Abstract
Overexpression of the cyclin D1/PRAD1 oncogene has been observed in a number of tumorigenic cell lines, suggesting that regulation of D1 expression may represent an important step in the control of cellular proliferation. We have examined the mRNA expression of cyclin D1, as well as two related D-type cyclins, D2 and D3, in response to defined growth factors that control the growth of Balb/c-3T3 fibroblasts. Transcripts for all three D-type cyclins were expressed during the G1 phase of the Balb cell cycle, however only D1 and D3 exhibited periodic induction. Although redundantly expressed, message levels of cyclin D1 and D3 were differentially regulated in regard to kinetics of induction; a modest increase in D3 mRNA was detected near the G1/S boundary, 12 h after serum stimulation of quiescent cells, while abundance of D1 transcript increased 20 to 30-fold, peaking 6 h after addition of serum. Factors such as platelet-derived growth factor (PDGF) that induce competence formation in Balb cells, increased D1 message and protein levels to the same extent as serum but did not affect expression of cyclin D3 and did not stimulate entry into S phase. Progression factors contained within platelet-poor plasma stimulated D1 expression only weakly but acted synergistically with low concentrations of PDGF to increase D1 mRNA to maximum levels. Depletion of protein kinase C severely reduced the ability of PDGF and serum to induce D1 mRNA. PDGF- and serum-mediated elevation of steady-state D1 message levels was in part because of a transcriptional activation of the D1 gene that was independent of protein synthesis. However, protein synthesis was required 3-4 h after serum stimulation for the shut down of D1 transcription leading to the normal decline in message levels after peak induction. Our results indicate that overexpression of cyclin D1 message may result from a disruption of negative regulatory events that repress D1 transcription.