An update on transforming growth factor-β (TGF-β): sources, types, functions and clinical applicability for cartilage/bone healing.

Transforming growth factor-β (TGF-β) has been reviewed for its sources, types of isoforms, biochemical effects on cartilage formation/repair, and its possible clinical applications. Purification of three isoforms (TGF-β-1, β-2 and β-3) and their biochemical characterization revealed mainly their homo-dimer nature, with heterodimers in traces, each monomer comprised of 112 amino acids and MW. of 12 500 Da. While histo-chemical staining by a variety of dyes has revealed precise localization of TGF-β in tissues, immune-blot technique has thrown light on their expression as a function of age (neonatal vs. adult), as also on its quantum in an active and latent state. X-ray crystallographic studies and nuclear magnetic resonance (NMR) analysis have unraveled mysteries of their three-dimensional structures, essential for understanding their functions. Their similarities have led to interchangeability in assays, while differences have led to their specialized clinical applicability. For this purpose, their latent (inactive) form is changed to an active form through enzymatic processes of phosphorylation/glycosylation/transamination/proteolytic degradation. Their functions encompass differentiation and de-differentiation of chondrocytes, synthesis of collagen and proteoglycans (PGs) and thereby maintain homeostasis of cartilage in several degenerative diseases and repair through cell cycle signaling and physiological control. While several factors affecting their performance are already identified, their interplay and chronology of sequences of functions is yet to be understood. For its success in clinical applications, challenges in judicious dealing with the factors and their interplay need to be understood.