Clusterin/apolipoprotein J binds to aggregated LDL in human plasma and plays a protective role against LDL aggregation

Abstract

Clusterin/apolipoprotein J (apoJ) is an extracellular chaperone involved in the quality control system against protein aggregation. A minor part of apoJ is transported in blood bound to LDLs, but its function is unknown. Our aim was to determine the role of apoJ bound to LDLs. Total LDL from human plasma was fractionated into native LDL [LDL(+)] and electronegative LDL [LDL(-)]. The latter was separated into nonaggregated [nagLDL(-)] and aggregated LDL(-) [agLDL(-)]. The content of apoJ was 6-fold higher in LDL(-) than in LDL(+) and 7-fold higher in agLDL(-) than in nagLDL(-). The proportion of LDL particles containing apoJ (LDL/J+) was 3-fold lower in LDL(+) than in LDL(-). LDL/J+ particles shared several characteristics with agLDL(-), including increased negative charge and aggregation. apoJ-depleted particles (LDL/J-) showed increased susceptibility to aggregation, whether spontaneous or induced by proteolysis or lipolysis, as was revealed by turbidimetric analysis, gel filtration chromatography, lipoprotein precipitation, native gradient gel electrophoresis, circular dichroism, and transmission electronic microscopy. The addition of purified apoJ to total LDL also prevented its aggregation induced by proteolysis or lipolysis. These findings point to apoJ as a key modulator of LDL aggregation and reveal a putative new therapeutic strategy against atherosclerosis.

Thermal stability of human plasma electronegative low-density lipoprotein: A paradoxical behavior of low-density lipoprotein aggregation

“…1–3). This observation agrees with previous reports that LDL(−) has increased susceptibility to aggregation upon other perturbations, such as intense agitation, prolonged incubation at 37 °C, or phospholipase treatment [17,18,27,29,38,46]. Our TEM and NGGE data confirm the presence of aggregated particles in LDL(−) isolated from blood.…”

“…For instance, apoJ and apoA-I are known to prevent LDL aggregation, fusion and rupture [21,22]. Western blot analysis confirmed increased apoA-I and apoJ content in LDL(−) compared to LDL(total) or LDL(+) and showed that endogenous apoJ remained intact after 15 min of incubation at 82 °C, while endogenous apoA-I was largely intact even after 90 min (Fig.…”

“…LDL concentration is expressed as mg protein/mL, unless otherwise indicated. ApoJ-containing LDL (LDL/J+) and apoJ-depleted LDL (LDL/J-) were obtained by affinity chromatography in NHS-HiTrap columns to which an apoJ-specific antibody was attached as described [21]. For biochemical heat-induced changes, LDL samples (0.35 mg protein/mL) in standard phosphate buffer were incubated at 82 °C for 15, 30 or 90 min (end-point).…”

“…An unique feature of LDL(−) is a relatively high content of non-apoB proteins [20]. The role of these proteins in the aggregation behavior of LDL(−) is unknown, but some of them, such as apoJ and apoA-I, can prevent aggregation, fusion and rupture of total LDL [21,22]. In the current work, we analyzed the role of apoJ and apoA-I in aggregation of LDL subclasses.…”

Increased concentration of clusterin/apolipoprotein J (apoJ) in hyperlipemic serum is paradoxically associated with decreased apoJ content in lipoproteins

“…The function of apoJ in LDL is unclear, but it has been suggested that could inhibit LDL-mediated cytotoxicity [25]. More recently, we have reported that apoJ could play a protective role preventing LDL from aggregation [26].…”