Three forms of TGFβ are found in mammals, encoded by three separate genes. TGFβ-1, TGFβ-2, and TGFβ-3 have distinctive physical and biological properties but share the same general structure and are synthesized and secreted by the same pathway. These are shown in Figure 1. Pro-TGFβ1 is a peptide of 390 amino acids (TGFβ2 and TGFβ-3 are 24 and 22 amino acids longer, respectively) of which the first 23 comprise the signal sequence directing it to the proper site for further processing. Proteolytic processing results in the cleavage of the remaining transcript into the latency associated peptide, LAP, and the mature monomer of TGFβ. Four intramolecular disulfide bonds give this molecule its characteristic cysteine knot structure and a dimer is formed through the formation of an interchain disulfide bond near the carboxyl termini of the monomers. LAP also forms a homodimer with the formation of two adjacent disulfide bonds. While still within the cell, these two dimers associate with each other to form the small latency complex, as shown in Figure 1B. In this complex, stabilized by noncovalent interactions, TGFβ is surrounded by LAP and unavailable to interact with any receptors it might encounter when secreted; i.e., its activity is latent. As this complex is secreted, a third protein, the latent TGFβ binding protein (LTBP), is added forming the large latent complex. The N-terminal region of LTBP binds to the extracellular matrix of cells surrounding the site of TGFβ biosynthesis. This serves as a storage form of TGFβ so that the regulation of the availability of active hormone occurs at the step of its release from the latent complex.
Fig1. Formation and activation of TGFβ. A. Processing of pro-TGFβ. The primary transcript of TGFβ-1 is 390 amino acids long. The signal peptide directs it to the rough endoplasmic reticulum (RER) where it is cleaved by the proteolytic enzyme furin into the 112 amino acid TGFβ-1 (green) and a 257 amino acid peptide, known as latency-associated peptide, or LAP (blue). Dimers of each of the peptides are formed through one (TGFβ-1) or two (LAP) adjacent disulfide bonds. The pathways for TGFβ-2 and TGFβ-3 are the same as those for TGFβ-1. B. Formation of latent complexes, secretion and activation of TGFβ. While still in the RER, the small latent complex is formed, sequestering TGFβ (purple) within the LAP (red). This complex is secreted along with another peptide, the latent TGFβ-binding protein, LTBP (green), forming the extracellular large latent complex. The LTBP binds to the extracellular matrix of the cell of origin or nearby cells, particularly at its carboxyl terminal, forming a reservoir of inactive TGFβ. Depending on the cell types and regulatory events involved, the latent complex can be opened, releasing active TGFβ to bind to specific cell receptors (Figure 2). TSP, thrombospondin-1; ROS, reactive oxygen species (from irradiation).
Fig2. TGFβ signaling. TGFβ, like other members of its family, interacts sequentially with two different types of receptor, TGFβRI and TGFβRII. The type 1 receptor, ALK-5, is one of seven ALK (Activin receptor-Like Kinase) receptor proteins that serve as the Type I receptors for other members of the TGFβ family of growth factors (see Table 1). Dimerized active TGFβ binds to a dimer of TGFβRII (yellow), which recruits a dimer of TGFβRI (blue); the heterotetrameric complex is stabilized by the bound ligand. The constitutively active serine/threonine kinase of the Type II receptor phosphorylates the Type I receptor, which phosphorylates Smads 2 and 3, two of the regulatory Smads. These are joined by a co-regulating Smad, Smad 4, in a heterotrimer which enters the nucleus and interacts with transcriptional co-activators and co-repressors to modulate, along with other transcription factors, the rate of gene transcription.
Table1. TGFβ Family: Typical Signaling Components a
Several mechanisms exist to accomplish this release of TGFβ, depending on the target cells and the regulatory events to which they are subject. These include various proteolytic events, reactive oxygen species produced in response to radiation, and interaction of the complex with extracellular matrix proteins such as thrombospondin or cell surface integrins that can open the complex releasing TGFβ. Other signaling molecules, such as BMPs, can also play a specific role in releasing and therefore activating TGFβ at its site of action. This regulation of the activation of TGFβ is often the most important step, compared to, for example, the rate of transcription, at which the amounts of active growth factor are modulated.
