Collagen-dependent DDR1 ectodomain shedding regulates proliferative signaling in radiation-resistant glioblastoma
Glioblastoma (GBM) is characterized by near-universal recurrence and resistance to therapy. Although tumor-intrinsic mechanisms of radiation resistance have been extensively studied, how the extracellular matrix influences signaling in radiation-resistant GBM remains poorly understood. Here, we investigated discoidin domain receptor 1 (DDR1), a collagen-activated receptor tyrosine kinase, using matched radiation-sensitive parental and radiation-resistant patient-derived GBM models generated through serial in vivo irradiation. Within a three-dimensional collagen I matrix, radiation-resistant GBM cells exhibited distinct temporal patterns of Ki-67 positivity and ERK1/2 phosphorylation compared with parental cells. Despite increased DDR1 expression, radiation-resistant cells exhibited markedly reduced DDR1 phosphorylation and enhanced ectodomain shedding, characterized by accumulation of the membrane-associated C-terminal fragment and release of the N-terminal fragment. DDR1 depletion reduced ERK1/2 phosphorylation, supporting its contribution to ERK1/2 signaling under these conditions. ADAMTS1 was markedly upregulated in radiation-resistant cells, and its knockdown reduced DDR1 ectodomain shedding while increasing full-length DDR1 abundance, ERK1/2 and RB phosphorylation, cyclin D1 expression, and Ki-67 positivity. Together, these findings reveal a distinct DDR1 regulatory state in acquired radiation-resistant GBM, characterized by enhanced ADAMTS1-associated ectodomain shedding, reduced receptor phosphorylation, and attenuated proliferative signaling within a three-dimensional collagen I matrix.