Cholesterol regulates ABCD2 gene expression: implications for X-linked adrenoleukodstrophy.

Abstract

X-linked adrenoleukodystrophy (X-ALD) is a severe neurodegenerative disorder with impaired very long-chain fatty acid (VLCFA) metabolism (Moser et al 2001). The disease-associated ABCD1 (ALD) gene encodes a peroxisomal membrane protein, which belongs to the superfamily of ATP- binding cassette transporters. Currently, no satisfactory therapy is available for X-ALD. The only treatment with some documented benefit for X-ALD patients is bone marrow transplantation when performed at an early stage of the disease (Shapiro et al 2000). Recently, the cholesterol-lowering drug lovastatin, a 3-hydroxy-3-methylglutaryl-coenzyme A (HMGCoA) reductase inhibitor, was proposed as a new therapeutic agent for X-ALD and a cause-and-effect relationship between lovastatin therapy and plasma VLCFA levels was observed when plasma VLCFA and serum cholesterol levels were compared (Singh et al 1998b, Pai et al 2000). However, statin treatment successfully normalized VLCFA levels in the plasma of X-ALD patients in only two out of three studies (Pai et al 2000, Singh et al 1998a, Verrips et al 2000). 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 levels of VLCFA in patients with X-ALD and to allow more efficacious treatment. Overexpression of ABCD2, 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 demonstrate 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.KeywordsSterol Regulatory Element Binding ProteinProximal Promoter RegionPeroxisomal ProteinPeroxisomal Membrane ProteinDietary Cholesterol LoweringThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.