Retinoblastoma gene product activates expression of the human TGF-β2 gene through transcription factor ATF-2

Abstract

THE retinoblastoma susceptibility gene product (pRb) plays an important role in constraining cellular proliferation and in regulating the cell cycle^1,2. The pRb inhibits transcription of genes involved in growth control (reviewed in ref. 3) and can regulate transforming growth factor β1 (TGF-β1) gene expression^4,5. TGF-β isoforms also down-regulate cellular proliferation^6,7. To determine whether pRb also regulates expression of other TGF-β isoforms, we examined the effect of pRb on the expression of the human TGF-β2 gene. The human TGF-β2 promoter contains multiple elements including an ATF site, which is essential for basal promoter activity⁸. Here we report that pRb activates transcription of the human TGF-β2 gene. The promoter element responsible for pRb-mediated transcriptional regulation is a binding site for ATF proteins, an extensive transcription factor family⁹. We provide evidence that implicates ATF-2 in pRb-responsiveness. First, the ATF promoter element in the TGF-β2 gene is a high-affinity ATF-2-binding site. Second, a GAL4–ATF2 fusion protein can support pRb-mediated transcriptional activation of a promoter containing GAL4-binding sites. Third, ATF-2 in nuclear extracts can interact with pRb. Our results reveal a new mechanism by which pRb constrains cellular proliferation: by activating expression of the inhibitory growth factor, TGF-β2.