Abstract 1256: A role for lipoprotein lipase in fatty acid acquisition by breast, prostate and liposarcoma tumors

The importance of de novo fatty acid (FA) synthesis for tumor cell growth is well established, and we have now examined the hypothesis that the uptake of exogenous FA provides an alternative FA fuel source for tumors.

Background: Many tumor cells utilize FA as a major energy source, and they do not survive if the supply is interrupted. In order to obtain lipids, tumors can synthesize fatty acids from glucose-derived precursors using fatty acid synthase (FASN), or they could potentially hydrolyze diet-derived triglycerides from circulating lipoproteins using the enzyme lipoprotein lipase (LPL), and take up the free fatty acids using the cell surface channel CD36. It is also conceivable that they could obtain lipids by receptor-mediated endocytosis, using syndecan (Sdc1) and LPL. The latter two of these three mechanisms have not been examined.

Methods: We employed cDNA microarray and quantitative RT-PCR to study mRNA expression, measured secreted LPL enzyme activity, and produced a monoclonal antibody for LPL protein analysis and immunohistochemistry (IHC). We also assessed cellular uptake of fluorescently labeled very low density lipoproteins (VLDL). We studied cell lines and/or tumor tissues for expression of FASN, Spot 14 (a nuclear protein that drives FASN expression), LPL, CD36, and Sdc1.

Results:

1. By RT-PCR, FASN and Spot14, a driver of FASN gene expression, are expressed in all cell lines (45 breast cancer, 1 liposarcoma, 1 cervical carcinoma, 3 prostate cancer) and tumor tissues (160 breast cancer, 24 liposarcoma, 10 prostate cancer) examined.

2. By RT-PCR and enzyme activity assays, LPL is expressed only in liposarcoma and triple-negative breast cancer cell lines. Prostate cancer cells secrete negligible LPL.

3. Addition of exogenous LPL and triglyceride substrate to culture media enhanced the growth of breast cancer and liposarcoma cell lines, but not prostate cancer cell lines.

4. Provision of exogenous LPL, however, rescued prostate cancer cell lines from the cytotoxicity of FASN inhibition.

5. siRNA-mediated knockdown of LPL impaired the growth of HeLa cells,

6. In contradistinction to observations in cell lines, IHC demonstrated brisk expression of LPL in all liposarcomas, prostate tumors, and breast cancer tumors examined, irrespective of ER/PR or HER2/neu status.

7. CD36 is expressed in the majority of tumors examined by IHC, including 98% of breast cancers.

8. Sdc1 and CD36 are expressed in triple-negative DU4475 breast cancer cells, but they did not endocytose labeled VLDL particles.

Conclusions: These data demonstrate that, in addition to de novo FA synthesis, tumors can utilize diet-derived fat, and that this can fuel their growth. Our findings provide a mechanism, namely lipolysis by LPL and uptake by CD36, for the link between dietary fat intake and tumor progression. Inhibition of lipolysis, as well as lipogenesis, may be a necessary strategy to target the FA requirement of aggressive tumors.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1256. doi:10.1158/1538-7445.AM2011-1256

Lipoprotein lipase links dietary fat to solid tumor cell proliferation

Many types of cancer cells require a supply of fatty acids (FA) for growth and survival, and interrupting de novo FA synthesis in model systems causes potent anticancer effects. We hypothesized that, in addition to synthesis, cancer cells may obtain preformed, diet-derived FA by uptake from the bloodstream. This would require hydrolytic release of FA from triglyceride in circulating lipoprotein particles by the secreted enzyme lipoprotein lipase (LPL), and the expression of CD36, the channel for cellular FA uptake. We find that selected breast cancer and sarcoma cells express and secrete active LPL, and all express CD36. We further show that LPL, in the presence of triglyceride-rich lipoproteins, accelerates the growth of these cells. Providing LPL to prostate cancer cells, which express low levels of the enzyme, did not augment growth, but did prevent the cytotoxic effect of FA synthesis inhibition. Moreover, LPL knockdown inhibited HeLa cell growth. In contrast to the cell lines, immunohistochemical analysis confirmed the presence of LPL and CD36 in the majority of breast, liposarcoma, and prostate tumor tissues examined (n = 181). These findings suggest that, in addition to de novo lipogenesis, cancer cells can use LPL and CD36 to acquire FA from the circulation by lipolysis, and this can fuel their growth. Interfering with dietary fat intake, lipolysis, and/or FA uptake will be necessary to target the requirement of cancer cells for FA.

Fatty acid synthesis is a therapeutic target in human liposarcoma

Liposarcomas (LS) are mesenchymal tumors that can recur after surgical resection and often do not respond to presently available medical therapies. This study demonstrates the dependence of LS on de novo long-chain fatty acid synthesis for growth. Lipogenesis can be impaired by inhibiting the activities of lipogenic enzymes, including acetyl CoA-carboxylase (ACC) and fatty acid synthase (FASN), or by suppressing the expression of key genes involved in the pathway and its regulation. The FASN inhibitors cerulenin and orlistat reduced the growth of two LS cell lines (LiSa2, SW872), as did inhibition of ACC with soraphen A. CDDO-Me, a synthetic triterpenoid, suppressed expression of Spot 14 and FASN genes and likewise inhibited LS cell growth. Importantly, the anti-proliferative effect of each agent was prevented by the co-administration of palmitate, the major product of cellular long-chain fatty acid synthesis. In stark contrast to LS cells, these compounds had no effect on the growth of fibroblasts. Four biochemically distinct agents that target critical points in the fatty acid synthetic pathway exert anti-proliferative effects on LS cells, and rescue of cell growth by palmitic acid suggests that reduced tumor cell lipogenesis mediates the growth inhibition. These findings warrant further studies aimed at the clinical exploitation of the dependence of LS cell growth on fatty acids.

Inhibition of arterial lesion progression in CD16-deficient mice: evidence for altered immunity and the role of IL-10

AIMS

Given the importance of IgG Fc receptors in immune regulation, we hypothesized that Fcg receptor type III (FcgRIII, CD16) plays an important role in atherogenesis. We therefore analysed the formation of arterial lesions in LDL receptor-deficient (LDLR(-/-)) and FcgRIII(-/-)xLDLR(-/-) double knockout mice at three different points up to 24 weeks of exposure to a high-fat diet.

METHODS AND RESULTS

Analysis of Oil Red-O-stained sections revealed no difference in lesion formation between strains after 6 weeks of a high-fat diet, and a modest decrease after 14 weeks in double knockouts relative to LDLR(-/-) controls. After 24 weeks, lesion formation was decreased in the aortic root (30%) and innominate artery (50%) in FcgRIII double knockouts relative to LDLR(-/-) controls. Analysis of peripheral CD4+ T-cells by intracellular flow cytometry from double knockouts after 24 weeks of a high-fat diet revealed statistically significant increases in the percentages of cells producing interferon-gamma, interleukin (IL)-10, and IL-4 relative to controls, differences that were also observed by analyses of whole aortas for cytokine mRNA levels. As determined by flow cytometry, FcgRIII deficiency resulted in an expansion of CD4+ cells and an increase in the CD4 to CD8 ratio. Analysis of plasma anti-oxidized LDL (OxLDL) antibodies by chemiluminescent assay revealed that IgG1 and IgG2c titers to OxLDL were increased in FcgRIII (-/-)xLDLR(-/-) double knockouts relative to LDLR(-/-) controls, while total IgG levels were similar.

CONCLUSION

These results reveal altered immunity in FcgRIII(-/-)xLDLR(-/-) mice and a reduction in lesion formation associated with increased production of IL-10 by an expansion of CD4+ T-cells. The reduction in lesion formation was manifest well after evidence of an immune response to OxLDL, suggesting that FcgRIII contributes to lesion progression in murine atherosclerosis.

Expression of “Spot 14” (THRSP) predicts disease free survival in invasive breast cancer: immunohistochemical analysis of a new molecular marker

Most breast cancers are "lipogenic", defined by high fatty acid synthase (FAS) content and dependence on fatty acid synthesis for growth and survival. S14 (Spot 14; THRSP) is a nuclear protein that activates genes required for fatty acid synthesis. The S14 gene is amplified in approximately 15% of breast cancers, but clinical correlates of its expression were unknown. We analyzed 131 breast cancers by immunohistochemistry for S14 and FAS. Staining was graded 0, 1, or 2+, and scores were correlated with traditional tumor markers, histological features, and outcome. S14 and FAS staining were related to tumor size (p=0.05 for S14, p=0.038 for FAS), but not to stage. S14 but not FAS scores correlated with tumor grade in both DCIS (p=0.003) and invasive cases (p<0.001). Invasive cases (pooled node - and +) with weak S14 staining (n=21) showed no recurrence over 3000 d follow-up, including 10 cases with lymph node involvement, whereas 32% of 67 strongly-staining tumors recurred (log rank p<0.0001). S14 scores did not cosegregate with sex steroid receptors, Her2/neu, or cyclin D1. Low level S14 expression is associated with prolonged disease-free survival in invasive cases, including those with nodal metastasis. High-level expression of S14 identifies a subset of high-risk breast cancers that is not specified by analysis of sex steroid receptors, Her2/neu, or cyclin D1, and provides a molecular correlate to histologic features that predict recurrence.

S14 protein in breast cancer cells: direct evidence of regulation by SREBP-1c, superinduction with progestin, and effects on cell growth

Most breast cancers exhibit brisk lipogenesis, and require it for growth. S14 is a lipogenesis-related nuclear protein that is overexpressed in most breast cancers. Sterol response element-binding protein-1c (SREBP-1c) is required for induction of lipogenesis-related genes, including S14 and fatty acid synthase (FAS), in hepatocytes, and correlation of SREBP-1c and FAS expression suggested that SREBP-1c drives lipogenesis in tumors as well. We directly tested the hypothesis that SREBP-1c drives S14 expression and mediates lipogenic effects of progestin in T47D breast cancer cells. Dominant-negative SREBP-1c inhibited induction of S14 and FAS mRNAs by progestin, while active SREBP-1c induced without hormone and superinduced in its presence. Changes in S14 mRNA were reflected in protein levels. A lag time and lack of progestin response elements indicated that S14 and FAS gene activation by progestin is indirect. Knockdown of S14 reduced, whereas overexpression stimulated, T47D cell growth, while nonlipogenic MCF10a mammary epithelial cells were not growth-inhibited. These data directly demonstrate that SREBP-1c drives S14 gene expression in breast cancer cells, and progestin magnifies that effect via an indirect mechanism. This supports the prediction, based on S14 gene amplification and overexpression in breast tumors, that S14 augments breast cancer cell growth and survival.

Acyl-coenzyme A:cholesterol acyltransferase 2 (ACAT2) is induced in monocyte-derived macrophages: in vivo and in vitro studies

To test the possibility that acyl-coenzyme A:cholesterol acyltransferase 2 (ACAT2) may be expressed in human macrophages under pathologic conditions, we employed specific anti-ACAT2 antibodies and found clear ACAT2 signals in lipid-laden as well as lipid-free macrophages under various disease conditions, including atherosclerosis. However, no ACAT2 signal was detectable in macrophages under normal physiologic conditions. Using cultured human macrophages derived from blood-borne monocytes, immunoblot and RT-PCR analyses demonstrated that immature macrophages expressed only ACAT1, but the fully differentiated macrophages expressed both ACAT1 and ACAT2. Furthermore, RT-PCR clearly revealed the presence of both ACAT1 and ACAT2 mRNAs in human atherosclerotic aorta. Double immunohistochemical staining indicated that in human atherosclerotic aorta, all macrophages expressed ACAT1, while approximately 70% to 80% of macrophages also expressed ACAT2. In congenital hyperlipidemic mice, immunohistochemistry and RT-PCR demonstrated that ACAT2 was also present in lipid-laden cells of the atheromatous plaques. Our results suggest that in atherosclerotic plaque, the ability of macrophage foam cell transformation may be augmented by the dual expressions of ACAT1 and ACAT2. Additional immunoblot and RT-PCR experiments showed that the ACAT2 signal was clearly detectable in thioglycollate-elicited exudate mouse macrophages but not in peritoneal resident macrophages. We conclude that under various pathologic conditions, fully differentiated macrophages express ACAT2 in addition to ACAT1.

Endotoxin induces rapid metalloproteinase-mediated shedding followed by up-regulation of the monocyte hemoglobin scavenger receptor CD163

CD163, a monocyte and macrophage-specific surface glycoprotein, which is increased by interleukin-10 and glucocorticoids, is a scavenger receptor for hemoglobin/haptoglobin complexes. We report a rapid and highly reproducible rise in soluble CD163 in the plasma of human volunteers given intravenous lipopolysaccharide (LPS). We also show that LPS induces shedding of CD163 from the surface of isolated monocytes, identifying shedding from monocytes and macrophages as a likely mechanism for the endotoxemia-associated rise in plasma CD163 in vivo. Studies using the inhibitor TAPI-0 indicate that a metalloproteinase is responsible for LPS-mediated shedding of CD163. Finally, we demonstrate a marked increase in surface CD163 expression on circulating monocytes 24 h following experimental endotoxemia. These findings show that CD163 is rapidly mobilized in response to bacterial endotoxin. As hemoglobin can bind LPS and enhance its toxicity, it will be important to determine how cell surface and soluble CD163 influence inflammatory processes during sepsis.

Immunocytochemical detection of Fcgamma receptors in human atherosclerotic lesions

Much evidence indicates that atherosclerotic lesions are largely of an inflammatory nature. Activated macrophages and macrophage-derived foam cells laden with cholesterol esters are a major constituent of these lesions and can influence lesion formation via several potential mechanisms. One such mechanism is Fcgamma receptor activation and/or Fcgamma receptor-mediated clearance of immune complexes containing cholesterol, such as lipoprotein immune complexes. That this mechanism contributes to lesion formation would be further supported if Fcgamma receptor expression in arterial lesions were demonstrated. We therefore used monoclonal antibodies and immunocytochemical methods to analyze frozen sections of human arterial lesions for expression of each of the three primary classes of mononuclear phagocyte Fcgamma receptors. Approximately 800 sections of aorta, carotid, and coronary arteries obtained from five elderly donors were analyzed. The presence of macrophages was determined by assaying reactivity of a monoclonal antibody specific to CD163, which is expressed only on cells of the human mononuclear phagocyte lineage. Results indicate that highly cellular preatheromatous lesions contained numerous macrophages in the zone of proliferation that expressed each class of Fcgamma receptor (FcgammaRIA, FcgammaRIIA, and FcgammaRIIIA). Fcgamma receptor-positive cells were also present in medial and adventitial areas. Fcgamma receptor staining was both punctate and diffuse, the latter suggesting that soluble receptors were present in the extracellular matrix. These data further support that Fcgamma receptor-mediated clearance of immune complexes can occur in arterial lesions during atherogenesis. Expression of both the high affinity (FcgammaRIA) and lower affinity (FcgammaRIIA/FcgammaRIIIA) receptors indicates that mono- and multivalent IgG-containing immune complexes could engage Fcgamma receptors and influence lesion formation through several different inflammatory mechanisms triggered by receptor activation.

Levels of expression of Fcgamma receptor IIA (CD32) are decreased on peripheral blood monocytes in patients with severe atherosclerosis

To obtain information in vivo concerning the role of Fcgamma receptors (FcgammaR) in atherosclerosis, we used quantitative flow cytometry to measure the levels of expression of FcgammaRI and FcgammaRIIA on peripheral monocytes in patients with severe atherosclerosis. Expression of several other markers was also measured. We found that differences in the levels of expression of FcgammaRI were not statistically significant when compared between patients and control subjects. For FcgammaRIIA, levels of expression were decreased in the patient group, a difference that was statistically significant. Levels of expression of CD14 and CD36 were also significantly decreased in the patient group. The decrease in expression of FcgammaRIIA was statistically significant when the effects of current cigarette smoking status or medication use, including statins, were taken into account. There was also a positive and statistically significant correlation between high-density lipoprotein-cholesterol and levels of expression of FcgammaRIIA for all subjects. In contrast, decreased levels of expression of CD14 and CD36 were strongly associated with current smoking status or statin use. In summary, levels of expression of FcgammaRIIA on peripheral blood monocytes were significantly decreased in patients with clinical atherosclerosis. Additional studies are warranted to determine if levels of expression of FcgammaRIIA have utility as a phenotypic marker for assessing relative risk of atherosclerotic disease.

Human monocytes express CD163, which is upregulated by IL-10 and identical to p155

CD163 is a glucocorticoid-inducible member of the scavenger receptor cysteine-rich family of proteins. Previous reports have indicated that CD163 is highly expressed on human macrophages, but found on less than 50% of peripheral blood monocytes. We now show that >99% of all CD14 positive monocytes express CD163 and that monocyte derived dendritic cells express low levels of CD163. We also show that IL-10, like glucocorticoids, induces high CD163 expression on cultured human monocytes. Glucocorticoid induced CD163 expression was not inhibited by anti-IL-10 and was additive with IL-10 treatment, suggesting that glucocorticoids increase CD163 expression by an IL-10 independent mechanism. Other anti-inflammatory cytokines (IL-4 and IL-13) did not increase CD163 expression. In addition, we show that p155 (a previously identified monocyte/macrophage marker of unknown function) shares identity with CD163. Western blots and flow cytometric analysis of HEK 293 cells transfected with the cDNA for CD163 were positive when probed with either mAb RM3/1 (which recognizes CD163) or Mac 2-48 (which defines p155).

Differential effects of interferon-gamma on metabolism of lipoprotein immune complexes mediated by specific human macrophage Fcgamma receptors

The objectives were to determine whether there are differences in the mechanisms of lipoprotein metabolism associated with different FcgammaRs and how metabolism associated with FcgammaRs compares to that mediated by scavenger receptors (SRA). To analyze lipoprotein metabolism in a receptor-specific manner, bispecific antibodies were used to target low density lipoproteins (LDL) labeled with (125)I or [(3)H]cholesterol linoleate to FcgammaRI or FcgammaRIIA in human macrophages. Interferon-gamma (IFN-gamma), which stimulates expression of FcgammaRI while inhibiting expression of SRA, was used to help delineate differences in metabolism between each receptor. For each receptor, the total amount of lipoprotein degradation paralleled changes in receptor expression induced by IFN-gamma. In particular, while SRA-mediated degradation typically exceeded degradation mediated by FcgammaRI, in IFN-gamma-treated cells degradation associated with FcgammaRI and SRA was similar. Assay of [(3)H]cholesterol linoleate-labeled lipoproteins indicated that total uptake and hydrolysis of [(3)H]cholesterol linoleate was similar for each class of receptor, and inhibited by IFN-gamma. For FcgammaRI versus FcgammaRIIA, in the presence or absence of IFN-gamma, the [(3)H]cholesterol derived from FcgammaRIIA-mediated uptake was preferentially targeted for re-esterification to [(3)H]cholesterol oleate, in comparison to that resulting from hydrolysis of [(3)H]cholesterol linoleate incorporated by selective uptake. For SRA, the formation of [(3)H]cholesterol oleate, which was substantial in control cells, was significantly inhibited in the presence of IFN-gamma. We conclude that there may be differences in cholesterol trafficking with respect to lipoprotein immune complex metabolism mediated by different classes of FcgammaRs.

A caspase inhibitor decreases oxidized low-density lipoprotein-induced apoptosis in bovine endothelial cells

BACKGROUND

Apoptosis is a pathway of cell death orchestrated by a family of proteases called caspases. Oxidized low density lipoprotein (oxLDL) is a putative cause of atherogenesis. We examined the effect of oxLDL on endothelial cell (EC) apoptosis and the ability of a caspase antagonist to inhibit oxLDL-induced EC injury.

METHODS

Bovine ECs were plated at a concentration of 5.0 x 10(5) cells/ml and exposed to LDL oxidized by ultraviolet radiation at a concentration of 100 microgram oxLDL/ml for 20 h. Some ECs were pretreated with an irreversible caspase inhibitor (ZVAD). Samples were analyzed histologically. Apoptosis was measured using the Annexin V assay (flow cytometry) which detects phosphatidylserine on plasma membranes and confirmed by TUNEL assay (flow cytometry). Statistical assessments were performed using ANOVA.

RESULTS

ECs treated with LDL were morphologically similar to untreated cells. Cells treated with oxLDL demonstrated cytoplasmic shrinkage, plasma membrane blebbing, chromatin condensation, and loss of adhesion. These effects were diminished after pretreatment with the caspase inhibitor ZVAD. The Annexin V assay showed: (a) cells exposed to LDL had a 12 +/- 1% apoptosis rate, (b) exposure to oxLDL induced apoptosis in 30 +/- 0.3% of the cells, and (c) pretreatment with the caspase inhibitor ZVAD decreased the oxLDL-induced apoptosis to 16 +/- 1% (P < 0.05). This decrease in apoptosis was also reflected by an increase in the percentage of alive cells from 34 +/- 7% after oxLDL exposure to 55 +/- 6% after apoptosis inhibition with ZVAD. TUNEL assay demonstrated a 2.5-fold reduction in mean fluorescence intensity between cells treated with oxLDL alone and those treated with ZVAD, suggesting a significant decrease in apoptosis in the latter group.

CONCLUSIONS

We conclude that treatment of bovine ECs with oxLDL induces apoptosis which can be significantly reduced by a specific caspase inhibitor.

Expression of ACAT-1 protein in human atherosclerotic lesions and cultured human monocytes-macrophages

The acyl coenzyme A:cholesterol acyltransferase (ACAT) gene was first cloned in 1993 (Chang et al, J Biol Chem. 1993;268:20747-20755; designated ACAT-1). Using affinity-purified antibodies raised against the N-terminal portion of human ACAT-1 protein, we performed immunohistochemical localization studies and showed that the ACAT-1 protein was highly expressed in atherosclerotic lesions of the human aorta. We also performed cell-specific localization studies using double immunostaining and showed that ACAT-1 was predominantly expressed in macrophages but not in smooth muscle cells. We then used a cell culture system in vitro to monitor the ACAT-1 expression in differentiating monocytes-macrophages. The ACAT-1 protein content increased by up to 10-fold when monocytes spontaneously differentiated into macrophages. This increase occurred within the first 2 days of culturing the monocytes and reached a plateau level within 4 days of culturing, indicating that the increase in ACAT-1 protein content is an early event during the monocyte differentiation process. The ACAT-1 protein expressed in the differentiating monocytes-macrophages was shown to be active by enzyme assay in vitro. The high levels of ACAT-1 present in macrophages maintained in culture can explain the high ACAT-1 contents found in atherosclerotic lesions. Our results thus support the idea that ACAT-1 plays an important role in differentiating monocytes and in forming macrophage foam cells during the development of human atherosclerosis.

Evidence that human Fc gamma receptor IIA (CD32) subtypes are not receptors for oxidized LDL

Several lines of evidence suggest that clearance of oxidized LDL (oxLDL) immune complexes by macrophage IgG Fc receptors (Fc gamma Rs) plays a role in atherogenesis. Ox-LDL may also be cleared directly by Fc gamma Rs, as shown for murine Fc gamma RII-B2. In humans, the homologous Fc gamma R is Fc gamma RIIA (CD32), which is abundantly expressed on monocytes and macrophages and shares 60% sequence identity with murine Fc gamma RII-B2. As murine Fc gamma RII-B2 and human Fc gamma RIIA also share similar IgG ligand-binding properties, the purpose of this study was to test the hypothesis that human CD32 is a receptor for oxLDL. For these studies we used transfected Chinese hamster ovary (CHO) cells, monocytes, and cell lines that functionally express either of two Fc gamma RIIA subtypes (R131 or H131) and assayed binding or degradation of several preparations of oxLDL. The integrity of all oxLDL preparations was checked by studying their ability to react with CHO cells expressing human type I scavenger receptors and by other characteristics of lipoprotein oxidation. Although we showed that each preparation of oxLDL could recognize class A or class B scavenger receptors, we did not detect any differences in the binding or degradation of any type of oxLDL preparation among control versus CHO cell transfectants. Using monocytes that express Fc gamma RIIA and CD36, we showed that the binding of oxLDL was inhibited by antibodies to CD36, but not by Fc gamma RIIA antibodies. Thus, the data do not support the hypothesis that human Fc gamma RIIA is by itself a receptor for oxLDL. We conclude that human CD32 can mediate uptake of lipoprotein immune complexes, but does not mediate uptake of oxLDL in the absence of anti-oxLDL antibodies. OxLDL may interact with human mononuclear phagocytes directly via other types of receptors, such as class A and class B scavenger receptors or CD68.

Increased potency of Fc-receptor-targeted antigens

A major challenge for using native and modified T cell epitopes to induce or suppress immunity relates to achieving efficient uptake and processing by antigen-presenting cells (APC) in vivo. IgG Fc receptors, which are expressed constitutively by professional APC including monocytes and dendritic cells, have long been known to mediate antigen uptake in a manner leading to efficient T cell activation. We have previously demonstrated enhanced presentation of antigenic and antagonistic peptides by targeting them to the type I Fc receptor for IgG (Fc gamma RI, CD64) on human monocytes. In the present report we review the literature suggesting that CD64-targeted antigens are likely to be effective in vivo, and present data demonstrating enhanced immunogenicity in CD64 transgenic mice of a fusion protein that combines the specificities of HIV gp120 and the humanized anti-CD64 monoclonal antibody H22. Overall, these studies suggest that targeting antigens to CD64 represents an effective approach to enhancing the effectiveness of vaccines in vivo.

Targeting lipoproteins to Fc gamma receptors with bispecific antibodies

The data presented here support use of bispecific antibodies (BsAb) for studying the role of each of the different types of IgG Fc receptors (Fc gamma Rs) in uptake and metabolism of low-density lipoprotein (LDL) immune complexes. The bispecific anti-Fc gamma R x anti-LDL antibodies used in these studies were effective in specifically triggering metabolic uptake and degradation of LDL immune complexes (LDL-IC) through each type of Fc gamma R. Using LDL-IC prepared with LDL aggregates, foam cell formation was induced with relatively acute stimulation. Thus, these conditions will be appropriate for studying lipoprotein metabolism in the context of specific Fc gamma R under a variety of conditions to determine if, in fact, there are differences in sterol metabolism associated with the different types of Fc gamma R and for comparing metabolism to that mediated by the other important pathways of lipoprotein uptake (native LDL receptors and scavenger receptors). The results of these studies will reveal which of these pathways are potentially most important in foam cell formation and might suggest the possibility that macrophage foam cell formation could be altered by redirecting LDL to particular pathways.

Enhanced metabolism of LDL aggregates mediated by specific human monocyte IgG Fc receptors

Macrophage-derived foam cells are important constituents of atheromatous lesions. In addition to the scavenger receptor pathway, uptake of immune complexed lipoproteins through IgG Fc receptors (Fc gamma receptors) represents an additional pathway of macrophage foam cell development that may be important during atherogenesis. The importance of this mechanism is suggested by studies showing that the titer of autoantibodies to modified lipoproteins correlated with the extent of occlusive disease in patients, and that those antibodies exist in human lesions. Human mononuclear phagocytes possess three structurally and functionally distinct classes of Fc gamma receptors, each of which could be associated with a unique pathway of lipoprotein metabolism. In order to determine whether uptake of an acute lipid load through each type of Fc gamma receptor was associated with foam cell development, we used bispecific antibodies consisting of anti-LDL monoclonal antibodies conjugated to anti-Fc gamma receptor monoclonal antibodies to study the effects of targeting LDL aggregates to each specific type of Fc gamma receptor on freshly isolated adherent human monocytes. Relative to appropriate controls, LDL degradation, cellular sterol mass, and foam cell development of monocytes were enhanced by targeting LDL aggregates to Fc gamma RI or Fc gamma RII, and this was accompanied by an apparent impairment of LDL degradation. Uptake was specific to the Fc gamma receptors and was not influenced by the presence of scavenger receptor ligands. Thus, with the bispecific approach, the functions of each class of Fc gamma receptor can be studied on an individual basis with respect to several aspects of cellular cholesterol metabolism. This will be critical for determining which of these receptors are potentially most important in the clearance of lipoprotein immune complexes during atherogenesis.

Use of bispecific antibodies in the therapy of tumors

Progress toward an understanding of the construction and use of BsAb in therapy has been considerable. The importance of accessory (adhesion) molecules as well as the requirements for killing and the mechanisms by which cytotoxicity is mediated are being clarified. New approaches to simultaneous activation and targeting of effector cells have been developed. Most important, limited clinical trials have demonstrated little toxicity and in several instances promising responses and long-term survivals, if not cures. It seems likely, therefore, that BsAb will be very useful tools for therapy of tumors that may be most efficacious as an adjunct tumor therapy after surgery, chemotherapy, and/or irradiation in order to further reduce, and to potentially eliminate, tumor cells in the patient. Clearly, much remains to be done before BsAb are used routinely for therapy, but the results thus far demonstrate the considerable potential of BsAb to redirect and focus natural immune mechanisms in the treatment of tumors.

Redirected targeting of LDL to human monocyte Fc gamma receptors with bispecific antibodies

Recent studies indicate that low density lipoprotein (LDL)-immune complexes consisting of anti-LDL antibodies bound to LDL may contribute to macrophage foam cell development by uptake through immunoglobulin G (IgG) Fc receptors. As human mononuclear phagocytes possess three structurally and functionally distinct classes of IgG Fc receptors, we developed a system whereby the effects of LDL-immune complexes could be studied with respect to each type of IgG Fc receptor. Novel bispecific antibodies consisting of anti-Fc gamma receptor antibodies linked to anti-LDL antibodies were used to prepare bispecific LDL-immune complexes for targeting to specific Fc gamma receptors. In this report, the effects of bispecific LDL-immune complexes directed to Fc gamma receptor types I, II, and III were studied primarily with monocytes and were compared with the effects of similarly prepared bispecific complexes that targeted LDL to human leukocyte antigen (HLA) class I antigens. Each type of bispecific antibody was effective in targeting 125I-LDL to its respective site on the cell surface. Using fluorophore-labeled LDL and flow cytometry, bispecific complexes directed to Fc gamma receptor types I or II but not to HLA class I antigens caused a two- to sevenfold increase in cell-associated fluorescence relative to control cells treated with LDL in the absence of bispecific antibody. Uptake occurred in the presence of excess unlabeled LDL, acetylated LDL, and antioxidants. That the bispecific complexes triggered metabolic uptake was supported by studies of kinetics and temperature dependence. Using 125I-labeled complexes, metabolic degradation of LDL was demonstrated in association with each of the three types of Fc gamma receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

The renin-angiotensin system during pregnancy in chronically instrumented, conscious rats

Whether the renin-angiotensin system is activated during rat gestation is controversial. Therefore we serially assessed plasma renin activity in unrestrained, chronically instrumented conscious rats during pregnancy and the postpartum period. Plasma renin activity was 3.26 +/- 0.30, 2.80 +/- 0.31, and 2.70 +/- 0.26 ng.ml-1.hr-1 on gestational days 6, 12, and 20, respectively. When the same rats were studied after delivery, plasma renin activity was 1.87 +/- 0.29, 1.81 +/- 0.09, and 2.31 +/- 0.35 ng.ml-1.hr-1 on postpartum days 3, 6, and 11, respectively. Levels measured during pregnancy were significantly greater than in the postpartum period (p less than 0.05 or less than 0.01). We then evaluated potential functional consequences of the renin-angiotensin system in gravid rats. Near term, renal hemodynamics fall from the peak levels of midgestation; we tested whether angiotensin II mediates this apparent vasoconstriction. Captopril (1.5 mg/kg, 1.5 mg.kg-1.hr-1) was acutely administered to lower circulating angiotensin II. The drug produced an 80% inhibition of angiotensin I pressor response, a tenfold elevation in plasma renin activity, but caused the same degree of mild renal vasodilation in rats whether they were virgin or pregnant. We also tested whether prior occupancy of receptors by endogenous hormone or receptor downregulation mediates the attenuated pressor response to angiotensin II observed during late pregnancy. Acute administration of captopril failed to augment refractory pressor responsiveness. Chronic treatment with enalaprilat (2.0 mg.kg-1.day-1 by osmotic minipump) also did not restore pressor responsiveness. But, in our hands, chronic administration of enalaprilat most likely failed to lower plasma angiotensin II. In summary, we suggest that during rat gestation (1) the renin-angiotensin system is activated, (2) angiotensin II does not mediate the apparent renal vasoconstriction observed near term, (3) prior receptor occupancy by endogenous hormone is not responsible for the attenuated pressor response to angiotensin II, and (4) long-term treatment with enalaprilat can produce hypotension without reducing plasma concentrations of angiotensin II.

Monoclonal antibodies that bind to distinct epitopes on Fc gamma RI are able to trigger receptor function

The human monocyte and macrophage Fc receptor that binds human IgG with high affinity is a surface glycoprotein with a relative molecular mass of approximately 70 kDa. This receptor (Fc gamma RI) has been partially characterized using mAb 32 which binds outside the Fc binding domain of the receptor, but nonetheless triggers Fc receptor-dependent functions. In this study, we describe the properties of four new antibodies with specificity for Fc gamma RI. Based on additivity and cross-blocking studies, we conclude that two of these antibodies (mAb 22 and 44) define a third epitope which is distinct from the binding sites for both mAb 32 and the Fc portion of human IgG. Each Fc gamma RI-specific hybridoma was selected for stable sublines expressing high levels of mAb on the cell surface, and then tested for the ability of this surface mAb to trigger antibody-dependent cell cytotoxicity. All sublines were killed by human monocytes when used as targets in a 51Cr-release assay, whereas hybridomas specific for myeloid Ag other than Fc gamma RI were not killed. We conclude that Fc receptor function is triggered through binding to each of the three epitopes of Fc gamma RI that we have defined. These mAb will be useful for additional characterization of Fc gamma RI, and may, when incorporated into tumor-directed heteroantibodies, enhance tumor cell killing by human monocytes and macrophages.

Monoclonal antibodies that bind to distinct epitopes on Fc gamma RI are able to trigger receptor function

The human monocyte and macrophage Fc receptor that binds human IgG with high affinity is a surface glycoprotein with a relative molecular mass of approximately 70 kDa. This receptor (Fc gamma RI) has been partially characterized using mAb 32 which binds outside the Fc binding domain of the receptor, but nonetheless triggers Fc receptor-dependent functions. In this study, we describe the properties of four new antibodies with specificity for Fc gamma RI. Based on additivity and cross-blocking studies, we conclude that two of these antibodies (mAb 22 and 44) define a third epitope which is distinct from the binding sites for both mAb 32 and the Fc portion of human IgG. Each Fc gamma RI-specific hybridoma was selected for stable sublines expressing high levels of mAb on the cell surface, and then tested for the ability of this surface mAb to trigger antibody-dependent cell cytotoxicity. All sublines were killed by human monocytes when used as targets in a 51Cr-release assay, whereas hybridomas specific for myeloid Ag other than Fc gamma RI were not killed. We conclude that Fc receptor function is triggered through binding to each of the three epitopes of Fc gamma RI that we have defined. These mAb will be useful for additional characterization of Fc gamma RI, and may, when incorporated into tumor-directed heteroantibodies, enhance tumor cell killing by human monocytes and macrophages.

Single step screening of monoclonal antibodies against interferon-gamma-induced surface molecules on human monocytes

Interferon-gamma (IFN-gamma) activation of human monocytes in vitro results in enhanced phagocytosis and cellular cytotoxicity. These enhanced effector functions are attributable, at least in part, to increased expression of recognition molecules on the plasma membrane. In this article we report a rapid screening procedure for the primary selection of monoclonal antibodies (mAbs) which bind to cell surface molecules, the expression of which is increased or decreased by IFN-gamma. The procedure is based on flow cytometric analysis of mixed cell populations. Mouse mAbs were prepared using human monocytes cultured for 40 h with 400 U/ml of IFN-gamma as the immunogen. The hybridoma supernatants were screened using mixtures of six cell populations, some of which were pretreated with IFN-gamma for 40 h. Cells included in the mixture were chosen for their distinctive light scatter profiles. mAbs of interest were identified by preferential binding to monocytes and increased or decreased binding to monocytes treated with IFN-gamma. This procedure allowed us to screen several hundred clones per day, and to immediately eliminate mAbs that bound to B cells, T cells, neutrophils, and several cell lines. We selected ten mAbs which bound to surface molecules on monocytes that were modulated by IFN-gamma. Further characterization of five of the initial ten mAbs revealed that mAb gamma M phi 22.2 and mAb gamma M phi 197.1 bind to the high affinity Fc receptor for IgG (Fc gamma RI). mAb gamma M phi 28.3 appears to bind to a class II histocompatibility antigen and mAb gamma M phi 150.3 and mAb gamma M phi 195.18 appear to have binding patterns to human leukocytes and cell lines which are distinct from previously described mAbs. This rapid and specific procedure for screening mAbs has broad application for selecting mAbs that are specific for any given cell type and/or for surface molecules that are modulated by any cytokine and other hormone.

IFN-gamma plus glucocorticoids stimulate the expression of a newly identified human mononuclear phagocyte-specific antigen

Glucocorticoid hormones, although able to exert profound immunosuppressive effects, do not suppress mononuclear phagocyte activation by IFN-gamma and may even enhance it. For example, expression and functional activity of the high affinity FcR for IgG on human mononuclear phagocytes (FcR gamma I) is increased by IFN-gamma and is maximal after co-treatment with IFN-gamma plus the glucocorticoid dexamethasone (DEX). To determine whether there are other mononuclear phagocyte surface Ag that are regulated in this manner, hybridomas were prepared using IFN-gamma-plus-DEX-treated human monocytes as immunogen. Five IgG1 mAb (Mac 2-8, 2-38, 2-48, 2-49, and 2-158) were developed that recognize a trypsin-sensitive mononuclear phagocyte-specific surface Ag of Mr 155,000. There was no detectable reactivity of these mAb to lymphocytes or granulocytes or to several cell lines, including U-937 and HL-60. The p155 Ag was detected on monocytes and increased significantly with time of culture or after treatment with DEX. Expression was maximal after co-treatment with rIFN-gamma plus DEX, but was inhibited or unaffected by treatment with IFN-gamma alone. For freshly isolated cells, expression of the p155 Ag was highest on peritoneal macrophages. Our results indicate that the p155 Ag is a newly identified Ag of the human mononuclear phagocyte lineage and may represent, in the least, a phenotypic marker of monocyte differentiation or maturation.

IFN-gamma plus glucocorticoids stimulate the expression of a newly identified human mononuclear phagocyte-specific antigen

Glucocorticoid hormones, although able to exert profound immunosuppressive effects, do not suppress mononuclear phagocyte activation by IFN-gamma and may even enhance it. For example, expression and functional activity of the high affinity FcR for IgG on human mononuclear phagocytes (FcR gamma I) is increased by IFN-gamma and is maximal after co-treatment with IFN-gamma plus the glucocorticoid dexamethasone (DEX). To determine whether there are other mononuclear phagocyte surface Ag that are regulated in this manner, hybridomas were prepared using IFN-gamma-plus-DEX-treated human monocytes as immunogen. Five IgG1 mAb (Mac 2-8, 2-38, 2-48, 2-49, and 2-158) were developed that recognize a trypsin-sensitive mononuclear phagocyte-specific surface Ag of Mr 155,000. There was no detectable reactivity of these mAb to lymphocytes or granulocytes or to several cell lines, including U-937 and HL-60. The p155 Ag was detected on monocytes and increased significantly with time of culture or after treatment with DEX. Expression was maximal after co-treatment with rIFN-gamma plus DEX, but was inhibited or unaffected by treatment with IFN-gamma alone. For freshly isolated cells, expression of the p155 Ag was highest on peritoneal macrophages. Our results indicate that the p155 Ag is a newly identified Ag of the human mononuclear phagocyte lineage and may represent, in the least, a phenotypic marker of monocyte differentiation or maturation.

Glucocorticoid effects on the production and actions of immune cytokines

The immunosuppressive actions of glucocorticoids form the basis for their use in treatment of autoimmune diseases and prevention of allograft rejection. However, the mechanisms responsible for glucocorticoid-induced immunosuppression are still poorly understood. It is now clear that glucocorticoids do not inhibit all aspects of the immune response and, in some cases, may enhance certain functions of immune effector cells. One example is that of the dramatic increase induced by IFN-gamma in the number of IgG Fc receptors on human mononuclear phagocytes, which is enhanced rather than inhibited by glucocorticoids. An aspect of the immune response which appears to be consistently suppressed by glucocorticoids is the production of immune cytokines. Since these hormones appear to be essential mediators for a vigorous immune response, inhibition of their production may be an effective way for glucocorticoids to block the immune response.

Nerve terminals and synapses in the cardiac ganglion of the adult lobster Homarus americanus

The cardiac ganglion in the lobster Homarus americanus was examined with a transmission electron microscope. Nerve terminals often existed in large aggregations surrounded by glial and connective tissue elements. Axo-axonic and axo-dendritic synapses were present. Six ultrastructurally different types of nerve terminal, each containing an abundance of vesicles, were distinguished: three formed discrete chemical synapses as indicated by typical release site morphology; three did not. The latter appear to be neurosecretory axon terminals of extrinsic neurons. More than one morphologically distinct type of synaptic vesicle occurred commonly in a given terminal, suggesting the presence of coexisting neurotransmitters and/or neuroregulatory factors. Symmetrical chemical synapses and electrotonic junctions between axons were present.

Recombinant immune interferon increases immunoglobulin G Fc receptors on cultured human mononuclear phagocytes

Although recent studies suggest that interferons can increase the number of IgG Fc receptor (FcR gamma) sites on mouse macrophages, direct assessment of similar effects on human mononuclear phagocytes is lacking. We therefore measured the specific binding of 125I- and fluorescein-labeled IgG1 to human monocytes and leukemic cell lines after culture in vitro with highly purified human interferons. We report that natural and recombinant human gamma-interferon causes a dramatic (nearly 10-fold) increase in the number of FcR gamma on normal human monocytes and on the human cell lines HL-60 and U-937. Alpha and beta-interferons cause a modest but significant increase in these receptors. This report demonstrates that gamma-interferon acts directly on human mononuclear phagocytes to increase FcR gamma sites, it identifies a qualitative difference in the physiologic actions of human type I and type II interferons, and it suggests that HL-60 and U-937 cells will be important models for further study of the molecular mechanisms of interferon action. The results reported here could also be the basis for a bioassay to assess the pharmacokinetics and variability of gamma-interferon action on monocytes of individual patients during treatment in vitro and in vivo.