Platelet-derived Growth Factor Signaling Pathway Research Articles

Analysis of messenger ribonucleic acid and protein for the ligands and receptors of the platelet-derived growth factor signaling pathway in the placenta, extraembryonic membranes, and uterus during the latter half of murine gestation.

Previous investigation of ligand and receptor messenger RNA (mRNA) expression implicated the platelet-derived growth factor (PDGF) pathway as a participant in the maintenance of pregnancy and fetal development during the first half of murine gestation. We extended these studies using Northern and in situ RNA hybridization and immunohistochemical detection of protein to evaluate the expression kinetics and cell-specific localization of PDGF-A, PDGF-B, PDGF alpha-receptor, and PDGF beta-receptor in mouse placenta, extraembryonic membranes, and uterus during the second half of gestation (days 9.5-18.5). Northern blotting experiments reveal that mRNAs for the PDGF signaling components exhibit unique time-dependent and tissue-specific expression in the placenta and uterus, being progressively and coordinately up-regulated as gestation proceeds. Cell-specific localization of mRNA and protein by in situ hybridization and immunohistochemistry demonstrates widespread expression in multiple cell types of the placenta, gravid uterus, and extraembryonic membranes. Abundant PDGF protein and mRNA expression is exhibited in the nucleated fetal erythroid progenitor cells that originate in the extraembryonic membranes and circulate throughout the developing conceptus. Our data together with those of previous studies demonstrate that PDGF ligands and receptors are globally expressed in many cell types within fetal and maternal tissues during murine gestation and, thus, imply a potential role for PDGF in fetal development and maternal-fetal interactions.

A Requirement for Fibroblast Growth Factor in Regulation of Skeletal Muscle Growth and Differentiation Cannot Be Replaced by Activation of Platelet-Derived Growth Factor Signaling Pathways

The distinct effects of cytokines on cellular growth and differentiation suggest that specific signaling pathways mediate these diverse biological activities. Fibroblast growth factors (FGFs) are well-established inhibitors of skeletal muscle differentiation and may operate via activation of specific signaling pathways distinct from recently identified mitogen signaling pathways. We examined whether platelet-derived growth factor (PDGF)-activated signaling pathways are sufficient to mediate FGF-dependent repression of myogenesis by introducing the PDGF beta receptor into a mouse skeletal muscle cell line. Addition of PDGF-BB to cells expressing the PDGF beta receptor activated the PDGF beta receptor tyrosine kinase, stimulated mitogen-activated protein (MAP) kinase, and increased the steady-state levels of junB and c-fos mRNAs. Despite the activation of these intracellular signaling molecules, PDGF beta receptor activation elicited no detectable effect on cell proliferation or differentiation. In contrast to PDGF-BB, addition of FGF-2 to myoblasts activated signaling pathways that resulted in DNA synthesis and repression of differentiation. Because of the low number of endogenous FGF receptors expressed, FGF-stimulated signaling events, including tyrosine phosphorylation and activation of MAP kinase, could be detected only in cells expressing higher levels of a transfected FGF receptor cDNA. As the PDGF beta receptor- and FGF receptor-stimulated signaling pathways yield different biological responses in these skeletal muscle cells, we hypothesize that FGF-mediated repression of skeletal muscle differentiation activates signaling pathways distinct from those activated by the PDGF beta receptor. Activation of PDGF beta receptor tyrosine kinase activity, stimulation of MAP kinase, and upregulation of immediate-early gene expression are not sufficient to repress skeletal muscle differentiation.

Dissociation of platelet-derived growth factor (PDGF) receptor autophosphorylation from other PDGF-mediated second messenger events

Activated p21ras alters the platelet-derived growth factor (PDGF) signal transduction pathway in fibroblasts by inhibiting autophosphorylation of the receptor as well as by inhibiting the induction of the growth-related genes c-myc, c-fos, and JE. To elucidate the cause and effect relationships between receptor autophosphorylation and other second messenger events in the PDGF signaling pathway we created revertants of v-ras transformed cells by two methods: 1) the use of cAMP analogues, and 2) the introduction of a gene, Krev-1, which has been reported previously to revert ras transformed cells to normal morphology. Analysis of the revertants shows that the PDGF-mediated tyrosine phosphorylation of the 180-kDa PDGF receptor remains inhibited; however, the PDGF-mediated activation of phospholipase C and the induction of the growth-related genes c-myc, c-fos, and JE have been restored. These data suggest the presence of parallel pathways for PDGF signal transduction which are not dependent on autophosphorylation of the PDGF receptor.

Analysis of platelet-derived growth factor receptor domain function using a novel chimeric receptor approach

A chimeric receptor consisting of an epidermal growth factor (EGF) receptor ligand-binding domain and platelet-derived growth factor (PDGF) receptor transmembrane and cytoplasmic signalling domains has been constructed and shown to be fully functional in phosphorylation, mitogenesis, transformation, Ca2+ release, and pH change assays. Expression of this receptor in EGF receptor-deficient, PDGF-responsive NIH 3T3 cells allows the activation of PDGF signalling pathways by EGF. This system was used to examine the function of kinase insertion sequences (KIS). While a mutant with a KIS deletion of 83 amino acids displayed a significant but reduced ability to induce mitogenic, transforming, and Ca2+ release responses in transfected cells, deletion of 20 additional amino acids resulted in abolishment of such activities. This differential loss of signalling potential correlated with the reduced or abolished potential of these receptor mutants to phosphorylate cellular substrates such as PLC gamma. Our results suggest an integral role for KIS in PDGF receptor cytoplasmic domain conformation and an involvement in substrate interaction, but provide no evidence for an exclusive role of KIS in the mediation of biological signals.

PDGF β-receptor stimulates tyrosine phosphorylation of GAP and association of GAP with a signaling complex

Platelet-derived growth factor (PDGF) stimulated the tyrosine phosphorylation of the GTPase activating protein (GAP) in 3T3 cells and in CHO cells expressing wild-type PDGF receptors, but not in several CHO cell lines expressing mutant receptors defective in transmitting mitogenic signals. Following PDGF treatment of cells, GAP physically associated with the PDGF receptor and with Raf-1, phospholipase c-γ, and Pl-3 kinase, suggesting that PDGF induced the formation of complexes of signaling molecules. The association of GAP with the PDGF receptor and the phosphorylation of GAP were reconstituted in vitro using purified protein and in insect cells expressing murine PDGF receptor and human GAP. However, in cells transformed by activated c-Ha- ras, which are defective in certain responses to PDGF, GAP failed to associate with the PDGF receptor or increase its phosphotyrosine content in response to PDGF. The association of GAP with ligand-activated PDGF receptors may directly link PDGF and ras signaling pathways.