PPARα, β/δ, and γ regulate genes involved in the control of lipid metabolism and inflammation and are expressed in all major cell types of atherosclerotic lesions. In vitro studies have suggested that PPARs exert antiatherogenic effects by inhibiting the expression of proinflammatory genes and enhancing cholesterol efflux via activation of the liver X receptor-ABCA1 (LXR-ABCA1) pathway. To investigate the potential importance of these activities in vivo, we performed a systematic analysis of the effects of PPARα, β, and γ agonists on foam-cell formation and atherosclerosis in male LDL receptor-deficient (LDLR -/-) mice. Like the PPARγ agonist, a PPARα-specific agonist strongly inhibited atherosclerosis, whereas a PPARβ-specific agonist failed to inhibit lesion formation. In concert with their effects on atherosclerosis, PPARα and PPARγ agonists, but not the PPARβ agonist, inhibited the formation of macrophage foam cells in the peritoneal cavity. Unexpectedly, PPARα and PPARγ agonists inhibited foam-cell formation in vivo through distinct ABCA1-independent pathways. While inhibition of foam-cell formation by PPARα required LXRs, activation of PPARγ reduced cholesterol esterification, induced expression of ABCG1, and stimulated HDL-dependent cholesterol efflux in an LXR-independent manner. In concert, these findings reveal receptor-specific mechanisms by which PPARs influence macrophage cholesterol homeostasis. In the future, these mechanisms may be exploited pharmacologically to inhibit the development of atherosclerosis.

Incubation of bone marrow macrophages with lipopolysaccharide (LPS) or interferon gamma (IFN gamma) blocks macrophage proliferation. LPS treatment or M-CSF withdrawal arrests the cell cycle at early G1 and induces apoptosis. Treatment of macrophages with IFN gamma stops the cell cycle later, at the G1/S boundary, induces p21Waf1, and does not induce apoptosis. Moreover, pretreatment of macrophages with IFN gamma protects from apoptosis induced by several stimuli. Inhibition of p21Waf1 with antisense oligonucleotides or using KO mice shows that the induction of p21Waf1 by IFN gamma mediates this protection. Thus, IFN gamma makes macrophages unresponsive to apoptotic stimuli by inducing p21Waf1 and arresting the cell cycle at the G1/S boundary. Therefore, the cells of the innate immune system could only survive while they were functionally active.

Liver X receptors (LXRs) regulate the expression of genes involved in cholesterol and fatty acid homeostasis, including the genes for ATP-binding cassette transporter A1 (ABCA1) and sterol response element binding protein 1 (SREBP1). Loss of LXR leads to derepression of the ABCA1 gene in macrophages and the intestine, while the SREBP1c gene remains transcriptionally silent. Here we report that high-density-lipoprotein (HDL) cholesterol levels are increased in LXR-deficient mice, suggesting that derepression of ABCA1 and possibly other LXR target genes in selected tissues is sufficient to result in enhanced HDL biogenesis at the whole-body level. We provide several independent lines of evidence indicating that the repressive actions of LXRs are dependent on interactions with the nuclear receptor corepressor (NCoR) and the silencing mediator of retinoic acid and thyroid hormone receptors (SMRT). While dissociation of NCoR and SMRT results in derepression of the ABCA1 gene in macrophages, it is not sufficient for derepression of the SREBP1c gene. These findings reveal differential requirements for corepressors in the regulation of genes involved in cholesterol and fatty acid homeostasis and raise the possibility that these interactions may be exploited to develop synthetic ligands that selectively modulate LXR actions in vivo.

Microbe-macrophage interactions play a central role in the pathogenesis of many infections. The ability of some bacterial pathogens to induce macrophage apoptosis has been suggested to contribute to their ability to elude innate immune responses and successfully colonize the host. Here, we provide evidence that activation of liver X receptors (LXRs) and retinoid X receptors (RXRs) inhibits apoptotic responses of macrophages to macrophage colony-stimulating factor (M-CSF) withdrawal and several inducers of apoptosis. In addition, combined activation of LXR and RXR protected macrophages from apoptosis caused by infection with Bacillus anthracis, Escherichia coli, and Salmonella typhimurium. Expression-profiling studies demonstrated that LXR and RXR agonists induced the expression of antiapoptotic regulators, including AIM͞ CT2, Bcl-XL, and Birc1a. Conversely, LXR and RXR agonists inhibited expression of proapoptotic regulators and effectors, including caspases 1, 4͞11, 7, and 12; Fas ligand; and Dnase1l3. The combination of LXR and RXR agonists was more effective than either agonist alone at inhibiting apoptosis in response to various inducers of apoptosis, and it acted synergistically to induce expression of AIM͞CT2. Inhibition of AIM͞CT2 expression in response to LXR͞RXR agonists partially reversed their antiapoptotic effects. These findings reveal unexpected roles of LXRs and RXRs in the control of macrophage survival and raise the possibility that LXR͞RXR agonists may be exploited to enhance innate immunity to bacterial pathogens that induce apoptotic programs as a strategy for evading host responses.oxysterol ͉ transcription M acrophages serve essential functions as regulators of immunity and homeostasis (1, 2). As participants in native immunity, macrophages phagocytose and kill invading microorganisms and elaborate signaling molecules that amplify acute inflammatory responses. Macrophages also contribute to acquired immune responses by means of specialized functions that include antigen presentation and regulation of T cell responses. Thus, regulation of macrophage differentiation and survival is critical to the overall control of the magnitude, duration, and characteristics of immune responses. Programmed cell death, or apoptosis, of lymphocyte and myeloid cells is regulated tightly through cell death receptor and mitochondrial pathways to limit amplification of immune responses and facilitate resolution of inflammation (3). Apoptosis and survival pathways are also targeted by pathogens as a means of either escaping immune surveillance or establishing residence within host cells (4). The inhibition of macrophage apoptosis may offer a strategy for augmenting innate immunity to highly virulent bacterial pathogens, such as Bacillus anthracis, Yersinia pestis, Salmonella spp., and Shigella flexneri, which have evolved various ways to kill host macrophages. The execution of all forms of programmed cell death involves the proteolytic activation of a cascade of intracellular cysteine proteases, known as caspases....

Decorin is a small proteoglycan that is ubiquitous in the extracellular matrix of mammalian tissues. It has been extensively demonstrated that decorin inhibits tumor cell growth; however, no data have been reported on the effects of decorin in normal cells. Using nontransformed macrophages from bone marrow, results of this study showed that decorin inhibits macrophage colony-stimulating factor (M-CSF)-dependent proliferation by inducing blockage at the G 1 phase of the cell cycle without affecting cell viability. In addition, decorin rescues macrophages from the induction of apoptosis after growth factor withdrawal. Decorin induces the expression of the cdk inhibitors p21 Waf1 and p27 Kip1 . Using macrophages from mice where these genes have been disrupted, inhibition of proliferation mediated by decorin is related to p27 Kip1 expression, whereas p21 Waf1 expression is necessary to protect macrophages from apoptosis. Decorin also inhibits M-CSF-dependent expression of MKP-1 and extends the kinetics of ERK activity, which is characteristic when macrophages become activated instead of proliferating. The effect of decorin on macrophages is not due to its interaction with epidermal growth factor or interferon-␥ receptors. Furthermore, decorin increases macrophage adhesion to the extracellular matrix, and this may be partially responsible for the expression of p27 Kip1 IntroductionStimulated monocytes and macrophages secrete a diverse set of mediators that influence cellular immune functions and inflammation. These mediators include proinflammatory and anti-inflammatory cytokines, prostaglandins, leukotrienes, and reactive oxygen metabolites. 1 At the inflammatory sites, proteoglycans are both secreted by activated mononuclear leukocytes and released as a result of extracellular matrix (ECM) degradation. Thus, proteoglycans, which are major constituents of the ECM, are another class of molecules produced by monocytes and macrophages 2,3 that are potential modulators of the immune response.Decorin belongs to a family of small leucine-rich proteoglycans 4,5 and is found in the ECM of several of tissues such as skin, 6,7 cartilage, 8,9 and bone. 10 The biologic importance of these molecules is unclear. In vitro binding studies have shown that some of them interact with several types of collagen 11,12 and act as important regulators of collagen fibrillogenesis. In support of this hypothesis, a decorin-deficient mouse was found to have fragile skin with an abnormal organization of collagen fibers. 13 Decorin may also affect the production of other ECM components by regulating the activity of transforming growth factor-␤ (TGF-␤). 14,15 Additionally, decorin can modulate the interactions of matrix molecules (eg, fibronectin) with cells. [16][17][18] These observations suggest that decorin and perhaps other proteoglycans regulate the production and assembly of the ECM at several levels and hence the remodeling of connective tissue.Different observations have revealed that decorin is involved in the control of cell prolif...