receptor Flk-1, CD309, type III receptor tyrosine kinase, FLK1, VEGFR-2.
References:
[1] de Vries C, Escobedo JA, Ueno H, Houck K, Ferrara N, Williams LT. The fms-like tyrosine kinase, a receptor for vascular endothelial growth factor. Science. 1992;255(5047):989-991. doi:10.1126/science.1312256
[2] Terman BI, Carrion ME, Kovacs E, Rasmussen BA, Eddy RL, Shows TB. Identification of a new endothelial cell growth factor receptor tyrosine kinase. Oncogene. 1991;6(9):1677-1683.
[3] Shibuya M. Vascular endothelial growth factor and its receptor system: physiological functions in angiogenesis and pathological roles in various diseases. J Biochem. 2013;153(1):13-19. doi:10.1093/jb/mvs136
[4] Shibuya M, Claesson-Welsh L. Signal transduction by VEGF receptors in regulation of angiogenesis and lymphangiogenesis. Exp Cell Res. 2006;312(5):549-560. doi:10.1016/j.yexcr.2005.11.012
[5] Lugano, R., Huang, H., Dimberg, A. (2018). Vascular Endothelial Growth Factor Receptor (VEGFR). In: Choi, S. (eds) Encyclopedia of Signaling Molecules. Springer, Cham. https://doi.org/10.1007/978-3-319-67199-4_101914
[6] Folkman J. Tumor angiogenesis: therapeutic implications. N Engl J Med. 1971;285(21):1182-1186. doi:10.1056/NEJM197111182852108
[7] Alitalo K, Carmeliet P. Molecular mechanisms of lymphangiogenesis in health and disease. Cancer Cell. 2002;1(3):219-227. doi:10.1016/s1535-6108(02)00051-x
[8] Shibuya M. Involvement of Flt-1 (VEGF receptor-1) in cancer and preeclampsia. Proc Jpn Acad Ser B Phys Biol Sci. 2011;87(4):167-178. doi:10.2183/pjab.87.167
[9] Carmeliet P, Moons L, Luttun A, et al. Synergism between vascular endothelial growth factor and placental growth factor contributes to angiogenesis and plasma extravasation in pathological conditions. Nat Med. 2001;7(5):575-583. doi:10.1038/87904
[10] Hattori K, Heissig B, Wu Y, et al. Placental growth factor reconstitutes hematopoiesis by recruiting VEGFR1(+) stem cells from bone-marrow microenvironment. Nat Med. 2002;8(8):841-849. doi:10.1038/nm740
[11] Maynard SE, Min JY, Merchan J, et al. Excess placental soluble fms-like tyrosine kinase 1 (sFlt1) may contribute to endothelial dysfunction, hypertension, and proteinuria in preeclampsia. J Clin Invest. 2003;111(5):649-658. doi:10.1172/JCI17189
[12] Irrthum A, Karkkainen MJ, Devriendt K, Alitalo K, Vikkula M. Congenital hereditary lymphedema caused by a mutation that inactivates VEGFR3 tyrosine kinase. Am J Hum Genet. 2000;67(2):295-301. doi:10.1086/303019
[13] Rockson SG. Lymphedema. Am J Med. 2001;110(4):288-295. doi:10.1016/s0002-9343(00)00727-0
[14] Gerber HP, Ferrara N. Pharmacology and pharmacodynamics of bevacizumab as monotherapy or in combination with cytotoxic therapy in preclinical studies. Cancer Res. 2005;65(3):671-680.
[15] Quesada AR, Muñoz-Chápuli R, Medina MA. Anti-angiogenic drugs: from bench to clinical trials. Med Res Rev. 2006;26(4):483-530. doi:10.1002/med.20059
[16] Piao Y, Henry V, Tiao N, et al. Targeting intercellular adhesion molecule-1 prolongs survival in mice bearing bevacizumab-resistant glioblastoma. Oncotarget. 2017;8(57):96970-96983. Published 2017 Jun 29. doi:10.18632/oncotarget.18859
[17] Calvete O, Garcia-Pavia P, Domínguez F, et al. POT1 and Damage Response Malfunction Trigger Acquisition of Somatic Activating Mutations in the VEGF Pathway in Cardiac Angiosarcomas. J Am Heart Assoc. 2019;8(18):e012875. doi:10.1161/JAHA.119.012875
