Effect of LPS on basal and induced apo E secretion by 25-OH chol and 9cRA in differentiated CaCo-2

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Abstract
The infection and inflammation process is associated with disturbances in lipid and lipoprotein metabolism. The apolipoprotein E (apo E) plays an important role in the lipoprotein metabolism and has been linked to inflammatory disease such as atherosclerosis and Alzheimer disease. An anti‐inflammatory effect has also been suggested. The heterodimer nuclear receptor Liver‐X‐Receptorα/Retinoid‐X‐Receptor (LXRα/RXR) is considered to be a transcription factor for apo E. The aim of this study was to determine whether lipopolysaccharide (LPS) (principal component of the outer membrane Gram‐negative bacteria) has an effect on apo E secretion by intestinal mucosa cells, using the Caco‐2 cell line. Differentiated Caco‐2 cells grown on filter inserts were incubated apically with LPS and/or 25‐hydroxycholesterol (25‐OH chol) and 9 cis retinoic acid (9cRA), ligands of LXR and RXR, respectively. The apical and basolateral media were separately collected. Apo E was detected by specific antibodies after protein separation by Two‐dimensional nondenaturing gradient gel electrophoresis and apo E secreted in the cell culture media was measured by enzyme linked immunosorbent assay (ELISA). Apo E mRNA was analyzed by reverse transcription‐polymerase chain reaction (RT‐PCR). LXRα and RXR mass was analyzed by Western Blot. We demonstrate here that CaCo‐2 cells secrete apo E, by either apical or basolateral sides, associated with a high‐density like lipoprotein, with a stoke's diameter comprised between 7.10 and 8.16 nm. We show that only apical secretion is decreased by LPS in a dose and time dependent manner. This is associated with a decrease in apo E gene expression contrasting with an increase of Il‐8, a chemokine factor. Moreover, we demonstrate that only basolateral apo E secretion by CaCo‐2 is significantly increased by 25‐OH chol and 9cRA while apical secretion remains unchanged. LPS does not decrease the 25‐OH chol and 9cRA mediated apo E secretion in basolateral compartment, while apical secretion is diminished under these circumstances. Our results provide evidence for the polarized secretion of apo E by intestinal epithelium. They also demonstrate that apo E secretion by CaCo‐2 cell line is decreased by LPS through an LXRα/RXR independent signaling pathway.