br Conclusion br Introduction Breast cancer is the most

Conclusion
Introduction Breast cancer is the most common cancer and second leading cause of cancer-related deaths in women [1,2]. There were about 1.7 million new cases of breast cancer worldwide in 2012 [3], and the number of new cases of female breast cancer is expected to increase to 3.2 million by 2050 [4], representing a serious threat to female health. Traditional treatments for breast cancer include surgery, chemotherapy [5,6], radiotherapy [7], endocrine therapy [8], and molecular targeted therapy [9,10]. However, although these treatments have good therapeutic effects, they are also associated with numerous side effects that can seriously affect patient prognosis and quality of life. It is therefore essential to develop novel effective and adjuvant therapies with minimal side effects. Xihuang pill (XHP) is a traditional Chinese anti-cancer medicine initially recorded in the ‘surgical syndrome all-life set’ by the Chinese medical scientist Hong-xu Wang in the Qing dynasty in China. XHP is composed of Niu Huang (Calculus bovis), She Xiang (Moschus), Ru Xiang (olibanum), and Mo Yao (Commiphoramyrrha). Numerous reports have demonstrated that XHP may enhance the curative effects, reduce side effects, and improve survival and quality of life in patients with breast cancer when combined with surgery, chemotherapy, endocrine therapy, or other drugs [[11], [12], [13]]. However, the specific mechanisms of XHP in the treatment of breast cancer remain unclear. Guo et al. [12] suggested that the mechanism of XHP in breast cancer may be related to its regulatory effect on the tumor microenvironment. We also previously showed that XHP had a regulatory effect on the T cell receptor (TCR) signaling pathway in the tumor microenvironment in a 4T1 mouse breast cancer model, and many studies have demonstrated roles for this signaling pathway in the growth, differentiation, and function of regulatory T (Treg) hinokitiol [14,15]. Treg cells are an important component of the tumor microenvironment [16] and comprise a T lymphocyte subset with immunosuppressive function [17]. These cells can widely inhibit the function of dendritic cells, natural killer cells, macrophages, Th1 cells, cytotoxic T lymphocytes, and other immune cells, to weaken the body’s anti-tumor immune response and promote tumor development [[18], [19], [20], [21]]. Many studies have showed increased numbers and proportions of Treg cells in the tumor microenvironment in breast cancer, which could inhibit the anti-tumor immune response in breast cancer patients and increase the body’s immune tolerance to tumors [[22], [23], [24], [25], [26], [27]]. Inhibiting the proliferation or inducing apoptosis of Treg cells in the breast tumor microenvironment may thus activate the body’s anti-tumor immune function and help to inhibit the development of breast cancer. The mitogen-activated protein kinase kinase kinase 1 (MEKK1)/SAPK Erk kinase 1 (SEK1)/c-Jun N-terminal kinase 1 (JNK1)/activating protein-1 (AP-1) pathway is an important TCR signaling pathway [[28], [29], [30]]. Activation of this pathway can promote T cell apoptosis [[30], [31], [32], [33]]. Faris et al. showed that MEKK1 promoted the activation and transcription of FasL promoter in Jurkat T lymphocytes and induced Fas–FasL-mediated apoptosis of T cells [31]. Meanwhile, MEKK1 activation has also been shown to increase the expression of the dual-specific serine/threonine protein kinase, SEK1 [34]. SEK1 can in turn activate the JNK apoptotic signaling pathway and further promote FasL, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), and TCR-mediated T cell apoptosis [33,[35], [36], [37], [38]]. In addition, activated JNK can translocate to the nucleus and activate the nuclear transcription factor AP-1(c-Jun) [39], which can bind to DNA in the T cell nucleus and initiate the transcription of apoptotic genes such as FasL and Bim, which then induce t cell apoptosis [40,41]. XHP has been shown to upregulate the expression of the MEKK1/SEK1/JNK1/AP-1 TCR signaling pathway in the tumor microenvironment in 4T1 mouse breast cancer, using mRNA high-throughput screening. However, no studies have yet demonstrated the effects of XHP on apoptosis and the MEKK1/SEK1/JNK1/AP-1 pathway in Treg cells in the 4T1 mouse breast tumor microenvironment.