John Glomset

Dialyzed serum from clotted monkey blood ("blood serum") promotes the proliferation of monkey arterial smooth muscle cells in culture, but dialyzed serum prepared from recalcified platelet-poor plasma ("plasma serum") is much less effective. Addition of platelets and calcium to platelet-poor plasma increases the activity of plasma serum to the same level achieved with blood serum. Furthermore, addition to plasma serum of a platelet-free supernatant prepared by exposing purified platelets to thrombin also stimulates the proliferation of smooth muscle cells. Thus, much of the growth-promoting activity of dialyzed serum is directly or indirectly derived from platelets. This finding has important implications for the response of arteries to localized injury and provides a key to further understanding of the role of factors derived from blood serum in promoting cell proliferation in vitro.

We postulate that the lesions of atherosclerosis arise as a result of some form of "injury" to arterial endothelium. This injury somehow results in alteration in endothelial cell-cell attachment or endothelial cell-connective tissue attachment, so that forces such as those derived from the shear in the flow of blood result in focal desquamation of endothelium. This is followed by adherence, aggregation, and release of platelets at the sites of focal injury. During the process of release, a mitogenic factor is secreted from the platelets which, together with plasma constituents, gains entry into the artery wall, resulting in focal intimal proliferation of smooth muscle cells. This intimal proliferation is accompanied by the synthesis of new connective tissue matrix proteins and often by the deposition of intracellular and extracellular lipids. Studies in cell culture of arterial smooth muscle have demonstrated that the principle mitogen present in blood serum is a platelet-derived factor that is present in all whole blood sera and missing in serum derived from platelet-free plasma. In the absence of the platelet factor, smooth muscle cells are quiescent in culture. This platelet mitogen is also active in vivo, since experimentally produced lesion of atherosclerosis induced mechanically by diet or by homocystine can be prevented if platelets are missing, as in thrombocytopenia, or if platelet function is impaired as a result of the use of platelet inhibitors such as dipyridamole. These studies point to the key role of the platelet in the stimulation of intimal smooth muscle proliferation that leads to the development of lesions of atherosclerosis.

The cholesteryl esters of very low density lipoproteins become labeled when human plasma is incubated with cholesterol-(14)C. The relative order of magnitude of the specific activity of the cholesteryl esters of the major lipoprotein fractions is: high density lipoproteins >> very low density lipoproteins > low density lipoproteins. This pattern of labeling is similar to that found by others in experiments performed in vivo. Very low density lipoprotein cholesteryl esters are probably not formed by direct action of the plasma lecithin:cholesteryl acyltransferase, since significant esterification of cholesterol does not occur when very low density lipoproteins are incubated separately with the enzyme. Instead, labeled cholesteryl esters formed in the other lipoprotein fractions transfer to the very low density lipoproteins, the relative amount of monounsaturated esters transferred being slightly greater than that of saturated and polyunsaturated esters. The results support the possibility that the acyltransferase indirectly increases the concentration of very low density lipoprotein cholesteryl esters in vivo.