A narrative review on tumor microenvironment in malignant tumors

Four important characteristics are required for tumor cells to progress. These characteristics can be defined as movement, breaking down the extracellular matrix (ECM), surviving in the blood, and eventually adapting to a new environment. In particular, recent scientific studies have stated that cancer cells activate various transcription factors that play a role in the embryonic progression process and gain pleiotropic properties. In this process, the tumor microenvironment is critical [1].

It is recommended that 8 common points unite the cancer cell at the cellular phenotype level in 2022. These properties are summarized as maintaining proliferative signaling, avoiding growth suppressors, resisting cell death, achieving replicative immortality, inducing vasculature, activating invasion and metastasis, reprogramming cellular metabolism, and avoiding immune destruction. Also, tumor cells produce most of their growth signals, reducing their dependence on the normal tissue microenvironment [2]. These cells also disrupt the hemostatic mechanism, as they are not dependent on externally generated signals [3]. Neovascularization following tumor development is controlled by a complex biological rheostat that includes cancer cells and associated stromal TME [4].

It is known that TME, which is formed not only by tumor cells but also by ECM and stromal cells, which are in close interaction with these cells, plays a critical role in tumor progression [5]. The interactions between tumor cells and their microenvironment, vital in cancer growth, contribute to a hallmark of cancer [6]. The normal stroma has an inherited plasticity ability to respond rapidly to neoplastic stimuli and form the “reactive stroma” in concert with the adjacent epithelium. While the ability of stromal cells to suppress the carcinogenesis process under normal conditions is associated with organismal survival and longevity, when cells in the stroma are stimulated and transformed in various ways, their anti-carcinogenic properties are reversed and begin to contribute to cancer development. In this case, cells in the stroma develop together with cancer cells and differentiate to synthesize various cytokines, chemokines, growth factors, and proteinases [7]. Fibroblast cells and myofibroblasts, adipose (fat) cells, immune system cells and inflammatory cells, blood cells, lymphatic vascular network, and ECM are structural and functional elements in the TME stroma (Figure 1) [8].

Figure 1.

Cell Populations in the Tumor Microenvironment (created by BioRender.com). The tumor microenvironment comprises cancer cells, extracellular matrix, and stromal tissue. In addition, there are macrophage cells, dendritic cells, natural killer cells, and T and B lymphocyte cells around cancer cells in the tumor microenvironment. When you move towards the center of the tumor microenvironment, the oxygen level decreases, and by-products such as anti-inflammatory cytokines and chemokines accumulate in the environment. A. CD8⁺T cells diversify into cytotoxic T cells and enter the tumor microenvironment [9]. These cells then exhibit cytotoxicity against tumor cells and provide immune control of the tumor microenvironment. B. Subpopulations of tumor-associated macrophages, anti-tumor M1-like and pre-tumor M2-like, tumor-associated macrophages coexist within the tumor. Contrasting effects of M1/M2 subpopulations on tumors modulate anti-tumor immune control. Moreover, T_reg cells in the tumor microenvironment can be differentiated induced by conventional T cells with a potent immunosuppressive function, promoting the formation and development of tumors [10]. As cancer cells proliferate, they pass mutations that cause them to metastasize to other sites. In this context, DNA damage of cancer cells in the tumor microenvironment creates genomic instability (Treg, regulatory T cells). Genomic instability is characterized by events such as DNA damage, chromosomal aberrations, and mutations in cancer cells, which increase tumor heterogeneity and alter how the immune system interacts with the tumor. This figure was created by Biorender (https://biorender.com/)

Fibroblasts are one of the most important cell types in TME [11]. They are the most abundant cell group in connective tissue and secrete ECM components [12]. These cells ensure the continuity of the structures and functions of healthy tissues with their roles in ECM remodeling and tissue repair [13]. Fibroblasts both provide a gateway for endothelial cells undergoing angiogenesis in the tumor and let cancer cells migrate from the prime tumor space to the bloodstream for systemic metastasis [14].

Data obtained from studies conducted in recent years show that fibroblasts have vital roles in tumorigenesis [13]. These cells, which are located in the tumor stroma and called “carcinoma-associated fibroblasts (CAF)”, have similar characteristics to myofibroblasts, which are involved in wound healing and inflammation processes. When tissue damage occurs, fibroblasts in that area transform into myofibroblasts in response to paracrine signals [15]. Organ fibrosis, which can develop with the stimulation of myofibroblasts, also increases the risk of cancer growth. Endothelial cells, myoepithelial cells, smooth muscle cells, and mesenchymal cells can potentially be precursor cell groups for CAFs [16].

Deposition of the ECM, regulation of epithelial differentiation, regulation of inflammation, and wound healing are among the most important functions of CAFs [17]. These cells can induce proliferation in cancer cells by secreting hepatocyte growth factor (HGF), epidermal growth factor (EGF), insulin-like growth factor-1 (IGF-1), stromal cell-derived factor-1 (SDF1), and various fibroblast growth factors (FGF) [18].

The immune system consists of various cell groups and mediators interacting in a complex and dynamic network between non-immune cells and other cells to protect living organisms against pathogens. In healthy individuals, the innate immune system is defensive against internal or external dangers [19]. In recent years, it has been shown that immune cells contribute to cancer initiation and metastasis processes [20], and it is thought that the “polarization” feature is the basis of the bilateral effects of these cells in cancer progression [21].

Studies have indicated that cellular interplays in TME influence carcinoma growth and progression. In the early stages of tumor development, malignant cells in TME are weak stimuli and then weak targets of the immune response. As time passes, these resist the innate immune reply and then begin to impair the adaptive immune response [22]. This process of dampening immune responses requires obstacles to the function and maturation of T lymphocytes, which at last make up a prominent part of TME. However, the literature we reviewed conflicts with the precise impacts of specific T cells. Some T cells are reported to enhance ongoing tumorigenesis, while others are tumor-restrictive [23]. It is known that various T cells that invade the tumor microenvironment control the immune response thanks to the cytokines they secrete [24]. These cell groups include cytotoxic CD8⁺ memory T cells, which are talented at killing tumor cells and have been associated with a good prognosis. These cells are supported by CD4⁺, T-helper 1 (Th1) cells, which are characterized by the production of interleukin-2 (IL-2) and interferon-gamma (IFN-γ). The high expression of these cytokines in TME is also related to a good prognosis. Other CD4+ cell groups, such as Th2 cells that induce B-cell response and secrete Interleukin-4 (IL-4), Interleukin-5 (IL-5), and Interleukin-13 (IL-13), are thought to be mostly associated with tumor growth [25].

Macrophages are one of the major immune system cells in TME. Macrophages, which play an important role in the transition from the inflammatory phase to the proliferative phase, attract other cell groups, such as fibroblasts, to the damaged area with the release of various growth factors and cytokines; they contribute to angiogenesis as well as organizing the formation of new tissue matrix [26]. Two types of macrophage activation have been identified: Cytokines such as IFN-γ and tumor necrosis factor-alpha (TNF-α), which are actively involved in the activation of macrophages to “classically activated” M1-type macrophages, are mostly released from Th1 cells, while IL-4 and interleukin-10 released from Th2 cells. Interleukin-10 (IL-10) inhibits macrophage activation, and these types of macrophages are called “alternatively activated/anti-inflammatory” M2-type macrophages [27]. As TME decides on M1 and M2 macrophage transformation, M2 macrophages can transform into M1 macrophages under the influence of exogenous factors in this process [28]. The most important function of M1 macrophages is to present antigens and support Th1 activation [29]. M1 macrophages can also secrete pro-inflammatory cytokines and immune activation factors to contribute to inflammation and exhibit anti-tumoral properties [30]. M2 macrophages can be activated by various cytokines (IL-4, IL-13), prostaglandin E2 (PGE2), vitamin D3, transforming growth factor-beta (TGF-β), and glucocorticoids [31]. The main function of these cells is to limit the immune response and contribute to tumor growth, invasion and metastasis by obstructing T cells by secreting inhibitory cytokines such as IL-10 and TGF-β [32]. M2 macrophages can secrete a variety of chemokine receptor agonists and matrix metalloproteinases but do not play an effective role in antigen presentation [33].

Cells in the monocyte-macrophage origin involved in TME are termed “tumor-associated macrophages (TAM)”. These cells mostly have an M2-type phenotype and are functionally similar to M2 macrophages. TAMs develop as circulating monocytes from hematopoietic bone marrow precursors and then actively accumulate in tumor tissues [34]. Both tumor cells and other cells produce signals regulating the presence and accumulation of TAMs in TME [35]. Chemotactic factors, vascular endothelial growth factor (VEGF), granulocyte colony-stimulating factor (G-CSF), and placental growth factors can cause monocytes to accumulate in tumor tissues [36].

Reprogramming, activation, and recruitment of stromal and immune cells in the extracellular environment result from the interaction between cancer cells and the tumor microenvironment [37]. In addition, it has been stated that cancer progression and progression are affected by components of TME and are controlled by the host immune system [38]. Therefore, TME components and immune system biomarkers are significant for cancer findings and evaluation of prognoses and treatment response [22, 38]. Examination of immune TME has crucial prognostic value and may support histopathological and molecular biomarkers for the evaluation of the response of a patient to therapy. For cancer progression, cells must escape epithelial compartments and invade surrounding spaces. Tumor growth is regulated by the TME through paracrine and juxtacrine interactions, with tumor-associated stroma supplying nutrients, enzymes, oxygen, and growth factors to support cancer cell proliferation [38].

Adipocytes contribute to malignant cell growth by supplying fatty acids used as fuel for cancer cells and taking part in the accumulation of malignant cells with the secretion of adipokines in some types of cancer (such as intra-abdominal tumors) [39]. The increase in white adipose tissue in obesity affects insulin resistance. This contributes to carcinogenesis by causing excessive release of cytokines and adipokines [40]. In a study in which adipose stromal cells were transplanted into mice to determine the role of white adipose tissue in cancer progression, adipocytes were shown to act as a source of stromal and vascular progenitors to support tumor vascularization and growth [41]. Studies are showing that endogenous adipose tissue-derived stromal progenitor cells contribute to pericytes and adipocytes in TME and provide these cells with growth factors and cytokines that are effective in tumor growth, chemotherapy resistance, and tumor recurrence [40].

Among the important cell groups in TME, blood endothelial cells and lymphatic endothelial cells can be counted. Tumor blood vessels, including blood endothelial cells, contribute to tumor growth and hematogenous tumor spread. In contrast, lymphatic vessels, including lymphatic endothelial cells, are weaker compared to blood vessels, but these cells also play an active role in the spread of tumor cells by lymph [42]. The lymphatic vessels are more permeable than blood vessels because they are sparsely surrounded by pericyte and smooth muscle cells. This is especially important for the metastasis process [43]. TME includes endothelial cells, which aid in tumor development and immune evasion, as well as immune cells like lymphocytes, granulocytes, and macrophages. Macrophages are the most significant immune cells, contributing to tumor-associated inflammation and cancer cell survival [6].

Macrophages have several functions linked to cancer growth and progression; they promote the getaway of tumor cells into the circulatory system and can push down anti-tumor immune mechanisms and responses. The data obtained from the literature reviews we conducted has revealed that macrophages can assist in the extravasation of circulating cancer cells at remote sites like the lungs, resulting in the persistent growth of metastatic colonies [44].

ECM combines molecules with biochemical and physical properties, such as proteins, glycoproteins, proteoglycans, and polysaccharides. It is one of the most important structures in TME [45]. The ECM forms a three-dimensional structure with various physiological, biochemical, and biomechanical properties, including cell growth, survival, motility, and cell differentiation, by ligating various receptors such as integrin, syndecan, and discoidin [46]. ECM provides a structural framework for cells in TME and plays an active role in cancer progression. It also contains many vital growth factors, such as angiogenic factors and chemokines, that interact with cell surface receptors and confer contractile and elasticity properties to the cell. Because CAFs cause ECM deposition in tumor tissues, tumor tissues are much stiffer than surrounding tissues [47]. Cell adhesion in the ECM is mediated by ECM receptors (such as integrins, discoidin domain receptors, and syndecan receptors) [46]. Tumors often feature desmoplasia, and this fibrotic state is associated with increased post-translational modifications and potentially altered structure of ECM proteins [48].

The involvement of adhesion and its molecules in immune cell interactions related to tumor activity is crucial. The presence of adhesion molecules is necessary to establish the immunological synapse [49]. Irregular adhesion molecule presentation on malignant cells permits evasion of immune control. In contrast, the atypical presentation of adhesion molecules on vasculature associated with tumors can make the entire tumor mass impervious to the immune system [50]. Additionally, extracellular vesicles associated with cell adhesion molecules are essential in intratumor communication. These extracellular vesicles are associated with cell adhesion molecules to transport. Hence, cell adhesion molecules that exhibit significant heterogeneity can be used as markers to indicate the origin or function of a particular cell type [51].

The ECM, an acellular tissue component, provides structural and biochemical support to cells, influencing adhesion, communication, and proliferation. Composed of water, proteoglycans, fibrous proteins, and minerals, its unique composition results from interactions between cells and their microenvironment [38, 52]. The components of the ECM can vary depending on the sedentary cells and the needs of the particular tissue. Studies show that the ECM arranges the production of different fibrous proteins, including laminin, collagen, and elastin [53].

ECM, one of the most important structural elements of TME, comprises various growth factors, cytokines, and chemokines [54], and these molecules are released into the microenvironment by cancer cells and stromal cells. Cytokines are small proteins that have specific effects on communication and interaction between cells [55], and they can directly affect carcinogenesis and metastasis processes by modifying the tumor phenotype [56], as well as increasing their recognition by cytotoxic effector cells by directly stimulating stromal and immune effector cells in the tumor region. Numerous studies with experimental animals have shown that cytokines have anti-tumoral activity. Chemokines that can be expressed by tumor cells migrate to the TME and regulate tumor immune responses. Additionally, chemokines can directly target non-immune cells (including tumor cells and vascular endothelial cells) in the TME. Hence, it can regulate tumor cell proliferation and cancer metastasis [57]. Chemokines, called chemotactic cytokines, regulate cell traffic and position by activating G-protein coupled chemokine receptors [58]. Chemokines, which play an active role in the development of various cancers such as pancreatic cancer, breast, ovarian, and lung cancer, have critical importance in tumor progression and metastasis.

Cancer is a disease of altered signaling and metabolism, causing uncontrolled division and survival of transformed cells [59]. However, cancer has been characterized by massive and prominent metabolic programming that negatively impacts normal bodily functions [60]. This reprogramming is often complex and may include metabolic cooperation between the surrounding stroma and cancer cells [61]. Inflammatory environment, hypoxia, and acidosis are among the most important features of TME. One of the most fundamental characteristics of cancer is that it is an environment that supports inflammation. Cancer cells influence inflammation by increasing the number of cytokines that lead to a state of immunosuppression and modifying various molecules, including cytokines responsible for the anti-tumor activity of immune cells [54,55,56].

As tumor cells develop, the supporting vasculature is frequently restrictive, disordered, and non-functional, leading to inadequate blood supply to the tissues and, thus, to a lack of oxygen (hypoxia) and nutrients. Regardless of the oncogene mutation, two factors are always strongly associated with malignancy: Acidic pH and lack of oxygen. Oxygen deficiency occurs in and around the tumor as the tumor volume increases. This is called hypoxia. Increased transcription is one of the major mechanisms driving tumor glycolytic shift in hypoxia and resulting in acidosis [62]. Hence, different proteins, growth factors, and enzymes become active in oxygen deficiency. Cancer cell gains the ability to metastasize. At the same time, the increase in protein activity called Hypoxia-inducible factor-1 (HIF-1) enables the cancer cell to develop new vessel extensions from existing vessels.

Stabilization of HIF-1 followed by dimerization with HIF-1 leads to transcriptional activation of several genes involved in decreased respiration, oxygen sensing, erythropoiesis, angiogenesis, glycolytic metabolism, pHi and pHo regulation, autophagy (cell survival) and migration [63]. As a result, cycles of hypoxia/reoxygenation, that is, survival/proliferation states, make tumor cells more aggressive [64].

Tumor acidosis has been recognized as an important feature of cancer growth, which can affect treatment response and severity of symptoms [61, 65]. Moreover, acidosis is no longer perceived as a passive side effect of tumor growth. In contrast, acidosis is recognized as an essential regulator of tumor progression. A review of research evidence suggests that tumor acidosis may be linked to extracellular lactic acid accumulation and hypoxia [60]. The high metabolic demands of tumor cells often lead to significant H⁺ accumulation in TME. In addition, the disordered nature of tumor vasculature often prevents the effective and timely elimination of H⁺ ions from the extracellular environment [61]; this leads to the development of hypoxic zones in TME and a shift in glycolytic metabolism. In other cases, the accumulation of H⁺ ions is associated with the hydration of CO₂ at oxidative tumor sites. These events usually occur at a high rate to meet tumors’ biosynthetic and bioenergetic needs.

Tumor cells have to separate, migrate, invade, adapt, and reattach for their mechanical processes, such as cell adhesion, changes in cadherin, cell movements, and motility. To ensure these processes are effective, the tumor cell interacts with the TME (Figure 2). These multiple interactions with tumor stroma determine cancer growth and metastasis and may also ensure protective effects according to the drug sensitivity/resistance of tumor cells [66]. Structural and biochemical features of the ECM (fiber network morphology, collagen content, fiber thickness, extent of intrafibrillary crosslinks, and mesh size-diameter ratio of the migrating cell) determine the degree of TME [67].

Figure 2.

Interaction of ECM with Tumor Cells. Tumor cells suppress the response of T cells, leading to immunosuppression. These cells remodel the ECM by activating fibroblasts, which are the major cells of the TME. Macrophage polarization, cancer cells, and fibroblast cells contribute to ECM remodeling about matrix stiffness. Matrix stiffness is mainly related to excess collagen in the TME. (ECM, Extracellular matrix, TME, Tumor microenvironment). This figure was created by Biorender (https://biorender.com/)

Tumor cells use distinct strategies for migration. Individual cell migration strategies may be performed mesenchymal-like or amoebic. Growth factors control these strategies. Fibrillary collagen deposition in the ECM can promote tumor cell motility by providing one-dimensional or two-dimensional “marks” for cell movement [68]. Further cross-linking of collagen fibrils by enzymes such as lysyl oxidase can cause cell invasion by increasing the alignment and stiffness of these marks [69]. Hence, the cellular mechanism, which recognizes the biochemical diversity of the ECM as well as its physical and topographic properties, such as stiffness, dimensionality, and ligand spacing, may be important for the response of cells to the ECM [70].