Abstract Introduction: Glioblastoma (GBM) is the most common malignant brain tumor in adults, and resistance to radiation therapy remains a major therapeutic obstacle. Although genetic alterations contribute to GBM progression, they do not fully explain the emergence of radiation resistance, suggesting contributions from non-genetic mechanisms. Notably, radiation-resistant GBM exhibits elevated collagen deposition. Discoidin domain receptor tyrosine kinase 1 (DDR1), a collagen-binding receptor regulated by ectodomain shedding, is highly expressed in GBM. Here, we demonstrate that radiation-resistant GBM alters DDR1 activity and shedding in response to collagen to promote tumor cell proliferation. Methods and Results: To investigate DDR1 regulation, we utilized a GBM patient-derived xenoline pair, JX39P (radiation-sensitive) and JX39P-RT (radiation-resistant). JX39P-RT displayed significantly higher DDR1 mRNA and protein expression. When cells were embedded in collagen I to mimic the extracellular environment, DDR1 phosphorylation at Y792 increased more rapidly and to a greater extent in JX39P-RT than in JX39P. Collagen embedment also led to the accumulation of a short DDR1 fragment in JX39P-RT, indicating enhanced ectodomain shedding. Western blotting of conditioned media confirmed increased release of the DDR1 N-terminal fragment and confirmed that ectodomain shedding occurred only in JX39P-RT in response to collagen. Treatment of JX39P-RT cells with marimastat, a broad-spectrum MMP inhibitor, diminished both the N-terminal fragment shed in the media and the short C-terminal fragment in cell lysates. Consistent with elevated DDR1 expression and activation, JX39P-RT exhibited increased phospho-ERK1/2 levels when embedded in collagen I, indicating enhanced downstream signaling pathways. Furthermore, cell proliferation assays and Ki67 staining showed that JX39P cells proliferate more rapidly than JX39P-RT in liquid culture; however, when embedded in collagen I, JX39P showed no significant change in Ki67 staining, whereas JX39P-RT demonstrated a time-dependent increase, indicating that radiation-resistant GBM cells proliferate in response to collagen- and DDR1-mediated signaling. Conclusions: Together, our results identify DDR1 upregulation and ectodomain shedding as key adaptive mechanisms that promote proliferation and survival of radiation-resistant GBM cells. These regulatory processes enable resistant cells to sustain growth under therapeutic stress, providing a potential mechanism of therapeutic evasion. Future studies will investigate how DDR1 shedding and downstream signaling integrate to maintain radiation resistance and explore strategies to sensitize GBM to radiation therapy. Citation Format: Sophia Dunlap, Caitlin A. Harvey, Christopher D. Willey, Ahn Erin. The role of DDR1 ectodomain shedding in glioblastoma radiation resistance abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3306.
Dunlap et al. (Fri,) studied this question.