Targeting NF-κB pathway for treating ulcerative colitis: comprehensive regulatory characteristics of Chinese medicines

In recent years, the incidence and prevalence of global ulcerative colitis (UC) have shown a rapidly increasing trend. It suggests that the highest prevalence rate of UC (505/100,000 in Norway) exists in western Europe, and the prevalence rate of UC in Korea increases from 0.34/100,000 in 1986–1990 to 3.08/100,000 in 2001–2005 [1, 2]. Statistical data show that the prevalence rates of Heilongjiang, Xian, Guangzhou, Zhongshan, Hong Kong and Taiwan are 1.77/100,000, 0.42/100,000, 3.44/100,000, 3.14/100,000, 3.06/100,000 and 4.59/100,000, respectively [3‐5]. Moreover, compared with UC in western countries, the disease in China is found to have some differences in clinical characteristics [6].

Typical symptoms of UC include diarrhea, mucus pus, bloody stools and abdominal pain, and its time of onset is more than 4 to 6 weeks, simultaneously, extraintestinal manifestations involve skin mucosa, joints, eyes, lungs and nervous system [7‐15]. Additionally, diarrhea, which does not exceed 6 weeks of disease process, needs to be distinguished from infectious enteritis [16]. Presently, western medicine generally thinks that there are several pathological factors related to UC, including genetic factor, infectious factor, intestinal microbiota imbalance, immune response, dietary factor and so on. Study evidences that less than 20% of susceptibility genes are associated with UC and 28 functional chromosomal loci have been reported [17]. A 10-year study on cytomegalovirus colitis shows that it has been a poor prognostic indicator among patients with UC onset [18]. Probiotics have been demonstrated to be able to protect intestinal mucosal barrier and reduce intestinal inflammation, and evidences indicate that a protective role of bifidobacteria in experimental induced colitis is related to regulation of an innate immune response of the host, and immune homeostasis contributes to the reduction of inflammatory bowel disease (IBD) onset [19, 20]. With the improvement of people’s living standards, the dietary structure begins to change. The intakes of meat and egg-milk products gradually increase and dietary fiber-based foods decreases. This high-protein and low-vegetable fiber diet has become one of the risk factors for enhancing prevalence of UC [21]. Moreover, excessive intakes of high-calorie and heterologous proteins may stimulate the body’s immune response, leading to aggravation or recurrence of UC, while diet strategy of reducing unsaturated fatty acids in food is effective on UC treatment [22].

Current therapeutic drugs for UC include 5-aminosalicylic acid (5-ASA), glucocorticoids, immunomodulators, biological agents and traditional Chinese medicines. The evidence-based consensus released by European Cron’s and Colitis Organisation (ECCO) recommends 5-ASA as the primary drug for the treatment of mild to moderate UC and maintaining drug during remission [23]. An analysis on mild to moderate UC and remission of UC shows that 50.5% patients with local treatment and 41.3% patients with oral treatment achieve clinical amelioration, but the incidence of adverse reactions in local and oral treatments was 22/105 (21.0%) and 36/109 (33.0%), respectively [24]. Glucocorticoids are prone to hormone dependent and resistant. In the treatment using budesonide foam enema for mild to moderate UC, the rate of side effects was 45.9% [25]. Therefore, glucocorticoids are not suitable for long-term application for maintaining treatment. Immunosuppressive agents have more significant adverse reactions such as nausea, vomiting, abdominal pain, severe diarrhea, myelosuppression, hepatotoxicity and pancreatitis, and in the treatment of biological agents, a large number of patients do not get mucosal healing and the mucosal healing also does not mean that the symptoms disappear [26, 27]. Although the above drugs have achieved a certain extent effects on clinical remission of UC and promoting mucosal healing, it still cannot meet the requirements of UC treatment. Clinically, Chinese medicines not only obviously improve symptoms and accelerate ulcer healing, but also alleviate adverse reactions of western medicine and enhance patients’ quality of life. Therefore, Chinese medicines have become a hotspot in UC treatment in recent years [28‐32].

Data show that 20.1% of UC patients in China accept the treatment of Chinese herbs, and 59.1% are administered with combined Chinese with western medicines [33]. Moreover, Chinese medicines are reported to be satisfied efficacious in the treatment for mild to moderate UC [34]. It shows that combination treatment of Chinese medicine formula and mesalazine notably improves the intestinal symptoms, mucosal condition, and extraintestinal characterizations compared to mesalazine alone in UC patients after 8 weeks [35]. Another study indicates that the therapeutic effect of curcumin combined with mesalamine is superior to that of mesalamine in patients with mild-to-moderate UC, and no apparent side effects emerge [36].

Although precise mechanisms are still not clear at present, numerous literature suggest that NF-κB pathway plays a central role in regulating the release of cytokines in patients with UC and participates in the inflammation and immune response in the intestinal tract of UC [37‐39]. The imbalance between excessive secretion of pro-inflammatory cytokines and relative insufficient secretion of anti-inflammatory cytokines is linked to the development of non-specific inflammatory responses in the intestine [40]. It indicates that NF-κBp65 highly expresses in intestinal mucosal epithelium, crypt epithelial cells and lamina propria monocytes of patients with UC, and the expression of NF-κB in the nucleus is significantly higher than that in cytoplasm [41]. NF-kappaB p65 antisense oligonucleotides blocks NF-κB pathway and down-regulates NF-κB-dependent IL-1beta and IL-8 mRNA expressions, which attenuates the productions of pro-inflammatory cytokines in lamina propria mononuclear cells from patients with UC [42]. In this review, based on the therapeutic superiorities of Chinese medicines and the important role of NF-κB pathway in UC pathogenesis, we summarize Chinese medicines and their active compounds as well as Chinese medicine formulae targeting NF-κB signaling pathway, and further analyze the characteristics and advantages of Chinese medicines during the process of UC treatment.

According to the etiology and pathogenesis theory of traditional Chinese medicine, the pathogens of dampness-heat and dampness-cold account for predominant status in the onset of UC. Subsequently, qi stagnation and blood stasis present in intestine, which exacerbates the symptoms of UC, e.g., abdominal pain, diarrhea, mucus pus and bloody stools. Additionally, emotion disorder and unhealthy diet lead to the disharmonization of liver and spleen functions, triggering cold and heat in complexity resulting in the change of stomach-intestine environment. Consequently, the pathological development injures spleen qi and kidney yang, causing the deficiency of spleen and kidney functions. Patients with chronic UC are prone to attract complications by exopathogenic factors, because it always exists deficiency in spleen qi and kidney yang. Therefore, UC is commonly divided into “syndrome of dampness-heat”, “syndrome of deficiency-cold of spleen and stomach”, “syndrome of deficiency of spleen and kidney Yang” and “syndrome of cold and heat in complexity”, “syndrome of qi stagnation and blood stasis” and so on [43]. In the treatment of UC, clearing heat, drying dampness, detoxifying, and activating qi and blood, harmonization of cold and warm, warming yang, tonifying qi are regular therapeutic methods.

At present, regulation of NF-κB pathway has become a key strategy in the treatment of UC. According to traditional Chinese medicinal functions, we categorize Chinese medicines and their active compounds targeting NF-κB pathway into seven types.

The formulae consists of multi-Chinese medicinals and exerts synergistic effects according to the Chinese medicine therapeutic pillars based on syndrome differentiation. By focusing on the traditional functions of the formulae targeting NF-κB pathway for UC treatment, we categorized these formulas into five types.

Numerous literature have demonstrated that Chinese medicines targeting NF-κB play a vital role in UC treatment. In present study, it suggests Chinese medicines with different traditional functions exert direct and indirect regulatory efficacies on NF-κB pathway in different pathological models of UC. The definitions of direct and indirect regulatory efficacies respectively refer to action on NF-κBp65 and influence on NF-κB pathway via NF-κBp65′s up-stream or other signaling pathways.

In terms of direct inhibition of NF-κB in the treatment of UC, we summarized 85 kinds of Chinese medicines and their active compounds, as well as formulae. By analysis, we find that four categories of heat-clearing and dampness-drying medicine, interior-warming and drying dampness medicine, detoxificating and purgative medicine and tonifying medicine account for primary status, which embodies prevention of phosphorylated and nuclear transcriptional NF-κBp65 or enhancement of the level of p65 in the cytoplasm. Simultaneously, we also discover that the number of formulae with function of heat-clearing and drying dampness targeting direct inhibition of NF-κB is maximum.

On the other hand, we summarized 58 Chinese medicines and their active compounds, as well as formulae for indirect inhibition of NF-κB. Among them, the categories of heat-clearing and dampness-drying medicine and formulae, and detoxificating and purgative medicine exert distinguished actions, which embodies enhancing the activity of IκBα to decrease phosphorylated p65 or inhibiting the degradation of IκBα binding to p65. Apart from affecting IκBα, some Chinese medicines and their active compounds are able to regulate the activity of NF-κB by affecting AKT, TLR4/MyD88, PPARγ/SIRT1, p-IKKβ/IKKβ, p38MAPK/ERK/JNK, TLR5/MyD88, p-STAT3. As shown in Tables 1 and 2, from the analysis on the frequency of Chinese medicines with different therapeutic function for treating UC, it suggests that the screening of Chinese medicines for UC treatment might be focused on the candidate agents with dampness-drying and detoxificating effects in future.

Furthermore, cell-specific role of NF-κB has been demonstrated to involve in the pathogenesis of IBD [158]. It suggests the enhancement of NF-κBp65 exists in macrophages and epithelial cells isolated from inflamed gut specimens from IBD patients [159]. Except for macrophages and epithelial cells, lamina propria fibroblasts also plays a NF-κB mediated pro-inflammatory role in IBD [160]. Increased NF‐κB expression in mucosal macrophages causes the productions of pro-inflammatory cytokines such as TNF‐α, IL‐1 and IL‐6 directly resulting in the mucosal tissue damage, and in colonic epithelial cells it is related to an increased expression of intercellular adhesion molecule‐1 that contributes to the recruitment of neutrophil granulocytes to the site of inflammation [161]. NF‐κB‐induced cytokines further stimulate, activate and differentiate lamina propria immune cells, which aggravates the perpetuation of mucosal inflammation. In this review, we find that 55% of Chinese medicines having the abilities of regulating NF‐κB expression in macrophages (e.g., SM934, artesunate, aesculin, gallic acid and epicatechin) repress the nucleus translocation and phosphorylation of NF-κBp65, inhibit the protein expressions and phosphorylation of NF-κB, whereas 45% (e.g., berberine, Total flavonoids of H. diffusa Willd, F. cymosum (Trev.) Meisn and osthole) block the phosphorylation of IκB, reduce the degradation of IκBα protein. It suggests that Chinese medicines with detoxificating and purgative function have the most obvious inhibited effects of NF-κB pathway on macrophages. Additionally, other active compounds or extracts of Chinese medicines, such as Chinese herb pair Paeoniae Radix Alba and Atractylodis Macrocephalae Rhizoma, Corni Fructus aqueous extract and cornuside, Siegesbeckia pubescens Makino, which can inactivate NF-κB pathway in macrophage, perhaps became candidate drugs for treating UC in future [162‐164].

Apart from NF-κB, Nrf2 is also a key transcription factor that protects cells and tissues from inflammation, and it has been found to be implicated in the pathogenesis of UC [165]. Study reports that when Nrf2 expression rises in colon tissue, the inflammatory injury significantly attenuates, demonstrating that Nrf2 has a protective effect on the colon [166]. Meanwhile, scholars find that there are multiple associations between Nrf2 and NF-κB [167]. For example, the absence of Nrf2 leads to oxidative stress activation, which contributes to increasing NF-κB-mediated cytokines expression. In present study, we find some active compounds of Chinese medicines and formulae, e.g., galangin, oligonol, bai-xia-xie-xin decoction, can either increase the nuclear translocation of Nrf2 or inhibit NF-κB pathway, suggesting the advantages of Chinese medicines’ multi-targets and comprehensive regulated abilities for UC treatment.

Additionally, numerous literature have reported Chinese herbal drugs administered by both, oral intake and retention enema, exert significant therapeutic actions in the treatment of IBD, and in order to understand the detailed mechanisms of Chinese medicines therapy on UC, the application of experimental rodent models is essential [168, 169]. In this review, we summarized the researches mainly from animal or human cell models, e.g. DSS-induced model is common pathological UC model. It suggests that the establishment of chronic colitis and the development of colorectal dysplasias and cancers with pathological features by DSS-induced rodent models resemble those of human colitis, suggesting that DSS-induced colitis may be an appropriate model to evaluate the clinical efficacy of candidate agents for UC [170, 171]. Therefore, we can infer some characteristics of Chinese medicine from analyzing UC animal models based on traditional Chinese herbal therapeutic functions. In addition, the development of novel deliver Chinese medicines for the treatment of IBD will more greatly enhance the therapeutic effects and safety of Chinese medicines, as well as reduce the cost of treatment [172].

In summary, evidences show that Chinese medicines exert a vital action on the treatment of UC by direct and indirect regulation of NF-κB pathway (Fig. 1), and the category of dampness-drying and detoxificating medicine accounts for the main status. This review not only further validates the scientificity of Chinese medicine on the pathogenesis of UC by categorized therapeutic agents and formulae according to their traditional functions, but also suggests the characteristics of comprehensive modulated abilities of Chinese medicines. Moreover, by careful analysis on cell-specific role of NF-κB in UC, novel Chinese medicines precisely modulate NF-κB becomes possible. Simultaneously, this review contributes to the choices of Chinese medicine category and provides curative potential of Chinese medicines for clinical UC treatment. Of course, more systematic and detailed pharmacological studies on Chinese medicines need to be fulfilled and provide more evidences to validate Chinese medicines’ efficacy and safety in the future.