3.1. Bupleurum falcatum 

The root of Bupleurum falcatum L. (Umbelliferae), especially B. chinense from mainland China and B. falcatum from Japan [28, 29], is also called Bupleuri Radix ((BR) Chaihu, in Chinese and Saiko in Japanese) and is one of the principal herbs in XCHT. Early study examined the hepatoprotective effect of several BR extracts with dimethylnitrosamine- (DMN-) induced hepatic fibrosis rat model and these extracts appeared to prevent fibrosis by improving liver function and modulating the levels of relevant cytokines [29]. Recent studies also suggest BR extracts as potent antioxidant agents because they are able to decrease L-thyroxine-induced hypothyroidism and to enhance the liver antioxidant defense systems [30]. 

Some saikosaponins, which are the active ingredients of BR, have been found to suppress hepatic fibrosis [31, 32], hepatocarcinoma [33–36], and HBV infection [37] and improve chemotherapy [38]. The different mechanisms through which saikosaponins suppressed hepatocarcinoma were summarized in Figure 1. Saikosaponin a (SSa) was shown to effectively inhibit CCl4-induced liver inflammation and fibrosis in SD rats by simultaneously blocking the production of hepatic proinflammatory cytokines/growth factors (TGFβ1 and hydroxyproline) and increasing the expression of anti-inflammatory cytokine IL-10 [31]. Saikosaponin d (SSd) was found to suppress hepatic fibrosis through the downregulation of TNF-α, IL-6, and NF-κB activities [32]. SSa may inhibit HepG2 growth by increasing the levels of p-15INK4a and p-16INK4b (cyclin-dependent kinase inhibitors) in a protein kinase C (PKC) [33] and/or extracellular signal-regulated kinase (ERK) signaling pathway-dependent manner [34]. SSd markedly reduced the liver nodule, tumor cell invasion while increased cellular atypia in xenograft model [35]. It appeared that SSd exerted its action by diminishing the expression of highly expressed cyclooxygenase 2 (COX-2) and CCAAT/enhancer-binding protein β (C/EBPβ) in tumor cells and macrophages of liver tumors [35]. In a study reported by Zhu et al. [36], SSd was shown capable of reversing the malignant phenotype of HepG2 cells. SSd-treated HepG2 cells grew and migrated at slower rate, had decreased volume ratios of nucleus to plasma and small round cell shape. At molecular level, SSd decreased the level of alpha-fetoprotein (AFP) and enhanced the expression of cell cycle inhibitor p27. Moreover, Chiang et al. showed that HBV-containing human hepatoma cells (2.2.15 cells) treated with saikosaponin c (SSc) secreted significantly less HBeAg into culture medium and had reduced HBV DNA replication [37]. Although not directly using liver cancer cells, SSa and SSd were also shown to sensitize cervical (HeLa and Siha), ovarian (SKOV3), and lung cancer cells (A549) to cisplatin-induced cell death by inducing the production of reactive oxygen species (ROS) and activation of caspases [38].

Figure 1

Antitumorigenesis effect of active components in Bupleuri radix.

3.2. Scutellaria baicalensis 

The dry root of Scutellaria baicalensis, Scutellaria radix ((SR) Hangqin, in Chinese ) is another principal herb in XCHT. Accumulating evidences indicate that wogonin, baicalein, and baicalin are the principal active components in SR [39]. SR has been widely used to treat hyperlipemia, atherosclerosis, and hypertension. Recent studies with various model systems suggest that SR also possesses a potent cytostatic [40–42], anti-inflammatory [43] and antiviral capabilities [44, 45]. 

Antitumorigenesis ability of SR was reported by Gao et al. in human lung cancer cells (SK-MES-1, SK-LU-1, and A549) [42]. Their study showed that the absolute ethanol extracts of Scutellaria baicalensis, baicalin, baicalein, and wogonin all displayed a concentration- and time-dependent cytotoxicity to lung cancer cells while were only weakly cytotoxic to the normal human lung fibroblasts. Jung et al. later discovered that Scutellaria baicalensis is an anti-inflammatory agent because it decreases histamine release and inhibits the passive cutaneous anaphylaxis reaction in SD rats [43]. Antiviral effect of Scutellaria baicalensis was shown by Tang et al., in which it was shown to significantly inhibit the replication of HCV RNA in HCV-infected nude mice [44]. Later study revealed that the aqueous extract of Scutellaria baicalensis was also able to suppress the replication of lamivudine-resistant HBV mutant in human hepatoma cells by suppressing HBV core promoter activity [45].

Besides the studies performed with Scutellaria baicalensis, active components of it have also been extensively investigated (Figure 2). TNF-related apoptosis-inducing ligand (TRAIL) has been recognized as a promising anticancer agent because it kills tumor cells without damaging normal tissues [46, 47]. However, resistance to TRAIL is frequently seen in various tumor types. Ding et al. found that wogonin and structurally related natural flavones apigenin and chrysin overcame TRAIL resistance by downregulating the level of c-FLIP (a key inhibitor of death receptor signaling) and up-regulating TRAIL receptor 2 (TRAIL-R2) expression in human T-cell leukemia virus type 1- (HTLV-1-) associated adult T leukemia/lymphoma (ATL) cells [48]. They further showed that these flavones could enhance TRAIL-mediated apoptosis in a wide variety of cancer cell types including hepatocellular carcinomas (HepG2), breast (MDA-MB-231), colon (HT-29), and pancreatic cancer cells (Capan-1) as well as melanoma cells (SK-MEL-37) [48], implicating the use of flavones as an adjuvant for TRAIL-mediated anticancer therapy. In another study, Polier et al. initially showed that wogonin and flavones are inhibitors of cyclin-dependent kinase 9 (CDK9) and can effectively block phosphorylation of the carboxy-terminal domain of RNA polymerase II at Ser2, which in turn reduces RNA synthesis and subsequent downregulation of antiapoptotic protein myeloid cell leukemia 1 (Mcl-1), leading to significant apoptosis in a variety of human cancer cells [49]. However, wogonin-induced apoptosis of human hepatocarcinoma cells was found to be accompanied with Bax increase and Bc1-2 decrease [50, 51]. Anti-HBV effect of wogonin was also found in vitro and in vivo [52], with the HBV antigen and HBV DNA level reduction.

Figure 2

Suppressive effect of active components of Scutellaria radix on liver tumorigenesis and fibrosis.

Baicalein, a flavonoid extracted from SR, has been shown to possess potent antitumorigenesis capability toward liver cancer cells. For example, baicalein is highly cytotoxic to HCC cell lines and exerts its cytotoxicity by reducing mitochondrial transmembrane potential and subsequent cytochrome c release and caspase-3/9 activation. Disruption of MEK-ERK signaling pathway is at least partially responsible for baicalein-induced cytotoxicity [53, 54]. When used in vivo, baicalein can significantly inhibit tumor growth of HCC xenografts [53]. In another study, baicalein was reported to block cell migration and invasion of human hepatoma cells through multiple mechanisms including the suppression of MMP-2, MMP-9, and uPA expressions, blockage of NF-κB activation, and decreasing the phosphorylation levels of PKCα and p38 MAPK activities [55]. In a recent study, Sun et al. showed that baicalein dose dependently decreased AST, ALT, hyaluronic acid, laminin, and procollagen type III (PCIII) in serum as well as hydroxyproline and MMPs in liver in CCl4-induced liver fibrosis model [56]. Moreover, baicalein also alleviated inflammation, destruction of liver architecture, collagen accumulation and expression of PDGFβ receptor, thus preventing the activation of stellate cells and liver fibrosis [56].

Baicalin is also an important active component included in SR. Zhang et al. [57] found that baicalin induced apoptosis with downregulation of glycosylated immunoglobulin superfamily transmembrane protein CD147 expression in SMMC-7721 cells, and interestingly, this effect was accompanied with cell autophagy. This study firstly suggested that baicalin induced autophagy cell death in SMMC-7721 cells and revealed a new mechanism for the anticancer effects of baicalin. Qiao et al. [58] studied the antihepatic fibrosis effect of baicalin and found that transplantation with baicalin-treated mesenchymal stem cells in combination with baicalin administration had the best therapeutic effect for hepatic fibrosis. This may further introduce a new therapeutic regimen for some liver diseases. Baicalin combined with oxymatrine showed better effect against HBV replication than oxymatrine in vitro, which was proved by Cheng et al. [59]. In aspects of the potential protective effect on liver injury, baicalin was also researched in many experiments [60–62]. Activation of peroxisome proliferator-activated receptor (PPARγ) signaling pathway, and Toll-like receptor 4- (TLR4-) mediated inflammatory responses were involved in the protective effect.

3.3. Panax ginseng 

Ginseng products are regularly consumed worldwide for the purpose of increasing vitality [63]. Recently, many studies have shown the chemopreventive or adjuvant effect of it [64]. A study involving two cases of control (905 pairs and 1987 pairs, resp.) and a cohort (4675 subjects) demonstrated the benefit of ginseng use for cancer prevention as ginseng use was found to be nonorgan-specific cancer preventive, and its effect depends on the frequency of ginseng intake [65].

In addition to ginseng's preventive effect toward cancer, evidences from experimental studies also suggest its direct role to suppress liver tumorigenesis. Wu et al. showed that ginseng lowered the rate of hepatoma development and prolonged life span on diethylnitrosamine (DEN) rat liver cancer model [66]. Kwon et al. found that oral administration of ginseng decreased&