Cross-species analyses implicate Lipin 1 involvement in human glucose metabolism

Therapeutic potential of lipin inhibitors for the treatment of cancer

Lipins are phosphatidic acid phosphatases (PAP) that catalyze the conversion of phosphatidic acid (PA) to diacylglycerol (DAG). Three lipin isoforms have been identified: lipin-1, -2 and -3. In addition to their PAP activity, lipin-1 and -2 act as transcriptional coactivators and corepressors. Lipins have been intensely studied for their role in regulation of lipid metabolism and adipogenesis; however, lipins are hypothesized to mediate several pathologies, such as those involving metabolic diseases, neuropathy and even cognitive impairment. Recently, an emerging role for lipins have been proposed in cancer. The study of lipins in cancer has been hampered by lack of inhibitors that have selectivity for lipins, that differentiate between lipin family members, or that are suitable for in vivo studies. Such inhibitors have the potential to be extremely useful as both molecular tools and therapeutics. This review describes the expression and function of lipins in various tissues and their roles in several diseases, but with an emphasis on their possible role in cancer. The mechanisms by which lipins mediate cancer cell growth are discussed and the potential usefulness of selective lipin inhibitors is hypothesized. Finally, recent studies reporting the crystallization of lipin-1 are discussed to facilitate rational design of novel lipin inhibitors.

Association of three single nucleotide polymorphisms in the LPIN1 gene with milk production traits in cows of the Yaroslavl breed

Lipin-1 is a member of the evolutionarily conserved family of proteins and is expressed predominantly in adipose tissue and skeletal muscle. On the one hand, lipin-1 is an enzyme that catalyzes the dephosphorylation of phosphatidic acid to diacylglycerol (DAG) and thus participates in the metabolic pathways of biosynthesis of storage lipids in the cell, membrane phospholipids, and intracellular signaling molecules. On the other hand, lipin-1 is able to be transported from the cytoplasm to the nucleus and is a coactivator of lipid metabolism gene transcription. It was shown, using the analysis of single nucleotide polymorphism (SNP) associations, that the lipin-1 coding gene (LPIN1) is a promising candidate gene for milk production traits in Holstein and Brown Swiss cows. However, it is unclear how much of its effect depends on the breed. The Yaroslavl dairy cattle breed was created in the 18-19 centuries in Russia by breeding northern Great Russian cattle, which were short and poor productive, but well adapted to local climatic conditions and bad food base. It was shown by whole genome genotyping and sequencing that the Yaroslavl breed has unique genetics compared to Russian and other cattle breeds. The aim of the study was to assess the frequency of alleles and genotypes of three SNPs in the LPIN1 gene and to study the association of these SNPs with milk production traits in Yaroslavl cows. Blood samples from 142 cows of the Yaroslavl breed were obtained from two farms in the Yaroslavl region. Genotyping of SNPs was carried out by polymerase chain reaction-restriction fragment length polymorphism method. Associations of SNPs with 305-day milk yield, fat yield, fat percentages, protein yield, and protein percentages were studied from the first to the fourth lactation. Statistical tests were carried out using a mixed linear model, taking into account the relationship between individuals. We identified three SNPs - rs110871255, rs207681322 and rs109039955 with a frequency of a rare allele of 0.042-0.261 in Yaroslavl cows. SNP rs110871255 was associated with fat yield during the third and fourth lactations. SNP rs207681322 was associated with milk yield for the second, third and fourth lactations, as well as protein yield for the third lactation. Thus, we identified significant associations of SNPs rs207681322 and rs110871255 in the LPIN1 gene with a number of milk production traits during several lactations in Yaroslavl cows.

Role of lipins in cardiovascular diseases

Lipin family members in mammals include lipins 1, 2, and 3. Lipin family proteins play a crucial role in lipid metabolism due to their bifunctionality as both transcriptional coregulators and phosphatidate phosphatase (PAP) enzymes. In this review, we discuss the structural features, expression patterns, and pathophysiologic functions of lipins, emphasizing their direct as well as indirect roles in cardiovascular diseases (CVDs). Elucidating the regulation of lipins facilitates a deeper understanding of the roles of lipins in the processes underlying CVDs. The activity of lipins is modulated at various levels, e.g., in the form of the transcription of genes, post-translational modifications, and subcellular protein localization. Because lipin characteristics are undergoing progressive clarification, further research is necessitated to then actuate the investigation of lipins as viable therapeutic targets in CVDs.

Adipocyte lipin 1 is positively associated with metabolic health in humans and regulates systemic metabolism in mice

Dysfunctional adipose tissue is believed to promote the development of hepatic steatosis and systemic insulin resistance, but many of the mechanisms involved are still unclear. Lipin 1 catalyzes the conversion of phosphatidic acid to diacylglycerol (DAG), the penultimate step of triglyceride synthesis, which is essential for lipid storage. Herein we found that adipose tissue LPIN1 expression is decreased in people with obesity compared to lean subjects and low LPIN1 expression correlated with multi-tissue insulin resistance and increased rates of hepatic de novo lipogenesis. Comprehensive metabolic and multi-omic phenotyping demonstrated that adipocyte-specific Lpin1-/- mice had a metabolically-unhealthy phenotype, including liver and skeletal muscle insulin resistance, hepatic steatosis, increased hepatic de novo lipogenesis, and transcriptomic signatures of nonalcoholic steatohepatitis that was exacerbated by high-fat diets. We conclude that adipocyte lipin 1-mediated lipid storage is vital for preserving adipose tissue and systemic metabolic health and its loss predisposes mice to nonalcoholic steatohepatitis.

Recent advances in our understanding of genetic rhabdomyolysis

PURPOSE OF REVIEW

This review summarizes recent advances in our understanding of the genetics of rhabdomyolysis.

RECENT FINDINGS

Rhabdomyolysis is the acute breakdown of myofibres resulting in systemic changes that can be life-threatening. Environmental triggers, including trauma, exercise, toxins and infections, and/or gene defects can precipitate rhabdomyolysis. A schema (aptly titled RHABDO) has been suggested for evaluating whether a patient with rhabdomyolysis is likely to harbour an underlying genetic defect. It is becoming increasingly recognized that defects in muscular dystrophy and myopathy genes can trigger rhabdomyolysis, even as the sole or presenting feature. Variants in genes not previously associated with human disease have been identified recently as causative of rhabdomyolysis, MLIP , MYH1 and OBSCN . Our understanding of the pathomechanisms contributing to rhabdomyolysis have also improved with an increased awareness of the role of mitochondrial dysfunction in LPIN1 , FDX2 , ISCU and TANGO2 -mediated disease.

SUMMARY

An accurate genetic diagnosis is important for optimal clinical management of the patient, avoiding associated triggers and genetic counselling and cascade screening. Despite recent advances in our understanding of the genetics contributing to rhabdomyolysis, many patients remain without an accurate genetic diagnosis, suggesting there are many more causative genes, variants and disease mechanisms to uncover.

Sequence Variation in the Bovine Lipin-1 Gene (LPIN1) and Its Association with Milk Fat and Protein Contents in New Zealand Holstein-Friesian × Jersey (HF × J)-cross Dairy Cows

Lipin-1 is known to play a regulatory role in tissues that function in lipid metabolism. In dairy cows, the lipin-1 gene (LPIN1) is highly expressed in the mammary gland, but its function in milk production is less understood. In this study, we used PCR-single strand conformation polymorphism analysis to investigate sequence variation in three regions of bovine LPIN1 in New Zealand Holstein-Friesian × Jersey (HF × J)-cross dairy cows, including part of the 5' non-coding region, the region containing the LPIN1β-spliced exon, and the sixth coding exon that encodes the putative transcriptional activating domain of the protein. No variation was found in the LPIN1β-spliced exon, but two sequence variants containing one single nucleotide polymorphism (SNP) were identified in the 5' non-coding region and four sequence variants containing four non-synonymous SNPs were identified in the sixth coding exon. Among the three common variants of the sixth coding exon, variant C was found to be associated with an increase in milk fat percentage (presence 4.96 ± 0.034% vs. absence 4.81 ± 0.050%; p = 0.006) and milk protein percentage (presence 4.09 ± 0.017% vs. absence 3.99 ± 0.025%; p = 0.001), but no associations (p > 0.01) were detected for milk yield. These results suggest that variation in LPIN1 affect the synthesis of fat and proteins in milk and has potential as a gene-marker to improve milk production traits.

Genomic adaptations to cereal-based diets contribute to mitigate metabolic risk in some human populations of East Asian ancestry

Adoption of diets based on some cereals, especially on rice, signified an iconic change in nutritional habits for many Asian populations and a relevant challenge for their capability to maintain glucose homeostasis. Indeed, rice shows the highest carbohydrates content and glycemic index among the domesticated cereals and its usual ingestion represents a potential risk factor for developing insulin resistance and related metabolic diseases. Nevertheless, type 2 diabetes and obesity epidemiological patterns differ among Asian populations that rely on rice as a staple food, with higher diabetes prevalence and increased levels of central adiposity observed in people of South Asian ancestry rather than in East Asians. This may be at least partly due to the fact that populations from East Asian regions where wild rice or other cereals such as millet have been already consumed before their cultivation and/or were early domesticated have relied on these nutritional resources for a period long enough to have possibly evolved biological adaptations that counteract their detrimental side effects. To test such a hypothesis, we compared adaptive evolution of these populations with that of control groups from regions where the adoption of cereal-based diets occurred many thousand years later and which were identified from a genome-wide dataset including 2,379 individuals from 124 East Asian and South Asian populations. This revealed selective sweeps and polygenic adaptive mechanisms affecting functional pathways involved in fatty acids metabolism, cholesterol/triglycerides biosynthesis from carbohydrates, regulation of glucose homeostasis, and production of retinoic acid in Chinese Han and Tujia ethnic groups, as well as in people of Korean and Japanese ancestry. Accordingly, long-standing rice- and/or millet-based diets have possibly contributed to trigger the evolution of such biological adaptations, which might represent one of the factors that play a role in mitigating the metabolic risk of these East Asian populations.

Caenorhabditis elegans Lipin 1 moderates the lifespan-shortening effects of dietary glucose by maintaining ω-6 polyunsaturated fatty acids

Excessive glucose causes various diseases and decreases lifespan by altering metabolic processes, but underlying mechanisms remain incompletely understood. Here, we show that Lipin 1/LPIN-1, a phosphatidic acid phosphatase and a putative transcriptional coregulator, prevents life-shortening effects of dietary glucose on Caenorhabditis elegans. We found that depletion of lpin-1 decreased overall lipid levels, despite increasing the expression of genes that promote fat synthesis and desaturation, and downregulation of lipolysis. We then showed that knockdown of lpin-1 altered the composition of various fatty acids in the opposite direction of dietary glucose. In particular, the levels of two ω-6 polyunsaturated fatty acids (PUFAs), linoleic acid and arachidonic acid, were increased by knockdown of lpin-1 but decreased by glucose feeding. Importantly, these ω-6 PUFAs attenuated the short lifespan of glucose-fed lpin-1-inhibited animals. Thus, the production of ω-6 PUFAs is crucial for protecting animals from living very short under glucose-rich conditions.

Variation in the Lipin 1 Gene Is Associated with Birth Weight and Selected Carcass Traits in New Zealand Romney Sheep

Lipin 1 plays an important role in lipid metabolism. In this study; we searched for variation in the ovine lipin 1 gene (LPIN1) in three gene regions (a 5' non-coding region; a region containing an alternatively spliced exon in intron 4; and a region containing coding exon 6) using polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis. The greatest amount of alleles was found in coding exon 6; with five sequences being detected. The effect of variation in this exon was investigated in 242 New Zealand Romney lambs derived from 12 sire-lines. The presence of variant E₃ was associated with a decrease in birth weight (p = 0.005) and the proportion of leg yield (p = 0.045), but with an increase in hot carcass weight (p = 0.032) and the proportion of loin yield (p = 0.014). The presence of variant B₃ was associated with an increased pre-weaning growth rate (p = 0.041), whereas the presence of variant C₃ was associated with an increase in shoulder yield (p < 0.001). These results suggest that ovine LPIN1 variation may have value as a genetic marker for improving meat production and carcass traits.

RNA-Seq based transcriptome analysis during bovine viral diarrhoea virus (BVDV) infection

BACKGROUND

Bovine viral diarrhoea virus (BVDV) is the member of the genus Pestivirus within the Flaviviridae family and responsible for severe economic losses in the cattle industry. BVDV can employ 'infect-and-persist' strategy and 'hit-and-run' strategy to remain associated with hosts and thus contributes to BVDV circulation in cattle herds. BVDV have also evolved various strategies to evade the innate immunity of host. To further understand the mechanisms by which BVDV overcomes the host cell innate immune response and provide more clues for further understanding the BVDV-host interaction, in this descriptive study, we conducted a investigation of differentially expressed genes (DEGs) of the host during BVDV infection by RNA-Seq analysis.

RESULTS

Our analysis identified 1297, 1732, 3072, and 1877 DEGs in the comparison groups mock vs. MDBK cells infected with BVDV post 2 h (MBV2h), mock vs. MBV6h, mock vs. MBV12h, and mock vs. MBV24h, respectively. The reproducibility and repeatability of the results were validated by RT-qPCR. Enrichment analyses of GO annotations and KEGG pathways revealed the host DEGs that are potentially induced by BVDV infection and may participate in BVDV-host interactions. Protein-protein interaction (PPI) network analyses identified the potential interactions among the DEGs. Our findings suggested that BVDV infection induced the upregulation of genes involved in lipid metabolism. The expression of genes that have antiviral roles, including ISG15, Mx1, OSA1Y, were found to be downregulated and are thus potentially associated with the inhibition of host innate immune system during BVDV infection. The expression levels of F3, C1R, KNG1, CLU, C3, FB, SERPINA5, SERPINE1, C1S, F2RL2, and C2, which belong to the complement and coagulation signalling cascades, were downregulated during BVDV infection, which suggested that the complement system might play a crucial role during BVDV infection.

CONCLUSION

In this descriptive study, our findings revealed the changes in the host transcriptome expression profile during BVDV infection and suggested that BVDV-infection induced altering the host's metabolic network, the inhibition of the expression of antiviral proteins and genes within the complement system might be contributed to BVDV proliferation. The above findings provided unique insights for further studies on the mechanisms underlying BVDV-host interactions.

Phenotypic Modulation of Skeletal Muscle Fibers in LPIN1-Deficient Lipodystrophic ( fld) Mice

Lipin-1 ( Lpin1)-deficient lipodystrophic mice have scant and immature adipocytes and develop transient fatty liver early in life. Unlike normal mice, these mice cannot rely on stored triglycerides to generate adenosine triphosphate (ATP) from the β-oxidation of fatty acids during periods of fasting. To compensate, these mice store much higher amounts of glycogen in skeletal muscle and liver than wild-type mice in order to support energy needs during periods of fasting. Our studies demonstrated that there are phenotypic changes in skeletal muscle fibers that reflect an adaptation to this unique metabolic situation. The phenotype of skeletal muscle (soleus, gastrocnemius, plantaris, and extensor digitorum longus [EDL]) from Lpin1^-/- was evaluated using various methods including immunohistochemistry for myosin heavy chains (Myh) 1, 2, 2a, 2b, and 2x; enzyme histochemistry for myosin ATPase, cytochrome-c oxidase (COX), and succinyl dehydrogenase (SDH); periodic acid-Schiff; and transmission electron microscopy. Fiber-type changes in the soleus muscle of Lpin1^-/- mice were prominent and included decreased Myh1 expression with concomitant increases in Myh2 expression and myosin-ATPase activity; this change was associated with an increase in the presence of Myh1/2a or Myh1/2x hybrid fibers. Alterations in mitochondrial enzyme activity (COX and SDH) were apparent in the myofibers in the soleus, gastrocnemius, plantaris, and EDL muscles. Electron microscopy revealed increases in the subsarcolemmal mitochondrial mass in the muscles of Lpin1^-/- mice. These data demonstrate that lipin-1 deficiency results in phenotypic fiber-specific modulation of skeletal muscle necessary for compensatory fuel utilization adaptations in lipodystrophy.

Micro-ribonucleic acid-binding site variants of type 2 diabetes candidate loci predispose to gestational diabetes mellitus in Chinese Han women

AIMS/INTRODUCTION

Emerging evidence has suggested that the genetic background of gestational diabetes mellitus (GDM) was analogous to type 2 diabetes mellitus. In contrast to type 2 diabetes mellitus, the genetic studies for GDM were limited. Accordingly, the aim of the present study was to extensively explore the influence of micro-ribonucleic acid-binding single-nucleotide polymorphisms (SNPs) in type 2 diabetes mellitus candidate loci on GDM susceptibility in Chinese.

MATERIALS AND METHODS

A total of 839 GDM patients and 900 controls were enrolled. Six micro-ribonucleic acid-binding SNPs were selected from 30 type 2 diabetes mellitus susceptibility loci and genotyped using TaqMan allelic discrimination assays.

RESULTS

The minor allele of three SNPs, PAX4 rs712699 (OR 1.366, 95% confidence interval 1.021-1.828, P = 0.036), KCNB1 rs1051295 (OR 1.579, 95% confidence interval 1.172-2.128, P = 0.003) and MFN2 rs1042842 (OR 1.398, 95% confidence interval 1.050-1.862, P = 0.022) were identified to significantly confer higher a risk of GDM in the additive model. The association between rs1051295 and increased fasting plasma glucose (b = 0.006, P = 0.008), 3-h oral glucose tolerance test plasma glucose (b = 0.058, P = 0.025) and homeostatic model assessment of insulin resistance (b = 0.065, P = 0.017) was also shown. Rs1042842 was correlated with higher 3-h oral glucose tolerance test plasma glucose (b = 0.056, P = 0.028). However, no significant correlation between the other included SNPs (LPIN1 rs1050800, VPS26A rs1802295 and NLRP3 rs10802502) and GDM susceptibility were observed.

CONCLUSIONS

The present findings showed that micro-ribonucleic acid-binding SNPs in type 2 diabetes mellitus candidate loci were also associated with GDM susceptibility, which further highlighted the similar genetic basis underlying GDM and type 2 diabetes mellitus.

Albiflorin ameliorates obesity by inducing thermogenic genes via AMPK and PI3K/AKT in vivo and in vitro

OBJECTIVE

Brown adipose tissue (BAT) activation has been identified as a possible target to treat obesity and to protect against metabolic diseases by increasing energy consumption. We explored whether albiflorin (AF), a natural compound, could contribute to lowering the high risk of obesity with BAT and primary brown preadipocytes in vivo and in vitro.

MATERIALS/METHODS

Human adipose tissue-derived mesenchymal stem cells (hAMSCs) were cultured with adipogenic differentiation media with or without AF. Male C57BL/6J mice (n=5 per group) were fed a high-fat diet (HFD) for six weeks with or without AF. Brown preadipocytes from the interscapular BAT of mice were cultured with or without AF.

RESULTS

In white adipogenic differentiation of hAMSCs, AF treatment significantly reduced the formation of lipid droplets and the expression of adipogenesis-related genes. In HFD-induced obese C57BL/6J mice, AF treatment significantly reduced body weight gain as well as the weights of the white adipose tissue, liver and spleen. Furthermore, AF induced the expression of genes involved in thermogenic function in BAT. In primary brown adipocytes, AF effectively stimulated the expressions of thermogenic genes and markedly up-regulated the AMP-activated protein kinase (AMPK) signaling pathway. Pretreatment with phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 nullified the induction of the thermogenic genes by AF in primary brown adipocytes. Moreover, AF activated beige cell marker genes induced by the pharmacological activation of peroxisome proliferator-activated receptor γ in hAMSCs.

CONCLUSION

This study shows that AF prevents the development of obesity in hAMSCs and mice fed an HFD and that it is also capable of stimulating the differentiation of brown adipocytes through the modulation of thermogenic genes by AMPK and PI3K/AKT.

Arginine supplementation modulates pig plasma lipids, but not hepatic fatty acids, depending on dietary protein level with or without leucine

Background

In the present study, the effect of arginine and leucine supplementation, and dietary protein level, were investigated in commercial crossbred pigs to clarify their individual or combined impact on plasma metabolites, hepatic fatty acid composition and mRNA levels of lipid sensitive factors. The experiment was conducted on fifty-four entire male pigs (Duroc × Pietrain × Large White × Landrace crossbred) from 59 to 92 kg of live weight. Each pig was randomly assigned to one of six experimental treatments (n = 9). The treatments followed a 2 × 3 factorial arrangement, providing two levels of arginine supplementation (0 vs. 1%) and three levels of basal diet (normal protein diet, NPD; reduced protein diet, RPD; reduced protein diet with 2% of leucine, RPDL).

Results

Significant interactions between arginine supplementation and protein level were observed across plasma lipids. While dietary arginine increased total lipids, total cholesterol, HDL-cholesterol, LDL-cholesterol, VLDL-cholesterol and triacylglycerols in NPD, the inverse effect was observed in RPD. Overall, dietary treatments had a minor impact on hepatic fatty acid composition. RPD increased 18:1c9 fatty acid while the combination of leucine and RPD reduced 18:0 fatty acid. Arginine supplementation increased the gene expression of FABP1, which contributes for triacylglycerols synthesis without affecting hepatic fatty acids content. RPD, with or without leucine addition, upregulated the lipogenic gene CEBPA but downregulated the fat oxidation gene LPIN1.

Conclusions

Arginine supplementation was responsible for a modulated effect on plasma lipids, which is dependent on dietary protein level. It consistently increased lipaemia in NPD, while reducing the correspondent metabolites in RPD. In contrast, arginine had no major impact, neither on hepatic fatty acids content nor on fatty acid composition. Likewise, leucine supplementation of RPD, regardless the presence of arginine, promoted no changes on total fatty acids in the liver. Ultimately, arginine, leucine and dietary protein reduction seem to be unrelated with fatty liver development.

The pro-fibrotic role of dipeptidyl peptidase 4 in carbon tetrachloride-induced experimental liver injury

Liver fibrosis is a progressive pathological process involving inflammation and extracellular matrix deposition. Dipeptidyl peptidase 4 (DPP4), also known as CD26, is a cell surface glycoprotein and serine protease. DPP4 binds to fibronectin, can inactivate specific chemokines, incretin hormone and neuropeptides, and influences cell adhesion and migration. Such properties suggest a pro-fibrotic role for this peptidase but this hypothesis needs in vivo examination. Experimental liver injury was induced with carbon tetrachloride (CCl₄) in DPP4 gene knockout (gko) mice. DPP4 gko had less liver fibrosis and inflammation and fewer B cell clusters than wild type mice in the fibrosis model. DPP4 inhibitor-treated mice also developed less liver fibrosis. DNA microarray and PCR showed that many immunoglobulin (Ig) genes and some metabolism-associated transcripts were differentially expressed in the gko strain compared with wild type. CCl₄-treated DPP4 gko livers had more IgM⁺ and IgG⁺ intrahepatic lymphocytes, and fewer CD4⁺, IgD⁺ and CD21⁺ intrahepatic lymphocytes. These data suggest that DPP4 is pro-fibrotic in CCl₄-induced liver fibrosis and that the mechanisms of DPP4 pro-fibrotic action include energy metabolism, B cells, NK cells and CD4⁺ cells.

Three indel variants in chicken LPIN1 exon 6/flanking region are associated with performance and carcass traits

LPIN1 is a Mg(2+)-dependent phosphatidic acid phosphatase. Variation in chicken LPIN1 exon 6 and its flanking regions were identified and three indel variants in 6 breeds and their associations with performance traits were studied. Seven variants were detected from 6 breeds, which contained a synonymous tri-allelic variant (c.924A/T/C) and three indels. The exon 6 variants detected from chicken breeds were conserved among bird species. The indel variation frequency presented clear differences among breeds. Two coding indels (c.1014-1018del3 and c.1125-1138del12) were multiples of three nucleotides and maintained the open reading frames of LPIN1 proteins. However, they were predicted to result in the clear change of the RNA secondary structure of chicken LPIN1 exon 6 and LPIN1 protein conformation. The association analysis showed that c.871-15-22del6 variation had a significant effect on body weight at hatch (BW0) and 2 weeks (BW2); c. 1014-1018del3 variation had a significant effect on BW4, BW6, caecum length and gizzard weight (GW) traits; c.1125-1138del12 variation had a significant effect on BW12, shank length at 4 weeks (SL4), carcass weight, lactate dehydrogenase traits (LDH), glucose (GLU) and albumin (ALB) traits. The genotype combination for c.1014-1018del3 and c.1125-1138del12 also presented significant effects on SL4, SL8, GW, leg muscle weight, ALB, GLU and LDH. The study demonstrated that chicken LPIN1 has an important effect on body, carcass and organ weight, serum LDH, GLU and ALB level.

Redundant roles of the phosphatidate phosphatase family in triacylglycerol synthesis in human adipocytes

AIMS/HYPOTHESIS

In mammals, the evolutionary conserved family of Mg(2+)-dependent phosphatidate phosphatases (PAP1), involved in phospholipid and triacylglycerol synthesis, consists of lipin-1, lipin-2 and lipin-3. While mutations in the murine Lpin1 gene cause lipodystrophy and its knockdown in mouse 3T3-L1 cells impairs adipogenesis, deleterious mutations of human LPIN1 do not affect adipose tissue distribution. However, reduced LPIN1 and PAP1 activity has been described in participants with type 2 diabetes. We aimed to characterise the roles of all lipin family members in human adipose tissue and adipogenesis.

METHODS

The expression of the lipin family was analysed in adipose tissue in a cross-sectional study. Moreover, the effects of lipin small interfering RNA (siRNA)-mediated depletion on in vitro human adipogenesis were assessed.

RESULTS

Adipose tissue gene expression of the lipin family is altered in type 2 diabetes. Depletion of every lipin family member in a human Simpson-Golabi-Behmel syndrome (SGBS) pre-adipocyte cell line, alters expression levels of adipogenic transcription factors and lipid biosynthesis genes in early stages of differentiation. Lipin-1 knockdown alone causes a 95% depletion of PAP1 activity. Despite the reduced PAP1 activity and alterations in early adipogenesis, lipin-silenced cells differentiate and accumulate neutral lipids. Even combinatorial knockdown of lipins shows mild effects on triacylglycerol accumulation in mature adipocytes.

CONCLUSIONS/INTERPRETATION

Overall, our data support the hypothesis of alternative pathways for triacylglycerol synthesis in human adipocytes under conditions of repressed lipin expression. We propose that induction of alternative lipid phosphate phosphatases, along with the inhibition of lipid hydrolysis, contributes to the maintenance of triacylglycerol content to near normal levels.

Molecular analysis of LPIN1 in Jordanian patients with rhabdomyolysis

Recessive mutations in LPIN1, which encodes a phosphatidate phosphatase enzyme, are a frequent cause of severe rhabdomyolysis in childhood. Hence, we sequenced the 19 coding exons of the gene in eight patients with recurrent hereditary myoglobinuria from four unrelated families in Jordan. The long-term goal is to facilitate molecular genetic diagnosis without the need for invasive procedures such as muscle biopsies. Three different mutations were detected, including the novel missense mutation c.2395G>C (Gly799Arg), which was found in two families. The two other mutations, c.2174G>A (Arg725His) and c.1162C>T (Arg388X), have been previously identified, and were found to cosegregate with the disease phenotype in the other two families. Intriguingly, patients homozygous for Arg725His were also homozygous for the c.1828C>T (Pro610Ser) polymorphism, and were exercise-intolerant between myoglobinuria episodes. Notably, patients homozygous for Arg388X were also homozygous for the c.2250G>C silent variant (Gly750Gly). Taken together, the data provide family-based evidence linking hereditary myoglobinuria to pathogenic variations in the C-terminal lipin domain of the enzyme. This finding highlights the functional significance of this domain in the absence of structural information. This is the first analysis of LPIN1 in myoglobinuria patients of Jordanian origin, and the fourth such analysis worldwide.

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LPIN1 mutations were cataloged in families with hereditary myoglobinuria.

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A novel missense Gly799Arg mutation was identified.

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Arg725His, the only other known missense mutation, was confirmed to be pathogenic.

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Arg388X, a known nonsense mutation, was the most common among Arabic patients.

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Patients exercise-intolerant between myoglobinuria episodes have a second mutation.

Drosophila Lipin interacts with insulin and TOR signaling pathways in the control of growth and lipid metabolism

Lipin proteins have key functions in lipid metabolism, acting as both phosphatidate phosphatases (PAPs) and nuclear regulators of gene expression. We show that the insulin and TORC1 pathways independently control functions of Drosophila Lipin (dLipin). Reduced signaling through the insulin receptor strongly enhanced defects caused by dLipin deficiency in fat body development, whereas reduced signaling through TORC1 led to translocation of dLipin into the nucleus. Reduced expression of dLipin resulted in decreased signaling through the insulin-receptor-controlled PI3K-Akt pathway and increased hemolymph sugar levels. Consistent with this, downregulation of dLipin in fat body cell clones caused a strong growth defect. The PAP but not the nuclear activity of dLipin was required for normal insulin pathway activity. Reduction of other enzymes of the glycerol-3 phosphate pathway affected insulin pathway activity in a similar manner, suggesting an effect that is mediated by one or more metabolites associated with the pathway. Taken together, our data show that dLipin is subject to intricate control by the insulin and TORC1 pathways, and that the cellular status of dLipin impacts how fat body cells respond to signals relayed through the PI3K-Akt pathway.

The Transcriptional Effects of PCB118 and PCB153 on the Liver, Adipose Tissue, Muscle and Colon of Mice: Highlighting of Glut4 and Lipin1 as Main Target Genes for PCB Induced Metabolic Disorders

Epidemiological studies have associated environmental exposure to polychlorinated biphenyls (PCBs) with an increased risk of type 2 diabetes; however, little is known about the underlying mechanisms involved in the metabolic side-effects of PCB. Our study evaluated the transcriptional effects of a subchronic exposure (gavage at Day 0 and Day 15 with 10 or 100 μmol/Kg bw) to PCB118 (dioxin-like PCB), PCB153 (non-dioxin-like PCB), or an equimolar mixture of PCB118 and PCB153 on various tissues (liver, visceral adipose tissue, muscle, and colon) in mice. Our results showed that a short-term exposure to PCB118 and/or PCB153 enhanced circulating triglyceride levels but did not affect glycemia. Among the studied tissues, we did not observe any modification of the expression of inflammation-related genes, such as cytokines or chemokines. The main transcriptional effects were observed in visceral adipose and liver tissues. We found a downregulation of lipin1 and glut4 expression in these two target organs. In adipose tissue, we also showed a downregulation of Agpat2, Slc25a1, and Fasn. All of these genes are involved in lipid metabolism and insulin resistance. In muscles, we observed an induction of CnR1 and Foxo3 expression, which may be partly involved in PCB metabolic effects. In summary, our results suggest that lipin1 and glut4, notably in adipose tissue, are the main targeted genes in PCB-induced metabolic disorders, however, further studies are required to fully elucidate the mechanisms involved.

Phosphatidate phosphatase-1 is functionally conserved in lipid synthesis and storage from human to yeast

Phosphatidate phosphatase-1 (PAP1) enzymes (yeast Pah1p/Smp2p, mammalian lipin1-3) have a key role in lipid homeostasis by controlling the relative proportions of its substrate phosphatidate (PA) and its product diacylglycerol (DAG). Recent investigation shows that mammalian lipin-1 complements phenotypes exhibited by yeast pah1Δ mutant cells, which indicates the functions of PAP1 enzymes are evolutionarily conserved. The observation was confirmed after transformation of human LPIN1 into PAH1-defective yeast, which resulted in human LPIN1-induced accumulation of triacylglycerol (TAG )and lipid droplet formation. In double mutants lacking Tgl3p and Tgl4p, overexpression of PAH1 or LPIN1 induced TAG accumulation and excessive obesity. Furthermore, the obese yeast was used as a model to study the anti-obesity effects of PAP1 activity inhibitors, including propranolol and clenbuterol. The data showed that the inhibitors significantly suppressed TAG accumulation and lipid droplets formation. These findings demonstrate that LPIN1 plays a functional role in lipid synthesis and storage, a role which is highly conserved from human to yeast. Inhibition of TAG synthesis will become an efficacious treatment strategy for obesity and our excessive obesity model will provide a very useful tool for discovery of new anti-obesity drugs in the future.

Rubi Fructus (Rubus coreanus) activates the expression of thermogenic genes in vivo and in vitro

OBJECTIVE

To investigate the anti-obesity effect of Rubi Fructus (RF) extract using brown adipose tissue (BAT) and primary brown preadipocytes in vivo and in vitro.

METHODS

Male C57BL/6 J mice (n=5 per group) were fed a high-fat diet (HFD) for 10 weeks with or without RF. Brown preadipocytes from the interscapular BAT of mice (age, post-natal days 1-3) were cultured with differentiation media (DM) including isobutylmethylxanthine, dexamethasone, T3, indomethacin and insulin with or without RF.

RESULTS

In HFD-induced obese C57BL/6 J mice, long-term RF treatment significantly reduced weight gain as well as the weights of the white adipose tissue, liver and spleen. Serum levels of total cholesterol and low-density lipoprotein cholesterol were also reduced in the HFD group which received RF treatment. Furthermore, RF induced thermogenic-, adipogenic- and mitochondria-related gene expressions in BAT. In primary brown adipocytes, RF effectively stimulated the expressions of thermogenic- and mitochondria-related genes. In addition, to examine whether LIPIN1, a regulator of adipocyte differentiation, is regulated by RF, Lipin1 small interfering RNA (siRNA) and RF were pretreated in primary brown adipocytes. Pretreatment with Lipin1 siRNA and RF downregulated the DM-induced expression levels of thermogenic- and mitochondria-related genes. Moreover, RF markedly upregulated AMP-activated protein kinase. Our study shows that RF is capable of stimulating the differentiation of brown adipocytes through the modulation of thermogenic genes.

CONCLUSIONS

This study demonstrates that RF prevents the development of obesity in mice fed with a HFD and that it is also capable of stimulating the differentiation of brown adipocytes through the modulation of thermogenic genes, which suggests that RF has potential as a therapeutic application for the treatment or prevention of obesity.

Whole blood transcriptomics and urinary metabolomics to define adaptive biochemical pathways of high-intensity exercise in 50-60 year old masters athletes

Exercise is beneficial for a variety of age-related disorders. However, the molecular mechanisms mediating the beneficial adaptations to exercise in older adults are not well understood. The aim of the current study was to utilize a dual approach to characterize the genetic and metabolic adaptive pathways altered by exercise in veteran athletes and age-matched untrained individuals. Two groups of 50–60 year old males: competitive cyclists (athletes, n = 9; VO_2peak 59.1±5.2 ml·kg⁻¹·min⁻¹; peak aerobic power 383±39 W) and untrained, minimally active individuals (controls, n = 8; VO_2peak 35.9±9.7 ml·kg⁻¹·min⁻¹; peak aerobic power 230±57 W) were examined. All participants completed an acute bout of submaximal endurance exercise, and blood and urine samples pre- and post-exercise were analyzed for gene expression and metabolic changes utilizing genome-wide DNA microarray analysis and NMR spectroscopy-based metabolomics, respectively. Our results indicate distinct differences in gene and metabolite expression involving energy metabolism, lipids, insulin signaling and cardiovascular function between the two groups. These findings may lead to new insights into beneficial signaling pathways of healthy aging and help identify surrogate markers for monitoring exercise and training load.

Chemical modulation of glycerolipid signaling and metabolic pathways

Thirty years ago, glycerolipids captured the attention of biochemical researchers as novel cellular signaling entities. We now recognize that these biomolecules occupy signaling nodes critical to a number of physiological and pathological processes. Thus, glycerolipid-metabolizing enzymes present attractive targets for new therapies. A number of fields-ranging from neuroscience and cancer to diabetes and obesity-have elucidated the signaling properties of glycerolipids. The biochemical literature teems with newly emerging small molecule inhibitors capable of manipulating glycerolipid metabolism and signaling. This ever-expanding pool of chemical modulators appears daunting to those interested in exploiting glycerolipid-signaling pathways in their model system of choice. This review distills the current body of literature surrounding glycerolipid metabolism into a more approachable format, facilitating the application of small molecule inhibitors to novel systems. This article is part of a Special Issue entitled Tools to study lipid functions.

Adiposopathy: how do diet, exercise and weight loss drug therapies improve metabolic disease in overweight patients?

An increase in bodyweight is generally associated with an increased risk of excessive fat-related metabolic diseases (EFRMD), including Type 2 diabetes mellitus, hypertension and dyslipidemia. However, not all patients who are overweight have EFRMD, and not all patients with EFRMD are significantly overweight. The adipocentric paradigm provides the basis for a unifying, pathophysiological process whereby fat gain in susceptible patients leads to fat dysfunction ('sick fat'), and wherein pathological abnormalities in fat function (adiposopathy) are more directly related to the onset of EFRMD than increases in fat mass (adiposity) alone. But just as worsening fat function worsens EFRMD, improved fat function improves EFRMD. Peroxisome proliferator-activated receptor-gamma agonists increase the recruitment, proliferation and differentiation of preadipocytes ('healthy fat') and cause apoptosis of hypertrophic and dysfunctional (including visceral) adipocytes resulting in improved fat function and improved metabolic parameters associated with EFRMD. Weight loss interventions, such as a hypocaloric diet and physical exercise, in addition to agents such as orlistat, sibutramine and cannabinoid receptor antagonists, may have favorable effects upon fat storage (lipogenesis and fat distribution), nutrient metabolism (such as free fatty acids), favorable effects upon adipose tissue factors involved in metabolic processes and inflammation, and enhanced 'cross-talk' with other major organ systems. In some cases, weight loss therapeutic agents may even affect metabolic parameters and adipocyte function independently of weight loss alone, suggesting that the benefit of these agents in improving EFRMD may go beyond their efficacy in weight reduction. This review describes how adiposopathy interventions may affect fat function, and thus improve EFRMD.

The role of genetic variants in CYP2C8, LPIN1, PPARGC1A and PPARγ on the trough steady-state plasma concentrations of rosiglitazone and on glycosylated haemoglobin A1c in type 2 diabetes

OBJECTIVE

The aim of this study was to examine the effect of single nucleotide polymorphisms in CYP2C8, LPIN1, PPARGC1A and PPARγ on rosiglitazone's (i) trough steady-state plasma concentration (C(ss,min)), (ii) on glycosylated haemoglobin A1c (HbA1c) and (iii) the risk of developing adverse events, mainly oedema, in patients with type 2 diabetes mellitus (T2D).

METHODS

The data used in this study were obtained from the South Danish Diabetes Study including 371 T2D patients with a focus on the 187 patients who were treated with rosiglitazone. The study was a placebo-controlled, partly blinded and multicentre clinical trial. The C(ss,min) of rosiglitazone and HbA1c was determined and the genotype of the patients was identified.

RESULTS

The mean C(ss,min) of rosiglitazone was 21.3 ng/ml (95% confidence interval 18.8; 24.2 ng/ml), with observations ranging from 1 to 296 ng/ml. Carriers of CYP2C8*3 (n=32) (rs10509681 and rs11572080) had a statistically significantly lower mean C(ss,min) than wild types (n=106), and they also had a statistically significantly lower mean absolute difference in HbA1c during rosiglitazone treatment. Finally, the carriers of CYP2C8*3 had a lower odds ratio of developing oedema.

CONCLUSION

We showed that CYP2C8*3 was associated with lower plasma levels of rosiglitazone and hence a reduced therapeutic response but also a lower risk of developing oedema during treatment with rosiglitazone. Individualized treatment with rosiglitazone on the basis of the CYP2C8 genotype may therefore be possible.

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.

Lipins, lipinopathies, and the modulation of cellular lipid storage and signaling

Members of the lipin protein family are phosphatidate phosphatase (PAP) enzymes, which catalyze the dephosphorylation of phosphatidic acid to diacylglycerol, the penultimate step in TAG synthesis. Lipins are unique among the glycerolipid biosynthetic enzymes in that they also promote fatty acid oxidation through their activity as co-regulators of gene expression by DNA-bound transcription factors. Lipin function has been evolutionarily conserved from a single ortholog in yeast to the mammalian family of three lipin proteins-lipin-1, lipin-2, and lipin-3. In mice and humans, the levels of lipin activity are a determinant of TAG storage in diverse cell types, and humans with deficiency in lipin-1 or lipin-2 have severe metabolic diseases. Recent work has highlighted the complex physiological interactions between members of the lipin protein family, which exhibit both overlapping and unique functions in specific tissues. The analysis of "lipinopathies" in mouse models and in humans has revealed an important role for lipin activity in the regulation of lipid intermediates (phosphatidate and diacylglycerol), which influence fundamental cellular processes including adipocyte and nerve cell differentiation, adipocyte lipolysis, and hepatic insulin signaling. The elucidation of lipin molecular and physiological functions could lead to novel approaches to modulate cellular lipid storage and metabolic disease.

Endoplasmic reticulum stress suppresses lipin-1 expression in 3T3-L1 adipocytes

Lipin-1 plays crucial roles in the regulation of lipid metabolism and cell differentiation in adipocytes. In obesity, adipose lipin-1 mRNA expression is decreased and positively correlated with systemic insulin sensitivity. Amelioration of the lipin-1 depletion might be improved dysmetabolism. Although some cytokines such as TNF-α and interleukin-1β reduces adipose lipin-1 expression, the mechanism of decreased adipose lipin-1 expression in obesity remains unclear. Recently, endoplasmic reticulum (ER) stress is implicated in the pathogenesis of obesity. Here we investigated the role of ER stress on the lipin-1 expression in 3T3-L1 adipocytes. We demonstrated that lipin-1 expression was suppressed by the treatment with ER stress inducers (tunicamycin and thapsigargin) at transcriptional level. We also showed that constitutive lipin-1 expression could be maintained by peroxisome proliferator-activated receptor-γ in 3T3-L1 adipocytes. Activation of peroxisome proliferator-activated receptor-γ recovered the ER stress-induced lipin-1 suppression. These results suggested that ER stress might be involved in the pathogenesis of obesity through lipin-1 depletion.

[Research and development of Lipin family.]

Lipin family including at least three members Lipin 1, Lipin 2, and Lipin 3 is a critical regulatory enzyme identified recently, which plays dual roles in lipid metabolisms. Lipin family has physiological effects not only on regulating lipid metabolism, but also on maintaining normal peripheral nervous functions, liver lipoprotein secretion, cell morphous, reproductive functions, and energy homeostasis. Since mutations in Lipin gene express may be associated with AIDS, insulin resistance, obesity, diabetes mellitus, and the other diseases of metabolic syndrome, Lipin may be a new useful target in treatment of above-mentioned clinical-related diseases. In this article, we focused on discovery, construction features, expression, regulatory mechanism, and biological functions of Lipin, as well as its correlation research with clinical-related diseases.

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.

Yeast lipin 1 orthologue pah1p regulates vacuole homeostasis and membrane fusion

Vacuole homotypic fusion requires a group of regulatory lipids that includes diacylglycerol, a fusogenic lipid that is produced through multiple metabolic pathways including the dephosphorylation of phosphatidic acid (PA). Here we examined the relationship between membrane fusion and PA phosphatase activity. Pah1p is the single yeast homologue of the Lipin family of PA phosphatases. Deletion of PAH1 was sufficient to cause marked vacuole fragmentation and abolish vacuole fusion. The function of Pah1p solely depended on its phosphatase activity as complementation studies showed that wild type Pah1p restored fusion, whereas the phosphatase dead mutant Pah1p(D398E) had no effect. We discovered that the lack of PA phosphatase activity blocked fusion by inhibiting the binding of SNAREs to Sec18p, an N-ethylmaleimide-sensitive factor homologue responsible for priming inactive cis-SNARE complexes. In addition, pah1Δ vacuoles were devoid of the late endosome/vacuolar Rab Ypt7p, the phosphatidylinositol 3-kinase Vps34p, and Vps39p, a subunit of the HOPS (homotypic fusion and vacuole protein sorting) tethering complex, all of which are required for vacuole fusion. The lack of Vps34p resulted in the absence of phosphatidylinositol 3-phosphate, a lipid required for SNARE activity and vacuole fusion. These findings demonstrate that Pah1p and PA phosphatase activity are critical for vacuole homeostasis and fusion.

Suppression of lipin-1 expression increases monocyte chemoattractant protein-1 expression in 3T3-L1 adipocytes

Lipin-1 plays a crucial role in the regulation of lipid metabolism and cell differentiation in adipocytes. Expression of adipose lipin-1 is reduced in obesity, and metabolic syndrome. However, the significance of this reduction remains unclear. This study investigated if and how reduced lipin-1 expression affected metabolism. We assessed mRNA expression levels of various genes related to adipocyte metabolism in lipin-1-depleted 3T3-L1 adipocytes by introducing its specific small interfering RNA. In lipin-1-depleted adipocytes, mRNA and protein expression levels of monocyte chemoattractant protein-1 (MCP-1) were significantly increased, although the other genes tested were not altered. The conditioned media from the cells promoted monocyte chemotaxis. The increase in MCP-1 expression was prevented by treatment with quinazoline or salicylate, inhibitors of nuclear factor-κB activation. Because MCP-1 is related to adipose inflammation and systemic insulin resistance, these results suggest that a reduction in adipose lipin-1 in obesity may exacerbate adipose inflammation and metabolism.

Expression of the splicing factor gene SFRS10 is reduced in human obesity and contributes to enhanced lipogenesis

Alternative mRNA splicing provides transcript diversity and may contribute to human disease. We demonstrate that expression of several genes regulating RNA processing is decreased in both liver and skeletal muscle of obese humans. We evaluated a representative splicing factor, SFRS10, downregulated in both obese human liver and muscle and in high-fat-fed mice, and determined metabolic impact of reduced expression. SFRS10-specific siRNA induces lipogenesis and lipid accumulation in hepatocytes. Moreover, Sfrs10 heterozygous mice have increased hepatic lipogenic gene expression, VLDL secretion, and plasma triglycerides. We demonstrate that LPIN1, a key regulator of lipid metabolism, is a splicing target of SFRS10; reduced SFRS10 favors the lipogenic β isoform of LPIN1. Importantly, LPIN1β-specific siRNA abolished lipogenic effects of decreased SFRS10 expression. Together, our results indicate that reduced expression of SFRS10, as observed in tissues from obese humans, alters LPIN1 splicing, induces lipogenesis, and therefore contributes to metabolic phenotypes associated with obesity.

Genetics of adipose tissue biology

Adipose tissue morphology and release of free fatty acids, as well as peptide hormones, are believed to contribute to obesity and related metabolic disorders. These adipose tissue phenotypes are influenced by adiposity, but there is also a strong hereditary impact. Polymorphisms in numerous adipose-expressed genes have been evaluated for association with adipocyte and clinical phenotypes. In our opinion, some results are convincing. Thus ADRB2 and GPR74 genes are associated with adipocyte lipolysis, GPR74 also with BMI; PPARG and SREBP1, which promote adipogenesis and lipid storage, are associated with T2D and possible adiposity; ADIPOQ and ARL15 are associated with circulating levels of adiponectin, ARL15 also with coronary heart disease. We anticipate that the use of complementary approaches such as expression profiling and RNAi screening, and studies of additional levels of gene regulation, that is, miRNA and epigenetics, will be important to unravel the genetics of adipose tissue function.

Dual function lipin proteins and glycerolipid metabolism

Lipin family proteins are emerging as crucial regulators of lipid metabolism. In triglyceride synthesis, lipins act as lipid phosphatase enzymes at the endoplasmic reticular membrane, catalyzing the dephosphorylation of phosphatidic acid to form diacylglycerol, which is the penultimate step in this process. However, lipin proteins are not integral membrane proteins, and can rapidly translocate within the cell. In fact, emerging evidence suggests that lipins also play crucial roles in the nucleus as transcriptional regulatory proteins. Thus, lipins are poised to regulate cellular lipid metabolism at multiple regulatory nodal points. This review summarizes the history of lipin proteins, and discusses the current state of our understanding of lipin biology.

Lipins: multifunctional lipid metabolism proteins

The lipin proteins are evolutionarily conserved proteins with roles in lipid metabolism and disease. There are three lipin protein family members in mammals and one or two orthologs in plants, invertebrates, and single-celled eukaryotes. Studies in yeast and mouse led to the identification of two distinct molecular functions of lipin proteins. Lipin proteins have phosphatidate phosphatase activity and catalyze the formation of diacylglycerol in the glycerol-3-phosphate pathway, implicating them in the regulation of triglyceride and phospholipid biosynthesis. Mammalian lipin proteins also possess transcriptional coactivator activity and have been implicated in the regulation of metabolic gene expression. Here we review key findings in the field that demonstrate roles for lipin family members in metabolic homeostasis and in rare human diseases, and we examine evidence implicating genetic variations in lipin genes in common metabolic dysregulation such as obesity, hyperinsulinemia, hypertension, and type 2 diabetes.

Lpin1 in human visceral and subcutaneous adipose tissue: similar levels but different associations with lipogenic and lipolytic genes

LPIN1 is a gene with important effects on lipidic and metabolic homeostasis. Human subcutaneous LPIN1 expression levels in adipose tissue are related with a better metabolic profile, including insulin sensitivity markers. However, there are few data on the regulation of LPIN1 in visceral adipose tissue (VAT). Our aim was to perform a cross-sectional analysis of VAT compared with subcutaneous (SAT) LPIN1 expression in a well-characterized obese cohort, its relation with the expression of genes involved in lipid metabolism, and the in vitro response to lipogenic and lipolytic stimuli. A downregulation of total LPIN1 mRNA expression in subjects with obesity was found in VAT similarly to that in SAT. Despite similar total LPIN1 mRNA levels in SAT and VAT, a close relationship with clinical parameters and with many lipogenic and lipolytic genes was observed primarily in SAT depot. As shown in the in vitro analysis, the low-grade proinflammatory environment and the insulin resistance associated with obesity may contribute to downregulate LPIN1 in adipose tissue, leading to a worse metabolic profile.

Studies of association between LPIN1 variants and common metabolic phenotypes among 17,538 Danes

OBJECTIVE

Lipin-1, encoded by LPIN1, is expressed in the major metabolically active tissues. Decreased expression of lipin-1 in adipose tissue correlates with increased insulin resistance, and tagging of the LPIN1 locus has shown that rs33997857, rs6744682, and rs6708316 associate with metabolic phenotypes, specifically body mass index (BMI) and fasting serum lipid levels, both on the individual single-nucleotide polymorphism level and with a three-marker haplotype. Our aim was to validate the reported findings in the Danish population.

DESIGN

In the present study, variants were analyzed in LPIN1 using case-control studies, haplotype analyses, and quantitative trait analyses in a population of 17,538 Danes.

METHODS

The three LPIN1 variants were genotyped in 17,538 Danes from four study populations of middle-aged people. This provided us with a statistical power >99% to replicate previous findings. Variants were analyzed individually and in haplotype combinations in studies of quantitative metabolic traits and in case-control studies.

RESULTS

None of the three variants were associated with the examined quantitative traits including BMI, waist circumference, blood pressure, fasting serum lipid concentrations, or plasma glucose or serum insulin concentrations in the fasting state and following an oral glucose tolerance test. Haplotypes were tested for association with quantitative traits; however, only nominal association with blood pressure (P=0.04) and waist circumference (P=0.04) was observed. In case-control studies, no association was found for individual variants or the three-marker haplotype.

CONCLUSION

LPIN1 rs33997857, rs6744682, and rs6708316 did not associate with type 2 diabetes, obesity, or related quantitative metabolic phenotypes in the Danish population examined.

The mRNA of lipin1 and its isoforms are differently expressed in the longissimus dorsi muscle of obese and lean pigs

Lipin1 has been documented to play an important role in adipogenesis. In the present study, the mRNA expression level of lipin1 and its isoforms in longissimus dorsi muscle were determined by semi-quantification RT-PCR in lean PIC and obese Rongchang pigs. Further, we determined mRNA expression for lipin1 and its two isoforms in Rongchang obese pigs which had either a high or low intramuscular fat content. We demonstrate for the first time that porcine lipin1 has two alternative forms, lipin-α and lipin-β. Unlike mice and humans where the lipin-β has 99 more nucleotides than lipin-α, we found that in swine, lipin-β has 108 more nucleotides than lipin-α. Our results indicate that the longissimus dorsi muscle of Rongchang obese pigs have a higher level of mRNA expression for lipin1 and its isoforms than PIC lean pigs. Furthermore, Rongchang pigs with higher intramuscular fat content had a higher lipin1 and lipin-β mRNA expression in longissimus dorsisi muscle than Rongchang pigs with lower intramuscular fat content (P<0.05), whereas no difference was seen in lipin-α mRNA expression between Rongchang pigs with high or low intramuscular fat. The ratio of lipin-β mRNA to lipin-α mRNA was also significantly different between Rongchang pigs distinguished by a high intramuscular fat content compared with those with low intramuscular fat (P<0.05). These data suggested that the lipin1 gene may have a crucial effect on body lipid accumulation in pigs, whereas the lipin-β isoform may play an important role in intramuscular fat deposition in obese pigs.

The associations of LPIN1 gene expression in adipose tissue with metabolic phenotypes in the Chinese population

The LPIN1 gene, encoding lipin-1 protein, plays critical roles in adipocyte differentiation and lipid metabolism. This study aimed to analyze the association of LPIN1 mRNA levels in human adipose tissue with metabolic phenotypes. We also examined the association of LPIN1 genetic variation with type 2 diabetes and related metabolic phenotypes in the Chinese population. The relative LPIN1 mRNA levels were measured in abdominal visceral (VAT) and subcutaneous adipose tissue (SAT) obtained from 102 nondiabetic Chinese females. Seven single-nucleotide polymorphisms (SNPs) spanning from the 5'-upstream region to the 3'-end of the LPIN1 gene were genotyped in 1,520 Chinese (760 type 2 diabetic cases and 760 controls). LPIN1 mRNA levels in VAT were negatively correlated with BMI (r = -0.21, P = 0.03), body fat percentage (r = -0.22, P = 0.02), plasma triglycerides levels (r = -0.21, P = 0.03), and plasma leptin levels (r = -0.63, P = 0.0002). LPIN1 mRNA levels were positively correlated with PPARG and ADIPOQ mRNA levels in both VAT and SAT. No single SNP of the LPIN1 gene was associated with type 2 diabetes in our population. One rare haplotype showed a significant association with type 2 diabetes (odds ratio (OR), 4.35; 95% confidence interval, 1.86-11.75; P = 4 x 10(-4)). No SNP or haplotype of the LPIN1 gene was associated with quantitative metabolic traits in the nondiabetic subjects. The results confirmed the association of LPIN1 gene expression in adipose tissue with lower adiposity and favorable metabolic profiles in the Chinese population. However, the LPIN1 gene seemed not to be a major susceptibility gene for type 2 diabetes or related metabolic phenotypes in the Chinese population.

Concurrent Lpin1 and Nrcam mouse mutations result in severe peripheral neuropathy with transitory hindlimb paralysis

Peripheral neuropathy is a broad category of disorders with a diverse etiology, grouped together by their common pathogenic effect on the peripheral nervous system (PNS). Because of the heterogeneity observed to be responsible for these disorders, a forward genetics method of gene discovery was used to identify additional affected pathways. In this report, we describe the mutant mouse line 20884, generated by N-ethyl-N-nitrosourea mutagenesis, which is characterized by adult-onset transitory hindlimb paralysis. Linkage mapping revealed that two point mutations are responsible for the phenotype: a partial loss-of-function mutation in the gene for phosphatidate phosphatase Lpin1 and a truncation mutation in the gene that encodes the neuronal cell adhesion molecule NrCAM. To investigate how the 20884 Lpin1 and Nrcam mutations interact to produce the paralysis phenotype, the double mutant and both single mutants were analyzed by quantitative behavioral, histological, and electrophysiological means. The Lpin1(20884) mutant and the double mutant are characterized by similar levels of demyelination and aberrant myelin structures. Nevertheless, the double mutant exhibits more severe electrophysiological abnormalities than the Lpin1(20884) mutant. The Nrcam(20884) mutant is characterized by normal sciatic nerve morphology and a mild electrophysiological defect. Comparison of the double mutant phenotype with the two single mutants does not point to an additive relationship between the two defects; rather, the Lpin1(20884) and Nrcam(20884) defects appear to act synergistically to produce the 20884 phenotype. It is proposed that the absence of NrCAM in a demyelinating environment has a deleterious effect, possibly by impairing the process of remyelination.

Biochemistry, physiology, and genetics of GPAT, AGPAT, and lipin enzymes in triglyceride synthesis

Triacylglycerol (TAG) synthesis and storage in tissues such as adipose tissue and liver have important roles in metabolic homeostasis. The molecular identification of genes encoding enzymes that catalyze steps in TAG biosynthesis from glycerol 3-phosphate has revealed an unexpected number of protein isoforms of the glycerol phosphate acyltransferase (GPAT), acylglycerolphosphate acyltransferase (AGPAT), and lipin (phosphatidate phosphatase) families that appear to catalyze similar biochemical reactions. However, on the basis of available data for a few members in which genetic deficiencies in mouse and/or human have been studied, we postulate that each GPAT, AGPAT, and lipin family member likely has a specialized role that may be uncovered through careful biochemical and physiological analyses.

The lipin family: mutations and metabolism

PURPOSE OF REVIEW

The family of three lipin proteins act as phosphatidate phosphatase (PAP) enzymes required for glycerolipid biosynthesis, and also as transcriptional coactivators that regulate expression of lipid metabolism genes. The genes for lipin-1, lipin-2 and lipin-3 are expressed in key metabolic tissues, including adipose tissue, skeletal muscle and liver, but the physiological functions of each member of the family have not been fully elucidated. Here we examine the most recent studies that provide information about the roles of lipin proteins in metabolism and human disease.

RECENT FINDINGS

Recent studies have identified mutations that cause lipin-1 or lipin-2 deficiency in humans, leading to acute myoglobinuria in childhood or the inflammatory disorder Majeed syndrome, respectively. The effects of lipin-1 deficiency appear to include both the loss of glycerolipid building blocks and the accumulation of lipid intermediates that disrupt cellular function. Several studies have demonstrated that polymorphisms in the LPIN1 and LPIN2 genes are associated with metabolic disease traits, including insulin sensitivity, diabetes, blood pressure and response to thiazolidinedione drugs. Furthermore, lipin-1 expression levels in adipose tissue and/or liver are positively correlated with insulin sensitivity. Studies of lipin-1 in adipocytes have shed some light on its relationship with insulin sensitivity.

SUMMARY

Lipin-1 and lipin-2 are required for normal lipid homeostasis and have unique physiological roles. Future studies, for example using engineered mouse models, will be required to fully elucidate their specific roles in normal physiology and disease.