The yeast two-hybrid system was used to isolate novel cellular factors that interact with the mouse peroxisome proliferator-activated receptor alpha (PPARalpha). One of the interacting clones isolated encoded LXRalpha, a recently described human orphan nuclear hormone receptor. LXRalpha bound directly to PPARalpha, as well as to the common heterodimerization partner 9-cis-retinoic acid receptor (RXRalpha). LXRalpha did not form a DNA binding complex with PPARalpha on synthetic hormone response elements composed of direct repeats of the TGACCT consensus half-site or on naturally occurring peroxisome proliferator response elements (PPREs) or LXRalpha response elements. However, LXRalpha inhibited binding of PPARalpha/RXRalpha heterodimers to PPREs, and coexpression of LXRalpha in mammalian cells antagonized peroxisome proliferator signaling mediated by PPARalpha/RXRalpha in vivo. These findings identify a novel partner for PPARalpha and suggest that LXRalpha plays a role in modulating PPAR-signaling pathways in the cell.

Liver X receptors (LXR␣ (NR1H3) and LXR␤ (NR1H2)) 1 are recently described members of the nuclear hormone receptor superfamily of ligand-activated transcription factors that are important in the regulation of genes that govern cholesterol homeostasis in the liver and cholesterol efflux from peripheral tissues. LXRs are bound by and activated by specific cholesterol metabolites, including oxysterols, and regulate the expression of target genes by binding to specific promoter response elements (LXREs) in association with the obligate heterodimerization partner, retinoid X receptor (RXR), the receptor for 9-cis-retinoic acid (9cRA) (1). Natural ligands for LXR include 22(R)-hydroxycholesterol (22(R)-HC), 24(S),25-epoxycholesterol, and 25-hydroxycholesterol (2-4), compounds that have been found free in serum and in association with atherogenic oxidized low density lipoprotein (oxLDL) particles (5-7). In the liver, LXR␣ serves as a sterol sensor and regulates the expression of genes that influence cholesterol metabolism and homeostasis, including the genes encoding cholesterol 7␣-hydroxylase, which controls the cholesterol/bile acid synthetic pathway, and sterol regulatory element-binding protein-1c, a key transcription factor that regulates expression of genes important in fatty acid biosynthesis (6, 8, 9 -11). Studies using lxr null mice have shown that LXR␣ is essential for normal cholesterol homeostasis and secretion of excess cholesterol in vivo (9).Recent work has shown that LXR␣ plays a fundamental role in macrophage biology by regulating cholesterol efflux from lipid-loaded cells. This is manifested by LXR␣-mediated induction of genes encoding the ATP-binding cassette proteins ABC-1 and ABCG1, which encode plasma membrane-associated reverse cholesterol transport proteins that mediate cholesteryl ester and free cholesterol efflux from monocytes and lipid-loaded macrophages (12-15). Effluxed cholesterol is subsequently delivered to extracellular acceptors, especially high density lipoprotein/ApoE, and transported back to the liver where it is converted to bile acids and excreted (1). The key role of LXR as a master regulator in the overall process governing cholesterol efflux and reducing intracellular cholesterol levels is underscored by the findings that the genes encoding ApoE, which is necessary for high density lipoprotein formation is also a transcriptional target of LXR (16). The pivotal role of LXR␣ in regulating reverse cholesterol transport in macrophages is of particular disease relevance, because lipid accumulation in these cells, through the uptake of oxLDL, is of fundamental importance to the etiology and pathogenesis of atherogenesis and atherosclerosis and other chronic inflammatory diseases. oxLDL particles accumulate in macrophages that have infiltrated the arterial intimal space, subsequently developing into lipid-loaded foam cells that comprise the characteristic fatty streak of early atherosclerotic lesions (17). In this context, LXR␣, by reducing intracellular cholesterol and lipid accumul...

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Peroxisome proliferator-activated nuclear hormone receptors (PPAR) are ligand-activated transcription factors that play pivotal roles in governing metabolic homeostasis and cell growth. PPARs are primarily in the nucleus but, under certain circumstances, can be found in the cytoplasm. We show here that PPARα interacts with the centrosome-associated protein CAP350. CAP350 also interacts with PPARδ, PPARγ and liver-X-receptor α, but not with the 9-cis retinoic acid receptor, RXRα. Immunofluorescence analysis indicated that PPARα is diffusely distributed in the nucleus and excluded from the cytoplasm. However, in the presence of coexpressed CAP350, PPARα colocalizes with CAP350 to discrete nuclear foci and to the centrosome, perinuclear region and intermediate filaments. In contrast, the subcellular distribution of RXRα or of thyroid hormone receptor α was not altered by coexpression of CAP350. An amino-terminal fragment of CAP350 was localized exclusively to nuclear foci and was sufficient to recruit PPARα to these sites. Mutation of the single putative nuclear hormone receptor interacting signature motif LXXLL present in this fragment had no effect on its subnuclear localization but abrogated recruitment of PPARα to nuclear foci. Surprisingly, mutation of the LXXLL motif in this CAP350 subfragment did not prevent its binding to PPARα in vitro, suggesting that this motif serves some function other than PPARα binding in recruiting PPARα to nuclear spots. CAP350 inhibited PPARα-mediated transactivation in an LXXLL-dependent manner, suggesting that CAP350 represses PPARα function. Our findings implicate CAP350 in a dynamic process that recruits PPARα to discrete nuclear and cytoplasmic compartments and suggest that altered intracellular compartmentalization represents a regulatory process that modulates PPAR function.