Action of cortisol on the proliferation of rainbow trout fibroblasts

The effect of cortisol on the proliferation of the rainbow trout fibroblast cell line, RTG-2, was examined in synchronous and asynchronous cultures. When the transition from G1 to S was synchronized by restoring serum to serum-deprived cultures, the addition of cortisol at the time of serum restoration delayed the entry of cells into S phase. However, if cortisol was added 24 h after serum restoration, at the G1/S transition point, the subsequent peak of DNA synthesis was unaffected. In asynchronous cultures cortisol inhibited [3H]-thymidine and [3H]-uridine but not [3H]-leucine incorporation into acid-insoluble material. If the exogenous nucleoside concentration was raised, [3H]-thymidine but not [3H]-uridine incorporation continued to be inhibited by cortisol. This suggested that cortisol's effect on [3H]-thymidine incorporation reflected a change in entry into S phase and not just on thymidine uptake and metabolism. Cortisol inhibited the proliferation of RTG-2 in asynchronous cultures. At 1000 ng/ml of cortisol a reduction in cell number became apparent before the RTG-2 cultures were confluent, whereas at 100 ng/ml the reduction only became evident in confluent cultures. The synthetic antiglucocorticoid, RU 486, which acts at the level of the corticosteroid receptor, blocked the growth inhibition by cortisol. These results suggest that cortisol regulates rainbow trout fibroblast proliferation via the corticosteroid receptor and that the G1/S transition is one point at which this regulation occurs.