A great variety of infections are accompanied by increases in anti-PL mostly in viral infections Hepatitis C virus, Epstein-Barr virus, Varicella virus, human immunodeficiency virus reviewed in [ ]. The avoidance of autoreactivity to the broadly distributed self-PL is a key instrument to prevent autoimmune diseases. However, little is known on the molecular mechanisms responsible of T cell autoreactivity on lipid antigens reviewed in [ ].
Protein prenylation is a ubiquitous covalent post-translational modification. It is the first critical step for membrane targeting and binding and mediates protein-protein interactions [ ]. This modification increases the hydrophobicity of the protein [ ], favors its temporary attachment to the membrane [ ] and is essential for a correct protein activity [ ].
Protein prenylation consists of the attachment of farnesyl 15 carbon or geranylgeranyl 20 carbon groups to the protein [ ]. First described in fungi [ ], farnesylation was described in mammals on lamin B [ , ]. Defective prenylation of proteins is linked to tumor pathogenesis among others [ ]. They are heterodimers recognising C-terminal consensus sequences in the target proteins. First, farnesyl residue binds to FTase [ , ], secondly the protein with a CaaX-box binds and the C-S bond is formed.
While the prenylated protein remains bound to the enzyme, another prenyl group binds to the enzyme and, lastly, the product prenylated protein is dissociated before or while a new CaaX-box substrate binds [ ]. As a consequence, stereochemistry at C-1 position of the isoprenoid is inverted [ , , ] but this causes minimal differences in protein conformation [ ].
Its enzymology is more complex and needs an escort protein to catalyze the reaction [ ]. Although, only a few prenyltransferase substrates e. This is the basis for the efforts made in the therapeutic targeting of protein prenylation.
Inhibitors of prenylation have been tested for the treatment of cancer, parasite causing diseases, viral and bacterial infections, multiple sclerosis or progeria [ ]. Proteins involved in signal transduction pathways require prenylation. An interesting example of these proteins is Ras protein family.
Many cancers present activating mutations in the RAS genes causing malignant activity of the protein [ ]. For this reason, prenyltransferase inhibitors have been tested in vitro and in animals for the inhibition of Ras proteins, showing promising results. Firstly, farnesyltransferase inhibitors FTIs were used to inhibit the farnesylation of proteins in cancer cells.
Unfortunately, clinical studies did not show robust anticancer activity [ ] with only several patients responding to FTIs both alone and in combination with other agents. In sum, some tumors depend on farnesylated proteins for survival, but further development is needed to apply this knowledge to the clinic [ ]. More recently, geranylgeranyltranseferase inhibitors GGTIs were developed. Ras protein isoforms were found to be geranylgeranylated when FTase was inhibited and some pathways downstream Ras were mediated by geranylgeranylated proteins [ ].
In cell and animal models, GGTIs block cell cycle in the G1 phase [ ], induce apoptosis, and inhibit tumor cell growth [ ]. Other protein susceptible to be targeted by prenylation inhibitors include proteins being exclusively farnesylated e. In the above sections it was made clear that many cell functions take place in membranes and their surroundings [ ].
Furthermore, it was reviewed how membrane components participate in functions altered in many human diseases [ 19 ].
Those functions include changing the localization of key proteins in the cell, in turn, altering key protein-protein interactions in membrane microdomains.
Subsequently, signaling cascades are altered and correct functioning of the cell is compromised. Hence, the new therapeutic approach, membrane lipid therapy MLT , aims to target membrane components to modify membrane composition or structure, i. This strategy has shown not only efficacy, but also higher specificity and safety, and has become a potential alternative to conventional drugs [ 6 ].
The variety of membrane microdomains based on different compositions and bearing specific biological properties was already introduced [ 45 ]. Furthermore, membrane microdomain composition, and subsequently function, may be modified by both lipid intake and enzymatic regulation [ 19 ].
Proteins involved in cell signaling interact with these microdomains and assemble crucial signaling platforms whose alteration might be pathological [ , ]. Thus, MLT aims to alter this protein signaling by modifying the composition and structure of the membrane microdomains and subsequently modulate cell signaling, offering potentially effective treatments for a variety of conditions [ , , , ].
The overall membrane lipid composition, the composition of membrane microdomains or the composition of internal organelles can be targeted by natural or modified lipids or drugs [ , , ]. For instance, proteins such as G proteins or protein kinase C PKC , as well as stress response proteins or enzymes such as SMS alter their binding to membrane according to membrane composition and structure [ 19 ]. Different regulatory effects might be obtained when targeting plasma membrane for therapy Figure 5 :.
Direct regulation through membrane structure modification: Dietary lipids and environmental changes modify cell membranes changing their properties and microdomain organization, thus controlling the localization and activity of proteins such as G proteins interaction with membrane and downstream signaling [ , ], the transcription of proteins involved in stress response such as heat shock proteins Hsp [ ], or the production of second messengers such as Cer [ ]. Regulation of enzymatic activity to alter membrane lipid levels: Enzymatic activity of SMS and other enzymes from SL metabolism are altered in cancer and consequently modify membrane composition and structure.
Modification of gene expression that results in alterations of membrane lipid composition: this gene expression change might affect the activity of an enzyme, protein-lipid interactions or protein-protein interaction. DNA-associated PL are found in nuclear membrane [ ] and their effects on nuclear functions have been documented [ ].
Evidence of this type of MLT include the increased SM levels in differentiated cells in contrast to low levels of this lipid on proliferating cells [ ]. This increase does not take place with SM addition but for SMS increased activity [ , ], which supports the concept of MLT-induced gene expression alteration. Lipid alterations that affect protein-protein interactions in specific membrane microdomains: The alteration of lipid ratios or the presence of particular lipids in membranes cause changes in protein-protein interactions.
Direct MLT-drug binding to a protein that alters its membrane binding affinity or that of other signaling proteins: In that case, the MLT formulated molecule binds to a protein rather than a lipid.
This is the case of prenylation inhibitors, which prevent Ras from binding to membrane and subsequently inhibit cancer cell proliferation while inducing cell differentiation and cell death [ ]. Molecular bases of Membrane-Lipid Therapy. Snapshot of the rationale behind targeting membrane composition or structure i. In MLT, synthetic fatty acids are newly designed with the aim of regulating membrane microdomain distribution mimicking natural lipids [ ].
Thus, MLT is a sophisticated therapeutic approach to make protein—protein interactions targetable in an environment where those interactions are not amenable to manipulation [ , , ]. Many diseases are already being targeted by MLT. The most relevant fields in which MLT molecules are being developed are listed below and further reviewed in [ 19 ] Figure 6. The basis of developing MLT in oncology comes from the finding that membrane lipid composition might work as a switch allowing or compromising propagation of proliferation signals received at the PM in tumor cells [ , ].
Many molecules have been developed since the discovery membrane-altering mechanism of action of doxorubicin [ ]. One of the most promising molecules is rationally-designed 2-OHOA 2-hydroxyoleic acid [ ], which is currently being tested in clinical trials for the treatment of glioma. It has shown good pharmaceutical efficacy and safety against cancer in humans ClinicalTrials. Other molecules being explored in oncology follow here: 2-hydroxylinoleic acid is on phase II clinical trial.
Worth mentioning are propofol-docosahexaenoic acid P-DHA and its analogue edelfosine reviewed in [ 6 ]. Metabolic and cardiovascular diseases: The use of dietary lipids for the treatment of diabetes or obesity is a clear case of MLT. Furthermore, oleic acid analogues induce reduction of body weight in rats by promoting overexpression of uncoupling proteins UCP1 and UCP3 and decreasing food intake [ ]. A high intake of oleic acid has also been shown to improve glycemic status and reduce saturated FA levels of diabetic patients while increasing those of unsaturated FA [ ].
This reduction is even greater with 2-hydroxioleic acid treatment [ , ]. Furthermore, unsaturated FA are cardioprotective [ ]. Neurodegenerative disorders: Behind adipose tissue, the central nervous system concentrates the largest depot of lipids in the body making it a primary target for MLT strategies.
Spinal cord injury might also benefit from MLT approaches reviewed in [ 19 ]. Other conditions that are bound to be ameliorated using MLT include infectious diseases, chemotherapeutic neuropathy, wound healing, retinopathies, nephropathies, acetaminophen liver toxicity, sunburn, ischemia reperfusion, intracranial hemorrhage, atrial fibrillation, vascular hypertension damage, and myocardial infarction as reviewed in [ 19 ].
Membrane-Lipid Therapy molecules are currently being developed for several medical conditions. Initially described as pure barriers, biological membranes are currently viewed as active players in cell and organelle architecture and physiology. Lipid composition peculiarities of the different cell compartments is behind their specific structural properties and functionalities. Indeed, lipid dysregulation is a relevant factor in the etiology of many diseases.
To exploit these new therapeutic avenues, lipids both natural and synthetic are being developed as potential drugs for different conditions using the MLT approach and are bound to constitute alternative or innovative therapies for fields in which there are no available or promising treatments. We apologize to all authors whose work could not be cited due to space constraints. The authors thank C Hoffmann for editorial assistance.
National Center for Biotechnology Information , U. Int J Mol Sci. Published online May 1. Doralicia Casares , 1, 2 Pablo V. Pablo V. Author information Article notes Copyright and License information Disclaimer. Received Apr 1; Accepted Apr This article has been cited by other articles in PMC. Abstract Biological membranes are key elements for the maintenance of cell architecture and physiology.
Keywords: membrane, lipid, therapy, endomembrane, structure, composition, 2OHOA. Introduction Biological membranes define cell boundaries and internal organelles in eukaryotes. Repertoire of Membrane Lipids Membrane lipids are the least studied biomolecules.
Nonetheless, cells depend upon lipids for three main functions, namely energy storage, compartmentalization and signaling reviewed in [ 16 ] : Energy storage: lipid droplets used for this function contain mainly triacylglycerol and steryl esters thanks to their relatively reduced state.
Open in a separate window. Figure 1. Figure 2. Endoplasmic Reticulum The secretory pathway is the best characterized model of lipid localization and involves the endoplasmic reticulum ER , Golgi apparatus, and PM.
Plasma Membrane Different cell types contain different lipid and protein compositions of their PM. Endosomes PM lipid domains are indispensable for the assembly of signalosomes and subsequently play a key role in their internalization [ 64 , 65 ]. Mitochondria Mitochondria rely on lipid and protein import for proper function [ 71 ]. Lysosomes Lysosomes contain the enzymatic tools for the degradation of extracellular molecules.
Nuclear Membrane Eukaryotic nucleus encloses this chromatin using a double membrane nuclear envelope, NE. Nuclear Size During NE reassembly after cell division, cell needs to adjust the proper nuclear size. Nuclear Phospholipid Regulation of Chromatin Lipid—nucleic acid interactions have been hypothesized to be the basis for the organization and expression of cellular genome reviewed in [ ].
Cellular Mechanisms of Physicochemical Membrane Homeostasis The amphiphilic nature of PL leads their spontaneous distribution into bilayers in aqueous environments, which are the basic component of all cellular membranes.
Figure 3. Lipid Imbalances and Human Pathologies The lipidome of cellular organelles is remodeled not only upon physiological conditions but also under pathological conditions Figure 4. Figure 4. Cancer Cancer cells require significant cellular growth and therefore membrane biosynthesis to survive.
Several alterations in membrane composition and PL synthesis, distribution, or catabolism reported in cancer are listed below: PtdIns 3,4,5 P3 concentration is also elevated in numerous cancers.
Polarized epithelia turn into disorganized structures that can occupy the adjacent tissues. In epithelial tissues, aberrant FA synthesis was linked to the loss of cell polarity.
In addition, other findings suggest that dietary-derived lipids might enlarge the overall lipid composition of malignant cells and so influence multiple signaling events within tumors [ ]. Examples include the support to cancer-associated fibroblasts by the expression of FASN [ ], the compromise of proper macrophage functioning upon FA biosynthesis [ , ] and of proper immune response upon prostaglandin presence [ ]. Highly proliferative cancer cells display a strong lipid and Chol avidity, which they fulfill by raising the incorporation of exogenous or dietary lipids or increasing their endogenous synthesis.
Excessive lipids and cholesterol in cancer cells stored in lipid droplets are considered marks of cancer aggressiveness reviewed in [ ].
Metabolic Diseases Several lysosomal storage disorders LSDs affect lysosomal hydrolases, membrane transporters, accessory or trafficking proteins and derive into lipid storage disorders sphingolipidoses, gangliosidoses, leukodystrophies reviewed in [ 95 ]. In parallel, an elevated degree of membrane saturation disrupts calcium homeostasis and triggers liver ER stress [ ]. Chronic activation of ER stress dysregulates lipid homeostasis and might lead to dyslipidemia, insulin resistance, type II diabetes, and obesity [ ].
As above mentioned, TFEB participates in autophagy and in the clearance of lipid droplets. In obese animals, TFEB overexpression rescues obesity and associated metabolic syndrome by promoting lipophagy [ ]. Neurological Disorders The neural membrane lipid composition has been reported to change in neurodegeneration. Immunological Disorders Anti-phospholipid syndrome APS is an autoimmune disease caused by the appearance of anti-PL antibodies and developed with vascular thromboembolism and miscarriages reviewed in [ ].
Regulation of Cellular Activities by Prenylation Protein prenylation is a ubiquitous covalent post-translational modification. Molecular Bases of Targeting the Plasma Membrane In the above sections it was made clear that many cell functions take place in membranes and their surroundings [ ].
Different regulatory effects might be obtained when targeting plasma membrane for therapy Figure 5 : Direct regulation through membrane structure modification: Dietary lipids and environmental changes modify cell membranes changing their properties and microdomain organization, thus controlling the localization and activity of proteins such as G proteins interaction with membrane and downstream signaling [ , ], the transcription of proteins involved in stress response such as heat shock proteins Hsp [ ], or the production of second messengers such as Cer [ ].
Figure 5. Development of Membrane Lipid Therapy in Different Therapeutic Areas In MLT, synthetic fatty acids are newly designed with the aim of regulating membrane microdomain distribution mimicking natural lipids [ ]. Figure 6. Future Directions and Conclusions Initially described as pure barriers, biological membranes are currently viewed as active players in cell and organelle architecture and physiology.
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B is the major site of carbohydrate synthesis in eukaryotic cells. C produces proteins for cell membranes. D helps assemble ribosomes for protein synthesis. B chemically modified in the nucleus. C produced by the cell for internal use. D released from the cell through the plasma membrane. B lysosomes. C endoplasmic reticulum. D nucleoli. B involves damage to liver cells. C is due to the absence of an enzyme that digests polysaccharides. D prevents the breakdown of glycogen. B The central vacuole of a plant cell may store waste products.
D The central vacuole of a plant cell may store poisons. B watery fluid enclosed by the inner membrane of a mitochondrion. C space between the inner and outer membranes of a chloroplast. D space between the inner and outer membranes of a mitochondrion. B surround the nucleus. C guide the movements of organelles. D support the inner mitochondrial membrane. B the function of lysosomes. C contraction of muscle cells. D the movements of sperm cells.
C cilia are typically more numerous and shorter than flagella. B similar in structure to centrioles. D identical in structure to cilia. D are the anchoring proteins in basal bodies. B helps to keep phospholipids from being too close to one another. C is an abnormality resulting from a diet high in cholesterol. A Membrane proteins serve as enzymes.
B Membrane proteins act as receptors to molecules like hormones. C Membrane proteins form junctions between cells. D Membrane proteins transfer genetic information to the cytoplasm. A spontaneous degradation of the intracellular environment B self-assembly into a simple membrane C ability to form an impermeable membrane D formation of a semi-solid membrane.
B very slowly diffuse through a membrane's lipid bilayer. D are actively transported across cell membranes. The presence of these organelles tells you that the environment A is isotonic to the protozoan.
B is hypotonic to the protozoan. C contains a higher concentration of solutes than the protozoan. D is hypertonic to the protozoan. A isotonic B hypotonic C hypertonic D hydrophilic. A energy and transport proteins; down B transport proteins; down C energy and transport proteins; against D transport proteins; against. B drinking. C chewing. D lysis. B violate the second law of thermodynamics. D are examples of a closed system.
A the burning of wood B the synthesis of glucose from carbon dioxide and water C the breakdown of glucose D cellular respiration. B is higher than the activation energy of a reaction. C prevents the spontaneous breakdown of molecules in the cell. D can only be overcome with the use of enzymes.
B a noncompetitive inhibitor is involved.
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