Measurements of Active TGF Generated by Culture Cells

The transforming growth factors-β (TGF-βs) constitute a family of potent regulators of cellular differentiation, proliferation, migration, and protein expression (1 ,2 ). Three isoforms of TGF-β have been described in mammals: TGF-β1, 2, and 3 (3 –5 ). Most cell lines and tissues secrete TGF-β as a large latent complex formed by three components: TGF-β, LAP (latency-associated protein), and LTBP (latent TGF-β binding proteins). TGF-β is noncovalently associated to its prodomain LAP (6 –8 ), and LAP is disulfide-bonded to LTBP (9 ). Four LTBPs (LTBP-1, 2, 3, and 4) have been described (10 –14 ). Mature TGF-β must be released from the complex to bind to its high-affinity receptor and elicit its biological functions (15 ). This process, called TGF-β activation, appears to be a critical step in the control of TGF-β activity (16 ). An additional regulatory step involved in the activation process is the LTBP-mediated incorporation of latent TGF-β into the extracellular matrix (2 ). Activation of latent TGF-β has been described in various cell systems (17 –19 ). However, the molecular mechanisms involved in extracellular TGF-β activation are not fully understood. It also remains to be elucidated whether latent TGF-β incorporation into the extracellular matrix regulates TGF-β activation in a positive or negative manner (2 ).