Up-regulation of NG2 proteoglycan and interferon-induced transmembrane proteins 1 and 3 in mouse astrocytoma: A membrane proteomics approach

Although brain tumors are classified as if their lineage were well understood, the relationship between the molecular events that specify neural cell lineage and brain tumors remains enigmatic. Traditionally, cell surface membrane antigens have served as biomarkers that distinguish brain tumor origin and malignancy. In this study, membrane proteins were identified from a terminally differentiated mouse astrocyte (AC) and CT-2A astrocytoma (CT-2A) cell line using liquid-chromatography coupled with tandem mass spectrometry (LC-MS/MS). A total of 321 and 297 protein groups with at least one unique peptide were identified in the AC and CT-2A cells. Using a label-free quantitative MS approach, 25 plasma membrane proteins in CT-2A were found significantly up- or down-regulated compared with those in AC. Three of the up-regulated proteins, chondroitin sulfate proteoglycan-4 (Cspg4), interferon-induced transmembrane protein-2 (IFITM2) and -3 (IFITM3) were further validated by semi-quantitative RT-PCR analysis. In addition, a third member of the IFITM family, interferon-induced transmembrane protein-1 (IFITM1) was also analyzed. Expression of Cspg4, IFITM1 and IFITM3 was significantly greater in the CT-2A cells than that in the AC cells. Interestingly, Cspg4, also known as neuronal/glial 2 (NG2) proteoglycan in human, is an oligodendrocyte progenitor marker. Therefore, our data suggest that the CT-2A tumor may be derived from NG2 glia rather than from fully differentiated astrocytes. Moreover, the CT-2A cells also express a series of interferon-induced signature proteins that may be specific to this tumor. These data highlight the utility of LC-MS/MS for the identification of brain tumor membrane biomarkers.