Research Status and Molecular Mechanism of the Traditional Chinese Medicine and Antitumor Therapy Combined Strategy Based on Tumor Microenvironment

Treatment of malignant tumors encompasses multidisciplinary comprehensive diagnosis and treatment and reasonable combination and arrangement of multidisciplinary treatment, which is not a simple superimposition of multiple treatment methods, but a comprehensive consideration of the characteristics and specific conditions of the patients and the tumor. The mechanism of tumor elimination by restoring the body’s immune ability is consistent with the concept of “nourishing positive accumulation and eliminating cancer by itself” in traditional Chinese medicine (TCM). The formation and dynamic changes in the tumor microenvironment (TME) involve many different types of cells and multiple signaling pathways. Those changes are similar to the multitarget and bidirectional regulation of immunity by TCM. Discussing the relationship and mutual influence of TCM and antitumor therapy on the TME is a current research hotspot. TCM has been applied in the treatment of more than 70% of cancer patients in China. Data have shown that TCM can significantly enhance the sensitivity to chemotherapeutic drugs, enhance tumor-suppressing effects, and significantly improve cancer-related fatigue, bone marrow suppression, and other adverse reactions. TCM treatments include the application of Chinese medicine monomers, extracts, classic traditional compound prescriptions, listed compound drugs, self-made compound prescriptions, as well as acupuncture and moxibustion. Studies have shown that the TCM functional mechanism related to the positive regulation of cytotoxic T cells, natural killer cells, dendritic cells, and interleukin-12, while negatively regulating of regulatory T cells, tumor-associated macrophages, myeloid-derived suppressive cells, PD-1/PD-L1, and other immune regulatory factors. However, the application of TCM in cancer therapy needs further study and confirmation. This article summarizes the existing research on the molecular mechanism of TCM regulation of the TME and provides a theoretical basis for further screening of the predominant population. Moreover, it predicts the effects of the combination of TCM and antitumor therapy and proposes further developments in clinical practice to optimize the combined strategy.

Treatment of malignant tumors encompasses multidisciplinary comprehensive diagnosis and treatment and reasonable combination and arrangement of multidisciplinary treatment, which is not a simple superimposition of multiple treatment methods, but a comprehensive consideration of the characteristics and specific conditions of the patients and the tumor. The mechanism of tumor elimination by restoring the body's immune ability is consistent with the concept of "nourishing positive accumulation and eliminating cancer by itself" in traditional Chinese medicine (TCM). The formation and dynamic changes in the tumor microenvironment (TME) involve many different types of cells and multiple signaling pathways. Those changes are similar to the multitarget and bidirectional regulation of immunity by TCM. Discussing the relationship and mutual influence of TCM and antitumor therapy on the TME is a current research hotspot. TCM has been applied in the treatment of more than 70% of cancer patients in China. Data have shown that TCM can significantly enhance the sensitivity to chemotherapeutic drugs, enhance tumor-suppressing effects, and significantly improve cancer-related fatigue, bone marrow suppression, and other adverse reactions. TCM treatments include the application of Chinese medicine monomers, extracts, classic traditional compound prescriptions, listed compound drugs, self-made compound prescriptions, as well as acupuncture and moxibustion. Studies have shown that the TCM functional mechanism related to the positive regulation of cytotoxic T cells, natural killer cells, dendritic cells, and interleukin-12, while negatively regulating of regulatory T cells, tumor-associated macrophages, myeloid-derived suppressive cells, PD-1/PD-L1, and other immune regulatory factors. However, the application of TCM in cancer therapy needs further study and confirmation. This article summarizes the existing research on the molecular mechanism of TCM regulation of the TME and provides a theoretical basis for further screening of the predominant population.

INTRODUCTION
Cancer is one of the major noncommunicable chronic diseases that seriously affect human health (1)(2)(3). Although treatment methods and drug research and development continue to improve, many problems such as drug resistance and recurrence still hinder progress (4)(5)(6)(7)(8)(9). The treatment of malignant tumors encompasses multidisciplinary comprehensive diagnosis and treatment (10)(11)(12) and reasonable combination and arrangement of multiple treatment methods including surgery, radiotherapy, chemotherapy, targeted therapy, immunotherapy, endocrine therapy, interventional therapy (13)(14)(15)(16)(17)(18)(19). Multidisciplinary treatment is not a simple superimposition of multiple treatment methods, but a comprehensive consideration of the characteristics and specific conditions of the patients and the tumor, leading to a planned and reasonable choice (20). While pursuing prolonged survival (21), attention should also be given to improving the quality of life of the patients (22).
Due to its huge population, China accounts for about a quarter of the world's new tumors and deaths, leading to a serious disease burden (23)(24)(25). Traditional Chinese medicine (TCM) is a unique diagnosis and treatment method with thousands of years of history (26,27). It is reported that most Chinese cancer patients have used TCM during the diagnosis and treatment process (28,29). TCM is mostly used in the form of compound prescriptions in clinical oncology, including oral herbal medicines, granules or capsules, and injections (30). There are many pieces of research on Chinese medicine monomers and their active ingredients (31). Many researches have shown that TCM combined with antitumor therapy can achieve significant tumor suppression effects, reduce drug resistance, and improve adverse reactions and patient quality of life (32)(33)(34)(35)(36). In recent years, targeting the immune checkpoints CTLA-4, PD-1, and PD-L1 has led to breakthroughs in a variety of cancer types (37)(38)(39). The mechanism of tumor eradication by restoring the body's immune ability is consistent with the concept of "nourishing positive accumulation and eliminating cancer by itself" or "strengthening vital Qi to treat cancer" in TCM (40,41).
The tumor microenvironment (TME) (42,43) is formed by the structural components such as tumor cells, endothelial cells, fibroblasts, immune cells, extracellular matrix, and secreted cytokines. It has three main roles: inhibiting the immune response, promoting angiogenesis, and growing cancer stem cells. Chronic inflammation (44,45) and immunosuppression (46,47) are the core features of the TME. Chronic inflammation leads to low oxygen levels, low pH, high pressure in the microenvironment, and the prolonged existence of inflammatory factors such as tumor necrosis factor (TNF) that maintain and continuously aggravate the inflammatory features of the TME. The hypoxic microenvironment increases hypoxia inducible factor (HIF) levels, induces the formation of new blood vessels, modifies the vascular endothelial growth factor (VEGF), and recruits bone marrow-derived endothelial progenitor cells to form new blood vessels. The TME enables a large number of regulatory T cells (Tregs) that penetrate and accumulate in tumor tissues, inhibit the differentiation and maturation of effector cells such as lymphocytes, macrophages, dendritic cells (DC), and isolate them from tumor tissues to inhibit immune responses. The immunosuppressive microenvironment is closely related to the "deficiency of vital Qi" in Chinese medicine (48). The "syndrome" of TCM involves multiple systems and levels of Western medicine. TCM treatment of cancer pays attention to overall regulation whether it is to strengthen the body (Fu Zheng) or eliminate evil (Qu Xie). Its advantage lies in regulating the tumor-host microenvironment, allowing normal immune cells to perform their duties, so that there is no environment for tumor cells to survive, and causing apoptosis or autophagy (49)(50)(51).
In this review, we mainly discuss, from the perspective of TME regulation, the studies on the combined application of TCM and anticancer treatments. Further screening of dominant populations and predictors will help optimize the joint strategy and provide a theoretical basis for clinical practice.

TCM COMBINED WITH CHEMOTHERAPY
Chemotherapy is still the cornerstone of anticancer therapy. As one of the most important treatments for advanced stage cancer, chemotherapy compatibility ensures the correct combination of chemotherapeutic drugs and the combination of chemotherapy and other types of treatment. There are many clinical and preclinical studies on the combination of TCM treatment and chemotherapy.
The clinical studies on TCM combined with chemotherapy for anticancer treatment research have been mainly published in Chinese journals ( Table 1). The studied cancer types include lung cancer (52)(53)(54)(55)(56)(57)(58), digestive system cancer (gastric/liver/ esophageal cancer) (59)(60)(61)(62)(63), gynecological cancer (ovarian    (79)], and self-made compound prescriptions (80,81). The most representative ones are curcumin, PHY906, and tonic Chinese medicines. The combination of curcumin and chemotherapy has been proven to overcome multidrug resistance [FOLFOX (70), oxaliplatin (71,73), 5-Fu (72)] through a number of in vivo and in vitro studies. The effect of this combined therapy may upregulate Bax, caspase-3, and PARP and downregulate EGFRs (such as IGF-1R), Bcl-2, survivin, HSP70, Nrf2, Bcl-2/Bax, NF-kB, p-p65, and TGF-b/ Smad2/3. PHY906 is derived from the classic formula prescription Huangqin decoction; however, instead of the separation and purification of the possible active compounds, it is taken as a whole. In-depth research via animal experiments, clinical trials, and quality control of PHY906 have been conducted. Results showed that PHY906 could significantly increase the antitumor activity of CPT-11, decreasing toxicity in normal tissues while promoting cell death within the TME, and that its effect may be upregulated by IRF-1, IRF-5, CCL-2/ MCP-1, and CCL-5/RANTES. Tonic Chinese medicines include monomers, water extracts, the classic compound Shiquan Dabu decoction, or self-made prescriptions, and their mechanism of action may be related to the regulation of macrophage polarization, the reduction of epithelial cell-mesenchymal transition, and cell stemness.

TCM COMBINED WITH TARGETED THERAPY OR IMMUNOTHERAPY
Unlike the destructive antitumor effects of traditional chemotherapy, new molecular targeted therapies target specific molecular changes in cancer (16). They have achieved significant effects in clinical practice in recent years and have also triggered a change in the concept of anticancer treatment. The immunotherapeutic approach involves the restart and maintenance of the tumor immune cycle, the restoration of the body's normal antitumor immune response, and the control and elimination of cancer, by means such as monoclonal antibody immune checkpoint inhibitors, therapeutic antibodies, tumor vaccines, cell therapy, and small molecule inhibitors. Among them, PD-1 inhibitors lead the treatment of malignant tumors into a new era of immunotherapy (17,38,39). There are currently many studies on the combined application of TCM and targeted drugs or immunotherapy, focusing on improving efficacy, reversing drug resistance, and reducing adverse reactions. Some research results describing the impact of TCM combined with targeted drugs ( Table 3) and immunotherapy ( Table 4) on the TME have been released; however, there are still more treatment aspects that need further clarification.
In addition to evaluating the immune function of peripheral blood, some studies involved the detection of tumor-specific markers and tumor tissue-related factors to evaluate the invasion ability of tumors. Clinical studies on non-small-cell lung carcinoma (NSCLC) (82), hepatocellular carcinoma (83), and ovarian cancer (84) use compound preparations, including classic traditional formula Xuefu Zhuyu decoction (82) and listed compound herbal medicine Shenqi Fuzheng injection (83). During in vitro and in vivo studies, apart from observing the effects on tumor cell proliferation, the related signaling pathways were explored, including the NF-kB and p38 MAPK signaling cascades mediated by TNFR1 in hepatocellular carcinoma cells (compound Kushen injection) (85) and the AKT pathway in gastric cancer and pancreatic cancer cells (Astragalus polysaccharide) (86,87). YIV-906, which is based on PHY906 (88), is a clinical observation drug that ensures >90% consistency in product quality (89). It is also the first Chinese medicinal project to be awarded a grant from the PO1 program of the National Cancer Center of the United States. Animal experiments show that PHY906 may potentiate sorafenib action and that its mechanism of action involves an increase in hMCP1 expression, enhanced infiltration of macrophages into tumors with a higher M1/M2 expression pattern, and upregulation of AMPKa-P and ULK1-S555-P. Computer simulation methods have also been used in the analysis of its mechanism of action and key components (90).   Whether tonic herbal medicine be used in combination with immunotherapy is one of the issues that Chinese cancer patients are extremely concerned about; moreover, it is a very controversial issue for cancer clinicians. Research on the combination of TCM and immunotherapy mainly includes in vivo and in vitro studies, while clinical studies are rarely conducted. The TCM studied mostly include tonic drugs or their components: Astragalus (91), ginsenoside Rg3 (92), Glycyrrhiza uralensis water extract (93), and bisdemethoxycurcumin (94). Most of the compound prescriptions are classic medicines, including Gegen Qinlian decoction (95), Renshen Yangrong decoction (96), Shiquan Dabu decoction (97), Guipi decoction (98), and Buzhong Yiqi decoction (99). The components of Astragalus can downregulate PD-L1 on the tumor cell surface, which may be related to the AKT/mTOR/ p70S6K pathway (91). In vivo studies have shown that TCM combinations have a positive effect on therapeutic curative potential and tumor inhibition. Some studies also explored the intestinal flora; however, in clinical observation, the main observed effect remains the improvement of symptoms. Both TCM treatment and immunotherapy have systematic and complex characteristics. Determining whether TCM affects the efficacy or the adverse effects of immunotherapy by regulating the TME and related factors necessitates further research.

TCM COMBINED WITH LOCAL TREATMENT
Malignant tumors require different treatment strategies according to the different stages of the disease. Additionally, local treatment plays an important role in the treatment of cancer. Early radical surgery is the most effective way to obtain a curative effect and long-term survival. Radiotherapy and interventional therapy can obtain survival benefits and symptom improvement through the control of local lesions. The combined citation of TCM and local treatment have been clinically observed to reduce perioperative complications, promote the recovery of immune function, reduce recurrence and metastasis, and improve long-term prognosis. The TCM involved are mostly listed drugs ( Table 5), and research on their mechanism of action is relatively lacking and limited to peripheral blood immune function detection.

DISCUSSION
The clinical application of TCM has a long history, and its treatment principles and philosophy have a unique system. With the continuous improvement of research methods, our understanding of TCM is deepening. The study of herbal medicine monomers and their components is relatively easy to explain; however, compound prescription and compatibility are more characteristic of TCM holistic thinking. TCM has its advantages and specifics in the treatment of cancer. In addition to reducing the side effects of antitumor treatment and improving the symptoms and patient quality of life, it also "supports the healthy Qi" and restores the body's own immune system. It can improve efficacy and prolong survival in the comprehensive treatment of cancer. Many studies on the monomers or components of herbal medicine have confirmed that they affect related factors in the TME; however, their effects in a more complex system are relatively unexplored. This review summarizes and analyzes the influence and effect of TCM in combination with antitumor therapy, including chemotherapy, targeted therapy, immunotherapy, the perioperative period, radiotherapy, and interventional therapy. Relevant Chinese medicines include marketed drugs (injections, oral liquids, and tablets), traditional prescriptions, and self-developed experiential prescriptions, as well as many Chinese medicinal monomers or ingredients. Tonic drugs are the main active agents, including multiple treatments such as replenishing Qi, invigorating the spleen, promoting blood circulation, eliminating phlegm, clearing heat, and dispelling stagnation. It is well known that the immune system of body, plays defensive, protective and eliminative roles on tumor cells. For example, NK cells can directly recognize and eradicate tumor cells; Dendritic cells (DCs) can activate adaptive immunity; macrophages (M) can kill tumor cells by generating cytotoxicity, which related to the production of effector molecules and accompanying phagocytosis. Clinical studies have shown that adding TCM to the treatment strategy can significantly improve patient symptoms without increasing adverse reactions, with a tendency to prolong survival. The detection of peripheral blood-related immune factors suggests that TCM has a regulatory effect on immune function and that it can promote a healthy Th1/Th2 balance and regulate the polarization of macrophages. Peripheral blood is the most commonly used medium for disease diagnosis and has been widely accepted by patients for noninvasive molecular diagnosis. In addition, compared with the tumor tissue sample, the dynamic change of microenvironment is ignored, and the peripheral blood can be sampled for many times regularly, which is convenient for monitoring. In relevant in vivo and in vitro studies, possible mechanisms of action have been discussed, including the classical NF-kB, AKT, and TLR4 signaling pathways and the intestinal flora. However, TCM treatment still needs to go through top-level design, good quality control, reverse verification, and in-depth research that can reproduce results to demonstrate the role of TCM in the comprehensive treatment of tumors and clarify its therapeutic mechanism.

CONCLUSIONS
Cancer treatment has multiple stages and high complexity, and the optimal approach includes multidisciplinary comprehensive diagnosis and treatment. TCM has its unique advantages and characteristics that are different from other types of antineoplastic treatment, and these should not be ignored. However, current research results cannot clearly explain the dominant population and mechanism of effect of TCM combined with antitumor therapy; however, the impact on the TME may be the core principle of this approach. More evidencebased experimental research is still needed to provide a basis for formulating better combined strategies for cancer treatment.