TRANSFORMING GROWTH FACTOR-BETA-1 SUPPRESSION OF C-MYC GENE-TRANSCRIPTION - ROLE IN INHIBITION OF KERATINOCYTE PROLIFERATION

Transforming growth factor β1 (TGF-β1) is a potent growth inhibitor for many cell types, including most epithelial cells. However, the mechanism of growth inhibition is unknown. In skin keratinocytes, TGF-β1 has been shown to inhibit growth and to rapidly reduce c-myc expression. It has been demonstrated that protein synthesis is required for TGF-β1 regulation of c-myc in keratinocytes. Here we present evidence that treatment of mouse BALB/MK keratinocyte cells with either antisense c-myc oligonucleotides or TGF-β1 inhibited cell entry into S phase. These results suggest that TGF-β inhibition of c-myc expression may be essential for growth inhibition by TGF-β1. The block in c-myc expression by TGF-β1 occurred at the level of transcriptional initiation. Studies with a series of 5'deletion c-myc/chloramphenicol acetyltransferase constructs indicated that a cis regulatory elements(s), which resides between positions -100 and +71 relative to P1 transcription start site, is responsible for the TGF-β1 responsiveness. Based on these data, it is proposed that the mechanism of TGF-β1 growth inhibition involves synthesis or modification of a protein that may interact with a specific element(s) in the 5'regulatory region of the c-myc gene, resulting in inhibition of transcriptional initiation.