Shuzhen Wng, Ting Zeng, Shung Zho, Yu Zhu, Chnghun Feng,Jinfeng Zhn, Shiming Li,*, Chi-Tng Ho, Alexnder Gosslu,d,*

a Huanggang Normal University, College of Life Science; Hubei Key Laboratory of Economic Forest Germplasm Improvement and Resources Comprehensive Utilization; Hubei Collaborative Innovation Center for the Characteristic Resources Exploitation of Dabie Mountains;Hubei Zhongke Research Institute of Industrial Technology, Huanggang 438000, China

b College of Pharmacy, Shandong University of Traditional Chinese Medicine, Jinan 250355, China

c Department of Food Science, Rutgers University, NJ 08901, USA

d Department of Science (Biology), City University of New York, NY 10007, USA

Keywords:

Oolong tea

Anticancer potential

Antioxidative activity

Microbiota modulating activities

Synergistic effects

A B S T R A C T

Oolong tea, partially fermented from Camellia sinensis leaves, exhibits significant antioxidative, anti-inflammatory, and anti-cancer activities as indicated in several in vitro and in vivo studies.However, studies on health promoting effects of oolong tea and its characteristic compounds are limited.The potential efficacy of bioactives derived from oolong tea and their roles as promising anticancer agents, their cardio-protective benefits during hypoxic conditions,effects in treating allergic disorders, potential prebiotic activities, improvement of blood lipid status in human beings, effectivity as oral hypoglycemic agent in the treatment of type 2 diabetes,and their potentials to reduce the risk of obesity have been discussed in this review.These promising studies mainly gained from animal studies might broaden the consumption and usage of the phenolic-enriched oolong tea and its products in food and pharmaceutical industries.However,potential health beneficial effects of oolong tea in humans should be further complemented by large-sized, randomized double-blind and placebo-controlled trials to consolidate potential therapeutic applications.

1.Introduction

Tea beverages, brewed fromCamellia sinensisleaves, have been consumed all over the world because of potential health promoting benefits and high-quality flavor and aroma.The well-known healthpromoting functions of tea comprise anticancer, antioxidant, anticardiovascular, antimicrobial, hypoglycemic, and hepatoprotective effects [1].Historically, the origin of tea is considered to be India and China [2].Nowadays, more than 25 countries produce different teas.Based on manufacturing processes and global consumption,tea beverages are divided into black tea (78%), green tea (20%),and oolong tea (less than 2%) [3,4].Compared with green tea(unfermented) and black tea (fully fermented), oolong tea is partially fermented during processing.Generally, green tea and oolong tea are more popular in oriental countries, while black tea is more preferred in western countries [2].

Oolong tea originated from Fujian province of China, but is also produced in some other Southeast Asia regions, such as India,Nepal, Thailand, Vietnam, and Indonesia [5].Original tea source of oolong tea prefers warm climate, proximately 10-30 °C, foggy,and drizzly seasons.Places with 1 000-1 250 mm annual rainfall are ideal for cultivating oolong tea [6].Oolong tea contains three typical kinds, represented by Tieguanyin, Fenghuang Dancong, and Dahongpao with the lightest, intermediate, and the highest degree of fermentation, respectively [7].The processing of oolong tea can be generally classified into seven steps, including sunning and withering,fermenting, panning, rolling, firing, final-firing, and packing [8].During processing, oolong tea is oxidized in the range of 10%-70% [9].The semi-oxidized oolong tea gives a taste between black tea and green tea [7].Oolong tea produced in China is famous due to its elegant fruity and floral aroma, as gradual dehydration and moderate bruising during the manufacturing process could clarify and intensify its special flavor [7].The content of the major catechins in oolong tea leaves is lower than that of green tea, as catechins are partially oxidized by polyphenol oxidase during the fermentation process [9].Moreover, an increase of polymerized polyphenols also occur [9].

In spite of its popularity in Asian countries, studies on health promoting effects of oolong tea and its characteristic compounds, such as theasinesins, have attracted limited attention as compared to the knowledge of preventive and therapeutic effects of green and black teas.Theasinesins exhibit antioxidative, anti-inflammatory, and anticancer activities bothin vitroandin vivo.In addition, the synergistic effects of polyphenols and polysaccharides towards cancer cells are barely understood.In this review, miscellaneous research has been comprehensively summarized, comprising processing, phytochemistry,health promoting properties, and potential side effects.Although more studies are needed, these biological effects are encouraging in the development of oolong tea as bioactive beverage and functional food.

2.Functional components of oolong tea

Oolong tea leaves contain more than 100 chemical entities which potentially can improve human health, such as polyphenols,polysaccharides, alkaloids, vitamins, flavonoids, amino acids,chlorophyll, minerals, volatile compounds, organic acids, trace elements, aromas, lignins, and proteins [8,10].

2.1 Polyphenols

Polyphenols extracted from oolong tea leaves can be categorized into several sub-groups: catechins, tannins, flavonol glycosides and other flavonoids [10].As illustrated in Fig.1, the catechins contain (–)-epicatechin (EC), (–)-epicatechin gallate (ECG),(–)-epigallocatechin (EGC), (–)-epigallocatechin gallate (EGCG),(–)-gallocatachin (GC), (–)-gallocatechin gallate (GCG), (+)-catechin(C), (–)-catechin gallate (CG), and (–)-epigallocatechin 3-O-(3-O-methyl) gallate (EGCG3”Me) [10-12].In aged oolong tea leaves,three flavonols, including myricetin, quercetin, and kaempferol have been detected [13].Moreover, proanthocyanidin type tannins,hydrolyzable tannins, and other tea pigments have also been isolated and identified [8].In total, 19 flavonol glycosides have been identified,containing myricetin rhamnodiglucoside, myricetin 3-O-galactoside,myricetin 3-O-rutinoside, quercetin glucorhamnoglucoside,quercetin 3-O-rhamnodiglucoside, quercetin 3-O-rutinoside,myricetin 3-O-glucoside, quercetin rhamnogalactoside, quercetin 3-O-galactoside, quercetin 3-O-glucoside, teaghrelin-1, teaghrelin-2,quercetin dirhamnoglucoside kaempferol glucorhamnoglucoside,kaempferol 3-O-rhamnodiglucoside, kaempferol 3-O-galactoside,kaempferol dirhamnoglucoside, kaempferol 3-O-rutinoside,kaempferol 3-O-glucoside, prechafurosides A, and prechafurosides B [14-16].

Fig.1 Chemical structures of oolong tea polyphenols.

Fenghuang Dancong, the typical oolong tea, possesses minor amounts of thea flavins [17].Dimeric and oligomeric polyphenols, such as oolong tea theasinensins, are newly formed during the fermentation process [18].Both in blood system and small intestine, theasinensins have more than a ten-fold lower absorption than EGCG even after the large-scale administration [19].In particular, theasinensin A (TSA),an EGCG dimer with an R-biphenyl bond, is the most abundant theasinensin prevalent in oolong tea [20].Oolong tea possesses about (8.1 ± 0.2)% (m/m) of total catechins and (5.1 ± 0.1)% (m/m)of polymerized polyphenols, representing an intermediate between green and black tea [21].The uniqueO-methylated forms of EGCG and ECG, EGCG3”Me and (–)-epicatechin 3-O-(3-O-methyl) gallate(ECG3”Me) are also present in oolong tea (Fig.1) [22].

2.2 Polysaccharides

Tea polysaccharides (TPS) possess various biological effects,including anticancer, antidiabetic, hypolipidemic, antioxidant, and immunomodulatory activities [23].Among these bioactivities, the antidiabetic potential is the most prominent as tea drink has been well recognized as a popular folk medicine in treating diabetes in China and Japan.Polysaccharides could act as functional delivery materials with other constituents [24].Moreover, the bioactivities of polysaccharides rely on its physicochemical properties and spatial conformation.Distinctive manufacturing processes such as the degree of fermentation, extraction, and purification conditions (solvents,enzyme, and temperature among others) all affect the properties of TPS [25].Up to now, only a few studies explored the bioactivity of polysaccharides derived from oolong tea leaves [25].Oolong tea polysaccharides (OTPSs) are water-soluble and all three OTPSs,isolated from Tieguanyin, Fenghuang Dancong and Dahongpao named TTPS, FTPS and DTPS, respectively, showed significantly healthpromoting activities [26].Through chemical composition analysis(protein, uronic acid, rot neutral sugars, and etc.), TTPS, FTPS and DTPS turned out to be acid heteropolysaccharides conjugated with proteins [26].Furthermore, TTPS, FTPS, and DTPS all contained seven monosaccharides with different molar ratio, includingD-fucose,L-rhamnose,L-arabinose,D-xylose,D-mannose,D-galactose,andD-glucose [26].Moreover,L-arabinose andD-rhamnose were also found in other OTPSs [1,25].Based on high performance gel permeation chromatography (HPGPC) analysis, 92.9% of TTPS had a peak in molecular weight of 0.82 × 106, 94.4% of DTPS peaked at 2.64 × 106as molecular weight.In relation to FTPS, 34.2% peaked at 0.93 × 106, and 68.8% peaked at 0.01 × 106, respectively [26].Fourier Transform IR spectra showed that TTPS, FTPS and DTPS all had similar characteristic absorption bands of proteins, uronic acid,polysaccharides, and pyran-glycosides, respectively [26].

2.3 Proteins and amino acids

‘Oriental Beauty’, a variety of oolong tea native to Taiwan, is highly prized due to the unique fruity aroma and sweet taste.The sweet taste might be partially influenced by protein expression profiles, most of which participate in biosynthesis, transport, responses to stress, carbohydrate metabolism, and amino acid metabolism [27].Some proteins isolated from oolong tea leaves contain amino acids as follows: alanine (0.19-0.76 mg/g), arginine (0.21-0.69 mg/g),asparagine (0.11-0.33 mg/g), aspartic acid (0.25-1.37 mg/g),GABA (0.09-0.97 mg/g), glutamic acid (0.92-1.49 mg/g), glutamine(0.26-0.76 mg/g), glycine (0.06-0.08 mg/g), histidine (0.14-0.31 mg/g),isoleucine (0.15-0.37 mg/g), leucine (0.12-1.22 mg/g),L-theanine(3.97-14.57 mg/g), lysine (0.11-0.43 mg/g), phenylalanine(0.10-0.71 mg/g), proline (0.08-0.51 mg/g), threonine (0.15-0.345 mg/g),tryptophan (0.09-0.48 mg/g), serine (0.31-0.81 mg/g), and valine(0.14-0.72 mg/g) [28-30].

2.4 Others

In regards to alkaloids, theobromine, theophylline, and caffeine have been found in oolong tea leaves [10].In addition, oolong tea leaves contains minerals and metals such as aluminum (Al), magnesium (Mg),copper (Cu), sodium (Na), calcium (Ca), iron (Fe), potassium (K),manganese (Mn), zinc (Zn), barium (Ba), strontium (Sr), chromium (Cr),cadmium(Cd), lead (Pb), titanium (Ti), cobalt (Co), selenium (Se), nickel(Ni), and semi-metals such as arsenic (As) [31,32].

Main organic acids extracted from oolong tea leaves contain gallic acid, quinic acid, glutamic acid, 5-galloylquinic acid, 3-p-coumaroylquinic acid, 5-caffeoylquinic acid,4-p-coumaroylquinic acid, 5-p-coumaroylquinic acid, and strictinin [33].Vitamins, including hydrophilic vitamin B, vitamin C,S-methylmethionine (vitamin U), and hydrophobic vitamin A, E and K,have also been identified in oolong tea leaves [8].

A list of aroma substances derived fromβ-primeverosides have been identified, such as geranyl 6-O-β-xylopyranosyl-β-D-glucopyranoside, benzyl disaccharide glycosides, 2-phenylethyl,(S)-linalyl,trans-/cis-linalool 3,6-oxide6-O-β-D-xylopyranosylβ-D-glucopyranosides, nerolidol, linalool,β-ionone, jasmine lactone,α-farnesene, indole, dihydroactinidiolide, geranylacetone,geraniol,γ-decalactone, hexanoic acid-3-hexene ester, and methyl jasmonate [34-37].

3.Health benefits of oolong tea

Several studies indicate that oolong tea could improve heart and vascular disease, protect teeth and bones, and may serve as antioxidative, anticancer, hypoglycemic, anti-inflammatory, antiallergic,and antimicrobial agent [8].Majorly, OTPSs and oolong tea polyphenols (OTP) contribute to these health-promoting properties.

3.1 Anticancer potentials of oolong tea extracts

Cancer, the uncontrolled cell division, results in the aggregation of cells to form malignant tumors [3].It is still one of the main causes of mortality despite great efforts to decode corresponding molecular mechanisms and to develop therapeutic strategies [38].Currently, surgery, radiotherapy, and chemotherapy are three main approaches to treat cancer patients.Single cancer therapy is often discounted due to genetic and immunologic complexity of tumor microenvironment [39].Small chemotherapeutic drug molecules are able to penetrate indiscriminately all cell types without specific targeting, resulting in serious systemic toxicity [24].Therefore,lower toxic treatments using alternative and highly specific chemopreventive agents against carcinoma are greatly needed.The anticancer effects of oolong tea and their proposed mechanisms are summarized and illustrated in Table 1 and Fig.2.

Table 1Anticancer activity of oolong tea extracts.

Fig.2 Molecular mechanism of oolong tea in inducing cancer cell apoptosis.

For instance, regular oolong tea consumption could significantly reduce the risk of ovarian cancer for southern Chinese women, with an adjusted odds ratio of 0.29 (95% confidence interval: 0.22-0.39) [13].Moreover, inverse dose–response relationships were found for years of drinking with numbers of cups and quantity of tea consumed, as well as the amount of dried tea leaves brewed (P< 0.01) [40].In human stomach cancerous KATO III cells, oolong tea polyphenol extracts mainly containing polyphenol trimers [OTP trimer]inhibited cell growth and induced cell apoptosis in dose- and kineticdependent manners, as apoptotic bodies and fragmentation of DNA to oligonucleosomal sized fragments were observed [41].Compared with a phenolic-enrichedn-butanol fraction (TBF) and water fraction of Tieguanyin oolong tea (TWF), the ethyl acetate fraction (TEF)with the highest content of phenolics, flavonoids, procyanidins, and catechin monomers exhibited the strongest antioxidant capacity and inhibition of the growth of 4T1 murine breast cancer cells [42].As popular oolong tea, the Fenghuang Dancong aqueous extracts could significantly induce cell apoptosis in MDA-MB231 cells mainly through death-receptor-mediated extrinsic apoptotic signaling pathway, as the internalized PDCe not only downregulated intracellular reactive oxygen species (ROS) levels but also induced oxidative damage to mitochondria [17].The boiling water extracts of oolong tea leaves (1%-2%) were able to induce DNA damage and cleavage, reduced cell growth, proliferation, and tumorigenesis in six different breast cancer cell lines (MCF-7, T47D, SKBR3, MDAMB-231, MDA-MB-436, and MDA-MB-468 cells) [43].

In human histolytic lymphoma U937 cells, TSA showed strong growth inhibitory effects via mitochondrial transmembrane potential (MTP) loss, elevating the production of ROS, as well as mitochondrial cytochrome c release into cytosol, which consequently induced rapid activation of caspase-9 and caspase-3 [44].In particular, TSA allowed caspase-activated deoxyribonuclease entering nucleus, thus fragmenting chromosomal DNA, and inducing DFF-45(DNA fragmentation factor) degradation [44].Unlike strong growth inhibitory effects against U937 (IC50= 12 μmol/L), TSA were less effective against human acute T cell leukemia Jurkat [44].In SKOV3 cells, the flavonoid fractions from oolong tea infusion showed significant anticancer activities with 62.78% inhibition rates at the dose of 100 μg/mL [45].

In male Kunming mice (18-22 g), a co-administration of OTPS1 and oolong polyphenols (OTPs) exhibited synergistic effects on tumor proliferation and growth with the combination index (CI) values of 0.34 and 0.39, respectively [46].Moreover, OTPS1 and OTP showed a strong synergistic ability on SMMC7721 cells (CI < 0.2) [46].

3.2 Anti-obesity properties

Obesity is characterized as an abnormally large amount of fat stored in adipose tissue leading to a significant increase in body weight, which has caused public health problems worldwide.Excess energy intake and reduced energy expenditure might lead to abnormal excessive growth of adipose tissue, and even result in obesity [47].Obesity is strongly associated with metabolic syndrome, such as insulin resistance, hypertension, and hyperlipidemia.In addition,excess weight can cause cardiovascular diseases and cancers, as evidenced by increasing morbidity and mortality.Now, great attention have been paid to natural products as potential functional ingredients with anti-obese activity.Several studies demonstrated that oolong tea bioactive ingredients could effectively reduce obesity and control diabetes (Fig.3).

Fig.3 The mechanism of oolong tea towards obesity and related diseases.

In 3T3-L1 preadipocytes, EGCG3”Me (20-80 μg/mL)significantly reduced the accumulation of triglycerides in a concentration-dependent manner (20-80 μg/mL) [47].The 20, 40,and 80 μg/mL EGCG3”Me led to a decrease of the accumulation of lipids to (89.42 ± 2.66)%, (64.36 ± 3.13)%, and (39.37 ± 2.79)%,respectively [47].Moreover, both EGCG3”Me and EGCG treatments showed a significant inhibitory effect on adipogenesis.Noteworthy,EGCG3”Me showed a relatively higher effect than EGCG in 3T3-L1 preadipocytes, which might be due to the structural difference,as hydroxyl group at C-3 in D ring was substituted by a methoxy group in EGCG3”Me [47].Oolong tea polyphenol phospholipids,prepared with OTPs and phospholipids, exhibited pronounced antiobesity effects in a human flora-associated (HFA) mice model [48].Moreover, the administration of both OTP and OTC dramatically alleviated the changes of serum and liver induced by high fat diet [48].In 12 healthy adults (3 males and 9 females with mean age of(22.0 ± 1.8) years) under high-lipid diet, oolong tea consumption significantly increased the fecal lipid and cholesterol excretion [49].Oolong tea contributed significantly to weight loss through increasing fat oxidation and energy expenditure, and the effect was about doubled as compared to green tea [50].In a small study with 11 healthy Japanese females (age (20 ± 1) years, body mass index(21.2 ± 2.5) kg/m2), the energy expenditures (EE) for 120 min were significantly increased by 10% after the consumption of oolong tea [50].In a double-blinded clinical trial, supplementation of catechin-enriched oolong tea could increase antioxidant and hepatoprotective activities in hypercholesterolemic subjects, and showed an effective reduction in hepatic lipid storage [51].In another mice model fed with a high-fat/high-sucrose diet, oolong tea extracts could inhibit fat deposition and significantly decrease liver total lipid and triacylglycerol content [52].

Hyperglycemia, the excessive amount of glucose circulating in blood, is considered to be the hallmark in diabetes [18].In type 2 diabetes patients, oolong tea could markedly lower the concentrations of plasma glucose and fructosamine from (229 ± 53.9) mg/dL and (409.9 ± 96.1) mg/dL to (162.2 ± 29.7) mg/dL and(323.3 ± 56.4) mg/dL, respectively [11].In KK-Ay mice and Sprague-Dawley rats, TSA remarkably reduced serum glucose levels, lowered hepatic triacylglycerol, and increased fecal fat excretion [53].Oolong tea was effective in lowering plasma glucose levels of subjects with type 2 diabetes, and was even more effective when conjugated with antihyperglycemic agents [11].Therefore, oolong tea could be an effective adjunct to oral hypoglycemic agents in treating type 2 diabetes.

3.3 Antiallergic and anti-inflammatory properties

Food allergy is a condition of immediate hypertensive (type I)allergy, and the release of chemical mediators from mast cellsis could trigger type I allergic disorders.Oolong tea infusions could significantly inhibit the release of histamine from mouse mast cells, and the inhibitory activity was higher than that of green tea infusions prepared from cultivar ‘Yabukita’ [54].In another study, tea catechins were effective in inhibiting histamine release from mast cells, which were activated by cross-linking specific allergens with IgE [55].Particularly, EGCG could strongly inhibit type I allergic reactions in rats through inhibiting histamine release from rat peritoneal exudate cells [56,57].The gallo-type structure in catechins appears to be critical in inhibiting histamine release,since the effect of EGCG (containing two gallo-type structures) was stronger than ECG (containing onegallate group) [56].Two catechin derivatives, (–)-epigallocatechin 3-O-(3-O-methyl)-gallate (C-1) and(–)-epigallocatechin 3-O-(4-O-methyl)-gallate (C-2), isolated from Taiwan oolong tea leaves, showed an obvious antiallergic activity via a significant inhibition of type I allergic (anaphylactic) reactions in mice sensitized with ovalbumin and Freund’s incomplete adjuvant [54].In particular, the oolong tea leaves contained 0.34% (dry weight)C-1 and 0.20% C-2, which wereO-methylated forms of galloyl moiety in EGCG [54].The antiallergic effects of C-1 and C-2 even exceeded that of EGCG, which might be due to the hydroxymoiety on B ring [54].In particular, C-2 could restrain 45% histamine release from IgE or antigen-stimulated mouse mast cells (PT-18) at a concentration of 50 μg/mL reaction mixture, higher than the inhibitory effect of EGCG (22%) at 100 μg/mL [54].On the other hand, the inhibitory effect of C-1 was almost the same with that of EGCG [54].The 3-O-/4-O-methylation of the galloyl moiety in EGCG might enhance the antiallergic response towards mouse mast cells, andO-methylated forms of EGCG might be associated with the increased stabilityin vivo.EGCG may be partly converted to C-2 or C-1 by a specific methyltransferasein vivo.The pure form of EGCG could be eliminated more easily from body than EGCG in decaffeinated tea extracts, inferring that other constituents in tea extract competed with EGCG for binding to plasma and tissue proteins/metabolic enzymes [54].

Inflammation, a complex process regulated by proinflammatory cytokines and mediators, occurs as an innate immune response towards irritation and infection caused by pathogens in infected tissues.Moreover, inflammation is characterized by the recruitment of several immune cells to the inflamed sites, including neutrophils,macrophages, and monocytes.As the primary polyphenols in oolong tea, theasinensins could potentially inhibit cyclooxygenase-2 (COX-2)expression as demonstrated in LPS-activated mouse macrophages(RAW 264.7) [58].OTEE have great potentials for use as antiinflammatory drug through reducing the production of nitric oxide(NO), COX-2, IL-6, IL-1b, and tumor necrosis factor alpha (TNF-α)in active RAW 264.7 cells [59].In the RAW264.7cell model,TSA reduced the levels of pro-inflammatory mediators, such as inducible nitric oxide synthase (iNOS), NO, interleukin-12 (IL-12),TNF-α, and monocyte chemotactic protein-1 (MCP-1) induced by lipopolysaccharide (LPS) [58].In particular, the study showed that TSA downregulated MAPK/ERK kinase (MEK)-extracellular signalregulated kinase (ERK) signaling through its binding activity [58].In male ICR mice model (4 weeks old), TSA suppressed the production of IL-12, TNF-α, and MCP-1, as well as attenuated mouse paw edema induced by LPS [58].InL-carnitine feeding mice, oolong tea extracts reduced the transformation ofL-carnitine to trimethylamine-N-oxide(TMAO), and significantly decreased vascular inflammation [9].

3.4 Antibacterial and intestinal microbiota modulating activities

The consumption of oolong tea infusion, mainly containing phenolic acids, flavonoids, alkaloids, and amino acids, could substantially increase diversity and alter the structure of gut microbiota in obese C57BL/6J mice induced by high-fat-diet [60].The positive modulatory effects of OTP on gut flora was observed,with a high expansion in the relative abundance of Bacteroidetes and a decrease in Firmicutes after 8 weeks of OTP administration [61].In addition, most of the differentially expressed proteins were related to metabolism, genetic information processing, and environmental information processing.Interestingly, OTP also regulated circadian rhythm via enhancing beneficial intestinal microbiota and affecting metabolic pathways, contributing to the improvement of host microecology [61].

In specific pathogen-free Sprague-Dawley rats, OTE (5 mg/mL in drinking water or 10 mg/g in diet) significantly inhibited dental caries, and the cariostatic activity was still effective even after the establishment ofS.sobrinusin oral cavity [62].Noteworthy, the cariostatic activity of oolong tea was more effective in drinking water than in diet [62].The chromatographically isolated polyphenol fractions OTF1 and OTF6 also showed significant inhibition of caries induction, with the minimum inhibitory concentration of OTF6 and OTF1 as low as 50 μg/mL in drinking water and 100 μg/g in diet,respectively [62].Interestingly, reduction in caries development was also observed even when OTE was supplied 1 day after inoculation ofS.sobrinus6715 [62].

In Caco-2 transwell system, OTP prepared by extraction with hot water modulated the intestinal flora and generated short-chain fatty acids (SCFA), showing great potentials in improving human health [63].In particular, OTP could induce the proliferation of certain beneficial bacteria, as well as inhibitingBacteroides-PrevotellaandClostridium histolyticum[63].In another study, oolong tea extract showed antibacterial activities againstStreptococcus mutansandS.sobrinus[64].Noteworthy, the antibacterial activities of oolong tea extract were induced by a synergism of monomeric polyphenols, which can easily bind to proteins.In mice obesity models supplied with OTC, the numbers (lg cell/mL) ofBifidobacterium,Lactobacillus/Enterococcusspp.,Bacteroides-Prevotella, andC.histolyticumwere 8.28 ± 0.03,8.21 ± 0.01, 7.23 ± 0.02, and 7.05 ± 0.03, while the numbers were 8.21 ± 0.02, 8.07 ± 0.02, 7.31 ± 0.02, and 7.10 ± 0.01 in the control sample, respectively [48].As a major bioactive in oolong tea, TSA could effectively suppress the oxacillin resistance of methicillin resistantS.aureus(MRSA) for at least 10 h.Repeated administration of TSA and oxacillin could prolong the effect [65].Furthermore,oolong tea enriched with EGCG showed antifungal activity againstC.glabratawith the MIC value of 0.156 mg/mL (Table 2) [66].

Table 2Antimicrobial activity of oolong tea extracts.

3.5 Antioxidant activities of oolong tea

It is well accepted that the accumulation of ROS and reactive nitrogen species (RNS) in tissues could lead to oxidative stress, which are key modulators of human carcinogenesis, arthritis, and metabolic syndrome [67,68].The ingestion of oolong tea had beneficial effects on plasma antioxidant defenses in healthy humans [69].A human study with 22 male rugby athletes showed that oolong tea could significantly lower plasma malondialdehyde (MDA) levels and superoxide dismutase (SOD) activity in resting and post-exhaustive exercise [70].The decrease of oxidative stress by oolong tea extracts was observed as lipid peroxidation was reduced [70].These effects were confirmed by another study where hot water extracts of oolong tea could effectively inhibit peroxidation of peanut oil [71].Consistent with these studies, oolong tea infusions showed strong DPPH radical scavenging activity, with SC50(scavenging-percentage) of DPPH radical by 0.119 mL of extract (3 min soaking at 100 °C), versus SC50of 0.109 mL (2 mg/mL) by vitamin C [71].Furthermore, DPPH radical scavenging activity of oolong tea extracts increased with the elevated temperatures of soaking water [71].

The core of LDL is composed by cholesterol esters and triacylglycerols covered by a phospholipid monolayer containing unesterified cholesterol and apolipoprotein B-100 (apoB-100).Importantly, the unsaturated fatty acid moieties of lipids in LDL can be oxidized by ROS and RNS [72].The unique polyphenol extracted from oolong tea, oolonghomobis flavan A (OFA),exhibited antioxidant activity against lipid peroxidation and oxidative modification of apoB-100 as measured by the thiobarbituric acid assay [72].In addition, OFA inhibited fragmentation, carbonylation,and nitration of apoB-100 [72].Moreover, OFA could protect the heparin binding activity of apoB-100 [72].Fenghuang Dancong tea aqueous extracts scavenged ABTS+and DPPH free radicals in a dose-dependent manner [17].In another study, OTF showed significant antioxidant activity, as the IC50of DPPH scavenging activity, ferric ion reducing power, and hydroxyl radical scavenging activity were(17.63 ± 0.45), (21.08 ± 0.17), (24.45 ± 0.59) μg/mL, respectively [45].

3.6 Hepatoprotective and cardioprotective effects

Besides viral hepatitis, the incidence of alcoholic liver disease(ALD) has become a vital risk factor formorbidity and mortality.Particularly, oxidative stress and generation of free radicals play critical roles in the development of ALD [22].In China, tea has been used to detoxify alcoholic intoxication for more than a millennium,as tea extracts can reduce alcohol toxicity.In alcohol-induced liver injury mice, the administration of EGCG3″Me (100 mg/kg BW per day) could significantly decrease serum levels of aspartate aminotransferase (AST) and alanine aminotransferase (ALT).Moreover, the content of liver MDA was significantly reduced and activities of SOD and glutathione peroxidase (GSH-Px) were restored in human hepatoma cells (HepG2) [22].Importantly, the oolong tea bioactive EGCG3″Me showed remarkable hepatoprotective activityin vitro, which could effectively alleviate the damage induced by alcohol in a concentration-dependent manner [22].Supplementation of TSA could ameliorate liver tissue injury and prevent liver fibrosis via the inhibition of hepatic stellate cells activation (HSC) [20].The oral administration of TSA relieved CCl4-induced liver injury and ameliorated liver functions [20].In particular, collagen deposition was profoundly reduced due to the supplementation of TSA [20].Thus,TSA may serve as a potent bioactive constituent against liver fibrosis.Moreover, the expression of hepatic α-smooth muscle actin (α-SMA)and matrix metallopeptidase 9 (MMP-9) was suppressed through inhibition of transforming growth factor β (TGF-β) [20].

OTE treatment (100-400 ng/mL) could also suppress cardiomyocyte hypertrophy and cellular apoptosis, as well as enhance cell survival in H9c2 cells and neonatal cardiomyocytes [73].In these cells, oolong tea provided cytoprotective effects via JNK-mediated signaling, and enhanced innate antioxidant mechanisms.In particular,OTE effectively attenuated 24 h hypoxia-triggered cardiomyocyte loss through a suppression of caspase-3-cleavage and apoptosis in a dosedependent manner.The molecular mechanisms might be associated with an enhanced IGFIR/p-Akt associated survival viap-Badser136 in a dose-dependent manner, thus fortifying the NRF-2 antioxidative system [73].

3.7 Other health promoting activities

In mice induced by constant dark treatment, OTP could significantly alleviate the disrupted diurnal oscillation, phase shift of specific intestinal microbiota and liver clock genes [61].In a long-term moderate consumption study of oolong tea, beneficial effects on bone health in postmenopausal women (southern China)were observed, as oolong tea drinkers (1–5 cups/day) had higher bone mineral density (BMD) in the calcaneus compared with nontea drinkers [74].In relation to diabetic conditions, OTP, EGCG,and EGCG3”Me, all exhibited inhibitory effects against pancreaticα-amylase with half inhibitory concentration (IC50) values of 0.375,0.350, and 0.572 mg/mL, respectively.Moreover, induced red-shift of UV absorbance and quenching of fluorescence ofα-amylase were also observed [12].In the context of cardiovascular disorders, OTF showed inhibitory activities against angiotensin I-converting enzyme(ACE) with the IC50of 58.32 μg/mL [45].

4.Discussion and conclusion

Regular tea consumption shows great benefits to human health,including preventing chronic diseases, reducing inflammation,preventing cancer, and even delaying aging [75].In tea, phenolics are the main bioactive components, which greatly contribute to human health.As partially fermented, oolong tea contains high levels of catechins, and great alterations in polyphenols often lead to the differences in active metabolites.Consequently, health benefits of oolong tea are closely related to the composition of polyphenols [18].In particular, the beneficial effects of oolong tea are mainly attributed to its catechins, theasinensins and polysaccharides.

Many studies have been focused on the prevention and management of cancer by the use of natural products, especially polyphenols and polysaccharides, which show better effects and lower toxicity.The boiling water extracts of oolong tea have a great potential as a chemo-preventive agent against breast cancer, as indicated by anticancer effects comparable to green tea [43].EGCG and ECG, the most concentrated catechins in green tea, greatly contribute to the health benefits, which might also be the main active components in oolong tea [76-78].ECG and EGCG can interfere with multiple cell signaling pathways, such as Nrf2/ARE, ERK, and cAMP signaling pathways [79-83].In different cancer cell models, green tea EGCG and ECG show great potentials in modulation of cell cycle progression via regulating proliferative genes and growth factors, and/or inducing apoptosis through activation of pro-apoptotic proteins, which might also be the main mechanism for oolong tea EGCG and ECG [84-90].EGCG could sensitize the efficacy of several chemotherapy agents,such as vardenafil and cisplatin [91].The combination of EGCG with vardena fil showed great potentials after tumor resection through killing the remaining cancer cells and preventing their recurrence [46].Moreover, the combination of EGCG and cisplatin could strongly repress tumor growth and exhibit protective effects on the nephrotoxicity [46].Interestingly, oolong tea derived polysaccharides and polyphenols showed synergistic effects towards tumor growth in a hepatocellular carcinoma model [42].

Though oolong tea contains many compounds, the putative antiobesity effects are mostly attributed to the polyphenolic fraction,specifically tea catechins [92].For ECG and ECG3”Me isolated from oolong tea leaves, ECG3”Me showed a relatively higher antiobesity effect and lower cytotoxic activity in 3T3-L1 preadipocytes as compared to ECG [47].Another study showed that oolong tea polyphenol-phospholipid complexes reduced obesity in a high fat diet-induced mice model [48].OTC might have prebiotic-like activity to prevent obesity-related metabolic disorders effectively.In a mouse model, phospholipids in OTC promoted the intestinal transport of polyphenols [48].Oolong tea may reduce body weight also via inhibition of pancreaticα-amylase activity [12].As a major oolong tea ingredient, TSA showed antihyperglycemic and hypotriacylglycerolemic effects via a decrease of serum triacylglycerol [53].Oral administration of TSA effectively suppressed intestinal fat absorption and glucose availability through inhibition of carbohydrate digestion and absorption [53].In rat skeletal muscle cells (L6 myotubes), antihyperglycemic activities of TSA and TSB were performed through glucose transporter 4 (GLUT4) translocation to plasma membrane and increased glucose uptake through CaMKK/AMPK signaling pathway [79].The number of galloyl moieties might contribute to the cellular glucose uptake, with the abilities of TSA >TSB = EGCG > EGC [93].

SinceS.aureushas the ability to evolve and become resistant to nearly all antibiotics currently used, MRSA became a significant clinical challenge in treating serious infections worldwide [94].For oolong tea, prominent antibacterial and intestinal microbiota modulating activities had been demonstrated.In particular, TSA showed great potentials towardsS.aureus.Interestingly, OTP regulated mouse circadian rhythms through exerting influence on host gut microbiota, as microbiome is a key element involved in maintaining circadian rhythms [61].Oolong tea may contain certain anticaries substances which could affect the virulence ofS.sobrinusother than glucosyltransferases [62].Galloylated catechins might agitate structural feature of bacterial cellular membrane by impairing the microbial cell wall [18].EGCG might inhibit penicillinase, which could degrade penicillin, therefore EGCG could harm bacteria through prooxidative mechanisms [95].Antibacterial activity of oolong tea extracts may also be caused by a synergistic effect of monomeric polyphenols, as they can easily bind to proteins [64].

Oxidative stress is caused by an imbalance between the antioxidant defense and an increase in prooxidative free radicals, which play important roles in the pathogenesis of alcohol-induced liver injury [22].Therefore, antioxidant properties of oolong tea might attribute to protective effects against ethanol-induced lipid peroxidation.Oxidative stress, chronic inflammation and hypertension could lead to cardiovascular diseases and cancer [45].It had been demonstrated that flavonoids could lower blood pressure, and inhibit both cancer cells growth and metastasis of tumor cells via apoptosis [96].Oolong tea contains unique polyphenols, such as OFA, which may prevent atherosclerosis by reducing oxidative stress [72].In apoB-100, cysteine, histidine, lysine, and tyrosine residues are main targets of ROS and RNS, while oxidative modification of apoB-100 may deteriorate the function of apoB-100 as ligand to LDL receptors [72].The inhibition of peanut oil oxidation and DPPH radical scavenging activity by oolong tea water extracts were mainly associated with polyphenol concentrations, thus emphasizing their preventive role as antioxidants [97].Noteworthy, the antioxidant activity of oolong tea infusions increased with the elevation of extraction temperature and duration of soaking [97].

Pathological heart conditions induced by hypoxia followed by reperfusion often results in adverse cardiac hypertrophy and apoptosis.Protective effects of oolong tea against cardiomyocyte loss have been demonstrated via inhibition ofp-JNK, thus fortifying the NRF2 antioxidative system [73].Oxidation of low-density lipoprotein (LDL) in blood by ROS and RNS is often associated with the onset of cardiovascular diseases, such as atherosclerosis.Oolong tea with its unique OFA could reduce lipid peroxidation and oxidative modification of apoB-100 in LDL, therefore preventing atherosclerosis [72].Furthermore, protective effects of EGCG3”Me against alcohol-induced liver injury were possibly due to its antioxidative effects [22].EGCG, as one of the main bioactive catechin in oolong tea, might contribute mostly to the ethanol and CCl4-induced liver injury [98].

In a mouse model, oolong tea derived polysaccharides and polyphenols showed synergistic effects in improving antioxidant and immune responses [46].Besides their immunomodulatory effects, tea polysaccharides have the ability to inhibit pathogenic bacteria adhesion [87-89].OTPS1 in combination with OTP might be functional supplements for the treatment of hepatocellular carcinoma (HCC) [42].In mice, the antioxidative and immune levels were significantly increased after the combinatory administration of OTPS1 (more than 80 kDa) and OTP [46].Synergistic effects of tea polysaccharides and polyphenols were also observed by the reduction of serum leptin levels and anti-inflammatory activities [99].The approach of synergism to enhance the antioxidant,anti-diabetic, and antitumor activities was demonstrated by a study where catechins were grafted onto polysaccharides (chitosan, dextran,and inulin) [60].Noteworthy, the stability of OTC was higher than OTPin vitro[48].The catechin-polysaccharide complex was vital in the immunomodulating activity of tea extracts, as uronic acid in TPS might have contributed significantly to its effect [100-102].In particular, high uronic acid content (32.96%), viscosity (239.56 mL/g),and multicavity structure of OTPS1 all contributed significantly to the synergistic effects [46].

In summary, as one of the most widely consumed beverages in the world, health-related benefits of tea have gained great recognition from both consumers and scientists.The bioactive constituents make tea an ideal functional food.Oolong tea has high efficiency against oxidation, inflammation, endothelial dysfunction, and cancer.It also provides cardio-protective benefits during hypoxic conditions, is effective in treating allergic disorders, has potential prebiotic activity,can improve glucose and lipid blood chemistry, and demonstrates a promising dietary intervention strategy to reduce the risk of obesity.Therefore, oolong tea might be also a promising candidate as an effective adjunct to oral hypoglycemic agents in the treatment of type 2 diabetes.Moreover, the induction of apoptosis by TSA could provide an important mechanism for the cancer chemopreventive function of oolong tea.Oolong tea with its high content of bioactives represents a very promising health-promoting agent with a great potential for the food and pharmaceutical industry.In future, welldesigned randomized controlled trials focusing on potential health beneficial effects of oolong tea in humans should be assessed for potential therapeutic applications.

Conflicts of interests

The authors declare that they have no competing interests.

Funding acknowledgement statement

This research reported in this paper are funded by Hubei Science and Technology Plan Key Project (G2019ABA100), Open fund of Hubei Key Laboratory of Economic Forest Germplasm Improvement and Resources Comprehensive Utilization (201932103), and fund from Assessment and Comprehensive Utilization of Characteristic Biological resources in Dabie Mountains (4022019006).