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STAYING CURRENT
Division of Basic Biomedical Sciences, Sanford School of Medicine, University of South Dakota, Vermillion, South Dakota
Address for reprint requests and other correspondence: K. M. Eyster, Div. of Basic Biomedical Sciences, Sanford School of Medicine, Univ. of South Dakota, 414 E. Clark St., Vermillion, SD 57069 (e-mail: Kathleen.Eyster{at}usd.edu)
Abstract
Reviews of signal transduction have often focused on the cascades of protein kinases and protein phosphatases and their cytoplasmic substrates that become activated in response to extracellular signals. Lipids, lipid kinases, and lipid phosphatases have not received the same amount of attention as proteins in studies of signal transduction. However, lipids serve a variety of roles in signal transduction. They act as ligands that activate signal transduction pathways as well as mediators of signaling pathways, and lipids are the substrates of lipid kinases and lipid phosphatases. Cell membranes are the source of the lipids involved in signal transduction, but membranes also constitute lipid barriers that must be traversed by signal transduction pathways. The purpose of this review is to explore the magnitude and diversity of the roles of the cell membrane and lipids in signal transduction and to highlight the interrelatedness of families of lipid mediators in signal transduction.
Key words: phospholipase; sphingomyelin; ceramide; fatty acid; lysophospholipid; phosphatidylinositol
THE CELLS OF A MULTICELLULAR ORGANISM use chemical messengers to communicate with each other, and single-celled organisms respond to chemical messengers in their environment. Moreover, the intracellular compartments and organelles of a cell must communicate with each other. The chemical messengers used in cellular communication are either water soluble or lipid soluble. Cells, themselves, are defined by the physical presence of the lipid bilayer membrane barrier that separates the inside of a cell from the outside of a cell, and intracellular compartments are defined by lipid membrane barriers as well. Extracellular and intracellular water-soluble chemical messengers cannot cross cellular membranes, so their messages must be transduced across the membrane. Lipid-soluble messengers can cross cell membranes and communicate directly with the contents of the cell by binding to intracellular receptors. Signal transduction is the field of science that seeks to comprehend the mechanisms that cells have developed to interpret the messages carried by extra- and intracellular chemicals into messages that the cell can understand. Thus, the field of signal transduction is important because of its fundamental role in cellular communication and regulation of cellular responses.
The field of signal transduction is also important because cellular communication often goes awry in pathological situations. Many diseases result from aberrant communication among cells or from problems with the machinery of signaling pathways (6, 27, 28, 54, 67). For example, mutations in components of signal transduction pathways that regulate mitosis can result in tumorigenesis (6, 34, 46, 89, 90).
Cellular physiology requires the presence of a barrier between cells, but the cell membrane is not merely a barrier that must be traversed; rather, the membrane and its constituent lipids are also indispensable participants in many events of signal transduction. Reviews of signal transduction have often focused on the cascades of protein kinases and protein phosphatases and their cytoplasmic substrates that become activated in response to extracellular signals. Lipids, lipid kinases, and lipid phosphatases have not received the same amount of attention as proteins in studies of signal transduction. It is important to not only acknowledge the contribution of the membrane and lipids to signal transduction but also to recognize that this contribution is substantial. For example, membrane lipids participate as components of signal transduction pathways and as docking sites for cytoplasmic signaling proteins, and they give rise to cleavage products that act as ligands or substrates for other signaling molecules. Nonmembrane lipids have a role in signal transduction as well; lipids serve as ligands, and posttranslational lipid modifications provide a means for proteins to associate intimately with the membrane. The magnitude and diversity of the roles of the membrane and lipids in signal transduction can be easily overlooked. Therefore, this review focused on the role of the cell membrane and its constituent lipids in signal transduction.
A look at the big picture sets the stage for a discussion of the details of lipid signal transduction. The first important aspect of the big picture is the interrelatedness among the lipid signaling pathways (Fig. 1). The details of these complex interactions are described in the following text, but several examples can be pointed out here. As shown in Fig. 1, phosphatidylcholine is the parent molecule for a number of lipid messengers. The presence in a given cell of one phospholipase (PL) enzyme versus another determines whether a parent lipid molecule such as phosphatidylcholine gives rise to arachidonic acid (AA), phosphatidic acid (PA), or platelet-activating factor (PAF). The interrelatedness of lipids involved in signal transduction is further illustrated by the fact that the polar head group of sphingomyelin, phosphorylcholine, is the same as the polar head group of phosphatidylcholine. Many other examples of the interrelatedness of lipid signaling pathways are described in the following text.
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Phospholipids comprise the most abundant class of membrane lipids. Phospholipids are composed of two fatty acid tails, glycerol, a phosphate group, and a polar head group (Fig. 3). Of the phospholipids, phosphatidylethanolamine, phosphatidylserine, and PI are found primarily in the inner leaflet of the membrane, whereas phosphatidylcholine is found primarily in the outer leaflet of the membrane. Phospholipids do not flip flop from one leaflet to the other independently. A class of enzymes called phospholipid scramblases catalyze the movement of phospholipids from one leaflet to the other (77).
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A fatty acid is composed of a long-chain aliphatic carboxylic acid. Fatty acids may be saturated (no double bonds in the hydrocarbon chain), monounsaturated, or polyunsaturated (one or more double bonds in the hydrocarbon chain, respectively). The presence of a double bond in a fatty acid adds a kink. Fatty acids that have more double bonds have more kinks in their structure and take up more space. Thus, as the number of double bonds increases, the membrane becomes more fluid because the fatty acids fit less closely together. Similarly, the fewer the number of double bonds (saturated fatty acids), the more tightly the fatty acid tails fit together and the less fluid the membrane. As shown in Fig. 3, many membrane lipids contain fatty acid chains. The fatty acid components of membrane lipids vary widely. Two lipids with the same parent structure may have very different fatty acids attached even though they come from the same source or from the same membrane. Physiologically, this means that hydrolysis of a given species of membrane lipid may yield different fatty acids. Also, the identity of a lipid is determined by its parent structure and not by its fatty acids, since the fatty acids may vary.
Phospholipases
The family of PL enzymes cleaves membrane phospholipids; each of the PLs acts at a different site on the phospholipid (Fig. 4). Both cell membrane and intracellular membrane phospholipids are substrates for PLs. The products that result from these cleavages are involved in a variety of aspects of signal transduction.
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, PGE2 (via the EP3D receptor), prostacyclin, and thromboxane (55). In contrast, stimulation of the 
-isoform of PLC (PLC-) is achieved by ligand-activated enzyme-linked tyrosine kinase receptors (7) (Fig. 5A). Examples of ligands that activate PLC- via enzyme-linked tyrosine kinase receptors include EGF, PDGF, FGF, and NGF (7). Two second messengers are released by the PLC cleavage reaction. The polar PI head group [PI (1,4,5)-trisphosphate (IP3)] that is liberated upon cleavage by PLC is released into the cytoplasm, where it acts as a second messenger (Fig. 5). IP3 releases Ca2+ from intracellular stores (4) (Fig. 2). Ca2+, in turn, activates its own series of signaling functions (78), including the activation of cytoplasmic PLA2 (cPLA2) (8). DAG is the additional second messenger. DAG was the first membrane lipid to be identified as a direct component of a signal transduction pathway (39). The first of the cellular messenger functions of DAG to be identified was the activation of the conventional (, ßI, ßII, and ) and novel (
, 
, 
, and 
) isoforms of PKC (50) (the atypical isoforms of PKC, PKC-
and -
, are not activated by DAG). In addition, DAG activates chimaerins, proteins with GTPase-activating protein activity toward Rac (87). DAG functions are confined to the lipid bilayer (Fig. 5A). Interestingly, the conventional and novel isoforms of PKC bind to the membrane phospholipid phosphatidylserine as well as to DAG to achieve activation (82). Since DAG and phosphatidylserine are confined to the membrane, the conventional and novel isoforms of PKC must translocate to the membrane for activation.
The body of literature describing the family of PLD isoforms is much smaller than that for PLC. The two isoforms of PLD (PLD1 and PLD2) are both palmitoylated as a posttranslational modification and both contain pleckstrin homology (PH) and phox homology (PX) lipid binding domains. The palmitoylation and two lipid binding domains all contribute to the association of PLD isoforms with membrane lipids (21). PLD cleaves the polar head group from phospholipids, leaving PA behind. Thus, PA is composed of the fatty acid chains, glycerol, and phosphate group of the initial phospholipid (Fig. 4). Activation of PLD1 requires association with a complex of proteins and lipids. These include two of the small GTPases, RalA and ARF6, the conventional PKC isoform PKC-, and the membrane phospholipid PI(4,5)P2 (introduced above as the substrate for PLC) (25) (Figs. 2 and 6). PLD does not appear to be directly associated with a receptor; that is, PLD is not activated by a GPCR/G protein or by an enzyme-activated receptor as are the PLC isoforms described above. Rather, PLD isoforms are activated by downstream effectors of other signaling pathways. The activation of PKC by PLC-ß or PLC-, as described above, and activation of guanine nucleotide exchange factors that activate RalA and ARF6 result in the activation of PLD (36). Therefore, PLD responds to activation by extracellular ligands, but the response is more indirect than direct.
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The PLA2 family is much larger than the PLC or PLD families, with multiple forms of both cytosolic and secretory isoforms (53). Secretory PLA2 (sPLA2) isoforms are stored in secretory granules and secreted in response to stimuli (53). sPLA2 isoforms are activated by Ca2+. Since the levels of Ca2+ in the extracellular space are high enough to maintain the activity of sPLA2, the actual regulation must occur at the points of gene transcription (regulation of availability of the gene product) and of protein secretion. Factors that increase the transcription and secretion of sPLA2 include inflammatory mediators such as TNF- and IL-1ß and downstream mediators of the 12/15-lipoxygenase (LOX) pathway such as 12(S)- or 15(S)-hydroxyeicosatetraenoic acid (HETE) and 12(S)-hydroxyoctadecadienoic acid (HODE) (44). cPLA2 isoforms are activated by increases in intracellular Ca2+ and by phosphorylation by several MAPKs (48). Therefore, an increase in intracellular Ca2+ by activation of the PLC-IP3 pathway to release intracellular Ca2+ (Fig. 2) or by opening membrane Ca2+ channels to allow Ca2+ entry from the extracellular space activates cPLA2. Also, activation of the MAPK pathway by growth factors (such as EGF, PDGF, FGF, and NGF) (7) activates cPLA2 through phosphorylation by MAPK. Moreover, the literature indicates that multiple levels of interaction exist among the various isoforms of PLA2 in both regulation and function. For example, the activation of cPLA2 increases the concentrations of HETE and HODE, which stimulate the transcription of sPLA2 (44), and cPLA2 and sPLA2 interact in the cleavage of lipids (8).
All of the isoforms of PLA2 cleave membrane phospholipids to yield one free fatty acid plus lysophospholipid (Fig. 4). The most physiologically important substrate for PLA2 is phosphatidylcholine (Figs. 1 and 7). The fatty acids that are incorporated into phosphatidylcholine vary, but the most common and most physiologically important fatty acid that can be released from phosphatidylcholine by PLA2 is AA (eicosatetraenoic acid) (53). The resulting lysophospholipid is lysophosphatidylcholine (LPC) (69). LPC binds as a ligand at specific GPCRs (33) and activates PLC-ß to release IP3 and DAG, with resultant increases in intracellular Ca2+ and activation of PKC (91) (Fig. 2). In addition, LPC regulates MAPK in a cell type-dependent manner (91). The activation of MAPK by LPC occurs by a link between the GPCR and phosphatidylinositol 3-kinase (PI3K) (93). The GPCR for LPC is found at highest levels in the spleen and thymus, so it is considered to play an important role in immune system regulation (94). AA released by PLA2 is the substrate for three different pathways: the cyclooxygenase (COX), LOX, and cytochrome P-450 2C (CYP2C) pathways (Figs. 1, 2, and 7). The two isoforms of COX (COX1 and COX2) give rise to PGs (27, 56, 66). The COX1 isoform is constitutively expressed, whereas the COX2 isoform is induced by extracellular ligands. Each of the PGs has its own cell type-specific effects. The isoforms of LOX (5-, 12-, and 15-LOX) give rise to leukotrienes, HETE, HODE, hydroxyperoxyeicosatetraenoic acids, and lipoxins (67). The CYP2C pathway gives rise to epoxyeicosatrienoic acids (EETs) (35). EETs are metabolized to dihydroxyepoxyeicosatrienoic acid (DHETE) by soluble epoxide hydrolase. Each of these pathways yields multiple products, some of which act as intracellular messengers (47, 94) and others that leave the cell and act as ligands on G protein-coupled membrane receptors (26, 55, 94). The array of specific synthase enzymes for each of the PGs and leukotrienes that is present in a given cell varies and determines which PGs or leukotrienes are produced in that cell. These lipid messengers are involved in a variety of physiological and pathological processes. For example, PGs are involved in ovulation (20), gastrointestinal tract function (86), and inflammation (94) among other functions. Leukotrienes are involved in the respiratory tract, and their overactivation is implicated in asthma (67). EETs have vasodilatory effects, whereas their metabolites, DHETEs, are vasoconstrictive (79).
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Platelet-Activating Factor
PAF is a modified phospholipid whose potent biological activities have been recognized for several decades (68). PAF is derived from the membrane phospholipid phosphatidylcholine. cPLA2 cleaves one fatty acid from phosphatidylcholine to yield LPC. Acetyl-CoA:lysoPAF acetyltransferase (lysoPAF AT) then acetylates LPC to yield PAF (61) (Figs. 4 and 7). Both cPLA2 and lysoPAF AT are activated by Ca2+ and via phosphorylation by MAPK family members such as ERK1, ERK2, and p38. However, specific MAPKs are responsible for phosphorylating the two enzymes in different cellular responses. For example, ERK1/2 phosphorylates cPLA2, whereas p38 MAPK phosphorylates lysoPAF AT (61). In cellular stress responses, p38 MAPK activates both enzymes in the PAF synthetic pathway (61). PAF can be found in the circulation, but data have suggested that the PAF that remains associated with the membrane of the cell of synthesis is the more bioactive fraction. Membrane-associated PAF acts in a paracrine manner at GPCRs on neighboring cells (12, 68). Receptor binding by PAF activates PLC and cPLA2. The result of PLC activation is a downstream increase in intracellular Ca2+ and the activation of PKC (Fig. 4). The result of PLA2 activation is increased substrate for the COX, LOX, and CYP2C pathways as well as for the PAF pathway (Fig. 7). The result is a positive feedback effect in that the increased intracellular Ca2+ contributes to the activation of both cPLA2 and lysoPAF AT. This enzyme activation coupled with increased substrate results in greater PAF synthesis (Fig. 2). The functions of PAF include inflammatory responses, stress responses, ovulation, and blastocyst implantation (12, 61, 68).
Lipid Kinases and Phosphatases
The focus in signal transduction, both in teaching and experimental design, is usually on protein kinases and protein phosphatases. As the understanding of lipid signaling molecules expands, the functions of lipid kinases and lipid phosphatases are proving to be equally important to those of proteins. The PI3K pathway is the most thoroughly studied of the lipid phosphorylation pathways. PI3K is a downstream effector of mitogens (90). Mutations of this pathway that result in unregulated activation have been implicated in cancer (90); hence, the broad interest in the PI3K pathway. Under normal circumstances, PI3K is activated by extracellular ligands such as insulin (19, 72) and other mitogenic growth factors (90) (Fig. 2). The parent substrate for PI3K is PI, the same phospholipid that is the substrate for the PLC and PLD pathways. There are many isoforms of PI3K; some of them prefer PI(4,5)P2 as a substrate to yield PI (3,4,5)-trisphosphate [PI(3,4,5)P3] as the end product (Figs. 5B and 8) (84). Other isoforms of PI3K prefer PI as substrate to yield PI (3)-monophosphate [PI(3)P] as the end product (84). Both PI(3)P and PI(3,4,5)P3 act as docking sites for proteins that translocate to the plasma membrane (Fig. 8). Proteins with FYVE or PX domains (named for the first four proteins identified with the domain: Fab1, YOTB, Vac1p, and EEA) (18, 45) bind to PI(3)P. PI(3,4,5)P3 serves as the binding site for proteins containing PH domains [such as Akt/PKB, phosphoinositide-dependent protein kinase 1 (PDK1), Btk family tyrosine kinases, and PLC-] (17, 45, 70). Thus, the activity of PI3K results in the formation of binding sites for proteins at the intracellular face of the plasma membrane (Fig. 8).
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The family of isoforms of Ca2+- and lipid-dependent protein kinase (PKC) also requires phosphorylation of the activation loop by PDK1 while bound to PI(3,4,5)P3 (58). However, phosphorylation of PKC in the activation loop is a maturational step rather than a direct activator of the enzyme. After proteins are synthesized in the cell, they must undergo processing before they are mature and can carry out their cellular functions. During maturation, PKC isoforms translocate to the cell membrane and bind to PI(3,4,5)P3. This brings them into juxtaposition with PDK1, which phosphorylates PKC in the activation loop. The conventional and novel isoforms of PKC must be phosphorylated in this manner before they can be activated by binding to Ca2+ and/or lipids (11, 58). The atypical isoforms of PKC also undergo phosphorylation in the activation loop while bound to PI(3,4,5)P3. However, the subsequent activational steps for atypical PKC isoforms remain obscure. Some evidence has suggested that phosphorylation of atypical PKC isoforms (PKC- and PKC-) (50) by PDK1 is sufficient for activation (14, 58), whereas other data have suggested that activation involves the PB1 protein-protein interacting domain (32, 62, 95). Thus, PI(3,4,5)P3 is critical for the maturation and activation of cytoplasmic serine/threonine protein kinases through both direct (Akt/PKB) and indirect (PKC) models.
In the same manner that protein phosphatases are critical to balance the function of protein kinases (22), lipid phosphatases are critical to balance the function of lipid kinases. For example, phosphatase and tensin homolog deleted on chromosome 10 is a lipid phosphatase that dephosphorylates the three position of PI(3,4,5)P3 (46, 89) and thereby reverses the lipid phosphorylation step of PI3K on PI(3,4,5)P3. Myotubularin dephosphorylates the three position of PI(3)P (89), and the five position of PI(3,4,5)P3 is dephosphorylated by Src homology domain 2 (SH2)-containing inositol phosphatase-1 and -2 (6).
Sphingomyelin, Ceramide, and Lipid Rafts
Sphingomyelin is an important structural component of the cell membrane and of lipid rafts. Sphingomyelin serves an important functional role as well, as it is the parent compound of several lipid mediators (15) (Fig. 9). The family of sphingomyelinase isoforms cleaves sphingomyelin to yield ceramide and phosphorylcholine (16). There are at least five isoforms of acidic, neutral, and basic sphingomyelinases. The pH designation refers to the optimal pH for association of the substrate with the enzyme rather than the pH for optimal activity of the enzyme (41). Despite extensive studies, details of the location and physiological regulation of sphingomyelinase isoforms have remained incomplete (49). An example of the complexity of lipid signaling is illustrated by our present knowledge about the differential regulation of sphingomyelinase isoforms by TNF. The binding of TNF to its 55-kDa receptor results in the stimulation of inflammatory pathways if neutral sphingomyelinase is activated but results instead in apoptosis if acidic sphingomyelinase is activated (40, 71). In both cases, the mechanism of action is release of ceramide from sphingomyelin. Because ceramide is extremely hydrophobic, it remains in the membrane compartment in which it is formed. Neutral and acidic sphingomyelinases are in separate membrane compartments, so the ceramide they produce stays in those separate compartments. This compartmentalization isolates the inflammatory pathway from the apoptosis pathway. Signaling by Fas ligand also activates sphingomyelinase activity and leads to apoptosis (41).
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Ceramide, C1P, sphingosine, and S1P have all been shown to carry out second messenger functions. Ceramide can directly activate PKC- (38) and other signaling pathway components (30, 40, 71), although the mechanisms of activation have remained in question, whereas sphingosine can inhibit PKC (29). Ceramide is a direct activator of type 1 and 2A protein phosphatases (9), whereas C1P is a potent inhibitor of these serine/threonine protein phosphatases (10). Both ceramide and sphingosine stimulate cell cycle arrest and apoptosis (10). In contrast, both C1P and S1P stimulate cell survival and proliferation and are antiapoptotic (10). Both C1P and S1P are involved in inflammatory reactions but at different points. S1P induces the transcription of COX2, whereas C1P activates cPLA2 to release AA, the substrate for COX2 (2, 10). C1P appears to have only intracellular messenger functions, whereas S1P has intracellular messenger functions and can also be released from the cell and bind to G protein-linked edg/S1P receptors (42, 43). As an intracellular messenger, S1P has been proposed to release intracellular Ca2+ in a manner similar to but independent of IP3 (2) (Fig. 2). Since ceramide, C1P, sphingosine, and S1P are interconvertible, the specific array of lipid mediators in this pathway in a given cell depends on which enzymes are available and have been activated in that cell. The biological effects of these lipid mediators are still under investigation, and it is expected that other cellular targets remain to be discovered.
In an alternative pathway, sphingomyelin deacylase converts sphingomyelin to sphingosylphosphorylcholine (Fig. 9) (51). Sphingosylphosphorylcholine has intracellular messenger functions and can also leave the cell to bind to GPCRs (42, 51) (Fig. 2). Sphingosylphosphorylcholine increases the intracellular concentration of Ca2+, activates ERK, and inhibits cell proliferation (51).
Within the sphingomyelin family of lipid mediators, the reactions are reversible and the lipids are interconvertible. The discussion above focuses on the breakdown of a parent lipid to a product lipid that carries out a specific function as is most likely to occur in a rapid signal transduction event. However, the reverse reactions are utilized in the de novo synthesis of the lipids. For example, the de novo synthesis of sphingomyelin proceeds from sphingosine through ceramide to sphingomyelin. An increase of ceramide in a lipid raft may come from sphingomyelin or sphingosine.
Fatty Acids
Fatty acids are integral structural elements of many of the lipids that have been discussed herein including phospholipids, sphingomyelin, ceramide, lysophospholipids, DAG, phosphatidic acid, LPA, PAF, C1P, and sphingosylphosphorylcholine (Figs. 3, 4, and 9). Fatty acids can be released from these lipids by PLA2 and other deacylases. In the past, the focus of signaling research has typically been on the parent compounds rather than on the fatty acids. However, the focus has been shifting to include fatty acids as we learn more about the functions of fatty acids beyond their role in energy metabolism. Fatty acids have been identified as ligands for members of the steroid superfamily of intracellular receptors (37, 47, 94) as well as for membrane GPCRs (43). Fatty acids have also been implicated in the accumulation of ceramide by stimulating de novo synthesis (31) or by increasing neutral sphingomyelinase activity (41). The role of AA as a precursor for multiple lipid mediators is another fatty acid function that is important in signal transduction. It is likely that additional functions of fatty acids in signal transduction have yet to be discovered.
Role of Dietary Lipids in Signal Transduction
The fatty acids that become incorporated into membrane lipids can come from the diet or can be synthesized by cells. A very interesting current topic in both the lay and scientific press is the role of dietary lipids in health and disease. Saturated fatty acids include palmitic and stearic acids (80) and are found in coconut oil, butter, and red meat. Monounsaturated fatty acids include palmitic and oleic acids (80) and are found in olive and peanut oils. Polyunsaturated fatty acids are categorized as 
-6 (linoleic, -linolenic, and AA) or -3 (-linolenic, eicosapentaenoic, and docosahexaenoic acid). The designation refers to the location of the first double bond in the hydrocarbon chain relative to the methyl end of the molecule (i.e., the sixth carbon or third carbon) (92). The -6 polyunsaturated fatty acids are found in many plant oils such as olive, canola, soybean, corn, cottonseed, sunflower, and palm. The primary source of long-chain -3 polyunsaturated fatty acids is fatty fish (3, 80) such as salmon and tuna. The sources of saturated and -6 polyunsaturated fatty acids are more common in Western diets than those of -3 fatty acids. A large body of evidence supports the concept of healthy effects of polyunsaturated -3 fatty acids such as those found in fatty fish (28, 60). When -3 fatty acids are included in the diet, those fatty acids are involved in cellular functions in competition with the more common -6 fatty acids because the same enzymes are used for the processing of -3 and -6 fatty acids. The -3 fatty acids can participate in the structures (i.e., membrane lipids) and functions (i.e., signal transduction) described in this review for fatty acids in general, but the structural differences of -3 and -6 fatty acids result in functional differences as well. The -3 fatty acids are released into the cell when membrane lipids containing them are cleaved by enzymes such as PLA2 or sphingomyelin deacylase. The -3 fatty acids are poorer substrates for COX, LOX, and CYP2C enzymes than are -6 fatty acids such as AA (37). Since COX and LOX mediate inflammatory pathways, the reduced efficiency of these enzymes with -3 fatty acids as substrates allows -3 fatty acids to have an anti-inflammatory effect when they have been incorporated into cell membranes (37). As mentioned above, fatty acids have been shown to act as ligands at both intracellular (37, 47, 94) and membrane receptors (43) and can increase the concentration of ceramide in cell membranes (31, 41). Whether the ability of -3 fatty acids to regulate the activity of these pathways differs from that of -6 fatty acids has not been thoroughly examined, but it is expected that their effects will differ. The -3 fatty acids also change the composition of the lipid rafts and the fluidity of the membranes into which they are incorporated (37). Signal transduction proteins that typically associate with lipid rafts may be excluded when the content of -3 fatty acids is increased because -3 fatty acids make the lipid rafts thicker or because the lipid environment is more unsaturated than when -6 fatty acids are high (37). Lipid-anchored proteins may no longer associate with the lipid raft if the lipid attachment is an -3 fatty acid instead of an -6 fatty acid. For example, the nonreceptor tyrosine kinase Fyn may be unable to associate with lipid rafts when it is anchored with a -3 fatty acid instead of an -6 fatty acid (37). Thus, many mechanisms by which dietary polyunsaturated fatty acids may affect signal transduction have been proposed. However, the specific mechanisms involved have not yet been rigorously demonstrated.
Lipids as Ligands
The above discussion describes specific lipid mediators that leave the cells of origin and bind to GPCRs in the membranes of neighboring cells or on the cell of origin. These lipid mediators include PGs, leukotrienes, lysophospholipids, sphingosylphosphorylcholine, and PAF (42) (Fig. 2). Another class of lipid-soluble ligands acts as conventional hormones; they travel in the blood to distant sites, where they bind to receptors and carry out their biological activity. The steroid hormones estradiol, progesterone, testosterone, aldosterone, and cortisol are all derived from cholesterol. 1,25-Dihydroxyvitamin D3 (cholecalciferol) is not a vitamin at all; rather, it too is a hormone derived from cholesterol. In contrast, the thyroid hormones triiodothyronine and tetraiodothyronine are synthesized from the amino acid tyrosine; the biosynthesis of the hormones from tyrosine renders them lipid soluble. These lipid-soluble ligands bind to intracellular receptors in the steroid receptor superfamily. This family of receptors is much larger than the number of ligands known to bind to them (1). Receptors for which no ligands have yet been identified are called orphan receptors. For example, peroxisome proliferator-activated receptors (PPARs) were previously considered as orphan receptors of the steroid receptor superfamily. PPAR- was identified as the binding site for the thiazolidinedione class of insulin sensitizers (54) before the endogenous ligands for the receptor were identified. It is now recognized that a variety of lipids/lipid-derived ligands bind to orphan receptors as in the case of PPAR. For example, derivatives of androgens are ligands for the constitutive androstane receptor (24, 76). Fatty acids are ligands for all isoforms of PPAR (37, 47, 94), and PGs have been proposed as ligands for PPARs as well (37, 47, 94). Fatty acids have been proposed as ligands at hepatic nuclear factor-4 and retinoic acid X receptors and as antagonists of the liver X receptor, but the supporting data have not been conclusive (37).
The classical mechanism of action of ligands bound to the steroid superfamily of receptors is to modify DNA transcription of specific genes. The ligand/receptor complex dimerizes and binds to DNA as a transcription factor (1). This is called the genomic mechanism of action and takes time to develop. The steps involved include DNA transcription to mRNA, processing of mRNA, movement of mRNA into the cytoplasm, translation of mRNA into protein, and proper folding and posttranslational modification of the protein. Thus, there is a time delay of hours to days from the time of the initiation of the signal until a processed protein that is capable of changing the biological activity of the cell is available. This is in dramatic contrast to ligands that bind to membrane receptors and initiate reversible phosphorylation cascades that result in measurable changes in the biological activity of their target cells within milliseconds to seconds. Rapid responses to steroid family hormones have been reported in addition to the classical delayed genomic responses mentioned here. Indeed, nongenomic effects of lipid-soluble steroid hormones were first reported decades ago (63, 64). However, at the time of their discovery (1960s), the genomic mechanisms for steroid hormones were quite novel and therefore so compelling that the nongenomic mechanisms received little attention. Although the literature on nongenomic effects of steroid and thyroid hormones has grown rapidly (85, 88), there is little consensus on which specific signaling pathways are activated by individual lipid-soluble ligands in a physiological manner. There are reports suggesting activation of nearly every signal transduction pathway known by nearly every steroid/thyroid hormone identified. However, we know that the effects of a given ligand are very specific. Therefore, continued refinement of our understanding of the activation of nongenomic signaling pathways by steroid/thyroid hormones will be required to clarify the mechanisms by which ligand specificity of action is maintained.
Lipid-Modified Proteins in Signal Transduction
Many of the components of signal transduction pathways are modified posttranslationally by the addition of lipid moieties. For example, -subunits of heterotrimeric G proteins may be myristoylated or palmitoylated (57) and -subunits of heterotrimeric G proteins are prenylated (57). Small G proteins such as the Ras, Rab, and Rho/Rac/Cdc42 families undergo palmitoylation, farnesylation, or geranylgeranylation (81), and nonreceptor tyrosine kinases of the Src family may be myristoylated or palmitoylated (52). Palmitoylation of signaling proteins may be reversible such that the protein undergoes cycles of lipid attachment and disattachment with concomitant cyclic association with the membrane when attached to the lipid. Gs is an example of a protein that undergoes reversible palmitoylation (52).
Extracellular ligands for membrane receptors may also bear lipid modifications. For example, ligands for two of the more recently described pathways, Hedgehog and Wnt, are both palmitoylated (34, 59). In addition, Hedgehog enjoys the distinction of being the only known protein to which a cholesterol group is attached (34). Other extracellular proteins are held in association with the membrane through GPI-anchoring tails. GPI-anchored proteins are involved in signal transduction and associate with lipid rafts (75). Examples of GPI-anchored proteins involved in signal transduction include PH-20 and GFR1. PH-20 is a multifunctional protein found in mammalian sperm membranes (13). The signaling function of PH-20, an increase in intracellular Ca2+ levels, is activated by hyaluronic acid (13). The GPI-anchored protein GFR1 is a coreceptor with the receptor tyrosine kinase Ret. In the absence of the ligand, glial-derived neurotrophic factor (GDNF), GFR1 is associated with the lipid raft. Ret cannot associate with the lipid raft by itself but needs to do so to achieve efficient tyrosine kinase signaling. In the presence of GDNF, GFR1 recruits Ret to the lipid raft so that it is active (83).
Themes in Lipid-Mediated Signal Transduction
As indicated at the beginning of this review, the cell membrane is a structural barrier that is necessary to cell function, but it has important physiological roles in signal transduction as well. The field of lipid signal transduction has received only a fraction of the attention of that of protein signal transduction, and there are many unanswered questions in this field. Several themes have emerged in this discussion of lipid mediators in signal transduction. The first theme is the relatedness and, often, the interconvertibility of families of lipid mediators (Fig. 1). A second common theme is the relatedness of lipids that act as ligands and the signal transduction pathways that utilize lipid mediators. Lipid mediators that leave the cell of origin and bind to GPCRs in the cell membrane of the same or neighboring cells include PGs (PGF2, PGE2, prostacyclin, and thromboxane), S1P, PAF, LPC, sphingosylphosphorylcholine, and LPA (Fig. 2). Most of these GPCRs are linked to PLC-ß, so they stimulate a lipid-mediated signaling pathway within the cell that leads to increased activity of PKC and to increased concentration of intracellular Ca2+. The central role of Ca2+ in lipid-mediated signaling is another important theme as many of the enzymes involved in the lipid signaling pathways are Ca2+ regulated (Fig. 2). Moreover, the enzyme systems that require Ca2+ for activation and that produce lipid mediators that increase Ca2+ obtain a degree of positive feedback through this interaction. A final important theme is the role of lipid mediators in inflammatory reactions. PGs, leukotrienes, PAF, and lysophospholipids are all lipid mediators in various aspects of inflammatory reactions.
The field of lipid signal transduction is complicated. The difficulty of visualizing lipid biochemistry, the lower profile of lipid signaling compared with protein signaling, and the many "black holes" that remain in our understanding of lipid signaling all contribute to the complicated nature of lipid signal transduction. Certainly, there are many details of lipid signaling that must be worked out. The relatively recent identification of messenger functions of C1P and sphingosylphosphorylcholine support the concept that other lipid mediators remain to be discovered. This review provides a framework in which to place those new details of lipid signaling as they emerge. Moreover, this review illustrates the magnitude and diversity of lipid mediators in signal transduction and highlights the importance of staying current in the roles of lipid mediators in cellular physiology.
GRANTS
This work was supported in part by National Science Foundation Grant IBN-0315717.
Footnotes
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received for publication August 24, 2006. Accepted for publication November 20, 2006.
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