Our results suggest that the gradual exposure of RPE cells to AGEs may impair their ability to maintain a highly polarized state and secretion program, which in turn could, in time, affect the overall transport and, potentially, barrier function of the monolayer. This is underscored by the alteration in the directional secretion of not just cystatin C but also PEDF, indicating a wider effect on the cells. Indeed, it was demonstrated that attainment of a fully polarized conformation in culture is enough to augment overall protein secretion by hfRPE,
60 highlighting the importance of this state for correct functional secretion patterns. Furthermore, in a study comparing the cell line ARPE19 (suggested by the authors to more closely mimic an aged phenotype) to hfRPE, a decrease in total VEGF secretion was observed, and also alterations in its polarized secretion were noted.
61 These findings pointed out the potential of the aging process to alter the polarized state of cells. In alveolar type II cells, varying components of the substratal ECM can affect intracellular organization, cell polarity, and TER measurements.
62 Advanced glycation end product–induced changes in the BrM could have similar consequences on RPE cells. ARPE19 cells cultured on an AGE-treated BrM mimic for 4 weeks displayed marked ultrastructural differences to those cultured on untreated ECM.
23 Perhaps the most relevant of these changes, in terms of maintained barrier function, was in the apparent reduction in tight-junction formation in AGE-treated cells. Studies in retinal endothelial cells demonstrated that short-term exposure to AGEs can increase permeability across the monolayer, possibly by affecting tight junctions.
63 A similar short-term study carried out in ARPE19 cells was also able to show an AGE-dependent decrease in the barrier function of the monolayer, resulting in decreased TER measurements.
64 Our study represents the first analysis of the long-term effects of AGEs on well-polarized RPE function, applied to a specific gene-expression product.