Origin of platelet-derived growth factor in megakaryocytes in guinea pigs

Expression of bone morphogenetic proteins and their receptors in the bone marrow megakaryocytes of GATA-1(low) mice: a possible role in osteosclerosis

The mechanism of osteosclerosis associated with myelofibrosis in megakaryocyte (MK)-related myeloproliferative disorders is largely unknown. However, growth factors released from the bone marrow cells, including from MKs, have been implicated in myelofibrosis, osteosclerosis, and angiogenesis. GATA-1 is a transcription factor required for normal MK development. GATA-1 deficiency in mice (GATA-1(low)) leads to increased megakaryocytic proliferation, followed by osteosclerosis and myelofibrosis. In this study we investigated the expression of bone morphogenetic proteins (BMPs) and BMP receptors and their possible role in the development of osteosclerosis in the MKs of 12-month-old GATA-1(low) mice by immunohistochemistry, cytomorphometry, and quantitative real-time PCR. Marrow MKs from both wild-type and GATA-1(low) mice showed moderate to intense staining for BMP-2, -4, and -6 and BMPR-IA and BMPR-II, whereas splenic MKs showed no BMP immunostaining. Presence of BMP protein in the bone marrow of GATA-1(low) mice was more than that seen in controls, owing to an increased number of MKs and osteoblasts. The osteosclerosis seen in GATA-1(low) mice appeared not to be due to a reduced number of functional osteoclasts because the number of tartrate-resistant acid phosphatase-positive osteoclasts was greater in GATA-1(low) mice than in controls. Our findings demonstrate the presence of significant amounts of BMP-2, -4, and -6 along with their receptors in bone marrow MKs of WT and GATA-1(low) mice. The increased levels of BMPs appear to be a result of increased numbers of MKs in GATA-1(low) mice and may, in part, account for the stimulation of osteoblastic activity and resulting osteosclerosis.

Pharmacodynamic modeling of thrombopoietin, platelet, and megakaryocyte dynamics in patients with acute myeloid leukemia undergoing dose intensive chemotherapy

A proposed model of thrombopoietin (TPO) regulation is that of a constitutive production of TPO with circulating levels being predominately regulated by changes in platelet and megackaryocyte mass. Using a pharmacodynamic (PD) approach, the authors examined the validity of this model for patients with acute myeloid leukemia (AML) undergoing dose-intensive postinduction chemotherapy (HDT). TPO and platelet values were assayed weekly in AML patients undergoing HDT. A parsimonious dynamic model was then applied to these experimental data. The results (1) support the proposed model of TPO regulation, (2) model and quantify the effects of HDT on the megakaryocyte compartment, (3) characterize variables not amenable to direct measurement, and (4) have clinical utility as this model predicted that TPO given after HDT would not have a significant effect on platelet recovery, a finding borne out in clinical trials. This model provides information relevant to the interpretation of clinical trials of hematopoietic growth factors.

Platelets and stroke

Platelets are anucleate cells with little or no capacity for de novo protein synthesis. Their potential haemostatic reactivity is established at or before thrombopoiesis by their precursor cell, the bone marrow megakaryocyte. In some pathologic conditions, the megakaryocyte-platelet-haemostatic axis (MPHA) becomes perturbed, resulting in the formation of hyperfunctional platelets which may contribute to the development of vascular disease or an acute thrombotic event such as ischaemic stroke or myocardial infarction. Laboratory measurements of platelet function have established that platelet reactivity is accentuated in acute ischaemic stroke, particularly following cortical rather than lacunar infarction. Whether accentuated platelet function is a cause or a consequence of stroke is not yet clear, but it is likely that patients with certain risk factor profiles have some degree of platelet activation preceding the stroke. Further work into the MPHA is required to establish whether enhanced post-stroke platelet reactivity can be referred to the megakaryocyte. The antiplatelet agents tested to date are effective in secondary but not primary prevention of stroke. This probably reflects the diverse pathophysiology of stroke: accentuated platelet function is only likely to be a significant factor in cortical stroke.

Megakaryocyte ploidy and platelet changes in human diabetes and atherosclerosis

Altered platelet morphology and function have been reported in patients with diabetes. They are likely to be associated with the pathological processes and increased risk of vascular disease seen in these patients. Mean platelet volume (MPV), platelet count, and megakaryocyte (MK) ploidy (DNA content) were measured in (1) nondiabetics with normal coronary arteries, (2) nondiabetics with coronary artery atherosclerosis, (3) diabetics without evidence of vascular complications, and (4) diabetics with vascular disease. The platelet count (+/- SD) was increased in all groups but only significantly in the diabetics with vascular disease (236 +/- 65 versus 250 +/- 54 versus 257 +/- 64 versus 295 +/- 90 [P < or = .05] x 10(9)/L, for groups, I, II, II, and IV, respectively). The MPV was significantly increased in patients with atherosclerosis (7.0 +/- 0.4 versus 8.0 +/- 1.2 [P < or = .05] versus 7.2 +/- 0.9 versus 8.1 +/- 0.9 [P < or = .05] IL). Geometric mean MK ploidy was significantly increased in all groups compared with controls (16 +/- 1.5 versus 18.7 +/- 1.8 [P < or = .05] versus 19.8 +/- 1.6 [P < or = .05] versus 20.1 +/- 2.7 [P < or = .05]). Furthermore, some patients with vascular disease and/or diabetes had a modal ploidy shift from 16 (the normal mammalian modal ploidy) to 32, with a concomitant reduction of MKs in the 8 and 16 ploidy classes. This shift was seen particularly in the diabetics with vascular disease (P = .007). Interleukin-6 (IL-6) levels were measured and were elevated in patients with atherosclerosis; the highest levels were found in the diabetic patients (0.7 +/- 0.9 versus 5.3 +/- 5.5 [P < or = .05] versus 2.5 +/- 2.8 versus 6.7 +/- 5.5 [P < or = .05] ng/L). In the diabetic patients with atherosclerosis, fibrinogen levels were also increased (2.85 +/- 0.76 versus 3.34 +/- 1.32 versus 2.43 +/- 1.50 versus 5.59 +/- 1.72 [P < or = .05] g/L). Furthermore, IL-6 levels correlated with MK ploidy (r = .45, P = .009) and fibrinogen levels (r = .5, P = .0001). This study demonstrates that patients with vascular disease, particularly diabetics, have an altered MK ploidy distribution, showing a shift toward higher ploidy in association with an increased platelet mass (count x volume). Changes in platelets in diabetes probably reflect MK changes, which themselves are a response to systemic change.

Clinicopathological impact of the interaction between megakaryocytes and myeloid stroma in chronic myeloproliferative disorders: a concise update

In this report an attempt has been made to discuss some of the issues pertinent to myelofibrosis complicating chronic myeloproliferative disorders (CMPDs) that are significantly associated with megakaryocyte function. In this context, biochemical, clinical and particularly morphological features were reviewed. Morphological findings based on elaborate techniques were in keeping with the assumption that in chronic myeloid leukemia (1) the number of CD61-positive megakaryocytes, and in particular their precursors were the parameters most closely associated with myelofibrosis (2) an increased content of reticulin fibers in follow-up biopsies significantly correlated with laboratory data indicative of a high tumor burden (anemia, peripheral blasts, hepatosplenomegaly) and thus a more advanced stage of the disease process (3) even a slight increase in reticulin, i.e. doubling of the normal fiber density was associated with a worse prognosis independent of therapeutic regimens given (4) Dynamics of myelofibrosis was significantly influenced by treatment. In this context, calculation of the myelofibrosis progression index (MPI) revealed a higher score following interferon therapy compared with busulfan. In addition, in idiopathic myelofibrosis (5) the evolution of myelofibrosis was unpredictable and according to the MPI, progression occurred at a relatively low rate (6) proliferation and dilatation of sinusoids accompanying intravascular hematopoiesis and collagen type IV deposits were predominant features in later (fibro-osteosclerotic) stages in the course of disease (7) transmural migration of megakaryocytes demonstrated by three dimensional reconstruction revealed a mole-like tunneling through the thickened sinusoidal wall. A very careful assessment of the numerous correlations between bone marrow features and laboratory data will allow clinicians and pathologists to gain a better insight into the mutual relationships between hematological and morphological findings in CMPDs.

Megakaryocytes and fibroblasts--interactions as determined in normal human bone marrow specimens

An in vitro study was performed to investigate possible interactions between megakaryocytes and bone marrow fibroblasts, both obtained from healthy donors. We were able to demonstrate that the proliferation of fibroblasts increased significantly by co-culturing these cells with megakaryocytes for 6 days. Addition of neutralizing antibodies for PDGF and TGF beta 1, caused a significant reduction of fibroblast growth. Inhibition of cell to cell contacts via tissue culture inserts generated a conspicuous impairment of fibroblast proliferation compared with megakaryocyte-fibroblast co-cultures, where contact was allowed. Hence, our findings suggest that a close spatial relationship between megakaryocytes and fibroblasts is needed for the activation of growth in normal human bone marrow. Neighbouring of megakaryocytes and fibroblasts seems to be necessary in order to achieve a certain threshold of local growth factor concentration. Our results are in keeping with the assumption that PDGF and TGF beta 1, are secreted by normal human megakaryocytes in very low concentrations and promote significantly fibroblast proliferation.

Influence of cytokines (IL-1 alpha, IL-3, IL-11, GM-CSF) on megakaryocyte-fibroblast interactions in normal human bone marrow

The evolution of myelofibrosis accompanying chronic myeloproliferative disorders (CMPDs) is often linked with megakaryopoiesis. However, it is not known whether or to what extent megakaryocytes of normal human bone marrow are capable of stimulating fibroblast growth. For this reason, an in vitro study was performed to elucidate possible cytokine-dependent interactions between megakaryocytes and fibroblasts derived from healthy volunteers. Fibroblast growth was significantly promoted by the presence of megakaryocytes and modulated by additional application of various cytokines. While recombinant human (rh) interleukin (IL)-1 alpha had no obvious effect on fibroblast proliferation, a slight increase was detected on adding granulocyte-macrophage colony stimulating factor (rhGM-CSF). Application of rhIL-3 caused a significant increase in the number of fibroblasts. In contrast, administration of rhIL-11 suppressed the megakaryocyte-dependent growth-promoting effect and co-stimulation with rhIL-3 led to a significant decrease of fibroblast number in comparison to rhIL-3-stimulated co-cultures. Inhibition of cell-cell contact in unstimulated, as well as in rhIL-3-stimulated co-cultured led to a conspicuous impairment of fibroblast growth. A similar effect was observed when neutralizing antibodies directed against platelet-derived growth factor (PDGF) and transforming growth factor (TGF)beta 1 were added to rhIL-3-stimulated cultures. Our findings are in keeping with the assumption that interactions between megakaryocytes and fibroblasts involve in cytokine-mediated functional network regulated by factors such as spatial relationship, cytokine stimulation, and low concentrations of mediators, particularly PDGF and TGF beta. In this complex system rhIL-3 seems to play a crucial role in the promotion of these various interrelationships.

The megakaryocyte platelet system and vascular disease

Platelets form a heterogeneous population of cells produced from the uniquely large polyploid cell found in the bone marrow, the megakaryocyte. The platelet megakaryocyte axis forms a dynamic equilibrium varying in normal biology and in disease. Prolonged platelet destruction leads to the production of large platelets from large, high ploidy megakaryocytes. In vivo and ex vivo studies show that such platelets have more haemostatic potential than smaller less dense platelets. The evidence suggesting that prothrombotic changes in the megakaryocyte platelet axis precede coronary artery thrombosis and the importance of platelet reactivity in atherosclerosis will be reviewed.

Calcium signalling in platelets and other nonexcitable cells

By virtue of their biological simplicity and widespread availability, platelets frequently have been used as a model system to study signal transduction. Such studies have revealed that changes in intracellular free calcium concentration are central to platelet functioning. The following article reviews current concepts of platelet structure and function, with particular emphasis on the mechanisms involved in platelet Ca2+ signalling.

Circulating megakaryocytes: delivery of large numbers of intact, mature megakaryocytes to the lungs

To determine the locus of platelet production, we sought to determine if sufficient megakaryocytes reach the lungs in a state that could produce platelets. Elutriation was used to isolate megakaryocytes from blood reaching and leaving the lungs of 20 patients undergoing routine cardiac catheterizations. A mean of 5.0 intact megakaryocytes/ml were found in pulmonary artery blood, compared to only 0.5 megakaryocytes/ml, with partial cytoplasmic content, in aortic samples. The megakaryocytes in central venous and aortic samples were all mature. The identity of these cells as megakaryocytes, their maturity and normal morphology were confirmed by standard and immunoelectron microscopy. Cardiac outputs were obtained for each patient at the time of blood sampling, allowing an extrapolation that 40 x 10(6) intact, mature megakaryocytes were being delivered to the lungs every day in the average patient, compared to only 4.0 x 10(6) partially spent megakaryocytes exiting the lungs daily. About 98% of megakaryocyte cytoplasm reaching the lungs did not exit as recognizable megakaryocytes or fragments. The number and state of the megakaryocytes apparently filtered in the lungs is consistent with the hypothesis that megakaryocytes may shed platelets within the pulmonary microvasculature, which may be the primary site of platelet production.

Mitochondrial DNA topoisomerase I from human platelets

An anucleated cell system has been used for the first time to study mitochondrial topoisomerase activity. Mitochondrial extracts from human blood platelets contained type I topoisomerase. The type I classification was based on ATP-independent activity, inhibition by ATP or camptothecin, and the lack of inhibition by novobiocin. Platelet mitochondrial topoisomerase I relaxation activity was inhibited linearly by increasing concentrations of EGTA. Topoisomerase activity greater than 90% inhibited by 175 microM EGTA was partially restored to 16 and 50% of the initial level of activity by the subsequent addition of 50 and 100 microM Ca2+, respectively. Additionally, results from studies of partially purified platelet mitochondrial topoisomerase I were consistent with the crude extract data. This work supports the hypothesis that platelet mitochondria contain a type I topoisomerase that is biochemically distinct from that previously isolated and characterized from cell nuclei.

Identification of mRNA for PDGF B-chain in human megakaryocytes isolated using a novel immunomagnetic separation method

A rapid and simple method has been developed for the separation of pure populations of intact human megakaryocytes from whole bone marrow. Megakaryocytes were specifically recognized using monoclonal antibodies coupled to magnetizable articles. Cells labelled with the magnetizable probe were separated from unlabelled cells by introduction of a magnetic field. The technique yields megakaryocyte suspensions with a purity of greater than 98%. Electron microscope examination showed that the ultrastructure of the isolated megakaryocytes was well preserved. Using this method of cell purification, we have investigated expression of the gene for platelet-derived growth factor (PDGF). RNA was isolated from cells harvested from either ribs or posterior iliac crest. The RNA was spotted onto nitrocellulose, and then hybridized using a c-sis riboprobe specific for PDGF B chain mRNA. We demonstrate that mRNA for PDGF B-chain is identifiable in samples of 50,000 cells. We conclude that PDGF is synthesized by the megakaryocyte.

Accelerated atherogenesis occurs following platelet destruction and increases in megakaryocyte size and DNA content

Platelets and megakaryocytes have a role in atherogenesis. In this study the effect of platelet destruction, caused by injection of serum containing anti-platelet antibodies, on the development of atheroma and on megakaryocyte size and DNA content was studied in rabbits on a high cholesterol diet. Seven days after destruction of the circulating platelets a significant (150% and 300% in two separate studies) increase in the area of atheroma in the aorta was demonstrated. Histological examination revealed that large quantities of extracellular lipid and connective tissue were present in these plaques. The acceleration of atherogenesis in this model was found to be preceded by a significant increase in megakaryocyte size and in the frequency of megakaryocytes with a high DNA content. Platelets derived from large, high ploidy megakaryocytes may be of importance for the development of the atherosclerotic lesion. Neither the endothelial permeability as assessed by injection of Evans blue nor the prostacyclin production by the aorta of animals injected with anti-platelet serum were significantly different from the aortas of control animals.

Platelet-derived growth factor--a growth factor with an expanding role in health and disease

Platelet-derived growth factor (PDGF) is the principal mitogen for connective tissue-derived cells such as fibroblasts, smooth muscle cells, and glial cells. It is synthesized by a variety of cell types and the synthesis of PDGF and its receptors is tightly controlled. Accumulating evidence obtained in vitro and in vivo suggests that PDGF plays important roles in the pathogenesis of clinically important diseases such as atherogenesis and cancer. Moreover, PDGF is an important research tool to study the signal transmission pathway of growth factors and other hormones.

Megakaryocyte function and dysfunction

More than a hundred years have passed since platelets were recognized as cells and their haemostatic functions discovered. However, the process of platelet production is still not understood. The location, the mechanism and the regulation of thrombopoiesis remain elusive. Megakaryocytes are known to be the source of platelets. Investigations of megakaryocytes have revealed their normal functions and some of the abnormalities present in various diseases which affect platelets. In recent years, new techniques of cell isolation and tissue culture have been developed and have made possible advances in characterizing megakaryocyte precursors and differentiation. The primary function of megakaryocytes is to synthesize and assemble platelet components and organelles. Although debated for a long time, new data seems to indicate that the lung may be a central locus of platelet production. The new techniques for megakaryocyte investigations have barely begun to be of use in the study of abnormal platelet production in disease.

Megakaryocyte and vascular changes in rabbits on a short-term high cholesterol diet

The effect of a short-term high cholesterol diet on thrombopoiesis and vascular ultrastructure was evaluated in rabbits. Six pairs of male litter-mate rabbits were randomized pairwise to feeding with either 2 g of cholesterol daily in addition to their normal diet or normal diet alone for 7 days. A significant 12-fold increase in median serum cholesterol (P less than 0.035) and an insignificant decrease in platelet count (P = 0.07) were found in the animals fed a high cholesterol diet. In these animals the total and cytoplasmic megakaryocyte size measured as planimetric areas in bone marrow sections were significantly decreased (P less than 0.035). No statistically significant difference in the megakaryocyte DNA content measured by flow cytometry in marrow suspensions enriched for megakaryocytes by density gradient centrifugation and centrifugal elutriation was observed between the cholesterol-fed animals and controls. Light microscopic, transmission and scanning electron microscopic examination of the aorta in both groups of animals showed a morphologically intact endothelium without any adhesion of blood-borne cells to the luminal surface. Transmission electron microscopic studies showed that cells with ultrastructural features resembling smooth muscle cells were present in the intima of the aortas of the animals on the high cholesterol diet, but not in control animals. A decrease in the size of bone marrow megakaryocytes and the occurrence of intimal smooth muscle cells are found in rabbits fed a high cholesterol diet for 7 days. These cellular events may be important features in early atherogenesis.

Role of platelets and thrombosis in mechanisms of acute occlusion and restenosis after angioplasty

The vascular disruption produced by angioplasty initiates platelet deposition through the processes of platelet adhesion and recruitment of circulating platelets to form an enlarging mural platelet thrombus. Thrombin produced by simultaneous activation of the coagulation cascade by subendothelial connective tissue structures enhances platelet deposition and stabilizes the forming thrombus with enmeshing fibrin. Platelet recruitment involves the expression of the glycoprotein IIb/IIIa receptor for fibrinogen and other cytoadhesive proteins including fibronectin, thrombospondin and von Willebrand factor. Platelet deposition and thrombus formation caused by angioplasty appear to be important in the development of 2 complications: acute thrombotic occlusion and restenosis. Experimental mechanical vascular injury produces a predictable, although rather variable, amount of vascular narrowing due to transient smooth muscle cell proliferative intimal lesion formation. This intimal thickening by proliferating smooth muscle cells is in part mediated by platelet mitogens, particularly platelet-derived growth factor, which are released into the damaged vessel from platelets at the time of angioplasty. Platelet-derived growth factor may also be released from other associated vascular and blood cells in response to mechanical injury, e.g., endothelium, monocyte/macrophage and smooth muscle cells themselves. The actual mitogens, and their cells of origin, that mediate restenosis after therapeutic angioplasty remain to be established. Various oral antiplatelet agents have been shown to reduce arterial thrombotic occlusion in a number of controlled clinical trials, e.g., aspirin in transient ischemic attacks and unstable angina, aspirin and dipyridamole in saphenous vein coronary artery bypass and progression of peripheral vascular disease and dipyridamole in artificial heart valves. Acute arterial thrombosis may require more potent, immediate and transient intervention, e.g., monoclonal antibody to platelet receptor expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Constitutive and inducible secretion of platelet-derived growth factor analogs by human leukemic cell lines coexpressing erythroid and megakaryocytic markers

We have examined the constitutive and inducible secretion of platelet-derived growth factor (PDGF)-like proteins in a variety of human hemopoietic cell lines. The highest levels of secreted protein were noted in four human erythroleukemia lines which, in addition to erythroid lineage markers, express one or more megakaryocytic lineage markers. Induction of these lines by 12-O-tetradecanoylphorbol-13-acetate enhanced the expression of megakaryocytic markers and increased secretion of PDGF-like proteins several fold. In concert with these changes, there was significant induction of c-sis/PDGF-B messenger RNA (mRNA) expression in all lines, whereas one line showed significant concurrent induction of PDGF-A mRNA expression. Whether PDGF-like secretion is part of the stem cell-like phenotype displayed by these lines or is secondary to their leukemic transformation remains to be determined. Nevertheless, these lines provide new cellular models for studying the expression and function of PDGF analogs in hemopoietic cells.

Inverse relationship between megakaryocyte buoyant density and maturity

We examined the relationship between rat megakaryocyte buoyant density and maturation stage in continuous Percoll density gradients. An average of 88% of megakaryocytes had buoyant densities less than 1.054 g/ml. There was an inverse relationship between megakaryocyte buoyant density and maturation. Morphologically mature forms comprised 90% of the megakaryocytes with buoyant densities of 1.030-1.033 g/ml. In contrast, immature morphology was present in three-quarters of megakaryocytes with buoyant densities of 1.042-1.046 g/ml. These morphological findings were confirmed by [3H]thymidine labelling studies. Cell viability assessed by trypan blue exclusion was highest among more dense megakaryocytes of which the majority were immature. The lowest trypan blue exclusion was found in the less dense, predominantly mature megakaryocytes indicating that these cells are more susceptible to membrane damage during marrow suspension. Megakaryocyte DNA content distributions and platelet antigen levels, determined by two-colour flow cytometry, were also related to megakaryocyte density; the more dense megakaryocytes showed an approximately two-fold higher proportion of 8N cells and less platelet antibody binding than did less dense megakaryocytes. These studies suggest that megakaryocytes can be fractionated according to their buoyant densities into immature and mature populations suitable for molecular studies of differentiation.

Megakaryoblastic transformation of myelofibrosis with expression of the c-sis oncogene

We describe a case of primary myelofibrosis which terminated in an acute megakaryoblastic leukaemia with massive marrow fibrosis and osteosclerosis. The megakaryocyte lineage of the terminal phase was confirmed by ultrastructural and surface marker studies of the blast cells. The leukaemic phase was associated with the presence of large numbers of progressively more immature megakaryocyte progenitors in the peripheral blood. The expression of c-sis mRNA in these blast cells was significantly higher than in normal mononuclear cells. Activation of the c-sis protooncogene leading to increased production of platelet-derived growth factor could be related to the progressive fibrosis observed.

In vitro synthesis of low molecular weight proteins in human platelets: absence of labelled release products

Isolated human blood platelets incubated at 37 degrees C in vitro incorporated labelled amino acids into compounds which included some low molecular weight (less than 80KDa) proteins, as determined by autoradiography after sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions. Non-dialysable plasma factor(s) inhibited both uptake and incorporation, which although unaffected by Actinomycin D, was inhibited partly by Chloramphenicol and almost completely by Puromycin and Cycloheximide, results which confirm that synthesis is directed by pre-existing mRNAs, some of which is mitochondrial. Assuming that the mRNA coding for proteins which are truly "platelet specific" must be present in megakaryocyte cytoplasm, we investigated the possibility that such RNA may be sufficiently stable for its translation to continue in platelets. Although leakage from platelet alpha-granules and cytoplasm during incubation was negligible and platelets retained their secretory potential we were unable to detect radiolabelled proteins in thrombin-released material after incubation. We conclude that either alpha-granule proteins are not synthesised in platelets or their megakaryocyte progenitors, or that their mRNAs become degraded by the time platelets reach the peripheral circulation. Alternatively, the mechanism which concentrates these proteins in granules does not function in circulating platelets.

Comparison of intracellular and extracellular mitogenic activity

Endothelial cell-derived growth factor (ECDGF) is a soluble mitogen secreted in vitro by bovine aortic endothelium. ECDGF is a mixture of at least two distinct heat-stable and trypsin-sensitive mitogens. Large amounts of mitogenic activity were found in lysates prepared from cultured endothelial cells. Other nonmitogen-secreting cells in culture, including bovine dermal fibroblasts and vascular smooth muscle cells, also contained a similar activity. In contrast to ECDGF, the lysate mitogenic activities were sensitive to heat (56 degrees C) and were not inactivated by trypsin. Similar to platelet-derived growth factor (PDGF), ECDGF and cell lysate mitogens promoted cell proliferation in the absence of other defined mitogens when added to culture medium and after exposure to plastic. The cytoplasmic mitogens, however, were distinct from PDGF by receptor competition assays and other criteria.

Therapy-related leukemia associated with myelofibrosis. Blast cell characterization in six cases

Six patients exhibiting severe pancytopenia or overt leukemia associated with myelofibrosis after chemotherapy for malignant disease have been investigated by immunologic techniques and ultrastructural cytochemistry. Initially, five patients displayed severe thrombocytopenia contrasting with mild neutropenia and anemia. Bone marrow biopsies showed a clear megakaryocytic proliferation and an excess of immature mononuclear cells. The demonstration of peroxidase activities at the ultrastructural level and immunofluorescence labeling with a panel of monoclonal antibodies, including an antiplatelet glycoprotein Ib and an antiglycoprotein IIb-IIIa complex, on blood or marrow cells, permitted identification of otherwise unidentifiable promegakaryoblastic proliferation. In two patients, the use of an immunoperoxidase technique with an antifactor VIII-R-Ag antibody has allowed direct confirmation of this diagnosis on bone marrow sections. This megakaryoblastic proliferation was not pure and was variably associated with blasts of other cell lines (erythroblasts or myeloblasts). Changes in the population of blasts were observed during evolution in two patients. The sixth patient had a mild thrombocytopenia associated with severe neutropenia and anemia. Bone marrow biopsy displayed a myelofibrosis and immature cells, without megakaryocytic proliferation. Ultrastructural study revealed a pure basophil-mast cell proliferation. In conclusion, in five of six patients with secondary acute leukemia associated with myelofibrosis, a proliferation of promegakaryoblasts was demonstrated using both immunofluorescent and ultrastructural cytochemical techniques.

Optimal techniques for the immunocytochemical demonstration of beta-thromboglobulin, platelet factor 4, and fibrinogen in the alpha granules of unstimulated platelets

The distribution of beta-thromboglobulin, platelet factor 4, and fibrinogen in unstimulated platelets was investigated by several immunocytochemical techniques. All three substances were found to be localized in the majority of platelet alpha granules either by immunoperoxidase methods on saponin-treated platelets or by colloidal gold immunoconjugates on frozen thin sections. The optimal conditions for preparing and fixing platelets for immunocytochemistry were also determined. Platelets obtained from blood dripped directly into fixative or anticoagulated blood were compared systematically with respect to shape. Temperature was found to be the most important variable. Immediately fixed platelets were generally disc-shaped, regardless of the temperature of the fixative. Reducing the temperature of blood (stored with anticoagulant) before fixation resulted in more swollen and fewer disc-shaped platelets. However, if the blood was mixed with an anticoagulant and maintained at 37 degrees C for 1 h before fixation, the same number of disc-shaped platelets were present as in samples from blood fixed immediately. The intracellular localization of beta-thromboglobulin, platelet factor 4, and fibrinogen was consistent regardless of platelet preparatory procedure, but several technical problems were encountered with respect to plasma membrane labelling when control experiments were analysed. Immediately fixed, non-permeabilized platelet plasma membranes were always labelled, no matter which control substances or immunoperoxidase markers were used. However, when platelets were washed by centrifugation, the plasma membranes were negative. Exposure to saponin markedly diminished labelling of the plasma membranes. Optimal techniques for the immunocytochemical demonstration of these alpha granule proteins in platelets are presented in this report.

Platelet factor-4 excretion in myeloproliferative disease: implications for the aetiology of myelofibrosis

Experimental evidence suggests that the fibroblastic proliferation often associated with the myeloproliferative disorders is not part of the neoplastic process, but is secondary to an unknown stimulus. This stimulus may be a factor derived from platelets which promotes the proliferation of fibroblasts in vitro (PDGF). Platelet-derived growth factor is localized to platelet alpha-granules together with PF4 and beta-TG. As an indicator of alpha-granule release, we have measured PF4 levels in plasma, platelets and urine in 46 normal subjects and 49 patients with myeloproliferative disorders, secondary thrombocytosis and miscellaneous malignancies. All 11 patients with elevated urinary PF4 excretion exhibited myelofibrosis, whereas 11 of 22 patients with documented myelofibrosis had urinary PF4 excretion in the normal range. No correlation was seen between marrow fibrosis and plasma levels or the platelet content of PF4. The data are consistent with the possibility that release of mitogen(s) from platelet or megakaryocyte alpha-granules in some patients with myeloproliferative disorders is pathogenetically related to the development of marrow fibrosis.

Partial purification and characterization of a growth factor present in goat's colostrum. Similarities with platelet-derived growth factor

A factor in goat's colostrum which stimulates DNA synthesis and cell proliferation in Swiss 3T3 fibroblasts has been purified approx. 350-fold by a sequence of acid precipitation, cation-exchange chromatography and gel filtration. The growth factor is a highly basic, heat stable (100 degrees C for 5 min) polypeptide with Mr approx. 35000. The polypeptide resists denaturation by guanidinium chloride or urea but is totally inactivated by treatment with reducing agents. The factor, which we have termed colostric basic growth factor ( CBGF ), inhibits the binding of 125I-labelled epidermal growth factor (125I-EGF) to Swiss 3T3 fibroblasts but does not inhibit 125I-EGF binding to epidermoid A431 cells. CBGF interacts synergistically with plasma in stimulating DNA synthesis in quiescent Swiss 3T3 cells. The chemical and biological properties of CBGF are thus very similar to the properties reported for the human platelet-derived growth factor. Although high concentrations of CBGF are present in the colostrum of goats, cows, and sheep, the milk of these species contains little or no factor. The origin and possible functions of CBGF are unknown.

[New aspects of the biochemistry and biology of the arterial wall]

Early lesions of arteriosclerosis are characterized by proliferating smooth muscle cells, macrophages,and foam cells. In addition, large amounts of connective tissue components and cholesterol esters are found. These changes are primarily located in the intima of the arterial wall. The initial mechanisms responsible for lesion formation are largely unknown. In recent years progress has been made particularly in fields of research related to the biochemistry of arterial wall cells in tissue culture. The findings obtained allow us to deepen our knowledge of the pathophysiology of arteriosclerosis. Of special interest are mechanisms involved in the maintenance of the thromboresistant endothelium, the factors triggering proliferation of intimal smooth muscle cells, and the transformation of macrophages to foam cells.

Platelet-derived growth factor

Platelet-derived growth factor (PDGF) is a basic (pI congruent to 10) 30 000 molecular weight protein circulating in normal blood sequestered within the platelet alpha-granules. It binds with high affinity (Kd = 10(-11) M) to a specific cell-surface receptor found on many connective tissue cell types in culture. It is active in stimulating the metabolism and multiplication of connective tissue cells at very low concentrations (ED50 = 10(-11) M). It is likely that PDGF is released from platelets at sites of vascular damage and that it contributes toward the cell proliferation and connective tissue formation seen in healing wounds and in arteriosclerotic lesions. PDGF which does not bind to responsive cells at the wound site is largely inactivated by a plasma binding protein and is rapidly cleared from the circulation.

New synthesis of a platelet-specific protein: platelet factor 4 synthesis in a megakaryocyte-enriched rabbit bone marrow culture system

The site of synthesis of platelet-specific proteins remains to be established. With the use of short-term megakaryocyte-enriched cultures, direct evidence was obtained to show that megakaryocytes synthesize the platelet-specific protein, platelet factor 4. A megakaryocyte-enriched fraction of rabbit bone marrow for culture was obtained by centrifugal elutriation and cultured with [3H]leucine. Newly synthesized 3H-platelet factor 4 was sought by copurification with added carrier rabbit platelet factor 4, using heparin agarose affinity chromatography and immunoprecipitation with specific goat anti-rabbit platelet factor 4 antisera. SDS PAGE of the washed immunoprecipitates demonstrated a [3H]leucine-containing peak which migrated identically with purified homogeneous rabbit platelet factor 4. A second, slightly larger molecular-weight protein was identified in the gels also, suggesting that rabbit platelet factor 4 may be synthesized as a larger molecular-weight precursor in rabbit megakaryocytes. These results provide direct evidence that the platelet-specific protein, platelet factor 4, is synthesized in rabbit megakaryocytes before it is packaged into alpha-granules for release in circulating platelets.

Myelofibrosis and acute megakaryoblastic leukemia in a child: topographic relationship between fibroblasts and megakaryocytes with an alpha-granule defect

In a child with acute megakaryoblastic leukemia--severe thrombocytopenia and myelofibrosis, EM studies on bone marrow showed a strict topographic relationship between the presence of clusters of abnormal megakaryocytes and the increased number of fibroblasts and extracellular fibers. Megakaryocytes and platelets lacked alpha-granules while the plasma thromboglobulin level was three times the normal level. This suggested that the alpha-granular proteins were synthesized but not retained in alpha-granules. If this occurs, the increased marrow levels of platelet-derived growth factor and factor 4 would favor the proliferation of fibroblasts and the synthesis of collagen, and thereby promote myelofibrosis. After therapy-induced remission, the number of marrow megakaryocytes decreased, the alpha-granules were normally produced, the plasma beta-thromboglobulin level was normal and the myelofibrosis disappeared. These observations suggest that during acute megakaryoblastic leukemia, an acquired gray-platelet syndrome occurs and that the local excretion of alpha-granule proteins triggers the myelofibrosis.

Effect of platelet-derived growth factor on DNA and protein synthesis in cultured rat calvaria

Platelet-derived growth factor (PDGF), a peptide isolated from human platelets, was studied for its effects on bone formation by examining the synthesis of DNA, collagen and noncollagen protein in cultures of 21 day fetal rat calvaria. PDGF, at concentrations of 0.6-200 ng/ml, increased the incorporation of 3H-thymidine into acid insoluble residues in calvaria cultured for 24-96 hr; PDGF also increased the bone DNA content. After 24 hr of culture, PDGF caused a dose-dependent stimulation of 3H-proline incorporation into noncollagen protein (NCP) and a small stimulation of its incorporation into collagenase-digestible protein (CDP). After 96 hr of culture, PDGF did not affect NCP but inhibited the labeling of CDP. The PDGF stimulatory effect on the incorporation of 3H-thymidine was seen in both the periosteum and periosteum-free calvarium while that on the labeling of NCP was seen in the central nonperiosteal bone. Histological sections showed a threefold increase in the mitotic index after colcemid arrest in PDGF-treated bones. The stimulatory effect of PDGF on the incorporation of 3H-thymidine was enhanced by cortisol. These studies indicate that PDGF stimulates bone DNA synthesis and has a no specific stimulatory effect on protein synthesis in short-term cultures.

Cholesterol and phospholipid biosynthesis in guinea pig megakaryocytes

Megakaryocytes are the bone marrow cells which produce the blood platelets. Platelet lipids are thought to be important determinants of platelet activity in thrombosis and hemostasis. We have investigated lipid synthesis from [U-14C]-acetate in isolated guinea pig megakaryocytes with the goal of elucidating the genesis of platelet lipids. Cholesterol was the major product of megakaryocyte lipid synthesis from [U-14C]acetate. Incorporation of acetate (0.1 mM) into cholesterol per 10(6) megakaryocytes (1.1 mg protein) was 0.14 nmol in 0.5 h, 0.95 nmol in 1.5 h and 3.2 nmol in 4.0 h. Megakaryocytes also synthesized cholesterol from [U-14C]glucose. In contrast, platelet sterol synthesis was negligible. Both megakaryocytes and platelets synthesized phospholipids from [U-14C]acetate. Megakaryocytes incorporated 0.07 nmol acetate per 10(6) cells into phospholipid in 0.5 h, 0.38 nmol in 1.5 h and 1.8 nmol in 4.0 h. Platelets (10(9) cells, 1.3 mg protein) incorporated 0.15 nmol acetate into phospholipids in 1.5 h. Phosphatidylcholine accounted for 62% of the phospholipid radioactive label in megakaryocytes and 78% in platelets. Phospholipid radioactivity was associated with the fatty acids. We hypothesize that the megakaryocyte may synthesize a major portion of platelet cholesterol and that the phospholipid and fatty acid synthetic pathways available to the platelet are derived from the megakaryocytes.