1. Introduction
The spleen, derived from mesenchymal tissue, is the largest lymphatic organ found in all vertebrates [
1] . Spleen (the multipurpose internal organ) have very important physiological roles in regard to red blood cells, storage of blood, the center of the blood defense system, the immune system as well as formation of lymphocytes and eliminating senescent erythrocyte cells [
2,
3,
4]. Moreover, spleen controls physiologically essential processes, e.g. metabolism of metals, albuminoids and, what recently is gaining increasing attention, metabolism of lipids. The latter encompasses digestion and absorption, transportation through the blood, as well as biosynthesis [
5].
There are several mechanisms presumably implicated in spleen regulation of lipid metabolism. The most well-known is theory of splenic lipid reservoir, which covers both the volume of spleen and the activity of macrophage. Hence, the presence of a mononuclear system of phagocytes active against fractions of lipids; biosynthesis of anti-oxLDL antibodies together with the removal of antigen-antibody species, interference with the lipid peroxidation in the liver (liver-spleen axis), activity of lipoprotein lipase, shifts in expression of microRNAs involved in regulation of genes correlated with high density lipoprotein (HDL) metabolism, platelet pathway and immune-mediated mechanism are also involved in lipid metabolism regulation [
5]. Additionally, splenic connection with the propagation of certain diseases connected with the lipid disorders (like Niemann-Pick’s disease, Gaucher’s disease, Fabry disease or gangliosidoses) were also reported [
6]. Total splenectomy may unfavorably affect levels of plasma lipids (triglycerides, cholesterols, fatty acids (FA)) and lipoproteins [
7,
8], and thus lead to the development of atherosclerosis and other cardiovascular disorders. The influence of spleen in lipids metabolism was confirmed in rats, rabbits and dogs models [
9,
10,
11]. However, to the Authors best knowledge, no research were undertaken in ruminants’ model. That is why our current experiment, aiming on evaluation of lipidomic profile of ovine spleen, seems to be up to date and valid.
Determinations of FA profiles of lambs’ spleen were previously performed e.g. to confirm the possible routes of supply of FA to lymph. The most abundant lipid classes were phospholipids (phosphatidylethanolamine, phosphatidylcholine), free cholesterol and triacylglycerols. In each lipid class, the amounts of the essential fatty acids (EFA) were lower than in the corresponding lipids of plasma or lymph [
12].
Lymph nodes and the spleen jointly make up the majority of peripheral immune tissues, and earlier investigations have documented that spleen tissue is very sensitive to changes of concentrations of selenium (Se) in diets [
4]. In fact, this element is the part of glutathione peroxidases (GPx), which selenoenzymes functioning as stimulators of the immune system, responsible for scavenging if free radicals in tissues, thus reducing oxidative damage [
13]. In fact, dietary deficiency of Se stimulated the inflammation and oxidative stress in spleen tissues, and so, disturbances immune activity of the spleen [
4,
14]. Low levels of Se in diet reduced the expression of seleno-proteins and Se contents, obstructed the thioredoxin and glutathione antioxidant systems, as well as caused disturbance of the redox balance in the spleen. Too low Se contents in tissues stimulated the HIF-1α and NF-κB transcription factors, increasing pro-inflammatory cytokines (like IL-1β, IL-6, IL-8, IL-17 or TNF-α), reducing anti-inflammatory cytokines (e.g.: IL-10, IL-13 or TGF-β) and stimulating expression of the downstream genes iNOS and COX-2, thus causing inflammation [
4,
14,
15]. Additionally, insufficient doses of Se stimulated apoptosis (
via the mitochondrial apoptotic pathway), up-regulated apoptotic genes, and down-regulated anti-apoptotic genes (
Bcl-2) (at the mRNA level). On the other hand, diets containing high concentrations of Se stimulated oxidative stress (i.e. oxidative damage) as well as reduced immune responses in splenocytes [
16,
17,
18]. Therefore, the optimal concentration of Se-compounds in diets is very important for proper functioning spleen.
Recent studies have shown that dietary supplementation with n-3 polyunsaturated FA (n-3PUFA), especially n-3 long-chain PUFA (n-3LPUFA), suppressed pro-inflammatory cytokine production, increased the number of lymphocyte cells and stimulated formation of immunocompetent cytokines in the spleen, which have a crucial role in anti-tumor and anti-infection activities [
19,
20,
21]. However, higher contents of unsaturated fatty acids (UFA) in internal organs and tissues stimulated the oxidative stress in animals’ body [
22,
23]. Oxidative stress caused by reactive nitrogen (RNS) and oxygen (ROS) species damage lipids, proteins as well as cellular RNA and DNA. Carbonyl compounds, especially malondialdehyde (MDA), the naturally occurring by-products of PUFA peroxidation and prostaglandin synthesis, are known to be detrimental for health [
22,
24]. Taking into account that the principal physiological functions of more than half of Se-enzymes (e.g., GPx, selenoprotein P or thioredoxin reductase) are to maintain the proper oxidative-antioxidant balance, low contents of ROS and RNS and free radicals within cells [
25], an adequate amount of Se should be delivered in ration.
However, not only quantity but also chemical form of Se is of utmost importance. Organoselenium compounds (particularly seleno-methionine derived from Se
Ye) are more efficiently incorporated in the mammalian organisms than inorganic seleno-compounds (like Se
4 or Se
6) [
24]. Seleno-methionine (Se-Met) derived from Se
Ye is metabolized to inorganic seleno-compounds or accumulated into ruminal microobiota and animal tissue proteins as Se-Met (as a replacement of methionine (Met)) or seleno-cysteine [
26].
Considering the different metabolism of seleno-compounds and thus, their physiological role, which cannot be limited only to antioxidant properties, we claim that an additional antioxidant should be added to diets rich in n-3LPUFA, e.g. containing fish oil (F-O). The herbal nutraceutical, carnosic acid (CA), which is one of the diterpenes present in rosemary, [
27] was chosen in this study. As it has been found previously CA introduced in the ruminants ration improved production parameters (like diet intake, feed conversion efficiency or live weight gain) and the growth and/or activity of rumen microorganisms [
28,
29,
30]. Furthermore, our previous studies showed that simultaneous supplementation of Se (as Se
Ye or Se
6) with CA affected the biosynthesis yield of Se-proteins as well as the profile of lipid compounds in rumen microbiota [
26,
31,
32], blood [
28], muscles [
33], adipose tissues [
34,
35] and internal organs like the brain [
36], kidneys [
37], heart [
38] and pancreas [
39] of lambs. That is why we also hypothesized that the bioaccumulation of FA, total cholesterol (TCh), tocopherols (Ts) and the MDA concentration in the ovine spleen depend upon seleno-compound added to the diet containing F-O and CA. Therefore, the principal objective of these studies was to evaluate the effect of Se
Ye and Se
6 on the lipidomic profile in the ovine spleen. It is of importance not only from the point of view of ruminant physiology and welfare but also human nutrition, especially in undeveloped countries, as the spleen, classified as giblets, may be considered as inexpensive source of bioactive lipids for humans at risk of malnutrition.
4. Discussion
The control diet containing F-O and R-O as well as all experimental diets including CA, F-O and R-O without/with Se (as Se
6 or Se
Ye) did not affect negatively the total health conditions and especially welfare of sheep. Furthermore, the results of current investigations were confirmed by previous research documented that neither spleen injuries symptoms nor toxic symptoms and macroscopic lesions of 10 g of F-O, 20 g of R-O and 0.35 mg of Se (as Se
6 or Se
Ye) added to 1 kg of the BD were observed in lambs [
28,
35,
36,
39,
46]. In fact, dietary supplementation with 1% F-O and 2% plant sunflower oil reduced the numbers of ruminal
Butyrivibrio C18:0-producers, and influenced the numbers of
Streptococcus bovis,
Selenomonas ruminantium, methanogens and protozoa, but not the total number of bacteria in a rumen [
47]. Moreover, previous research showed that dietary supplementation of 1% F-O without or with 0.1% CA, regardless of the presence of Se
6 or Se
Ye, changes composition of ruminal microbiota and, so, FA metabolism, so, decreased the biohydrogenation yield of C18-UFA and stimulated bacterial isomerization of UFA [
28,
46]. Similarly, diets including up to 2 mg of Se in kilogram of the BD would not be toxic for sheep and cows whereas long-term administration of diets containing Se
6 or especially Se
4 (selenite) or selenides at doses of more than 5 mg of Se per kilogram of the BD can be teratogenic as well as hepatotoxic [
24,
48]. Indeed, these physiological effect of dietary Se were confirmed in the spleen, as well as previously in adipose tissues, muscles and other internal organs (like kidneys, pancreas, heart or brain) [
28,
32,
33,
36,
37,
39,
46]. Furthermore, lack of the detrimental health impact of dietary Se can be due to the fact, Se
6 used in our investigation, is relatively less reactive and toxic in mammals [
24]. Moreover, Se-Met (the main Se-compound in Se
Ye) is the less physiologically-active chemical form of Se, therefore, dietary supplementation with Se
Ye is considered as safe storage mode for Se [
24].
The mammals’ spleen is the very important immune organ, possessing different immunocompetent cytokines, which effectively stimulate anti-cancer as well as anti-infective functions. The higher spleen weight as well as the higher value of the spleen index observed in the lambs receiving the Se
YeCA diet may suggests that Se-Met (derived from Se
Ye) is predominantly accumulated in spleen proteins instead of Met [
24]. These Se-Met containing proteins in the spleen have no impact on important biochemical reactions, particularly protein or enzyme biosynthesis [
49]. Moreover, Se-Met containing proteins in the spleen are less effective in the detoxification of RNS, ROS or other radicals in comparison to Se-Cys-enzymes biosynthesized primarily from SeVI supplemented to the SeVICA diet [
50]. As a consequence, compared to sheep receiving the Se
6CA diet, Se-Cys-enzymes deficiency in the spleen of sheep receiving the Se
YeCA diet caused redox imbalance. Therefore, the current researches are consistent with studies Yan et al. [
44] showing that the value of the spleen index decreased significantly (p ≤ 0.05) with increasing dietary supplementation with superoxide dismutase (SOD).
Moreover, the present researches showed that especially the Se
YeCA diet significantly stimulated the TCh incorporation in the ovine spleen, which was also observed in pancreases and kidneys [
37,
39]. It is well established that elevated levels of cholesterol and atherogenic SFA (as well atherogenic index) in tissues are associated with increases oxidative stress, LDL-cholesterol concentration as well as they activate the inflammation process [
51,
52]. Similarly, a high-cholesterol diet increased ROS generation and formation in mitochondria and decreased the level of glutathione (efficient a free radical scavenger and a key the antioxidant) [
52]. These was confirmed by our results obtained on lambs fed particularly with the Se
YeCA diet. Indeed, the level of MDA tended to decrease (p > 0.05) and the index values of
ΣPUFAMDA
index and
indexA
SFA in the spleen of animals receiving the Se
YeCA diet were lower (p ≤ 0.05) than in the spleen of ewes receiving the Se
6CA diet. The earlier results [
37,
39] obtained for pancreas and kidneys also seem to support above mentioned assumptions. Thus, we argued that the higher levels of ROS, RNS or other radicals in the spleen of the Se
YeCA treated lambs may cause inflammation of this internal organ [
20]. Therefore, we supposed that the levels of TNF-α, IL-1β, IL-6, IL-8 or IL-17 (the pro-inflammatory cytokines [
4]) in the lambs’ spleen of the Se
YeCA group were higher than that in the Se
6CA group. So, the higher spleen index observed in the Se
YeCA-treated lambs may suggests the increased amounts of splenocytes, and thus, the enhanced immunoreaction, or state similar to hypersplenism, when macrophages in the spleen contain a large amount of fat due to hyper-active phagocytosis [
5].
Our current results indicated that diets enriched with extra Se-compounds exert the lipogenic effects. In fact, the Se
6CA and Se
YeCA diets significantly stimulated or tend to stimulate the incorporation of TCh in the ovine spleens when compared to the CA and control diets, which was observed also in kidneys [
37]. Indeed, cholesterol as well as seleno-proteins used isopentenyl pyrophosphate for Sec-tRNA and isoprenoid synthesis [
53]. In contrast, compared to the Se
YeCA and Se
6CA diets, the CA diet reduced the concentration of TCh in the spleen, kidneys, heart, subcutaneous fat and fat located between thigh muscles of sheep [
34,
37,
38]. So, these results is consistent with previous studies [
54] indicating that CA reduced the BWG and concentrations of triglyceride, TCh and glucose in experimental animals. Really, CA reduces the nuclear level of SREBPs (i.e., sterol regulatory element-binding proteins) as well as downregulates their target genes, hence reducing the yield of the de novo-biosynthesis of cholesterol and fatty acids; dietary CA stimulates the degradation of mature SREBPs-form [
55].
This study revealed that the Se
YeCA and Se
6CA diets statistically significant increase the yield of Δ9-desaturation of C16:0 in the ovine spleen in comparison with the CA and control diets. So, the present research indicated that the Se
YeCA diet and particularly the Se
6CA diet affected a substrate preference in the Δ9-desaturase in the ovine spleens. In fact, the Δ9-desaturase prefers acyl-CoA [
56] with lengths of saturated fatty acid containing 16-carbons (i.e. formation of palmitoyl-CoA desaturase) in the spleen of animals receiving the Se
YeCA and Se
6CA diets as compared with the CA and control diets. Additionally, the Se
YeCA and Se
6 CA diets increased the concentrations of α-T and stimulated DPA preference in the Δ4-desaturase, hence the higher level of DHA was found in the spleen of the Se
6 or Se
Ye-treated lambs than in animals fed the CA or control groups. Thus, our current study is in agreement with earlier researches [
57,
58] in which the Δ4-desaturase capacity and DHA content correlated with the level of Se-dependent hormones and enzymes, as well as tocopherol concentrations in animal tissues [
59]. Indeed, tocopherols and Se-compounds play the essential roles in Δ4-, Δ5- and Δ6-desaturations of UFA by involving in the microsomal electron transport chain and in a peroxidase group of the desaturase complex [
59].
In contrast, the CA diet decreased C14:0 and C16:0 (as the substrates) preference in the Δ9-desaturation, whereas increased t11C18:1 preference in the Δ9-desaturation in the ovine spleens in comparison with the Se
YeCA and Se
6CA diets. Thus, we claimed that compared to C14:0 and C16:0, t11C18:1 shows greater affinity to Δ9-desaturase in the spleen of ewes receiving the CA diet. However, Se
Ye and particularly Se
6 added to experimental diets with CA stimulated C14:0 and C16:0 preference in the Δ9-desaturation in the spleen as well as values of Δ9-desaturase index in body fat of sheep [
34] as compared with the CA and control diets.
The present and our previous researches showed that the Se
YeCA and Se
6CA diets increased the accumulation of TCh and Σall-Ts in the spleen, heart and subcutaneous fat [
34,
38] in comparison with the CA and control diets. So, we claimed that dietary SeY or SeVI spared of tocopherols as well as easy peroxidized long-chain highly UFA in lambs’ tissues. In fact, dietary Se
Ye and Se
6 are utilized for biosynthesis of Se-dependent antioxidant enzymes which prevent against peroxidation of UFA (particularly highly unsaturated long-chain FA) in mammalian organisms [
49,
58,
60]. Thus, Se-enzymes decreased content of free radicals mediated peroxidation and synergistically with tocopherols regulated lipid peroxidation in mammalian tissues [
61].
In this study it was shown, that all experimental diets decreased spleen content of fatty acids responsible for atherogenesis [
51]. Indeed, particularly the Se
6CA diet, decreased the content of A-SFA, values of the modified atherogenic index (
indexA
SFA/Toc) and
indexT
SFA, whereas improved the value of the h/H-Ch ratio in the ovine spleen in comparison with the control diet.
Author Contributions
Conceptualization, M.C.; methodology, M.C.; software, M.C., M.B., and A.B.; validation, M.C., M.B., and A.B., ; formal analysis, M.C., M.B., A.B., and W.W.; investigation, M.C., M.B., A.B., and W.W.; resources, M.C.; data curation, M.C., M.B., A.B., and W.W.; writing—original draft preparation, M.C.; writing—review and editing, M.C., M.B., A.B., and W.W.; visualization, M.C., M.B., A.B., and W.W; supervision, M.C., A.B., and A.B.; project administration, M.C.; funding acquisition, M.C., M.B., and A.B. All authors have read and agreed to the published version of the manuscript.