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Review
. 2024 Apr 20;17(4):533.
doi: 10.3390/ph17040533.

Advances and Challenges in Targeting TGF-β Isoforms for Therapeutic Intervention of Cancer: A Mechanism-Based Perspective

Affiliations
Review

Advances and Challenges in Targeting TGF-β Isoforms for Therapeutic Intervention of Cancer: A Mechanism-Based Perspective

David Danielpour. Pharmaceuticals (Basel). .

Abstract

The TGF-β family is a group of 25 kDa secretory cytokines, in mammals consisting of three dimeric isoforms (TGF-βs 1, 2, and 3), each encoded on a separate gene with unique regulatory elements. Each isoform plays unique, diverse, and pivotal roles in cell growth, survival, immune response, and differentiation. However, many researchers in the TGF-β field often mistakenly assume a uniform functionality among all three isoforms. Although TGF-βs are essential for normal development and many cellular and physiological processes, their dysregulated expression contributes significantly to various diseases. Notably, they drive conditions like fibrosis and tumor metastasis/progression. To counter these pathologies, extensive efforts have been directed towards targeting TGF-βs, resulting in the development of a range of TGF-β inhibitors. Despite some clinical success, these agents have yet to reach their full potential in the treatment of cancers. A significant challenge rests in effectively targeting TGF-βs' pathological functions while preserving their physiological roles. Many existing approaches collectively target all three isoforms, failing to target just the specific deregulated ones. Additionally, most strategies tackle the entire TGF-β signaling pathway instead of focusing on disease-specific components or preferentially targeting tumors. This review gives a unique historical overview of the TGF-β field often missed in other reviews and provides a current landscape of TGF-β research, emphasizing isoform-specific functions and disease implications. The review then delves into ongoing therapeutic strategies in cancer, stressing the need for more tools that target specific isoforms and disease-related pathway components, advocating mechanism-based and refined approaches to enhance the effectiveness of TGF-β-targeted cancer therapies.

Keywords: TGF-β; fibrosis; oncogene; therapeutics; tumor progression.

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Conflict of interest statement

The author declares no conflicts of interest.

Figures

Figure 1
Figure 1
Regulators of the transcriptional expression of TGF-βs 1, 2, and 3. TGF-β1 emerges as the predominant isoform upregulated in tumors, correlating with increased cell proliferation activity and malignant transformation. Inducers of proliferation typically induce the expression of TGF-β1 expression while inhibiting the expression of TGF-β2 and TGF-β3. Conversely, conditions promoting growth arrest and differentiation typically selectively induce the expression of TGF-βs 2 and 3 over that of TGF-β1. Abbreviations: AKT (Akt/PKB serine-threonine kinase), RA (retinoic acid), ATF2 (activating transcription factor 2), CREB-1 (cAMP-responsive element binding protein-1), CREBH (cAMP-responsive element-binding hepatocyte protein), DHT (dihydrotestosterone), E2 (estradiol), HoxB7 (Homeobox B7 protein), RFX (regulatory factor x), MSS (mechanical sheer stress), and VD (1,25-dihydroxyvitamin D3). These data provide potential triggers for isoform induction in cancer, offering opportunities for TGF-β isoform-targeted therapeutic approaches.
Figure 2
Figure 2
The intricate processes of TGF-β activation within the intracellular and extracellular environments. Initially synthesized as homodimers with pro-peptides, mature TGF-βs are cleaved from latency-associated proteins (LAPs) by furin-like enzymes in the trans-Golgi. They are then secreted as small latent complexes (SLCs) bound to LAPs, often associating with latent TGF-β binding proteins (LTBPs) or glycoprotein A repetitions predominant (GARP) to form large latency complexes (LLCs) anchored to the extracellular matrix (ECM), or in the case of GARP, on the surface of specific cells. Activation of TGF-βs can occur via proteolytic cleavage or conformational changes induced by mechanical forces, integrins, reactive oxygen species (ROS), and other effectors. Notably, different isoforms of TGF-β are activated by distinct factors. Once activated, TGF-βs either bind to TGF-β receptors or are sequestered in an inactive form bound to extracellular matrix proteins such as decorin or the plasma protein alpha-2 macroglobulin (α2M), the latter of which has a 10-fold higher affinity for TGF-β2 than TGF-β1. Understanding the complexities of TGF-β activation offers insights into potential therapeutic interventions targeting aberrant TGF-β signaling.
Figure 3
Figure 3
TGF-β receptor binding and downstream canonical and non-canonical signaling pathways. Upon encountering TGF-β1, the TGF-β type II receptor (TβRII) prompts a conformational change that allows for the recruitment of the TGF-β type I receptor (TβRI), forming a complex comprising two TβRIIs and two TβRIs. Conversely, TGF-β2 requires TβRIII (also called β-glycan) for cellular responses due to its inability to directly bind TβRII or TβRI. The formation of the TβRII-TβRI-ligand complex triggers the phosphorylation of TβRI by TβRII. This event activates TβRI, leading to the phosphorylation of downstream Smads, particularly Smads 2 and 3 (the canonical pathway). SARA (Smad anchor for receptor activation) and Hrs/Hgr (hepatocyte growth factor-regulated tyrosine kinase substrate) are crucial for the delivery of R-Smad to the TβRII-TβRI complex for R-Smad activation. Additional proteins involved in delivering R-Smad to the TGF-β receptors include DAB2 (Disabled-2) and cPML (cytoplasmic promyelocytic leukemia protein). Normally confined to the nucleus, cPML is sequestered in a tertiary complex with transcription factor c-Jun and the transcriptional repressor TGIF (TG-interacting factor). Upon TGF-β stimulation, PCTA (PML competitor for TGIF association) translocates into the nucleus, where it competes with cPML for TGIF binding. This competition leads to the export of cPML to the cytoplasm, where it interacts with R-Smads, thereby promoting R-Smad-TβRI interaction. After phosphorylation, Smads 2 and 3 form heterotrimeric complexes with Smad4 and translocate into the nucleus, where they regulate the transcription of target genes by interacting with other transcription factors and co-regulators. Meanwhile, inhibitory mechanisms, including the action of Smad7, ubiquitin ligases, and the nuclear phosphatase PPM1A (magnesium-dependent protein phosphatase A1) work in concert to deactivate TGF-β signaling, ensuring its dynamic control. In the non-canonical pathways of TGF-β signaling, various adapters are recruited to the activated TβRI-TβRII complex independent of Smads, triggering various kinase signaling cascades that ultimately promote cell growth, survival, cell migration, and invasion. Other abbreviations: TSC1/2 (tuberous and tuberin sclerosis complexes 1 and 2); JNK (c-Jun N-terminal kinase); c-Jun (cellular Jun transcription factor, subunit of the AP-1 complex); c-Fos (cellular Fos proto-oncogene, AP-1 transcription factor subunit); ELK (E26 transformation-specific (ETS)-like protein); Rheb (Ras homologue enriched in brain).
Figure 4
Figure 4
Role of TGF-β1 in immune regulation. The intricate role of TGF-β1 in immune regulation is depicted, showcasing its dual nature as both an inducer of immune tolerance and a regulator of immune effector functions. TGF-β1 transforms CD4+ T cells into regulatory T cells (Tregs), essential for immune homeostasis. TGF-β1 also exerts inhibitory effects on CD8+ cytotoxic T cells, natural killer cells, and antigen-presenting cells, dampening their effector functions. Additionally, it suppresses B cell differentiation and antibody production, further contributing to immune regulation. On the other hand, TGF-β released by tumor cells promotes angiogenesis, promotes leukocyte chemotaxis, and promotes the differentiation of macrophages from an M1 to an M2 phenotype. Both M2 macrophages and angiogenesis promote tumor growth.
Figure 5
Figure 5
The intricate involvement of TGF-β in wound healing, shedding light on its diverse roles and the complex interplay with various cellular processes. Platelets emerge as pivotal players, releasing TGF-β1 upon degranulation at the wound site, where it orchestrates a cascade of events. Tissue plasminogen activator (tPA) cleaves plasminogen into plasmin, which not only acts to limit the size of a blood clot by breaking down fibrin but also functions to activate TGF-β1 by cleaving it from its large latency complex (LLC). Here, activated TGF-β1 then acts as a chemoattractant for immune cells while simultaneously stimulating fibroblast proliferation and differentiation into myofibroblasts, which contribute to extracellular matrix deposition and wound repair. Despite its role in promoting wound repair, TGF-β1 induces immunotolerance, crucial for dampening autoimmunity triggered by tissue damage in normal tissue repair. TGF-β1 also drives the transcriptional induction of PAI-1 (tPA inhibitor-1), which functions to block the activation of plasmin, thereby limiting the extent of fibrin degradation. Tumors in which TGF-β1 is overexpressed/overactivated likely result in excess PAI-1 induction, which inhibits fibrin dissolution, thereby contributing to increased hypoxia and tissue damage.
Figure 6
Figure 6
The central role of TGF-βs in driving fibrosis. TGF-βs induce extracellular matrix (ECM) production by driving the transcription of genes for the expression of ECM proteins such as collagen, fibronectin, laminin, tenascin, and proteoglycans, while it also inhibits ECM breakdown by inhibiting the transcription of ECM proteases and inducing the expression of ECM protease inhibitors. Elevated TGF-β levels in certain pathologies including cancers contribute to tissue fibrosis, by overproduction and over-activation of TGF-β and TGF-β signaling. TGF-β also induces the expression of lysyl oxidase (LOX) genes, which promotes the crosslinking between EMC proteins, contributing to ECM rigidity. LOX and superoxide (ROS) promote TGFβ-induced fibrosis. In desmoplastic cancers, excess ECM promotes metastasis and activates latent TGF-βs, further exacerbating fibrosis and impeding the efficacy of chemotherapeutic drug access and drug resistance.
Figure 7
Figure 7
The intricate relationship between chronic inflammation, coagulation, and TGF-β1, particularly in the context of malignancies. Chronic inflammation can trigger the coagulation cascade through the activation of factors like tissue factor and Factor XII. This leads to the formation of microthrombi within blood vessels, causing hypoxia and tissue damage, further exacerbating inflammation and clotting. Platelets play a pivotal role in this process by releasing various substances, including pro-fibrotic factors like TGF-β1. Additionally, hypoxia-inducible factors (HIFs) driven by tumor hypoxia promote the stabilization of HIF-1α, which cooperates with TGF-β1 to drive fibrosis and tumor progression.
Figure 8
Figure 8
Chemical structures of various ALK5 kinase (TβRI) inhibitors used in preclinical and clinical studies.

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