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.
MMPs
Matrix metalloproteinases (MMPs) are a large multigene family of zinc-dependent endopeptidases. The collective MMP family is capable of degrading all known ECM macromolecules. MMP activity is regulated at the level of transcription, post-translationally by proteolytic cleavage, and by endogenous inhibitors known as tissue inhibitors of metalloproteinases (TIMPs). The role of matrix metalloproteinases and TIMPs in several pathological conditions including angiogenesis, tumor growth, and metastasis has been investigated and very well described.
Matrix metalloproteinases contain five conserved domains/sequence motifs: Signal peptide sequence, which directs the enzyme into the rough endoplasmic reticulum during synthesis Propeptide domain, which is cleaved to activate the enzyme Catalytic domain, which contains the conserved Zn2+ binding region and mediates enzyme activity Hemopexin domain, which provides the substrate specificity Small hinge region, which allows the hemopexin domain to bring the substrate to the active core of the catalytic domain
There is also a subfamily of the matrix metalloproteinases, the membrane-type MMPs (MT-MMPs) which contain an additional transmembrane domain and a short cytoplasmic domain. After activation of MMPs by removal of the propeptide domain, their activity is regulated by TIMPs. TIMPs specifically and reversibly inhibit the activity of MMPs. So far there have been identified four members of the family, TIMP1–4. All TIMPs contain twelve conserved cystein residues, which form six disulfide bonds. The C-terminal domains of TIMPs are highly variable and confer their specificity towards preferred MMP targets.
ADAM/ADAMTS
ADAMs comprise a family of integral membrane as well as secreted glycoproteins which are related to snake venom metalloproteinases and MMPs. Like MMPs, ADAMs are composed of multiple conserved domains. They contain propeptide, metalloproteinase, disintegrin-like, cystein-rich, and epidermal growth factor like domains, although variations in domain composition have been observed in non-animal organisms. Membrane anchored ADAMs contain a transmembrane and cytoplasmic domain. The domains contained within the ADAMs family have been characterized, uncovering their functional and structural roles. ADAMs contain a consensus sequence which has three histidine residues that bind to the catalytically essential zinc ion. The propeptide is removed through cleavage by a furin type protease yielding the active enzyme. The propeptide of most MMPs is cleavable by proteases such as trypsin, plasmin, chymotrypsin and other MMPs. ADAMs participate in a wide variety of cell surface remodeling processes, including ectodomain shedding, regulation of growth factor availability and mediating cell-matrix interactions. ADAM17 and ADAM15 have recently been identified in endothelial cells (EC).
ADAMTS are a subfamily of ADAM related metalloproteinases that contain at least one thrombospondin type I sequence repeat motif (TSR). They are secreted proteins; and the TSR facilitates their localization to the ECM placing it in close proximity to their substrates. Functionally, ADAMTS can be divided into three groups: procollagen aminopeptidase, aggrecanase, and ADAMTS13 which cleaves von Willebrand factor. Unlike with MMPs, TIMPs are more selective in their ability to inhibit ADAMs and ADAMTSs. TIMP3 is able to inhibit ADAM17 and 12 as well as ADAMTS4 and 5. ADAM8 and ADAM9 are not susceptible to inhibition by TIMPs.