The three isoforms of TGF-Beta expressed in mammals are 65-85% homologous to each other. Although they are functionally interchangeable in most in vitro assay systems, they have distinctive activities on certain cells such as endothelial cells where the activities of TGF-Beta 1 and TGF-Beta 3 on inhibition of growth and migration are approximately 100-fold more potent than that of TGF-Beta 2. To attempt to define specific regions of the TGF-Beta molecule responsible for these differences in activities, we have developed a system for expression of recombinant TGF-Betas engineered by recombinant DNA methodology to have portions of different isoforms spliced into a single chimeric molecule. Most interesting to date have been chimeras in which a region of the amino acid sequence of mature TGF- Beta 1 has been spliced into the TGF-Beta 2 molecule. Each recombinant TGF-Beta chimera has been purified to homogeneity using a series of high pressure liquid chromatography steps. We have shown that the middle third of the TGF-Beta molecule is sufficient to confer isoform-specific biological activity on endothelial cells. By making chimeras with only a few selected amino acid changes, we hope to identify specific amino acids responsible for this selective response. In a related aspect of this problem of isoform specificity, we are attempting to characterize the expression of TGF-Beta receptors and possible down-stream signalling intermediates to determine whether these will be common to all TGF-Beta isoforms or whether they will mediate isoform-specific effects.
