An endothelial cell-derived growth factor

Mitogenic proteins of the 3T3 plasma membrane

A mitogenic factor from 3T3 plasma membranes has been identified and partially characterized. The factor appears to be a peripheral membrane protein that can be released by mild trypsin, chymotrypsin, or plasmin treatment. This component is sensitive to heat and acid, and has a molecular weight in the range of 150,000-200,000 daltons as determined by gel filtration. A similar mitogenic activity has also been found on the membranes of both SV-40-transformed 3T3 cells and human fibroblasts. The factor appears to be distinct from all previously described mitogenic components.

Growth factors from platelets, monocytes, and endothelium: their role in cell proliferation

Of the various biological roles assigned to growth factors at the beginning of this article, the factors described here are largely associated with the response to injury. These represent a special type of factor since two of them, PDGF and MDGF, are carried in the circulation by the platelet and the monocyte respectively, and can therefore be delivered to sites where a proliferative response would be an important event in the restitution of tissue continuity. The role of the endothelial-derived growth factor in these phenomena is not clear at present. Atherosclerosis has been suggested to represent a protective proliferative response that has gone awry and become disease. In this instance both PDGF and MDGF could play important roles, since platelets have been associated with the early injury phenomenon and macrophages appear to be present in virtually all phases of the development of the lesions of atherosclerosis from the fatty streak to the fibrous plaque and the complicated lesion. In each of these circumstances the macrophage may be important in lesion progression and possibly in lesion initiation. PDGF may also be important in initiation of some lesions, and in some instances would undoubtedly participate in the fibroproliferative response that occurs during organization of a thrombus.

Organ culture of rat carotid artery: maintenance of morphological characteristics and of pattern of matrix synthesis

Segments of rat carotid artery were maintained in serum-free or serum-supplemented medium for 2 wk, and at intervals of 3, 7, and 14 d the morphology and pattern of matrix synthesis were compared to those in vivo. In serum-free medium and 0.2% serum both the endothelium and the smooth muscle cells (SMC) could be maintained with a minimum of change for 7 d and without substantial change for 14 d. In 2% and 10% serum there was little change for the first 3 d, but subsequently there was a progressive overlapping of the endothelial cells to produce a 3 to 4 layered cell sheet, often separated from the subendothelial matrix; the SMC, however, did not appear to proliferate or migrate and in general retained their typical cellular features for the full time in culture. The synthesis of matrix components was measured by autoradiographic detection of incorporated [H]glucosamine and 35S. At all time periods and serum concentrations the percentage distribution for each label across the arterial wall was found to be similar to that in live animals injected with the same labels. [3H]Glucosamine predominated in the endothelium and the narrow subendothelial layer, which together make up the intima, whereas 35S predominated in the media. In vitro more than 50% of the [3H]glucosamine in the intima and 40% in the adjacent first layer of the media was susceptible to Streptomyces hyaluronidase. As the morphology of both cell types and their synthesis of matrix components could be maintained in organ culture without substantial change we believe that the rat carotid artery may be a suitable model for the investigation of factors affecting arterial structure.

Smooth muscle and endothelial cell function in the pathogenesis of atherosclerosis

Although clinical studies have been very useful in identifying factors that accelerate the development of atherosclerotic vascular disease, the pathogenesis of the vascular lesions remains unknown. Studies carried out in the last 10 years have shown that smooth muscle and endothelial cells of the vascular wall play a very important role in atherogenesis. It has become apparent that these cells are very active metabolically during the initiation and subsequent growth of the plaques, and that the materials that these cells produce and secrete are important in the composition and growth of the plaques. In addition, there are important interactions at the vessel wall-blood interface that involve endothelial cells, hemodynamic forces and many constituents of the blood, including platelets, lipoproteins, coagulation factors, fibrinolytic agents and leukocytes. In this article the numerous functions of both smooth muscle and endothelial cells are discussed and the effects of known atherogenic agents on these cellular functions are reviewed. Emphasis is placed on the important interactions that take place both within the vessel wall and at the vessel wall-blood interface. Understanding of the regulation of smooth muscle and endothelial cell function during the development and subsequent growth of fibrofatty plaques may be useful in designing appropriate therapeutic interventions to control atherosclerotic disease.

Plasma-derived serum as a selective agent to obtain endothelial cultures from swine aorta

Endothelial cell and smooth muscle cell cultures from artery wall provide a potential model system for studying cellular processes involved in atherogenesis. To prepare serial subcultures of swine arterial endothelial cells that are free of smooth muscle cells without either selecting a small population or subjecting the cells to cytotoxic conditions, we used swine plasma-derived serum (SPDS) to establish conditions in which endothelial cells have a growth advantage. Endothelial cells were collected by collagenase digestion and smooth muscle cell cultures were prepared by outgrowth from explants of arterial medial segments. Growth rates were compared when each cell type was maintained on SPDS, or fetal bovine serum (FBS), or swine whole serum (SWS). When 20% FBS or SWS were used the doubling times were less than 30 h for both endothelial cells and smooth muscle cells. On 20% SPDS the doubling time for endothelial cells was 32 h, but for smooth muscle cells it was at least 168 h. Using SPDS, we prepare endothelial subcultures from swine aorta that express principally polygonal morphology at confluence. Endothelial cell cultures grown on SPDS have higher angiotensin-converting enzyme than those grown on FBS.

The evolution of experimental endarteritis in the rabbit abdominal aorta. Light and transmission electron microscopy

Experimental aortic intimal thickening has been induced in rabbits by sheathing the vessel with a polyethylene cuff. The alterations have been examined by light and transmission electron microscopy, during 12 months. An irregular intimal thickening develops as soon as the 15th day and includes numerous myofibroblasts with some other cells of monocytic or endothelial type. Microfibrils, elastic aggregates and collagen fibers are found in the intercellular space. Simultaneously, the media undergoes a fragmentation of the elastic laminae and the adventitia shows a capillary angiectasis and a granuloma. After 3 months there is, between the intimal smooth muscle cells, a progressive increase of elastic and collagenous material. In the media, elastic break up becomes more frequent after the 4th month and myocytes appear increasingly atrophic, which facilitates the extension of fibrosis. This is accompanied at times by a thinning of the arterial wall with or without localized disappearance of the media. All these modifications are discussed and compared to what we had previously found in the femoral artery [12].

Ability of endothelial cells to condition culture medium

Endothelial cell-conditioned medium contains two classes of factors distinguishable by behavior during dialysis and on specificity for cell type. One species, which diffuses through dialysis tubing with an exclusion limit of 6,000 to 8,000 daltons, supports growth of bovine aortic endothelial (BAE) cells in medium containing a growth limiting concentration of serum (0.2% serum). The production of this material appears to depend upon the presence of serum in the medium being conditioned. The activity increases with time of exposure of BAE cells to serum and with increasing concentration of serum present in the incubation medium. This activity cannot be replaced by exogenous epidermal growth factor, fibroblast growth factor, platelet-derived growth factor, insulin, or thymidine. The second species, the endothelial cell-derived growth factor (ECDGF), is retained by dialysis tubing with an exclusion limit of 6,000 to 8,000 daltons. ECDGF stimulates the growth of smooth muscle cells but does not support BAE cell growth in limiting serum concentrations. unlike the dialyzable species, the production of ECDGF is independent of previous incubation of BAE cell cultures in serum. These studies suggest that BAE cells are able to utilize serum components to produce conditioning factors for their own growth that are distinct from the higher molecular weight ECDGF.

Endothelial--vascular smooth muscle cell interactions. Rous--Whipple Award Lecture

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Growth stimulation of bovine endothelial cells by vitamin A

Retinol at concentrations of 10(-6) and 10(-5) M stimulated growth of bovine aortic endothelial cells maintained in Eagle's MEM supplemented with delipidized serum. In addition to retinol, retinal, retinoic acid, and retinyl acetate were also growth stimulatory. At very low inoculum densities (4-40 cells/cm2) the growth promoting effect could be demonstrated only in the presence of conditioned medium from macrophage-like culture P388D1. When added to media containing whole (nondelipidized) serum, retinol was growth inhibitory at 10(-6) and 10(-5) M concentrations.

What controls smooth muscle phenotype?

Enzyme-dispersed smooth muscle cells from the adult pig aortic media in the first few days of primary culture are in the contractile phenotype and do not divide when challenged with 5% WBS. After 6--8 days the isolated cells spontaneously undergo a change in phenotype where contraction cannot be stimulated and the cells respond to mitogens in WBS by logarithmic growth. The change in phenotype is reversible if the cells are seeded sufficiently densely (5 x 10(4) to 1 x 10(5)/ml) that a confluent monolayer results after less than 1 week of proliferation, but is irreversible if the cells are seeded sparsely (1 x 10(3) to 5 x 10(3)/ml) and take more than 2 weeks of proliferation to reach confluence. When the cells are seeded so densely (10(6)/ml) that a confluent monolayer is present from day 1, the cells do not undergo a change in phenotype but remain indefinitely in the contractile state. The spontaneous modulation of phenotype of isolated smooth muscle cells can be inhibited by a confluent monolayer of contractile smooth muscle or endothelial cells in co-culture with the sparsely seeded smooth muscle such that the two cell layers are not in contact but bathed by the same nutrient medium. Smooth muscle modulation can also be inhibited by a factor extracted from pig and rabbit aortic tissue, and its effects mimicked by commercially available sodium heparin at 50 units/ml.

Cultured endothelial cells produce a heparinlike inhibitor of smooth muscle cell growth

Using cultured cells from bovine and rat aortas, we have examined the possibility that endothelial cells might regulate the growth of vascular smooth muscle cells. Conditioned medium from confluent bovine aortic endothelial cells inhibited the proliferation of growth-arrested smooth muscle cells. Conditioned medium from exponential endothelial cells, and from exponential or confluent smooth muscle cells and fibroblasts, did not inhibit smooth muscle cell growth. Conditioned medium from confluent endothelial cells did not inhibit the growth of endothelial cells or fibroblasts. In addition to the apparent specificity of both the producer and target cell, the inhibitory activity was heat stable and not affected by proteases. It was sensitive flavobacterium heparinase but not to hyaluronidase or chondroitin sulfate ABC lyase. It thus appears to be a heparinlike substance. Two other lines of evidence support this conclusion. First, a crude isolate of glycosaminoglycans (TCA-soluble, ethanol-precipitable material) from endothelial cell-conditioned medium reconstituted in 20 percent serum inhibited smooth muscle cell growth; glycosaminoglycans isolated from unconditioned medium (i.e., 0.4 percent serum) had no effect on smooth muscle cell growth. No inhibition was seen if the glycosaminoglycan preparation was treated with heparinase. Second, exogenous heparin, heparin sulfate, chondroitin sulfate B (dermatan sulfate), chondroitin sulfate ABC, and hyaluronic acid were added to 20 percent serum and tested for their ability to inhibit smooth muscle cell growth. Heparin inhibited growth at concentrations as low as 10 ng/ml. Other glycosaminoglycans had no effect at doses up to 10 mug/ml. Anticoagulant and non- anticoagulant heparin were equally effective at inhibiting smooth muscle cell growth, as they were in vivo following endothelial injury (Clowes and Karnovsk. Nature (Lond.). 265:625-626, 1977; Guyton et al. Circ. Res. 46:625-634, 1980), and in vitro following exposure of smooth muscle cells to platelet extract (Hoover et al. Circ. Res. 47:578-583, 1980). We suggest that vascular endothelial cells may secrete a heparinlike substance in vivo which may regulate the growth of underlying smooth muscle cells.

Interaction of aortic endothelial and smooth muscle cells in culture. Effect on glycosaminoglycan levels

Co-cultivation of various intra- and interspecific combinations of pig and rat aortic endothelial cells (AEC) and smooth muscle cells (SMC) resulted in a marked increase in hyaluronic acid (HA) levels, a smaller but significant increase in sulphated glycosaminoglycans (GAG), and an increase in the HA/sulphated GAG ratio, compared with the separate culture of the two cell types. Culture of SMC in AEC-conditioned medium produced similar changes in GAG levels whereas SMC-conditioned medium had no effect on AEC GAG levels. These results add further support for the concept that epithelial cells in general can modulate the GAG composition of adjacent connective tissue and thereby influence its morphological and physiological properties. It is suggested that the normal amounts, types and distribution of GAG in the arterial wall, and especially in the intima, may be partly dependent on interaction between the endothelium and SMC. It is further suggested that injury to endothelium, with a consequent failure in this interaction, could lead directly to changes in intimal GAG composition that contribute to lesion development.

Vascular wall growth control: the role of the endothelium

The current state of our knowledge of the control of endothelial growth and the role of endothelial injury in the pathogenesis of atherosclerosis can be summarized as follows: 1. Endothelial cells can be grown in plasma-derived serum in the absence of exogenous growth factors. This is quite different from the growth requirements of most other nontransformed cells. These factors may, however, prolong replicative life span and increase the ability of endothelium to grow at sparse density. The relevance of these phenomena to the control of endothelial growth in vivo is unclear. There is no evidence that exogenous growth factors are required for wound edge regeneration. In view of the relative lack of growth factor requirements, it is intriguing to consider the possibility that the critical control factor for endothelial cell growth is cell contact. 2. Endothelial cell regeneration may be dependent on endothelial cell motility. The nature of this relationship may be important in controlling the ability of the endothelium to regenerate itself under different flow conditions around lesions or in different parts of the vessel tree and in determining the ability of the endothelium to respond to changes in the connective tissue overlying lesions. 3. Endothelial cells in vivo are able to regenerate small areas of denudation extremely rapidly. This process may be sufficiently rapid to permit the endothelium to replace dying cells as they are being lost, resulting in desquamation without denudation. 4. We have little evidence for endothelial denudation either spontaneously or in response to atherosclerosis risk factors until after lesion formation has begun. This does not rule out the possibility that small, repeated, transient episodes of denudation occur and play a role in the initiation of atherosclerotic lesions. It is important, however, to begin considering the role of nondenuding injuries in atherosclerosis. 5. The fact that thrombosis occurs in atherosclerosis implies an eventual breakdown of endothelial integrity. The mechanism of that breakdown remains unknown. 6. Finally, there is the question of interactions between smooth muscle cells and endothelial cells at the level of growth control. This includes the evidence that there is a critical amount of endothelium that must be lost before lesion formation is stimulated and the recent evidence that endothelial cells produce substances able to regulate growth of smooth muscle cells.

Angiogenesis in vitro

Cloned capillary endothelial cells, cultured in tumour-conditioned medium, form capillary tubes. By light and electron microscopy these tubes resemble capillaries in vivo. This first demonstration of angiogenesis in vitro: (1) shows that all the information necessary to develop an entire capillary network in vitro is expressed by one cell type; (2) suggests a mechanism for lumen formation; and (3) offers a possibility of distinguishing between direct and indirect angiogenesis factors.

Potent stimulation of vascular endothelial cell growth by differentiated 3T3 adipocytes

3T3 cells that have undergone adipose differentiation in vitro secrete into the culture medium a potent growth stimulatory activity for bovine aortic endothelial cells. When medium containing 2% fetal calf serum, which does not support significant endothelial cell growth, is conditioned by 3T3-F442A adipocytes, the endothelial cells grow rapidly (doubling time, 24 hr) at a rate equal to the growth rate in 20% fetal calf serum. The potency of the conditioned medium is further shown by the fact that it can be diluted 1:5 with little apparent loss of activity and shows a half-maximal stimulation at 10 microliter/ml. Serum is not required for either the secretion of this mitogen by the adipocytes or its action on the endothelial cells, as shown by the fact that the latter are stimulated to divide in serum-free medium conditioned by the adipocytes. The growth stimulatory activity appears to be specific for vascular endothelial cells in that no other cell type examined, including vascular smooth muscle cells and pericytes, are significantly stimulated by medium conditioned by 3T3-F442A cells. Similarly, medium conditioned by no other cell type examined has more than 10% of the activity of medium conditioned by the adipocytes. The specificity and potency of the adipocyte-derived factor suggest that it may play a role in the vascularization of this tissue during development. Preliminary biochemical analysis indicates that the adipocyte factor is nondialyzable and is not inactivated by heat or proteases. The protease insensitivity distinguishes the adipocyte growth stimulatory activity from the low levels of activity secreted by fibroblasts and preadipocytes, suggesting that the adipocyte mitogen is a product specifically related to the differentiation process.