**3.4 Occludin and PKA pathway**

PKA, a cAMP-dependent kinase involved in the regulation of vascular endothelium, belongs to AGA kinase. Recently, it was reported in the literature that a novel β-adrenergic receptor agonist [complex 49b]-treated diabetic retinal endothelial cells could activate the PKA signaling pathway, promote the expression of occludin in retinal vascular endothelium, increase vascular tight junctions, and reduce endothelial cell apoptosis [27]. cAMP/PKA signal transduction is involved in the increase of blood-tumor barrier permeability mediated by bradykinin and promotes the up-regulation of occludin expression [28]. Glucagon-like peptide-1 (GLP-1) activates the cAMP/PKA signaling pathway to promote occludin expression and maintain the integrity of the blood-brain barrier in rat primary brain capillary endothelial cells [29].

### **3.5 Occludin and AMP-activated protein kinase [AMPK] pathway**

AMPK is a serine/threonine protein kinase involved in the regulation of cellular and body metabolism. AMPK activation counteracts oxidative stress by inhibiting the production of reactive oxygen derived by NAD[P]H oxidase in endothelial cells [30]. Stimulation of lipopolysaccharide in aging mice can significantly inhibit the activation of AMPK pathway in cerebral vascular endothelial cells, up-regulate the production of NAD[P]H oxidase, and reduce the expression of occludin protein, leading to blood-brain barrier disorders [31]. AMPK kinase inhibits the activation of inflammasome NLRP3 through the mTOR/ULK1 pathway-mediated autophagy, promotes the expression of occludin, and protects the blood-brain barrier in human brain capillary endothelial cells cultured *in vitro* [18]. Studies reported that occludin can negatively regulate AMPK activity to affect blood glucose uptake and energy production [32]. In conclusion, there is a strong connection between the energy metabolism pathway AMPK and occludin.
