We previously identified mutations in the Lpin1 gene, encoding lipin-1, as the underlying cause of lipodystrophy in the fatty liver dystrophy (fld) mutant mouse. Lipin-1 is normally expressed at high levels in adipose tissue and skeletal muscle, and deficiency in the fld mouse causes impaired adipose tissue development, insulin resistance, and altered energy expenditure. We also identified two additional lipin protein family members of unknown function, lipin-2 and lipin-3. Han et al. Triacylglycerol (TAG)3 plays a key role in metabolic homeostasis, serving as the major energy storage molecule that allows organisms to survive periods of food deprivation. The regulation of TAG storage is important in human disease because both excessive and inadequate fat storage is associated with dyslipidemia, insulin resistance, and diabetes (reviewed in Refs. 1-3). We previously characterized the fatty liver dystrophy mouse, a model of generalized lipodystrophy with impaired TAG storage in adipose tissue, insulin resistance, and increased susceptibility to atherosclerosis (4, 5). Lipodystrophy in the fld mouse results from mutation in the Lpin1 (lipin-1) gene, the founding member of a family of three genes of previously unknown function (6). Genes for lipin-1, lipin-2, and lipin-3 occur in mammals and other vertebrates, whereas a single lipin gene ortholog can be detected in evolutionarily distant organisms including fruit fly, nematode, plants, and yeast (6). This suggests a fundamental function for lipin that is conserved from single celled eukaryotes to mammals.In the mouse, lipin-1 is expressed at high levels in adipose tissue and skeletal muscle, consistent with a role in lipid metabolism in these tissues. Indeed, adipocytes in lipin-1-deficient mice fail to accumulate TAG and do not develop mature adipocyte function (7). By contrast, transgenic mice with enhanced lipin-1 expression in adipocytes accumulate more TAG per cell and are prone to obesity (7-9). Furthermore, lipin-1 expression levels are reduced in adipose tissue of human lipodystrophic patients concomitantly with reduced fat mass (10). A role for lipin-1 in muscle metabolism is suggested by increased energy expenditure and fatty acid oxidation in the muscle of lipin-1-deficient mice and the opposite effects in muscle-specific lipin-1 transgenic mice (8). Thus, alterations in lipin-1 expression levels in either adipose tissue or skeletal muscle produce important physiological effects on energy storage and expenditure.In mammalian cells, the de novo biosynthesis of TAG, PC, and phosphatidylethanolamine is catalyzed mainly through the glycerol phosphate pathway (11). Several enzymes in this pathway have been characterized, but not all of these have been identified at the molecular level. Among those for which a gene has not been isolated is the phosphatidate phosphohydrolase (phosphatase) type-1 that converts the PA formed from glycerol phosphate and lysoPA to DAG (12). There are two main types of PA phosphatase. The first is the type-1 activity (PAP1) that is ...

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Recent studies have identified factors responsible for angiogenesis within developing tumors, but mediators of vessel formation at sites of trauma, injury, and wound healing are not clearly established. Here we show that sphingosine 1-phosphate (S1P) released by platelets during blood clotting is a potent, specific, and selective endothelial cell chemoattractant that accounts for most of the strong endothelial cell chemotactic activity of blood serum, an activity that is markedly diminished in plasma. Preincubation of endothelial cells with pertussis toxin inhibited this effect of S1P, demonstrating the involvement of a Galphai-coupled receptor. After S1P-induced migration, endothelial cells proliferated avidly and differentiated forming multicellular structures suggestive of early blood vessel formation. S1P was strikingly effective in enhancing the ability of fibroblast growth factor to induce angiogenesis in the avascular mouse cornea. Our results show that blood coagulation initiates endothelial cell angiogenic responses through the release of S1P, a potent endothelial cell chemoattractant that exerts its effects by activating a receptor-dependent process.

To determine the mechanism by which the (n-3) fatty acids (FA) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) decrease proliferation and induce apoptosis in MDA-MB-231 human breast cancer cells, we examined the effects of EPA and DHA on the lipid composition of lipid rafts as well as epidermal growth factor receptor (EGFR) raft localization and phosphorylation. (n-3) FA (a combination of EPA and DHA) inhibited (P < 0.05) the growth of MDA-MB-231 cells by 48-62% in the presence and absence, respectively, of linoleic acid (LA). More EPA and DHA were incorporated into lipid rafts isolated from MDA-MB-231 cells after treatment with (n-3) FA compared with cells treated with LA (P < 0.05). EPA and DHA treatment decreased (P < 0.05) lipid raft sphingomyelin, cholesterol, and diacylglycerol content and, in the absence of LA, EPA and DHA increased (P < 0.05) raft ceramide levels. Furthermore, there was a marked decrease in EGFR levels in lipid rafts, accompanied by increases in the phosphorylation of both EGFR and p38 mitogen-activated protein kinase (MAPK), in EPA+DHA-treated cells (P < 0.05). As sustained activation of the EGFR and p38 MAPK has been associated with apoptosis in human breast cancer cells, our results indicate that (n-3) FA modify the lipid composition of membrane rafts and alter EGFR signaling in a way that decreases the growth of breast tumors.

Lipid phosphates initiate key signaling cascades in cell activation. Lysophosphatidate (LPA) and sphingosine 1-phosphate (S1P) are produced by activated platelets. LPA is also formed from circulating lysophosphatidylcholine by autotaxin, a protein involved tumor progression and metastasis. Extracellular LPA and S1P stimulate families of G-protein coupled receptors that elicit diverse responses. LPA is involved in wound repair and tumor growth. Exogenous S1P is a potent stimulator of angiogenesis, a process vital in development, tissue repair and the growth of aggressive tumors. Inside the cell, phosphatidate (PA), ceramide 1-phosphate (C1P), LPA, and S1P act as signaling molecules with distinct functions including the stimulation of cell division, cytoskeletal rearrangement, Ca(2+) transients, and membrane movement. These observations imply that phosphatases that degrade lipid phosphates on the cell surface, or inside the cell, regulate cell signaling under physiological and pathological conditions. This occurs through attenuation of signaling by the lipid phosphates and by the production of bioactive products (diacylglycerol, ceramide, and sphingosine). Three lipid phosphate phosphatases (LPPs) and a splice variant dephosphorylate LPA, PA, CIP, and S1P. Two S1P phosphatases (SPPs) act specifically on S1P. In addition, there is family of four LPP-related proteins (LPRs, or plasticity-related genes, PRGs). PRG-1 expression in neurons has been reported to increase extracellular LPA breakdown and attenuate LPA-induced axonal retraction. It is unclear whether the LRPs dephosphorylate LPA directly, stimulate LPP activity, or bind LPA and S1P. Also, the importance of extra- versus intra-cellular actions of the LPPs and SPPs, and the individual roles of different isoforms is not firmly established. Understanding the functions and regulation of the LPPs, SPPs and related proteins will hopefully contribute to interventions to correct dysfunctions in conditions such as wound repair, inflammation, angiogenesis, tumor growth, and metastasis.