Classical Phospholipase D isoforms in retinal pigment epithelium cells response to stress

Abstract

Inflammation and oxidative stress (OS) are key factors in the pathogenesis of many ocular and retinal neurodegenerative disea- ses that eventually end in vision loss and blindness, such as age-related macular degeneration, diabetic retinopathy and uveitis, among others. For all these retinal inflammatory conditions, pharmacological treatment options are limited. Therefore, to provide new insights for the treatment of ocular inflammatory diseases, it is important to elucidate the molecular mechanisms involved in these disorders. Findings from our laboratory described for the first time the participation of classical Phospholipase D isoforms (PLD1 and PLD2) in the inflammatory responses of human retinal pigment epithelium (RPE) cells exposed to lipopolysaccharide (LPS) and also to high glucose levels (HG). Classical PLD isoforms catalyze phosphatidylcholine (PC) hydrolysis to generate phosphatidic acid (PA) which can be further dephosphorylated by lipid phosphate phosphatases (LPPs) to diacylglycerol (DAG). Thus, the PLD pathway modulates the activities of PA-responding proteins, as well as DAG-responding proteins. In recent years, we have reported the participation of classical PLDs in the LPS-induced inflammatory response of human RPE cells (ARPE-19 and D407 cells) through extracellular regulated kinase 1/2 (ERK1/2) activation and cyclooxygenase-2 (COX-2) induction. We also showed that even though LPS treatment promoted an inflammatory response in human RPE cells, it also triggered autophagy which served as a cell pro- tective mechanism, and that the PLD pathway modulates this autophagic process. Moreover, we described that PLD1 and PLD2 mediate the inflammatory response generated by exposing RPE cells to HG concentration. This response involved concatenated PLDs and ERK1/2 activation, NFκB (nuclear factor kappa B) nuclear translocation, expression of pro-inflammatory interleukins (IL-6 and IL-8) and COX-2, and reduced cell viability. Specific PLD1 and PLD2 pharmacological inhibitors were able to prevent the LPS-induced inflammatory responses in RPE cells and also the HG-induced inflammatory responses, OS and phagocytic function loss in RPE cells. Our findings postulate that PLD inhibition presents a novel pharmacological tool to prevent at the same time oxidative stress and the inflammatory responses, the two hallmarks of several retinal diseases that are usually addressed by different therapeutic strategies.