Plants belonging to the family Oleaceae, which consists of 27 genera and 400 species worldwide, have important applications in the daily life of people living in developing countries. Plants of many well-known genera, including Forsythia, Syringa, and Osmanthus, have been widely used for medicinal and industrial purposes. For instance, the stems and roots of S. pinnatifolia var. alashanensis is the major composition of atraditional formula ‘Ba wei chenxiang’ powder that is used for treatment of asthma, cardiopalmus, and angina [1].
Most Syringa plants are deciduous shrubs and arbors and include more than 40 species distributed around Europe and Asia [2]. At present, 22 species are found in China, of which 18 are endemic species that are mainly distributed in the southwestern part of Sichuan, Yunnan, Tibet, and other Northwestern regions. Many Syringa species, such as S. chinensis, S. meyeri, and S. pekinensis, are used for making ornaments. Flowers of S. oblata and S. reticulata var. mandshurica are an ideal source of aroma oils or nectar. Some Syringa plants are also used for construction purposes or for manufacturing furniture [1].
Previous phytochemical studies on Syringa species have revealed the presence of more than 140 secondary metabolites, including iridoids, lignans, phenylethanoids, their glycosides, minor organic acids, and essential oils [3,4]. Modern pharmacological studies have shown the bioactivities of these metabolites, such as antitumor, antihypertensive, anti-oxidant, anti-inflammatory activities, and so on [5]. However, a systematic review of these studies has not been performed to date. This review summarizes the phytochemical and pharmacological progress on Syringa to date by focusing on its chemical classification, structural features, and biological and pharmacological applications to provide information for further research on this genus
Iridoids
Iridoids are one of the most important natural compounds that are widely distributed in various plant families such as Plantaginaceae, Rubiaceae, and Scrophulariaceae [6]. Iridoids are extensively present in almost all Syringa species and have antitumor, antihypertensive, anti-inflammatory, anti-oxidant, and antifungal activities. In addition, iridoids play an important role in defense mechanism of ants [7]. Among all the iridoids reported in this genus, secoiridoids are the most abundant and have been shown to have antitumor activity. To date, 46 iridoids (1–46) have been described, including secoiridoids (1–30 and 40–44), eight typical iridoids (32–39), and three minor dimers (31, 45, and 46). Most iridoids exist as glycosides and are mainly produced by the glycosylation of glucose and galactose. Syringa iridoids are generally substituted by various acid fragments and phenolic moieties such as 1-O-cinnamoyl-β-d-glucopyranosyl, p-hydroxphenethyl, 3, 4-dihydroxy-phenethyl, and caffeic acid, which contribute to their low polarity. Syringa iridoids have antitumor (33 and 40) [8,9], antihypertensive (4), and anti-oxidant (4 and 31) activities [10].
Lignans
Lignans are another major compounds in this genus, particularly in S. komarowii [27], S. pubescens [3], S. reticulata [10], S. velutina [28], S. patula [5], S. vulgaris [29], S. pinnatifolia var. alashanensis [30,31], and S. reticulata var. mandshurica [32]. Syringa species have 34 lignans and their glycosides (47–80), including monoepoxylignans (47–60, 62) and their dimers (63 and 64), neolignans (61, 73–74), cyclolignans (65 and 66), simple lignans (67–72), and bisepoxylignans (75–80). Lignans also exhibit many bioactivities. For example, compound 50 has anti-oxidant activity [10]; compounds 57 and 58 have antifungal activities [32]; and compound 75 has significant cytotoxic, antihypertensive, anti-inflammatory, and anti-oxidant activities [5].
Other compounds
Phenylethanoids (81–105), phenylpropanoids and their analogues (106–121), flavonoids (122–128), sesquiterpenes (129 and 130), and other minor compounds have been described in Syringa plants. Of these, phenylethanoids are predominant, particularly in S. reticulata [10,12,35], S. vulgaris [29], S. pubescens [3], S. oblata var. alba [36], S. reticulata var. mandshurica [35], S. afghanica [13], and S. komarowii [27]. Sesquiterpenes (129 and 130) are present in the stems of S. pinnatifolia var. alashanensis [37]. These miscellaneous compounds have cytotoxic, anti-inflammatory, antihypertensive, anti-oxidant, and antifungal properties.
Besides the abovementioned compounds, Syringa plants contain essential oils that form the most important constituents not only because of their economic utility but also because of their potential medicinal value as antimicrobial, antipyretic, and antiviral agents. Multiple analytical techniques such as headspace solid-phase microextraction, gas chromatography–mass spectrometry (GC–MS), GC–MS coupled with heuristic evolving latent projections, moving subwindow searching, nuclear magnetic resonance spectroscopy, and X-ray single-crystal diffraction analysis have been used to identify essential oils from fresh flowers of S. oblata var. alba. For instance, 39 volatile oil constituents were identified, including four characteristic isomers of lilac alcohols (lilac alcohols A–D) and lilac aldehydes A–D [38]. Ninety-five components, including 15 terpenes, 14 oxygenated terpenes, 10 aromatic compounds, and 13 n-alkanes were quantitatively analyzed from S. oblata buds [39]. Forty-nine components were described from essential oil of S. pubescens flowers, most of which are monoterpenes and sesquiterpenes [40]. Thirty-four volatile oil components, accounting for around 64.7% (zerumbone) of the toil oil, were identified from roots and barks of S. pinnatifolia var. alashanensis [4]. These data imply that Syringa plants could be considerably different from each other in terms of their essential oil components.
Pharmacological activities
Various crude extracts and isolated compounds from Syringa plants have shown significant antitumor, antihypertensive, anti-inflammatory, anti-oxidant, and antifungal activities.
Antitumor activity
Cytotoxic activities of crude extracts and chemicals obtained from Syringa plants have been extensively evaluated against various tumor cell lines. Aqueous extracts from the flowers and leaves of S. pubescens inhibited the growth of L2215 (hepatitis B virus) cells, with a 50% inhibitory concentration (IC50) value of 78 μg/mL [51]. Hydrolysis of isoligustroside (1) and isooleuropein (2) were assayed using a disease-oriented panel of 39 human cancer cell lines. The results showed that the hydrolysis product of compound 2 had moderate cytotoxic activity against lung cancer cell lines DMS273 [log GI50 = 5.19 (6.4 μM)] and DMS114 [log GI50 = 5.06 (8.7 μM)]. Preliminary analysis of structure–activity relationship suggested that C-5′-OH plays an important role in this cytotoxic activity [11]. Isooleoacteoside (40) showed weak cytotoxicity against LOX-IMVI melanoma cell line, with GI50 value of 16 μM, and syringopicroside B (33) showed weak cytotoxic activity against NCI-H522 lung cancer cell line, with GI50 value of 13 μM [9]. MTT assay used to assess the cytotoxicities of syringaresinol (78) and oleoside 11-methyl ester (3) showed that compound 78 had a strong dose-dependent effect on HepG2 cell line, with an IC50 value of 94.6 μM, and compound 3 has a dose–response curve of low slope, with a high IC50 value of 186.5 μM, compared with positive controls dexamethasone (IC50 14.2 μM) and paclitaxel (IC50 700 nM). However, compound 78 was cytotoxic even at the lowest concentration of 29.9 μM. β-Amyrin acetate (139) showed weak cytotoxicity against A2780 human ovarian cancer and HepG2 cell lines [5]. Oleuropein (4) and 2-(3, 4-dihydroxy)-phenylethyl-β-d-glucopyranoside (83) showed evident cytotoxicities against P-388, L-1210, SNU-5, and HL-60 cell lines, with IC50 values varying from 8.5 to 139.8 μM [12]. Verbascoside (86) showed moderate cytotoxic activity against SNB-75 (brain cancer) and SNB-78 cell lines, with GI50 values of 7.4 and 7.7 μM, respectively [9]. A pharmacokinetic study showed that compound 86 interacted with the catalytic domain of PKC and acted as a competitive inhibitor of adenosine triphosphate (Ki = 22 μM) and non-competitive inhibitor of phosphate acceptor (histone III). Because 83 is one part of 86 in its molecular structure, the cytotoxic effect could be attributed to 3, 4-dihydroxyphenylethoxy moiety, which may act as a competitive inhibitor to the catalytic domain of PKC. Therefore, 83 is a potentially essential skeleton of most cytotoxic phenylethanoid glycosides [12].
Hypotensive activity
Syringin (110) and kaempferol-3-O-rutinoside (125) showed antihypertensive activity. Intravenous injection of 10 mg/kg of compound 86 significantly decreased systolic, diastolic, and mean arterial blood pressure in Pentothal-anesthetized rats. Moreover, the depressor effect of compound 86 was independent of muscarinic and histaminergic receptors because it did not block the effect of atropine (an antimuscarinic agent) and chlorpheniramine/cimetidine (antihistaminergic agents) [36]. In vitro studies showed that oleuropein (4) significantly lowered blood pressure. It is interesting to note that antihypertensive effect of compound 4 (33% at 30 mg/kg dose) on the blood pressure of anesthetized rats was similar to that of compound 86 (39.04% ± 2.38% at 10 mg/kg dose) [14,36], which is probably because of the similarity in their structures, with both possessing the same aromatic fragment having two hydroxy groups.
Anti-inflammatory activity
Iridoid glycosides (IGs) exerted obvious anti-inflammatory effects on ulcerative colitis in vivo by inhibiting relative proinflammatory cytokines [53]. IGs significantly ameliorated macroscopic damages and histological changes, reduced the activity of myeloperoxidase, and strongly inhibited epithelial cell apoptosis. Moreover, IGs markedly decreased the levels of tumor necrosis factor-α, interleukin-8, cyclooxygenase-2, and transforming growth factor-β1 in colonic tissues in a dose-dependent manner. Moreover, effects of IGs (160 and 240 mg/kg) were superior to those of positive control salicylazosulfapyridine (150 mg/kg). Furthermore, IGs significantly blocked NF-κB signaling by inhibiting inflammatory bowel phosphorylation/degradation and inhibitor kappa B kinase β activity; downregulated protein and mRNA expressions of Fas/FasL, Bax, and caspase-3; and activated Bcl-2 in intestinal epithelial cells [53,54]. β-Amyrin acetate (139) and syringaresinol (78) at a dose of 20 μg/mL evidently inhibited lipopolysaccharide-induced nitric oxide (NO) production, with inhibition rates of 49.97% and 33.21%, respectively [5].
Liver-protective and cholagogic effects
Crude extract of Syringa species, interferon (IFN), and an injection of “Gan-Yan-Ling” were compared to evaluate their liver-protective effects on the survival rates of HepG2.215 cells and secretion of hepatitis B surface antigen (HBsAg) and HBeAg. The results indicated that all the three assayed drugs may suppress the secretion of HBsAg and HBeAg from HepG2.215 cells in a dose-dependent manner, with the effect of crude extract of Syringa being intermediate those of IFN and Gan-Yan-Ling. Therefore, extracts of Syringa plant could be used to develop effective and less toxic antihepatitis B medicines [55].
Aqueous extracts of S. reticulata var. mandshurica significantly decreased the levels of alanine transaminase and aspartate transaminase and the concentration of malondialdehyde in the serum but increased the activity of superoxide dismutase (SOD) in the liver. These extracts showed protective effects on acute liver injury induced by CCl4 in mice [56]. In addition, the essential oils of Syringa exerted protective effects on the liver and cholecyst [39].
Antifungal activity
Phenylpropanoids such as verbascoside (86) and forsythiaside (82) exhibit significant antimicrobial activity [29]. Compounds 93 and 94 at 1- mM concentration inhibited the radial growth of Phytophthora capsici after 6 days of incubation, with inhibition rates 59.1% and 72.5%, respectively [43]. Two sesquiterpenes, guai-9-en-4β-ol (129) and 4, 15-dinorguai-1, 11-dien-9, 10-dione (130), have antibacterial and antifungal properties. Compound 129 was active against Bacillus coagulans [inhibition zone (IZ) = 15.34 mm] and Aspergillus niger (IZ = 13.20 mm) while compound 130 significantly inhibited Escherichia coli (IZ = 15.34 mm) and Fusarium oxysporum (IZ = 15.32 mm) [37].
Compound 3 showed effective antimicrobial activity against Lactobacillus pentosus (IZ = 1 mm), and compound 139 inhibited the growth of Candida species at concentrations of 30–250 μg/mL [5].
Antioxidant activity
A 70% EtOH extract of S. reticulata barks showed potent superoxide anion and DPPH free radical scavenging activities, with EC50 values of 5.88 and 38.10 μg/mL, respectively [10].
Among the compounds isolated from the bark of S. reticulata, six (4, 31, 50, 77, 83, and 111) showed significant superoxide anion scavenging activity, with EC50 values of 2.57, 4.97, 10.64, 15.98, 4.97, and 14.14 μg/mL, respectively. Compound 4 also interacted with the stable free radical DPPH, with an IC50 value of 40.4 μM [8,10]. These different anti-oxidant activities are closely related to their structural features. Presence of 2-(3, 4-dihydroxyphenyl)-ethoxy moiety might be important for a higher activity because the most potent compounds (EC50 = 2.57–4.97 μM), including the two secoiridoid glycosides (31 and 4) and a phenylethanoid glycoside (83), possess the same structural features. Comparison of the structures of compounds 4 and 83 with those of 8(Z)-ligstroside (21) and salidroside (89) showed that presence of ortho-coupling hydroxyl group at C-2 might be responsible for their different activities. It has been previously reported that 1, 2-dihydroxybenzene moiety is crucial to its DPPH scavenging activity [10].
Syringaresinol (78) showed a strong scavenging activity against DPPH, with EC50 value as low as 12.5 μg/mL, which might be responsible for its strong inhibition of NO production [5].
Eugenol (112) inhibited the catalytic activity of H2O2/Ca2+ human erythrocyte membrane lipid peroxidation at a concentration of 200 μmol/L, with an inhibition rate of 62%, and completely suppressed the catalytic activity of dibenzoyl peroxide/Ca2+ human erythrocyte membrane lipid peroxidation at a concentration of 100 μmol/L. Compound 112 exerted its effect in a non-competitive manner by reacting with Ca2+ and inhibiting the formation of hydroxyl radicals, thus, protecting the cell membrane lipid from oxidation [2].
Inhibition of platelet aggregation
Aqueous extract of S. aramaticum significantly inhibited adenosine diphosphate (ADP) and collagen-induced platelet aggregation, with inhibition rates of 37.4% and 69.7%, respectively [57]. Mandshuricols A (57) and B (58) showed antagonistic activities on platelet-activating factor (PAF) in [3H]PAF receptor binding assay, with IC50 values of 4.8 × 10−5 and 3.5 × 10−5 M, respectively [32].
Others
Essential oils from the stems and roots of S. pinnatifolia var. alashanensis (SPEO) reduced the deviation of ST segment; decreased the levels of lactate dehydrogenase, creatine kinase, and troponin T; and increased the activity of SOD. These protective effects were further confirmed by histopathological examination [58]. Treatment with both 8 and 32 mg/kg SPEO prolonged the survival of mice under hypoxia conditions, showing a remarkable protective effect against H2O2-induced death in cultured rat myocytes. Moreover, 5, 2.5 and 1.25 μg/mL doses of SPEO inhibited ADP-induced rat platelet aggregation by 47.4%, 37.0%, and 32.9%, respectively [58], implying that SPEO exerted protective effects against myocardial ischemia.
Oral and intraperitoneal administration of 0.2–0.4 g of leaf extract of S. vulgaris in cats or rabbits exerted an antipyretic effect that was equal to the effect of 0.1–0.3 g of aminopyrine administered orally or intraperitoneally. However, leaf extracts of S. vulgaris are considerably more toxic than aminopyrine, with their toxic dosages being 0.4 and 1.2 g/kg, respectively [59]. In vitro evaluation of leaf extract of S. aramaticum showed its antiviral activity against herpes simplex virus at concentrations 1.25%–2.5%. The protective effect was more obvious when controlling the amount of virus attacks at 9.2–92 tissue culture infective dose (TCID50), suggesting that S. aramaticum effectively killed the virus without any harmful side effects [60-62].
Studies have reported that leaf extracts of S. aramaticum could be used for treating hemorrhoids [63]. Eugenol (112) inhibited the metabolism of arachidonic acid. Extracts of S. reticulata var. mandshurica have been used for treating bronchitis, and one of its constituents 2-(3, 4-dihydroxyphenyl) ethanol (91) significantly inhibited the production of phlegm [2].
Review and conclusions
This review describes phytochemical and pharmacological progress on the genus Syringa in the recent 20 years and discusses the future research prospects.
Syringa plants are used not only as traditional medicines to treat rheumatoid arthritis, asthma, cardiopalmus, and angina pectoris by natives in China but also for making ornaments, volatile oils, food additives, and bactericides worldwide, particularly in developing countries. Previous phytochemical studies on crude extracts from various species of this genus have identified iridoids, lignans, phenylpropanoids, and phenylethanoids having antitumor, antihypertensive, anti-oxidant, and anti-inflammatory activities. Iridoids, lignans, and phenylethanoids are the most predominant compounds in Syringa plants that probably contribute independently or synergistically to their main biological activities.
To the best of our knowledge, 46 iridoid representatives have been reported in Syringa plants, with high concentrations present in the leaves of S. vulgaris, S. pubescens, S. afghanica, S. reticulata, and S. velutina and barks of S. vulgaris and S. reticulata and low concentrations present in the flowers (S. pubescens), seeds, and seeds crust (S. oblata). This difference may be associated with their ecological roles, because iridoids are produced mainly to fight predators and/or microbes. Moreover, high concentrations of lignans in the stems and roots can be attributed to the rigidity of these plants. This may be the reason for the absence of iridoids in S. pinnatifolia var. alashanensis because materials used for chemical investigation included peeled stems and roots. Anti-inflammatory effects of extracts from these plants are mainly responsible for their applications in traditional medicine. However, only preliminary work has been performed on most isolated compounds, such as in vitro cytotoxicity screening (1, 2, 78, and 139). Limited studies have been performed on the in vivo effects of these compounds; thus, providing opportunities for further detailed research. It is particularly worthy to mention that China has an abundant resource of Syringa, with many endemic species. For instance, S. pinnatifolia var. alashanensis is a well-known Mongolian medicine traditionally used for myocardial ischemia in clinical practice. However, no substantial evidence is available on its bioactive ingredients and mechanisms of action underlying this effect. Therefore, it deserves further phytochemical and pharmacological studies.
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