Malignant transformation of human skin fibroblasts by two alternative pathways

We developed a telomerase-positive, infinite life span human fibroblast cell strain (MSU-1.0) by transfection of a v-MYC oncogene and spontaneous over-expression of transcription factors SP1/SP3. Loss of expression of p14(ALT) and enhanced expression of SPRY2 gave rise to the MSU-1.1 cell strain. Unlike MSU-1.0 cells, the MSU-1.1 cells can be malignantly transformed by expression of N-RAS(LYS61) or H-Ras(v12) oncoproteins (driven by their original promoters) and expression of a SRC-family protein, v-FES. MSU-1.1 cells can also be malignantly transformed by high expression of these RAS oncogenes or the v-K-RAS oncogene. PDGF-B transformed MSU-1.1 cells give rise to benign tumors (fibromas) in athymic mice. A second route to malignant transformation of the MSU-1.1 cells involves loss of functional TP53 protein by carcinogen treatment and loss of expression of wild type p16(INK). These studies indicate 6-8 "hits" are required to activate the oncogenes and inactivate the suppressor genes we identified.