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M.–A. Laurin, C. Salesse; Role of Phospholipases A2 in the Phagocytosis of Rod Outer Segments by RPE Cells . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2870.
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RPE cells express two forms of secreted phospholipases A2 (sPLA2) : types IIA and V. These enzymes have been shown to be involved in macrophage endocytosis by interacting with a sPLA2 180 kD receptor (M–type receptor). At the cellular level, the endocytosis and phagocytosis mechanisms are known to be very closely related. These enzymes are also implicated in the cleavage of membrane phospholipids and were reported to facilitate fusion between cell membranes. The purpose of this study is to determine if the sPLA2s secreted by RPE cells are involved in photoreceptor outer segment (OS) phagocytosis and if it so, by which mechanisms.
RT–PCR experiments were performed to determine if the sPLA2 180 kD receptor is expressed by RPE/ARPE–19 cells. Transient mammalian vectors expressing types IIA and V sPLA2s were constructed using RPE cDNA and were transfected in ARPE–19 cells. Phagocytosis assays, using bovine OS bound with fluorescein, were performed with ARPE–19 transfected with types IIA and V sPLA2s. Assays were also conducted with exogenous types IB and IA sPLA2s that are known to respectively bind and not to bind the sPLA2 receptor.
The 180 kD sPLA2 receptor transcript is strongly expressed by both primary cultures of RPE cells as well as by ARPE–19 cell line. The overexpression of type IIA, V as well as the addition of exogenous IB sPLA2s in the culture media have been found to enhance OS phagocytosis rate by ARPE–19 cells whereas type IA sPLA2 has no effect.
The data suggest that the phagocytosis rate of OS by ARPE–19 is enhanced by different sPLA2s. These sPLA2 are well known to bind the sPLA2 180 kD receptor which is expressed by RPE cells. Furthermore, type IA sPLA2, which does not bind the receptor, does not enhance phagocytosis. Altogether, these data suggest that sPLA2s are implicated in the phagocytosis of OS through their interaction with a receptor and not by their enzymatic activity.
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