3.1. Histopathological Tests of Arteries
In our study, we evaluated the anti-atherogenic effect of polyphenol-rich extract of bee pollen on the development of atherosclerosis induced by a high-fat diet in ApoE-knockout mice. We showed that supplementation of a high-fat diet with EEP in a dose of 1 g/kg BM protected heart arteries from development of atherosclerotic plaque. Histopathological presentation did not reveal any proliferative changes which would be evidence of atherosclerosis development (Figure 1
C). Supplementing a high-fat diet with EEP in a dose of 0.1 g/kg BM significantly limited the growth of atherosclerotic plaque (Figure 1
B), while severe atherosclerotic changes occurred in mice on a high-fat diet without supplementation. Near the aortic arch, atherosclerotic plaque almost completely filled the vascular lumen (Figure 1
A). In mice on a standard diet, the control group SD (Figure 1
F), and in the mice on a standard diet supplemented with EEP in a dose of 0.1 g/kg BM (Figure 1
D), and 1 g/kg BM (Figure 1
E) atherosclerotic characteristics were not observed.
The mechanism of inhibitory capacity of EEP, as far as atherosclerosis is concerned, is difficult to explain. Our study has led to a conclusion that the mechanism is related to a significant decrease in total cholesterol, oxidatively modified pro-atherogenic ox-LDL molecules, and the decrease of ADMA and ANG II level. To the best of our knowledge, the current study is the first report on the subject.
EEP used in our study was characterized by a high content of polyphenols (27 mg GAE/g) and flavonoids (20 mg QE/g), which corresponded to a strong antioxidant effect resulting from reduction of DPPH (EC50
= 57.5 μmol/g), free radicals, and ABTS•+
(TEAC = 0.692 mmol/g). Rutin was a dominant flavonoid, which is a glycoside of quercetin. Other flavonoids present in the extract are: mireycetin > quercetin > isorhamnetin > kaempferol. The main phenolic acids which are present in EEP are: gallic acid > trans-cinnamic acid > 4-hydroxycinnaminic acid > felluric acid > 4-trans-p-coumaric acid > caffeic acid. Detailed characteristics of particular phenolic acid and flavonoid content in EEP used in research were presented in a previously published paper [22
]. These compounds are characterized by a strong antioxidant effect reducing oxidative stress, and strongly inhibiting lipid peroxidation, which probably was crucial for preventing the formation of atherosclerotic plaque in ApoE-knockout mice.
In vitro studies showed that quercetin and catechin inhibit platelet aggregation, limit pro-clotting activity and block phosphoinositide cascade [24
]. Flavonoids interact with platelet receptors. Quercetin decreases reactivity of platelets by blocking activation that depends on GPVI receptors [25
]. Quercetin and catechin inhibit oxidative stress in platelets, and simultaneously limit activation of a fibrogen receptor, and increase NO synthesis [26
]. In studies conducted on mice with apolipoprotein E deficiency, it was shown that polyphenol mixture, e.g., catechin, caffeic acid and resveratrol, decreases progression of atherosclerotic changes in the aortic arch. According to authors, the inhibition of endothelin 1 synthesis is one of the mechanisms in which polyphenols affect the balance between vasoconstrictive and vasodilating factors [27
]. Hayek et al. [28
] obtained significant reduction of atherosclerotic plaque on the surface of the aortic intima-media in ApoE-knockout mice after administration of quercetin.
The anti-atherosclerotic effect of propolis, an important apitherapeutic, was described in the literature. The reduction of early and advanced atherosclerotic changes due to a polyphenol fraction from green, brown, and red propolis was confirmed in in vitro studies conducted on genetically modified mice without LDL receptor (LDLr-/- mice). Polyphenols from red propolis had the strongest effect, and they reduced atherosclerotic changes in the aortic sinus. This was related to improving the lipid profile, reducing the level of pro-inflammatory cytokines, monocyte chemotactic protein-1 (MCP1) and interleukin 6 (IL6), chemokine, and angiogenic factors [29
]. Limitation of atherosclerotic progression in the aortic sinus was observed in the case of a polyphenol fraction from Chilean propolis. Authors claim that this can result from the synergic effect of a polyphenol complex present in propolis that involves a significant reduction of the expression of a pro-angiogenic vascular endothelial growth factor A (VEGF-A) [30
3.2. Effect of EEP on Total Cholesterol (TC)
Improper diet can impair lipid management and contribute to the development of cardio-vascular diseases. Disorders of lipid metabolism, manifested by hypercholesterolemia, are recognized and important factors which increase the risk of atherosclerosis [31
In the current study, we have evaluated the effect of polyphenol-rich extract of bee pollen (EEP) on total cholesterol level in ApoE-knockout mice on high-fat and standard diets. Total cholesterol level in ApoE-knockout mice is presented in Figure 2
After five weeks, a high-fat diet in genetically modified animals resulted in an increase of TC to 779 mg/dL (Figure 2
A), i.e., by 101% compared with the average level (388 mg/dL, Figure 2
B) in the controls. The use of a high-fat diet for 16 weeks further increased the level of TC to 1303 mg/dL (Figure 2
A), i.e., by 67% compared with values recorded after five weeks. In this group, the average TC level was the highest and the difference in comparison with all remaining experimental groups was statistically significant (p
< 0.05, Figure 2
Supplementing a high-fat diet with EEP in a dose of 0.1 g/kg BM decreased TC level, which reached the value of 856 mg/dL (Figure 2
A) in the 16th week, and was 34% lower when compared with an unsupplemented high-fat diet. Supplementation of a high-fat diet with EEP in a dose of 1 g/kg BM stabilized TC level as early as in the 5th week. This parameter level in all experimental periods was similar to the control group level, and the difference was not statistically significant (Figure 2
B). In the 16th week, TC level was 398 md/dL (Figure 2
A), i.e., 69% lower than in an unsupplemented high-fat diet.
Supplementation of a standard diet with EEP in a dose of 0.1 g/kg did not significantly change TC level (p
> 0.05), but a dose of 1 g/kg MB resulted in a high decrease in TC level (p
< 0.05) when compared with the control group (Figure 2
A significant decrease of TC level due to polyphenol fraction in bee pollen extract (EEP), which was recorded in our study, can be an effect of various mechanisms. It can be a sign of decreased biosynthesis of cholesterol in the liver. It can be a result of increased excretion of cholesterol with bile, or a result of the activation of peroxisome proliferator-activated receptors (PPAR), which regulate adipocyte maturation and lipid storage; therefore, they are regulators of hepatic metabolism of lipids.
It can be concluded from published data that polyphenols from natural products affect lipid metabolism as well as having cardio- and angio-protective effect. During in vitro studies, quercetin activates PPAR-γ receptors by increasing their expression. It also increases the expression of ATP-binding cassette transporter (ABCA1), which plays a key role in reverse cholesterol transport. Quercetin-induced modulation of ABCA1 expression reduces cholesterol build-up in macrophages, and increases cholesterol outflow from macrophages, and therefore reduces formation of foam cells, lowering the risk of atherosclerosis [32
]. The lipid level in LDLr-knockout mice is normalized by a polyphenol fraction from different types of propolis. The polyphenol fraction from red propolis had the strongest protective effect. This fraction caused the most significant reduction of triglyceride level, and TC level as well as an increase in HDL. The anti-atherogenic effect was weaker in the case of green and brown propolis. This may be related to different content of main polyphenol components, in particular, types of propolis. The following were mostly present in red propolis: 3-hydroxy-8,9-dimethoxypterocarpan, medicarpin, daidzein, whereas artepellin C, pinocembrin, kampferol in green propolis, and pinocembrin, caffeic acid phenyl ester, quercetin, galangin in brown propolis [29
The current study shows that EEP in a dose of 1 g/kg BM significantly decreases TC levels in ApoE-knockout mice on both, high-fat and standard diets. Consequently, it reduces hypercholesterolemia, which is a risk factor for atherosclerosis.
3.3. Effect of EEP on Oxidized Low Density Lipoprotein (Ox-LDL)
Oxidized LDL molecules are one of the main factors responsible for atherosclerosis development. They are formed due to free radical activity during the process of chemical modification, namely, oxidation of low density lipoproteins (LDL). Ox-LDL are highly atherogenic molecules, since they have pro-inflammatory activity and they cause focal inflammation of arterial vascular endothelium [33
In our study, we have investigated the effect of polyphenol-rich EEP on ox-LDL levels in ApoE-knockout mice on high-fat and standard diets. Ox-LDL level in ApoE-knockout mice is presented in Figure 3
The highest levels of ox-LDL in all experimental periods were recorded in mice on a high-fat diet, whereas the lowest ones were noted when a standard diet was supplemented with EEP at a dose of 1 g/kg BM (Figure 3
A,B). Since the fifth week, a high-fat diet resulted in a significant increase of ox-LDL level, which was 867 ng/mL in the 16th week (Figure 3
A) and was 91% higher than the average level (453 ng/mL, Figure 3
B) in the controls. Supplementing a high-fat diet with EEP as early as in the fifth week resulted in lowering ox-LDL levels, which diminished to 526 ng/mL (Figure 3
A), i.e., by 40% in the case of a dose of 0.1 g/kg BM, and to 355 ng/mL (Figure 3
A), i.e., by 59% when the dose was 1 g/kg BM compared with an unsupplemented high-fat diet. In Apo-E-knockout mice, supplementation of a high-fat diet with EEP regardless of the dose, causes statistically significant reduction of average ox-LDL level to a level similar to the one of the controls (Figure 3
The literature indicates that polyphenols are inhibitors of oxidative modification of LDL. They play an important role in prevention of many degenerative diseases, including cardio-vascular diseases. Polyphenols inhibit progression of atherosclerosis by reducing oxidative stress, and inflammatory biomarkers of atherosclerosis, as well as by improving lipid profile and insulin sensitivity, and by making endothelial function more efficient [17
Loke et al. [37
], in their studies on ApoE-knockout mice showed that quercetin reduced hydrogen peroxide and leukotrien B4 in vessels, and P-selectin serum level as well as increased the activity of endothelial nitric oxide synthase (eNOS). Theaflavin (catechin dimer) and epicatechin have a similar but weaker effect. In vitro studies of olive oil polyphenols have shown that they reduce intracellular levels of reactive oxygen species (ROS) and the expression of nuclear factor-κB (NFκB) transcription. Reduction of NFκB expression has been related to lowering metalloproteinase 9 level [36
]. Oxidative stress in ApoE-knockout mice is also reduced by cocoa polyphenols. This effect has resulted from reducing expression of vascular cell adhesion molecule-1 (VCAM1) and intercellular adhesion molecule-1 (ICAM1) [38
]. Olive oil supplemented with epigallocatechin gallate (EGCG) significantly improves vascular endothelium function in patients with early atherosclerotic dysfunction of endothelium due to the levels of ICAM, monocytes and lymphocytes [39
]. Polyphenols inhibit LDL oxidation, reduce thrombocyte aggregation, inhibit the activity of enzymes that mediate the immune cells response to LDL, and reduce TC levels [40
The presented effect of the polyphenol fraction of bee pollen extract on ox-LDL level in ApoE-knockout mice has been reported for the first time. Any information on the effect of polyphenols from bee pollen on ox-LDL level has not been found in the literature. Our study can lead to the conclusion that a polyphenol fraction from bee pollen significantly reduces ox-LDL level, and it results, most probably, from high antioxidant capacity of EEP. Reduction of proatherogenic ox-LDL level and consequently, limitation of the development of atherosclerotic changes are the signs that ethanol extract of bee pollen is highly efficient at reducing oxidative stress. Therefore, it can be useful as a potential anti-atherogenic substance.
3.4. Effect of EEP on Asymmetric Dimethylarginine (ADMA)
ADMA is an endogenous inhibitor of endothelial nitric oxide synthase (eNOS). ADMA participates in one of the mechanisms limiting bioavailability of nitric oxide (NO), which is an endogenous strongly anti-atherogenic substance [41
]. Impairment of endothelial functions, which is observed in various disorders, results from high levels of ADMA. This allows us to single out ADMA as an early biochemical marker of endothelial dysfunction in the prophylaxis of cardio-vascular diseases and insulin resistance [42
We have studied the effect of bee pollen extract on ADMA level in ApoE-knockout mice on both, high-fat and standard diets. ADMA level is presented in Figure 4
After five weeks, a standard diet led to an increases in ADMA level to 0.653 μmol/L (Figure 4
A), i.e., by 26%, and up to 0.704 μmol/L (Figure 4
A), i.e., by 36% after 16 weeks when compared with an average level (0.519 μmol/L, Figure 4
B) in the controls.
Sixteen-week supplementation of a high-fat diet with EEP in a dose of 0.1 g/kg BM did not significantly change this parameter level. When a high-fat diet was supplemented with EEP in a dose of 1 g/kg BM, the lowest ADMA level was recorded in the 14th week (0.431 μmol/L, Figure 4
A), while in the 16th week ADMA level was 0.648 μmol/L (Figure 4
A), 8% lower than in the unsupplemented group. The lowest levels of this parameter were obtained for a standard diet supplemented with bee pollen extract in a dose of 1 g/kg BM (Figure 4
A,B). A high-fat diet in ApoE-knockout mice caused a statistically significant increase of ADMA level. Supplementing a high-fat diet with EEP in a dose of 1 g/kg BM in ApoE-knockout mice resulted in a statistically significant decrease of average ADMA levels (Figure 4
According to the literature, polyphenols added to diet lower ADMA. Li Volti’s et al. [45
] in vitro studies showed that supplementation with silibinin—flavonoid belonging to the group of flavonolignans—lowered ADMA serum level in mice, limited endothelial dysfunction, and reduced insulin resistance. A significant decrease of ADMA level, ox-LDL level and C-reactive protein (CRP) level, as a result of polyphenol-rich olive oil consumption, was observed in studies carried out in a group of young women with slight hypertension or hypertension stage 1. A significant decrease of hypertension and improving endothelial function was recorded [46
There are no published reports on the effect of bee pollen extract on ADMA level. Our results show that a polyphenol fraction from bee pollen reduces ADMA level, and consequently increases NO bioavailability, by limiting endothelial dysfunction and development of atherosclerotic changes. It is very important for prevention of cardio-vascular diseases, because an increase of ADMA level is an early risk factor of vascular endothelium dysfunction, and limiting its excessive synthesis can be used in atherosclerotic prevention.
3.5. Effect of EEP on Angiotensin-Converting Enzyme (ACE) and Angiotensin II (ANG II)
The renin-angiotensin-aldosterone system (RAA) is a key mechanism of physiologic regulation of blood pressure and electrolyte balance. Individual constituents of the system can participate in pathogenesis of hypertension and organ changes related to hypertension such as cardiac and vascular remodeling, atherosclerosis, myocardial fibrosis, and renal fibrosis [47
ACE belongs to zinc metalloproteinases. It is an enzyme that participates in blood pressure regulation through converting inactive angiotensin I into biologically active angiotensin II, with a vassopresor-like activity [48
ANG II has a harmful effect on vascular endothelium and cardiac muscle. It causes endothelial dysfunction, increases oxidative stress, and contributes to atherosclerotic plaque rupture. ANG II also accelerates apoptosis of cardiac muscle cells, and increases tissue-specific insulin resistance [49
]. By contributing to ox-LDL formation, ANG II can indirectly lead to dysfunctions of endothelial function related to clotting and fibrinolysis [50
]. ANG II can also cause a change in endothelial function from anti-adhesive to pro-adhesive one, thus affecting the expression of endothelial adhesion molecules (ICAM, VCAM) [51
In our study, we have investigated the effect of EEP on ACE and ANG II level in ApoE-knockout mice on high-fat and standard diets. ACE level is presented in Figure 5
The highest levels of ACE in particular experimental periods were recorded in animals on a high-fat diet, whereas the lowest ones were observed for a standard diet supplemented with bee pollen extract in a dose of 1 g/kg BM (Figure 5
A). After five weeks, a high-fat diet increased ACE level, which was 161 ng/mL (Figure 5
A) in the 16th week, i.e., 30% higher than the average level (124 ng/mL, Figure 5
B) in the control group. Supplementing a high-fat diet with EEP resulted in lowering this parameter level to 152 ng/mL after 16 weeks, i.e., by 6% for 0.1 g/kg BM, and to 124 ng/mL (Figure 5
A) i.e., by 23% for a dose of 1 g/kg BM when compared with the unsupplemented group.
The highest level of ANG II was recorded in mice on a high-fat diet. In the fifth and sixth week, ANG II level was 2.16 ng/mL (Figure 6
A), i.e., 95% higher than the average level (1.11 ng/mL, Figure 6
B) in the control group. Supplementing a high-fat diet with EEP decreased this parameter as early as in the fifth week, and after 16 weeks ANG II level was reduced to 1.31 ng/mL (Figure 6
A), i.e., by 39% for a dose of 0.1 g/kg BM, and to 1.09 ng/mL (Figure 6
A), i.e., by 50% for a dose of 1 g/kg BM compared with the unsupplemented group. The lowest ANG II level was recorded in mice on a standard diet supplemented with EEP in a dose of 1 g/kg BM (Figure 6
A). Supplementation of a high-fat diet with bee pollen extract, regardless of the dose, caused a statistically significant decrease of ANG II level (Figure 6
The published data reveals that polyphenols are hypotensive, which results from their antioxidant capacity. This feature leads to lowering ROS concentration and to weakening ACE activity [19
]. Xu et al. [53
], both in studies on rats and in vitro studies showed that genistein had reduced expression and activity of ACE in endothelium and serum. The estrogen receptor activating a pathologic path of ERK1/2 participated in this activity. Genistein stimulates the synthesis of nitrogen oxide in endothelial cells through a mechanism dependent on cyclic 3′5′-adenosine monophosphate. Taxifolin inhibits ACE activity in the rat aorta [52
], while quercetin lowers blood pressure in spontaneously hypertensive rats (SHR), and eliminates all pathological changes in their blood vessels [54
]. The antihypertensive effect of polyphenols in SHRs was related to activation of eNOS, and inhibition of metalloproteinase-2 [55
]. Lowering blood pressure by polyphenols can also result from modelling the renin-angiotensin-aldosterone system through reducing oxidative stress [56
]. Polyphenols from various tea types inhibit ACE activity in in vivo studies [57
]. Furthermore, also bee pollen extract has inhibited ACE activity in in vitro studies, which results from its high antioxidant potential [58
Based on our study, it can be concluded that supplementing a high-fat diet and a standard diet with EEP decreases angiotensin II level in ApoE-knockout mice. This is related to weakening the activity of ACE. The available literature does not offer any data on the effect of bee pollen on ANG II level; this issue has been presented in our study for the first time. Our study may lead to a conclusion that a polyphenol fraction from bee pollen, due to its high antioxidant capacity, affects the modulation of the renin-angiotensin-aldosterone system; therefore, it improves endothelial function, and consequently it can inhibit the development of atherosclerotic changes.
In spite of the fact that many studies have been conducted, the mechanism of the anti-atherosclerotic effect has not been yet determined. According to the published data, it can be assumed that the main protective effect from the development of atherosclerosis consists improvement of endothelial function, oxidative reduction, LDL modification, decrease of total cholesterol, inhibiting the synthesis of pro-inflammatory cytokines such as TNF-α, IL-6, IL-8, and inhibiting the adhesion of ICAM-1, VCAM-1 molecules as well as the stimulation of eNOS synthase, and intensification of NO synthase [59