The regulation of gene expression by oncogene Myc and cell growth state: The involvement of transcription and DNA-replication factor CTF/NF-1.

Abstract

Regulation of gene expression is a fundamental event in the control of life processes. In an effort to understand cell proliferation and transformation, we investigated the effect of the c-Myc oncogene on cellular gene expression. Using differential filter hybridization assay against 7,000 clones in a c-DNA library, we cloned four genes, all of whose expression were suppressed in c-Myc transfected cells. Three of them were identified as different types of collagen genes. It seems that suppression of pro alpha 2(I) collagen gene, which is one of the four genes, occurs at the transcriptional level and requires the biochemical activity of c-Myc protein, based on the fact that this gene promoter activity is down-regulated by c-Myc and the deletion mutant of c-Myc protein in the leucine-zipper domain failed to repress it. Genetic and biochemical studies reveal that the activity of one of pro alpha 2(I) collagen promoter binding proteins, CTF/NF-1 is affected by cell-growth state and c-Myc. Most of CTF/NF-1 proteins in actively growing cells are more negatively charged species, compared to the proteins in quiescent cells. This suggests that cell-growth state affects in vitro biochemical property of CTF/NF-1. In vivo transcriptional activity of CTF/NF-1 is higher in quiescent cells than in actively growing cells. Thus, there is a good correlation between the abundance of less negatively charged CTF/NF-1 species and its differing transcriptional activity. Deregulated expression of c-Myc affects cell-growth state associated CTF/NF-1 properties as well. CTF/NF-1 properties in highly Myc-expressing quiescent state are closer to actively growing cells rather than to quiescent cells. The down-modulation of CTF/NF-1 transcriptional activity in quiescent cells expressing high level of c-Myc suggests that the suppression of pro alpha 2(I) collagen gene by c-Myc is mediated through CTF/NF-1 binding site. The CTF/NF-1 antisense oligomer inhibits cell transition from G0/G1 to S phase, suggesting a possibility that its activity might be directly involved in cell proliferation. This study suggests that the regulation of gene expression and the cell proliferation and transformation might be closely linked processes.