Tumor promoters retard the loss of a transient subpopulation of cells in low passage Syrian hamster cell cultures

Early passage normal diploid Syrian hamster (SH) fetal cell cultures contain a transient subpopulation of contact‐insensitive (CS − ) cells which lack density‐dependent inhibition of cell division. The size of this CS − subpopulation decreases during in vitro passage by conversion of the CS − cells to contact‐sensitive (CS + ) cells. Approximately 10—15 population doublings after the frequency of the CS − cells has declined to below 0.001%, mass cultures cease proliferating and exhibit cellular senescence. Cultures with higher initial numbers of CS − cells exhibit longer in vitro proliferative life spans than cultures with smaller initial numbers of CS − cells. Active tumor promoting phorbol esters (12‐O‐tetra‐decanoyl‐phorbol‐13‐acetate [TPA] and phorbol‐12, 13‐didecanoate [PDD]) retard the decline in the proportion of CS − cells during in vitro passage, while the inactive tumor promoting phorbol ester, 4α‐phorbol‐12, 13‐didecanoate (4αPDD) has no effect on the rate of loss of the CS − cells. In addition, continuous treatment from secondary culture with TPA or PDD extends by approximately twofold the in vitro proliferative life span of SH fetal cell cultures. Treatment must, however, begin at passage 1 or 2 when the CS − cells are still present. After the proportion of the CS − cells has decreased to <0.001%, as in passage 6 cultures, promoters have no effect on the life span of the culture. This finding that exposure to promoters results in both a prolonged maintenance of the CS − cellular subpopulation, as well as an extention of in vitro proliferative life span, suggests that the conversion of CS − cells to CS + cells is involved in the mechanism of in vitro senescence.