Modified lipoproteins influence the biosynthetic activity of vascular smooth muscle cells

Abstract

Ultrastructural investigations have shown that in the atherosclerotic plaque, smooth muscle cells (SMC) like macrophages can transform to lipid-enriched foam cells. To date, it is not clear by which mechanisms the lipid-loading of SMC occurs and how lipoproteins, particularly atherogenic lipoproteins, influence the metabolism of SMC. In this study we have analyzed the effects of LDL and acetylLDL, atherogenic lipoproteins that lead to foam cell formation in macrophages in vitro, on cholesterol storage, proliferation, DNA-synthesis and protein synthesis of monolayer-cultured SMC. In some sets of experiments cells were additionally co-cultured with phorbol esters in order to stimulate scavenger receptor expression or with PDGF to raise the LDL-uptake. Lipid accumulation in SMC was judged by fluorescence microscopy after Nile Red staining. The results demonstrate that lipid-loading of SMC may occur upon acetyl-LDL incubation as well as upon LDL-incubation, particularly under the influence of PDGF. Phorbol esters, alone or in combination with LDL do not influence proliferation of SMC. Acetyl-LDL-incubation in combination with phorbol esters results in a slight reduction of cell proliferation. As expected, incubation of cell monolayers with PDGF alone stimulates cell proliferation. Acetyl-LDL in contrast to LDL has an inhibitory effect on the stimulation of cell proliferation by PDGF. Our data suggest that there is a specific effect of modified LDL on the growth factor induced regulation of SMC proliferation. To correlate cell proliferation with DNA content nucleic acids were stained with propidium iodide and fluorescence intensity was measured with a flow cytometer. The data gained suggest that under the influence of acetyl-LDL the percentage of cells in the S phase of the cell cycle increases. A further increase of cells in the S phase is observed when acetyl-LDL is used in combination with phorbol esters or PDGF. LDL shows no effects, neither alone nor in combination with phorbol esters or growth factors. The influence of lipoproteins on protein synthesis was investigated by incorporation of ¹⁴C-proline. There is evidence for a stimulation of total protein synthesis and an inhibition of collagen synthesis by acetyl-LDL in monolayer-cultured SMC.

Zusammenfassung

Die Atherogenese ist verknüpft mit Veränderungen der Eigenschaften der glatten Muskelzellen (SMC) in der Gefäßwand, wie z. B. gesteigerter Proliferationsaktivität, Lipidakkumulation (Schaumzellbildung) [2] und erhöhter Synthese von Matrixproteinen, v. a. Kollagenen [1]. Die biosynthetische Aktivität glatter Muskelzellen kann von verschiedenen Faktoren beeinflußt werden. Zu diesen Faktoren zählen u. a. die aus dem Plasma in die Gefäßwand gelangenden, z. T. modifizierten Lipoproteine, verschiedene Wachstumsfaktoren und die die Gefäßwand aufbauenden Matrixproteine selbst.