CAD gene expression in serum-starved and serum-stimulated hamster cells

The enzymes in the pathway for de novo pyrimidine biosynthesis, including those associated with the tri-functional CAD protein, show a marked increase in activity in rapidly growing cells and tissues. To learn more about the relationship of this pathway to cellular proliferation, we have studied changes in levels of CAD RNA, rates of CAD protein synthesis, and levels of aspartate transcarbamylase activity in Syrian hamster ts13 cells in response to serum starvation and serum stimulation. The steady-state level of CAD RNA and the synthetic rate of CAD protein decrease by 12- to 15-fold following 24 hr of serum starvation, as compared to exponentially growing cells. Upon serum stimulation of quiescent cells, steady-state CAD RNA levels increase substantially (13-fold), peaking during mid to late G1. Parallel increases occur in the synthesis of new CAD protein and in aspartate transcarbamylase activity. At the same time, the rate of CAD transcription increases only about twofold. These findings indicate that regulation of CAD expression in this system is primarily at the post-transcriptional level. This is in contrast to the transcriptional regulation of CAD previously reported in terminally differentiating HL60 cells (Rao et al., Mol. Cell. Biol. 7, 1961-1966, 1987). While both systems indicate that CAD gene expression is dependent on cell growth, there apparently are alternative mechanisms that can produce the same effect. Evidence is also presented that indicates that the accumulation of CAD transcripts during serum stimulation requires the synthesis of new proteins.