Platelet-derived growth factor. Phorbol ester induces the expression of the B-chain but not of the A-chain in HEL cells

p22phox-dependent NADPH oxidase activity is required for megakaryocytic differentiation

Transient reactive oxygen species (ROS) production is currently proving to be an important mechanism in the regulation of intracellular signalling, but reports showing the involvement of ROS in important biological processes, such as cell differentiation, are scarce. In this study, we show for the first time that ROS production is required for megakaryocytic differentiation in K562 and HEL cell lines and also in human CD34(+) cells. ROS production is transiently activated during megakaryocytic differentiation, and such production is abolished by the addition of different antioxidants (such as N-acetyl cysteine, trolox, quercetin) or the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor diphenylene iodonium. The inhibition of ROS formation hinders differentiation. RNA interference experiments have shown that a p22(phox)-dependent NADPH oxidase activity is responsible for ROS production. In addition, the activation of ERK, AKT and JAK2 is required for differentiation, but the activation of phosphatidylinositol 3-kinase and c-Jun N-terminal kinase seems to be less important. When ROS production is prevented, the activation of these signalling pathways is partly inhibited. Taken together, these results show that NADPH oxidase ROS production is essential for complete activation of the main signalling pathways involved in megakaryocytopoiesis to occur. We suggest that this might also be important for in vivo megakaryocytopoiesis.

Protein kinase C-dependent regulation of human erythroleukemia (HEL) cell sphingosine kinase activity

Sphingosine kinase functions in both the catabolism of sphingosine and in signal transduction pathways utilizing sphingosine-1-phosphate. The regulation of sphingosine kinase activity in human erythroleukemia (HEL) cells was investigated by treatment with several bioactive agents. Treatment of HEL cells with phorbol 12-myristate 13-acetate (PMA) caused a time- and concentration-dependent increase in sphingosine kinase activity measured in vitro. Sphingosine kinase activity increased in a phorbol ester- and diacylglycerol-specific manner. Staurosporine and calphostin C, protein kinase C (PKC) inhibitors, blocked the increased in sphingosine kinase activity, suggesting a PKC-dependent regulation. The effects of PMA on sphingosine kinase were dependent on transcription and translation. Purified PKC had no direct effect on sphingosine kinase activity. However, these studies led to the observation that HEL cell sphingosine kinase activity is stimulated in vitro by phosphatidylserine. Interestingly, other inducers of HEL cell differentiation, dimethylsulfoxide and retinoic acid, did not affect sphingosine kinase activity. These results indicate a separate and distinct pathway of PKC-dependent sphingosine kinase activation, and suggest a role for sphingosine kinase in regulation of cell function.

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.

Megakaryocytic leukemia and platelet factor 4

The de novo megakaryocytic leukemia fulfilling the FAB criteria is still an uncommonly recognized variant of acute leukemia. Many studies have shown that the megakaryocytic leukemic events may occur at a pluripotent stem cell level and clinical observations reveal that the megakaryocytic leukemias are diverse entities. The immunophenotyping using monoclonal antibodies against platelet specific surface antigens and the ultrastructural detection of platelet peroxidase reaction do not provide sufficiently useful information to determine whether a megakaryocytic leukemia is chronic, acute, therapy-responsive or therapy-unresponsive. More sophisticated techniques are required to further characterize megakaryocytic leukemic cells. In this review, we emphasize that megakaryocytic leukemic cells can be categorized into two groups; one with the PF4 mRNA, and the other without it, and that the expression of PF4 mRNA in the blasts could be a useful marker for the identification of mature megakaryoblasts. It seems that the patients with blasts expressing PF4 mRNA will have a longer survival and a better response to chemotherapy than those without PF4. We further discuss the fact that the detection of mRNAs of the IL-6 receptor, PDGF A- and B-chains, and TGF beta 1 in megakaryocytic leukemic cells will be useful to clarify the mechanisms involved in the proliferation of megakaryocytic leukemic cells and fibroblasts in the bone marrow. Furthermore, we reviewed data showing that megakaryocytic erythroid, and mast cell lineages share the nuclear transcription factor known as GF-1 (NF-E1 or Erf-1). We suggest that characterization of megakaryocytic leukemia should be performed using monoclonal antibodies against erythroid, megakaryocytic and mast cell lineages.

Receptor-mediated increases in cytosolic Ca2+ in the human erythroleukaemia cell line involve pertussis toxin-sensitive and -insensitive pathways

The pluripotent human erythroleukaemia cell line, HEL, possesses erythrocytic, megakaryocytic and macrophage-like properties. With respect to signal transduction, HEL cells have been used as a model system for platelets, but little attention has been paid to their phagocytic properties. We studied the effects of various receptor agonists on the intracellular free Ca2+ concentration ([Ca2+]i) in HEL cells. Thrombin, platelet-activating factor (PAF), ATP, UTP, prostaglandins E1 and E2 (PGE1 and PGE2), the PGE2 analogue sulprostone and the stable PGI2 analogues iloprost and cicaprost increased [Ca2+]i. ADP was less effective than ATP, and UDP was unable to increase [Ca2+]i. The increases in [Ca2+]i induced by thrombin, PAF, ATP, UTP, iloprost and cicaprost were pertussis toxin-insensitive, whereas the increases induced by PGE2 and sulprostone were completely inhibited by the toxin. The increase in [Ca2+]i induced by PGE1 was partially inhibited by pertussis toxin. PGE2 did not desensitize the increase in [Ca2+]i induced by iloprost, and vice versa. PGE1 desensitized the response to PGE2 and iloprost but not vice versa. Adrenaline potentiated the iloprost- but not the PGE2-induced rise in [Ca2+]i. The phorbol ester phorbol 12-myristate 13-acetate completely blocked the rise in [Ca2+]i induced by ATP and PGE1, whereas the increases induced by thrombin and PAF were only partially inhibited. Agonists increased [Ca2+]i through release from internal stores and sustained Ca2+ influx. Thrombin stimulated Mn2+ influx, which was blocked by Ni2+. Diltiazem, isradipine, gramicidin and 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride (SK&F 96365) did not affect agonist-induced rises in [Ca2+]i. HEL cells contained substantial amounts of beta-glucuronidase which, however, could not be released, and they did not aggregate or generate superoxide. Our data suggest that: (1) HEL cells possess nucleotide receptors with properties similar to those of phagocytes; (2) they possess receptors for PGE2 and PGI2, and PGE1 is an agonist at both receptors; (3) agonist-induced increases in [Ca2+]i are mediated through pertussis toxin-sensitive as well as -insensitive signal transduction pathways; and (4) agonists increase [Ca2+]i by mobilization from internal stores and influx from the extracellular space through cation channels with properties similar to those of phagocytes and platelets.

Transcriptional regulation of basic fibroblast growth factor gene expression in capillary endothelial cells

The growth of capillary endothelial cells (BCE) is an important regulatory step in the formation of capillary blood vessels. In vivo, the proliferation of these cells is stringently controlled. In vitro they can be stimulated by polypeptide growth factors, such as acidic fibroblast growth factor (aFGF) and basic fibroblast growth factor (bFGF). Since bFGF is synthesized and stored by vascular endothelial cells, this mitogen may play an important role in an autocrine growth regulation during angiogenesis. Here, evidence is presented for induction of the mRNA of bFGF by bFGF itself. A similar increase of bFGF mRNA was observed in response to thrombin and after treatment with phorbol ester. These results suggest that an autocrine loop may exist that may serve to modulate the mitogenic response in BCE under various physiological conditions, (e.g., wound healing and new capillary formation).

Evidence for the calcium-dependent activation of phospholipase D in thrombin-stimulated human erythroleukaemia cells

Human erythroleukaemia (HEL) cells were exposed to thrombin and other platelet-activating stimuli, and changes in radiolabelled phospholipid metabolism were measured. Thrombin caused a transient fall in PtdInsP and PtdInsP2 levels, accompanied by a rise in diacylglycerol and phosphatidic acid, indicative of a classical phospholipase C/diacylglycerol kinase pathway. However, the rise in phosphatidic acid preceded that of diacylglycerol, which is inconsistent with phospholipase C/diacylglycerol kinase being the sole source of phosphatidic acid. In the presence of ethanol, thrombin and other agonists (platelet-activating factor, adrenaline and ADP, as well as fetal-calf serum) stimulated the appearance of phosphatidylethanol, an indicator of phospholipase D activity. The Ca2+ ionophore A23187 and the protein kinase C activator phorbol myristate acetate (PMA) also elicited phosphatidylethanol formation, although A23187 was at least 5-fold more effective than PMA. Phosphatidylethanol production stimulated by agonists or A23187 was Ca2(+)-dependent, whereas that with PMA was not. These result suggest that phosphatidic acid is generated in agonist-stimulated HEL cells by two routes: phospholipase C/diacylglycerol kinase and phospholipase D. Activation of the HEL-cell phospholipase D in response to agonists may be mediated by a rise in intracellular Ca2+.

Platelet-derived growth factors

Blood platelets are a rich source of growth factors, including platelet-derived growth factor, platelet-derived endothelial cell growth factor, and transforming growth factor beta. Platelet-derived growth factor stimulates the growth of mesenchymal cells such as fibroblasts and vascular smooth muscle cells, whereas platelet-derived endothelial cell growth factor is a mitogen for vascular endothelial cells. Transforming growth factor beta is a bifunctional regulator of cellular growth, but acts as a potent inhibitor for most cell types. Most of the growth regulatory substances in platelets have been reported to reside in platelet alpha-granules, but platelet-derived endothelial cell growth factor appears to be present in platelet cytoplasm. These growth factors may act at sites of injury as wound hormones. Moreover, they play important roles for some pathological conditions such as atherosclerosis, myelofibrosis, connective tissue diseases, and neoplastic disorders.

Large-scale preparation of recombinant platelet-derived growth factor AA secreted from recombinant baby hamster kidney cells

The short isoform of platelet-derived growth factor A (PDGF-A) was expressed in a mammalian host (BHK-21 cell). A cell line was obtained that secreted up to 0.3 micrograms/10(6) cells recombinant PDGF-A chain homodimer/day into the medium. For large-scale production of supernatant, cells were grown either in roller bottles or in 2.5-1 stirred tank fermenters. A simple two-step procedure was developed to purify recombinant PDGF-AA (rPDGF-AA). The first step was adsorption onto porous glass and the final step was reversed-phase high-performance liquid chromatography. The yield was 0.2 mg/l supernatant. A total amount of 20-30 mg pure rPDGF-AA may be obtained from a single fermenter run. Sequence analysis showed the correct amino terminus and no internal proteolytic cleavages. The specific activity was 5 ng/ml for mouse AKR-2B cells. [125I]rPDGF-AA had an affinity constant of approximately 0.5 nM to these cells and 25,000 binding sites were estimated/cell.

Expression and stability of c-sis mRNA in human glioblastoma cells

The production of platelet-derived growth factor like (PDGF-like) material by glioblastomas may be involved in the conversion of normal cells to tumor cells. In an investigation of this problem, we have examined some of the properties of the platelet-derived growth factor B-chain mRNA (c-sis mRNA) by a sensitive and quantitative RNA-RNA solution hybridization method. In 5 out of 8 human glioblastoma cell lines, c-sis mRNA was present, and in the line with the highest level, there were approximately 4-10 molecules per cell. The half-lives of the c-sis mRNA in two glioblastoma cell lines were 2.6 and 3.4 h, while in human umbilical vein endothelial (HUVE) and bladder carcinoma (T24) cells they were 1.6 and 2.5 h, respectively. Inhibiting protein synthesis produced no significant alteration of the c-sis mRNA half-lives in the glioblastoma or HUVE cells. The A-U-rich sequence at the 3' end of the c-sis mRNA therefore does not appear to affect the mRNA stability in the presence of cycloheximide as it does in other transcripts. The similarity of the c-sis mRNA half-lives in normal and tumor cells suggests that regulation of stability of c-sis mRNA is not a major factor in tumorigenesis in the glioblastoma cell lines examined.

The role of blood platelets in nucleoside metabolism: regulation of megakaryocyte development and platelet production

In higher vertebrates, different types of blood cells develop from common precursors. Mammals are unique in possessing two types of blood cells--erythrocytes and platelets--which lack nuclei. Although platelets display consistent and easily-recognisable morphological and ultrastructural characteristics and show extreme metabolic and functional versatility, they are not true cells, being produced by fragmentation of giant polyploid precursors called megakaryocytes. At present, the physiological mechanisms which regulate megakaryocyte development and platelet production are not well understood. Platelets are actively involved in metabolism of purine derivatives and a significant platelet role in pyrimidine metabolism has also been demonstrated (see previous papers). Here an attempt is made to integrate information about platelet involvement in nucleic acid precursor metabolism with current concepts of haematopoiesis, particularly megakaryocyte development and platelet production. It is concluded (i) that megakaryocytic cells are immediate descendents of haematopoietic stem cells which have become polyploid as a result of genetic damage or metabolic imbalances, (ii) megakaryocytes and platelets are the ultimate regulators of stem cell development because they control the availability of thymidine and (iii) that the production of megakaryocytes and platelets is a physiological safety mechanism which prevents fixation of genetic damage and protects other cells from potentially cytotoxic and genotoxic stimuli.

The long 3'-untranslated regions of the PDGF-A and -B mRNAs are only distantly related

A cDNA clone of about 2300 base pairs was prepared from the human osteosarcoma cell line U-2 OS by hybridization with a 22-mer oligonucleotide complementary to the NH2-terminus of PDGF-A. Restriction and sequence analysis showed that this clone contains the entire coding region for PDGF-A and a long 3'-untranslated region which is only distantly related to that in the mRNA of PDGF-B.