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IntroductionLipoprotein(a) (Lp(a)) is an LDL-like particle with an additional apolipoprotein (apo)(a) covalently attached. Elevated levels of circulating Lp(a) are a risk factor for atherosclerosis. A proinflammatory role for Lp(a) has been proposed, but its molecular details are incompletely defined.Methods and resultsTo explore the effect of Lp(a) on human macrophages we performed RNA sequencing on THP-1 macrophages treated with Lp(a) or recombinant apo(a), which showed that especially Lp(a) induces potent inflammatory responses. Thus, we stimulated THP-1 macrophages with serum containing various Lp(a) levels to investigate their correlations with cytokines highlighted by the RNAseq, showing significant correlations with caspase-1 activity and secretion of IL-1β and IL-18. We further isolated both Lp(a) and LDL particles from three donors and then compared their atheroinflammatory potentials together with recombinant apo(a) in primary and THP-1 derived macrophages. Compared with LDL, Lp(a) induced a robust and dose-dependent caspase-1 activation and release of IL-1β and IL-18 in both macrophage types. Recombinant apo(a) strongly induced caspase-1 activation and IL-1β release in THP-1 macrophages but yielded weak responses in primary macrophages. Structural analysis of these particles revealed that the Lp(a) proteome was enriched in proteins associated with complement activation and coagulation, and its lipidome was relatively deficient in polyunsaturated fatty acids and had a high n-6/n-3 ratio promoting inflammation.DiscussionOur data show that Lp(a) particles induce the expression of inflammatory genes, and Lp(a) and to a lesser extent apo(a) induce caspase-1 activation and IL-1 signaling. Major differences in the molecular profiles between Lp(a) and LDL contribute to Lp(a) being more atheroinflammatory.

Abstract Background The focus of studies on high-density lipoproteins (HDL) has shifted from HDL-cholesterol (HDL-C) to HDL function. We recently demonstrated that low USF1 expression in mice and humans associates with high plasma HDL-C and low triglyceride levels, as well as protection against obesity, insulin resistance, and atherosclerosis. Here, we studied the impact of USF1 deficiency on HDL functional capacity and macrophage atherogenic functions, including inflammation, cholesterol efflux, and cholesterol accumulation. Methods We used a congenic Usf1 deficient mice in C57Bl/6JRccHsd background and blood samples were collected to isolate HDL for structural and functional studies. Lentiviral preparations containing the USF1 silencing shRNA expression vector were used to silence USF1 in human THP-1 and Huh-7 cells. Cholesterol efflux from acetyl-LDL loaded THP-1 macrophages was measured using HDL and plasma as acceptors. Gene expression analysis from USF1 silenced peritoneal macrophages was carried out using Affymetrix protocols. Results We show that Usf1 deficiency not only increases HDL-C levels in vivo, consistent with elevated ABCA1 protein expression in hepatic cell lines, but also improves the functional capacity of HDL particles. HDL particles derived from Usf1 deficient mice remove cholesterol more efficiently from macrophages, attributed to their higher contents of phospholipids. Furthermore, silencing of USF1 in macrophages enhanced the cholesterol efflux capacity of these cells. These findings are consistent with reduced inflammatory burden of USF1 deficient macrophages, manifested by reduced secretion of pro-inflammatory cytokines MCP-1 and IL-1β and protection against inflammation-induced macrophage cholesterol accumulation in a cell-autonomous manner. Conclusions Our findings identify USF1 as a novel factor regulating HDL functionality, showing that USF1 inactivation boosts cholesterol efflux, reduces macrophage inflammation and attenuates macrophage cholesterol accumulation, linking improved macrophage cholesterol metabolism and inflammatory pathways to the antiatherogenic function of USF1 deficiency.