Cell components of tumor microenvironment in lung adenocarcinoma: Promising targets for small-molecule compounds

Abstract

Lung cancer is one of the most lethal tumors in the world with a 5-year overall survival rate of less than 20%, mainly including lung adenocarcinoma (LUAD). Tumor microenvironment (TME) has become a new research focus in the treatment of lung cancer. The TME is heterogeneous in composition and consists of cellular components, growth factors, proteases, and extracellular matrix. The various cellular components exert a different role in apoptosis, metastasis, or proliferation of lung cancer cells through different pathways, thus contributing to the treatment of adenocarcinoma and potentially facilitating novel therapeutic methods. This review summarizes the research progress on different cellular components with cell–cell interactions in the TME of LUAD, along with their corresponding drug candidates, suggesting that targeting cellular components in the TME of LUAD holds great promise for future theraputic development.

Introduction

Lung cancer is the fourth most common cancer worldwide, ^[1] with a 5-year survival rate of 4–17%. Lung cancer primarily includes two subtypes: non-small cell lung cancer (NSCLC) (80%), which includes lung adenocarcinoma (LUAD) (40%), lung squamous cell carcinoma (LUSC, 25–30%), and large cell lung carcinoma (LCLC) and small cell lung cancer. Supplementary Table 1, http://links.lww.com/CM9/C188 displays the classification, high-risk groups, and common treatment methods of lung cancer. Significant differences exist in the histopathology, genetic drivers, prognostic indicators, tumor progression pathways, and the proportion of cell types between LUAD and LUSC [Supplementary Table 2, http://links.lww.com/CM9/C188]. Although the treatment of LUAD [Supplementary Table 3, http://links.lww.com/CM9/C188] has advanced in recent decades, greatly improving the prognosis of early-stage LUAD, the overall outcomes of late-stage LUAD remain poor.

The tumor microenvironment (TME) refers to the complex and abundant multicellular environment in which tumors grow. TME plays a crucial role in tumor proliferation, migration, and apoptosis. It responds to internal or external pressures, stimuli, and treatments, ultimately allowing the cancer cells to survive and migrate in the body. The TME includes immune cells, including T and B cells, tumor-associated macrophages (TAMs), dendritic cells (DCs), natural killer (NK) cells, tumor-associated neutrophils (TANs), and innate lymphoid cells (ILCs); stromal cells, such as cancer-associated fibroblasts (CAFs), endothelial cells (ECs), smooth muscle cells (SMCs), and pericytes; and tissue-specific cell types such as neurons [Figure 1]. Besides, the TME also includes extracellular matrix (ECM), other secretory molecules, and tumor angiogenesis, which are intertwined and interconnected in TME.

Figure 1.

Main components of TME created with BioRender.com (both cellular and non-cellular components). The TME contains different cell types and secretes factors, representing a target for anticancer therapy. TME usually includes immune cells, including T and B cells, TAMs, DCs, NK cells, TANs, and ILCs; stromal cells, such as CAFs, ECs, SMCs, and pericytes; ECM and other secretory molecules; and tumor angiogenesis, which are intertwined and interconnected. AKT: protein kinase B; ANXA2: Annexin A2; CAFs: Cancer-associated fibroblasts; CCL2: Chemokine (C−C motif) ligand 2; CXCL13: C–X–C motif chemokine ligand 13; DCs: Dendritic cells; ECM: Extracellular matrix; ECs: Endothelial cells; EMT: Epithelial–mesenchymal transition; FAM72B: Neuronal production of family 72B; GFPT2: Glutamine-fructose-6-phosphate aminotransferase 2; IGF-1: Insulin-like growth factor-1; ILCs: Innate lymphoid cells; LAG-3: Lymphocyte-activation gene 3; LUAD: Lung adenocarcinoma; MAPK: Mitogen-activated protein kinase; MMPs: Matrix metalloproteinase; MPO: Myeloperoxidase; mTOR: Mammalian target of rapamycin; NE: Neutrophil elastase; NK: Natural killer; NRK: Nik-related kinase; PD-1: Programmed cell death-1; PD-L1: Programmed cell death ligand-1; ROS: Reactive oxygen species; SMCs: Smooth muscle cells; SNAIL1: Snail family transcriptional repressor 1; STAT3: Signal transducer and activator of transcription 3; TAMs: Tumor-associated macrophages; TANs: Tumor-associated neutrophils; TGF-β: Transforming growth factor-β; TME: Tumor microenvironment; TNF-α: Tumor necrosis factor-α; VCAM-1: Vascular cell adhesion molecule-1; VEGF: Vascular endothelial growth factor; ZC3H12D: Zinc finger CCCH type containing 12D.

Several differences exist in the cellular composition of the TME between LUAD and other tumors, as well as among different stages and subtypes of LUAD. The unique behavior of different cells in TME offers valuable insights for understanding and treating LUAD, such as the frequencies of cluster of differentiation (CD) 8⁺ T cells/cytotoxic T cells (CTL), regulatory T (Treg) cells, and macrophages increased significantly in advanced LUAD, whereas the frequencies of follicular B cells, Langerhans cells, and CD4⁺ T cells were significantly reduced in advanced LUAD.^[2] The TME plays a pivotal role in driving the LUAD heterogeneity and influences disease progression and treatment response. To date, studies have introduced mechanisms and pathways that regulate tumor proliferation, apoptosis, metastasis, angiogenesis, and distant metastasis in the LUAD. If we can gain a deeper comprehension of the functions performed by different cellular components and their interaction within the TME, it may facinate the small molecule targeted drugs development in LUAD.

Targeted therapy using small-molecule drugs for LUAD has made significant progress in recent years. LUAD is treated with various drugs, including both synthetic chemicals and natural small-molecule compounds. Natural small-molecule compounds are derived from animals, plants, and microorganisms following modern medical theory. They exhibit specific pharmacological activities in the human body. Current experimental studies have demonstrated that the active ingredients of Astragali radix, Rhodiola rosea L., and others can act on lung cancer via the TME and have the potential to be used as drugs for its treatment.

In this review, we summaried the effect of various cell components of TME in LUAD as promising targets and discussed the small molecules exerting their anti-LUAD activity through regulating cell components in the TME of LUAD. We summarized the current progress in the study of cellular components and cell–cell interactions in the TME, for the purpose of seeking future strategies in the treatment of LUAD.

Non-immune Cellular Components in LUAD and Drug-targeting Therapy

Non-immune cell components, including fibroblasts, endothelial cells, and pericytes and etc., impact LUAD therapy efficacy through remodeling of the TME. For an instance, both cancer-associated fibroblasts and endothelial cells were enriched in TME, affecting extracellular matrix organization, cell migration, and cell-matrix adhesion, which will lead to the failure of LUAD treatment. Studying their function in LUAD can help to better investigate the therapeutic approaches.

CAFs in LUAD and drug-targeting therapy

CAFs are the primary components of the TME and are required for ECM production and remodeling in interstitial tissues. They typically exhibit a uniform morphology and can be categorized into two types: myofibroblastic-CAFs (myoCAFs) and inflammatory-CAFs (iCAFs). Cancer cells can establish a cross-dialogue with CAFs, secreting cytokines, growth factors, CAF-specific proteins, and exosomes to enhance tumor metastasis, ^[3] tumor growth, formation of a stem cell niche, immunosuppression, prognosis, and chemotherapy resistance.

Interaction of CAFs and LUAD

Glutamine-fructose-6-phosphate aminotransferase 2 (GFPT2) in CAFs increases glucose uptake and metabolic reprogramming in the TME of LUAD via the phosphatidylinositol-3-kinase (PI3K)-protein kinase B (AKT)-mammalian target of rapamycin (mTOR) pathway.^[4] Some cancer cells even manifest the glutamine addiction. Liu et al^[5] found that LINCO1614 enhances the uptake of glutamine secreted by CAFs in LUAD and promotes LUAD growth. Nik-related kinase (NRK) in CAFs may play a role in treating LUAD by regulating the ECM and glycolysis/glycolysis pathways.^[6] CAFs enhance the metastatic potential of LUAD cells through the interleukin (IL)-6/signal transducer and activator of transcription 3 (STAT3) signaling pathway.^[7] In addition, insulin-like growth factor 1 (IGF-1) secreted by CAFs and hepatocyte growth factor co-induces the expression of membrane-linked Annexin A2 (ANXA2), promoting epithelial–mesenchymal transition (EMT) and drug resistance in LUAD.^[8] Recent studies have demonstrated that CAFs can transfer Snail family transcriptional repressor 1 (SNAIL1) to recipient cells through exosomes and induce EMT in LUAD.^[9] Extracellular vesicles of CAFs generate premetastatic niches in the lungs by activating the fibroblasts.^[10] Vascular cell adhesion molecule-1 (VCAM-1) secreted by CAFs enhances LUAD cell growth and invasion via the AKT and MAPK pathways.^[11]

Exosomes are an important part of the TME and act as messengers to transmit signals between cells. Micro RNA (miR)-1290 in the exosomes of LUAD cells inhibits cullin 3 (CUL3), which inhibits the expression of cyclooxygenase 2 (COX-2). COX-2 mediated activation of CAFs and production of ECM, which promotes LUAD growth.^[12] Moreover, circ_16601, increased in LUAD, promotes Hippo pathway signaling through the miR-5580-5p/FGB axis to facilitate CAFs recruitment in LUAD.^[13] The current reports demonstrate that CAFs can promote LUAD growth and invasion, and LUAD cells play a regulatory role for CAFs. These findings indicate CAFs as a promising target in LUAD.

Small-molecule compounds targeting CAFs in LUAD

CAFs are highly heterogeneous and current studies suggest personalized treatments based on CAFs (I, II, and III) in patients with tumor. Currently, the drugs targeting CAFs to treat LUAD include docetaxel.^[14] Meanwhile, a recent study reported that targeting the interaction of insulin-like growth factor 1R and focal adhesion kinase (FAK) with small-molecule could mimic CAF-mediated effects, improving the anti-cancer effect of osimertinib in LUAD.^[15]

ECs in LUAD and drug-targeting therapy

ECs are a thin layer of epithelial cells composed of a layer of flattened, polygonal cells with serrated edges. Vascular ECs provide nutrition and oxygen to tumors, and actively participate in and regulate the inflammatory response in normal and pathological tissues.

Interaction of ECs and LUAD

A high proportion of tip-like ECs in patients with LUAD is associated with poor clinical outcomes.^[16] The markers of ECs are glucose transporter type 1 (GLUT-1), CD71, lymphatic vessel endothelial receptor-1 (LYVE-1), CD34, CD276, etc.^[17] Nucleolin (NCL) is involved in the interaction between ECs and cancer cells. Upregulation of Collagen Type III Alpha 1 Chain (COL3A1) in ECs has an important role in driving LUAD carcinogenesis through the NCL-PI3K-AKT axis.^[18] In contrast, EC-derived exosome miR-30a-5p inhibits LUAD by targeting Cyclin E2 (CCNE2).^[19]

LUAD promotes the growth of ECs in the veins via kinase-associated proteins.^[20] SLC25A29 was underexpressed in LUAD tissues, and as SLC25A29 expression decreased, EC proliferation and migration increased while apoptosis decreased.^[21] Exosomal-miR-629-5p in LUAD promotes LUAD invasion by increasing tumor cell invasion and EC permeability.^[22] Calmodulin-regulated spectrin-associated protein 3 (CAMSAP3) levels are decreased in LUAD, while CAMSAP3 stably negatively regulates lung cancer cell invasion and angiogenesis through the NCL/HIF-1α mRNA complex, leading to further reduction of vascular endothelial growth factor A (VEGFA), matrix metalloproteinase 2 (MMP2), and MMP9.^[23]

Small-molecule compounds targeting ECs in LUAD

Bevacizumab mainly targets vascular endothelial growth factor (VEGF), with different hemorrhage outcomes between LUAD and LUSC patients. It is interesting to notice the differential expression of IRF7 and IFIT2 in the endothelium of LUSC and LUAD tumors may contribute to the differential hemorrhage outcomes in patients with NSCLC after bevacizumab treatment.^[24] Apigenin inhibits tumor growth and angiogenesis by suppressing the angiogenic capacity of ECs in early tumor and reducing the expression of hypoxia-inducible factor (HIF)-1 in LUAD.^[25]

The function of ECs is influenced by the interaction between tumor cells and ECs, suggesting that ECs could mediate the communication of related tumors, thus deserving further studies.

Pericytes in LUAD and drug-targeting therapy

Pericytes have a prominent nucleus containing a small amount of cytoplasm, several long projections covering the endothelial wall, and desmin or α-smooth muscle actin (α-SMA). Pericytes are an integral component of the vascular system. They participate in blood flow, maintain the tissue blood barrier, and contribute to tissue homeostasis, relate to ECs, and regulate blood pressure through systolic action and vascular remodeling.

Interaction of pericytes and LUAD

Studies using preclinical tumor models have demonstrated that blocking pericyte coverage increases hypoxia and metastasis.^[26] FAK negatively regulates the growth arrest-specific protein 6 (Gas6)/AXL receptor tyrosine kinase signaling and inhibits tumor angiogenesis and tumor growth.^[27] In the meantime, pericytes can produce growth factors, such as vascular endothelial growth factor (VEGF), or degrade enzymes, such as the basement membrane protein MMP-1/2, indicating their active role in promoting angiogenesis in LUAD. Pericytes not only secrete cytokines through specific pathways but also recruit ECs for tumor blood vessel angiogenesis, which is implicated in LUAD.

Small-molecule compounds targeting pericytes in LUAD

Melafolone exerts a dual inhibitory effect on both COX-2 and epidermal growth factor receptor (EGFR) in LUAD, improveing anti-PD-1 therapy. Mechanistically, dual inhibitions of COX-2 and EGFR in LUAD increased pericyte migration, decreased endothelial proliferation, and enhanced CD8⁺ T cell function, affecting tumor microenvironment with vascular normalization and PD-L1 downregulation.^[28] Apigenin not only treats LUAD through ECs, but also reduces pericyte coverage during late angiogenesis.^[25]

SMCs in LUAD

SMCs are elongated, spindle-shaped cells without transverse lines, predominantly found in the walls of blood vessels. The majority of them are distributed in bundles or layers and regulated by numerous factors, such as myosin light chain protein. Approximately 2% of benign lung tumors are pulmonary smooth muscle lesions. For instance, vascular SMCs (VSMCs) represent the middle layer of blood vessels, responsible for constriction and dilation of the vessel.^[29] The human actin α2 gene encodes SMC α-specific actin and its expression is intricately related to the development and function of SMCs. Actin α2-smooth muscle antisense RNA 1 has been found to inhibit cell migration and EMT in LUAD.^[30] SMCs, generally playing an assisting role, might develop as novel and specific targets for disease prevention and treatment in LUAD.

Neurons in LUAD

Neurons and nerve fibers are present in the TME, and mounting evidence indicates the involvement of neurons in tumorigenesis. Neurons release neurotransmitters, neurotrophins, and chemokines that stimulate cancer stemness, inhibit apoptosis, and enhance cell proliferation. In addition to directly stimulating cancer cells, nerves have a significant and far-reaching impact on the TME by establishing interactions with other cells in the TME, such as ECs and immune cells.^[31] Neuron-specific enolase (NSE) is specific to neurons and neuroendocrine cells. It is generally used as a sensitive indicator to evaluate the severity of nerve cell injury and determine its prognosis. For example, the current study reported that high NSE levels were associated with poor prognosis in LUAD.^[32] Neuronal production of family 72B (FAM72B) with sequence similarity may promote LUAD growth via lncRNA-AL360270.2/TMPO-AS1/AC125807.2/has-let 7a/7b/7c/7e/7f.^[33] Although a few studies have explored the direct impact neurons on LUAD, other cancer research has shown that neurons can influence functions through various interactions and may be linked to the core, which could be a significant factor for future investigations.

Immune Cellular Components in LUAD and Drug-targeting Therapy

The basic building blocks of TME are immune cells. Tumor-infiltrating immune cells comprise two distinct compartments: innate and adaptive immune responses. The first line of defense against foreign pathogens and transformed cells is the innate immune system of phagocytes, which include neutrophils, macrophages (CD68⁺), DCs, NK cells (CD56⁺ and CD3⁻), and NK T cells (CD3⁺ and CD56⁺). The adaptive immune system is mediated by two main T lymphocyte subsets, CD8⁺ T cells (cytotoxic T lymphocytes [CTLs]) and CD4⁺ T cells (T helper cells [Th]). The adaptive immune system is the second line of defense that recognizes and acts on antigen-specific molecules that require cloning and amplification following exposure to a foreign antigen. ILCs are a class of lymphocytes that do not express specific antigen recognition receptors; however, their specific effects remain to be analyzed.

T cells in LUAD and drug-targeting therapy

T cells are derived from pluripotent stem cells in the bone marrow (embryonic yolk sacs and liver). Tregs are predominant in patients with lung cancer. CD4⁺ Tregs (26%) are the most abundant T cell population, followed by CD8⁺ Tregs (22%). CD4⁺ Th lymphocyte is a kind of heterogenous T lymphocyte that secretes cytokines. CTLs are activated by Th1 lymphocytes, whereas humoral immunity is activated by Th2 lymphocytes. Except for Th1 and Th2 subsets, effector T lymphocytes are inhibited by CD4⁺ regulatory T lymphocyte subsets. CD4 antigens and T cell receptors on T lymphocytes jointly complete antigen presentation and recognition. CD8⁺ cytotoxic T lymphocytes directly kill tumor cells. γδT cells are T cells that perform innate immune functions, and their T cell receptor (TCR) consists of γ and δ chains. γδT cells are immune cells that can kill cancer cells as well as tumor stem cells, and recognize cancer antigens. γδT cells are CD4⁻/CD8⁻. Major γδT cells do not express CD4 and CD8 molecules on their cell surface. However, a few cells express CD4 or CD8 and are involved in immune regulation and immune responses.

Interaction of T cells and LUAD

A current study reports that in LUAD, γδT cells are associated with IL-17-mediated promotion of tumor cell proliferation and activation of symbiotic bacteria.^[34] Kagamu et al^[35] reported that immune monitoring of peripheral blood CD4⁺ Tregs could predict the anti-PD-1 treatment response in patients with lung cancer and others had demonstrated the importance of CD4 immunity for immunotherapy.^[36] Patients responding to treatment had a higher proportion of CD4⁺ Tregs before treatment. CD4⁺ Tregs proliferate at baseline and respond to PD-1 blockade.^[37] These findings support the use of vaccines to enhance antitumor immunity of CD4⁺ neoantigen-specific Tregs. Generating a durable response in subpopulations of patients with different solid tumors is one of the major immunosuppressive mechanisms of the TME (the upregulation of PD-1 expression in tumor-infiltrating lymphocytes, leading to CD8⁺ T cell suppression and Treg cell proliferation through interaction with its ligands [PD-L1/2]).^[38]

CircRNA-002178 can be detected in plasma exosomes of LUAD patients and can be used as a biomarker for early diagnosis of LUAD. Enhanced PD-L1 expression was observed by sponging miR-34 in cancer cells, thereby inducing T cell depletion.^[39] High expression level of beta-1,4-galactosyltransferase1 (B4GALT1) in early-stage LUAD suppresses CD8⁺ T cell abundance and activity and reduces anti-tumor immunity to anti-PD-1 therapy in vivo, and may be a novel target for LUAD intervention and immunotherapy.^[40]

Small-molecule compounds targeting T cells in LUAD

Several drugs function through T cells and other pathways. A recent study pointed out that interleukin-4-inducible (IL4I1) gene was highly expressed in LUAD tissue and cells. IL4I1 could increase PD-L1 expression through Janus kinase (JAK)/STAT signaling pathway, thus promoting LUAD progression. The inhibitory effect of IL4I1 silence on PD-L1 expression and immune escape of LUAD cells could be reversed by atezolizumab treatment. Besides, T cell-mediated cytotoxicity was notably enhanced after si-IL4I1 treatment.^[41] Certain studies have demonstrated that natural small-molecule compounds can be used to treat LUAD via immune cells. Resveratrol decreases M2 macrophages and increases CD8⁺ T cell population by activating IL-18.^[42] Ginsenosides can be used as potential immunomodulatory targets and transmembrane protease serine 2 (TMPRSS2) inhibitors (the correlation between TMPRSS2 expression and CD8⁺ and CD4⁺ T cells is stronger in LUAD than in LUSC) for combination immunotherapy in patients with LUAD with failed the anti-PD-19 therapy.^[43]

T cells differentiate into distinct subpopulations and collaborate with upstream and downstream factors or pathways to regulate tumors. T-cell research that links immunological principles with recent advances in synthetic biology and genetic engineering will propel the progress of next-generation T-cell therapies.

B cells in LUAD and drug-targeting therapy

B cells are present at all stages of lung cancer growth and are observed in approximately 35% of all lung cancers. After stimulation with antigens, they differentiate and multiply into plasma cells, synthesize antibodies, and participate in humoral immunity. The density of follicular B cells is a strong predictor of survival in NSCLC. According to the blood and lymph node from patients with NSCLC, the survival of plasma cells needs B-cell activating factor (BAFF) and proliferation-inducing ligand (APRIL).^[44] B cells have long been considered to be key players in the pathogenesis and in the response to checkpoint blockade in LUAD. B cells are ubiquitous in human cancers and have dual functions. B cells not only recognize multiple tumor antigens, but are also intricately related to T cells and other immune cells. They are associated with favorable outcomes, yet they also have a negative effect on protective antitumor responses and could promote tumor growth. At present, we are summarizing the research progress on B cells for lung cancer, which could assist in its treatment.

Interaction of B cells and LUAD

A recent study found a positive correlation between B cell infiltration and PD-L1 expression, and elevated PD-L1 expression could hinder the B cell infiltration-mediated immune response in patients with LUAD, leading to poor prognosis. The key enzyme in immunoglobulin somatic hypermutation and classification switch recombination is activation-induced cytidine deaminase, which is expressed by B cells derived from tertiary lymphoid structures in patients. Antitumor antigens such as Lang-1, TP-53, and New York esophageal squamous cell carcinoma 1 (NY-ESO-1) have been recognized and combined with tumor-specific antibodies produced by tumor-associated B cells in the lungs.^[45] Production of zinc finger CCCH-type containing 12D (ZC3H12D) in B cells inhibits LUAD growth via hsa-miR-4443-ENST00000630242 competing endogenous RNA (ceRNA).^[46]

B cell chemokine C–X–C motif chemokine ligand 13 (CXCL13) secreted by tumor cells, follicular DCs, and T follicular helper (TFH) cells in human lung tumor lesions has been demonstrated to be responsible for the influx of B cells into tumors.^[47] Pyruvate kinase M2 (PKM2) is highly expressed in LUAD tumor cells and negatively correlates with B cells and CD4⁺ T cells, thus elevated PKM2 expression may promote LUAD growth by impeding infiltration of tumor-antagonistic immune cells.^[48] The expression levels of INTS7^[49] and NEIL3^[50] mRNA were significantly upregulated in LUAD, regulating B cell infiltration.

Small-molecule compounds targeting B cells in LUAD

Certain small-molecule compounds have therapeutic effects on LUAD. The natural small-molecule compound ginsenoside acts as an inhibitor of TMPRSS2, which recruits different types of immune cells in LUAD and LUSC patients, including CD8⁺ and CD4⁺ T cells, B cells, and DCs.^[43]

TAMs in LUAD and drug-targeting therapy

TAMs can be divided into two types, that is, M1/M2-like TAMs. M1-type macrophages promote inflammation and inhibit tumor growth, whereas M2-type macrophages exert anti-inflammatory and immunosuppressive effects and promote tumor growth and metastasis while inhibiting DCs and CD8⁺ T lymphocytes.^[51]In vivo, M2-like TAMs promote tumor growth, angiogenesis, and metastasis. M1-like TAMs promote the release of pro-inflammatory cytokines including IL-1, IL-6, IL-12, IL-23, nitric oxide (NO), and tumor necrosis factor (TNF)-α. M2-like TAMs promote the expression of anti-inflammatory cytokines including IL-10, prostaglandin E2, transforming growth factor-β (TGF-β), MMP, and VEGF.

TAMs in inhibiting growth of LUAD

With the deeper understanding of TAMs, TAMs have become an important target for lung cancer immunotherapy. TAMs contribute to T-cell dysfunction. High macrophage infiltration tends to induce drug resistance to PD-1/PD-L1 immune suppressants. Therefore, TAMs could be considered as critial targets for reversing the resistance to anti-PD-1/PD-L1 therapy in LUAD. Exosomal circZNF451 in LUAD patients inhibits anti-PD1 therapy by remodeling the tumor immune microenvironment in association with macrophage polarization induced via the FXR1-ELF4-IRF4 axis.^[52]

Small-molecule compounds targeting TAMs in LUAD

Certain drugs such as arsenic trioxide inhibit M2 polarization of TAMs.^[53] Other drugs also target TAMs in other ways to treat lung cancer: (1) resistance against CSF-1R: IMC-CS4, ^[54] (2) CSF1/CSF1R signaling inhibitors: PLX3397^[55] and BLZ945, ^[56] (3) CCR2 antimicrobial agent: PF04136309 and CCX872, ^[54] (4) antiangiogenic drugs: lenvatinib^[57] and cabozantinib, ^[58] (5) CD47 resistane: Hu5F9-G4, ^[59] (6) competitive heavy SIRP1αFc: TTI621 and ALX148, ^[60] (7) PD-1/PD-L1 class inhibitors: keytruda^[61] and opdivo.^[62] Natural small-molecule compounds astragaloside IV can inhibit the growth of LUAD by inducing M2-type polarization of macrophages through adenosine 5′-monophosphate (AMP)-activated protein kinase (AMPK) pathway.^[63] Scutellariae Radix is a traditional Chinese medicine component, and its active components, namely wogonin and baicalein, have been demonstrated to promote macrophages to M1 polarization and activate Janus kinase 2-signal transducer and activators of transcription 1 (JAK2-STAT1) pathway to induce LUAD cell death.^[64]

TAM plays an important role in the immune response and in collaborating with other cells. At the same time, there are many drugs that target TAM and have gained some stage in the clinic. Therefore, studying TAM will be one of the ways to cure LUAD completely.

NK cells in LUAD and drug-targeting therapy

NK cells are innate immune cells and constitute the first line of defense against tumors that can kill tumor and virus-infected cells, regulate other immune cells, and promote tissue growth. Based on the expression of CD56 and CD16, human NK cells are divided into two functional subpopulations, namely, CD56^brightCD16⁻ and CD56^dimCD16⁺. CD56^brightCD16⁻ NK cells are receptor cytokine secretions but lack CD16a. CD56^dimCD16⁺ cells are highly cytotoxic and express CD16a. Most human NK cells express high levels of CD16 and low levels of CD56 and are called CD56^dimCD16⁺ NK cells.

Interaction of NK cells and LUAD

NK cells are rarely in direct contact with cancer cells and largely exert antitumor properties by expressing Fc receptors and releasing extracellular vesicles, such as CD56^dimCD16⁺ NK cells, which express numerous cytotoxic molecules, such as perforin and granzyme to dissolve target cells. Studies have demonstrated that the presence of NK cells does not affect the clinical efficacy of patients with LUAD, possibly because the TME can locally reshape the phenotype of NK cells in tumors, resulting in reduced expression of activated receptors and increased expression of inhibitory receptors such as cytotoxic T lymphocyte-associated protein-4 (CTLA4). NK cell surfaces can express immunosuppressive molecules such as PD-1, lymphocyte-activation gene 3 (LAG-3), T cell immunoglobulin, and mucin domain-3 (TIM-3). In addition, lung TMEs could induce NK cell inhibition; moreover, there is a positive correlation between CD8⁺ T cell infiltration and CTLA4 expression in NK cells.^[65]

Receptor-type tyrosine-protein phosphatase-like N (PTPRN) is a tyrosine phosphatase-like intrinsic membrane protein involved in the biological processes of insulin secretory granules in pancreatic β-cells and is overexpressed in LUAD cells, inhibiting NK cells cytotoxicity and thus facilitating LUAD metastasis.^[66]

Small-molecule compounds targeting NK cells in LUAD

Docetaxel, vinorelbine, and paclitaxel are commonly used for LUAD chemotherapy. However, prolonged administration of these drugs often leads to the development of acquired resistance to chemotherapy. Using bioinformatics with thousands of LUAD samples find that the number of NK cells is associated with drug sensitivity.^[67] Therefore, targeting NK cells is a feasible approach for the treatment of LUAD.

As an important component of the innate immune system, NK cells have been implicated in the host’s early anticancer defense. Studies have demonstrated that NK cells have a good but imperfect influence on LUAD prognosis, and target NK cell may provide a novel treatment method for NSCLC.

DCs in LUAD and drug-targeting therapy

DCs are a type of cells with branching or dendritic morphology and phagocytic functions. They play a crucial role in presenting antigens and central regulators of antitumor immunity. DCs can be divided into two types. Myeloid stem cells differentiate into myeloid DC under cytokine stimulation. The second type is derived from lymphoid stem cells and is called plasma DC.

DCs in LUAD regulating the tumor development

DCs regulate the tumor process through signaling pathways and cytokines (such as IL-10, IL-27, and TGF-β).^[68] Lu et al^[69] reported that LUAD cells downregulated the expression of co-stimulatory molecules (CD80 and CD86) and proinflammatory cytokines (IL-12 and IL-23) but promoted the secretion of anti-inflammatory cytokines (IL-10) by CD1c⁺ DC subtypes, thereby blocking antitumor immunity in vivo.

Small-molecule compounds targeting DCs in LUAD

Several drugs have been shown to treat LUAD by regulating DCs. As a third-generation tyrosine kinase inhibitor (TKI), osimertinib (OSI) is currently the first-line treatment of choice for patients with LUAD having EGFR activation mutations or acquired T790M mutations following first- or second-generation TKI therapy. Studies have demonstrated that it could significantly induce the expression of markers of DC maturation (CD40 and CD83) in patients with LUAD.^[70] These studies demonstrate the cellular and molecular mechanisms involved in the DC-mediated immunosuppressive microenvironment in lung cancer and provide an experimental basis for treatment.

TANs in LUAD and drug-targeting therapy

Neutrophils play a key role in inflammation and host defense against microbial infections. They are also believed to be involved in regulating tumor growth and the progression of metastasis. Recent studies have demonstrated that neutrophils are promising candidates for novel lung cancer treatments. TANs govern tumor progression, including tumorigenesis, ECM modification, angiogenesis, cell migration, and immunosuppression. TANs can exert dual effects on tumors, both combating cancer and facilitating its progression. Similar to the phenotype of TAMs, TANs can be divided into two types: (1) “N1” type that inhibits tumor growth and (2) “N2” type that promotes tumor growth and malignant metastasis.

TANs in LUAD regulating the tumor development

Overexpression of TNF Alpha-induced Protein 6 (TNFAIP6) in LUAD cancer cells may lead to “N2” polarization of neutrophils, which may exhibit pro-tumorigenic effects.^[71] “N1” TANs largely reduce tumor proliferation through antibody-dependent cell-mediated cytotoxicity. “N1” TANs kill tumor cells and inhibit their proliferation by directly producing cytotoxic mediators such as myeloperoxidase (MPO) and reactive oxygen species (ROS). “N2” TANs are intricately related to tumor proliferation, invasion, and metastasis through the synthesis and secretion of neutrophil elastase (NE), MMPs, and other related proteases.

Small-molecule compounds targeting therapy of TANs in LUAD

Salidroside is a potential therapeutic agent that specifically enhances the antitumor activity of pulmonary neutrophils, thereby inhibiting the transition of neutrophils from N1 to N2 and effectively suppressing nicotine-induced lung metastasis.^[72]

ILCs in LUAD

ILCs play a very important role in innate immunity, with a lymphocyte morphology independent of the antigen receptor rearrangement process of recombination-activating genes. ILCs are classified into ILC1, 2, 3, and ILCreg. ILC1 produces Interferon-γ (IFN-γ); ILC2 produces IL-5 and IL-13; ILC3 produces IL-17, IL-22, and IFN-γ; ILCreg produces IL-10 and TGF-β. Exomes can be used as reliable markers to distinguish NK and ILC1.^[73]

ILCs in LUAD regulating the tumor development

G protein-coupled receptor 35 (GPR35) is an orphan GPR that interacts with Na⁺/K⁺-ATPase. Studies have demonstrated that GPR35 enhances the production of IL-5 and IL-13 to promote tumor development, thus promoting the formation of the ILC2-myeloid-derived suppressor cells axis.^[74] IL-33 induced activation of ILC2 within the tumor, and activated ILC2s acted as novel enhancers of anti-PD-1 immunomodulatory properties, enhancing anti-tumor effects.^[75]

Moreover, ILCs can be targeted therapeutically, akin to other immune cells, to address human diseases and offer new approaches and strategies for treatment. However, due to the limited research, there are still several questions in the field of ILC cells, such as whether ILCs are interconnected with other immune cells.

Intercellular Interaction in LUAD as a Promising Target for Cancer Treatment

We illustrated the function of various cell in the TME of LUAD and the progress of small-molecule compound-targeting therapy. It would also be interesting to explore how cellular interactions affect LUAD [Figure 2].

Figure 2.

Interactions among immune cells within the microenvironment of lung adenocarcinoma (created with BioRender.com). This image shows the regulatory role of cellular interactions in LUAD. For example, stromal cells, ECs, and pericytes form tumor blood vessels through VEGF interaction and promote tumor growth. CAFs: Cancer-associated fibroblasts; CCL21: Chemokine (C−C motif) ligand 21; CCL5: Chemokine (C−C motif) ligand 5; CD: Cluster of differentiation; DCs: Dendritic cells; ECs: Endothelial cells; FLT3: FMS-associated tyrosine kinase 3; IFN-γ: Interferon-γ; LUAD: Lung adenocarcinoma; MIF: Migration inhibitory factor; NK: Natural killer; PD-1: Programmed cell death-1; PD-L1: Programmed cell death ligand-1; SIRPα: Signal regulatory protein α; TAMs: Tumor-associated macrophages; VEGF: Vascular endothelial growth factor.

Non-immune cell interactions

In LUAD, tumor angiogenesis is a hallmark of advanced cancer and is required for tumorigenesis, invasive tumor growth, and metastasis. Hypoxic tumors and stromal cells release a variety of pro-angiogenic factors in the TME, including VEGF. VEGF promotes the proliferation, migration, and sprouting of ECs derived from tumor growth, and interacts with pericytes to facilitate the final maturation of tumor vessels. Therefore, the inhibition of pro-angiogenic factors can halt the aggregation and growth of ECs and pericytes, thus hindering tumor angiogenesis and tumor growth.

Immune cell interaction

CD4 TFH cells as a subpopulation of CD4⁺ T cells can synergistically promote anti-tumor CD8 T cell responses with neoantigen-driven B cells and promote anti-tumor immunity by enhancing CD8 T cell effector function.^[76] Studies have demonstrated that increased levels of CD8⁺ and CD4⁺ tumour-infiltrating lymphocytes (TILs) are associated with high CD20⁺ B cell infiltration in LUAD.^[77] TAMs interact with and promote regulatory Tregs to inhibit immune responses. TAMs recruit Tregs through chemokine (C−C motif) ligand 22 (CCL22), which induces the expression of IL-6 and IL-10 and enhances PD-L1 expression on TAMs, thereby inhibiting CD8⁺ CTL responses.^[78] As a “don’t eat me” immune checkpoint signal transduction receptor, CD47 primarily binds to the signal regulatory protein α (SIRPα) expressed by TAMs and DCs.^[79] Blocking the CD47-SIRPα interaction not only enhanced macrophage-mediated cancer cell clearance, but also induced DCs endocytosis and activation, thereby stimulating T-cell-mediated tumor clearance. DCs produce CCL21 and recruit CD8⁺ cells to inhibit tumor growth. CD40 is generated by the activation in DCs, which not only interacts with the CD40 ligand expressed by CD4⁺ T cells to regulate T cell-dependent antitumor immunity, but also increases co-stimulatory molecules and cytokines such as CD80 and CD86 to reverse immunosuppression and drive antitumor T cell function. CD40 agonist antibodies induce IFN-γ production, causing TAM to reprogram to a tumor-killing phenotype. TAM reprogramming is more pronounced when CD40 agonists are combined with CSF1R inhibition, producing a pro-inflammatory TME that enhances an effective T cell response.^[80] NK cells not only receive stimuli leading to the recruitment and enhanced function of tumor-specific T cells but also increase the levels of conventional type 1 DCs in tumors by producing cytokines, including CCL5/FMS-related receptor tyrosine kinase 3 ligand (FIT3LG)^[81] to promote immune control of LUAD. NK cells further activate T cells via FMS-associated tyrosine kinase 3 (FLT3) axis-controlled DCs, thereby prolonging LUAD patient survival.^[82]

Interaction between immune cells and non-immune cells

Tang et al^[83] reported that M2 TAM undergoes macrophage-myofibroblast transformation (MMT) to promote CAF formation. Thrombospondin 2 (THBS2), a disulfide-bonded homologous trimer glycoprotein that is largely derived from a specific subgroup of CAF, largely affects B cells, CD8⁺ T cells, and TAMs, and is negatively correlated with CD8⁺ T cell infiltration.^[84] Moreover, Zhang et al^[16] reported that ECs could recruit TAMs via the macrophage migration inhibitory factor (MIF)-CD74 axis or directly inhibit T cell activation, thereby inhibiting the antitumor immune response.

Therapeutic targeting of the TME has long been considered a promising strategy for cancer treatment. Although these treatments have been somewhat successful, the treatment of LUAD is not well-documented. Therefore, targeting the interaction between TME and non-immune cells should be focused on developing strategies to alleviate immunosuppression, activate antitumor immunity, and/or improve the efficacy of immune-targeted drugs.

Conclusion and Prospect

Lung cancer remains one of the leading causes of death in the world. With the development of technology in recent years, the number of drugs targeting lung cancer has increased, especially approved in market, such as PD-1/PD-L1 inhibitors: pembrolizumab (available for the treatment of NSCLC and advanced or stage III SCLC), nivolumab (available for the treatment of advanced NSCLC and SCLC), atezolizumab (for SCLC), durvalumab (for advanced NSCLC and SCLC), and the CTLA-4 inhibitor ipilimumab (for the treatment of advanced NSCLC and SCLC). However, the average 5-year survival rate for patients with late-stage lung cancer is still low Therefore, new therapeutic approaches and methods need to be explored. Exploring its internal role and increasing research on targeted therapies will become the future trend for LUAD. Targeting cells, biological processes, and signaling pathways within the TME has gradually emerged as a promising strategy for treating LUAD with a deeper understanding of the tumor ecosystem. Moreover, LUAD cells can also act on the cellular components of TME through some cytokines and pathways [Figure 3].

Figure 3.

Effects of LUAD on cellular components of TME (created with BioRender.com). CAF: Cancer-associated fibroblast; CAMSAP3: Calmodulin-regulated spectrin-associated protein 3; CCNE2: Cyclin E2; COX-2: cyclooxygenase 2; CUL3: Cullin 3; ECs: Endothelial cell; ERK: Extracellular signal-regulated kinase; HIF-1α: Hypoxia-inducible factor-1α; LUAD: Lung adenocarcinoma; MMP: Matrix metalloproteinase; MEK: Mitogen-activated protein kinase kinases; NK: Natural killer; PI3K: Phosphatidylinositol-3-kinase; PTPRN: Receptor-type tyrosine-protein phosphatase-like N; SLC25A29: Solute carrier family 25 member 29; TAN: Tumor-associated neutrophil; TME; Tumor microenvironment; TNFAIP6: TNF Alpha-induced protein 6; VAGFA: Vascular endothelial growth factor A.

It should be noted, however, that therapeutic strategies targeting cells in TME present many challenges. Firstly, standard treatments for LUAD, including chemotherapy and radiation, tend to make changes in the TME. For instance, radiation and specific chemotherapies can trigger immunological cell death, consequently influencing the therapeutic outcome, either augmenting or impeding the response. Secondly, the interaction among TME cells in LUAD and the function of certain cells remain unclear. According to the previous study, many cellular components in TME affect LUAD development in a dual role. It is also interesting to notice that LUAD could affect the function of cellular components in TME in different situations. Therefore, cellular components of TME in LUAD deserve further study for providing a comprehensive overview of cancer process and a promising direction of cancer therapy.

In this review, we summarized the research progress on several cellular components of the TME in LUAD, individually and mutually, as well as the research and development of drugs, on purpose of providing a “roadmap” for therapeutically targeting cellular components in TME and exploring novel potential approaches in the treatment of LUAD.

Funding

The study was supported by National Natural Science Foundation of China (Nos. 82003879 and U19A2010), National Natural Science Foundation of Science and Technology Department of Sichuan Province (Nos. 2023NSFSC1928 and 2023NSFSC1992), Young Elite Scientists Sponsorship Program by China Association for Science and Technology (No. CACM-2020-QNRC1-01), Project of State Administration of Traditional Chinese Medicine of China (No. ZYYCXTD-D-202209), Project of Sichuan Provincial Administration of Traditional Chinese Medicine (No. 2022C001), and the Open Research Fund of State Key Laboratory of Southwestern Chinese Medicine Resources (No. SKLTCM202205).

Conflicts of interest

None.