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LAMMP Seminar Video
Photoprotective and antioxidant functions of melanin in the eye
Tadeusz Sarna, PhD

Pigment epithelial cells and melanocytes synthesize melanin in the form as distinct pigment granules – the melanosomes. Although in human skin and eye melanocytes, melanosomes often contain a mixture of so-called eumelanin and pheomelanin, in retinal pigment epithelium (RPE) only eumelanin is present. Compared to eumelanin, pheomelanin is believed to be more photoreactive and phototoxic. Unlike in the skin, melanin in the eye, particularly in RPE cells, is formed early during fetal development and undergoes very little or no metabolic turnover afterward. However, being exposed to intense visible light from focal irradiation and high oxygen tension, melanin in the human RPE may undergo substantial oxidative modification that could affect its normal biological functions. In my talk, I will discuss data that were obtained in selected model systems of different complexity, in which we studied antioxidant and photoprotective properties of untreated bovine and porcine RPE melanosomes, as well as partially photobleached melanosomes – our experimental model of young and aged human RPE melanosomes, respectively. The data indicate that while untreated RPE melanosomes protect unsaturated lipids against peroxidation induced by iron/ascorbate and photosensitized reaction, experimentally photoaged melanosomes loose their antioxidant capability and even become prooxidant. The effect of untreated and photobleached melanosomes, introduced by phagocytosis into ARPE-19 cells, on survival of the cells subjected to hydrogen peroxide or photic stress, was tested by dynamic cytotoxicity assay. The results show that melanosomes perform a cytoprotective function within cultured cells by acting as an antioxidant. On the other hand photobleaching diminished the stress resistance conferred by the pigment granules. In addition, photobleached melanosomes photosensitized the cells making them more susceptible to photic stress.

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