Review

Volume 8, December 2019, pages 42-50

Chemistry, pharmacology and processing method of rhubarb (Rheum species): a review

As a widely used Chinese herb, rhubarb is officially documented in Chinese Pharmacopoeia as the dried root and rhizome of Rheum palmatum L., Rheum tanguticum Maxim. ex Balf. or Rheum officinale Baill. (Polygonaceae) (China, 2015). In traditional Chinese medicine, Rhubarb is effective, short-lived and painless for the treatment of purging accumulation (Qing-yu in Chinese), cooling blood (Liang-xue in Chinese), and draining damp-heat (Qu-nei-re in Chinese).

The components isolated from rhubarb have been classified into five primary categories, namely, anthraquinone, anthrones, stilbenes, flavonoids and acylglucosides (Nonaka et al., 1977; Nonaka et al., 1981; Kashiwada et al., 1984; Kashiwada et al., 1986; Komatsu et al., 2006; Xing-Sheng et al., 2011). Drying, a crucial step in the post-harvest processes, can limit enzymatic degradation and microbial growth while preserving the beneficial properties of the plant material. Generally, during the drying process, the chemical compositions, content and pharmacological activities of the active principles may change, thus affecting the quality of Chinese herbal preparations. (Zhu et al., 2014). Consequently, this contribution summarizes the progress made in the chemical composition, pharmacological activities and processing methods of rhubarb in recent years.

Up to date, a variety of constituents have been isolated from Rheum species with their structures being elucidated. As an important traditional Chinese herb, the components of rhubarb have been classified into five primary categories, namely, anthraquinone, anthrones, stilbenes, flavonoids and acylglucosides.

As an important type of components in rhubarb (compounds 1-30), the free anthraquinones, such as aloe-emodin, rhein, emodin, chrysophanol and physcion, are present in nearly all Rheum species. In addition, the anthraquinone glycosides, such as aloe-emodin-8-O-β-D-glucoside, rhein-8-O-β-D-glucoside, chrysophanol-8-O-β-D-glucoside, emodin-8-O-β-D-glucoside and physcion-8-O-β-D-glucoside, are isolated from different species of rhubarb. In addition to those anthraquinones, the conjugated anthraquinone derivatives are responsible for their cathartic effects (Table 1), isolated from several rhubarbs.

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Table 1. Anthraquinones and anthrone derivatives isolated from different species of rhubarb

Anthrones are less oxygenated than anthraquinones (compounds 31-56). Twenty-six anthrones were isolated from different species of rhubarb (Table 1). Among them, sennosides have been identified from Rheum palmatum L and include a number of anthraquinone derivatives and functioned as laxatives.

Stilbenes, important components in chemotaxonomy (compounds 57-76), have been reported to show inhibit nitric oxide production, anti-bacterial, anti-fungal, anti-oxidant, anti-inflammatory, anti-cancer, and anti-malarial activities. Twenty stilbenes isolated from different species of rhubarb (Table 2) have reported in studies.

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Table 2. Stilbene derivatives isolated from different species of rhubarb

Being a large class of compounds that exist ubiquitously in different species of rhubarb, flavonoids contain several phenolic hydroxyl groups attached to ring structures, conferring antioxidant activity (compounds 77-82). There are four main flavonoids isolated from rhubarbs (Table 3).

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Table 3. Flavonoids and acylglucosides isolated from different species of rhubarb

Acylglucosides are considered to be important bioactive components in rhubarb (compounds 83-94). Twelve acyglucosides have been isolated from rhubarbs (Table 3) with most of them exhibiting excellent pharmacological activities.

Rhubarb, the rhizome part of Rheum species including Rheum palmatum, Rheum tanguticum, Rheum undulatum, and Rheum officinale, is one of the oldest traditional medicines and is most commonly used as a purgative (Siegers et al., 1993). In addition, rhubarb has many other bioactive effects such as antibacterial, analgesic, and anti-inflammatory activities (Darias, 1987; Jong-Chol et al., 1987; Matsuda et al., 2001). Rhubarb possesses anticoagulant and anti-platelet activities have been reported and confirmed to be primarily associated with stilbene derivatives such as resveratrol, rhaponticin, and piceatannol (Aburjai, 2000; Park et al., 2002).

Previous studies have suggested that diarrhea is associated with decreased Na⁺/K⁺-ATPase activity (Ejderhamn et al., 1989). Inhibition of this intestinal enzyme may be critical to the regulation and absorption of Na⁺ and K⁺ in the intestine, leading to intestinal fluid accumulation and contributing to diarrhea (Yakubu and Salimon, 2015). It showed that the purgative activities of the crude rhubarb could be determined approximately by analysis of sennoside content, whereas the correlation between purgative activity and sennoside content is not very strong, suggesting that other factors might be involved (HARIMA et al., 1994). Recent studies on the relationship between the chemical structure and functionality indicated that “Watery Diarrhea” effect induced by rhubarb was concerned with the location alteration or the expression change of Aquaporins (Li et al., 2008).

As have been demonstrated in a number of studies, as the main anthraquinones of rhubarb, emodin, aloe-emodin, and rhein could inhibit the growth and proliferation of various cancer cells. For example, emodin has been reported to inhibit proliferation in breast, lung, cervical, colorectal, and prostate cancers cells (Chan et al., 1993; Zhang et al., 1995; Chang et al., 1996; Kuo et al., 1997; Cha et al., 2005).

Emodin showed less or no cytotoxic effect in several normal cells, including HBL-100 cells derived from normal human breast tissue (Zhang et al., 1995), human fibroblast like lung WI-38 cells, and three primary cultured rat normal cells (Shieh et al., 2004). As have also been demonstrated, normal cells are more resistant to emodin-induced cytotoxicity than cancer cells. Such specificity of emodin towards malignant cells might be due to its effect targeting some oncogene signaling transductions, which are constitutively active or amplified in cancer cells (Chan et al., 1993).

Aloe-emodin was also able to inhibit cell growth in several tumor cells, including human lung carcinoma (Lee et al., 2001), hepatoma (Kuo et al., 2002), and leukemia cell lines (Chen et al., 2004). In addition, it exhibited higher cytotoxicity against oral squamous cell carcinoma and salivary gland tumor than normal human gingival fibroblasts (HGF) (Shi et al., 2001). Interestingly, high specificity for neuroectodermal tumor cells was exhibited in aloe-emodin (Pecere et al., 2003). According to Pecere’ report, energy dependent incorporation of aloe-emodin may correspond to the higher sensitivity of neuroectodermal tumor cells (Pecere et al., 2000).

Rhein, another anthraquinone derivative of rhubarb, has also been reported to display an inhibitory effect on the proliferation of human breast, colon, lung, CNS, and glioma cancer cells (Cichewicz et al., 2004). In a recent study conducted by Zhou et al, rhein exhibited anti-fungal potential but was less cytotoxic than other anthraquinones in rhubarb (Zhou et al., 2006). Also, Huang et al. reported that rhein could effectively inhibit the uptake of glucose in tumor cells and induce cell necrosis (Huang et al., 2007).

Having been compared with other anthraquinone derivatives, such as emodin 1-O-β-D-glucoside, physcion, and physcion 1-O-β-D-glucoside, emodin with C1 and C3 position is believed to be important for the anti-tumor function (Kuo et al., 1997).

The hepatoprotective effect of rhubarb and its components has also been documented as antagonizing α-naphthylisothiocyanate (ANIT)- and concanavalin A-induced experimental liver injury (Mase et al., 2010). A hepatoprotective effect of rhubarb against infantile cholestatic hepatitis and acute icteric hepatitis (Huang et al., 1997) have been revealed by some clinical evidence. Research has shown that free anthraquinones extracted from rhubarb, such as rhein and emodin, exhibited protective activity against ANIT-induced cholestatic liver injury by reducing the serum levels of glutamate-pyruvate transaminase, glutamic oxaloacetic transaminase and total serum bilirubin, direct bilirubin, alkaline phosphatase, γ-glutamyltransferase and total bile acids. The morphological alterations induced by ANIT in rats, including the necrosis of hepatocytes and biliary epithelial cells, as well as neutrophil infiltration and sinusoid congestion, were also alleviated by concurrent intragastric administration of these free anthraquinones (Zhao et al., 2009).

As have been shown in research, via activating NO/cGMP signaling, vascular smooth muscle can be dilated in aqueous extract, whose rhubarb treatment not only suppressed TNF-α-induced increase of proinflammatory and adhesion molecules in HUVECs, but also reduced the adhesion between U937 cells and HUVECs. The aqueous extract of rhubarb might have the potential to dilate vascular tissues and suppress the vascular inflammatory process, which may be closely related to the activation of vascular NO/cGMP signaling (Moon et al., 2006). Emodin exerted an anti-inflammatory effect through blocking of MAPK and PI3K pathway signaling and inhibition of the activation of NF-κB and iNOS expression (Zhu et al., 2011). Meanwhile, following severe acute pancreatitis, emodin reduced the inflammatory response in the rat lung by decreasing the expression of tumor necrosis factor-α (TNF-α) and IL-6 (Zhang et al., 2005). Rhein also showed anti-inflammation effect through inhibiting the expression of iNOS (Wang et al., 2002). Aloe-emodin might inhibit the inflammatory mediators including decreasing the levels of PAF, IL-6, and TNF-α during the acute pancreatitis (AP) in rats (Chen et al., 2010).

Being able to promote blood circulation to remove blood stasis, Rhubarb was used as an ingredient in numerous prescriptions in the medical classic “Shang-Han-Lun” by Zhongjing Zhang (Han Dynasty). In thrombotic diseases, as a multifunctional serine protease, thrombin is generated in response to vascular injury and catalyzes the proteolytic cleavage of the soluble plasma-protein fibrinogen to form soluble fibrin, leading to clot formation. In addition, thrombin also serves as a potent platelet agonist and amplifies its own generation by a several-step feedback activation process as part of the coagulation cascade (Weitz, 2003). With thrombin playing a pivotal role in thrombogenesis, its inhibitors have been used in the treatment of thrombotic diseases, which have been proved to be effective (Hanessian et al., 2008). The anti-thrombin activities of various rhubarb samples have been investigated. The inhibitory activity of smoked rhubarb (inhibition percentage of 56.34 %) was significantly higher than those of the other samples (P < 0.05). Also, IC₅₀ values for the rhubarb samples on thrombin were determined. According to the IC₅₀ values, smoked rhubarb exhibited the best thrombin inhibition activity (IC₅₀ = 6.36 mg/mL) (Sun et al., 2018). The effects of anthraquinone derivatives isolated from rhubarb on platelet activity have been investigated. Among four anthraquinone derivatives isolated from rhubarb examined, chrysophanol-8-O-glucoside (CP-8-O-glc) had the most potent inhibitory effect on collagen- and thrombin-induced platelet aggregation with mice treated by it showing significantly prolonged bleeding times. Furthermore, CP-8-O-glc was found to have a significant inhibitory effect on rat platelet aggregation ex vivo and on thromboxane A2 formation in vitro. In coagulation tests, CP-8-O-glc prolonged the activated partial thromboplastin time instead of altering prothrombin time. However, CP-8-O-glc only inhibited platelet phosphatidylserine exposure without directly inhibiting intrinsic factors. This study has demonstrated the anti-platelet and anticoagulant effects of CP-8-O-glc and suggested that this compound might be of therapeutic benefit for the prevention of platelet-related cardiovascular diseases (Seo et al., 2012).

The aqueous extract of rhubarb has been reported to have an anti-platelet aggregation activity (Ko et al., 1999). Anti-platelet aggregation activity of stilbenes from Rheum undulatum was shown to be affected by the presence of methoxyl and free hydroxy groups of the structure. The inhibition activity on platelet aggregation induced by arachidonic acid is considered to be closely related to arachidonate metabolism. Consequently, these inhibitory effects may partially contribute to anti-blood stagnancy activity of rhubarb (Ko et al., 1999). Research suggested that rhaponticin and rhapontigenin exhibited an inhibitory effect toward platelet aggregation in vitro and ex vivo and protected the mice from thromboembolism. Among these compounds, rhapontigenin, a metabolite of rhaponticin transformed by human intestinal microflora prior to absorption in the intestine, was the more potent. Rhei Rhizoma also exhibited in vitro and ex vivo anti-platelet aggregation activity and protection against thromboembolism. Based on these findings, it is concluded that Rhei Rhizoma, particularly rhapontigenin, could prevent the development of thrombosis or its recurrence (Park et al., 2002). The phytochemical analysis of Rheum palaestinum revealed the presence of two stilbenes: piceid and rhaponticin, which may explain the use of this plant in traditional medicine for its anti-platelet activity as these compounds exhibited significant anti-platelet aggregation activity (Aburjai, 2000).

As one of the most important and fundamental steps in traditional Chinese medicine (TCM), drying operation is employed to maintain bioactive properties of medicinal products and cut down transport expenses as well as facilitating their consumption. The aim of drying TCM is to remove water and consequently prevent microbial and chemical decay, thus improving its shelf-life. As a result, efforts should be put on to maximizing the rate of drying through conveying heat and humidity.

Fresh crude Rheum palmatum was sliced and treated with the different drying methods such as sun drying, shady drying, microwave heating and various temperatures drying to choose the optimum initial processing method, and the content of anthraquinones derivatives, slicing colors and dried rates were used as evaluation indexes. Compared with traditional processing, sliced fresh crude Rheum palmatum had lower content of the anthraquinones derivatives and dry rates, slicing color had changed as well. For various drying methods, smoking drying was superior to sun drying, shady drying, microwave heating and various temperatures drying methods (Li et al., 2011). Due to processing-induced variations in the chemical composition, raw rhubarb samples subjected to different drying procedures have different therapeutic effects. The raw materials were processed by smoking, sun-drying, shade-drying and oven-drying at low, moderate and high temperatures. The total concentrations of twelve compounds, namely, gallic acid, catechins, epigallocatechin gallate, epicatechin, epicatechin gallate, sennoside B, sennoside A, aloe-emodin, rhein, emodin, chrysophanol, and physcion, in smoked rhubarb were higher than the concentrations of the same components in raw rhubarb and rhubarb products processed using other drying techniques. Smoked rhubarb was found to inhibit Na⁺/K⁺-ATPase and thrombin substantially. These results confirmed that post-harvest fresh plant materials, especially roots, were still physiologically active, and could undergo a series of anti-dehydration mechanisms, including the production of related secondary metabolites during the early stages of dehydration. Therefore, the proper design of drying processes could enhance the quality of rhubarb as well as other similar medicinal plants (Sun et al., 2018).

In the view of wide applications of rhubarb, although many studies on the chemical constituents, quality analysis, pharmacological activities, and clinical practices had been reported, there are still lots of problems to be solved. Firstly, there are about ninety-four compounds with five types of skeletons isolated from the genus Rheum, including anthraquinone, anthrones, stilbenes, flavonoids and acylglucosides (Gao et al., 2013). Most of the studies have focused on exploring the bioactivities of anthraquinones, with the precise mechanisms underlying their activities being not fully understood. Secondly, derived from long-term practices and experiences, smoking was a traditional technology primarily used for the processing of Chinese herbal medicines. A Chinese herbal medicine was baked with a small fire with the ambient temperature being between 12–18 °C, it typically took at least two weeks to reach complete dryness. This drying technique went slow, allowing the innate metabolic processes of the plant to continue after harvest. Therefore, it is imperative to study the drying mechanism based on secondary metabolism.

Acknowledgments

This study was supported by Tianjin Key R&D Program–Key Projects for Science and Technology Support (19YFZCSY00170), College Student Innovation and Entrepreneurship Training (201910069007, 201910069102), National Natural Science Foundation of China (No. 31571832), Tianjin Innovative Research Team Grant (TD13-5087), and Key Program of Tianjin Municipal Health Bureau (No. 2013-GG-05).

Conflict of interest

The authors declare no conflicts of interest.