Regulation of expression of c-myc protoocogene in a clonal line of mouse lens epithelial cells by serum growth factors.

Steady-state levels of c-myc mRNA were determined in a clonal line of mouse lens epithelial cells in quiescent and growth-stimulated states. Steady-state mRNA levels for c-myc increased rapidly from an undetectable amount in quiescent cells to the maximum level (8-fold) in growth-stimulated cells. In contrast to its steady-state mRNA levels, its rate of transcription increased by only 3.4-fold in serum-stimulated cells versus quiescent cells, indicating that the abundance of c-myc transcripts in lens epithelial cells during the serum-induced transition from quiescence to proliferation is regulated by both transcriptional and post-transcriptional mechanisms. Serum stimulation in combination with cycloheximide caused superinduction in the steady-state levels of c-myc mRNA in lens epithelial cells. These additive increases in c-myc mRNA levels in the presence of cycloheximide could be due to a decrease in the apparent turnover rate of c-myc mRNA, which, in fact, was observed in actively growing cells. DNA synthesis, as revealed by [3H]thymidine uptake, began 18 h after the addition of serum to quiescent cells and peaked at 24 h. From these results it is concluded that the expression of c-myc gene in mouse lens epithelial cells in response to serum induction is growth dependent.