Proteomic map of glioma biopsies reveals functional defects in endocytosis
Jean-Louis Boulay, Martin Spiess, Luigi Mariani, Paul Jenö, Marie-Françoise Ritz, Suzette Moes, Gregor Hutter, Dominik Buser, Stephan Frank, Cristobal Tostado
Introduction: Decades of molecular genetic analyses have shown that gliomas accumulate genetic alterations leading to enhanced activity of growth factor receptor tyrosine kinases (RTKs) and mediators of downstream pathways. These alterations result in gain-of-function of EGFR, PDGFR, PIK3CA or BRAF, and loss-of-function of PTEN or NF1, to exacerbate proliferative responses.
Objective: To determine the proteomic profile of diverse genetic subtypes of gliomas and potentially find a common trait among these subtypes.
Methods: We performed deep proteomic analysis of human gliomas of distinct genetic backgrounds, i.e. IDH, TERT statuses in combination with EGFR or PDGFRA amplification.
Results: Proteomic quantification revealed dramatically high EGFR protein levels in biopsies carrying an EGFR amplification but also in glioma biopsies devoided of EGFR amplification. Furthermore, high EGFR was paralleled with decreased levels of core components of clathrin-mediated endocytosis (CME), i.e. subunits of the endocytic adaptor AP-2 , clathrin (CLTC) and dynamin (DNM) proteins in a concerted manner, as confirmed by Western blotting. Functional binding assays confirmed higher cell surface EGFR levels when endocytic machinery proteins were reduced. Ongoing CpG methylation analyses of endocytosis gene promoters revealed that part of them are subject to epigenetic silencing, thereby providing clues on an orchestrated down-regulation of the corresponding genes.
Conclusions: Depletion of CME components increases the availability of RTKs at the plasma membrane to prolong exposure to growth factors. Subsequent enhanced and sustained growth factor response may result in selective advantage in tumorigenesis and progression. Restoration of CME may be a promising therapeutic strategy.