Clonal selection in cultured human fibroblasts: role of protein synthetic errors.

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Abstract
Protein synthetic error frequency, determined in cell-free extracts as .DELTA.leu/.DELTA.phe [the ratio of the change in leucine to the change in phenylalanine] incorporation following poly(U) stimulation, has been found to decrease progressively in several strains of human diploid fibroblasts during their limited replicative lifespan. To explore the basis of this phenomenon, a mass (uncloned) culture of 1 normal strain was followed at 13 stages of its replicative lifespan. A progressive 10-fold decline in error frequency was found that was inversely correlated with passage level. This could not be ascribed to the slow rates of replication associated with fibroblast senescence because slowing of growth by serum deprivation did not change error frequency. Additionally, terminal mass cultures maintained for 16 wk at saturation density to mimimize cell selection did not change error frequency over this time. Error frequencies in 12 individual clones purified from the parental culture did not decline on repeated passage, either remaining constant or, in 2 clones, rising abruptly 3- to 5-fold after initial assays. Error frequencies of clones showed a weak inverse correlation with growth vigor but not with the maximum doubling number. Evidently, selective pressures favor more vigorously dividing clones with low protein synthetic error frequencies leading to their predominance in mass cultures.