Growth-promoting actions of extracts from mouse submaxillary glands on human endothelial cells in culture

Myointimal hyperplasia: pathogenesis and implications. 1. In vitro characteristics

Myointimal hyperplasia (MIH) is an arterial wall smooth muscle cell (SMC) proliferative disorder. This process is responsible for a significant number of early and long-term arterial reconstructive and graft failures. Histopathologically, this process is characterized by a proliferation of SMC in the intima of traumatized arteries resulting in arterial and/or anastomatic stenosis with secondary thrombosis. In vitro studies of cultured SMC have allowed the evaluation of SMC response to factors suspected of being clinically associated with MIH. Principal among these is platelet derived growth factor (PDGF), which is known to be secreted by several cell types including endothelial cells (ECs) and monocytes as well as being stored and secreted by platelets. PDGF, somatomedin-C, epithelial growth factor, insulin, and other factors have been found to significantly increase SMC replication in vitro. Lipoproteins may be important substrates for SMC proliferation in contrast to heparin, which may directly inhibit SMC protein synthesis. Unlike SMCs, whose continued growth in culture is dependent on various growth factors and nutrients, ECs essentially cease to proliferate after the cells have formed a monolayer over the available surface. Extracellular matrix proteins, polypeptide mitogens, and heparin have been shown to modify EC migration and proliferation in vitro. Wounding of EC monolayers by scratching results in increased replication and migration, processes which require plasma factors that remain poorly defined. However, two general forms of EC growth factor have been isolated from many body tissues, are potent stimulators of capillary endothelial growth, and appear important both for normal EC monolayer homeostasis and for the response to injury. Cultured ECs produce mitogens for SMC. Production of the principal mitogen, PDGF, is significantly increased in sparse versus confluent cell cultures as well as by toxic agents such as endotoxin and phorbol esters. Acetyl low density lipoprotein as well as omega-3 fatty acids may significantly and selectively inhibit EC PDGF production, a finding with potentially profound implications for the clinical control of MIH in vivo.

Tumour angiogenesis

A variety of different factors has been implicated in inducing angiogenesis. Their identity and precise mechanisms of action remain elusive. The present treatise summarises the state of our knowledge not only in relation to tumour-induced capillary growth but also for non-neoplastic situations and mechanisms.

Dialyzable factor in human serum of platelet origin stimulates endothelial cell replication and growth

Porcine aortic endothelial cells were isolated and maintained in Dulbecco's modified Eagle's medium (DME medium)/10% citrate-treated human plasma. They were stimulated by DME medium/10% human serum to grow from a density of 10,100 +/- 500 per well to a final density of 83,000 +/- 1,800 per well over a 9-day period. On the other hand these cells grew poorly (11% increase) in DME medium/10% human platelet-poor plasma prepared without chelating agents and containing platelet factor 4 at 18 ng/ml by radioimmunoassay. Dialysis of the human serum (M(r) cutoff, 3,500) eliminated all the stimulatory activity. The activity recovered from the dialysate stimulated growth when added to endothelial cultures in conjunction with either dialyzed serum or platelet-poor plasma alone. The dialyzable factor could be obtained directly from platelets; both acetic acid extracts and boiled NaCl extracts stimulated porcine aortic endothelial cell replication. Gel filtration chromatography on Sephadex G-15 showed that the endothelial growth factor had a molecular weight of 700. Partially purified material induced a concentration-dependent stimulation of porcine aortic endothelial cell replication in the presence of DME medium alone; however, simultaneous incubation with platelet-poor plasma resulted in a much greater response. Fibroblast growth factor isolated from bovine brain was found to be mitogenic only in the presence of nondialyzed serum or of the dialyzable factor together with plasma. In the absence of this serum factor, fibroblast growth factor had no effect. We conclude that human serum contains a potent endothelial cell mitogen of platelet origin. Human plasma that is devoid of platelet content does not stimulate endothelial cell growth. This growth factor may be an important stimulant of the endothelial cell response to vascular wall injury.

Proliferative response of human endothelial cultures to various types and treatments of human sera, to culture treatments and to various oxygen concentrations

Endothelial cells from the human umbilical vein were exposed to different human sera, differently treated serum, to various oxygen concentrations, and various culture treatments. Endothelial proliferation was determined by measuring the uptake of [3H]-thymidine by means of autoradiography and presented as thymidine labeling index (TI) values. TI values differed according to different serum concentrations, serum types, serum preparations (WBS-PDS) and serum pretreatments. Low oxygen concentrations in the incubator atmosphere induced an early DNA synthesis response without an increase in cell number. The addition of the protease inhibitors aprotinin and soybean trypsin inhibitor to the culture medium resulted in a dose dependent TI decrease, whereas PMSF showed no influence.

Expression of angiotensin-converting enzyme activity in cultured pulmonary artery endothelial cells

Angiotensin-converting enzyme (EC 3.4.51.1) is a carboxyterminal dipeptidyl peptidase. The enzyme catalyzes the conversion of the decapeptide angiotensin I to the octapeptide angiotensin II. In addition, the enzyme catabolizes bradykinin. Because of these actions, the enzyme is of pivotal importance in blood pressure homeostasis. Numerous investigators have demonstrated the presence of the enzyme in association with endothelial cells but relatively little is known concerning the factors controlling the expression enzyme activity by endothelial cells in culture. We have demonstrated that endothelial cells in culture do not express significant amounts of enzyme activity until several days after growth ceases due to high cell density. This is important because it demonstrates a change in function with stage of growth in culture and a possible difference in functional capabilities between nondividing endothelial cells and cells that are dividing in response to injury. Since density-dependent expression of differentiated traits does not appear to be unique to endothelial cells an understanding of the mechanisms underlying this phenomenon may provide a general explanation for the expression of differentiated traits by cultured cells.

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.