Astragalus polysaccharide as a potential antitumor immunomodulatory drug (Review)

Introduction

In recent years, the incidence of new global cancer cases has been consistently increasing. As indicated in the World Health Organization report, malignant neoplasms are among the predominant causes of mortality worldwide, and it is estimated that the global population of patients with cancer will reach 28.4 million by 2040 (1). The onset and progression of malignant tumors are closely associated with the tumor microenvironment, which comprises tumor cells, immune cells, fibroblasts, the extracellular matrix and a variety of cytokines that enhance the proliferation, migration and immune evasion capabilities of tumor cells (2). As research into the tumor microenvironment has increased, tumor immunotherapy has emerged as a novel and promising therapeutic modality for the management of cancer. This type of therapy uniquely activates the immune microcirculation, thereby eliciting a targeted immune response against tumor cells and facilitating their destruction. Traditional Chinese Medicine (TCM) has been used in the treatment of tumors, with an emphasis on individualized patient care (3). Throughout the course of tumor progression, TCM has demonstrated efficacy in bolstering immunity, thereby restoring and maintaining normal immune functions, preventing the invasion of exogenous pathogens and mitigating the detrimental effects of tumor cells. These principles are consistent with the modern medical concept of tumor immunotherapy. Astragalus polysaccharide (APS) can modulate various immune cells, including macrophages, dendritic cells (DCs), myeloid-derived suppressor cells (MDSCs), T lymphocytes and natural killer (NK) cells, thereby altering their functional activities and enhancing immune responsiveness. Consequently, the tumor immune microenvironment and the cytotoxic capability of immune cells against tumor cells are improved (4). In the present study, research on the antitumor immune properties of APS has been comprehensively reviewed, and its regulatory effects on immune cells and the underlying antitumor mechanisms are discussed, thereby providing a theoretical foundation for the development of novel Astragalus-based antitumor pharmacological agents.

Chemical structure of APS

Huangqi (Jianqi) is the dried root of the leguminous plant Astragalus mongholicus or Astragalus membranaceus (5). As a common traditional Chinese herbal medicine, Astragalus has a history of more than 2,000 years of use in China, is used extensively in numerous countries and has been included in the pharmacopeias of the United States, Japan, and South Korea (6). The constituents of Astragalus encompass a variety of compounds such as APS, triterpenoid saponins, alkaloids, flavonoids and different mineral elements (7–10). APS, which serves as the principal active constituent within Astragalus, is characterized by its copious availability, affordability, and minimal toxic or adverse effects (11). The categorization of polysaccharides is performed in accordance with the structural classification principles employed for proteins and DNA. Investigating the constituent monosaccharides, relative molecular mass and sequential arrangement of monosaccharide residues is necessary to elucidate the structure of APS (12). APS is classified as an acidic complex polysaccharide, predominantly comprising glucose (Glc), galactose (Gal), arabinose (Ara), rhamnose (Rha) and mannose (Man). Infrared spectroscopic analysis has revealed that APS possesses the typical infrared absorption characteristics of sugar, along with distinct signals that are attributed to the carboxyl group, indicating the presence of glucuronic acid. The signals corresponding to sugar rings indicate that APS constitutes a pyranose ring structure with α and β configurations (13). Glucose is a monosaccharide based on a glucopyranose structure, including α-D-Glu, β-D-Glu and α, β-D-Glu, with β-D-Glu being the most extensively investigated monosaccharide (14).

Given the presence of numerous branched chains and a number of hydroxyl groups, APS exhibits a high degree of structural diversity and complexity (Table I). APS MAPS-5 is composed of an α-D-(1–4) Glc backbone, where, on average, approximately two out of every 15 sugar residues at the C-6 position are substituted with terminal glucose residues, with an approximate relative molecular weight of 1.32×104 (15). Furthermore, three distinct polysaccharides, namely APS I, II and III, have been isolated from the aqueous extract of Astragalus roots sourced from Inner Mongolia. APS I is a heteropolysaccharide composed of D-Glu, D-Gal, and L-Ara in a molar ratio of 1.75:1.63:1, with an average relative molecular mass of approximately 3.63×104. APS II and III are predominantly composed of D-Glu, with average relative molecular weights of ~1.23×104 and 3.46×104, respectively. Their structure primarily comprises α-(1→6)-D-Glc condensation linkages, with a minor fraction of α-(1→6)-D-Glc condensation bonds (16). Through complete acid hydrolysis, four types of APS have been differentiated, including two glucans (AG-1 and AG-2) and two heteropolysaccharides (AH-1 and AH-2) (17). AG-1 is a water-soluble glucan, whereas AG-2 is water insoluble and is characterized as an α-(1→4) glucan. AH-1 is an acidic, water-soluble polysaccharide; after hydrolysis, AH-1 contains hexuronic acids (galacturonic and glucuronic acids), glucose, rhamnose and arabinose, as identified through paper chromatography, with a molar ratio of 1:0.04:0.02:0.01. AH-2 is also water soluble, and the presence of glucose and arabinose can be confirmed via paper and gas chromatography post-hydrolysis, with a molar ratio of 1:0.15 (17).

Table I. Chemical structure of APS.

Table I.

Chemical structure of APS.

First author, year	Polysaccharide name	Monosaccharide composition	Relative molecular mass	Quantity ratio of matter	(Refs.)
Lin, 2009	APS MAPS-5	α-D-(1–4) Glc	1.32×104	-	(15)
Fang, 1982	APS I	D-Glu, D-Gal, L-Ara	3.63×104	1.75:1.63:1	(16)
Fang, 1982	APS II	D-Glu	1.23×104	-	(16)
Fang, 1982	APS III	D-Glu	3.46×104	-	(16)
Huang, 1982	AG-1	Water-soluble glucan	-	-	(17)
Huang, 1982	AG-2	α-(1→4) glucan	-	-	(17)
Huang, 1982	AH-1	Hexuronic acids (Glc UA and Gal UA),	-	1:0.04:0.02:0.01	(17)
		Glc, Rha and Ara
Huang, 1982	AH-2	Glc and Ara	-	1:0.15	(17)
Tang, 2014	APS-I	Man, Rha, Glc UA, Gal UA, Glc, Gal	1.06×104	29.12:1.89:4.00:1.35:1:81.97	(18)
Tang, 2014	APS-II	Man, Rha, Glc UA, Gal UA, Glc, Gal, Xyl	2.47×106	50.46:1.16:1:2.27: 2.66:15.72:7.86	(18)
Cao, 2020	APS	Rha, Gal UA, Glc, Gal, Ara	300-2×106	0.43:0.23:19.36:0.69:1	(19)
Liu, 2020	APS I	Gal, Glc	-	1:49.76	(20)
Liu, 2020	APS II	Rha, Gal, Glc	-	1:2.99:16.26	(20)

[i] ‘-’ indicates that it is not mentioned in the literature. APS, Astragalus polysaccharide; Glc, glucose; Gal, galactose; Ara, arabinose; Man, mannose; Rha, rhamnose; Glc UA, glucuronic acid; Gal UA, galacturonic acid; Xyl, xylose.

Tang et al (18) extracted and isolated polysaccharides from Astragalus, and characterized their physicochemical properties and structure. This previous study revealed that the composition of monosaccharides in APS-I was mannose (Man), rhamnose (Rha), Glc uronic acid (Glc UA), Gal UA, Glc and Gal, with a substance amount ratio of 29.12:1.89:4.00:1.35:1:81.97, whereas the composition of monosaccharides in APS-II was Man, Rha, Glc UA, Gal UA, Glc, Gal and xylose, with a substance amount ratio of 50.46:1.16:1:2.27:2.66:15.72:7.86. The relative molecular mass of APS-I was ~1.06×104, and the relative molecular mass of APS-II was ~2.47×106. Cao et al (19) extracted from the experiment that the total polysaccharide content of APS was 74.6%, the protein content was 0.42%, and the relative molecular weight distribution was 300–2×106. In addition, its monosaccharide composition was Rha, Gal UA, Glc, Gal and Ara, with a substance amount ratio of 0.43:0.23:19.36:0.69:1. Furthermore, a previous study characterized the monosaccharide group of two polysaccharides obtained from the isolation and purification of Astragalus membranaceus from Mongolia using ion chromatography. The results showed that APS I was composed of Gal and Glc, with a substance amount ratio of 1:49.76, whereas APS II mainly consisted of Rha, Gal and Glc, with a substance amount ratio of 1:2.99:16.26 (20). Although a number of technical methods have been used to analyze APS, the specific molecular structure of APS remains difficult to elucidate, which results in certain obstacles in the study of the molecular mechanism underlying APS bioactivity.

APS regulates tumor immune function

As academic research on tumor immunotherapy continues, the successful development of immune checkpoint inhibitors that target PD-1, CTLA-4 and PD-L1 has led to the gradual emergence of immunotherapy as a potential method to treat tumors (21). Body immunity is divided into active and passive immunity. Tumor cells evade elimination by the immune system, which not only promotes the state of immunosuppression and the production of regulatory immune cells, but also recruits a large number of protumor cells to establish a tumor microenvironment (22). In the tumor microenvironment, tumor cells inhibit the antigen presentation function of DCs; promote the polarization of tumor-associated macrophages from M1 type to M2 type; release the immune-suppressive factors TGF-β, VEGF and IL-10; deliver the abnormally expressed tumor-associated antigens to T cells; induce T-cell apoptosis; and inhibit the function of cytotoxic T lymphocytes, which together affect the immune system and result in tumor immune escape and the acceleration of tumor progression (23–26). In determining the mechanism underlying tumor immune escape, current immunotherapeutic drugs, including drugs that block negative regulatory signals, immunostimulatory drugs, tumor vaccines and exogenous recombinant cytokines, have been used (27), among which immune checkpoint inhibitors, such as anti CTLA-4 and anti-PD-1/PD-L1 antibodies, have shown good therapeutic effects in clinical tumor therapy. A number of active ingredients of TCM have shown effects on regulating immune cells and improving the tumor microenvironment, indicating their therapeutic potential in synergistic radiotherapy and chemotherapy against tumors (28).

APS, as a main active ingredient of Astragalus, has important pharmacological effects, including immune-regulatory and antitumor properties, which can regulate the microenvironment of numerous solid tumors, improve the state of immune suppression and inhibit tumor growth. APS inhibits the M2-type polarization of tumor-associated macrophages, increases cytotoxic T lymphocyte infiltration, and promotes the maturation of DCs and the function of NK cells (29). In addition, APS not only reduces the release of the immunosuppressive factors IL-10, TGF-β and VEGF in the tumor microenvironment, but also increases the levels of the immune activating factors TNF-α, IL-2 and IL-12 (30). In clinical trials for patients with cancer, APS has been shown to relieve pain, nausea and vomiting symptoms, enhance immune function, reduce the toxic side effects of chemotherapeutic drugs and improve quality of life (31) (Table II; Figs. 1 and 2).

Figure 1. Effects of APS on tumor immune cells and their associated factors. APS, Astragalus polysaccharide; CTL, cytotoxic T lymphocyte; IFN, interferon; iNOS, inducible NO synthase; NK, natural killer; NO, nitric oxide; ROS, reactive oxygen species; Th, helper T; Treg, regulatory T.

Figure 2. Mechanisms of APS interference with tumor cell-associated pathways. APS, Astragalus polysaccharide; eNOS, endothelial nitric oxide synthase; ER, endoplasmic reticulum.

Table II. Antitumor immune mechanism of APS.

Table II.

Antitumor immune mechanism of APS.

Pharmacological action	Function	Intervention model	(Refs.)
Regulation of macrophage M1/M2 type polarization	Promoted the production of cytokines, such as IL-6, TNF-α and inducible NO synthase, through the Notch signaling pathway to activate M1 macrophages, while inhibiting macrophage M2-type polarization	Non-small cell lung cancer; mouse macrophages	(36)
	Influence cell-cell interactions, specifically through modulation of macrophage M2 polarization and CD8+	Macrophages	(37)
	T-cell exhaustion, thereby overcoming DDP resistance
	Increased the proportion of M1 macrophages in HCC tumor cell tissues and decreased macrophage M2 type polarization	Macrophages in HCC tumor cell tissues	(38)
	Increased cellular energy metabolism and enhanced phagocytosis of RAW 264.7 mouse monocyte macrophages	RAW 264.7 monocyte macrophages	(39,40)
	Activated mouse macrophages through TLR4-mediated signaling pathways, upregulated the expression of p-p38, p-ERK1/2 and p-JNK, induced the degradation of IκB-α and NF-κB translocation, and ultimately produced TNF-α, IL-6 and nitric oxide	Macrophages	(41,42)
	Enhanced the proliferation and phagocytic function of mouse macrophages; increased the content of IL-2, TNF-α and IFN-γ in the peripheral blood of mice	Macrophages	(43)
Induces activation of DCs	Induced the DC cell surface molecule CD80 through the TLR2/4 receptor-mediated MyD88 signaling pathway, CD86 and IL-12 expression, increased T lymphocyte activity and released more IL-2	DCs	(49,50)
	Activated DCs through the TLR-mediated signaling pathway, increased the expression of co-stimulatory molecules on the surface of DCs, and enhanced the antigen-presenting ability of tumor cells, which led to the rapid migration of DCs to lymph nodes, activated the initial T lymphocytes in the lymph nodes and inhibited the growth of secondary lymph node tumors	DCs	(51)
	After APS treatment, DCs promoted the proliferation of CD4+ and CD8+ T cells in mice with breast cancer; DC-based antitumor vaccine increased the expression of CD40, CD80 and CD86 molecules on the surface of DCs, and inhibited the proliferation and metastasis of breast cancer tumor cells	DCs	(52)
	APS activated BDCA1 and BDCA3 peripheral blood dendritic cells elicited proliferative activation of autologous T cells	Monocyte-derived DCs	(53)
Reduces the number and activity of MDSCs	Suppress MDSC generation and reduce the MDSC population in the spleens of tumor-bearing mice.	MDSCs in the spleen of tumor-bearing mice	(57)
	Concurrently, APS downregulates immunosuppressive cytokines (IL-10, TGF-β) while upregulating pro-inflammatory cytokines (IFN-γ, TNF-α), thereby enhancing overall immune function
	Reduced the number of MDSCs, decreased the immunosuppressive activity of MDSCs in mice with melanoma, decreased the expression of Arg-1, IL-10 and TGF-β, and enhanced the killing ability of CD8+ T cells on tumor cells	MDSCs in mice with melanoma	(58)
	Reduced the level of myeloid-derived suppressor cells in the peripheral circulation of lung cancer, continuously improve imune function, and increas teh overall clinical treatment efficacy	MDSCs	(59)
	The protein and gene expression of S1PR1, STAT3 and p-STAT3 in the S1PR1/STAT3 signaling pathway was inhibited by APS, which interfered with the S1PR1/STAT3 signaling pathway and inhibited the accumulation of MDSCs in the pre-metastatic niche	MDSCs	(61)
Reversal of Th1 to Th2 conversion	Increased the expression of Th1 cytokines IL-2 and IFN-γ, decreased the expression of Th2 cytokines IL-4 and IL-10, regulated the ratio of Th1/Th2 cells, reversed the conversion of Th1 to Th2 cells through the TLR4/ MyD88/NF-κB signaling pathway	Th1 cells and Th2 cells in the spleen tissue of mice with lung cancer	(62)
	Not only increased the immune organ index in mice with lung cancer, but also increased the ratio of CD4+/CD8+ cells in mouse serum and significantly enhanced cellular immune activity	CD4+/CD8+ T cells	(63,64)
	Protected the immune organs and regulated the ratio of CD4+/CD8+ T lymphocyte subpopulations	CD4+/CD8+ T lymphocyte subpopulations	(65)
	Increases the number of allogeneic T lymphocytes and the expression level of IL-12 and IFN-γ. In addition, cytotoxic T lymphocytes activated by sensitized DCs kill tumor cells	T lymphocyte	(66)
Suppression of Treg cells	Reduced the number of Treg cells, decreased the expression of	Treg cells	(69)
	splenic TGF-β and IL-10 in mice with malignant melanoma,
	and inhibited tumor cell metastasis of malignant melanoma in
	mice Repaired cytokine imbalance, inhibited forkhead box	Treg cells	(70)
	protein P3 expression, and suppressed Treg cell activity and function
	Inhibited the expression of TGF-β by activating the TLR4-mediated signaling pathway, which reduced the number of Treg cells	Treg cells	(71)
	Reduced Treg frequency, decreased anti-inflammatory cytokines including IL-10, TGF-β, and VEGF-A, and incresaed the pro-inflammatory cytokine IL-6	Treg cells	(72)
Enhances NK cell killing activity	Inhibited the proliferation of leukemia cells by activating MHC class I polypeptide-related sequence A molecules on the surface of leukemia cells and enhanced the killing sensitivity of NK cells to HL-60 leukemia cells	NK cells	(78)
	Protected immune organs and stimulated NK cells to kill tumor cells; promoted the proliferation of splenic lymphocytes, improved NK cell activity and enhanced the immune response ability of rats with cancer	Lymphocytes and NK cells	(79,80)
Inhibition of tumor cell proliferation	Inhibited the proliferation of ovarian cancer cells through the miR-27a/F-box and WD-40 structural domain protein 7 signaling pathway	Ovarian cancer cells	(81)
	Inhibited the proliferation and migration of NSCLC cells and its mechanism of action was related to the regulation of the miR-195-5p signaling pathway; improved the morphology of lung cancer cells by inhibiting the lung cancer cell cycle	NSCLC cells	(83,84)
	Inhibited TGF-β1-induced EMT of A549/DDP cell xenografts of lung adenocarcinoma, which was associated with the inhibition of PI3K/ Akt pathway protein activation	A549/DDP lung adenocarcinoma cell graft tumor	(85)
	APS had an inhibitory effect on the cell proliferation of nasopharyngeal carcinoma cell lines in a time-and dose-dependent manner; the combination of APS and cisplatin inhibited the migration and invasion of CNE-1 cells	CNE-1 nasopharyngeal carcinoma cells	(86)
	Inhibited the proliferation of SW620 human colorectal cancer cells, which were subjected to cyclic inhibition, with a predominant G2/M phase block	SW620 colorectal cancer cells	(87)
	Inhibited the proliferation and invasion of PCa cells in a time-and dose-dependence manner; inhibited tumorigenesis and lipid metabolism by modulating the miR-138-5p/SIRT1/SREBP1 pathway in PCa	PCa cells	(88)
	Enhanced miR-133a expression and inhibited the JNK signaling pathway in MG63 osteosarcoma cells, which suppressed proliferation, migration and invasion, and induced apoptosis in MG63 cells	MG63 osteosarcoma cells	(89)
Induces apoptosis in tumor cells	Promotes the anti-proliferative and apoptotic effects of cisplatin on nasopharyngeal carcinoma cells, by regulating the expression levels of Bax/Bcl-2, caspase-3, and caspase-9	Nasopharyngeal carcinoma cells	(94)
	Decreased the viability of GC cells and accelerated their apoptosis in a time- and dose-dependent manner; MGC-803 cells treated with APS showed typical apoptotic morphology and cell cycle arrest in S phase	GC cells	(95)
	Promoted apoptosis in NSCLC cell lines by inhibiting the expression of Notch1 and Notch3, and upregulating tumor suppressors	NSCLC cells	(97–101)
	Induced apoptosis of tumor cells and prevented cell transformation from G1 phase to S phase of cell cycle	-	(102)
	Decreased the number of HepG2 liver cancer cells in S phase, and differentiated the cells into G0/G1 and G2/M phases, which blocked the cellular proliferation cycle and induceed cancer cell apoptosis	HepG2 cells	(103,104)
	Promoted the apoptosis of HepG2 cells, increased the activity of caspase-3, decreased the expression of ERK1/2, inhibited the ERK1/2 signaling pathway and promoted the apoptosis of tumor cells	HepG2 cells	(105)
	APS dose-dependently promoted Dox-induced apoptosis and enhanced ER stress; decreased O-GlcNAc transferase mRNA levels and protein stability, and increased O-GlcNAcase expression; promoted Dox-induced apoptosis in HCC cells by decreasing O-GlcNAcylation, leading to increased ER stress and activation of apoptotic pathways	HCC cells	(114)
Inhibits tumor cell invasion and metastasis	Inhibited the growth and metastasis of Lewis lung cancer in mice, improved the function of the immune organs, and inhibited the expression of VEGF and EGFR proteins in tumor tissues	Lewis lung cancer	(116)
	Downregulated NF-κB transcriptional activity, and significantly inhibited the proliferation and metastasis of A549 and NCI-H358 NSCLC cells	A549 and NCI-H358 NSCLC cells	(117,118)
	Inhibited angiogenesis of human colorectal cancer tumors and decreased VEGF expression in tumor tissue	Colorectal cancer tumor	(119)
	Inhibited the proliferation and invasion of HS-746T cells, suppress the expression of miR-25 mRNA, and promote the expression of FBXW7 mRNA. It can also reverse the increase in MiR-25 and the suppression of FBXW7 caused by transfection with miR-25 minic. APS inhibits the proliferation and invasion of gastric cancer HS-746-T cells through the miR-25/FBXW7 signaling pathway.	Gastric cancer HS-746T cells	(120)
	Promoted inactivation of the JNK signaling pathway through the upregulation of miR-133a, which inhibited the proliferation, metastasis and invasion, and induced the apoptosis of MG63 osteosarcoma cells	MG63 osteosarcoma cells	(89,121)
	Inhibited the Wnt/β-catenin signaling pathway and reduced breast cancer cell proliferation and EMT-mediated migration and invasion	Breast cancer cells	(122,123)
	Moesin is the target of miR-133a-3p, miR-133a-3p negatively regulates MSN; APS attenuated PD-L1-mediated immunosuppression through the miR-133a-3p/MSN pathway	HCC	(124)
Removal and inhibition of free radicals	Increased superoxide dismutase activity, reduced malondialdehyde content, and scavenged reactive oxygen species and reactive nitrogen species generated by intracellular oxidative stress, but also reduced free radical generation	-	(127)
Reversal or escape of MDR	Inhibited TGF-β1 overexpression-mediated EMT, and suppressed the proliferation of cisplatin-resistant A549 cell xenograft tumors	A549-cell transplanted tumors	(131)
	Induced apoptosis in GC cells and enhanced the pro-apoptotic effect of adriamycin on GC cells; APS acted as a chemotherapeutic sensitizer to a certain extent	GC cells	(95)
	Induced apoptosis in HL-60/A cells, activated the caspase cascade reaction, significantly decreased the expression of MDR-associated protein and increased the concentration of intracellular antitumor drugs	HL-60/A cells	(132)
	Improved the sensitivity of tumor cells to chemotherapeutic drugs, such as apatinib, escaped MDR and strengthened the antitumor effect	-	(133)
	Increased the expression levels of IL-6 and TNF-α, and decreased the expression of IL-10, P-glycoprotein and MDR1.	H22 hepatocellular carcinoma mice	(134)
	APS enhanced the antitumor effect of adriamycin and the sensitization effect of chemotherapy drugs
	Activated the JNK signaling pathway and enhanced the sensitivity of SKOV3 cells to cisplatin	SKOV3 cells	(135)
	Promoted gefitinib-resistant cell apoptosis and decreased cell proliferation and migration; inhibited the PD-L1/SREBP-1/EMT signaling pathway, increased E-cadherin expression, decreased N-cadherin and vimentin protein expression, and reversed acquired resistance to gefitinib in lung cancer cells	Lung cancer cells	(136)

[i] ‘-’ indicates that it is not mentioned in the literature. APS, Astragalus polysaccharide; BDCA, blood DC antigen; DC, dendritic cell; Dox, doxorubicin; EMT, epithelial-mesenchymal transition; ER, endoplasmic reticulum; GC, gastric cancer; HCC, hepatocellular carcinoma; IFN, interferon; MDR, multi drug resistance; MDSC, myeloid-derived suppressor cell; miR, microRNA; NK, natural killer; NSCLC, non-small cell lung cancer; p-, phosphorylated; PCa, prostate cancer; Th, helper T; TLR, Toll-like receptor; Treg, regulatory T.

Regulation of M1/M2 macrophage polarization

Macrophages develop from bone marrow precursor cells and serve an important role in the immune system. They are present in almost all tissues of the body, and they directly phagocytose and kill foreign pathogens. In addition, they can activate lymphocytes or other immune cells in the body to indirectly respond to pathogens, and protect tissues and organs from foreign pathogens (32). Macrophages are classified into M1 and M2 types. M1 macrophages, as classically activated macrophages, produce nitric oxide (NO), reactive oxygen species (ROS), inducible NO synthase and a large number of pro-inflammatory cytokines, such as IL-1β, interferon (IFN)-γ and TNF-α, which participate in the positive immune response. M1 macrophages also have a role in phagocytosis and tumor cell killing (33).

By contrast, M2 macrophages, as alternatively activated macrophages, produce a large number of anti-inflammatory cytokines, such as IL-4, IL-10 and IL-13, which suppress the immune response to a certain extent. Tumor cells selectively recruit bone marrow-derived macrophages to differentiate into M2 macrophages, that is, tumor-associated macrophages (34), which promote tumor cell proliferation, neovascularization, invasion and metastasis (35). Adjustment of M1/M2 macrophage polarization is key to improve the effectiveness of antitumor therapy. In an experimental study in mice with non-small cell lung cancer (NSCLC), APS modulates the Notch signaling pathway, promotes the production of IL-6, TNF-α and iNOS cytokines, activates M1 macrophages, inhibits macrophage M2-type polarization, and enhances phagocytosis and tumor cell killing (36). Bioinformatics analysis indicates that APS may remodel the tumor microenvironment (TME) and influence cell-cell interactions, specifically through modulation of macrophage M2 polarization and CD8+ T cell exhaustion, thereby overcoming DDP resistance (37). Li et al (38) reported that APS can increase the proportion of M1 macrophages in hepatocellular carcinoma cell-derived xenograft tumors, reduce macrophage M2-type polarization and inhibit the proliferation of hepatocellular carcinoma tumor cells. Furthermore, APS improves the energy metabolism of mouse macrophages in vivo, and enhances the phagocytic activity of RAW 264.7 monocyte macrophages and the immune response in mice (39,40). Mouse macrophages activated by APS are triggered through the Toll-like receptor (TLR)4-mediated signaling pathway; this activation event results in the upregulation of the levels of phosphorylated (p)-p38, p-ERK1/2 and p-JNK. Concurrently, it induces the degradation of IκB-α and the subsequent translocation of NF-κB. Eventually, these molecular events culminate in the production of TNF-α, IL-6 and NO, thereby exerting an inhibitory effect on the proliferation of mouse tumor cells (41,42). Li et al (43) demonstrated that APS enhances the proliferation and phagocytic functions of mouse macrophages, increases the peripheral blood levels of IL-2, TNF-α and IFN-γ, and improves the ability of macrophages to phagocytose tumor cells and inhibit the proliferation of tumor cells. Therefore, APS may enhance macrophage phagocytosis and tumor cell killing, and inhibit tumor cell proliferation and metastasis by increasing M1 macrophage activity, regulating M1/M2-type macrophage polarization and altering the levels of immune cytokines.

Induced activation of DCs

DCs are derived from bone marrow CD34+ hematopoietic stem cells and are a specialized class of antigen-presenting cells, which are divided into immature DCs (imDCs), mature DCs (mDCs) and regulatory DCs, in accordance with the different stages of cell differentiation (44). ImDCs capture tumor antigens and are stimulated to differentiate into mature mDCs. The mDCs migrate to draining lymph nodes via lymphatic vessels, where they present the antigens to T cells. They secrete various co-stimulatory molecules such as CD80, CD86, CD40 and CD70, which further activate effector T cells, thereby inducing and maintaining anti-tumor immunity (45,46). Enhancing the function of DCs and promoting T-cell activation in patients with cancer are an effective means of antitumor immunotherapy. APS promotes DC maturation, enhances DC antigen-presenting ability, improves the accuracy of tumor cell killing and inhibits tumor cell proliferation (47). When DCs are induced to mature further, the expression of the DC surface co-radical molecules CD80 and CD86 are enhanced. DCs present antigens to T lymphocytes, T lymphocytes increase IL-12 and IFN-γ secretion, improve antitumor immunity and inhibit the proliferation of tumor cells (48).

APS induces the DC cell surface molecule CD80, CD86 and IL-12 expression through the TLR2/4 receptor-mediated MyD88 signaling pathway, increases T lymphocyte activity, releases more IL-2 and enhances antitumor ability (49,50). APS nanoparticles have been shown to modulate the TLR-mediated signaling pathway to activate DCs, to increase the expression levels of co-stimulatory molecules on the surface of DCs and enhance their tumor cell antigen-presenting ability. Subsequently, DCs migrate rapidly to lymph nodes, activate the initial T lymphocytes at lymph nodes and inhibit secondary lymph node tumor growth (51). Furthermore, after APS treatment, DCs have been shown to promote the proliferation of CD4+ and CD8+ T cells in mice with breast cancer, and a DC-based antitumor vaccine can increase the expression levels of CD40, CD80 and CD86 molecules on the surface of DCs, and inhibit the growth and metastasis of breast cancer cells (52). APS induces morphological changes, the expression of activation markers and the upregulation of inflammatory cytokines in human blood monocyte-derived DCs. In addition, APS promotes the activation of blood DC antigen (BDCA)1 and BDCA3 peripheral blood dendritic cells (pBDCs), which results in the proliferative activation of T cells (53). Therefore, APS may induce the activation of DCs, increased the expression levels of co-stimulatory factors on the surface of DCs, and accelerate the maturation of DCs and antigen presentation. Subsequently, this may enhance the proliferation and activation of CD4+ and CD8+ T cells, and thus the activation of immune function under the stimulation of cytokines, thereby effectively inhibiting the proliferation and metastasis of tumor cells.

Reduced activity of MDSCs

MDSCs are a group of heterogeneous cells derived from the bone marrow; the number of MDSCs in the peripheral blood and spleen is small, accounting for only 4% of cells (54). Under pathological conditions, these immature myeloid-derived heterogeneous cells stop differentiating and become immunosuppressive MDSCs (55,56). MDSCs promote tumor cell survival, angiogenesis, and invasion and metastasis to healthy tissues.

APS inhibit MDSC activity, promote MDSC differentiation and reduce MDSC population. The underlying mechanisms represent a current research hotspot in immunotherapy (51). APS suppress MDSC generation and reduce the MDSC population in the spleens of tumor-bearing mice. Concurrently, APS downregulates immunosuppressive cytokines (IL-10, TGF-β) while upregulating pro-inflammatory cytokines (IFN-γ, TNF-α), thereby enhancing overall immune function (57). Ding et al (58) revealed that APS exert a marked regulatory effect on gut microbiota. APS effectively remodels the gut microbiota environment, and elevates the levels of glutamate and creatine metabolites. Notably, glutamate and creatine serve crucial roles in controlling tumor growth. Simultaneously, APS contributes to the reduction in the number of MDSCs. Furthermore, in mice with melanoma, APS has been shown to curtail the immunosuppressive activity of MDSCs, as manifested by decreased expression levels of Arg-1, IL-10 and TGF-β; this series of changes bolsters the cytotoxic capacity of CD8+ T cells against tumor cells. Consequently, this previous study demonstrated that the tumor inhibition rate in mice with melanoma was significantly increased, highlighting the potential of APS in antitumor regulation. APS has been reported to effectively reduce the level of myeloid-derived suppressor cells in the peripheral circulation of lung cancer, continuously improve immune function, and increase the overall clinical treatment efficacy (59). As a common malignant tumor, lung cancer has shown an increasing trend in morbidity and mortality in recent years (60). Multi-targeted, low-toxicity Chinese medicines intervene in the ‘pre-metastatic niche’, a potentially effective means of preventing and treating tumor metastasis. APS has been reported to inhibit the formation of the lung pre-metastatic niche and suppress the recruitment of lung MDSCs. Mechanistically, the expression of S1PR1, STAT3 and p-STAT3 in the S1PR1/STAT3 signaling pathway have been shown to be inhibited by APS, which interferes with the S1PR1/STAT3 signaling pathway and inhibits the accumulation of MDSCs in the pre-metastatic niche to achieve antitumor effects (61). Therefore, APS promote the rapid differentiation of MDSCs, reduce the number and activity of MDSCs, influence the release of immune cytokines, enhance immune functions, and inhibit the proliferation and metastasis of tumor cells.

Reversal of Th1 to Th2 conversion

T lymphocytes develop from bone marrow pluripotent stem cells, differentiate and mature under the induction of thymic hormone, and become immunologically active T cells, playing antitumor specific immunity. T lymphocytes are classified into CD4+ T and CD8+ T cells in accordance with their surface molecular types. CD4+ and CD8+ are the two functionally different subpopulations of T lymphocytes. CD4+ T cells are also known as helper T cells, which play a role in assisting the induction of body immunity. Th cells are differentiated into functional subpopulations such as Th1 cells, Th2 cells, and Th17 cells. The main functional subpopulation of CD8+ T cells is cytotoxic T lymphocytes. Th1 cells secrete IL-2 and TNF-α and stimulate the development of cytotoxic T lymphocytes. TNF-α, which stimulates the activation of macrophages and NK cells, also secretes IFN-γ to kill tumor cells. Th2 cells secrete IL-4, IL-6, and IL-10, which assist B lymphocytes to produce specific antibodies and participate in humoral immunity. Under normal circumstances, the number of Th1 and Th2 cells remains relatively balanced. However, in patients with tumor, a transformation from Th1 cells to Th2 cells occurs, which suppresses the body's cellular immunity. APS enhances the expression of Th1 cytokines such as IL-2 and IFN-γ in the spleen tissues of mice with lung cancer through the TLR4/MyD88/NF-κB signaling pathway while reducing the expression of Th2 cytokines, including IL-4 and IL-10. It also regulates the ratio of Th1/Th2 cells, reverses the transformation from Th1 to Th2, improves the immunity of mice with lung cancer, and inhibits the growth of tumor cells (62). APS not only improves the immune organ index in mice with lung cancer, but also increases the ratio of CD4+/CD8+ T cells in mouse serum and enhances cellular immune activity (63,64). APS protects immune organs, regulates the ratio of CD4+/CD8+ T lymphocyte subpopulations, and inhibits the growth of mouse tumors in vivo; APS also activates antitumor immune cells, promotes the tumor microenvironment of anaerobic metabolism, and effectively induced apoptosis in tumor cells (65). APS increases the number of allogeneic T lymphocytes and the expression level of IL-12 and IFN-γ. In addition, cytotoxic T lymphocytes activated by sensitized DCs kill tumor cells (66). APS reverses Th1-to-Th2 conversion, maintains Th1/Th2 cell balance, regulates CD4+/CD8+ T cell ratio and T cell-related cytokine expression, enhances cellular immunity, and inhibits tumor growth.

Suppression of regulatory T cells (Tregs)

Tregs are a group of T lymphocytes that can regulate the immune response and maintain immune tolerance. Tumor immunotherapy reduces the number of Tregs, inhibits their cellular activity, maintains immune function of the body and achieves the killing effects against tumor cells. DCs are a key target for Tregs to inhibit the tumor immune response (67), and Tregs interact with tumor-associated CD11c+ DCs to reduce the expression of DC co-stimulatory ligands and inhibit T-cell proliferation. Conversely, the clearance of Tregs leads to the restoration of CD11c+ DC immunogenicity, co-stimulatory ligand expression and CD8+ T-cell activation, which in turn inhibits tumor growth (68).

APS has been reported to reduce the number of Tregs, decrease the expression levels of splenic TGF-β and IL-10, and inhibit the proliferation and metastasis of malignant melanoma tumor cells in mice (69). APS also repairs cytokine imbalance, inhibits forkhead box protein P3 expression, and suppresses Treg activity and function (70). Through the CXCR4/CXCL12 signaling pathway, APS has been shown to inhibit the recruitment of Tregs by stromal cell-derived factor-1, and to attenuate the proliferation and migration of Tregs. Furthermore, APS activates the TLR4-mediated signaling pathway, thereby inhibiting the expression of TGF-β, decreasing the number of Tregs and sustaining the killing effect of immune cells on tumor cells (71). APS significantly enhanced the PBMC proliferation, reduced Treg frequency, decreased anti-inflammatory cytokines including IL-10, TGF-β, and VEGF-A, and increased the pro-inflammatory cytokine IL-6 (72). Therefore, APS may inhibit Treg activity by reducing the number of Tregs and their cytokine secretion, thus restoring and maintaining the normal immune function of T lymphocytes, achieving the killing effect of T cells on tumor cells, and inhibiting tumor growth and metastasis.

Enhanced NK cell killing activity

NK cells are derived from bone marrow lymphoid stem cells, which are important members of the lymphocyte family, and they are not controlled by high-resolution antigen specificity. NK cells can rapidly recognize and remove heterogeneous cells, and the degree of attenuation in vivo is closely related to the malignant progression of tumors (73). NK cells secrete cytokines, regulate adaptive immunity and kill tumor cells, and they have killing and inhibitory effects on hepatocellular carcinoma, NSCLC, breast cancer and ovarian cancer cells (74–77). APS has been reported to inhibit the proliferation of leukemia cells by activating MHC class I polypeptide-related sequence A molecules on the surface of leukemia cells, and enhances the sensitivity of HL-60 cells to NK cell killing activity and inhibit leukemia cell growth (78). Low-molecular-weight APS can protect the immune organs of mice with hepatocellular carcinoma and stimulate NK cells to kill tumor cells. In addition, APS promotes the proliferation of splenic lymphocytes, increases the activity of NK cells and enhances the immune response of rats with cancer (79,80).

Antitumor effects of APS

Inhibition of tumor cell proliferation

Tumor cell proliferation primarily aggravates cancer progression, and pharmacological intervention in tumor cell proliferation serves an important role in tumor therapy. F-box and WD-40 structural domain protein 7 (FBXW7) is a classical tumor suppressor, the translation of which is inhibited by microRNA (miR)-27a in ovarian cancer cells. APS inhibits the proliferation of ovarian cancer cells through the miR-27a/FBXW7 signaling pathway, revealing the therapeutic potential of APS for ovarian cancer (81). Lung cancer is the second most common and the deadliest type of cancer worldwide. Clinically, non-small cell lung cancer (NSCLC) is the most common pathological type of lung cancer (82). APS can inhibit the proliferation and migration of NSCLC cells, and its mechanism of action is related to the regulation of the miR-195-5p signaling pathway; in addition, APS inhibits the lung cancer cell cycle and improves the morphology of lung cancer cells (83,84). APS inhibit TGF-β1-induced epithelial-mesenchymal transition (EMT) in A549/DDP cell xenografts of lung adenocarcinoma, which is associated with inhibition of PI3K/Akt pathway protein activation (85). In a study exploring the synergistic inhibitory effect of APS and cisplatin on the growth of human carcinoma nasopharyngeal cell (CNE), APS was shown to exert an inhibitory effect on the proliferation of nasopharyngeal carcinoma cells in a time- and dose-dependent manner, and the combined application of APS and cisplatin could inhibit the migration and invasion of CNE-1 cells, which was more effective than the use of either drug alone with regard to the antitumor effect of either drug (86). In addition, the study demonstrated that APS at varying concentrations inhibited the proliferation of human colorectal cancer SW620 cells, inducing SW620 cell cycle arrest predominantly at the G2/M phase (87). In addition, APS can markedly inhibit the proliferation and invasion of prostate cancer (PCa) cells in a time- and dose-dependent manner. Under APS treatment, the cellular triacylglycerol and cholesterol levels of patients were revealed to be markedly reduced, and APS can regulate the miR-138-5p/SIRT1/SREBP1 pathway in PCa to inhibit lipid metabolism and tumorigenesis (88). Furthermore, APS enhances the expression of miR-133a in MG63 osteosarcoma cells, inhibits the JNK signaling pathway, the proliferation, migration and invasion of MG63 cells, and induces the apoptosis of MG63 cells (89).

Inducing tumor cell apoptosis

Apoptosis is a type of programmed cell death regulated by various genes, which serves an important role in tumorigenesis and development (90). The occurrence of malignant tumors is due to the uncontrolled and excessive proliferation of tumor cells. In recent years, promoting the apoptosis of tumor cells has become a focus of research for the treatment of tumors. There are two main apoptotic pathways: i) The extrinsic pathway, which is mediated by the activation of caspases through the cell membrane death receptor; and ii) the intrinsic pathway, which is mediated by mitochondria in the cytoplasm that release apoptotic factors and activate caspases. These activated caspases degrade the key proteins in the cell, causing apoptotic cell death. The genes and proteins that have been studied include caspase-3, caspase-8, caspase-9, the Bcl-2 family and cyclooxygenase-2 (91). In the mitochondrion-mediated intrinsic apoptotic pathway, proteins belonging to the Bcl-2 family alter mitochondrial membrane permeability, regulate cytochrome c release and modulate the apoptotic process (92). Bax and Bcl-2 belong to the Bcl-2 superfamily, which are key genes in the regulation of apoptosis, and they are in a balanced state in normal organisms. Notably, apoptosis can be induced by a decrease in the expression levels of the apoptosis inhibitory protein Bcl-2, or an increase in the expression levels of the pro-apoptotic protein Bax (93).

APS markedly promotes the anti-proliferative and apoptotic effects of cisplatin on nasopharyngeal carcinoma cells, by regulating the expression levels of Bax/Bcl-2, caspase-3, and caspase-9 (94). APS also markedly decreases gastric cancer (GC) cell viability, and accelerates GC cell apoptosis in a time- and dose-dependent manner; these effects are associated with an increase in p-AMPK. APS-treated MGC-803 cells have been shown to exhibit the typical morphological features of apoptosis, alongside cell cycle arrest at the S phase (95). A novel cold-water-soluble polysaccharide (APS4) was isolated from Astragalus membranaceus, induce mitochondria-dependent apoptosis, which is associated with intracellular ROS accumulation, mitochondrial membrane potential imbalance, increased pro-apoptotic/anti-apoptotic Bax/Bcl-2 ratio and cytochrome c release (96).

The Notch signaling has an important role in the transformation and proliferation of human malignant tumors, and it is readily activated in human non-small cell lung carcinoma, whereas the inhibitors of Notch signaling can attenuate cell proliferation and induce apoptosis in lung cancer cell lines. APS has been shown to inhibit the expression of Notch1 and Notch3 in NSCLC cell lines to upregulate the pro-apoptotic Bax and caspase 8, promoting apoptosis in NSCLC cell lines (97-101). APS may also inhibit tumor cells of A549, MC38 and B16, induce apoptosis and prevent cell transformation from the G1 phase to the S phase of the cell cycle, and is associated with the expression of molecules related to immunogenic cell death and the regulation of immune cells (102). APS has also been reported to decrease the number of HepG2 liver cancer cells in S-phase, and cells were arrested at the G0-G1 and G2-M phases, block the cellular proliferation cycle and induce apoptosis in cancer cells (103,104). Wang (105) investigated the role of ERK1/2 in the promotion of apoptosis in HepG2 cells by APS and found that caspase-3 activity is increased, ERK1/2 expression is decreased and APS inhibits the ERK1/2 signaling pathway to promote apoptosis in tumor cells.

Protein O-GlcNAc modification is a unique post-translational modification, and various nuclear and cytoplasmic proteins can be modified by the O-GlcNAcylation of free hydroxyl groups of selected serine and threonine residues (106). The cell modification cycle is mediated by O-GlcNAc transferase, which transfers N-acetylglucosamine to the protein substrate, and the O-GlcNAcase enzyme, which removes this modification from the protein (107–109). O-GlcNAcylation affects a wide range of protein functions, including transcription, subcellular localization, protein-protein interactions and protein stability (110). In numerous types of cancer, O-GlcNAc modification is associated with endoplasmic reticulum (ER) stress, and reduced O-GlcNAcylation leads to the activation of the ER stress response in a variety of cancer cells (111–113). APS promotes doxorubicin (Dox)-induced apoptosis and ER stress response in a dose-dependent manner. In addition, APS decreases O-GlcNAc transferase mRNA levels and protein stability. and increases O-GlcNAcase expression level. APS thus reduces O-GlcNAcylation and promotes Dox-induced apoptosis in hepatocellular carcinoma cells, leading to the exacerbation of ER stress and the activation of apoptotic pathways (114).

Suppression of tumor cell invasion and metastasis

Invasion and metastasis are crucial biological characteristics of malignant tumors, as well as important factors contributing to tumor recurrence following surgery. Tumor cells disseminate to new organs and tissues through the blood and lymphatic systems, subsequently proliferating. Notably, tumor vasculature serves as a conduit for metastasizing tumor cells; therefore, inhibiting tumor cell angiogenesis can impede the progressive deterioration of tumors and aid in the treatment of neoplastic diseases. Vascular endothelial growth factor (VEGF) is widely recognized as a pivotal mediator of tumor angiogenesis (115). APS effectively inhibits tumor cell metastasis, with APS efficiently suppressing Lewis lung cancer growth and metastasis in mice while enhancing immune organ function. Moreover, it suppresses the protein expression levels of VEGF and EGFR in tumor tissues, exhibiting a concentration-dependent relationship (116). The NF-κB signaling pathway has a critical role in lung cancer prevention and treatment; APS significantly downregulates NF-κB transcriptional activity, thereby inhibiting the proliferation and metastasis of A549 and NCI-H358 NSCLC cells (117,118). Furthermore, the angiogenesis of human colorectal cancer tumors is markedly inhibited by APS, which is closely associated with the reduction in VEGF expression within tumor tissues (119). APS can inhibit the proliferation and invasion of HS-746T cells, suppress the expression of miR-25 mRNA, and promote the expression of FBXW7 mRNA. It can also reverse the increase in miR-25 and the suppression of FBXW7 caused by transfection with miR-25 mimic. APS can inhibit the proliferation and invasion of gastric cancer HS-746T cells through the miR-25/FBXW7 signaling pathway (120). miR-133, which serves as a tumor suppressor, inhibits proliferation, migration in tumor cells and promotes their apoptosis. APS has been shown to upregulate the expression levels of miR-133a, promote the inactivation of the JNK signal transduction pathway, and inhibit the proliferation, metastasis and invasion of MG63 osteosarcoma cells while inducing apoptosis (89,121). EMT serves a crucial role in tumor migration and invasion. APS can serve as a therapeutic agent for breast cancer by inhibiting the Wnt/β-catenin signaling pathway to reduce proliferation and the EMT-mediated migration and invasion of breast cancer cells (122,123). Moesin (MSN) serves as a potential biomarker associated with tumor invasion and metastasis while maintaining PD-L1 stability. In hepatocellular carcinoma, MSN is targeted by miR-133a-3p, which negatively regulates its expression; notably, APS attenuates PD-L1-mediated immunosuppression by modulating the miR-133a-3p/MSN pathway (124).

Scavenging and inhibiting free radicals

Under physiological conditions, free radicals enhance immune function; by contrast, under pathological conditions, free radicals cause damage to normal tissues and cells, leading to diseases (125). Scavenging free radicals is another important tool for tumor prevention and treatment. The activities of free radical-scavenging enzymes, such as catalase and superoxide dismutase (SOD), are markedly decreased in patients with tumors, whereas the content of lipid peroxides and lipid peroxidation products such as malondialdehyde (MDA) are elevated (126). In addition, the anti-oxidative stress function is decreased. APS have been reported to reduce free radical production, increase SOD activity, decrease MDA content, and scavenge ROS and reactive nitrogen species generated by intracellular oxidative stress (127).

Reversal or escape of multidrug resistance (MDR)

In patients with cancer who are receiving chemotherapy, most treatment failures or deaths are attributed to tumor MDR (128). Modern pharmaceutical research focuses on drugs that can reverse or escape MDR, and the key to reversing tumor drug resistance lies in blocking the MDR pathway, reducing drug efflux and improving the chemosensitivity of tumor cells (129). APS markedly increases tumor response to chemotherapeutic drugs and reverses the MDR effect, while reducing the toxicity response induced by chemotherapeutic drugs (130). APS can also inhibit TGF-β1 overexpression-mediated EMT, and inhibit the proliferation of cisplatin-resistant A549 cell xenograft tumors (131). In vitro experiments confirmed that APS induces apoptosis in GC cells and enhances the pro-apoptotic effect of Adriamycin on GC cells, and that APS serves as a chemotherapeutic sensitizer to a certain extent (95). In addition, in the HL-60/A cell line with high drug resistance, APS induces apoptosis, activates the caspase cascade reaction, decreases the expression levels of MDR-associated protein and increases the concentration of intracellular antitumor drugs (132). Furthermore, APS increases the sensitivity of tumor cells to chemotherapeutic drugs, such as apatinib, escapes MDR and strengthens the antitumor effect (133). In the experimental study using H22 hepatoma cell xenograft mouse models, the expression levels of IL-6 and TNF-α were increased, whereas those of IL-10, P-glycoprotein and MDR1 were decreased in the APS treatment group. In addition, APS was shown to enhance the antitumor effect of Adriamycin (134). APS can also serve as a chemosensitive enhancer by activating the JNK signaling pathway and enhancing the sensitivity of SKOV3 ovarian cancer cells to cisplatin (135). Furthermore, APS has been shown to enhance the sensitivity of HeLa cervical cancer cells to cisplatin chemotherapy by upregulating the autophagy-related protein beclin1, promoting the conversion of LC3 I to LC3 II, and downregulating the marker protein p62 to enhance the autophagic activity of HeLa cells (133). APS promotes the apoptosis of gefitinib-resistant cells, and reduces cell proliferation and migration. Moreover, APS inhibits the PD-L1/SREBP-1/EMT signaling pathway, increases the expression levels of E-cadherin, reduces the expression levels of N-cadherin and vimentin, and reverses the acquired resistance of lung cancer cells to gefitinib (136).

Summary and outlook

Astragalus membranaceus has a medicinal history of >2,000 years in China. As an active ingredient with the highest content in Astragalus membranaceus, APS has shown great application potential in regulating the tumor microenvironment and improving immunity. Numerous previous experimental studies have confirmed that APS can regulate the activation of immune cells and the release of cytokines in the tumor microenvironment, as well as inhibit tumor immune escape. The combination of APS and chemotherapeutic drugs may alleviate the adverse reactions of patients after chemotherapy and improve their quality of life, which provides great reference for the use of APS as an adjuvant in clinical antitumor medication. APS also has certain effects on the treatment of other diseases, such as diabetes and inflammatory diseases (137). Therefore, the therapeutic use of APS for the treatment of other diseases requires further exploration.

The structure of APS is complex; thus, its bioavailability can be further improved through chemical means, such as structural modification, so that it can serve a better targeted role in cancer. Although APS has been proven to have good application potential in antitumor immunity, it still faces a number of problems: i) The structure of APS is complex, and it contains a large number of chemical components. Its extraction, separation, purification and structural analysis are currently the biggest challenges, which require further in-depth exploration. ii) Tumor immune escape is a complex process in the tumor microenvironment. Tumor cells can not only directly affect the function of immune cells, but also indirectly inhibit immunity by regulating immune checkpoints and cytokines. Although APS can improve the function of immune cells in the tumor microenvironment, the specific mechanism by which APS regulates immune cells remains unclear. iii) At present, most of the studies on the antitumor effect of APS immunotherapy are limited to cell experiments or in vivo experiments using mice, and evidence from clinical-related trials is lacking. Therefore, more abundant clinical trials are needed for verification. In conclusion, the research on the mechanism of action of APS in regulating immune cells, improving the tumor immune microenvironment and resisting tumors, as well as further research on the mechanism by which APS serves a regulatory role in the immunotherapy of malignant tumors, is of great importance to the research and development of clinical antitumor drugs. This research may improve the treatment of tumor-related diseases by combining traditional Chinese and Western medicine.

Acknowledgements

Not applicable.

Funding

The present study was supported by the National Natural Science Foundation of China (grant no. 82173996).

Availability of data and materials

Not applicable.

Authors' contributions

ZQ, ZW and YW conceived and designed the study. ZQ wrote the manuscript. YW and ZW revised the manuscript. WC and XX conducted literature organization and proofreading Data authentication is not applicable. All authors read and approved the final manuscript.

Ethics approval and consent to participate

Not applicable.

Patient consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

References