Potential Role for Ceramide in Mitogen-activated Protein Kinase Activation and Proliferation of Vascular Smooth Muscle Cells Induced by Oxidized Low Density Lipoprotein

Abstract

Proliferation of vascular smooth muscle cells (SMC) isAtherosclerosis, and its complications, namely myocardial infarction, stroke, and peripheral vascular diseases, is one of the most prevalent cause of morbidity and mortality in Western countries. During atherogenesis, focal lesions spread out progressively and lead to the formation of fibro-atheroma plaques, in which smooth muscle cell (SMC) 1 proliferation plays a critical role (1, 2). Among the risk factors identified, low density lipoprotein (LDL) cholesterol level is strongly predictive of coronary heart disease. LDL are believed to have an important role in atherogenesis (3), following oxidative modifications (4 -6), because oxidized LDL are present in atherosclerotic lesions (7) and possess a wide range of biological properties potentially occurring during atherogenesis in vivo (8). Oxidized LDL have recently been shown to be mitogenic to vascular SMC (9 -11). These studies suggest that oxidized LDL may be considered as an additional mitogenic factor, alongside the classical growth factors implicated in SMC proliferation during atherogenesis (6). To date, the mechanism of the oxidized LDL proliferative effect is poorly elucidated and may result from the triggering of a mitogenic intracellular signal either directly by oxidized LDL or indirectly through an autocrine effect involving growth factor secretion and/or growth factor receptor over-expression.Recently, sphingolipids have emerged as key signaling molecules involved in the regulation of cell growth and differentiation (for reviews, see . In particular, the sphingomyelin (SM; ceramide phosphocholine)-ceramide pathway appears as a prototypic sphingolipid signaling pathway implicated in the positive or negative regulation of cell growth. Activation of this pathway leads to SM hydrolysis and subsequent generation of ceramide, the backbone of all sphingolipids, which serves as an intracellular second messenger. To date, several agents have been described to stimulate the SMceramide pathway (reviewed in Refs. 12 and 14 -17), including cytokines such as TNF␣, interleukin-1␤, interferon ␥, nerve growth factor, anti-CD28, anti-Fas antibodies, anticancer drugs, and ionizing radiations (18 -21). Cell-permeant ceram-

Native LDL Induces Proliferation of Human Vascular Smooth Muscle Cells via Redox-Mediated Activation of ERK 1/2 Mitogen-Activated Protein Kinases

“…This finding is consistent with work by Augé and coworkers using oxLDL as a stimulus in bovine VSMC. 48 PD98059 did not completely inhibit native LDLinduced proliferation, which could be due to incomplete inhibition of the enzyme at the dose used. Alternatively, other growth-promoting protein kinases such as p38 MAPK, which contributes to human VSMC proliferation under certain conditions, 29 or Akt 49 could also be involved in the inhibitory effects observed with the antioxidants used in this study.…”

Pancreatic Bile Salt-Dependent Lipase Induces Smooth Muscle Cells Proliferation

“…The amounts of cellular SPM and ceramide do not vary on SMC incubation with proliferating dose of BSDL, allowing us to rule out the SPM-ceramide activation pathway. 9,10 Cellular phospholipids and cell-free medium lysoPC significantly decrease, whereas free oleic acid appears in the medium, suggesting that this mitogenic fatty acid 12 is generated by BSDL from PC and lysoPC. Consistently, BSDL, which may hydrolyze PC preferentially exposed to the outer leaflet of plasma membrane, could (through the generation of lipid second messenger) induce the release of the bFGF of the pericellular compartment.…”

“…9,10 Cells were seeded at a density of 25 000 cells/mL in 24- ]-choline, 0.5 Ci/mL). Cells were washed with PBS and treated with BSDL in the presence or absence of effectors.…”

“…Lipids from cell lysate or cell-free culture medium were extracted by chloroform/ methanol. 9,10 The lipidic phase was evaporated under nitrogen. [ 3 H]-choline-labeled sphingomyelin was quantified as described.…”

Carboxyl ester lipase

“…The hydrolysis of sphingolipids in LDL also resulted in the accumulation of 10-50-fold excess amounts of ceramide in the vessel wall (146). Ceramide is another atherogenic lipid signaling molecule present in oxidized LDL that has previously been shown to promote vascular smooth muscle cell proliferation (147,148). A direct contributory role of ceramide in vascular occlusive disease was also supported by experiments showing that oxidative stress-induced vascular changes are related to redox stress-induced proliferation of smooth muscle cells through ceramide-induction of the mitogen activated protein kinase pathway (149).…”