Experimental radiobiology Endoglin haploinsufficiency reduces radiation-induced fibrosis and telangiectasia formation in mouse kidneys Marion Scharpfenecker a , Ben Floot a , Nicola S. Russell b , Peter ten Dijke c , Fiona A. Stewart a, * a Department of Experimental Therapy, The Netherlands Cancer Institute, Amsterdam, The Netherlands b Department of Radiotherapy, The Netherlands Cancer Institute, Amsterdam, The Netherlands c Department of Molecular Cell Biology and Centre for Biomedical Genetics, Leiden University Medical Centre, Leiden, The Netherlands article info Article history: Received 27 April 2009 Received in revised form 2 June 2009 Accepted 5 June 2009 Available online 1 July 2009 Keywords: Endoglin Irradiation TGF-b Telangiectasia Fibrosis HHT abstract Background and purpose: Endoglin is a transforming growth factor beta (TGF-b) co-receptor mainly expressed in dividing endothelial cells. It regulates cell proliferation and survival and is upregulated at sites of vessel repair. Mutations in endoglin have been linked to the vascular disease hereditary hemor- rhagic telangiectasia (HHT). HHT patients display dilated capillaries (telangiectasia) that are prone to rup- ture. Cancer patients receiving radiotherapy develop similar vascular damage in normal tissues lying in the irradiation field. If located in the mucosa, irradiation-induced telangiectasia can lead to severe bleed- ing. Therefore, this study was aimed at investigating the role of endoglin in radiation-induced telangiec- tasia formation. Materials and methods: Kidneys of endoglin heterozygous (Eng +/À ) or wild type mice were irradiated with 16 Gy. Mice were sacrificed after 20 weeks and changes in gene expression and protein levels were ana- lysed. Results: Expression of TGF-b target genes involved in radiation-induced fibrosis and fibrosis development in the kidney decreased in Eng +/À compared to wild type mice. Unexpectedly, Eng +/À mice also displayed reduced telangiectasia formation in the irradiated kidney. Conclusions: Endoglin plays an important role in the development of irradiation-induced normal tissue damage. Future studies will show whether interfering with endoglin functions protects tissues from late radiation toxicity. Ó 2009 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 92 (2009) 484–491 Despite improved radiation techniques, vascular injury is still a major cause of late radiation morbidity, developing slowly and progressively over many years. Vascular lesions manifest in micro- vessels as telangiectasia, which are characterised as dilated, tortu- ous and thin-walled blood vessels. Especially mucosal telangiectasia are prone to rupture causing excessive bleeding and limiting the radiation dose for treatment [1]. Fibrosis is a com- plex process initiated as a repair response in damaged tissues. Yet, chronic, unregulated activation of profibrotic pathways after irra- diation results in accelerated fibroblast differentiation and massive deposition of extracellular matrix thereby impairing organ func- tion [2,3]. TGF-b is considered a master switch of the fibrotic programme and also plays a role in the repair of vascular damage. Activated TGF-b first binds to the TGF-b type II receptor (TGFRII) which then forms heteromeric complexes with type I receptors [4–6]. Signals are subsequently transduced by Smad molecules that translocate into the nucleus where they participate in target gene transcrip- tion. In endothelial cells, TGF-b signals through two different type I receptors: activin receptor-like kinase 1 (ALK1) and 5 (ALK5). Smad1 and Smad5 are molecules transducing ALK1 signals leading to activation of the downstream target gene inhibitor of differenti- ation-1 (Id-1), a regulator of endothelial cell proliferation and migration. ALK5 signals are transduced by Smad2 and Smad3 lead- ing to transcription of genes that inhibit extracellular matrix deg- radation and neovessel formation. Smad3 activation seems to be especially important for the development of fibrosis. TGF-b induc- tion of profibrotic genes such as plasminogen activator inhibitor-1 (PAI-1), connective tissue growth factor (CTGF) or collagens are dependent on Smad3 activation [7–9]. Moreover, mice lacking Smad3 do not develop fibrosis [10]. Endoglin is a co-receptor of TGF-b signalling [11]. It is mainly expressed in endothelial cells and is important for endothelial cell survival and vessel repair. It has been shown that endoglin is required for TGF-b/ALK1 signalling in endothelial cells. Endoglin is also needed for Smad2-mediated ALK5 signals [12]. On the other hand, overexpression of endoglin in endothelial cells and non-endothelial cells inhibits Smad3 transcriptional activity [13,14] and reduces expression of collagens and CTGF [10,15,16]. 0167-8140/$ - see front matter Ó 2009 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2009.06.013 * Corresponding author. Address: The Netherlands Cancer Institute (NKI-AVL), Department of Experimental Therapy (B8), Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands. E-mail address: f.stewart@nki.nl (F.A. Stewart). Radiotherapy and Oncology 92 (2009) 484–491 Contents lists available at ScienceDirect Radiotherapy and Oncology journal homepage: www.thegreenjournal.com