Melanovytic tumor development in transgenic mice carrying the ret oncogene

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We generated four transgenic mouse lines which showed severe melanosis of the whole body by introducing the ret oncogene fused to the mouse metallothionein-I promoter-enhancer (MT/ret). Melanocytic tumors frequently developed in three of the four lines. Mice of one line developed tumors in the dermis of the face and neck, the leg muscle, the mediastinum and the retroperitoneal cavity while mice of the other two lines developed them predominantly in the dermis of the face. Northern hybridization and in situ hybridization analyses showed that tumors cells and nontumorous melanin-producing cells expressed the transgene at high levels. To analyze the signal transduction pathway of the ret oncogene product, we established a cell line (Mel-ret) from a melanocytic tumor developed in a MT/ret transgenic mouse. Unlike primary melanocytic tumors which did not showed malignant features, the Mel-ret cells had the metastatic ability when they were transplanted into nude mice. We detected 100kd, 125kd and 135kd tyrosine phosphorylated proteins in both cell lysates of melanocytic tumors and Mel-ret cells. In addition, an 85kd tyrosine phosphorylated band was present specifically in the Mel-ret cells. The overall level of tyrosine phosphorylation in the Mel-ret cells was much higher than that in the primary tumors, suggesting that the increase of tyrosine phosphorylation may be responsible for malignant transformation. Immunofluorescence and cell fractionation studies showed that the ret protein and most of tyrosine phosphorylated proteins in the Mel-ret cells localized in the membrane fraction.