The liver is the most common site of metastatic disease1. While this metastatic tropism may reflect mechanical trapping of circulating tumor cells, liver metastasis is also dependent, at least in part, on formation of a “pro-metastatic” niche that supports tumor cell spread to the liver2,3. Mechanisms that direct formation of this niche, though, 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. Early during pancreatic tumorigenesis, hepatocytes demonstrate activation of Signal Transducer and Activator of Transcription 3 (STAT3) signaling and increased production of serum amyloid A1 and A2 (SAA). Overexpression of SAA by hepatocytes also occurs in pancreatic and colorectal cancer patients with liver metastases, and many patients with locally advanced and metastatic disease display elevated levels of circulating SAA. STAT3 activation in hepatocytes and the subsequent production of SAA are dependent on interleukin 6 (IL-6) that is released into the circulation by non-malignant cells. Genetic ablation or blockade of components of IL-6/STAT3/SAA signaling prevents establishment of a pro-metastatic niche and inhibits liver metastasis. Our data reveal an intercellular network underpinned by hepatocytes that forms the basis for a pro-metastatic niche in the liver and identify new therapeutic targets.

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 Ilabeled 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.

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. Key Words: SAA Ⅲ HDL Ⅲ CETP Ⅲ apoA-I Ⅲ inflammation I nflammation induces major changes in HDL levels and composition. Mediators of inflammation such as tumor necrosis factor (TNF)-␣ and interleukin (IL)-6 induce expression of serum amyloid A 1 and group IIA secretory phospholipase A 2 (sPLA 2 -IIA), 2 which dramatically alter HDL apolipoprotein content and levels, respectively. Acute phase SAA in the plasma is associated with HDL, where it can comprise the major apolipoprotein. 3 The increase in sPLA 2 -IIA activity results in hydrolysis of HDL surface phospholipids and a decrease in HDL particle size. 4 The plasma cholesteryl ester transfer protein (CETP) is an integral component of reverse cholesterol transport and regulates HDL cholesterol concentrations. By promoting the transfer of cholesteryl esters (CE) from HDL to apoB-containing lipoprotein particles, HDL-derived CE is taken up via the LDL receptor and cleared by the liver. 5 An additional result of CETP action is the generation of lipid-poor apoA-I, 6 a key acceptor in ATP-binding cassette transporter AI (ABCA1)-mediated lipid efflux. 7 The presence of SAA on HDL holds the potential to impact both the CE transfer and the apoA-I liberating ability of CETP. sPLA 2 -IIA could also impact the latter action of CETP as apoA-I was shown to dissociate more readily from CETP-remodeled reconstituted HDL after hydrolysis by bee venom phospholipase A 2 . 8 Given the centrality of inflammation in atherogenesis, there is a paucity of information regarding CETP function when acute phase HDL is the "substrate." In the present study, we used plasma from patients undergoing cardiac surgery with cardiopulmonary bypass as a "standardized" insult where the oxygenator membrane activates macrophages to produce cytokines. 9 We characterized the SAAcontaining acute phase (AP) HDL during the acute phase to define the polydisperse HDL "substrate" that CETP would encounter. We further investigated CETP function in the acute phase, particularly as it relates to the presence of SAA and sPLA 2 on AP HDL, with respect to its CE transfer and apoA-I liberating functions.Teleologically, the dramatic changes in HDL composition and metabolism during inflammation must serve a short-term purpose to allow the organism to survive a noxious assault. Acute tissue injury results in cell death with large quantities of cell membranes rich in phospholipids and cholesterol generated. Macrophages are mobilized to such sites, ingest these fragments, and acquire considerable lipid load. 10 We thus e...

BackgroundSerum 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.MethodsIntestinal 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.ResultsSignificant 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.ConclusionsIntestinal 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..

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-␤, 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. Key Words: chemokines Ⅲ cytokines Ⅲ interferon regulatory factor 7 Ⅲ lipid cell membrane Ⅲ myeloid differentiation factor 88 H igh-density lipoprotein (HDL) protects against vascular disease by removing cholesterol from artery wall macrophages through reverse cholesterol transport. [1][2][3][4] There is mounting evidence, however, that it has additional antiatherosclerotic effects. 2,[5][6][7][8] One such activity may be modulation of the inflammatory response of the innate immune system. 9 -14 Editorial see p 1900 Clinical Perspective on p 1927Two cholesterol-transporting proteins, ABCA1 and ABCG1, might link HDL to both cholesterol removal and regulation of inflammation. 10 -12 Both membrane-associated ATP-binding cassette transporters are found in macrophages. ABCA1 promotes cholesterol efflux to lipid-poor apolipoprotein (apo) A-I, the major HDL protein, and ABCG1 induces cholesterol efflux to intact HDL particles. 10 Macrophages isolated from mice with genetically engineered deficiencies in ABCA1 and/or ABCG1 overexpress well-known inflammatory genes such as tumor necrosis factor-␣ (TNF-␣), interleukin (IL) -1␤, and IL-8 when the cells are stimulated with bacterial lipopolysaccharide (LPS). 11 In both macrophages and endothelial cells, the antiinflammatory effects of HDL have been proposed to reflect changes in membrane chole...