Antisense oligonucleotide targeting hepatic Serum Amyloid A limits the progression of angiotensin II-induced abdominal aortic aneurysm formation

BACKGROUND AND AIMS

Obesity increases the risk for abdominal aortic aneurysms (AAA) in humans and enhances angiotensin II (AngII)-induced AAA formation in C57BL/6 mice. We reported that deficiency of Serum Amyloid A (SAA) significantly reduces AngII-induced inflammation and AAA in both hyperlipidemic apoE-deficient and obese C57BL/6 mice. The aim of this study is to investigate whether SAA plays a role in the progression of early AAA in obese C57BL/6 mice.

METHODS

Male C57BL/6J mice were fed a high-fat diet (60% kcal as fat) throughout the study. After 4 months of diet, the mice were infused with AngII until the end of the study. Mice with at least a 25% increase in the luminal diameter of the abdominal aorta after 4 weeks of AngII infusion were stratified into 2 groups. The first group received a control antisense oligonucleotide (Ctr ASO), and the second group received ASO that suppresses SAA (SAA-ASO) until the end of the study.

RESULTS

Plasma SAA levels were significantly reduced by the SAA ASO treatment. While mice that received the control ASO had continued aortic dilation throughout the AngII infusion periods, the mice that received SAA-ASO had a significant reduction in the progression of aortic dilation, which was associated with significant reductions in matrix metalloprotease activities, decreased macrophage infiltration and decreased elastin breaks in the abdominal aortas.

CONCLUSIONS

We demonstrate for the first time that suppression of SAA protects obese C57BL/6 mice from the progression of AngII-induced AAA. Suppression of SAA may be a therapeutic approach to limit AAA progression.

Antisense Oligonucleotide Targeting Hepatic Serum Amyloid A Limits the Progression of Angiotensin II-Induced Abdominal Aortic Aneurysm Formation

OBJECTIVE

Obesity increases the risk for abdominal aortic aneurysms (AAA) in humans and enhances angiotensin II (AngII)-induced AAA formation in C57BL/6 mice. Obesity is also associated with increases in serum amyloid A (SAA). We previously reported that deficiency of SAA significantly reduces AngII-induced inflammation and AAA in both hyperlipidemic apoE-deficient and obese C57BL/6 mice. In this study, we investigated whether SAA plays a role in the progression of early AAA in obese C57BL/6 mice.

APPROACH AND RESULTS

Male C57BL/6J mice were fed a high-fat diet (60% kcal as fat) throughout the study. After 4 months of diet, the mice were infused with angiotensin II (AngII) until the end of the study. Mice with at least a 25% increase in the luminal diameter of the abdominal aorta after 4 weeks of AngII infusion were stratified into 2 groups. The first group received a control antisense oligonucleotide (Ctr ASO), and the second group received ASO that suppresses SAA (SAA-ASO) until the end of the study. Plasma SAA levels were significantly reduced by the SAA ASO treatment. While mice that received the control ASO had continued aortic dilation throughout the AngII infusion periods, the mice that received SAA-ASO had a significant reduction in the progression of aortic dilation, which was associated with significant reductions in matrix metalloprotease activities, decreased macrophage infiltration and decreased elastin breaks in the abdominal aortas.

CONCLUSION

We demonstrate for the first time that suppression of SAA protects obese C57BL/6 mice from the progression of AngII-induced AAA. Suppression of SAA may be a therapeutic approach to limit AAA progression.

Serum Amyloid A augments the atherogenic effects of Cholesteryl Ester Transfer Protein

Serum Amyloid A (SAA) is predictive of cardiovascular disease (CVD) in humans and causes atherosclerosis in mice. SAA has many pro-atherogenic effects in vitro. However, HDL, the major carrier of SAA in the circulation, masks these effects. The remodeling of HDL by CETP liberates SAA restoring its pro-inflammatory activity. Here, we investigated whether deficiency of SAA suppresses the previously described pro-atherogenic effect of Cholesteryl Ester Transfer Protein (CETP). ApoE^-/- mice and apoE^-/- mice deficient in the three acute-phase isoforms of SAA (SAA1.1, SAA2.1, and SAA3; "apoE^-/- SAA-TKO") with and without AAV-mediated expression of CETP were studied. There was no effect of CETP expression or SAA genotype on plasma lipids or inflammatory markers. Atherosclerotic lesion area in the aortic arch of apoE^-/- mice was 5.9 ± 1.2%, CETP expression significantly increased atherosclerosis in apoE^-/- mice (13.1 ± 2.2%). However, atherosclerotic lesion area in the aortic arch of apoE^-/- SAA-TKO mice (5.1±1.1%) was not significantly increased by CETP expression (6.2 ± 0.9%). The increased atherosclerosis in apoE^-/- mice expressing CETP was associated with markedly increased SAA immunostaining in aortic root sections. Thus, SAA augments the atherogenic effects of CETP, which suggests that inhibiting CETP may be of particular benefit in patients with high SAA.

Adipocyte-Derived Serum Amyloid A Promotes Angiotensin II-Induced Abdominal Aortic Aneurysms in Obese C57BL/6J Mice

BACKGROUND

Obesity increases the risk for human abdominal aortic aneurysms (AAAs) and enhances Ang II (angiotensin II)-induced AAA formation in C57BL/6J mice. Obesity is also associated with increases in perivascular fat that expresses proinflammatory markers including SAA (serum amyloid A). We previously reported that deficiency of SAA significantly reduces Ang II-induced inflammation and AAA in hyperlipidemic apoE-deficient mice. In this study. we investigated whether adipose tissue-derived SAA plays a role in Ang II-induced AAA in obese C57BL/6J mice.

METHODS

The development of AAA was compared between male C57BL/6J mice (wild type), C57BL/6J mice lacking SAA1.1, SAA2.1, and SAA3 (TKO); and TKO mice harboring a doxycycline-inducible, adipocyte-specific SAA1.1 transgene (TKO-Tgfat; SAA expressed only in fat). All mice were fed an obesogenic diet and doxycycline to induce SAA transgene expression and infused with Ang II to induce AAA.

RESULTS

In response to Ang II infusion, SAA expression was significantly increased in perivascular fat of obese C57BL/6J mice. Maximal luminal diameters of the abdominal aorta were determined by ultrasound before and after Ang II infusion, which indicated a significant increase in aortic luminal diameters in wild type and TKO-TGfat mice but not in TKO mice. Adipocyte-specific SAA expression was associated with MMP (matrix metalloproteinase) activity and macrophage infiltration in abdominal aortas of Ang II-infused obese mice.

CONCLUSIONS

We demonstrate for the first time that SAA deficiency protects obese C57BL/6J mice from Ang II-induced AAA. SAA expression only in adipocytes is sufficient to cause AAA in obese mice infused with Ang II.

Serum Amyloid A Promotes Inflammation-Associated Damage and Tumorigenesis in a Mouse Model of Colitis-Associated Cancer

BACKGROUND & AIMS

Identifying new approaches to lessen inflammation, as well as the associated malignant consequences, remains crucial to improving the lives and prognosis of patients diagnosed with inflammatory bowel diseases. Although it previously has been suggested as a suitable biomarker for monitoring disease activity in patients diagnosed with Crohn's disease, the role of the acute-phase protein serum amyloid A (SAA) in inflammatory bowel disease remains unclear. In this study, we aimed to assess the role of SAA in colitis-associated cancer.

METHODS

We established a model of colitis-associated cancer in wild-type and SAA double-knockout (Saa1/2-/-) mice by following the azoxymethane/dextran sulfate sodium protocol. Disease activity was monitored throughout the study while colon and tumor tissues were harvested for subsequent use in cytokine analyses, Western blot, and immunohistochemistry +experiments.

RESULTS

We observed attenuated disease activity in mice deficient for Saa1/2 as evidenced by decreased weight loss, increased stool consistency, decreased rectal bleeding, and decreased colitis-associated tissue damage. Macrophage infiltration, including CD206+ M2-like macrophages, also was attenuated in SAA knockout mice, while levels of interleukin 4, interleukin 10, and tumor necrosis factor-ɑ were decreased in the distal colon. Mice deficient for SAA also showed a decreased tumor burden, and tumors were found to have increased apoptotic activity coupled with decreased expression for markers of proliferation.

CONCLUSION

Based on these findings, we conclude that SAA has an active role in inflammatory bowel disease and that it could serve as a therapeutic target aimed at decreasing chronic inflammation and the associated risk of developing colitis-associated cancer.

Serum amyloid A is not incorporated into HDL during HDL biogenesis

Liver-derived serum amyloid A (SAA) is present in plasma where it is mainly associated with HDL and from which it is cleared more rapidly than are the other major HDL-associated apolipoproteins. Although evidence suggests that lipid-free and HDL-associated forms of SAA have different activities, the pathways by which SAA associates and disassociates with HDL are poorly understood. In this study, we investigated SAA lipidation by hepatocytes and how this lipidation relates to the formation of nascent HDL particles. We also examined hepatocyte-mediated clearance of lipid-free and HDL-associated SAA. We prepared hepatocytes from mice injected with lipopolysaccharide or an SAA-expressing adenoviral vector. Alternatively, we incubated primary hepatocytes from SAA-deficient mice with purified SAA. We analyzed conditioned media to determine the lipidation status of endogenously produced and exogenously added SAA. Examining the migration of lipidated species, we found that SAA is lipidated and forms nascent particles that are distinct from apoA-I-containing particles and that apoA-I lipidation is unaltered when SAA is overexpressed or added to the cells, indicating that SAA is not incorporated into apoA-I-containing HDL during HDL biogenesis. Like apoA-I formation, generation of SAA-containing particles was dependent on ABCA1, but not on scavenger receptor class B type I. Hepatocytes degraded significantly more SAA than apoA-I. Taken together, our results indicate that SAA's lipidation and metabolism by the liver is independent of apoA-I and that SAA is not incorporated into HDL during HDL biogenesis.

Hepatocytes direct the formation of a pro-metastatic niche in the liver

The liver is the most common site of metastatic disease1. Although this metastatic tropism may reflect the mechanical trapping of circulating tumour cells, liver metastasis is also dependent, at least in part, on the formation of a 'pro-metastatic' niche that supports the spread of tumour cells to the liver2,3. The mechanisms that direct the formation of this niche are poorly understood. Here we show that hepatocytes coordinate myeloid cell accumulation and fibrosis within the liver and, in doing so, increase the susceptibility of the liver to metastatic seeding and outgrowth. During early pancreatic tumorigenesis in mice, hepatocytes show activation of signal transducer and activator of transcription 3 (STAT3) signalling and increased production of serum amyloid A1 and A2 (referred to collectively as SAA). Overexpression of SAA by hepatocytes also occurs in patients with pancreatic and colorectal cancers that have metastasized to the liver, and many patients with locally advanced and metastatic disease show increases in circulating SAA. Activation of STAT3 in hepatocytes and the subsequent production of SAA depend on the release of interleukin 6 (IL-6) into the circulation by non-malignant cells. Genetic ablation or blockade of components of IL-6-STAT3-SAA signalling prevents the establishment of a pro-metastatic niche and inhibits liver metastasis. Our data identify an intercellular network underpinned by hepatocytes that forms the basis of a pro-metastatic niche in the liver, and identify new therapeutic targets.

Serum Amyloid A Is an Exchangeable Apolipoprotein

Objective- SAA (serum amyloid A) is a family of acute-phase reactants that have proinflammatory and proatherogenic activities. SAA is more lipophilic than apoA-I (apolipoprotein A-I), and during an acute-phase response, <10% of plasma SAA is found lipid-free. In most reports, SAA is found exclusively associated with high-density lipoprotein; however, we and others have reported SAA on apoB (apolipoprotein B)-containing lipoproteins in both mice and humans. The goal of this study was to determine whether SAA is an exchangeable apolipoprotein. Approach and Results- Delipidated human SAA was incubated with SAA-free human lipoproteins; then, samples were reisolated by fast protein liquid chromatography, and SAA analyzed by ELISA and immunoblot. Both in vitro and in vivo, we show that SAA associates with any lipoprotein and does not remain in a lipid-free form. Although SAA is preferentially found on high-density lipoprotein, it can exchange between lipoproteins. In the presence of CETP (cholesterol ester transfer protein), there is greater exchange of SAA between lipoproteins. Subjects with diabetes mellitus, but not those with metabolic syndrome, showed altered SAA lipoprotein distribution postprandially. Proteoglycan-mediated lipoprotein retention is thought to be an underlying mechanism for atherosclerosis development. SAA has a proteoglycan-binding domain. Lipoproteins containing SAA had increased proteoglycan binding compared with SAA-free lipoproteins. Conclusions- Thus, SAA is an exchangeable apolipoprotein and increases apoB-containing lipoproteins' proteoglycan binding. We and others have previously reported the presence of SAA on low-density lipoprotein in individuals with obesity, diabetes mellitus, and metabolic syndrome. We propose that the presence of SAA on apoB-containing lipoproteins may contribute to cardiovascular disease development in these populations.

Abstract 1102: IL-6/STAT3 activation in hepatocytes drives pro-metastatic niche formation in the liver

The liver is the most common site of metastasis in pancreatic ductal adenocarcinoma (PDAC). This metastatic tropism is dependent, at least in part, on the formation of a “pro-metastatic” niche that supports tumor cell seeding and colonization in the liver. However, mechanisms that direct the formation of this niche remain poorly understood. We show using the LSL-KrasG12D/+;LSL-Trp53R172H/+;Pdx-1-Cre (KPC) model of PDAC that pancreatic tumor development enhances the susceptibility of the liver to metastatic seeding by inducing recruitment of F4/80+ and Ly6G+ myeloid cells and fibrosis within the liver. 3' mRNA sequencing (QuantSeq) on RNA isolated from the liver of KPC mice versus control PC mice revealed that the liver produces a specific set of myeloid chemoattractants, particularly serum amyloid A1 and A2 (SAA1/2), early during PDAC development. In addition, gene set enrichment analysis (GSEA) on genes upregulated in the liver of KPC mice demonstrated a significant enrichment of the interleukin 6 (IL-6)/Signal Transducer and Activator of Transcription 3 (STAT3) signaling pathway. Consistent with this finding, phosphorylation of STAT3 was detected in 20-30% of F4/80+ myeloid cells and 80-90% of hepatocytes. A requirement for IL-6/STAT3 signaling in the formation of a pro-metastatic niche was determined by comparing the metastatic potential of wild type mice, Il-6 knockout (Il-6-/-) mice, and mice treated with anti-IL-6 receptor (IL-6R) antibody after orthotopic implantation of KPC-derived PDAC cells. Compared to wild type mice, the liver of Il-6-/- mice and mice treated with anti-IL-6R antibody was less susceptible to metastatic seeding and showed significantly less accumulation of myeloid cells, fibrosis, and production of SAA1/2 in the liver. We obtained similar results with mice that lack Stat3 specifically in hepatocytes (Stat3flox/flox Alb-Cre), demonstrating that IL-6/STAT3 signaling in hepatocytes is necessary for the formation of a pro-metastatic niche in the liver. Further, using Saa1/2 double knockout (Saa-/-) mice, we found that SAA1/2 production by hepatocytes was required for formation of the pro-metastatic niche in the liver and increased susceptibility to metastatic seeding. Patients with a history of liver metastasis also showed higher levels of SAA1/2 in the plasma compared to normal donors, and SAA overexpression was detected in hepatocytes in liver biopsy samples collected from PDAC patients. Collectively, our study reveals a novel role for hepatocytes in directing the formation of a pro-metastatic niche in the liver during PDAC development and identifies IL-6/STAT3/SAA1/2 signaling as a promising therapeutic target for prevention of metastasis in PDAC.

Citation Format: Jae W. Lee, Stacy K. Thomas, Chad A. Komar, Whitney L. Gladney, Xia Hua, Dong Xin, Abraham Shaked, Mitesh J. Borad, Ramesh K. Ramanathan, Ailing Ji, Nancy R. Webb, Maria C. de Beer, Frederick C. de Beer, Paige M. Porrett, Gregory L. Beatty. IL-6/STAT3 activation in hepatocytes drives pro-metastatic niche formation in the liver [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1102.

Serum amyloid A3 is pro-atherogenic

BACKGROUND AND AIMS

Serum amyloid A (SAA) predicts cardiovascular events. Overexpression of SAA increases atherosclerosis development; however, deficiency of two of the murine acute phase isoforms, SAA1.1 and SAA2.1, has no effect on atherosclerosis. SAA3 is a pseudogene in humans, but is an expressed acute phase isoform in mice. The goal of this study was to determine if SAA3 affects atherosclerosis in mice.

METHODS

ApoE-/- mice were used as the model for all studies. SAA3 was overexpressed by an adeno-associated virus or suppressed using an anti-sense oligonucleotide approach.

RESULTS

Over-expression of SAA3 led to a 4-fold increase in atherosclerosis lesion area compared to control mice (p = 0.01). Suppression of SAA3 decreased atherosclerosis in mice genetically deficient in SAA1.1 and SAA2.1 (p < 0.0001).

CONCLUSIONS

SAA3 augments atherosclerosis in mice. Our results resolve a previous paradox in the literature and support extensive epidemiological data that SAA is pro-atherogenic.

Abstract 15: Serum Amyloid A is Not Just an HDL-Associated Lipoprotein

Serum Amyloid A (SAA) is traditionally thought to be only found with HDL; however, recently several groups have found that SAA is found on apoB-containing lipoproteins (LDL and VLDL) under some circumstances. The goal of this study was to determine the relative lipoprotein association of SAA. Native human acute phase SAA was isolated from acute phase plasma collected from patients undergoing cardiovascular bypass surgery; the SAA was purified and delipidated. Plasma was collected from a group of healthy, non-obese humans with low levels of SAA (< 2 mg/L) and lipoproteins (VLDL, LDL and HDL) were isolated by density gradient ultracentrifugation. The delipidated SAA (at 2 concentrations) was incubated in vitro with the various lipoprotein preparations then samples were re-isolated by fast protein liquid chromatography and SAA was analyzed by ELISA and immunoblot. When SAA was incubated with any single lipoprotein all of the SAA was found associated with that lipoprotein and none remained in a lipid-free form. When SAA was incubated with a mixture of VLDL, LDL and HDL (based on equal protein and corresponding to concentrations in plasma) a majority of SAA (50-60%) was found on HDL with the remaining SAA found on VLDL and LDL. When SAA first complexed to HDL was added to a mixture of SAA-free LDL and VLDL the majority of SAA remained with the HDL (76-86%) but 5-10% of the SAA was found on each of LDL and VLDL. When SAA first complexed to either apoB-containing lipoprotein was then added to a mixture of HDL and the other apoB-containing lipoprotein most SAA moved to HDL (55-70%) but the remainder was found on the apoB-containing particles. Thus, SAA can move between lipoprotein particles in vitro. To determine if the presence of SAA on apoB-containing lipoproteins had a functional effect we evaluated lipoprotein-proteoglycan binding affinity. Proteoglycan mediated lipoprotein retention in the vessel wall is thought to be one of the key steps in initiation of atherosclerosis. Compared to SAA-free LDL or VLDL, the presence of SAA on apoB-containing lipoproteins caused increased proteoglycan binding affinity. Thus, SAA is not simply an HDL lipoprotein, but SAA can move between lipoprotein particles, and the presence of SAA on apoB particles may increase their atherogenicity.

Abstract 380: SAA Lipidation and Delipidation by Hepatocytes

Serum amyloid A (SAA) is one of the most striking acute phase reactants that can rapidly increase 1000-fold in plasma concentration in response to inflammatory cytokines. SAA in lipid-free form exhibits pro-inflammatory activities, but its putative physiological function(s) are poorly understood. SAA is produced and secreted largely by the liver and is present in plasma mainly as an HDL apolipoprotein. The pathways by which SAA is lipidated and incorporated into HDL are poorly understood. Plasma SAA is cleared more rapidly than the other major HDL apolipoproteins, but pathways involved in its delipidation and plasma clearance have also not been defined. In this study we examined how SAA is lipidated in primary hepatocytes and how such lipidation relates to the formation of nascent HDL particles. Endogenous hepatocyte SAA was lipidated and released from cells as large particles that were distinct from apoA-I-containing nascent HDL’s. Unlike apoA-I, formation of these SAA-containing particles was independent of ABCA-I. Similarly, when SAA was exogenously added to cells, SAA was lipidated to form nascent particles that were distinct from apoA-I-containing particles. We further studied the interaction of lipid-free and HDL-bound SAA with hepatocytes. Both in lipid-free form and as part of HDL, SAA exhibited significantly greater binding to cells than apoA-I or apoA-II. Binding studies were also carried out with normal and acute phase HDL’s isolated from control and SAA-deficient mice. Together, the results suggested that SAA, unlike apoA-I, is selectively removed from HDL by binding to hepatocytes. These findings may provide new insights into SAA metabolism and function.

Impact of individual acute phase serum amyloid A isoforms on HDL metabolism in mice

The acute phase (AP) reactant serum amyloid A (SAA), an HDL apolipoprotein, exhibits pro-inflammatory activities, but its physiological function(s) are poorly understood. Functional differences between SAA1.1 and SAA2.1, the two major SAA isoforms, are unclear. Mice deficient in either isoform were used to investigate plasma isoform effects on HDL structure, composition, and apolipoprotein catabolism. Lack of either isoform did not affect the size of HDL, normally enlarged in the AP, and did not significantly change HDL composition. Plasma clearance rates of HDL apolipoproteins were determined using native HDL particles. The fractional clearance rates (FCRs) of apoA-I, apoA-II, and SAA were distinct, indicating that HDL is not cleared as intact particles. The FCRs of SAA1.1 and SAA2.1 in AP mice were similar, suggesting that the selective deposition of SAA1.1 in amyloid plaques is not associated with a difference in the rates of plasma clearance of the isoforms. Although the clearance rate of SAA was reduced in the absence of the HDL receptor, scavenger receptor class B type I (SR-BI), it remained significantly faster compared with that of apoA-I and apoA-II, indicating a relatively minor role of SR-BI in SAA's rapid clearance. These studies enhance our understanding of SAA metabolism and SAA's effects on AP-HDL composition and catabolism.

Serum amyloid A impairs the antiinflammatory properties of HDL

HDL from healthy humans and lean mice inhibits palmitate-induced adipocyte inflammation; however, the effect of the inflammatory state on the functional properties of HDL on adipocytes is unknown. Here, we found that HDL from mice injected with AgNO3 fails to inhibit palmitate-induced inflammation and reduces cholesterol efflux from 3T3-L1 adipocytes. Moreover, HDL isolated from obese mice with moderate inflammation and humans with systemic lupus erythematosus had similar effects. Since serum amyloid A (SAA) concentrations in HDL increase with inflammation, we investigated whether elevated SAA is a causal factor in HDL dysfunction. HDL from AgNO3-injected mice lacking Saa1.1 and Saa2.1 exhibited a partial restoration of antiinflammatory and cholesterol efflux properties in adipocytes. Conversely, incorporation of SAA into HDL preparations reduced antiinflammatory properties but not to the same extent as HDL from AgNO3-injected mice. SAA-enriched HDL colocalized with cell surface-associated extracellular matrix (ECM) of adipocytes, suggesting impaired access to the plasma membrane. Enzymatic digestion of proteoglycans in the ECM restored the ability of SAA-containing HDL to inhibit palmitate-induced inflammation and cholesterol efflux. Collectively, these findings indicate that inflammation results in a loss of the antiinflammatory properties of HDL on adipocytes, which appears to partially result from the SAA component of HDL binding to cell-surface proteoglycans, thereby preventing access of HDL to the plasma membrane.

Abstract 114: Serum Amyloid A Plays an Important Role in Making HDL Lose Its Anti-inflammatory Effect on Adipocytes

Adipose tissue inflammation associates with insulin resistance and increased cardiovascular disease risk. HDL has both anti-inflammatory and atheroprotective properties. We recently showed that HDL from healthy humans and lean mice inhibits palmitate-induced monocyte chemotactic factor expression by decreasing lipid raft (LR) content in adipocytes. However, little is known regarding whether inflammation would cause HDL to lose these anti-inflammatory effects on adipocytes. To generate HDL under conditions of sterile inflammation, HDL was purified from plasma of AgNO 3 -injected C57BL/6 mice. In 3T3L-1 adipocytes, “inflamed HDL” did not disrupt the formation of LR or inhibit monocyte chemotactic factor expression. Moreover, inflamed HDL lost its ability to facilitate cholesterol efflux in adipocytes. To determine causal factors of HDL that might be responsible for this loss of anti-inflammatory effect on adipocytes, we investigated role of serum amyloid A (SAA), levels of which are increased up to 1000 fold in HDL by acute inflammation. In a loss of function study, inflamed HDL from AgNO 3 -injectd SAA1/2 double knockout (DKO) mice restored part of its anti-inflammatory and cholesterol efflux capacity. Conversely, in a gain of function study, increasing the SAA content in uninflamed HDL by exposure to 293 HEK cells with lentiviral overexpression of SAA2 resulted in a partial loss of its anti-inflammatory effect and cholesterol efflux capacity. To determine whether HDL from moderately inflamed humans had similar effects on adipocyte inflammation, HDL isolated from patients with systemic lupus erythematosus (n=18) also demonstrated impaired anti-inflammatory effects in adipocytes. The extent of impairment associated positively with plasma SAA levels ( r =0.80; p <0.001), and negatively with cholesterol efflux capacity of the HDL ( r =-0.74; p <0.001). These findings indicate that inflamed HDL loses its anti-inflammatory effect and cholesterol efflux capacity on adipocytes. The presence of SAA in HDL may contribute to the generation of dysfunctional HDL and should be considered as a causal factor, not simply as a biomarker of dysfunctional HDL.

Nascent HDL formation by hepatocytes is reduced by the concerted action of serum amyloid A and endothelial lipase

Inflammation is associated with significant decreases in plasma HDL-cholesterol (HDL-C) and apoA-I levels. Endothelial lipase (EL) is known to be an important determinant of HDL-C in mice and in humans and is upregulated during inflammation. In this study, we investigated whether serum amyloid A (SAA), an HDL apolipoprotein highly induced during inflammation, alters the ability of EL to metabolize HDL. We determined that EL hydrolyzes SAA-enriched HDL in vitro without liberating lipid-free apoA-I. Coexpression of SAA and EL in mice by adenoviral vector produced a significantly greater reduction in HDL-C and apoA-I than a corresponding level of expression of either SAA or EL alone. The loss of HDL occurred without any evidence of HDL remodeling to smaller particles that would be expected to have more rapid turnover. Studies with primary hepatocytes demonstrated that coexpression of SAA and EL markedly impeded ABCA1-mediated lipidation of apoA-I to form nascent HDL. Our findings suggest that a reduction in nascent HDL formation may be partly responsible for reduced HDL-C during inflammation when both EL and SAA are known to be upregulated.

ATP binding cassette G1-dependent cholesterol efflux during inflammation

ATP binding cassette transporter G1 (ABCG1) mediates the transport of cellular cholesterol to HDL, and it plays a key role in maintaining macrophage cholesterol homeostasis. During inflammation, HDL undergoes substantial remodeling, acquiring lipid changes and serum amyloid A (SAA) as a major apolipoprotein. In the current study, we investigated whether remodeling of HDL that occurs during acute inflammation impacts ABCG1-dependent efflux. Our data indicate that lipid free SAA acts similarly to apolipoprotein A-I (apoA-I) in mediating sequential efflux from ABCA1 and ABCG1. Compared with normal mouse HDL, acute phase (AP) mouse HDL containing SAA exhibited a modest but significant 17% increase in ABCG1-dependent efflux. Interestingly, AP HDL isolated from mice lacking SAA (SAAKO mice) was even more effective in promoting ABCG1 efflux. Hydrolysis with Group IIA secretory phospholipase A(2) (sPLA(2)-IIA) significantly reduced the ability of AP HDL from SAAKO mice to serve as a substrate for ABCG1-mediated cholesterol transfer, indicating that phospholipid (PL) enrichment, and not the presence of SAA, is responsible for alterations in efflux. AP human HDL, which is not PL-enriched, was somewhat less effective in mediating ABCG1-dependent efflux compared with normal human HDL. Our data indicate that inflammatory remodeling of HDL impacts ABCG1-dependent efflux independent of SAA.

Intestinal epithelial serum amyloid A modulates bacterial growth in vitro and pro-inflammatory responses in mouse experimental colitis

BACKGROUND

Serum Amyloid A (SAA) is a major acute phase protein of unknown function. SAA is mostly expressed in the liver, but also in other tissues including the intestinal epithelium. SAA reportedly has anti-bacterial effects, and because inflammatory bowel diseases (IBD) result from a breakdown in homeostatic interactions between intestinal epithelia and bacteria, we hypothesized that SAA is protective during experimental colitis.

METHODS

Intestinal SAA expression was measured in mouse and human samples. Dextran sodium sulfate (DSS) colitis was induced in SAA 1/2 double knockout (DKO) mice and in wildtype controls. Anti-bacterial effects of SAA1/2 were tested in intestinal epithelial cell lines transduced with adenoviral vectors encoding the CE/J SAA isoform or control vectors prior to exposure to live Escherichia coli.

RESULTS

Significant levels of SAA1/SAA2 RNA and SAA protein were detected by in situ hybridization and immunohistochemistry in mouse colonic epithelium. SAA3 expression was weaker, but similarly distributed. SAA1/2 RNA was present in the ileum and colon of conventional mice and in the colon of germfree mice. Expression of SAA3 was strongly regulated by bacterial lipopolysaccharides in cultured epithelial cell lines, whereas SAA1/2 expression was constitutive and not LPS inducible. Overexpression of SAA1/2 in cultured epithelial cell lines reduced the viability of co-cultured E. coli. This might partially explain the observed increase in susceptibility of DKO mice to DSS colitis. SAA1/2 expression was increased in colon samples obtained from Crohn's Disease patients compared to controls.

CONCLUSIONS

Intestinal epithelial SAA displays bactericidal properties in vitro and could play a protective role in experimental mouse colitis. Altered expression of SAA in intestinal biopsies from Crohn's Disease patients suggests that SAA is involved in the disease process..

High-density lipoprotein suppresses the type I interferon response, a family of potent antiviral immunoregulators, in macrophages challenged with lipopolysaccharide

BACKGROUND

High-density lipoprotein (HDL) protects the artery wall by removing cholesterol from lipid-laden macrophages. However, recent evidence suggests that HDL might also inhibit atherogenesis by combating inflammation.

METHODS AND RESULTS

To identify potential antiinflammatory mechanisms, we challenged macrophages with lipopolysaccharide, an inflammatory microbial ligand for Toll-like receptor 4. HDL inhibited the expression of 30 (277 of 911) of the genes normally induced by lipopolysaccharide, microarray analysis revealed. One of its major targets was the type I interferon response pathway, a family of potent viral immunoregulators controlled by Toll-like receptor 4 and the TRAM/TRIF signaling pathway. Unexpectedly, the ability of HDL to inhibit gene expression was independent of macrophage cholesterol stores. Immunofluorescent studies suggested that HDL promoted TRAM translocation to intracellular compartments, which impaired subsequent signaling by Toll-like receptor 4 and TRIF. To examine the potential in vivo relevance of the pathway, we used mice deficient in apolipoprotein A-I, the major protein of HDL. After infection with Salmonella typhimurium, a Gram-negative bacterium that expresses lipopolysaccharide, apolipoprotein A-I-deficient mice had 6-fold higher plasma levels of interferon-β, a key regulator of the type I interferon response, than did wild-type mice.

CONCLUSIONS

HDL inhibits a subset of lipopolysaccharide-stimulated macrophage genes that regulate the type I interferon response, and its action is independent of sterol metabolism. These findings raise the possibility that regulation of macrophage genes by HDL might link innate immunity and cardioprotection.

Group X secretory phospholipase A(2) augments angiotensin II-induced inflammatory responses and abdominal aortic aneurysm formation in apoE-deficient mice

OBJECTIVE

Abdominal aortic aneurysm (AAA) is a complex vascular disease characterized by matrix degradation and inflammation and is a major cause of mortality in older men. Specific interventions that prevent AAA progression remain to be identified. In this study, we tested the hypothesis that Group X secretory phospholipase A(2) (GX sPLA(2)), an enzyme implicated in inflammatory processes, mediates AAA.

METHODS AND RESULTS

GX sPLA(2) was detected by immunostaining in human aneurysmal tissue and in angiotensin II (Ang II)-induced AAAs in apolipoprotein E-deficient (apoE(-/-)) mice. GX sPLA(2) mRNA was increased significantly (11-fold) in abdominal aortas of apoE(-/-) mice in response to Ang II infusion. To define the role of GX sPLA(2) in experimental AAAs, apoE(-/-) and apoE(-/-) x GX sPLA(2)(-/-) (GX DKO) mice were infused with Ang II for either 10 (n=7) or 28 (n=24-26) days. Deficiency of GX sPLA(2) significantly reduced the incidence and severity of AAAs, as assessed by ultrasound measurements in vivo of aortic lumens and by computer-assisted morphometric analyses ex vivo of external diameter. Results from gene expression profiling indicated that the expression of specific matrix metalloproteinases and inflammatory mediators was blunted in aortas from GX DKO mice compared to apoE(-/-) mice after 10-day Ang II infusion. Ang II induction of cyclooxygenase-2, interleukin-6, matrix metalloproteinase (MMP)-2, MMP-13 and MMP-14 was reduced significantly in GX DKO mice compared to apoE(-/-) mice.

CONCLUSION

GX sPLA(2) promotes Ang II-induced pathological responses leading to AAA formation.

Impact of serum amyloid A on high density lipoprotein composition and levels

Serum amyloid A (SAA) is an acute-phase protein mainly associated with HDL. To study the role of SAA in mediating changes in HDL composition and metabolism during inflammation, we generated mice in which the two major acute-phase SAA isoforms, SAA1.1 and SAA2.1, were deleted [SAA knockout (SAAKO) mice], and induced an acute phase to compare lipid and apolipoprotein parameters between wild-type (WT) and SAAKO mice. Our data indicate that SAA does not affect apolipoprotein A-I (apoA-I) levels or clearance under steady-state conditions. HDL and plasma triglyceride levels following lipopolysaccharide administration, as well as the decline in liver expression of apoA-I and apoA-II, did not differ between both groups of mice. The expected size increase of WT acute-phase HDL was surprisingly also seen in SAAKO acute-phase HDL despite the absence of SAA. HDLs from both mice showed increased phospholipid and unesterified cholesterol content during the acute phase. We therefore conclude that in the mouse, SAA does not impact HDL levels, apoA-I clearance, or HDL size during the acute phase and that the increased size of acute-phase HDL in mice is associated with an increased content of surface lipids, particularly phospholipids, and not surface proteins. These data need to be transferred to humans with caution due to differences in apoA-I structure and remodeling functions.

A murine model of obesity with accelerated atherosclerosis

The epidemic of obesity sweeping developed nations is accompanied by an increase in atherosclerotic cardiovascular diseases. Dyslipidemia, diabetes, hypertension, and obesity are risk factors for cardiovascular disease. However, delineating the mechanism of obesity-accelerated atherosclerosis has been hampered by a paucity of animal models. Similar to humans, apolipoprotein E-deficient (apoE(-/-)) mice spontaneously develop atherosclerosis over their lifetime. To determine whether apoE(-/-) mice would develop obesity with accelerated atherosclerosis, we fed mice diets containing 10 (low fat (LF)) or 60 (high fat (HF)) kcal % from fat for 17 weeks. Mice fed the HF diet had a marked increase in body weight and atherosclerotic lesion formation compared to mice fed the LF diet. There were no significant differences between groups in serum total cholesterol, triglycerides, or leptin concentrations. Plasma concentrations of the acute-phase reactant serum amyloid A (SAA) are elevated in both obesity and cardiovascular disease. Accordingly, plasma SAA concentrations were increased fourfold (P < 0.01) in mice fed the HF diet. SAA was associated with both pro- and antiatherogenic lipoproteins in mice fed the HF diet compared to those fed the LF diet, in which SAA was primarily associated with the antiatherogenic lipoprotein high-density lipoprotein (HDL). Moreover, SAA was localized with apoB-containing lipoproteins and biglycan in the vascular wall. Taken together, these data suggest male apoE-deficient mice are a model of metabolic syndrome and that chronic low level inflammation associated with increased SAA concentrations may mediate atherosclerotic lesion formation.

SR-BI selective lipid uptake: subsequent metabolism of acute phase HDL

OBJECTIVE

The purpose of this study was to investigate the interaction of SAA and SR-BI in remodeling of acute phase HDL (AP HDL).

METHODS AND RESULTS

We used SAA and SR-BI adenoviral vector expression models to study the interaction between these entities. SR-BI processing of mouse AP HDL generated progressively smaller discreet HDL particles with distinct apolipoprotein compositions. SR-BI actions segregated apolipoproteins with the smallest particles containing only apoA-I. Larger remnants contained apoA-I, apoA-II, and SAA. Small apoA-I only particles failed to associate with preformed HDL, whereas larger remnants readily did. The presence of SAA on SR-BI-processed HDL particles propelled apoA-I to a small lipid-poor form and accelerated apoA-I catabolism.

CONCLUSIONS

Data indicate that after core and surface HDL lipid perturbation by SR-BI, SAA propels apoA-I to a small lipid-poor form while accelerating HDL metabolism.

HDL remodeling during the acute phase response

OBJECTIVE

The purpose of this study was to examine the interactive action of serum amyloid A (SAA), group IIA secretory phospholipase A(2) (sPLA(2)-IIA), and cholesteryl ester transfer protein (CETP) on HDL remodeling and cholesterol efflux during the acute phase (AP) response elicited in humans after cardiac surgery.

METHODS AND RESULTS

Plasma was collected from patients before (pre-AP), 24 hours after (AP-1 d), and 5 days after cardiac surgery (AP-5 d). SAA levels were increased 16-fold in AP-1 d samples. The activity of sPLA(2)-IIA was increased from 77.7+/-38.3 U/mL (pre-AP) to 281.4+/-57.1 U/mL (AP-1 d; P<0.001). CETP mass and activity reduction was commensurate to the reduction of HDL cholesterol levels. The combined action of SAA, sPLA(2)-IIA, and CETP in vitro markedly remodeled HDL with the generation of lipid-poor apoA-I from both pre-AP and AP-1 d HDL. The net result of this remodeling was a relative preservation of ABCA1- and ABCG1-dependent cholesterol efflux during the acute phase response.

CONCLUSIONS

Our results show that the many and complex changes in plasma proteins during the acute phase response markedly remodel HDL with functional implications, particularly the relative retention of cholesterol efflux capacity.

Serum amyloid A, but not C-reactive protein, stimulates vascular proteoglycan synthesis in a pro-atherogenic manner

Inflammatory markers serum amyloid A (SAA) and C-reactive protein (CRP) are predictive of cardiac disease and are proposed to play causal roles in the development of atherosclerosis, in which the retention of lipoproteins by vascular wall proteoglycans is critical. The purpose of this study was to determine whether SAA and/or CRP alters vascular proteoglycan synthesis and lipoprotein retention in a pro-atherogenic manner. Vascular smooth muscle cells were stimulated with either SAA or CRP (1 to 100 mg/L) and proteoglycans were then isolated and characterized. SAA, but not CRP, increased proteoglycan sulfate incorporation by 50 to 100% in a dose-dependent manner (P < 0.0001), increased glycosaminoglycan chain length, and increased low-density lipoprotein (LDL) binding affinity (K(d), 29 microg/ml LDL versus 90 microg/ml LDL for SAA versus control proteoglycans; P < 0.005). Furthermore, SAA up-regulated biglycan via the induction of endogenous transforming growth factor (TGF)-beta. To determine whether SAA stimulated proteoglycan synthesis in vivo, ApoE(-/-) mice were injected with an adenovirus expressing human SAA-1, a null virus, or saline. Mice that received adenovirus expressing SAA had increased TGF-beta concentrations in plasma and increased aortic biglycan content compared with mice that received either null virus or saline. Thus, SAA alters vascular proteoglycans in a pro-atherogenic manner via the stimulation of TGF-beta and may play a causal role in the development of atherosclerosis.

SR-BI protects against endotoxemia in mice through its roles in glucocorticoid production and hepatic clearance

Septic shock results from an uncontrolled inflammatory response, mediated primarily by LPS. Cholesterol transport plays an important role in the host response to LPS, as LPS is neutralized by lipoproteins and adrenal cholesterol uptake is required for antiinflammatory glucocorticoid synthesis. In this study, we show that scavenger receptor B-I (SR-BI), an HDL receptor that mediates HDL cholesterol ester uptake into cells, is required for the normal antiinflammatory response to LPS-induced endotoxic shock. Despite elevated plasma HDL levels, SR-BI-null mice displayed an uncontrollable inflammatory cytokine response and a markedly higher lethality rate than control mice in response to LPS. In addition, SR-BI-null mice showed a lack of inducible glucocorticoid synthesis in response to LPS, bacterial infection, stress, or ACTH. Glucocorticoid insufficiency in SR-BI-null mice was due to primary adrenal malfunction resulting from deficient cholesterol delivery from HDL. Furthermore, corticosterone supplementation decreased the sensitivity of SR-BI-null mice to LPS. Plasma from control and SR-BI-null mice exhibited a similar ability to neutralize LPS, whereas SR-BI-null mice showed decreased plasma clearance of LPS into the liver and hepatocytes compared with normal mice. We conclude that SR-BI in mice is required for the antiinflammatory response to LPS-induced endotoxic shock, likely through its essential role in facilitating glucocorticoid production and LPS hepatic clearance.

HDL cholesterol transport during inflammation

PURPOSE OF REVIEW

The aim of this article is to review recent advances made towards understanding how inflammation and acute phase proteins, particularly serum amyloid A and group IIa secretory phospholipase A2, may alter reverse cholesterol transport by HDL during inflammation and the acute phase response.

RECENT FINDINGS

Findings suggest that the decreased apoA-I content and markedly increased serum amyloid A content in HDL during the acute phase response result from reciprocal and coordinate transcriptional regulation of these proteins as well as HDL remodeling by group IIa secretory phospholipase A2. Serum amyloid A functions efficiently in a lipid-free or lipid-poor form to promote cholesterol efflux by ATP binding cassette protein ABCA1, evidently by functioning directly as an acceptor for cholesterol efflux as well as by increasing the availability of cellular free cholesterol. Serum amyloid A increases the ability of acute phase HDL to serve as an acceptor for SR-BI-dependent cellular cholesterol efflux. Altered remodeling of HDL by group IIa secretory phospholipase A2 in concert with cholesterol ester transfer protein may contribute to the generation of lipid-poor apoA-I and serum amyloid A acceptors for cholesterol efflux.

SUMMARY

Current data support a model for the acute phase response in which serum amyloid A and sPLA2-IIa, present at sites of inflammation and tissue damage, play a protective role by enhancing cellular cholesterol efflux, thereby promoting the removal of excess cholesterol from macrophages.

Group V Secretory Phospholipase A2Promotes Atherosclerosis

OBJECTIVE

Group V secretory phospholipase A2 (GV sPLA2) has been detected in both human and mouse atherosclerotic lesions. This enzyme has potent hydrolytic activity towards phosphatidylcholine-containing substrates, including lipoprotein particles. Numerous studies in vitro indicate that hydrolysis of high density lipoproteins (HDL) and low density lipoproteins (LDL) by GV sPLA2 leads to the formation of atherogenic particles and potentially proinflammatory lipid mediators. However, there is no direct evidence that this enzyme promotes atherogenic processes in vivo.

METHODS AND RESULTS

We performed gain-of-function and loss-of-function studies to investigate the role of GV sPLA2 in atherogenesis in LDL receptor-deficient mice. Compared with control mice, animals overexpressing GV sPLA2 by retrovirus-mediated gene transfer had a 2.7 fold increase in lesion area in the ascending region of the aortic root. Increased atherosclerosis was associated with an increase in lesional collagen deposition in the same region. Mice deficient in bone marrow-derived GV sPLA2 had a 36% reduction in atherosclerosis in the aortic arch/thoracic aorta.

CONCLUSIONS

Our data in mouse models provide the first in vivo evidence that GV sPLA2 contributes to atherosclerotic processes, and draw attention to this enzyme as an attractive target for the treatment of atherosclerotic disease.

SR-BI-mediated selective lipid uptake segregates apoA-I and apoA-II catabolism

The HDL receptor scavenger receptor class B type I (SR-BI) binds HDL and mediates the selective uptake of cholesteryl ester. We previously showed that remnants, produced when human HDL(2) is catabolized in mice overexpressing SR-BI, become incrementally smaller, ultimately consisting of small alpha-migrating particles, distinct from pre-beta HDL. When mixed with mouse plasma, some remnant particles rapidly increase in size by associating with HDL without the mediation of cholesteryl ester transfer protein, LCAT, or phospholipid transfer protein. Here, we show that processing of HDL(2) by SR-BI-overexpressing mice resulted in the preferential loss of apolipoprotein A-II (apoA-II). Short-term processing generated two distinct, small alpha-migrating particles. One particle (8.0 nm diameter) contained apoA-I and apoA-II; the other particle (7.7 nm diameter) contained only apoA-I. With extensive SR-BI processing, only the 7.7 nm particle remained. Only the 8.0 nm remnants were able to associate with HDL. Compared with HDL(2), this remnant was more readily taken up by the liver than by the kidney. We conclude that SR-BI-generated HDL remnants consist of particles with or without apoA-II and that only those containing apoA-II associate with HDL in an enzyme-independent manner. Extensive SR-BI processing generates small apoA-II-depleted particles unable to reassociate with HDL and readily taken up by the liver. This represents a pathway by which apoA-I and apoA-II catabolism are segregated.

Serum Amyloid A Promotes Cholesterol Efflux Mediated by Scavenger Receptor B-I

Serum amyloid A (SAA) is an acute phase protein whose expression is markedly up-regulated during inflammation and infection. The physiological function of SAA is unclear. In this study, we reported that SAA promotes cellular cholesterol efflux mediated by scavenger receptor B-I (SR-BI). In Chinese hamster ovary cells, SAA promoted cellular cholesterol efflux in an SR-BI-dependent manner, whereas apoA-I did not. Similarly, SAA, but not apoA-I, promoted cholesterol efflux from HepG2 cells in an SR-BI-dependent manner as shown by using the SR-BI inhibitor BLT-1. When SAA was overexpressed in HepG2 cells using adenovirus-mediated gene transfer, the endogenously expressed SAA promoted SR-BI-dependent efflux. To assess the effect of SAA on SR-BI-mediated efflux to high density lipoprotein (HDL), we compared normal HDL, acute phase HDL (AP-HDL, prepared from mice injected with lipopolysaccharide), and AdSAA-HDL (HDL prepared from mice overexpressing SAA). Both AP-HDL and AdSAA-HDL promoted 2-fold greater cholesterol efflux than normal HDL. Lipid-free SAA was shown to also stimulate ABCA1-dependent cholesterol efflux in fibroblasts, in line with an earlier report (Stonik, J. A., Remaley, A. T., Demosky, S. J., Neufeld, E. B., Bocharov, A., and Brewer, H. B. (2004) Biochem. Biophys. Res. Commun. 321, 936-941). When added to cells together, SAA and HDL exerted a synergistic effect in promoting ABCA1-dependent efflux, suggesting that SAA may remodel HDL in a manner that releases apoA-I or other efficient ABCA1 ligands from HDL. SAA also facilitated efflux by a process that was independent of SR-BI and ABCA1. We conclude that the acute phase protein SAA plays an important role in HDL cholesterol metabolism by promoting cellular cholesterol efflux through a number of different efflux pathways.

Low-Density Lipoprotein From Apolipoprotein E-Deficient Mice Induces Macrophage Lipid Accumulation in a CD36 and Scavenger Receptor Class A-Dependent Manner

Objective— To investigate the potential of circulating low-density lipoprotein (LDL), isolated from apolipoprotein E (apoE)-deficient mice (E−/−LDL) and from LDL receptor-deficient mice (Lr−/−LDL), to induce foam cell formation.

Methods and Results— Binding studies using COS-7 cells overexpressing CD36, J774 cells, and mouse peritoneal macrophages (MPMs) unexpectedly showed for the first time that E−/−LDL, which is enriched in cholesterol, is a high-affinity ligand for CD36 and exhibited greater macrophage uptake than Lr−/−LDL or normal LDL. Minimal copper-mediated oxidization of Lr−/−LDL or C57LDL in vitro resulted in increased ligand internalization, although cell uptake of these oxidized LDLs was lower than that of E−/−LDL, even at oxidation levels similar to that found in E−/−LDL. Treatment of MPMs with E−/−LDL and Lr−/−LDL (to a 2- to 3-fold lesser extent), but not normal LDL, resulted in significant cellular cholesteryl ester accumulation and foam cell formation. Experiments using MPMs lacking CD36, scavenger receptor class A (SR-A), or both, indicated a major contribution of CD36 (≈50%), and to a lesser extent, SR-A (24% to 30%), to E−/−LDL uptake.

Conclusions— Because of its increased state of oxidation and high cholesterol content, LDL in apoE-deficient mice acts in a proatherogenic manner, without requiring further modification in the vascular wall, to induce foam cell formation through its uptake by scavenger receptors.

Serum amyloid A is a ligand for scavenger receptor class B type I and inhibits high density lipoprotein binding and selective lipid uptake

Serum amyloid A is an acute phase protein that is carried in the plasma largely as an apolipoprotein of high density lipoprotein (HDL). In this study we investigated whether SAA is a ligand for the HDL receptor, scavenger receptor class B type I (SR-BI), and how SAA may influence SR-BI-mediated HDL binding and selective cholesteryl ester uptake. Studies using Chinese hamster ovary cells expressing SR-BI showed that (125)I-labeled SAA, both in lipid-free form and in reconstituted HDL particles, functions as a high affinity ligand for SR-BI. SAA also bound with high affinity to the hepatocyte cell line, HepG2. Alexa-labeled SAA was shown by fluorescence confocal microscopy to be internalized by cells in a SR-BI-dependent manner. To assess how SAA association with HDL influences HDL interaction with SR-BI, SAA-containing HDL was isolated from mice overexpressing SAA through adenoviral gene transfer. SAA presence on HDL had little effect on HDL binding to SR-BI but decreased (30-50%) selective cholesteryl ester uptake. Lipid-free SAA, unlike lipid-free apoA-I, was an effective inhibitor of both SR-BI-dependent binding and selective cholesteryl ester uptake of HDL. We have concluded that SR-BI plays a key role in SAA metabolism through its ability to interact with and internalize SAA and, further, that SAA influences HDL cholesterol metabolism through its inhibitory effects on SR-BI-mediated selective lipid uptake.

Remodeling of HDL remnants generated by scavenger receptor class B type I

Scavenger receptor class B type I (SR-BI) mediates the selective transfer of cholesteryl ester from HDL to cells. We previously established that SR-BI overexpressed in livers of apolipoprotein A-I-deficient mice processes exogenous human HDL2 to incrementally smaller HDL particles. When mixed with normal mouse plasma either in vivo or ex vivo, SR-BI-generated HDL "remnants" rapidly remodel to form HDL-sized lipoproteins. In this study, we analyzed HDLs throughout the process of HDL remnant formation and investigated the mechanism of conversion to larger particles. Upon interacting with SR-BI, alpha-migrating HDL2 is initially converted to a prealpha-migrating particle that is ultimately processed to a smaller alpha-migrating HDL remnant. SR-BI does not appear to generate prebeta-1 HDL particles. When incubated with isolated lipoprotein fractions, HDL remnants are converted to lipoprotein particles corresponding in size to the particle incubated with the HDL remnant. HDL remnant conversion is not altered in phospholipid transfer protein (PLTP)-deficient mouse plasma or by the addition of purified PLTP. Although LCAT-deficient plasma promoted only partial conversion, this deficiency was attributable to the nature of HDL particles in LCAT-/- mice rather than to a requirement for LCAT in the remodeling process. We conclude that HDL remnants, generated by SR-BI, are converted to larger particles by rapidly reassociating with existing HDL particles in an enzyme-independent manner.

Influence of apoA-I and apoE on the formation of serum amyloid A-containing lipoproteins in vivo and in vitro

Serum amyloid A (SAA) circulates bound to HDL3 during the acute-phase response (APR), and recent evidence suggests that elevated levels of SAA may be a risk factor for cardiovascular disease. In this study, SAA-HDL was produced in vivo during the APR and without the APR by injection of an adenoviral vector expressing human SAA-1. SAA-HDL was also produced in vitro by incubating mouse HDL with recombinant mouse SAA and by SAA-expressing cultured hepatoma cells. Whether produced in vivo or in vitro, SAA-HDL floated at a density corresponding to that of human HDL3 (d 1.12 g/ml) separate from other apolipoproteins, including apolipoprotein A-I (apoA-I; d 1.10 g/ml) when either apoA-I or apolipoprotein E (apoE) was present. In the absence of both apoA-I and apoE, SAA was found in VLDL and LDL, with low levels in the HDL and the lipid-poor fractions suggesting that other HDL apolipoproteins are incapable of facilitating the formation of SAA-HDL. We conclude that SAA does not exist in plasma as a lipid-free protein. In the presence of HDL-associated apoA-I or apoE, SAA circulates as SAA-HDL with a density corresponding to that of human HDL3. In the absence of both apoA-I and apoE, SAA-HDL is not formed and SAA associates with any available lipoprotein.

ApoA-II modulates the association of HDL with class B scavenger receptors SR-BI and CD36

The class B scavenger receptors SR-BI and CD36 exhibit a broad ligand binding specificity. SR-BI is well characterized as a HDL receptor that mediates selective cholesteryl ester uptake from HDL. CD36, a receptor for oxidized LDL, also binds HDL and mediates selective cholesteryl ester uptake, although much less efficiently than SR-BI. Apolipoprotein A-II (apoA-II), the second most abundant HDL protein, is considered to be proatherogenic, but the underlying mechanisms are unclear. We previously showed that apoA-II modulates SR-BI-dependent binding and selective uptake of cholesteryl ester from reconstituted HDL. To investigate the effect of apoA-II in naturally occurring HDL on these processes, we compared HDL without apoA-II (from apoA-II null mice) with HDLs containing differing amounts of apoA-II (from C57BL/6 mice and transgenic mice expressing a mouse apoA-II transgene). The level of apoA-II in HDL was inversely correlated with HDL binding and selective cholesteryl ester uptake by both scavenger receptors, particularly CD36. Interestingly, for HDL lacking apoA-II, the efficiency with which CD36 mediated selective uptake reached a level similar to that of SR-BI. These results demonstrate that apoA-II exerts a marked effect on HDL binding and selective lipid uptake by the class B scavenger receptors and establishes a potentially important relationship between apoA-II and CD36.

ApoB-containing lipoproteins in apoE-deficient mice are not metabolized by the class B scavenger receptor BI

The scavenger receptor class B type I (SR-BI) recognizes a broad variety of lipoprotein ligands, including HDL, LDL, and oxidized LDL. In this study, we investigated whether SR-BI plays a role in the metabolism of cholesterol-rich lipoprotein remnants that accumulate in apolipoprotein E (apoE)(-/-) mice. These particles have an unusual apolipoprotein composition compared with conventional VLDL and LDL, containing mostly apoB-48 as well as substantial amounts of apoA-I and apoA-IV. To study SR-BI activity in vivo, the receptor was overexpressed in apoE(-/-) mice by adenoviral vector-mediated gene transfer. An approximately 10-fold increase in liver SR-BI expression resulted in no detectable alterations in VLDL-sized particles and a modest depletion of cholesterol in intermediate density lipoprotein/LDL-sized lipoprotein particles. This decrease was not attributable to altered secretion of apoB-containing lipoproteins in SR-BI-overexpressing mice. To directly assess whether SR-BI metabolizes apoE(-/-) mouse lipoprotein remnants, in vitro assays were performed in both CHO cells and primary hepatocytes expressing high levels of SR-BI. This analysis showed a remarkable deficiency of these particles to serve as substrates for selective lipid uptake, despite high-affinity, high-capacity binding to SR-BI. Taken together, these data establish that SR-BI does not play a direct role in the metabolism of apoE(-/-) mouse lipoprotein remnants.

Macrophage-expressed group IIA secretory phospholipase A2 increases atherosclerotic lesion formation in LDL receptor-deficient mice

OBJECTIVE

Transgenic mice expressing human group IIA secretory phospholipase A(2) (group IIA sPLA(2)) spontaneously develop atherosclerotic lesions. The mechanism for this proatherogenic effect is likely multifactorial, because HDL-cholesterol is significantly lower and LDL/VLDL cholesterol is slightly higher in transgenic mice compared with nontransgenic littermates. In the present study, we show for the first time that elicited peritoneal macrophages from transgenic mice express human group IIA sPLA(2). This study tested whether macrophage-expressed sPLA(2) contributes to atherogenesis.

METHODS AND RESULTS

Bone marrow cells from either sPLA(2) transgenic mice or control C57BL/6 mice were transplanted into LDL receptor-deficient mice. After hematopoietic engraftment, animals were fed a diet enriched with saturated fat and cholesterol for 12 weeks. Despite a lack of effect on serum lipoprotein concentrations, the presence of bone marrow-derived cells expressing human group IIA sPLA(2) resulted in a significant increase in the extent of atherosclerosis in the aortic arch (12.8+/-1.4% versus 7.4+/-0.9%; P<0.005) and aortic sinus (0.3+/-0.03 mm(2) versus 0.2+/-0.04 mm(2); P<0.05).

CONCLUSIONS

Group IIA sPLA(2) can contribute to atherosclerotic lesion development through a mechanism that is independent of systemic lipoprotein metabolism.

The fate of HDL particles in vivo after SR-BI-mediated selective lipid uptake

Scavenger receptor class B type I (SR-BI) delivers cholesterol ester from HDL to cells via a selective uptake mechanism, whereby lipid is transferred from the core of the particle without concomitant degradation of the protein moiety. The precise metabolic fate of HDL particles after selective lipid uptake is not known. To characterize SR-BI-mediated HDL processing in vivo, we expressed high levels of this receptor in livers of apoA-I(-/-) mice by adenoviral vector gene transfer, and then injected the mice with a bolus of human HDL(2) traced with (125)I-dilactitol tyramine. HDL recovered from apoA-I(-/-) mice over-expressing SR-BI was significantly smaller than HDL recovered from control mice as measured by non-denaturing gel electrophoresis. When injected into C57BL/6 mice, these HDL "remnants" were rapidly converted to HDL(2)-sized lipoprotein particles, and were cleared from the plasma at a rate similar to HDL(2). In assays in cultured cells, HDL remnants did not stimulate ATP-binding cassette transporter A1-dependent cholesterol efflux. When mixed with mouse plasma ex vivo, HDL remnants rapidly converted to larger HDL particles. These studies identify a previously ill-defined pathway in HDL metabolism, whereby SR-BI generates small, dense HDL particles that are rapidly remodeled in plasma. This remodeling pathway may represent a process that is important in determining the rate of apoA-I catabolism and HDL-mediated reverse cholesterol transport.

Overexpression of SR-BI by adenoviral vector promotes clearance of apoA-I, but not apoB, in human apoB transgenic mice

Scavenger receptor BI (SR-BI) is a multi-ligand lipoprotein receptor that mediates selective lipid uptake from HDL, and plays a central role in hepatic HDL metabolism. In this report, we investigated the extent to which SR-BI selective lipid uptake contributes to LDL metabolism. As has been reported for human LDL, mouse SR-BI expressed in transfected cells mediated selective lipid uptake from mouse LDL. However, LDL-cholesteryl oleoyl ester (CE) transfer relative to LDL-CE bound to the cell surface (fractional transfer) was approximately 18-fold lower compared with HDL-CE. Adenoviral vector-mediated SR-BI overexpression in livers of human apoB transgenic mice ( approximately 10-fold increased expression) reduced plasma HDL-cholesterol (HDL-C) and apolipoprotein (apo)A-I concentrations to nearly undetectable levels 3 days after adenovirus infusion. Increased hepatic SR-BI expression resulted in only a modest depletion in LDL-C that was restricted to large LDL particles, and no change in steady-state concentrations of human apoB. Kinetic studies showed a 19% increase in the clearance rate of LDL-CE in mice with increased SR-BI expression, but no change in LDL apolipoprotein clearance. Quantification of hepatic uptake of LDL-CE and LDL-apolipoprotein showed selective uptake of LDL-CE in livers of human apo B transgenic mice. However, such uptake was not significantly increased in mice over-expressing SR-BI. We conclude that SR-BI-mediated selective uptake from LDL plays a minor role in LDL metabolism in vivo.