Multiple Forms of TGF‐β: Distinct Promoters and Differential Expression

Abstract

There are now five known distinct isoforms of TGF‐β with 64–82% identity. Of these, only TGF‐β1, 2 and 3 thus far have been demonstrated to be expressed in mammalian tissues; TGF‐β4 has been described only in chicken and TGF‐β5 only in frog. Although the biological activities of these five isoforms of TGF‐β are indistinguishable in most in vitro assays their sites of synthesis and localization in vivo are often distinct. Expression of the various isoforms is differentially controlled both in vivo, as in development, and in vitro after treatment of cells with steroids, such as oestrogen or tamoxifen, or with retinoids. To investigate the basis of these observations we have cloned and characterized the promoters for the human TGF‐β1, 2 and 3 genes. Significant differences have been found: whereas the TGF‐β1 promoter has no TATAA box and is regulated principally by AP‐1 sites, both the TGF‐β2 and 3 promoters have TATAA boxes as well as AP‐2 sites and cAMP‐responsive elements. Accordingly, TGF‐β1 gene expression is induced strongly by phorbol esters whereas that of TGF‐β2 and 3 is induced by forskolin, an activator of adenylate cyclase. Expression of TGF‐β2 and 3 is often coordinately regulated in vivo in a pattern distinct from that of TGF‐β1.