Thyroid hormones and thyroid hormone receptors: Effects of thyromimetics on reverse cholesterol transport

Reverse cholesterol transport (RCT) is a complex process which transfers cholesterol from peripheral cells to the liver for subsequent elimination from the body via feces. Thyroid hormones (THs) affect growth, develop- ment, and metabolism in almost all tissues. THs exert their actions by binding to thyroid hormone receptors (TRs). There are two major subtypes of TRs, TRα and TRβ, and several isoforms (e.g. TRα1, TRα2, TRβ1, and TRβ2). Activation of TRα1 affects heart rate, whereas activation of TRβ1 has positive effects on lipid and lipoprotein metabolism. Consequently, particular interest has been focused on the development of thyromimetic compounds targeting TRβ1, not only because of their ability to lower plasma cholesterol but also due their ability to stimulate RCT, at least in pre-clinical models. In this review we focus on THs, TRs, and on the effects of TRβ1-modulating thyromimetics on RCT in various animal models and in humans.

A new framework for reverse cholesterol transport: Non-biliary contributions to reverse cholesterol transport

Reduction of low-density lipoprotein-cholesterol through statin therapy has only modestly decreased coronary heart disease (CHD)-associated mortality in developed countries, which has prompted the search for alternative therapeutic strategies for CHD. Major efforts are now focused on therapies that augment high-density lipoprotein (HDL)-mediated reverse cholesterol transport (RCT), and ultimately increase the fecal disposal of cholesterol. The process of RCT has long been thought to simply involve HDL-mediated delivery of peripheral cholesterol to the liver for biliary excretion out of the body. However, recent studies have revealed a novel pathway for RCT that does not rely on biliary secretion. This nonbiliary pathway rather involves the direct excretion of cholesterol by the proximal small intestine. Compared to RCT therapies that augment biliary sterol loss, modulation of non-biliary fecal sterol loss through the intestine is a much more attractive therapeutic strategy, given that excessive biliary cholesterol secretion can promote gallstone formation. However, we are at an early stage in understanding the molecular mechanisms regulating the non-biliary pathway for RCT, and much additional work is required in order to effectively target this pathway for CHD prevention. The purpose of this review is to discuss our current understanding of biliary and nonbiliary contributions to RCT with particular emphasis on the possibility of targeting the intestine as an inducible cholesterol secretory organ.

Reverse cholesterol transport revisited

Reverse cholesterol transport was originally described as the high-density lipoprotein-mediated cholesterol flux from the periphery via the hepatobiliary tract to the intestinal lumen, leading to fecal excretion. Since the introduction of reverse cholesterol transport in the 1970s, this pathway has been intensively investigated. In this topic highlight, the classical reverse cholesterol transport concepts are discussed and the subject reverse cholesterol transport is revisited.

Oligomeric procyanidins combined with Parabacteroides distasonis ameliorate high-fat diet-induced atherosclerosis by regulating lipid metabolism,inflammation reaction and bile acid metabolism in ApoE^(-/-)mice

Atherosclerosis(AS)is the main pathological basis of cardiovascular diseases.Hence,the prevention and treatment strategies of AS have attracted great research attention.As a potential probiotic,Pararabacteroides distasonis has a positive regulatory effect on lipid metabolism and bile acids(BAs)profile.Oligomeric procyanidins have been confirmed to be conducive to the prevention and treatment of AS,whose antiatherosclerotic effect may be associated with the promotion of gut probiotics.However,it remains unclear whether and how oligomeric procyanidins and P.distasonis combined(PPC)treatment can effectively alleviate high-fat diet(HFD)-induced AS.In this study,PPC treatment was found to significantly decrease atherosclerotic lesion,as well as alleviate the lipid metabolism disorder,inflammation and oxidative stress injury in ApoE^(-/-)mice.Surprisingly,targeted metabolomics demonstrated that PPC intervention altered the BA profile in mice by regulating the ratio of secondary BAs to primary BAs,and increased fecal BAs excretion.Further,quantitative polymerase chain reaction(qPCR)analysis showed that PPC intervention facilitated reverse cholesterol transport by upregulating Srb1 expression;In addition,PPC intervention promoted BA synthesis from cholesterol in liver by upregulating Cyp7a1 expression via suppression of the farnesoid X receptor(FXR)pathway,thus exhibiting a significant serum cholesterol-lowering effect.In summary,PPC attenuated HFD-induced AS in ApoE^(-/-)mice,which provides new insights into the design of novel and efficient anti-atherosclerotic strategies to prevent AS based on probiotics and prebiotics.

Biliary cholesterol secretion: More than a simple ABC

Biliary cholesterol secretion is a process important for 2 major disease complexes, atherosclerotic cardiovascular disease and cholesterol gallstone disease. With respect to cardiovascular disease, biliary cholesterol secretion is regarded as the f inal step for the elimination of cholesterol originating from cholesterol-laden macrophage foam cells in the vessel wall in a pathway named reverse cholesterol transport. On the other hand, cholesterol hypersecretion into the bile is considered the main pathophysiological determinant of cholesterol gallstone formation. This review summarizes current knowledge on the origins of cholesterol secreted into the bile as well as the relevant processes and transporters involved. Next to the established ATP-binding cassette (ABC) transporters mediating the biliary secretion of bile acids (ABCB11), phospholipids (ABCB4) and cholesterol (ABCG5/G8), special attention is given to emerging proteins that modulate or mediate biliary cholesterol secretion. In this regard, the potential impact of the phosphatidylserine flippase ATPase class Ⅰ type 8B member 1, the Niemann Pick C1-like protein 1 that mediatescholesterol absorption and the high density lipoprotein cholesterol uptake receptor, scavenger receptor class B type Ⅰ, is discussed.

Emerging roles of the intestine in control of cholesterol metabolism

The liver is considered the major “control center” for maintenance of whole body cholesterol homeostasis. This organ is the main site for de novo cholesterol synthesis, clears cholesterol-containing chylomicron remnants and low density lipoprotein particles from plasma and is the major contributor to high density lipoprotein (HDL; good cholesterol) formation. The liver has a central position in the classical definition of the reverse cholesterol transport pathway by taking up periphery-derived cholesterol from lipoprotein particles followed by conversion into bile acids or its direct secretion into bile for eventual removal via the feces. During the past couple of years, however, an additional important role of the intestine in maintenance of cholesterol homeostasis and regulation of plasma cholesterol levels has become apparent. Firstly, molecular mechanisms of cholesterol absorption have been elucidated and novel pharmacological compounds have been identified that interfere with the process and positively impact plasma cholesterol levels. Secondly, it is now evident that the intestine itself contributes to fecal neutral sterol loss as a cholesterol-secreting organ. Finally, very recent work has unequivocally demonstrated that the intestine contributes significantly to plasma HDL cholesterol levels. Thus, the intestine is a potential target for novel anti-atherosclerotic treatment strategies that, in addition to interference with cholesterol absorption, modulate direct cholesterol excretion and plasma HDL cholesterol levels.

A Possible Mechanism Linking Hyperglycemia and Reduced High-density Lipoprotein Cholesterol Levels in Diabetes

This study investigated the role of glucose in the biogenesis of high-density lipoprotein cholesterol(HDL-C).Mouse primary peritoneal macrophages were harvested and maintained in Dulbecco’s modified Eagle’s medium(DMEM) containing glucose of various concentrations.The cells were divided into 3 groups in terms of different glucose concentrations in the cultures:Control group(5.6 mmol/L glucose),high glucose concentration groups(16.7 mmol/L and 30 mmol/L glucose).ATP-binding cassette transporter A1(ABCA1) mRNA expression in the macrophages was detected by semi-quantitative RT-PCR 24,48 and 72 h after glucose treatment.The results showed that ABCA1 mRNA expression in the 16.7 mmol/L glucose group was not significantly different from that in the control group at all testing time points(P>0.05 for each).In the 30 mmol/L glucose group,macrophage ABCA1 mRNA expression was not changed significantly at 24 h(P=0.14),but was substantially decreased by 40.4% at 48 h(P=0.009) and by 48.1% at 72 h(P=0.015) as compared with that in the control group.It was concluded that ABCA1 is of vital importance for HDL-C biogenesis.High glucose may hamper HDL-C biogenesis by decreasing ABCA1 expression,which contributes to low HDL-C level in diabetes.

Effect of Huxin Formula(护心方) on Reverse Cholesterol Transport in ApoE-Gene Knockout Mice

Objective： TO observe the effect of Huxin Formula （护心方, HXF） on expressions of the chief reverse cholesterol transport （RCT） associated genes, caveolin-1 and scavenger receptor-B I （SR-B I ） in ApoE-gene knockout [ApoE （-/-）] mice. Methods： Thirty ApoE （-/-） mice of 4-6 weeks old were randomly divided into three groups （A-C）. After being fed with high-fat diet for 16 weeks, they were treated with HXF （1 mL/100 g）, pravachol （0.3 mg/100 g）, and saline in equal volume respectively for 16 weeks successively; in addition, a blank group was set up with 10 C57BL/6J mice of 6-week old received 16-week high-fat feeding and saline treatment. Animals were sacrificed at the termination of the experiment, their paraffin sections of aortic tissue were used to measure the size of plaque, expressions of cavolin-1 and SR-B I were detected by immunological histochemical method. Results： As compared with the blank group, levels of caveolin-1 and SR-B I were increased in Groups A and B （P〈0.01）; but the increase in Group A was more significant than that in Group B （P〈0.05）. The plaque/aorta area ratio decreased significantly in Groups A and B, but showed insignificant difference between the two groups. Conclusion： HXF could obviously increase the expressions of RCT associated genes, caveolin-1 and SR-B I, promote the RCT process, so as to reduce the formation of aorta atherosclerotic plaque in ApoE （-/-） mice.

New insights into the regulation of cholesterol eft from the sperm membrane

Cholesterol is an essential component of the mammalian plasma membrane because it promotes membrane stability without comprising membrane fluidity. Given this important cellular role, cholesterol levels are tightly controlled at multiple levels. It has been clearly shown that cholesterol redistribution and depletion from the sperm membrane is a key part of the spermatozoon＇s preparation for fertilization. Some factors that regulate these events are described （e.g., bicarbonate, calcium） but the mechanisms underlying cholesterol export are poorly understood. How does a hydrophobic cholesterol molecule inserted in the sperm plasma membrane enter the energetically unfavorable aqueous surroundings？ This review will provide an overview of knowledge in this area and highlight our gaps in understanding. The overall aim is to better understand cholesterol redistribution in the sperm plasma membrane, its relation to the possible activation of a cholesterol transporter and the role of cholesterol acceptors. Armed with such knowledlze, sl）erm handlin~ techniques can be adapted to better prepare spermatozoa for in vitro and in vivo fertilization.

Danggui Shaoyao powder improves hepatic lipid metabolism in atherosclerosis mice via PPARγ-LXRα-ABCA1 pathway regulation

Objective:To observe the effects of Danggui Shaoyao powder(DSP)on hepatic lipid metabolism and further explore its mechanism of action by peroxisome proliferator-activated receptor(PPARγ)-liver X receptor(LXRα)-adenosine triphosphate(ATP)-binding cassette transporter A1(ABCA1)pathway regulation.Methods: Eight C57BL/6J male mice were selected as the control group,and 24 ApoE^(−/−)male mice were randomly divided into the atherosclerosis model(AS)group,atorvastatin calcium(AC)group,and DSP group(n=8 each group).To establish an AS model,ApoE^(−/−)mice were fed a high-fat diet for 16 weeks.Pathologic changes in the aortic vasculature and liver were identified using Oil Red O staining.Triglyceride(TG),cholesterol(TC),and low-density lipoprotein cholesterol(LDL-C)levels were determined in the livers using a single-reagent GPO-PAP method.Fluorescence quantitative polymerase chain reaction and western blot were used to observe and evaluate the mRNA and protein expression of the PPARγ-LXRα-ABCA1 intermediates in the liver.Results: After 16 weeks of a high-fat diet,ApoE^(−/−)mice showed more Oil Red O staining in the aorta and liver compared to the CONT group.Compared to the AS group,the DSP and AC treatment reduced aortic plaque and hepatic lipid deposition to varying degrees.Furthermore,DSP significantly reduced the hepatic lipid area in ApoE^(−/−)mice(P<.001)and decreased the levels of TG,TC,and LDL-C in liver(P<.001,P=.027,P<.001,respectively).DSP also significantly increased the levels of PPARγ,LXRα,ABCA1,and ABCG1 mRNA expression,as well as the PPARγ,LXRα,ABCA1,and ABCG1 protein expression in liver.Conclusion: DSP improved hepatic lipid metabolism via PPARγ-LXRα-ABCA1 pathway modulation for AS treatment.