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Zahra Faghiri, Nicolas G. Bazan; Selective Relocalization and Proteasomal Downregulation of PKCα Induced by Platelet-Activating Factor in Retinal Pigment Epithelium. Invest. Ophthalmol. Vis. Sci. 2006;47(1):397-404. doi: 10.1167/iovs.05-0290.
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purpose. Protein kinases C (PKCs) are key cell-signaling mediators in retinal physiology and pathophysiology. The cellular localization of PKC isoforms is important in defining their activity and specificity; the present study investigated the modulatory potential of the proinflammatory mediator platelet-activating factor (PAF) on the subcellular distribution of PKCα, β, and δ isotypes.
methods. This study used real-time visualization of green fluorescent protein fused to PKCα, β, or δ in the human retinal pigment epithelial (RPE) cell line ARPE-19.
results. In PAF-stimulated ARPE-19 cells, PKCα translocated to the plasma membrane and then colocalized with Golgi markers p230 and GM130; PKCβ translocated to the plasma membrane but not to the Golgi; and PKCδ translocated to the Golgi. Pretreatment with PKC inhibitor calphostin C abolished the PAF-induced translocation of PKCα to the plasma membrane or to the Golgi, but the Golgi inhibitor Brefeldin A only prevented the accumulation of PKCα in Golgi, without affecting its membrane relocalization. PAF promoted depletion of PKCα and δ isoforms but not that of PKCβ. Proteasome inhibitors lactacystin and MG-132 prevented the PAF-induced depletion of PKCα, but the inhibitor of lysosomal proteolysis E-64d was ineffective in rescuing PKCα.
conclusions. These results suggest that the PAF-induced downregulation of PKCα occurs principally through the proteasomal pathway. This remarkable PAF-mediated diversity in PKC translocation and downregulation highlights the significance of isotype-specific PKC activation in signaling pathways in ARPE-19 cells. These signaling events may be critical during RPE responses to oxidative stress, inflammation, and retinal degenerations, when PAF production is enhanced.
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