Clonal sublines of rat neurotumor RT4 and cell differentiation: IV. Cell surface proteins

There are stem cells in RT4 neurotumor that undergo spontaneous differentiation into three distinct cell types in culture (cell type conversion). Stem cells (RT4-AC) and one of the differentiated cell types (RT4-D) are tumorigenic and synthesize glial-specific S100 protein, while the others (RT4-B and RT4-E) are nontumorigenic and demonstrate some neuronal function. Interrelationships between cell surface proteins and differentiation of RT4 cells were analyzed. The surface proteins are radioiodinated by a lactoperoxidase method, separated by gel electrophoresis in two dimensions, visualized by autoradiography, and quantitated according to the relative radioactivity associated with each protein species. Integral surface proteins are compared in the present study which remain associated with plasma membrane after the extraction of radioiodinated cells with 0.1 N NaOH. The extraction also helps to remove serum proteins in the medium which are absorbed on cell surfaces and may be artifactually recognized as surface proteins. All RT4 cell types are complex in cell surface protein composition and nearly 100 integral surface proteins have been identified when all RT4 cell types are combined. Many unique proteins as well as common proteins have been identified. Considerable similarity exists between RT4-AC and RT4-D and between RT4-B and RT4-E. The two cell types of each pair are distinct yet share some common neurological and tumorigenic characteristics. In contrast, little similarity exists in other combinations of the two cell types, e.g., RT4-AC and RT4-E, etc. The results support the notion that the pattern of cell surface protein expression is a stable differentiation property and a characteristic set of proteins corresponds to each stage of differentiation.