Adhesion- and stress-related adaptation mechanisms eliciting glioblastoma radiochemoresistance can be effectively circumvented by beta1 integrin/JNK co-targeting

Glioblastoma multiforme (GBM) is the most common brain tumor in adults and characterized by poor clinical outcome due to genetic and epigenetic alterations in resistance-mediating genes and destructive infiltration into the normal brain. Upon therapy, malignant tumors show adaptation to maintain their homeostasis. Two critical determinants of this adaptation process are cell adhesion by beta1 integrins and stress signaling via c-Jun N-terminal kinases (JNK). Here, we evaluated the potential of simultaneous beta1 integrin/JNK targeting to overcome GBM adaptation controlling radiochemoresistance and invasion.

Comparative Oncomine data base analysis was performed on the expression of JNK1/2/3 isoforms, beta1 integrin and its ligands in GBM with normal brain. Different human GBM cell populations (patient-derived, stem-like, established) were analyzed for sphere formation, clonogenicity, 3D collagen type-1 invasion, cell cycling, chromatin organization, DNA double strand break (DSB) repair (γH2AX foci assay), broad-spectrum phosphoproteome analysis, FACS analysis and protein expression/phosphorylation upon irradiation (0-6 Gy X-rays) and chemotherapy (Temozolomide) with and without single and simultaneous inhibition of beta1 integrin (AIIB2) and JNK (SP600125, JNKi). The radiochemosensitizing potential of AIIB2/JNKi was also investigated in an orthotopic GBM mouse model using stem-like cells.

In contrast to JNK isoforms, beta1 integrin and col1 showed significant overexpression in GBM compared with normal brain. While single inhibition of beta1 integrin and JNK mediated cytotoxicity, only combined targeting resulted in radiochemosensitization. Intriguingly, double AIIB2/JNKi treatment abrogated GBM cell invasion. Importantly, dual beta1 integrin/JNK inhibition elicited a significant reduction in tumor growth and longer survival of mice concomitantly treated with radiotherapy/Temozolomide. Mechanistically, JNK blocking induced beta1 integrin expression for stimulating diverse signaling pathways controlling cell cycling, invasion and radiochemosensitivity. Radiosensitization by AIIB2/JNKi is caused by enhanced ATM phosphorylation and prolonged G2/M cell cycle arrest as well as impaired DNA double strand break repair in the context of elevated levels of euchromatin.

In summary, our data reveal that dual beta1 integrin/JNK targeting efficiently impairs adhesion and stress-related adaptation mechanisms involved in radiochemoresistance and invasion. More in-depth evaluation is warranted to clarify the potential of this kind of beta1 integrin/JNK multi-targeting strategy administrated concomitantly to standard radiochemotherapy in patients suffering from GBM.