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Biomedical subjects

D R Clemmons

Publications and source records attributed to D R Clemmons.

At least 199 records · Page 11Linked to original sources
PubMed1987

The effect of exercise on plasma somatomedin-C/insulinlike growth factor I concentrations.

Because inadequate intake of energy or protein causes reduction in the plasma concentrations of somatomedin-C/insulinlike growth factor I (Sm-C/IGF-I), we have completed studies to determine whether the energy deficit induced by vigorous exercise sustained for 1 week would also be associated with a reduction in this peptide. When six healthy, exercise-conditioned males were fed a diet consisting of 35 kcal/kg ideal body weight (IBW) and 1.0 g protein/kg and exercised so that 14.1 to 16.3 kcal/kg of energy was expended each day, their plasma Sm-C/IGF-I concentrations declined from a mean basal value of 1.15 +/- 0.78 U/mL (1 +/- SD) to 0.62 +/- 0.41 U/mL during the last two days of a seven-day exercise period (P less than .05). This decrease in Sm-C/IFG-I occurred almost entirely in the first four days of the study period. After 3 weeks for reacclimation, the calorie intake of each volunteer was reduced by the same number of calories as had been expended in the form of exercise during each day of the exercise week (mean of 15.1 kcal/kg IBW/d). On this regimen the mean plasma Sm-C/IGF-I declined from 1.12 +/- 0.57 U/mL, to 0.69 +/- 0.37 U/mL on the last two days of dietary restriction. The magnitude of fall with dietary restriction was not significantly different from that observed during exercise. During the last three days of the exercise period the mean daily nitrogen balance was -1.60 +/- 1.70 g/d, while this value was -3.50 +/- 1.73 g/d during the last three days of dietary restriction (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1987

An insulin-like growth factor (IGF) binding protein enhances the biologic response to IGF-I.

The insulin-like growth factors IGF-I and IGF-II circulate in blood bound to carrier proteins. The higher molecular mass IGF-binding protein complex (150 kDa) is composed of subunits, and one subunit that forms this complex is growth hormone dependent. In addition, many cell types and tissues secrete another form of IGF binding protein that is not growth hormone dependent. Both forms of the IGF binding protein are believed to inactivate the IGFs and to function as delivery systems to tissues. This conclusion was based on studies that determined the effects of impure preparations of these binding proteins or that examined the effect of these proteins only on the insulin-like actions of the IGFs. We report here that a pure preparation of the extracellular form of the IGF binding protein (purified from human amniotic fluid) markedly potentiated replication of several cell types in response to human IGF-I. Secondary cultures of human, mouse, and chicken embryo fibroblasts as well as porcine aortic smooth muscle cells showed marked enhancement of their DNA synthesis response (2.8- to 4.4-fold increases) to IGF-I in the presence of this protein. These responses were synergistic since the sum of the responses to either IGF-I or to the binding protein alone was between 8 and 17% of the increase obtained in cultures exposed to both peptides. The binding protein not only potentiated the DNA synthesis response but also enhanced the increase in cell number in response to IGF-I. This stimulation is specific for growth factors that bind to the binding protein since incubation with insulin, which binds to the type I IGF receptor but not to the binding protein, did not result in potentiation of this response. We conclude that a form of IGF binding protein that is present in extracellular fluids and is secreted by many types of cells can markedly potentiate the cellular response to IGF-I.

Animals↗
PubMed1987

Reduced oxygen tension induces pulmonary endothelium to release a pulmonary smooth muscle cell mitogen(s).

Bovine pulmonary endothelial cells grown in vitro were shown to release a factor that was mitogenic for pulmonary smooth muscle cells during exposure to a reduced oxygen tension atmosphere. The addition of hypoxic endothelium-derived medium resulted in a 60% increase in smooth muscle cell number after 24 h of exposure. Addition of medium from pulmonary endothelium exposed to normoxic conditions or medium derived from either hypoxic or normoxic aortic endothelium did not result in significant increases in smooth muscle cell number. Physicochemical characterization of the hypoxic pulmonary endothelial cell-derived factor(s) showed that it was resistant to heat and reducing agent treatment and stable between pH 3 and 10. The mitogenic activity decreased by 71% at pH 2 and by 68% after treatment with trypsin. The activity adhered to DEAE Sephadex. Gel filtration chromatography of the hypoxic-conditioned medium demonstrated 2 major peaks of smooth muscle cell growth factor activity corresponding to molecular weights between 6,000 and 14,000 and 20,000 and 65,000 daltons, respectively. These data suggest that this pulmonary endothelial cell-derived smooth muscle cell peptide mitogen(s) may be involved in the smooth muscle cell proliferative response seen with chronic alveolar hypoxia.

Animals↗
PubMed1987

Growth hormone administration conserves lean body mass during dietary restriction in obese subjects.

Because weight-reducing diets result in loss of lean body tissue as well as fat, we sought to determine whether injections of GH might facilitate the preservation of nitrogen and accelerate the loss of body fat during dietary restriction. The dietary intake of 8 obese subjects was restricted to 24 Cal/kg ideal BW and 1 g protein/kg for 11 weeks. During weeks 3-5, 4 subjects were given a total of 10 im injections of recombinant methionyl human GH, 1 morning injection every 48 h in a dosage of 0.1 mg/kg ideal BW. The other 4 subjects were given injections of vehicle. During weeks 8-10, the subjects who had received GH previously were given vehicle and vice versa. While receiving GH injections the mean daily nitrogen deficit [0.35 +/- 2.14 (+/- SD) g/day] was significantly less than the loss during injection of vehicle (2.21 +/- 1.45 g/day; P less than 0.001). Although three of six subjects lost 190% more fat (as determined by hydrostatic weighing) while receiving GH, the difference in group mean fat loss during GH injection was not significantly greater than that during injection of vehicle (3.06 +/- 1.39 kg lost with GH vs. 2.64 +/- 1.08 kg lost with vehicle; P = NS). In parallel with the changes in nitrogen balance, GH produced a significant increase in the mean plasma somatomedin-C/insulin-like growth factor I concentration. From a mean pretreatment value of 1.06 +/- 0.28 U/mL, a maximal value of 3.20 +/- 1.60 U/mL was achieved after 12 days of GH injection (P less than 0.001). Somatomedin-C/insulin-like growth factor I concentrations did not change during injection of vehicle. During GH injection weight loss was attenuated because of fluid retention. If weight loss was determined 1 week after the end of GH administration, however, the total weight loss (3.42 +/- 1.73 kg) was not significantly different from that during the 3 weeks of vehicle administration and the following week (4.16 +/- 1.30 kg). Fasting blood glucose and serum insulin concentrations did not change during GH administration, and no glycosuria was detected in morning urine samples. Short term GH administration is effective in decreasing the loss of lean body mass in individuals ingesting restricted diets. However, fat loss was not accelerated.

Adipose Tissue↗
PubMed1987

Alterations in the synthesis of a fibroblast surface associated 35 K protein modulates the binding of somatomedin-C/insulin-like growth factor I.

Human fibroblasts, a cell type that is used extensively to determine the pleiotypic effects of the insulin-like growth factors, have been shown to secrete a 35K protein that binds somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I) but not insulin. This 35 K protein is associated with the fibroblast surface and following transfer to the surface of cell types that do not have this protein on their surfaces, it alters the binding of radiolabeled Sm-C/IGF-I. In this study human fibroblast monolayers that were incubated with cyclohexamide (50 micrograms/ml) for 14 h at 37 C had no detectable 35 K protein on their cell surface, but type I Sm-C/IGF-I receptors were still present. Loss of the 35 K protein was associated with 60-70% increase in binding of Sm-C/IGF-I to type I receptors. The relative affinity of the type I receptor for Sm-C/IGF-I was apparently increased because unlabeled Sm-C/IGF-I (12 ng/ml) competitively displaced 63% of radiolabeled Sm-C/IGF-I after cycloheximide exposure, whereas in cultures not exposed to cycloheximide [125I]Sm-C/IGF-I binding was increased by 11%. Coincubation of fibroblast conditioned media containing the 35 K protein with cycloheximide-treated fibroblast monolayers resulted in restoration of the paradoxical increase in Sm-C/IGF-I binding and loss of sensitivity to competition by unlabeled Sm-C/IGF-I. Exposure of suspended fibroblasts, which do not have 35 K on their cell surface, to media conditioned by fibroblast monolayers also induced both of these changes.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell Membrane↗
PubMed1986

Purification and biologic properties of fibroblast somatomedin.

Cultured human fibroblasts produce a peptide growth factor that cross-reacts with antisera to human somatomedin-C (Sm-C). To determine the identity of this species and compare its molecular properties to pure Sm-C, 2 liters of conditioned medium derived from human fibroblast monolayers were concentrated (X10) by ultrafiltration. The concentrated conditioned medium was purified further by CM-Sephadex ion-exchange chromatography. Following elution in 1.0 M NaCl, pH 8.0, the active material was purified by gel filtration on Sephadex G-150. The active fractions which eluted at Kd 0.45 (Mr estimated at 32,000) were further purified by isoelectric focusing. Two peaks of activity electrofocused at pI 5.4 and 7.2, respectively. The pI 5.4 peak contained only binding protein activity. The active fractions from the neutral pool were further purified by reverse-phase high pressure liquid chromatography on a C-18 Bondapak with a linear gradient of acetonitrile (10-60%). The active single peak which eluted at 55% acetonitrile gave a single band when analyzed by polyacrylamide gel electrophoresis. This material stimulated [3H]thymidine incorporation into human fibroblast DNA with approximately 3.2 times the potency of pure Sm-C but was equipotent in stimulating BALB/c 3T3 fibroblasts. It was degraded by fibroblast cultures at a slower rate compared to Sm-C, although it had a similar affinity for Sm-C-binding protein. We conclude that human fibroblasts produce two peptides that react with anti-Sm-C antibody but are chemically distinct from Sm-C. The greater response to fibroblast somatomedin may be due to its affinity for somatomedin-binding protein and slower degradation. These findings may have implications for understanding the regulation of human fibroblast replication.

Animals↗
PubMed1986

Paracrine functions of somatomedins.

Evidence is growing that the somatomedins act by a paracrine and/or autocrine mechanism. The importance of these mechanisms relative to the traditional endocrine actions is not clear, and it is possible that these growth factors act through all three mechanisms. Supporting the possible paracrine/autocrine mechanisms are reports that production of somatomedins or somatomedin-like peptides is widespread throughout the body. Additionally, the somatomedins have biological actions on remarkably diverse cell types, and these responsive cells are found in close proximity to cells known to produce somatomedin. Finally, factors that alter the growth rate of cultured cells produce parallel changes in somatomedin secretion, suggesting that these phenomena are closely linked.

Animals↗
PubMed1986

Platelets contain a peptide inhibitor of endothelial cell replication and growth.

Platelets release specific growth factors that stimulate division of aortic endothelial cells. Acidification or heating to 56 degrees C of platelet extracts is required to detect these factors. Nonheated platelet extracts inhibit endothelial growth. To determine if this inhibitory effect is due to a discrete chemical substance, a crude extract obtained by freezing and thawing human platelets was incubated with an endothelial cell mitogen and found to inhibit the endothelial cell incorporation of [3H]thymidine by greater than 95%. The inhibitor was partially purified by ion-exchange and gel-filtration chromatography. The partially purified material inhibited endothelial cell division as well as DNA synthesis. Inhibition caused by the partially purified material was concentration dependent, reversible, and not due to cytotoxicity. Further purification by heparin-Sepharose chromatography resulted in removal of greater than 95% of the contaminating substances without a loss of inhibitory activity. Physical characterization revealed that heparin-Sepharose-purified factor was heat labile, had a molecular size estimated at 35-40 kDa, and was stable between pH 4.5 and 9.0. Treatment with proteolytic enzymes destroyed all biologic activity, suggesting a peptide composition. These data support the conclusion that platelets contain a potent inhibitor of endothelial replication and growth. Since platelet deposition and release occur after endothelial denudation, release of this platelet-derived growth inhibitor may be an important regulator of reendothelialization that occurs after vessel injury.

Animals↗
PubMed1986

Regulation of somatomedin-C/insulin-like growth factor I by nutrients.

Nutritional intake is an important regulator of plasma somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I) concentrations in plasma. Concentrations in humans are reduced to the hypopituitary range by fasting for only a few days, and their normalization after fasting depends on the adequacy of energy and protein in the refeeding diet. The changes correlate with changes in nitrogen balance. In rats we have observed a close relationship between change in plasma Sm-C/IGF-I and hepatic GH binding with fasting and refeeding, suggesting that alterations in GH binding might be responsible partly for changes in Sm-C/IGF-I. When malnourished humans are given nutrient repletion, the increase in Sm-C/IGF-I is far more dramatic than changes in other nutrient-related serum proteins.

Adult↗
PubMed1986

Cultured fibroblast monolayers secrete a protein that alters the cellular binding of somatomedin-C/insulinlike growth factor I.

We studied somatomedin-C/insulinlike growth factor (Sm-C/IGF-I) binding to human fibroblasts in both adherent monolayers and in suspension cultures. The addition of Sm-C/IGF-I in concentrations between 0.5 and 10 ng/ml to monolayers cultures resulted in a paradoxical increase in 125I-Sm-C/IGF-I binding and concentrations between 25 and 300 ng/ml were required to displace the labeled peptide. The addition of unlabeled insulin resulted in no displacement of labeled Sm-C/IGF-I from the adherent cells. When fibroblast suspensions were used Sm-C/IGF-I concentrations between 1 and 10 ng/ml caused displacement, the paradoxical increase in 125I-Sm-C/IGF-I binding was not detected, and insulin displaced 60% of the labeled peptide. Affinity cross-linking to fibroblast monolayers revealed a 43,000-mol wt 125I-Sm-C-binding-protein complex that was not detected after cross-linking to suspended cells. The 43,000-mol wt complex was not detected after cross-linking to smooth muscle cell monolayers, and binding studies showed that 125I-Sm-C/IGF-I was displaced greater than 90% by Sm-C/IGF-I using concentrations between 0.5 and 10 ng/ml. Because fibroblast-conditioned medium contains the 43,000-mol wt complex, smooth muscle cells were incubated with conditioned medium for 24 h prior to initiation of the binding studies. 125I-Sm-C/IGF-I-binding increased 1.6-fold compared to control cultures and after cross-linking the 43,000-mol wt complex could be detected on the smooth muscle cell surface. Human fibroblast monolayers secrete a protein that binds 125I-Sm-C/IGF-I which can be transferred to the smooth muscle cell surface and alters 125I-Sm-C/IGF-I binding.

Carrier Proteins↗
PubMed1986

Somatomedin-C binding to cultured human fibroblasts is dependent on donor age and culture density.

Cultured fibroblasts have been used extensively to study age-related changes in the cellular response to serum stimulation. Since somatomedin-C (Sm-C) is an important growth factor in serum, we determined if there were age-related changes in Sm-C fibroblast receptor number or affinity and if culture density influenced these changes. Skin fibroblasts were obtained from six normal donors in three separate age groups and tested for their capacity to bind Sm-C. Sparse cultures (10-15K cells/well) derived from fetal donors had an affinity for Sm-C that was 4.7-fold greater than that of cultures derived from elderly (74-96 yr old) donors [9.4 +/- 0.2 (+/- SD) compared to 2.0 +/- 0.2 X 10(10) M-1]). When grown to high density (60-100K cells/well), the affinity of the fetal cultures was significantly reduced to 2.2 +/- 0.2 X 10(10) M-1 (P less than 0.001) and was not significantly different from the affinity of high density elderly donor cultures (2.3 +/- 0.4 X 10(10) M-1). Intermediate age donors (3-14 yr old) also had a significant reduction in receptor affinity with increasing density. Fetal donor cultures showed no density-dependent changes in receptor number. Fetal donor cells at low density had 5.2 +/- 1.0 X 10(4) receptors/cell compared to 5.9 +/- 0.6 X 10(4) receptors/cell in the high density cultures. In contrast, cells derived from donors aged 3-14 yr had 12.0 +/- 1.6 X 10(4) receptors/cell at 15K cells/well and 5.1 +/- 0.6 X 10(4) at 80K cells/well (P less than 0.05). Cultures from elderly donors had significantly greater mean receptor numbers per cell compared to fetal donor cells at four of five densities tested and had a significantly lower receptor number per cell with increasing culture density 25.2 +/- 1.2 X 10(4) (10-15K cells/well) compared to 5.2 +/- 0.2 X 10(4) (60-100K) cells/well. Thus, increasing donor age at low culture density was associated with an increase in receptor number per cell and a decrease in receptor affinity. At high culture densities, these differences were not detected. These changes in Sm-C receptor number and affinity at low density could lead to donor age-related changes in the cellular response to Sm-C.

Adolescent↗
PubMed1985

Supplemental essential amino acids augment the somatomedin-C/insulin-like growth factor I response to refeeding after fasting.

Plasma somatomedin-C (Sm-C)/insulin-like growth factor I (IGF-I) concentrations have been shown to reflect changes in nitrogen balance induced by manipulation of nutrient intake. To assess the Sm-C/IGF-I response to refeeding a protein restricted diet in which the nitrogen source was supplemented with essential amino acids, six normal adults were fasted for five days, then refed for nine days diets consisting of 35 kcals/kg and 0.48 g protein/kg body weight. In one diet, 80% of the nitrogen was supplied as essential amino acids, and in the other, 80% was supplied as nonessential amino acids. Following the first fast/refeed cycle, a control diet was eaten for two weeks before the fast was repeated and the other test diet was ingested. The refeeding diets were given a random order. Plasma Sm-C/IGF-I fell from a pre-fast mean of 1.64 +/- 0.24 U/mL (mean +/- 1 SEM) to 0.67 +/- 0.18 (P less than 0.001) following fasting, and rose to 1.41 +/- 0.19 U/mL (P less than 0.001) following ingestion of the diet with supplemental essential amino acids. In contrast, when the same subjects were refed a diet in which 80% of the nitrogen was in the form of nonessential amino acids, the plasma Sm-C/IGF-I concentrations rose from 0.74 +/- 0.17 to 1.15 +/- 0.15 U/mL (P less than 0.01). This increase was significantly less than that observed after ingestion of the essential amino acid supplemented diet (0.74 +/- 0.10 U/mL v 0.40 +/- 0.11 U/mL; P less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1985

Use of plasma somatomedin-C/insulin-like growth factor I measurements to monitor the response to nutritional repletion in malnourished patients.

Changes in plasma somatomedia-C/insulin-like growth factor I (Sm-C/IGF-I) concentrations are a sensitive indicator of the anabolic response of normal human volunteers to alterations in nutritional intake. To determine if measurement of this peptide could be used to monitor the response to nutritional repletion in malnourished patients, six patients were studied while receiving nutritional support for periods of 10-16 days. Plasma Sm-C/IGF-I increased from a mean basal level of 0.67 +/- 0.15 U/ml (+/-1 SD) to a peak of 1.80 +/- 0.44 U/ml on day 10, then declined to a concentration of 1.28 +/- 0.49 U/ml by day 16. All patients entered positive nitrogen balance by day 2 and nitrogen accretion continued throughout the study. Changes in serum concentrations of prealbumin, transferrin, and retinol-binding protein were compared to changes in Sm-C/IGF-I during nutritional support. Prealbumin increased to a posttreatment mean of 121 +/- 23% of control by the end of the study (p greater than 0.05, NS). Likewise, there was minimal change in retinol-binding protein, a peak value of 118 +/- 21% of control being reached by day 12 of treatment (p greater than 0.05, NS). Transferrin also showed minimal change, increasing to a mean value of 110% +/- 13% of control by day 12 (p greater than 0.05, NS). Measurement of plasma Sm-C/IGF-I concentrations appears to be a much more sensitive index of acute directional changes in nutritional status than other plasma proteins commonly used to monitor nutritional responses. The increase of Sm-C/IGF-I correlated temporally with entry into positive nitrogen balance.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1985

Variables controlling the secretion of a somatomedin-like peptide by cultured porcine smooth muscle cells.

Somatomedin-C, a peptide growth factor that is also termed insulin-like growth factor I, stimulates smooth muscle cell replication; however, these cells proliferate in somatomedin-C-deficient medium. We postulated, therefore, that these cells might release a somatomedin-like molecule into the surrounding culture medium. Porcine aortic smooth muscle cells that were exposed to serum-free Dulbecco's modified minimum essential medium for 24 hours were found to release a somatomedin-like molecule that could be detected by a specific radioimmunoassay for somatomedin-C. Several variables in the design of tissue culture conditions were found to regulate the secretion of the somatomedin-like peptide. There was an inverse relationship between somatomedin-like peptide secretion and culture density. Cultures grown to a density of 110,000 cells/well secreted 0.18 +/- 0.02 U/ml per 10(5) cells, whereas cultures grown to 38,000 cells/well secreted 0.59 +/- 0.06 U/ml per 10(5) cells (P less than 0.001). Serum deprivation and days of preincubation before initiation of an experiment were found to influence somatomedin-like peptide secretion significantly. Platelet-derived growth factor, which stimulates smooth muscle cell replication, was a potent stimulant of somatomedin-like peptide secretion. Exposure to a concentration of 500 ng/ml platelet-derived growth factor induced a 2.8-fold increase in somatomedin-like peptide secretion over basal levels, (i.e., 0.37 +/- 0.05 to 0.98 +/- 0.7 U/ml, P less than 0.001). The effect of platelet-derived growth factor was enhanced (41% increase) by concomitant incubation with 1% somatomedin-C-deficient platelet poor plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1985

Evidence for a functional role of endogenously produced somatomedinlike peptides in the regulation of DNA synthesis in cultured human fibroblasts and porcine smooth muscle cells.

Cultured porcine aortic smooth muscle cells and human fibroblasts produce somatomedinlike peptides and secrete them into the surrounding microenvironment. This production has been linked to their ability to replicate. The objective of this study was to determine if a specific anti-somatomedin-C (Sm-C) monoclonal antibody that binds the somatomedinlike peptides could inhibit replication by porcine aortic smooth muscle cells and human fibroblasts. To determine if the antibody could inhibit the effect of endogenously produced somatomedinlike peptide, increasing concentrations of antibody were co-incubated with platelet-derived growth factor, a known stimulant of somatomedinlike peptide secretion, and Sm-C-deficient platelet-poor plasma. Addition of the antibody reduced fibroblast [3H]thymidine incorporation from 35,100 +/- 500 to 10,600 +/- 700 cpm (P less than 0.001), and in smooth muscle cells from 29,600 +/- 1,800 to 10,800 +/- 1,100 cpm (P less than 0.001). Co-incubation of exogenously added Sm-C (20 ng/ml) with maximally inhibitory dilutions of antibody increased [3H]thymidine incorporation in fibroblasts from 7,800 +/- 1,000 to 18,900 +/- 800 cpm (P less than 0.01), and in smooth muscle cells from 9,800 +/- 1,200 to 17,200 +/- 1,100 cpm (P less than 0.01). Insulin, which can substitute for Sm-C as a mitogen and does not bind to the antibody, stimulated DNA synthesis when co-incubated with the antibody, thereby excluding the possibility of nonspecific cytotoxicity. These results strengthen the hypothesis that the rate of DNA synthesis of these two cell types in vitro is directly linked to their capacity to produce somatomedinlike peptides. They further support the cellular production of somatomedinlike peptides as examples of the autocrine model of growth regulation.

Animals↗
PubMed1985

Exposure to platelet-derived growth factor modulates the porcine aortic smooth muscle cell response to somatomedin-C.

Platelet-derived growth factor (PDGF) is a potent mitogen for smooth muscle cells, but to detect maximal stimulation by PDGF, the cells must be incubated with plasma. Somatomedin-C (Sm-C), a peptide growth factor that is present in plasma, has been shown to interact with PDGF synergistically to stimulate DNA synthesis in cultured fibroblasts. These studies were designed to test the hypothesis that PDGF interacted with Sm-C to stimulate smooth muscle cell replication and to compare the response of this cell type to that of fibroblasts. When PDGF or Sm-C was added individually to smooth muscle cell cultures, each growth factor induced only minimal increases in [3H]thymidine incorporation into DNA (i.e. PDGF, 3,400 +/- 1,120 to 54,900 +/- 1,550 cpm; Sm-C, 3,400 +/- 1,120 to 10,950 +/- 980 cpm). In contrast, addition of increasing concentrations of Sm-C to cultures that were continuously exposed to PDGF in the presence of Sm-C-deficient plasma resulted in a synergistic increase in [3H]thymidine incorporation (3,400 +/- 1,120 to 54,500 +/- 1,800 cpm; P less than 0.001). To determine if Sm-C was required for smooth muscle cell replication, cultures were sequentially exposed to PDGF, followed by Sm-C-deficient plasma. The rate at which the non-Sm-C-exposed cells synthesized DNA was retarded compared to that of cells exposed to Sm-C-containing plasma; however, 68% nuclear labeling was present after 44 h of incubation. To exclude the possibility that some cellular secretory product was substituting for Sm-C, the medium was changed every 2 h and replaced by fresh Sm-C-deficient medium. Using these test conditions, exposure to PDGF and Sm-C-deficient plasma induced only 11% labelling. Readdition of pure Sm-C to this medium restored nuclear labeling to 82% at 44 h. Other variables that appeared to modulate the cellular response to Sm-C were culture density and simultaneous PDGF exposure. Sm-C and PDGF both appear to be potent mitogens for porcine aortic smooth muscle cells, and when added together to quiescent cultures, their effects are synergistic. Smooth muscle cells appear to require Sm-C to initiate DNA synthesis, and in its absence produce a Sm-like factor that can partially compensate for Sm-C deficiency and allow replication.

Animals↗
PubMed1984

Interaction of circulating cell-derived and plasma growth factors in stimulating cultured smooth muscle cell replication.

Multiple growth factors that circulate in plasma have been shown to stimulate cellular growth in vitro. The plasma growth factors appear to stimulate DNA synthesis in cultured fibroblasts only after prior exposure of cell growth factors derived from circulating cell types, such as platelets and macrophages. The purpose of these studies was to investigate the role of the plasma growth factors in stimulating smooth muscle cell replication following exposure to platelet-derived growth factor (PDGF). Following transient exposure to PDGF, insulin stimulated smooth muscle cell replication but only when supraphysiologic concentrations were used (i.e., greater than 1.0 micrograms/ml). Somatomedin-C (Sm-C), in contrast, was found to stimulate a 320% increase in [3H]thymidine incorporation when concentrations that are present in extracellular fluids were used (i.e., 0.5-10 ng/ml). Epidermal growth factor (EGF), an important mitogen for multiple cell types, caused a 70% increase in [3H]thymidine incorporation when added to quiescent cells following PDGF exposure, and EGF caused a substantial increase in the absolute level of [3H]thymidine incorporation when coincubated with Sm-C. When EGF (1 ng/ml) was incubated simultaneously with concentrations of Sm-C between 1 and 10 ng/ml plus Sm-C-deficient plasma, maximal [3H]thymidine incorporation was 2.1-fold greater in the presence of EGF. In contrast, insulin (20 ng/ml), when coincubated with Sm-C under similar conditions, had no enhancing effect on the cellular response to Sm-C. None of the plasma factors tested was an effective stimultant of replication when incubated either in serum-free medium or in the presence of Sm-C-deficient plasma without prior PDGF exposure. Hydrocortisone was shown to inhibit smooth muscle cell replication in concentrations between 10(-7) and 10(-5) M. In summary, multiple plasma growth factors can stimulate the smooth muscle cell replication, and Sm-C appears to be most effective of those tested. Insulin and EGF appear to work by different mechanisms; that is, EGF can facilitate the cellular response to Sm-C, whereas insulin is effective only at supraphysiologic concentrations at which it will directly bind to Sm-C receptors.

Animals↗