TGF-β family
The transforming growth factor beta (TGF-β) group of proteins is made up of at least five multifunctional proteins that have functions ranging from modification of the ECM, to induction of differentiation of target cells, to stimulation of proliferation in multiple cell types and tissues.
Based on continuing genetic and molecular studies, it is now known that TGF-βs are only part of a superfamily of at least 40 members structurally related proteins that include the TGF-βs, activins/inhibins (important in gamete production, i.e., first identified based on their activity in regulation of FSH secretion), and bone morphogenetic proteins (BMPs). At least 28 genes encode for various elements of this family of proteins and companion receptors. Three of the variants, TGF-βl, TGF-β2, and TGF-β-3, stimulate connective tissue formation and are chemotactic for fibroblasts. They can indirectly promote proliferation of mesenchymal cells but can inhibit growth of epithelial cells in vivo and in vitro. These varied effects of TGF-βs suggests that they are likely important in tissue development and function. TGF-βl is the best described of these proteins related to mammary function. Blood platelets provide the most concentrated source of TGF-βl, but it is believed to be produced by nearly every cell type in the body. Biologically active TGF-βl is a 25-kDa disulfide-linked homodimer. When secreted, it is bound to a large 75-kDa glycoprotein called the latency-associated peptide (LAP). Activation of the latent form by proteases, alka- Iinization, or chaotropic agents is necessary for TGF- βl to bind to its receptor, so that control of this reaction is an important regulator of TGF-βl action. Unregulated epithelial cell proliferation is obviously an undesirable trait of tumor formation, so it should not be surprising that activity-growing cells must be controlled to prevent hyperplasia. For example, most of the mammary-associated effects of TGF-βs are inhibitory (Plaut et al., 2003).Effects of TGF-β are mediated by binding to specific cell surface receptors (designated type I, II, or III receptors) present on most cell types. The type I and type II receptors are directly involved in signal transduction, while the type III receptor is thought to enhance binding of TGF-β to one of the other receptor subtypes. In heifers, for example, the ductal epithelial cells of the mammary gland show extensive presence of type I and type II receptors by immunocytochemical localization of antibody to the receptors.
The specific role of TGF-βl in ruminant mammary development is unknown but concentrations of TGF- βl in serum ranged from 7 to 30ng∕mL and receptors for TGF-βl are increased during the peripubertal period corresponding with rapid mammary development. Related studies show that TGF-βl inhibits the proliferation due to addition of IGF-I, IGF-II, des (l-3)-IGF-I, EGF, or amphiregulin. TGF-βl also affects the morphology of bovine mammary organoids in culture (Ellis et al., 2000). The possibility that IGFBP-3 (or fragments) might have IGF-I receptor-independent actions in mammary cells, via binding to the type V TGF-β receptor, coupled with TGF-β induction of IGFBP-3, makes for an intriguing overlap between the growth-stimulating actions of the IGF-I axis and the inhibitory effects of the TGF-β family of molecules. TGF-β members are important in early embryonic development and maintenance of homeostasis in adult tissue by affecting cell growth, differentiation of epithelial cells, and apoptosis.
The signaling cascade for TGF-β, like that of the other growth factors, involves binding of the ligand to cell surface receptors. These receptors are transmembrane serine/threonine kinases. A current model is that binding induces the creation of a receptor complex composed of the type I and type II receptor.
Receptor II then acts to phosphorylate receptor I. The phosphorylated form of receptor I is activated to generate the intracellular signal responsible for the effect of the growth factor. Specifically, cytoplasmic Smad proteins in target cells are substrates for the activated receptor and these serve as signaling modulators. Interestingly, Smad proteins were named based on work that arose from comparative molecular studies. Specifically, Drosophila geneticists isolated a gene called Mad while others working on Caenorhabditis elagans identified a gene they called Sma. It was soon realized that these were the same gene products so that a combined naming convention was evolved—Smad. Smads are very widely expressed throughout development and in virtually all tissues.Regulation of specific genes can be either positive or negative, depending on the conditions specific to a particular target cell. In TGF-β signaling, I-Smads block signaling by recruiting so-called Smurf ubiquitin ligases to capture various Smad proteins and target then for degradation. Generally, the activity of many cellular proteins is controlled by selective proteolysis through the ubiquitin-proteasome pathway. In summary, Smads can be thought of as transcriptional co-modulators whose activity is controlled by various receptors of the TGF-β superfamily of receptors via induction of nuclear accumulation of Smads (Moustakas et al., 2003).