Mechanism of regulation of PDGF-A chain gene expression by serum and TPA

Transcription of the platelet-derived growth factor A-chain gene

The molecular mechanisms which control the transcription of growth factor genes underlie such diverse biological processes as embryonic development, cellular differentiation and wound healing. Moreover, disruption of these controls is implicated in the development and progression of a wide variety of human diseases, including cancer, atherosclerosis and fibrotic disease. This review highlights progress made in the study of the gene encoding platelet-derived growth factor A-chain (PDGF-A) from the perspective of its normal patterns of expression, as well as possible mechanisms leading to dysregulation and disease. A particular focus has been placed on the identification and characterization of specific DNA elements, DNA-binding proteins and other aspects of transcriptional regulation involved in activation and repression of the human PDGF-A promoter.

Mechanism of action and in vivo role of platelet-derived growth factor

Platelet-derived growth factor (PDGF) is a major mitogen for connective tissue cells and certain other cell types. It is a dimeric molecule consisting of disulfide-bonded, structurally similar A- and B-polypeptide chains, which combine to homo- and heterodimers. The PDGF isoforms exert their cellular effects by binding to and activating two structurally related protein tyrosine kinase receptors, denoted the alpha-receptor and the beta-receptor. Activation of PDGF receptors leads to stimulation of cell growth, but also to changes in cell shape and motility; PDGF induces reorganization of the actin filament system and stimulates chemotaxis, i.e., a directed cell movement toward a gradient of PDGF. In vivo, PDGF has important roles during the embryonic development as well as during wound healing. Moreover, overactivity of PDGF has been implicated in several pathological conditions. The sis oncogene of simian sarcoma virus (SSV) is related to the B-chain of PDGF, and SSV transformation involves autocrine stimulation by a PDGF-like molecule. Similarly, overproduction of PDGF may be involved in autocrine and paracrine growth stimulation of human tumors. Overactivity of PDGF has, in addition, been implicated in nonmalignant conditions characterized by an increased cell proliferation, such as atherosclerosis and fibrotic conditions. This review discusses structural and functional properties of PDGF and PDGF receptors, the mechanism whereby PDGF exerts its cellular effects, and the role of PDGF in normal and diseased tissues.

Signals controlling the expression of PDGF

PDGF is an important polypeptide growth factor that plays an essential role during early vertebrate development and is associated with tissue repair and wound healing in the adult vertebrate. Moreover, PDGF is thought to play a role in a variety of pathological phenomena, such as cancer, fibrosis and atherosclerosis. PDGF is expressed as a dimer of A and/or B chains, the precursors of which are encoded by two single copy genes. Although the PDGF genes are expressed coordinately in a number of cell types, they are independently expressed in a majority of cell types. The expression of either PDGF gene can be affected by very diverse extracellular stimuli and the type of response is dependent on the cell type that is exposed to the stimulus. Expression of the PDGF chains can be modulated at every imaginable level: by regulating accessibility of the transcription start site, by varying the transcription initiation rate, by using alternative transcription start sites, by alternative splicing, by using alternative polyadenylation signals, by varying mRNA decay rates, by regulating efficiency of translation, by protein modification, and by regulating secretion. Even upon secretion, the activity of PDGF can be modulated by non-specific or specific PDGF-binding proteins. This review provides an overview of the cell types in which the PDGF genes are expressed, of the factors that are known to affect the expression of PDGF, and of the various levels at which the expression of PDGF genes can be regulated.

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.