To begin to understand the mechanism of the different regulation of expression of the TGF-beta isoforms, an analysis was undertaken of the 5' flanking regions of the human genes for TGF-beta1, TGF-beta2, and TGF- beta3. Although the TGF-beta1, TGF-beta2, and TGF-beta3 amino acid sequences are highly conserved, further analysis has revealed that there is substantial divergence among the nucleotide sequences of their respective gene promoters. The TGF-beta2 and TGF-beta3 genes contain classic TATA box elements found 20-30 nucleotides upstream of the transcriptional start sites, whereas the 5' flanking region of the TGF- beta1 gene lacks a TATA box and contains high GC-rich regions containing multiple Sp1 binding sites. We have shown that TGF-beta2 is expressed from multiple promoters, one of which is regulated, in part, through a CRE/ATF-like element. Recent studies demonstrate that pRb is a transcriptional activator for the TGF-beta2 gene. Because TGF-beta inhibits proliferation of many cell types and arrests growth in the late G1 phase of the cell cycle, induction of TGF-beta2 by pRb may play an important role in cell cycle control. Recently, we have also characterized that TGF-beta1 is a target for the Wilms' tumor suppressor gene (WT1) product through a specific promoter element. In addition to transcriptional regulation, numerous reports suggest that expression of TGF-betas may also be regulated post-transcriptionally. The TGF-beta1 mRNA contains an unusually long 5' - untranslated sequence, which is rich in GC content. We have shown that a 137 nucleotide region of the TGF- beta1 5'-untranslated region (UTR) potently inhibits the expression of a heterologous reporter gene and in vitro gel retardation; and cross-linking assays, using a radiolabelled RNA probe transcribed from this region of the TGF-beta1 5'-UTR, demonstrate the specific binding of a cytosolic factor.
