The Biology of TGF-beta
Transforming growth factor beta (TGF-beta) was originally identified, and obtained its name, from studies on the soluble factors present in cell culture media that could induce changes in cells that were characteristic of transformation (Cancer Res. 42: 4776). It has since been established that TGF-beta has effects on the proliferation, differentiation, migration and apoptosis of many cell types, and is involved in many biological processes and implicated in a number of pathological processes, including atherogenesis.
The pleiotropic effects of TGF-beta necessitate very tight control of its activity. TGF-beta is among the most highly regulated of all proteins, with complex regulatory mechanisms operating at all steps from transcription through to signaling events downstream of receptor binding. Regulation of TGF-beta activity has been a focus of work in the laboratory, and we have a particular interest in the regulation of TGF-beta production and activation.
Five isoforms of TGF-beta have been identified and these belong to the TGF-beta superfamily of proteins; a family of cytokines that share a unique structural motif known as the cysteine knot. Members of this superfamily are produced as latent preproproteins that must undergo proteolytic processing, mediated by furin or other members of the proprotein convertase family, to generate the active ligands that are capable of receptor binding. In the case of TGF-beta, an additional activation step is required to release the prodomain, termed the latency associated peptide or LAP, from the active protein domain. Many activators of TGF-beta have been identified, including interaction with other proteins (there is convincing evidence that interactions with integrins are critical for TGF-beta1 activation (J. Cell. Biol. 176:787)), enzymatic processes (such as proteolysis within the LAP region) and physiochemical conditions (such as extremes pf pH and temperature). Which of these mechanisms of activation are physiologically relevant may be context dependent. The active proteins of the three mammalian TGF-beta isoforms (TGF-beta1, 2 and 3) are highly homologous and have similar biological activities, but are differentially expressed and regulated. TGF-beta1, which is an essential regulator of VSMCs and cells of the immune system, has been the focus of most, but not all, of our research.
In our laboratory, Dr Gemma Bourne is currently extending the work she carried out for her PhD thesis, investigating the proteolytic processing step in TGF-beta1 production; an under-researched area in the field of TGF-beta biology. It has previously been assumed that this processing step occurs in a constitutive manner and intracellularly, during transit through the secretory pathway. However, Gemma has shown that, in culture, most cells secrete proTGF-beta1, which can be processed by recombinant furin to generate active TGF-beta1. This raises questions about the role of proTGF-beta and mechanisms of processing in vivo. Using a variety of techniques, including a functional assay to measure furin activity, work is ongoing to determine whether proteolytic processing of proTGF-beta is a step at which TGF-beta activity can be modulated, and how this step is normally achieved and regulated.
Cells secrete proTGF-beta1 that can be processed by recombinant furin in vitro and activated to generate active TGF-beta1.