Peroxisome proliferator activated-receptor (PPAR) isoforms, ␣ and ␥, function as important coregulators of energy (lipid) homeostasis. PPAR␣ regulates fatty acid oxidation primarily in liver and to a lesser extent in adipose tissue, whereas PPAR␥ serves as a key regulator of adipocyte differentiation and lipid storage. Of the two PPAR␥ isoforms, PPAR␥1 and PPAR␥2 generated by alternative splicing, PPAR␥1 isoform is expressed in liver and other tissues, whereas PPAR␥2 isoform is expressed exclusively in adipose tissue where it regulates adipogenesis and lipogenesis. Since the function of PPAR␥1 in liver is not clear, we have, in this study, investigated the biological impact of overexpression of PPAR␥1 in mouse liver. Adenovirus-PPAR␥1 injected into the tail vein induced hepatic steatosis in PPAR␣ ؊/؊ mice. Northern blotting and gene expression profiling results showed that adipocyte-specific genes and lipogenesis-related genes are highly induced in PPAR␣ ؊/؊ livers with PPAR␥1 overexpression. These include adipsin, adiponectin, aP2, caveolin-1, fasting-induced adipose factor, fat-specific gene 27 (FSP27), CD36, ⌬ 9 desaturase, and malic enzyme among others, implying adipogenic transformation of hepatocytes. Of interest is that hepatic steatosis per se, induced either by feeding a diet deficient in choline or developing in fasted PPAR␣ ؊/؊ mice, failed to induce the expression of these PPAR␥-regulated adipogenesis-related genes in steatotic liver. These results suggest that a high level of PPAR␥ in mouse liver is sufficient for the induction of adipogenic transformation of hepatocytes with adipose tissue-specific gene expression and lipid accumulation. We conclude that excess PPAR␥ activity can lead to the development of a novel type of adipogenic hepatic steatosis.

Peroxisome proliferator-activated receptor ␣ (PPAR␣) plays a central role in the cell-specific pleiotropic responses induced by structurally diverse synthetic chemicals designated as peroxisome proliferators. Transcriptional regulation by liganded nuclear receptors involves the participation of cofactors that form multiprotein complexes to achieve cell-and gene-specific transcription. Here we report the identification of such a transcriptionally active PPAR␣-interacting cofactor (PRIC) complex from rat liver nuclear extracts that interacts with full-length PPAR␣ in the presence of ciprofibrate, a synthetic ligand, and leukotriene B 4, a natural ligand. The liganded PPAR␣-PRIC complex enhanced transcription from a peroxisomal enoyl-CoA hydratase͞L-3-hydroxyacyl-CoA dehydrogenase bifunctional enzyme gene promoter template that contains peroxisome proliferator response elements. Rat liver PRIC complex comprises some 25 polypeptides, and their identities were established by mass spectrometry and limited sequence analysis. Eighteen of these peptides contain one or more LXXLL motifs necessary for interacting with nuclear receptors. PRIC complex includes known coactivators or coactivator-binding proteins (CBP, SRC-1, PBP, PRIP, PIMT, TRAP100, SUR-2, and PGC-1), other proteins that have not previously been described in association with transcription complexes (CHD5, TOG, and MORF), and a few novel polypeptides designated PRIC300, -285, -215, -177, and -145. We describe the cDNA for PRIC285, which contains five LXXLL motifs. It interacts with PPAR␣ and acts as a coactivator by moderately stimulating PPAR␣-mediated transcription in transfected cells. We conclude that liganded PPAR␣ recruits a distinctive multiprotein complex from rat liver nuclear extracts. The composition of this complex may provide insight into the basis of tissue and species sensitivity to peroxisome proliferators.

Cidea (cell death-inducing DNA fragmentation factor ␣-like effector A), a member of a novel family of proapoptotic proteins, is expressed abundantly in the brown adipose tissue of the mouse. Although Cidea mRNA is not detectable in the mouse liver, we now show that peroxisome proliferator-activated receptor (PPAR) ␣ ligands Wy-14,643 and ciprofibrate increase the Cidea mRNA level in a PPAR␣-dependent manner, whereas Cidea induction in liver by PPAR␥ overexpression is PPAR␣ independent. Increase in Cidea mRNA content in liver did not alter the expression of uncoupling protein 1 (Ucp1) gene, which regulates thermogenesis, lipolysis, and conservation of energy. Although Cidea is considered to be a proapoptotic factor, Cidea induction in liver did not result in increased apoptosis. To elucidate the mechanism by which PPAR␣ and PPAR␥ regulate Cidea gene expression in the liver, we analyzed the promoter region of the Cidea gene. Three putative peroxisome proliferator response elements (PPREs) are found in the Cidea gene promoter. Transactivation, gel-shift, and chromatin immunoprecipitation assays indicated that the proximal PPRE in Cidea gene (Cidea-PPRE1 at ؊680/؊668) is functional for both PPAR␣ and -␥. We conclude that Cidea is a novel target gene for both PPAR␣ and -␥ in the liver where these two transcription factors utilize the same PPRE region for dual regulation. The induction of Cidea in liver with these PPAR␣ and -␥ agonists suggests a possible role for Cidea in energy metabolism and a less likely role in hepatocyte apoptosis.

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Disruption of the gene encoding for the transcription coactivator peroxisome proliferator-activated receptor (PPAR)-binding protein (PBP/TRAP220/DRIP205/Med1) in the mouse results in embryonic lethality. Here, we have reported that targeted disruption of the Pbp/Pparbp gene in hepatocytes (Pbp ⌬Liv ) impairs liver regeneration with low survival after partial hepatectomy. Analysis of cell cycle progression suggests a defective exit from quiescence, reduced BrdUrd incorporation, and diminished entry into G 2 /M phase in Pbp ⌬Liv hepatocytes after partial hepatectomy. Pbp ⌬Liv hepatocytes failed to respond to hepatocyte growth factor/scatter factor, implying that hepatic PBP deficiency affects c-met signaling. Pbp gene disruption also abolishes primary mitogen-induced liver cell proliferative response. Striking abrogation of CCl 4 -induced hepatocellular proliferation and hepatotoxicity occurred in Pbp ⌬Liv mice pretreated with phenobarbital due to lack of expression of xenobiotic metabolizing enzymes necessary for CCl 4 activation. Pbp ⌬Liv mice, chronically exposed to Wy-14,643, a PPAR␣ ligand, revealed a striking proliferative response and clonal expansion of a few Pbp fl/fl hepatocytes that escaped Cre-mediated gene deletion in Pbp ⌬Liv livers, but no proliferative expansion of PBP null hepatocytes was observed. In these Pbp ⌬Liv mice, none of the Wy-14,643-induced hepatic adenomas and hepatocellular carcinomas was derived from PBP ⌬Liv hepatocytes; all liver tumors developing in Pbp ⌬Liv mice maintained non-recombinant Pbp alleles and retained PBP expression. These studies provide direct evidence in support of a critical role of PBP/TRAP220 in liver regeneration, induction of hepatotoxicity, and hepatocarcinogenesis.Transcription cofactors/coregulators consist of corepressors, coactivators, and coactivator-or corepressor-associated proteins, which participate in nuclear receptor-directed transcription (1-3). The functional significance for the existence of Ͼ200 nuclear receptor cofactors is not readily evident, but there is an increasing recognition of the general importance of some of these molecules in gene expression, embryogenesis, cell growth, and oncogenesis, as well as energy and xenobiotic metabolism (1, 3). Emerging gene knock-out mouse models show that some of the coactivators that directly bind to transcription factors to enhance gene expression are essential for embryonic growth and survival (see Ref. 3 for review). For example, the disruption of a coactivator gene such as peroxisome proliferated-activated receptor (PPAR) 2 -binding protein (PBP; also known as TRAP (thyroid hormone receptor-associated protein) 220/DRIP (vitamin D 3 receptor-interacting protein) 205)/Med1 (Mediator 1)) is embryonically lethal between gestational days 11.5 and 12.5, implying that this coactivator is widely involved in the transcriptional activity of many transcription factors (4, 5-7). To define the in vivo role of this coactivator, we used conditional mutagenesis in mice and found that deletion of the Pbp/Pparbp gen...