A Deadly Gift

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Although many malignant gliomas express EGFRvIII, a constitutively active oncogenic mutant form of the epidermal growth factor receptor (EGFR), the mutant receptor may actually be present in only a subset of the tumor cells. Al-Nedawi et al . found that a human glioma cell line (U373 cells) stably transfected with and made to express EGFRvIII (U373vIII cells) produced more membrane-derived microvesicles than did the parent cell line. Increased formation of vesicular membrane protrusions was apparent in U373vIII cells by scanning electron microscopy, and the abundance of total protein in the microvesicle fraction of medium in which cells had grown was increased. U373vIII cell-derived microvesicles contained EGFRvIII, as well as the lipid raft marker flotillin-1. Unlike U373 cells, U373vIII cells form subcutaneous tumors when injected into immunodeficient mice; these EGFRvIII-immunoreactive tumors released EGFRvIII-containing microvesicles into the circulatory system. Experiments in which U373 cells were incubated with vesicles derived from U373vIII cells indicated that microvesicular EGFRvIII was transferred to the U373 cells, where it appeared on the cell surface. A day later, U373 cells that had been exposed to EGFRvIII-containing microvesicles showed increased phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2). Pharmacological treatment of microvesicles to inhibit signaling by EGFR-family receptors attenuated this increase in ERK1/2 phosphorylation, as did treatment of the microvesicles with annexin V (to prevent their uptake). EGFRvIII-containing microvesicles also stimulated other events downstream of EGFRvIII: Akt phosphorylation, release of vascular endothelial growth factor, abundance of the antiapoptotic protein Bcl-x L , and the ability of U373 cells to grow in soft agar (indicative of malignant transformation). The authors thus conclude that membrane microvesicles provide a mechanism for horizontal transfer of the transformed phenotype. K. Al-Nedawi, B. Meehan, J. Micallef, V. Lhotak, L. May, A. Guha, J. Rak, Intercellular transfer of the oncogenic receptor EGFRvIII by microvesicles derived from tumour cells. Nat . Cell Biol . 10 , 619-624 (2008). [PubMed]