Collagen type I modulates the platelet-derived growth factor (PDGF) regulation of the growth and expression of beta1 integrins by rat mesangial cells

Mesangial cell (MC) proliferation and the deposition of collagen type I (collagen I) are the major pathological features in many types of glomerulonephritis (GN). Recent work suggested that beta-integrins play a critical role in the cell proliferation and extracellular matrix (ECM) remodeling observed in tissue repair after injury. To examine the involvement of beta-integrins in MC proliferation in association with the interaction of MCs with pathological collagen I, we investigated the effect of a prominent mitogen, platelet-derived growth factor-BB (PDGF-BB) on the growth and expression of beta-integrins by MCs cultured on plastic or in a three-dimensional collagen I gel. Immunoprecipitation using 35S-metabolic labeling, flow cytometry and a 3H-thymidine-uptake analysis demonstrated that PDGF-BB stimulated the cell mitogenicity and the expression of alpha5beta1 integrin (a fibronectin receptor), but not alpha1beta1 integrin (a collagen and laminin receptor) of MCs on plastic, in a dose-dependent manner. In contrast, MCs in the collagen I gels showed no significant changes in mitogenicity or alpha1beta1 and alpha5beta1 integrin expression, but increased alpha1beta1 integrin-mediated gel contraction was observed after PDGF-BB stimulation. Thus, the parallel up-regulation of MC-mitogenicity and alpha5beta1 integrin expression by PDGF-BB suggested that alpha5beta1 integrin is an important ECM receptor involved in the proliferative phenotype of MC. A spatial interaction between MCs and pathological collagen I in GN may influence the PDGF regulation of the MC phenotype regarding the cell growth and the expression of beta1 integrins.     

High glucose-induced transforming growth factor beta1 production is mediated by the hexosamine pathway in porcine glomerular mesangial cells

Previous studies revealed that exposure of mesangial cells to high glucose concentration induces the production of matrix proteins mediated by TGF-beta1. We tested if structural analogues of D-glucose may mimic the high glucose effect and found that D-glucosamine was strikingly more potent than D-glucose itself in enhancing the production of TGF-beta protein and subsequent production of the matrix components heparan sulfate proteoglycan and fibronectin in a time- and dose-dependent manner. D-Glucosamine also promoted conversion of latent TGF-beta to the active form. Therefore, we suggested that the hexosamine biosynthetic pathway (the key enzyme of which is glutamine:fructose-6-phosphate amidotransferase [GFAT]) contributes to the high glucose-induced TGF-beta1 production. Inhibition of GFAT by the substrate analogue azaserine or by inhibition of GFAT protein synthesis with antisense oligonucleotide prevented the high glucose-induced increase in cellular glucosamine metabolites and TGF-beta1 expression and bioactivity and subsequent effects on mesangial cell proliferation and matrix production. Overall, our study indicates that the flux of glucose metabolism through the GFAT catalyzed hexosamine biosynthetic pathway is involved in the glucose-induced mesangial production of TGF-beta leading to increased matrix production.     

Inhibition of platelet-derived growth factor autocrine growth stimulation by a monoclonal antibody to the human alpha platelet-derived growth factor receptor

A potent neutralizing monoclonal antibody to the human alpha platelet-derived growth factor (PDGF) receptor (alpha PDGFR) was raised by immunizing BALB/c mice with 32D cells expressing the human alpha PDGFR. This monoclonal antibody, designated alpha R1, immunoprecipitated human, monkey, rabbit, pig, dog, and cat, but not hamster, rat, or mouse alpha PDGFRs. Comparison with PR292, a monoclonal antibody previously generated against the alpha PDGFR, showed that both recognized alpha PDGFR extracellular domains, but neither demonstrated reactivity against the beta PDGFR. In vitro binding studies revealed that alpha R1, but not PR292, detection of the alpha PDGFR was blocked by either PDGF AA or PDGF BB. These results strongly suggest that the receptor ligand-binding domain spatially overlapped with the alpha R1 epitope. Monoclonal antibody alpha R1 also inhibited PDGF stimulation of [3H]thymidine uptake by 32D cells expressing the alpha PDGFR (32D alpha R) as well as autocrine growth stimulation of 32D alpha R cells transfected with and expressing PDGF AA or PDGF BB. Therefore, monoclonal antibody alpha R1 may be useful in the detection and growth inhibition of malignancies in which PDGF autocrine stimulation and/or alpha PDGFR overexpression plays an important role(s).     

Insulin potentiates platelet-derived growth factor action in vascular smooth muscle cells

Correlative studies have indicated that hyperinsulinemia is present in many individuals with atherosclerosis. Insulin resistance has also been linked to cardiovascular disease. It has proved to be difficult to decipher whether hyperinsulinemia or insulin resistance plays the most important role in the pathogenesis of atherosclerosis and coronary artery disease. In this study, we demonstrate that insulin increases the amount of farnesylated p21Ras in vascular smooth muscle cells (VSMC), thereby augmenting the pool of cellular Ras available for activation by platelet-derived growth factor (PDGF). In VSMC incubated with insulin for 24 h, PDGF's influence on GTP-loading of Ras was significantly increased. Furthermore, in cells preincubated with insulin, PDGF increased thymidine incorporation by 96% as compared with a 44% increase in control cells (a 2-fold increment). Similarly, preincubation of VSMC with insulin increased the ability of PDGF to stimulate gene expression of vascular endothelial growth factor 5- to 8-fold. The potentiating influence of insulin on PDGF action was abrogated in the presence of a farnesyltransferase inhibitor. Thus, the detrimental influence of hyperinsulinemia on the arterial wall may be related to the ability of insulin to augment farnesyltransferase activity and provide greater amounts of farnesylated p21Ras for stimulation by various growth promoting agents.     

Regulation of chemotaxis by the platelet-derived growth factor receptor-beta

Chemotaxis is an important component of wound healing, development, immunity and metastasis, yet the signalling pathways that mediate chemotaxis are poorly understood. Platelet-derived growth factor (PDGF) acts both as a mitogen and a chemoattractant. Upon stimulation, the tyrosine kinase PDGF receptor-beta (PDGFR-beta) autophosphorylates and forms a complex that includes SII2(Src homology 2)-domain-containing proteins such as the phosphatidylinositol-specific phospholipase C-gamma, Ras-GTPase-activating protein (GAP), and phosphatidylinositol-3-OH kinase. Specific tyrosine-to-phenylalanine substitutions in the PDGFR-beta can prevent binding of one SH2-domain-containing protein without affecting binding of other receptor-associated proteins. Here we use phospholipase C-gamma and PDGFR-beta mutants to map specific tyrosines involved in both positive and negative regulation of chemotaxis towards the PDGF-BB homodimer. Our results indicate that a delicate balance of migration-promoting (phospholipase C-gamma and phosphatidylinositol-3-OH kinase) and migration-suppressing (GAP) activities are recruited by the PDGFR-beta to drive chemotaxis towards PDGF-BB.     

Evidence for a functional hepatocyte growth factor receptor in human mesangial cells

In a previous study, mu-opioid receptor binding was decreased by chronic treatment of rats with a mu-opioid receptor-selective agonist [CH3Phe3, D-Pro4]morphiceptin (PL-017) [Tao, P.L., Lee, H.Y., Chang, L.R., Loh, H.H., 1990. Decrease in mu-opioid receptor binding capacity in rat brain after chronic PL-017 treatment. Brain Res. 526, 270-275]. However, there was a lack of correlation between the time course of receptor down-regulation and the loss of pharmacological effects of the drug. In the current study, we used immunohistochemistry to reinvestigate this issue. Male Sprague-Dawley rats were chronically treated with PL-017 i.c.v. for 1, 3 or 5 days, using an escalating dosage paradigm (0.75-6.0 microg), which resulted in a 1.4 to 32-fold increase in the AD50. Rat brains were removed, frozen, coronally sectioned (14 microm) and processed for mu-, delta- or kappa-opioid receptor immunohistochemistry by the avidin-biotin complex (ABC) method. Significant decreases in OP3 immunodensity were found in many brain regions which are enriched with OP3 after chronic treatment of PL-017. Time-dependent decreases in OP3 were detected and reached a plateau around 3 days of PL-017 treatment. No significant change in OP1 or OP2 immunodensity after chronic treatment with PL-017 was found. Our conclusion is that chronic treatment with PL-017 of rats selectively down-regulates mu-opioid receptors in the brain. This may be an important mechanism for PL-017 tolerance.     

Embryonic mesoderm cells spread in response to platelet-derived growth factor and signaling by phosphatidylinositol 3-kinase

Abnormal mesoderm movement, leading to defects in axial organization, is observed in mouse and Xenopus laevis embryos deprived of platelet-derived growth factor (PDGF) AA signaling. However, neither the cellular response to PDGF nor the signaling pathways involved are understood. Herein we describe an in vitro assay to examine the direct effect of PDGF AA on aggregates of Xenopus embryonic mesoderm cells. We find that PDGF AA stimulates aggregates to spread on fibronectin. This behavior is similar to that of migrating mesoderm cells in vivo that spread and form lamellipodia and filipodia on contact with fibronectin-rich extracellular matrix. We go on to show two lines of evidence that implicate phosphatidylinositol 3-kinase (PI3K) as an important component of PDGF-induced mesoderm cell spreading. (i) The fungal metabolite wortmannin, which inhibits signaling by PI3K, blocks mesoderm spreading in response to PDGF AA. (ii) Activation of a series of receptors with specific tyrosine-to-phenylalanine mutations revealed PDGF-induced spreading of mesoderm cells depends on PI3K but not on other signaling molecules that interact with PDGF receptors including phospholipase C gamma, Ras GTPase-activating protein, and phosphotyrosine phosphatase SHPTP2. These results indicate that a PDGF signal, medicated by PI3K, can facilitate embryonic mesoderm cell spreading on fibronectin. We propose that PDGF, produced by the ectoderm, influences the adhesive properties of the adjacent mesoderm cells during gastrulation.     

Quantitation of platelet-derived growth factor receptors in human arterial smooth muscle cells in vitro

Platelet-derived growth factor (PDGF) is suggested to play an important role in the development of atherosclerosis as a migratory and mitogenic stimulus to arterial smooth muscle cells (ASMCs). Stimulated and unstimulated ASMCs were studied with respect to PDGF receptor (PDGF-R) mRNA and protein expression. Quantitative RT-PCR was developed for simultaneous evaluation of both PDGF-R alpha and -R beta mRNA expression and a quantitative ELISA for estimation of corresponding PDGF-R subunits. On the mRNA level, the overall PDGF-R beta expression was approximately 100 times lower than that of PDGF-R alpha. Furthermore, although PDGF-R alpha mRNA levels were high irrespective of hASMC phenotype, PDGF-R beta mRNA was influenced by serum stimulation with lower copy numbers in proliferating and confluent cells compared with quiescent cells. On the protein level, quiescent hASMCs expressed 10 times more PDGF-R beta than PDGF-R alpha. Serum stimulation decreased cell surface PDGF-Rs, with most prominent loss of PDGF-R alpha (ELISA and immunohistochemistry). Our results suggest a differential regulatory pattern for PDGF-R alpha and -R beta and are compatible with the usage of alternative promoters for regulation of -R alpha expression. Further, it seems that the number of available receptor subunits is not the only determinant of variations in cell stimulation with different PDGF isoforms.     

Signal transduction via platelet-derived growth factor receptors

Platelet-derived growth factor (PDGF) exerts its stimulatory effects on cell growth and motility by binding to two related protein tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation, allowing binding and activation of cytoplasmic SH2-domain containing signal transduction molecules. Thereby, a number of different signaling pathways are initiated leading to cell growth, actin reorganization migration and differentiation. Recent observations suggest that extensive cross-talk occurs between different signaling pathways, and that stimulatory signals are modulated by inhibitory signals arising in parallel.     

SPARC is expressed by mesangial cells in experimental mesangial proliferative nephritis and inhibits platelet-derived-growth-factor-medicated mesangial cell proliferation in vitro

Mesangial cell proliferation is a characteristic feature of many glomerular diseases and often precedes extracellular matrix expansion and glomerulosclerosis. This study provides the first evidence that SPARC (secreted protein acidic and rich in cysteine) could be an endogenous factor mediating resolution of experimental mesangial proliferative nephritis in the rat. SPARC is a platelet-derived-growth-factor-binding glycoprotein that inhibits proliferation of endothelial cells and fibroblasts. We now show that SPARC is synthesized by mesangial cells in culture and that SPARC mRNA levels are increased by platelet-derived growth factor and basic fibroblast growth factor. Recombinant SPARC or the synthetic SPARC peptide 2.1 inhibited platelet-derived-growth-factor-induced mesangial cell DNA synthesis in vitro. In a model of experimental mesangioproliferative glomerulonephritis, SPARC mRNA was increased 5-fold by day 7 and was identified in the mesangium by in situ hybridization. Similarly, SPARC was increased in glomerular mesangial cells and visceral epithelial cells by day 5 and reached maximal expression levels by day 7. Mesangial cell proliferation increased by 36-fold on day 5 and decreased abruptly on day 7. Maximal expression of SPARC was correlated with the resolution of mesangial cell proliferation. We propose that SPARC functions in part as an endogenous inhibitor of platelet-derived-growth-factor-mediated mesangial cell proliferation in glomerulonephritis and that it could account for the resolution of cellular proliferation in this disease.     

Isoform-specific induction of nuclear free calcium oscillations by platelet-derived growth factor

Confocal laser scanning microscopy was used to analyze alterations in nuclear free calcium (Ca2+n) levels induced by platelet-derived growth factor (PDGF) isoforms in BALB/c3T3 fibroblasts loaded with the calcium-sensitive fluorescent indicator Fluo-3. Both AA-PDGF and BB-PDGF caused a transient increase in Ca2+n. Analysis of PDGF-induced Ca2+n alterations as a function of time revealed that BB-PDGF stimulation resulted in the generation of Ca2+n oscillations that diminished over time. The frequency of BB-PDGF-stimulated oscillations was modulated by extracellular Ca2+ and could not be mimicked by increasing intracellular inositol 1,4,5-trisphosphate levels in the absence of growth factor stimulation. Caffeine alone had no effect on Ca2+n levels, but exposure of cells to caffeine after BB-PDGF stimulation augmented Ca2+n oscillations, either by increasing the frequency or reinitiating preexisting oscillations. The genesis of these oscillations in Ca2+n appears to be in the region just outside of the nucleus, as perinuclear cytoplasmic free calcium (Ca2+i) increased just prior to Ca2+n. In contrast, AA-PDGF stimulation resulted in the generation of one or two irregular, transient Ca2+n spikes. Caffeine pretreatment followed by AA-PDGF stimulation resulted in Ca2+n oscillations very similar to those produced by BB-PDGF alone. Additionally, the AA-PDGF and BB-PDGF isoforms appeared to modulate distinct pools of cellular Ca2+, as BB-PDGF was still capable of inducing Ca2+n oscillations subsequent to prior induction of oscillations by AA-PDGF/caffeine. These PDGF isoform-specific changes in nuclear free Ca2+ could serve as a mechanism by which isoform-specific cellular signaling pathways may be manifested by the growth factors.     

Visceral glomerular epithelial cells can proliferate in vivo and synthesize platelet-derived growth factor B-chain

In glomerular diseases associated with antibody- and complement-mediated injury to endothelial and mesangial cells, cell proliferation is an important early response that precedes matrix accumulation and glomerulosclerosis. In contrast, in diseases in which the visceral glomerular epithelial cell (vGEC) is the principal target of injury, cell proliferation is not a recognized consequence, although vGECs respond in vitro to a variety of growth factors and inflammatory mediators. To explore the possibility that low levels of vGEC proliferation may occur and participate in such diseases, serial studies were done in the passive Heymann nephritis model of membranous nephropathy, in which the vGEC is the primary target of antibody- and C5b-9-mediated injury. The results showed mitotic figures and positive staining for the proliferating cell nuclear antigen in cells whose location defined them as vGECs. The proliferating cell nuclear antigen-positive cells at the edge of the capillary wall were confirmed to be vGECs by double-immunostaining with antibodies to SPARC/osteonectin, which preferentially label vGECs in passive Heymann nephritis. Proliferation of vGECs in vivo was preceded by increased glomerular expression of platelet-derived growth factor (PDGF) B-chain protein and messenger RNA, both of which localized to vGECs. PDGF B-chain protein and messenger RNA were also detected in cultured vGECs. PDGF receptor beta-subunit protein or messenger RNA could not be demonstrated in vGECs in vivo or in vitro, and no growth response of cultured vGECs to PDGF was noted. These results suggest that proliferation of vGECs does occur in a model of glomerular injury induced by antibody and C5b-9 on vGECs. VGEC proliferation and production of PDGF may be involved in the restoration of the capillary wall but could also contribute to local capillary wall injury and proliferation of other cells in Bowman's capsule, interstitium, and tubules.     

Activated mesangial cells produce vascular permeability factor in early-stage mesangial proliferative glomerulonephritis

Vascular permeability factor (VPF), also known as vascular endothelial growth factor (VEGF), is a multifunctional cytokine involved in angiogenesis, inflammation, and wound healing. Although VPF/VEGF has been reported to be produced only by glomerular podocytes in glomeruli, it was found that it is produced by human cultured mesangial cells (MC). Therefore, immunohistochemical analysis (using indirect immunofluorescence and in situ hybridization) of VPF/VEGF in normal kidneys (n = 7) and biopsy specimens taken from 83 patients with renal diseases, including mesangial proliferative glomerulonephritis (PGN) (n = 58), was performed to examine whether VPF/VEGF is produced by MC in human PGN. In all of the healthy subjects and all of the patients except those with PGN (disease control subjects), VPF/VEGF protein and mRNA were detected mainly in podocytes. However, in some PGN patients, VPF/VEGF protein was demonstrated clearly in MC as well as in podocytes, as some of the VPF/VEGF was colocalized with alpha-smooth muscle actin, a marker of activated MC, and VPF/VEGF mRNA was expressed by MC and podocytes. Mesangial VPF/VEGF expression level increased significantly in PGN patients with early lesions (P < 0.01 versus healthy subjects or disease control subjects, P < 0.05 versus PGN with later lesions). The time between biopsy and disease onset was significantly shorter in PGN patients with than in those without mesangial VPF/VEGF expression (P < 0.01). These findings provide the first evidence that activated MC are a source of VPF/VEGF in human PGN, and indicate that mesangial VPF/VEGF expression is characteristic of early lesions of PGN. Because VPF/ VEGF plays a pivotal role in tissue repair, MC-produced VPF/VEGF may play pathophysiologic roles, including promoting recovery from glomerular injuries, in early-stage PGN.     

Regulation of the growth hormone-insulin-like growth factor I axis in developing and adult monkeys is affected by estradiol replacement and supplementation with insulin-like growth factor I

Developmental changes in the GH-insulin-like growth factor I (IGF-I) axis were evaluated in female rhesus monkeys to test the hypothesis that estradiol differentially regulates IGF-I secretion and molar ratios of IGF-I to IGF-binding protein-3 (IGFBP-3) from adolescence into adulthood and that estradiol can reestablish GH secretion in the face of enhanced IGF-I negative feedback inhibition of GH. Adult ovariectomized females were compared to ovariectomized adolescent females studied from 18-36 months of age, a period encompassing the juvenile phase through the expected age at first ovulation. A subgroup of adult (n = 5) and adolescent females (n = 5) was treated continuously with human IGF-I (110 micrograms/kg.day, s.c.) throughout the study period and were compared to age-matched, untreated adults (n = 5) and adolescent animals (n = 6). To further understand how IGF-I affects the GH-IGF-I axis, the acute response to IGF-I (100 micrograms/kg, s.c.) was assessed in adults and at two ages in developing females. Furthermore, all females were treated periodically with estradiol (4 micrograms/kg.day) to assess the effects on the parameters of the GH-IGF-I axis from adolescence into adulthood. Finally, the response to GHRH (1.0 microgram/kg, i.v.) was assessed in adult females and in adolescent females at 18 and 24 months during no estradiol and estradiol replacement. Serum IGF-I and IGFBP-3, in the absence of estradiol replacement, increased significantly throughout puberty before declining from late adolescence into adulthood. Supplementation with IGF-I resulted in a significant increase in both serum IGF-I and IGFBP-3 concentrations at all ages, although the effect was less in juvenile females. Nevertheless, the age-dependent increase and decline in IGF-I and IGFBP-3 were maintained in these supplemented animals. Estradiol replacement significantly increased both serum IGF-I and IGFBP-3 through adolescence, even in IGF-I-supplemented animals. However, with the transition from adolescence, estradiol suppressed serum IGF-I secretion, yet continued to increase IGFBP-3 in young adult and fully adult females. This change in proportionately less IGF-I compared with IGFBP-3 resulted in a significant age-dependent decrease in the molar ratio of IGF-I to IGFBP-3. Indeed, the molar ratio was highest during midadolescence, when both IGF-I and IGFBP-3 were at their zeniths. Serum IGFBP-1 was significantly higher in adolescent compared with adult females. However, estradiol replacement significantly elevated serum IGFBP-1 in adult, but not adolescent, females, abolishing the age differences observed under no estradiol conditions. Serum GH was significantly higher in adolescent compared with adult females; levels in juvenile animals were intermediate. Replacement with estradiol significantly elevated serum GH in adolescent and adult females, particularly in females supplemented with IGF-I. In contrast, estradiol had no effect on serum GH during the juvenile phase. Supplementation with IGF-I significantly dampened the response to GHRH in young and fully adult females, but not in juvenile animals. However, estradiol replacement restored the response to GHRH in these adult, IGF-I-supplemented females. These data indicate that in the absence of any ovarian influence, the decline in serum IGF-I and IGFBP-3 begins in postpubertal, young adult females and is not necessarily a consequence of old age. Furthermore, there is an age-dependent uncoupling of estradiol regulation of the GH-IGF-I axis, as estradiol stimulates GH and IGFBP-3 at all ages but increases serum IGF-I only during adolescent and decreases IGF-I in postpubertal, young adult females. Furthermore, IGF-I has a greater suppressive effect on GH secretion with advancing age, an effect reversed by estradiol replacement. These data suggest that the deficits in the GH-IGF-I axis observed in aged individuals may reflect a continuation of the regulatory changes that begin in young adult females.     

Insulin-like growth factor I in fetal serum obtained by cordocentesis is correlated with intrauterine growth retardation

We examined whether insulin-like growth factor-I (IGF-I) and one of its binding proteins (IGFBP-1) in fetal serum obtained by cordocentesis is correlated with intrauterine growth retardation (IUGR) and weight estimation by ultrasound. Cordocentesis sera from 27 fetuses suspected of having IUGR were analysed for IGF-I and IGFBP-1 by radioimmunoassay. The results showed that IGF-I concentrations were correlated significantly with birth weight (P < 0.001) and placenta weight (P < 0.05). Mean fetal concentrations of IGF-I were 38 +/- 18 microg/l. In patients (n = 11) with a weight deviation at delivery <-33%, IGF-I concentrations were 24.1 +/- 13.2 microg/l. IGFBP-1 was inversely correlated with birth weight (P < 0.006) and concentrations of IGF-I. Mean plasma concentrations of IGFBP-1 were 234.2 +/- 161.4 microg/l. Furthermore, IGF-I concentrations were correlated with the weight deviation estimated by ultrasonography at the time of cordocentesis (P < 0.007), as well as with the weight deviation at delivery (P < 0.0001). The actual weight deviation at delivery was correlated more strongly with fetal IGF-I concentrations than with the estimated weight deviation at cordocentesis. The lowest concentrations of IGF-I were found in patients with a weight deviation <-33%. Very low concentrations of IGF-I are thus associated with IUGR, indicating that IGF-I measured in fetal serum may increase the predictive value of ultrasonographic weight estimation.     

Usefulness of platelet-derived growth factor as a prognostic factor in pulmonary adenocarcinoma

The effects of 6-methyluracil given in single intraperitoneal doses of 50 and 2.7 mg/kg on the healing of burn wounds and some physicochemical parameters of lipid peroxidation regulation were studied in the liver and erythrocytes of noninbred albino rats with thermal burns. 6-Methyluracil was shown to alter the time course of a wound process and to accelerate the healing of burn wounds. The drug doses under study were found to exert a great effect on the level of lipid antioxidative activity and the composition of phospholipids of the liver and erythrocytes, which remained for a long time after burn. The findings suggest the hypothesis that the capacity of 6-methyluracil to be involved in the regulation of lipid peroxidation processes underlies its therapeutic effect.     

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

Growth factor activity, as determined by the stimulation of [3H]thymidine incorporation into the DNA of quiescent 3T3 cells in culture, was found in lysates of guinea pig platelets and megakaryocytes. Quantitative dilution studies demonstrated that, of the cells present in the guinea pig bone marrow, only the megakaryocyte possessed quantitatively significant growth factor activity. The amount of activity present in one megakaryocyte was equivalent to that present in 1,000-5,000 platelets, a value approximately comparable to the number of platelets shed from a single megakaryocyte. It is suggested that guinea pig platelet-derived growth factor has its origin in the megakaryocyte.