HDL-apoA-I induces the expression of angiopoietin like 4 (ANGPTL4) in endothelial cells via a PI3K/AKT/FOXO1 signaling pathway

Abstract

Background: High Density Lipoprotein (HDL) and its main protein component, apolipoprotein A-I (apoA-I), have numerous atheroprotective functions on various tissues including the endothelium. Therapies based on reconstituted HDL containing apoA-I (rHDL-apoA-I) have been used successfully in patients with acute coronary syndrome, peripheral vascular disease or diabetes but very little is known about the genomic effects of rHDL-apoA-I and how they could contribute to atheroprotection.

Objective: The present study aimed to understand the endothelial signaling pathways and the genes that may contribute to rHDL-apoA-I-mediated atheroprotection.

Methods: Human aortic endothelial cells (HAECs) were treated with rHDL-apoA-I and their total RNA was analyzed with whole genome microarrays. Validation of microarray data was performed using multiplex RT-qPCR. The expression of ANGPTL4 in EA.hy926 endothelial cells was determined by RT-qPCR and Western blotting. The contribution of signaling kinases and transcription factors in ANGPTL4 gene regulation by HDL-apoA-I was assessed by RT-qPCR, Western blotting and immunofluorescence using chemical inhibitors or siRNA-mediated gene silencing.

Results: It was found that 410 transcripts were significantly changed in the presence of rHDL-apoA-I and that angiopoietin like 4 (ANGPTL4) was one of the most upregulated and biologically relevant molecules. In validation experiments rHDL-apoA-I, as well as natural HDL from human healthy donors or from transgenic mice overexpressing human apoA-I (TgHDL-apoA-I), increased ANGPTL4 mRNA and protein levels. ANGPTL4 gene induction by HDL was direct and was blocked in the presence of inhibitors for the AKT or the p38 MAP kinases. TgHDL-apoA-I caused phosphorylation of the transcription factor forkhead box O1 (FOXO1) and its translocation from the nucleus to the cytoplasm. Importantly, a FOXO1 inhibitor or a FOXO1-specific siRNA enhanced ANGPTL4 expression, whereas administration of TgHDL-apoA-I in the presence of the FOXO1 inhibitor or the FOXO1-specific siRNA did not induce further ANGPTL4 expression. These data suggest that FOXO1 functions as an inhibitor of ANGPTL4, while HDL-apoA-I blocks FOXO1 activity and induces ANGPTL4 through the activation of AKT.

Conclusion: Our data provide novel insights into the global molecular effects of HDL-apoA-I on endothelial cells and identify ANGPTL4 as a putative mediator of the atheroprotective functions of HDL-apoA-I on the artery wall, with notable therapeutic potential.

Keywords: Angiopoietin like 4; Apolipoprotein A-I; Endothelial cells; Forkhead box 1; High Density Lipoprotein; Microarrays.