TGF‐β regulation of epithelial cell proliferation

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Abstract
The predominant effect of TGF‐β1 on cell proliferation is inhibition. Earlier studies demonstrated that TGF‐β1 inhibition of skin keratinocyte proliferation involves suppression of c‐myc transcription and indirect evidence suggested that the protein product of the retinoblastoma gene (pRB) may be involved in this process. Skin keratinocytes transformed by SV40 and human papilloma virus‐16 (HPV‐16) or HPV‐18 resisted growth inhibition and suppression of c‐myc mRNA by TGF‐β. Transient expression of HPV‐16 E7 gene, adenovirus E1A, and SV40 large T antigen (TAg) blocked the TGF‐β1 suppression of c‐myc transcription. Studies with transformation‐defective mutants of E1A and TAg suggested that a cellular protein(s) that interacts with a conserved domain of the DNA tumor virus oncoproteins mediates TGF‐β1 suppression of c‐myc transcription and keratinocyte growth. Transient expression of pRB in skin keratinocytes repressed human c‐myc promoter/CAT transcription as effectively as TGF‐β1. The same c‐myc promoter region, termed the TGF‐β Control Element (TCE), was required for regulation by both TGF‐β1 and pRB. TCE bound a cellular protein of approximately 106 kDa and this binding was decreased by TGF‐β1 treatment. Our data indicate that pRB can inhibit c‐myc transcription and suggest the involvement of cellular factor(s) in addition to pRB in the TGF‐β1 pathway for the suppression of c‐myc transcription and growth inhibition. The possible involvement of pRB in the TGF‐β1 pathway for suppression of c‐myc transcription has a number of implications; c‐myc is a cellular proto‐oncogene involved in positively regulating cell proliferation. TGF‐β1 may therefore act through the tumor suppressor gene product, pRB, to negatively regulate c‐myc transcription and subsequently cell growth. This would implicate tumor suppressor genes in the response pathway for diffusible growth inhibitors, perhaps analogous to nuclear proto‐oncogene involvement in the growth factor pathway.