Three distinct isoforms of TGF-beta are expressed in mammals. Although each of these isoforms shows 65-85% homology to each other, the amino acid sequence of any particular isoform is conserved greater than 98% between mammalian, avian, and amphibian species. This strongly suggests that the mature peptides will have certain unique biological activities. Only TGF-beta's 1, and 2 have been purified from natural sources. So that we might be able to compare the activities of all three isoforms, we developed a system for expression of recombinant TGF-beta 3. Moreover, to begin to explore exactly which regions of the molecule are responsible for isoform-specific binding and activity, we have used this same expression system to express specific recombinant chimeric TGF-beta's in which regions of the amino acid sequence of TGF-beta's 1, 2, and 3 are substituted for each other in the mature peptide. Each of these recombinant TGF-beta's has been purified to homogeneity using a sequence of high pressure liquid chromatography steps. The availability of TGF-beta 3 has permitted us to raise antibodies against this peptide which can now be used to detect its presence in a variety of experimental systems. Moreover, selective effects of TGF-beta 3 have been identified in inhibition of the growth of hematopoietic cells, in migration of fetal fibroblasts, and in inhibiting the survival of certain neuronal cells. Using the chimeric TGF-beta constructs, we have been able to deduce that the middle third of the TGF-beta molecule is sufficient to confer isoform-specific biological activity. In a related aspect of this project, we are exploring the effects of the different TGF-beta isoforms on protection of cardiac myocytes from damage resulting from the action of growth factors such as interleukin-I and tumor necrosis factor-alpha.
