Yo Cheng, Jichen Sun,*, Hui Zho*, Hongxing Guo, Jinying Li

a School of Biotechnology and Food Science, Tianjin University of Commerce, Tianjin 300134, China

b The Third Central Clinical College, Tianjin Medical University, Tianjin 300170, China

Keywords:

Functional mechanism

Stem cell

Camellia sinensis

Bioactive compounds

A B S T R A C T

Camellia sinensis (tea), one of the most popular commercial crops, is commonly applied in all parts of the world.The main active ingredients of tea include polyphenols, alkaloids, polysaccharides, amino acids,aroma and volatile constitutes, all of which are potentially responsible for the activities of tea.Stem cells(SCs) are the immature and undifferentiated cells by a varying capacity for proliferation, self-renewal and the capability to differentiate into one or more different derivatives with specialized function or maintain their stem cell phenotype.Herein, a thorough review is conducted of the functional mechanism on SCs by tea bioactive compounds.

1.Introduction

Camellia sinensis, namely tea, is one of the most popular commercial crops across the world.It originates from the Yunnan-Guizhou Plateau located in the southwest of China and has been cultivated in various Asian countries such as China, Japan, India and Thailand for over 5 000 years to date [1,2].Tea has now received growing popularity on a global scale due to its attractive flavor and excellent bioactivity.Similar to most crude extracts from natural materials, tea is a complex mix of various compounds.Generally,polyphenols, alkaloids, polysaccharides, amino acids, aroma and volatile constitutes are the main active ingredients contained in tea [3,4].The concentrations of individual constituent contrast differ on plant variety, climatic conditions, season, the conditions of growth, the age of leaves, and the process methods post harvesting.Over the past several decades, research has made some pivotal progress on biological activities of tea, which has been demonstrated as capable to possess a variety of biological activities, for example, antioxidant,anti-diabetic, anti-inflammatory, antibacterial, lipid lowering, antiatherosclerosis, anti-hyperlipidemia activity and others [5,6].

To be specific, stem cells (SCs) are those immature and undifferentiated cells generally characterized by a varying capacity of growth (“immortal” in the case of embryonic SCs) and the capacity of differentiating into one or more different derivatives with specialized function or maintain their SCs phenotype (i.e., selfrenewal) [7].There are 4 defined types of SCs: 2 physiological types that are present at different stages of life-embryonic SCs and adult SCs; engineered or “induced” type (induced pluripotent SCs); and pathological type (SCs present in cancer).SCs do not age and can self-renew indefinitely while maintaining their capacity to differentiate into all cell lineages [8].Throughout the lifespan,SCs play a key role in both the generation and regeneration of the tissues in all living organisms including humans.SCs show a unique potential to restore health because they not only replenish the tissue they originally resided but also may contribute to repair another tissue.For example, a single neural stem cell is capable to generate various distinct cells within the central nervous system, including neurons, astrocytes, and oligodendrocytes [9].Mesenchymal SCs possess the capability of differentiating into mesoderm adipocytes and osteoblasts and the potential to regulate osteoclast differentiation [10].At the present time, adipose-derived SCs have been verified as more robust tools used for regenerative medicine due to their ease of isolation, wide proliferation capacity, lower donor-site morbidity and hypoimmunogenic nature [11].Nevertheless, classed as pathological type SCs, cancer SCs have such capabilities as self-renewal and multipotent differentiation, and are involved in chemo-resistance as well as the relapse of malignancies [12,13].

For the past few years, an increasing number of studies suggested that the extracts and bioactive ingredients obtained from tea can produce a variety of therapeutic effects on SCs, for instance,suppressing the characteristic and self-renewal, promoting the differentiation and proliferation of SCs.In this review, we discuss recent advances of functional mechanism on SCs by tea bioactive compounds (Fig.1).

Fig.1 An overview of the functional effects on stem cells by tea bioactive compounds.

2.Main bioactive compounds of tea

Up to now, there have been various constituents isolated and identified from tea, including polyphenols, alkaloids, polysaccharides,amino acids, aroma and volatile constitutes.

The most desirable effects produced by teaare attributed to the tea polyphenols, which make up 35% of the dry weight of tea leaves.Polyphenols includes catechin, epicatechin, catechin gallate, epigallocatechin (EGC), epigallocatechin gallate (EGCG),gallocatechin gallate, flavonoids and phenolic acid.It has been reported that tea contains about 2%–5% caffeine and much lower concentrations of theobromine and theophylline.Alkaloid is considered as a variety of neuroactive ingredient that has impact on the central nervous system.Besides, polysaccharides as a structurally diversified class of macromolecules, play various essential roles in different biological processes.Tea polysaccharides also represent the main components contained in tea extracts.Amino acids are essential precursors of aromatics in tea and play an important role in the aromatic quality of tea.Aroma and volatile compounds are the significant influencing factors in the quality of tea, especially for the sensory properties of each tea type.Additionally, tea contains a certain amount of saponins (theasaponins), trace elements (Cr, Mn,Se, and Zn), chlorophyll and other nutrients.

3.Therapeutic effect of compounds in tea on SCs

3.1 Cancer stem cells (CSCs)

In recent decades, CSCs have been confirmed to play a key role in the initiation, unlimited growth, recurrence, and metastasis of cancers.The active ingredients and action mechanisms are summarized in Table 1.

Table 1Action mechanisms of C.sinensis for prevention and therapy of CSCs.

In a recently-conducted study, tea is confirmed as a functional food in inhibiting the propagation of breast CSCs, with an IC50of 0.2 mg/mL, in tissue culture [14].Tea polyphenols, the main compounds of tea, can inhibit Wnt signaling in breast CSCs [15].Furthermore,tea polyphenols can prevent and treat prostate cancer by changing the characteristics of SCs, with particular molecular mechanisms, such as influencing tumor growth, apoptosis, androgen receptor signaling, cell cycle, and various malignant behaviors [16].Tea polyphenols also can effectively inhibit pluripotency maintaining transcription factors(including Nanog, c-Myc, and Oct-4), thereby decreasing the selfrenewal capacity of pancreatic CSCs.Besides, it also inhibits epithelialmesenchymal transition by decreasing Snail, Slug, and ZEB-1,diminishing the migration and invasion of pancreatic CSCs [17].Additionally, tea polyphenols can also lead to synergistic inhibitory effects on the self-renewal capacity of pancreatic CSCs through the attenuation of TCF/LEF and Gli activities, apart from inhibiting prostate CSCs characteristics, invasion, migration and epithelial mesenchymal transition [18].

Many studies have been carried out to indicate that EGCG, one of the main active ingredients of tea, could modify the properties of breast CSCs by modulating STAT3-NFkB signaling pathways [19].EGCG inhibits the growth of ER-negative human breast CSCs through down-regulating of ER-α36 expression, suggesting that EGCG treatment can extend the survival of mammary cancer patients [20].In addition, EGCG demonstrates an inhibitory effect on the characteristics of breast CSCs via selectively blocking signal transducer and activator of transcription 3 phosphorylation and down-regulating expression of the key cell surface biomarkers for tumor initiating cells in breast cancer [21].Activation of AMPK by EGCG resulted in inhibition of cell proliferation,up-regulation of the cyclin-dependent kinase inhibitor p21, downregulation of mTOR pathway, and suppression population of human breast CSCs [22].The combination of EGCG and quercetin attest an inhibitory effect on the characteristics of human prostate CSCs [23], while EGCG alone can reduce the viability of human CSCs in primary and secondary spheroids in a dose dependent manner [24].EGCG and quercetin synergistically inhibit the property of self-renewal of prostate CSCs, induce apoptosis,and block the migration and invasion of CSCs.Furthermore,the application of EGCG alone or combined with quercetin possess the ability of eliminating cancer stem cell-properties [25].EGCG and extracts of tea can inhibit epithelial-mesenchymal transition in lung cancer cells, increase cell stiffening, and suppress the self-renewal of lung CSCs, thus leading to the apoptosis of lung CSCs [26].Moreover, EGCG effectively diminish lung CSCs activity by inhibiting tumorsphere formation, decreasing lung CSCs markers (CD133, CD44, ALDH1A1, Nanog, and Oct4), suppressing proliferation and inducing apoptosis, which demonstrates that EGCG can inhibit the proliferation of lung CSCs and further induce the apoptosis [27].EGCG also inhibits CSCs-like properties by targeting hsa-mir-485-5p/RXRα in lung cancer [28].Also, EGCG can inhibit the self-renewal potential of hepatoma and colon CSCs, downregulate the expression of ATP-binding cassette transporter genes and lower the levels of never in mitosis A (NIMA)-related kinase 2(Nek2) [29].EGCG is confirmed as effective in inhibiting colorectal CSCs by down-regulating the activation of Wnt/β-catenin pathway.Moreover, EGCG also inhibits the spheroid formation capability of colorectal cancer cells as well as the expression of colorectal CSCs markers, along with suppression of cell proliferation and induction of apoptosis [30].Besides, EGCG inhibits the self-renewal capacity of head and neck squamous carcinoma CSCs by suppressing their sphere forming capacity, and attenuating the expression of stem cell markers,such as Oct4, Sox2, Nanog and CD44 [31].EGCG also inhibits nasopharyngeal CSCs self-renewal and migration and reverses the epithelial mesenchymal transition via NF-κB p65 inactivation [32].In a recent study, EGCG and theanine can modify self-renewal properties of CSCs, which can influence the balance between proliferative and quiescent cells by regulating critical feedback molecules in the network including dickkopf 1 and secreting frizzledrelated protein 2 [33].EGCG inhibits the CSCs phenotype by downregulating of STAT3-NF?B signaling [34].

As a minor constituent of tea, quercetin possess the capability of reversing multidrug resistance in breast cancer cells by eliminating breast CSCs mediated via Y-box binding protein 1 nuclear translocation and down-regulating P-glycoprotein expression [35].Moreover, some studies have demonstrated that breast CSCs (CD44+/CD24?) could be suppressed by quercetin through inhibiting the cell viability, clone formation, ammo sphere generation, nude mice tumor metastasis and the expression of CyclinD1 and B cell lymphoma-2,and enhancing Bcl-2-like protein-4 expression [36].Quercetin can alter the properties of pancreatic CSCs by affecting self-renewal potential, ALDH1 activity, apoptosis induction, inhibition of angiogenesis, NF-κB and epithelial-mesenchymal transition (EMT)processesin vitroand in mice [37].By the induction of apoptosis,quercetin could inhibit the growth of pancreatic CSCsin vitroandin vivo[38].Quercetin possesses a unique capability to suppress the nuclear translocation ofβ-catenin, downstream of the Wnt signaling pathway, one of the therapeutic targets of pancreatic CSCs [39].By down-regulating the expression of Bcl-2 and XIAP, phosphorylation of FKFR and activating caspase-3, quercetin can reduce the self-renewal capacity of pancreatic CSCs [40].The mechanism investigation of the inhibitory effect of quercetin on the growth of human gastric CSCs revealed the induction of mitochondrialdependent apoptosis through the inhibition of phosphoinositide 3-kinase/Akt signaling [41].

3.2 Neural stem cells (NSCs)

NSCs are the pluripotent cells with the capability of self-renewal and differentiation into neurons and glial cells, which are required to replace dead neural cells and repair the damage caused to the nervous system.The effects of tea constituents on NSCs are detailed in Table 2.

Table 2Action mechanisms of C.sinensis for prevention and therapy of NSCs.

Some studies have been conducted to suggest that different concentrations of tea polyphenols (10, 20 and 40 μg/mL) could enhance the rate of NSCs survival rate and protect the integrity of cell morphology, among which the protective effect of 20 μg/mL tea polyphenols on damaged cells were extremely significant(P< 0.01) [42].EGCG can also promote proliferation and neurospheres formation in isolated mouse cochlear NSCsin vitro,at a non-toxic concentration by PI3K/Akt signaling pathway [43].Specifically, EGCG can promote NSCs proliferation and inhibit the free radical-induced degradation of NSCs, leading to the potential to differentiate into neurons and glia around the damaged area following traumatic brain injury [44].In addition, both theanine and caffeine at a high dose (348 and 50 μg/mL, respectively) failed to inhibit neurospheres adhesion or migration activities.EGCG possess a unique capability to suppress neurospheres adhesion at a concentration of 20 μg/mL.In the neurospheres migration assay, cell migration and neurite outgrowth from neurospheres were significantly reduced with EGCG treated for 72 h, suggesting that EGCG has a potential impact on NSCs survival or differentiation [9].EGCG also can increase the number of NSCs around the damaged area after rat traumatic brain injury [45].

Theanine, a unique free amino acid in tea, can promote the proliferation of NSCs and their differentiation into neurons through a mechanism relevant to up-regulation ofSlc38a1gene in undifferentiated neural progenitor cells [46,47].Recent research also showed that theanine attenuate isoflurane-induced NSCs injury, increase NSCs viability and proliferation, decrease NSCs apoptosis, and improve learning and memory function in young mice.Additionally, it has demonstrated that theanine pretreatment provides neuroprotection against isoflurane-induced injury in NSCs and cognitive impairment in young mice possibly by activating Akt/GSK-3β pathway [48].

3.3 Mesenchymal stem cells (MSCs)

MSCs are the adult pluripotent progenitor cells, could develop into osteoblasts, adipocytes, chondrocytes, and myocytes [49].Table 3 pools the mechanisms of bioactivity ingredients to MSCs.

Table 3Action mechanisms of C.sinensis for prevention and therapy of MSCs.

According to some studies, tea polyphenols can produce a cytoprotective effect on oxidative-stressed bone marrow-derived MSCs, promote osteoblastogenesis, suppress osteoclastogenesis and activate the differentiation of MSCs into osteoblasts rather than adipocytes by modulating the signaling pathways [50].Tea polyphenols can induce mouse bone marrow MSCs for the differentiation into neuron-like cellsin vitro[51].Plenty of research indicated that catechins, the main compounds of tea, are the most potent tea polyphenols evaluated in promoting adipocyte differentiation in human bone marrow MSCs in a dose-dependent manner [52].Catechins also can enhance osteogenesis by upregulating the level of protein phosphatase 2A (PP2A) that inhibits the dephosphorylates ERK kinase (MEK) and extracellular signalregulated kinase (ERK) signaling in human MSCs [53].Additionally,some studies have indicated that the stimulatory effects of EGCG on osteogenesis of MSCs may be one of the mechanisms behind the phenomenon that tea drinkers show a higher level of bone mineral density [54].EGCG could increase the formation of mineralized bone nodules by human osteoblast-like, SaOS-2 cells, enhance osteogenesis in a bone marrow MSCs, and induce apoptotic cell death of osteoclasts differentiated from RAW 264.7 cellsin vitro[55].In addition, EGCG enhances osteogenic differentiation of human bone marrow MSCs by increasing the mRNA expression of BMP2, Runx2,alkaline phosphatase (ALP), osteonectin and osteocalcin [56].EGCG can inhibit mitogen-activated protein (MAP)/ERK kinase (MEK)and ERK signaling as a result of the increasing the level of protein phosphatase PP2A in human MSCs to enhance osteogenesis [53].EGCG can also enhance the osteogenic differentiation of murine bone marrow MSCs and inhibit the osteoclastogenesis in RAW264.7 cells by the modulation of the receptor activator of nuclear factor-κB/RANK ligand/osteoprotegrin pathway [57].EGCG ameliorates the hypoxia-induced apoptosis and osteogenic differentiation reduction of MSCs via up-regulating miR-210 [58].EGC, one of the bioactive constituents of tea, has been identified as effective in promoting osteogenesis in MSCs by improving ALP and calcium deposition by 2.3 and 1.7 times, respectively.Furthermore, the mRNA expression of bone formation markers runt-related transcription factor 2, ALP,osteonectin, and osteopontin are improved [49].

Caffeine, the bioactive ingredient from tea, produces an inhibitory effect on the osteogenic differentiation of MSCs derived from the offspring of rats during pregnancy and lactation by significantly reducing the activity of alkaline phosphatase and the expression of collagen I [59].Recent study observed that caffeine can inhibit the adipocyte differentiation in MSCs by suppressing the expression of adipogenic genes, and reducing lipid storage and accumulation in 3T3-L1 adipocytes [60].The least effective dose concentration of caffeine that can induce potent anti-inflammatory property in the MSCs population is 0.5 mmol/L.At this concentration, caffeine could induce lower levels of IFN-γ, IL-6, and IL-1β and a higher level of IDO, TGF-β, and IL-10 compared to other groups [61].Conditioned medium of MSCs treated with caffeine significantly enhanced phagocytosis and simultaneously regressed expression of reactive oxygen species and nitric oxide as well as IL-12 by macrophages compared to the supernatants of MSCs alone [62].

Furthermore, quercetin promotes the osteogenic differentiation of rat MSCs via mitogen-activating protein kinase signaling, thus increasing the protein expression levels of alkaline phosphatase,collagen type I and bone morphogenetic protein, the mNRA expression levels of transforming growth factor-β1, bone morphogenetic protein-2 and core binding factor α1 [63].Quercetin has been identified as effective in reducing the tumor necrosis factor-alpha-induced impairments in bone marrow-derived MSCs osteogenesis and improving osteoporosis in rats by inhibiting the activation of NF-κB and the degradation ofβ-catenin [64].Moreover, quercetin can suppress erastin-induced ferroptosis in bone marrow-derived MSCs through antioxidant pathways [65].

3.4 Adipose-derived stem cells (ADSCs)

ADSCs have been verified as more robust tools used for regenerative medicine compared with bone marrow MSCs owing to their ease of isolation, wide proliferation capacity, high yield efficiency, low donor-site morbidity and hypoimmunogenic nature [11].The bioactive constituents obtained from tea are believed as capable to produce a desirable pharmacological effect on ADSCs(Table 4).

Table 4Action mechanisms of C.sinensis for prevention and therapy of ADSCs.

Recent research has demonstrated that EGCG can increase the expression of survival marker in ADSCs under high-glucose environment and reduce serum oxidative stress in diabetic group rats [66].Moreover, EGCG at the concentration of 5 μmol/L can significantly enhance cell proliferation, ALP activity and the osteoblastogenic differentiation of human ADSCs, which is correlated to the considerably lower levels of phosphorylated STAT3, C/EBP-x,and PPAR-γ [67].

Caffeine is confirmed as effective in inhibiting the expression of adipogenesis-related genes including peroxisome proliferator-activated receptor-γ, CCAAT/enhancer-binding protein-α, adipocyte lipid binding protein, lipoprotein lipase, leptin and tumor necrosis factor-α in a dose-dependent manner, as inhibiting the adipogenic differentiation of ADSCs [68].In addition, caffeine can also promote osteogenic differentiation and mineralization of primary ADSCs and a bone marrow stromal cell line by significantly enhancing mineralization and ALP activity, up-regulating the osteogenic differentiation marker genes ALP and osteocalcin, as well as elevating osteoprotegerin, Runt-related transcription factor 2 and Sirtuin 1 levels [69].

By up-regulating the RUNX2-BMP2 mediated osteogenic pathway and suppressing PPARy-induced signaling of adipogenesis,tea polyphenols have been identified as effective in promoting osteogenesis and suppressing ADSCs differentiation into the adipogenic lineage at the same time [70].

4.Perspectives and conclusion

As tea has been receiving growing popularity among ordinary consumers, many productive studies on the isolation, identification and evaluation of chemical compounds in tea have been performed.Emerging studies have indicated that the extracts and bioactive ingredients obtained from tea can produce a variety of therapeutic effects on SCs, for instance, suppressing the properties, growth and self-renewal, promoting the differentiation and proliferation.At present, SCs have been used preliminarily in the therapeutic treatment of some rarely-seen diseases, due to their adjustment and regenerative potentials.This article provides some practical guidance on the clinical applications of tea in SCs.

Declaration of competing interest

The authors have declared no conflict of interest.

Acknowledgments

This study was supported by National College Students Innovation and Entrepreneurship Training Program (201910069007,201910069102) and Tianjin Key R&D Plan-Key Projects Supported by Science and Technology (19YFZCSN00010).