At first, proteins in the TGF-β family were isolated as a group of compounds called by them the sarcoma growth factor (SGF) at the end of the 1970s. However, later studies showed that SGF is a mixture of two different compounds, transforming growth factor alpha (TGF-α) and transforming growth factor beta (TGF-β). TGF-β family is consisting of three isoforms, TGF-β1, TGF-β2 and TGF-β3. Their structure is formed by interrelated dimeric polypeptide chains.
TGF-β family is related to many cell processes, including proliferation, differentiation, and migration. The TGF-β family is considered to play key roles in control of physiological phenomena about maintenance of metabolic homeostasis in the bone tissue. Once the continuity of bone tissue is broken, a bone healing process is started, and that is considered a recapitulation process of embryonic intracartilaginous ossification. Many in vivo and in vitro studies in kinds of models involving cytokines and growth factors show that TGF-β is essential factors in the fracture healing process.
An active form of TGF-β1 compound found in human tissues after a complete post-translation processing is a homodimer consisting of two polypeptide chains. Every peptide contains 112 amino acid residues and they are combined by a disulphide bond to form a 25 kDa complex.
The TGF-β signaling is one of the major pathways essential for fundamental cell fate decisions during embryonic development and adult tissue homeostasis. These proteins are initially released into the extracellular matrix as latent pre-propeptide precursors. After activation through proteolysis, the ligands dimerize and elicit the assembly of heteromeric receptor complexes. Once activated, the receptor in turn propagates the signal through intracellular transcription factors known as Smad proteins, the primary intracellular proteins used for TGF-β signal transduction.