Protective effects of PACAP against high glucose-induced inflammation on air-liquid interface corneal epithelium barrier

Diabetes mellitus (DM) is a chronic metabolic disease considered the "epidemic" of the new millennium, with devastating impacts on quality of life and global prevalence. One of the most common ocular complications is diabetic keratopathy (DK), often overlooked compared to other diabetes-related diseases. This disease causes significant corneal damage and is characterized by an overactive inflammatory condition, alteration of the corneal epithelial barrier, and delayed wound healing. Current therapies do not always ensure effective restoration of corneal function. In our previous study, we found that changes in the endogenous pituitary adenylate cyclase-activating polypeptide (PACAP) expression can concur for delayed epithelial wound healing in diabetic cornea. Moreover, the peptide showed cytoprotective effects on the corneal epithelium, promoting cell viability and wound healing under high glucose conditions, suggesting its potential effect in this diabetes-related disease. Therefore, this study aims to investigate the effect of PACAP against hyperglycemia-induced inflammation. To better resemble the natural conditions of corneal epithelium in vivo, an air-liquid interface (ALI) culture of the corneal epithelial cells was performed. The ALI corneal epithelium was cultured under high-glucose conditions to generate a model of DK. Our model reproduced well-established molecular and cellular characteristics of this pathology, including barrier thickness decrease and inflammation, with increased expression of IL-1β, TNF-α, and p-NF-kB. Our results indicated that PACAP significantly reduced the levels of proinflammatory cytokines IL-1β and TNF-α and inhibited the activation of NF-kB, an important mediator of inflammation. Moreover, PACAP improved epithelial morphology and corneal barrier thickness, suggesting its therapeutic potential in the treatment of DK.