Characterization of the human LPIN1-encoded phosphatidate phosphatase isoforms

Architecture and function of yeast phosphatidate phosphatase Pah1 domains/regions

Phosphatidate (PA) phosphatase, which catalyzes the Mg2+-dependent dephosphorylation of PA to produce diacylglycerol, provides a direct precursor for the synthesis of the storage lipid triacylglycerol and the membrane phospholipids phosphatidylcholine and phosphatidylethanolamine. The enzyme controlling the key phospholipid PA also plays a crucial role in diverse aspects of lipid metabolism and cell physiology. PA phosphatase is a peripheral membrane enzyme that is composed of multiple domains/regions required for its catalytic function and subcellular localization. In this review, we discuss the domains/regions of PA phosphatase from the yeast Saccharomyces cerevisiae with reference to the homologous enzyme from mammalian cells.

Exploring the impact of estrogen-related receptor gamma on metabolism and disease

Estrogen-related receptor gamma (ERRγ) is a member of the ERR orphan nuclear receptor family which possesses three subtypes, α, β, and γ. ERRγ is reportedly predominantly expressed in metabolically active tissues and cells, which promotes positive and negative effects in different tissues. ERRγ overexpression in the liver, pancreas, and thyroid cells is related to liver cancer, oxidative stress, reactive oxygen species (ROS) regulation, and carcinoma. Reduced ERRγ expression in the brain, immune cells, tumor cells, and energy metabolism causes neurological dysfunction, gastric cancer, and obesity. ERRγ is a constitutive receptor; however, its transcriptional activity also depends on co-regulators, agonists, and antagonists, which, when after forming a complex, can play a role in targeting and treating diseases. Moreover, ERRγ has proven crucial in regulating cellular and metabolic activity. However, many functions mediated via ERRγ remain unknown and require further exploration. Hence, considering the importance of ERRγ, this review focuses on the critical findings and interactions between ERRγ and co-regulators, agonists, and antagonists alongside its relationship with downstream and upstream signaling pathways and diseases. This review highlights new findings and provides a path to understanding the current ideas and future studies on ERRγ-mediated cellular activity.

Protein kinase Hsl1 phosphorylates Pah1 to inhibit phosphatidate phosphatase activity and regulate lipid synthesis in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, Pah1 phosphatidate (PA) phosphatase, which catalyzes the Mg2+-dependent dephosphorylation of PA to produce diacylglycerol, plays a key role in utilizing PA for the synthesis of the neutral lipid triacylglycerol and thereby controlling the PA-derived membrane phospholipids. The enzyme function is controlled by its subcellular location as regulated by phosphorylation and dephosphorylation. Pah1 is initially inactivated in the cytosol through phosphorylation by multiple protein kinases and then activated via its recruitment and dephosphorylation by the protein phosphatase Nem1-Spo7 at the nuclear/endoplasmic reticulum membrane where the PA phosphatase reaction occurs. Many of the protein kinases that phosphorylate Pah1 have yet to be characterized with the identification of the target residues. Here, we established Pah1 as a bona fide substrate of septin-associated Hsl1, a protein kinase involved in mitotic morphogenesis checkpoint signaling. The Hsl1 activity on Pah1 was dependent on reaction time and the amounts of protein kinase, Pah1, and ATP. The Hsl1 phosphorylation of Pah1 occurred on Ser-748 and Ser-773, and the phosphorylated protein exhibited a 5-fold reduction in PA phosphatase catalytic efficiency. Analysis of cells expressing the S748A and S773A mutant forms of Pah1 indicated that Hsl1-mediated phosphorylation of Pah1 promotes membrane phospholipid synthesis at the expense of triacylglycerol, and ensures the dependence of Pah1 function on the Nem1-Spo7 protein phosphatase. This work advances the understanding of how Hsl1 facilitates membrane phospholipid synthesis through the phosphorylation-mediated regulation of Pah1.

SAH is a major metabolic sensor mediating worsening metabolic crosstalk in metabolic syndrome

In this study, we observed worsening metabolic crosstalk in mouse models with concomitant metabolic disorders such as hyperhomocysteinemia (HHcy), hyperlipidemia, and hyperglycemia and in human coronary artery disease by analyzing metabolic profiles. We found that HHcy worsening is most sensitive to other metabolic disorders. To identify metabolic genes and metabolites responsible for the worsening metabolic crosstalk, we examined mRNA levels of 324 metabolic genes in Hcy, glucose-related and lipid metabolic systems. We examined Hcy-metabolites (Hcy, SAH and SAM) by LS-ESI-MS/MS in 6 organs (heart, liver, brain, lung, spleen, and kidney) from C57BL/6J mice. Through linear regression analysis of Hcy-metabolites and metabolic gene mRNA levels, we discovered that SAH-responsive genes were responsible for most metabolic changes and all metabolic crosstalk mediated by Serine, Taurine, and G3P. SAH-responsive genes worsen glucose metabolism and cause upper glycolysis activation and lower glycolysis suppression, indicative of the accumulation of glucose/glycogen and G3P, Serine synthesis inhibition, and ATP depletion. Insufficient Serine due to negative correlation of PHGDH with SAH concentration may inhibit the folate cycle and transsulfurarion pathway and consequential reduced antioxidant power, including glutathione, taurine, NADPH, and NAD+. Additionally, we identified SAH-activated pathological TG loop as the consequence of increased fatty acid (FA) uptake, FA β-oxidation and Ac-CoA production along with lysosomal damage. We concluded that HHcy is most responsive to other metabolic changes in concomitant metabolic disorders and mediates worsening metabolic crosstalk mainly via SAH-responsive genes, that organ-specific Hcy metabolism determines organ-specific worsening metabolic reprogramming, and that SAH, acetyl-CoA, Serine and Taurine are critical metabolites mediating worsening metabolic crosstalk, redox disturbance, hypomethylation and hyperacetylation linking worsening metabolic reprogramming in metabolic syndrome.

Nonspecific phospholipases C3 and C4 interact with PIN-FORMED2 to regulate growth and tropic responses in Arabidopsis

The dynamic changes in membrane phospholipids affect membrane biophysical properties and cell signaling, thereby influencing numerous biological processes. Nonspecific phospholipase C (NPC) enzymes hydrolyze common phospholipids to release diacylglycerol (DAG), which is converted to phosphatidic acid (PA) and other lipids. In this study, 2 Arabidopsis (Arabidopsis thaliana) tandemly arrayed genes, NPC3 and NPC4, were identified as critical factors modulating auxin-controlled plant growth and tropic responses. Moreover, NPC3 and NPC4 were shown to interact with the auxin efflux transporter PIN-FORMED2 (PIN2). The loss of NPC3 and NPC4 enhanced the endocytosis and vacuolar degradation of PIN2, which disrupted auxin gradients and slowed gravitropic and halotropic responses. Furthermore, auxin-triggered activation of NPC3 and NPC4 is required for the asymmetric PA distribution that controls PIN2 trafficking dynamics and auxin-dependent tropic responses. Collectively, our study reveals an NPC-derived PA signaling pathway in Arabidopsis auxin fluxes that is essential for fine-tuning the balance between root growth and environmental responses.

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.

Identification of propranolol and derivatives that are chemical inhibitors of phosphatidate phosphatase as potential broad-spectrum fungicides

Plant diseases cause enormous economic losses in agriculture and threaten global food security, and application of agrochemicals is an important method of crop disease control. Exploration of disease-resistance mechanisms and synthesis of highly bioactive agrochemicals are thus important research objectives. Here, we show that propranolol, a phosphatidate phosphatase (Pah) inhibitor, effectively suppresses fungal growth, sporulation, sexual reproduction, and infection of diverse plants. The MoPah1 enzyme activity of the rice blast fungus Magnaporthe oryzae is inhibited by propranolol. Alterations in lipid metabolism are associated with inhibited hyphal growth and appressorium formation caused by propranolol in M. oryzae. Propranolol inhibits a broad spectrum of 12 plant pathogens, effectively inhibiting infection of barley, wheat, maize, tomato, and pear. To improve antifungal capacity, we synthesized a series of propranolol derivatives, one of which shows a 16-fold increase in antifungal ability and binds directly to MoPah1. Propranolol and its derivatives can also reduce the severity of rice blast and Fusarium head blight of wheat in the field. Taken together, our results demonstrate that propranolol suppresses fungal development and infection through mechanisms involved in lipid metabolism. Propranolol and its derivatives may therefore be promising candidates for fungicide development.

Catalytic core function of yeast Pah1 phosphatidate phosphatase reveals structural insight into its membrane localization and activity control

The PAH1-encoded phosphatidate (PA) phosphatase is a major source of diacylglycerol for the production of the storage lipid triacylglycerol and a key regulator for the de novo phospholipid synthesis in Saccharomyces cerevisiae. The catalytic function of Pah1 depends on its membrane localization which is mediated through its phosphorylation by multiple protein kinases and dephosphorylation by the Nem1-Spo7 protein phosphatase complex. The full-length Pah1 is composed of a catalytic core (N-LIP and HAD-like domains, amphipathic helix, and the WRDPLVDID domain) and non-catalytic regulatory sequences (intrinsically disordered regions, RP domain, and acidic tail) for phosphorylation and interaction with Nem1-Spo7. How the catalytic core regulates Pah1 localization and cellular function is not clear. In this work, we analyzed a variant of Pah1 (i.e., Pah1-CC (catalytic core)) that is composed only of the catalytic core. Pah1-CC expressed on a low-copy plasmid complemented the pah1Δ mutant phenotypes (e.g., nuclear/ER membrane expansion, reduced levels of triacylglycerol, and lipid droplet formation) without requiring Nem1-Spo7. The cellular function of Pah1-CC was supported by its PA phosphatase activity mostly associated with the membrane fraction. Although functional, Pah1-CC was distinct from Pah1 in the protein and enzymological properties, which include overexpression toxicity, association with heat shock proteins, and significant reduction of the Vmax value. These findings on the Pah1 catalytic core enhance the understanding of its structural requirements for membrane localization and activity control.

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.

A membrane-sensing mechanism links lipid metabolism to protein degradation at the nuclear envelope

Lipid composition determines organelle identity; however, whether the lipid composition of the inner nuclear membrane (INM) domain of the ER contributes to its identity is not known. Here, we show that the INM lipid environment of animal cells is under local control by CTDNEP1, the master regulator of the phosphatidic acid phosphatase lipin 1. Loss of CTDNEP1 reduces association of an INM-specific diacylglycerol (DAG) biosensor and results in a decreased percentage of polyunsaturated containing DAG species. Alterations in DAG metabolism impact the levels of the resident INM protein Sun2, which is under local proteasomal regulation. We identify a lipid-binding amphipathic helix (AH) in the nucleoplasmic domain of Sun2 that prefers membrane packing defects. INM dissociation of the Sun2 AH is linked to its proteasomal degradation. We suggest that direct lipid-protein interactions contribute to sculpting the INM proteome and that INM identity is adaptable to lipid metabolism, which has broad implications on disease mechanisms associated with the nuclear envelope.

Lipin-1 stability and its adipogenesis functions are regulated in contrasting ways by AKT1 and LKB1

Lipin-1 is a protein that plays a critical role in many cellular functions. At molecular level, it acts as a phosphatidic acid phosphohydrolase and a transcriptional coactivator. The functions of lipin-1 are largely dependent upon its subcellular localization, post-translational modifications like phosphorylation and acetylation, and also on its interaction with other proteins such as 14-3-3. However, the kinases and phosphatases that are responsible for these post translational modifications are not entirely known. Using bioinformatics and other biochemical approaches, we demonstrate lipin-1 as a novel target for AKT1 and LKB1. While AKT1 stabilizes lipin-1, LKB1 causes its degradation. Interestingly, our findings further show that lipin-1 enhances AKT1 activity as can be seen by increased phosphorylation of the substrates of AKT1. Taken together, our results suggest that lipin-1 plays an important role in the regulation of PI3K-AKT-mTOR pathway, which is dysregulated in majority of cancers. Therefore, understating the role of lipin-1 may provide new and important insights into the regulation and functions of the PI3K-mTOR pathway, which is one of the major targets for anti-cancer drug development strategies.

Acute Deletion of the Glucocorticoid Receptor in Hepatocytes Disrupts Postprandial Lipid Metabolism in Male Mice

Hepatic lipid metabolism is highly dynamic, and disruption of several circadian transcriptional regulators results in hepatic steatosis. This includes genetic disruption of the glucocorticoid receptor (GR) as the liver develops. To address the functional role of GR in the adult liver, we used an acute hepatocyte-specific GR knockout model to study temporal hepatic lipid metabolism governed by GR at several preprandial and postprandial circadian timepoints. Lipidomics analysis revealed significant temporal lipid metabolism, where GR disruption results in impaired regulation of specific triglycerides, nonesterified fatty acids, and sphingolipids. This correlates with increased number and size of lipid droplets and mildly reduced mitochondrial respiration, most noticeably in the postprandial phase. Proteomics and transcriptomics analyses suggest that dysregulated lipid metabolism originates from pronounced induced expression of enzymes involved in fatty acid synthesis, β-oxidation, and sphingolipid metabolism. Integration of GR cistromic data suggests that induced gene expression is a result of regulatory actions secondary to direct GR effects on gene transcription.

Phosphatidate phosphatase Pah1 contains a novel RP domain that regulates its phosphorylation and function in yeast lipid synthesis

The Saccharomyces cerevisiae PAH1-encoded phosphatidate (PA) phosphatase, which catalyzes the Mg2+-dependent dephosphorylation of PA to produce diacylglycerol, is one of the most highly regulated enzymes in lipid metabolism. The enzyme controls whether cells utilize PA to produce membrane phospholipids or the major storage lipid triacylglycerol. PA levels, which are regulated by the enzyme reaction, also control the expression of UASINO-containing phospholipid synthesis genes via the Henry (Opi1/Ino2-Ino4) regulatory circuit. Pah1 function is largely controlled by its cellular location, which is mediated by phosphorylation and dephosphorylation. Multiple phosphorylations sequester Pah1 in the cytosol and protect it from 20S proteasome-mediated degradation. The endoplasmic reticulum-associated Nem1-Spo7 phosphatase complex recruits and dephosphorylates Pah1 allowing the enzyme to associate with and dephosphorylate its membrane-bound substrate PA. Pah1 contains domains/regions that include the N-LIP and haloacid dehalogenase-like catalytic domains, N-terminal amphipathic helix for membrane binding, C-terminal acidic tail for Nem1-Spo7 interaction, and a conserved tryptophan within the WRDPLVDID domain required for enzyme function. Through bioinformatics, molecular genetics, and biochemical approaches, we identified a novel RP (regulation of phosphorylation) domain that regulates the phosphorylation state of Pah1. We showed that the ΔRP mutation results in a 57% reduction in the endogenous phosphorylation of the enzyme (primarily at Ser-511, Ser-602, and Ser-773/Ser-774), an increase in membrane association and PA phosphatase activity, but reduced cellular abundance. This work not only identifies a novel regulatory domain within Pah1 but emphasizes the importance of the phosphorylation-based regulation of Pah1 abundance, location, and function in yeast lipid synthesis.

Systematic crosstalk in plasmalogen and diacyl lipid biosynthesis for their differential yet concerted molecular functions in the cell

Plasmalogen is a major phospholipid of mammalian cell membranes. Recently it is becoming evident that the sn-1 vinyl-ether linkage in plasmalogen, contrasting to the ester linkage in the counterpart diacyl glycerophospholipid, yields differential molecular characteristics for these lipids especially related to hydrocarbon-chain order, so as to concertedly regulate biological membrane processes. A role played by NMR in gaining information in this respect, ranging from molecular to tissue levels, draws particular attention. We note here that a broad range of enzymes in de novo synthesis pathway of plasmalogen commonly constitute that of diacyl glycerophospholipid. This fact forms the basis for systematic crosstalk that not only controls a quantitative balance between these lipids, but also senses a defect causing loss of lipid in either pathway for compensation by increase of the counterpart lipid. However, this inherent counterbalancing mechanism paradoxically amplifies imbalance in differential effects of these lipids in a diseased state on membrane processes. While sharing of enzymes has been recognized, it is now possible to overview the crosstalk with growing information for specific enzymes involved. The overview provides a fundamental clue to consider cell and tissue type-dependent schemes in regulating membrane processes by plasmalogen and diacyl glycerophospholipid in health and disease.

A Comparative Study on Growth Performance, Body Composition, and Liver Tissue Metabolism Rearing on Soybean Lecithin-Enriched Artemia Nauplii and Microdiet in Rock Bream (Oplegnathus fasciatus) Larvae

This study is aimed at establishing optimal soybean lecithin (SL) enrichment protocols in Artemia nauplii and at comparing the growth performance, body composition, and liver tissue metabolism in rock bream (Oplegnathus fasciatus) larvae reared on SL-enriched Artemia nauplii or SL-enriched microdiet (MD). The enrichment protocol experiment results indicated 12 h enrichment, and 10 g SL/m³ seawater could obtain desirable results. Rock bream larvae (25 days posthatching (dph)) were fed Artemia nauplii or MD for 30 days with three replicates. At stage 1 (larval 25-40 dph), significantly higher growth performance was observed in larvae fed the live prey (P < 0.05). Conversely, at stage 2 (41-55 dph), feeding with MD significantly increased larval standard length, and specific growth rate compared with those of larvae fed live prey. Larvae fed a MD showed decreased lipolysis-related lipase activity as well as decreased amino acid catabolism-related alanine aminotransferase and aspartate aminotransferase enzyme activities in liver tissue. RNA sequencing revealed that feeding with the MD primarily increased the expression of lipogenesis-related genes and protein translation-related gene expression in the liver tissue. Notably, feeding with MD significantly increased ribosome biogenesis-related genes as well as mitochondria synthesis-related gene expression, indicating a high protein anabolism rate and high energy production in liver tissue. In conclusion, 10 g SL/m³ seawater and 12 h could effectively enrich SL in Artemia nauplii. Retard weaning onto MD led to lower growth performance, which was likely due to the diversity of lipid and protein metabolism.

Identification of ferroptosis related biomarkers and immune infiltration in Parkinson's disease by integrated bioinformatic analysis

BACKGROUND

Increasing evidence has indicated that ferroptosis engages in the progression of Parkinson's disease (PD). This study aimed to explore the role of ferroptosis-related genes (FRGs), immune infiltration and immune checkpoint genes (ICGs) in the pathogenesis and development of PD.

METHODS

The microarray data of PD patients and healthy controls (HC) from the Gene Expression Omnibus (GEO) database was downloaded. Weighted gene co-expression network analysis (WGCNA) was processed to identify the significant modules related to PD in the GSE18838 dataset. Machine learning algorithms were used to screen the candidate biomarkers based on the intersect between WGCNA, FRGs and differentially expressed genes. Enrichment analysis of GSVA, GSEA, GO, KEGG, and immune infiltration, group comparison of ICGs were also performed. Next, candidate biomarkers were validated in clinical samples by ELISA and receiver operating characteristic curve (ROC) was used to assess diagnose ability.

RESULTS

In this study, FRGs had correlations with ICGs, immune infiltration. Then, plasma levels of LPIN1 in PD was significantly lower than that in healthy controls, while the expression of TNFAIP3 was higher in PD in comparison with HC. ROC curves showed that the area under curve (AUC) of the LPIN1 and TNFAIP3 combination was 0.833 (95% CI: 0.750-0.916). Moreover, each biomarker alone could discriminate the PD from HC (LPIN1: AUC = 0.754, 95% CI: 0.659-0.849; TNFAIP3: AUC = 0.754, 95% CI: 0.660-0.849). For detection of early PD from HC, the model of combination maintained diagnostic accuracy with an AUC of 0.831 (95% CI: 0.734-0.927), LPIN1 also performed well in distinguishing the early PD from HC (AUC = 0.817, 95% CI: 0.717-0.917). However, the diagnostic efficacy was relatively poor in distinguishing the early from middle-advanced PD patients.

CONCLUSION

The combination model composed of LPIN1 and TNFAIP3, and each biomarker may serve as an efficient tool for distinguishing PD from HC.

Identification of Selection Signatures and Loci Associated with Important Economic Traits in Yunan Black and Huainan Pigs

Henan Province is located in central China and rich in domestic pig populations; Huainan (HN) pigs are one of three Henan indigenous breeds with great performance, including early maturation, strong disease resistance and high meat quality. Yunan (YN) black pigs are a typical, newly cultivated breed, synthesized between HN pigs and American Duroc, and are subjected to selection for important traits, such as fast growth and excellent meat quality. However, the genomic differences, selection signatures and loci associated with important economic traits in YN black pigs and HN pigs are still not well understood. In this study, based on high-density SNP chip analysis of 159 samples covering commercial DLY (Duroc × Landrace × Large White) pigs, HN pigs and YN black pigs, we performed a comprehensive analysis of phylogenetic relationships and genetic diversity among the three breeds. Furthermore, we used composite likelihood ratio tests (CLR) and F-statistics (Fst) to identify specific signatures of selection associated with important economic traits and potential candidate genes. We found 147 selected regions (top 1%) harboring 90 genes based on genetic differentiation (Fst) in the YN-DLY group. In the HN-DLY group, 169 selected regions harbored 58 genes. In the YN-HN group, 179 selected regions harbored 77 genes. In addition, the QTLs database with the most overlapping regions was associated with triglyceride level, number of mummified pigs, hemoglobin and loin muscle depth for YN black pigs, litter size and intramuscular fat content for HN pigs, and humerus length, linolenic acid content and feed conversion ratio mainly in DLY pigs. Of note, overlapping 14 tissue-specific promoters’ annotation with the top Fst 1% selective regions systematically demonstrated the muscle-specific and hypothalamus-specific regulatory elements in YN black pigs. Taken together, these results contribute to an accurate knowledge of crossbreeding, thus benefitting the evaluation of production performance and improving the genome-assisted breeding of other important indigenous pig in the future.

Two tales of LPIN1 deficiency: from fatal rhabdomyolysis to favorable outcome of acute compartment syndrome

LPIN1 deficiency is an autosomal recessive disease caused by biallelic mutations in LPIN1, where impaired fatty acid metabolism leads to stress in skeletal muscle, resulting in severe rhabdomyolysis, often triggered by fever, exercise, fasting, and anesthesia. It is the second most common cause of severe, recurrent episodes of rhabdomyolysis in early childhood which can result in serious morbidity and mortality. To date, 71 patients have been published in 20 clinical studies in the form of case series. We describe two previously unreported cases, one with a novel LPIN1 mutation that resulted in mortality, and another, to the best of our knowledge, with the first reported compartment syndrome managed with a favorable outcome in this disorder. Recognition of the complications including ventricular arrythmias, acute renal failure and compartment syndrome on the severe end of the spectrum may change the outcome and prognosis of this devastating condition.

Metabolic routing maintains the unique fatty acid composition of phosphoinositides

Phosphoinositide lipids (PPIn) are enriched in stearic- and arachidonic acids (38:4) but how this enrichment is established and maintained during phospholipase C (PLC) activation is unknown. Here we show that the metabolic fate of newly synthesized phosphatidic acid (PA), the lipid precursor of phosphatidylinositol (PI), is influenced by the fatty acyl-CoA used with preferential routing of the arachidonoyl-enriched species toward PI synthesis. Furthermore, during agonist stimulation the unsaturated forms of PI(4,5P)₂ are replenished significantly faster than the more saturated ones, suggesting a favored recycling of the unsaturated forms of the PLC-generated hydrolytic products. Cytidine diphosphate diacylglycerol synthase 2 (CDS2) but not CDS1 was found to contribute to increased PI resynthesis during PLC activation. Lastly, while the lipid transfer protein, Nir2 is found to contribute to rapid PPIn resynthesis during PLC activation, the faster re-synthesis of the 38:4 species does not depend on Nir2. Therefore, the fatty acid side-chain composition of the lipid precursors used for PI synthesis is an important determinant of their metabolic fates, which also contributes to the maintenance of the unique fatty acid profile of PPIn lipids.

Phosphorylation-mediated regulation of the Nem1-Spo7/Pah1 phosphatase cascade in yeast lipid synthesis

The PAH1-encoded phosphatidate phosphatase, which catalyzes the dephosphorylation of phosphatidate to produce diacylglycerol, controls the divergence of phosphatidate into triacylglycerol synthesis and phospholipid synthesis. Pah1 is inactive in the cytosol as a phosphorylated form and becomes active on the nuclear/endoplasmic reticulum membrane as a dephosphorylated form by the Nem1-Spo7 protein phosphatase complex. The phosphorylation of Pah1 by protein kinases, which include casein kinases I and II, Pho85-Pho80, Cdc28-cyclin B, and protein kinases A and C, controls its cellular location, catalytic activity, and susceptibility to proteasomal degradation. Nem1 (catalytic subunit) and Spo7 (regulatory subunit), which form a protein phosphatase complex catalyzing the dephosphorylation of Pah1 for its activation, are phosphorylated by protein kinases A and C. In this review, we discuss the functions and interrelationships of the protein kinases in the control of the Nem1-Spo7/Pah1 phosphatase cascade and lipid synthesis.

Mutant phosphatidate phosphatase Pah1-W637A exhibits altered phosphorylation, membrane association, and enzyme function in yeast

The Saccharomyces cerevisiae PAH1-encoded phosphatidate (PA) phosphatase, which catalyzes the dephosphorylation of PA to produce diacylglycerol, controls the bifurcation of PA into triacylglycerol synthesis and phospholipid synthesis. Pah1 is inactive in the cytosol as a phosphorylated form and becomes active on the membrane as a dephosphorylated form by the Nem1-Spo7 protein phosphatase. We show that the conserved Trp-637 residue of Pah1, located in the intrinsically disordered region, is required for normal synthesis of membrane phospholipids, sterols, triacylglycerol, and the formation of lipid droplets. Analysis of mutant Pah1-W637A showed that the tryptophan residue is involved in the phosphorylation-mediated/dephosphorylation-mediated membrane association of the enzyme and its catalytic activity. The endogenous phosphorylation of Pah1-W637A was increased at the sites of the N-terminal region but was decreased at the sites of the C-terminal region. The altered phosphorylation correlated with an increase in its membrane association. In addition, membrane-associated PA phosphatase activity in vitro was elevated in cells expressing Pah1-W637A as a result of the increased membrane association of the mutant enzyme. However, the inherent catalytic function of Pah1 was not affected by the W637A mutation. Prediction of Pah1 structure by AlphaFold shows that Trp-637 and the catalytic residues Asp-398 and Asp-400 in the haloacid dehalogenase-like domain almost lie in the same plane, suggesting that these residues are important to properly position the enzyme for substrate recognition at the membrane surface. These findings underscore the importance of Trp-637 in Pah1 regulation by phosphorylation, membrane association of the enzyme, and its function in lipid synthesis.

Ribosomal Protein S6: A Potential Therapeutic Target against Cancer?

Ribosomal protein S6 (RPS6) is a component of the 40S small ribosomal subunit and participates in the control of mRNA translation. Additionally, phospho (p)-RPS6 has been recognized as a surrogate marker for the activated PI3K/AKT/mTORC1 pathway, which occurs in many cancer types. However, downstream mechanisms regulated by RPS6 or p-RPS remains elusive, and the therapeutic implication of RPS6 is underappreciated despite an approximately half a century history of research on this protein. In addition, substantial evidence from RPS6 knockdown experiments suggests the potential role of RPS6 in maintaining cancer cell proliferation. This motivates us to investigate the current knowledge of RPS6 functions in cancer. In this review article, we reviewed the current information about the transcriptional regulation, upstream regulators, and extra-ribosomal roles of RPS6, with a focus on its involvement in cancer. We also discussed the therapeutic potential of RPS6 in cancer.

NLIP and HAD-like Domains of Pah1 and Lipin 1 Phosphatidate Phosphatases Are Essential for Their Catalytic Activities

Saccharomyces cerevisiae Pah1 phosphatidate phosphatase (PAP) catalyzes the dephosphorylation of phosphatidate to yield diacylglycerol, controlling phospholipids and triacylglycerol metabolisms. Pah1 and human Lipin 1 are intrinsically disordered proteins with 56% and 43% unfolded regions, respectively. Truncation analysis of the conserved and non-conserved regions showed that N- and C-conserved regions are essential for the catalytic activity of Pah1. PAP activities can be detected in the conserved N-terminal Lipin (NLIP) domain and C-terminal Lipin (CLIP)/haloacid dehalogenase (HAD)-like domain of Pah1 and Lipin 1, suggesting that the evolutionarily conserved domains are essential for the catalytic activity. The removal of disordered hydrophilic regions drastically reduced the protein solubility of Pah1. Thioredoxin is an efficient fusion protein for production of soluble NLIP–HAD recombinant proteins in Escherichia coli.

The middle lipin domain adopts a membrane-binding dimeric protein fold

Phospholipid synthesis and fat storage as triglycerides are regulated by lipin phosphatidic acid phosphatases (PAPs), whose enzymatic PAP function requires association with cellular membranes. Using hydrogen deuterium exchange mass spectrometry, we find mouse lipin 1 binds membranes through an N-terminal amphipathic helix, the Ig-like domain and HAD phosphatase catalytic core, and a middle lipin (M-Lip) domain that is conserved in mammalian and mammalian-like lipins. Crystal structures of the M-Lip domain reveal a previously unrecognized protein fold that dimerizes. The isolated M-Lip domain binds membranes both in vitro and in cells through conserved basic and hydrophobic residues. Deletion of the M-Lip domain in lipin 1 reduces PAP activity, membrane association, and oligomerization, alters subcellular localization, diminishes acceleration of adipocyte differentiation, but does not affect transcriptional co-activation. This establishes the M-Lip domain as a dimeric protein fold that binds membranes and is critical for full functionality of mammalian lipins.

Nuclear lipid droplets form in the inner nuclear membrane in a seipin-independent manner

Nuclear lipid droplets (LDs) in hepatocytes are derived from precursors of very-low-density lipoprotein in the ER lumen, but it is not known how cells lacking the lipoprotein secretory function form nuclear LDs. Here, we show that the inner nuclear membrane (INM) of U2OS cells harbors triglyceride synthesis enzymes, including ACSL3, AGPAT2, GPAT3/GPAT4, and DGAT1/DGAT2, and generates nuclear LDs in situ. mTOR inhibition increases nuclear LDs by inducing the nuclear translocation of lipin-1 phosphatidic acid (PA) phosphatase. Seipin, a protein essential for normal cytoplasmic LD formation in the ER, is absent in the INM. Knockdown of seipin increases nuclear LDs and PA in the nucleus, whereas seipin overexpression decreases these. Seipin knockdown also up-regulates lipin-1β expression, and lipin-1 knockdown decreases the effect of seipin knockdown on nuclear LDs without affecting PA redistribution. These results indicate that seipin is not directly involved in nuclear LD formation but instead restrains it by affecting lipin-1 expression and intracellular PA distribution.

Lipid metabolism has been good to me

My career in research has flourished through hard work, supportive mentors, and outstanding mentees and collaborators. The Carman laboratory has contributed to the understanding of lipid metabolism through the isolation and characterization of key lipid biosynthetic enzymes as well as through the identification of the enzyme-encoding genes. Our findings from yeast have proven to be invaluable to understand regulatory mechanisms of human lipid metabolism. Several rewarding aspects of my career have been my service to the Journal of Biological Chemistry as an editorial board member and Associate Editor, the National Institutes of Health as a member of study sections, and national and international scientific meetings as an organizer. I advise early career scientists to not assume anything, acknowledge others’ accomplishments, and pay it forward.

Lipin-1, a Versatile Regulator of Lipid Homeostasis, Is a Potential Target for Fighting Cancer

The rewiring of lipid metabolism is a major adaptation observed in cancer, and it is generally associated with the increased aggressiveness of cancer cells. Targeting lipid metabolism is therefore an appealing therapeutic strategy, but it requires a better understanding of the specific roles played by the main enzymes involved in lipid biosynthesis. Lipin-1 is a central regulator of lipid homeostasis, acting either as an enzyme or as a co-regulator of transcription. In spite of its important functions it is only recently that several groups have highlighted its role in cancer. Here, we will review the most recent research describing the role of lipin-1 in tumor progression when expressed by cancer cells or cells of the tumor microenvironment. The interest of its inhibition as an adjuvant therapy to amplify the effects of anti-cancer therapies will be also illustrated.

Sphingomyelin synthase-related protein generates diacylglycerol via the hydrolysis of glycerophospholipids in the absence of ceramide

Diacylglycerol (DG) is a well-established lipid second messenger. Sphingomyelin synthase (SMS)-related protein (SMSr) produces DG and ceramide phosphoethanolamine (CPE) by the transfer of phosphoethanolamine from phosphatidylethanolamine (PE) to ceramide. We previously reported that human SMSr overexpressed in COS-7 cells significantly increased DG levels, particularly saturated and/or monounsaturated fatty acid-containing DG molecular species, and provided DG to DG kinase (DGK) δ, which regulates various pathophysiological events, including epidermal growth factor-dependent cell proliferation, type 2 diabetes, and obsessive-compulsive disorder. However, mammalian SMSr puzzlingly produces only trace amounts of CPE/DG. To clarify this discrepancy, we highly purified SMSr and examined its activities other than CPE synthase. Intriguingly, purified SMSr showed a DG-generating activity via hydrolysis of PE, phosphatidic acid (PA), phosphatidylinositol (PI), and phosphatidylcholine (PC) in the absence of ceramide. DG generation through the PA phosphatase (PAP) activity of SMSr was approximately 300-fold higher than that with PE and ceramide. SMSr hydrolyzed PI ten times stronger than PI(4,5)bisphosphate (PI(4,5)P₂). The PAP and PC-phospholipase C (PLC) activities of SMSr were inhibited by propranolol, a PAP inhibitor, and by D609, an SMS/PC-PLC inhibitor. Moreover, SMSr showed substrate selectivity for saturated and/or monounsaturated fatty acid-containing PA molecular species, but not arachidonic-acid-containing PA, which is exclusively generated in the PI(4,5)P₂ cycle. We confirmed that SMSr expressed in COS-7 cells showed PAP and PI-PLC activities. Taken together, our study indicated that SMSr possesses previously unrecognized enzyme activities, PAP and PI/PE/PC-PLC, and constitutes a novel DG/PA signaling pathway together with DGKδ, which is independent of the PI(4,5)P₂ cycle.

Metabolic Alterations in Myotonic Dystrophy Type 1 and Their Correlation with Lipin

Myotonic dystrophy type 1 (DM1) is an autosomal dominant hereditary and multisystemic disease, characterized by progressive distal muscle weakness and myotonia. Despite huge efforts, the pathophysiological mechanisms underlying DM1 remain elusive. In this review, the metabolic alterations observed in patients with DM1 and their connection with lipin proteins are discussed. We start by briefly describing the epidemiology, the physiopathological and systemic features of DM1. The molecular mechanisms proposed for DM1 are explored and summarized. An overview of metabolic syndrome, dyslipidemia, and the summary of metabolic alterations observed in patients with DM1 are presented. Patients with DM1 present clinical evidence of metabolic alterations, namely increased levels of triacylglycerol and low-density lipoprotein, increased insulin and glucose levels, increased abdominal obesity, and low levels of high-density lipoprotein. These metabolic alterations may be associated with lipins, which are phosphatidate phosphatase enzymes that regulates the triacylglycerol levels, phospholipids, lipid signaling pathways, and are transcriptional co-activators. Furthermore, lipins are also important for autophagy, inflammasome activation and lipoproteins synthesis. We demonstrate the association of lipin with the metabolic alterations in patients with DM1, which supports further clinical studies and a proper exploration of lipin proteins as therapeutic targets for metabolic syndrome, which is important for controlling many diseases including DM1.

LPIN1 downregulation enhances anticancer activity of the novel HDAC/PI3K dual inhibitor FK-A11

Phosphatidylinositol-3 kinase (PI3K) inhibitor and histone deacetylase (HDAC) inhibitor have been developed as potential anticancer drugs. However, the cytotoxicity of PI3K inhibitor or HDAC inhibitor alone is relatively weak. We recently developed a novel HDAC/PI3K dual inhibitor FK-A11 and confirmed its enhanced cytotoxicity when compared to that of PI3K inhibitor or HDAC inhibitor alone on several cancer cell lines. However, the in vivo antitumor activity of FK-A11 was insufficient. We conducted high-throughput RNA interfering screening and identified gene LPIN1 which enhances the cytotoxicity of FK-A11. Downregulation of LPIN1 enhanced simultaneous inhibition of HDAC and PI3K by FK-A11 and enhanced the cytotoxicity of FK-A11. Propranolol, a beta-adrenoreceptor which is also a LPIN1 inhibitor, enhanced the in vitro and in vivo cytotoxicity and antitumor effect of FK-A11. These findings should help in the development of FK-A11 as a novel HDAC/PI3K dual inhibitor.

Excess Lipin enzyme activity contributes to TOR1A recessive disease and DYT-TOR1A dystonia

TOR1A/TorsinA mutations cause two incurable diseases: a recessive congenital syndrome that can be lethal, and a dominantly-inherited childhood-onset dystonia (DYT-TOR1A). TorsinA has been linked to phosphatidic acid lipid metabolism in Drosophila melanogaster. Here we evaluate the role of phosphatidic acid phosphatase (PAP) enzymes in TOR1A diseases using induced pluripotent stem cell-derived neurons from patients, and mouse models of recessive Tor1a disease. We find that Lipin PAP enzyme activity is abnormally elevated in human DYT-TOR1A dystonia patient cells and in the brains of four different Tor1a mouse models. Its severity also correlated with the dosage of Tor1a/TOR1A mutation. We assessed the role of excess Lipin activity in the neurological dysfunction of Tor1a disease mouse models by interbreeding these with Lpin1 knock-out mice. Genetic reduction of Lpin1 improved the survival of recessive Tor1a disease-model mice, alongside suppressing neurodegeneration, motor dysfunction, and nuclear membrane pathology. These data establish that TOR1A disease mutations cause abnormal phosphatidic acid metabolism, and suggest that approaches that suppress Lipin PAP enzyme activity could be therapeutically useful for TOR1A diseases.

Lipin 1 deficiency causes adult-onset myasthenia with motor neuron dysfunction in humans and neuromuscular junction defects in zebrafish

Lipin 1 is an intracellular protein acting as a phosphatidic acid phosphohydrolase enzyme controlling lipid metabolism. Human recessive mutations in LPIN1 cause recurrent, early-onset myoglobinuria, a condition normally associated with muscle pain and weakness. Whether and how lipin 1 deficiency in humans leads to peripheral neuropathy is yet unclear. Herein, two novel compound heterozygous mutations in LPIN1 with neurological disorders, but no myoglobinuria were identified in an adult-onset syndromic myasthenia family. The present study sought to explore the pathogenic mechanism of LPIN1 in muscular and neural development.

Methods: The clinical diagnosis of the proband was compared to the known 48 cases of LPIN1 recessive homozygous mutations. Whole-exome sequencing was carried out on the syndromic myasthenia family to identify the causative gene. The pathogenesis of lipin 1 deficiency during somitogenesis and neurogenesis was investigated using the zebrafish model. Whole-mount in situ hybridization, immunohistochemistry, birefringence analysis, touch-evoke escape response and locomotion assays were performed to observe in vivo the changes in muscles and neurons. The conservatism of the molecular pathways regulated by lipin 1 was evaluated in human primary glioblastoma and mouse myoblast cells by siRNA knockdown, drug treatment, qRT-PCR and Western blotting analysis.

Results: The patient exhibited adult-onset myasthenia accompanied by muscle fiber atrophy and nerve demyelination without myoglobinuria. Two novel heterozygous mutations, c.2047A>C (p.I683L) and c.2201G>A (p.R734Q) in LPIN1, were identified in the family and predicted to alter the tertiary structure of LPIN1 protein. Lipin 1 deficiency in zebrafish embryos generated by lpin1 morpholino knockdown or human LPIN1 mutant mRNA injections reproduced the myotomes defects, a reduction both in primary motor neurons and secondary motor neurons projections, morphological changes of post-synaptic clusters of acetylcholine receptors, and myelination defects, which led to reduced touch-evoked response and abnormalities of swimming behaviors. Loss of lipin 1 function in zebrafish and mammalian cells also exhibited altered expression levels of muscle and neuron markers, as well as abnormally enhanced Notch signaling, which was partially rescued by the specific Notch pathway inhibitor DAPT.

Conclusions: These findings pointed out that the compound heterozygous mutations in human LPIN1 caused adult-onset syndromic myasthenia with peripheral neuropathy. Moreover, zebrafish could be used to model the neuromuscular phenotypes due to the lipin 1 deficiency, where a novel pathological role of over-activated Notch signaling was discovered and further confirmed in mammalian cell lines.

A review of phosphatidate phosphatase assays

Phosphatidate phosphatase (PAP) catalyzes the penultimate step in the synthesis of triacylglycerol and regulates the synthesis of membrane phospholipids. There is much interest in this enzyme because it controls the cellular levels of its substrate, phosphatidate (PA), and product, DAG; defects in the metabolism of these lipid intermediates are the basis for lipid-based diseases such as obesity, lipodystrophy, and inflammation. The measurement of PAP activity is required for studies aimed at understanding its mechanisms of action, how it is regulated, and for screening its activators and/or inhibitors. Enzyme activity is determined through the use of radioactive and nonradioactive assays that measure the product, DAG, or Pi However, sensitivity and ease of use are variable across these methods. This review summarizes approaches to synthesize radioactive PA, to analyze radioactive and nonradioactive products, DAG and Pi, and discusses the advantages and disadvantages of each PAP assay.

Interface of Phospholipase Activity, Immune Cell Function, and Atherosclerosis

Phospholipases are a family of lipid-altering enzymes that can either reduce or increase bioactive lipid levels. Bioactive lipids elicit signaling responses, activate transcription factors, promote G-coupled-protein activity, and modulate membrane fluidity, which mediates cellular function. Phospholipases and the bioactive lipids they produce are important regulators of immune cell activity, dictating both pro-inflammatory and pro-resolving activity. During atherosclerosis, pro-inflammatory and pro-resolving activities govern atherosclerosis progression and regression, respectively. This review will look at the interface of phospholipase activity, immune cell function, and atherosclerosis.

The Spo7 sequence LLI is required for Nem1-Spo7/Pah1 phosphatase cascade function in yeast lipid metabolism

The Nem1-Spo7 complex in the yeast Saccharomyces cerevisiae is a protein phosphatase that catalyzes the dephosphory-lation of Pah1 phosphatidate phosphatase, required for its translocation to the nuclear/endoplasmic reticulum membrane. The Nem1-Spo7/Pah1 phosphatase cascade plays a major role in triacylglycerol synthesis and in the regulation of phospholipid synthesis. In this work, we examined Spo7, a regulatory subunit required for Nem1 catalytic function, to identify residues that govern formation of the Nem1-Spo7 complex. By deletion analysis of Spo7, we identified a hydrophobic Leu-Leu-Ile (LLI) sequence comprising residues 54-56 as being required for the protein to complement the temperature-sensitive phenotype of an spo7Δ mutant strain. Mutational analysis of the LLI sequence with alanine and arginine substitutions showed that its overall hydrophobicity is crucial for the formation of the Nem1-Spo7 complex as well as for the Nem1 catalytic function on its substrate, Pah1, in vivo Consistent with the role of the Nem1-Spo7 complex in activating the function of Pah1, we found that the mutational effects of the Spo7 LLI sequence were on the Nem1-Spo7/Pah1 axis that controls lipid synthesis and related cellular processes (e.g. triacylglycerol/phospholipid synthesis, lipid droplet formation, nuclear/endoplasmic reticulum membrane morphology, vacuole fusion, and growth on glycerol medium). These findings advance the understanding of Nem1-Spo7 complex formation and its role in the phosphatase cascade that regulates the function of Pah1 phosphatidate phosphatase.

MAPK-interacting kinase 2 (MNK2) regulates adipocyte metabolism independently of its catalytic activity

The mitogen-activated protein kinase (MAPK)-interacting kinases (MNKs) are serine/threonine protein kinases that are activated by the ERK1/2 (extracellular regulated kinase) and p38α/β MAPK pathways. The MNKs have previously been implicated in metabolic disease and shown to mediate diet-induced obesity. In particular, knockout of MNK2 in mice protects from the weight gain induced by a high-fat diet. These and other data suggest that MNK2 regulates the expansion of adipose tissue (AT), a stable, long-term energy reserve that plays an important role in regulating whole-body energy homeostasis. Using the well-established mouse 3T3-L1 in vitro model of adipogenesis, the role of the MNKs in adipocyte differentiation and lipid storage was investigated. Inhibition of MNK activity using specific inhibitors failed to impair adipogenesis or lipid accumulation, suggesting that MNK activity is not required for adipocyte differentiation and does not regulate lipid storage. However, small-interfering RNA (siRNA) knock-down of MNK2 did reduce lipid accumulation and regulated the levels of two major lipogenic transcriptional regulators, ChREBP (carbohydrate response element-binding protein) and LPIN1 (Lipin-1). These factors are responsible for controlling the expression of genes for proteins involved in de novo lipogenesis and triglyceride synthesis. The knock-down of MNK2 also increased the expression of hormone-sensitive lipase which catalyses the breakdown of triglyceride. These findings identify MNK2 as a regulator of adipocyte metabolism, independently of its catalytic activity, and reveal some of the mechanisms by which MNK2 drives AT expansion. The development of an MNK2-targeted therapy may, therefore, be a useful intervention for reducing weight caused by excessive nutrient intake.

Phosphatidic acid: an emerging versatile class of cellular mediators

Lipids function not only as the major structural components of cell membranes, but also as molecular messengers that transduce signals to trigger downstream signaling events in the cell. Phosphatidic acid (PA), the simplest and a minor class of glycerophospholipids, is a key intermediate for the synthesis of membrane and storage lipids, and also plays important roles in mediating diverse cellular and physiological processes in eukaryotes ranging from microbes to mammals and higher plants. PA comprises different molecular species that can act differently, and is found in virtually all organisms, tissues, and organellar membranes, with variations in total content and molecular species composition. The cellular levels of PA are highly dynamic in response to stimuli and multiple enzymatic reactions can mediate its production and degradation. Moreover, its unique physicochemical properties compared with other glycerophospholipids allow PA to influence membrane structure and dynamics, and interact with various proteins. PA has emerged as a class of new lipid mediators modulating various signaling and cellular processes via its versatile effects, such as membrane tethering, conformational changes, and enzymatic activities of target proteins, and vesicular trafficking.

Yeast phosphatidic acid phosphatase Pah1 hops and scoots along the membrane phospholipid bilayer

PA phosphatase, encoded by PAH1 in the yeast Saccharomyces cerevisiae, catalyzes the Mg²⁺-dependent dephosphorylation of PA, producing DAG at the nuclear/ER membrane. This enzyme plays a major role in triacylglycerol synthesis and in the regulation of phospholipid synthesis. As an interfacial enzyme, PA phosphatase interacts with the membrane surface, binds its substrate, and catalyzes its reaction. The Triton X-100/PA-mixed micellar system has been utilized to examine the activity and regulation of yeast PA phosphatase. This system, however, does not resemble the in vivo environment of the membrane phospholipid bilayer. We developed an assay system that mimics the nuclear/ER membrane to assess PA phosphatase activity. PA was incorporated into unilamellar phospholipid vesicles (liposomes) composed of the major nuclear/ER membrane phospholipids, PC, PE, PI, and PS. We optimized this system to support enzyme-liposome interactions and to afford activity that is greater than that obtained with the aforementioned detergent system. Activity was regulated by phospholipid composition, whereas the enzyme's interaction with liposomes was insensitive to composition. Greater activity was attained with large (≥100 nm) versus small (50 nm) vesicles. The fatty-acyl moiety of PA had no effect on this activity. PA phosphatase activity was dependent on the bulk (hopping mode) and surface (scooting mode) concentrations of PA, suggesting a mechanism by which the enzyme operates along the nuclear/ER membrane in vivo.

Lipin-1 Deficiency-Associated Recurrent Rhabdomyolysis and Exercise-Induced Myalgia Persisting into Adulthood: A Case Report and Review of Literature

Phosphatidate phosphatase-1 (lipin-1) is encoded by LPIN1 gene. Lipin-1 deficiency has been reported as the second most common cause of early-onset rhabdomyolysis after primary fatty acid oxidation disorders. We report a case of a 32-year-old Sri Lankan female with a history of more than 10 episodes of rhabdomyolysis and exercise intolerance since childhood. These episodes were triggered by infections and exercise. A temporal relationship between the acute episodes and use of drugs such as theophylline, mefenamic acid, co-trimoxazole, and combined oral contraceptive pills was also noted. There was marked elevation of serum creatine kinase and transaminases during acute episodes. Family history revealed parental consanguinity and an affected sibling who died of an acute episode associated with muscle weakness, dark coloured urine, and cyanosis, at the age of 2 years. The histochemical findings of the patient under discussion were consistent with a metabolic myopathy affecting membrane integrity. A homozygous, likely pathogenic variant c.1684G>T encoding p.(Glu562∗) was identified by clinical exome sequencing. Even though the studies to date give no convincing evidence of a possible causal or contributory relationship between the drugs under discussion and lipin-1 related rhabdomyolysis, this case highlights the importance of pharmacovigilance and reporting adverse drug reactions in patients with lipin-1 deficiency.

Yck1 casein kinase I regulates the activity and phosphorylation of Pah1 phosphatidate phosphatase from Saccharomyces cerevisiae

The PAH1-encoded phosphatidate phosphatase in Saccharomyces cerevisiae plays a major role in triacylglycerol synthesis and the control of phospholipid synthesis. For its catalytic function on the nuclear/endoplasmic reticulum membrane, Pah1 translocates to the membrane through its phosphorylation/dephosphorylation. Pah1 phosphorylation on multiple serine/threonine residues is complex and catalyzed by diverse protein kinases. In this work, we demonstrate that Pah1 is phosphorylated by the YCK1-encoded casein kinase I (CKI), regulating Pah1 catalytic activity and phosphorylation. Phosphoamino acid analysis coupled with phosphopeptide mapping of the CKI-phosphorylated Pah1 indicated that it is phosphorylated mainly on multiple serine residues. Using site-directed mutagenesis and phosphorylation analysis of Pah1, we identified eight serine residues (i.e. Ser-114, Ser-475, Ser-511, Ser-602, Ser-677, Ser-705, Ser-748, and Ser-774) as the target sites of CKI. Of these residues, Ser-475 and Ser-511 were specific for CKI, whereas the others were shared by casein kinase II (Ser-705), Cdc28-cyclin B (Ser-602), Pho85-Pho80 (Ser-114, Ser-602, and Ser-748), protein kinase A (Ser-667 and Ser-774), and protein kinase C (Ser-677). CKI-mediated phosphorylation of Pah1 stimulated both its phosphatidate phosphatase activity and its subsequent phosphorylation by casein kinase II. However, the CKI-mediated phosphorylation of Pah1 strongly inhibited its subsequent phosphorylation by Pho85-Pho80, protein kinase A, and protein kinase C. In a reciprocal analysis, Pah1 phosphorylation by Pho85-Pho80 inhibited subsequent phosphorylation by CKI. CKI-mediated Pah1 phosphorylation was also inhibited by a peptide containing the Pah1 residues 506-517, including the kinase-specific Ser-511 residue. These findings advance our understanding of how Pah1 catalytic activity and phosphorylation are regulated by multiple protein kinases.

Probing the Global Cellular Responses to Lipotoxicity Caused by Saturated Fatty Acids

Excessive levels of saturated fatty acids are toxic to cells, although the basis for this lipotoxicity remains incompletely understood. Here, we analyzed the transcriptome, lipidome, and genetic interactions of human leukemia cells exposed to palmitate. Palmitate treatment increased saturated glycerolipids, accompanied by a transcriptional stress response, including upregulation of the endoplasmic reticulum (ER) stress response. A comprehensive genome-wide short hairpin RNA (shRNA) screen identified >350 genes modulating lipotoxicity. Among previously unknown genetic modifiers of lipotoxicity, depletion of RNF213, a putative ubiquitin ligase mutated in Moyamoya vascular disease, protected cells from lipotoxicity. On a broader level, integration of our comprehensive datasets revealed that changes in di-saturated glycerolipids, but not other lipid classes, are central to lipotoxicity in this model. Consistent with this, inhibition of ER-localized glycerol-3-phosphate acyltransferase activity protected from all aspects of lipotoxicity. Identification of genes modulating the response to saturated fatty acids may reveal novel therapeutic strategies for treating metabolic diseases linked to lipotoxicity.

Signal Transduction Mechanisms of Alcoholic Fatty Liver Disease: Emer ging Role of Lipin-1

Lipin-1, a mammalian phosphatidic acid phosphatase (PAP), is a bi-functional molecule involved in various signaling pathways via its function as a PAP enzyme in the triglyceride synthesis pathway and in the nucleus as a transcriptional co-regulator. In the liver, lipin-1 is known to play a vital role in controlling the lipid metabolism and inflammation process at multiple regulatory levels. Alcoholic fatty liver disease (AFLD) is one of the earliest forms of liver injury and approximately 8-20% of patients with simple steatosis can develop into more severe forms of liver injury, including steatohepatitis, fibrosis/ cirrhosis, and eventually hepatocellular carcinoma (HCC). The signal transduction mechanisms for alcohol-induced detrimental effects in liver involves alteration of complex and multiple signaling pathways largely governed by a central and upstream signaling system, namely, sirtuin 1 (SIRT1)-AMP activated kinase (AMPK) axis. Emerging evidence suggests a pivotal role of lipin-1 as a crucial downstream regulator of SIRT1-AMPK signaling system that is likely to be ultimately responsible for development and progression of AFLD. Several lines of evidence demonstrate that ethanol exposure significantly induces lipin-1 gene and protein expression levels in cultured hepatocytes and in the livers of rodents, induces lipin-1-PAP activity, impairs the functional activity of nuclear lipin-1, disrupts lipin-1 mRNA alternative splicing and induces lipin-1 nucleocytoplasmic shuttling. Such impairment in response to ethanol leads to derangement of hepatic lipid metabolism, and excessive production of inflammatory cytokines in the livers of the rodents and human alcoholics. This review summarizes current knowledge about the role of lipin-1 in the pathogenesis of AFLD and its potential signal transduction mechanisms.

A Genetic Variant in GPR126 Causing a Decreased Inclusion of Exon 6 Is Associated with Cartilage Development in Adolescent Idiopathic Scoliosis Population

Adolescent idiopathic scoliosis (AIS) is the most common spinal deformity disease in adolescents but its etiology and pathogenesis are still unclear. The current study aims to identify the relationship between single nucleotide polymorphisms (SNPs) of G protein-coupled receptor 126 (GPR126) gene and AIS predisposition. GPR126 contains 26 exons and alternative splicing of exon 6 and exon 25 produces 4 protein-coding transcripts. We genotyped SNPs of GPR126 gene around exon 6 and exon 25 in 131 Chinese AIS patients and 132 healthy controls and provided evidence that SNP rs41289839 G>A is strongly associated with AIS susceptibility. Linkage disequilibrium analysis suggests that rs41289839 and other AIS-related SNPs were in strong LD. Next, we demonstrated that rs41289839 G>A inhibits the inclusion of exon 6 during alternative splicing, resulting in a decreased expression level of exon 6-included transcript (GPR126-exon6ⁱⁿ) relative to the exon 6 excluded transcript (GPR126-exon6^ex) by minigene assay. Chondrogenic differentiation experiment showed that GPR126-exon6ⁱⁿ has a high expression level relative to GPR126-exon6^ex during chondrogenic differentiation of hMSCs. Our findings indicate that newly discovered SNP is related to cartilage development and may provide valuable insights into the etiology and pathogenesis of adolescent idiopathic scoliosis.

Casein kinase II-mediated phosphorylation of lipin 1β phosphatidate phosphatase at Ser-285 and Ser-287 regulates its interaction with 14-3-3β protein

The mammalian lipin 1 phosphatidate phosphatase is a key regulatory enzyme in lipid metabolism. By catalyzing phosphatidate dephosphorylation, which produces diacylglycerol, the enzyme plays a major role in the synthesis of triacylglycerol and membrane phospholipids. The importance of lipin 1 to lipid metabolism is exemplified by cellular defects and lipid-based diseases associated with its loss or overexpression. Phosphorylation of lipin 1 governs whether it is associated with the cytoplasm apart from its substrate or with the endoplasmic reticulum membrane where its enzyme reaction occurs. Lipin 1β is phosphorylated on multiple sites, but less than 10% of them are ascribed to a specific protein kinase. Here, we demonstrate that lipin 1β is a bona fide substrate for casein kinase II (CKII), a protein kinase that is essential to viability and cell cycle progression. Phosphoamino acid analysis and phosphopeptide mapping revealed that lipin 1β is phosphorylated by CKII on multiple serine and threonine residues, with the former being major sites. Mutational analysis of lipin 1β and its peptides indicated that Ser-285 and Ser-287 are both phosphorylated by CKII. Substitutions of Ser-285 and Ser-287 with nonphosphorylatable alanine attenuated the interaction of lipin 1β with 14-3-3β protein, a regulatory hub that facilitates the cytoplasmic localization of phosphorylated lipin 1. These findings advance our understanding of how phosphorylation of lipin 1β phosphatidate phosphatase regulates its interaction with 14-3-3β protein and intracellular localization and uncover a mechanism by which CKII regulates cellular physiology.

A putative NEM1 homologue regulates lipid droplet biogenesis via PAH1 in Tetrahymena thermophila

Nuclear envelope morphology protein 1 (NEM1) along with a phosphatidate phosphatase (PAH1) regulates lipid homeostasis and membrane biogenesis in yeast and mammals. We investigated four putative NEM1 homologues (TtNEM1A, TtNEM1B, TtNEM1C and TtNEM1D) in the Tetrahymena thermophila genome. Disruption of TtNEM1B, TtNEM1C or TtNEM1D did not compromise normal cell growth. In contrast, we were unable to generate knockout strain of TtNEM1A under the same conditions, indicating that TtNEM1A is essential for Tetrahymena growth. Interestingly, loss of TtNEM1B but not TtNEM1C or TtNEM1D caused a reduction in lipid droplet number. Similar to yeast and mammals, TtNem1B of Tetrahymena exerts its function via Pah1, since we found that PAH1 overexpression rescued loss of Nem1 function. However, unlike NEM1 in other organisms, TtNEM1B does not regulate ER/nuclear morphology. Similarly, neither TtNEM1C nor TtNEM1D is required to maintain normal ER morphology. While Tetrahymena PAH1 was shown to functionally replace yeast PAH1 earlier, we observed that Tetrahymena NEM1 homologues did not functionally replace yeast NEM1. Overall, our results suggest the presence of a conserved cascade for regulation of lipid homeostasis and membrane biogenesis in Tetrahymena. Our results also suggest a Nem1-independent function of Pah1 in the regulation of ER morphology in Tetrahymena.

Discoveries of the phosphatidate phosphatase genes in yeast published in the Journal of Biological Chemistry

This JBC Review on the discoveries of yeast phosphatidate (PA) phosphatase genes is dedicated to Dr. Herbert Tabor, Editor-in-Chief of the Journal of Biological Chemistry (JBC) for 40 years, on the occasion of his 100th birthday. Here, I reflect on the discoveries of the APP1, DPP1, LPP1, and PAH1 genes encoding all the PA phosphatase enzymes in yeast. PA phosphatase catalyzes PA dephosphorylation to generate diacylglycerol; both substrate and product are key intermediates in the synthesis of membrane phospholipids and triacylglycerol. App1 and Pah1 are peripheral membrane proteins catalyzing an Mg²⁺-dependent reaction governed by the DXDX(T/V) phosphatase motif. Dpp1 and Lpp1 are integral membrane proteins that catalyze an Mg²⁺-independent reaction governed by the KX ₆RP-PSGH-SRX ₅HX ₃D phosphatase motif. Pah1 is PA-specific and is the only PA phosphatase responsible for lipid synthesis at the nuclear/endoplasmic reticulum membrane. App1, Dpp1, and Lpp1, respectively, are localized to cortical actin patches and the vacuole and Golgi membranes; they utilize several lipid phosphate substrates, including PA, lyso-PA, and diacylglycerol pyrophosphate. App1 is postulated to be involved in endocytosis, whereas Dpp1 and Lpp1 may be involved in lipid signaling. Pah1 is the yeast lipin homolog of mice and humans. A host of cellular defects and lipid-based diseases associated with loss or overexpression of PA phosphatase in yeast, mice, and humans, highlights its importance to cell physiology.

Purification of Lipin and Measurement of Phosphatidic Acid Phosphatase Activity from Liposomes

The lipin family of enzymes are phosphatidic acid (PA) phosphatases responsible for converting PA to diacylglycerol (DAG). Lipins therefore occupy a central node in the synthesis of triacylglycerol (TAG) and phospholipids, and may play a role in regulating the levels of PA and DAG as signaling molecules. Some enzymatic assays used to measure PA phosphatase activities use detergents above their critical micelle concentration to present substrate; however, these methods do not represent the physiological membrane bilayers found in cells and these conditions can drastically alter phosphatase activities. Other assays use poorly defined mixtures of phosphatidylcholine (PC), PA, and high concentrations of BSA to present substrate. In this chapter, we describe methods for affinity purification of FLAG-tagged lipin proteins, and an alternative enzymatic assay using small unilamellar vesicles, also known as liposomes, to investigate specific activities of PA phosphatases. These activities are measured using an acidified Bligh-Dyer extraction to separate the water-soluble, radiolabeled, inorganic phosphate released during the assay from the chloroform-soluble PA.