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Whereas the physiological significance of microsomal fatty acid elongation is generally appreciated, its molecular nature is poorly understood. Here, we describe tissue-specific regulation of a novel mouse gene family encoding components implicated in the synthesis of very long chain fatty acids. The Ssc1 gene appears to be ubiquitously expressed, whereas Ssc2 and Cig30 show a restricted expression pattern. Their translation products are all integral membrane proteins with five putative transmembrane domains. By complementing the homologous yeast mutants, we found that Ssc1 could rescue normal sphingolipid synthesis in the sur4/elo3 mutant lacking the ability to synthesize cerotic acid (C26:0). Similarly, Cig30 reverted the phenotype of the fen1/elo2 mutant that has reduced levels of fatty acids in the C20–C24 range. Further, we show that Ssc1 mRNA levels were markedly decreased in the brains of myelin-deficient mouse mutants known to have very low fatty acid chain elongation activity. Conversely, the dramatic induction of Cig30 expression during brown fat recruitment coincided with elevated elongation activity. Our results strongly implicate this new mammalian gene family in tissue-specific synthesis of very long chain fatty acids and sphingolipids.

Very little is known about the in vivo regulation of mammalian fatty acid chain elongation enzymes as well as the role of specific fatty acid chain length in cellular responses and developmental processes. Here, we report that the Elovl3 gene product, which belongs to a highly conserved family of microsomal enzymes involved in the formation of very long chain fatty acids, revealed a distinct expression in the skin that was restricted to the sebaceous glands and the epithelial cells of the hair follicles. By disruption of the Elovl3 gene by homologous recombination in mouse, we show that ELOVL3 participates in the formation of specific neutral lipids that are necessary for the function of the skin. The Elovl3-ablated mice displayed a sparse hair coat, the pilosebaceous system was hyperplastic, and the hair lipid content was disturbed with exceptionally high levels of eicosenoic acid (20:1). This was most prominent within the triglyceride fraction where fatty acids longer than 20 carbon atoms were almost undetectable. A functional consequence of this is that Elovl3-ablated mice exhibited a severe defect in water repulsion and increased trans-epidermal water loss.Fatty acids consisting of up to 16 carbons are synthesized by the well studied fatty acid synthase complex (1). However, a significant amount of the fatty acids produced by fatty acid synthase are further elongated into very long chain fatty acids (VLCFA). 1 VLCFA have been recognized as structural components in a variety of fat molecules such as glycerolipids and sphingolipids. They are found in virtually all cells and are major constituents of the brain, skin, and testis (2-4). Depending on their chain length and degree of unsaturation, they contribute to membrane fluidity and other chemical properties of the cell.Formation of VLCFA is performed in the endoplasmic reticulum, in the early Golgi, and in mitochondria by membranebound enzymes, the former being more prominent (5, 6).Recently, five mammalian genes, Elovl1-5, 2 whose protein products belong to a highly conserved family of microsomal enzymes involved in the formation of VLCFA, have been identified (7-10). All five genes show a diverse tissue-specific expression pattern indicating a unique role for different VLCFA in different cell types.Although the general functions of the Elovl genes are partially understood (8 -14), very little is known about the role of specific fatty acid chain length in cellular responses and developmental processes. The ELOVL3 protein has been suggested to be involved in the formation of saturated and monounsaturated fatty acyl chains containing up to 24 carbon atoms (9). Elovl3 gene expression has only been detected in brown adipose tissue, liver, and skin (7, 9). To assess the in vivo role of ELOVL3, we disrupted the Elovl3 gene by homologous recombination in mouse. Here we describe the characterization of Elovl3 expression in skin and present evidence that ELOVL3 participates in the formation of certain VLCFA and triglycerides in certain cells of the hair follicles and...

During the recruitment process of brown adipose tissue, the mRNA level of the fatty acyl chain elongase Elovl3 is elevated more than 200-fold in cold-stressed mice. We have obtained Elovl3-ablated mice and report here that, although cold-acclimated Elovl3-ablated mice experienced an increased heat loss due to impaired skin barrier, they were unable to hyperrecruit their brown adipose tissue. Instead, they used muscle shivering in order to maintain body temperature. Lack of Elovl3 resulted in a transient decrease in the capacity to elongate saturated fatty acyl-CoAs into very long chain fatty acids, concomitantly with the occurrence of reduced levels of arachidic acid (C20:0) and behenic acid (C22:0) in brown adipose tissue during the initial cold stress. This effect on very long chain fatty acid synthesis could be illustrated as a decrease in the condensation activity of the elongation enzyme. In addition, warmacclimated Elovl3-ablated mice showed diminished ability to accumulate fat and reduced metabolic capacity within the brown fat cells. This points to ELOVL3 as an important regulator of endogenous synthesis of saturated very long chain fatty acids and triglyceride formation in brown adipose tissue during the early phase of the tissue recruitment.Brown adipose tissue is the only tissue that functions exclusively to combust fat for heat production (1-3). When mammals encounter cold, there is an induced synthesis of specific mRNA species in brown adipose tissue, which encode enzymes regulating energy expenditure and lipid metabolism (4 -6). Most notable is the increased mRNA level of the mitochondrial uncoupling protein 1 (UCP1), which can uncouple the mitochondrial respiratory chain and thereby dissipate heat instead of conserving energy in the form of ATP. The induction of processes needed for increased brown adipose tissue activity is referred to as brown fat recruitment.The process is controlled by norepinephrine release from sympathetic nerve endings found in the tissue (7, 8) and can roughly be divided into two major phases, depending on how the animal experiences the cold (4°C) (i.e. cold stress or cold acclimation). During the first days of cold exposure (i.e. cold stress), the major effect on brown adipose tissue is the initiation of hypertrophy, and a few days later, during the onset of cold acclimation, hyperplasia occurs (9, 10). After 4 days of cold exposure, there is a doubling in DNA amount, and after 3 weeks, a 3-fold increase is detected, which is the maximal level achieved with this degree of cold (11,12). The increase in the total amount of protein in cold-exposed mice occurs also as a two-phase phenomenon (5). The first phase is completed within 3 days, and the second phase of protein increase (i.e. during cold acclimation) is found in the tissue after 2-3 weeks of cold exposure. During these events, protein synthesis mainly reflects an increase in mRNA levels (5).During the process of identifying specific cDNA molecules corresponding to mRNA species that are induced in the brown adipose ti...

Despite the significance of mitochondrial ATP synthase for mammalian metabolism, the regulation of the amount of ATP synthase in mammalian systems is not understood. As brown adipose tissue mitochondria contain very low amounts of ATP synthase, relative to respiratory chain components, they constitute a physiological system that allows for examination of the control of ATP synthase assembly. To examine the role of the expression of the P1-isoform of the c-Fo subunit in the biogenesis of ATP synthase, we made transgenic mice that express the P1-c subunit isoform under the promoter of the brown adipose tissue-specific protein UCP1. In the resulting UCP1p1 transgenic mice, total P1-c subunit mRNA levels were increased; mRNA levels of other F1Fo-ATPase subunits were unchanged. In isolated brown-fat mitochondria, protein levels of the total c-Fo subunit were increased. Remarkably, protein levels of ATP synthase subunits that are part of the F1-ATPase complex were also increased, as was the entire Complex V. Increased ATPase and ATP synthase activities demonstrated an increased functional activity of the F1Fo-ATPase. Thus, the levels of the c-Fo subunit P1-isoform are crucial for defining the final content of the ATP synthase in brown adipose tissue. The level of c-Fo subunit may be a determining factor for F1Fo-ATPase assembly in all higher eukaryotes.

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The development of the dual peroxisome proliferator-activated receptor (PPAR) alpha/gamma agonist tesaglitazar as an oral antidiabetic was recently discontinued. Here we present tumor data from a 2-year carcinogenicity study in rats given 0.3, 1, 3, and 10 micromol/kg tesaglitazar is presented with focus on the findings of subcutaneous fibrosarcomas. To investigate the mechanism for induction of fibrosarcomas, replicative DNA synthesis (immunohistochemical detection of BrdU-labeled cells) and expression of PPARgamma (immunohistochemistry and reverse transcription-polymerase chain reaction) in subcutaneous adipose tissues was assessed in rats administered 1 or 10 micromol/kg for 2 weeks or 3 months. Poorly differentiated subcutaneous mesenchymal sarcomas with a predominant spindle cell appearance occurred at the highest dose level of 10 micromol/kg in both sexes, and these tumors were diagnosed as fibrosarcomas. The 10-micromol/kg dose was at or above the maximum tolerated dose and caused considerable cardiovascular mortality. Tesaglitazar stimulated DNA synthesis mainly in subcutaneous interstitial mesenchymal cells. The percentage of BrdU-labeled interstitial cells was increased at 1 and 10 micromol/kg after 2 weeks. The increase in DNA synthesis was still significant at the end of the 12-week treatment at 10 mumol/kg, the dose producing fibrosarcoma. However, at 1 micromol/kg, a dose below the no-observed-effect level for fibrosarcoma, the level of DNA synthesis was similar to control levels at 12 weeks. Immunohistochemical analyses showed no detectable PPARgamma protein in the majority of BrdU-labeled interstitial mesenchymal cells in white and brown fat. This indicates that stimulation of DNA synthesis is not mediated via direct activation of PPARgamma in these cells. The results suggest that the induction of rat fibrosarcoma by tesaglitazar, at exposures 100-fold above the human therapeutic exposure, may involve proliferation of undifferentiated mesenchymal cells in subcutaneous tissues.

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