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The mouse acyl-CoA-binding protein (ACBP) 5 /diazepam binding inhibitor is a 10-kDa intracellular protein consisting of 86 amino acids. It is highly conserved throughout evolution and expressed in all cell types in the eukaryotes investigated (1, 2). This, together with the characteristics of the ACBP promoter (3, 4), implies a housekeeping function of the gene. However, expression levels vary markedly between tissues (5) and in response to different metabolic stimuli (6 -9), thereby indicating that ACBP might perform more specialized functions in some cell types. The ACBP protein binds C 14 -C 22 acyl-CoA esters with high affinity and specificity (10, 11) and has very little or no affinity toward other ligands (11-13). From in vitro studies, ACBP is known to protect acyl-CoA esters from hydrolysis (14 -16) and to relieve acyl-CoA inhibition of a number of enzymes, including long chain acyl-CoA synthetase, acetyl-CoA carboxylase (ACC), adenine nucleotide translocase, fatty acid synthetase (FAS), carnitine palmitoyltransferase, and acyl-CoA:cholesterol acyltransferase (9, 16 -18). In addition, ACBP is known to donate acyl-CoA esters to phospholipid, glycerolipid, and cholesteryl ester (CE) synthesis (14, 18 -21). Finally, proteolytic products of secreted ACBP have been shown to have signaling functions in Dictyostelium as well as mammalian cells (22). Targeted disruption of the yeast ACBP gene (ACB1) revealed that ACBP deficiency results in increased levels of C18:0 acyl-CoA esters and a decrease in the amount of total C26:0 fatty acids, indicating that transport of FA toward elongation is impaired by lack of ACBP. Furthermore, sphingolipid and ceramide amounts were reduced, membrane structure was altered, and vesicular transport was compromised (23-25).The functions of ACBP in lipid metabolism have been further studied in different mammalian cell culture systems and animal models by both knockdown strategies and overexpression of the protein. It has been reported that knockdown of ACBP by small interfering RNA causes growth arrest and lethality in three different mammalian cell lines (26); however, data from our laboratory show that ACBP can be knocked down in many different cell systems without affecting growth and survival (27). 6 Recently, knockdown of ACBP in HepG2 cells was shown to suppress the expression of a number of genes involved in lipid biosynthesis and lead to decreased levels of saturated and monounsaturated fatty acids (28). In 3T3-L1 preadipocytes, knockdown of ACBP caused a mild impairment of adipocyte differentiation and accumulation of triacylglycerol (TAG) (27), whereas overexpression of ACBP in McA-RH7777 rat hepatoma cells resulted in increased intracellular TAG accumulation (29). Overexpression of ACBP in transgenic mice resulted in accumulation of different lipid classes, including TAG in the liver (30). These results suggest *

This article is available online at http://www.jlr.org Supplementary key words stratum corneum • very long chain fatty acids • mono alkyl diacyl glycerol • transepidermal water loss • multiphoton excitation microscopy • lipid mass spectrometryThe acyl-CoA binding protein (ACBP)/diazepam binding inhibitor (DBI) (Entrez Gene ID: 13167) is a 10 kDa intracellular protein that specifi cally binds medium and long chain acyl-CoA esters (C 14 -C 22 ) with very high affi nity (K d ‫ف‬ 1-10 nM) ( 1, 2 ). The protein is expressed in all eukaryotic species and in all mammalian tissues investigated ( 3-5 ); however, expression levels differs markedly between different tissues and cell types examined, with particularly high levels in epithelial cell types and in cells with a high turn-over of fatty acids (FA) ( 6 ). Consistently with this, we have shown that the ACBP gene is activated by lipogenic transcription factors, such as the peroxisome proliferatoractivated receptor ␥ ( 7 ) and members of the sterol-regulatory element binding protein family ( 7-10 ). In vitro studies indicate that ACBP plays a role in transport of acyl-CoA esters and may deliver acyl-CoA esters to phospholipid

We previously reported that mice deficient in acyl-CoA-binding protein (ACBP) display a delayed metabolic adaptation to weaning. This includes a delayed activation of the hepatic lipogenic gene program, which may result from hepatic accumulation of triacylglycerol and/or cholesteryl esters in the late suckling period. To further investigate the basis for this phenotype, we generated mice deficient in ACBP in hepatocytes (Alb-ACBP(-/-)) and keratinocytes (K14-ACBP(-/-)). Surprisingly, the delayed adaptation to weaning, including hepatic lipid accumulation, is caused by ACBP deficiency in the skin rather than in the liver. Similarly to ACBP(-/-) mice, K14-ACBP(-/-) mice exhibit an increased transepidermal water loss, and we show that the hepatic phenotype is caused specifically by the epidermal barrier defect, which leads to increased lipolysis in white adipose tissue. Our data demonstrate that an imperfect epidermal barrier leads to profound suppression of the hepatic SREBP gene program and lipid accumulation in the liver.

The acyl-CoA binding protein (ACBP) is a small intracellular protein that specifically binds and transports medium to long-chain acyl-CoA esters. Previous studies have shown that ACBP is ubiquitously expressed but found at particularly high levels in lipogenic cell types as well as in many epithelial cells. Here we show that ACBP is widely expressed in human and mouse kidney epithelium, with the highest expression in the proximal convoluted tubules. To elucidate the role of ACBP in the renal epithelium, mice with targeted disruption of the ACBP gene (ACBP(-/-)) were used to study water and NaCl balance as well as urine concentrating ability in metabolic cages. Food intake and urinary excretion of Na(+) and K(+) did not differ between ACBP(-/-) and (+/+) mice. Interestingly, however, water intake and diuresis were significantly higher at baseline in ACBP(-/-) mice compared with that of (+/+) mice. Subsequent to 20-h water deprivation, ACBP(-/-) mice exhibited increased diuresis, reduced urine osmolality, elevated hematocrit, and higher relative weight loss compared with (+/+) mice. There were no significant differences in plasma concentrations of renin, corticosterone, and aldosterone between mice of the two genotypes. After water deprivation, renal medullary interstitial fluid osmolality and concentrations of Na(+), K(+), and urea did not differ between genotypes and cAMP excretion was similar. Renal aquaporin-1 (AQP1), -2, and -4 protein abundances did not differ between water-deprived (+/+) and ACBP(-/-) mice; however, ACBP(-/-) mice displayed increased apical targeting of pS256-AQP2. AQP3 abundance was lower in ACBP(-/-) mice than in (+/+) control animals. Thus we conclude that ACBP is necessary for intact urine concentrating ability. Our data suggest that the deficiency in urine concentrating ability in the ACBP(-/-) may be caused by reduced AQP3, leading to impaired efflux over the basolateral membrane of the collecting duct.

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