- Research Article
- Open Access
Genome-wide expression profiling in muscle and subcutaneous fat of lambs in response to the intake of concentrate supplemented with vitamin E
© The Author(s). 2017
- Received: 15 April 2016
- Accepted: 9 December 2016
- Published: 17 January 2017
The objective of this study was to acquire a broader, more comprehensive picture of the transcriptional changes in the L. Thoracis muscle (LT) and subcutaneous fat (SF) of lambs supplemented with vitamin E. Furthermore, we aimed to identify novel genes involved in the metabolism of vitamin E that might also be involved in meat quality. In the first treatment, seven lambs were fed a basal concentrate from weaning to slaughter (CON). In the second treatment, seven lambs received basal concentrate from weaning to 4.71 ± 2.62 days and thereafter concentrate supplemented with 500 mg dl-α-tocopheryl acetate/kg (VE) during the last 33.28 ± 1.07 days before slaughter.
The addition of vitamin E to the diet increased the α-tocopherol muscle content and drastically diminished the lipid oxidation of meat. Gene expression profiles for treatments VE and CON were clearly separated from each other in the LT and SF. Vitamin E supplementation had a dramatic effect on subcutaneous fat gene expression, showing general up-regulation of significant genes, compared to CON treatment. In LT, vitamin E supplementation caused down-regulation of genes related to intracellular signaling cascade. Functional analysis of SF showed that vitamin E supplementation caused up-regulation of the lipid biosynthesis process, cholesterol, and sterol and steroid biosynthesis, and it down-regulated genes related to the stress response.
Different gene expression patterns were found between the SF and LT, suggesting tissue specific responses to vitamin E supplementation. Our study enabled us to identify novel genes and metabolic pathways related to vitamin E metabolism that might be implicated in meat quality. Further exploration of these genes and vitamin E could lead to a better understanding of how vitamin E affects the oxidative process that occurs in manufactured meat products.
- Vitamin E
- Genome-wide expression profiling
- Subcutaneous fat
- Meat quality
Vitamin E is a lipid-soluble essential component of human and animal diets due to its powerful antioxidant activity. There are eight different isoforms of vitamin E, and the most active isoform for cell protection appears to be α-tocopherol . Alpha-tocopherol is a cell signaling molecule involved in a broad range of effects on cellular systems  and specific regulation of gene expression [2–6]; therefore, α-tocopherol can influence a number of biological functions by regulating cell signaling at both the mRNA and microRNA (miRNA) levels . So far, the impact and beneficial effects of vitamin E in the prevention of chronic diseases have been mainly associated with its antioxidant properties .
Vitamin E is extensively used in animal diets as an antioxidant supplement. Feeding strategy is an important tool to manipulate intramuscular α-tocopherol content in animals. An increase in α-tocopherol intake can be achieved through grazing systems  or by feeding animals with concentrate supplemented with α-tocopherol, which can be used as an effective method to reduce the oxidative processes reported in meat products [10, 11].
During oxidative processes, the muscle haeminic pigment changes from red oxymyoglobin to brown metmyoglobin, giving the meat an undesirable brownish color . Moreover, lipid oxidation results in the production of free radicals, which are linked to the formation of off-flavors and odors, a reduction in polyunsaturated fatty acids and the production of undesirable compounds, such as potentially toxic peroxides and aldehydes . All of these modifications cause decreases in the freshness and quality of meat and lower consumer acceptance, resulting in considerable economic loss for the meat industry.
So far, efforts to discover the genes and metabolic pathways involved in vitamin E metabolism have mainly focused on rodents. These studies have reported important long-term effects of vitamin E deficiency on liver gene expression, up-regulation of genes involved in cholesterol synthesis and steroidogenesis, lipid uptake, and anti-oxidative mechanisms [3, 13]. In addition, vitamin E in the rat brain affected genes associated with hormones, nerve growth, apoptosis, dopaminergic neurotransmission, and clearance of amyloid-α . Furthermore, genes encoding for muscle structure and extra cellular matrix and those involved in anti-oxidative and anti-inflammatory processes were up-regulated in rat skeletal muscle in response to vitamin E deficiency .
Although nutritional science has embraced the tools of genomics, including cDNA arrays in rodent models, few attempts at large-scale or global evaluation of nutritional gene regulation have mainly used rodents as a model. Only a small number of genes are currently known to be related to vitamin E metabolism in muscle or fat. In the last few years, several studies have explored the transcriptomic adaptations of skeletal muscle in response to different nutrition variables in ruminants [16–19]. Data from these studies have allowed scientists to identify the biochemical mechanisms that could be associated with key physiological processes in animals and to define specific markers for evaluating meat quality.
A better understanding of the genes and metabolic pathways associated with vitamin E metabolism is critical for identifying the key physiological processes associated with vitamin E content, oxidative stress and other metabolic pathways associated with meat quality. The main effect of vitamin E on meat is to change the lipid oxidation and thus to increase shelf-life of meat. Because lipid oxidation depends mainly on enzyme activity, post-transcriptional and post-translational changes are processes implicated in the shelf-life of meat. However, the identification of genes related to vitamin E content would be a good starting point for candidate gene selection and single nucleotide polymorphism (SNP) association studies with variations in vitamin E content and therefore implicated in meat quality. Furthermore, non-antioxidant roles of vitamin E, serving as a regulator of gene/protein, have been described. In this sense, transcriptomic analysis of skeletal muscle could identify metabolic pathways modified by vitamin E that could influence meat traits.
Therefore, the main objective of this study was to investigate transcriptional changes in the L. Thoracis muscle (LT) and subcutaneous fat (SF) of lambs supplemented with vitamin E using the Affymetrix Ovine Gene 1.1 ST whole-genome array. Furthermore, we aimed to identify novel genes that could play important roles in the metabolism of vitamin E and that might be associated with meat quality traits.
Alpha-tocopherol muscle content, intramuscular fat, TBARS and metmyoglobin formation
Effect of the dietary treatment on slaughter age and weight, growth rate, alpha-tocopherol muscle content, intramuscular fat, TBARS and metmyoglobin formation
Weaning Weight (kg)
Slaughter Age (days)
Slaughter Weight (kg)
Average Daily Gain (g/day)
- From birth to weaning
- Experimental period2
- From birth to slaughter
L. Thoracis muscle
α-Tocopherol, mg/kg muscle
IMF (% fresh matter)
TBARS (7 d)3
K/S572/525 (7 d)4
Microarray gene expression results
Identification and classification of differentially expressed genes by microarray analysis in LT and SF
The significant features identified by SAM in VE vs. CON contrast in L. Thoracis muscle
aldo-keto reductase family 7, member A2
differentially expressed in FDCP 8 homolog
Protein Phosphatase 6, Regulatory Subunit 3
spermidine/spermine N1-acetyltransferase 1
ATP-binding cassette, sub-family C (CFTR/MRP), member 4
cytokine inducible SH2-containing protein
A kinase (PRKA) anchor protein 7
acetyl-Coenzyme A carboxylase beta
v-maf musculoaponeurotic fibrosarcoma oncogene homolog B
H1 histone family, member 0
myosin light chain kinase 2
dual specificity phosphatase 26 (putative)
regulatory solute carrier protein, family 1, member 1
four and a half LIM domains 3
acetyl-Coenzyme A acetyltransferase 1
family with sequence similarity 151, member A
zinc finger protein 777
DEP Domain Containing MTOR-Interacting Protein
leucine rich repeat transmembrane neuronal 2
DEAD (Asp-Glu-Ala-Asp) Box Polypeptide 39B
F-box and leucine-rich repeat protein 4
insulin-like growth factor 1 receptor
ST6-N-acetylgalactosaminide alpha-2,6-sialyltransferase 6
chromosome 10 open reading frame 104
Top 50 genes identify with SAM in VE vs. CON contrast in subcutaneous fat
DEAD (Asp-Glu-Ala-Asp) box polypeptide 47
UTP14, U3 small nucleolar ribonucleoprotein, homolog C
signal recognition particle receptor, B subunit
tetratricopeptide repeat domain 37
Ras-related GTP binding A
leucine rich repeat containing 59
Ras association domain family (N-terminal) member 8
elongation protein 3 homolog
SEC23 interacting protein
transmembrane protein 181
cullin-associated and neddylation-dissociated 1
phosphatidylinositol glycan anchor biosynthesis, class M
transmembrane protein 203
similar to ATP-binding cassette, sub-family E, member 1
WD repeat domain 55
solute carrier family 35, member A4
actin related protein 2/3 complex, subunit 5, 16 kDa
multiple inositol polyphosphate histidine phosphatase, 1
RER1 retention in endoplasmic reticulum 1 homolog
translocase of inner mitochondrial membrane 22 homolog
exostoses (multiple)-like 3
guanine monphosphate synthetase
solute carrier family 39 (zinc transporter), member 7
RAB22A, member RAS oncogene family
immediate early response 3
ring finger protein 121
DnaJ (Hsp40) homolog, subfamily C, member 5
nudix (nucleoside diphosphate linked moiety X)-type motif 9
wings apart-like homolog
F-box protein 28
adaptor-related protein complex 1, sigma 1 subunit
canopy 4 homolog
Phosphatase Domain Containing, Paladin 1
A kinase (PRKA) anchor protein 10
ribosomal protein L36a pseudogene 51
mitochondrial calcium uniporter
solute carrier family 39 (zinc transporter), member 3
ER membrane protein complex subunit 8
proline synthetase co-transcribed homolog
ATP-binding cassette, sub-family F (GCN20), member 3
chromosome 5 open reading frame, human C22orf28
PIN2-interacting protein 1
ceramide synthase 6
dolichyl pyrophosphate phosphatase 1
ribosomal protein S4, Y-linked 1
translocase of inner mitochondrial membrane 8 homolog A
Treatment-dependent multivariate analysis results of gene expression
Moreover, we investigated patterns in gene expression changes in subcutaneous fat (Fig. 3d). For example, the gene expressions of ALG11, SRPBR, and PPX47 were positively correlated with each other, being up-regulated in VE treatment.
Hierarchical clustering analysis (HCA)
Regarding subcutaneous fat, the results of clustering analysis of top 50 genes are presented in Fig 4b. Analyzing the heatmap, it can be observed that the CON group showed very different gene expression patterns from the VE group. There is a general down-regulation of gene expression in the CON group and an up-regulation of gene expression in the VE group. The results of clustering analysis of the total 330 genes in subcutaneous fat were similar to the 50 genes analysis results, but one VE animal was clustered in the CON group (Additional file 2: Figure S1). More specifically, 34 genes were down- regulated in the VE group (SOD3, CLEC3B, METTL7A, IER3, PLK2, CLEC1A, HSPA1A, TNFSF10, MERTK, MLLT3, TPPP2, HMGB2, PECAM1, IFITM3, H1F0, HMG20B, GALK1, NFIL3, CDH5, ERG, CEBPA, ANKRD44, HCST, UACA, IFITM1, RPL10, PID1, LUC7L3, H3F3A, AMOTL2, PSIP1, RPL17, TRA2A, and RPS259).
Functional clustering annotation
In the LT muscle, the results of Database for Annotation, Visualization and Integrated Discovery (DAVID) functional annotation clustering (FAC), including the 29 significant genes in the VE-CON contrast, showed that 4 genes from the “intracellular signaling cascade” (IGF1R, DEF8, AKAP7 and CISH) had the most enrichment and were all down-regulated in the VE treatment (Additional file 3: Table S2). However, the confident enrichment scores were less than 1.3 in both cases.
DAVID Functional Annotation Clustering of SAM genes in VE vs. CON subcutaneous fat
Annotation Cluster 1
Enrichment Score: 5.82
GO:0016126 ~ sterol biosynthetic process
EBP, MVD, CYP51A1, SQLE, DHCR7, HMGCS1, LSS, SIGMAR1, FDFT1
GO:0016125 ~ sterol metabolic process
SREBF1, EBP, LDLR, MVD, CYP51A1, SQLE, DHCR7, INSIG1, HMGCS1, LSS, SIGMAR1, FDFT1
GO:0008610 ~ lipid biosynthetic process
PGAP3, EBP, CRLS1, MVD, CYP51A1, GNE, HMGCS1, DPAGT1, LSS, SIGMAR1, LPCAT3, FDFT1, DOLK, PIGM, SQLE, DHCR7, AGPAT9, LTA4H, PCYT2, HSD17B7
GO:0008203 ~ cholesterol metabolic process
SREBF1, EBP, LDLR, MVD, CYP51A1, SQLE, DHCR7, INSIG1, HMGCS1, LSS, FDFT1
GO:0006695 ~ cholesterol biosynthetic process
EBP, MVD, CYP51A1, DHCR7, HMGCS1, LSS, FDFT1
GO:0006694 ~ steroid biosynthetic process
EBP, MVD, CYP51A1, SQLE, DHCR7, HMGCS1, LSS, SIGMAR1, HSD17B7, FDFT1
GO:0008202 ~ steroid metabolic process
SREBF1, EBP, MVD, LDLR, CYP51A1, HMGCS1, LSS, SIGMAR1, FDFT1, SQLE, DHCR7, INSIG1, HSD17B7
Annotation Cluster 2
Enrichment Score: 3.59
GO:0007005 ~ mitochondrion organization
NDUFAF4, CAV2, DNM1L, MTX2, TOMM40, TIMM44, TIMM8A, TRNT1, GFM2, MRPL12, FXC1, PTCD2, DNAJA3
GO:0015031 ~ protein transport
SNX19, MTX2, SNX4, RAB1A, COPB2, AP1S1, FXC1, VPS4A, NUP35, SEC23IP, ZW10, RAB2B, SRP54, VPS45, TOMM40, NUP85, AP4S1, TIMM44, TIMM8A, TIMM22, TRNT1, YWHAG, PLEKHA8, RAB22A, KPNA6, NOP58, NUP43, KPNA1, SNX11
GO:0045184 ~ establishment of protein localization
SNX19, MTX2, SNX4, RAB1A, COPB2, AP1S1, FXC1, VPS4A, NUP35, SEC23IP, ZW10, RAB2B, SRP54, VPS45, TOMM40, NUP85, AP4S1, TIMM44, TIMM8A, TIMM22, TRNT1, YWHAG, PLEKHA8, RAB22A, KPNA6, NOP58, NUP43, KPNA1, SNX11
GO:0008104 ~ protein localization
SNX19, MTX2, SNX4, AKAP10, RAB1A, COPB2, AP1S1, FXC1, PIKFYVE, VPS4A, NUP35, SEC23IP, ZW10, RAB2B, SRP54, VPS45, TOMM40, NUP85, AP4S1, TIMM44, TIMM8A, TIMM22, TRNT1, YWHAG, PLEKHA8, RAB22A, KPNA6, NOP58, NUP43, KPNA1, SNX11
GO:0033365 ~ protein localization in organelle
TRNT1, SRP54, MTX2, FXC1, PIKFYVE, KPNA6, TOMM40, NOP58, TIMM44, KPNA1, TIMM8A
GO:0006626 ~ protein targeting to mitochondrion
TRNT1, MTX2, FXC1, TOMM40, TIMM44, TIMM8A
GO:0070585 ~ protein localization in mitochondrion
TRNT1, MTX2, FXC1, TOMM40, TIMM44, TIMM8A
GO:0034613 ~ cellular protein localization
SRP54, VPS45, MTX2, TOMM40, AP4S1, TIMM44, TIMM8A, TRNT1, COPB2, YWHAG, AP1S1, FXC1, PIKFYVE, KPNA6, NOP58, SEC23IP, KPNA1, ZW10
GO:0070727 ~ cellular macromolecule localization
SRP54, VPS45, MTX2, TOMM40, AP4S1, TIMM44, TIMM8A, TRNT1, COPB2, YWHAG, AP1S1, FXC1, PIKFYVE, KPNA6, NOP58, SEC23IP, KPNA1, ZW10
Annotation Cluster 3
Enrichment Score: 3.00
GO:0008299 ~ isoprenoid biosynthetic process
MVD, HMGCS1, DPAGT1, FDFT1, DOLK
GO:0006720 ~ isoprenoid metabolic process
RDH11, MVD, HMGCS1, DPAGT1, FDFT1, DOLK
GO:0016090 ~ prenol metabolic process
DPAGT1, FDFT1, DOLK
GO:0016093 ~ polyprenol metabolic process
DPAGT1, FDFT1, DOLK
Annotation Cluster 4
Enrichment Score: 1.72
GO:0034660 ~ ncRNA metabolic process
GARS, EPRS, IARS, TRNT1, WDR55, NOLC1, METTL1, ADAT2, NOP58, ALG11, NSUN2, FTSJ2, MARS
GO:0034470 ~ ncRNA processing
TRNT1, WDR55, NOLC1, METTL1, ADAT2, NOP58, ALG11, FTSJ2, NSUN2
GO:0042254 ~ ribosome biogenesis
WDR55, TSR1, NOLC1, NOP58, ALG11, FTSJ2
Annotation Cluster 5
Enrichment Score: 1.68
GO:0006399 ~ tRNA metabolic process
TRNT1, IARS, METTL1, ADAT2, GARS, EPRS, NSUN2, MARS
GO:0043039 ~ tRNA aminoacylation
IARS, GARS, EPRS, MARS
GO:0043038 ~ amino acid activation
IARS, GARS, EPRS, MARS
GO:0006418 ~ tRNA aminoacylation for protein translation
IARS, GARS, EPRS, MARS
Annotation Cluster 6
Enrichment Score: 1.43
GO:0016192 ~ vesicle-mediated transport
CAV2, RAB2B, LDLR, CNIH, VPS45, GARS, RER1, SNX4, AP4S1, RAB1A, COPB2, AP1S1, TFRC, CDC42SE2, ACTR1A, PIKFYVE, RAB22A, VPS4A, TRAPPC3, ZW10
GO:0016044 ~ membrane organization
CAV2, COPB2, AP1S1, DNM1L, LDLR, TFRC, CDC42SE2, FXC1, RAB22A, PIKFYVE, SNX4, TIMM8A
Validation of microarrays results using qPCR
Real-time PCR confirmation of the microarray results
VE vs CON
VE vs CON
The aim of the present study was to assess the effects of adding VE to the fattening concentrate, fed between weaning and slaughter, on the transcriptional changes in the LT and SF. The experimental period ended when the lambs reached the target slaughter LW (22–24 kg), according to the specifications of Ternasco de Aragón Protected Geographical Indication (Regulation (EC) No. 1107/96). The light lamb production is based on two periods: suckling and fattening. The suckling period is usually limited by the time, usually lasting approximately 45 d (in the present study, it was 48 d), while the fattening period is limited by the weight (22–24 kg LW). Thus, lambs with high ADG during lactation period spent fewer days in the fattening period than those with low ADG. To be able to evaluate the potential effects of the inclusion of VE on the transcriptome of the L. Thoracis muscle and subcutaneous fat, we planned to feed at least 30 days of VE concentrate. According to Ripoll et al. , supplementation with VE for 30 days was found to increase noticeably concentration of the α-tocopherol content in muscle. In the present study, the ADG during the experimental period was not different, however during the suckling period it was lower in VE treatment (p < 0.01; Table 1). This implies that the lambs of the VE treatment during the suckling period presented lower growth compared to the CON group and needed more days to reach the target weight. Moreover, there was a strong correlation between ADG from birth to slaughter and SA (r 2 =−0.9). Therefore, to overcome this unbalance between treatments ADG values were included as covariates in the statistical model used to validate gene expression differences by qPCR, thus avoiding estimation biases. We analyzed LT muscle and SF because meat cuts for human consumption include both intramuscular and subcutaneous fat , constituting the total amount of meat fat purchased at retail. The results related to the content of α-tocopherol in the muscle and TBARS confirm what was planned in the methodology. Vitamin E has been successfully used to increase the muscle α-tocopherol and to reduce lipid oxidation in beef and chicken meat . In this study, as expected, there were significant differences between both diets in the α-tocopherol content, lipid oxidation and MMb formation in the LT muscle, as previously found by Ripoll et al. . Moreover, TBARS values were lower in VE lambs which showed that the lipid oxidation process was slower in animals fed this type of diet.
In the present study, we used microarray technology to study changes in gene expression profiles in LT muscle and SF in response to vitamin E supplementation in lambs. Our results, showed that vitamin E supplementation caused different responses in gene expression in LT muscle and SF, suggesting a specific response of tissue to vitamin E supplementation. In our study, we did not measure the concentration of VE in the SF, however the gene expression results might be related to the greater α-tocopherol accumulation in adipose tissues than in skeletal muscles [22, 23].
It has been reported that α-tocopherol can influence a number of biological functions by regulating cell signaling at both the mRNA and miRNA levels . Indeed, our results showed that vitamin E supplementation affected the expression of 29 genes in the LT muscle. The results of functional analysis showed that genes related to the intracellular signaling cascade (CISH, IGF1R, DEF8, and AKAP7) and metabolic processes (ZNF79, MAFB, MYLK2, ACACB, ACAT1, CISH, IGF1R, PGLS, DUSP26, AKR7A2, FBXL4, AKAP7, and RSC1A1) were down-regulated. The enrichment scores were less than 1.3, likely because the number of significant genes in this contrast was low. The most down-regulated gene by vitamin E in LT was CISH (Cytokine Inducible SH2 containing protein). Chen et al.  reported that CISH activates protein kinase C (PKC) activity by G-protein coupled receptor protein, which is important for the activation of both the activator protein 1 (AP-1) and nuclear factor- κB (NF-κB) pathways. In a previous study Boscoboinik et al. , demonstrated that PKC is inhibited by α-tocopherol. NF-κB proteins are a family of transcription factors and are of central importance in inflammation, immunity and apoptosis . Evidence suggests a role for reactive oxidative intermediates (ROIs) as a common and critical intermediate for various NF-κB-activating signals, based on inhibition of NF-κB activation by a variety of antioxidants [27, 28]. There is a bidirectional relationship between cytokines and oxidative stress. Exposure of myotubes to reactive oxygen species (ROS)-producing agents resulted in an increase in interleukin 6 (IL-6) release through the activation of the redox-sensitive transcription factor, NF-κB . In this sense, IL-6 induces marked increases in expression of CISH, SOCS-1, SOCS-2, and SOCS-3 in tissues, which in turn result in inhibition of the signaling of wide range of cytokines . Thus, CISH expression could be related to oxidative status in the muscle. In this sense, low levels of ROS and ROIs in muscle caused by α-tocopherol could be associated with low expression of CISH. In this sense, lipid oxidation in the LT muscle and metmyoglobin formation were lower when lambs were supplemented with VE. In the same manner, RSC1A, which inhibits a dynamin and PKC-dependent exocytotic pathway of SLC5A1 gene, was down-regulated in the VE group. Interestingly enough CISH and MYLK, also down-regulated in the VE treatment, are involved in inflammation mediated by chemokine and cytokines signaling pathway, which could suggest another possible role for VE that of decreased inflammation.
In our study, alterations of the Ras homolog gene family, member A (RhoA) and actin cytoskeleton signaling were identified (IGF1R and MYLK2 genes). Both IGF1R and MYLK2 were down-regulated in VE treatment. Insulin-like growth hormone 1 (IGF-1) is a protein structurally similar to insulin, and it regulates tissue growth and development in several vertebrates . As a main receptor of IGFs, IGF1R mediates the transduction of metabolic signals of cell proliferation, bone growth, and protein synthesis in the GH/IGF pathway . In our study, IGF1R was down-regulated in VE treatment, and because of IGF1R polymorphisms have been associated to growth traits [32–36], we thought that this effect could be due to the higher ADG in CON animals. However, ADG values were included as a covariate in the statistical model used to validate expression differences by qPCR, thus avoiding estimation biases related to ADG. Therefore these results suggest that supplementation with VE causes down-regulation of IGF1R in LT muscle. Our findings are in agreement with Araujo et al. , who showed that VE supplementation reduces IGF1R expression by 17% in hyperthyroid of Wistar rats. In addition, Chuang et al.  also found that VE alone significantly and dose-dependently reduced the cell surface expression of IGFIR in HL-60 cells. Moreover, Holzenberger et al.  reported that Igf1r +/−mice display greater resistance to oxidative stress.
In addition, vitamin E down-regulated two genes related to lipid metabolisms (ACAT1 and ACACB) in the LT muscle. ACAT1 (acetyl-coenzyme A acetyltransferase 1) encodes a mitochondrial localized enzyme that catalyzes the reversible formation of acetoacetyl- CoA from two molecules of acetyl-CoA. ACAT1 is responsible for cholesterol homeostasis and maintain appropriate cholesterol availability in cell membranes, whereas ACACB is the key regulator of the fatty acid oxidation pathways . It controls fatty acid oxidation by means of the ability of malonyl-CoA to inhibit carnitine-palmitoyl-CoA transferase I (CPT1B). As in our work, Shige et al.  showed that vitamin E reduced the uptake of modified low-density lipoprotein (LDL) and suppressed ACAT activity, resulting in less cholesterol esterification in macrophages. Interestingly enough ABCC4, an ATP-binding cassette (ABC) transporter, AKR7A2 which is involved in the detoxification of aldehydes and ketones, and finally RSC1A1 which transport carbohydrate across the plasma membrane, were all down- regulated with VE treatment. In hepatocytes, ABCC4 was shown to be induced by oxidative stress through binding of the oxidative sensor nuclear factor E2-related factor 2 (Nrf2) to antioxidant-responsive element sequences in the promoter of ABCC4 . Our results showed for the first time that vitamin E down-regulates ABCC4 expression. Because of significant differences in ADG between treatments, we validated the expression results of the CISH, ABCC4, ACAT1 and IGFR1 genes by qPCR, including in the statistical model ADG as a covariate. ADG was not significant, with a similar FC between the microarray and qPCR results (r 2 = 0.99; P = 0.008). Therefore, treatment was the main effect over gene expression.
In addition, we found that the transcription factors FHL3, ZNF777 and MAFB were down-regulated. Considering all together, our results showed that supplementation with VE increased the content of α-tocopherol in the LT muscle and decreased metmyoglobin formation and lipid oxidation. We speculate that α-tocopherol in the LT muscle causes a decrease in the catalytic activity of enzymes involved in cellular transport of fatty acids and carbohydrates, and fatty acid oxidation in the mitochondria. Decreased IGF1R expression might be related to the lower lipid oxidation levels in VE animals. We also found that IGFR1, ABCC4 ACAT1, CISH, ACACB, MYLK2, ZNF777 and MAFB were positively correlated with each other and with the diet, which suggest co-expression processes of these genes. We speculate that transcription factors ZNF777, MAFB and FHL3 could be important players in mediating the effects of VE in regulating the expression of these genes.
To establish whether IGFR1, ABCC4 and ACAT1, CISH are markers of meat oxidation or indirect markers of meat quality, further studies with greater numbers of animals are necessary.
A most dramatic effect of VE was observed on SF gene expression. ALG11, SRPBR, DDX47, SEC23ID and TTC37 were among the most important genes in discriminate fed treatments. These genes were up-regulated and positively correlated with each other and with the diet, which might suggest co-expression processes. Four of the up-regulated genes in the VE group were related to heat shock proteins (HSPs) or chaperonin activity (TTC37, DNAJC16, HSB8, AHSA1). Some HSPs are characterized by various specific functions such as anti-apoptotic or anti-inflammatory effects . In our study, these genes were up-regulated and positively correlated, suggesting putative increased stress protection in VE lambs.
Surprisingly VE treatment showed general up-regulation of almost all significant genes, compared to CON treatment. Lipid biosynthetic processes were among the most enriched functional clusters with major biological significance and importance (PGAP3, EBP, CRLS1, MVD, CYP51A1, GNE, HMGCS1, DPAGT1, LSS, SIGMAR1, LPCAT3, FDFT1, DOLK, PIGM, SQLE, DHCR7, AGPAT9, LTA4H, PCYT2, and HSD17B7). Moreover, genes implicated in sterol, steroid and cholesterol biosynthesis processes (SREBF1, EBP, LDLR, MVD, CYP51A1, SQLE, DHCR7, INSIG1, HMGCS1, LSS, and FDFT1) were up-regulated in the SF of VE animals, compared to the CON group. It has been previously reported that tocopherols inhibited de novo cholesterol synthesis within enterocytes , and cause repression of genes (DHCR7 and HMGCS1) involved in the de novo synthesis of cholesterol in testes and adrenal glands . The differences reported in previous studies and ours might be due to the different tissues analyzed. This supports even more our idea that there is a tissue specific response in response to VE supplementation. On the other hand, Wang et al.  found oxysterol-specific repressive effects in the CYP51A1 and FDFT1 genes, mediated via direct binding of the ligands to liver X receptor (LXR). Because of α–tocopherol and other antioxidants can inhibit the oxidation of cholesterol , the VE group could have a lower quantity of oxysterols and then have inhibited the repression of cholesterol biosynthesis via LXR and oxysterols and up-regulated the sterol, steroid and cholesterol biosynthesis processes. In this sense, several genes implicated in “the stress response” (SOD3, IER3, HMGB2, UACA, LUC7L3) were down-regulated in the VE group, compared to the CON group (Table 3 and Supplementary Table S1). Among them, SOD3 showed higher values of FC. This gene encodes a member of the superoxide dismutase (SOD) protein family that protects the extracellular space from the toxic effects of reactive oxygen intermediates by converting superoxide radicals into hydrogen peroxide and oxygen. This finding contradicts the results of previous studies, which suggested that grass-based treatments elevated the activity of antioxidants, such as glutathione and superoxide dismutase (SOD), compared to grain-feeding . However, Kumar et al.  in the myocardium muscle and Strobel et al.  in the skeletal muscle of exercise-trained and sedentary rats, found that antioxidant supplements reduced the endogenous antioxidants SOD2 gene and protein and the glutathione peroxidase (GPx) gene and enzyme activity.
Another down-regulated gene in VE treatment was IER3. IER3-deficient NCM460 cells exhibited reduced reactive oxygen species levels, indicating increased antioxidative protection . In our study, IER3 was down regulated in the VE group suggesting an increase in antioxidative protection. The role of HMGB1 in recognizing aberrant or damaged DNA has been shown in multiple in vitro experiments. A recent study directly showed the accumulation of HMGB1 at sites of oxidative DNA damage in live cells, thus defining HMGB1 as a component of an early DNA damage response . A similar function has been attributed to HMGB2 . These authors hypothesized that HMGB1/2 proteins act as a sensor of DNA modification, and their interaction with chemically altered DNA changes the chromatin structure, thus inducing DNA damage responses.
Finally, a cluster related to tRNA metabolic processes was also significant (TRNT1, METTL1, ADAT2, NSUN2, IARS, GARS, EPRS and MARS). Aminoacyl-tRNA synthetases perform an essential function in protein synthesis by catalyzing the esterification of an amino acid to its cognate tRNA (IARS, GARS, EPRS and MARS). Considering all together, we speculate that in the SF, vitamin E exerts anti-inflammatory effect and stress protection by increasing heat shock protein expression. Vitamin E reduces stress response probably as results of reduced reactive oxygen species levels. In addition animals supplemented with VE might have inhibited cholesterol oxidation in SF and enhanced sterol, steroid and cholesterol biosynthesis processes. And finally, we speculate that regulation of these gene expressions it could be mediated through tRNAs IARS, GARS, EPRS and MARS.
Regarding LT, we included ADG values as a covariate in the statistical model used to validate the expression differences by qPCR to avoid estimation biases. In this case, ADG was not significant, and a similar FC between microarray and qPCR results was found (r 2 = 0.99; p < 0.0001). Thus, the treatment was also the main effect over gene expression of SF.
Although we found differences in mRNA activity, it did not necessarily cause differences in metabolic processes. An increase in gene expression is not necessarily correlated with an increase in protein concentrations or enzyme activities. There are many processes between transcription and translation, including post-transcriptional, translational and protein degradation regulation, in controlling steady-state protein abundances . Moreover, DNA microarray has been the technology of choice for transcriptome analysis in recent years. Nonetheless, array technology has several limitations which include: using microarray technology limits the researcher to detecting transcripts that correspond to existing genomic sequencing information; and background hybridization limits the accuracy of expression measurements, particularly for transcripts present in low abundance.
This study demonstrated the beneficial effects of vitamin E supplementation during fattening period in lambs by increasing α-tocopherol content in the LT muscle and diminishing drastically the lipid oxidation of the meat. We observed a tissue-specific response to vitamin E supplementation. The gene expression profiles for VE and CON treatments were different in both LT and SF. Vitamin E supplementation had a dramatic effect on subcutaneous fat gene expression, showing a general up-regulation of genes, compared to CON treatment. Our study enabled us to identify novel genes (for example, IGF1R, ACAT1, ABCC4, ACACB, SOD3, and IER3) and metabolic pathways related to vitamin E metabolism that might be implicated in meat quality. To the best of our knowledge, this study was the first to report the effect of vitamin E supplementation on gene expression in the LT muscle and SF of lambs. Future exploration of these genes is necessary for a better understanding of how vitamin E affects the oxidative processes that occur in meat products.
All experimental procedures including the care of animals and euthanasia were performed in accordance with the guidelines of the European Union and with Spanish regulations for the use and care of animals in research and were approved by the Animal Welfare Committee of the Centro de Investigación y Tecnología agroalimentaria (CITA) (protocol number 2009-01_MJT). In all cases, euthanasia was performed by penetrating captive bolt followed by immediate exsanguination.
Animals and sample collection
Ingredients and chemical composition of the feedstuffs used in the experiment
CP, g/kg DM
NDF, g/kg DM
ADF, g/kg DM
α-tocopherol, mg/kg DM
All of the lambs were slaughtered when they attained 22–24 kg of slaughter weight (SW), according to the specifications of Ternasco de Aragón Protected Geographical Indication (Regulation (EC) No. 1107/96), which stipulates that lambs must be less than 90 days old with a SW between 22 and 24 kg. The lambs were slaughtered using EU laws in the same commercial abattoir, and the carcasses were hung by the Achilles tendon and chilled for 24 h at 4 °C in total darkness. The slaughter age, slaughter weight, and growth rate of the 2 management strategies are presented in Table 1.
Immediately after slaughter, a piece of LT muscle from the 12th thoracic vertebra and a piece of SF between the atlas and axis cervical vertebrae were cut, frozen in liquid nitrogen and stored at−80 °C until RNA isolation.
Analysis of α-tocopherol, intramuscular fat content, TBARS and metmyoglobin formation
After chilling, a piece of the LT muscle between the 4th and the 6th lumbar vertebrae was vacuum-packed and kept at−20 °C in darkness until the α-tocopherol analysis. The α-tocopherol concentration was determined by liquid extraction as described in González-Calvo et al. . A portion of the loin between the 7th and the 13th thoracic vertebrae was used to measure the LT muscle oxidative processes. The color (metmyoglobin content, MMb) and lipid oxidation analysis (thiobarbituric acid-reactive substance, TBARS) were quantified after 7 days of being maintained in darkness at 4 °C. The LT muscle color and LT intramuscular fat oxidation were measured as described in González-Calvo et al. . Briefly, the relative content of metmyoglobin (MMb) was estimated by the K/S572/525 ratio . This ratio decreases when the MMb content increases. Intramuscular fat oxidation of the M. Longissimus thoracis was determined using the procedure reported by Pfalzgraf et al. . The TBARS values are expressed as milligrams of malonaldehyde (MDA) kilogram−1 of muscle.
RNA isolation and assessment of RNA integrity
Total RNA was extracted from approximately 500 mg of LT muscle or SF using RNeasy Tissue mini kits (QIAGEN, Madrid, Spain), following the manufacturer’s protocol. Prior to microarray analysis, RNA integrity and quality were assessed by a RNA 6000 Nano LabChip on the Agilent 2100 Bioanalyzer (Agilent, Madrid, Spain) and were quantified using a nanophotometric spectrophotometer (Implen, Madrid, Spain). All RNA integrity number (RIN) values were greater than 8.
Microarray hybridization and data processing
RNA samples (n = 14, 7 samples from each treatment) were analyzed using Ovine Gene 1.1 ST Array Strip (Affymetrix, High Wycombe, UK). The Ovine gene 1.1 ST Array Strip allows for processing of four samples in parallel. This array contains a collection of 508538 probes that interrogate up to 26 unique sequences of each transcript (median probes/transcript = 23). These probes correspond to 22047 ovine genes. This study was part of a larger study in which we analyzed 84 samples corresponding to 3 treatments and 4 tissues (7 animals per treatment). In each strip, we hybridized the four tissues, selecting the animal tissue for each strip by random sampling. Microarray hybridization and scanning were performed at the Functional Genomics Core facility (Institute for Research in Biomedicine, IRB Barcelona, Spain), following the recommendations of the manufacturer. Scanned images (DAT files) were transformed into intensities (CEL files) by Affymetrix GeneChip Operating Software (GCOS). Overall array intensity was normalized between arrays to correct for systematic bias in the data and to remove the impact of non-biological influences on biological data. The imported data were analyzed at the gene-level, with exons summarized to genes, using the mean expression of all of the exons of a gene. Normalization was performed with the Robust Multi-Array Average (RMA) algorithm using quantile normalization, median polish probe summarization, and log2 probe transformation. The data sets supporting the results and discussed in this publication have been deposited in NCBI’s Gene Expression Omnibus repository  and are accessible through GEO Series accession number GSE63774 (https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE63774? acc = GSE63774).
Validation of microarray data by qPCR
Primers forward and reverse used in RT-PCR
Primers forward and reverse
Cytokine inducible SH2-containing protein
ATP-binding cassette, sub-family C (CFTR/MRP), member 4
R: 5′- gcaaaacatacggctcatcat-3′
A cetyl-Coenzyme A acetyltransferase 1
R: 5′- cctcctcctccgttgcaaat-3′
Insulin-like growth factor 1 receptor
R: 5′- ctggacacacattcccgct-3′
Low density lipoprotein receptor
F:5′- gcatcaacttcgacaaccct -3′
R:5′- tcctccaagctgaccatctg -3′
R:5′- tagactgcgacgccaaagaa -3′
Sterol regulatory element binding transcription factor 1
F:5′- ctgctatgcaggcagcac -3′
R:5′- ggttgatgggcagcttgt -3′
Superoxide dismutase 3, extracellular
HCLS1 associated protein X-1
High-mobility group box 2
Heat shock 22 kDa protein 8
Immediate early response 3
Methyltransferase like 1
Tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein
R:5′ - ttctctctgtattctcgagccatct-3
Statistical analysis of performance, α-tocopherol and lipid oxidation in the LT muscle
Statistical analysis was performed using a general lineal model (GLM). Treatment (CON and VE) was included as the fixed factor for weaning age (WA), slaughter age (SA), slaughter weight, intramuscular fat content (IMF) and average daily gain (ADG). For muscle α-tocopherol, myoglobin and TBARS, the model also included intramuscular fat content and slaughter age as covariates. The results are expressed as least square means ± the standard error (SE) values and the differences were tested at a level of significance of 0.05 with the t statistic.
Statistical analysis for the identification of differentially expressed genes by microarray analysis in LT and SF
Normalized data were further analyzed using Babelomics (http://babelomics.bioinfo.cipf.es/) and MetaboAnalyst software . Genes showing a statistically significant value of the Limma-test; (P < 0.01) were screened out as differentially expressed between the treatments. Significant genes were annotated based on similarity scores in blastn comparisons of Affymetrix Transcript cluster sequences against ovine sequences in GenBank. In addition, a second method, significance analysis of microarray (SAM), was used to identify and reconfirm differentially expressed genes in VE-CON contrasts. Furthermore, SAM was used to detect false positive significant genes from Limma-testing. SAM is a well-established statistical method for the identification of differentially expressed genes in microarray data analysis . SAM is designed to address the FDR when running multiple tests on high-dimensional microarray data. First, it assigns a significance score to each variable based on its relative change from the standard deviation of repeated measurements. Then, it chooses variables with scores greater than an adjustable threshold Δ and compares their relative differences from the distribution estimated by random permutations of the class labels. For each Δ, a certain proportion of the variables in the permutation set will be found to be significant by chance. This number is used to calculate the FDR. Underestimation of the variability inflates the value of the statistic and can result in an increased number of false positives.
Multivariate analysis of gene expression
Multivariate analysis was performed using MetaboAnalyst, according to Xia et al. . Principal component analysis (PCA), partial least squares discriminate analysis (PLS-DA), and variable importance of projection (VIP) were used to cluster the samples based on the selected gene expression profiles. Principal component analysis was used to reduce the large set of variables (genes) into 2 principal components (PCA 1 and 2). PLS-DA was used to enhance the separation between the groups by summarizing the data into a few latent variables that maximized covariance between the response and the predictors. The corresponding loading plot was used to determine the genes most responsible for separation in the PLS-DA score plot. Based on the PLS-DA results, genes were plotted according to their importance in separating the dietary groups and each gene received a value called the variable importance in the projection. Variable importance in the projection values >1 suggests that the variable is significantly involved in the separation of groups . Variables with the highest VIP values were the most powerful group of discriminators. We also investigated trends or patterns in gene expression changes.
Hierarchical clustering analysis (HCA)
Cluster analysis was performed using MetaboAnalyst. In (agglomerative) hierarchical cluster analysis, each sample begins as a separate cluster and the algorithm proceeds to combine them until all of the samples belong to one cluster. Two parameters must be considered when performing hierarchical clustering. The first is the similarity measure, and the other is the clustering algorithm. For distance measurements we used the Euclidian and Ward algorithm for clustering. The results are shown as a heatmap.
Statistical analysis of gene expression validated by qPCR
where, y rigkm is the C q value (transformed data taking into account E < 2) of the gth gene (GOIs and housekeeping) from the rth well in the kth plate collected from the mth animal subjected to the ith treatment (CON, VE and ALF); TGgi is the fixed interaction among the ith treatment and the gth gene; IMFm (only used in L. Thoracis muscle tissue gene expression), and ADGm are the effects of intramuscular fat, and the average daily gain of the mth animal included as covariates; Pk is the fixed effect of the kth plate; Am is the random effect of the mth animal from which samples were collected (Am ~ (0,σ2 A)); and erigkm is the random residual. Gene specific residual variance (heterogeneous residual) was fitted to the gene by treatment effect (e rigkm ~ N (0, σ2 egi).
To test differences (diff GOI ) in the expression rates of the target genes between treatments in terms of fold changes (FCs), the approach suggested by Steibel et al.  was used. The significance of the diff GOI estimates was determined with the t statistic.
Functional annotation analyses
The Database for Annotation, Visualization and Integrated Discovery (DAVID) v6.7b , was used to determine the pathways and processes of major biological significance and importance through the Functional Annotation Cluster (FAC) tool based on the Gene Ontology (GO) annotation function. DAVID FAC analysis was performed with the gene lists obtained after SAM analysis. Medium stringency EASE score parameters were selected to indicate confident enrichment scores of functional significance and the importance of the given pathways and processes investigated. An enrichment score of 1.3 was employed as a threshold for cluster significance .
The authors wish to thank the staff of “CITA de Aragón” for its assistance in sample collection and analysis.
This study was funded by the Ministry of Education and Science of Spain and the European Union Regional Development funds (INIA-RTA 2012-0041 and RZP2013-0001) and the Research Group Funds of the Aragón Government (A49). L. González-Calvo was supported by a doctoral grant from INIA.
Availability of data and materials
The data sets supporting the results and discussed in this publication have been deposited in NCBI’s Gene Expression Omnibus repository and are accessible through GEO Series accession number GSE63774.
LGC and PS performed the experiments. MJ, MS and JHC designed the research and obtained funding for this research. LGC, ED, MS, JMO and JHC wrote the paper. LGC, MS, ED, MJ, RMH, JMO and JHC analyzed the data. MJ provided the animals. JHC had primary responsibility for the final content. All of the authors contributed to the manuscript discussion. All of the authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Ethics approval and consent to participate
All of the experimental procedures, including the care of animals and euthanasia, were performed in accordance with the guidelines of the European Union and with Spanish regulations for the use and care of animals in research and were approved by the Animal Welfare Committee of the Centro de Investigación y Tecnología Agroalimentaria (CITA) (protocol number 2009-01_MJT). In all cases, euthanasia was performed by penetrating captive bolt followed by immediate exsanguination.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
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