Sphingomyelinase, an Enzyme Implicated in Atherogenesis, Is Present in Atherosclerotic Lesions and Binds to Specific Components of the Subendothelial Extracellular Matrix

Abstract

Abstract-Atherosclerotic lesions contain an extracellular sphingomyelinase (SMase) activity that hydrolyzes the sphingomyelin of subendothelial low density lipoprotein (LDL). This SMase activity may promote atherosclerosis by enhancing subendothelial LDL retention and aggregation, foam cell formation, and possibly other atherogenic processes.The results of recent cell-culture studies have led to the hypothesis that a specific molecule called secretory SMase (S-SMase) is responsible for the SMase activity known to be in lesions, although its presence in atheromata had not been examined directly. Herein we provide immunohistochemical and biochemical support for this hypothesis. First, 2 different antibodies against S-SMase detected extracellular immunoreactive protein in the intima of mouse, rabbit, and human atherosclerotic lesions. Much of this material in lesions appeared in association with the subendothelial matrix. Second, binding studies in vitro demonstrated that 125 I-S-SMase adheres to the extracellular matrix of cultured aortic smooth muscle and endothelial cells, specifically to the laminin and collagen components. Third, in its bound state, S-SMase retains substantial enzymatic activity against lipoprotein substrates. Overall, these data support the hypothesis that S-SMase is an extracellular arterial wall SMase that contributes to the hydrolysis of the sphingomyelin of subendothelial LDL. S-SMase may therefore be an important participant in atherogenesis through local enzymatic effects that stimulate subendothelial retention and aggregation of atherogenic lipoproteins. (Arterioscler Thromb Vasc

Acute systemic inflammation up-regulates secretory sphingomyelinase in vivo : A possible link between inflammatory cytokines and atherogenesis

“…Cultured endothelial cells are by far the most abundant source of S-SMase among the many cell types studied by us, a finding supported by immunohistochemical analysis of animal aortae. Moreover, S-SMase from endothelial cells, but not from macrophages, is induced by cytokines (32,34). Approximately half of the secretion of S-SMase from cultured endothelial cells is from the apical surface (32).…”

“…First, inflammation plays an important role in atherogenesis (13)(14)(15)(16)(17), and, given the evidence cited above, ASM-mediated ceramide signaling may be responsible for certain inflammatory responses in the developing lesion, such as smooth muscle cell proliferation (25) and apoptosis of macrophages and smooth muscle cells (26,27). Second, one of the products of the ASM gene-secretory SMase (SSMase; see below)-has been linked directly to extracellular subendothelial lipoprotein aggregation, an event that promotes both retention of lipoproteins in the arterial wall and macrophage foam cell formation (25,(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39).…”

“…Furthermore, several cell types involved in atherosclerosis and the inflammatory response are rich sources of S-SMase, such as endothelial cells and macrophage (32,41). Endothelial cells, in particular, have the highest levels of S-SMase secretion of any cultured cell type tested in vitro (32) and the most intense immunostaining of any cell type within vascular tissue samples (34). Finally, several inflammatory cytokines, such as IL-1␤, tumor necrosis factor ␣ (TNF-␣), and IFN-␥ (46), increase the already high levels of S-SMase secretion from cultured endothelial cells and, importantly, decrease the levels of L-SMase in these cells (32).…”

Proteolysis sensitizes LDL particles to phospholipolysis by secretory phospholipase A2 group V and secretory sphingomyelinase

“…The presence of secretory acid sphingomyelinase in human atherosclerotic lesions has also been confi rmed ( 17 ). The human enzyme has an acidic pH optimum and, accordingly, at neutral pH is capable of yielding only a limited degree of hydrolysis of LDL particles ( 18 ).…”

Atherosclerotic Plaque Progression and Vulnerability to Rupture

“…The level of sphingomylenase activity in plaques should be sufficient to catalyze the release of cholesterol from RBC membranes. 27,28 In addition, excess membrane cholesterol can phase separate and form metastable membrane domains consisting of pure cholesterol arranged in a tail-to-tail orientation, creating a nidus for nucleation to crystalline cholesterol. 29 In the early to mid-20th century, several leading pathologists forwarded the hypothesis that intraplaque hemorrhage is a major contributor to the progression of coronary atherosclerosis; however, the precise nature of this relationship was not well understood.…”