4.6.1 Effects on Ang-2

4.6.1 Effects on Ang-2

Angiopoietins (Angs) are a family of protein molecules discovered in recent years, with two main members: Ang-1 and Ang-2. They are primarily expressed during embryonic development to promote the completion and maturation of the cardiovascular system. In adulthood, only the female reproductive system—ovaries and uterus—exhibits high expression, while other tissues show low expression. Tumor formation involves the generation of new blood vessels, which differs from normal angiogenesis. New blood vessels grow on the existing tumor vascular bed, originating from angioblasts. The process of angiogenesis is divided into two phases: the pre-angiogenic phase and the angiogenic phase. During the angiogenic phase, endothelial cells proliferate and migrate, the extracellular matrix is remodeled, and newly formed capillaries differentiate and connect with each other. In this process, Ang-1 regulates the number and diameter of new blood vessels, influencing the adhesion between perivascular stromal cells and endothelial cells, thereby maintaining the stability and maturity of the vessels. Ang-2, on the other hand, is the natural antagonist of Ang-1 [97] and is closely related to malignant tumors. Studies have shown [98–99] that Ang-2 can promote tumor angiogenesis, facilitating tumor metastasis and invasion. Below is a summary of the structure of Ang-2 and how it promotes tumor angiogenesis.

In 1996, researchers discovered Ang-2 with a molecular weight of 75 kDa, consisting of 496 amino acids and a total length of 149 base pairs. Ang-1 and Ang-2 share 60% homology, while human and mouse Ang-2 share 85% homology. Ang-2 has a similar structure to Ang-1, with a coiled-coil domain at the amino end and a fibrinogen-like domain at the carboxyl end. A common characteristic of Angs is their ability to bind Tie-1 or Tie-2, inducing sprouting [100]. The Tie receptor family is a type of receptor tyrosine kinase superfamily (Receptor Tyrosin Kinases RTKs). This family consists of cell surface proteins that can mediate intercellular signaling into the cytoplasm, such as maintaining and establishing blood vessels [101–102]. The Tie receptor family includes two subtypes: Tie-1 and Tie-2. Under normal physiological conditions, Tie-2 mainly binds Ang-1, which is synthesized by supporting cells around the endothelial cells and then binds to the Tie-2 receptors on the surrounding endothelial cell membranes via paracrine signaling, triggering phosphorylation of the Tie-2 receptors to exert their effects—allowing extracellular signals to enter the cell through the Ang-1/Tie-2 system [103]. Ang-1 helps endothelial cells survive, inhibits apoptosis, promotes endothelial cell sprouting and migration, and maintains the maturity and stability of the vessels. Therefore, the Ang-1/Tie-2 system mainly mediates vessel maturation. Ang-2, however, is the antagonist of Ang-1. At the site of vascular invasion, Ang-2 and Tie-2 bind extensively within the endothelial cells, inhibiting the biological effects of Ang-1 and directly causing endothelial cell apoptosis and vascular shrinkage. The endothelial cells become separated from each other and from the surrounding stromal cells, destroying the vascular stability maintained by Ang-1. At this point, the vessels become highly elastic and more responsive to sprouting signals from endothelial growth factors such as VEGF, thereby promoting tumor angiogenesis and remodeling [104]. Studies have found that Ang-2/Tie-2 expression in tumor endothelial cells is significantly higher than in surrounding tissues [105], and the expression level is positively correlated with the number of tumor vessels and the malignancy of tumor development [106]. Moreover, the highest specific expression of Ang-2 occurs in the tumor edge vascular sprouting area [107], where it participates in the generation and continuation of tumor vessels, directly affecting tumor growth and metastasis.

Using immunohistochemistry to observe Ang-2 expression in tumor tissue: In all dosage groups of Pei’s Soft Liver and Anti-Bloating Pill, the number of Ang-2-positive cells was lower than in the model group, with the large- and medium-dose groups showing significant differences compared with the model group (p<0.05). The results indicate that this drug can reduce Ang-2 expression in tumor tissue of tumor-bearing mice, thereby inhibiting tumor angiogenesis and exerting an anti-tumor effect.