A Conserved Serine Residue Is Required for the Phosphatidate Phosphatase Activity but Not the Transcriptional Coactivator Functions of Lipin-1 and Lipin-2

Abstract

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...

Lipin 2 Binds Phosphatidic Acid by the Electrostatic Hydrogen Bond Switch Mechanism Independent of Phosphorylation

“…Nuclear lipin 1 inhibits sterol regulatory element-binding protein transcriptional activity, which places lipin 1 within the mTOR/sterol regulatory element-binding protein signaling axis. Lipin 2 has been clearly shown to coactivate transcription through peroxisome proliferator-activated receptor ␣-PGC1-␣ (40). Considering that the localization of lipin 2 does not change with Torin1 treatment suggests that it is not a player in mTOR's role in lipogenic control, further distinguishing lipin 1 from lipin 2.…”

Hypoxia causes triglyceride accumulation via HIF-1-mediated stimulation of lipin 1 expression

“…Mutations in the lipin 1 gene (Lpin1), which is expressed predominantly in adipose tissue and skeletal muscle, are the cause of lipodystrophy in the fatty liver dystrophic (fld) mouse and, conversely, overexpression of lipin 1 in the adipose tissue of transgenic mice promotes obesity (Péterfy et al, 2001;Phan and Reue, 2005). Moreover, lipin 1 as well as lipin 2, a liver-enriched isoform, interact with and/or modulate the activity of several transcription factors, including members of the peroxisome proliferator-activated receptor (PPAR) family and SREBP that control the expression of genes involved in fatty acid and lipid metabolism (Donkor et al, 2009;Finck et al, 2006;Koh et al, 2008;Peterson et al, 2011). Apart of its apparently important role in adipogenesis, relatively little is known about the function of lipin 1 in cells other than adipocytes.…”

A Hypomorphic Mutation in Lpin1 Induces Progressively Improving Neuropathy and Lipodystrophy in the Rat

“…The amino-terminal and carboxyl-terminal regions of Lipin 1 (NLIP and CLIP, respectively), and a predicted nuclear localization signal are highly conserved among the three mammalian Lipin family members and among species (9). The CLIP domain contains multiple key protein functional domains: four haloacid dehalogenase motifs and a transcription factor-binding motif (LXXIL) (1,6,11). Moreover, Lipin 1 and Lipin 2 have been predicted to possess the same structural organization as previously characterized HAD protein family members (11).…”

“…The CLIP domain contains multiple key protein functional domains: four haloacid dehalogenase motifs and a transcription factor-binding motif (LXXIL) (1,6,11). Moreover, Lipin 1 and Lipin 2 have been predicted to possess the same structural organization as previously characterized HAD protein family members (11). In addition to its enzymatic function, Lipin 1 also has the ability to regulate gene expression, and it is likely that this ability is shared by Lipin 2 and Lipin 3 (1,12,13).…”