IDENTIFICATION OF GENETIC EVENTS INVOLVED IN EARLY STEPS OF IMMORTALIZATION OF MOUSE FIBROBLASTS

The spontaneously immortalized early passaged fibroblasts from three d ifferent strains of mouse are observed to represent two distinct stage s of immortalization. The cells at stage I are characterized by slow g rowth rate, contact inhibition and requisition of serum factors for th eir growth and proliferation. Stage II cells are marked by fast, multi layer growth that is independent of serum supplementation in growth me dium and by the elevated levels of the two marker proteins, i.e., p53 and p81. The change from cytosolic distribution of mortalin, a senesce nce inducing protein (J. Biol. Chem. (1993) 268, 6615-6621; 22239-2224 2) to the perinuclear locale is detected as an early event during cell ular immortalization. Furthermore, the distinct stages could be charac terized by thermal analysis of intact cells, that to the best of our k nowledge is employed for the first time for the analysis of cellular m ortal and immortal phenotypes. The study characterizes at least two di stinct end points in rodent transformation suggesting that there are m ultiple routes to immortalization.