Increased Transcription of IL-8 in Endothelial Cells Is Differentially Regulated by TNF-α and Oxidized Phospholipids

Abstract

Oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (Ox-PAPC) upregulates a spectrum of inflammatory cytokines and adhesion molecules different from those induced by classic inflammatory mediators such as tumor necrosis factor-α (TNF-α) or lipopolysaccharide. Interestingly, Ox-PAPC also induces the expression of a set of proteins similar to those induced by TNF-α or lipopolysaccharide, which include the chemokines monocyte chemotactic protein-1 (MCP-1) and interleukin (IL)-8. To elucidate the molecular mechanisms of Ox-PAPC-induced gene expression and to determine whether Ox-PAPC and other inflammatory mediators such as TNF-α utilize common signaling pathways, we examined the transcriptional regulation of IL-8 by Ox-PAPC and TNF-α in human aortic endothelial cells. Both Ox-PAPC and TNF-α induced the expression of IL-8 mRNA in a dose-dependent fashion; however, the kinetics of IL-8 mRNA accumulation between the 2 ligands differed. Ox-PAPC-induced IL-8 mRNA was seen as early as 30 minutes, peaked between 4 and 8 hours, and decreased substantially by 24 hours. In contrast, TNF-α-induced IL-8 mRNA synthesis was elevated at 30 minutes, peaked at 2 hours, and reached basal/undetectable levels by 6 hours. Actinomycin D experiments suggested that both Ox-PAPC and TNF-α regulate the expression of IL-8 at the transcriptional level. Furthermore, the half-life of IL-8 mRNA for both ligands was similar (<30 minutes), suggesting that mRNA stability was not responsible for the differences in the kinetics of IL-8 accumulation between the 2 ligands. Transient transfection studies with reporter constructs containing 1.48 kb of the IL-8 promoter identified an Ox-PAPC-specific response region between −133 and −1481 bp of the IL-8 promoter. In contrast, TNF-α activation of the IL-8 promoter was mediated almost entirely through the nuclear factor-κB and activation protein-1 response elements present between −70 and −133 bp of the IL-8 promoter. Thus, although Ox-PAPC and TNF-α both induced IL-8 synthesis, our data suggest that the 2 ligands utilize different mechanisms in the regulation of IL-8 transcription.