**3. The signaling pathway of occludin exerting the protective effect of vascular endothelium**

In recent years, the study of cellular tight junction proteins has increased dramatically. Occludin, the most typical cell tight junction protein, has attracted much attention. A large number of studies have shown that many classical signaling pathways are involved in the regulation of occludin, affecting the distribution and expression of occludin.

### **3.1 Occludin and mTOR pathway**

mTOR, consisting of two distinct complexes (mTORC1 and mTORC2), is a sensor of ATP. mTOR1, a classical metabolic pathway in mammals, is involved in the proliferation and migration of vascular endothelial cells [16] and the occurrence and development of various cardiovascular diseases. Currently, a large number of studies have focused on the role of the mTOR pathway in the regulation of occludin expression [1]. In diabetic rat model, as the phosphorylation level of mTOR increases, the downstream 4EBP1 and S6K1 proteins are activated, and the expression level of ROS is increased, which leads to the reduction of NO production in vascular endothelial cells and the decrease of the expression of occludin protein, and the vascular endothelium is damaged. However, the expression of occludin increased after adding the mTOR inhibitor rapamycin [2, 17] inhibiting the production of occludin *via* the inhibition of the PI3K/Akt/mTOR signaling pathway in the cerebral vascular endothelium may lead to the age-related leakage of the blood-brain barrier [3, 18]. Hepatocyte growth factor [HGF] secreted by mesenchymal stromal cells can activate endothelial cell mTOR/ STAT3 signaling pathway to promote endothelial occludin expression, maintaining vascular endothelial permeability homeostasis and reducing endothelial cell apoptosis [19]. In conclusion, mTOR signaling pathway is involved in the regulation of occludin expression.

#### **3.2 Occludin and VEGF pathway**

The VEGF family consists of five vascular growth factors: VEGFA, VEGFB, VEGFC, VEGFD, and placental growth factor (PIGF). VEGF binds to tyrosine kinase cell receptors (VEGFR1/fms-like tyrosine kinase 1(FLT1), VEGFR2/human kinase insertion domain receptor (KDR)/mouse fetal liver kinase 1 (FLK1), and VEGFR3/ fms-like tyrosine kinase 4 (FLT4)) to exert biological effects. Under physiological conditions, VEGF may cause neovascularization, and aggravate vascular inflammation, vascular endothelial cell proliferation, migration and invasion, and endothelial cell survival [20]. A variety of studies revealed the relationship between the VEGF pathway and occludin: (1) Phosphorylation of occludin S490 could induce endothelial cell VEGF expression and promote endothelial cell proliferation and angiogenesis both *in vivo* and *in vitro* [6]; (2) in rat model of cerebral artery occlusion, the lack of VEGF expression in microvascular endothelial cells can prevent the expression of occluding *via* inhibiting the VEGFR2/eNOS signaling pathway to further affect the permeability of the blood-brain barrier [21]; and (3) in mouse mammary cancer model, VEGF secreted by cancer cells can inhibit the expression of occludin in pulmonary vascular endothelium, increase pulmonary vascular permeability, and induce cancer cell metastasis, while overexpression of occludin can alleviate vascular endothelial disorder [22]. In conclusion, the interaction between VEGF and occluding could affect the occurrence and development of the disease.

#### **3.3 Occludin and PKC pathway**

PKC, a second messenger-regulated serine/threonine kinase, belongs to the AGA kinase family. Studies have shown that PKC can participate in the regulation of vascular endothelial integrity by interacting with the vascular endothelial marker tight junction protein [23]. Presently, a variety of *in vivo* and *in vitro* disease models have been studied to explore the role of the PKC pathway in regulating the expression and *The Role of Occludin in Vascular Endothelial Protection DOI: http://dx.doi.org/10.5772/intechopen.107479*

distribution of occludin: (1) In diabetes, metformin improves TJ barrier function by promoting the abundance and assembly of full length occludin at the TJ and that this process involves phosphorylation of the protein *via* an AMPK-PKCζ pathway [24]; (2) high glucose/ethanol induction increases the activity of NAD(P)H and promotes the phosphorylation level of subunit p47phox *via* inhibiting the activity of PKCα and PKCβ to increase the activity of matrix metalloproteinase 2 and reduce the expression of occluding, ultimately increasing vascular endothelial permeability, leading to the loss of blood-brain barrier integrity [25]; (3) in rat model of hypoxia and pulmonary ischemia-reperfusion injury, PKCα inhibits the expression of occludin in cerebral blood vessels and affects cerebral angiogenesis [26]; and (4) endothelial monocyteactivating polypeptide-II (endothelial monocyte-activating polypeptide II, EMAP-II) induced-redistribution of occludin by activating the PKCζ/PP2A signaling pathway is another mechanism in the impairment of the blood-tumor barrier [27].
