Liver X Receptor α Interferes with SREBP1c-mediated Abcd2 Expression

Abstract

The peroxisomal ATP binding cassette (ABC) transporter adrenoleukodystrophy-related protein, encoded by ABCD2, displays functional redundancy with the X-linked adrenoleukodystrophyassociated 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 LXR␣ and SREBP1c. In cell culture, SREBP1c-mediated Abcd2 induction is counteracted by ligand-activated LXR␣. Finally, hepatic Abcd2 expression in LXR␣,␤-deficient mice is inducible to levels vastly exceeding wild type. Together, we identify LXR␣ as negative modulator of Abcd2, acting through a novel regulatory mechanism involving overlapping SREBP and LXR␣ binding sites. X-linked adrenoleukodystrophy (X-ALD2 ; OMIM 300100) is a neurodegenerative disorder caused by mutations in the ABCD1 (ALD) gene, encoding the peroxisomal ATP binding cassette (ABC) half-transporter adrenoleukodystrophy protein (ALDP) (1). Currently, no satisfying therapy is available for X-ALD patients.Next to ALDP, three additional mammalian peroxisomal ABC halftransporters have been identified: adrenoleukodystrophy-related protein (ALDRP), 70-kDa peroxisomal membrane protein (PMP70), and PMP70-related protein (P70R), sharing 66, 39, and 25% amino acid identity with ALDP, respectively. Thus, the ABCD2-encoded ALDRP is the protein most closely related to ALDP and upon overexpression can functionally compensate for ALDP deficiency in X-ALD fibroblasts and Abcd1-deficient mice (2, 3). Therefore, pharmacological stimulation of ABCD2 expression has been targeted as an alternative therapeutic strategy for X-ALD (4), requiring detailed knowledge about how the ABCD2 gene is transcriptionally regulated.We recently showed that human ABCD2 is induced upon cholesterol depletion in cultured cells via a mechanism requiring the activation and binding of sterol regulatory element (SRE)-binding proteins (SREBPs) (5). SREBPs, which are synthesized as membrane-bound precursors and cleaved to generate the active nuclear form, are a class of transcription factors known to play a major role in regulating genes involved in fatty acid and cholesterol synthesis (reviewed in Ref. 6). To date, three SREBPs have been identified: SREBP1a and SREBP1c, which are produced from a single gene and preferentially regulate fatty acid synthesis, and SREBP2, encoded by a separate gene and controlling expression of ...

The absence of ABCD2 sensitizes mice to disruptions in lipid metabolism by dietary erucic acid

“…Conversely, the expression of D2 is relatively low in liver, but is up-regulated by fasting and peroxisome proliferator activated receptor (PPAR) ␣ agonists ( 9 ). Hepatic expression of D2 is suppressed by the liver X receptor (LXR), suggesting a role for D2 in cholesterol metabolism ( 16 ). However, the functional signifi cance of this observation is unclear and has not been investigated.…”

“…D2 mRNA and protein have been detected in brain, liver, lung, adrenal gland, and skeletal muscle but are most abundant in adipose tissue ( 9,15 ). Within adipose, D2 is restricted to adipocytes and is upregulated during adipogenesis, presumably by the lipogenic transcription factor sterol receptor element binding protein 1 (SREBP1) ( 15,16 ). Conversely, the expression of D2 is relatively low in liver, but is up-regulated by fasting and peroxisome proliferator activated receptor (PPAR) ␣ agonists ( 9 ).…”

ABCD2 is abundant in adipose tissue and opposes the accumulation of dietary erucic acid (C22:1) in fat

“…However, its expression is induced by fasting, feeding fi brates, and statin treatment, while expression levels in brain remain constant (13)(14)(15)(16)(17). More recently, D2 mRNA was shown to be expressed in adipose tissue where its mRNA levels are highest in the fed state, decline in the fasted state, and return during refeeding ( 17 ).…”

“…However, its expression is induced by fasting, feeding fi brates, and statin treatment, while expression levels in brain remain constant (13)(14)(15)(16)(17). More recently, D2 mRNA was shown to be expressed in adipose tissue where its mRNA levels are highest in the fed state, decline in the fasted state, and return during refeeding ( 17 ). The promoter of D2 contains a functional sterol response element that interacts with both regulatory element binding protein (SREBP-1a) and -1c, suggesting that it is a component of the lipogenic program (17)(18)(19).…”

The peroxisomal ABC transporter family

“…Interestingly, SREBP2 mRNA levels were markedly induced by fenofibrate in the wild-type (+/+) mice, but not in the PPARalpha (−/−) mice, whereas SREBP1c expression was downregulated by fenofibrate in a PPARalpha-independent way. These data suggested that the PPARalpha [97] showed that the regulation of the ABCD2 gene is even more complicated as originally believed and involves different players, including LXR, RXR, TR, and SREBP. One puzzling finding has been the lack of response in the brain by fenofibrate and other PPARalpha ligands.…”