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Identification of Intracellular Phospholipases A₂ in the Human Eye: Involvement in Phagocytosis of Photoreceptor Outer Segments

¹From the Eye Pathology Institute, the ³Department of Medical Anatomy, The Panum Institute, and the ⁵Institute of Molecular Biology and Physiology, University of Copenhagen, Copenhagen, Denmark; the ²Department of Ophthalmology, Glostrup University Hospital, Copenhagen, Denmark; the ⁴Department of Ophthalmology, Second Clinical Hospital, Jilin University, Changchun, Peoples Republic of China; and the ⁶Neuroscience Center of Excellence and Department of Ophthalmology, Louisiana State University Health Sciences Center School of Medicine, New Orleans, Louisiana.

PURPOSE. To identify intracellular phospholipases A₂ (PLA₂) in the human retina and to explore the role of these enzymes in human retinal pigment epithelium (RPE) phagocytosis of photoreceptor outer segments (POS).

METHODS. PCR amplification and Western blot analysis were used to identify mRNA and protein expression of intracellular PLA₂ subtypes in the retinal pigment epithelial cell line ARPE-19. Immunohistochemical staining of normal human eye sections was performed to reveal the cellular location of the enzymes. A model of RPE phagocytosis of POS was used to explore the role of intracellular PLA₂ in phagocytosis. An activity assay was used to evaluate PLA₂ activity, and inhibitors of specific PLA₂ were applied to evaluate the role of PLA₂ in RPE phagocytosis.

RESULTS. Genes encoding calcium-independent (i)PLA₂, group VIA; calcium-dependent cytosolic (c)PLA₂, groups IVA, IVB, and IVC; and iPLA₂, group VIB, were identified in the human RPE cell line ARPE-19. Furthermore, protein of iPLA₂-VIA, cPLA₂-IVA, and iPLA₂-VIB were identified in ARPE-19 cells and in various parts of the normal human eye. iPLA₂-VIA protein levels were upregulated during phagocytosis, and iPLA₂-VIA activity was found to be specifically increased 12 hours after ARPE-19 cells were fed with POS. Finally, RPE phagocytosis was inhibited by the iPLA₂-VIA inhibitor bromoenol lactone.

CONCLUSIONS. Various intracellular PLA₂ subtypes are present in the human retina. iPLA₂-VIA may play an important role in the regulation of RPE phagocytosis of POS and may also be involved in the regulation of photoreceptor cell renewal.

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