Dental Hypotheses

ORIGINAL HYPOTHESIS
Year
: 2013  |  Volume : 4  |  Issue : 4  |  Page : 122--126

Transforming growth factor-β1/Smad/connective tissue growth factor axis: The main pathway in radiation-induced fibrosis of osteoradionecrosis?


Qian Wei Zhuang, Zhi Yuan Zhang, Guang Long Liu, Shui Ting Fu, Yue He 
 Department of Oro-Maxillofacial Head and Neck Oncology, Ninth People's Hospital Affiliated to Shanghai Jiao Tong University, School of Medicine, Shanghai - 200 011, China

Correspondence Address:
Yue He
Department of Oro-Maxillofacial Head and Neck Oncology, Ninth People«SQ»s Hospital Affiliated to Shanghai Jiao Tong University, School of Medicine, 639 Zhi Zao Ju Road, Shanghai - 200 011
China

Abstract

Introduction: Osteoradionecrosis (ORN) of the mandible is a serious complication following radiation therapy for malignancies of the head and neck. Radiation-induced fibrosis (RIF) is a new theory that accounts for the damage to normal tissues after radiotherapy, and the radiation-induced fibroatrophic mechanism includes the free-radical formation, endothelial dysfunction, inflammation, microvascular thrombosis, fibrosis and remodeling, and finally bone and tissue necrosis. The Hypothesis: Previous studies revealed that transforming growth factor-β1 (TGF-β1) is the master switch cytokine responsible for the regulation of fibroblast proliferation and differentiation that result in RIF. Among the targets of TGF-β1, connective tissue growth factor (CTGF) is a downstream mediator through the Smad3/4 pathway and plays an important role in connective tissue homeostasis and fibroblast proliferation. Studies have proved that the TGF-β1/Smad/CTGF signaling pathway is involved in the RIF of soft tissues, so the authors put forward a hypothesis that the TGF-β1/Smad/CTGF axis is also the main pathway in RIF of ORN. Evaluation of the Hypothesis: The validation of our hypothesis may provide new insights for better understanding the pathogenesis of ORN and open new perspectives for anti-fibrotic therapies, and pioneer novel approaches to treat ORN.



How to cite this article:
Zhuang QW, Zhang ZY, Liu GL, Fu ST, He Y. Transforming growth factor-β1/Smad/connective tissue growth factor axis: The main pathway in radiation-induced fibrosis of osteoradionecrosis?.Dent Hypotheses 2013;4:122-126


How to cite this URL:
Zhuang QW, Zhang ZY, Liu GL, Fu ST, He Y. Transforming growth factor-β1/Smad/connective tissue growth factor axis: The main pathway in radiation-induced fibrosis of osteoradionecrosis?. Dent Hypotheses [serial online] 2013 [cited 2019 Aug 17 ];4:122-126
Available from: http://www.dentalhypotheses.com/text.asp?2013/4/4/122/122673


Full Text

 Introduction



Osteoradionecrosis (ORN) is one of the most severe serious complications of head and neck cancer therapy. [1] In the maxillofacial region, jaw bone ORN patients present with chronic exposure of necrotic bone orosteosynthesis devices, mucosal necrosis, ulceration or persistent pain. [2] In these irradiated patients, the common histological characteristics of bone are radiation-induced fibrosis (RIF) and atrophy. At a later stage, necrosis will predominate in bone. RIF is an occasional irreversible damage, which is unavoidable and may last for years after radiotherapy (RT). [3] At the molecular level, transforming growth factor-β1(TGF-β1) has been reported to be able to stimulate radiation-induced endothelial proliferation, fibroblast proliferation, collagen deposition, and fibrosis. [4] TGF-β acts with TGF-β receptors (TβRs), which contain serine/threonine kinase domains in their intracellular portions, [5] and then TGF-β Type I receptor (TβR-I) phosphorylate the major downstream molecules-Smads. [6],[7] Activated Smads formed hetero-trimers, containing two R-Smad molecules and one co-Smad molecule, which translocate into the nucleus and regulate the transcription of target genes. [8] Signaling mainly proceeds through Smad3 pathway, [9],[10] then leads to the development and maintenance of fibrosis and controls the differentiation of smooth muscle cells, subepithelial myofibroblasts and extracellular matrix (ECM) synthesis. [3]

 TGF-β1 and RIF of ORN



ORN is a severe delayed radiation-induced injury, characterized by bone tissue necrosis and failure to heal. [11] ORN is best defined as low-healing radiation-induced ischemic necrosis of bone with associated soft tissue necrosis of variable extent occurring in the absence of local primary tumor necrosis, recurrence, or metastatic disease. [12] And the incidence was reported as 8.2% by Reuther et al. [13] Over the previous three decades, the development of ORN has been accepted as being caused by a combination of hypovascularity, osteoblast death (hypocellularity), and hypoxia. [14] However, the mechanisms underlying ORN in bone are likely to be similar to those occurring in other low-turnover tissues. In such tissues, a RIF develops. [15] RIF and radionecrosis are late complications that are usually considered irreversible. [16] A key event in the progression of RIF is the activation and dysregulation of fibroblastic activity leading to tissue atrophy within the previously irradiated region, and unsurprisingly this has been termed the fibroatrophic theory of radiation damage. [17]

The radiation-induced fibroatrophic mechanism includes the free-radical formation, endothelial dysfunction, inflammation, microvascular thrombosis, fibrosis and remodeling, and finally bone and tissue necrosis. [15] Three distinct phases are seen: [18] The initial prefibrotic phase in which changes in endothelial cells predominate, together with the acute inflammatory response; the constitutive organized phase in which abnormal fibroblastic activity predominates, and there is disorganization of the ECM; and the late fibroatrophic phase, when attempted tissue remodeling occurs with the formation of fragile healed tissues that carry a serious inherent risk of late reactivated inflammation in the event of local injury. Interestingly, Marx had reached similar conclusions, but thought that the driving force of the sequence of events was persistent tissue hypoxia. [19]

Numerous cytokines such as tumor necrosis factor-α, interleukin-1, platelet-derived growth factor, fibroblast growth factor, and TGF-β1 have been reported to be increased within irradiated tissue. [20] TGF-β1 is the master switch cytokine [21] responsible for the regulation of fibroblast proliferation and differentiation [22],[23],[24] that result in RIF. [21],[25] Expression of TGF-β after radiation is a consistent finding. TGF-β levels are increased as early as 6 h after irradiation and remain elevated in fibrotic lesions as long as 20 years. [21],[26] TGF-β has chemotactic properties to recruit neutrophils and activate macrophages. Activated macrophages can secrete an array of cytokines, including TGF-β itself, which can stimulate fibroblasts to secrete matrix proteins that may well lead toward fibrosis.

 TGF-β1/Smads/Connective Tissue Growth Factor (CTGF) Axis



Cytokines of the TGF-β superfamily are dimeric proteins with conserved structures and have pleiotropic functions in vitro and in vivo. [27] TGF-βs are the prototype of the TGF-β superfamily. [28] Among its three isoforms (TGF-β1, -β2 and -β3), TGF-β1, the central player in the fibrogenic process, is a key mediator in promoting ECM production while inhibiting its degradation. [29]

TGF-β1 is the major cytokine responsible for the regulation of fibroblast proliferation and differentiation. Differentiated fibroblasts synthesize the collagens and proteoglycans in the ECM, and it has been suggested that an increase in these fibroblasts may cause the fibrotic phenotype. [30] Radiation induces long-term TGF-β1 over expression, probably owing in part to oxidative stress and an inflammatory response. [15],[21],[23] Interestingly, elevated serum TGF-β1 levels were correlated with an increased risk of fibrosis in patients with breast and lung cancer. [31]

Members of the TGF-β superfamily bind to two distinct receptor types, known as Type II and Type I receptors. [6],[32] Both Type II and Type I receptors are required for signal transduction. Type I receptors act as downstream components of Type II receptors in the signaling pathways and determine the specificity of the intracellular signals. Of the seven Type I receptors, the activin receptor-like kinase-5, also known as TβR-I, serves as a specific receptor for TGF-βs.

Smad proteins are major signaling molecules acting downstream of the serine/threonine kinase receptors. [6],[7] Of all the Smads, Smad3 is activated by TβR-Is and mediates all the biological effects by the activation of the Smad3/4 pathway [33] and subsequent transactivation of various genes including CTGF [34],[35] and ECM molecules. [36]

There are at least 60 ECM-related genes that are downstream targets of TGF-β1. [36] CTGF is a downstream mediator of TGF-β1 activity and plays an important role in connective tissue homeostasis and fibroblast proliferation, migration, adhesion, and ECM accumulation. [37],[38]

CTGF belongs to the family of the immediate early genes CCN (CYR 61, CTGF, and NOV) and is secreted as a 38-kDa cysteine-rich peptide by fibroblasts and endothelial cells. It is selectively induced by the TGF-β through a specific TGF-β response element located in its promoter. [39] Furthermore, CTGF is thought to be the autocrine agent responsible for mediating TGF-β fibrogenic effects by a direct induction of the α-5 integrin, Type I collagen, and fibronectin genes. [40]

Many experimental approaches used to study fibrosis focused on the initiation steps of the fibrogenic process, in which a specific role for Smad3 (especially inradiation injury) has been demonstrated in mice lacking Smad3. [41] CTGF (recently named CCN2) is thought to be the main downstream fibrogenic effect or of TGF-β1 because it controls both fibronectin and collagen gene expression. [42],[43] Zhou et al, [44] reported that TGF-β1/Smad/CTGF signaling pathway plays a role in fibrosis induction in the corpus cavernosum of diabetic models. In radiation enteritis, a high-expression level of α-smooth muscle actin was found associated with increased collagen deposition and increased expression of the fibrogenic growth factor CTGF in the muscularis propria. In late intestinal radiation fibrosis, the increased level of CTGF protein and messenger ribonucleic acid was found associated with the accumulation of fibroblasts/myofibroblasts and collagen deposition. [18] This suggests that CTGF could be associated with radiation induced fibrogenic differentiation and thus understanding the mechanisms that TGF-β1/Smad/CTGF signaling pathway plays a role in fibrosis induction. [45]

 The Hypothesis



ORN of the jaw is a metabolic dysfunction in irradiated bones where radioactive rays affect the differentiation from hematopoietic stem cells and osteoprecursors [4],[46] into endothelial cells and osteoblasts, and activate myofibroblasts through kinds of cytokines, especially of TGF-β1. After RT, the endothelial cells are injured. Injured endothelial cells produce chemotactic cytokines that trigger an acute inflammatory response and then generate a further release of reactive oxygen species from polymorphs and other phagocytes. The destruction of endothelial cells, coupled with vascular thrombosis, lead to necrosis of microvessels, local ischemia, and tissue loss. Loss of the natural endothelial cell barrier allows seepage of various cytokines that cause fibroblasts to become myofibroblasts. [47] Degeneration of normal bone tissues and excessive formation of pathologically fibrotic tissues result in inactive and fragile bone, also called ORN. RIF is a new theory that accounts for the damage to normal tissues after RT. [48] The theory of RIF suggests that the key event in the progression of ORN is the activation and dysregulation of fibroblastic activity that leads to fibrosis and remodeling, and finally tissue atrophy and bone necrosis within a previously irradiated area. TGF-β1 is today considered a primary inducer of tissue fibrosis including RIF. [21] Its fibrogenic signal is mediated through two TGF-specific serine/threonine kinase receptors, downstream activation of the Smad3/4 pathway and subsequent transactivation of various genes including the CTGF and ECM molecules. CTGF is a fibrogenic cytokine in radiation fibrosis and thought to be the main downstream fibrogenic effector of TGF-β1 because it controls both fibronectin and collagen gene expression [14] and collagen deposition. TGF-β1/Smad/CTGF signaling pathway is demonstrated to play a role in fibrosis induction in the corpus cavernosum of diabetic models and maintenance of radiation enteritis. Since the TGF-β1/Smad/CTGF signaling pathway is involved in the RIF of soft tissues, so the authors hypothesize that the TGF-β1/Smad/CTGF axis is the main pathwayin RIF of ORN.

 Evaluation of the Hypothesis



As described above, TGF-β1/Smad/CTGF axis plays an important role in the RIF of soft tissues; as in bone tissue, RIF is the radiation-induced fibroatrophic mechanism of ORN, in which the study of TGF-β1/Smad/CTGF axis is still blank. We believe that TGF-β1/Smad/CTGF axis plays the main role in the ORN. The methods to testify our hypothesis are technically feasible. In vitro studies should be performed at first. Cells pertaining to osteogenesis, such as osteoblasts, can be obtained and cultured. In the experiment group, the TGF-β1/Smad/CTGF axis is blocked. All these cells will be irradiated and subsequently, a series of assays will be conducted for cell proliferation, apoptosis, collagen synthesis, and so on. On the basis of in vitro experiments, in vivo studies in animals can be performed. The validation of our hypothesis will provide new insights for better understanding the pathogenesis of ORN and open new perspectives for anti-fibrotic therapies. Targeting the TGF-β1/Smad/CTGF pathway may become a novel therapeutic approach to treat ORN, and may even help providing valid protection against the occurrence of ORN.

 Acknowledgments



This work was supported by grants of the National Natural Science Foundation of China (NSFC 81271112), and "ShuGuang" project (10SG19) supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation.

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