Growth differentiation factor 15 as a radiation-induced marker in oral carcinoma increasing radiation resistance Eik Schiegnitz 1 , Peer W. K€ ammerer 1,2 , Katharina Rode 1 , Thomas Schorn 1 ,J€ urgen Brieger 3 , Bilal Al-Nawas 1 1 Department of Oral and Maxillofacial Surgery, Plastic Surgery, University Medical Center of the Johannes Gutenberg-University, Mainz, Germany; 2 Department of Oral and Maxillofacial Surgery, Plastic Surgery, University Medical Centre, Rostock, Germany; 3 Department of Otorhinolaryngology, Molecular Tumor Biology Laboratory, University Medical Center of the Johannes Gutenberg-University, Mainz, Germany BACKGROUND: Growth differentiation factor 15 (GDF15) is involved in tumor pathogenesis of oral squamous cell carcinoma (OSCC). The aim of this study was an investigation of the potential influence of GDF15 on radioresistance of OSCC cells in vitro. METHODS: Oral squamous cell carcinoma cell lines were irradiated with 0, 2, or 6 Gy, and GDF15 expression in the supernatant per survived cell colony was examined with ELISA. Non-irradiated and OSCC cell lines irradiated with 6 Gy were evaluated for GDF15 expression using immunofluorescent staining. For further investigation of GDF15 effects on radioresistance, a GDF15 knockdown model in a human OSCC cell line was established, and apoptotic activity after radiation was measured using the Caspase-Glo 3/7 system. RESULTS: ELISA and immunofluorescent staining indi- cated an increased GDF15 expression in 5 OSCC cell lines compared with human gingival epithelial cells. Irradiation with two and six gray resulted in a significant elevation of GDF15 expression per survived cell colony in the irradiated OSCC cell lines (P < 0.001). Furthermore, a dose-dependent expression of GDF15 was seen. Immu- nofluorescent staining confirmed an elevated GDF15 expression in irradiated OSCC cell lines (n = 10; P ≤ 0.001). Apoptotic activity was significantly increased after irradiation in the GDF15 knockdown group com- pared with control cells (n = 24; P < 0.001). CONCLUSION: This study describes for the first time the vital role of GDF15 both in tumorigenesis and in radioresistance of OSCC cells. With its anti-apoptotic effects, GDF15 possibly promotes tumor progression and might protect carcinoma cells against irradiation effects. Consequently, GDF15 may be a promising therapeutic target in oral cancer. J Oral Pathol Med (2015) Keywords: biomarker; GDF15; oral cancer; oral squamous cell carcinoma; radiation resistance Introduction Oral squamous cell carcinoma (OSCC) is one of the most common malignancies in the head and neck region (1). The 5-year survival rate of patients with OSCC remains, in spite of progress in surgery techniques, radiation and chemother- apy protocols, at a low level of approximately 50–60% (2). Therefore, the understanding of molecular mechanisms of OSCC oncogenesis will generate precious information on diagnosis, prognosis, and development of novel therapy strategies. OSCC, like other cancers, is specified by the shifted expression of cytokines and growth factors. One such cytokine is growth differentiation factor 15 (GDF15), a divergent member of the transforming growth factor-b (TGF-beta) superfamily (3). Under physiological condi- tions, GDF15 is marginally expressed (3–5). However, in response to pathologic or environmental stress, GDF15 synthesis may increase considerably (6–9). In some cancer tissues and cancer cell lines, an enhanced GDF15 expres- sion was seen as well (10–14). In most cancers, the major origin of GDF15 expression is the malignant epithelia itself, although there may be an involvement of cancer stroma cells and infiltrating phagocytes (15–19). GDF15 is released as both processed and unprocessed forms (17). Examination of GDF15 secretion showed that unprocessed GDF15 precursor is quickly secreted. In contrast, processed GDF15 produced within the cell by intracellular processing is released slower, using an alternate secretory route (17). Bioavailability of GDF15 in the cancer microenvironment is controlled by the formation of latent stromal stores of Correspondence: Eik Schiegnitz, Department of Oral and Maxillofacial Surgery, Plastic Surgery, University Medical Centre of the Johannes Gutenberg-University, Augustusplatz 2, 55131 Mainz, Germany. Tel: 00496131/17-5084, Fax: 00496131/17-6602, E-mail: eik.schiegnitz@unimedizin-mainz.de Accepted for publication March 23, 2015 doi: 10.1111/jop.12323 J Oral Pathol Med © 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd wileyonlinelibrary.com/journal/jop