Inhibition of N-linked glycosylation disrupts receptor tyrosine kinase signaling in tumor cells

Unrestrictive identification of non-phosphorylation PTMs in yeast kinases by MS and PTMap

A few protein PTMs, such as phosphorylation and ubiquitination, are known to be critical in regulation of protein kinase activities. However, the roles of other PTMs have not been extensively studied in kinases. Development of a comprehensive description of all types of PTMs and discovering novel in vivo PTMs in low abundance represent major analytical challenges. Toward this goal, we have developed a strategy for systematic and accurate identification of the full-spectrum of PTMs in yeast protein kinases. Our strategy involves isolation of GST-fused kinase proteins, MS analysis, and unrestrictive PTM identification by PTMap algorithm. Among the 30 purified yeast kinases, we identified 27 different types of PTMs, and 53 PTM sites, among which are 13 novel mass shifts that have not been previously reported, likely representing novel PTMs. These results represent a significant expansion of our current understanding of PTMs in kinases and suggest highly complex regulation of kinase function.

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

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.

Mass spectrometry (LC-MS/MS) site-mapping of N-glycosylated membrane proteins for breast cancer biomarkers

Cancer cell membrane proteins are released into the plasma/serum by exterior protein cleavage, membrane sloughing, cellular secretion or cell lysis, and represent promising candidates for interrogation. Because many known disease biomarkers are both glycoproteins and membrane bound, we chose the hydrazide method to specifically target, enrich, and identify glycosylated proteins from breast cancer cell membrane fractions using the LTQ Orbitrap mass spectrometer. Our initial goal was to select membrane proteins from breast cancer cell lines and then to use the hydrazide method to identify the N-linked proteome as a prelude to evaluation of plasma/serum proteins from cancer patients. A combination of steps facilitated identification of the glycopeptides and also defined the glycosylation sites. In MCF-7, MDA-MB-453 and MDA-MB-468 cell membrane fractions, use of the hydrazide method facilitated an initial enrichment and site mapping of 27 N-linked glycosylation sites in 25 different proteins. However, only three N-linked glycosylated proteins, galectin-3 binding protein, lysosome associated membrane glycoprotein 1, and oxygen regulated protein, were identified in all three breast cancer cell lines. In addition, MCF-7 cells shared an additional 3 proteins with MDA-MB-453. Interestingly, the hydrazide method isolated a number of other N-linked glycoproteins also known to be involved in breast cancer, including epidermal growth factor receptor (EGFR), CD44, and the breast cancer 1, and early onset isoform 1 (BRCA1) biomarker. Analyzing the N-glycoproteins from membranes of breast cancer cell lines highlights the usefulness of the procedure for generating a practical set of potential biomarkers.

Targeting EGFR resistance networks in head and neck cancer

A core set of oncoproteins is overexpressed or functionally activated in many types of cancer, and members of this group have attracted significant interest as subjects for development of targeted therapeutics. For some oncoproteins such as EGFR/ErbB1, both small molecule and antibody agents have been developed and applied in the clinic for over a decade. Analysis of clinical outcomes has revealed an initially unexpected complexity in the response of patients to these agents. Diverse factors, including developmental lineage of the tumor progenitor cell, co-mutation or epigenetic modulation of genes encoding proteins in an extended EGFR signaling network or regulating core survival responses in individual tumors, and environmental factors including inflammatory agents and viral infection, all have been identified as modulating response to treatment with EGFR-targeted drugs. Second and third generation therapeutic strategies increasingly incorporate knowledge of cancer type-specific signaling environments, in a more personalized treatment approach. This review takes squamous cell carcinoma of the head and neck (SCCHN) as a specific example of an EGFR-involved cancer with idiosyncratic biological features that influence design of treatment modalities, with particular emphasis on commonalities and differences with other cancer types.

Identification of predictive biomarkers for response to trastuzumab using plasma FUCA activity and N-glycan identified by MALDI-TOF-MS

The aim of this study was to identify glycobiological biomarkers that indicate sensitivity to trastuzumab, a humanized monoclonal antibody against HER2 in plasma samples from breast cancer patients. Plasma samples were obtained from 24 breast cancer patients treated with trastuzumab monotherapy. The catalytic activities of plasma alpha1-6, fucosyltransferase (FUT8) and alpha-L fucosidase (FUCA) were analyzed using high-performance liquid chromatography (HPLC) and spectrophotometer, respectively. The plasma N-glycan profiles were investigated using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Plasma FUT8 activity was not significantly correlated with either the clinical response or progression-free survival (PFS). On the other hand, plasma FUCA activity was significantly correlated with PFS (p < 0.05). The MALDI-TOF-MS analysis of the plasma N-glycan profile revealed that the expression of 2534 m/z N-glycan was lower in patients with progressive disease (PD) and was correlated with PFS. Low expression of 2534 m/z N-glycan discriminated between PD and non-PD with 75% sensitivity and 82% specificity. We demonstrated that the plasma FUCA activity and 2534 m/z N-glycan may be predictive biomarkers of sensitivity to trastuzumab. Our results suggest that glycosylation analysis may provide useful information for determining clinical cancer therapy and provide novel insight into biomarker studies using glycobiological tools in the field of breast cancer.