👤 Song-Kun Shyue

Excess nitric oxide (NO) deregulates cholesterol metabolism in macrophage foam cells, yet the underlying molecular mechanism is incompletely understood. To investigate the mechanism, we found that in macrophages, treatment with NO donors S-nitroso-N-acetyl-D,L-penicillamine (SNAP) or diethylenetriamine/nitric oxide induced LXRα degradation and reduced the expression of the downstream target of LXRα, ATP-binding cassette transporter A1 (ABCA1), and cholesterol efflux. In addition, SNAP induced calcium (Ca(2+)) influx into cells, increased calpain activity and promoted the formation of calpain-LXRα complex. Pharmacological inhibition of calpain activity reversed the SNAP-induced degradation of LXRα, down-regulation of ABCA1 and impairment of cholesterol efflux in macrophages. SNAP increased the formation of calpain-LXRα complex in a Pro-Glu-Ser-Thr (PEST) motif-dependent manner. Truncation of the PEST motif in LXRα abolished the calpain-dependent proteolysis. Removal of extracellular Ca(2+) by EGTA or pharmacological inhibition of TRPV1 channel activity diminished SNAP-induced increase in intracellular Ca(2+), calpain activation, LXRα degradation, ABCA1 down-regulation and impaired cholesterol efflux. In conclusion, excess NO may activate calpain via TRPV1-Ca(2+) signaling and promote the recognition of calpain in the PEST motif of LXRα, thereby leading to degradation of LXRα and, ultimately, downregulated ABCA1 expression and impaired ABCA1-dependent cholesterol efflux in macrophages. Show less

Soluble guanylyl cyclase (sGC) is a key modulator in the regulation of vascular tone. However, its role and involving mechanism in cholesterol metabolism of macrophages and atherosclerosis remain unclear. Oil red O staining, Dil-oxidized low-density lipoprotein (oxLDL)-binding assay and cholesterol efflux assay were performed in biology of foam cells. Levels of cytokines or intracellular lipid were evaluated by ELISA or colorimetric kits. Expression of gene or protein was determined by quantitative real-time PCR or Western blotting. Histopathology was examined by haematoxylin and eosin staining. Soluble guanylyl cyclase was expressed in macrophages of mouse atherosclerotic lesions. Treatment with 1H-[1, 2, 4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, sGC inhibitor) exacerbated oxLDL-induced cholesterol accumulation in macrophages. In contrast, 3-(5'-hydroxymethyl-2'furyl)-1-benzyl indazole (YC-1, sGC activator) attenuated the oxLDL-induced cholesterol accumulation because of increased cholesterol efflux. Additionally, YC-1 dose dependently increased the protein expression of ATP-binding cassette transporter A1 (ABCA1) but did not alter that of scavenger receptor class A (SR-A), CD36, SR-BI or ABCG1. Moreover, YC-1-upregulated ABCA1 level depended on liver X receptor α (LXRα). Inhibition of the LXRα-ABCA1 pathway by LXRα small interfering RNA (siRNA), ABCA1 neutralizing antibody or ABCA1 siRNA abolished the effect of YC-1 on cholesterol accumulation and cholesterol efflux. In vivo, YC-1 retarded the development of atherosclerosis, accompanied by reduced serum levels of cholesterol and pro-inflammatory cytokines, in apolipoprotein E-deficient mice. Activation of sGC by YC-1 leads to LXRα-dependent upregulation of ABCA1 in macrophages and may confer protection against atherosclerosis. Show less

Excess nitric oxide (NO) promotes the progression of atherosclerosis by increasing the oxidation of low-density lipoprotein (LDL) and inflammatory responses. However, little is known about the impact of NO and its underlying molecular mechanism on lipid metabolism of macrophage foam cells. In this study, Oil-red O staining, cholesterol and triglyceride assay, Dil-oxidized LDL (oxLDL) binding assay, cholesterol efflux assay, real-time RT-PCR and Western blot analysis were used for in vitro experiments. Apolipoprotein E-deficient (apoE(-/-) ) and apoE and inducible nitric oxide synthase-deficient (apoE(-/-) iNOS(-/-) ) mice were as our in vivo models. Treatment with S-nitroso-N-acetyl-D,L-penicillamine (SNAP), an NO donor, exacerbated oxLDL-induced cholesterol accumulation in macrophages, because of reduced efficacy of cholesterol efflux. In addition, SNAP decreased the protein level of ATP-binding cassette transporter A1 (ABCA1) without affecting scavenger receptor type A (SR-A), CD36, ABCG1, or SR-B1 levels. This SNAP-mediated downregulation of ABCA1 was mainly through the effect of NO but not peroxynitrite. Furthermore, the SNAP-downregulated ABCA1 was due to the decrease in the liver X receptor α (LXRα)-dependent transcriptional regulation. Moreover, genetic deletion of iNOS increased the serum capacity of reverse cholesterol efflux and protein expression of LXRα, ABCA1, and SR-BI in aortas and retarded atherosclerosis in apoE(-/-) mice. Our findings provide new insights in the pro-atherogenic effect of excess NO on cholesterol metabolism in macrophages. Show less

Paeonol, a phenolic component purified from Paeonia suffruticosa (Cortex Moutan), is used in traditional Chinese medicine to treat inflammatory diseases. However, little is known about the effect of paeonol on cholesterol metabolism. We investigated the efficacy of paeonol on cholesterol metabolism and the underlying mechanism in macrophages and apolipoprotein E deficient (apoE(-/-)) mice. Treatment with paeonol markedly attenuated cholesterol accumulation induced by oxidized LDL in macrophages, which was due to increased cholesterol efflux. Additionally, paeonol enhanced the mRNA and protein expression of ATP-binding membrane cassette transport protein A1 (ABCA1) but did not alter the protein level of ABCG1 or other scavenger receptors. Inhibition of ABCA1 activity with a pharmacological inhibitor, neutralizing antibody or small interfering RNA (siRNA), negated the effects of paeonol on cholesterol efflux and cholesterol accumulation. Furthermore, paeonol induced the nuclear translocation of liver X receptor α (LXRα) by increasing its activity. siRNA knockdown of LXRα abolished the paeonol-induced upregulation of ABCA1, promotion of cholesterol efflux and suppression of cholesterol accumulation. Moreover, atherosclerotic lesions, hyperlipidemia and systemic inflammation were reduced and the protein expression of ABCA1 was increased in aortas of paeonol-treated apoE(-/-) mice. Paeonol may alleviate the formation of foam cells by enhancing LXRα-ABCA1-dependent cholesterol efflux. Show less

The transient receptor potential vanilloid type 1 (TRPV1) is crucial in the pathogenesis of atherosclerosis; yet its role and underlying mechanism in the formation of macrophage foam cells remain unclear. Here, we show increased TRPV1 expression in the area of foamy macrophages in atherosclerotic aortas of apolipoprotein E-deficient mice. Exposure of mouse bone-marrow-derived macrophages to oxidized low-density lipoprotein (oxLDL) upregulated the expression of TRPV1. In addition, oxLDL activated TRPV1 and elicited calcium (Ca(2+)) influx, which were abrogated by the pharmacological TRPV1 antagonist capsazepine. Furthermore, oxLDL-induced lipid accumulation in macrophages was ameliorated by TRPV1 agonists but exacerbated by TRPV1 antagonist. Treatment with TRPV1 agonists did not affect the internalization of oxLDL but promoted cholesterol efflux by upregulating the efflux ATP-binding cassette (ABC) transporters ABCA1 and ABCG1. Moreover, the upregulation of ABC transporters was mainly through liver X receptor α-(LXRα-) dependent regulation of transcription. Moreover, the TNF-α-induced inflammatory response was alleviated by TRPV1 agonists but aggravated by the TRPV1 antagonist and LXR α siRNA in macrophages. Our data suggest that LXR α plays a pivotal role in TRPV1-activation-conferred protection against oxLDL-induced lipid accumulation and TNF-α-induced inflammation in macrophages. Show less

Curcumin, a potent antioxidant extracted from Curcuma longa, confers protection against atherosclerosis, yet the detailed mechanisms are not fully understood. In this study, we examined the effect of curcumin on lipid accumulation and the underlying molecular mechanisms in macrophages and apolipoprotein E-deficient (apoE⁻/⁻) mice. Treatment with curcumin markedly ameliorated oxidized low-density lipoprotein (oxLDL)-induced cholesterol accumulation in macrophages, which was due to decreased oxLDL uptake and increased cholesterol efflux. In addition, curcumin decreased the protein expression of scavenger receptor class A (SR-A) but increased that of ATP-binding cassette transporter (ABC) A1 and had no effect on the protein expression of CD36, class B receptor type I (SR-BI), or ATP-binding cassette transporter G1 (ABCG1). The downregulation of SR-A by curcumin was via ubiquitin-proteasome-calpain-mediated proteolysis. Furthermore, the curcumin-induced upregulation of ABCA1 was mainly through calmodulin-liver X receptor α (LXRα)-dependent transcriptional regulation. Curcumin administration modulated the expression of SR-A, ABCA1, ABCG1, and SR-BI in aortas and retarded atherosclerosis in apoE⁻/⁻ mice. Our findings suggest that inhibition of SR-A-mediated oxLDL uptake and promotion of ABCA1-dependent cholesterol efflux are two crucial events in suppression of cholesterol accumulation by curcumin in the transformation of macrophage foam cells. Show less

α-Lipoic acid (α-LA), a key cofactor in cellular energy metabolism, has protective activities in atherosclerosis, yet the detailed mechanisms are not fully understood. In this study, we examined whether α-LA affects foam cell formation and its underlying molecular mechanisms in murine macrophages. Treatment with α-LA markedly attenuated oxidized low-density lipoprotein (oxLDL)-mediated cholesterol accumulation in macrophages, which was due to increased cholesterol efflux. Additionally, α-LA treatment dose-dependently increased protein levels of ATP-binding cassette transporter A1 (ABCA1) and ABCG1 but had no effect on the protein expression of SR-A, CD36, or SR-BI involved in cholesterol homeostasis. Furthermore, α-LA increased the mRNA expression of ABCA1 and ABCG1. The upregulation of ABCA1 and ABCG1 by α-LA depended on liver X receptor α (LXRα), as evidenced by an increase in the nuclear levels of LXRα and LXRE-mediated luciferase activity and its prevention of the expression of ABCA1 and ABCG1 after inhibition of LXRα activity by the pharmacological inhibitor geranylgeranyl pyrophosphate (GGPP) or knockdown of LXRα expression with small interfering RNA (siRNA). Consistently, α-LA-mediated suppression of oxLDL-induced lipid accumulation was abolished by GGPP or LXRα siRNA treatment. In conclusion, LXRα-dependent upregulation of ABCA1 and ABCG1 may mediate the beneficial effect of α-LA on foam cell formation. Show less

Berberine, a botanical alkaloid purified from Cortidis rhizoma, has effects in cardiovascular diseases, yet the mechanism is not fully understood. Foam cells play a critical role in the progression of atherosclerosis. This study aimed to investigate the effect of berberine on the formation of foam cells by macrophages and the underlying mechanism. Treatment with berberine markedly suppressed oxidized low-density lipoprotein (oxLDL)-mediated lipid accumulation, which was due to an increase in cholesterol efflux. Berberine enhanced the mRNA and protein expression of ATP-binding membrane cassette transport protein A1 (ABCA1) but did not alter the protein level of ABCG1 or other scavenger receptors. Additionally, functional inhibition of ABCA1 with a pharmacological inhibitor or neutralizing antibody abrogated the effects of berberine on cholesterol efflux and lipid accumulation. Moreover, berberine induced the nuclear translocation and activation of liver X receptor alpha (LXRalpha) but not its protein expression. Knockdown of LXRalpha mRNA expression by small interfering RNA abolished the berberine-mediated protective effects on ABCA1 protein expression and oxLDL-induced lipid accumulation in macrophages. These data suggest that berberine abrogates the formation of foam cells by macrophages by enhancing LXRalpha-ABCA1-dependent cholesterol efflux. Show less