Fatty acid acylation of proteins: specific roles for palmitic, myristic and caprylic acids

Fatty acid acylation of proteins corresponds to the coor post-translational covalent linkage of an acyl-CoA, derived from a fatty acid, to an amino-acid residue of the substrate protein. The cellular fatty acids which are involved in protein acylation are mainly saturated fatty acids. Palmitoylation (S-acylation) corresponds to the reversible attachment of palmitic acid (C16:0) via a thioester bond to the side chain of a cysteine residue. N-terminal myristoylation refers to the covalent attachment of myristic acid (C14:0) by an amide bond to the N-terminal glycine of many eukaryotic and viral proteins. Octanoylation (O-acylation) typically concerns the formation of an ester bond between octanoic acid (caprylic acid, C8:0) and the side chain of a serine residue of the stomach peptide ghrelin. An increasing number of proteins (enzymes, hormones, receptors, oncogenes, tumor suppressors, proteins involved in signal transduction, eukaryotic and viral structural proteins) have been shown to undergo fatty acid acylation. The addition of the acyl moiety is required for the protein function and usually mediates protein subcellular localization, protein-protein interaction or protein-membrane interaction. Therefore, through the covalent modification of proteins, these saturated fatty acids exhibit emerging specific and important roles in modulating protein functions. This review provides an overview of the recent findings on the various classes of protein acylation leading to the biological ability of saturated fatty acids to regulate many pathways. Finally, the nutritional links between these elucidated biochemical mechanisms and the physiological roles of dietary saturated fatty acids are discussed.


Introduction
Fatty acid acylation of proteins (Towler et al., 1988) corresponds to the co-or post-translational covalent linkage of a fatty acid, activated in the form of acyl-CoA, to an aminoacid residue of the substrate protein (Fig. 1).The cellular fatty acids covalently bound to proteins are mainly saturated fatty acids (SFAs).Palmitoylation (S-acylation) corresponds to the reversible attachment of palmitic acid (C16:0) to the side chain of a cysteine residue via a thioester bond (Blaskovic et al., 2014).N-terminal myristoylation refers to the covalent attachment of myristic acid (C14:0) by an amide bond to the N-terminal glycine of many eukaryotic and viral proteins (Johnson, Bhatnagar, et al., 1994).Octanoylation (O-acylation) typically concerns the formation of an ester bond between octanoic acid (caprylic acid, C8:0) (Lemarié, Beauchamp, Legrand, et al., 2015) and the side chain of a serine residue of the stomach ghrelin peptide (Kojima et al., 1999).An increasing number of proteins (enzymes, hormones, receptors, oncogenes, tumor suppressors, proteins involved in signal transduction, eukaryotic and viral structural proteins) have been shown to undergo fatty acid acylation.The acyl moiety can mediate protein subcellular localization, protein-protein interaction or protein-membrane interaction.Therefore, through the covalent modification of proteins, these particular saturated fatty acids exhibit emerging specific and important roles in modulating protein functions (Ezanno et al., 2013).

Palmitic acid and protein palmitoylation
Protein S-acylation (Fig. 1) is also called palmitoylation because palmitic acid (C16:0) is the main SFA involved in this posttranslational thioester linkage with the side chain of cysteine residues (Mitchell et al., 2006), but other SFAs like myristic (Rioux et al., 2002) and lauric acids (Rioux et al., 2003) have also been found.Protein palmitoylation is catalyzed by a family of palmitoyltransferases sharing a DHHC motif and including 20 to 24 members in humans (Greaves and Chamberlain, 2011).Palmitoylation is involved in regulatory mechanisms because the association of the protein with the palmitoyl moiety is reversible and facilitates proteinmembrane interactions and subcellular trafficking of proteins.Proteins that undergo this modification span almost all cellular functions.Several signal transductions depend for instance on palmitic acid, including proteins that have been shown to undergo successive myristoylation and palmitoylation, like the α subunit of many heterotrimeric G proteins (Chen and Manning, 2001).

Myristic acid and protein N-terminal myristoylation
Protein N-myristoylation (Fig. 1) specifically involves myristic acid (C14:0) (Beauchamp, Rioux, et al., 2009;Rioux and Legrand, 2001).Myristoyl-CoA: protein N-myristoyltransferase (NMT), the enzyme catalyzing this stable acylation, has been identified in many organisms.In mammals, two distinct NMT genes referred to as type 1 and 2 have been described (Giang and Cravatt, 1998;Rioux et al., 2006;Rundle et al., 2002).The contribution of each gene transcript to NMT expression and activity in vivo, and the specific role of each NMT isoform in cellular replication, proliferation, and other cellular processes, is however still not clearly described (Ducker et al., 2005;Selvakumar et al., 2006;Yang et al., 2005).Both isoforms seem to have a similar high substrate selectivity for myristic acid (Giang and Cravatt, 1998;Rioux et al., 2006).The myristoyl moiety has been shown to mediate protein subcellular localization, protein-protein interaction or protein-membrane interactions required for the biological activities of the myristoylated proteins (Johnson, Bhatnagar, et al., 1994).Initially described as a co-translational modification, N-myristoylation has more recently been shown to also occur as a post-translational mechanism in apoptotic cells (Martin et al., 2011), after proteolytic cleavage by caspases exposing a previously hidden N-terminal glycine residue.and DES2), rat Δ6-desaturase (Fatty Acid Desaturase 2 or FADS2) and rat NADH-cytochrome b5 reductase (NCb5R).Among these putative myristoylation candidates, only FADS2 was shown to be preclude from myristoylation.
The proteins that are myristoylated all possess an N-terminal glycine residue, but the subsequent so-called "myristoylation consensus sequence" is less well-defined (Fig. 2).Computational prediction suggested that about 0.5% of all proteins in the human genome could be myristoylated (Maurer-Stroh et al., 2004).Indeed, the N-myristoylated proteome was recently studied in human cells, leading to the identification of more than 100 N-myristoylated proteins (Thinon et al., 2014).The myristoylated proteins include key components in intracellular signaling pathways, oncogenes, structural viral proteins but also common constitutive eukaryotic proteins.
Among this last category, we noticed throughout the past years that several members of the mammalian family of membrane-bound desaturases possess a potential site of myristoylation.Both isoforms of dihydroceramide Δ4-desaturase (DES1 and DES2) indeed present a site of myristoylation in rats (Fig. 2), mice and humans (Beauchamp et al., 2007;Mizutani et al., 2004;Ternes et al., 2002).DES1 catalyzes the last step of de novo ceramide biosynthesis which consists in the introduction of a trans Δ4-double bond in the carbon chain of the dihydroceramide.DES2 possesses a bifunctionnal Δ4-desaturase/C4-hydroxylase activity (Omae et al., 2004).The presence of the trans Δ4-double bond seems to be critical for the acquisition of the biological activities of ceramide (Bielawska et al., 1993).Indeed, ceramide is able to induce apoptosis (Garcia-Ruiz et al., 1997;Gudz et al., 1997;Siskind et al., 2002), which is not the case of its precursor dihydroceramide.We showed that both DES1 and DES2 are myristoylated and that this N-terminal modification significantly increased the activity of the recombinant DES1 when expressed in COS-7 cells (Beauchamp et al., 2007).Compared to a recombinant unmyristoylable mutant form of DES1 (N-terminal glycine replaced by an alanine), the desaturase activity of the myristoylable wild-type DES1 was two times higher, in the presence of myristic acid incubated with the cells.The description of this regulatory mechanism highlighted a new potential relationship between myristic acid, the saturated fatty acid capable of binding and activating the enzyme involved in the final de novo ceramide biosynthesis step, and lipoapoptosis induced through the ceramide pathway.Indeed, we subsequently showed that the myristoylation of recombinant DES1 can target part of the enzyme to the mitochondria, leading to an increase in ceramide levels (specifically in the mitochondria) which in turn leads to apoptosis in the COS-7 cell model (Beauchamp, Tekpli, et al., 2009).Finally, myristic acid also increased native DES1 activity in cultured rat hepatocytes (Ezanno et al., 2012).
A second example of a membrane-bound desaturase which has been studied for its potential myristoylation is the Δ6desaturase (Fatty Acid Desaturase 2: FADS2) involved in essential polyunsaturated fatty acid synthesis (Aki et al., 1999;Cho et al., 1999;D'Andrea et al., 2002).Several years ago, myristic acid was shown to trigger a specific and dosedependent increasing effect on Δ6-desaturase activity in cultured rat hepatocytes (Jan et al., 2004) whatever the substrate used to measure this enzyme activity (oleic acid, linoleic or α-linolenic acid).Because the FADS2 enzyme exhibits an N-terminal glycine residue (Fig. 2), the increase in the activity of Δ6-desaturase by myristic acid was first postulated to be mediated by N-myristoylation.However, bioinformatic predictions indicated and biological experiments confirmed that FADS2 is not myristoylated (Beauchamp et al., 2007).Nevertheless, FADS2 is believed to cooperate with NADHcytochrome b5 reductase (NCb5R) in the endoplasmic reticulum membrane (Guillou et al., 2004) and this last enzyme is also known (Fig. 2) to be N-terminally myristoylated (Borgese et al., 1996;Colombo et al., 2005;Ozols et al., 1984).The hypothesis according to which the myristoylation of NADH cytochrome b5 reductase could account for the increased Δ6-desaturase activity was therefore proposed (Rioux et al., 2011).Although its linkage with myristic acid is not absolutely required for its association with endoplasmic reticulum membranes (Strittmatter et al., 1993), myristoylation of NCb5R may modify the transfer by lateral diffusion of electrons from NCb5R to the heme of cytochrome b5 and then to the terminal desaturase.It may also change the interaction between NCb5R and the desaturase.Moreover, it may modify the conformation of the whole complex, as the analysis of the relative contribution of the myristoyl moiety in membrane binding in a model of phospholipid vesicles suggests (Strittmatter et al., 1993).In such a hypothesis, not only the Δ6-desaturase but also all the membrane-bound desaturases which are associated with NCb5R would be affected by this regulatory mechanism.This regulation may explain the effect of dietary myristic acid on the overall conversion of α-linolenic acid to longer highly unsaturated fatty acids, like eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids, shown in rat nutritional experiments (Legrand et al., 2010;Rioux et al., 2005Rioux et al., , 2008)).
These two detailed examples show that through the myristoylation of the dihydroceramide Δ4-desaturase (DES) and of the NADH-cytochrome b5 reductase, myristic acid may therefore be considered as one of the regulators of cellular bioactive lipid concentration such as ceramide and polyunsaturated fatty acids.

Caprylic acid and ghrelin octanoylation
Fifteen years ago, caprylic acid (C8:0) was surprisingly found attach to the ghrelin (Fig. 1) purified from rat stomach (Kojima et al., 1999), but only recently the presence of the octanoyl moiety appeared crucial for this peptide hormone (Lemarié, Beauchamp, Legrand, et al., 2015).Ghrelin is a 28 amino acid peptide expressed in the digestive tract and mainly in the stomach.Its octanoylated form binds to the growth hormone secretagogue receptor (GHSR-1a) located in the pituitary gland and hypothalamus (Howard et al., 1996).Preproghelin is first translated from its corresponding gene and then cleaved to produce the proghrelin.Proghrelin is octanoylated on its 3rd amino-acid residue which corresponds to a serine.The octanoylated proghrelin is finally cleaved a 2nd time and secreted in the plasma.Both octanoylated and non-octanoylated ghrelin may co-exist but only the octanoylated form is able to bind to its hypothalamic receptor.
Octanoylated ghrelin is therefore suspected to regulate many relevant biological processes including the secretion of the growth hormone (GH), the stimulation of appetite and food intake, the modulation of gastric acid secretion and motility, the regulation of glucose homeostasis and adiposity (Delporte, 2013).During its maturation in the gastric mucosa and before secretion in the blood (Fig. 3), the preproghrelin is first cleaved and part of the proghrelin is octanoylated on its N-terminal 3rd serine residue, in the endoplasmic reticulum lumen.The stomach enzyme involved in ghrelin octanoylation is called ghrelin O-acyltransferase (GOAT) and belongs to the family of membrane bound O-acyltransferases (MBOAT), a group of proteins involved in acetyltransferase and acyltransferase activity (Gutierrez et al., 2008).This enzyme is described by some authors as a "fat sensor" informing the brain, via the octanoylated ghrelin level, of the nutrient availability in the digestive tract (Taylor et al., 2013).The GOAT is an endoplasmic reticulum membrane protein with 11 transmembrane helices and one reentrant loop (Taylor et al., 2013).The GOAT expression corresponds predominantly to the distribution of the ghrelin (Lim et al., 2011) with a strong co-expression in the mucosa of the digestive tract.Studies showed that GOAT displays a high affinity for caprylic acid but can also link ghrelin to C6:0, C10:0 and C10:1 to a lesser extent (Darling et al., 2015).Little is however still known about the GOAT enzymatic mechanism and the analysis of native stomach GOAT activity still remains a challenge (Taylor et al., 2012).
Whether octanoylated or not, the proghrelin is then subjected to a second cleavage under the action of a prohormone convertase (Fig. 3) which leads to the production of the native ghrelin peptide (octanoylated or not) and to an additional C-terminal peptide which can itself undergo further proteolytic processes generating a smaller peptide called obestatin (Zhang et al., 2005).The mature acylated ghrelin secreted in the plasma is the only form that can bind to its GHSR-1a receptor.However, unacylated ghrelin can also be secreted and seems involved in the regulation of several physiological functions (Delhanty et al., 2013).Ghrelin is for now the only known case of a peptide covalently linked to caprylic acid.
As mentioned above, plasma ghrelin exists in both unacylated and acylated forms, but only the active acylated form can bind its GHSR-1a receptor (Howard et al., 1996).Octanoylated ghrelin is best known for its orexigenic actions in the central nervous system, involved in the regulation of food intake and thereby in weight control (Kamegai et al., 2001).The first studies showed that intracerebral chronic acylated ghrelin administration increased body weight by stimulating food intake and by inhibiting energy expenditure and fat catabolism (Tschöp et al., 2000).However, as demonstrated in recent studies showing that ghrelin-null mice did not exhibit altered food intake nor altered expression of hypothalamic neuropeptides involved in the regulation of appetite, the essentiality of endogenous ghrelin in the regulation of food intake appeared controversial (Albarran-Zeckler et al., 2011).

Impact of dietary SFAs on cellular protein acylation
Because of their potential dual origin (diet and endogenous synthesis), the impact of dietary SFAs on the regulation of the protein acylation processes is still questioned.More specifically, human data reporting the balance between intake and de novo synthesis of SFAs are not available.In addition, the optimal cellular concentration of saturated fatty acyl-CoAs required for each type of protein acylation is not known and data available suggest that the free cytosolic concentration of fatty acyl-CoA esters is in the low nanomolar range (Faergeman and Knudsen, 1997).
Palmitic acid is universally found in natural fats, representing 15-25% of total fatty acids.Therefore, due to its high dietary level (30-38 g/day in humans) (Katan et al., 1994) and well-described predominant synthesis by the Fatty Acid Synthase (Singh et al., 1984), palmitic acid intracellular concentration may not be considered as the rate-limiting molecule for the palmitoylation mechanism.
Concerning now myristic acid, its endogenous biosynthesis (Rioux et al., 2007) appeared very low in cultured rat hepatocytes.If it is also the case in humans, the diet is therefore the main source (4-8 g/day) for this particular fatty acid (Wolk et al., 2001) which represents about 10% of FAs in ruminant milk fat.Very low dietary intakes of myristic acid may likely lead to insufficient intracellular concentration of myristoyl-CoA to ensure the proper activity of N-myristoyltransferase (NMT), when considering the whole pool of myristoylable proteins.One may therefore wonder about the risk of total eviction of dairy products as the unique source of dietary myristic acid.In yeast, studies analyzing the activity of NMT have suggested that the enzyme able to use both exogenous and endogenous myristic acid as substrate (Duronio et al., 1991(Duronio et al., , 1992;;Johnson, Knoll, et al., 1994).The requirement for myristic acid suggests that in certain cases, it could be the rate-limiting molecule in this mechanism or that competition could occur.In addition, the mechanism by which myristic acid initially esterified in the TAG or PL is used for myristoylation is unknown, too.
Concerning finally caprylic acid, no endogenous biosynthesis of this fatty acid has been described in animals except in the lactating mammary gland (Fernando-Warnakulasuriya et al., 1981).On the other hand, natural food sources of caprylic acid are restricted to specific vegetable oils and milk products.Caprylic acid is abundant in coconut oil (6-10% of FAs, with C8:0 mainly in sn-1 and -3 positions on the triglycerides, TG) and in palm kernel oil (2-5% of FAs).Milk is the only natural source of animal caprylic acid with strong differences between mammalian species.C8:0 represents about 0.5% of FAs in human milk (Jensen, 1996), but is higher in cow milk (1-2%) (Jensen et al., 1990), in goat milk (3%) (Alonso et al., 1999), in rat milk (5-6%) (Fernando-Warnakulasuriya et al., 1981) and reaches up to 15-18% in rabbit milk (Perret, 1980).Caprylic acid is primarily esterified in sn-3 position of the TGs in cow (Jensen et al., 1990), rat (Staggers et al., 1981) and human (Jensen, 1996) milks.In western countries, dietary MCFAs represent less than 2% of total dietary energy and caprylic acid is only a minor part of these MCFAs in milk fat (1-2% of cow milk FAs).
Part of caprylic acid coming from dietary medium chain TG (MCTs) can be early released during digestion through the action of preduodenal lipase (Clark et al., 1969), leading to its potential and yet not clearly quantified direct absorption by the stomach mucosa (Lai and Ney, 1998;Perret, 1980).Dietary caprylic acid is therefore suspected to directly provide GOAT enzyme with octanoyl-CoA co-substrates (Fig. 3) necessary for the acyl modification of ghrelin.Indeed, ingestion by mice of either MCFAs or MCTs increased the stomach concentration of acylated ghrelin (Nishi, Hiejima, Hosoda, et al., 2005), without changing the total ghrelin amounts.Nishi et al. detected heptanoylghrelin (Nishi, Hiejima, Hosoda, et al., 2005) or decanoylghrelin (Nishi et al., 2013) in the stomachs of mice fed with triheptanoin or tricaprin, confirming that at least part of the ingested MCFAs was directly used for ghrelin acylation.These results are consistent with the hypothesis of gastric absorption of MCTs but part of the caprylic acid present in the stomach may also come from intestinal absorption followed by uncompleted uptake by the liver.In ruminants, the ingestion of MCFAs during 2 weeks by lactating dairy cows increased the plasma acylated ghrelin concentrations (Fukumori et al., 2013).In cachectic patients, a 2-week administration of an enteral nutrition formula containing 3 g/day of caprylic acid enhanced plasma octanoylated ghrelin and also improved the body mass index (Ashitani et al., 2009).A single administration of the formula also increased plasma octanoylated ghrelin 5 h after administration.Conversely, in another study on the role of the gustatory G-protein in the sensing of FAs for octanoylation of ghrelin, ingested MCFAs increased stomach acylated ghrelin but did not change the plasma ghrelin concentration (Janssen et al., 2012).
To further understand the effect of dietary caprylic acid on the concentration of circulating plasma acylated and unacylated ghrelin, we recently designed a nutritional study (Lemarié, Beauchamp, Dayot, et al., 2015) including Sprague-Dawley male rats which were fed during 6 weeks with three dietary C8:0 levels (0, 8 and 21% of FAs).A specific doseresponse enrichment of the stomach tissue C8:0 was observed.However, the acylated ghrelin concentration in the plasma was unchanged in spite of the increased C8:0 availability.Conversely, a reproducible decrease in the plasma concentration of unacylated ghrelin was observed, which was consistent with a decrease in the stomach preproghrelin mRNA and stomach ghrelin expression.Additionally, we measured high levels of acylated ghrelin in the plasma of rats receiving no dietary C8:0.Thus, the low stomach C8:0 level observed in these rats could be enough to supply the octanoyl-CoA co-substrate used to acylate the proghrelin (Lemarié, Beauchamp, Dayot, et al., 2015).
In addition, the maturation and secretion of stomach acylated ghrelin are complex processes potentially regulated by dietary LCFAs, MCFAs and GPR120 (Gong et al., 2014).The circulating ghrelin is additionally submitted to clearance and rapid de-acylation or degradation.For instance, a ghrelin deacylation enzyme (acyl-protein thioesterase-1, APT1) has recently been described, that can des-acylate ghrelin in the plasma (Chen and Enriori, 2014).Moreover, acylated ghrelin has been shown to form a complex with larger proteins like immunoglobulins (Ghrelin-reactive IgG) (Takagi et al., 2013) that protects the acylated form from degradation.Some studies assaying total ghrelin in plasma have also reported lower levels in obese subjects, due to lower levels of unacylated ghrelin, whereas acylated ghrelin remained stable, suggesting a specific decreased degradation of acylated ghrelin in obese (Takagi et al., 2013).In humans, it has also been shown that the GOAT enzyme was present in the blood, which could modify the balance between de-acylation and re-acylation (Goebel-Stengel et al., 2013).For all these reasons, the concentration of both the acylated and unacylated plasma ghrelin may not simply reflect the stomach concentration (Nishi, Hiejima, Mifune, et al., 2005b).

Conclusion
Focusing on fatty acid acylation of proteins, this review reports new knowledge on cellular and physiological functions of individual SFAs.This review particularly emphasizes that palmitic, myristic and caprylic acids, through their capacity to acylate different proteins, have important and specific roles for which they cannot be a substitute for each other and that cannot be assumed by other fatty acids.For this reason, like for other physiological and pathophysiological aspects (Legrand, 2013;Legrand and Rioux, 2015) not detailed in the present review, SFAs should no longer be considered as a single group in terms of structure, metabolism and functions.

Fig. 1 .
Fig. 1.Classification of the saturated fatty acid protein acylation and examples of acylated proteins.Palmitoylation (S-acylation) corresponds to the reversible attachment of palmitic acid (C16:0) to the side chain of a cysteine residue via a thioester bond.N-terminal myristoylation refers to the covalent attachment of myristic acid (C14:0) by an amide bond to the N-terminal glycine of many eukaryotic and viral proteins.Octanoylation (O-acylation) typically concerns the formation of an ester bond between octanoic acid (caprylic acid, C8:0) and the side chain of a serine residue of the stomach ghrelin peptide.

Fig. 3 .
Fig.3.Ghrelin synthesis, maturation and post-translational octanoylation in stomach cells.Preproghelin is first translated from its corresponding gene and then cleaved to produce the proghrelin.Proghrelin is octanoylated on its 3rd amino-acid residue which corresponds to a serine.The octanoylated proghrelin is finally cleaved a 2nd time and secreted in the plasma.Both octanoylated and non-octanoylated ghrelin may co-exist but only the octanoylated form is able to bind to its hypothalamic receptor.