Measurement of Active TGF-β Generated by Cultured Cells

The transforming growth factors-β (TGF-βs) constitute a family of potent regulators of cellular differentiation, proliferation, migration, and protein expression (,). Three isoforms of TGF-β have been described in mammals: TGF-β1, 2, and 3 (, , ). 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 (, , ), and LAP is disulfide-bonded to LTBP (). Four LTBPs (LTBP-1, 2, 3, and 4) have been described (, , , , ). Mature TGF-β must be released from the complex to bind to its high-affinity receptor and elicit its biological functions (). This process, called TGF-β activation, appears to be a critical step in the control of TGF-β activity (). An additional regulatory step involved in the activation process is the LTBP-mediated incorporation of latent TGF-β into the extracellular matrix (). Activation of latent TGF-β has been described in various cell systems (, , ). 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 ().