The lipin family: mutations and metabolism

The role of lipin-1 in the pathogenesis of alcoholic fatty liver

AIMS

The aim of this review was to focus on the knowledge of the role of lipin-1 in the pathogenesis of alcoholic fatty liver.

METHODS

Systematic review of animal clinical and cell level studies related to the function of lipin-1 on alcoholic fatty liver, alcoholic hepatitis and alcoholic liver cirrhosis disease.

RESULT

Ethanol could increase the expression of lipin-1 through the AMPK-SREBP-1 signaling and dramatically increase the ratio of Lpin1β to Lpin1α by SIRT1-SFRS10-Lpin1β/α axis in the liver. Moreover, research has shown that over-expression of lipin-1 could also remarkably suppress very low density lipoprotein-triacylglyceride secretion. Last, lipin-1 has potent anti-inflammatory property.

CONCLUSION

In conclusion, lipin-1 has dual functions in lipid metabolism. In the cytoplasm, lipin-1β functions as a Mg(2+)-dependent phosphatidic acid phosphohydrolase (PAP) enzyme in triglyceride synthesis pathways. In the nucleus, lipin-1α acts as a transcriptional co-regulator to regulate the capacity of the liver for fatty acid oxidation and activity of the lipogenic enzyme. In hepatocytes of alcoholic fatty liver disease (AFLD), ethanol increases the expression of lipin-1 through the AMPK-SREBP-1 signaling and the Lpin1β/α ratio by SIRT1-SFRS10- Lpin1β/α axis. Of course, in addition to that, ethanol could also produce the PAP activity and interrupt the nucleus function of lipin-1. Furthermore, over-expression of lipin-1 could remarkably suppress very low-density lipoprotein-triacylglyceride (VLDL-TAG) secretion. In the end, endogenous lipin-1 has potent anti-inflammatory property. Increased synthesis of TAG, decreased fatty acid oxidation, impaired VLDL-TAG secretion and activated inflammatory factors act together to exacerbate the development of AFLD.

Adipose tissue changes in obesity and the impact on metabolic function

Obesity is associated with adverse alterations in adipose tissue that predispose to metabolic dysregulation. These adverse alterations include accumulation of inflammatory macrophages leading to the activation of inflammation pathways, reduction in lipid turnover, and deposition of fat in ectopic locations. These alterations are precursors to the development of insulin resistance and metabolic dysfunction.

TORC1 regulates Pah1 phosphatidate phosphatase activity via the Nem1/Spo7 protein phosphatase complex

The evolutionarily conserved target of rapamycin complex 1 (TORC1) controls growth-related processes such as protein, nucleotide, and lipid metabolism in response to growth hormones, energy/ATP levels, and amino acids. Its deregulation is associated with cancer, type 2 diabetes, and obesity. Among other substrates, mammalian TORC1 directly phosphorylates and inhibits the phosphatidate phosphatase lipin-1, a central enzyme in lipid metabolism that provides diacylglycerol for the synthesis of membrane phospholipids and/or triacylglycerol as neutral lipid reserve. Here, we show that yeast TORC1 inhibits the function of the respective lipin, Pah1, to prevent the accumulation of triacylglycerol. Surprisingly, TORC1 regulates Pah1 in part indirectly by controlling the phosphorylation status of Nem1 within the Pah1-activating, heterodimeric Nem1-Spo7 protein phosphatase module. Our results delineate a hitherto unknown TORC1 effector branch that controls lipin function in yeast, which, given the recent discovery of Nem1-Spo7 orthologous proteins in humans, may be conserved.

A 4-polymorphism risk score predicts steatohepatitis in children with nonalcoholic fatty liver disease

OBJECTIVE

Nonalcoholic fatty liver disease (NAFLD) has become the most common cause of chronic liver disease in industrialized countries in adults and children, following the trail of the epidemic diffusion of obesity. Nonalcoholic steatohepatitis (NASH) is a potentially serious form of NAFLD linked with a significant increase in overall and liver-related morbidity and mortality. Because diagnosis still requires liver biopsy, there is urgent need of developing noninvasive early markers. The aim of the present study was to assess whether the simultaneous detection of genetic risk factors could predict NASH.

METHOD

We enrolled 152 untreated, consecutive obese children and adolescents with biopsy-proven NAFLD and increased liver enzymes. The PNPLA3 rs738409 C>G (I148 M), SOD2 rs4880 C>T, KLF6 rs3750861 G>A, and LPIN1 rs13412852 C>T polymorphisms were detected by Taqman assays.

RESULTS

A multivariate logistic model based on the genetic risk factors significantly predicted NASH (area under the receiver-operating characteristic curve [AUC] 0.75, 95% confidence interval [CI] 0.67-0.82, P < 0.0001), performing better than a clinical risk score identified at stepwise regression based on age, aspartate aminotransferase levels, and diastolic blood pressure (AUC 0.66, 95% CI 0.57-0.75). A single cutoff value of the genetic risk score had 90% sensitivity and 36% specificity for NASH. A risk score combining the clinical and genetic risk factors resulted in an AUC of 0.80 (95% CI 0.73-0.87).

CONCLUSIONS

A score based on genetic risk factors significantly predicts NASH in obese children with increased liver enzymes, representing a proof-of-principle that genetic scores may be useful to predict long-term outcomes of the disease and guide clinical management.

Spatial control of phospholipid flux restricts endoplasmic reticulum sheet formation to allow nuclear envelope breakdown

The nuclear envelope is a subdomain of the endoplasmic reticulum (ER). Here, Oegema and colleagues characterize the conserved lipin-activating protein phosphatase CNEP-1, which is specifically enriched on the nuclear envelope. The data show that CNEP-1 spatially controls lipin-dependent phospholipid flux to limit phosphatidylinositol levels and restrict ER sheet formation in the vicinity of the nuclear envelope. This study reveals coupling between control of phospholipid synthesis and ER subdomain structure.

The nuclear envelope is a subdomain of the endoplasmic reticulum (ER). Here we characterize CNEP-1 (CTD [C-terminal domain] nuclear envelope phosphatase-1), a nuclear envelope-enriched activator of the ER-associated phosphatidic acid phosphatase lipin that promotes synthesis of major membrane phospholipids over phosphatidylinositol (PI). CNEP-1 inhibition led to ectopic ER sheets in the vicinity of the nucleus that encased the nuclear envelope and interfered with nuclear envelope breakdown (NEBD) during cell division. Reducing PI synthesis suppressed these phenotypes, indicating that CNEP-1 spatially regulates phospholipid flux, biasing it away from PI production in the vicinity of the nuclear envelope to prevent excess ER sheet formation and NEBD defects.

Autoinflammatory bone disorders: update on immunologic abnormalities and clues about possible triggers

PURPOSE OF REVIEW

To provide an update on the genetics and immunologic basis of autoinflammatory bone disorders including chronic recurrent multifocal osteomyelitis including the monogenic forms of the disease.

RECENT FINDINGS

Ongoing research in murine, canine and human models of sterile bone inflammation has solidified the hypothesis that sterile bone inflammation can be genetically driven. Mutations in Pstpip2, LPIN2 and IL1RN have been identified in monogenic autoinflammatory bone disorders that have allowed more detailed dissection of the immunologic defects that can produce sterile osteomyelitis. Recent studies in murine chronic multifocal osteomyelitis, deficiency of the interleukin-1 receptor antagonist (DIRA), Majeed syndrome and SAPHO syndrome reveal abnormalities in innate immune system function. IL-1 pathway dysregulation is present in several of these disorders and blocking IL-1 therapeutically has resulted in control of disease in DIRA, Majeed syndrome and in some cases of SAPHO and CRMO. Basic research demonstrates the importance of the innate immune system in disease pathogenesis and offers clues about potential disease triggers.

SUMMARY

Research and clinical data produced over the last several years support the important role of innate immunity in sterile osteomyelitis. Based on what has been learned in the monogenic autoinflammatory bone disorders, IL-1 is emerging as an important pathway in the development of sterile bone inflammation.

Genetic predisposition in NAFLD and NASH: impact on severity of liver disease and response to treatment

Liver fat deposition related to systemic insulin resistance defines non-alcoholic fatty liver disease (NAFLD) which, when associated with oxidative hepatocellular damage, inflammation, and activation of fibrogenesis, i.e. non-alcoholic steatohepatitis (NASH), can progress towards cirrhosis and hepatocellular carcinoma. Due to the epidemic of obesity, NAFLD is now the most frequent liver disease and the leading cause of altered liver enzymes in Western countries. Epidemiological, familial, and twin studies provide evidence for an element of heritability of NAFLD. Genetic modifiers of disease severity and progression have been identified through genome-wide association studies. These include the Patatin-like phosholipase domain-containing 3 (PNPLA3) gene variant I148M as a major determinant of inter-individual and ethnicity-related differences in hepatic fat content independent of insulin resistance and serum lipid concentration. Association studies confirm that the I148M polymorphism is also a strong modifier of NASH and progressive hepatic injury. Furthermore, a few large multicentre case-control studies have demonstrated a role for genetic variants implicated in insulin signalling, oxidative stress, and fibrogenesis in the progression of NAFLD towards fibrosing NASH, and confirm that hepatocellular fat accumulation and insulin resistance are key operative mechanisms closely involved in the progression of liver damage. It is now important to explore the molecular mechanisms underlying these associations between gene variants and progressive liver disease, and to evaluate their impact on the response to available therapies. It is hoped that this knowledge will offer further insights into pathogenesis, suggest novel therapeutic targets, and could help guide physicians towards individualised therapy that improves clinical outcome.

Analyzing the functions and structure of the human lipodystrophy protein seipin

Disruption of the gene BSCL2, which encodes the protein seipin, causes severe generalized lipodystrophy in humans with a near complete absence of adipose tissue. Moreover, cell culture studies have demonstrated that seipin plays a critical cell-autonomous role in adipocyte differentiation. These observations reveal seipin as a critical regulator of human adipose tissue development; however, until recently very little has been known about the potential molecular functions of this intriguing protein. Despite significant recent interest in the function of seipin, our understanding of its molecular role(s) remains limited. The topology of seipin and lack of evidence for any enzymatic domains or activity indicate that it may act principally as a scaffold for other proteins or play a structural role in altering membrane curvature and/or budding. Work in this area has been hampered by several factors, including the lack of homology that might imply testable functions, the poor availability of antibodies to the endogenous protein and the observation that this hydrophobic ER membrane-resident protein is difficult to analyze by standard Western blotting techniques. Here we summarize some of the techniques we have applied to investigate the association of seipin with a recently identified binding partner, lipin 1. In addition, we describe the use of atomic force microscopy (AFM) to image oligomers of the seipin protein. We believe that AFM will offer a valuable tool to examine the association of candidate binding proteins with the seipin oligomer.

Genome wide association study (GWAS) of Chagas cardiomyopathy in Trypanosoma cruzi seropositive subjects

BACKGROUND

Familial aggregation of Chagas cardiac disease in T. cruzi-infected persons suggests that human genetic variation may be an important determinant of disease progression.

OBJECTIVE

To perform a GWAS using a well-characterized cohort to detect single nucleotide polymorphisms (SNPs) and genes associated with cardiac outcomes.

METHODS

A retrospective cohort study was developed by the NHLBI REDS-II program in Brazil. Samples were collected from 499 T. cruzi seropositive blood donors who had donated between 1996 and 2002, and 101 patients with clinically diagnosed Chagas cardiomyopathy. In 2008-2010, all subjects underwent a complete medical examination. After genotype calling, quality control filtering with exclusion of 20 cases, and imputation of 1,000 genomes variants; association analysis was performed for 7 cardiac and parasite related traits, adjusting for population stratification.

RESULTS

The cohort showed a wide range of African, European, and modest Native American admixture proportions, consistent with the recent history of Brazil. No SNPs were found to be highly (P<10(-8)) associated with cardiomyopathy. The two mostly highly associated SNPs for cardiomyopathy (rs4149018 and rs12582717; P-values <10(-6)) are located on Chromosome 12p12.2 in the SLCO1B1 gene, a solute carrier family member. We identified 44 additional genic SNPs associated with six traits at P-value <10(-6): Ejection Fraction, PR, QRS, QT intervals, antibody levels by EIA, and parasitemia by PCR.

CONCLUSION

This GWAS identified suggestive SNPs that may impact the risk of progression to cardiomyopathy. Although this Chagas cohort is the largest examined by GWAS to date, (580 subjects), moderate sample size may explain in part the limited number of significant SNP variants. Enlarging the current sample through expanded cohorts and meta-analyses, and targeted studies of candidate genes, will be required to confirm and extend the results reported here. Future studies should also include exposed seronegative controls to investigate genetic associations with susceptibility or resistance to T. cruzi infection and non-Chagas cardiomyopathy.

PAH1-encoded phosphatidate phosphatase plays a role in the growth phase- and inositol-mediated regulation of lipid synthesis in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, the synthesis of phospholipids in the exponential phase of growth occurs at the expense of the storage lipid triacylglycerol. As exponential phase cells progress into the stationary phase, the synthesis of triacylglycerol occurs at the expense of phospholipids. Early work indicates a role of the phosphatidate phosphatase (PAP) in this metabolism; the enzyme produces the diacylglycerol needed for the synthesis of triacylglycerol and simultaneously controls the level of phosphatidate for the synthesis of phospholipids. Four genes (APP1, DPP1, LPP1, and PAH1) encode PAP activity in yeast, and it has been unclear which gene is responsible for the synthesis of triacylglycerol throughout growth. An analysis of lipid synthesis and composition, as well as PAP activity in various PAP mutant strains, showed the essential role of PAH1 in triacylglycerol synthesis throughout growth. Pah1p is a phosphorylated enzyme whose in vivo function is dependent on its dephosphorylation by the Nem1p-Spo7p protein phosphatase complex. nem1Δ mutant cells exhibited defects in triacylglycerol synthesis and lipid metabolism that mirrored those imparted by the pah1Δ mutation, substantiating the importance of Pah1p dephosphorylation throughout growth. An analysis of cells bearing PPAH1-lacZ and PPAH1-DPP1 reporter genes showed that PAH1 expression was induced throughout growth and that the induction in the stationary phase was stimulated by inositol supplementation. A mutant analysis indicated that the Ino2p/Ino4p/Opi1p regulatory circuit and transcription factors Gis1p and Rph1p mediated this regulation.

Role of PPARα and HNF4α in stress-mediated alterations in lipid homeostasis

Stress is a risk factor for several cardiovascular pathologies. PPARα holds a fundamental role in control of lipid homeostasis by directly regulating genes involved in fatty acid transport and oxidation. Importantly, PPARα agonists are effective in raising HDL-cholesterol and lowering triglycerides, properties that reduce the risk for cardiovascular diseases. This study investigated the role of stress and adrenergic receptor (AR)-related pathways in PPARα and HNF4α regulation and signaling in mice following repeated restraint stress or treatment with AR-antagonists administered prior to stress to block AR-linked pathways. Repeated restraint stress up-regulated Pparα and its target genes in the liver, including Acox, Acot1, Acot4, Cyp4a10, Cyp4a14 and Lipin2, an effect that was highly correlated with Hnf4α. In vitro studies using primary hepatocyte cultures treated with epinephrine or AR-agonists confirmed that hepatic AR/cAMP/PKA/CREB- and JNK-linked pathways are involved in PPARα and HNF4α regulation. Notably, restraint stress, independent of PPARα, suppressed plasma triglyceride levels. This stress-induced effect could be attributed in part to hormone sensitive lipase activation in the white adipose tissue, which was not prevented by the increased levels of perilipin. Overall, this study identifies a mechanistic basis for the modification of lipid homeostasis following stress and potentially indicates novel roles for PPARα and HNF4α in stress-induced lipid metabolism.

Making, baking, and breaking: the synthesis, storage, and hydrolysis of neutral lipids

The esterification of amphiphilic alcohols with fatty acids is a ubiquitous strategy implemented by eukaryotes and some prokaryotes to conserve energy and membrane progenitors and simultaneously detoxify fatty acids and other lipids. This key reaction is performed by at least four evolutionarily unrelated multigene families. The synthesis of this "neutral lipid" leads to the formation of a lipid droplet, which despite the clear selective advantage it confers is also a harbinger of cellular and organismal malaise. Neutral lipid deposition as a cytoplasmic lipid droplet may be thermodynamically favored but nevertheless is elaborately regulated. Optimal utilization of these resources by lipolysis is similarly multigenic in determination and regulation. We present here a perspective on these processes that originates from studies in model organisms, and we include our thoughts on interventions that target reductions in neutral lipids as therapeutics for human diseases such as obesity and diabetes.

Genetic variants of LPIN1 indicate an association with Type 2 diabetes mellitus in a Chinese population

AIMS

Metabolic disorders are independent risk factors for the development of Type 2 diabetes. The aim of the study is to test the association of LPIN1 variants with Type 2 diabetes and clinical characteristics in large samples of the Chinese population.

METHODS

In the first stage, 15 single nucleotide polymorphisms within the LPIN1 region were selected and genotyped in 3700 Chinese Han participants. In the second stage, the single nucleotide polymorphisms showing significant association or trends towards association were genotyped in an additional 3122 samples for replication. Meta-analyses and genotype-phenotype association studies were performed after combining the data from the two stages.

RESULTS

In the first stage, we detected that rs16857876 was significantly associated with Type 2 diabetes with an odds ratio of 0.806 (95% CI 0.677-0.958, P = 0.015), while rs11695610 showed a trend with Type 2 diabetes (odds ratio 0.846, 95% CI 0.709-1.009, P = 0.062). In the second stage, a similar effect of rs11695610 on Type 2 diabetes was observed (odds ratio 0.849, 95% CI 0.700-1.030, P = 0.096). The meta-analyses combining the information from the two stages showed a significant effect of rs11695610 on Type 2 diabetes with an odds ratio of 0.847 (95% CI 0.744-0.965, P = 0.012). Finally, the phenotype-genotype association analyses showed that rs11695610 was associated with 2-h plasma glucose (P = 0.040) and triglyceride levels (P = 0.034).

CONCLUSIONS

Our data implied that common single nucleotide polymorphisms within the LPIN1 region were associated with Type 2 diabetes and metabolic traits in the Chinese population.

Negative control of mast cell degranulation and the anaphylactic response by the phosphatase lipin1

Mast cells play a critical role in the pathogenesis of allergic diseases; however, how mast cell function is regulated is still not well understood. Both phosphatidic acid (PA) and diacylglycerol (DAG) are important secondary messengers involved in mast cell activ-ation. Lipin1 is a phosphatidate phosphatase that hydrolyzes PA to produce DAG, but the role of lipin1 in mast cell function has been thus far unknown. Here we show that lipin1 is an important and selective inhibitor of mast cell degranulation. Lipin1 deficiency enhanced FcεRI-mediated β-hexosaminidase and prostaglandin D2 release from mast cells in vitro and exacerbated the passive systemic anaphylaxis reaction in vivo. Lipin1 deficiency, however, did not exert obvious effects on IL-6 or TNF-α production following FcεRI engagement. FcεRI-induced PKC and SNAP-23 phosphorylation were augmented in the lipin1-deficient mast cells. Moreover, inhibition of PKC activity reduced SNAP-23 phosphorylation and mast cell degranulation in lipin1-deficient mast cells. Together, our findings suggest that lipin1 may negatively control mast cell degranulation and the anaphylactic response through inhibiting the PKC-SNAP-23 pathway.

Efficacy of anti-IL-1 treatment in Majeed syndrome

BACKGROUND AND OBJECTIVE

Majeed syndrome is an autosomal recessive disorder characterised by the triad of chronic recurrent multifocal osteomyelitis, congenital dyserythropoietic anaemia and a neutrophilic dermatosis that is caused by mutations in LPIN2. Long-term outcome is poor. This is the first report detailing the treatment of Majeed syndrome with biological agents and demonstrates clinical improvement with IL-1blockade.

METHODS

We describe the clinical presentation, genetic analysis, cytokine profiles and response to biological therapy in two brothers with Majeed syndrome.

RESULTS

Both boys were homozygous for a novel 2-base pair deletion in LPIN2 (c.1312_1313delCT; p.Leu438fs+16X), confirming the diagnosis. Their bone disease and anaemia were refractory to treatment with corticosteroids. Both siblings had elevated proinflammatory cytokines in their serum, including tumour necrosis factor α (TNF-α), however a trial of the TNF inhibitor etanercept resulted in no improvement. IL-1 inhibition with either a recombinant IL-1 receptor antagonist (anakinra) or an anti-IL-1β antibody (canakinumab) resulted in dramatic clinical and laboratory improvement.

CONCLUSIONS

The differential response to treatment with TNF-α or IL-1 blocking agents sheds light into disease pathogenesis; it supports the hypothesis that Majeed syndrome is an IL-1β dependent autoinflammatory disorder, and further underscores the importance of IL-1 in sterile bone inflammation.

Cell autonomous lipin 1 function is essential for development and maintenance of white and brown adipose tissue

Through analysis of mice with spatially and temporally restricted inactivation of Lpin1, we characterized its cell autonomous function in both white (WAT) and brown (BAT) adipocyte development and maintenance. We observed that the lipin 1 inactivation in adipocytes of aP2(Cre/+)/Lp(fEx2)(-)(3/fEx2)(-)(3) mice resulted in lipodystrophy and the presence of adipocytes with multilocular lipid droplets. We further showed that time-specific loss of lipin 1 in mature adipocytes in aP2(Cre-ERT2/+)/Lp(fEx2)(-)(3/fEx2)(-)(3) mice led to their replacement by newly formed Lpin1-positive adipocytes, thus establishing a role for lipin 1 in mature adipocyte maintenance. Importantly, we observed that the presence of newly formed Lpin1-positive adipocytes in aP2(Cre-ERT2/+)/Lp(fEx2)(-)(3/fEx2)(-)(3) mice protected these animals against WAT inflammation and hepatic steatosis induced by a high-fat diet. Loss of lipin 1 also affected BAT development and function, as revealed by histological changes, defects in the expression of peroxisome proliferator-activated receptor alpha (PPARα), PGC-1α, and UCP1, and functionally by altered cold sensitivity. Finally, our data indicate that phosphatidic acid, which accumulates in WAT of animals lacking lipin 1 function, specifically inhibits differentiation of preadipocytes. Together, these observations firmly demonstrate a cell autonomous role of lipin 1 in WAT and BAT biology and indicate its potential as a therapeutical target for the treatment of obesity.

Activation of cannabinoid receptor type 1 (Cb1r) disrupts hepatic insulin receptor signaling via cyclic AMP-response element-binding protein H (Crebh)-mediated induction of Lipin1 gene

Activation of hepatic cannabinoid 1 receptor (Cb1r) signaling has been implicated in the development of phenotypes associated with fatty liver, hypertriglyceridemia, and insulin resistance. In the current study, we have elucidated the critical role of endoplasmic reticulum-bound transcription factor cyclic AMP-response element-binding protein H (Crebh) in mediating activated Cb1r signaling in inducing phosphatidic acid phosphatase Lipin1 gene expression and subsequently deregulating hepatic insulin receptor signaling. Cb1r agonist (2-arachidonoylglycerol (2-AG)) treatment induced Lipin1 gene expression in a Crebh-dependent manner via recruiting CREBH to the endogenous Lipin1 gene promoter. Adenoviral overexpression of Crebh or 2-AG treatment in mice induced Lipin1 gene expression to increase the hepatic diacylglycerol (DAG) level and phosphorylation of protein kinase Cε (PKCε). This in turn inhibited hepatic insulin receptor signaling. Knockdown of Crebh or Cb1r antagonism attenuated 2-AG-mediated induction of Lipin1 gene expression and decreased DAG production in mouse liver and subsequently restored insulin receptor signaling. Similarly, knockdown of Lipin1 attenuated the 2-AG-induced increase in the DAG level and PKCε phosphorylation. Finally, shRNA-mediated knockdown of Crebh partially but significantly blunted Lipin1 expression and the DAG level in db/db mice. These results demonstrate a novel mechanism by which Cb1r signaling induces Lipin1 gene expression and increases DAG production by activating Crebh, thereby deregulating insulin receptor signaling pathway and lipid homeostasis.

Current understanding of the pathogenesis and management of chronic recurrent multifocal osteomyelitis

Chronic recurrent multifocal osteomyelitis (CRMO) is an inflammatory disorder that primarily affects children. Its hallmark is recurring episodes of sterile osteomyelitis. The clinical presentation is insidious onset of bone pain with or without fever. Laboratory studies typically reveal nonspecific evidence of inflammation. Radiologic imaging and histologic appearance resemble those of infectious osteomyelitis. There is a strong association with inflammatory disorders of the skin and intestinal tract in affected individuals and their close relatives, suggesting a shared pathophysiology and supporting a genetic component to disease susceptibility. Two genetic syndromes have CRMO as a prominent phenotype-Majeed syndrome and deficiency of the interleukin-1 receptor antagonist-and suggest that interleukin-1 may be a key cytokine in disease pathogenesis. This review briefly summarizes the main clinical and radiologic aspects of the disease and then focuses on genetics and pathophysiology and provides an update on treatment.

Synphilin-1 alters metabolic homeostasis in a novel Drosophila obesity model

AIMS

The pathogenesis of obesity remains incompletely understood. Drosophila have conserved neuroendocrine and digestion systems with human and become an excellent system for studying energy homeostasis. Here, we reported a novel obesity Drosophila model, in which expression of human protein, synphilin-1 (SP1), in neurons fosters positive energy balance.

SUBJECTS AND METHODS

To further understand the actions of SP1 in energy balance control, the upstream activation sequence UAS/GAL4 system was used to generate human SP1 transgenic Drosophila. We characterized a human SP1 transgenic Drosophila by assessing SP1 expression, fat lipid deposition, food intake and fly locomotor activity to determine the major behavioral changes and their consequences in the development of the obesity-like phenotype.

RESULTS

Overexpression of SP1 in neurons, but not peripheral cells, increased the body weight of flies compared with that of non-transgenic controls. SP1 increased food intake but did not affect locomotor activity. SP1 increased the levels of triacylglycerol, and the size of fat body cells and lipid droplets, indicating that SP1 increased lipid-fat disposition. Survival studies showed that SP1 transgenic flies were more resistant to food deprivation. SP1 regulated lipin gene expression that may participate in SP1-induced fat deposition and starvation resistance.

CONCLUSION

These studies demonstrate that SP1 expression affects energy homeostasis in ways that enhance positive energy balance and provide a useful obesity model for future pathogenesis and therapeutic studies.

Human HAD phosphatases: structure, mechanism, and roles in health and disease

Phosphatases of the haloacid dehalogenase (HAD) superfamily of hydrolases are an ancient and very large class of enzymes that have evolved to dephosphorylate a wide range of low- and high molecular weight substrates with often exquisite specificities. HAD phosphatases constitute approximately one-fifth of all human phosphatase catalytic subunits. While the overall sequence similarity between HAD phosphatases is generally very low, family members can be identified based on the presence of a characteristic Rossmann-like fold and the active site sequence DxDx(V/T). HAD phosphatases employ an aspartate residue as a nucleophile in a magnesium-dependent phosphoaspartyl transferase reaction. Although there is genetic evidence demonstrating a causal involvement of some HAD phosphatases in diseases such as cancer, cardiovascular, metabolic and neurological disorders, the physiological roles of many of these enzymes are still poorly understood. In this review, we discuss the structure and evolution of human HAD phosphatases, and summarize their known functions in health and disease.

Regulation of lipid metabolism: a tale of two yeasts

Eukaryotic cells synthesize multiple classes of lipids by distinct metabolic pathways in order to generate membranes with optimal physical and chemical properties. As a result, complex regulatory networks are required in all organisms to maintain lipid and membrane homeostasis as well as to rapidly and efficiently respond to cellular stress. The unicellular nature of yeast makes it particularly vulnerable to environmental stress and yeast has evolved elaborate signaling pathways to maintain lipid homeostasis. In this article we highlight the recent advances that have been made using the budding and fission yeasts and we discuss potential roles for the unfolded protein response (UPR) and the SREBP-Scap pathways in coordinate regulation of multiple lipid classes.

Thematic review series: Lipid droplet synthesis and metabolism: from yeast to man. Lipid droplet-based storage fat metabolism in Drosophila

The fruit fly Drosophila melanogaster is an emerging model system in lipid metabolism research. Lipid droplets are omnipresent and dynamically regulated organelles found in various cell types throughout the complex life cycle of this insect. The vital importance of lipid droplets as energy resources and storage compartments for lipoanabolic components has recently attracted research attention to the basic enzymatic machinery, which controls the delicate balance between triacylglycerol deposition and mobilization in flies. This review aims to present current insights in experimentally supported and inferred biological functions of lipogenic and lipolytic enzymes as well as regulatory proteins, which control the lipid droplet-based storage fat turnover in Drosophila.

LPIN1 rs13412852 polymorphism in pediatric nonalcoholic fatty liver disease

OBJECTIVES

The aim of the present study was to evaluate whether the lipin1 rs13412852 C>T polymorphism is associated with nonalcoholic steatohepatitis and fibrosis in pediatric Italian patients with nonalcoholic fatty liver disease (NAFLD).

METHODS

A total of 142 untreated, consecutive children and 115 adults with biopsy-proven NAFLD and 337 healthy controls without steatosis were studied. Liver histology was assessed by the NAFLD activity score and the rs13412852 polymorphism by a 5' nuclease Taqman assay.

RESULTS

Homozygosity for the rs13412852 T allele was underrepresented in pediatric, but not adult, patients with NAFLD compared with healthy controls (7% vs 14%; odds ratio [OR] 0.58, 95% confidence interval [CI] 0.35-0.91), and it was associated with lower triglycerides both in pediatric patients and healthy controls (P ≤ 0.01). Affected children carrying the rs13412852 TT genotype had a trend for a lower prevalence of nonalcoholic steatohepatitis, and significantly less severe liver damage, as indicated by NAFLD activity score severity (P = 0.026) and a lower prevalence of liver fibrosis (P = 0.012). The negative association between rs13412852 TT genotype and fibrosis was independent of Patatin-like phospholipase domain-containing-3 genotype and other clinical risk factors, including age, waist circumference, the presence of hyperglycemia, and alanine transaminase levels (OR 0.29; 95% CI 0.11-0.66), and it was confirmed at multivariate analysis in adults (OR 0.15; 95% CI 0.02-0.67).

CONCLUSIONS

Lipin1 rs13412852 single nucleotide polymorphism is associated with the severity of liver damage and fibrosis progression in pediatric patients with histological NAFLD.

Lipin-2 reduces proinflammatory signaling induced by saturated fatty acids in macrophages

Lipin-2 is a member of the lipin family of enzymes, which are key effectors in the biosynthesis of lipids. Mutations in the human lipin-2 gene are associated with inflammatory-based disorders; however, the role of lipin-2 in cells of the immune system remains obscure. In this study, we have investigated the role of lipin-2 in the proinflammatory action of saturated fatty acids in murine and human macrophages. Depletion of lipin-2 promotes the increased expression of the proinflammatory genes Il6, Ccl2, and Tnfα, which depends on the overstimulation of the JNK1/c-Jun pathway by saturated fatty acids. In contrast, overexpression of lipin-2 reduces the release of proinflammatory factors. Metabolically, the absence of lipin-2 reduces the cellular content of triacylglycerol in saturated fatty acid-overloaded macrophages. Collectively, these studies demonstrate a protective role for lipin-2 in proinflammatory signaling mediated by saturated fatty acids that occurs concomitant with an enhanced cellular capacity for triacylglycerol synthesis. The data provide new insights into the role of lipin-2 in human and murine macrophage biology and may open new avenues for controlling the fatty acid-related low grade inflammation that constitutes the sine qua non of obesity and associated metabolic disorders.

Nuclear envelope phosphatase 1-regulatory subunit 1 (formerly TMEM188) is the metazoan Spo7p ortholog and functions in the lipin activation pathway

Lipin-1 catalyzes the formation of diacylglycerol from phosphatidic acid. Lipin-1 mutations cause lipodystrophy in mice and acute myopathy in humans. It is heavily phosphorylated, and the yeast ortholog Pah1p becomes membrane-associated and active upon dephosphorylation by the Nem1p-Spo7p membrane complex. A mammalian ortholog of Nem1p is the C-terminal domain nuclear envelope phosphatase 1 (CTDNEP1, formerly "dullard"), but its Spo7p-like partner is unknown, and the need for its existence is debated. Here, we identify the metazoan ortholog of Spo7p, TMEM188, renamed nuclear envelope phosphatase 1-regulatory subunit 1 (NEP1-R1). CTDNEP1 and NEP1-R1 together complement a nem1Δspo7Δ strain to block endoplasmic reticulum proliferation and restore triacylglycerol levels and lipid droplet number. The two human orthologs are in a complex in cells, and the amount of CTDNEP1 is increased in the presence of NEP1-R1. In the Caenorhabditis elegans embryo, expression of nematode CTDNEP1 and NEP1-R1, as well as lipin-1, is required for normal nuclear membrane breakdown after zygote formation. The expression pattern of NEP1-R1 and CTDNEP1 in human and mouse tissues closely mirrors that of lipin-1. CTDNEP1 can dephosphorylate lipins-1a, -1b, and -2 in human cells only in the presence of NEP1-R1. The nuclear fraction of lipin-1b is increased when CTDNEP1 and NEP1-R1 are co-expressed. Therefore, NEP1-R1 is functionally conserved from yeast to humans and functions in the lipin activation pathway.

Identification of possible genetic polymorphisms involved in cancer cachexia: a systematic review

Cancer cachexia is a polygenic and complex syndrome. Genetic variations in regulation of the inflammatory response, muscle and fat metabolic pathways, and pathways in appetite regulation are likely to contribute to the susceptibility or resistance to developing cancer cachexia. A systematic search of Medline and EmBase databases, covering 1986-2008 was performed for potential candidate genes/genetic polymorphisms relating to cancer cachexia. Related genes were then identified using pathway functional analysis software. All candidate genes were reviewed for functional polymorphisms or clinically significant polymorphisms associated with cachexia using the OMIM and GeneRIF databases. Genes with variants which had functional or clinical associations with cachexia and replicated in at least one study were entered into pathway analysis software to reveal possible network associations between genes. A total of 184 polymorphisms with functional or clinical relevance to cancer cachexia were identified in 92 candidate genes. Of these, 42 polymorphisms (in 33 genes) were replicated in more than one study with 13 polymorphisms found to influence two or more hallmarks of cachexia (i.e. inflammation, loss of fat mass and/or lean mass and reduced survival). Thirty-three genes were found to be significantly interconnected in two major networks with four genes (ADIPOQ, IL6, NFKB1 and TLR4) interlinking both networks. Selection of candidate genes and polymorphisms is a key element of multigene study design. The present study provides an initial framework to select genes/polymorphisms for further study in cancer cachexia, and to develop their potential as susceptibility biomarkers of developing cachexia.

Recent insights into the structure and function of comparative gene identification-58

PURPOSE OF REVIEW

Comparative gene identification-58 (CGI-58) is an important player in lipid metabolism. It acts as activator of triglyceride hydrolases and as acyl-CoA-dependent lysophosphatidic acid acyltransferase. This review aims at establishing a structure-function relationship of this still rather enigmatic protein based on recent studies characterizing different functions of CGI-58.

RECENT FINDINGS

Novel studies confirm the important regulatory role of CGI-58 as activator of the triglyceride hydrolase adipose triglyceride lipase. New evidence, corroborated by the characterization of a CGI-58 knockout mouse model, also suggests the existence of yet unknown lipases that are activated by CGI-58. Additionally, CGI-58 was identified to exert acyl-CoA-dependent lysophosphatidic acid acyltransferase activity, which implies possible roles in triglyceride or phospholipid synthesis or signaling processes. Unlike mammalian CGI-58 proteins, orthologs from plants and yeast additionally act as weak triglyceride and phospholipid hydrolases. A first three-dimensional model was calculated and allows preliminary structural considerations for the functions of CGI-58.

SUMMARY

Despite important progress concerning the different biochemical functions of CGI-58, the physiological importance of these activities requires better characterization. Furthermore, three-dimensional structural data for CGI-58 are required to unveil the molecular mechanism of how CGI-58 acts as activator of lipases and exerts its enzymatic functions.

Endoplasmic reticulum stress promotes LIPIN2-dependent hepatic insulin resistance

OBJECTIVE

Diet-induced obesity (DIO) is linked to peripheral insulin resistance-a major predicament in type 2 diabetes. This study aims to identify the molecular mechanism by which DIO-triggered endoplasmic reticulum (ER) stress promotes hepatic insulin resistance in mouse models.

RESEARCH DESIGN AND METHODS

C57BL/6 mice and primary hepatocytes were used to evaluate the role of LIPIN2 in ER stress-induced hepatic insulin resistance. Tunicamycin, thapsigargin, and lipopolysaccharide were used to invoke acute ER stress conditions. To promote chronic ER stress, mice were fed with a high-fat diet for 8-12 weeks. To verify the role of LIPIN2 in hepatic insulin signaling, adenoviruses expressing wild-type or mutant LIPIN2, and shRNA for LIPIN2 were used in animal studies. Plasma glucose, insulin levels as well as hepatic free fatty acids, diacylglycerol (DAG), and triacylglycerol were assessed. Additionally, glucose tolerance, insulin tolerance, and pyruvate tolerance tests were performed to evaluate the metabolic phenotype of these mice.

RESULTS

LIPIN2 expression was enhanced in mouse livers by acute ER stress-inducers or by high-fat feeding. Transcriptional activation of LIPIN2 by ER stress is mediated by activating transcription factor 4, as demonstrated by LIPIN2 promoter assays, Western blot analyses, and chromatin immunoprecipitation assays. Knockdown of hepatic LIPIN2 in DIO mice reduced fasting hyperglycemia and improved hepatic insulin signaling. Conversely, overexpression of LIPIN2 impaired hepatic insulin signaling in a phosphatidic acid phosphatase activity-dependent manner.

CONCLUSIONS

These results demonstrate that ER stress-induced LIPIN2 would contribute to the perturbation of hepatic insulin signaling via a DAG-protein kinase C ε-dependent manner in DIO mice.

Homo sapiens dullard protein phosphatase shows a preference for the insulin-dependent phosphorylation site of lipin1

Human lipin1 catalyzes the highly regulated conversion of phosphatidic acids to diacylglycerides. Lipin's cellular location, protein partners, and biological function are directed by phosphorylation-dephosphorylation events catalyzed by the phosphoserine phosphatase dullard. To define the determinants of dullard substrate recognition and catalysis, and hence, lipin regulation, steady-state kinetic analysis was performed on phosphoserine-bearing nonapeptides based on the phosphorylation sites of lipin. The results demonstrate that dullard shows specificity for the peptide corresponding to the insulin-dependent phosphorylation site (Ser106) of lipin with a k(cat)/K(m) of 2.9 × 10(4) M(-1) s(-1). These results are consistent with a coil-loop structure for the insulin-dependent phosphorylation site on human lipin1 and make unlikely the requirement for an adaptor protein to confer activity such as that proposed for the yeast homologue.

Lipin is a central regulator of adipose tissue development and function in Drosophila melanogaster

Lipins are evolutionarily conserved proteins found from yeasts to humans. Mammalian and yeast lipin proteins have been shown to control gene expression and to enzymatically convert phosphatidate to diacylglycerol, an essential precursor in triacylglcerol (TAG) and phospholipid synthesis. Loss of lipin 1 in the mouse, but not in humans, leads to lipodystrophy and fatty liver disease. Here we show that the single lipin orthologue of Drosophila melanogaster (dLipin) is essential for normal adipose tissue (fat body) development and TAG storage. dLipin mutants are characterized by reductions in larval fat body mass, whole-animal TAG content, and lipid droplet size. Individual cells of the underdeveloped fat body are characterized by increased size and ultrastructural defects affecting cell nuclei, mitochondria, and autophagosomes. Under starvation conditions, dLipin is transcriptionally upregulated and functions to promote survival. Together, these data show that dLipin is a central player in lipid and energy metabolism, and they establish Drosophila as a genetic model for further studies of conserved functions of the lipin family of metabolic regulators.

Drosophila melanogaster lipins are tissue-regulated and developmentally regulated and present specific subcellular distributions

Lipins constitute a novel family of Mg(2+)-dependent phosphatidate phosphatases that catalyze the dephosphorylation of phosphatidic acid to yield diacylglycerol, an important intermediate in lipid metabolism and cell signaling. Whereas a single lipin is detected in less complex organisms, in mammals there are distinct lipin isoforms and paralogs that are differentially expressed among tissues. Compatible with organism tissue complexity, we show that the single Drosophila Lpin1 ortholog (CG8709, here named DmLpin) expresses at least three isoforms (DmLpinA, DmLpinK and DmLpinJ) in a temporal and spatially regulated manner. The highest levels of lipin in the fat body, where DmLpinA and DmLpinK are expressed, correlate with the highest levels of triacylglycerol (TAG) measured in this tissue. DmLpinK is the most abundant isoform in the central nervous system, where TAG levels are significantly lower than in the fat body. In the testis, where TAG levels are even lower, DmLpinJ is the predominant isoform. Together, these data suggest that DmLpinA might be the isoform that is mainly involved in TAG production, and that DmLpinK and DmLpinJ could perform other cellular functions. In addition, we demonstrate by immunofluorescence that lipins are most strongly labeled in the perinuclear region of the fat body and ventral ganglion cells. In visceral muscles of the larval midgut and adult testis, lipins present a sarcomeric distribution. In the ovary chamber, the lipin signal is concentrated in the internal rim of the ring canal. These specific subcellular localizations of the Drosophila lipins provide the basis for future investigations on putative novel cellular functions of this protein family.

A phosphatidic acid binding/nuclear localization motif determines lipin1 function in lipid metabolism and adipogenesis

A polybasic motif in the metabolic regulator lipin1 is both a membrane anchor and a nuclear localization sequence required for lipin1 function in phospholipid metabolism and adipogenesis.

Lipins are phosphatidic acid phosphatases with a pivotal role in regulation of triglyceride and glycerophospholipid metabolism. Lipin1 is also an amplifier of PGC-1α, a nuclear coactivator of PPAR-α responsive gene transcription. Lipins do not contain recognized membrane-association domains, but interaction of these enzymes with cellular membranes is necessary for access to their phospholipid substrate. We identified a role for a conserved polybasic amino acid motif in an N-terminal domain previously implicated as a determinant of nuclear localization in selective binding of lipin1β to phosphatidic acid, using blot overlay assays and model bilayer membranes. Studies using lipin1β polybasic motif variants establish that this region is also critical for nuclear import and raise the possibility that nuclear/cytoplasmic shuttling of lipin1β is regulated by PA. We used pharmacological agents and lipin1β polybasic motif mutants to explore the role of PA-mediated membrane association and nuclear localization on lipin1β function in phospholipid metabolism and adipogenic differentiation. We identify a role for the lipin1 polybasic motif as both a lipid binding motif and a primary nuclear localization sequence. These two functions are necessary for full expression of the biological activity of the protein in intracellular lipid metabolism and transcriptional control of adipogenesis.

Phospholipase D in brain function and Alzheimer's disease

Alzheimer's disease is the most common neurodegenerative disorder. Although lipids are major constituents of brain, their role in Alzheimer's disease pathogenesis is poorly understood. Much attention has been given to cholesterol, but growing evidence suggests that other lipids, such as phospholipids, might play an important role in this disorder. In this review, we will summarize the evidence linking phospholipase D, a phosphatidic acid-synthesizing enzyme, to multiple aspects of normal brain function and to Alzheimer's disease. The role of phospholipase D in signaling mechanisms downstream of beta-amyloid as well as in the trafficking and processing of amyloid precursor protein will be emphasized.

Lipin 1 represses NFATc4 transcriptional activity in adipocytes to inhibit secretion of inflammatory factors

Lipin 1 is a bifunctional protein that regulates gene transcription and, as a Mg(2+)-dependent phosphatidic acid phosphatase (PAP), is a key enzyme in the biosynthesis of phospholipids and triacylglycerol. We describe here the functional interaction between lipin 1 and the nuclear factor of activated T cells c4 (NFATc4). Lipin 1 represses NFATc4 transcriptional activity through protein-protein interaction, and lipin 1 is present at the promoters of NFATc4 transcriptional targets in vivo. Catalytically active and inactive lipin 1 can suppress NFATc4 transcriptional activity, and this suppression may involve recruitment of histone deacetylases to target promoters. In fat pads from mice deficient for lipin 1 (fld mice) and in 3T3-L1 adipocytes depleted of lipin 1 there is increased expression of several NFAT target genes including tumor necrosis factor alpha, resistin, FABP4, and PPARgamma. Finally, both lipin 1 protein and total PAP activity are decreased with increasing adiposity in the visceral, but not subcutaneous, fat pads of ob/ob mice. These observations place lipin 1 as a potentially important link between triacylglycerol synthesis and adipose tissue inflammation.

Triacylglycerol homeostasis: insights from yeast

The endemic increase in lipid-associated disorders such as obesity and type 2 diabetes mellitus has placed triacylglycerol metabolism and its associated organelle, lipid droplets, in the spotlight of biomedical research. Key enzymes of triacylglycerol metabolism are structurally and functionally conserved between yeast and mammalian cells, and studies in yeast have contributed significantly to the understanding of their biological function(s). Based on these similarities, studies performed in yeast may provide further significant mechanistic insight into the molecular basis of triacylglycerol homeostasis and its important physiological roles in healthy and diseased cells.

Insulin-stimulated interaction with 14-3-3 promotes cytoplasmic localization of lipin-1 in adipocytes

Lipin-1 is a bifunctional protein involved in lipid metabolism and adipogenesis. Lipin-1 plays a role in the biosynthesis of triacylglycerol through its phosphatidate phosphatase activity and also acts as a transcriptional co-activator of genes involved in oxidative metabolism. Lipin-1 resides in the cytoplasm and translocates to the endoplasmic reticulum membrane to catalyze the phosphatidate phosphatase reaction. It also possesses a nuclear localization signal, which is required for its translocation to the nucleus and may therefore be important for lipin-1 co-activator function. Thus, subcellular localization may be an important factor in the regulation of this protein. Here, we show that the nuclear localization signal alone is not sufficient for lipin-1 nuclear localization, and identify lipin-1 interaction with 14-3-3 as a determinant of its subcellular localization. We demonstrate that lipin-1 interacts with 14-3-3 proteins and that overexpression of 14-3-3 promotes the cytoplasmic localization of lipin-1 in 3T3-L1 adipocytes. The effect of 14-3-3 is mediated through a serine-rich domain in lipin-1. Functional mapping of the 14-3-3-interacting region within the serine-rich domain indicates redundancy and cooperativity among several sites, including five phosphorylated serine and threonine residues. Insulin stimulation of 3T3-L1 adipocytes results in increased lipin-1 phosphorylation, enhanced interaction with 14-3-3, and predominantly cytoplasmic localization. In summary, our studies suggest that insulin may modulate the cellular function of lipin-1 by regulating its subcellular localization through interactions with 14-3-3 proteins.