Glucocorticoids and cyclic AMP selectively increase hepatic lipin-1 expression, and insulin acts antagonistically

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.

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.

Nutrient-sensing AgRP neurons relay control of liver autophagy during energy deprivation

Autophagy represents a key regulator of aging and metabolism in sensing energy deprivation. We find that fasting in mice activates autophagy in the liver paralleled by activation of hypothalamic AgRP neurons. Optogenetic and chemogenetic activation of AgRP neurons induces autophagy, alters phosphorylation of autophagy regulators, and promotes ketogenesis. AgRP neuron-dependent induction of liver autophagy relies on NPY release in the paraventricular nucleus of the hypothalamus (PVH) via presynaptic inhibition of NPY1R-expressing neurons to activate PVH^CRH neurons. Conversely, inhibiting AgRP neurons during energy deprivation abrogates induction of hepatic autophagy and rewiring of metabolism. AgRP neuron activation increases circulating corticosterone concentrations, and reduction of hepatic glucocorticoid receptor expression attenuates AgRP neuron-dependent activation of hepatic autophagy. Collectively, our study reveals a fundamental regulatory principle of liver autophagy in control of metabolic adaptation during nutrient deprivation.

Steroid-mediated liver steatosis is CD1d-dependent, while steroid-induced liver necrosis, inflammation, and metabolic changes are CD1d-independent

Introduction

Glucocorticoids contribute to the pathogenesis of non-alcoholic fatty liver disease (NAFLD). Natural killer T cells play a role in the pathogenesis of NAFLD and response to steroids. The present study aimed to determine the role of CD1d in steroid-mediated metabolic derangement and the steroid-protective effect of glycosphingolipids.

Methods

Ten groups of mice were studied. Steroids were orally administered to C57BL/6 mice to assess the therapeutic effect of β-glucosylceramide (GC) on the development of steroid-mediated liver damage and metabolic derangements. The role of CD1d in the pathogenesis of steroid-induced liver damage and in mediating the hepatoprotective effect of GC was studied in CD1d^−/− mice.

Results

A model of oral administration of steroids was established, resulting in insulin resistance, hyperinsulinemia, hypertriglyceridemia, liver steatosis, and hepatocellular injury. Steroid administration to CD1d^−/− mice was associated with hyperglycemia and hypertriglyceridemia. However, CD1d^−/− mice did not manifest marked steroid-induced steatosis. GC treatment alleviated steroid-associated metabolic derangements and liver injury independent of CD1d expression.

Conclusion

A steroid-mediated model of NAFLD and metabolic derangements was established in which steroid-mediated steatosis was CD1d-dependent while steroid-induced liver necrosis, inflammation, and metabolic changes were CD1d-independent, which may support a dichotomy between steatosis and steatohepatitis in NAFLD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12876-022-02242-9.

Impaired glucocorticoid receptor expression in liver disrupts feeding-induced gene expression, glucose uptake, and glycogen storage

The transition from a fasted to a fed state is associated with extensive transcriptional remodeling in hepatocytes facilitated by hormonal- and nutritional-regulated transcription factors. Here, we use a liver-specific glucocorticoid receptor (GR) knockout (L-GRKO) model to investigate the temporal hepatic expression of GR target genes in response to feeding. Interestingly, in addition to the well-described fasting-regulated genes, we identify a subset of hepatic feeding-induced genes that requires GR for full expression. This includes Gck, which is important for hepatic glucose uptake, utilization, and storage. We show that insulin and glucocorticoids cooperatively regulate hepatic Gck expression in a direct GR-dependent manner by a 4.6 kb upstream GR binding site operating as a Gck enhancer. L-GRKO blunts preprandial and early postprandial Gck expression, which ultimately affects early postprandial hepatic glucose uptake, phosphorylation, and glycogen storage. Thus, GR is positively involved in feeding-induced gene expression and important for postprandial glucose metabolism in the liver.

2-Adrenergic receptor agonist induced hepatic steatosis in mice: modeling nonalcoholic fatty liver disease in hyperadrenergic states

Nonalcoholic fatty liver disease (NAFLD) is a spectrum of disorders ranging from hepatic steatosis [excessive accumulation of triglycerides (TG)] to nonalcoholic steatohepatitis, which can progress to cirrhosis and hepatocellular carcinoma. The molecular pathogenesis of steatosis and progression to more severe NAFLD remains unclear. Obesity and aging, two principal risk factors for NAFLD, are associated with a hyperadrenergic state. β-Adrenergic responsiveness in liver increases in animal models of obesity and aging, and in both is linked to increased hepatic expression of β2-adrenergic receptors (β2-ARs). We previously showed that in aging rodents intracellular signaling from elevated hepatic levels of β2-ARs may contribute to liver steatosis. In this study we demonstrate that injection of formoterol, a highly selective β2-AR agonist, to mice acutely results in hepatic TG accumulation. Further, we have sought to define the intrahepatic mechanisms underlying β2-AR mediated steatosis by investigating changes in hepatic expression and cellular localization of enzymes, transcription factors, and coactivators involved in processes of lipid accrual and disposition-and also functional aspects thereof-in livers of formoterol-treated animals. Our results suggest that β2-AR activation by formoterol leads to increased hepatic TG synthesis and de novo lipogenesis, increased but incomplete β-oxidation of fatty acids with accumulation of potentially toxic long-chain acylcarnitine intermediates, and reduced TG secretion-all previously invoked as contributors to fatty liver disease. Experiments are ongoing to determine whether sustained activation of hepatic β2-AR signaling by formoterol might be utilized to model fatty liver changes occurring in hyperadrenergic states of obesity and aging, and thereby identify novel molecular targets for the prevention or treatment of NAFLD.NEW & NOTEWORTHY Results of our study suggest that β2-adrenergic receptor (β2-AR) activation by agonist formoterol leads to increased hepatic TG synthesis and de novo lipogenesis, incomplete β-oxidation of fatty acids with accumulation of long-chain acylcarnitine intermediates, and reduced TG secretion. These findings may, for the first time, implicate a role for β2-AR responsive dysregulation of hepatic lipid metabolism in the pathogenetic processes underlying NAFLD in hyperadrenergic states such as obesity and aging.

Lack of Multidrug Resistance-associated Protein 4 Prolongs Partial Hepatectomy-induced Hepatic Steatosis

Multidrug resistance-associated protein 4 (Mrp4) is an efflux transporter involved in the active transport of several endogenous and exogenous chemicals. Previously, we have shown that hepatic Mrp4 expression increases following acetaminophen overdose. In mice, these increases in Mrp4 expression are observed specifically in hepatocytes undergoing active proliferation. From this, we hypothesized that Mrp4 plays a key role in hepatocyte proliferation and that lack of Mrp4 impedes liver regeneration following liver injury and/or tissue loss. To evaluate the role of Mrp4 in these processes, we employed two-third partial hepatectomy (PH) as an experimental liver regeneration model. In this study, we performed PH-surgery on male wildtype (C57BL/6J) and Mrp4 knockout mice. Plasma and liver tissues were collected at 24, 48, and 72 h postsurgery and evaluated for liver injury and liver regeneration endpoints, and for PH-induced hepatic lipid accumulation. Our results show that lack of Mrp4 did not alter hepatocyte proliferation and liver injury following PH as evaluated by Ki-67 antigen staining and plasma alanine aminotransferase levels. To our surprise, Mrp4 knockout mice exhibited increased hepatic lipid content, in particular, di- and triglyceride levels. Gene expression analysis showed that lack of Mrp4 upregulated hepatic lipin1 and diacylglycerol O-acyltransferase 1 and 2 gene expression, which are involved in the synthesis of di- and triglycerides. Our observations indicate that lack of Mrp4 prolonged PH-induced hepatic steatosis in mice and suggest that Mrp4 may be a novel genetic factor in the development of hepatic steatosis.

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.

Deficiency of Lipin2 Results in Enhanced NF-κB Signaling and Osteoclast Formation in RAW-D Murine Macrophages

Lipin2 is a phosphatidate phosphatase that plays critical roles in fat homeostasis. Alterations in Lpin2, which encodes lipin2, cause the autoinflammatory bone disorder Majeed syndrome. Lipin2 limits lipopolysaccharide (LPS)-induced inflammatory responses in macrophages. However, little is known about the precise molecular mechanisms underlying its anti-inflammatory function. In this study, we attempted to elucidate the molecular link between the loss of lipin2 function and autoinflammatory bone disorder. Using a Lpin2 knockout murine macrophage cell line, we showed that lipin2 deficiency enhances innate immune responses to LPS stimulation through excessive activation of the NF-κB signaling pathway, partly because of TAK1 signaling upregulation. Lipin2 depletion also enhanced RANKL-mediated osteoclastogenesis and osteoclastic resorption activity accompanied by NFATc1 dephosphorylation and increased nuclear accumulation. These results suggest that lipin2 suppresses the development of autoinflammatory bone disorder by fine-tuning proinflammatory responses and osteoclastogenesis in macrophages. Therefore, this study provides insights into the molecular pathogenesis of monogenic autoinflammatory bone disorders and presents a potential therapeutic intervention.

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.

Role of bioactive sphingolipids in physiology and pathology

Sphingolipids are a class of complex lipids containing a backbone of sphingoid bases, namely the organic aliphatic amino alcohol sphingosine (Sph), that are essential constituents of eukaryotic cells. They were first described as major components of cell membrane architecture, but it is now well established that some sphingolipids are bioactive and can regulate key biological functions. These include cell growth and survival, cell differentiation, angiogenesis, autophagy, cell migration, or organogenesis. Furthermore, some bioactive sphingolipids are implicated in pathological processes including inflammation-associated illnesses such as atherosclerosis, rheumatoid arthritis, inflammatory bowel disease (namely Crohn's disease and ulcerative colitis), type II diabetes, obesity, and cancer. A major sphingolipid metabolite is ceramide, which is the core of sphingolipid metabolism and can act as second messenger, especially when it is produced at the plasma membrane of cells. Ceramides promote cell cycle arrest and apoptosis. However, ceramide 1-phosphate (C1P), the product of ceramide kinase (CerK), and Sph 1-phosphate (S1P), which is generated by the action of Sph kinases (SphK), stimulate cell proliferation and inhibit apoptosis. Recently, C1P has been implicated in the spontaneous migration of cells from some types of cancer, and can enhance cell migration/invasion of malignant cells through interaction with a Gi protein-coupled receptor. In addition, CerK and SphK are implicated in inflammatory responses, some of which are associated with cancer progression and metastasis. Hence, targeting these sphingolipid kinases to inhibit C1P or S1P production, or blockade of their receptors might contribute to the development of novel therapeutic strategies to reduce metabolic alterations and disease.

Fructose Consumption Affects Glucocorticoid Signaling in the Liver of Young Female Rats

The effects of early-life fructose consumption on hepatic signaling pathways and their relation to the development of metabolic disorders in later life are not fully understood. To investigate whether fructose overconsumption at a young age induces alterations in glucocorticoid signaling that might contribute to development of metabolic disturbances, we analysed glucocorticoid receptor hormone-binding parameters and expression of its target genes involved in gluconeogenesis (phosphoenolpyruvate carboxykinase and glucose-6-phosphatase) and lipid metabolism (lipin-1), as well as redox and inflammatory status in the liver of female rats subjected to a fructose-rich diet immediately after weaning. The fructose diet increased hepatic corticosterone concentration, 11β-hydroxysteroid dehydrogenase type 1 level, glucocorticoid receptor protein level and hormone-binding activity, as well as lipin-1 level. The expression of glucose-6-phosphatase was reduced in fructose-fed rats, while phosphoenolpyruvate carboxykinase remained unaltered. The fructose-rich diet increased the level of fructose transporter GLUT2, while the expression of fructolytic enzymes fructokinase and aldolase B remained unaltered. The diet also affected pro-inflammatory pathways, but had no effect on the antioxidant defence system. In conclusion, a fructose-rich diet applied immediately after weaning promoted lipogenesis and enhanced hepatic glucocorticoid signaling, possibly to protect against inflammatory damage, but without an effect on gluconeogenesis and antioxidant enzymes. Yet, prolonged treatment might ultimately lead to more pronounced metabolic disturbances.

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.

Lipin 2/3 phosphatidic acid phosphatases maintain phospholipid homeostasis to regulate chylomicron synthesis

The lipin phosphatidic acid phosphatase (PAP) enzymes are required for triacylglycerol (TAG) synthesis from glycerol 3-phosphate in most mammalian tissues. The 3 lipin proteins (lipin 1, lipin 2, and lipin 3) each have PAP activity, but have distinct tissue distributions, with lipin 1 being the predominant PAP enzyme in many metabolic tissues. One exception is the small intestine, which is unique in expressing exclusively lipin 2 and lipin 3. TAG synthesis in small intestinal enterocytes utilizes 2-monoacylglycerol and does not require the PAP reaction, making the role of lipin proteins in enterocytes unclear. Enterocyte TAGs are stored transiently as cytosolic lipid droplets or incorporated into lipoproteins (chylomicrons) for secretion. We determined that lipin enzymes are critical for chylomicron biogenesis, through regulation of membrane phospholipid composition and association of apolipoprotein B48 with nascent chylomicron particles. Lipin 2/3 deficiency caused phosphatidic acid accumulation and mammalian target of rapamycin complex 1 (mTORC1) activation, which were associated with enhanced protein levels of a key phospholipid biosynthetic enzyme (CTP:phosphocholine cytidylyltransferase α) and altered membrane phospholipid composition. Impaired chylomicron synthesis in lipin 2/3 deficiency could be rescued by normalizing phospholipid synthesis levels. These data implicate lipin 2/3 as a control point for enterocyte phospholipid homeostasis and chylomicron biogenesis.

The phosphatidic acid phosphatase lipin-1 facilitates inflammation-driven colon carcinogenesis

Colon cancer is a devastating illness that is associated with gut inflammation. Here, we explored the possible role of lipin-1, a phosphatidic acid phosphatase, in the development of colitis-associated tumorigenesis. Azoxymethane and dextran sodium sulfate-treated (DSS-treated) animals deficient in lipin-1 harbored fewer tumors and carcinomas than WT animals due to decreased cellular proliferation, lower expression of antiapoptotic and protumorigenic factors, and a reduced infiltration of macrophages in colon tumors. They also displayed increased resistance to DSS-induced colitis by producing less proinflammatory cytokines and experiencing less immune infiltration. Lipin-1-deficient macrophages from the colon were less activated and displayed lower phosphatidic acid phosphatase activity than WT macrophages isolated from DSS-treated animals. Transference of WT macrophages into lipin-1-deficient animals was sufficient to increase colitis burden. Furthermore, treatment of lipin-1-deficient mice with IL-23 exacerbated colon inflammation. Analysis of human databases from colon cancer and ulcerative colitis patients showed that lipin-1 expression is increased in those disorders and correlates with the expression of the proinflammatory markers CXCL1 and CXCL2. And finally, clinically, LPIN1 expression had prognostic value in inflammatory and stem-cell subtypes of colon cancers. Collectively, these data demonstrate that lipin-1 is a critical regulator of intestinal inflammation and inflammation-driven colon cancer development.

Triglyceride Metabolism in the Liver

Triglyceride molecules represent the major form of storage and transport of fatty acids within cells and in the plasma. The liver is the central organ for fatty acid metabolism. Fatty acids accrue in liver by hepatocellular uptake from the plasma and by de novo biosynthesis. Fatty acids are eliminated by oxidation within the cell or by secretion into the plasma within triglyceride-rich very low-density lipoproteins. Notwithstanding high fluxes through these pathways, under normal circumstances the liver stores only small amounts of fatty acids as triglycerides. In the setting of overnutrition and obesity, hepatic fatty acid metabolism is altered, commonly leading to the accumulation of triglycerides within hepatocytes, and to a clinical condition known as nonalcoholic fatty liver disease (NAFLD). In this review, we describe the current understanding of fatty acid and triglyceride metabolism in the liver and its regulation in health and disease, identifying potential directions for future research. Advances in understanding the molecular mechanisms underlying the hepatic fat accumulation are critical to the development of targeted therapies for NAFLD. © 2018 American Physiological Society. Compr Physiol 8:1-22, 2018.

How lipid droplets "TAG" along: Glycerolipid synthetic enzymes and lipid storage

Triacylglycerols (TAG) serve as the predominant form of energy storage in mammalian cells, and TAG synthesis influences conditions such as obesity, fatty liver, and insulin resistance. In most tissues, the glycerol 3-phosphate pathway enzymes are responsible for TAG synthesis, and the regulation and function of these enzymes is therefore important for metabolic homeostasis. Here we review the sites and regulation of glycerol-3-phosphate acyltransferase (GPAT), acylglycerol-3-phosphate acyltransferase (AGPAT), lipin phosphatidic acid phosphatase (PAP), and diacylglycerol acyltransferase (DGAT) enzyme action. We highlight the critical roles that these enzymes play in human health by reviewing Mendelian disorders that result from mutation in the corresponding genes. We also summarize the valuable insights that genetically engineered mouse models have provided into the cellular and physiological roles of GPATs, AGPATs, lipins and DGATs. Finally, we comment on the status and feasibility of therapeutic approaches to metabolic disease that target enzymes of the glycerol 3-phosphate pathway. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Hepatic lipid accumulation: cause and consequence of dysregulated glucoregulatory hormones

Fatty liver can be diet, endocrine, drug, virus or genetically induced. Independent of cause, hepatic lipid accumulation promotes systemic metabolic dysfunction. By acting as peroxisome proliferator-activated receptor (PPAR) ligands, hepatic non-esterified fatty acids upregulate expression of gluconeogenic, beta-oxidative, lipogenic and ketogenic genes, promoting hyperglycemia, hyperlipidemia and ketosis. The typical hormonal environment in fatty liver disease consists of hyperinsulinemia, hyperglucagonemia, hypercortisolemia, growth hormone deficiency and elevated sympathetic tone. These endocrine and metabolic changes further encourage hepatic steatosis by regulating adipose tissue lipolysis, liver lipid uptake, de novo lipogenesis (DNL), beta-oxidation, ketogenesis and lipid export. Hepatic lipid accumulation may be induced by 4 separate mechanisms: (1) increased hepatic uptake of circulating fatty acids, (2) increased hepatic de novo fatty acid synthesis, (3) decreased hepatic beta-oxidation and (4) decreased hepatic lipid export. This review will discuss the hormonal regulation of each mechanism comparing multiple physiological models of hepatic lipid accumulation. Nonalcoholic fatty liver disease (NAFLD) is typified by increased hepatic lipid uptake, synthesis, oxidation and export. Chronic hepatic lipid signaling through PPARgamma results in gene expression changes that allow concurrent activity of DNL and beta-oxidation. The importance of hepatic steatosis in driving systemic metabolic dysfunction is highlighted by the common endocrine and metabolic disturbances across many conditions that result in fatty liver. Understanding the mechanisms underlying the metabolic dysfunction that develops as a consequence of hepatic lipid accumulation is critical to identifying points of intervention in this increasingly prevalent disease state.

Dysregulation of hepatic cAMP levels via altered Pde4b expression plays a critical role in alcohol-induced steatosis

Alcohol-induced hepatic steatosis is a significant risk factor for progressive liver disease. Cyclic adenosine monophosphate (cAMP) signalling has been shown to significantly regulate lipid metabolism; however, the role of altered cAMP homeostasis in alcohol-mediated hepatic steatosis has never been studied. Our previous work demonstrated that increased expression of hepatic phosphodiesterase 4 (Pde4), which specifically hydrolyses and decreases cAMP levels, plays a pathogenic role in the development of liver inflammation/injury. The aim of this study was to examine the role of PDE4 in alcohol-induced hepatic steatosis. C57BL/6 wild-type and Pde4b knockout (Pde4b(-/-) ) mice were pair-fed control or ethanol liquid diets. One group of wild-type mice received rolipram, a PDE4-specific inhibitor, during alcohol feeding. We demonstrate for the first time that an early increase in PDE4 enzyme expression and a resultant decrease in hepatic cAMP levels are associated with the significant reduction in carnitine palmitoyltransferase 1A (Cpt1a) expression. Notably, alcohol-fed (AF) Pde4b(-/-) mice and AF wild-type mice treated with rolipram had significantly lower hepatic free fatty acid content compared with AF wild-type mice. Importantly, PDE4 inhibition in alcohol-fed mice prevented the decrease in hepatic Cpt1a expression via the Pparα/Sirt1/Pgc1α pathway. These results demonstrate that the alcohol- induced increase in hepatic Pde4, specifically Pde4b expression, and compromised cAMP signalling predispose the liver to impaired fatty acid oxidation and the development of steatosis. Moreover, these data also suggest that hepatic PDE4 may be a clinically relevant therapeutic target for the treatment of alcohol-induced hepatic steatosis. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

The SCFβ-TRCP E3 ubiquitin ligase complex targets Lipin1 for ubiquitination and degradation to promote hepatic lipogenesis

The SCF^β-TRCP E3 ubiquitin ligase complex plays pivotal roles in normal cellular physiology and in pathophysiological conditions. Identification of β-transducin repeat-containing protein (β-TRCP) substrates is therefore critical to understand SCF^β-TRCP biology and function. We used a β-TRCP-phosphodegron motif-specific antibody in a β-TRCP substrate screen coupled with tandem mass spectrometry and identified multiple β-TRCP substrates. One of these substrates was Lipin1, an enzyme and suppressor of the family of sterol regulatory element-binding protein (SREBP) transcription factors, which activate genes encoding lipogenic factors. We showed that SCF^β-TRCP specifically interacted with and promoted the polyubiquitination of Lipin1 in a manner that required phosphorylation of Lipin1 by mechanistic target of rapamycin 1 (mTORC1) and casein kinase I (CKI). β-TRCP depletion in HepG2 hepatocellular carcinoma cells resulted in increased Lipin1 protein abundance, suppression of SREBP-dependent gene expression, and attenuation of triglyceride synthesis. Moreover, β-TRCP1 knockout mice showed increased Lipin1 protein abundance and were protected from hepatic steatosis induced by a high-fat diet. Together, these data reveal a critical physiological function of β-TRCP in regulating hepatic lipid metabolic homeostasis in part through modulating Lipin1 stability.

Decreased Gluconeogenesis in the Absence of Cystathionine Gamma-Lyase and the Underlying Mechanisms

AIMS

To investigate the regulation of hepatic glucose production by cystathionine γ-lyase (CSE)-generated hydrogen sulfide (H2S) in hepatic glucose production under physiological conditions.

RESULTS

We found that CSE knockout (KO) mice had a reduced rate of gluconeogenesis, which was reversed by administration of NaHS (an H2S donor) (i.p.). Interestingly, isolated CSE KO hepatocytes exhibited a reduced glycemic response to chemical-induced activation of the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) and glucocorticoid pathways compared with wild-type (WT) hepatocytes. Treatment with the inhibitors for PKA (KT5720) or glucocorticoid receptor (GR) (RU-486) significantly reduced H2S-stimulated glucose production from both WT and CSE KO mouse hepatocytes. NaHS treatment upregulated the protein levels of key gluconeogenic transcription factors, such as peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) and CCAAT-enhancer-binding protein-β (C/EBP-β). Moreover, exogenous H2S augmented the S-sulfhydration of the rate-limiting gluconeogenic enzymes and PGC-1α and increased their activities, which were lower in untreated CSE KO hepatocytes. Finally, knockdown of PGC-1α, but not C/EBP-β, significantly decreased NaHS-induced glucose production from the primary hepatocytes.

INNOVATION

This study demonstrates the stimulatory effect of endogenous H2S on liver glucose production and reveals three underlying mechanisms; that is, H2S upregulates the expression levels of PGC-1α and phosphoenolpyruvate carboxykinase via the GR pathway; H2S upregulates the expression level of PGC-1α through the activation of the cAMP/PKA pathway as well as PGC-1α activity via S-sulfhydration; and H2S upregulates the expression and the activities (by S-sulfhydration) of glucose-6-phosphatase and fructose-1,6-bisphosphatase.

CONCLUSION

This study may offer clues for the homeostatic regulation of glucose metabolism under physiological conditions and its dysregulation in metabolic syndrome.

Non-alcoholic fatty liver disease as a consequence of autonomic imbalance and circadian desynchronization

The circadian system, headed by the suprachiasmatic nucleus, synchronizes behaviour and metabolism according to the external light-dark cycle through neuroendocrine and autonomic signals. Metabolic diseases, such as steatosis, obesity and glucose intolerance, have been associated with conditions of circadian misalignment wherein the feeding schedule has been moved to the resting phase. Here we describe the physiological processes involved in liver lipid accumulation and show how they follow a circadian pattern importantly regulated by both the autonomic nervous system and the feeding-fasting cycle. We propose that an unbalanced activity of the sympathetic-parasympathetic branches between organs induced by circadian misalignment provides the conditions for the development and progression of non-alcoholic fatty liver disease.

Sirtuin 1 signaling and alcoholic fatty liver disease

Alcoholic fatty liver disease (AFLD) is one of the most prevalent forms of liver disease worldwide and can progress to inflammation (hepatitis), fibrosis/cirrhosis, and ultimately lead to end stage liver injury. The mechanisms, by which ethanol consumption leads to AFLD, are complicated and multiple, and remain incompletely understood. Nevertheless, understanding its pathogenesis will facilitate the development of effective pharmacological or nutritional therapies for treating human AFLD. Chronic ethanol consumption causes steatosis and inflammation in rodents or humans by disturbing several important hepatic transcriptional regulators, including AMP-activated kinase (AMPK), lipin-1, sterol regulatory element binding protein 1 (SREBP-1), PPARγ co-activator-1α (PGC-1α), and nuclear transcription factor-κB (NF-κB). Remarkably, the effects of ethanol on these regulators are mediated in whole or in part by inhibition of a central signaling molecule, sirtuin 1 (SIRT1), which is a nicotinamide adenine dinucleotide (NAD(+), NADH)-dependent class III protein deacetylase. In recent years, SIRT1 has emerged as a pivotal molecule controlling the pathways of hepatic lipid metabolism, inflammatory responses and in the development of AFLD in rodents and in humans. Ethanol-mediated SIRT1 inhibition suppresses or stimulates the activities of above described transcriptional regulators and co-regulators, thereby deregulating diverse lipid metabolism and inflammatory response pathways including lipogenesis, fatty acid β-oxidation, lipoprotein uptake and secretion and expression of pro-inflammatory cytokines in the liver. This review aims to highlight our current understanding of SIRT1 regulatory mechanisms and its response to ethanol-induced toxicity, thus, affirming significant role of SIRT1 signaling in the development of AFLD.

Liver-specific loss of lipin-1-mediated phosphatidic acid phosphatase activity does not mitigate intrahepatic TG accumulation in mice

Lipin proteins (lipin 1, 2, and 3) regulate glycerolipid homeostasis by acting as phosphatidic acid phosphohydrolase (PAP) enzymes in the TG synthesis pathway and by regulating DNA-bound transcription factors to control gene transcription. Hepatic PAP activity could contribute to hepatic fat accumulation in response to physiological and pathophysiological stimuli. To examine the role of lipin 1 in regulating hepatic lipid metabolism, we generated mice that are deficient in lipin-1-encoded PAP activity in a liver-specific manner (Alb-Lpin1(-/-) mice). This allele of lipin 1 was still able to transcriptionally regulate the expression of its target genes encoding fatty acid oxidation enzymes, and the expression of these genes was not affected in Alb-Lpin1(-/-) mouse liver. Hepatic PAP activity was significantly reduced in mice with liver-specific lipin 1 deficiency. However, hepatocytes from Alb-Lpin1(-/-) mice had normal rates of TG synthesis, and steady-state hepatic TG levels were unaffected under fed and fasted conditions. Furthermore, Alb-Lpin1(-/-) mice were not protected from intrahepatic accumulation of diacylglycerol and TG after chronic feeding of a diet rich in fat and fructose. Collectively, these data demonstrate that marked deficits in hepatic PAP activity do not impair TG synthesis and accumulation under acute or chronic conditions of lipid overload.

Lipin Family Proteins - Key Regulators in Lipid Metabolism

Background: Proteins in the lipin family play a key role in lipid synthesis due to their phosphatidate phosphatase activity, and they also act as transcriptional coactivators to regulate the expression of genes involved in lipid metabolism. The lipin family includes three members, lipin1, lipin2, and lipin3, which exhibit tissue-specific expression, indicating that they may have distinct roles in mediating disease. To date, most studies have focused on lipin1, whereas the roles of lipin2 and lipin3 are less understood. Summary: This review introduces the structural characteristics, physiological functions, relationship to lipid metabolism, and patterns of expression of the lipin family proteins, highlighting their roles in lipid metabolic homeostasis. © 2014 S. Karger AG, Basel

Conserved residues in the N terminus of lipin-1 are required for binding to protein phosphatase-1c, nuclear translocation, and phosphatidate phosphatase activity

Lipin-1 is a phosphatidate phosphatase in glycerolipid biosynthesis and signal transduction. It also serves as a transcriptional co-regulator to control lipid metabolism and adipogenesis. These functions are controlled partly by its subcellular distribution. Hyperphosphorylated lipin-1 remains sequestered in the cytosol, whereas hypophosphorylated lipin-1 translocates to the endoplasmic reticulum and nucleus. The serine/threonine protein phosphatase-1 catalytic subunit (PP-1c) is a major protein dephosphorylation enzyme. Its activity is controlled by interactions with different regulatory proteins, many of which contain conserved RVXF binding motifs. We found that lipin-1 binds to PP-1cγ through a similar HVRF binding motif. This interaction depends on Mg(2+) or Mn(2+) and is competitively inhibited by (R/H)VXF-containing peptides. Mutating the HVRF motif in the highly conserved N terminus of lipin-1 greatly decreases PP-1cγ interaction. Moreover, mutations of other residues in the N terminus of lipin-1 also modulate PP-1cγ binding. PP-1cγ binds poorly to a phosphomimetic mutant of lipin-1 and binds well to the non-phosphorylatable lipin-1 mutant. This indicates that lipin-1 is dephosphorylated before PP-1cγ binds to its HVRF motif. Importantly, mutating the HVRF motif also abrogates the nuclear translocation and phosphatidate phosphatase activity of lipin-1. In conclusion, we provide novel evidence of the importance of the lipin-1 N-terminal domain for its catalytic activity, nuclear localization, and binding to PP-1cγ.

Combination of lipid metabolism alterations and their sensitivity to inflammatory cytokines in human lipin-1-deficient myoblasts

Lipin-1 deficiency is associated with massive rhabdomyolysis episodes in humans, precipitated by febrile illnesses. Despite well-known roles of lipin-1 in lipid biosynthesis and transcriptional regulation, the pathogenic mechanisms leading to rhabdomyolysis remain unknown. Here we show that primary myoblasts from lipin-1-deficient patients exhibit a dramatic decrease in LPIN1 expression and phosphatidic acid phosphatase 1 activity, and a significant accumulation of lipid droplets (LD). The expression levels of LPIN1-target genes [peroxisome proliferator-activated receptors delta and alpha (PPARδ, PPARα), peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), acyl-coenzyme A dehydrogenase, very long (ACADVL), carnitine palmitoyltransferase IB and 2 (CPT1B and CPT2)] were not affected while lipin-2 protein level, a closely related member of the family, was increased. Microarray analysis of patients' myotubes identified 19 down-regulated and 51 up-regulated genes, indicating pleiotropic effects of lipin-1 deficiency. Special attention was paid to the up-regulated ACACB (acetyl-CoA carboxylase beta), a key enzyme in the fatty acid synthesis/oxidation balance. We demonstrated that overexpression of ACACB was associated with free fatty acid accumulation in patients' myoblasts whereas malonyl-carnitine (as a measure of malonyl-CoA) and CPT1 activity were in the normal range in basal conditions accordingly to the normal daily activity reported by the patients. Remarkably ACACB invalidation in patients' myoblasts decreased LD number and size while LPIN1 invalidation in controls induced LD accumulation. Further, pro-inflammatory treatments tumor necrosis factor alpha+Interleukin-1beta(TNF1α+IL-1ß) designed to mimic febrile illness, resulted in increased malonyl-carnitine levels, reduced CPT1 activity and enhanced LD accumulation, a phenomenon reversed by dexamethasone and TNFα or IL-1ß inhibitors. Our data suggest that the pathogenic mechanism of rhabdomyolysis in lipin-1-deficient patients combines the predisposing constitutive impairment of lipid metabolism and its exacerbation by pro-inflammatory cytokines.

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.

Differential regulation of the expressions of the PGC-1α splice variants, lipins, and PPARα in heart compared to liver

Peroxisome proliferator-activated receptor α (PPARα) and PPARγ coactivator 1α (PGC-1α) are crucial transcriptional regulators for genes involved in FA oxidation. Lipin-1 is essential for this increased capacity for β-oxidation in fasted livers, and it is also a phosphatidate phosphatase involved in triacylglycerol and phospholipid synthesis. Little is known about the regulation of these proteins in the heart during fasting, where there is increased FA esterification and oxidation. Lipin-1, lipin-2, lipin-3, carnitine palmitoyltransferase-1b (Cpt1b), and PGC-1α-b mRNA were increased by glucocorticoids and cAMP in neonatal rat cardiomyocytes. However, Cpt1b upregulation was caused by increased PPARα activation rather than expression. By contrast, the effects of PPARα in fasted livers are mediated through increased expression. During fasting, the expressions of PGC-1α-b and PGC-1α-c are increased in mouse hearts, and this is explained by increased cAMP-dependent signaling. By contrast, PGC-1α-a expression is increased in liver. Contrary to our expectations, lipin-1 expression was decreased and lipin-2 remained unchanged in hearts compared with increases in fasted livers. Our results identify novel differences in the regulation of lipins, PPARα, and PGC-1α splice variants during fasting in heart versus liver, even though the ultimate outcome in both tissues is to increase FA turnover and oxidation.

[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.

Phosphatidate phosphatase, a key regulator of lipid homeostasis

Yeast Pah1p phosphatidate phosphatase (PAP) catalyzes the penultimate step in the synthesis of triacylglycerol. PAP plays a crucial role in lipid homeostasis by controlling the relative proportions of its substrate phosphatidate and its product diacylglycerol. The cellular amounts of these lipid intermediates influence the synthesis of triacylglycerol and the pathways by which membrane phospholipids are synthesized. Physiological functions affected by PAP activity include phospholipid synthesis gene expression, nuclear/endoplasmic reticulum membrane growth, lipid droplet formation, and vacuole homeostasis and fusion. Yeast lacking Pah1p PAP activity are acutely sensitive to fatty acid-induced toxicity and exhibit respiratory deficiency. PAP is distinguished in its cellular location, catalytic mechanism, and physiological functions from Dpp1p and Lpp1p lipid phosphate phosphatases that utilize a variety of substrates that include phosphatidate. Phosphorylation/dephosphorylation is a major mechanism by which Pah1p PAP activity is regulated. Pah1p is phosphorylated by cytosolic-associated Pho85p-Pho80p, Cdc28p-cyclin B, and protein kinase A and is dephosphorylated by the endoplasmic reticulum-associated Nem1p-Spo7p phosphatase. The dephosphorylation of Pah1p stimulates PAP activity and facilitates the association with the membrane/phosphatidate allowing for its reaction and triacylglycerol synthesis. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Myocardial fatty acid metabolism and lipotoxicity in the setting of insulin resistance

Management of diabetes and insulin resistance in the setting of cardiovascular disease has become an important issue in an increasingly obese society. Besides the development of hypertension and buildup of atherosclerotic plaques, the derangement of fatty acid and lipid metabolism in the heart plays an important role in promoting cardiac dysfunction and oxidative stress. This review discusses the mechanisms by which metabolic inflexibility in the use of fatty acids as the preferred cardiac substrate in diabetes produces detrimental effects on mechanical efficiency, mitochondrial function, and recovery from ischemia. Lipid accumulation and the consequences of toxic lipid metabolites are also discussed.

Regulation of triglyceride metabolism by glucocorticoid receptor

Glucocorticoids are steroid hormones that play critical and complex roles in the regulation of triglyceride (TG) homeostasis. Depending on physiological states, glucocorticoids can modulate both TG synthesis and hydrolysis. More intriguingly, glucocorticoids can concurrently affect these two processes in adipocytes. The metabolic effects of glucocorticoids are conferred by intracellular glucocorticoid receptors (GR). GR is a transcription factor that, upon binding to glucocorticoids, regulates the transcriptional rate of specific genes. These GR primary target genes further initiate the physiological and pathological responses of glucocorticoids. In this article, we overview glucocorticoid-regulated genes, especially those potential GR primary target genes, involved in glucocorticoid-regulated TG metabolism. We also discuss transcriptional regulators that could act with GR to participate in these processes. This knowledge is not only important for the fundamental understanding of steroid hormone actions, but also are essential for future therapeutic interventions against metabolic diseases associated with aberrant glucocorticoid signaling, such as insulin resistance, dyslipidemia, central obesity and hepatic steatosis.

Regulation of hepatic lipin-1 by ethanol: role of AMP-activated protein kinase/sterol regulatory element-binding protein 1 signaling in mice

Lipin-1 is a protein that exhibits dual functions as a phosphatidic acid phosphohydrolase enzyme in the triglyceride synthesis pathways and a transcriptional coregulator. Our previous studies have shown that ethanol causes fatty liver by activation of sterol regulatory element-binding protein 1 (SREBP-1) and inhibition of hepatic AMP-activated protein kinase (AMPK) in mice. Here, we tested the hypothesis that AMPK-SREBP-1 signaling may be involved in ethanol-mediated up-regulation of lipin-1 gene expression. The effects of ethanol on lipin-1 were investigated in cultured hepatic cells and in the livers of chronic ethanol-fed mice. Ethanol exposure robustly induced activity of a mouse lipin-1 promoter, promoted cytoplasmic localization of lipin-1, and caused excess lipid accumulation, both in cultured hepatic cells and in mouse livers. Mechanistic studies showed that ethanol-mediated induction of lipin-1 gene expression was inhibited by a known activator of AMPK or overexpression of a constitutively active form of AMPK. Importantly, overexpression of the processed nuclear form of SREBP-1c abolished the ability of 5-aminoimidazole-4-carboxamide ribonucleoside to suppress ethanol-mediated induction of lipin-1 gene-expression level. Chromatin immunoprecipitation assays further revealed that ethanol exposure significantly increased the association of acetylated histone H3 at lysine 9 with the SRE-containing region in the promoter of the lipin-1 gene.

CONCLUSION

In conclusion, ethanol-induced up-regulation of lipin-1 gene expression is mediated through inhibition of AMPK and activation of SREBP-1.

Relationship of glucose and oleate metabolism to cardiac function in lipin-1 deficient (fld) mice

Lipin-1 is the major phosphatidate phosphatase (PAP) in the heart and a transcriptional coactivator that regulates fatty acid (FA) oxidation in the liver. As the control of FA metabolism is essential for maintaining cardiac function, we investigated whether lipin-1 deficiency affects cardiac metabolism and performance. Cardiac PAP activity in lipin-1 deficient [fatty liver dystrophy (fld)] mice was decreased by >80% compared with controls. Surprisingly, oleate oxidation and incorporation in triacylglycerol (TG), as well as glucose oxidation, were not significantly different in perfused working fld hearts. Despite this, [³H]oleate accumulation in phosphatidate and phosphatidylinositol was increased in fld hearts, reflecting the decreased PAP activity. Phosphatidate accumulation was linked to increased cardiac mammalian target of rapamycin complex 1 (mTORC1) signaling and endoplasmic reticulum (ER) stress. Transthoracic echocardiography showed decreased cardiac function in fld mice; however, cardiac dysfunction was not observed in ex vivo perfused working fld hearts. This showed that changes in systemic factors due to the global absence of lipin-1 could contribute to the decreased cardiac function in vivo. Collectively, this study shows that fld hearts exhibit unchanged oleate esterification, as well as oleate and glucose oxidation, despite the absence of lipin-1. However, lipin-1 deficiency increases the accumulation of newly synthesized phosphatidate and induces aberrant cell signaling.

Dietary cholesterol reduces plasma triacylglycerol in apolipoprotein E-null mice: suppression of lipin-1 and -2 in the glycerol-3-phosphate pathway

Background

Cholesterol metabolism is tightly regulated by both cholesterol and its metabolites in the mammalian liver, but the regulatory mechanism of triacylglycerol (TG) synthesis remains to be elucidated. Lipin, which catalyzes the conversion of phosphatidate to diacylglycerol, is a key enzyme involved in de novo TG synthesis in the liver via the glycerol-3-phosphate (G3P) pathway. However, the regulatory mechanisms for the expression of lipin in the liver are not well understood.

Methodology/Principal Findings

Apolipoprotein E-knock out (apoE-KO) mice were fed a chow supplemented with 1.25% cholesterol (high-Chol diet). Cholesterol and bile acids were highly increased in the liver within a week. However, the amount of TG in very low-density lipoprotein (VLDL), but not in the liver, was reduced by 78%. The epididymal adipose tissue was almost eradicated in the long term. DNA microarray and real-time RT-PCR analyses revealed that the mRNA expression of all the genes in the G3P pathway in the liver was suppressed in the high-Chol diet apoE-KO mice. In particular, the mRNA and protein expression of lipin-1 and lipin-2 was markedly decreased, and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), which up-regulates the transcription of lipin-1, was also suppressed. In vitro analysis using HepG2 cells revealed that the protein expression of lipin-2 was suppressed by treatment with taurocholic acid.

Conclusions/Significance

These data using apoE-KO mice indicate that cholesterol and its metabolites are involved in regulating TG metabolism through a suppression of lipin-1 and lipin-2 in the liver. This research provides evidence for the mechanism of lipin expression in the liver.

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.

Shedding light on the enigma of myocardial lipotoxicity: the involvement of known and putative regulators of fatty acid storage and mobilization

Excessive fatty acid (FA) uptake by cardiac myocytes is often associated with adverse changes in cardiac function. This is especially evident in diabetic individuals, where increased intramyocardial triacylglycerol (TG) resulting from the exposure to high levels of circulating FA has been proposed to be a major contributor to diabetic cardiomyopathy. At present, our knowledge of how the heart regulates FA storage in TG and the hydrolysis of this TG is limited. This review concentrates on what is known about TG turnover within the heart and how this is likely to be regulated by extrapolating results from other tissues. We also assess the evidence as to whether increased TG accumulation protects against FA-induced lipotoxicity through limiting the accumulations of ceramides and diacylglycerols versus whether it is a maladaptive response that contributes to cardiac dysfunction.

Recent progress in understanding protein and lipid factors affecting hepatic VLDL assembly and secretion

Excess lipid induced metabolic disorders are one of the major existing challenges for the society. Among many different causes of lipid disorders, overproduction and compromised catabolism of triacylglycerol-rich very low density lipoproteins (VLDL) have become increasingly prevalent leading to hyperlipidemia worldwide. This review provides the latest understanding in different aspects of VLDL assembly process, including structure-function relationships within apoB, mutations in APOB causing hypobetalipoproteinemia, significance of modulating microsomal triglyceride-transfer protein activity in VLDL assembly, alterations of VLDL assembly by different fatty acid species, and hepatic proteins involved in vesicular trafficking, and cytosolic lipid droplet metabolism that contribute to VLDL assembly. The role of lipoprotein receptors and exchangeable apolipoproteins that promote or diminish VLDL assembly and secretion is discussed. New understanding on dysregulated insulin signaling as a consequence of excessive triacylglycerol-rich VLDL in the plasma is also presented. It is hoped that a comprehensive view of protein and lipid factors that contribute to molecular and cellular events associated with VLDL assembly and secretion will assist in the identification of pharmaceutical targets to reduce disease complications related to hyperlipidemia.

Characterization of the human LPIN1-encoded phosphatidate phosphatase isoforms

The human LPIN1 gene encodes the protein lipin 1, which possesses phosphatidate (PA) phosphatase (3-sn-phosphatidate phosphohydrolase; EC 3.1.3.4) activity (Han, G.-S., Wu, W.-I., and Carman, G. M. (2006) J. Biol. Chem. 281, 9210-9218). In this work, we characterized human lipin 1 alpha, beta, and gamma isoforms that were expressed in Escherichia coli and purified to near homogeneity. PA phosphatase activities of the alpha, beta, and gamma isoforms were dependent on Mg(2+) or Mn(2+) ions at pH 7.5 at 37 degrees C. The activities were inhibited by concentrations of Mg(2+) and Mn(2+) above their optimums and by Ca(2+), Zn(2+), N-ethylmaleimide, propranolol, and the sphingoid bases sphingosine and sphinganine. The activities were thermally labile at temperatures above 40 degrees C. The alpha, beta, and gamma activities followed saturation kinetics with respect to the molar concentration of PA (K(m) values of 0.35, 0.24, and 0.11 mm, respectively) but followed positive cooperative (Hill number approximately 2) kinetics with respect to the surface concentration of PA (K(m) values of 4.2, 4.5, and 4.3 mol %, respectively) in Triton X-100/PA-mixed micelles. The turnover numbers (k(cat)) for the alpha, beta, and gamma isoforms were 68.8 + or - 3.5, 42.8 + or - 2.5, and 5.7 + or - 0.2 s(-1), respectively, whereas their energy of activation values were 14.2, 15.5, and 18.5 kcal/mol, respectively. The isoform activities were dependent on PA as a substrate and required at least one unsaturated fatty acyl moiety for maximum activity.

Skeletal muscle lipogenic protein expression is not different between lean and obese individuals: a potential factor in ceramide accumulation

CONTEXT

Skeletal muscle lipid content is increased in obesity. Recent evidence suggests that fatty acid (FA) storage as triacylglycerol (TAG) represents a metabolically safe pool compared to the more bioactive diacylglycerol (DAG) and ceramide.

OBJECTIVE/DESIGN

The purpose of this study was to compare the expression of lipogenic proteins and ceramide and DAG content in skeletal muscle of lean and obese humans. We hypothesized that lipogenic protein expression would be increased in obese to facilitate the storage of excess FA as TAG.

PARTICIPANTS

Eighteen lean (BMI < or = 26 kg/m(2)) and 15 obese (BMI > 29 kg/m(2)) women participated in this study.

RESULTS

There was no difference in the expression of any lipogenic (stearoyl-CoA desaturase-1, stearoyl retinol binding protein-1c, mitochondrial glycerol-3-phosphate acyltransferase, diacylglycerol acyltransferase-1) or sphingolipid proteins measured between lean and obese humans. Total ceramide was increased in muscle from obese humans (lean vs. obese, 529.4 +/- 54.8 vs. 672.4 +/- 57.4 nmol/g; P < 0.05), but there was no difference in total DAG content (lean vs. obese, 2244.1 +/- 278.2 vs. 1941.4 +/- 165.0 nmol/g). Content of protein phosphatase 2A, a ceramide target, was increased in muscle of obese humans (P < 0.05).

CONCLUSIONS

We propose that in muscle of obese humans there is an insufficient lipogenic response to the lipid oversupply, allowing more FA to be stored as reactive lipid species, particularly ceramide, potentially contributing to subsequent metabolic complications.

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.

A conserved serine residue is required for the phosphatidate phosphatase activity but not the transcriptional coactivator functions of lipin-1 and lipin-2

Mammalian lipins (lipin-1, lipin-2, and lipin-3) are Mg2+-dependent phosphatidate phosphatase (PAP) enzymes, which catalyze a key reaction in glycerolipid biosynthesis. Lipin-1 also functions as a transcriptional coactivator in conjunction with members of the peroxisome proliferator-activated receptor family. An S734L mutation in LPIN2 causes Majeed syndrome, a human inflammatory disorder characterized by recurrent osteomyelitis, fever, dyserythropoietic anemia, and cutaneous inflammation. Here we demonstrate that mutation of the equivalent serine in mouse lipin-1 and lipin-2 to leucine or aspartate abolishes PAP activity but does not impair lipin association with microsomal membranes, the major site of glycerolipid synthesis. We also determined that lipin-2 has transcriptional coactivator activity for peroxisome proliferator-activated receptor-response elements similar to lipin-1 and that this activity is not affected by mutating the conserved serine. Therefore, our results indicate that the symptoms of the Majeed syndrome result from a loss of lipin-2 PAP activity. To characterize sites of lipin-2 action, we detected lipin-2 expression by in situ hybridization on whole mouse sections and by quantitative PCR of tissues relevant to Majeed syndrome. Lipin-2 was most prominently expressed in liver, where levels were much higher than lipin-1, and also in kidney, lung, gastrointestinal tract, and specific regions of the brain. Lipin-2 was also expressed in circulating red blood cells and sites of lymphopoiesis (bone marrow, thymus, and spleen). These results raise the possibility that the loss of lipin-2 PAP activity in erythrocytes and lymphocytes may contribute to the anemia and inflammation phenotypes observed in Majeed syndrome patients.

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.