Oligosaccharyltransferase inhibition Reduces Receptor Tyrosine Kinase Activation and Enhances Glioma Radiosensitivity

Purpose: Parallel signaling reduces the effects of receptor tyrosine kinase (RTK) targeted therapies in glioma. We hypothesized that inhibition of protein N-linked glycosylation, an endoplasmic reticulum co- and post-translational modification crucial for RTK maturation and activation, could provide a new therapeutic approach for glioma radiosensitization. Experimental design: We investigated the effects of a small molecule inhibitor of the oligosaccharyltransferase (NGI-1) on EGFR family receptors, MET, PDGFR, and FGFR1. The influence of glycosylation state on tumor cell radiosensitivity, chemotherapy induced cell toxicity, DNA damage, and cell cycle arrest were determined and correlated with glioma cell receptor expression profiles. The effects of NGI-1 on xenograft tumor growth were tested using a nanoparticle formulation validated by in vivo molecular imaging. A mechanistic role for RTK signaling was evaluated through the expression of a glycosylation independent CD8-EGFR chimera. Results: NGI-1 reduced glycosylation, protein levels, and activation of most RTKs. NGI-1 also enhanced the radiosensitivity and cytotoxic effects of chemotherapy in those glioma cells with elevated ErbB family activation, but not in cells without high levels of RTK activation. NGI-1 radiosensitization was associated with increases in both DNA damage and G1 cell cycle arrest. Combined treatment of glioma xenografts with fractionated radiation and NGI-1 significantly reduced tumor growth compared to controls. Expression of the CD8-EGFR eliminated NGI-1's effects on G1 arrest, DNA damage, and cellular radiosensitivity, identifying RTK inhibition as the principal mechanism for the NGI-1 effect. Conclusion: This study suggests that OST inhibition with NGI-1 is a novel approach to radiosensitize malignant gliomas with enhanced RTK signaling.

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