Cancer-associated fibroblasts: Secretions,interactions, and therapy

Tumor microenvironment (TME) could impose a great challenge for cancer targeted therapies. Immunosuppression within the TME creates a barrier between cancer cells and therapeutic approaches. A number of cells are hosted within this milieu, among them cancer-associated fibroblasts (CAFs) are the most abundant cell populations playing major roles in mediating an immunosuppressive TME. CAFs have cross-talks with almost all cells within the TME for reprogramming them into being tumorigenic. This reprogramming reduces the pre-existing tumor immunity and dampens the efficacy of chemotherapeutic approaches. CAFs would do this through releasing a myriad of factors to the TME making it an appropriate nest for tumor growth. The cells degrade and deposit extracellular matrix components, both of which are tumorigenic. Therefore, disruption of cross-talks between CAFs with other cells within the TME would be a promising approach in cancer targeted therapies. This approach is applicable through dampening dominant signals mediated by CAFs. Another interesting approach would be reprogramming of CAFs toward their normal counterpart. This would need identification of different subtypes for these cells and their functions. More knowledge is also required about selective markers for each CAF subtype.     

Transforming growth factor-β signaling: Tumorigenesis and targeting for cancer therapy

Transforming growth factor (TGF)-β is a multitasking cytokine such that its aberrant expression is related to cancer progression and metastasis. TGF-β is produced by a variety of cells within the tumor microenvironment (TME), and it is responsible for regulation of the activity of cells within this milieu. TGF-β is a main inducer of epithelial–mesenchymal transition (EMT), immune evasion, and metastasis during cancer progression. TGF-β exerts most of its functions by acting on TβRI and TβRII receptors in canonical (Smad-dependent) or noncanonical (Smad-independent) pathways. Members of mitogen-activated protein kinase, phosphatidylinositol 3-kinase/protein kinase B, and nuclear factor κβ are involved in the non-Smad TGF-β pathway. TGF-β acts by complex signaling, and deletion in one of the effectors in this pathway may influence the outcome in a diverse way by taking even an antitumor role. The stage and the type of tumor (contextual cues from cancer cells and/or the TME) and the concentration of TGF-β are other important factors determining the fate of cancer (progression or repression). There are a number of ways for targeting TGF-β signaling in cancer, among them the special focus is on TβRII suppression.     

Extracellular vesicle-orchestrated crosstalk between cancer-associated fibroblasts and tumors

Communication networks in the tumor microenvironment (TME) play a crucial role in tumor progression. Cancer-associated fibroblasts (CAFs) are among the most abundant stromal cells in the TME. Bidirectional signal transduction between cancer cells and CAFs within the TME is important for cancer development and treatment responsiveness. Extracellular vesicles (EVs) carrying proteins, miRNAs, and other biomolecules are secreted into the extracellular matrix (ECM), which has been demonstrated to be an important communication medium between tumors and CAFs. Tumors regulate the activation of CAFs by secreting EVs. Conversely, CAFs can also affect tumor proliferation, metastasis, and therapeutic resistance through EVs. Here, we will classify EV cargoes and discuss the role of EV-mediated interactions between CAFs and tumors, reviewing current knowledge in combination with our confirmed results.     

Extracellular vesicle-orchestrated crosstalk between cancer-associated fibroblasts and tumors

Communication networks in the tumor microenvironment (TME) play a crucial role in tumor progression. Cancer-associated fibroblasts (CAFs) are among the most abundant stromal cells in the TME. Bidirectional signal transduction between cancer cells and CAFs within the TME is important for cancer development and treatment responsiveness. Extracellular vesicles (EVs) carrying proteins, miRNAs, and other biomolecules are secreted into the extracellular matrix (ECM), which has been demonstrated to be an important communication medium between tumors and CAFs. Tumors regulate the activation of CAFs by secreting EVs. Conversely, CAFs can also affect tumor proliferation, metastasis, and therapeutic resistance through EVs. Here, we will classify EV cargoes and discuss the role of EV-mediated interactions between CAFs and tumors, reviewing current knowledge in combination with our confirmed results.     

Targeting Tumor Microenvironment by Small-Molecule Inhibitors

The tumor microenvironment (TME) is a hypoxic, acidic, and immune/inflammatory cell–enriched milieu that plays crucial roles in tumor development, growth, progression, and therapy resistance. Targeting TME is an attractive strategy for the treatment of solid tumors. Conventional cancer chemotherapies are mostly designed to directly kill cancer cells, and the effectiveness is always compromised by their penetration and accessibility to cancer cells. Small-molecule inhibitors, which exhibit good penetration and accessibility, are widely studied, and many of them have been successfully applied in clinics for cancer treatment. As TME is more penetrable and accessible than tumor cells, a lot of efforts have recently been made to generate small-molecule inhibitors that specifically target TME or the components of TME or develop special drug-delivery systems that release the cytotoxic drugs specifically in TME. In this review, we briefly summarize the recent advances of small-molecule inhibitors that target TME for the tumor treatment.     

The cancer-inhibitory effects of proliferating tumor-residing fibroblasts

Initiation, local progression, and metastasis of cancer are associated with specific morphological, molecular, and functional changes in the extracellular matrix and the fibroblasts within the tumor microenvironment (TME). In the early stages of tumor development, fibroblasts are an obstacle that cancer cells must surpass or nullify to progress. Thus, in early tumor progression, specific signaling from cancer cells activates bio-pathways, which abolish the innate anticancer properties of fibroblasts and convert a high proportion of them to tumor-promoting cancer-associated fibroblasts (CAFs). Following this initial event, a wide spectrum of gene expression changes gradually leads to the development of a stromal fibroblast population with complex heterogeneity, creating fibroblast subtypes with characteristic profiles, which may alternate between being tumor-promotive and tumor-suppressive, topologically and chronologically in the TME. These fibroblast subtypes form the tumor's histological landscape comprising areas of cancer growth, inflammation, angiogenesis, invasion fronts, proliferating and non-proliferating fibroblasts, cancer-cell apoptosis, fibroblast apoptosis, and necrosis. These features reflect general deregulation of tissue homeostasis within the TME. This review discusses fundamental and current knowledge that has established the existence of anticancer fibroblasts within the various interacting elements of the TME. It is proposed that the maintenance of fibroblast proliferation is an essential parameter for the activation of their anticancer capacity, similar to that by which normal fibroblasts would be activated in wound repair, thus maintaining tissue homeostasis. Encouragement of research in this direction may render new means of cancer therapy and a greater understanding of tumor progression.     

Colorectal cancer: Metabolic interactions reshape the tumor microenvironment

Colorectal cancer (CRC) is one of the most common cancers worldwide, which ranks third in terms of incidence and the second leading cause of cancer-related mortality. Metabolic reprogramming within the tumor microenvironment (TME) has been proved intimately involved in the initiation and malignant progression of CRC. Signal messengers, including cytokines, metabolites, and exosomes among others, derived from cancer cells can be utilized by the surrounding cells within the TME to induce metabolic alteration and cancer-associated transformation. In turn, the cargos secreted from cancer-associate cells further provide the nutrition and energy supply for cancer cells, supporting their metabolic reprogramming to promote proliferation, migration, metastasis, and radiochemoresistance.In this review, we focus on the main cellular components in the TME: CAFs, TAMs, lymphocytes and neutrophils, and enumerate and integrate how the metabolic interactions between these components and cancer cells reshape TME to foster CRC malignancy.     

Intratumoral lipid metabolic reprogramming as a pro-tumoral regulator in the tumor milieu

Reprogramming of the tumor microenvironment (TME) is a hallmark of cancer. Metabolic reprogramming is a vital approach to sustaining the energy supply in the TME. This alteration exists in both cancer cells and TME cells, collectively establishing an immunotolerant niche to facilitate tumor progression. Limited resources lead to metabolic competition and hinder the biological functions of anti-tumoral immunity. Reprogramming of lipid metabolism and tumor progression is closely related to each other. Due to the complexity of fatty acid (FA) types and the lack of an effective approach for detection, the mechanisms and effects of FA metabolic reprogramming have been unclear. Herein, we review FA metabolism in the tumor milieu, summarize how FA metabolic reprogramming influences antitumor immune response, suggest the mechanisms by which FAs affect immunotherapy against cancer, and discuss the potential of FA metabolism-based drugs in cancer treatment.     

Insights into the role of components of the tumor microenvironment in oral carcinoma call for new therapeutic approaches

The research on oral cancer has focused mainly on the cancer cells, their genetic changes and consequent phenotypic modifications. However, it is increasingly clear that the tumor microenvironment (TME) has been shown to be in a dynamic state of inter-relations with the cancer cells. The TME contains a variety of components including the non-cancerous cells (i.e., immune cells, resident fibroblasts and angiogenic vascular cells) and the ECM milieu [including fibers (mainly collagen and fibronectin) and soluble factors (i.e., enzymes, growth factors, cytokines and chemokines)]. Thus, it is currently assumed that TME is considered a part of the cancerous tissue and the functionality of its key components constitutes the setting on which the hallmarks of the cancer cells can evolve. Therefore, in terms of controlling a malignancy, one should control the growth, invasion and spread of the cancer cells through modifications in the TME components. This mini review focuses on the TME as a diagnostic approach and reports the recent insights into the role of different TME key components [such as carcinoma-associated fibroblasts (CAFs) and inflammation (CAI) cells, angiogenesis, stromal matrix molecules and proteases] in the molecular biology of oral carcinoma. Furthermore, the impact of TME components on clinical outcomes and the concomitant need for development of new therapeutic approaches will be discussed.     

Colon cancer therapy by focusing on colon cancer stem cells and their tumor microenvironment

Despite many advances and optimization in colon cancer treatment, tumor recurrence and metastases make the development of new therapies necessary. Colon cancer stem cells (CCSCs) are considered as the main triggering factor of cancer progression, recurrence, and metastasis. CCSCs as a result of accumulated genetic and epigenetic alterations and also complex interconnection with the tumor microenvironment (TME) can evolve and convert to full malignant cells. Mounting evidence suggests that in cancer therapy both CCSCs and non-CCSCs in TME have to be regarded to break through the limitation of current therapies. In this regard, stem cell capabilities of some non-CCSCs may arise inside the TME condition. Therefore, a deep knowledge of regulatory mechanisms, heterogeneity, specific markers, and signaling pathways of CCSCs and their interconnection with TME components is needed to improve the treatment of colorectal cancer and the patient's life quality. In this review, we address current different targeted therapeutic options that target cell surface markers and signaling pathways of CCSCs and other components of TME. Current challenges and future perspectives of colon cancer personalized therapy are also provided here. Taken together, based on the deep understanding of biology of CCSCs and using three-dimensional culture technologies, it can be possible to reach successful colon cancer eradication and improvise combination targeted therapies against CCSCs and TME.     

Fibroblasts Influence Survival and Therapeutic Response in a 3D Co-Culture Model

In recent years, evidence has indicated that the tumor microenvironment (TME) plays a significant role in tumor progression. Fibroblasts represent an abundant cell population in the TME and produce several growth factors and cytokines. Fibroblasts generate a suitable niche for tumor cell survival and metastasis under the influence of interactions between fibroblasts and tumor cells. Investigating these interactions requires suitable experimental systems to understand the cross-talk involved. Most in vitro experimental systems use 2D cell culture and trans-well assays to study these interactions even though these paradigms poorly represent the tumor, in which direct cell-cell contacts in 3D spaces naturally occur. Investigating these interactions in vivo is of limited value due to problems regarding the challenges caused by the species-specificity of many molecules. Thus, it is essential to use in vitro models in which human fibroblasts are co-cultured with tumor cells to understand their interactions. Here, we developed a 3D co-culture model that enables direct cell-cell contacts between pancreatic, breast and or lung tumor cells and human fibroblasts/ or tumor-associated fibroblasts (TAFs). We found that co-culturing with fibroblasts/TAFs increases the proliferation in of several types of cancer cells. We also observed that co-culture induces differential expression of soluble factors in a cancer type-specific manner. Treatment with blocking antibodies against selected factors or their receptors resulted in the inhibition of cancer cell proliferation in the co-cultures. Using our co-culture model, we further revealed that TAFs can influence the response to therapeutic agents in vitro. We suggest that this model can be reliably used as a tool to investigate the interactions between a tumor and the TME.     

13C metabolic flux analysis clarifies distinct metabolic phenotypes of cancer cell spheroid mimicking tumor hypoxia

Cancer cells adapt their intracellular energy metabolism to the oxygen-deprived tumor microenvironment (TME) to ensure tumor progression. This adaptive mechanism has focused attention on the metabolic phenotypes of tumor cells under hypoxic TME for developing novel cancer therapies. Although widely used monolayer (2D) culture does not fully reflect in vivo hypoxic TME, spheroid (3D) culture can produce a milieu similar to the TME in vivo. However, how different metabolic phenotypes are expressed in 3D cultures mimicking tumor hypoxia compared with 2D cultures under hypoxia remains unclear. To address this issue, we investigated the metabolic phenotypes of 2D- and 3D-cultured cancer cells by ¹³C-metabolic flux analysis (¹³C-MFA). Principal component analysis of ¹³C mass isotopomer distributions clearly demonstrated distinct metabolic phenotypes of 3D-cultured cells. ¹³C-MFA clarified that 3D culture significantly upregulated pyruvate carboxylase flux in line with the pyruvate carboxylase protein expression level. On the other hand, 3D culture downregulated glutaminolytic flux. Consistent with our findings, 3D-cultured cells are more resistant to a glutaminase inhibitor than 2D-cultured cells. This study suggests the importance of considering the metabolic characteristics of the particular in vitro model used for research on cancer metabolism.     

Fibroblast-Derived Extracellular Matrices: An Alternative Cell Culture System That Increases Metastatic Cellular Properties

Poor survival rates from lung cancer can largely be attributed to metastatic cells that invade and spread throughout the body. The tumor microenvironment (TME) is composed of multiple cell types, as well as non-cellular components. The TME plays a critical role in the development of metastatic cancers by providing migratory cues and changing the properties of the tumor cells. The Extracellular Matrix (ECM), a main component of the TME, has been shown to change composition during tumor progression, contributing to cancer cell invasion and survival away from the primary cancer site. Although the ECM is well-known to influence the fate of tumor progression, little is known about the molecular mechanisms that are affected by the cancer cell-ECM interactions. It is imperative that these mechanisms are elucidated in order to properly understand and prevent lung cancer dissemination. However, common in vitro studies do not incorporate these interactions into everyday cell culture assays. We have adopted a model that examines decellularized human fibroblast-derived ECM as a 3-dimensional substrate for growth of lung adenocarcinoma cell lines. Here, we have characterized the effect of fibroblast-derived matrices on the properties of various lung-derived epithelial cell lines, including cancerous and non-transformed cells. This work highlights the significance of the cell-ECM interaction and its requirement for incorporation into in vitro experiments. Implementation of a fibroblast-derived ECM as an in vitro technique will provide researchers with an important factor to manipulate to better recreate and study the TME.     

The role of the microenvironment in tumor growth and invasion

Mathematical modeling and computational analysis are essential for understanding the dynamics of the complex gene networks that control normal development and homeostasis, and can help to understand how circumvention of that control leads to abnormal outcomes such as cancer. Our objectives here are to discuss the different mechanisms by which the local biochemical and mechanical microenvironment, which is comprised of various signaling molecules, cell types and the extracellular matrix (ECM), affects the progression of potentially-cancerous cells, and to present new results on two aspects of these effects. We first deal with the major processes involved in the progression from a normal cell to a cancerous cell at a level accessible to a general scientific readership, and we then outline a number of mathematical and computational issues that arise in cancer modeling. In Section 2 we present results from a model that deals with the effects of the mechanical properties of the environment on tumor growth, and in Section 3 we report results from a model of the signaling pathways and the tumor microenvironment (TME), and how their interactions affect the development of breast cancer. The results emphasize anew the complexities of the interactions within the TME and their effect on tumor growth, and show that tumor progression is not solely determined by the presence of a clone of mutated immortal cells, but rather that it can be ‘community-controlled’.     

The roles of mast cells in anticancer immunity

The tumor microenvironment (TME), which is composed of stromal cells such as endothelial cells, fibroblasts, and immune cells, provides a supportive niche promoting the growth and invasion of tumors. The TME also raises an immunosuppressive barrier to effective antitumor immune responses and is therefore emerging as a target for cancer immunotherapies. Mast cells (MCs) accumulate in the TME at early stages, and their presence in the TME is associated with poor prognosis in many aggressive human cancers. Some well-established roles of MCs in cancer are promoting angiogenesis and tumor invasion into surrounding tissues. Several mouse models of inducible and spontaneous cancer show that MCs are among the first immune cells to accumulate within and shape the TME. Although MCs and other suppressive myeloid cells are associated with poor prognosis in human cancers, high densities of intratumoral T effector (T(eff)) cells are associated with a favorable prognosis. The latter finding has stimulated interest in developing therapies to increase intratumoral T cell density. However, cellular and molecular mechanisms promoting high densities of intratumoral T(eff) cells within the TME are poorly understood. New evidence suggests that MCs are essential for shaping the immune-suppressive TME and impairing both antitumor T(eff) cell responses and intratumoral T cell accumulation. These roles for MCs warrant further elucidation in order to improve antitumor immunity. Here, we will summarize clinical studies of the prognostic significance of MCs within the TME in human cancers, as well as studies in mouse models of cancer that reveal how MCs are recruited to the TME and how MCs facilitate tumor growth. Also, we will summarize our recent studies indicating that MCs impair generation of protective antitumor T cell responses and accumulation of intratumoral T(eff) cells. We will also highlight some approaches to target MCs in the TME in order to unleash antitumor cytotoxicity.     

Mesenchymal stem cells in tumor development

Mesenchymal stem cells (MSCs) are multipotent progenitor cells that participate in the structural and functional maintenance of connective tissues under normal homeostasis. They also act as trophic mediators during tissue repair, generating bioactive molecules that help in tissue regeneration following injury. MSCs serve comparable roles in cases of malignancy and are becoming increasingly appreciated as critical components of the tumor microenvironment. MSCs home to developing tumors with great affinity, where they exacerbate cancer cell proliferation, motility, invasion and metastasis, foster angiogenesis, promote tumor desmoplasia and suppress anti-tumor immune responses. These multifaceted roles emerge as a product of reciprocal interactions occurring between MSCs and cancer cells and serve to alter the tumor milieu, setting into motion a dynamic co-evolution of both tumor and stromal tissues that favors tumor progression. Here, we summarize our current knowledge about the involvement of MSCs in cancer pathogenesis and review accumulating evidence that have placed them at the center of the pro-malignant tumor stroma.     

Mesenchymal stem cells in tumor development: Emerging roles and concepts

Mesenchymal stem cells (MSCs) are multipotent progenitor cells that participate in the structural and functional maintenance of connective tissues under normal homeostasis. They also act as trophic mediators during tissue repair, generating bioactive molecules that help in tissue regeneration following injury. MSCs serve comparable roles in cases of malignancy and are becoming increasingly appreciated as critical components of the tumor microenvironment. MSCs home to developing tumors with great affinity, where they exacerbate cancer cell proliferation, motility, invasion and metastasis, foster angiogenesis, promote tumor desmoplasia and suppress anti-tumor immune responses. These multifaceted roles emerge as a product of reciprocal interactions occurring between MSCs and cancer cells and serve to alter the tumor milieu, setting into motion a dynamic co-evolution of both tumor and stromal tissues that favors tumor progression. Here, we summarize our current knowledge about the involvement of MSCs in cancer pathogenesis and review accumulating evidence that have placed them at the center of the pro-malignant tumor stroma.     

The dynamic interactions between the stroma,pancreatic stellate cells and pancreatic tumor development: Novel therapeutic targets

The stroma is a main driver of metastasis and aggressiveness in pancreatic cancer (PC), one of the deadliest malignancies worldwide. Pancreatic stellate cells (PSCs) form approximately 50% of the pancreatic tumor stroma, causing desmoplasia, extracellular matrix (ECM) deposition, epithelial-to-mesenchymal transition (EMT) and metastatic spread. Furthermore, activated PSCs can remodel the pancreatic tumor microenvironment (TME) via dynamic and complex interactions and feedback loops with PC cells, thus facilitating tumor growth through various signalling and immune pathways. Hence, increased understanding of these cellular cross-talks and how they shape the TME in PC might guide the development of novel treatment approaches against this stubborn and deadly malignancy that has so far resisted therapeutic advances. In this review, we will explore the role of the stroma and PSCs in PC development, invasion and metastasis, examine their interaction with PC cells and discuss potential treatment approaches aimed at targeting PSCs in order to reprogram the pancreatic tumor environment.     

Breast Tumor Microenvironment: Emerging target of therapeutic phytochemicals

Triple negative breast cancer (TNBC) is the most aggressive and challenging form of breast cancers. Tumor microenvironment (TME) of TNBC is associated with induction of metastasis, immune system suppression, escaping immune detection and drug resistance. TME is highly complex and heterogeneous, consists of tumor cells, stromal cells and immune cells. The rapid expansion of tumors induce hypoxia, which concerns the reprogramming of TME components. The reciprocal communication of tumor cells and TME cells predisposes cancer cells to metastasis by modulation of developmental pathways, Wnt, notch, hedgehog and their related mechanisms in TME. Dietary phytochemicals are non-toxic and associated with various human health benefits and remarkable spectrum of biological activities. The phytochemicals serve as vital resources for drug discovery and also as a source for breast cancer therapy. The novel properties of dietary phytochemicals propose platform for modulation of tumor signaling, overcoming drug resistance, and targeting TME. Therefore, TME could serve as promising target for the treatment of TNBC. This review presents current status and implications of experimentally evaluated therapeutic phytochemicals as potential targeting agents of TME, potential nanosystems for targeted delivery of phytochemicals and their current challenges and future implications in TNBC treatment. The dietary phytochemicals especially curcumin with significant delivery system could prevent TNBC development as it is considered safe and well tolerated in phase II clinical trials.     

Color-coded imaging of splenocyte-pancreatic cancer cell interactions in the tumor microenvironment

In spite of advances in surgical and medical care pancreatic cancer remains a leading cause of cancer-related death in the United States. An understanding of cancer cell interactions with host cells is critical to our ability to develop effective antitumor therapeutics for pancreatic cancer. We report here a color-coded model system for imaging cancer cell interactions with host immune cells within the native pancreas. A human pancreatic cancer cell line engineered to express green fluorescent protein (GFP) in the nucleus and red fluorescent protein (DsRed2) in the cytoplasm was orthotopically implanted into the pancreas of a nude mouse. After 10-14 days red or green fluorescent splenocytes from immune-competent donors were delivered systemically to the pancreatic cancer-bearing nude mice. Animals were imaged after splenocyte delivery using high-resolution intravital imaging systems. At 1 day after iv injection red or green fluorescent spleen cells were found distributed in lung, liver, spleen and pancreas. By 4 days after cell delivery, however, the immune cells could be clearly imaged surrounding the tumor cells within the pancreas as well as collecting within lymphatic tissues such as lymph nodes and spleen. With the high-resolution intravital imaging afforded by the Olympus IV100 and OV100 systems the interactions of the dual-colored cancer cells and the red fluorescent spleen cells could be clearly imaged in this orthotopic pancreatic cancer model. This color-coded in vivo imaging technology offers a novel approach to imaging the interactions of cancer and immune cells in the tumor microenvironment (TME).