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 ...

Mammalian lipins (lipin-1, lipin-2, and lipin-3) are Mg 2؉ -dependent phosphatidate phosphatase (PAP) enzymes, which catalyze a key reaction in glycerolipid biosynthesis. Lipin-1 also functions as a transcriptional coactivator in conjunction with members of the peroxisome proliferator-activated receptor family. An S734L mutation in LPIN2 causes Majeed syndrome, a human inflammatory disorder characterized by recurrent osteomyelitis, fever, dyserythropoietic anemia, and cutaneous inflammation. Here we demonstrate that mutation of the equivalent serine in mouse lipin-1 and lipin-2 to leucine or aspartate abolishes PAP activity but does not impair lipin association with microsomal membranes, the major site of glycerolipid synthesis. We also determined that lipin-2 has transcriptional coactivator activity for peroxisome proliferator-activated receptor-response elements similar to lipin-1 and that this activity is not affected by mutating the conserved serine. Therefore, our results indicate that the symptoms of the Majeed syndrome result from a loss of lipin-2 PAP activity. To characterize sites of lipin-2 action, we detected lipin-2 expression by in situ hybridization on whole mouse sections and by quantitative PCR of tissues relevant to Majeed syndrome. Lipin-2 was most prominently expressed in liver, where levels were much higher than lipin-1, and also in kidney, lung, gastrointestinal tract, and specific regions of the brain. Lipin-2 was also expressed in circulating red blood cells and sites of lymphopoiesis (bone marrow, thymus, and spleen). These results raise the possibility that the loss of lipin-2 PAP activity in erythrocytes and lymphocytes may contribute to the anemia and inflammation phenotypes observed in Majeed syndrome patients.The mammalian lipin protein family is composed of three members, lipin-1, lipin-2, and lipin-3, each of which are ϳ100 kDa in size and have 44 -48% amino acid similarity (reviewed in Ref. 1). Orthologous lipin genes are present in plants, invertebrates, and single cell eukaryotes such as yeast and plasmodium (2), suggesting that lipin proteins play a fundamental cellular role that has been conserved in evolution. In particular, extended stretches of 100 -200 amino acids at the N-terminal and C-terminal regions of the protein (the N-LIP and C-LIP domains, respectively) are highly conserved among the three mammalian lipin family members and among species. Within the C-LIP domain are two key protein functional motifs as follows: a haloacid dehalogenase motif (DXDXT) found in a superfamily of Mg 2ϩ -dependent phosphatases (3, 4), and a transcription factor-binding motif (LXXIL) (5). These motifs confer two distinct molecular functions on members of the lipin family. All three mammalian lipins are Mg 2ϩ -dependent phosphatidate phosphatase (PAP) 4 enzymes, which catalyze the conversion of phosphatidate (PA) to diacylglycerol, a key step in the biosynthesis of triacylglycerol, phosphatidylcholine, and phosphatidylethanolamine (3, 4, 6, 7). Lipin-1 also acts as a transcriptional coact...

The three lipin phosphatidate phosphatase (PAP) enzymes catalyze a step in glycerolipid biosynthesis, the conversion of phosphatidate to diacylglycerol. Lipin-1 is critical for lipid synthesis and homeostasis in adipose tissue, liver, muscle, and peripheral nerves. Little is known about the physiological role of lipin-2, the predominant lipin protein present in liver and the deficient gene product in the rare disorder Majeed syndrome. By using lipin-2–deficient mice, we uncovered a functional relationship between lipin-1 and lipin-2 that operates in a tissue-specific and age-dependent manner. In liver, lipin-2 deficiency led to a compensatory increase in hepatic lipin-1 protein and elevated PAP activity, which maintained lipid homeostasis under basal conditions, but led to diet-induced hepatic triglyceride accumulation. As lipin-2–deficient mice aged, they developed ataxia and impaired balance. This was associated with the combination of lipin-2 deficiency and an age-dependent reduction in cerebellar lipin-1 levels, resulting in altered cerebellar phospholipid composition. Similar to patients with Majeed syndrome, lipin-2–deficient mice developed anemia, but did not show evidence of osteomyelitis, suggesting that additional environmental or genetic components contribute to the bone abnormalities observed in patients. Combined lipin-1 and lipin-2 deficiency caused embryonic lethality. Our results reveal functional interactions between members of the lipin family in vivo, and a unique role for lipin-2 in central nervous system biology that may be particularly important with advancing age. Additionally, as has been observed in mice and humans with lipin-1 deficiency, the pathophysiology in lipin-2 deficiency is associated with dysregulation of lipid intermediates.

Glucocorticoids (GCs) increase hepatic phosphatidate phosphatase (PAP1) activity. This is important in enhancing the liver's capacity for storing fatty acids as triacylglycerols (TAGs) that can be used subsequently for b-oxidation or VLDL secretion. PAP1 catalyzes the conversion of phosphatidate to diacylglycerol, a key substrate for TAG and phospholipid biosynthesis. PAP1 enzymes in liver include lipin-1A and -1B (alternatively spliced isoforms) and two distinct gene products, lipin-2 and lipin-3. We determined the mechanisms by which the composite PAP1 activity is regulated using rat and mouse hepatocytes. Levels of lipin-1A and -1B mRNA were increased by dexamethasone (dex; a synthetic GC), and this resulted in increased lipin-1 synthesis, protein levels, and PAP1 activity. The stimulatory effect of dex on lipin-1 expression was enhanced by glucagon or cAMP and antagonized by insulin. Lipin-2 and lipin-3 mRNA were not increased by dex/cAMP, indicating that increased PAP1 activity is attributable specifically to enhanced lipin-1 expression.This work provides the first evidence for the differential regulation of lipin activities. Selective lipin-1 expression explains the GC and cAMP effects on increased hepatic PAP1 activity, which occurs in hepatic steatosis during starvation, diabetes, stress, and ethanol

Lipin-1 expression levels in adipose tissue of healthy young subjects and in mice are correlated with a favorable metabolic profile and expression of fatty acid oxidation genes.