Little is known of the transcriptional events controlling the differentiation and function of dendritic cells (DC). We found that the ligand-activated transcription factor Peroxisome Proliferator Activated Receptor gamma (PPARgamma) is immediately upregulated after the induction of monocyte-derived DC differentiation. Activation of PPARgamma changed the expression pattern of cell surface receptors and enhanced the internalizing activity of DC. Unexpectedly, we found that CD1 glycoproteins, a class of molecules responsible for the presentation of self and foreign modified lipids, were coordinately regulated by PPARgamma activation. CD1a levels were reduced, while CD1d expression was induced. Enhanced expression of CD1d was coupled to the selective induction of invariant natural-killer T cell (iNKT cell) proliferation in the presence of alpha-GalCer. These results suggest that PPARgamma orchestrates a transcriptional response leading to the development of a DC subtype with increased internalizing capacity, efficient lipid presentation, and the augmented potential to activate iNKT cells.

A key issue in the immune system is to generate specific cell types, often with opposing activities. The mechanisms of differentiation and subtype specification of immune cells such as macrophages and dendritic cells are critical to understand the regulatory principles and logic of the immune system. In addition to cytokines and pathogens, it is increasingly appreciated that lipid signaling also has a key role in differentiation and subtype specification. In this review we explore how intracellular lipid signaling via a set of transcription factors regulates cellular differentiation, subtype specification, and immune as well as metabolic homeostasis. We introduce macrophages and dendritic cells and then we focus on a group of transcription factors, nuclear receptors, which regulate gene expression upon receiving lipid signals. The receptors we cover are the ones with a recognized physiological function in these cell types and ones which heterodimerize with the retinoid X receptor. These are as follows: the receptor for a metabolite of vitamin A, retinoic acid: retinoic acid receptor (RAR), the vitamin D receptor (VDR), the fatty acid receptor: peroxisome proliferator-activated receptor γ (PPARγ), the oxysterol receptor liver X receptor (LXR), and their obligate heterodimeric partner, the retinoid X receptor (RXR). We discuss how they can get activated and how ligand is generated and eliminated in these cell types. We also explore how activation of a particular target gene contributes to biological functions and how the regulation of individual target genes adds up to the coordination of gene networks. It appears that RXR heterodimeric nuclear receptors provide these cells with a coordinated and interrelated network of transcriptional regulators for interpreting the lipid milieu and the metabolic changes to bring about gene expression changes leading to subtype and functional specification. We also show that these networks are implicated in various immune diseases and are amenable to therapeutic exploitation.

Dendritic cells (DCs) expressing CD1d, a molecule responsible for lipid antigen presentation, are capable of enhancing natural killer T (iNKT) cell proliferation. The signals controlling CD1 expression and lipid antigen presentation are poorly defined. We have shown previously that stimulation of the lipid-activated transcription factor, peroxisome proliferator-activated receptor (PPAR)γ, indirectly regulates CD1d expression. Here we demonstrate that PPARγ, turns on retinoic acid synthesis by inducing the expression of retinol and retinal metabolizing enzymes such as retinol dehydrogenase 10 and retinaldehyde dehydrogenase type 2 (RALDH2). PPARγ-regulated expression of these enzymes leads to an increase in the intracellular generation of all-trans retinoic acid (ATRA) from retinol. ATRA regulates gene expression via the activation of the retinoic acid receptor (RAR)α in human DCs, and RARα acutely regulates CD1d expression. The retinoic acid–induced elevated expression of CD1d is coupled to enhanced iNKT cell activation. Furthermore, in vivo relevant lipids such as oxidized low-density lipoprotein can also elicit retinoid signaling leading to CD1d up-regulation. These data show that regulation of retinoid metabolism and signaling is part of the PPARγ-controlled transcriptional events in DCs. The uncovered mechanisms allow the DCs to respond to altered lipid homeostasis by changing CD1 gene expression.

SummaryPPARγ is essential for adipogenesis and metabolic homeostasis. We describe mutations in the DNA and ligand binding domains of human PPARγ in lipodystrophic, severe insulin resistance. These receptor mutants lack DNA binding and transcriptional activity but can translocate to the nucleus, interact with PPARγ coactivators and inhibit coexpressed wild-type receptor. Expression of PPARγ target genes is markedly attenuated in mutation-containing versus receptor haploinsufficent primary cells, indicating that such dominant-negative inhibition operates in vivo. Our observations suggest that these mutants restrict wild-type PPARγ action via a non-DNA binding, transcriptional interference mechanism, which may involve sequestration of functionally limiting coactivators.

Accumulating data have shown that the microenvironment of dendritic cells modulates subtype differentiation and CD1 expression, but the mechanisms by which exogenous factors confer these effects are poorly understood. Here we describe the dependence of CD1a- monocyte-derived dendritic cell (moDC) development on lipids associated with the expression of peroxisome proliferator-activated receptor-gamma (PPARgamma). We also show the consecutive differentiation of immature CD1a-PPARgamma+ moDCs to CD1a+PPARgamma- cells limited by serum lipoproteins and terminated by proinflammatory cytokines. Immature CD1a- moDCs possess higher internalizing capacity than CD1a+ cells, whereas both activated subtypes have similar migratory potential but differ in their cytokine and chemokine profiles, which translates to distinct T-lymphocyte-polarizing capacities. CD1a+ moDCs stand out by their capability to secrete high amounts of IL-12p70 and CCL1. As lipoproteins skew moDC differentiation toward the generation of CD1a-PPARgamma+ cells and inhibit the development of CD1a+PPARgamma- cells, we suggest that the uptake of lipids results in endogenous PPARgamma agonists that induce a cascade of gene transcription coordinating lipid metabolism, the expression of lipid-presenting CD1 molecules, subtype dichotomy, and function. The presence of CD1a-PPARgamma+ and CD1a+PPARgamma- DCs in lymph nodes and in pulmonary Langerhans cell histiocytosis confirms the functional relevance of these DC subsets in vivo.

No abstract

ABCG2, a member of the ATP-binding cassette transporters has been identified as a protective pump against endogenous and exogenous toxic agents. ABCG2 was shown to be expressed at high levels in stem cells and variably regulated during cell differentiation. Here we demonstrate that functional ABCG2 is expressed in human monocyte-derived dendritic cells by the activation of a nuclear hormone receptor, PPAR␥. We identified and characterized a 150-base pair long conserved enhancer region, containing three functional PPAR response elements (PPARE), upstream of the human ABCG2 gene. We confirmed the binding of the PPAR␥⅐RXR heterodimer to this enhancer region, suggesting that PPAR␥ directly regulates the transcription of ABCG2. Consistent with these results, elevated expression of ABCG2 mRNA was coupled to enhanced protein production, resulting in increased xenobiotic extrusion capacity via ABCG2 in PPAR␥-activated cells. Furthermore PPAR␥ instructed dendritic cells showed increased Hoechst dye extrusion and resistance to mitoxantrone. Collectively, these results uncovered a mechanism by which up-regulation of functional ABCG2 expression can be achieved via exogenous or endogenous activation of the lipid-activated transcription factor, PPAR␥. The increased expression of the promiscuous ABCG2 transporter can significantly modify the xenobiotic and drug resistance of human myeloid dendritic cells.

Cholesterol uptake and efflux are key metabolic processes associated with macrophage physiology and atherosclerosis. Peroxisome proliferator-activated receptor gamma (PPAR␥) and liver X receptor alpha (LXR␣) have been linked to the regulation of these processes. It remains to be identified how activation of these receptors is connected and regulated by endogenous lipid molecules. We identified CYP27, a p450 enzyme, as a link between retinoid, PPAR␥, and LXR signaling. We show that the human CYP27 gene is under coupled regulation by retinoids and ligands of PPARs via a PPAR-retinoic acid receptor response element in its promoter. Induction of the enzyme's expression results in an increased level of 27-hydroxycholesterol and upregulation of LXR-mediated processes. Upregulated CYP27 activity also leads to LXR-independent elimination of CYP27 metabolites as an alternative means of cholesterol efflux. Moreover, human macrophage-rich atherosclerotic lesions have an increased level of retinoid-, PPAR␥-, and LXR-regulated gene expression and also enhanced CYP27 levels. Our findings suggest that nuclear receptor-regulated CYP27 expression is likely to be a key integrator of retinoic acid receptor-PPAR␥-LXR signaling, relying on natural ligands and contributing to lipid metabolism in macrophages.Handling of lipids by macrophages is an important metabolic process in the context of hypercholesterolemia and the development of atherosclerotic lesions (20,32,44). For this reason it is critical to understand the regulatory processes associated with cholesterol and fatty acid uptake and release (efflux) in this cell type. A regulatory network has been associated with macrophage lipid metabolism in recent years. First, it has been shown that peroxisome proliferator-activated receptor gamma (PPAR␥), a member of the nuclear receptor superfamily, can be linked to macrophage maturation and uptake of modified (oxidized) low-density lipoprotein (LDL) (35,45). Later, the oxysterol receptor liver X receptor (LXR) was linked to macrophage lipid metabolism by showing that LXR␣ is a direct transcriptional target of PPAR␥ and could induce lipid transporters such as ABCA1 (9, 40) and ABCG1 (26). A coordinated lipid transport is likely to be regulated by these receptors. Linking of the two receptor systems (PPAR␥ and LXR␣) provides an attractive but not well understood pathway to explain lipid and cholesterol uptake and efflux from macrophages.