Effects of atherogenic diet
When livers of mice in the AD + DW group were compared to those in the ND + DW group, the fatty acid beta oxidation process was found to be up-regulated, presumably to metabolize extra fatty acids obtained from the atherogenic diet. When challenged with the atherogenic diet, the liver thus adjusts its metabolic processes in relation to lipid metabolism and energy production [
44]. Nuclear receptors involved in tissue growth and genes involved in cell death were also up-regulated in the present study, thus suggesting that the atherogenic diet triggered hepatic inflammatory reprogramming and liver regeneration in the mice. This also explains the enlargement of livers that was observed in these animals. An example of a nuclear receptor up-regulated by the atherogenic diet in the present study is the hepatocyte nuclear factor 4-alpha (
Hnf4a) (FC 2.57), which was also found to be up-regulated when ApoE3Leiden (E3L) mice (which have lipid profiles resembling those of humans) were fed an atherogenic diet [
45]. Among the genes involved in cell death that were up-regulated in the present study were those encoding cytochrome c oxidases belonging to the mitochondrial electron transport chain, complement genes and caspases. The up-regulation of these genes suggests that cell death occurred via apoptosis as a result of complement-mediated cell damage. Interestingly, Recinos et al. [
46] also showed that induction of the complement pathway in the liver was associated with lesion development in atherosclerosis-prone LDL receptor-deficient (LDLr
−/−) mice when they were fed a high-fat Western style diet. As the atherogenic diet provided dietary cholesterol that further increased cholesterol levels in the blood circulation, genes involved in hepatic cholesterol biosynthesis were down-regulated in this study. This observation was not unexpected as de novo cholesterol biosynthesis is down-regulated when cholesterol is available from dietary intake [
44,
47].
The immune system has long been implicated in atherosclerosis [
48‐
50], due to the presence of inflammation. In response to the atherogenic diet, the spleen showed an up-regulation in the production and turnover of immune cells in the present study. A network significantly up-regulated involved the
Stat3 gene (FC 1.59). It is interesting to note that the
Stat3 gene was discovered because of its role in the acute phase response and that this is the only capacity in which its function in vivo can be clearly ascribed to its activity as a transcription factor [
51]. Although apoptosis was up-regulated in this study, the tumor suppressor
Tp53 (FC −1.53) and other genes linked to it were down-regulated. Together with the up-regulation of the
Stat3 network and apoptosis, the down-regulation of the tumor suppressor
Tp53 implies that the atherogenic diet caused an increased turnover of immune cells in the spleen. This thus explains the increased production and deployment of immune cells in the blood circulation, which may further exacerbate the inflammatory effects of the atherogenic diet.
This study also revealed that two important networks were regulated by the atherogenic diet in the heart, with the first involving an up-regulated
Jun oncogene (FC 1.67) and the second involving a down-regulated
Tgfb1 (FC −6.65). The JUN protein forms part of the transcription factor activator protein-1, which is pro-inflammatory as it has been implicated in oxidative stress [
52]. Binding sites of the redox-regulated transcription factor activator protein-1 are located in the promoter region of a large variety of genes that are directly involved in the pathogenesis of diseases, including atherosclerosis. Activation of
Jun via Jun amino-terminal kinase (
Jnk) in response to various forms of stress causes arterial injury [
53] and heart disease [
54‐
59]. The down-regulation of the
Tgfb1 gene by the atherogenic diet in the present study also implies a pro-inflammatory response toward the diet in the heart. This is because
Tgfb is anti-inflammatory in atherosclerosis [
60], as it plays an important role in the maintenance of normal blood vessel structure, and defects in this gene have been linked to a range of cardiovascular syndromes including loss of healthy vessel architecture and aneurysm [
61]. Microarray profiling carried out by Tabibiazar et al. [
62] on the aortas of apolipoprotein E-deficient (apoE
−/−) mice on a high-fat diet compared with control C57BL/6J and C3H mice across time also showed a decreased expression of an isoform of
Tgfb.
In the present study, preliminary physiology studies carried out during animal feeding in order to identify the effects of the atherogenic diet on the well-being of mice showed several adverse effects of the diet, which include increases in inflammation and oxidative stress, similar to the observations found in previous studies [
63‐
65].
Effects of OPP
In the livers of mice belonging to the AD + OPP group, genes up-regulated when compared to those of mice belonging to the AD + DW group were found to be involved in the unfolded protein response. These genes include
Herpud1 (homocysteine-inducible, endoplasmic reticulum stress-inducible, ubiquitin-like domain member 1) (FC 1.51),
Tra1 (tumor rejection antigen gp96) (FC 1.35) and
Vcp (valosin containing protein) (FC 1.23). The unfolded protein response can be promoted by the buildup of unfolded proteins in the endoplasmic reticulum, and it constitutes a mechanism to reduce this burden. The unfolded protein response acutely reduces translation of new proteins, followed by increased expression of chaperones to aid folding of existing proteins and enhanced elimination of proteins that cannot be refolded [
66]. Endoplasmic reticulum stress responsive genes have been suggested to be a protective response to protein unfolding or protein damage resulting from cellular stress signals. In addition, accumulation of oxidatively modified proteins can elicit cellular damage and this is curtailed under normal conditions by intracellular protein degradation systems such as the ubiquitin–proteasome system [
67]. Thus, OPP may help to reduce the amount of damaged proteins caused by the atherogenic diet in the liver.
Transketolase (
Tkt), which controls the non-oxidative branch of the pentose phosphate pathway, provides reduced nicotinamide adenine dinucleotide phosphate (NADPH) for biosynthesis and reducing power of several antioxidant systems [
68]. It was up-regulated in the spleens of mice by OPP (FC 1.85), together with glucose-6-phosphate dehydrogenase (X-linked) (
G6pdx) (FC 3.59) and phosphogluconate dehydrogenase (
Pgd) (FC 2.82), all of which are involved in the pentose phosphate pathway. The products of the pentose phosphate pathway are important for the biosynthesis of purine and for stimulating antioxidant response pathways in conjunction with the action of dietary phenolic antioxidants. This may also explain the up-regulation of antioxidant genes including
Mgst1 (microsomal glutathione S-transferase 1) (FC 1.79),
Mgst2 (microsomal glutathione S-transferase 2) (FC 3.08),
Gsr (glutathione reductase 1) (FC 2.49) and
Gstm1 (glutathione S-transferase, mu 1) (FC 1.89) in the spleens of mice given OPP. Genes encoding MHC molecules such as
H2-
Ab1 (FC −2.32) and
H2-
Eb1 (FC −2.36), which have been implicated in atherosclerosis [
62], were down-regulated in the spleens of mice, thus suggesting that OPP was able to attenuate the inflammatory response brought about by the atherogenic diet. Activated macrophages and smooth muscle cells express class II histocompatibility antigens such as HLA-DR that allow them to present antigens to T cells, which cause atherosclerosis [
69]. The gene expression of MHC II molecules is transcriptionally regulated by the class II transcriptional activator (CIITA or
C2ta) (FC −2.58). CIITA activates the expression of MHC II in all types of professional antigen-presenting cells (macrophages, dendritic cells and B lymphocytes), of which dendritic cells are the most potent among the three [
70]. In line with the down-regulation of MHCs, the
C2ta gene was down-regulated by OPP in mice fed the atherogenic diet in the present study. A mechanism of anti-inflammation brought about by antioxidants is through the modulation of cytokine induction during inflammation [
71]. In agreement with this, cytokines and cytokine receptors such as
Ccl5,
Ccl19 and
Ccr7 were down-regulated by OPP in the present study (FC −3.25, −2.89 and −2.28, respectively). The CCR7 receptor present on the surface of secondary lymphoid cells for instance functions to attract dendritic cells, which migrate to secondary lymphoid organs to present antigens for the activation of naive T cells. Hence, the down-regulation of cytokines and cytokine receptors by OPP in the present study suggests anti-inflammatory effects of the extract. Additionally, cluster of differentiation (CD) antigenic markers such as
Cd3d,
Cd24a,
Cd59b,
Cd72,
Cd79a,
Cd79b,
Cd83 and
Cd86 were also down-regulated by OPP (FC −1.55, −1.37, −3.02, −2.17, −1.97, −2.28, −3.14 and −2.00, respectively).
Cd83 and
Cd86 are specific markers of mature dendritic cells, which are up-regulated by oxidative stress through a nuclear factor kappa-B-dependent mechanism [
70]. The down-regulation of MHC II genes and genes encoding antigenic markers in this study further suggests that OPP suppressed the inflammatory response associated with the atherogenic diet, and this may constitute a mechanism by which OPP ameliorates atherosclerosis.
In the hearts of mice belonging to the AD + OPP group, genes up-regulated when compared to those of mice belonging to the AD + DW group include antioxidant genes, such as
Mgst1 (microsomal glutathione
S-transferase 1) (FC 1.71) and
Gpx1 (glutathione peroxidase 1) (FC 1.24). These antioxidant genes are essential in the detoxification of carcinogens and the scavenging of reactive oxygen species [
72].
Despite the fact that mice in the AD + OPP group did not show significant changes in terms of body and liver weights as well as the hematology and clinical biochemistry parameters when compared to mice in the AD + DW group, further cytokine profiling and antioxidant analysis on the blood serum samples of these mice supported the in vivo anti-inflammatory and antioxidant effects of the extract. In contrast to the effects of OPP that down-regulated hepatic cholesterol biosynthesis genes in mice fed the normal diet found in our previous study [
27], the extract did not down-regulate this group of hepatic genes in mice fed the atherogenic diet in the present study. This makes sense as administration of the atherogenic diet has already down-regulated cholesterol biosynthesis, and thus further down-regulation of the pathway would be futile to prevent atherosclerosis. On the other hand, OPP acted as an anti-inflammatory agent and an antioxidant in mice given the atherogenic diet to prevent oxidative stress and inflammation caused by the diet, and this is considered important in the prevention of atherosclerosis and cardiovascular disease.
As a component of the immune response, cytokines play an important role in mediating the inflammatory response in atherosclerosis. Atherosclerosis is normally associated with cytokines that promote a Type 1 helper T-cell (Th1) cellular immune response rather than a Type 2 helper T-cell (Th2) humoral immune response [
73]. The modulation of the Th1/Th2 axis toward the latter may thus be atheroprotective [
74]. In mice belonging to the AD + OPP group, a decrease in the pro-inflammatory IL-12 (p40 subunit) cytokine and an increase in the anti-inflammatory IL-13 cytokine in the sera were observed compared to the AD + DW group. This is believed to be an attenuation of the inflammatory response toward atherosclerosis. IL-12 is a cytokine of innate immunity, which is secreted by activated macrophages and dendritic cells, and is a key inducer of cell-mediated immunity as it stimulates the production of IFN-γ, stimulates the differentiation of CD4 + helper T lymphocytes into Th1 cells as well as enhances the cytolytic functions of activated natural killer cells and CD8 + cytolytic T lymphocytes [
75]. It has been implicated in atherosclerosis [
74,
76,
77] and other inflammatory diseases [
78,
79]. IL-13 is a cytokine of adaptive immunity, which is secreted by CD4 + helper T lymphocytes (Th2 cells), and it inhibits macrophages and antagonizes IFN-γ [
75]. In the present study, the anti-inflammatory effects observed in the serum samples were consistent with the gene expression changes seen in the spleens of mice given OPP, which indicate attenuation of the inflammatory response.
In addition, antioxidant analysis carried out on the mouse blood serum samples showed that OPP restored the antioxidant capacity of animals fed the atherogenic diet. This is in line with the gene expression changes observed in the liver, spleen and heart, in which antioxidant genes were up-regulated by OPP. While the effects observed in the present study are mainly attributed to phenolic compounds, the possible effects of other components in OPP cannot be discounted. What is important here is that the extract in its entirety confers the positive outcomes reported in the present study.
Limitations of study
We acknowledge that the biggest limitation in this study is the fact that BALB/c mice were used as biological models for atherosclerosis, although rodents that are HDL animals in general are not suitable as they do not mimic the human atherosclerotic disease [
80]. Nonetheless, microarray studies in which normal rodent models were used to test for the effects of high-fat or atherogenic diets have been carried out before [
62,
81]. It was thus reasoned that OPP might still bring about gene expression changes in major organs of BALB/c mice (which have intermediate susceptibility to atherosclerosis compared to the C57BL/6 mice) on an atherogenic diet. It was also easier to compare the effects of the extract in this study with a previous one involving the normal diet, as animals with the same genetic background were used [
27]. Thus, it would be interesting to extend this study to other mouse models of atherosclerosis, such as apoE
−/− and LDLr
−/− mice in the future.
Another limitation of this study is that fact that the aorta as a primary target of atherosclerosis was not subjected to atheroslerotic lesion and transcriptomic analyses to establish the anti-atherosclerotic mechanisms of OPP. Nevertheless, we have previously shown that OPP reduced atherosclerotic plaques in the aortas of atherogenic diet-fed rabbits [
25]. When we first initiated this transcriptomic analysis, however, no commercial whole genome rabbit microarrays were available. Hence, we did not carry out transcriptomic analysis on the aortas of rabbits. We then decided to use whole genome mouse microarrays, as a first step toward identifying the gene expression changes caused by OPP. In relation to this, we previously published a transcriptomic analysis study on the effects of OPP in mice on a normal diet, in which we analyzed three particular organs, liver, spleen and heart [
27]. This present study was not a standalone project but a part of this previous study, as we were interested to explore the gene expression changes caused by OPP when the mice were on an atherogenic diet, rather than on a normal diet. During the course of the present study, although we did intend to isolate aortas from the mice for atherosclerotic lesion and transcriptomic analyses, we faced technical difficulties in doing so due to the size limitation of this mouse model. Moreover, the animals that we had were not enough for pooling enough samples to obtain sufficient total RNA. Thus, we decided to carry out transcriptomic analysis on the three organs instead, and identify the gene expression changes that may provide initial clues to help explain how OPP confers protection against the effects of an atherogenic diet. Regardless, transcriptomic analysis on the aortas of any biological model is deemed necessary in the future to provide conclusive insights into the anti-atherosclerotic mechanisms of OPP.