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432

G. Malek et al.

Fig. 49.1 Protein (a) and RNA (b) expression of three PPAR isoforms in extracts of human cultured ARPE19 cells

homeostasis in skeletal muscle. We have found that all three isoforms of PPARs are expressed in an immortalized human RPE cell line (ARPE19; Fig. 49.1a, b). Furthermore, robust PPAR-α, -β and -γ DNA binding activity in nuclear fractions of ARPE19 was seen using a PPAR transcription factor assay (Cayman Chemical-data not shown)

49.2 LcPUFA Regulates Gene Expression in ARPE19 Cells

49.2.1 Purpose and Methods

To obtain an overview of PUFA-regulated genes in ARPE19 cells, relevant to lipid metabolism, we grew ARPE19 cells (ATCC) in media supplemented with 20 μM PUFAs [n-6: arachidonic acid (AA) or n-3: docosahexanoic acid (DHA)] or 1% FBS supplemented DMEM/F12 media for over 3 weeks and post-confluence. PUFA and vehicle treated cells were harvested and total RNA was isolated using Qiagen micro RNeasy kit. Total RNA was reverse transcribed using the iScript cDNA synthesis kit (BioRad). Lipid-pathway focused gene expression kit (SABiosciences) along with real-time PCR was used to evaluate 80 genes in RPE cells. Quantitative PCR was performed using the BioRad icycler Realtime PCR system using cDNA, appropriate primers and iQ SYBR Green Supermix detection reagent. The expression data were normalized to 3 different housekeeping genes.

49.2.2 Results

Chronic lipid loading of ARPE19 cells with AA or DHA at concentrations of greater than 50 μM was toxic to the cells (data not shown). Amongst the 80 genes

49 PPAR Nuclear Receptors and Altered RPE Lipid Metabolism in ARMD

433

Fig. 49.2 Polyunsaturated fatty acids regulate gene expression in ARPE19 cells. These genes can broadly be categorized as extracellular matrix molecules, genes involved in lipid sequestration, synthesis and metabolism, inflammatory-related genes and growth factors

examined 18 were upregulated and 11 were downregulated with a 1.5-fold or greater difference between either PUFA treated group versus control. PUFAs were seen to differentially regulate lipid-related, extracellular matrix (ECM), inflammatory and growth factor genes (Fig. 49.2).

PUFA feeding of cells resulted in upregulation of genes implicated in atherosclerosis such as Selectin P-ligand (SELPLG) and adipogenic genes including adipose differentiation-related protein (ADRP). ADRP is a ubiquitously expressed protein, found in the highest levels in adipose tissue. Its expression is increased in cells under conditions that increase lipid accumulation and triacylglycerol synthesis. Another gene of interest is TGFβ which was also detected at higher levels in PUFA treated cells. Overproduction of TGFβ may stimulate production of ECMs such as collagen, fibronectin and elastin as seen in wound healing (Sime 2008). PPRE, the location on genes to which the PPAR-RXR heterodimer binds to in order to initiate gene transcription, is not located in the promoter of ECM, supporting the idea that intermediary gene(s) may be involved in this pathway.

49.2.3 Discussion

Lipid dysregulation plays a role in the pathogenesis of ARMD, a disease characterized in part by alterations in the content and composition of extracellular matrix. We believe that this altered matrix has the ability to promote further deposit formation and potentially neovascularization. Lower dietary intake of n-3 PUFAs is associated with decreased risk for progression of ARMD. PUFAs have been shown to affect gene expression through at least three different nuclear receptors; PPAR, liver X receptor and hepatocyte nuclear factor-4 (Sampath and Ntambi 2005). Nuclear PPARs in RPE cells are constitutively activated by lipids from adjacent photoreceptors and diet and have a detectable baseline activity. Following PUFA treatment we found elevated levels of ADRP, a ubiquitously expressed protein increased in cells under conditions of lipid sequestration as well several other genes involved in lipid