New insights into the role of ubiquitination in angiogenesis (Review)

Abstract

Angiogenesis is a dynamic and complex mechanism for generating new blood vessels from existing ones. Angiogenesis occurs through all life stages and involves several physiological processes. It has an important physiological and pathological role including in cancer, wound healing and inflammation. The emerging role of ubiquitination in regulating angiogenesis highlights the importance of studying this pathway in an angiogenic setting. In angiogenic events, imbalances between pro‑ and anti‑angiogenic factors, induction of hypoxic signaling and stimulation of angiogenic signaling pathways play a central role. This review provides a comprehensive overview of the role of ubiquitination in angiogenesis. This includes angiogenic factors [VEGF, platelet‑derived growth factor, (basic) fibroblast growth factor and angiopoietin], vascular cells (pericytes, endothelial cells, vascular smooth muscle cells) and extracellular matrix and cell adhesion molecules, all of which have important roles in angiogenesis, hypoxic signaling (hypoxia‑inducible factor), which induces angiogenesis, and important vascular signaling pathways (Wnt and Notch). In addition, the molecular biological basis of angiogenesis is discussed and the potential therapeutic value of ubiquitination in angiogenesis‑related diseases is highlighted.

Keywords: Notch signaling molecules; Wnt signaling molecules; angiogenesis; angiogenic factors; cell adhesion molecules; extracellular matrix; hypoxia signaling; ubiquitination; vascular cells.

Figure 1

Ubiquitination modification process. The process of ubiquitination typically requires the synergistic action of three enzymes: E1 Ub-activating enzyme, E2 Ub-conjugating enzyme and E3 Ub ligase. First, the E1 Ub-activating enzyme activates the Ub molecule in the presence of ATP-supplied energy. Next, the activated Ub molecule is delivered to the E2 Ub-conjugating enzyme. Finally, the E3 Ub ligase attaches the Ub molecule to the target protein. Proteins to be degraded are first modified by ubiquitination and then degraded by the proteasome. Ubiquitination is a reversible process and specific DUBs remove ubiquitin from the target protein. DUB, deubiquitinating enzymes; Ub, ubiquitin.

Figure 2

The process of sprouting angiogenesis. (A) Sprouting angiogenesis is the process by which new shoots form from existing vessels. The key biological process of neovascularization involves a balance between the formation of 'tip' and 'stalk' ECs. (B) Tip cells primarily play the role of directional navigation, guiding the direction of microvascular outgrowth. The tip cells maintain their phenotypic stability through VEGF and Notch signaling pathways in response to various factors in the extracellular environment. In addition, tip cells can specifically enhance glycolytic pathways to adapt to the low-oxygen environment during neovascularization, while non-tip cells can specifically use fatty acid metabolites to maintain their proliferative capacity, which drives the tip cells to extend forward. VEGF, vascular endothelial growth factor; VEGFR, VEGF receptor; EC, endothelial cell; ECM, extracellular matrix; PDGF, platelet-derived growth factor; ANG, angiopoietin; HIF, hypoxia-inducible factor; DLL4, delta-like 4; TGF, transforming growth factor; PC, pericyte; TIMP, tissue inhibitor of metalloproteinases.

Figure 3

Angiogenesis is involved in human diseases. Angiogenesis is involved in the majority of diseases, including tumors, cardiovascular diseases, neurological diseases, metabolic diseases, infectious diseases, diseases of the bones, and physiological processes such as wound healing. Theoretically, targeted stimulation or inhibition of angiogenesis may provide novel therapeutic options for these diseases.

Figure 4

Ubiquitination during angiogenesis. During angiogenesis, imbalances between pro- and anti-angiogenic factors, increased hypoxic signals and stimulation of angiogenic signaling pathways have a central role, and the three enzymes of ubiquitination (E1-activating enzyme, E2-binding enzyme and E3 ligase) regulate the angiogenic process through various mechanisms. Angiogenesis begins with pro-angiogenic signaling in vascular ECs by growth factors such as VEGF, PDGF and EGF. Degradation of pro-angiogenic signals is inducible by ubiquitinating enzymes in the process. VEGF increases EC permeability and forms a temporary ECM skeleton. ECs migrate to the surface of the ECM. Proteases result in the release of angiogenic molecules stored in the ECM and remodel the ECM into a vasoactive environment. Cells neighboring the tip cells occupy auxiliary positions as stem cells, dividing to extend the stem and establish a lumen. A hypoxia-induced program driven by HIF-1α renders the ECs responsive to angiogenic signals, and the stability and transcriptional activity of HIF-1α is regulated by ubiquitination. By contrast, signals such as PDGF-B, Ang-1 and NOTCH induce the coverage of ECs by pericytes. As a consequence of the response to VEGF, activation of VEGFR-2 upregulates DLL4 expression in tip cells. VEGFR-2 is primarily degraded via the ubiquitin-proteasome pathway. Jagged1, another NOTCH ligand expressed by stem cells, promotes tip cell selection by interfering with reciprocal DLL4 and NOTCH signaling from stem cells to tip cells, and NOTCH and ligand formation can be ubiquitination-regulated. Ligand-activated Wnt signaling pathways are also activated in response to VEGF. Overactivation of Wnt-activated signaling pathways consistently induces upregulation of pro-angiogenic factors (VEGF-A and VEGF-C), which can be degraded by the proteasome complex. EC, endothelial cell; ECM, extracellular matrix; VEGF, vascular endothelial growth factor; VEGFR, VEGF receptor; PDGF, platelet-derived growth factor; EGF, epithelial growth factor; HIF-1α, hypoxia-inducible factor-1α; Ang, angiopoietin; DLL4, delta-like 4; ICAM-1, intercellular adhesion molecule-1; VCAM-1, vascular cell adhesion molecule-1; MMPs, matrix metalloproteinases; UPS, ubiquitin-proteasome system; bFGF, basic fibroblast growth factor; ANG, angiopoietin; TRIM, tripartite motif protein; UBE2D, ubiquitin-conjugating enzyme E2D; Tie2, tyrosine kinase receptor 2; Grb, GF receptor-bound protein; SMURF, SMAD ubiquitination regulatory factor; SMAD7, recombinant mothers against decapentaplegic homolog 7.