The transforming growth factors-beta (TGF-beta 1-5) constitute of a family of multi-functional cytokines that regulate cell growth and differentiation []. Many cells synthesise TGF-beta, and essentially all have specific receptors for this peptide []. TGF-beta regulates the actions of many other peptide growth factors and determines a positive or negative direction of their effects. The protein functions as a disulphide-linked homodimer. Its sequence is characterised by the presence of several C-terminal cysteine residues, which form interlocking disulphide links arranged in a knot-like topology. A similar cystine-knot arrangement has been noted in the structures of some enzyme inhibitors and neurotoxins that bind to voltage-gated Ca2+ channels, although the precise topology differs.TGF-beta genes are expressed differentially, suggesting that the various TGF- beta species may have distinct physiological roles in vivo. Examination of TGF-beta 3 mRNA levels in adult murine tissues indicated that expression is predominant in brain, heart, adipose tissue and testis []. TGF beta-3 mRNA is also observed in adult mouse lung and placenta. Research on rat foetal lung fibroblasts has shown that exposure to cortisol increases TGF-beta 3 mRNA expression in a time- and dose-dependent manner, suggesting that glucocorticoids may mediate their stimulatory effect on lung maturation by inducing TGF-beta 3 expression in foetal lung fibroblasts [].The three-dimensional structures of several members of the TGF-beta super-family have been deduced. The crystal structure of TGF-beta 3 reveals a near identical central core to that of TGF-beta 2 []. The principal differences are witnessed in the conformations of the N-terminal α-helix and in the β-sheet loops, which could account for the individual cellular responses, if these differences are recognised by the TGF-beta receptors.
