AP-1 binding transcriptionally regulates human neutral ceramidase

Lysophospholipids and Their G-Coupled Protein Signaling in Alzheimer's Disease: From Physiological Performance to Pathological Impairment

Lysophospholipids (LPLs) are bioactive signaling lipids that are generated from phospholipase-mediated hydrolyzation of membrane phospholipids (PLs) and sphingolipids (SLs). Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are two of the best-characterized LPLs which mediate a variety of cellular physiological responses via specific G-protein coupled receptor (GPCR) mediated signaling pathways. Considerable evidence now demonstrates the crucial role of LPA and S1P in neurodegenerative diseases, especially in Alzheimer’s disease (AD). Dysfunction of LPA and S1P metabolism can lead to aberrant accumulation of amyloid-β (Aβ) peptides, the formation of neurofibrillary tangles (NFTs), neuroinflammation and ultimately neuronal death. Summarizing LPA and S1P signaling profile may aid in profound health and pathological processes. In the current review, we will introduce the metabolism as well as the physiological roles of LPA and S1P in maintaining the normal functions of the nervous system. Given these pivotal functions, we will further discuss the role of dysregulation of LPA and S1P in promoting AD pathogenesis.

Role of Ceramidases in Sphingolipid Metabolism and Human Diseases

Human pathologies such as Alzheimer’s disease, type 2 diabetes-induced insulin resistance, cancer, and cardiovascular diseases have altered lipid homeostasis. Among these imbalanced lipids, the bioactive sphingolipids ceramide and sphingosine-1 phosphate (S1P) are pivotal in the pathophysiology of these diseases. Several enzymes within the sphingolipid pathway contribute to the homeostasis of ceramide and S1P. Ceramidase is key in the degradation of ceramide into sphingosine and free fatty acids. In humans, five different ceramidases are known—acid ceramidase, neutral ceramidase, and alkaline ceramidase 1, 2, and 3—which are encoded by five different genes (ASAH1, ASAH2, ACER1, ACER2, and ACER3, respectively). Notably, the neutral ceramidase N-acylsphingosine amidohydrolase 2 (ASAH2) shows considerable differences between humans and animals in terms of tissue expression levels. Besides, the subcellular localization of ASAH2 remains controversial. In this review, we sum up the results obtained for identifying gene divergence, structure, subcellular localization, and manipulating factors and address the role of ASAH2 along with other ceramidases in human diseases.

Sphingosine 1-Phosphate Receptors and Metabolic Enzymes as Druggable Targets for Brain Diseases

The central nervous system is characterized by a high content of sphingolipids and by a high diversity in terms of different structures. Stage- and cell-specific sphingolipid metabolism and expression are crucial for brain development and maintenance toward adult age. On the other hand, deep dysregulation of sphingolipid metabolism, leading to altered sphingolipid pattern, is associated with the majority of neurological and neurodegenerative diseases, even those totally lacking a common etiological background. Thus, sphingolipid metabolism has always been regarded as a promising pharmacological target for the treatment of brain disorders. However, any therapeutic hypothesis applied to complex amphipathic sphingolipids, components of cellular membranes, has so far failed probably because of the high regional complexity and specificity of the different biological roles of these structures. Simpler sphingosine-based lipids, including ceramide and sphingosine 1-phosphate, are important regulators of brain homeostasis, and, thanks to the relative simplicity of their metabolic network, they seem a feasible druggable target for the treatment of brain diseases. The enzymes involved in the control of the levels of bioactive sphingoids, as well as the receptors engaged by these molecules, have increasingly allured pharmacologists and clinicians, and eventually fingolimod, a functional antagonist of sphingosine 1-phosphate receptors with immunomodulatory properties, was approved for the therapy of relapsing-remitting multiple sclerosis. Considering the importance of neuroinflammation in many other brain diseases, we would expect an extension of the use of such analogs for the treatment of other ailments in the future. Nevertheless, many aspects other than neuroinflammation are regulated by bioactive sphingoids in healthy brain and dysregulated in brain disease. In this review, we are addressing the multifaceted possibility to address the metabolism and biology of bioactive sphingosine 1-phosphate as novel targets for the development of therapeutic paradigms and the discovery of new drugs.

The Cross-Talk Between Sphingolipids and Insulin-Like Growth Factor Signaling: Significance for Aging and Neurodegeneration

Bioactive sphingolipids: sphingosine, sphingosine-1-phosphate (S1P), ceramide, and ceramide-1-phosphate (C1P) are increasingly implicated in cell survival, proliferation, differentiation, and in multiple aspects of stress response in the nervous system. The opposite roles of closely related sphingolipid species in cell survival/death signaling is reflected in the concept of tightly controlled sphingolipid rheostat. Aging has a complex influence on sphingolipid metabolism, disturbing signaling pathways and the properties of lipid membranes. A metabolic signature of stress resistance-associated sphingolipids correlates with longevity in humans. Moreover, accumulating evidence suggests extensive links between sphingolipid signaling and the insulin-like growth factor I (IGF-I)-Akt-mTOR pathway (IIS), which is involved in the modulation of aging process and longevity. IIS integrates a wide array of metabolic signals, cross-talks with p53, nuclear factor κB (NF-κB), or reactive oxygen species (ROS) and influences gene expression to shape the cellular metabolic profile and stress resistance. The multiple connections between sphingolipids and IIS signaling suggest possible engagement of these compounds in the aging process itself, which creates a vulnerable background for the majority of neurodegenerative disorders.

The Role of Ceramide and Sphingosine-1-Phosphate in Alzheimer's Disease and Other Neurodegenerative Disorders

Bioactive sphingolipids-ceramide, sphingosine, and their respective 1-phosphates (C1P and S1P)-are signaling molecules serving as intracellular second messengers. Moreover, S1P acts through G protein-coupled receptors in the plasma membrane. Accumulating evidence points to sphingolipids' engagement in brain aging and in neurodegenerative disorders such as Alzheimer's, Parkinson's, and Huntington's diseases and amyotrophic lateral sclerosis. Metabolic alterations observed in the course of neurodegeneration favor ceramide-dependent pro-apoptotic signaling, while the levels of the neuroprotective S1P are reduced. These trends are observed early in the diseases' development, suggesting causal relationship. Mechanistic evidence has shown links between altered ceramide/S1P rheostat and the production, secretion, and aggregation of amyloid β/α-synuclein as well as signaling pathways of critical importance for the pathomechanism of protein conformation diseases. Sphingolipids influence multiple aspects of Akt/protein kinase B signaling, a pathway that regulates metabolism, stress response, and Bcl-2 family proteins. The cross-talk between sphingolipids and transcription factors including NF-κB, FOXOs, and AP-1 may be also important for immune regulation and cell survival/death. Sphingolipids regulate exosomes and other secretion mechanisms that can contribute to either the spread of neurotoxic proteins between brain cells, or their clearance. Recent discoveries also suggest the importance of intracellular and exosomal pools of small regulatory RNAs in the creation of disturbed signaling environment in the diseased brain. The identified interactions of bioactive sphingolipids urge for their evaluation as potential therapeutic targets. Moreover, the early disturbances in sphingolipid metabolism may deliver easily accessible biomarkers of neurodegenerative disorders.

Sphingolipid abnormalities in cancer multidrug resistance: Chicken or egg?

The cancer multidrug resistance (MDR) phenotype encompasses a myriad of molecular, genetic and cellular alterations resulting from progressive oncogenic transformation and selection. Drug efflux transporters, in particular the MDR P-glycoprotein ABCB1, play an important role in MDR but cannot confer the complete phenotype alone indicating parallel alterations are prerequisite. Sphingolipids are essential constituents of lipid raft domains and directly participate in functionalization of transmembrane proteins, including providing an optimal lipid microenvironment for multidrug transporters, and are also perturbed in cancer. Here we postulate that increased sphingomyelin content, developing early in some cancers, recruits and functionalizes plasma membrane ABCB1 conferring a state of partial MDR, which is completed by glycosphingolipid disturbance and the appearance of intracellular vesicular ABCB1. In this review, the independent and interdependent roles of sphingolipid alterations and ABCB1 upregulation during the transformation process and resultant conferment of partial and complete MDR phenotypes are discussed.

Neutral ceramidase-enriched exosomes prevent palmitic acid-induced insulin resistance in H4IIEC3 hepatocytes

Oversupply of free fatty acids such as palmitic acid (PA) from the portal vein may cause liver insulin resistance. Production of reactive oxygen species plays a pivotal role in PA‐induced insulin resistance in H4IIEC3 hepatocytes. Recently, we found that exosomes secreted from INS‐1 cells that were transfected with neutral ceramidase (NCDase) plasmids had raised NCDase activity; these NCDase‐enriched exosomes could inhibit PA‐induced INS‐1 cell apoptosis. Here, we showed that PA reduced insulin‐stimulated tyrosine phosphorylation of insulin receptor substrate 2 and decreased insulin‐stimulated uptake of the fluorescent glucose analog 2‐NBDG, confirming that insulin resistance occurred in PA‐treated H4IIEC3 cells. Moreover, NCDase‐enriched exosomes from INS‐1 cells rescued PA‐induced H4IIEC3 insulin resistance and blocked PA‐induced reactive oxygen species production in which ceramide was involved.

p38 mitogen-activated protein kinase/activator protein-1 involved in serum deprivation-induced human alkaline ceramidase 2 upregulation

Our previous study revealed that serum deprivation upregulated human alkaline ceramidase 2 (haCER2) activity and mRNA in HeLa cells, but the mechanism remains unknown. In the present study, serum deprivation also upregulated haCER2 activity in HepG2 human hepatoma cell line cells due to an increase in haCER2 mRNA, in which mRNA transcription, not mRNA stability, is involved. Furthermore, p38 mitogen-activated protein kinase (MAPK)/activator protein-1 (AP-1) signaling pathway is involved in haCER2 mRNA upregulation by serum deprivation, and this mechanism may explain why haCER2 is upregulated in human liver cancer. In conclusion, p38 MAPK, AP-1 or haCER2 may be used as targets in liver cancer therapy.

Essential roles of neutral ceramidase and sphingosine in mitochondrial dysfunction due to traumatic brain injury

In addition to immediate brain damage, traumatic brain injury (TBI) initiates a cascade of pathophysiological events producing secondary injury. The biochemical and cellular mechanisms that comprise secondary injury are not entirely understood. Herein, we report a substantial deregulation of cerebral sphingolipid metabolism in a mouse model of TBI. Sphingolipid profile analysis demonstrated increases in sphingomyelin species and sphingosine concurrently with up-regulation of intermediates of de novo sphingolipid biosynthesis in the brain. Investigation of intracellular sites of sphingosine accumulation revealed an elevation of sphingosine in mitochondria due to the activation of neutral ceramidase (NCDase) and the reduced activity of sphingosine kinase 2 (SphK2). The lack of change in gene expression suggested that post-translational mechanisms are responsible for the shift in the activities of both enzymes. Immunoprecipitation studies revealed that SphK2 is complexed with NCDase and cytochrome oxidase (COX) subunit 1 in mitochondria and that brain injury hindered SphK2 association with the complex. Functional studies showed that sphingosine accumulation resulted in a decreased activity of COX, a rate-limiting enzyme of the mitochondrial electron transport chain. Knocking down NCDase reduced sphingosine accumulation in mitochondria and preserved COX activity after the brain injury. Also, NCDase knockdown improved brain function recovery and lessened brain contusion volume after trauma. These studies highlight a novel mechanism of secondary TBI involving a disturbance of sphingolipid-metabolizing enzymes in mitochondria and suggest a critical role for mitochondrial sphingosine in promoting brain injury after trauma.

Secreted hCLCA1 is a signaling molecule that activates airway macrophages

The CLCA gene family produces both secreted and membrane-associated proteins that modulate ion-channel function, drive mucus production and have a poorly understood pleiotropic effect on airway inflammation. The primary up-regulated human CLCA ortholog in airway inflammation is hCLCA1. Here we show that this protein can activate airway macrophages, inducing them to express cytokines and to undertake a pivotal role in airway inflammation. In a U-937 airway macrophage-monocyte cell line, conditioned media from HEK 293 cells heterologously expressing hCLCA1 (with or without fetal bovine serum) increased the levels of pro-inflammatory cytokines (IL-1β, IL-6, TNF-α and IL-8). This effect was independent of the metalloprotease domain of hCLCA1. Primary porcine alveolar macrophages were similarly activated, demonstrating the effect was not cell line dependent. Similarly, immuno-purified hCLCA1 at physiologically relevant concentration of ~100 pg/mL was able to activate macrophages and induce pro-inflammatory response. This cytokine response increased with higher concentration of immuno-purified hCLCA1. These findings demonstrate the ability of hCLCA1 to function as a signaling molecule and activate macrophages, central regulators of airway inflammation.

New insight into the structure, reaction mechanism, and biological functions of neutral ceramidase

Ceramidase (CDase) is an enzyme that hydrolyzes the N-acyl linkage between the sphingoid base and fatty acid of ceramide. These enzymes are classified into three distinct groups, acid (Asah1), neutral (Asah2), and alkaline (Asah3) CDases, based on their primary structure and optimum pH. Acid CDase catabolizes ceramide in lysosomes and is found only in vertebrates. In contrast, the distribution of neutral and alkaline CDases is broad, with both being found in species ranging from lower eukaryotes to mammals; however, only neutral CDase is found in prokaryotes, including some pathogenic bacteria. Neutral CDase is thought to have gained a specific domain (mucin box) in the N-terminal region after the vertebrate split, allowing the enzyme to be stably expressed at the plasma membrane as a type II membrane protein. The X-ray crystal structure of neutral CDase was recently solved, uncovering a unique structure and reaction mechanism for the enzyme. Neutral CDase contains a zinc ion in the active site that functions as a catalytic center, and the hydrolysis of the N-acyl linkage in ceramide proceeds through a mechanism that is similar to that described for zinc-dependent carboxypeptidase. This review describes the structure, reaction mechanism, and biological functions of neutral CDase in association with the molecular evolution, topology, and mechanical conformation. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Radiation-induced acid ceramidase confers prostate cancer resistance and tumor relapse

Escape of prostate cancer (PCa) cells from ionizing radiation-induced (IR-induced) killing leads to disease progression and cancer relapse. The influence of sphingolipids, such as ceramide and its metabolite sphingosine 1-phosphate, on signal transduction pathways under cell stress is important to survival adaptation responses. In this study, we demonstrate that ceramide-deacylating enzyme acid ceramidase (AC) was preferentially upregulated in irradiated PCa cells. Radiation-induced AC gene transactivation by activator protein 1 (AP-1) binding on the proximal promoter was sensitive to inhibition of de novo ceramide biosynthesis, as demonstrated by promoter reporter and ChIP-qPCR analyses. Our data indicate that a protective feedback mechanism mitigates the apoptotic effect of IR-induced ceramide generation. We found that deregulation of c-Jun induced marked radiosensitization in vivo and in vitro, which was rescued by ectopic AC overexpression. AC overexpression in PCa clonogens that survived a fractionated 80-Gy IR course was associated with increased radioresistance and proliferation, suggesting a role for AC in radiotherapy failure and relapse. Immunohistochemical analysis of human PCa tissues revealed higher levels of AC after radiotherapy failure than those in therapy-naive PCa, prostatic intraepithelial neoplasia, or benign tissues. Addition of an AC inhibitor to an animal model of xenograft irradiation produced radiosensitization and prevention of relapse. These data indicate that AC is a potentially tractable target for adjuvant radiotherapy.

Transcriptional regulation of the human neutral ceramidase gene

Ceramidases play a critical role in generating sphingosine-1-phosphate by hydrolyzing ceramide into sphingosine, a substrate for sphingosine kinase. In order to elucidate its transcriptional regulation, we identify here a putative promoter region in the 5'-UTR of the human neutral CDase (nCDase) gene. Using human genomic DNA, we cloned a 3000 bp region upstream of the translational start site of the nCDase gene. Luciferase reporter analyses demonstrated that this 3000 bp region had promoter activity, with the strongest induction occurring within the first 200 bp. Computational analysis revealed the 200 bp essential promoter region contained several well-characterized promoter elements, lacked a conical TATA box, but did contain a reverse oriented CCAAT box, a feature common to housekeeping genes. Electrophoretic mobility shift assays demonstrated that the identified candidate transcriptional response elements (TRE) bind their respective transcription factors, including NF-Y, AP-2, Oct-1, and GATA. Mutagenic analyses of the TRE revealed that these sites regulated promoter activity and mutating an individual site decreased promoter reporter activity by up to 50%. Together, our findings suggest that regulation of nCDase expression involves coordinated TATA-less transcriptional activity.