Stromal Modulators of TGF-β in Cancer

Abstract

Transforming growth factor-β (TGF-β) is an intriguing cytokine exhibiting dual activities in malignant disease. It is an important mediator of cancer invasion, metastasis and angiogenesis, on the one hand, while it exhibits anti-tumor functions on the other hand. Elucidating the precise role of TGF-β in malignant development and progression requires a better understanding of the molecular mechanisms involved in its tumor suppressor to tumor promoter switch. One important aspect of TGF-β function is its interaction with proteins within the tumor microenvironment. Several stromal proteins have the natural ability to interact and modulate TGF-β function. Understanding the complex interplay between the TGF-β signaling network and these stromal proteins may provide greater insight into the development of novel therapeutic strategies that target the TGF-β axis. The present review highlights our present understanding of how stroma modulates TGF-β activity in human cancers.

Keywords: TGF-β; cancer-associated fibroblasts; proteases; proteoglycans; stroma.

Figure 1

Canonical and non-canonical TGF-β signaling pathways. (A) In the canonical signaling pathway, biologically active TGF-β ligands bind to TGFβRII, which in turn activates TGFβRI. TGFβRI-regulated SMAD2/3 proteins are phosphorylated at their C-terminal serine residues and form complexes with SMAD4 (co-SMAD), initiating a number of biological processes through transcriptional regulation of target genes. (B) In the non-canonical signaling pathways, the TGF-β receptor complex transmits its signal through other factors, such as the mitogen-activated protein kinases (MAPKs), phosphatidylinositide 3-kinase (PI3K), TNF receptor-associated factor 4/6 (TRAF4/6) and Rho family of small GTPases. Activated MAPKs can exert transcriptional regulation either through direct interaction with the nuclear SMAD protein complex or via other downstream proteins. Moreover, activated JNK/p38/ERK act in concert with SMADs to regulate cellular apoptosis and proliferation, whereas they mediate metastasis, angiogenesis and cellular growth through other transcription factors, such as c-JUN and ATF. RhoA/ROCK can be activated by TGF-β to induce actin stress fiber formation during EMT via a non-transcriptional mechanism. TGF-β can activate PI3K and AKT by inducing a physical interaction between the PI3K p85 subunit and the receptor complex leading to translational responses via mTOR/S6kinase activation. TGF-β activation of the TRAF proteins can initiate nuclear factor-κB (NF-κB) signaling activity, leading to the inflammatory response among other processes. The arrows indicate activation/signaling direction of the respective pathway.

Figure 2

TGF-β-mediated cancer cell/stromal cell crosstalk. (A) TGF-β can activate resident stromal cells giving rise to cancer-associated fibroblasts (CAFs). In cancer cells, TGF-β promotes the transcription of SNAIL, the functional loss of E-cadherin, the acquisition of an EMT phenotype and the recruitment of SMAD/AKT signaling proteins. The process of metastasis is further supported by activated CAFs through secretion of IL-11 or IL-6, which further promotes STAT3 signaling in cancer cells. (B) TGF-β can trigger angiogenesis in endothelial cells through activation of VEGFR2 by VEGF. The TGF-β-mediated angiogenic effect on cancer cells is regulated by TGFβRII/SMAD3-dependent upregulation of fibroblast growth factor-2 (FGF2) expression and release in the stroma. (C) Cancer cells via the induction of aberrant TGF-β signaling can induce the down-regulation of CAV1 in adjacent fibroblasts leading to a CAF phenotype. The loss of CAV1 has been observed to lead to an increase in oxidative stress, activation of HIF-1α and the induction of aerobic glycolysis. Under these conditions, CAF have been reported to produce and secrete lactate, which is used as fuel by cancer cells. Blue arrows indicate proteins secreted by cancer cells. Magenta arrows indicate proteins secreted by stromal cells. Black arrows indicate overexpression (upward pointing) and down-regulation (downward pointing) of target proteins.

Figure 3

Stromal activators of TGF-β in the tumor microenvironment. (A) MT1-MMP, MMP2 and MMP9 proteolytically cleave latent transforming growth factor-β binding protein (LTBP), thereby releasing latent TGF-β from the extracellular matrix. Plasmin, thrombin, BMP1 and fibulin-2 also activate TGF-β through cleavage or interaction with LTBP1. (B) MMP2, MMP3, MMP9 and MMP13 activate latent TGF-β via proteolytic cleavage of the latency-associated peptide (LAP), while integrins expressed on fibroblasts (αvβ3, αvβ5 and αvβ8) bind to the large latent complex (LLC) and activate latent TGF-β through MT1-MMP-dependent cleavage of LAP. (C) Integrins αvβ-1 and 5 bind to the LLC and induce conformational changes in the latent complex via contractile action from activated fibroblasts. (D) ROS produced by activated fibroblasts via the induction of oxidative stress from adjacent cancer cells can lead to the oxidation of the LAP domain and induce allosteric changes that release mature TGF-β from LAP. The loss of CAV1 expression in activated fibroblasts is also associated with enhanced oxidative stress and increased production of ROS. (E) Thrombospondin-1 (TSP-1) directly interacts with the LAP domain, inducing conformational rearrangement of LAP and altering the interaction of LAP with the mature domain of TGF-β. The mature (active) form of TGF-β can then bind to its cognate receptor and exert its tumor promoting and tumor suppressive properties. Dashed arrow indicates recruitment of the mature TGF-β protein to its cognate receptor.

Figure 4

Stromal inhibitors of TGF-β in the tumor microenvironment. (A) The SLRPs, decorin, biglycan, lumican and asporin, bind with high affinity to TGF-β, preventing the biologically-active protein from binding to its cognate receptor. (B) Fibulin-3 and -4indirectly inhibit TGF-β activity by interacting with TGF-β RI, leading to a decrease in TGFβRI/TGFβRII complex formation. (C) Decorin can indirectly disrupt TGF-β activity by negative regulation of SMAD2 phosphorylation. (D) Nephrocan indirectly regulates TGF-β activity by inhibiting canonical SMAD3 signaling. (E) Fibromodulin and fibrilin-1 and -2 bind to the TGF-β LLC, preventing its release from the ECM. Fibronectin mediates the regulation of LTBP1 in the ECM, thereby suppressing TGF-β1 bioavailability. Solid arrows indicate activation and/or enzymatic activity while blunted arrows indicate inhibitory activity (solid—direct; interrupted—indirect activity).