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Isabelle Weinhofer

Publications and source records attributed to Isabelle Weinhofer.

6 recordsLinked to original sources
PubMed2006

Peroxisomal cholesterol biosynthesis and Smith-Lemli-Opitz syndrome.

Smith-Lemli-Opitz syndrome (SLOS), caused by 7-dehydrocholesterol-reductase (DHCR7) deficiency, shows variable severity independent of DHCR7 genotype. To test whether peroxisomes are involved in alternative cholesterol synthesis, we used [1-(14)C]C24:0 for peroxisomal beta-oxidation to generate [1-(14)C]acetyl-CoA as cholesterol precursor inside peroxisomes. The HMG-CoA reductase inhibitor lovastatin suppressed cholesterol synthesis from [2-(14)C]acetate and [1-(14)C]C8:0 but not from [1-(14)C]C24:0, implicating a peroxisomal, lovastatin-resistant HMG-CoA reductase. In SLOS fibroblasts lacking DHCR7 activity, no cholesterol was formed from [1-(14)C]C24:0-derived [1-(14)C]acetyl-CoA, indicating that the alternative peroxisomal pathway also requires this enzyme. Our results implicate peroxisomes in cholesterol biosynthesis but provide no link to phenotypic variation in SLOS.

Acetyl Coenzyme A↗
PubMed2005

Liver X receptor alpha interferes with SREBP1c-mediated Abcd2 expression. Novel cross-talk in gene regulation.

The peroxisomal ATP binding cassette (ABC) transporter adrenoleukodystrophy-related protein, encoded by ABCD2, displays functional redundancy with the X-linked adrenoleukodystrophy-associated protein, making ABCD2 up-regulation of therapeutic value. Cholesterol lowering activates human ABCD2 in cultured cells. To investigate in vivo regulation by sterols, we first characterized a sterol regulatory element (SRE) in the murine Abcd2 promoter that is directly bound by SRE-binding proteins (SREBPs). Intriguingly, this element overlaps with a direct repeat 4, which serves as binding site for liver X receptor (LXR)/retinoid X receptor heterodimers, suggesting novel cross-talk between SREBP and LXR/retinoid X receptor in gene regulation. Using fasting-refeeding and cholesterol loading, SREBP accessibility to the SRE/direct repeat 4 was tested. Results suggest that adipose Abcd2 is induced by SREBP1c, whereas hepatic Abcd2 expression is down-regulated by concurrent activation of LXRalpha and SREBP1c. In cell culture, SREBP1c-mediated Abcd2 induction is counteracted by ligand-activated LXRalpha. Finally, hepatic Abcd2 expression in LXRalpha,beta-deficient mice is inducible to levels vastly exceeding wild type. Together, we identify LXRalpha as negative modulator of Abcd2, acting through a novel regulatory mechanism involving overlapping SREBP and LXRalpha binding sites.

ATP Binding Cassette Transporter, Subfamily D↗
PubMed2005

X-linked adrenoleukodystrophy mice demonstrate abnormalities in cholesterol metabolism.

The neurodegenerative disorder X-linked adrenoleukodystrophy (X-ALD) is caused by ABCD1 mutations and characterized by very long-chain fatty acid (VLCFA) accumulation. Cholesterol-lowering normalized VLCFA in fibroblasts and plasma of X-ALD patients. We show that in cultured cells, cholesterol-loading induces ABCD1. In X-ALD mice, plasma cholesterol is elevated and not further increasable by cholesterol-feeding, whereas hepatic HMG-CoA reductase and Abcd2 are downregulated. Upon cholesterol modulation, brain VLCFA increased in X-ALD mice, but decreased in controls. In murine X-ALD fibroblasts, cholesterol-lowering did not normalize VLCFA. Thus, ALDP-deficiency and VLCFA are linked to cholesterol but species differences complicate evaluating cholesterol-lowering drugs in X-ALD mice.

ATP Binding Cassette Transporter, Subfamily D, Mem↗
PubMed2003

Evaluation of the therapeutic potential of PPARalpha agonists for X-linked adrenoleukodystrophy.

Adrenoleukodystrophy protein (ABCD1), a peroxisomal membrane protein, is mutated in patients affected by X-linked adrenoleukodystrophy (X-ALD). Adrenoleukodystrophy-related protein (ABCD2) is the closest relative of ABCD1. Pharmacological induction of ABCD2 gene expression has been proposed as a novel therapy strategy for X-ALD. Fibrates induce peroxisome proliferation and Abcd2 expression in rodent liver. Here we evaluate the possibility of using peroxisome proliferator-activated receptor alpha (PPARalpha) agonists for pharmacological induction of ABCD2 expression. In the liver of PPARalpha-deficient mice, both the constitutive and the fenofibrate-inducible Abcd2 gene expression was found to be PPARalpha-dependent. In the brain, PPARalpha-deficiency has no effect on Abcd2 expression. In mice orally treated with the novel, highly selective, and potent PPARalpha agonists GW 7647, GW 6867, and tetradecylthioacetic acid, Abcd2 expression was induced in liver and adrenal glands, but not in brain and testis. None of four putative PPREs identified in the 5(')-flanking DNA and in intron 1 of the Abcd2 gene conferred fibrate response in luciferase reporter assays. Thus, although fibrate-mediated Abcd2 induction is PPARalpha-dependent, it appears to be an indirect mechanism. Within the mouse Abcd2 promoter, a putative sterol regulatory element (SRE) similar in sequence and position to the characterized SRE sequence of the human ABCD2 promoter, was identified. A PPARalpha dependent induction of the sterol regulatory-binding protein 2 (SREBP2) and a down-regulation of SREBP1c mRNA levels could be demonstrated after fenofibrate treatment of mice. Our results suggest that the PPARalpha agonist-mediated induction of Abcd2 expression seems to be indirect and possibly mediated by SREBP2.

ATP Binding Cassette Transporter, Subfamily D↗
PubMed2002

Cholesterol regulates ABCD2 expression: implications for the therapy of X-linked adrenoleukodystrophy.

X-linked adrenoleukodystrophy (X-ALD) is a severe neurodegenerative disorder with impaired very long-chain fatty acid (VLCFA) metabolism. The disease-associated ABCD1 (ALD) gene encodes a peroxisomal membrane protein, which belongs to the superfamily of ATP-binding cassette transporters. Several treatment regimes have been tried without satisfactory clinical benefit. Recently, the cholesterol-lowering drug lovastatin was reported to normalize VLCFA levels in two out of three clinical studies. This investigation aimed to disclose the molecular mechanism of successful reduction of VLCFA accumulation in order to fill in the gap in the understanding how dietary cholesterol lowering affects the levels of VLCFA in patients with X-ALD and to allow more efficacious treatment. Overexpression of ABCD2 (ALDR), the closest relative of ABCD1, restores VLCFA accumulation in cultured ABCD1-deficient cells. Here we show by real-time PCR that the ABCD2 gene is induced in cultured human fibroblasts and monocytes upon sterol depletion via a mechanism requiring the activation of sterol regulatory element-binding proteins (SREBPs), a family of transcription factors that control the metabolism of cholesterol and fatty acids. This is unexpected and the first report that extends the mechanism of transcriptional regulation by SREBPs to a peroxisomal protein, thus providing a closer link between peroxisomes, cholesterol and fatty acid biosynthesis. Using reporter gene studies, site-directed mutagenesis and gel shift assays, we identified a functional sterol regulatory element in the proximal promoter region of ABCD2. Finally, we demonstrated that ABCD2 induction by sterol depletion significantly reduced the accumulation of VLCFA in X-ALD fibroblasts. Thus, lowering cholesterol leads to SREBP maturation, increased ABCD2 expression and reduced VLCFA accumulation.

ATP Binding Cassette Transporter, Subfamily D↗