Matrix metalloproteinases (MMPs) are zinc endopeptidases that degrade extracellular matrix (ECM) components during normal and pathogenic tissue remodeling. Inappropriate expression of these enzymes contributes to the development of vascular pathology, including atherosclerosis. MMP-9 is expressed in its active form in atherosclerotic lesions and is believed to play an important role in vascular remodeling, smooth muscle cell migration, and plaque instability. We demonstrate here that the liver X receptors (LXRs) LXR␣ and LXR␤ inhibit basal and cytokine-inducible expression of MMP-9. Treatment of murine peritoneal macrophages with the synthetic LXR agonists GW3965 or T1317 reduces MMP-9 mRNA expression and blunts its induction by pro-inflammatory stimuli including lipopolysaccharide, interleukin-1␤, and tumor necrosis factor ␣. In contrast, macrophage expression of MMP-12 and MMP-13 is not altered by LXR ligands. We further show that the ability of LXR ligands to regulate MMP-9 expression is strictly receptor-dependent and is not observed in macrophages obtained from LXR␣␤ null mice. Analysis of the 5-flanking region of the MMP-9 gene indicates that LXR/RXR heterodimers do not bind directly to the MMP-9 promoter. Rather, activation of LXRs represses MMP-9 expression, at least in part through antagonism of the NFB signaling pathway. These observations identify the regulation of macrophage MMP-9 expression as a mechanism whereby activation of LXRs may impact macrophage inflammatory responses. Matrix metalloproteinases (MMPs)1 are a large family of Zn 2ϩ -containing endopeptidases that play a key role in degrading extracellular matrix (ECM) components. MMP activation has been implicated in physiological and pathological tissue remodeling in multiple contexts, including cancer invasion, inflammation, organ development, angiogenesis, and wound healing (1). The MMP family contains more than 20 members, including collagenases, gelatinases, matrilysins, stromelysins, and metalloelastases. Active MMPs trigger the degradation of ECM components such as collagens, elastin, gelatin, fibronectin, laminin, and proteoglycans. In addition, MMP activity induces the processing of several non-ECM proteins, including ␣ 1 -antitrypsin, ␣ 2 -macroglobulin, plasminogen, and serum amyloid protein A (2). The MMP activity is regulated at several levels, including gene transcription, cellular secretion, activation of proenzymes, and binding to tissue inhibitors of metalloproteinases (TIMPs). Increasing experimental evidence supports a direct link between elevated MMP activity and diseases that involve enhanced turnover of the ECM, such as atherosclerosis and rheumatoid arthritis (3).Enhanced expression of several MMPs, including MMP-9 and MMP-1, has been documented in both murine and human atherosclerotic lesions. Macrophages are a primary source of MMP expression, and their elaboration of these enzymes is believed to contribute to smooth muscle cell migration, neointima formation, and plaque rupture (4). MMP-9 degrades vessel wall collagen...

Macrophages are phagocytic cells that play essential roles in innate immunity and lipid homeostasis. The uptake of modified lipoproteins is an important early event in the development of atherosclerosis. We analyzed the ability of modified low-density lipoprotein (LDL) (oxidized and acetylated) to alter the expression and activity of arginases (ArgI and ArgII) in macrophages. We show that ArgI expression is potently induced by both oxidized and acetylated LDL in macrophages. We further show that this effect is mediated by peroxisome proliferator-activated receptors (PPAR). ArgI expression is highly responsive to agonists for PPARgamma and PPARdelta but not PPARalpha. Moreover, the induction of ArgI by both PPAR agonists and IL-4 is blocked in macrophages from PPARgamma- and PPARdelta-deficient mice. Functionally, PPAR activity induces macrophage activation toward a more Th2 immune phenotype in a model of Leishmania major infection. We show that PPARgamma and -delta ligands promote intracellular amastigote growth in infected macrophages, and this effect is dependent on both PPAR expression and Arg activity. Collectively, our results strongly suggest that ArgI is a key marker of the alternative program triggered by PPAR in macrophages.

The liver X receptors (LXRs) are ligand-dependent transcription factors that have been implicated in lipid metabolism and inflammation. LXRs also inhibit the expression of inflammatory genes in macrophages, including inducible nitric oxide synthase (iNOS). Some of these actions are mediated through LXR antagonism of NF-B activity. The potential for LXRs to positively regulate the expression of anti-inflammatory molecules, however, has not been explored. Here we show that the arginase II (ArgII) gene is a direct target for LXR regulation. ArgII catalyzes the conversion of L-arginine into L-ornithine and urea, leading to the synthesis of polyamines. Expression of ArgII is induced by LXR agonists in macrophage cell lines and primary murine macrophages in a receptor-dependent manner. The ArgII promoter contains a functional LXR response elements that mediates promoter induction by LXR/RXR (retinoid X receptor) in transfection assays. Since ArgII and iNOS utilize a common substrate, induction of ArgII expression has the potential to exert antiinflammatory effects by shifting arginine metabolism toward polyamine synthesis at the expense of NO production. In support of this hypothesis, we demonstrate that forced expression of ArgII mimics the inhibitory effect of LXR activation on macrophage NO production. Furthermore, inhibition of arginase activity partially reverses the inhibitory effect of LXR agonists on NO production. These studies suggest that regulation of ArgII may contribute to the immunomodulatory effects of LXRs.