The role of CAF derived exosomal microRNAs in the tumour microenvironment of melanoma

Exosomes play a crucial role in the crosstalk between cancer associated fibroblasts (CAFs) and cancer cells, contributing to carcinogenesis and the tumour microenvironment. Recent studies have revealed that CAFs, normal fibroblasts and cancer cells all secrete exosomes that contain miRNA, establishing a cell-cell communication network within the tumour microenvironment. For example, miRNA dysregulation in melanoma has been shown to promote CAF activation via induction of epithelial-mesenchymal transition (EMT), which in turn alters the secretory phenotype of CAFs in the stroma. This review assesses the roles of melanoma exosomal miRNAs in CAF formation and how CAF exosome-mediated feedback signalling to melanoma lead to tumour progression and metastasis. Moreover, efforts to exploit exosomal miRNA-mediated network communication between tumour cells and their microenvironment, and their potential as prognostic biomarkers or novel therapeutic targets in melanoma will also be considered.     

Gastric Cancer Exosomes Trigger Differentiation of Umbilical Cord Derived Mesenchymal Stem Cells to Carcinoma-Associated Fibroblasts through TGF-β/Smad Pathway

Background

Mesenchymal stem cells (MSCs) promote tumor growth by differentiating into carcinoma-associated fibroblasts (CAFs) and composing the tumor microenvironment. However, the mechanisms responsible for the transition of MSCs to CAFs are not well understood. Exosomes regulate cellular activities by mediating cell-cell communication. In this study, we aimed to investigate whether cancer cell-derived exosomes were involved in regulating the differentiation of human umbilical cord-derived MSCs (hucMSCs) to CAFs.

Methodology/Principal Findings

We first showed that gastric cancer cell-derived exosomes induced the expression of CAF markers in hucMSCs. We then demonstrated that gastric cancer cell-derived exosomes stimulated the phosphorylation of Smad-2 in hucMSCs. We further confirmed that TGF-β receptor 1 kinase inhibitor attenuated Smad-2 phosphorylation and CAF marker expression in hucMSCs after exposure to gastric cancer cell-derived exosomes.

Conclusion/Significance

Our results suggest that gastric cancer cells triggered the differentiation of hucMSCs to CAFs by exosomes-mediated TGF-β transfer and TGF-β/Smad pathway activation, which may represent a novel mechanism for MSCs to CAFs transition in cancer.     

Protein kinase C zeta regulates interleukin-8-mediated stromal-derived factor-1 expression and migration of human mesenchymal stromal cells

Mesenchymal stromal cells (MSCs) are bone marrow-derived cells with multipotent differentiation capability that are mobilized into the circulation in response to injury and localize to areas of tissue damage including solid tumors. They have the capacity to adopt a phenotype similar to carcinoma-associated fibroblasts (CAFs) and, like CAFs, promote tumor growth. The molecular communication between tumor cells and MSCs has not been well defined. However, MSCs have increased expression of the chemokine stromal-derived factor 1 (SDF-1) when exposed to conditioned medium from tumor cells. Additionally, SDF-1 has been shown to be important in the promotion of tumor growth by CAFs. These data suggest that the SDF-1 signaling axis is a key feature of the tumor microenvironment. In this report, we demonstrate that interleukin 8 (IL-8) induces an increase in SDF-1 expression by MSCs. The increase in SDF-1 expression in response to IL-8 is mediated by the activation of the protein kinase C (PKC) zeta isoform. In a functional assay, activation of PKC is required for in vitro MSC migration in response to tumor conditioned medium. These results indicate that IL-8-mediated SDF-1 production by MSCs requires PKC zeta activation. This signaling pathway provides insight into possible molecular targets for cancer therapy aimed at disrupting the interaction between components of the tumor microenvironment.     

Weighted gene co‐expression network analysis can sort cancer‐associated fibroblast‐specific markers promoting bladder cancer progression

The role of cancer‐associated fibroblasts (CAFs) has been thoroughly investigated in tumour microenvironments but not in bladder urothelial carcinoma (BLCA). The cell fraction of CAFs gradually increased with BLCA progression. Weighted gene co‐expression network analysis (WGCNA) revealed a specific gene expression module of CAFs that are relevant to cancer progression and survival status. Fifteen key genes of the module were consistent with a fibroblast signature in single‐cell RNA sequencing, functionally related to the extracellular matrix, and significant in survival analysis and tumour staging. A comparison of the luminal‐infiltrated versus luminal‐papillary subtypes and fibroblast versus urothelial carcinoma cell lines and immunohistochemical data analysis demonstrated that the key genes were specifically expressed in CAFs. Moreover, these genes are highly correlated with previously reported CAF markers. In summary, CAFs play a major role in the progression of BLCA, and the 15 key genes act as BLCA‐specific CAF markers and can predict CAF changes. WGCNA can, therefore, be used to sort CAF‐specific gene set in cancer tissues.     

HIF-1α is necessary for activation and tumour-promotion effect of cancer-associated fibroblasts in lung cancer

Cancer-associated fibroblasts (CAFs) activation is crucial for the establishment of a tumour promoting microenvironment, but our understanding of CAFs activation is still limited. In this study, we found that hypoxia-inducible factor-1α (HIF-1α) was highly expressed in CAFs of human lung cancer tissues and mouse spontaneous lung tumour. Accordingly, enhancing the expression of HIF-1α in fibroblasts via hypoxia induced the conversion of normal fibroblasts into CAFs. HIF-1α-specific inhibitor or HIF-1α knockout (KO) significantly attenuated CAFs activation, which was manifested by the decreased expression of COL1A2 and α-SMA. In vivo, during tumour formation, the expression of Ki-67 and proliferating cell nuclear antigen (PCNA) in the tumour tissue with HIF-1α KO fibroblasts was significantly lower than that of normal fibroblasts. Moreover, HIF-1α in fibroblasts could activate the NF-κB signalling pathway and enhance a subsequent secretion of CCL5, thus promoting the tumour growth. In conclusion, our results suggest that HIF-1α is essential for the activation and tumour-promotion function of CAFs in lung cancer (LC). And targeting HIF-1α expression on CAFs may be a promising strategy for LC therapy.     

Chinese Herbal Medicine Suppresses Invasion-Promoting Capacity of Cancer-Associated Fibroblasts in Pancreatic Cancer

Pancreatic cancer remains one of the leading causes of cancer-related deaths, due to aggressive growth, high metastatic rates during the early stage and the lack of an effective therapeutic approach. We previously showed that Qingyihuaji (QYHJ), a seven-herb Chinese medicine formula, exhibited significant anti-cancer effects in pancreatic cancer, associated with modifications in the tumor microenvironment, particularly the inhibition of cancer-associated fibroblast (CAF) activation. In the present study, we generated CAF and paired normal fibroblast (NF) cultures from resected human pancreatic cancer tissues. We observed that CAFs exhibited an enhanced capacity for inducing pancreatic cancer cell migration and invasion compared with NFs, while QYHJ-treated CAFs exhibited decreased migration and invasion-promoting capacities in vitro. The results of further analyses indicated that compared with NFs, CAFs exhibit increased CXCL1, 2 and 8 expression, contributing to the enhanced invasion-promoting capacities of these cells, while QYHJ treatment significantly suppressed CAF proliferation activities and the production of CAF-derived CXCL1, 2 and 8. These in vitro observations were confirmed in mice models of human pancreatic cancer. Taken together, these results suggested that suppressing the tumor-promoting capacity of CAFs through Chinese herbal medicine attenuates pancreatic cancer cell invasion.     

Markers of breast cancer stromal fibroblasts in the primary tumour site associated with lymph node metastasis: a systematic review including our case series

CAFs (cancer-associated fibroblasts), the most abundant cell type in breast cancer stroma, produce a plethora of chemokines, growth factors and ECM (extracellular matrix) proteins, that may contribute to dissemination and metastasis. Axillary nodes are the first metastatic site in breast cancer; however, to the present date, there is no consensus of which specific proteins, synthesized by CAFs, might be related with lymph node involvement. The purpose of this study was to perform a systematic review of CAF biomarkers associated with the presence of regional metastasis. PubMed was searched using the words: ‘breast cancer’ and ‘lymph node’ and fibroblast or stroma or microenvironment. After exclusions, eight studies evaluating biomarkers immunoexpression in CAFs and lymph node status were selected. Biomarkers evaluated in these studies may be divided in two groups, according to their ontology: extracellular matrix components [MMP13 (matrix metalloproteinase 13), TIMP2 (tissue inhibitor of metalloproteinases-2), THBS1 (thrombospondin 1), LGALS1 (lectin, galactoside-binding, soluble, 1)] and response to wounding [PDPN (podoplanin), PLAU (plasminogen activator, urokinase), PLAUR (plasminogen activator, urokinase receptor), CAV1 (caveolin 1), THBS1, LGALS1]. A positive expression of MMP13 and LGALS1 in CAFs was associated with enhanced OR (odds ratio) for regional metastasis. Contrariwise, CAV1 positive staining of fibroblasts was associated with decreased OR for nodal involvement. Expression of MMP13, PDPN and CAV1 was further tested in a new series of 65 samples of invasive ductal breast carcinomas by immunohistochemistry and no association between biomarkers expression in CAFs and nodal status was found. It was suggested that breast cancer subtypes may differentially affect CAFs behaviour. It would be interesting to evaluate the prognostic significance of these biomarkers in CAFs from different tumour types.     

Quantitative phosphoproteomics reveals ectopic ATP synthase on mesenchymal stem cells to promote tumor progression via ERK/c-Fos pathway activation

The tumor microenvironment (TME), which comprises cellular and noncellular components, is involved in the complex process of cancer development. Emerging evidence suggests that mesenchymal stem cells (MSCs), one of the vital regulators of the TME, foster tumor progression through paracrine secretion. However, the comprehensive phosphosignaling pathways that are mediated by MSC-secreting factors have not yet been fully established. In this study, we attempt to dissect the MSC-triggered mechanism in lung cancer using quantitative phosphoproteomics. A total of 1958 phosphorylation sites are identified in lung cancer cells stimulated with MSC-conditioned medium. Integrative analysis of the identified phosphoproteins and predicted kinases demonstrates that MSC-conditioned medium functionally promotes the proliferation and migration of lung cancer via the ERK/phospho-c-Fos-S374 pathway. Recent studies have reported that extracellular ATP accumulates in the TME and stimulates the P2X7R on the cancer cell membrane via purinergic signaling. We observe that ectopic ATP synthase is located on the surface of MSCs and excreted extracellular ATP into the lung cancer microenvironment to trigger the ERK/phospho-c-Fos-S374 pathway, which is consistent with these previous findings. Our results suggest that ectopic ATP synthase on the surface of MSCs releases extracellular ATP into the TME, which promotes cancer progression via activation of the ERK/phospho-c-Fos-S374 pathway.     

TIMP-1 Promotes Accumulation of Cancer Associated Fibroblasts and Cancer Progression

Treatment options for late stage prostate and colon cancer are limited and there is an urgent need to develop more effective and targeted novel therapies, which starts with identification and validation of novel therapeutic targets. Recent clinical studies have demonstrated that tissue inhibitor matrix metalloproteinase-1 (TIMP-1) levels are elevated in cancer patient plasma and elevated TIMP-1 levels are associated with worse clinical outcomes. However, it is unknown whether TIMP-1 serves merely as a biomarker of cancer progression or has a functional role in promoting cancer progression and can serve as a cancer therapeutic target, which is the main objective of this study. Here, we show that stroma of human prostate and colon cancer express higher levels of TIMP-1 compared to their normal counterparts and increased expression of TIMP-1 promotes in vivo growth of both cancer types. We demonstrate for the first time that increased TIMP-1 expression stimulates accumulation of cancer associated fibroblasts (CAFs) within prostate and colon cancer tissues and that TIMP-1 enhances prostate CAF proliferation and migration in vitro and promotes ERK1/2 kinase activation in these CAF cells. Our results establish the novel promotive effects of TIMP-1 on cancer progression and on accumulation of CAFs that in turn provides a pro-tumor microenvironment. Together, these results establish the potential of TIMP-1 as a novel target for cancer therapy and the mechanism underlying the pro-tumor activity of TIMP-1.     

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.     

Carcinoma‐associated fibroblasts: Non‐neoplastic tumour‐promoting mesenchymal cells

Cancerous stroma coevolves alongside tumour progression, thereby promoting the malignant conversion of epithelial carcinoma cells. To date, an abundance of data have supported crucial roles of the tumour microenvironment (TME) in providing cancer cells with proliferative, migratory, survival and invasive propensities favouring the processes of tumourigenesis. The cancerous reactive stroma is frequently populated by a large number of myofibroblasts (MFs), which are activated, non‐transformed fibroblasts expressing α‐smooth muscle actin (α‐SMA). MFs together with non‐MF cells present in the tumour‐associated stroma are collectively referred to as carcinoma‐associated fibroblasts (CAFs), one of the major stromal cell types recognised in various human carcinomas. Recruitment of fibroblasts and/or their progenitors to a tumour mass and their subsequent transdifferentiation into MFs, as well as ongoing maintenance of their activated state, are believed to be essential processes facilitating tumour progression. However, the complex networks of signalling pathways mediating the phenotypic conversion into CAFs, as well as those underlying their tumour‐promoting interactions with other tumour‐constituting cells, have yet to be fully explored. Histopathological confirmation of the presence of large numbers of CAF MFs within TME and their altered gene expression profiles are known to be associated with disease progression and to serve as independent negative prognostic factors for a wide range of tumour types. In this review, we examine the current evidence shedding light on the emerging roles of tumour‐promoting CAFs, cells that are pivotal for epithelial cancer development and progression, and discuss the therapeutic potential of targeting these cells. J. Cell. Physiol. 228: 1651–1657, 2013. © 2013 Wiley Periodicals, Inc.     

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.     

The Role of Fibroblasts in Pancreatic Cancer: Extracellular Matrix Versus Paracrine Factors

BACKGROUND AND AIM: Desmoplasia is a characteristic feature and a suspected mechanism of tumor progression in pancreatic ductal adenocarcinoma (PDAC). Main constituents of the stroma involve cancer-associated fibroblasts (CAFs) and extracellular matrix (ECM). The aim of this study was to dissect the interaction of CAFs, ECM, and PDAC cells in both an in vitro setting and a large-scale clinical cohort study. METHODS AND MATERIAL: Patients operated for PDAC were identified from our prospectively maintained clinical database. A standard pathology protocol was applied for pancreatoduodenectomy specimens also assessing CAF activation as either CAF grade 0 or CAF grade +. Interaction between a spectrum of pancreatic cancer cell lines (PCCs) and mouse embryonic fibroblasts (NIH 3T3) was assessed in a conditioned medium experimental setup. RESULTS: One hundred eleven patients operated for PDAC from 2001 to 2011 were identified. Univariate analysis disclosed CAF grade + (P = .030), positive M status (P < .001), and lymph node ratio (LNR) > 0.1 (P = .045) to impair overall survival. Independent prognostic factors were CAF grade (P = .050) and positive M status (P = .002). CAF grade correlated with N status (CC = 0.206, P = .030), LNR (CC = 0.187, P = .049), tumor size (CC = ?0.275, P = .003), and M status (CC = 0.190, P = .045). In the in vitro setting, paracrine effects of pancreatic cancer cell resulted in morphological activation of fibroblasts and tumor cell differentiation–dependent increase of fibroblast growth. Paracrine effects of poorly differentiated PCCs led to an upregulation of Vimentin in NIH 3T3 fibroblasts. Paracrine effects of fibroblasts on their part promoted cancer cell motility in all PCCs. As the second stromal component, fibroblast-derived ECM resulted in significantly decreased proliferation depending on density and led to upregulation of ZEB1 in poorly differentiated PCCs. CONCLUSION: In PDAC patients, positive CAF grading was identified as a negative prognostic parameter correlating with positive N status, high LNR, positive M status, and smaller tumor size. Whereas bilateral interaction of PCCs and CAFs promotes tumor progression, ECM poses PCC growth restrictions. In summary, our study discloses differential effects of stromal components and may help to interpret heterogeneous results of former studies.     

Fibroblast heterogeneity in pancreatic ductal adenocarcinoma: Perspectives in immunotherapy

Cancer-associated fibroblasts (CAFs), the key component in pancreatic tumor microenvironment (TME), originate from many sources and are naturally heterogeneous in phenotype and function. Numerous studies have identified their crucial role in promoting tumorigenesis through many routes including fostering cancer proliferation, angiogenesis, invasion, and metastasis. Conversely, research also indicates that subsets of CAFs express anti-tumor activity. These dual effects reflect the complexity of CAF heterogeneity and their interactions with other cells and factors in pancreatic TME. A critical component in this environment is infiltrated immune cells and immune mediators, which can communicate with CAFs. The crosstalk occurs via the production of various cytokines, chemokines, and other mediators and shapes the immunological state in TME. Comprehensive studies of the crosstalk between CAFs and tumor immune environment, particularly internal mechanisms interlinking CAFs and immune effectors, may provide new approaches for pancreatic ductal adenocarcinoma (PDAC) treatments. In this review, we explore the characteristics of CAFs, describe the interplay among CAFs, infiltrated immune cells, other mediators, and provide an overview of recent CAF-target therapies, their limitations, and potential research directions in CAF in the context of PDAC.     

Stromal miR-200s contribute to breast cancer cell invasion through CAF activation and ECM remodeling

The activation of cancer-associated fibroblasts (CAFs) is a key event in tumor progression, and alternative extracellular matrix (ECM) proteins derived from CAFs induce ECM remodeling and cancer cell invasion. Here we found that miR-200 s, which are generally downregulated in activated CAFs in breast cancer tissues and in normal fibroblasts (NFs) activated by breast cancer cells, are direct mediators of NF reprogramming into CAFs and of ECM remodeling. NFs with downregulated miR-200 s displayed the traits of activated CAFs, including accelerated migration and invasion. Ectopic expression of miR-200 s in CAFs at least partially restored the phenotypes of NFs. CAF activation may be governed by the targets of miR-200 s, Fli-1 and TCF12, which are responsible for cell development and differentiation; Fli-1 and TCF12 were obviously elevated in CAFs. Furthermore, miR-200 s and their targets influenced collagen contraction by CAFs. The upregulation of fibronectin and lysyl oxidase directly by miR-200 or indirectly through Fli-1 or TCF12 contributed to ECM remodeling, triggering the invasion and metastasis of breast cancer cells both in vitro and vivo. Thus, these data provide important and novel insights into breast CAF activation and ECM remodeling, which trigger tumor cell invasion.It has been well established that a reactive microenvironment induces cancer cells to proliferate, migrate and become invasive. Cancer-associated fibroblasts (CAFs) are thought to be the main players among the cohabitating stromal cell types, and they favor tumor progression. The cancer-promoting ability of CAFs is dependent on their activation; however, this process has not been fully explored.The extracellular matrix (ECM) is a complex mixture of structural proteins, proteoglycans and glycoproteins that exerts biochemical and mechanical effects on cells. An increasing body of evidence suggests that ECM remodeling has an important role in cell morphogenesis,¹ survival,² migration and invasion.³ CAFs can deposit certain ECM components and facilitate the directional migration and invasion of carcinoma cells through mechanotransduction-triggered architectural remodeling of the microenvironment.^{4, 5} However, the mechanism by which activated CAFs stimulate the dysregulation of ECM proteins, thus influencing cancer cell invasion, is not well understood.Previously, our team identified a set of dysregulated miRNAs in breast CAFs using a miRNA microarray, and it was found that the levels of miR-200 family members were noticeably suppressed,⁶ indicating their importance in CAF function. Whether these downregulated miR-200 s in the stroma drive the activated phenotype of CAFs as well as aberrant ECM protein expression to promote cancer cell invasion is an intriguing question.The miR-200 s family can be functionally divided into cluster 1 (miR-200a and miR-141) and cluster 2 (miR-200b and miR-200c) according to their ''seed'' region for binding to mRNA. The effects of the miR-200 s on fibrosis, epithelial cell characteristics, cell differentiation and tumor progression have been discussed. For example, miR-200b is essential for the regulation of renal fibrogenesis⁷ and the intestinal fibrosis of Crohn''s disease.⁸ In aggressive carcinoma cells, the maintenance of EMT,⁹ tumor growth,¹⁰ migration,¹¹ invasion,⁹ anoikis resistance¹² and poor response to chemotherapy¹³ are enhanced by the reduced expression of miR-200 s. Furthermore, miR-200 s are upregulated during mammary differentiation¹⁴ but are downregulated in breast cancer stem cells,¹⁵ and these molecules support the maintenance of pluripotent stem cells.¹⁶ These previous reports indicate that miR-200 s may have a significant role in CAF activation.In the current work, we first determined that miR-200 s were commonly downregulated in breast CAFs, and this result was also demonstrated in normal fibroblasts (NFs) co-cultured with breast cancer cells. miR-200 s induced the conversion of NFs into CAFs by targeting Fli-1 and TCF12. Re-expression of miR-200 s in CAFs attenuated the activation-associated CAF phenotype. In particular, miR-200 s and their targets all contributed to CAF-associated ECM remodeling through two key ECM remodeling proteins, fibronectin (FN) and lysyl oxidase, further fueling cancer cell invasion and metastasis. Therefore, our data provide new information regarding the role of CAF activation and function in the promotion of cancer cell invasion through ECM remodeling and provide a considerable amount of information that will be useful for the development of stromal therapeutic targets.     

Activation of fibroblasts in cancer stroma

Tumor microenvironment has emerged as an important target for cancer therapy. In particular, cancer-associated fibroblasts (CAF) seem to regulate many aspects of tumorigenesis. CAFs secrete a variety of soluble factors that act in a paracrine manner and thus affect not only cancer cells, but also other cell types present in the tumor stroma. Acting on cancer cells, CAFs promote tumor growth and invasion. They also enhance angiogenesis by secreting factors that activate endothelial cells and pericytes. Tumor immunity is mediated via cytokines secreted by immune cells and CAFs. Both immune cells and CAFs can exert tumor-suppressing and -promoting effects. CAFs, and the factors they produce, are attractive targets for cancer therapy, and they have proven to be useful as prognostic markers. In this review we focus mainly on carcinomas and discuss the recent findings regarding the role of activated fibroblasts in driving tumor progression.