Advanced glycation end products-related modulation of cathepsin L and NF-κB signalling effectors in retinal pigment epithelium lead to augmented response to TNFα

The retinal pigment epithelium (RPE) plays a central role in neuroretinal homoeostasis throughout life. Altered proteolysis and inflammatory processes involving RPE contribute to the pathophysiology of age-related macular degeneration (AMD), but the link between these remains elusive. We report for the first time the effect of advanced glycation end products (AGE)-known to accumulate on the ageing RPE's underlying Bruch's membrane in situ-on both key lysosomal cathepsins and NF-κB signalling in RPE. Cathepsin L activity and NF-κB effector levels decreased significantly following 2-week AGE exposure. Chemical cathepsin L inhibition also decreased total p65 protein levels, indicating that AGE-related change of NF-κB effectors in RPE cells may be modulated by cathepsin L. However, upon TNFα stimulation, AGE-exposed cells had significantly higher ratio of phospho-p65(Ser536)/total p65 compared to non-AGEd controls, with an even higher fold increase than in the presence of cathepsin L inhibition alone. Increased proportion of active p65 indicates an AGE-related activation of NF-κB signalling in a higher proportion of cells and/or an enhanced response to TNFα. Thus, NF-κB signalling modulation in the AGEd environment, partially regulated via cathepsin L, is employed by RPE cells as a protective (para-inflammatory) mechanism but renders them more responsive to pro-inflammatory stimuli.