HDL is essential for atherosclerotic lesion regression in Apoe knockout mice by bone marrow Apoe reconstitution.

BACKGROUND AND AIMS: Although studies in mice have suggested that lesion regression is feasible, the underlying mechanisms remain largely unknown. Here we determined the impact of high-density lipoprotein (HDL) on atherosclerosis regression outcome. METHODS: Atherosclerotic lesion dynamics were studied upon bone marrow transplantation-mediated re-introduction of apolipoprotein E (Apoe) in Apoe knockout mice. Probucol was used to pharmacologically deplete HDL. RESULTS: Restoration of Apoe function was associated with an initial growth of atherosclerotic lesions and parallel decrease in lesional macrophage foam cell content (47 ±â€¯4% at 4 weeks versus 72 ±â€¯2% at baseline: p < 0.001), despite the fact that cholesterol levels were markedly reduced. Notably, significant lesion regression was detected from 4 weeks onwards, when plasma cholesterol levels had returned to the normolipidemic range. As a result, lesions were 41% smaller (p < 0.05) at 8 weeks than at 4 weeks after bone marrow transplantation. Regressed lesions contained an even lower level of macrophage foam cells (33 ±â€¯5%: p < 0.001) and were rich in collagen. Probucol co-treatment was associated with a 3.2-fold lower (p < 0.05) plasma HDL-cholesterol level and a more pro-inflammatory (CCR2+) monocyte phenotype. Importantly, probucol-treated mice exhibited atherosclerotic lesions that were larger than those of regular chow diet-fed bone marrow transplanted mice at 8 weeks (186 ± 15*103 µm2 for probucol-treated versus 120 ± 19*103 µm2 for controls: p < 0.05). CONCLUSIONS: We have shown that probucol-induced HDL deficiency impairs the ability of established lesions to regress in response to reversal of the genetic hypercholesterolemia in Apoe knockout mice. Our studies thus highlight a crucial role for HDL in the process of atherosclerosis regression.

Prolactin receptor antagonism uncouples lipids from atherosclerosis susceptibility.

The pituitary-derived hormone prolactin has been suggested to stimulate the development of atherosclerosis and cardiovascular disease through its effects on metabolism and inflammation. In this study, we aimed to challenge the hypothesis that inhibition of prolactin function may beneficially affect atherosclerosis burden. Hereto, atherosclerosis-susceptible LDL receptor (Ldlr) knockout mice were transplanted with bone marrow from transgenic mice expressing the pure prolactin receptor antagonist Del1-9-G129R-hPRL or their non-transgenic littermates as control. Recipient mice expressing Del1-9-G129R-hPRL exhibited a decrease in plasma cholesterol levels (-29%; P<0.05) upon feeding a Western-type diet (WTD), which could be attributed to a marked decrease (-47%; P<0.01) in the amount of cholesterol esters associated with pro-atherogenic lipoproteins VLDL/LDL. By contrast, Del1-9-G129R-hPRL-expressing mice did not display any change in the susceptibility for atherosclerosis after 12 weeks of WTD feeding. Both the absolute atherosclerotic lesion size (223 ± 33 × 10(3) µm(2) for Del1-9-G129R-hPRL vs 259 ± 32 × 10(3) µm(2) for controls) and the lesional macrophage and collagen contents were not different between the two groups of bone marrow recipients. Importantly, Del1-9-G129R-hPRL exposure increased levels of circulating neutrophils (+91%; P<0.05), lymphocytes (+55%; P<0.05), and monocytes (+43%; P<0.05), resulting in a 49% higher (P<0.01) total blood leukocyte count. In conclusion, we have shown that prolactin receptor signaling inhibition uncouples the plasma atherogenic index from atherosclerosis susceptibility in Ldlr knockout mice. Despite an associated decrease in VLDL/LDL cholesterol levels, application of the prolactin receptor antagonist Del1-9-G129R-hPRL does not alter the susceptibility for initial development of atherosclerotic lesions probably due to the parallel increase in circulating leukocyte concentrations.

Elimination of macrophages drives LXR-induced regression both in initial and advanced stages of atherosclerotic lesion development.

While numerous studies have aimed to develop strategies to inhibit the development and progression of atherosclerosis, recent attention has focussed on the regression of pre-existing atherosclerotic plaques. As important regulator of total body cholesterol homeostasis, the liver X receptor (LXR) could possibly be an important target to induce regression. Here, we describe the effect of LXR activation by the synthetic agonist T0901317 on lesion regression in different mouse models with early fatty streak lesions or advanced collagen-rich lesions. Although T0901317 caused a dramatic increase in plasma (V)LDL levels in low-density lipoprotein (LDL) receptor knockout mice, no further increase in lesion size was observed, which points to beneficial LXR activity in the vascular wall. In normolipidemic C57BL/6 mice with cholate diet-induced atherosclerotic lesions, T0901317 treatment improved plasma lipoprotein levels and induced lesion regression (-43%, p<0.05). Apolipoprotein E (APOE) reconstitution in APOE knockout mice by means of bone marrow transplantation dramatically improved plasma lipoprotein profiles and resulted in a marked regression of initial (-45%, p<0.001) and advanced lesions (-23%, p<0.01). Atherosclerosis regression was associated with a decrease in the absolute macrophage content (-84%, p<0.001). T0901317 supplementation further decreased the size of early (-71%, p<0.001 vs baseline; -48%, p<0.01 vs chow diet alone) and more advanced atherosclerotic lesions (-36%, p<0.001 and -17%, p=0.06 respectively). In conclusion, our study highlights the potential of LXR agonist T0901317 to stimulate removal of macrophages from atherosclerotic lesions ultimately leading to a highly significant plaque regression of both early and advanced atherosclerotic lesions.

Niacin reduces plasma CETP levels by diminishing liver macrophage content in CETP transgenic mice.

The anti-dyslipidemic drug niacin has recently been shown to reduce the hepatic expression and plasma levels of CETP. Since liver macrophages contribute to hepatic CETP expression, we investigated the role of macrophages in the CETP-lowering effect of niacin in mice. In vitro studies showed that niacin does not directly attenuate CETP expression in macrophages. Treatment of normolipidemic human CETP transgenic mice, fed a Western-type diet with niacin for 4 weeks, significantly reduced the hepatic cholesterol concentration (-20%), hepatic CETP gene expression (-20%), and plasma CETP mass (-30%). Concomitantly, niacin decreased the hepatic expression of CD68 (-44%) and ABCG1 (-32%), both of which are specific markers for the hepatic macrophage content. The decrease in hepatic CETP expression was significantly correlated with the reduction of hepatic macrophage markers. Furthermore, niacin attenuated atherogenic diet-induced inflammation in liver, as evident from decreased expression of TNF-alpha (-43%). Niacin similarly decreased the macrophage markers and absolute macrophage content in hyperlipidemic APOE*3-Leiden.CETP transgenic mice on a Western-type diet. In conclusion, niacin decreases hepatic CETP expression and plasma CETP mass by attenuating liver inflammation and macrophage content in response to its primary lipid-lowering effect, rather than by attenuating the macrophage CETP expression level.