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TGF-beta 3 （transforming growth factor-beta 3） is a member of a TGF-beta superfamily subgroup that is defined by their structural and functional similarities （1-5）. TGF-beta 3 and its closely related proteins, TGF-beta 1 and --beta 2, act as cellular switches to regulate immune function, cell proliferation, and epithelial--mesenchymal transition （4, 6, 7）. The non-redundant biological effects of TGF-beta 3 include involvement in palatogenesis, chondrogenesis, and pulmonary development （1, 2, 7-9）. Human TGF--beta 3 cDNA encodes a 412 amino acid （aa） precursor that contains a 20 aa signal peptide and a 392 aa proprotein. The proprotein is processed by a furin-like convertase to generate a 220 aa latency-associated peptide （LAP） and a 112 aa mature TGF--beta 3 （10, 11）. Mature human TGF--beta 3 shows 100%, 99%, and 98% aa identity with mouse/dog/horse
, rat, and pig TGF--beta 3, respectively. TGF-beta 3 is secreted as a complex with LAP. This latent form of TGF-beta 3 becomes active upon cleavage by plasmin, matrix metalloproteases, thrombospondin-1, and a subset of integrins （12）. TGF-beta 3 binds with high affinity to TGF-beta RII, a type II serine/threonine kinase receptor. This receptor then phosphorylates and activates type I serine/threonine kinase receptors, TGF--beta RI or ALK--1, to modulate transcription through Smad phosphorylation
（13-15）. The divergent biological effects exerted by individual TGF-beta isoforms is dependent upon the recruitment of co-receptors （TGF--beta RIII and endoglin） and the subsequent initiation of Smad--dependent or -independent signaling pathways （14, 16, 17）.