ABCA1 deficiency causes tissue-specific dysregulation of the SREBP2 pathway in mice.

ABCA1 plays an essential role in the formation of high-density lipoprotein (HDL), and its mutations cause Tangier disease (TD), a familial HDL deficiency. In addition to the disappearance of HDL, TD patients exhibit cholesterol deposition in peripheral tissues through a mechanism poorly understood, which may contribute to the development of premature atherosclerosis. We and others previously showed that ABCA1 deficiency causes hyperactivation of the SREBP2 pathway in vitro. Here, we show using Abca1 knockout mice that ABCA1 deficiency leads to tissue-specific dysregulation of SREBP2 activity in a nutritional status-dependent manner, which may underlie the pathophysiology of TD.

Expression of ATP-binding cassette transporter A1 is induced by nerve injury and its deficiency affects neurite tip morphology and elongation in cultured neurons.

Axonal regeneration requires changes in the lipid dynamics of the axon membrane for growth and extension. Here, we examined the expression of genes associated with lipid transport after nerve injury. The expression of ATP-binding cassette transporter-A1 (ABCA1), which participates in the transport of cholesterol from the plasma membrane, was markedly upregulated in motor and sensory neurons after nerve injury. Stimulation of PC12 cells with the nerve growth factor induced neurite extension and ABCA1 expression predominantly in regions proximal to the neurite tip. To clarify the functional role of ABCA1 in neurite elongation, we examined the morphology of neurons cultured from conditionally-injured dorsal root ganglia from ABCA1-deficient mice. We found a significant increase in neurite branch formation in these neurons. In addition, the neurite tips of ABCA1-deficient neurons appeared excessively ruffled, and the direction of neurite elongation was unsteady. In contrast, the neurite tips of wild-type neurons were not excessively ruffled, and the neurites elongated rapidly in a stable directionally-oriented manner. Together, these findings suggest that ABCA1 plays an important role in regulating the membrane lipid composition of injured neurons and in axonal regeneration following nerve injury.

The effect of maltobionic acid on bone metabolism markers in healthy Japanese postmenopausal women: A randomized double-blind placebo-controlled crossover study.

Osteoporosis is characterized by compromised bone strengthpredisposing to an increased risk of fracture and is a disease with a high incidence in postmenopausal women. Frequent estrogen deficiency, particularly in postmenopausal women, induces osteoclast activation and is a major contributor to reduced bone mineral density. Maltobionic acid (MB) reportedly promotes mineral resorption and maintains bone mineral density in human clinical trials, although no studies have confirmed that MB improves bone metabolism in humans. Therefore, this study aimed to investigate the effects of MB administration on bone-resorption markers in healthy Japanese postmenopausal women. This was a randomized, double-blind, placebo-controlled, crossover trial. Twenty-six healthy adult Japanese women who realized that they had passed through more than 1 year of natural menopause and were aged 40-69 years were categorized into three groups. The experimental groups were allowed to consume maltobionic acid syrup 4 g (MB syrup 4 g group), maltobionic acid syrup 2 g plus maltose syrup 2 g (MB syrup 2 g group), and maltose syrup 4 g (placebo group) for 4 weeks. All 26 participants completed the intervention. Continuous ingestion of MB syrup 2 g or 4 g for 4 weeks significantly reduced the levels of bone-resorption markers deoxypyridinoline (DPD) and urinary N-telopeptide (u-NTx), and significantly increased the bone formation marker osteocalcin (OC) compared with the placebo group. Maltobionic acid (MB) intake may improve bone metabolism and reduce bone health problems, including osteoporosis, in postmenopausal, adult Japanese women. (UMIN-CTR ID: UMIN000038627).

Comparison of the Physical Characteristics and Behavior in ABC Transporter A1, A7 or Apolipoprotein E Knockout Mice with Lipid Transport Dysfunction.

The physical characteristics and behavior of the ATP-binding cassette (ABC) A1, A7, and apolipoprotein (apo) E knockout (KO) mice with lipid transport dysfunction were investigated. These KO mice exhibited adequate growth, and their body masses increased steadily. No remarkable changes were observed in their blood pressure and heart rate. However, there was a slight increase in the heart rate of the ABCA7 KO mice compared with that of the wild-type (WT) mice. ABCA1 and apoE KO mice showed hypo- and hyper-cholesterol concentrations in the plasma, respectively. With regard to the cerebrum, however, the weight of the ABCA1 KO mice was lighter than those of the other genotypes. Furthermore, the cholesterol, triglyceride and phospholipid concentrations, and fatty acid composition were generally similar. Compared with the WT mice, ABCA1 KO mice stayed for a shorter time in the closed arm of the elevated plus maze, and performed worse in the initial stage of the Morris water maze. To thermal stimuli, the ABCA1 and apoE KO mice showed hyper- and hypo-sensitivities, respectively. Only the response of the ABCA1 KO mice was significantly inhibited by pretreatment with indomethacin. A low concentration of the prostaglandin E metabolites was detected in the plasma of the ABCA1 KO mice. Thus, ABCA1 is thought to play a specific role in the neural function.

ABCA7 links sterol metabolism to the host defense system: Molecular background for potential management measure of Alzheimer's disease.

ATP-binding cassette transporter (ABC) A7 is a membrane protein that belongs to the large family of ABC transporters. It is 54% homologous in amino acid residue sequence to ABCA1 which mediates biogenesis of plasma high density lipoprotein (HDL) from cellular phospholipid and cholesterol with extracellular helical apolipoproteins such as apolipoprotein (apo) A-I. When transfected and expressed, ABCA7 also mediates generation of HDL-like particles but small and of less cholesterol content. However, endogenous ABCA7 is unlikely involved in HDL biogenesis and rather to regulate the host-defense system such as phagocytotic function of the cells. ABCA1 expression is regulated by cellular cholesterol levels, positively by the liver X receptor (LXR) in extrahepatic peripheral cells. However, it is modulated dually in the liver being relevant to transport of cholesterol for its catabolism; positively by LXR and negatively by sterol regulatory element binding protein (SREBP) or hepatic nuclear factor 4α (HNF4α). In contrast, ABCA7 expression was shown to be regulated negatively by the SREBP system so that decrease of cell cholesterol enhances ABCA7 function such as cellular phagocytotic reaction, suggesting that it links cholesterol metabolism to the host defense system. The interest is being build up in ABCA7 as its genomic diversity has been found related to a risk for late-onset Alzheimer's diseases. More recent findings indicate that ABCA7 is involved in metabolism of amyloid ß peptide including its phagocytotic clearance. Accordingly, modulation of ABCA7 activity by manipulating cholesterol metabolism may open a new path for management of Alzheimer's disease.

Molecular mechanism for nobiletin to enhance ABCA1/G1 expression in mouse macrophages.

BACKGROUND AND AIMS: Nobiletin (NOB), a functional ingredient found in citrus peel, is said to act against diabetes, obesity, and atherosclerosis. It has been reported to activate AMPK pathway, as well as increase SREBP1c, PPARα and PPARγ expression. However, no molecular mechanism has been elucidated to be able to integrate these sporadic findings with some controversies to lead to concrete outcomes. In this study, regulation of HDL biogenesis by NOB was investigated modulating ABCA1 and ABCG1 expression. METHODS AND RESULTS: Regulation of ABCA1/G1 by NOB was investigated in mouse macrophages J774.1. NOB increased mRNA and protein levels of ABCA1/G1, and cell cholesterol release by these factors. It also increased mRNA of PPARγ and LXRα but not PPARα. The increase in ABCA1/G1 mRNA levels by NOB was suppressed by antagonists of PPARγ and LXRα. The increase in PPARγ mRNA levels by NOB was suppressed by an LXRα antagonist, and the increase in LXRα mRNA levels was suppressed by a PPARγ antagonist. NOB increased CD36 mRNA and this was suppressed by an LXRα antagonist. The increase in ABCA1 mRNA by a PPARγ agonist was also suppressed by an LXRα antagonist. NOB did not influence LPL1 mRNA expression levels. NOB stimulated AMPK phosphorylation, and the increase in ABCA1/G1, LXRα and PPARγ mRNA levels and ABCA1/G1 protein levels by NOB was reversed by an AMPK inhibitor. AMPK siRNA suppressed ABCA1 expression. CONCLUSIONS: NOB activates AMPK and subsequently LXRα to promote the expression of ABCA1 and ABCG1, and an LXRα - PPARγ loop pathway amplifies these signals.

Sodium balance, circadian BP rhythm, heart rate variability, and intrarenal renin-angiotensin-aldosterone and dopaminergic systems in acute phase of ARB therapy.

We have revealed that even in humans, activated intrarenal renin-angiotensin-aldosterone system (RAAS) enhances tubular sodium reabsorption to facilitate salt sensitivity and nondipper rhythm of blood pressure (BP), and that angiotensin receptor blocker (ARB) could increase daytime urinary sodium excretion rate (UNaV) to produce lower sodium balance and restore nondipper rhythm. However, the sympathetic nervous system and intrarenal dopaminergic system can also contribute to renal sodium handling. A total of 20 patients with chronic kidney disease (61 ± 15 years) underwent 24-h ambulatory BP monitoring before and during two-day treatment with ARB, azilsartan. Urinary angiotensinogen excretion rate (UAGTV, µg/gCre) was measured as intrarenal RAAS; urinary dopamine excretion rate (UDAV, pg/gCre) as intrarenal dopaminergic system; heart rate variabilities (HRV, calculated from 24-h Holter-ECG) of non-Gaussianity index λ25s as sympathetic nerve activity; and power of high-frequency (HF) component or deceleration capacity (DC) as parasympathetic nerve activity. At baseline, glomerular filtration rate correlated inversely with UAGTV (r = -0.47, P = 0.04) and positively with UDAV (r = 0.58, P = 0.009). HF was a determinant of night/day BP ratio (ß = -0.50, F = 5.8), rather than DC or λ25s During the acute phase of ARB treatment, a lower steady sodium balance was not achieved. Increase in daytime UNaV preceded restoration of BP rhythm, accompanied by decreased UAGTV (r = -0.88, P = 0.05) and increased UDAV (r = 0.87, P = 0.05), but with no changes in HRVs. Diminished sodium excretion can cause nondipper BP rhythm. This was attributable to intrarenal RAAS and dopaminergic system and impaired parasympathetic nerve activity. During the acute phase of ARB treatment, cooperative effects of ARB and intrarenal dopaminergic system exert natriuresis to restore circadian BP rhythm.

Addition of hydrochlorothiazide to angiotensin receptor blocker therapy can achieve a lower sodium balance with no acceleration of intrarenal renin angiotensin system in patients with chronic kidney disease.

OBJECTIVE: Angiotensin receptor blockers (ARBs) produce a lower sodium (Na) balance, and the natriuretic effect is enhanced under Na deprivation, despite falls in blood pressure (BP) and glomerular filtration rate (GFR). METHODS: The effect of additional hydrochlorothiazide (HCTZ; 12.5 mg/day) to ARB treatment (valsartan; 80 mg/day) on glomerulotubular Na balance was evaluated in 23 patients with chronic kidney disease. RESULTS: Add-on HCTZ decreased GFR, tubular Na load, and tubular Na reabsorption (t(Na)), although 24-hour urinary Na excretion (U(Na)V) remained constant. Daily urinary angiotensinogen excretion (U(AGT)V, 152±10→82±17 µg/g Cre) reduced (p=0.02). Changes in tubular Na load (r(2)=0.26) and t(Na) (r(2)=0.25) correlated with baseline 24-hour U(AGT)V. Changes in filtered Na load correlated with changes in nighttime systolic BP (r(2)=0.17), but not with changes in daytime systolic BP. The change in the t(Na) to filtered Na load ratio was influenced by the change in daytime U(Na)V (ß=-0.67, F=16.8), rather than the change in nighttime U(Na)V. CONCLUSIONS: Lower Na balance was produced by add-on HCTZ to ARB treatment without an increase of intra-renal renin-angiotensin system activity, leading to restoration of nocturnal hypertension. A further study is needed to demonstrate that the reduction of U(AGT)V by additional diuretics to ARBs prevents the progression of nephropathy or cardiovascular events.

ATP-binding cassette transporter A7 (ABCA7) loss of function alters Alzheimer amyloid processing.

The ATP-binding cassette transporter A7 (ABCA7) has been identified as a susceptibility factor of late onset Alzheimer disease in genome-wide association studies. ABCA7 has been shown to mediate phagocytosis and affect membrane trafficking. The current study examined the impact of ABCA7 loss of function on amyloid precursor protein (APP) processing and generation of amyloid-ß (Aß). Suppression of endogenous ABCA7 in several different cell lines resulted in increased ß-secretase cleavage and elevated Aß. ABCA7 knock-out mice displayed an increased production of endogenous murine amyloid Aß42 species. Crossing ABCA7-deficient animals to an APP transgenic model resulted in significant increases in the soluble Aß as compared with mice expressing normal levels of ABCA7. Only modest changes in the amount of insoluble Aß and amyloid plaque densities were observed once the amyloid pathology was well developed, whereas Aß deposition was enhanced in younger animals. In vitro studies indicated a more rapid endocytosis of APP in ABCA7 knock-out cells that is mechanistically consistent with the increased Aß production. These in vitro and in vivo findings indicate a direct role of ABCA7 in amyloid processing that may be associated with its primary biological function to regulate endocytic pathways. Several potential loss-of-function ABCA7 mutations and deletions linked to Alzheimer disease that in some instances have a greater impact than apoE allelic variants have recently been identified. A reduction in ABCA7 expression or loss of function would be predicted to increase amyloid production and that may be a contributing factor in the associated Alzheimer disease susceptibility.

Deficiency in the Lipid Exporter ABCA1 Impairs Retrograde Sterol Movement and Disrupts Sterol Sensing at the Endoplasmic Reticulum.

Cellular cholesterol homeostasis involves sterol sensing at the endoplasmic reticulum (ER) and sterol export from the plasma membrane (PM). Sterol sensing at the ER requires efficient sterol delivery from the PM; however, the macromolecules that facilitate retrograde sterol transport at the PM have not been identified. ATP-binding cassette transporter A1 (ABCA1) mediates cholesterol and phospholipid export to apolipoprotein A-I for the assembly of high density lipoprotein (HDL). Mutations in ABCA1 cause Tangier disease, a familial HDL deficiency. Several lines of clinical and experimental evidence suggest a second function of ABCA1 in cellular cholesterol homeostasis in addition to mediating cholesterol efflux. Here, we report the unexpected finding that ABCA1 also plays a key role in facilitating retrograde sterol transport from the PM to the ER for sterol sensing. Deficiency in ABCA1 delays sterol esterification at the ER and activates the SREBP-2 cleavage pathway. The intrinsic ATPase activity in ABCA1 is required to facilitate retrograde sterol transport. ABCA1 deficiency causes alternation of PM composition and hampers a clathrin-independent endocytic activity that is required for ER sterol sensing. Our finding identifies ABCA1 as a key macromolecule facilitating bidirectional sterol movement at the PM and shows that ABCA1 controls retrograde sterol transport by modulating a certain clathrin-independent endocytic process.

Preferential incorporation of shorter and less unsaturated acyl phospholipids into high density lipoprotein-like particles in the ABCA1- and ABCA7-mediated biogenesis with apoA-I.

Molecular species of phosphatidylcholine (PC) and sphingomyelin (SPM) were globally analyzed for lipidomics in the nascent high-density lipoprotein (HDL)-like particles generated with human apolipoprotein A-I (apoA-I) form HEK293 cells where either human ATP binding cassette transporter (ABC) A1 or ABCA7 was transfected and overexpressed. SPM/PC ratio was higher in the ABCA1-mediated HDL than ABCA7-mediated HDL likely being related to their cholesterol content, while it was less than the ratio in the cell membrane in either case. Molecular species composition of hydrocarbon chain moiety in each phospholipid in the HDL largely reflected that in the cells the lipoprotein originated in, without remarkable difference between ABCA1 and ABCA7. Further analysis, however, revealed apparent preference for the molecules with shorter hydrocarbon chain length for both PC and SPM in their relative incorporation into HDL by ABCA1 and ABCA7. Likewise, it was in favor for less-unsaturated hydrocarbon chains of PC while this preference was not apparent for SPM. The results are consistent with the view that assembly of HDL particles with extracellular apoA-I is primarily with the cellular phospholipid molecules being regulated in part by their physicochemical nature.

Calpain-mediated ABCA1 degradation: post-translational regulation of ABCA1 for HDL biogenesis.

Helical apolipoproteins remove cellular phospholipid and cholesterol to generate nascent HDL and this reaction is the major source of plasma HDL. ABCA1 is mandatory and rate-limiting for this reaction. Besides regulation of the gene expression by transcriptional factors including LXR, AP2 and SREBP, the ABCA1 activity is regulated post-translationally by calpain-mediated proteolytic degradation of ABCA1 protein that occurs in the early endosome after its endocytosis. When the HDL biogenesis reaction is ongoing as helical apolipoproteins interact with ABCA1, ABCA1 becomes resistant to calpain and is recycled to cell surface after endocytosis. Biogenesis of HDL is most likely to take place on cell surface. Clearance rate of ABCA1 by this mechanism is also retarded by various factors that interact with ABCA1, such as α1-syntrophin, LXRß and calmodulin. Physiological relevance of the retardation by these factors is not entirely clear. Pharmacological inhibition of the calpain-mediated ABCA1 degradation results in the increase of the ABCA1 activity and HDL biogenesis in vitro and in vivo, and potentially suppresses atherogenesis. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

ApoA-I enhances generation of HDL-like lipoproteins through interaction between ABCA1 and phospholipase Cγ in rat astrocytes.

In the previous paper, we reported that apolipoprotein (apo) A-I enhances generation of HDL-like lipoproteins in rat astrocytes to be accompanied with both increase in tyrosine phosphorylation of phospholipase Cγ (PL-Cγ) and PL-Cγ translocation to cytosolic lipid-protein particles (CLPP) fraction. In this paper, we studied the interaction between apoA-I and ATP-binding cassette transporter A1 (ABCA1) to relate with PL-Cγ function for generation of HDL-like lipoproteins in the apoA-I-stimulated astrocytes. ABCA1 co-migrated with exogenous apoA-I with apparent molecular weight over 260kDa on SDS-PAGE when rat astrocytes were treated with apoA-I and then with a cross-linker, BS3. The solubilized ABCA1 of rat astrocytes was associated with the apoA-I-immobilized Affi-Gel 15. An LXR agonist, To901317, increased the cellular level of ABCA1, association of apoA-I with ABCA1 and apoA-I-mediated lipid release in rat astrocytoma GA-1/Mock cells where ABCA1 expression at baseline is very low. PL-Cγ was co-isolated by apoA-I-immobilized Affi-Gel 15 and co-immunoprecipitated by anti-ABCA1 antibody along with ABCA1 from the solubilized membrane fraction of rat astrocytes. The SiRNA of ABCA1 suppressed not only the PL-Cγ binding to ABCA1 but also the tyrosine phosphorylation of PL-Cγ. A PL-C inhibitor, U73122, prevented generation of apoA-I-mediated HDL-like lipoproteins in rat astrocytes. To901317 increased the association of PL-Cγ with ABCA1 in GA-1/Mock cells dependently on the increase of cellular level of ABCA1 without changing that of PL-Cγ. These findings suggest that the exogenous apoA-I augments the interaction between PL-Cγ and ABCA1 to stimulate tyrosine phosphorylation and activation of PL-Cγ for generation of HDL-like lipoproteins in astrocytes.

Impact of self-association on function of apolipoprotein A-I.

Self-association is an inherent property of the lipid-free forms of several exchangeable apolipoproteins, including apolipoprotein A-I (apoA-I), the main protein component of high density lipoproteins (HDL) and an established antiatherogenic factor. Monomeric lipid-free apoA-I is believed to be the biologically active species, but abnormal conditions, such as specific natural mutations or oxidation, produce an altered state of self-association that may contribute to apoA-I dysfunction. Replacement of the tryptophans of apoA-I with phenylalanines (ΔW-apoA-I) leads to unusually large and stable self-associated species. We took advantage of this unique solution property of ΔW-apoA-I to analyze the role of self-association in determining the structure and lipid-binding properties of apoA-I as well as ATP-binding cassette A1 (ABCA1)-mediated cellular lipid release, a relevant pathway in atherosclerosis. Monomeric ΔW-apoA-I and wild-type apoA-I activated ABCA1-mediated cellular lipid release with similar efficiencies, whereas the efficiency of high order self-associated species was reduced to less than 50%. Analysis of specific self-associated subclasses revealed that different factors influence the rate of HDL formation in vitro and ABCA1-mediated lipid release efficiency. The α-helix-forming ability of apoA-I is the main determinant of in vitro lipid solubilization rates, whereas loss of cellular lipid release efficiency is mainly caused by reduced structural flexibility by formation of stable quaternary interactions. Thus, stabilization of self-associated species impairs apoA-I biological activity through an ABCA1-mediated mechanism. These results afford mechanistic insights into the ABCA1 reaction and suggest self-association as a functional feature of apoA-I. Physiologic mechanisms may alter the native self-association state and contribute to apoA-I dysfunction.

HMG-CoA reductase inhibitors enhance phagocytosis by upregulating ATP-binding cassette transporter A7.

We recently reported that the endogenous ATP-binding cassette transporter (ABC) A7 strongly associates with phagocytosis, being regulated by sterol regulatory element binding protein 2. We therefore examined the effect of statins on phagocytosis in vitro and in vivo through the SREBP-ABCA7. Phagocytosis was found to be enhanced by pravastatin, rosuvastatin and simvastatin and cyclodextrin in J774 macrophages, as cellular cholesterol was reduced and expressions of the cholesterol-related genes were modulated, including an increase of ABCA7 mRNA and decrease of ABCA1 mRNA. Conversely, knock-down of ABCA7 expression by siRNA ablated enhancement of phagocytosis by statins. In vivo, pravastatin enhanced phagocytosis in wild-type mice, but not in ABCA7-knockout mice. We thus concluded that statins enhance phagocytosis through the SREBP-ABCA7 pathway. These findings provide a molecular basis for enhancement of the host-defense system by statins showing that one of their "pleiotropic" effects is in fact achieved through their reaction to a primary target.

Insulin down-regulates specific activity of ATP-binding cassette transporter A1 for high density lipoprotein biogenesis through its specific phosphorylation.

Insulin resistance/hyperinsulinism is one of the major risks for atherosclerotic vascular diseases and low HDL may be involved in pathogenesis. We examined direct effects of insulin on HDL biosynthesis focusing on the activity of ATP-binding cassette transporter A1 (ABCA1) in culture cells and in experimental animals. Insulin impairs HDL biosynthesis through modulation of ABCA1 activity by two different mechanisms. Insulin enhances degradation of ABCA1. However, even after this effect was cancelled by blocking its specific signal, insulin still reduces HDL biogenesis. This effect was found due to phosphorylation of ABCA1 that leads to decrease of its specific activity. We identified a novel insulin-specific phosphorylation site Tyr1206 of ABCA1 to regulate its specific activity. The observation in a rat model of insulin resistance was consistent with these results. The findings demonstrate a new mechanism for regulation of ABCA1 activity and provide new insights into the link between development of atherosclerosis, and insulin resistance/hyperinsulinism.

Roles of ATP-binding cassette transporter A7 in cholesterol homeostasis and host defense system.

ATP-binding cassette transporter (ABC) A7 is an ABC family protein that is a so-called full-size ABC transporter, highly homologous to ABCA1, which mediates the biogenesis of high-density lipoprotein (HDL) with cellular lipid and helical apolipoproteins. ABCA7 mediates the formation of HDL when exogenously transfected and expressed; however, endogenous ABCA7 was shown to have no significant impact on the generation of HDL and was found to be associated with phagocytosis regulated by sterol regulatory element binding protein 2. Since phagocytosis is one of the fundamental functions of animal cells as an important responsive reaction to infection, injury and apoptosis, ABCA7 seems to be one of the key molecules linking sterol homeostasis and the host defense system. In this context, HDL apolipoproteins were shown to enhance phagocytosis by stabilizing ABCA7 against calpain-mediated degradation and increasing its activity, shedding light on a new aspect of the regulation of the host-defense system.

Helical apolipoproteins of high-density lipoprotein enhance phagocytosis by stabilizing ATP-binding cassette transporter A7.

We previously reported that the endogenous ATP-binding cassette transporter (ABC)A7 strongly associates with phagocytic function rather than biogenesis of high-density lipoprotein (HDL), being regulated by sterol-regulatory element binding protein (SREBP)2. Phagocytic activity was found enhanced by apolipoprotein (apo)A-I and apoA-II more than twice the maximum in J774 and mouse peritoneal macrophages. Therefore we investigated the molecular basis of this reaction in association with the function of ABCA7. Similar to ABCA1, ABCA7 was degraded, likely by calpain, and apoA-I and apoA-II stabilize ABCA7 against degradation. Cell surface biotinylation experiments demonstrated that endogenous ABCA7 predominantly resides on the cell surface and that the apolipoproteins increase the surface ABCA7. The increase of phagocytosis by apolipoproteins was retained in the J774 cells treated with ABCA1 siRNA and in the peritoneal macrophages from ABCA1-knockout mice, but it was abolished in the J774 cells treated with ABCA7 siRNA and in the peritoneal macrophages from ABCA7-knockout mice. Phagocytosis was decreased in the cells in the peritoneal cavity of the ABCA7-knockout mouse compared with the wild-type control. We thus concluded that extracellular helical apolipoproteins augment ABCA7-associated phagocytosis by stabilizing ABCA7. The results demonstrated direct enhancement of the host defense system by HDL components.

Calmodulin interacts with ATP binding cassette transporter A1 to protect from calpain-mediated degradation and upregulates high-density lipoprotein generation.

OBJECTIVE: To investigate the interaction of ATP-binding cassette transporter A1 (ABCA1) with calmodulin in relation to its calpain-mediated degradation because many calpain substrates bind calmodulin to regulate cellular functions. METHODS AND RESULTS: The activity of ABCA1 is regulated through proteolysis by calpain. An immunoprecipitation and glutathione S-transferase pull-down assay revealed that ABCA1 directly binds calmodulin in a Ca(2+)-dependent manner. The cytoplasmic loop of ABCA1 contains a typical calmodulin binding sequence of 1-5-8-14 motifs (1245 to 1257 amino acids). The peptide of this region showed binding to calmodulin, and deletion of the 1-5-8-14 motif abolished this interaction. This motif is located near the ABCA1 Pro-Glu-Ser-Thr sequence, and the presence of calmodulin/Ca(2+) protected the peptides from proteolysis by calpain. The knockdown of calmodulin by a specific small and interfering RNA increased the degradation of ABCA1 and decreased ABCA1 protein and apolipoprotein A-I-mediated lipid release. Surprisingly, calmodulin inhibitor W7 increased calmodulin binding to ABCA1 and protected it from calpain-mediated degradation, consistent with our previous finding that this compound increased apolipoprotein A-I-mediated cell cholesterol release. CONCLUSIONS: Calmodulin directly binds and stabilizes ABCA1 in the presence of Ca(2+) and increases the generation of high-density lipoprotein.

Pharmacological inhibition of ABCA1 degradation increases HDL biogenesis and exhibits antiatherogenesis.

Expression of ABCA1 is regulated by transcription of the gene and calpain-mediated proteolytic degradation, and inhibition ABCA1 degradation results in increased ABCA1 and HDL biogenesis in vitro. We examined whether this approach could be a potential antiatherogenic treatment. Although probucol inhibits both the activity and degradation of ABCA1, its oxidized products, spiroquinone and diphenoquinone, reduce degradation of ABCA1 without inhibiting its activity or altering transcription of the ABCA1 gene. Accordingly, both compounds enhanced apolipoprotein A-I/ABCA1-dependent generation of HDL in vitro, and increased hepatic ABCA1 and plasma HDL without increasing antioxidant activity in plasma when given to rabbits. Both compounds also decreased vascular lipid deposition in cholesterol-fed rabbits. We therefore conclude that stabilization of ABCA1 against calpain-mediated degradation is a novel and potentially important strategy to increase HDL formation and prevent atherosclerosis. Spiroquinone and diphenoquinone are potential seeds for development of such drugs.