The Orphan Nuclear Receptor Small Heterodimer Partner as a Novel Coregulator of Nuclear Factor-κB in Oxidized Low Density Lipoprotein-treated Macrophage Cell Line RAW 264.7

Abstract

Small heterodimer partner (SHP), specifically expressed in liver and a limited number of other tissues, is an unusual orphan nuclear receptor that lacks the conventional DNA binding domain. In this work, we found that SHP expression is abundant in murine macrophage cell line RAW 264.7 but was suppressed by oxidized low density lipoprotein (oxLDL) and its constituent 13-hydroxyoctadecadienoic acid, a ligand for peroxisome proliferator-activated receptor ␥. Furthermore, SHP acted as a transcription coactivator of nuclear factor-B (NFB) and was essential for the previously described NFB transactivation by palmitoyl lysophosphatidylcholine, one of the oxLDL constituents. Accordingly NFB, which was transcriptionally active in the beginning, became progressively inert in oxLDL-treated RAW 264.7 cells as oxLDL decreased the SHP expression. Thus, SHP appears to be an important modulatory component to regulate the transcriptional activities of NFB in oxLDL-treated, resting macrophage cells.The nuclear receptor superfamily includes receptors for a variety of small hydrophobic ligands such as steroids, T3, and retinoids as well as a large number of related proteins that do not have known ligands, referred to as orphan nuclear receptors (for review, see Ref. 1). The receptor proteins are direct regulators of transcription that function by binding to specific DNA sequences named hormone response elements in promoters of target genes. Nearly all the superfamily members bind as dimers to DNA elements. Although some apparently bind only as homodimers, thyroid hormone receptors, vitamin D receptor, retinoic acid receptors (RARs), 1 the peroxisome proliferatoractivated receptors (PPARs), and several orphan nuclear receptors bind their specific response elements with high affinity as heterodimers with the retinoid X receptors. Receptors have dimerization interfaces in both their DNA binding domain and ligand binding domain. The DNA binding domain interfaces are quite different for each receptor, whereas the ligand binding domain interface is primarily based on a conserved motif referred to as the ninth heptad or the I-box (2). This motif is required for both heterodimerization and homodimerization of many nuclear receptors, including retinoid X receptor, thyroid hormone receptor, RAR, hepatocyte nuclear factor 4, and chicken ovalbumin upstream promoter transcription factor (2-4). Small heterodimer partner (SHP) is an orphan nuclear receptor specifically expressed in liver and a limited number of other tissues, and its activities are in some ways opposite to those of retinoid X receptor (5). SHP, like the orphan nuclear receptor DAX-1 (for review, see Ref. 6), lacks the conventional DNA binding domain. Both direct biochemical and the yeast two-hybrid results demonstrated that SHP interacts with many members of the receptor superfamily (5). As expected from its lack of a DNA binding domain, addition of SHP inhibited in vitro DNA binding by nuclear receptors with which it interacted, and in mammalian cell cotransfections, SHP repress...

Ox-LDL suppresses PMA-induced MMP-9 expression and activity through CD36-mediated activation of PPAR-γ

“…Short exposure (less than 4 h) of resting macrophages to ox-LDL leads to increased expression of MMP-9 mRNA through activation of NF-κB (Xu et al, 1999). In our experiment, however, U937 monocytic cells were incubated with ox-LDL for more than 24 h. In this case, PPAR-γ-mediated pathway overrides the NF-κB-mediated responses, leading to suppression of MMP-9 mRNA expression (Kim et al, 2001). In primary monocytes, ox-LDL (5-25 µg/ml) in the presence of tumor necrosis factor (TNF)-α and GM-CSF increased slightly MMP-9 activity (Ardans et al, 2002).…”

“…The differences between our data and the previous ones might be explained by considering context or concentration-dependent influence of ox-LDL on macrophage gene expression. Ox-LDL activates NF-κB first, then PPAR-γ (Han et al, 2000;Kim et al, 2001). Short exposure (less than 4 h) of resting macrophages to ox-LDL leads to increased expression of MMP-9 mRNA through activation of NF-κB (Xu et al, 1999).…”

SHP and Sin3A expression are essential for adamantyl-substituted retinoid-related molecule–mediated nuclear factor-κB activation, c-Fos/c-Jun expression, and cellular apoptosis

“…SHP was found to be a coactivator of NF-κB in macrophages and this activation of NF-κB was secondary to SHP binding to the NF-κB p65 subunit with enhanced binding of the p65/p50 complex to the NF-κB consensus sequence (14). We and others have previously shown that ARR activation of NF-κB is essential in ARR-mediated apoptosis (31,35).…”

“…SHP modulation of gene expression is due to its ability to associate with other proteins and it has been found to heterodimerize with a large number of nuclear receptors, including the thyroid hormone receptor, retinoic acid receptor, retinoid X receptor, estrogen receptor, glucocorticoid receptor, and hepatocyte nuclear factor-4 (13). In contrast, SHP has been found to activate the NF-κB and Peroxisome Proliferator-Activated Receptor γ (PPARγ) nuclear receptors (14,15). At least three separate and distinct pathways seem to be used by SHP to silence gene expression.…”

“…Whereas SHP has been shown to inhibit the induction of gene expression by a number of nuclear receptors, it has been shown to stimulate the transcriptional activity of NF-κB as well as PPARγ (14,15). We have previously shown that NF-κB is required for ARR-mediated apoptosis (31).…”