1. Introduction

In recent years, Vaccinium species have gained great attention for their potential health benefits. Vaccinium L. (Ericaceae) is a morphologically various genus of terrestrial or epiphytic shrubs and sub-shrubs, comprising approximately 450 species across Europe, North and Central America, South East and Central Africa, and Asia [1]. Deciduous or evergreen dwarf shrubs, shrubs, or small trees characterize the genus, and the fruits of each variety are edible. V. corymbosum was imported by North America, and now is cultivated in Europe for its big edible fruits [2]. V. myrtillus (bilberry) is a woody dwarf shrub, present in the forests of the Northern Hemisphere.

Fruits of several Vaccinium species have been extensively investigated for their chemical profile. They are described as being a rich source of polyphenols and carotenoids. Nevertheless, due to their high content of anthocyanins, these fruits are recognized for their bioactive properties, such as prevention or treatment of cardiovascular diseases, diabetes, obesity, cancer, urinary tract infections, and aging diseases [3,4].

Polyphenols are the subject of increasing interest because of their potential beneficial effects on human health [5,6,7,8,9]. In fact, several epidemiological studies suggested that long-term consumption of foods rich in polyphenols offers protection against the development of cardiovascular diseases, diabetes, cancers, and neurodegenerative diseases [5,6]. Polyphenols have been recognized due to their potent antioxidant activity and ability to modulate key signaling pathways of several inflammatory cytokines and enzymes [5]. Therefore, beyond these modulatory roles, their antioxidant activity related to the capacity to scavenge reactive oxygen species (ROS), or to activate cellular endogenous antioxidant systems, may be of importance in countering the oxidative stress in inflammatory diseases [5,6].

The antioxidant and anti-inflammatory activities of Vaccinum species are also reflected in a protective role for vascular endothelium against cardiovascular diseases linked to endothelial dysfunction [10,11].

2. Traditional Uses of Vaccinium Species

Fruits and leaves of different Vaccinium species are extensively used in traditional medicine, as summarized in Table 1  [12,13,14,15,16,17,18,19,20,21,22]. 

Table 1. Traditional uses of Vaccinium species

 Traditional uses of Vaccinium species.

3. Phytochemicals of Vaccinium Fruits and Leaves

Anthocyanins are present in the outer layer of fruits, together with polyphenolic compounds, and a small content was found also in pulp and seeds. Environmental factors can affect the content and composition of secondary metabolites in berries. Growing conditions also affect the content of anthocyanins and other phenolic compounds in the berries of wild and cultivated species [23]. Prior to berry ripening, proanthocyanidins, flavonols, and hydroxycinnamic acids are the major phenolic compounds. During the ripening process, flavonoid profiles vary, and anthocyanins accumulate in the skin. High levels—and a wide variety—of anthocyanins provide the red, blue, and purple colors that characterize berries of this genus.

Vaccinium berries have a well-deserved reputation as potential healthy products and functional foods, supported by many studies, which have identified and quantified various bioactive phytochemicals with known benefits for human health.

Many studies have demonstrated the benefits of anthocyanin-rich extracts of Vaccinium species in the prevention of several diseases [24]. Nonetheless, it is important to note that their efficacy is subject to their bioavailability. Once ingested, anthocyanins are metabolized into various conjugates, which are metabolized into phenolic acid degradation products. Accumulated evidence suggests synergistic effects between all possible metabolites to explain their health-promoting properties. Inter-individual and intra-individual variability in anthocyanins absorption, metabolism, distribution, and excretion is also evident.

Six anthocyanidins (cyanidin, delphinidin, malvidin, pelargonidin, petunidin, and peonidin), which are also the most common anthocyanidin skeletons in higher plants, have been isolated from Vaccinium species [25]. To date, more than 35 anthocyanin glycosides have been isolated from the genus Vaccinium.

Mono, di, or trisaccharide derivatives of delphinidin, cyanidin, peonidin, petunidin, and malvidin are common in Vaccinium berries [25]. The principal sugars are glucose, galactose, xylose, rhamnose, and arabinose.

The fruits of V. myrtillus are characterized by the presence of different types of anthocyanins. In particular, cyanidin 3-O-galactoside, cyanidin 3-O-glucoside, cyanidin 3-O-arabinoside, peonidin 3-O-galactoside, and peonidin 3-O-arabinoside were identified [26,27,28,29,30,31].

In V. myrtillus, cyanidin 3-O-xyloside, cyanidin 5-O-glucoside, cyanidin 3,5-O-diglucoside, cyanidin 3-O-(6”-O-2-rhamnopyranpsyl-2”-O-β-xylopranosyl-β-glucopyranoside), cyanidin, delphinidin 3-O-sambuobiside, and peonidin-3-glycoside have also been identified [31,32,33,34].

Malvidin and delphinidin derivatives represent about 75% of the total anthocyanins content of V. corymbosum fruits [35,36]. Cho et al. [29] reported percentages of 27–40% for delphinidin, 22–33% for malvidin, 19–26% for petunidin, 6–14% for cyanidin, and 1–5% for peonidin. Petunidin 3-O-glucoside has been also identified in V. corymbosum and V. myrtillus [27,31]. The 3-O-galactosides and 3-O-arabinosides of cyanidin and peonidin are the most abundant recognised anthocyanins in the fruits of V. oxycoccos [27,37,38].

Twelve anthocyanins, namely cyanidin 3-O-glucoside, delphinidin 3-O-glucoside cyanidin 3-O-arabinoside, peonidin 3-O-arabinoside, peonidin 3-O-glucoside, peonidin 3-O-galactoside, delphinidin 3-O-arabinoside 2-O-glucoside, malvidin 3-O-galactoside, and malvidin 3-O-glucoside, were isolated from the extract of the edible berries of V. vitis-idaea by a combination of chromatography techniques [39,40,41,42,43,44].

Delphinidin-3-O-xyloside, delphinidin-3-O-glucoside, malvidin-3-O-galactoside, malvidin-3-O-glucoside petunidin-3-O-galactoside, petunidin-2-O-glucoside, malvidin-3-O-xyloside, and petunidin-3-O-xyloside were isolated from V. arctostaphylos [45,46].

Except anthocyanins, to date, more than 50 other flavonoids (mainly flavanols and proanthocyanidins) have been isolated and identified from the genus Vaccinium [25,28,29,30,31,40,41,42,43,44].

Glycosides are usually O-glycosides, with the sugar moiety bound to the hydroxyl group at the C-3 or C-7 position. The most common sugar moieties include D-glucose, L-rhamnose, D-xylose, D-galactose, and L-arabinose [25].

Quercetin is the most common flavonoid isolated from Vaccinium species [25]. It was found in high quantities in V. uliginosum and V. myrtillus [29]; however, the richest source of quercetin is V. oxycoccos [38].

Several glycosides of myricetin (myricetin 3-glucoside, myricetin 3-arabinoside, myricetin 3-O-rhamnoside) and quercetin (quercetin 3-  O-arabinoside, quercetin 3-O-ramnoside, quercetin 3-O-galactoside, quercetin 3-O-glucoside, and quercetin 3-O-rutinoside) were identified in V. myrtillus [28,29,30,31]. Apigenin, chrysoeriol, myricetin, myricetin-3-xyloside, quercetin 3-O-glucuronide, luteolin are other flavonoids described in V. myrtillus [47].

Glycosides of quercetin, myricetin, and kaempferol are the main flavonoids identified in V. oxycoccosare [49]. Quercetin 3-O-galactoside is the dominant compound, but at least 11 other glycosides are present in lower concentrations [38].

Epicatechin is the dominant constitutive unit of V. oxycoccos, whereas catechin and (epi)gallocatechins are present only in trace amounts [24,40].

The major flavonoids described in V. vitis idea are kaempferol [41], quercetin [41,50], myricetin, myricetin 3-O-glucoside [44], quercetin derivatives (bond to glucose, galactose, glucuronide, rhamnose, arabinose, and xylose), kaempferol 3-O-rhamnoside , isorhamnetin 3-O-glucoside, syringetin-3-O-glucoside, kaempferol 3-O-glucoside, and rutin [51].

The fruits of V. uliginosum are characterized by the presence of kaempferol, laricitrin [50], quercetin [50,52,53,54], myricetin [54], syringetin, quercetin 3-O-glucoside, quercetin 3-O-galactoside, quercetin 3-O-glucuronide, isorhamnetin, syringetin 3-O-glucoside, myricetin 3-O-galactoside

Sellappan et al. [55] described, in V. corymbosum, the presence of catechin, myricetin, quercetin and kaempferol, but not the presence of epicatechin. Seventeen phenolic acids were identified in some varieties of V. myrtillus [56]. Phenolic acids including gallic, p-coumaric, ferulic, ellagic, and caffeic acids were found in V. corymbosum and V. oxycoccos [30, 55].  V. corymbosum was characterised by the presence of chlorogenic acid as a major phenolic acid, followed by caffeic, ferulic, p-coumaric, and traces of p-hydroxybenzoic acids, while p-coumaric acid was the principal phenolic acid of V. oxycoccos, followed by ferulic, chlorogenic, caffeic, and p-hydroxybenzoic acids.

Other studies have reported p-coumaric, sinapic, caffeic, and ferulic acids as the main hydroxycinnamic acids identified in V. oxycoccos [57,58,59]. Ellagic acid and ellagitannins have not been detected in significant amounts [24].

Thirteen phenolic acids (gallic, protocatechuic, p-hydroxybenzoic, m-hydroxybenzoic, gentisic, chlorogenic, p-coumaric, caffeic, ferulic, syringic, sinapic, salicylic, and trans-cinnamic acids) were identified in V. arctostaphylos. The dominant phenolic acids were caffeic and p-coumaric acids. The phenolic acid concentrations are mostly lower in V. arctostaphylos in comparison to the other berries of the Vaccinium genus [60].

Iridoids are a widespread group of monoterpenoids comprising a generally glycosylated cyclopentan[c]pyran skeleton. They are specifically produced by several botanical families and are a class of secondary metabolites that is characteristic of the Ericaceae. Iridoids from the Vaccinium genus have been less studied than anthocyanins and other phenolic compounds. However, iridoids have known human health benefits including anti-inflammatory, anticancer, antimicrobial, antioxidant, antispasmodic, cardioprotective, choleretic, hepatoprotective, hypoglycaemic, hypolipidemic, neuroprotective, and purgative activities [61,62,63]. In Vaccinium species, iridoids have often been identified in mixtures and have not always been isolated. The stereochemistry of the asymmetric carbons of some of them has not been elucidated. Asperuloside, scandoside, and monotropein, and their derivatives, seem to be representative of the genus [64,65].

Heffels et al. [64] have tentatively identified, in V. uliginosum and V. myrtillus, 14 iridoid glucosides, including vaccinoside, monotropein, p-coumaroyl-scandoside, deacetylasperulosidic acid (C6: (S)), scandoside (C6: (R)),p-coumaroyl-deacetylasperulosidic acid, p-coumaroyl-monotropein, and p-coumaroyldihydromonotropein (C6-C7hydrogenated). V. oxycoccos juice showed the presence of two new coumaroyl iridoid glycosides, namely 10-p-trans- and 10-p-cis-coumaroyl-1S-dihydromonotropein [66]. Detection and isolation of iridoids from fruits is not straightforward. Surprisingly, iridoid glycosides have not been identified in V. corymbosum [64,67,68], whereas scandoside, geniposide, vaccinoside, and dihydromonotropein have recently been identified in V. corymbosum extracts [65].

Ursolic acid, which showed to possess strong anti-inflammatory effects, is abundant in V. oxycoccos, which also contains two rare derivatives of ursolic acid: cis-3-O-p-hydroxycinnamoyl ursolic acid and trans-3-O-p-hydroxycinnamoyl ursolic acid [69].

Triterpenoids are the most predominant components in the cuticular wax of blueberry fruits, together with the triterpene alcohols α-amyrin, β-amyrin, and lupeol [70].

Ursolic acid was the dominant triterpene in V. corymbosum (southern highbush blueberry) cultivars, whereas oleanolic acid was the most abundant in northern highbush blueberry cultivars. Hentriacontan-10,12-dione was detected for the first time in V. corymbosum [70].

The non-volatile malic, citric, and quinic acids were identified and quantified in V. arctostaphylos and V. myrtillus. It is interesting to note that the level of malic acid in both berries increases gradually during maturation. In contrast, the level of citric and quinic acids, as well as the total acid level, decreases towards ripening in both species [71]. The major acids (organic and phenolic) present in V. corymbosum are citric, malic, quinic, and chlorogenic acids.

In addition to fruits, the leaves of Vaccinium species have also been used in traditional remedies (Table 1). Leaves are considered by-products of berries cultivation. Their traditional use against several diseases, such as inflammation, diabetes, and ocular dysfunction, has been almost forgotten nowadays. The scientific interest regarding the leaf composition and beneficial properties grows, documenting that leaves may be considered an alternative source of bioactive compounds. Analytical studies reveal that the chemical composition of leaves is similar to that of the fruits or even higher, indicating that they may be used as an alternative source of bioactive compounds for the development of functional foods, nutraceuticals, and/or food supplements.

4. Biological Properties of Vaccinium Species

Many biological properties have been reported for extracts and derivatives of different Vaccinium species, and the anti-inflammatory, antioxidant, anti-carcinogenic, cardiovascular and neurodegenerative protective effects have been extensively described [11,88,89,90]. High antioxidant activity has been demonstrated for V. corymbosum [76,91], V. oxycoccos [92], V. myrtillus [93], and many others. This activity appears to be linked to cultivar, genotype, growing site, cultivation techniques and conditions, processing, and storage.

Similarly, in different anti-inflammatory tests, Vaccinium exhibited high anti-inflammatory activity [11]. High concentrations of anthocyanins (such as cyanidin, delphinidin and malvidin) and flavonoids (such as astragalin, hyperoside, isoquercitrin, and quercitrin) appear to be related to the anti-inflammatory and antioxidant activities ascribed to these berries [94,95]. As Vaccinium berries are edible, their consumption may be helpful for the treatment of inflammatory illnesses.

The vascular endothelium occupies a catalogue of functions that contribute to the homeostasis of the cardiovascular system. Endothelial cells (ECs) play a variety of roles, including the control of tone regulation, blood coagulation and vascular permeability, and local regulation of coagulative, immune and inflammatory stimuli [96].

Indeed, many cardiovascular diseases are either a direct or indirect result of a dysfunction of the endothelium that fails to maintain body homeostasis [97,98]. Endothelial dysfunction (ED) is considered a predictor of cardiovascular events, and it is characterized by alterations in vascular tone and endothelial production of procoagulant and prothrombotic factors [97,98].

Several risk factors including smoking, obesity, insulin resistance, diabetes, hypercholesterolemia, and physical inactivity have been described for ED. In addition, ED occurs with aging, as a consequence of senescence processes [99,100]. Vaccinium extracts have long been used in traditional medicine and appear to be promising nutraceuticals to prevent endothelial dysfunction and cardiovascular diseases.

4.1. Vaccinium and diabetes

Several reports indicate a potential role of Vaccinium in the control of diabetes, and it has been used in traditional medicine to ameliorate its symptoms [101,102,103]. Approximately 90% of diabetic patients have type 2 diabetes that is characterized by peripheral insulin resistance and by a reduction in the number and the activity of pancreatic β-cells [104]. Anthocyanins from Vaccinium have potential in terms of lowering the risk of developing various chronic diseases due to their ability to regulate energy metabolism as well as through their anti-inflammatory and anti-oxidative effects [11]. Phenolic compounds affect key pathways of carbohydrate metabolism and hepatic glucose homeostasis including glycolysis, glycogenesis, and gluconeogenesis, which are usually impaired in diabetes.

In addition, Vaccinium extracts and derivatives protect pancreatic β-cells from glucose-induced oxidative stress, increase insulin secretion, possess glucose-lowering effects, restore glutathione concentration, inhibit DPP-4, enhance insulin response, and attenuate the secretion of glucose-dependent insulinotropic polypeptide and GLP-1 Blueberry metabolites reduce the expression of inflammatory markers and restore the glycosaminoglycan levels increased by high glucose in in vitro models of diabetic ECs [110]. Moreover, malvidin, a major anthocyanin present in blueberries, decreases ROS levels, increases the enzyme activity of catalase and superoxide dismutase, and downregulates NADPH oxidase 4 (NOX4) expression in ECs exposed to high glucose levels [111], indicating a protective role against diabetes-induced oxidative stress. In similar models, this compound also reduces vascular endothelial growth factor (VEGF) up-regulation, ICAM-1 expression, and NF-κB levels [112], and restores PI3K and Akt levels, which are reduced by high glucose [113]. 

These observations are also confirmed in the retina of diabetic rats, where blueberry anthocyanins reduce oxidative stress, vascular endothelial growth factor (VEGF) and interleukin 1β (IL-1β) expression, and activate the Nrf2-related/heme oxygenase 1 (Nrf2/HO-1) signalling pathway [114], suggesting that Vaccinium anthocyanin may be helpful in inhibiting diabetes-induced retinal abnormalities and preventing the development of diabetic retinopathy.

4.2. Vaccinium and atherosclerosis

Atherosclerosis is one of the major causes of cardiovascular diseases and is characterized by the accumulation of lipids and fibrous plaques in the large arteries, which may lead to heart attacks, strokes, and peripheral vascular diseases [115].

Hydroalcoholic extracts of V.myrtillus leaves showed lipid-lowering activity, while V. corymbosum berries decreased blood cholesterol levels, thus reducing cardiovascular risk and promoting atherosclerosis prevention [116,117,118]. In addition, consumption of cranberry anthocyanins improved lipid profiles, increasing HDL and decreasing LDL in rats, hamsters fed a high-fat diet, and hypercholesterolemic swine [119,120,121]. Blueberries showed to induce regression of atherosclerotic plaques in arteries, and to reduce total, HDL and LDL-VLDL blood cholesterol and triglycerides, as well as the hepatic expression of bile acid synthesis genes in mice models [122, 123].

Although published animal studies primarily focused on the specific cardiovascular disease risk factors or biomarkers, and the antioxidant and anti-inflammatory effects of Vaccinium and its derivatives, clinical data have also been published [10]. Indeed, good results were also observed with cranberry juice in obese men, and hyper-triglyceridemic or diabetic patients [24].

The molecular mechanisms of atheroprotective effects of Vaccinium are not completely understood and are often associated with antioxidant and anti-inflammatory activities. In fact, the protective activity in atherosclerosis development has been associated with the reduction in oxidative stress, inhibition of inflammation, and regulation of cholesterol accumulation and trafficking [10].

In apoE−/− mice, the treatment with 1% wild blueberries for 20 weeks modulated gene expression and protein levels of scavenger receptors CD36 and SR-A, the principal receptors responsible for the binding and uptake of modified LDL in macrophages [124]. CD36 and SR-A were found to be lower in peritoneal macrophages of blueberry-fed mice, and fewer ox-LDL-induced foam cells were formed, probably through a mechanism involving PPARγ [124]. In addition, Xie et al. [125] demonstrated that blueberry consumption increased the levels of the cholesterol transporter ABCA1, indicating that blueberries may facilitate cholesterol efflux and lowering cholesterol accumulation. Overall, it has been shown that blueberry consumption increased PPARα, PPARγ, ABCA1, and fatty acid synthase expression, while reducing SREBP-1 levels [10].

4.3. Vaccinium and endothelial dysfunction

Endothelial dysfunction is an early predictor of cardiovascular diseases, and it is well known that oxidative stress and low grade of inflammation contribute to endothelial cell activation, priming it for adhesion, infiltration, and immune cell activation [126].

In this context, data from the literature indicate that Vaccinium extracts and derivatives may prevent or delay cardiovascular diseases due to their capability to revert endothelial dysfunction. Very recently, Curtis et al. [127] showed that one cup of blueberries/day, for six months, promotes 12–15% reductions in cardiovascular disease risk, demonstrating that higher intakes of blueberries improve markers of vascular function and ameliorate lipid status. Similarly, the intake of blueberry acutely improved peripheral arterial dysfunction in smoker and in non-smoker subjects [128,129], improved endothelial function over six weeks in subjects with metabolic syndrome [130], and improved endothelium-dependent vasodilation in hypercholesterolemic individuals through the induction of the NO-cGMP signaling pathway [131].

In animal models, blueberry anthocyanin-enriched extracts were shown to be able to increase Bcl-2 protein expression, as well as to decrease interleukin 6, malondialdehyde, endothelin 1, and angiotensin II levels and to reduce Bax protein expression after rat exposure to fine particulate matter [132]. Blueberry consumption was also able to protect endothelial function in obese Zucker rats, through the attenuation of local inflammation in perivascular adipose tissue (PVAT) In diabetic rats, the Vaccinium treatment decreased markers of diabetic retinopathy, such as retinal VEGF expression and degradation of zonula occludens-1, occludin and claudin-5 [134]. Finally, in hypoperfusion-reperfusion experiments in rats, the administration of the extract of V. myrtillus protected pial microcirculation by preventing vasoconstriction, microvascular permeability, and leukocyte adhesion [135].

The endothelium protective role of Vaccinium has also been reported in in vitro experimental models. Human aortic endothelial cells (HAECs) treated with palmitate exhibited elevated ROS levels, and increased expression of several markers of endothelial dysfunction including NOX4, chemokines, adhesion molecules, and IκBα.

The effects of palmitate were ameliorated in HAECs previously treated with blueberry metabolites [136]. In human umbilical vein endothelial cells (HUVEC), pterostilbene, an active constituent of blueberries, is able to induce a concentration-dependent nitric oxide release via endothelial nitric oxide synthase (eNOS) phosphorylation, mediated by activation of the PI3K/Akt signaling pathway [137]. Similarly, blueberry anthocyanins protect endothelial cells from oxidative deterioration by decreasing the levels of ROS and Xanthine Oxidase -1 (XO-1) and increasing the levels of superoxide dismutase and HO-1 [138].

This entreviewy reports chemical composition of fruits and leaves of Vaccinium species and overviews their biological properties focusing on the activity of Vaccinium extracts and derivatives in cardiovascular diseases and endothelial dysfunctions, closely associated with inflammation processes and oxidative stress. 

Many evidences indicate that Vaccinium is an important source of bioactive molecules that appear to satisfy all the requirements to develop drugs and nutraceuticals against endothelial dysfunction, thus preventing cardiovascular disease onset and progression.