To gain insight into the function of peroxisome proliferator-activated receptor (PPAR) isoforms in rodents, we disrupted the ligand-binding domain of the alpha isoform of mouse PPAR (mPPAR alpha) by homologous recombination. Mice homozygous for the mutation lack expression of mPPAR alpha protein and yet are viable and fertile and exhibit no detectable gross phenotypic defects. Remarkably, these animals do not display the peroxisome proliferator pleiotropic response when challenged with the classical peroxisome proliferators, clofibrate and Wy-14,643. Following exposure to these chemicals, hepatomegaly, peroxisome proliferation, and transcriptional-activation of target genes were not observed. These results clearly demonstrate that mPPAR alpha is the major isoform required for mediating the pleiotropic response resulting from the actions of peroxisome proliferators. mPPAR alpha-deficient animals should prove useful to further investigate the role of this receptor in hepatocarcinogenesis, fatty acid metabolism, and cell cycle regulation.

The thyroid-specific enhancer-binding protein (T/ebp) gene was disrupted by homologous recombination in embryonic stem cells to generate mice lacking T/EBP expression. Heterozygous animals developed normally, whereas mice homozygous for the disrupted gene were born dead and lacked the lung parenchyma. Instead, they had a rudimentary bronchial tree associated with an abnormal epithelium in their pleural cavities. Furthermore, the homozygous mice had no thyroid gland but had a normal parathyroid. In addition, extensive defects were found in the brain of the homozygous mice, especially in the ventral region of the forebrain. The entire pituitary, including the anterior, intermediate, and posterior pituitary, was also missing. In situ hybridization showed that the T/ebp gene is expressed in the normal thyroid, lung bronchial epithelium, and specific areas of the forebrain during early embryogenesis. These results establish that the expression of T/EBP, a transcription factor known to control thyroid-specific gene transcription, is also essential for organogenesis of the thyroid, lung, ventral forebrain, and pituitary.[Key Words: T/EBP; gene targeting; thyroid; lung; ventral forebrain; pituitary] Received August 22, 1995; revised version accepted November 10, 1995.Thyroid-specific enhancer-binding protein (T/EBP) binds to an enhancer element located -5.5 kb upstream of the human thyroid peroxidase gene transcription start site and regulates thyroid-specific gene expression (Kikkawa et al. 1990;Mizuno et al. 1991). T/EBP, also named thyroid-specific transcription factor 1 (TTF-1)or Nkx-2.1, was originally described to govern thyroid-specific expression of the rat thyroglobulin gene (Civitareale et al. 1989). Several studies have established the role of T/EBP in expression of genes encoding thyroid peroxidase (Kikkawa et al. 1990;Mizuno et al. 1991;Abramowicz et al. 1992;Francis-Lang et al. 1992), thyroglobulin (Civitareale et al. 1989), and the thyrotropin (TSH) receptor tCivitareale et al. 1993;Shimura et al. 1994). All three proteins are essential for thyroid hormone biosynthesis (DeGroot and Niepomniszcze 1977). T/EBP is also expressed in the lung (Guazzi et al. 1990;Mizuno et al. 1991), and it has been recently demonstrated that the expression of genes encoding the lung surfactant proteins A and B is regulated by this DNA-binding protein (Bohinski et al. 1994;Bruno et al. 1995). T/ebp ) is the first member of the mouse Nkx-2 gene family that is closely related to Drosophila NK-2 in their homeo domain sequences (68%-95% similarity) (Kim and Nirenberg 1989;Guazzi et al. 1990;Price et al. 1992;Lints et al. 1993). Members of the Nkx-2 family also share a highly conserved 17-aminoacid motif that is located on the carboxyl-terminal side of the homeo domain. From the six members of this family characterized to date, the expression patterns of three genes, T/ebp (Nkx-2.1), Nkx-2.2, and Nkx-2.5, have been studied. Lazzaro et al. (1991) have established T/ebp(Nkx-2.1) gene expression at -10.5 days postcoitum (El0.5) in the ...

The aryl hydrocarbon (Ah) receptor (AHR) mediates many carcinogenic and teratogenic effects of environmentally toxic chemicals such as dioxin. An AHR-deficient (Ahr-/-) mouse line was constructed by homologous recombination in embryonic stem cells. Almost half of the mice died shortly after birth, whereas survivors reached maturity and were fertile. The Ahr-/- mice showed decreased accumulation of lymphocytes in the spleen and lymph nodes, but not in the thymus. The livers of Ahr-/- mice were reduced in size by 50 percent and showed bile duct fibrosis Ahr-/- mice were also nonresponsive with regard to dioxin-mediated induction of genes encoding enzymes that catalyze the metabolism of foreign compounds. Thus, the AHR plays an important role in the development of the liver and the immune system.

ObjectivePeroxisome proliferator-activated receptor α (PPARα) is a nuclear receptor expressed in tissues with high oxidative activity that plays a central role in metabolism. In this work, we investigated the effect of hepatocyte PPARα on non-alcoholic fatty liver disease (NAFLD).DesignWe constructed a novel hepatocyte-specific PPARα knockout (Pparαhep−/−) mouse model. Using this novel model, we performed transcriptomic analysis following fenofibrate treatment. Next, we investigated which physiological challenges impact on PPARα. Moreover, we measured the contribution of hepatocytic PPARα activity to whole-body metabolism and fibroblast growth factor 21 production during fasting. Finally, we determined the influence of hepatocyte-specific PPARα deficiency in different models of steatosis and during ageing.ResultsHepatocyte PPARα deletion impaired fatty acid catabolism, resulting in hepatic lipid accumulation during fasting and in two preclinical models of steatosis. Fasting mice showed acute PPARα-dependent hepatocyte activity during early night, with correspondingly increased circulating free fatty acids, which could be further stimulated by adipocyte lipolysis. Fasting led to mild hypoglycaemia and hypothermia in Pparαhep−/− mice when compared with Pparα−/− mice implying a role of PPARα activity in non-hepatic tissues. In agreement with this observation, Pparα−/− mice became overweight during ageing while Pparαhep−/− remained lean. However, like Pparα−/− mice, Pparαhep−/− fed a standard diet developed hepatic steatosis in ageing.ConclusionsAltogether, these findings underscore the potential of hepatocyte PPARα as a drug target for NAFLD.