Cytokine response to lipoprotein lipid loading in human monocyte-derived macrophages

Macrophage foam cell formation is a prominent feature of human atherosclerotic plaques, usually considered to be correlated to uptake of and inflammatory response to oxidized low density lipoproteins (OxLDL). However, there are alternative pathways for formation of macrophage foam cells and the effect of such lipid loading on macrophage function remains to be fully characterized. In the present study we investigated basal and inducible cytokine expression in primary human macrophages either loaded with triglycerides through incubation with very low density lipoproteins (VLDL) or with cholesterol through incubation with aggregated LDL (AgLDL). We then analyzed how foam cell lipid content affected secretion of three pro-inflammatory cytokines: interleukin-1beta (IL-1beta), interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha), and of one chemokine: interleukin-8 (IL-8), all of which are considered pro-inflammatory, pro-atherosclerotic, and are expressed by cells in atherosclerotic tissue.

Formation of triglyceride-loaded foam cells resulted in a four-fold increase in basal IL-1beta secretion, whereas cholesterol loading lacked significant effect on IL-1beta secretion. In contrast, secretion of TNF-alpha and IL-6 decreased significantly following both cholesterol and triglyceride loading, with a similar trend for secretion of IL-8. Lipid loading did not affect cell viability or expression of caspase-3, and did not significantly affect macrophage ability to respond to stimulation with exogenous TNF-alpha.

Lipid loading of primary human macrophages resulted in altered cytokine secretion from cells, where effects were similar regardless of neutral lipid composition of cells. The exception was IL-1beta, where triglyceride, but not cholesterol, lipid loading resulted in a stimulation of basal secretion of the cytokine. It is apparent that macrophage cytokine secretion is affected by lipid loading by lipoproteins other than OxLDL. As both VLDL and AgLDL have been found in the vessel wall, macrophage cytokine response to uptake of these lipoproteins may have a direct effect on atherosclerotic development in vivo. However, macrophage neutral lipid amount and composition did not affect cellular activation by exogenous TNF-alpha, making it likely that lipoprotein lipid loading can affect foam cell cytokine secretion during basal conditions but that the effects can be overruled by TNF-alpha during acute inflammation.