Acidic extracellular pH promotes accumulation of free cholesterol in human monocyte-derived macrophages via inhibition of ACAT1 activity.

Background And Aims: In focal areas of advanced human atherosclerotic lesions, the intimal fluid is acidic. An acidic medium impairs the ABCA1-mediated cholesterol efflux from macrophages, so tending to increase their content of free cholesterol, which is then available for esterification by the macrophage enzyme ACAT1. Here we investigated whether low extracellular pH would affect the activity of ACAT1.

Human mast cell neutral proteases generate modified LDL particles with increased proteoglycan binding.

Background And Aims: Subendothelial interaction of LDL with extracellular matrix drives atherogenesis. This interaction can be strengthened by proteolytic modification of LDL. Mast cells (MCs) are present in atherosclerotic lesions, and upon activation, they degranulate and release a variety of neutral proteases.

Acidification of the intimal fluid: the perfect storm for atherogenesis.

Atherosclerotic lesions are often hypoxic and exhibit elevated lactate concentrations and local acidification of the extracellular fluids. The acidification may be a consequence of the abundant accumulation of lipid-scavenging macrophages in the lesions. Activated macrophages have a very high energy demand and they preferentially use glycolysis for ATP synthesis even under normoxic conditions, resulting in enhanced local generation and secretion of lactate and protons.

Acidity increases the uptake of native LDL by human monocyte-derived macrophages.

The extracellular pH is locally decreased in advanced atherosclerotic lesions, particularly in lipid-rich areas of the lesions. Since accumulation of LDL-derived cholesterol and formation of foam cells are key processes in atherogenesis, we tested here the effects of acidic pH on the uptake of native LDL. First, human monocytes were differentiated into macrophages in the presence of granulocyte-monocyte-colony stimulating factor (GM-CSF) after which native LDL was applied to the monocyte-derived macrophages at pH 5.

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

LDL particles that enter the arterial intima become exposed to proteolytic and lipolytic modifications. The extracellular hydrolases potentially involved in LDL modification include proteolytic enzymes, such as chymase, cathepsin S, and plasmin, and phospholipolytic enzymes, such as secretory phospholipases A2 (sPLA2-IIa and sPLA2-V) and secretory acid sphingomyelinase (sSMase). Here, LDL was first proteolyzed and then subjected to lipolysis, after which the effects of combined proteolysis and lipolysis on LDL fusion and on binding to human aortic proteoglycans (PG) were studied.

Phospholipase A(2)-modified LDL particles retain the generated hydrolytic products and are more atherogenic at acidic pH.

Objective: Hydrolysis of LDL by phospholipase A(2) (PLA(2)) generates free fatty acids (FFAs) and lysophospholipids (lysoPCs). Binding of the PLA(2)-modified LDL to proteoglycans, and their uptake by macrophages are increased. Since the extracellular pH is locally decreased in advanced atherosclerotic plaques, we examined the effects of acidic pH on PLA(2)-induced LDL modification and its proatherogenic consequences.