Progress in growth factor research最新文献

Vascular endothelial growth factor (VEGF) is a highly specific mitogen for vascular endothelial cells and an angiogenic factor that is structurally related to platelet derived growth factor (PDGF). It is also known as the vascular permeability factor (VPF) because it efficiently potentiates the permeabilization of blood vessels. Five types of VEGF mRNA encoding VEGF species which differ in their molecular mass and in their biological properties are transcribed from a single gene as a result of alternative splicing. VEGFs are produced and secreted by several normal cell types including smooth muscle, luteal and adrenal cortex cells. VEGFs are also produced by different tumorigenic cells, and appear to play a major role in tumour angiogenesis. Antibodies directed against VEGF can inhibit the growth of a variety of VEGF producing tumours. Of the various VEGF species, the best characterized is the 165 amino acid long form (VEGF₁₆₅). VEGF₁₆₅ is a heparin binding growth factor, and its interaction with VEGF receptors on the cell surface of vascular endothelial cells depends on the presence of heparin-like molecules. Several cell types which do not proliferate in response to VEGF such as bovine corneal endothelial cells, HeLa cells and human melanoma cells also express cell surface VEGF receptors, but the function of the VEGF receptors in these cells is unclear. Recently, the tyrosine-kinase receptors encoded by the flt and $\text{KDR}\text{flk-1}$ genes were found to function as VEGF₁₆₅ receptors.

Cytokines are multifunctional regulators of cell behaviour affecting such diverse activities as cell proliferation, gene expression and motility. Matrix macromolecules influence a similarly wide range of cell functions. A review of the available literature suggests that cytokines may affect cell motility by (a) directly influencing the motility apparatus, and (b) indirectly as a consequence of the altered expression of genes coding for matrix macromolecules, their respective cell surface receptors and matrix degrading enzymes and their inhibitors. Conversely, the composition and supramolecular organisation of the matrix plays a central role in defining cellular response to potentially multifunctional cytokines. Such complex and reciprocal interactions between cytokines and the matrix elicit both positive and negative reiterative feedback loops which must be taken into account when interpreting the results of migration assays in vitro and extrapolating them to in vivo processes.

A new family of transmembrane receptors that contain intracellular serine/threonine kinase domains is emerging. Ligands for this class of receptors include members of the transforming growth factor-β (TGF-β) superfamily, e.g. TGF-βs and activins. TGF-βs exert their effects on target cells via formation of heteromeric serine/threonine kinase complexes (TGF-β type I and type II receptors). Other components, i.e. TGF-β type III receptor and endoglin, appear to have more indirect roles, e.g. to present ligands to the signalling receptors. Given the structural similarity between members of the TGF-β superfamily, other ligands in this family may act through structurally and functionally similar serine/threonine kinase receptors.

The peptide hormone angiotensin II (AngII) has clearly defined physiologic roles as a regulator of vasomotor tone and fluid homeostasis. In addition AngII has trophic or mitogenic effects on a variety of target tissues, including vascular smooth muscle and adrenal cells. More recent data indicate that AngII exhibits many characteristics of the ‘classical’ peptide growth factors such as $\text{EGF}\text{TGFα}$, PDGF and IGF-1. These include the capacity for local generation (‘autocrine or paracrine’ action) and the ability to stimulate tyrosine phosphorylation, to activate MAP kinases and to increase expression of nuclear proto-oncogenes. The type 1 AngII receptor, which is responsible for all known physiologic actions of AngII, has been cloned. Activation of this receptor leads to elevated phosphoinositide hydrolysis, mobilization of intracellular Ca²⁺ and diacylglycerol, and activation of $\text{Ca}{}^{\mathrm{2+}}\text{calmodulin}$ and $\text{Ca}{}^{\mathrm{2+}}\text{phospholipid}\text{-}\text{dependent}$$\text{Ser}\text{Thr}$ kinases, as well as Ca²⁺ regulated tyrosine kinases. The existence of other AngII receptor subtypes has been postulated, but the function(s) of these sites remains unclear. In vascular smooth muscle, AngII can promote cellular hypertrophy and/or hyperplasia, depending in part on the patterns of induction of secondary factors that are known to stimulate (PDGF, IGF-1, basic FGF) or inhibit (TGF-β) mitosis. Together, these findings have suggested that AngII plays important roles in both the normal development and pathophysiology of vascular, cardiac, renal and central nervous system tissues.

The three isoforms of PDGF bind with different affinities to two related tyrosine kinase receptors, denoted the PDGF α- and β-receptors. Ligand binding induces receptor dimerization, creating receptor homo- or heterodimers. Dimerization is accompanied by, and might be a prerequisite for, receptor autophosphorylation and kinase activation. Receptor autophosphorylation serves to regulate the kinase activity and to create binding sites on the receptor molecule for downstream signalling components. The activities of the signalling components are ultimately manifested as specific biological responses. All the currently described PDGF receptor-binding components, e.g. phospholipase C-γ, members of the src family of cytoplasmic tyrosine kinases, the rasGT-Pase activating protein and p85, the regulatory subunit of phosphatidylinositol 3′ kinase, contain a conserved src homology 2-domain, through which the association with the receptor takes place. The receptor-binding components appear to either possess an intrinsic enzymatic activity, or they function as adaptors, which may complex with catalytically active components. For most receptor-binding components, there is insufficient understanding of how binding to the receptor affects the catalytic function. Certain of these components become tyrosine-phosphorylated, i.e. they are substrates for the receptor tyrosine kinase. Moreover, the change in subcellular localization, which most of the receptor binding components undergo in conjunction with receptor binding, could play a critical role. The current efforts of many laboratories are aimed at delineating different PDGF receptor signal transduction pathways and what roles the different receptor-binding components play in the establishment of these pathways.

The stimulation of cellular metabolism by the nine fibroblast growth factors (FGFs) is mediated by a dual-receptor system. This comprises a family of four receptor tyrosine kinases (FGFR) and heparan sulphate proteoglycans (HSPG). The stimulation of cell division by FGFs has an obligate requirement for both partners of the dual-receptor system. The binding of the nine FGFs to the FGFRs is marked by a pattern of overlapping specificity despite alternative splicing events generating a large number of FGFR proteins. Thus many of the FGFR isoforms bind several FGFs. It is likely that each FGF requires a different pattern of sulphation within the heparan sulphate chains for binding. Therefore, the HSPG receptors may provide additional specificity, allowing a cell to fine tune its response to the FGFs present in the extracellular milieu. The HSPG receptors also control the availability of FGFs and hence regulate the transport of FGFs within a tissue. FGF-stimulated cell division would appear to have a mandatory requirement for the FGFs to be translocated to the nucleus via the cytosol after interacting with the dual-receptor system. The consequences of the potential direct action of FGFs in stimulating cell division are examined in the light of current models of signal transduction.

MAP kinases (MAPK) are serine/threonine kinases which are activated by a dual phosphorylation on threonine and tyrosine residues. Their specific upstream activators, called MAP kinase kinases (MAPKK), constitute a new family of dual-specific threonine/tyrosine kinases, which in turn are activated by upstream MAP kinase kinase kinases (MAPKKK). These three kinase families are successively stimulated in a cascade of activation described in various species such as mammals, frog, fly, worm or yeast.

In mammals, the MAP kinase module lies on the signaling pathway triggered by numerous agonists such as growth factors, hormones, lymphokines, tumor promoters, stress factors, etc. Targets of MAP kinase have been characterize tin all subcellular compartments. In yeast, genetic epistasis helped to characterize the presence of several MAP kinase modules in the same system. By complementation tests, the relationships existing between phylogenetically distant members of each kinase family have been described. The roles of the MAP kinase cascade have been analyzed by engineering various mutations in the kinases of the module. The MAP kinase cascade has thus been implicated in higher eukaryotes in cell growth, cell fate and differentiation, and in low eukaryotes, in conjugation, osmotic stress, cell wall constrct and mitosis.