Photoreceptor phagocytosis selectively activates PPAR? expression in retinal pigment epithelial cells

Abstract

Phagocytosis of tips of rod outer segments (ROS) by retinal pigment epithelial (RPE) cells is vitally important for maintaining structural and functional integrity of the retina. We previously reported that receptor-mediated specific phagocytosis of ROS induces expression of early response genes coding for transcription factors. Here we study the expression of peroxisome proliferator-activated receptors (PPAR) -alpha, -delta (beta) and -gamma during ROS phagocytosis of rat RPE cells in primary cell culture, using competitive quantitative RT-PCR. During phagocytosis of ROS (but not of latex particles) by RPE cells, RT-PCR revealed a transient increase in PPARgamma mRNA expression, that peaked at 4-6 hr. We sequenced and described two alternatively spliced variants of rat PPARgamma: rPPARgamma1a and rPPARgamma1b. Both of these, along with the recently described rPPARgamma2 were induced by ROS phagocytosis. PPARalpha and PPARdelta mRNA expression was also detected in RPE cells, but the level of expression did not change during ROS phagocytosis. All-trans-retinoic acid and prostaglandin E(2) (PGE(2)) selectively potentiated both basal and ROS-phagocytosis-induced PPARgamma expression. All-trans-retinoic acid had the opposite inhibitory effect on PPARalpha and PPARdelta expression. Cycloheximide had a dual action on PPARgamma expression in RPE cells: it enhanced expression under basal conditions but repressed expression induced by ROS phagocytosis. It also stimulated expression of PPARalpha but had no effect on PPARdelta. Selective activation of PPARgamma may play an important role in regulating the expression of target genes that are involved in lipid and fatty acid metabolism in the photoreceptor renewal process.

The lipofuscin fluorophore A2E perturbs cholesterol metabolism in retinal pigment epithelial cells

“…Several lines of evidence suggest that OS phagocytosis is tightly coupled to RPE lipid and cholesterol metabolism: (i) Phagocytosis triggers expression of the AP-2 transcription factor (25), which regulates acid lipase expression (26); (ii) OS phagocytosis induces PPAR␥ (25), which increases transcription of cholesterol transporters (21); and (iii) selective metabolism of OS lipids by the RPE leads to the conservation and recycling of docosahexaenoic acid (27). In addition, RPE cells express several receptors and cholesterol transporters (28) that participate in lipoprotein uptake and cholesterol efflux.…”

The Retinal Pigment Epithelium Utilizes Fatty Acids for Ketogenesis

“…During starvation, an influx of fatty acids into the liver induces activation of PPAR (10), which, in turn, promotes ketogenesis through the up-regulation of Hmgcs2 expression; of genes involved in fatty acid transport, including acyl-CoA synthetase and fatty acid-binding protein; of genes involved in fatty acid metabolism peroxisomal acyl-coenzyme A oxidase and acyl-CoA dehydrogenase; and of genes involved in carnitine transport (carnitine palmitoyltransferase and organic cation/ carnitine transporter (OCTN2, encoded by SLC22A5)). The RPE expresses all three isoforms of PPARs (␣, ␦, and ␥), and PPAR-␥ expression was induced in RPE by phagocytosis of POS (11). These findings suggest that an influx of fatty acids into the RPE from POS phagocytosis can act as a signal for PPAR-mediated activation of fatty acid metabolism.…”

“…In addition to intracellular metabolites, we also analyzed the Ringer's solution in the apical and basolateral compartments of these hfRPE for 13 C metabolites. We found that hfRPE cells incubated with [1][2][3][4][5][6][7][8][9][10][11][12][13] C]palmitate alone released significantly more (7-fold) labeled ␤-HB (75.2 Ϯ 49.7 pmol/cm 2 RPE in a 500-l volume over 3 h) into the apical chamber compared with the basal chamber (10.5 Ϯ 18.3 pmol/cm 2 RPE in a 1500-l volume over 3 h) (Fig. 3E).…”

Regulation of Stearoyl Coenzyme A Desaturase Expression in Human Retinal Pigment Epithelial Cells by Retinoic Acid

“…phagocytosis of rod outer segments undergoing circadian shedding (35). Thus, the identification of SCD in RPE cells is quite interesting because it may play an important role in the process of complex lipid metabolism and trafficking in the RPE.…”