NEOPLASTIC DEVELOPMENT - PARADOXICAL RELATION BETWEEN IMPAIRED CELL-GROWTH AT LOW POPULATION-DENSITY AND EXCESSIVE GROWTH AT HIGH-DENSITY

The role of heritable, population-wide cell damage in neoplastic devel opment was studied in the 28 L subline of NTH 3T3 cells, These cells d iffer from the 17(3c) subline used previously for such studies in thei r lower frequency of ''spontaneous'' transformation at high population density and their greater capacity to produce large, dense transforme d foci. Three cultures of the 28 L subline of NM 3T3 cells were held u nder the constraint of confluence for 5 wk (5 wk 1 degrees assay) and then assayed twice in succession (2 degrees and 3 degrees assays) for transformed foci and saturation density, After the 2 degrees assay, th e cells were also passaged at low density to determine their exponenti al growth rates and cloned to determine the size and morphological fea tures of the colonies, Concurrent measurements were made in each case with control cells that had beep kept only in frequent low-density pas sages and cells that had been kept at confluence for only 2 wk (2 wk 1 degrees), Two of the three cultures transferred from the 2 degrees as say of the 5 wk 1 degrees cultures produced light transformed foci, an d the third produced dense foci, The light focus-forming cultures grew to twice the control saturation density ip their 2 degrees assay and 6-8 times the control density in the 3 degrees assay; saturation densi ties for the dense focus formers were about 10 times the control value s in both assays, All three of the cultures transferred from the 2 deg rees assay of the 5 wk 1 degrees cultures multiplied at lower rates th an controls at low densities, but the dense focus formers multiplied f aster than the light focus formers, The reduced rates of multiplicatio n of the light focus formers persisted for >50 generations of exponent ial multiplication at low densities. Isolated colonies formed from sin gle cells of the light focus formers were of a lower population densit y than controls; colonies formed by the dense focus formers were sligh tly denser than the controls but occupied only half the area A much hi gher proportion of the colonies from the 5 wk 1 degrees cultures than the controls consisted of giant cells or mixtures of giant and normal- appearing cells. The results reinforce the previous conclusion that th e early increases in saturation density and light focus formation are associated with, and perhaps caused by, heritable, population-wide dam age to cells that is essentially epigenetic in nature. The more advanc ed transformation characterized by large increases in saturation densi ty and dense focus formation could have originated from rare genetic c hanges, such as chromosome rearrangements, known to occur at an elevat ed frequency in cells destabilized by antecedent cellular damage.