The TGF-beta s are potent inhibitors of proliferation of most cell types in culture and in vivo. Previous studies have demonstrated that TGF-beta inhibition of skin keratinocyte proliferation involves suppression of c-myc transcription. Evidence derived from use of expression plasmids for certain DNA viral oncoproteins has suggested that the retinoblastoma gene (RB) may be involved in this process. Transient expression of pRB, like TGF-beta 1, in skin keratinocytes represses expression of a human c-myc reporter plasmid, and the same c-myc promoter region (TCE) is required for repression by either TGF-beta 1 or pRB. We showed here that proliferation and c-myc expression in a cell line lacking normal pRB (DU145 human prostate adenocarcinoma cells) are not inhibited by TGF-beta 1. Oligonucleotides containing the TCE were found to bind to a cellular protein of approximately 106 kD (termed p106) in Southwestern assays, utilizing extracts from both the skin keratinocytes and DU145 cells. TCE binding to p106 was diminished by TGF-beta in TGF-beta-sensitive skin keratinocytes but not in TGF-beta-insensitive SV40-transformed keratinocytes. These data support the hypothesis that pRB is required for TGF-beta 1 suppression of c-myc transcription and suggest the involvement of a cellular factor(s) in addition to pRB in the TGF-beta 1 pathway for inhibition of c-myc transcription and growth inhibition.
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