1 Introduction

Breast cancer is one of the leading causes of cancer death in women. The latest epidemiological report shows that female breast cancer has surpassed lung cancer as the most commonly diagnosed cancer with 2.3 million new cases (11.7% of total cases) worldwide in 2020, closely followed by lung cancer (11.4%) and colorectal cancer (10.0%) [1]. The incidence rate of breast cancer has been decreased slightly in Canada over the past 30 years [2] but increased at 0.5% per year in the United States in the past years [3]. In addition, it has been noticed that the affected population is getting younger [4], which is highly concerning in the society. However, the mortality rate of breast cancer has been decreased steadily in both Canada and the United States since 1990s [3, 5], which is mainly attributed to advances in breast cancer diagnosis and treatment, such as mammography, targeted therapy and immunotherapy.

Although the 5-year survival rate for breast cancer is around 90%, prognosis for advanced stage and recurrent breast cancer remains poor [6, 7]. For example, the 5-year survival rate is around 100% for stage I patients but 22% for stage IV patients in Canada [6]. Chemotherapy of breast cancer usually faces drug resistance and severe adverse drug reactions, which, in turn, significantly affects the quality of life in breast cancer patients [8, 9]. Furthermore, the high cost of oncology drugs may limit their clinical application in certain countries [10]. Thus, many cancer patients, not just limited to advanced stage patients, seek complementary and alternative medicine (CAM) treatments in an attempt to improve therapeutic efficacy and/or reduce adverse drug reactions. A study of cancer patients in Northern Ontario, Canada showed that 51.8% of the patients used CAM products after diagnosis [11]. For patients’ safety, it is critical to enhance researches to understand the mechanism of action (MOA) and side effects of the CAM products.

Medicinal plants are a valuable resource for develo** anticancer therapeutics and widely used in CAM. For example, paclitaxel, a chemotherapy drug used to treat breast cancer and ovarian cancer, was discovered from the Pacific Yew tree. Solanum nigrum L., commonly known as black nightshade, is a folklore herb used in traditional Chinese medicine (TCM). It is usually used to treat ailments such as fever, pain and inflammation [12,13,14]. However, its anticancer function has attracted people’s attention in recent years [15,16,17,18]. The water extract of S. nigrum and various active ingredients such as polyphenols have shown potent in vitro anticancer activities [19,20,76]. SRC (encoding SRC or c-SRC) is a proto-oncogene that is frequently activated in solid tumors including breast cancer [77]. SRC can activate multiple signaling pathways to promote cancer cell proliferation, growth, survival, migration and invasion [78,79,80]. Both epidermal growth factor (EGF) and SRC can bind to EGFR and enhance each respective cancer-promoting functions [80,81,82]. Furthermore, SRC was observed to crosstalk with AR during prostate cancer progression [83].

From the above analyses, it is clear that S. nigrum can regulate multiple signaling pathways via the six critical hub proteins and quercetin, cholesterol and 3-epi-beta-sitosterol are likely to be the major active ingredients responsible for those regulatory actions. Molecular docking suggests that the three active ingredients could directly bind to the hub proteins to exert the anticancer activity of S. nigrum. Therefore, we may conclude that administration of S. nigrum is beneficial for breast cancer patients and the anticancer function of S. nigrum is via a network including multiple bioactive compounds and multiple protein targets. Synergistic effects could be achieved from regulating multiple pathways. However, caution should be taken in co-administration of chemotherapy drugs and medicinal herbs such as S. nigrum since the drug-herb interactions are basically unknown. These drug-herb interactions may reduce the therapeutic efficacy of chemotherapy drugs and could even be detrimental to cancer patients. We highly recommend avoiding co-administration of chemotherapy drugs and medicinal herbs unless the co-administration is clinically proved to be safe, or the drug-herb interaction is clearly defined.

5 Conclusion

In this study, we used network pharmacology and molecular docking approaches to illustrate that S. nigrum may exert its anticancer function via interactions between the 6 active ingredients and 80 protein targets. Out of these interactions, the interactions between quercetin, cholesterol and 3-epi-beta-sitosterol and 6 hub proteins (AKT1, ESR1, EGFR, SRC1, AR and MMP9) are more important. Synergistic effects are likely to be achieved from the multiple signaling pathways. Briefly, our current research indicates that the anticancer activity of S. nigrum is probably due to synergistic effects achieved from multiple bioactive compound-target interactions.

In addition, we would like to point out several limitations of the current study. First, because of the vast amounts of data from the literature, discrepancies may exist in various databases. Secondly, during the decocting process, active compounds with low concentrations may be dismissed and/or unidentified gradients may be generated. Finally, as genes are dynamically regulated by a variety of factors, gene expression information extracted from the databases may not be fully representative for the diseases.Despite several limitations, the current study sets the groundwork for further experimental and clinical confirmation of S. nigrum's molecular mechanism for fighting breast cancer.

Our present approach of determining the MOAs for S. nigrum including multi-component, multi-target, and multi-pathway may be also applicable for identifying the MOAs for other medicinal herbs.These medicinal herbs unquestionably merit further pharmacological and clinical research to validate theoretical predictions and standardize their applications in cancer treatment.