Angiostatin is a naturally occurring protein found in several animal species, including humans. It is an endogenous angiogenesis inhibitor (i.e., it blocks the growth of new blood vessels). Clinical trials have been undertaken for its use in anticancer therapy. [1]
Angiostatin is a 38 kDa fragment of a larger protein, plasmin (itself a fragment of plasminogen) enclosing three to five contiguous kringle modules. Each module contains two small beta sheets and three disulfide bonds. [2] [3]
There are four different structural variants to angiostatin differing in the combination of kringle domains: K1-3, K1-4, K1-5, K1-4 with a fragment of K-5. Each kringle domain contributes a different element of inhibition to the cytokine. Recent studies through recombinant angiostatin have shown however that K1-3 is pivotal is the inhibitory nature of angiostatin. [4]
K1-3 form the “triangular bowl-like structure” of angiostatin. [5] This structure is stabilized by interactions between inter-kringle peptides and kringles, although the kringle domains do not directly interact with each other. Angiostatin is effectively divided into two sides. The active site of K1 is found on one side, while the active sites of K2 and K3 are found on the other. This is hypothesized to result in the two different functions of angiostatin. The K1 side is believed to be primarily responsible for the inhibition of cellular proliferation, while the K2-K3 sides is believed to be primarily responsible for the inhibition of cell migration. [5]
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Angiostatin is produced, for example, by autoproteolytic cleavage of plasminogen, involving extracellular disulfide bond reduction by phosphoglycerate kinase. Furthermore, angiostatin can be cleaved from plasminogen by different metalloproteinases (MMPs), elastase, prostate-specific antigen (PSA), 13 KD serine protease, or 24KD endopeptidase.[ which? ]
Angiostatin is known to bind many proteins, especially to angiomotin and endothelial cell surface ATP synthase but also integrins, annexin II, C-met receptor, NG2 proteoglycan, tissue-type plasminogen activator, chondroitin sulfate proteoglycans, and CD26. Additionally, smaller fragments of angiostatin may bind several other proteins. There is still considerable uncertainty on its mechanism of action, but it seems to involve inhibition of endothelial cell migration, [6] proliferation and induction of apoptosis. It has been proposed that angiostatin activity is related, among other things, to the coupling of its mechanical and redox properties. [7]
Although the exact mechanisms of action of angiostatin has not been completely understood yet, there are three proposed mechanism of action. The first proposed mechanism of action is that angiostatin binds to F1-FoATP synthase found both in the mitochondria and on the cellular membrane of epithelial cells which not only inhibits ATP production in tumor cells but also inhibits the cell's ability to maintain the acidic pH of tumor cells. This inability to regulate the intracellular pH can initiate apoptosis. [8] Another proposed mechanism of action is that angiostatin is able to reduce epithelial cell migration by binding to avB3-integrins. [5] However studies have shown that avB3-integrins are not critically essential for angiogenesis, so more investigation is require to ascertain how the inhibition of avB3-integrins inhibit epithelial cell migration. [9] Another proposed mechanism of action is that angiostatin binds to Angiomotin (AMOT) and activating focal adhesion kinase (FAK). FAK has been shown to promote the inhibition of cell proliferation and cell migration, but lack of knowledge on how angiostatin and angiomotin function necessitate that addition research is required. [5]
Glycogen synthase kinase 3 (GSK-3) is a serine/threonine protein kinase that mediates the addition of phosphate molecules onto serine and threonine amino acid residues. First discovered in 1980 as a regulatory kinase for its namesake, glycogen synthase (GS), GSK-3 has since been identified as a protein kinase for over 100 different proteins in a variety of different pathways. In mammals, including humans, GSK-3 exists in two isozymes encoded by two homologous genes GSK-3α (GSK3A) and GSK-3β (GSK3B). GSK-3 has been the subject of much research since it has been implicated in a number of diseases, including type 2 diabetes, Alzheimer's disease, inflammation, cancer, addiction and bipolar disorder.
Urokinase, also known as urokinase-type plasminogen activator (uPA), is a serine protease present in humans and other animals. The human urokinase protein was discovered, but not named, by McFarlane and Pilling in 1947. Urokinase was originally isolated from human urine, and it is also present in the blood and in the extracellular matrix of many tissues. The primary physiological substrate of this enzyme is plasminogen, which is an inactive form (zymogen) of the serine protease plasmin. Activation of plasmin triggers a proteolytic cascade that, depending on the physiological environment, participates in thrombolysis or extracellular matrix degradation. This cascade had been involved in vascular diseases and cancer progression.
Granzyme B (GrB) is one of the serine protease granzymes most commonly found in the granules of natural killer cells and cytotoxic T cells. It is secreted by these cells along with the pore forming protein perforin to mediate apoptosis in target cells.
Protein kinase B (PKB), also known as Akt, is the collective name of a set of three serine/threonine-specific protein kinases that play key roles in multiple cellular processes such as glucose metabolism, apoptosis, cell proliferation, transcription, and cell migration.
Angiomotin (AMOT) is a protein that in humans is encoded by the AMOT gene. It belongs to the motin family of angiostatin binding proteins, which includes angiomotin, angiomotin-like 1 (AMOTL1) and angiomotin-like 2 (AMOTL2) characterized by coiled-coil domains at N-terminus and consensus PDZ-binding domain at the C-terminus. Angiomotin is expressed predominantly in endothelial cells of capillaries as well as angiogenic tissues such as placenta and solid tumor.
An angiogenesis inhibitor is a substance that inhibits the growth of new blood vessels (angiogenesis). Some angiogenesis inhibitors are endogenous and a normal part of the body's control and others are obtained exogenously through pharmaceutical drugs or diet.
Endostatin is a naturally occurring, 20-kDa C-terminal fragment derived from type XVIII collagen. It is reported to serve as an anti-angiogenic agent, similar to angiostatin and thrombospondin.
Nucleoside-diphosphate kinases are enzymes that catalyze the exchange of terminal phosphate between different nucleoside diphosphates (NDP) and triphosphates (NTP) in a reversible manner to produce nucleotide triphosphates. Many NDP serve as acceptor while NTP are donors of phosphate group. The general reaction via ping-pong mechanism is as follows: XDP + YTP ←→ XTP + YDP. NDPK activities maintain an equilibrium between the concentrations of different nucleoside triphosphates such as, for example, when guanosine triphosphate (GTP) produced in the citric acid (Krebs) cycle is converted to adenosine triphosphate (ATP). Other activities include cell proliferation, differentiation and development, signal transduction, G protein-coupled receptor, endocytosis, and gene expression.
Endoglin (ENG) is a type I membrane glycoprotein located on cell surfaces and is part of the TGF beta receptor complex. It is also commonly referred to as CD105, END, FLJ41744, HHT1, ORW and ORW1. It has a crucial role in angiogenesis, therefore, making it an important protein for tumor growth, survival and metastasis of cancer cells to other locations in the body.
Thrombospondin 1, abbreviated as THBS1, is a protein that in humans is encoded by the THBS1 gene.
Ephrins are a family of proteins that serve as the ligands of the Eph receptor. Eph receptors in turn compose the largest known subfamily of receptor protein-tyrosine kinases (RTKs).
Matrilysin also known as matrix metalloproteinase-7 (MMP-7), pump-1 protease (PUMP-1), or uterine metalloproteinase is an enzyme in humans that is encoded by the MMP7 gene. The enzyme has also been known as matrin, putative metalloproteinase-1, matrix metalloproteinase pump 1, PUMP-1 proteinase, PUMP, metalloproteinase pump-1, putative metalloproteinase, MMP). Human MMP-7 has a molecular weight around 30 kDa.
ROCK1 is a protein serine/threonine kinase also known as rho-associated, coiled-coil-containing protein kinase 1. Other common names are ROKβ and P160ROCK. ROCK1 is a major downstream effector of the small GTPase RhoA and is a regulator of the actomyosin cytoskeleton which promotes contractile force generation. ROCK1 plays a role in cancer and in particular cell motility, metastasis, and angiogenesis.
Pigment epithelium-derived factor (PEDF) also known as serpin F1 (SERPINF1), is a multifunctional secreted protein that has anti-angiogenic, anti-tumorigenic, and neurotrophic functions. Found in vertebrates, this 50 kDa protein is being researched as a therapeutic candidate for treatment of such conditions as choroidal neovascularization, heart disease, and cancer. In humans, pigment epithelium-derived factor is encoded by the SERPINF1 gene.
Vascular endothelial growth factor A (VEGF-A) is a protein that in humans is encoded by the VEGFA gene.
The Akt signaling pathway or PI3K-Akt signaling pathway is a signal transduction pathway that promotes survival and growth in response to extracellular signals. Key proteins involved are PI3K and Akt.
Collagen receptors are membrane proteins that bind the extracellular matrix protein collagen, the most abundant protein in mammals. They control mainly cell proliferation, migration and adhesion, coagulation cascade activation and they affect ECM structure by regulation of MMP.
Angiogenesis is the process of forming new blood vessels from existing blood vessels, formed in vasculogenesis. It is a highly complex process involving extensive interplay between cells, soluble factors, and the extracellular matrix (ECM). Angiogenesis is critical during normal physiological development, but it also occurs in adults during inflammation, wound healing, ischemia, and in pathological conditions such as rheumatoid arthritis, hemangioma, and tumor growth. Proteolysis has been indicated as one of the first and most sustained activities involved in the formation of new blood vessels. Numerous proteases including matrix metalloproteinases (MMPs), a disintegrin and metalloproteinase domain (ADAM), a disintegrin and metalloproteinase domain with throbospondin motifs (ADAMTS), and cysteine and serine proteases are involved in angiogenesis. This article focuses on the important and diverse roles that these proteases play in the regulation of angiogenesis.
Tumstatin is a protein fragment cleaved from collagen that serves as both an antiangiogenic and proapoptotic agent. It has similar function to canstatin, endostatin, restin, and arresten, which also affect angiogenesis. Angiogenesis is the growth of new blood vessels from pre-existing blood vessels, and is important in tumor growth and metastasis. Angiogenesis is stimulated by many growth factors, the most prevalent of which is vascular endothelial growth factor (VEGF).
In molecular biology, the protein domain VEK-30, is a 30-amino acid long, internal peptide present within bacterial organisms that acts as an epitope or antigenic determinant. It increases the pathogenicity of the cell. More specifically, it is found in streptococcal M-like plasminogen (Pg)-binding protein (PAM) from gram-positive group-A streptococci (GAS). VEK-30 represents an epitope within PAM that shows high affinity for the lysine binding site (LBS) of the kringle-2 (K2) domain of human (h)Pg.