Signaling pathways in cancer-associated fibroblasts: recent advances and future perspectives

Abstract

As a critical component of the tumor microenvironment (TME), cancer-associated fibroblasts (CAFs) play important roles in cancer initiation and progression. Well-known signaling pathways, including the transforming growth factor-β (TGF-β), Hedgehog (Hh), Notch, Wnt, Hippo, nuclear factor kappa-B (NF-κB), Janus kinase (JAK)/signal transducer and activator of transcription (STAT), mitogen-activated protein kinase (MAPK), and phosphoinositide 3-kinase (PI3K)/AKT pathways, as well as transcription factors, including hypoxia-inducible factor (HIF), heat shock transcription factor 1 (HSF1), P53, Snail, and Twist, constitute complex regulatory networks in the TME to modulate the formation, activation, heterogeneity, metabolic characteristics and malignant phenotype of CAFs. Activated CAFs remodel the TME and influence the malignant biological processes of cancer cells by altering the transcriptional and secretory characteristics, and this modulation partially depends on the regulation of signaling cascades. The results of preclinical and clinical trials indicated that therapies targeting signaling pathways in CAFs demonstrated promising efficacy but were also accompanied by some failures (e.g., NCT01130142 and NCT01064622). Hence, a comprehensive understanding of the signaling cascades in CAFs might help us better understand the roles of CAFs and the TME in cancer progression and may facilitate the development of more efficient and safer stroma-targeted cancer therapies. Here, we review recent advances in studies of signaling pathways in CAFs and briefly discuss some future perspectives on CAF research.

Keywords: Cancer-associated fibroblasts; Cell-cell interaction; Signaling pathways; Therapeutic targets; Tumor microenvironment.

FIGURE 1

The origins of CAFs. CAFs are formed from a wide range of cell precursors through specific mechanisms. Upon the stimulation of growth factors, cytokines and chemokines such as TGF‐β, FGF, PDGF, and IL‐6, as well as epigenetic modification mediated by non‐coding RNAs and DNA methylation, multiple signaling pathways in tissue‐resident NFs are activated, leading to CAF formation [24, 51, 139, 158]. Metabolic reprogramming caused by cancer cell‐derived ROS and other metabolites, senescent fibroblasts, inflammatory cells and mechanotransduction in ECM also mediated the transformation of NFs into CAFs [231, 335]. In addition, endothelial cells can be transformed into CAFs via the EndoMT, while epithelial cells can be transformed into CAFs via the EMT [33, 36]. BM‐MSCs and quiescent stellate cells are recruited and activated to become CAFs by growth factors, cytokines and chemokines such as TGF‐β, PDGF, IL‐1, CXCL12, CXCL16, CCL2, and CCL5 [285, 336]. Evidence suggests that adipocytes, pericytes and smooth muscle cells can also be transformed into CAFs [37, 41, 42]. Abbreviations: CAFs, cancer‐associated fibroblasts; TGF‐β, transforming growth factor‐β; FGF, fibroblast growth factor; PDGF, platelet‐derived growth factor; IL, interleukin; NFs, normal fibroblasts; ROS, reactive oxygen species; ECM, extracellular matrix; EndoMT, endothelial‐to‐mesenchymal transition; EMT, epithelial‐to‐mesenchymal transition; BM‐MSCs, Bone marrow‐derived mesenchymal stem cells; CXCL, C‐X‐C chemokine ligand; CCL, C‐C motif chemokine ligand

FIGURE 2

The heterogeneity and functions of CAFs. Due to the wide range of cellular precursors and the differences in activation mechanisms, CAFs show a high degree of heterogeneity and are generally classified into rCAFs, myCAFs, iCAFs and apCAFs [4, 51]. Among these CAFs, rCAFs play a role in cancer suppression; myCAFs mediate ECM remodeling by synthesizing collagen and regulating mechanotransduction; iCAFs perform immunomodulation by changing secretory characteristics; and apCAFs activate CD4⁺ T cells in an antigen‐specific manner [6, 51]. myCAFs and iCAFs contribute to tumor metabolic reprogramming and angiogenesis through various mechanisms. The joint actions of myCAFs, iCAFs and apCAFs ultimately promote the proliferation, migration, invasion, metastasis and therapeutic resistance of cancer cells, thus facilitating cancer progression. Abbreviations: CAFs, cancer‐associated fibroblasts; rCAFs, restraining CAFs; myCAFs, myofibroblasts; iCAFs, inflammatory CAFs; apCAFs, antigen‐presenting CAFs; ECM, extracellular matrix

FIGURE 3

HIF, HSF1, P53, and the EMT‐related TFs Snail, Twist and ZEB in CAFs. (A) HIF in CAFs is activated by the TGF‐β‐ or PDGF‐mediated IDH3α/α‐KG axis [60], MAOA/mTOR axis [61] and LPA [337]. Activated HIF increases the expression of key metabolic enzymes, such as PKM2, LDHA, and HK2 [59], at the transcriptional level to mediate the metabolic reprogramming of CAFs, thus providing the metabolites required for fast‐growing cancer cells and secreting ROS to promote cancer cell migration and invasion [61]. In addition, VEGF is a downstream target of HIF, suggesting that HIF is a key TF regulating tumor angiogenesis. (B) HSF1 activation in CAFs exerts paracrine effects through TGF‐β, SDF1, INHBA and THBS2 and activates related signaling pathways in cancer cells, thus promoting cancer progression [68, 338]. In addition, DKK3, an HSF1 effector, enhances canonical Wnt signaling, resulting in a decrease in YAP/TAZ degradation to subsequently increase ECM remodeling and promote cancer cell growth and invasion [70]. (C) P53 in CAFs are activated by the activin A/mDia2 axis, MDM2 inhibitor Nutlin‐3a, cancer cell‐derived miRNAs, and so on [77, 339]. Activated P53 suppresses the production of a series of growth factors, cytokines and chemokines, resulting in a decrease in the expression of CAF markers and hindering CAF formation. TSPAN12 is highly expressed after P53 activation, and it activates β‐catenin signaling in cancer cells and leads to the secretion of CXCL6, thus promoting tumor invasion [74]. (D) Snail in CAFs is activated by the TGF‐β1/USP27X axis [87], PDGF [86] or ECM stiffness‐mediated FAK/ERK2 axis [85]. Activated Snail feedback regulates ECM stiffness through the RhoA/α‐SMA axis, thus promoting cancer progression [84]. Additionally, Snail⁺ CAFs can promote the chemoresistance of cancer cells to 5‐fluorouracil and paclitaxel by secreting CCL1 [88]. Twist and ZEB in CAFs are activated under specific conditions and contribute to the regulation of malignant tumor biological behaviors, such as proliferation, invasion, and EMT. Abbreviations: HIF, hypoxia‐inducible factor; HSF1, heat shock transcription factor 1; EMT, epithelial‐to‐mesenchymal transition; TFs, transcription factors; ZEB, Zinc finger E‐box binding homeobox; CAFs, cancer‐associated fibroblasts; TGF‐β, transforming growth factor‐β; PDGF, platelet‐derived growth factor; IDH3α, isocitrate dehydrogenase 3α; α‐KG, α‐ketoglutarate; MAOA, monoamine oxidase A; mTOR, mammalian target of rapamycin; LPA, lysophosphatidic acid; PKM2, pyruvate kinase M2; LDHA, lactate dehydrogenase A; HK2, hexokinase 2; ROS, reactive oxygen species; VEGF, vascular endothelial growth factor; SDF1, stromal‐derived factor 1; INHBA, inhibin subunit beta A; THBS2, thrombospondin‐2; DKK3, Dickkopf‐3; YAP, Yes‐associated protein; TAZ, transcriptional coactivator with PDZ‐binding motif; ECM, extracellular matrix; TSPAN12, Tetraspanin 12; CXCL, C‐X‐C chemokine ligand; USP27X, ubiquitin specific peptidase 27 X‐linked; ERK, extracellular signal regulated kinase; RhoA, Ras homolog family member A; α‐SMA, α‐smooth muscle actin; CCL, C‐C motif chemokine ligand; IL, interleukin; FGF, fibroblast growth factor; IGF, insulin growth factor; STAT, signal transducer and activator of transcription; HGF, hepatocyte growth factor

FIGURE 4

The TGF‐β signaling pathway is involved in the crosstalk between CAFs and cancer cells. TGF‐β signals are transduced in CAFs through canonical and non‐canonical pathways; the former is a SMAD‐dependent pathway mediated by TGF‐β receptors or BMP receptors, while the latter does not require the participation of SMADs. Numerous factors are involved in the activation of TGF‐β signaling in CAFs, including paracrine signaling by the TGF‐β protein in the TME, hypoxic conditions, tumor‐derived exosomes or non‐coding RNAs, increased ROS levels induced by long‐term fractionated radiation, and the dysregulation of molecules such as DIAPH1 and LXRα [104, 105, 106, 107]. Activated TGF‐β signaling in CAFs exerts corresponding biological effects by directly or indirectly modulating the expression of target molecules. For example, TGF‐β signaling upregulates the expression of CAF markers such as α‐SMA and FAP, promoting the activation of CAFs; alters the secretion of proinflammatory factors, driving the acquisition of cell chemoresistance; and modulates a series of other target proteins, mediating ECM remodeling and immunomodulation. Ultimately, TGF‐β signaling in CAFs regulates cancer progression. Furthermore, activated CAFs are among the most important sources of the TGF‐β protein in the TME, and TGF‐β derived from CAFs exerts a pivotal function in initiating TGF‐β signal transduction in cancer cells, which contributes to cancer cell proliferation, stemness maintenance, migration, invasion, tumor angiogenesis, metastasis and the acquisition of chemoresistance. In addition, TGF‐β‐activated CAF‐derived factors, including various cytokines, chemokines, and specific proteins, such as VCAN, regulate cancer progression through various mechanisms. Abbreviations: TGF‐β, transforming growth factor‐β; CAFs, cancer‐associated fibroblasts; SMAD, Sma‐and Mad‐related protein; BMP, bone morphogenetic protein; TME, tumor microenvironment; ROS, reactive oxygen species; DIAPH1, diaphanous homolog 1; LXRα, liver X receptor α; α‐SMA, α‐smooth muscle actin; FAP, fibroblast activation protein; ECM, extracellular matrix; VCAN, versican; LAP: latency‐associated polypeptide; PI3K, phosphoinositide 3‐kinase; HIF, hypoxia‐inducible factor; NF‐κB, nuclear factor kappa‐B