Molecular imaging of N-linked glycosylation suggests glycan biosynthesis is a novel target for cancer therapy

Abstract

Purpose: Redundant receptor tyrosine kinase (RTK) signaling is a mechanism for therapeutic resistance to epidermal growth factor receptor (EGFR) inhibition. A strategy to reduce parallel signaling by coexpressed RTKs is inhibition of N-linked glycosylation (NLG), an endoplasmic reticulum (ER) cotranslational protein modification required for receptor maturation and cell surface expression. We therefore investigated the feasibility of blocking NLG in vivo to reduce overexpression of RTKs. Experimental Design: We developed a model system to dynamically monitor NLG in vitro and in vivo using bioluminescent imaging techniques. Functional imaging of NLG is accomplished with a luciferase reporter (ER-LucT) modified for endoplasmic reticulum translation and glycosylation. After in vitro validation, this reporter was integrated with D54 glioma xenografts to do noninvasive imaging of tumors, and inhibition of NLG was correlated with RTK protein levels and tumor growth. Results: The ER-LucT reporter shows the ability to sensitively and specifically detect NLG inhibition. Using this molecular imaging approach we carried out serial imaging studies to determine safe and efficacious in vivo dosing of the GlcNAc-1-phosphotransferase inhibitor tunicamycin, which blocks N-glycan precursor biosynthesis. Molecular analyses of tunicamycin-treated tumors showed reduced levels of EGFR and Met, two RTKs overexpressed in gliomas. Furthermore, D54 and U87MG glioma xenograft tumor experiments showed significant reductions in tumor growth following NLG inhibition and radiation therapy, consistent with an enhancement in tumor radiosensitivity. Conclusions: This study suggests that NLG inhibition is a novel therapeutic strategy for targeting EGFR and RTK signaling in both gliomas and other malignant tumors. ©2010 AACR.