Genetic variability of procolipase associates with altered insulin secretion in non-diabetic Caucasians.

AIMS: Procolipase (CLPS) is secreted from the exocrine pancreas into the gastrointestinal tract and becomes proteolytically cleaved into colipase and the pentapeptide enterostatin. While colipase is an indispensable cofactor for pancreatic lipase, enterostatin acts as a hormone that inhibits insulin secretion and confers satiety signals to the brain, thereby restricting further food intake in animal models. As both high fat diet and obesity contribute to insulin resistance, we investigated whether genetic variability of CLPS associates with metabolic traits in non-diabetic humans at diabetes risk. METHODS: Tagging single nucleotide polymorphisms (SNPs) in the human CLPS locus on chr6p21.1 were selected using HapMap data. 498 humans, phenotyped for different glucose and lipid metabolic traits, were genotyped by bidirectional sequencing and multivariate linear regression analyses were undertaken. RESULTS: 2 tagging SNPs (rs3748050 in the Kozak sequence: A/G and rs3748051 in intron 1: A/G), covering 100% of CLPS variability including 8 kb of its promoter, were genotyped for association analyses. The minor alleles of both tagging SNPs associated significantly with a reduced insulin secretion (-20.2%, both SNPs) in various estimation models derived from the oral glucose tolerance test (OGTT; rs3748050/51: 30 min C-peptide levels: p=0.001/0.01, insulinogenic index: p=0.02/0.02, AUC C-peptide/AUC glucose: p=0.01/0.01) after adjustment for relevant covariates. No significant associations with fasting total cholesterol (c), HDL-c, LDL-c, triglycerides and free fatty acids were found (all p > 0.11). CONCLUSIONS: CLPS genetic variability associates with insulin secretory function in non-diabetic humans and may represent a novel candidate gene for development of type 2 diabetes.

Fibrate induction of the adrenoleukodystrophy-related gene (ABCD2): promoter analysis and role of the peroxisome proliferator-activated receptor PPARalpha.

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 beta-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 peroxisome-proliferators in rodents. We provide evidence that the induction is dependent on peroxisome proliferator-activated receptor (PPARalpha) as both genes were not induced in fenofibrate-treated PPARalpha -/- knock-out mice. To further characterize the PPARalpha 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 PPARalpha 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 PPARalpha-dependent regulation of the ABCD2 gene.

Rat adrenoleukodystrophy-related (ALDR) gene: full-length cDNA sequence and new insight in expression.

X-linked adrenoleukodystrophy (X-ALD) is an inherited demyelinating disorder due to mutations in the ALD gene, which encodes a peroxisomal ABC half-transporter (ALDP). It has been suggested that ALDP assembles with ALDRP (adrenoleukodystrophy-related protein), a close homologous half-transporter, to form a functional heterodimer. For the first time full-length ALDRP cDNA (5.5 kb) was cloned, and 5' and 3' RACE analysis revealed that alternative usage of polyadenylation sites generates the two transcripts of 3.0 and 5.5 kb observed in the rat in Northern blot analysis. Southern blotting and chromosomal mapping demonstrated one ALDR locus in the rat genome. Characterisation of the 3' flanking region suggested that an ID sequence might be responsible for high expression of the 5.5 kb ALDRP transcript in rat brain. ALDR gene expression was found to be high in the liver of rats before weaning and very low in adult rats; the reverse developmental regulation was observed in the brain. Fenofibrate, which is a potent inducer of the ALDR gene in the liver of adult rats, could not induce the ALDR gene in suckling rats. The exact significance of this result with regard to development of an efficient pharmacological gene therapy for X-ALD is discussed.

Genomic organization, expression analysis, and chromosomal localization of the mouse PEX3 gene encoding a peroxisomal assembly protein.

The peroxin Pex3p has been identified as an integral peroxisomal membrane protein in yeast where pex3 mutants lack peroxisomal remnant structures. Although not proven in higher organisms, a role of this gene in the early peroxisome biogenesis is suggested. We report here the cDNA cloning and the genomic structure of the mouse PEX3 gene. The 2 kb cDNA encodes a polypeptide of 372 amino acids (42 kDa). The gene spans a region of 30 kb, contains 12 exons and 11 introns and is located on band A of chromosome 10. The putative promoter region exhibits characteristic housekeeping features. PEX3 expression was identified in all tissues analyzed, with the strongest signals in liver and in testis, and could not be induced by fenofibrate. The data presented may be useful for the generation of a mouse model defective in PEX3 in order to clarify the yet unknown functional impact of disturbances in early peroxisomal membrane assembly.

The four murine peroxisomal ABC-transporter genes differ in constitutive, inducible and developmental expression.

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 beta-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' 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.

The mouse gene encoding the peroxisomal membrane protein 1-like protein (PXMP1-L): cDNA cloning, genomic organization and comparative expression studies.

PXMPI-L (synonyms: PMP69, P70R) is a peroxisomal protein that belongs to the ABC-transporter superfamily. Its closest homolog is the peroxisomal membrane protein 1 (PMP70). We have cloned the mouse PXMP1-L gene. It encodes a 606 amino acid protein. In contrast to the human and the rat, mouse PXMP1-L is predominantly expressed in the liver. The mouse PXMP1-L gene consists of 19 exons and spans 21 kb of genomic sequence. No obvious peroxisome proliferator response element has been found in 1.1 kb of the putative promoter region. No coordination of constitutive or fenofibrate-induced expression of PXMP1-L with other peroxisomal ABC transporters was observed so that an obligate exclusive heterodimer formation is not likely to occur. The data presented will be particularly useful for the generation of a mouse model defective in PXMP1-L in order to elucidate the yet unknown function of this protein.

Copurification of dihydroxyacetone-phosphate acyl-transferase and other peroxisomal proteins from liver of fenofibrate-treated rats.

Dihydroxyacetone-phosphate acyl-transferase (DHAP-AT), a peroxisomal membrane-bound enzyme that catalyzes the first step of ether-glycerolipid synthesis, was purified from liver of rats treated with fenofibrate, a peroxisome proliferator. The protocol first included isolation of peroxisomes, their purification through a discontinuous gradient and solubilization of membranes in CHAPS. DHAP-AT was further purified by four chromatographic steps, namely low-pressure size-exclusion, cation-exchange, hydroxylapatite and chromatofocusing. The chromatofocusing step led to a 4000-fold increase in the specific activity of DHAP-AT with respect to the liver homogenate with a yield of about 0.2%. Trypsin digestion of a 64-kDa protein band upon SDS-PAGE resulted in a peptide sequence unknown in databases. A corresponding degenerated oligonucleotide was used as a probe in Northern blotting, and a transcript of 3.3 kb was detected in some rat tissues. Moreover, the overall procedure allowed co-purification of four major peroxisomal enzymes: urate-oxidase, catalase, multifunctional enzyme and palmitoyl-CoA oxidase, respectively.

Fenofibrate differently alters expression of genes encoding ATP-binding transporter proteins of the peroxisomal membrane.

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).