Four ATP-binding cassette (ABC) half-transporters have been identified in mammalian peroxisomes: adrenoleukodystrophy protein (ALDP), adrenoleukodystrophy-related protein (ALDRP), 70-kDa peroxisomal membrane protein (PMP70) and PMP70-related protein (P70R). Inherited defects in ALDP cause the neurodegenerative disorder X-linked adrenoleukodystrophy (X-ALD). By comparative Northern blot analyses we found each of the four murine peroxisomal ABC transporter mRNA species at maximum abundance only in a few tissues, which differed for each family member. The four genes were also regulated differentially during mouse brain development: ALDP mRNA was most abundant in embryonic brain and gradually decreased during maturation; ALDRP and P70R mRNA accumulated in the early postnatal period; and the amount of PMP70 transcript increased slightly during the second and third postnatal week. The different expression patterns could explain why b-oxidation is defective in X-ALD, although ALDRP and PMP70 can replace ALDP functionally in fibroblasts. Dietary fenofibrate had no effect on the ALD and P70R genes, but strongly increased expression of the ALDR and PMP70 genes in mouse liver. However, in P-glycoprotein Mdr1a-deficient mice fenofibrate treatment increased ALDR gene expression also in the brain, suggesting that the multidrug-transporter P-glycoprotein restricts entry of fenofibrate to the brain at the blood±brain barrier. Analysis of the promoter sequences revealed a cryptic nuclear hormone receptor response element of the DR+4 type in the ALDR promoter and a novel 18-bp sequence motif present only in the 5 H flanking DNA of the ALDR and PMP70 genes. The mouse ALDR gene uses a single transcription start site but alternative polyadenylation sites. These data are of importance for the use of ALDP-deficient mice as a model in pharmacological gene therapy studies.Keywords: adrenoleukodystrophy; ATP-binding cassette transporter; fenofibrate; gene expression; peroxisome.Four ABC-transporters have been identified in the mammalian peroxisomal membrane: adrenoleukodystrophy protein (ALDP), adrenoleukodystrophy-related protein (ALDRP), 70 kDa peroxisomal membrane protein (PMP70) and PMP70-related protein (P70R/PMP69). Relative to ALDP the human proteins display 63%, 36%, and 25% amino acid identity [1], respectively, and all have the predicted structure of a half-transporter with one membrane spanning domain and one nucleotide binding fold. As most of the half-transporters identified to date function as dimers, it has been suggested that the peroxisomal ABCtransporters also need to assemble as homo-or heterodimers in order to form a functional unit [2,3].Although the exact functions and substrates of the mammalian peroxisomal ABC-transporters have yet to be defined, the detrimental effects of a deficient transporter is demonstrated by mutations in the ALD gene, leading to the lipid storage disorder X-linked adrenoleukodystrophy (X-ALD, McKusick #300100). Biochemically X-ALD is characterized by defective peroxisomal b-oxidation of sat...

The 70-kDa peroxisomal membrane protein (PMP 70), adrenoleukodystrophy protein (ALDP) and adrenoleukodystrophy-related protein (ALDRP) belong to the ATP-binding transporter family, share a structure of half-transporters and are localized in the peroxisomal membrane of mammals. It was suggested that these proteins may heterodimerize to form functional transporters. The expression of the three genes was examined in various tissues of control or fenofibrate (a peroxisome proliferator)-treated rats using Northern and immuno-blotting techniques. The patterns of tissue expression were distinct for the three genes. Upon treatment, expression of the ALD gene was not altered while that of the PMP 70 and ALDR genes was strongly increased in intestine and liver, respectively. The absence of coordinated expression excludes that the three transporters function as exclusive and obligatory partners. We also report for the first time that the ALDR gene is inducible in rodents and that the corresponding mRNA is different in length in rat (3.0 and 5.5 kb) and in mouse and human (4.2 kb).

Most receptor-type protein-tyrosine phosphatases (RPTPs) contain two tandem PTP domains. For some RPTPs the enzymatically inactive membrane-distal phosphatase domains (D2) were found to bind enzymatically active membrane proximal PTP (D1) domains, and oligomerization has been proposed as a general regulatory mechanism. The RPTP-like proteins IA-2 and IA-2␤, major autoantigens in insulin-dependent diabetes mellitus, contain just a single enzymatically inactive PTPlike domain. Their physiological role is as yet enigmatic. To investigate whether the catalytically inactive cytoplasmic domains of IA-2 and IA-2␤ are involved in oligomerization, we exploited interaction trap assay in yeast and glutathione S-transferase pull-down and coimmunoprecipitation strategies on lysates of transfected COS-1 cells. The results show that IA-2 and IA-2␤ are capable of homo-and heterodimerization to which both the juxtamembrane region and the phosphataselike segment can contribute. Furthermore, they can form heterodimers with some other RPTP members, most notably RPTP␣ and RPTP⑀, and down-regulate RPTP␣ enzymatic activity. Thus, in addition to homodimerization, the enzymatic activity of receptor-type PTPs can be regulated through heterodimerization with other RPTPs, including the catalytically inactive IA-2 and IA-2␤.

X-linked adrenoleukodystrophy (X-ALD) is a neurodegenerative disease due to a defect in the ABCD1 (ALD) gene. ABCD1, and the two close homologues ABCD2 (ALDR) and ABCD3 (PMP70), are genes encoding ATP-binding cassette half-transporters of the peroxisomal membrane. As overexpression of the ABCD2 or ABCD3 gene can reverse the biochemical phenotype of X-ALD (reduced b-oxidation of very-long-chain fatty acids), pharmacological induction of these partially redundant genes may represent a therapeutic approach to X-ALD. We previously reported that the ABCD2 and ABCD3 genes could be strongly induced by fibrates, which are hypolipidaemic drugs and peroxisomeproliferators in rodents. We provide evidence that the induction is dependent on peroxisome proliferator-activated receptor (PPARa) as both genes were not induced in fenofibrate-treated PPARa ±/± knock-out mice. To further characterize the PPARa pathway, we cloned and analysed the promoter of the ABCD2 gene, the closest homologue of the ABCD1 gene. The proximal region (2 kb) of the rat promoter displayed a high conservation with the human and mouse cognate sequences suggesting an important role of the region in regulation of the ABCD2 gene. Classically, fibrate-induction involves interaction of PPARa with a response element (PPRE) characterized by a direct repeat of the AGGTCA-like motif. Putative PPRE motifs of the rat ABCD2 promoter were studied in the isolated form or in their promoter context by gel-shift assay and transfection of COS-7 cells. We failed to characterize a functional PPRE, suggesting a different mechanism for the PPARa-dependent regulation of the ABCD2 gene.