TGF-beta 2 is synthesized as a prepro-cytokine with a 19 amino acid (aa) signal sequence, a
283 aa pro-region, and a 112 aa mature segment (1-5). It dimerizes with formation of disulfide
bonds between the 'pro' regions and disulfide bonds between the 'mature' regions. The mature
region is 71% and 80% identical with human TGF-beta 1 and TGF-beta 3 (6, 7) and 97% identical with
the corresponding mouse protein (8). After proteolytic cleavage of the disulfide-linked mature
region, it remains hydrogen-bonded to the disulfide-linked prosegments (LAP or latencyassociated protein) (1, 2, 9). If secreted in this form, LAP keeps TGF-beta 2 in an inactive state until
dissociation, caused by proteases, glycosidases, or extreme pH (2, 9). In many types of cells,
an additional protein, latent TGF-beta binding protein (LTBP), is covalently linked to the LAP
homodimer prior to secretion. LTBP, a 130 kDa cysteine-rich glycoprotein, creates a 235 kDa
large latent complex that is secreted, most likely binding to the extracellular matrix (1, 9-11).
The latency components are believed to act as natural antagonists of TGF-beta activity, to target
TGF-beta to distinct tissues, and to maintain a reservoir of TGF-beta (1, 2, 12). On release from latency,
active homodimeric TGF-beta can bind to cell-surface receptors or to other proteins, such as
alpha
2-macroglobulin (13).
The signal transducing receptor for TGF-beta 2 is a heterotetrameric complex of two type I
signal-transducing receptors (53 kDa; TGF-beta RI) and two type II ligand-binding receptors
(75-85 kDa; TGF-beta RII) (14-19). The binding of TGF-beta 2 appears to initially involve a type III TGF-beta
receptor, either 300 kDa betaglycan (20) or 180 kDa endoglin (14, 21), which then "hands off"
to TGF-beta RII.
TGF-beta 2 is expressed by a variety of cells, including osteoclasts, thymic epithelium,
keratinocytes, hepatocytes, chief cells of the stomach, satellite cells, skeletal muscle cells,
prostatic epithelium, bronchial epithelium, neurons and astrocytes, fibroblasts and visceral
smooth muscle, and macrophages (14-19). TGF-beta 2 has marked cross-species bioactivity
(e.g., human TGF-beta 2 is active on mouse cells (33), while porcine TGF-beta 2 is active on rabbit
cells (34)). TGF-beta 2 has four fundamental activities: it is a growth inhibitor for most types of
cells; it enhances the deposition of extracellular matrix; it is immunosuppressive, suppressing
APC expression of both IL-12 and CD40L while upregulating IL-10 secretion; and during fetal
development, it is expressed in discrete areas, such as epithelium, myocardium, cartilage and
bone of extremities and in the nervous system, suggesting specific functions (1, 35-37).