Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) Promotes Regeneration of Corneal Epithelial Cells via the NR4A1 Signaling Pathway

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide that promotes tear secretion from the lacrimal gland. PACAP is known to be involved in corneal injury repair, but as the mechanism underlying its effects remains unclear, we used a mouse corneal wound-injury model and cultured human corneal epithelial cells to investigate further. PAC1 receptor (PAC1-R) mRNA and associated immunoreactivity were detected in the mouse corneal epithelium. In corneal wound-injury model mice, PACAP eye drops significantly reduced the injured area 12 hours after the wound insult compared to control, and this effect was abolished by co-treatment with a PACAP receptor antagonist. Corneal healing was delayed in the heterozygous PACAP knockout and PAC1-R knockout mice. Despite attenuation of corneal healing following the surgical removal of the lacrimal gland in wild-type (WT) mice, PACAP eye drops significantly improved corneal repair. In vitro, PACAP treatment significantly reduced the injury area in cultured human corneal epithelial cells, and this effect was abolished by Ara-C, a cell proliferation inhibitor. Results of DNA whole-genome microarray analysis suggested that the nuclear receptor NR4A1 is crucial for corneal epithelial proliferation. THPN, which induces the nuclear export of NR4A1, suppressed the PACAP-induced proliferation of human corneal epithelial cells and inhibited corneal repair in mice. These data suggest that PACAP stimulates corneal repair by promoting corneal epithelial proliferation via PAC1-R and NR4A1 transcriptional activity. These findings suggest that PACAP could be a promising candidate for eye-drop medication to treat corneal injuries, including dry eye syndrome.