Oxidative damage to retinal pigment epithelial (RPE) has been identified as one of the major regulatory factors in the pathogenesis of age-related macular degeneration (AMD). Catalpol is an iridoid glucoside compound that has been found to possess potential antioxidant activity. In the present study, we aimed to investigate the protective effect of catalpol on RPE cells under oxidative stress and to elucidate the potential molecular mechanism involved. We found that catalpol significantly attenuated hydrogen peroxide (H₂O₂)-induced cytotoxicity, G0/G1 phase cell cycle arrest, and apoptosis in RPE cells. The overproduction of reactive oxygen species (ROS) and malondialdehyde (MDA) stimulated by oxidative stress and the corresponding reductions in antioxidant glutathione (GSH) and superoxide dismutase (SOD) levels were largely reversed by catalpol pretreatment. Moreover, catalpol pretreatment markedly activated the expression of nuclear factor (erythroid-derived 2)-like 2 (Nrf2) and its downstream antioxidant enzymes, catalase (CAT), heme oxygenase-1 (HO-1), and NADPH dehydrogenase (NQO1). It also increased the expression levels of cyclin E, Bcl-2, cyclin A, and cyclin-dependent kinase 2 (CDK2) and decreased the expression levels of Bax, Fas, cleaved PARP, p-p53, and p21 cleaved caspase-3, 8, and 9. The oxidative stress-induced formation of the Keap1/Nrf2 complex in the cytoplasm was significantly blocked by catalpol pretreatment. These results indicate that catalpol protected RPE cells from oxidative stress through a mechanism involving the activation of the Keap1/Nrf2/ARE pathways and the inactivation of oxidative stress-mediated pathways of apoptosis.

The incidence of ocular surface disease (OSD) is increasing, with a trend towards younger ages. However, it is difficult for drugs to reach the deep layers of the cornea due to ocular surface barriers, and bioavailability is less than 5%. In this study, DSPE-PEG2000 was modified with L-valine (L-Val), and an HS15/DSPE-PEG2000-L-Val nanomicelle delivery system containing baicalin (BC) (BC@HS15/DSPE-PEG2000-L-Val) was constructed using thin-film hydration, with a high encapsulation rate, small particle size and no irritation to the ocular surface. Retention experiments on the ocular surface of rabbits and an in vivo corneal permeation test showed that, compared with the control, nanomicelles not only prolonged retention time but also enhanced the ability to deliver drugs to the deep layers of the cornea. The results of a protein inhibition and protein expression assay showed that nanomicelles could increase uptake in human corneal epithelial cells (HCEC) through energy-dependent endocytosis mediated by clathrin, caveolin and the carrier pathway mediated by PepT1 by inhibiting the overexpression of claudin-1 and ZO-1 and suppressing the expression of PepT1-induced by drug stimulation. These results indicate that BC@HS15/DSPE-PEG2000-L-Val is suitable for drug delivery to the deep layers of the ocular surface, providing a potential approach for the development of ocular drug delivery systems.