Platelet-derived Growth Factor and its Receptor: Structure and Roles in Normal Growth and Pathology

Mitogen activated protein kinase pathway-dependent effects of platelet-derived growth factor on migration of trophectoderm cells

Successful development of the conceptus and implantation requires an intimate trophic connection between maternal uterus and conceptus mediated by local regulators including growth factors. Platelet-derived growth factor (PDGF) acts as a chemotactic factor for a variety of cell types. Current studies have determined that PDGF participates in rapid growth and development of cleavage stage embryos, but PDGF-induced effects on the growth and development of peri-implantation conceptus remains unknown. In the present study, PDGF induced phosphorylation of ERK1/2, AKT and RPS6 proteins in porcine trophectoderm (pTr) cells in a dose- and time-dependent manner. Addition of U0126 (an inhibitor of ERK1/2) or LY294002 (a PI3K inhibitor) blocked PDGF-induced effects on phosphorylation of signaling proteins. Combinations of PDGF and U0126 decreased PDGF-induced p-ERK1/2 and p-AKT1, but combinations of PDGF and LY294002 blocked only PDGF-induced AKT phosphorylation. Furthermore, PDGF significantly induced pTr cell migration and these stimulatory effects were blocked by U0126 and LY294002. Immunoreactive p-ERK1/2 and p-RPS6 proteins were abundant in pTr cells treated with PDGF, but U0126 reduced PDGF-induced p-ERK1/2 and p-RPS6 levels to basal amounts. Present study suggests that PDGF secreted into the maternal-conceptus microenvironment stimulates pTr cell migration through signal transduction cascades mediated by the ERK1/2 MAPK and AKT1 pathways.

Histone deacetylase-1 is enriched at the platelet-derived growth factor-D promoter in response to interleukin-1beta and forms a cytokine-inducible gene-silencing complex with NF-kappab p65 and interferon regulatory factor-1

Understanding the mechanisms governing cytokine control of growth factor expression in smooth muscle cells would provide invaluable insight into the molecular regulation of vascular phenotypes and create future opportunities for therapeutic intervention. Here, we report that the proinflammatory cytokine interleukin (IL)-1beta suppresses platelet-derived growth factor (PDGF)-D promoter activity and mRNA and protein expression in smooth muscle cells. NF-kappaB p65, induced by IL-1beta, interacts with a novel element in the PDGF-D promoter and inhibits PDGF-D transcription. Interferon regulatory factor-1 (IRF-1) is also induced by IL-1beta and binds to a different element upstream in the promoter. Immunoprecipitation and chromatin immunoprecipitation experiments showed that IL-1beta stimulates p65 interaction with IRF-1 and the accumulation of both factors at the PDGF-D promoter. Mutation of the IRF-1 and p65 DNA-binding elements relieved the promoter from IL-1beta-mediated repression. PDGF-D repression by IL-1beta involves histone deacetylation and interaction of HDAC-1 with IRF-1 and p65. HDAC-1 small interfering RNA ablates complex formation with IRF-1 and p65 and abrogates IRF-1 and p65 occupancy of the PDGF-D promoter. Thus, HDAC-1 is enriched at the PDGF-D promoter in cells exposed to IL-1beta and forms a cytokine-inducible gene-silencing complex with p65 and IRF-1.

Lipoprotein (a) and stroke

Strokes are one of the most common causes of mortality and long term severe disability. There is evidence that lipoprotein (a) (Lp(a)) is a predictor of many forms of vascular disease, including premature coronary artery disease. Several studies have also evaluated the association between Lp(a) and ischaemic (thrombotic) stroke. Several cross sectional (and a few prospective) studies provide contradictory findings regarding Lp(a) as a predictor of ischaemic stroke. Several factors might contribute to the existing confusion--for example, small sample sizes, different ethnic groups, the influence of oestrogens in women participating in the studies, plasma storage before Lp(a) determination, statistical errors, and selection bias. This review focuses on the Lp(a) related mechanisms that might contribute to the pathogenesis of ischaemic stroke. The association between Lp(a) and other cardiovascular risk factors is discussed. Therapeutic interventions that can lower the circulating concentrations of Lp(a) and thus possibly reduce the risk of stroke are also considered.

Oxidized lipoprotein(a) increases the expression of platelet-derived growth factor-B in human umbilical vein endothelial cells

Lipoprotein(a) ¿Lp(a) has been demonstrated to be an independent risk factor for atherosclerosis and restenosis after angioplasty. However, the precise mechanism by which it contributes to the development of both remains unclear. Platelet-derived growth factor (PDGF) is one of the key factors that induce the proliferation and migration of vascular smooth muscle cells. The present study investigated the effects of native and oxidized LDL ¿n-LDL and ox-LDL and Lp(a) ¿n-Lp(a) and ox-Lp(a) on the expression of PDGF-B in cultured human umbilical vein endothelial cells (HUVECs). Results showed that PDGF-B expression was not influenced by n-LDL, but was moderately increased by ox-LDL and n-Lp(a). Ox-Lp(a) was the most potent stimulus for PDGF-B expression, increasing it in HUVECs by 156%+/-18% at 5 nmol/l and 219%+/-42% at 20 nmol/l. Northern Blot analysis demonstrated that the amount of PDGF-B mRNA was markedly increased after treatment with ox-Lp(a) but not n-LDL, ox-LDL and n-Lp(a). These results demonstrate that ox-Lp(a) can elicit PDGF-B expression in HUVECs, which may thereby influence the pathogenesis of atherosclerosis and restenosis after angioplasty.

Angiotensin II (ATII)-inducible platelet-derived growth factor A-chain gene expression is p42/44 extracellular signal-regulated kinase-1/2 and Egr-1-dependent and mediated via the ATII type 1 but not type 2 receptor. Induction by ATII antagonized by nitric oxide

Angiotensin II (ATII) and platelet-derived growth factor (PDGF) are two vasoconstrictors implicated in the maintenance of normal vascular homeostasis. PDGF A-chain levels increase in cultured vascular smooth muscle cells (SMCs) exposed to ATII. The molecular mechanisms underlying this induction are not known. We used transient transfection analysis to show that ATII can increase reporter gene activity driven by fragments of the PDGF-A promoter bearing recognition elements for the transcription factor, Egr-1. Nuclear run-off experiments indicate that ATII induces Egr-1 expression at the level of transcription. Gel shift and supershift studies show that Egr-1 protein accumulates in the nuclei of SMCs exposed to ATII and binds to the proximal region of the PDGF-A promoter in a specific, time-dependent manner. ATII induced extracellular-signal regulated kinase (p42/44 ERK) activity as did phorbol 12-myristate 13-acetate. The specific MEK1/2 inhibitor, PD98059, suppressed both PDGF-A and Egr-1 endogenous and promoter-dependent expression inducible by ATII. The ATII type 1 receptor (AT1) antagonist, Losartan, inhibited ATII-induction of p42/44 ERK, as well as Egr-1 and PDGF-A, whereas neither PD123319, an AT2 receptor antagonist, nor wortmannin, an inhibitor of phosphatidylinositol 3-kinase and c-Jun N-terminal kinase, had any effect. ATII-induction of Egr-1 and PDGF-A was blocked by SIN-1, a NO donor. In addition, this pathway was blocked by overexpression of NO synthase. Collectively, these findings demonstrate that ATII activation of the PDGF-A promoter is mediated via the MEK/ERK/Egr-1 pathway and AT1 receptor and that this process is antagonized by NO.

Interplay of Sp1 and Egr-1 in the proximal platelet-derived growth factor A-chain promoter in cultured vascular endothelial cells

The platelet-derived growth factor (PDGF) A-chain has been implicated in the initiation and progression of vascular occlusive lesions. The elements in the human PDGF-A promoter that mediate increased expression of the gene in vascular endothelial cells have not been identified. A potent inducer of PDGF-A expression in endothelial cells is phorbol 12-myristate 13-acetate (PMA). 5'-Deletion and transfection analysis revealed that a G+C-rich region in the proximal PDGF-A promoter is required for PMA-inducible gene expression. This region bears overlapping consensus recognition sequences for Sp1 and Egr-1. PMA induces Egr-1 mRNA expression within 1 h, whereas PDGF-A transcript levels increase after 2-4 h. Constitutive levels of Sp1 are not altered over 24 h. A specific nucleoprotein complex is formed when an oligonucleotide bearing the G+C-rich element is incubated with nuclear extracts from PMA-treated cells. The temporal appearance of this complex is consistent with the transient increase in Egr-1 transcripts. Antibodies to Egr-1 completely supershift the PMA-induced complex. Interestingly, increased nuclear levels of Egr-1 attenuate the ability of Sp1 to interact with the oligonucleotide, implicating competition between Egr-1 and Sp1 for the G+C-rich element. Binding studies with recombinant proteins demonstrate that Egr-1 can displace Sp1 from this region. Insertion of the G+C-rich element into a hybrid promoter-reporter construct confers PMA inducibility on the construct. Mutations that abolish Egr-1 binding also abrogate expression induced by PMA or overexpressed Egr-1. These findings demonstrate that PMA-induced Egr-1 displaces Sp1 from the G+C-rich element and activates expression driven by the PDGF-A proximal promoter in endothelial cells. The Sp1/Egr-1 displacement mechanism may be an important regulatory circuit in the control of inducible gene expression in vascular endothelial cells.