No abstract

Adipocyte differentiation involves the transcriptional activation of several genes in triglyceride metabolism, including the adipose P2 (aP2 or 422) gene that encodes the adipocyte lipid-binding protein ALBP. Within the mouse aP2 promoter region, the AE-1 sequence functions as either a positive or a negative element in the regulation of aP2 gene expression. The AE-1 sequence is the binding site for the positive murine (3T3) adipocyte factor C/EBP-alpha, several human preadipocyte factors, and a 3T3 preadipocyte factor(s) that has been implicated as a repressor of aP2 gene expression. Here we report the cloning of new complementary DNAs that encode the 3T3 preadipocyte factor (termed AEBP1) and demonstrate that AEBP1 expression is abolished during adipocyte differentiation. Furthermore, we show that an activity of a carboxypeptidase associated with AEBP1 is important in the transcriptional repression function of AEBP1. Thus AEBP1 might represent a new type of transcription factor that regulates transcription by cleavage of factors involved in transcription.

We have isolated the mouse gene encoding adipocyte P2, aP2, the differentiation-dependent adipocyte protein homologous to myelin P2. The aP2 gene is present in a single copy in the mouse and is present in single or few copies in species from human to Drosophila. The entire gene spans 4 kilobases and consists of four exons encoding 25, 57, 34, and 16 amino acids; the overall exon structure is similar to the gene encoding liver fatty acid binding protein. A plasmid vector was constructed containing the entire aP2 gene with flanking sequences, modified by linker insertion. When this gene is stably introduced into 3T3-F442A cells, it is expressed only upon adipose differentiation, with a time course of induction very similar to that of the endogenous aP2 gene. We have compared the DNA sequence of the 5'-flanking region of the aP2 gene to the promoter regions of two other genes activated during adipocyte differentiation, glycerol-3-phosphate dehydrogenase and adipsin, and find a 13-base region of homology (Formula: see text) present in multiple copies in the 5'-flanking region of each gene. An adjacent 15-base sequence is present only in glycerol-3-phosphate dehydrogenase and aP2 genes. Both of these elements share homology with putative viral enhancer core sequences. These results indicate that the aP2 gene contains sequence information necessary for differentiation-dependent expression in fat cells; common elements shared by adipocyte-specific genes may play a role in this process.

Mitogen-activated protein kinase (MAPK) is required for cell growth and cell differentiation. In adipogenesis, MAPK activation opposes the differentiation process. The regulatory mechanisms or the cellular factors that regulate the switch between growth and differentiation in the adipogenic lineage have been largely unelucidated. We show here that AEBP1, a transcriptional repressor that is down-regulated during adipogenesis, complexes and protects MAPK from its specific phosphatase in mammalian cells. We further show evidence that the modulation of MAPK activation by AEBP1 is a biologically relevant process in adipogenesis. Our results suggest that modulation of MAPK activation by the protective effect of AEBP1 may constitute a critical part in the determination between cell growth and differentiation in the adipogenic lineage. The proposed mode of action by which a transcription factor regulates MAPK activation is novel.

Peroxisome proliferator-activated receptor ␥1 (PPAR␥1) and liver X receptor ␣ (LXR␣) play pivotal roles in macrophage cholesterol homeostasis and inflammation, key biological processes in atherogenesis. Herein we identify adipocyte enhancer-binding protein 1 (AEBP1) as a transcriptional repressor that impedes macrophage cholesterol efflux, promoting foam cell formation, via PPAR␥1 and LXR␣ down-regulation. Contrary to AEBP1 deficiency, AEBP1 overexpression in macrophages is accompanied by decreased expression of PPAR␥1, LXR␣, and their target genes ATP-binding cassette A1, ATP-binding cassette G1, apolipoprotein E, and CD36, with concomitant elevation in IL-6, TNF-␣, monocyte chemoattractant protein 1, and inducible NO synthase levels. AEBP1, but not the C-terminally truncated DNA-binding domain mutant (AEBP1 ⌬Sty ), represses PPAR␥1 and LXR␣ in vitro. Expectedly, AEBP1-overexpressing transgenic (AEBP1 TG ) macrophages accumulate considerable amounts of lipids compared with AEBP1 nontransgenic macrophages, making them precursors for foam cells. Indeed, AEBP1-overexpressing transgenic macrophages exhibit diminished cholesterol efflux compared with AEBP1 nontransgenic macrophages, whereas AEBP1-knockout (AEBP1 ؊/؊ ) macrophages exhibit enhanced cholesterol efflux compared with wild-type (AEBP1 ؉/؉ ) macrophages. Our in vitro and ex vivo experimental data strongly suggest that AEBP1 plays critical regulatory roles in macrophage cholesterol homeostasis, foam cell formation, and proinflammation. Thereby, we speculate that AEBP1 may be critically implicated in the development of atherosclerosis, and it may serve as a molecular target toward developing antiinflammatory, antiatherogenic therapeutic approaches.atherogenesis ͉ cholesterol efflux ͉ liver X receptor ␣ ͉ peroxisome proliferator-activated receptor ␥

Nuclear factor B (NF-B) subunits comprise a family of eukaryotic transcription factors that are critically involved in cell proliferation, inflammation, and apoptosis. Under basal conditions, NF-B subunits are kept under inhibitory regulation by physical interaction with NF-B inhibitors (IB subunits) in the cytosol. Upon stimulation, IB subunits become phosphorylated, ubiquitinated, and subsequently degraded, allowing NF-B subunits to translocate to the nucleus and bind as dimers to B responsive elements of target genes. Previously, we have shown that AEBP1 enhances macrophage inflammatory responsiveness by inducing the expression of various proinflammatory mediators. Herein, we provide evidence suggesting that AEBP1 manifests its proinflammatory function by up-regulating NF-B activity via hampering IB␣, but not IB␤, inhibitory function through protein-protein interaction mediated by the discoidin-like domain (DLD) of AEBP1. Such interaction renders IB␣ susceptible to enhanced phosphorylation and degradation, subsequently leading to augmented NF-B activity. Collectively, we propose a novel molecular mechanism whereby NF-B activity is modulated by means of protein-protein interaction involving AEBP1 and IB␣. Moreover, our study provides a plausible mechanism explaining the differential regulatory functions exhibited by IB␣ and IB␤ in various cell types. We speculate that AEBP1 may serve as a potential therapeutic target for the treatment of various chronic inflammatory diseases and cancer.

Peroxisome proliferator-activated receptor γ1 (PPARγ1) and liver X receptor α (LXRα) are nuclear receptors that play pivotal roles in macrophage cholesterol homeostasis and inflammation; key biological processes in atherogenesis. The activation of PPARγ1 and LXRα by natural or synthetic ligands results in the transactivation of ABCA1, ABCG1, and ApoE; integral players in cholesterol efflux and reverse cholesterol transport. In this review, we describe the structure, isoforms, expression pattern, and functional specificity of PPARs and LXRs. Control of PPARs and LXRs transcriptional activity by coactivators and corepressors is also highlighted. The specific roles that PPARγ1 and LXRα play in inducing macrophage cholesterol efflux mediators and antagonizing macrophage inflammatory responsiveness are summarized. Finally, this review focuses on the recently reported regulatory functions that adipocyte enhancer-binding protein 1 (AEBP1) exerts on PPARγ1 and LXRα transcriptional activity in the context of macrophage cholesterol homeostasis and inflammation.

NF-κB comprises a family of transcription factors that are critically involved in various inflammatory processes. In this paper, the role of NF-κB in inflammation and atherosclerosis and the regulation of the NF-κB signaling pathway are summarized. The structure, function, and regulation of the NF-κB inhibitors, IκBα and IκBβ, are reviewed. The regulation of NF-κB activity by glucocorticoid receptor (GR) signaling and IκBα sumoylation is also discussed. This paper focuses on the recently reported regulatory function that adipocyte enhancer-binding protein 1 (AEBP1) exerts on NF-κB transcriptional activity in macrophages, in which AEBP1 manifests itself as a potent modulator of NF-κB via physical interaction with IκBα and a critical mediator of inflammation. Finally, we summarize the regulatory roles that recently identified IκBα-interacting proteins play in NF-κB signaling. Based on its proinflammatory roles in macrophages, AEBP1 is anticipated to serve as a therapeutic target towards the treatment of various inflammatory conditions and disorders.