Platelet-derived growth factors and their receptors: structural and functional perspectives.

The four types of platelet-derived growth factors (PDGFs) and the two types of PDGF receptors (PDGFRs, which belong to class III receptor tyrosine kinases) have important functions in the development of connective tissue cells. Recent structural studies have revealed novel mechanisms of PDGFs in propeptide loading and receptor recognition/activation. The detailed structural understanding of PDGF-PDGFR signaling has provided a template that can aid therapeutic intervention to counteract the aberrant signaling of this normally silent pathway, especially in proliferative diseases such as cancer. This review summarizes the advances in the PDGF system with a focus on relating the structural and functional understandings, and discusses the basic aspects of PDGFs and PDGFRs, the mechanisms of activation, and the insights into the therapeutic antagonism of PDGFRs. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.

Transforming growth factor-beta 1 regulates platelet-derived growth factor receptor beta subunit in human liver fat-storing cells.

Activated liver fat-storing cells (FSC) are known to play a key role in the development of liver fibrosis. An important element in FSC activation process is the increased expression of receptors for platelet-derived growth factor (PDGF), a potent mitogen for FSC. The aim of the present study was to evaluate the expression PDGF-receptor alpha and beta subunits in cultured human FSC and their regulation induced by transforming growth factor-beta 1 (TGF-beta), a cytokine potentially involved in an autocrine loop. TGF-beta induced a significant increase of the mitogenic effect of PDGF-BB and did not affect the mitogenicity of PDGF-AA and PDGF-AB, suggesting a selective action of the PDGF-receptor-beta subunit. This hypothesis was confirmed by regulation experiments showing selective and time-dependent upregulation of the messenger (m)RNA encoding for the PDGF-receptor-beta subunit and the relative protein induced by TGF-beta. In addition, binding studies showed a parallel increase of PDGF-BB binding sites after incubation of human FSC with TGF-beta. These studies provide evidence for an additional mechanism leading to the perpetuation of FSC activation and proliferation and contribute to a better understanding of the role of TGF-beta and PDGF in the development of liver fibrosis.

Differential regulation of neuronal sodium channel expression by endogenous and exogenous tyrosine kinase receptors expressed in rat pheochromocytoma cells.

The biological activity of growth factors that act through receptor tyrosine kinases (RTKs) can differ dramatically, depending on both the properties of the RTKs and the cellular environment in which the RTKs are expressed. To determine the ability of different RTKs to elicit ras-independent responses central to neuronal differentiation, we analyzed voltage-dependent sodium (Na) channel expression in rat pheochromocytoma (PC12) cells after activation of a variety of endogenously and exogenously expressed RTKs. In PC12 cells expressing trkB (Ip et al., 1993), the increase in Na current density caused by brain-derived neurotrophic factor (BDNF) was similar to that observed upon activation of endogenous trkA by NGF. BDNF also increased type II Na channel mRNA expression, as did neurotrophin-3 in PC12 cells expressing trkC (Tsoulfas et al., 1993). In contrast, insulin did not increase type II Na channel mRNA expression or Na current density in PC12 cells, while epidermal growth factor (EGF) elicited small, yet reproducible increases in type II Na channel mRNA and Na current density when compared to NGF, even upon coexpression of an EGF receptor/p75 receptor chimera (Yan et al., 1991). Finally, in PC12 cells expressing beta-platelet-derived growth factor (PDGF) receptors (Heasley and Johnson, 1992), PDGF increased type II Na channel mRNA and Na current density to the same extent as NGF. The results show the capabilities of these RTKs in eliciting Na channel expression and the specificity arising due to differences in their intrinsic properties.

Expression of platelet-derived growth factor in a model of acute liver injury.

Platelet-derived growth factor has been shown to play an important role in the repair process after acute tissue injury and in the pathogenesis of several fibrogenic disorders. The aim of this study was to evaluate whether increased expression of platelet-derived growth factor and its beta-receptor subunit occurs in a model of acute liver injury. Male Sprague-Dawley rats were given a single intragastric dose of carbon tetrachloride and killed at intervals of 24, 48 and 72 hr and 1 wk. Control animals were included in each group. Platelet-derived growth factor-B chain mRNA expression, analyzed by RNase protection assay, was not detectable in control samples or in samples obtained 24 hr or 1 wk after carbon tetrachloride. However, the presence of protected fragments of 130 kb was clearly detected after 48 hr and was still present, although less abundant, after 72 hr. The distribution of platelet-derived growth factor protein in liver tissue sections, evaluated by immunohistochemistry, was restricted to centrilobular veins and portal tracts in normal liver. In carbon tetrachloride-treated rats, prominent staining was observed in areas corresponding to hepatocellular necrosis and inflammatory infiltration. This feature, already present at 24 hr after carbon tetrachloride, became more marked at 48 hr with a gradual resolution after 72 hr. The expression of platelet-derived growth factor-receptor beta-subunit mRNA, evaluated by in situ hybridization, was markedly increased after carbon tetrachloride with a peak at 24 hr and was mainly localized over mesenchymal cells in the hepatic sinusoids.(ABSTRACT TRUNCATED AT 250 WORDS)

Platelet-derived growth factor (PDGF)-AB-mediated phosphorylation of PDGF beta receptors.

Platelet-derived growth factor (PDGF) stimulates the proliferation of Balb/c-3T3 fibroblasts through binding and subsequent activation of PDGF receptors. Activation of the PDGF receptors has been proposed to involve receptor dimerization. PDGF-AB has been shown to bind PDGF alpha and beta receptor subunits to form PDGF alpha beta and alpha alpha receptor dimers. In this paper we demonstrate that, following the down-regulation of PDGF alpha receptors, the binding of PDGF-AB to beta receptors occurred at 37 degrees C but not at 4 degrees C. PDGF-AB stimulated the phosphorylation of PDGF beta receptor monomers in cells depleted of PDGF alpha receptors by prior exposure to PDGF-AA.

Actions of platelet-derived growth factor isoforms in mesangial cells.

Platelet-derived growth factor (PDGF) occurs as homodimers or heterodimers of related polypeptide chains PDGF-BB, -AA, and -AB. There are two receptors that bind PDGF, termed alpha and beta. The beta receptor recognizes PDGF B chain and is dimerized in response to PDGF BB. The alpha receptor recognizes PDGF B as well as A chains and can be dimerized by the three dimeric forms of PDGF AA, AB, and BB. To characterize PDGF receptor signaling mechanisms and biologic activities in human mesangial cells (MC), we explored the effects of the three PDGF isoforms on DNA synthesis, phospholipase C activation, and PDGF protooncogene induction. PDGF-BB homodimer and AB heterodimer induced a marked increase in DNA synthesis, activation of phospholipase C, and autoinduction of PDGF A and B chain mRNAs, whereas PDGF-AA homodimer was without effect. The lack of response to PDGF AA could be accounted for by down-regulation of the PDGF-alpha receptor since preincubation of MC with suramin restored PDGF AA-induced DNA synthesis. Ligand binding studies demonstrate specific binding of labeled PDGF BB and AB and to a lower extent PDGF AA isoforms to mesangial cells. These results are consistent with predominant expression of PDGF beta receptor in MC, which is linked to phospholipase-C activation. The potent biologic effects of PDGF-AB heterodimer in cells that express very few alpha receptors and do not respond to PDGF AA are somewhat inconsistent with the currently accepted model of PDGF receptor interaction and suggest the presence of additional mechanisms for PDGF isoform binding and activation.

Biosynthesis and processing of the platelet derived growth factor type alpha receptor.

The homodimers (AA, BB) of the platelet derived growth factor (PDGF) differentially interact with two highly related PDGF receptors (alpha, beta) that appear to mediate different functional responses in different cell types. To seek a basis for these apparent functional differences, we investigated the processing of the PDGF alpha-receptor. The PDGF alpha-receptor is rapidly glycosylated to a 160 kD form and undergoes a number of intermediate glycosylation steps that result in a mature form of 185 kD that appears at the cell surface within 60-90 minutes. The alpha receptor has a half-life of approximately 4 1/2 hours without and approximately 20 minutes in the presence of ligand. The processing steps of the alpha-receptor are similar to the processing of the PDGF beta receptor, suggesting that differential binding of signalling molecules to activated receptors may be responsible for the apparent functional differences in cellular responses to PDGF AA and PDGF BB.

Chrysotile asbestos upregulates gene expression and production of alpha-receptors for platelet-derived growth factor (PDGF-AA) on rat lung fibroblasts.

PDGF isoforms have been postulated to serve as mediators of fibroblast proliferation and chemotaxis during lung fibrogenesis induced by asbestos inhalation. We have studied the interaction of chrysotile asbestos fibers with rat lung fibroblasts (RLF) in vitro and the consequent changes in PDGF receptor mRNA expression, PDGF binding, and mitogenic activity of PDGF isoforms. Northern blot analysis revealed that mRNA for the PDGF-receptor alpha subtype (PDGF-R alpha) on RLF was upregulated after a 24-h exposure to asbestos in culture (0.5-15 micrograms fibers/cm2). [125I]PDGF-BB receptor assays showed that normal RLF possess mainly PDGF-R beta and a paucity of PDGF-R alpha. In agreement with the Northern data, saturation binding of [125I]PDGF-BB to RLF exposed to asbestos demonstrated an approximately 40% increase in binding sites accompanied by a twofold decrease in receptor affinity. Treating asbestos-exposed RLF with PDGF-AA, which binds only PDGF-R alpha, blocked the PDGF binding sites that were upregulated by fiber exposure. PDGF-AA had increased mitogenic potency for fiber-exposed RLF, but PDGF-BB was a less potent mitogen for these RLF. Nonfibrogenic carbonyl iron spheres induced similar changes in PDGF growth responses. These data show that inorganic particulates alter the PDGF-R alpha population on RLF without significant change in PDGF-R beta.