Anticancer potentials of sinomenine from Sinomenium acutum: A mini-review

Sinomenine, an important isoquinoline from Sinomenium acutum (Menispermaceae), is currently considered as a reliable drug for treating various malignant cancers including breast cancer, colon cancer, lung cancer, liver cancer, gastric cancer, osteosarcoma cervical cancer, and esophageal cancer. The pharmacological mechanisms are probably related to induction of apoptosis and cell cycle arrest. This paper was aimed at reviewing the anticancer potential of sinomenine, which is expected to be beneficial in developing this compound as a candidate drug for cancer therapy.


INTRODUCTION
Sinomenium acutum (Menispermaceae), also called Qing-feng-teng in Chinese and first recorded in Compendium of Materia medica, has been successfully used for hundreds of years as a traditional Chinese medicine for rheumatism and neuralgia, and with low side effect [1,2]. Sinomenine (SIN, Figure 1), an isoquinoline, is the most important active constituent of this plant, and is currently considered as a reliable drug for treating rheumatoid arthritis (RA) in clinics [3,4]. Furthermore, increasing evidence has demonstrated that cancer has become the leading cause of disease-related death in recent years. Malignant tumors are among the diseases that endanger human life and cause death. Due to the increasing number of new cases every year, cancer also imposes a heavy burden on society and medical care.
Chemotherapy predominates in the treatment of cancer, but it has the disadvantages of toxic side effects and drug resistance. Therefore, research emphasis is shifting more and more to the anticancer effects of Chinese herbal medicine. Interestingly, studies have reported that besides its anti-RA, anti-inflammatory, analgesic, sedative and immunosuppressive effects [1], SIN has promising anticancer potential against various cancers ( Table 1, Table 2 and Table 3) [5][6][7]. Consequently, this paper was aimed at reviewing the anticancer effects of SIN and its potential molecular mechanisms, based on available literature, so as to provide information which might be beneficial for drug development and clinic applications of the SIN for treating cancers.

Breast cancer
Breast cancer, a common malignant cancer of the mammary gland epithelial tissues with an increasing incidence in recent years, is a predominant cause of cancer related death in women [8,9]. Nowadays, some interesting investigations have indicated that SIN exerts anticancer potential on breast cancer in vivo and in vitro.
In 2015, a study by Song et al revealed that SIN at concentrations ranging from 0.25 to 1 mM, suppressed the invasion and migration of MDAMB-231 and 4T1 breast cancer cell lines in vitro in a dose-dependent manner. The possible molecular mechanisms were related to suppression of NF-kB activation via regulation of the IL4/miR-324-5p/CUEDC2 axis [10]. Furthermore, another study in 2017 reported that sinomenine hydrochloride (SH) inhibited the proliferation, migration and invasion of two breast cancer cell lines (MDAMB-231 and 4T1 breast cancer cell lines) in vitro [11]. By using orthotopic mouse model of 4T1 and MDA-MB-231 cell lines, it was shown that SH inhibited breast cancer metastasis by suppressing epithelialmesenchymal transition (EMT) and cancer stem cell (CSC) properties via attenuation of inflammatory reactions, without obvious hepatotoxicity and renal toxicity [11]. In another study, Li et al reported that SH inhibited tumor growth by induction of apoptosis in human breast cancer cell lines (MDA-MB-231 and MCF-7 cells) in vitro without showing significant toxicity [12]. Moreover, it was reported that the potential mechanism involved might be closely related to up-regulation of MAPKs via ROS-dependent and -independent pathways [12]. Besides, Zhang et al reported that SH suppressed the proliferation and migration of human umbilical vein endothelial cells (HUVEC), and SH at dose of 100mg/kg inhibited the progression and metastasis of tumor in 4T1 murine breast cancer model and 168FARNcell 4T1 murine breast cancer model [13]. The inhibitory effect was attributed to the restoration of the balance between pro-angiogenic factor (bFGF) and antiangiogenic factor (PF4), resulting in the normalization of blood vessels [13].

Colon cancer
Colon cancer is one of the most common cancers. At present, its incidence and mortality are ranked third in both male and female patients worldwide [8]. Postoperative metastasis and multi drug-resistant (MDR) caused by chemotherapy are the bottlenecks of the treatment. Some studies have shown that SIN may be an auxiliary drug in the treatment of colon cancer. It has been reported that in vivo and in vitro, the inhibitory effect of SIN combined with the first-line chemotherapy drug 5fluorouracil (5-FU) on the growth of LoVo cells in colorectal cancer were significantly better than that of SIN or 5-FU when used alone [14]. There were no significant increases in side effects of chemotherapy in the study [14]. At the same time, SIN treatment up-regulated the expressions of Bax, whereas Bcl-2 was down-regulated, resulting in decline in the anti-apoptotic/proapoptotic (Bcl-2/Bax) protein ratio, and finally apoptosis. Thus, it can be said that SIN exhibits anticancer effects by controlling the permeability of mitochondrial membrane. Further studies [15] have found that SIN inhibited the survival of the human colon adenocarcinoma cell line SW1116 by inducingcell cycle arrest (mainly in G1 phase) and regulating the expression of cyclooxygenase-2 (COX-2). Consequently, COX-2 may be an important target for SIN in the treatment of colon cancer.
Studies using a multidrug-resistant Caco-2 (MDR-CaCO-2) cell model established with increasing concentrations of doxorubicin found that the expressions of COX-2 and MDR1 mRNA (MDR-1 genes) in the cells were enhanced, and the P-glycoprotein (P-gp) in the cells (MDR1 gene-encoding) was elevated, while the NF-κB signaling pathway was activated. It has been reported that SIN reduced the expressions of COX-2, MDR1 and P-gp through the NF-κB signaling pathway, thus enhancing the sensitivity of MDR-CaCO-2 cells to doxorubicin, which is undoubtedly a new strategy for effectively reversing drug resistance of cancer cells [16].

Lung cancer
Currently, SIN is often reported in the treatment of lung cancer. Lung cancer is a malignant tumor that originates from bronchial mucosa or glands. Although the incidence in some developed countries has decreased in recent years, it still accounts for the highest mortality rate in the world [8,17,18]. The pharmacological effects of SIN on the treatment of lung cancer are mainly manifested in inhibition cell proliferation and induction apoptosis. Previous studies also found that SIN induced apoptosis and inhibited the proliferation of non-small cell lung adenocarcinoma NCI -H460 cells in a time-and dose-dependent manner in vitro [19]. The mitochondrial membrane potential of the cells also decreased with the increase in SIN concentration, indicating that SIN may induce apoptosis in NCI-H460 cells by activating the mitochondrial pathway.
Previous studies found that SIN may inhibit the proliferation of human lung cancer cell line A549 cells in a dose-dependent fashion in vitro, probably due to the inhibition of the expression of alpha 7 nicotinic acetylcholine receptor (α7nAChR) and n-nitrosomorpholine (NNK) (nicotine metabolite), thereby increasing the proportion of early apoptotic cells and reducing the proliferation of nicotine [20]. Furthermore, it has been reported that SIN exerted cytotoxic effects on A549 cells, and reduced the protein expressions of Janus kinase 2 (JAK2), signal transducer, activator of transcription 3 (STAT3), and EMT marker [21]. This demonstrates that it may restrain the invasion of lung cancer by regulating STAT 3 signaling pathway and interfering EMT [21].
In further studies on the molecular mechanism of SIN-induced apoptosis in lung cancer cells, it was suggested that the activation of the two signal pathways of PI3K/Akt and MEK/ERK antagonized the apoptosis-inducing effect of SIN on 3 human lung cancer cell lines (NCL-H460, NCI-H 226 and NCI-H 522 cells) [22]. When the activation of two signal pathways is suppressed, SIN can synergistically enhance apoptosis induced by inhibitors. The synergistic effect of Akt and ERK pathway is an important route for SIN to induce apoptosis in multiple lung cancer cell lines. Thus, SIN is expected to be an effective drug for the treatment of lung cancer.

Liver cancer
In recent years, the therapeutic potential of SIN for liver cancer has also been demonstrated. Liver cancer is a malignant tumor that originates in hepatocytes or intrahepatic bile duct cells. In recent decades, the incidence of liver cancer has increased steadily [8]. The survival time after diagnosis is usually less than 6 months, and only 5 -9 % of the patients survive more than 5 years [23]. It has been found that SH (0.5-4.0 mM) not only inhibited the proliferation of human liver cancer cell SMMC-7721 in vitro, but also induced apoptosis, as was evident through MTT and Hoechst33258 staining [24]. At the same time, it increased the protein expression levels of death receptor DR4 and DR5 in a concentrationdependent manner, suggesting that exogenous apoptosis may be one of the mechanisms involved in the anticancer action of SH.
Previous investigations have revealed that the SIN inhibition and apoptosis induction on human hepatoma cell SMMC-7721 may be related to the inhibition of the transcription factor C/EBP β, and the expressions of inflammatory factors such as TNF-α and IL-1β, which may further damage the micro-environmental role of tumor inflammation [25]. Since C/EBP β is a downstream transcription factor of RAS signaling pathway and COX-2 has been shown to be closely related to cancer pathogenesis, it can be speculated that SIN may inhibit the downstream target molecule COX-2 through the RAS signaling pathway to regulate the expressions of inflammatory factors such as TNF-α and IL-1β, and then play an anticancer role by destroying the microenvironment of swelling and inflammation. However, there are no experimental evidence linking liver cancer cells, RAS signaling pathway and COX-2. The mechanism remains to be further verified.

Gastric cancer
Gastric cancer is also one of the common digestive system cancers. Every year around the world, about 990,000 people are diagnosed with gastric cancers, and about 738,000 affected people die, more than half of them in developing countries [26,27]. Some studies have suggested that SIN may be a chemotherapeutic adjunct to gastric cancer [28].
Some researchers have investigated the effect of combined use of SIN + 5 FU on gastric cancer. The results showed that when combined using the half effective doses of the two drugs (20 M SIN + 50 mg/ L 5 FU) to cultivate the human gastric cancer cell line MKN-28, the inhibitory effect was significantly higher than that obtained using the single complete effective dose of one drug (40 M SIN or 100 mg/L 5 FU) [28]. These observations were also confirmed in vivo using MKN-28 xenograft mouse model. It was suggested that SIN and 5-Fu exerted synergistic effect on the inhibition of the proliferation of gastric cancer cells through a mechanism involving SIN-assisted, 5-FU-induced release of cytochrome c from the mitochondria to cytoplasm through the mitochondrial pathway, and activation of the caspase-3 and caspase-9. This would enhance the apoptotic effect of 5-Fu on the cells. In addition, SIN also significantly inhibited the expression of thymidylate synthase (TS) mRNA (key gene of 5-Fu metabolism). These results indicate that SIN combined with 5-FU is expected to improve chemo-sensitivity of gastric cancer cells.

Osteosarcoma
Osteosarcoma occurs predominantly in children and adolescents, and its incidence accounts for approximately 3 -4 % of all childhood tumors [29,30]. The disease is a malignant bone tumor characterized by vascular invasion and local soft tissue infiltration. It is characteristically of high malignancy, early distant metastasis and high local recurrence [31]. Previous researchers have studied the effect of SIN on the treatment of osteosarcoma using human osteosarcoma (OS) cell lines HOS and U2OS, and found that SIN restrained the expressions of RANKL, VEGF, MMP-2 and MMP-9 via the CXCR4-STAT3 signaling pathway [32]. This reduced the bone destruction and osteolysis mediated by RANKL in osteosarcoma, and also decreased the formation of neovascularization associated with VEGF and the degradation of extracellular matrix (ECM) by the cancer cells [32].

Other cancers
Studies have reported that SIN induced apoptosis in cancer cells in common malignancies such as cervical cancer and esophageal cancer. At doses of 0.625 -2.5mmol/L, SIN inhibited the proliferation of human cervical cancer Hela cells [33]. This inhibition was achieved by blocking the expression of AKT signal transduction protein, promoting the formation of Caspase-3, and inducing apoptosis Hela cells. A previous study observed that SIN inhibited the proliferation of EC109 in the esophageal cancer cell line, and the expressions of COX-2 and survivin proteins in the cells were significantly weakened by SIN [34]. Therefore, it may be speculated that SIN may inhibit the expression of COX-2 and reduce the expression of survivin protein, thus promoting apoptosis.

CONCLUDING REMARKS
Cancer is a disease of immense concern in medical research [35]. In the past decade, the incidence of most cancers has continued to rise due to environmental pollution, aging population, and bad living habits. Among them, the rising trends in breast cancer, lung cancer and other cancers are particularly significant. Although there have been remarkable improvements in early diagnosis and treatment, the five-year survival for cancer patients is still not encouraging [8,36]. Thus, research has focused on the discovery of more effective and safer therapeutic drugs. Plants and their metabolites are the treasure-house of drug sources. Indeed, about 40 % of drugs or pharmaceutical raw materials are derived from plants [37,38]. Therefore, more and more researchers have turned their attention to a wide variety of medicinal plants with great potential.
The compound SIN is derived from the natural medicinal plant Sinomenium acutum. A large number of studies have shown that SIN is a potent anticancer principle from Traditional Chinese Medicine (TCM) [1][2][3][4][5]. It acts on different cancer types through multiple links, multiple pathways, and multiple targets. Studies have shown that SIN induces apoptosis in various cancers such as breast cancer, lung cancer, colon cancer, and gastric cancer. Moreover, SIN has fewer toxic and side effects and high safety. When combined with chemotherapy drugs, the side effects of SIN on colon cancer cells are not significantly increased, and the combined treatment effectively reverses drug resistance of cancer cells. Thus, its clinical application value is great. It is believed that with the deepening of research on the anticancer mechanism of SIN, it is likely to become a candidate drug for clinical treatment of cancers. However, more research works should be devoted to elucidating the exact mechanism(s) involved in the anticancer properties of SIN, as well as its toxicities and druggability.