1. Introduction
Fats belong to basic nutrients for human body, constituting the main source of energy. A profile of fatty acids is one of the factors affecting their health value. Besides the fatty acid composition, the ratios between saturated, monounsaturated, and polyunsaturated fatty acids (SFA, MUFA, and PUFA, respectively) play an important role. Fatty acids, particularly PUFA belonging to n-6 and n-3 families, are thought to participate in regulation of many physiological and pathological processes, such as inflammation, glycemic control, lipid metabolism, oxidative stress, cardiovascular diseases (CVD), skin changes, asthma, nervous system disturbances, or cancer [
1,
2,
3]. Thanks to research on the biological properties of individual fatty acids, especially PUFA, new mechanisms of their action are discovered. PUFA are believed to exert their effects directly or indirectly through various metabolites [
4,
5,
6]. Thus, they may be used to optimize a diet and prevent a variety of diseases.
Fatty acid contents in the body can be influenced by many factors such as everyday diet, the activity of PUFA converting enzymes (desaturases, elongases) and a general state of an organism. It has been observed that dietary fatty acids, including those originating from various vegetable oils used both in cooking and as dietary supplements, modify the body’s fatty acid profile [
7]. Numerous studies indicated the inverse correlation between increased fat intake, especially SFA, and the risk of coronary events [
8,
9,
10]. The replacement of some amounts of SFA with MUFA, administered as olive oil (up to 75% of oleic acid, OL, C18:1,
n-9), resulted in a significant risk reduction of cardiovascular mortality, various cardiovascular events, and stroke [
11]. Similarly, high-oleic acid soybean oil (over 70% of oleic acid vs. 28% in the standard soybean oil), oils rich in n-6 PUFA such as linoleic acid (LA, C18:2, n-6) or in n-3 PUFA like α-linolenic acid (ALA, C18:3, n-3) and its metabolites— eicosapentaenoic acid (EPA, C20:5, n-3), docosahexaenoic acid (DHA, C22:6, n-3)—used to substitute SFA showed significant decrease of total cholesterol (TC), LDL cholesterol, plasma triglycerides, and apolipoprotein B (apoB) as well as of a general reduction of CVD risk [
12,
13]. N-3 PUFA supplementation or moderate consumption of fish may also decrease the risk of Alzheimer’s disease and dementia [
14,
15]. Other studies indicate that the intake of n-3 PUFA is associated with a lower risk the head and neck, esophagus, colorectal, and breast cancer [
16,
17,
18,
19]. Studies concern fatty acid profile, their metabolites, and the activity of enzymes involved in their conversion.
PUFA metabolites synthesized on cyclooxygenase (COX) or lipoxygenase (LOX) pathways can have a significant effect on many processes in the body. They are referred to as eicosanoids. They are locally active compounds derived from arachidonic acid (AA, C20:4,
n-6), dihomo-γ-linolenoic acid (DGLA, C20:3,
n-6), and EPA. The group also includes octadecanoids and docosanoids, which are metabolites of linoleic acid and DHA, respectively [
20,
21]. Whereas arachidonic acid metabolites synthesized on the COX pathway, such as prostaglandins, belong to well-known compounds, hydroxyeicosatetraenoic acids (HETE) produced on the LOX pathway were initially considered biologically inactive [
22]. They are currently regarded as important lipid mediators influencing various pathological processes, such as inflammation, asthma, allergy, diabetes, hypertension, or cancer [
23,
24]. Depending on the LOX isoform engaged in AA metabolism, three main isomers of HETE exist: 5-, 12-, and 15-hydroxyeicosatetraenoic acids (5-, 12-, and 15-HETE). 12- and 5-HETE affect growth and development of different types of cancer [
23]. 12-HETE has been indicated to induce cancer cell proliferation, motility, invasiveness, and angiogenesis, as well as to enhance their adhesion leading to facilitated metastasis [
23,
25]. It also inhibited apoptosis. 5-HETE, alike 12-HETE isomer, was also noticed to stimulate proliferation and enhance growth of breast, prostate, and lung cancer, acting as a survival factor for tumor cells [
24,
26]. 15-HETE, another arachidonic acid derivative, seems to be acting in an opposite way, enhancing apoptosis, and inhibiting proliferation of prostate and colorectal cancer cells. Its lower concentration in lung tumors correlated with decreased activity of peroxisome proliferator-activated receptor gamma (PPARγ) and, as a result, with cancer development [
27].
12-HETE appears also to play role in human hypertension, atherosclerosis, and diabetes. Its significantly elevated level was observed in hypertension patients, comparing to control subjects [
28]. What is more, the urinary secretion of this isoform was enhanced in patients than in the control group. Similarly, increased plasma levels of 12-HETE were found both in diabetic patients and diabetic patients with coronary artery disease, when compared to the control group [
29]. HETE may also play an important role in the regulation of allergic reactions. Significantly higher sputum concentrations of 15-HETE were noted in asthma patients, then in healthy persons [
30].
In the light of above information, the aim of this study was to estimate the influence of dietary supplementation with six selected oils (carotino oil, linseed oil, olive oil, rice oil, sesame oil, sunflower oil), characterized by different composition and content of fatty acids, on fatty acid profile in serum of rats. Indices of fatty acid desaturase activities were assessed as the product/substrate ratio: Δ6-desaturase (D6D) [18:3 n-6/18:2 n-6], and Δ5-desaturase (D5D) [20:4 n-6/20:3 n-6].
The contents of lipoxygenase metabolites of arachidonic acid (5-, 12- 15-HETE), linoleic acid (HODE) and EPA (5-, 12-, 15-HEPE) in rat serum were also determined. Knowledge of the relationship between them and different components of the diet (e.g., edible oils), makes it possible to modify their biosynthesis and thus support the treatment of some diseases.
4. Discussion
The level of fatty acids in the body can be affected by many factors such as a diet (including intake of vegetable oils, characterized by a diverse fatty acid composition), the activity of PUFA transforming enzymes (desaturases and elongases), and overall health. The impact of various edible oils on fatty acid profile and metabolites of arachidonic, linoleic, and eicosapentaenoic acids in rat serum, as well as on activity of Δ6- and Δ5-desaturases was studied. This is not the first study on the effect of oils on fatty acids and their derivatives [
34,
37]. For the first time, however, it is so comprehensive and this is where its uniqueness lies. So far, the influence of popular, commonly used oils—such as sunflower, olive, or soybean oils—on the content of fatty acids and their derivatives has been studied. In our previous works we investigated the relation between conjugated linoleic acid (CLA) and fatty acid profile and metabolites [
38]. This time we focused on vegetable oils not so popular in Western countries as the above-mentioned ones, but of still increasing consumer interest. We chose cartino, rice, and sesame oils as such oils to compare their impact with well-known fats, like sunflower, olive, and linseed oils. What is more, prostaglandin E
2 (PGE
2), synthesized from arachidonic acid on the cyclooxygenase pathway, is the most often determined and well investigated metabolite of polyunsaturated fatty acids. On the contrary, in the current study, we analyzed the impact of the supplemented oils on PUFA metabolites synthesized on LOX pathway. They play an equally important role in the human body as COX derivatives. This is another aspect of uniqueness of this research.
The vegetable oils used in the study differ in the content of individual fatty acids as well as in the proportions of
n-6 and
n-3 PUFA. Some authors pointed out that this relationship may have a direct impact on the fatty acid profile in the blood serum [
33,
34,
39].
With regard to the saturated fatty acids, palmitic and stearic acids are the most common representatives of this group in the human body. As they are synthesized in humans, their dietary intake, especially of palmitic acid, may not always significantly influence serum contents [
40]. In our results, there were no significant differences in the percentage of these acids of all oil-fed groups compared to the control one. However, statistical disparities were found among the oil-supplemented groups. The most interesting observation concerns the group receiving rice oil. Serum palmitic acid concentration was the lowest (13.6%), whereas stearic acid was the highest (about 21.3%) in this group, although the rice oil contains nearly 20% of palmitic acid. This relation can be easily explained by palmitic acid metabolism. Under normal physiological conditions, C16:0 is elongated to stearic acid or desaturated to palmitooleic acid (C16:1), what also explicates elevated stearic acid level in the rice-oil-fed group [
40]. Nevertheless, the ratios among saturated, monounsaturated, and polyunsaturated fatty acids seem to remain unchanged regardless the supplementation used. Probably the profile of individual fatty acid families is quite stable and does not change after the introduction of dietary supplementation. The maintaining of the balance between saturated and unsaturated fatty acids is thought to be crucial for various physiological functions. The disruption of fatty acid ratios may lead to pathological states such as cardiovascular and neurodegenerative diseases and cancer [
40]. Imbalanced saturated and unsaturated proportions are considered to promote various mechanisms resulting in elevated LDL cholesterol level and suppression of LDL receptor expression [
41]. This results in the reduced removal of LDL from plasma. What is more, saturated fatty acids were found to stimulate expression of peroxisome-proliferator-activated receptor-γ coactivator 1β (PGC-1β) in liver, which activates expression of transcription factors related to the lipid synthesis in liver. Consequently, the synthesis, production and secretion of very-low-density lipoproteins (VLDL), rich in cholesterol and triglycerides, increase [
42]. Saturated fatty acids may also influence the inflammatory response activating nuclear factor κB (NF-κB). This leads to the increased synthesis of proinflammatory cytokines such as interleukin 6 (IL-6) or tumor necrosis factor α (TNF-α) as well as to overexpression of cyclooxygenase-2 (COX-2), involved in the biosysthesis of proinflammatory eicosanoids from arachidonic acid [
43,
44]. In our experiment, the rice oil given to rats contributed to the greatest changes in palmitic and stearic acid contents, despite not affecting the overall saturated fatty acid level. In the light of above information, the carotino oil seems to be an interesting proposition for consumers, because it decreases the total saturated fatty acid content and in this way may exert a protective activity.
Of the all fatty acids determined in rat serum, MUFA were found in the smallest proportion with oleic acid being the main representative of this family. The highest contents of oleic acid were noted in the carotino and olive oil-supplemented groups (12.7% and 9.9%, respectively). They were significantly higher comparing to the groups fed the rice, sesame, and linseed oils (
Table 3). Olive oil and carotino oil are the main sources of oleic acid (73–75% and 53%, respectively), leaving the remaining oils used in the study behind (
Table 2) [
45]. What is more, olive oil is the major fat of the Mediterranean countries and, together with the whole lifestyle of this area, has been the subject of many studies [
46,
47]. Oleic acid does not belong to essential fatty acids, because it is synthesized in humans by stearoyl-CoA desaturase 1 (SCD1) from stearic acid. Nevertheless, its health role appears to be very interesting and has been investigated by many authors [
48]. The beneficial impact of oleic acid was observed in cardiovascular diseases or rheumatoid arthritis. The consumption of this fatty acid increased the content of anti-inflammatory leukotriene A3 (LTA3), which is a potent inhibitor of pro-inflammatory leukotriene B4 (LTB4) and in this way prevent rheumatoid arthritis development [
49]. Oleic acid was also found to reduce organ dysfunction and mortality of experimental sepsis in mice by inducing fatty acid oxidation, decreasing plasma non-estrified fatty acid concentration, the reactive oxygen species synthesis, and production of pro-inflammatory cytokines [
50,
51]. Treatment with oleic acid also inhibited neutrophile migration and accumulation in infected sites in mice [
51]. The moderate consumption of olive oil seems to have the beneficial effect on the risk of breast cancer [
46].
As regards PUFA, arachidonic acid followed by linoleic acid, both belonging to the
n-6 family, dominated in the all groups. In humans, linoleic acid taken from the diet is metabolized first by D6D to GLA, which is elongated to DGLA. DGLA is then converted by D5D to arachidonic acid. The same enzymes convert ALA to EPA, which is further metabolized by D6D, elongases, and β-oxidated to DHA. Therefore, we expected the highest contents of arachidonic and linoleic acids in the groups supplemented with oils richest in linoleic acid—sunflower oil, followed by sesame and rice oils, i.e., the oils we have been particularly focused on. However, not entirely in line with our expectations and our previous studies [
33,
34], the highest arachidonic acid level was found in the rice oil-fed group. This value was significantly increased than in the carotino oil-supplemented group and was followed by the group fed with sesame oil. The arachidonic acid contents we have determined are higher compared to some other studies [
52]. However, results similar to ours were also described by other authors [
53]. This may be due to the increased endogenous synthesis of this fatty acid in rats, particularly under conditions of increased linoleic acid supply. The activity of enzymes involved in linoleic acid metabolic pathway can also influence arachidonic acid synthesis.
Regarding serum linoleic acid, its highest content was observed in the control group, whereas it was slightly lower in all the oil-supplemented groups. The lowest LA concentration was found in serum of rats receiving olive oil (11.3%), what reflects its composition. Olive oil contains the lowest amount of linoleic acid (6%) of all the oils tested [
54]. As far as
n-3 PUFA are concerned, DHA was present in the highest contents. However, they do not exceed 3.5%. DHA precursors—ALA and EPA—were detected in small amounts, below 1% even in the group fed with linseed oil, which is the best source of ALA of all oils tested [
55].
The results concerning PUFA may be explained by both desaturase activities and synthesis of eicosanoids. Supplementation with each of the oils significantly intensified D6D activity, comparing to the control group, which resulted in increased GLA contents in the oil-fed groups. Most of the oils also increased the D5D activity, which was, to our surprise, the highest in the olive oil-receiving group, followed by the groups supplemented with rice, linseed and sesame oils.
The serum fatty acid profile depends on their dietary composition. However, the results obtained in this study indicate that the other factors, such as endogenous synthesis or the activity of enzymes involved in PUFA metabolism, may also affect the fatty acid profile in the body. In the present study, the concentrations of five eicosanoids—5-, 12-, 15-HETE, HODE, and 12-HEPE—in serum of rats fed with one of vegetable oils were determined (
Table 4). 12-HETE was the major of detected compounds, regardless of the supplementation used. Its highest concentration was observed in serum of rats supplemented with linseed oil (857 ng/mL,
Table 4,
Figure 3). This result is rather unexpected because linseed oil is a very good source of α-linolenic acid and contains only approx. 15% of linoleic acid, which is the precursor of arachidonic acid. However, our finding may be explained by the not entirely obvious metabolism of linoleic and linolenic acids [
56]. ALA conversion to its metabolites EPA and DHA may be affected by various factors, such as gender or diet. It is well known that diet rich in linoleic acid decreases conversion of α-linolenic acid by even 40% [
57]. The conversion rate may be also decreased by high intake of EPA and DHA and by high intake of α-linolenic acid itself [
58]. This fact appears to refer to our results and probably helps to explain them. The high level of 12-HETE in serum of linseed oil fed rats corresponds to a relatively high concentration of linoleic and arachidonic acid (
Table 3,
Figure 1), as well as to increased D6D and D5D activities (
Figure 2). The similar situation concerns results from olive oil supplemented rats, where 12-HETE level was high (765 ng/mL serum), despite the low proportion of linoleic acid in olive oil (approx. 6%) (
Table 2).
Sunflower, sesame, and rice oils contain the highest contents of linoleic acid—approximately 60%, 44%, and 30%, from among tested oils respectively (
Table 2). In this case, we should have expected to find the higher levels of arachidonic acid metabolites in serum of rats supplemented with one of these oils. Meanwhile, this poor influence of dietary fats on the content of the main hydroxy fatty acid in serum may result from relatively short supplementation time (35 days) and considerable range of results within one group. Similar situation has been described by other authors who determined the content of eicosanoids in blood of salmon fed with diets containing either sunflower oil or fish oil [
59]. Sunflower oil administration, despite high content of linoleic acid, reduced not only the levels of EPA metabolites—12-HEPE and LTB5—but also arachidonic acid metabolite—12-HETE [
59]. The synthesis of eicosanoids derived from arachidonic acid can be inhibited by DGLA [
60]. The decreased content of arachidonic acid derivatives in sunflower oil-supplemented group may also be due to 12-HETE oxidation to 12-oxo-eicosatetraenoic acid (12-oxo-HETE) [
61].
Linoleic acid derivatives—HODE—were present in the samples in slightly smaller amounts than 12-HETE. The highest concentrations was found in the group of animals supplemented with sunflower oil (468 ng/mL,
Table 4,
Figure 3). It was significantly more than in serum of rats receiving olive oil (303 ng/mL) or carotino oil (260 ng/mL). Sunflower oil is a good source of linoleic acid (approximately 60%), which may be converted by 15-LOX-1 to HODE [
62]. For comparison, the content of linoleic acid in olive oil and carotino oil was 5.9% and 16.8%, respectively. HODE concentration in the group supplemented with carotino oil was also significantly lower in comparison to animals fed only with feed (401 ng/mL,
Table 4,
Figure 3). We can therefore conclude that serum HODE concentrations in the best way reflects linoleic acid content in the diet, comparing to other eicosanoids. A similar conclusion was produced by Ferdouse et al. who observed that higher dietary linoleic acid increased its hydroxy metabolites, but did not influence arachidonic acid derivatives [
63].
12-HEPE, which is an EPA derivative, was determined in the tested samples. It was the only detectable compound among HEPE isomers. Its concentrations appear to be probably modulated by dietary γ-linolenic acid, which corresponds to results received by other researchers [
63]. Ferdouse et al. observed that HEPE hydroxy derivatives can be increased not only by dietary EPA, but also by α-linolenic acid and DHA [
63].
What is more, we found strong correlations between serum PUFA content and their metabolite concentrations (
Table 5). However, other studies have shown that that there are not always correlations between PUFA and their hydroxyl metabolites [
64]. In our work 15-HETE, 5-HETE, and 12-HETE concentrations negatively correlated with linoleic acid, what results from their various metabolic pathways. 12-HEPE was also negatively correlated with arachidonic acid content, while strong positive correlation was observed between 12-HEPE and its precursor—EPA. Furthermore, we observed strong positive correlation between arachidonic acid and its three metabolites—15-HETE, 5-HETE, and 12-HETE (the last result despite of being not significant, it was still on a trend level). These correlations confirm different and complicated pathways of PUFA metabolism and indicate these processes are subjected to the strict endogenous control.
Fatty acids are thought to play a crucial role in people health. The disturbances of their profile and metabolites may be the basis of various disorders, like inflammation and its results—heart failure, asthma, a neoplastic process. What is more, the knowledge of these relations may help to predict the risk of one of above-mentioned diseases. Fatty acids derivatives appear to be influenced by nutrition. An appropriate composition of the diet affects directly or indirectly—through regulation of gene suppression—synthesis of fatty acid metabolites, such as eicosanoids. They may reflect the current state of an organism and constitute an interesting tool for foreseeing and inflammation-based disorders [
65]. Both experimental and epidemiological investigations suggest beneficial impact of
n-6 and
n-3 PUFA in arythmia, hypertension, and atherothrombotic cardiovascular disease. The results of a randomized, double-blind, placebo-controlled study performed on nearly 7000 participants confirmed that supplementation with 1 g of
n-3 PUFA daily significantly reduced cardiovascular mortality and admission to hospital for cardiovascular reasons in patients with heart failure [
66]. This and other studies have been recently summarized by Sakamoto et al. who highlighted some mechanisms of
n-3 PUFA protective role in heart failure [
67]. They involve direct and indirect actions, such as alterations of metabolic cardiomyocyte conditions or anti-inflammatory properties of PUFA metabolites.
N-3 PUFA also appeared to be protective in various neurodegenerative diseases, like Parkinson and Alzheimer’s diseases or depression, however the results are unequivocal [
68,
69]. Nevertheless, prospective studies carried out in Europe and USA showed decreased risk of Alzheimer’s disease and dementia due to increased
n-3 PUFA intake [
14,
70]. Consumption of fatty fish more often than twice a week also correlated with the reduced risk of Alzheimer’s disease (by 41%) [
71]. Carcinogenesis is another process in which fatty acids, especially unsaturated ones, play an important role. Lipids were observed to accumulate in many cancer tissues, including colorectal, breast, brain, and ovarian [
72]. It has been recently found that lipid droplets are sites where enzymes converting fatty acids to their metabolites—eicosanoids—are located and where the synthesis of eicosanoids takes place during inflammation and cancer [
73]. These discoveries make both enzymes involved in the metabolism of fatty acids and metabolites interesting factors that can modify the course of neoplastic activity.