Platelet-derived growth factor gene expression in cultured human retinal pigment epithelial cells.

Gene expression of platelet-derived growth factor (PDGF) and its receptors in cultured human retinal pigment epithelial (RPE) cells was studied by using semiquantitative polymerase chain reaction. The RPE cells were found to express PDGF A- and B-chain genes as well as alpha- and beta-receptor genes with dominant expression of B-chain and beta-receptor isoforms. Phorbol myristate acetate (PMA) and thrombin increased the expression of PDGF B-chain gene to 19.8 +/- 1.75 and 15.9 +/- 1.84 fold (n = 3) of the control without affecting beta-receptor gene expression. PDGF produced by the RPE cells may play an important role in the pathogenesis of some ocular proliferative diseases.     

Differential migratory response of U-2 OS osteosarcoma cell to the various forms of platelet-derived growth factor.

U-2 OS osteosarcoma cells are mesenchymal-derived transformed cells spontaneously expressing both platelet-derived growth factor (PDGF) A- and B-chain genes, and releasing PDGF AA dimers in culture. Using modified Boyden chemotactic chambers, platelet-purified PDGF was shown to be a chemoattractant for U-2 OS cells. More specifically, U-2 OS cells migrated in the presence of PDGF AB and BB dimers but not in the presence of PDGF AA dimers. This pattern of response was similar to that observed with human fibroblasts and this similarity is consistent with the fact that U-2 OS cells express PDGF receptor alpha- and beta-subunits in a similar fashion to human fibroblasts.     

Expression of recombinant platelet-derived growth factor A- and B-chain homodimers in rat-1 cells and human fibroblasts reveals differences in protein processing and autocrine effects.

The autocrine effects of platelet-derived growth factor (PDGF) A- and B-chain homodimers (PDGF-AA and PDGF-BB) on rat-1 cells and human fibroblasts have been investigated by using human PDGF A- and B-chain cDNA clones expressed in a retroviral vector. Infection with replication-defective virus carrying the B-chain cDNA resulted in a phenotypical transformation resembling that induced by simian sarcoma virus. The resulting cells were focus forming in monolayer cultures, grew to high saturation densities, and formed large colonies in soft agar. The PDGF A-chain transfectants showed no transformed morphology and lacked focus-forming activity but grew to high saturation density in monolayer culture and formed small colonies in soft agar. A similar but weaker effect was obtained with an A-chain cDNA variant containing a 69-base-pair insertion in the 3' end of the protein-coding domain. A- and B-chain transfectants released PDGF receptor-competing activity into the medium, but only the medium conditioned by the B-chain transfectants possessed potent mitogenic activity on human fibroblasts. Both types of transfectants had downregulated levels of PDGF receptors; however, the B-chain transfectants were downregulated to significantly lower levels. Metabolic labeling and immunoprecipitations with PDGF antiserum showed that the PDGF B-chain protein was processed to a 24-kilodalton cell-associated and a 30-kilodalton secreted dimeric protein. The A-chain protein was rapidly secreted as a 31-kilodalton dimeric protein. The present study shows a marked difference in the autocrine effects of PDGF-AA and -BB expressed under the control of a retroviral promoter and suggests that different biological properties may be assigned to these two PDGF isoforms.     

Induction of platelet-derived growth factor gene expression during megakaryoblastic and monocytic differentiation of human leukemia cell lines.

Platelet-derived growth factor (PDGF) is one of the most important polypeptide growth factors in human serum. It is composed of two polypeptide chains linked by disulfide bonds. The B-chain is encoded by the c-sis proto-oncogene, which is expressed in several malignant and non-malignant cells including K562 cells differentiating towards megakaryoblasts. Expression of the A-chain has been reported to occur in human solid tumor cell lines independently of c-sis expression. We report here the non-coordinate expression of the A- and B-chains in human leukemia cell lines. The PDGF-A and B-chain (c-sis) RNA expression as well as secretion of PDGF polypeptides are induced in the K562 cell line upon induction of megakaryoblastic differentiation with 12-O-tetradecanoyl phorbol-13-acetate (TPA) whereas erythroid differentiation induced with sodium butyrate is accompanied by c-sis expression only. Simultaneously with megakaryoblastic differentiation the RNA level for another platelet protein, the transforming growth factor-beta was also increased, but in a complex manner. The promyelocytic leukemia cell line HL-60 does not express PDGF-A RNA, whereas the promonocytic cell line U937 does. Preferential induction of the A-chain RNA is obtained in both cell lines after treatment with TPA which causes monocytic differentiation. PDGF-A expression in HL-60 cells is also observed after treatment with the tumor necrosis factor-alpha but granulocytic differentiation of HL-60 cells induced with dimethyl sulfoxide or the granulocyte colony-stimulating factor is not associated with PDGF gene expression.     

PDGF-AA and PDGF-BB biosynthesis: proprotein processing in the Golgi complex and lysosomal degradation of PDGF-BB retained intracellularly

Platelet-derived growth factor is a potent mitogen for cells of mesenchymal origin. It is made up of two polypeptide chains (A and B) combined in three disulfide-linked dimeric forms (AA, AB, and BB). Here, the biosynthesis and proteolytic processing of the two homodimeric forms of PDGF (AA and BB) were studied in CHO cells stably transfected with A-chain (short splice version) or B-chain cDNA. PDGF-AA was processed to a 30-kD molecule which was secreted from the cells. In contrast, PDGF-BB formed two structurally distinct end products; a minor secreted 30-kD form and a major cell-associated 24-kD form. Immunocytochemical studies at light- and electron-microscopical levels revealed presence of PDGF in the Golgi complex, in lysosomes, and to a smaller extent in the ER. From analysis of cells treated with brefeldin A, an inhibitor of ER to Golgi transport, it was concluded that dimerization occurs in the ER, whereas the proteolytic processing of PDGF-AA and PDGF-BB precursors normally occurs in a compartment distal to the ER. Exposure of the cultures to the lysosomal inhibitor chloroquine led to an increased cellular accumulation of PDGF-BB, as determined both by metabolic labeling experiments and immunocytochemical methods, indicating that the retained form of PDGF-BB is normally degraded in lysosomes. Structural analysis of the two end products of PDGF-BB revealed that the secreted 30-kD form is a dimer of peptides processed as the B-chain of PDGF purified from human platelets, and that the retained 24-kD form is made up of subunits additionally processed in the NH2-terminus. Also, the 24-kD form was shown to be composed of proteolytic fragments held together by disulfide bridges. Taken together these findings suggest that the newly synthesized PDGF A- and B-chains are dimerized in the ER and thereafter transferred to the Golgi complex for proteolytic processing. From there, PDGF-AA is carried in vesicles to the cell surface for release extracellularly by exocytosis. A smaller part of PDGF-BB (the 30-kD form) is handled in a similar way, whereas the major part (the 24-kD form) is generated by additional proteolysis in the Golgi complex, from which it is slowly carried over to lysosomes for degradation.     

Platelet-derived growth factor receptor beta regulates migration and DNA synthesis in metanephric mesenchymal cells

Platelet-derived growth factor (PDGF) B-chain and PDGF receptor beta (PDGFR beta) are essential for glomerulogenesis. Mice deficient in PDGF B-chain or PDGFR beta exhibit an abnormal glomerular phenotype characterized by total lack of mesangial cells. In this study, we localized PDGFR beta in the developing rat kidney and explored the biological effects of PDGF in metanephric mesenchymal cells in an attempt to determine the mechanism by which PDGF regulates mesangial cell development. Immunohistochemical and in situ hybridization studies of rat embryonic kidneys reveal that PDGFR beta localizes to undifferentiated metanephric mesenchyme and is later expressed in the cleft of the comma-shaped and S-shaped bodies and in more mature glomeruli in a mesangial distribution. We also isolated and characterized cells from rat metanephric mesenchyme. Metanephric mesenchymal cells express vimentin and alpha-smooth muscle actin but not cytokeratin. These cells also express functional PDGFR beta, as demonstrated by autophosphorylation of the receptor as well as activation of phosphatidylinositol 3 kinase in response to PDGF B-chain homodimer. PDGF B-chain also induces migration and proliferation of metanephric mesenchymal cells. Taken together with the fact that PDGF B-chain is expressed in the glomerular epithelium and mesangial area, as demonstrated in the human embryonic kidney, we suggest that PDGF B-chain acts in a paracrine fashion to stimulate the migration and proliferation of mesangial cell precursors from undifferentiated metanephric mesenchyme to the mesangial area. PDGF B-chain also likely stimulates proliferation of mesangial cell precursors in an autocrine fashion once these cells migrate to the glomerular tuft.     

Specific growth stimulation of cultured smooth muscle cells from spontaneously hypertensive rats by platelet-derived growth factor A-chain homodimer.

Cultured vascular smooth muscle cells (VSMC)1 from spontaneously hypertensive rats (SHR) possess specific cell surface receptors for both homodimeric forms of platelet-derived growth factor (PDGF-AA and PDGF-BB), in contrast to cells from normotensive Wistar Kyoto (WKY) animals, which express receptors only for the B-chain form of PDGF. Stimulation of quiescent VSMC from SHR with PDGF-AA resulted in activation of S6-kinase and induction of phosphoinositide catabolism, as well as cellular proliferation when cultures were maintained for prolonged periods with daily supplementation of the growth factor. WKY-derived VSMC showed no response to PDGF-AA, which was consistent with their lack of specific receptors for this homodimer. The responsiveness of quiescent cells from SHR and WKY to the B-chain homodimer was similar. The enhanced growth responsiveness of SHR-derived cells to fetal calf serum, as compared with cells from their normotensive counterparts, may be accounted for in part by their expression of receptors for the AA homodimer of PDGF.     

Agents that increase cAMP accumulation block endothelial c-sis induction by thrombin and transforming growth factor-beta

Endothelial cells express the product of the c-sis gene, which encodes the B-chain of platelet-derived growth factor (PDGF). Through local production of growth factors such as PDGF in vascular sites, endothelial cells may stimulate proliferation of adjacent cells through a paracrine mechanism. Previously, we have shown that the expression of c-sis mRNA and release of growth factor activity by human renal endothelial cells is induced by thrombin. We now show that another agent of possible importance in mediating proliferation of cells adjacent to the endothelial cell layer, transforming growth factor-beta (TGF-beta), also induced c-sis expression in these cells. In addition, we have studied the effect of agents that increase intracellular cAMP levels upon the induction of endothelial cell c-sis mRNA. The adrenergic agonists isoproterenol and norepinephrine blocked the elevation of cellular c-sis mRNA accompanying exposure to either thrombin or TGF-beta. This effect was mediated through beta-adrenergic receptors, since propranolol but not phentolamine reversed the inhibition. Forskolin, a direct activator of adenylate cyclase, also blocked induction of c-sis mRNA by thrombin and TGF-beta and inhibited the release of PDGF activity into the media of these cells. Basal, as well as stimulated c-sis mRNA levels were attenuated by these agents that increase cellular cAMP levels. These data suggest that increased cAMP production inhibits the expression of c-sis encoded mitogens by endothelial cells, and that c-sis expression is subject to bidirectional regulation in these cells.     

Cytokine-Regulated Expression of Platelet-Derived Growth Factor Gene and Protein in Cultured Human Astrocytes

Abstract: To elucidate mechanisms regulating the production of platelet-derived growth factor (PDGF) in the CNS, we analyzed the influence of a panel of cytokines on PDGF mRNA and protein levels in astrocyte-enriched cultures from the human embryonic brain and spinal cord. Using a specific ELISA, PDGF AB protein was detected in serum-free astrocyte supernatants and its levels were significantly increased after treatment of the cultures with transforming growth factor-β₁ (TGF-β₁) or tumor necrosis factor-α (TNF-α); the largest increase was detected after combined treatment with the two cytokines. Interleukin-1β (IL-1β) by itself had little or no effect but synergized with TGF-β₁ in enhancing PDGF AB production. Supernatants from human astrocyte cultures stimulated the proliferation of rat oligodendrocyte progenitors, and most of the mitogenic activity could be accounted for by PDGF. By northern blot analysis, both PDGF A- and PDGF B-chain mRNAs were detected in untreated astrocytes. PDGF B-chain mRNA levels were increased by TGF-β₁, TNF-α, TNF-α/TGF-β₁, or IL-1β/TGF-β₁, whereas PDGF A-chain mRNA levels were not consistently affected by cytokine treatments. These in vitro data indicate that TGF-β₁, TNF-α, and IL-1β are able to stimulate astrocyte PDGF production. This cytokine network could play a role in CNS development and repair after injury or inflammation.     

Thrombin stimulates c-sis gene expression in microvascular endothelial cells

We have determined whether expression of the c-sis gene product, platelet-derived growth factor (PDGF), is regulated in cultured renal microvascular endothelial cells by factors to which vascular endothelial cells may be exposed at sites of perivascular cellular proliferation. Thrombin exposure increased endothelial cell levels of c-sis message by 3-5-fold over a time course that peaked at 4 h after exposure. Similarly, thrombin-exposed microvascular endothelial cells released increased amounts of PDGF activity into their media. The thrombin effect was not mediated through the proteolytic activity of thrombin, as proteolytically inactive thrombin stimulated the c-sis expression as well as native thrombin. This stimulation was mimicked by exposure of cells to biologically active phorbol esters, suggesting that thrombin action may be mediated through activation of kinase C (Ca2+/phospholipid-dependent enzyme). Thus, thrombin regulates the expression and release of PDGF activity from endothelial cells in culture and may act in vivo to stimulate mitogen release from endothelial cells, thereby inducing proliferation of perivascular cells.