Apoprotein E isoform-dependent expression and secretion of pro-inflammatory cytokines TNF-α and IL-6 in macrophages

The anti-atherogenic properties of human apoprotein E-associated lipoproteins have been partially attributed to its anti-inflammatory properties. We studied if endogenously expressed apoprotein E (apoE) elicits isoform-dependent effects on pro-inflammatory cytokine expression and secretion. Mouse J774A.1 peritoneal macrophages without native expression of apoE were used to establish cell lines with stable expression of the three human apoE isoforms, apoE2, apoE3 and apoE4. In the presence of lipopolysaccharide (LPS), expression and secretion of TNF-α and IL-6 in cells expressing different apoE isoforms were determined by RT-PCR, immunoblotting and ELISA assays. ApoE3-expressing cells have significantly lower expression and secretion levels of the two cytokines as compared to cells with apoE2 and apoE4 expression. Such observations were accompanied with the lowest ERK1/2 activity in apoE3-expressing cells. Further study shows that the apoE isoform-dependent variations of TNF-α and IL-6 expression/secretion in macrophages are diminished in the presence of ERK1/2 inhibitor U0126. In conclusion, apoE elicits isoform-dependent effects on macrophage TNF-α and IL-6 expression as well as secretion. The ERK1/2 signaling pathways are involved in mediating such apoE isoform-dependent effects.