A 15-ketosterol is a liver X receptor ligand that suppresses sterol-responsive element binding protein-2 activity

The Cholesterol Metabolite Cholest-5-en-3-One Alleviates Hyperglycemia and Hyperinsulinemia in Obese (db/db) Mice

Dietary sterols are catabolized into various substances in the intestinal tract. Dietary 3-oxo derivatives of cholesterol and plant sterols (e.g., cholest-4-en-3-one and campest-5-en-3-one) have been shown to have anti-obesity effects. In this study, we tested whether feeding cholest-5-en-3-one (5-cholestenone), a cholesterol metabolite, to db/db mice protects them from obesity-associated metabolic disorders. In db/db mice, dietary 5-cholestenone significantly alleviated hepatomegaly and elevated serum triglyceride levels; however, the effect was not sufficient to improve hepatic steatosis and obesity. On the other hand, hyperglycemia and severe hyperinsulinemia in control db/db mice were markedly attenuated in 5-cholestenone-fed db/db mice. The production of inflammatory cytokines, such as monocyte chemoattractant protein-1, interleukin-6, and tumor necrosis factor-alpha (TNFα), was decreased, suggesting that the suppressive actions of 5-cholestenone were attributable to the alleviation of chronic inflammation in db/db mice. Additionally, 5-cholestenone showed an inhibitory effect on TNFα-induced nuclear factor kappa B (NFκB) activation in the NFκB luciferase gene reporter assay. These results suggest that obesity-induced abnormal glucose metabolism could be alleviated in 5-cholestenone-fed db/db mice by reducing the production of inflammatory cytokines through suppression of the NFκB signaling pathway.

Role of the liver X receptors in skin physiology: Putative pharmacological targets in human diseases

Liver X receptors (LXRs) are members of the nuclear receptor superfamily that have been shown to regulate various physiological functions such as lipid metabolism and cholesterol homeostasis. Concordant reports have elicited the possibility to target them to cure many human diseases including arteriosclerosis, cancer, arthritis, and diabetes. The high relevance of modulating LXR activities to treat numerous skin diseases, mainly those with exacerbated inflammation processes, contrasts with the lack of approved therapeutic use. This review makes an assessment to sum up the findings regarding the physiological roles of LXRs in skin and help progress towards the therapeutic and safe management of their activities. It focuses on the possible pharmacological targeting of LXRs to cure or prevent selected skin diseases.

Negative feedback loop of cholesterol regulation is impaired in the livers of patients with Alagille syndrome

AIM

To characterize cholesterol regulation in the liver of patients with Alagille syndrome (AGS).

METHODS

Serum total cholesterol (TC) and total bile acid (TBA) levels were measured in 23 AGS patients. The expressions of genes involved in cholesterol regulation, including low-density lipoprotein receptor (LDLR), scavenger receptor class B type I (SR-BI), 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR), cholesterol 7α-hydroxylase (CYP7A1), ATP-binding cassette transporter (ABC) A1, and ABCG1/5/8, were measured in liver tissues from five of these patients. Expression of regulators for these genes, including farnesoid X receptor/small heterodimer partner (SHP), liver X receptor α (LXRα) and mature Sterol regulatory element-binding protein 2 (SREBP2) was measured. The expression of mature SREBP2 protein was also examined.

RESULTS

Serum TC and TBA levels were correlated in the AGS patients. Liver cholesterol was also increased compared with controls, and correlated with bile acid contents. LDLR, SR-BI, HMGCR, and ABCGs mRNA expression were upregulated, while CYP7A1 mRNA expression was downregulated in AGS livers. SHP and LXRα mRNA expression was also increased, but maturation of SREBP2 was not suppressed in the patients.

CONCLUSIONS

The major upregulators of liver cholesterol might be increased in AGS patients, indicating an impaired negative feedback mechanism and accelerated liver cholesterol accumulation.

New LDL-cholesterol lowering therapies: pharmacology, clinical trials, and relevance to acute coronary syndromes

BACKGROUND

Reduction in plasma low-density lipoprotein cholesterol (LDL-C) is a fundamental treatment for the prevention of acute coronary syndromes (ACS). Although statin therapy confers significant protection against ACS in both primary and secondary prevention, a considerable residual risk remains after intensive therapy. In addition, a significant proportion of high-risk patients do not achieve the optimal LDL-C goal recommended in the current guidelines (<1.8 mmol/L). Hence, novel LDL-C-lowering agents that act via mechanisms distinct from HMG-CoA reductase inhibition are under investigation.

OBJECTIVE

We reviewed the recent literature on the development of novel LDL-C-lowering agents that could potentially be used as an alternative or adjunct to statin therapy in high-risk coronary patients.

METHODS

PubMed and Scopus databases were searched to retrieve studies on the efficacy and/or tolerability of novel LDL-C-lowering agents in animals and humans.

RESULTS

Agents that inhibit proprotein convertase subtilisin/kexin type 9 (PCSK9), apolipoprotein (apo) B, and microsomal triglyceride transfer protein (MTTP) are the most promising therapies. Inhibition of PCSK9, apoB, and MTTP has been achieved mostly via fully humanized monoclonal antibodies (mAbs), antisense oligonucleotides, and synthetic compounds, respectively. PCSK9 inhibitors increase the hepatic uptake of LDL-C, while apoB and MTTP inhibitors decrease the synthesis and secretion of apoB-containing lipoproteins. These 3 mechanisms lead to marked reductions in plasma LDL-C in patients with hypercholesterolemia at risk for ACS, particularly those with familial hypercholesterolemia. Moreover, these agents can exert additional benefits by decreasing plasma levels of apoB, triglycerides, and lipoprotein(a). Mipomersen and lomitapide have been approved by the United States Food and Drug Administration (US FDA) for use in patients with homozygous familial hypercholesterolemia. PCSK9 inhibitors are currently under final evaluation in clinical outcomes studies and are anticipated to find wide application either as monotherapy or as an adjunct to statins. A main safety concern is the risk for hepatic steatosis with apoB and MTTP inhibitors, which needs to be explored in prospective, long-term trials.

CONCLUSIONS

PCSK9, apoB, and MTTP inhibitors can exert potent reductions in plasma LDL-C and apoB concentrations, either as monotherapy or in combination with statins. These effects are particularly relevant to high-risk individuals with marked hypercholesterolemia, such as those with familial hypercholesterolemia. Although the use of mipomersen and lomitapide is limited to severe familial hypercholesterolemia as a replacement for LDL-apheresis, PCSK9 inhibitors are likely to be more widely prescribed in patients at high risk for CVD, especially those who are resistant to or intolerant of high-intensity statin therapy. PCSK9 mAbs are efficacious and have an excellent safety profile, but their long-term impact on cardiovascular events is currently under investigation. Whether PCSK9 mAbs decrease the rates of recurrent cardiovascular events within 3 months following ACS is questionable; however, these agents, unlike statins, may not have pleiotropic benefits on the unstable plaque.

Frontier of epilepsy research - mTOR signaling pathway

Studies of epilepsy have mainly focused on the membrane proteins that control neuronal excitability. Recently, attention has been shifting to intracellular proteins and their interactions, signaling cascades and feedback regulation as they relate to epilepsy. The mTOR (mammalian target of rapamycin) signal transduction pathway, especially, has been suggested to play an important role in this regard. These pathways are involved in major physiological processes as well as in numerous pathological conditions. Here, involvement of the mTOR pathway in epilepsy will be reviewed by presenting; an overview of the pathway, a brief description of key signaling molecules, a summary of independent reports and possible implications of abnormalities of those molecules in epilepsy, a discussion of the lack of experimental data, and questions raised for the understanding its epileptogenic mechanism.

Effects of currently prescribed LDL-C-lowering drugs on PCSK9 and implications for the next generation of LDL-C-lowering agents

Background

During the past decade, proprotein convertase subtilisin kexin type 9 (PCSK9) has been identified as a key regulator of serum LDL-cholesterol (LDL-C) levels. PCSK9 is secreted by the liver into the plasma and binds the hepatic LDL receptor, causing its subsequent degradation. In humans, gain-of-function mutations in PCSK9 cause a form of familial hypercholesterolemia that manifests with dramatically increased serum levels of LDL-C, while loss-of-function mutations in PCSK9 are associated with significantly decreased LDL-C and cardiovascular risk.

Results

Initial studies in animals and cultured cells demonstrated that statins increased PCSK9 mRNA expression, resulting in many research groups exploring the effect of statins on PCSK9 levels in humans. We first reported that statins increased human PCSK9 circulating protein levels. Additional researchers subsequently confirmed these observations, further prompting many laboratories including our own to examine the effect of other lipid lowering medications on PCSK9 levels. Our observation that fenofibrate (200 mg/day) significantly increased PCSK9 levels was confirmed by another laboratory, and an additional group demonstrated that ezetimibe also increased PCSK9 levels.

Conclusions

It has become clear that the major classes of commonly prescribed lipid-lowering medications increase serum PCSK9 levels. These observations almost certainly explain why these agents are not more effective in lowering LDL-C and suggest that efforts should be made toward the development of new LDL-C lowering medications that either do not increase circulating PCSK9 levels or work through decreasing or inhibiting PCSK9.

[Regulation of cholesterol biosynthesis and metabolism in Hep G2 cells by delta8(14)-15-ketoergostane derivatives]

The comparative study of effects of 5alpha-cholest-8(14)-en-15-on-3beta-ol (I), (22E)-5alpha-ergosta-8(14),22-dien-15-on-3beta-ol (II), (22S,23S)-22,23-oxido-5alpha-ergost-8(14)-en- 15-on-3beta-ol (III) and (22R,23R)-22,23-oxido-5alpha-ergost-8(14)-en-15-on-3beta-ol (IV) on HMG-CoA reductase, CYP27A1 and CYP3A4 genes expression in Hep G2 cells was performed. In the contrast to 15-ketocholestane derivative (I), 15-ketoergostane derivatives (II - IV) decreased the HMG- CoA reductase mRNA level; (22R,23R)-22,23-oxido-5alpha-ergost-8(14)-en-15-on-3beta-ol (IV) significantly increased CYP3A4 mRNA level (320% from control). Ketosterol (II) was found to be a more potent inhibitor of cholesterol biosynthesis in Hep G2 cells at a prolong incubation, compared with ketosterol (I). The side chain conformation of compounds (I) - (IV) was evaluated by computational modeling; the correlation between biological activity of these compounds and conformational flexibility of their side chains was found. The results obtained indicated that delta8(14)-15-ketoergostane derivatives may be used as a sterol biosynthesis and metabolism regulators in liver cells.

Strategies for proprotein convertase subtilisin kexin 9 modulation: a perspective on recent patents

IMPORTANCE OF THE FIELD

Proprotein convertase subtilisin kexin 9 (PCSK9) is a new actor discovered in 2003 that is implicated in autosomal dominant hypercholesterolemia, cholesterol homeostasis and coronary heart disease. It has been shown to degrade the low-density lipoprotein (LDL) receptor independently of its catalytic activity. Several pharmacological strategies to reduce PCSK9 are being thoroughly investigated.

AREAS COVERED IN THIS REVIEW

This article reviews all different strategies that are presently pursued to modulate the functional activity of PCSK9 which is a prime target for controlling LDL-cholesterol. It also provides a briefing of all the patents up to July 2010 from various organizations including pharmaceutical companies and academic institutions that have been submitted and/or approved.

WHAT THE READER WILL GAIN

This review is addressed to researchers from academia and pharmaceutical companies who are engaged in PCSK9 research/cholesterol regulation and in the development of cholesterol lowering drugs. Readers will gain an up-to-date overview of the different strategies that have been investigated to reduce PCSK9 including antisense technology and specific antibodies.

TAKE HOME MESSAGE

Clinical trials have been launched using RNA interference approaches to reduce PCSK9 expression or specific antibodies targeting and inhibiting PCSK9 interaction with the LDL receptor. They constitute very promising approaches to reducing cholesterol levels and coronary heart disease.

Chemical combinations elucidate pathway interactions and regulation relevant to Hepatitis C replication

SREBP-2, oxidosqualene cyclase (OSC) or lanosterol demethylase were identified as novel sterol pathway-associated targets that, when probed with chemical agents, can inhibit hepatitis C virus (HCV) replication.

Using a combination chemical genetics approach, combinations of chemicals targeting sterol pathway enzymes downstream of and including OSC or protein geranylgeranyl transferase I (PGGT) produce robust and selective synergistic inhibition of HCV replication. Inhibition of enzymes upstream of OSC elicit proviral responses that are dominant to the effects of inhibiting all downstream targets.

Inhibition of the sterol pathway without inhibition of regulatory feedback mechanisms ultimately results in an increase in HCV replication because of a compensatory upregulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) expression. Increases in HMGCR expression without inhibition of HMGCR enzymatic activity ultimately stimulate HCV replication through increasing the cellular pool of geranylgeranyl pyrophosphate (GGPP).

Chemical inhibitors that ultimately prevent SREBP-2 activation, inhibit PGGT or encourage the production of polar sterols have great potential as HCV therapeutics if associated toxicities can be reduced.

Chemical inhibition of enzymes in either the cholesterol or the fatty acid biosynthetic pathways has been shown to impact viral replication, both positively and negatively (Su et al, 2002; Ye et al, 2003; Kapadia and Chisari, 2005; Sagan et al, 2006; Amemiya et al, 2008). FBL2 has been identified as a 50 kDa geranylgeranylated host protein that is necessary for localization of the hepatitis C virus (HCV) replication complex to the membranous web through its close association with the HCV protein NS5A and is critical for HCV replication (Wang et al, 2005). Inhibition of the protein geranylgeranyl transferase I (PGGT), an enzyme that transfers geranylgeranyl pyrophosphate (GGPP) to cellular proteins such as FBL2 for the purpose of membrane anchoring, negatively impacts HCV replication (Ye et al, 2003). Conversely, chemical agents that increase intracellular GGPP concentrations promote viral replication (Kapadia and Chisari, 2005). Statin compounds that inhibit 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR), the rate-limiting enzyme in the sterol pathway (Goldstein and Brown, 1990), have been suggested to inhibit HCV replication through ultimately reducing the cellular pool of GGPP (Ye et al, 2003; Kapadia and Chisari, 2005; Ikeda et al, 2006). However, inhibition of the sterol pathway with statin drugs has not yielded consistent results in patients. The use of statins for the treatment of HCV is likely to be complicated by the reported compensatory increase in HMGCR expression in vitro and in vivo (Stone et al, 1989; Cohen et al, 1993) in response to treatment. Enzymes in the sterol pathway are regulated on a transcriptional level by sterol regulatory element-binding proteins (SREBPs), specifically SREBP-2 (Hua et al, 1993; Brown and Goldstein, 1997). When cholesterol stores in cells are depleted, SREBP-2 activates transcription of genes in the sterol pathway such as HMGCR, HMG-CoA synthase, farnesyl pyrophosphate (FPP) synthase, squalene synthase (SQLS) and the LDL receptor (Smith et al, 1988, 1990; Sakai et al, 1996; Brown and Goldstein, 1999; Horton et al, 2002). The requirement of additional downstream sterol pathway metabolites for HCV replication has not been completely elucidated.

To further understand the impact of the sterol pathway and its regulation on HCV replication, we conducted a high-throughput combination chemical genetic screen using 16 chemical probes that are known to modulate the activity of target enzymes relating to the sterol biosynthesis pathway (Figure 1). Using this approach, we identified several novel antiviral targets including SREBP-2 as well as targets downstream of HMGCR in the sterol pathway such as oxidosqualene cyclase (OSC) and lanosterol demethylase. Many of our chemical probes, specifically SR-12813, farnesol and squalestatin, strongly promoted replicon replication. The actions of both farnesol and squalestatin ultimately result in an increase in the cellular pool of GGPP, which is known to increase HCV replication (Ye et al, 2003; Kapadia and Chisari, 2005; Wang et al, 2005).

Chemical combinations targeting enzymes upstream of squalene epoxidase (SQLE) at the top of the sterol pathway (Figure 4A) elicited Bateson-type epistatic responses (Boone et al, 2007), where the upstream agent's response predominates over the effects of inhibiting all downstream targets. This was especially notable for combinations including simvastatin and either U18666A or squalestatin, and for squalestatin in combination with Ro48-8071. Treatment with squalestatin prevents the SQLS substrate, farnesyl pyrophosphate (FPP) from being further metabolized by the sterol pathway. As FPP concentrations increase, the metabolite can be shunted away from the sterol pathway toward farnesylation and GGPP synthetic pathways, resulting in an increase in host protein geranylgeranylation, including FBL2, and consequently replicon replication. This increase in replicon replication explains the source of the observed epistasis over Ro48-8071 treatment.

Combinations between probes targeting enzymes downstream of and including OSC produced robust synergies with each other or with a PGGT inhibitor. Figure 4B highlights examples of antiviral synergy resulting from treatment of cells with an OSC inhibitor in combination with an inhibitor of either an enzyme upstream or downstream of OSC. A combination of terconazole and U18666A is synergistic without similar combination effects in the host proliferation screen. Likewise, clomiphene was also synergistic when added to replicon cells in combination with U18666A. One of the greatest synergies observed downstream in the sterol pathway is a combination of amorolfine and AY 9944, suggesting that there is value in developing combinations of drugs that target enzymes in the sterol pathway, which are downstream of HMGCR.

Interactions with the protein prenylation pathway also showed strong mechanistic patterns (Figure 4C). GGTI-286 is a peptidomimetic compound resembling the CAAX domain of a protein to be geranylgeranylated and is a competitive inhibitor of protein geranylgeranylation. Simvastatin impedes the antiviral effect of GGTI-286 at low concentrations but that antagonism is balanced by comparable synergy at higher concentrations. At the low simvastatin concentrations, a compensatory increase in HMGCR expression leads to increased cellular levels of GGPP, which are likely to result in an increase in PGGT enzymatic turnover and decreased GGTI-286 efficacy. The antiviral synergy observed at the higher inhibitor concentrations is likely nonspecific as synergy was also observed in a host viability assay. Further downstream, however, a competitive interaction was observed between GGTI-286 and squalestatin, where the opposing effect of one compound obscures the other compound's effect. This competitive relationship between GGTI and SQLE explains the epistatic response observed between those two agents. For inhibitors of targets downstream of OSC, such as amorolfine, there are strong antiviral synergies with GGTI-286. Notably, combinations with OSC inhibitors and GGTI-286 were selective, in that comparable synergy was not found in a host viability assay. This selectivity suggests that jointly targeting OSC and PGGT is a promising avenue for future HCV therapy development.

This study provides a comprehensive and unique perspective into the impact of sterol pathway regulation on HCV replication and provides compelling insight into the use of chemical combinations to maximize antiviral effects while minimizing proviral consequences. Our results suggest that HCV therapeutics developed against sterol pathway targets must consider the impact on underlying sterol pathway regulation. We found combinations of inhibitors of the lower part of the sterol pathway that are effective and synergistic with each other when tested in combination. Furthermore, the combination effects observed with simvastatin suggest that, though statins inhibit HMGCR activity, the resulting regulatory consequences of such inhibition ultimately lead to undesirable epistatic effects. Inhibitors that prevent SREBP-2 activation, inhibit PGGT or encourage the production of polar sterols have great potential as HCV therapeutics if associated toxicities can be reduced.

The search for effective Hepatitis C antiviral therapies has recently focused on host sterol metabolism and protein prenylation pathways that indirectly affect viral replication. However, inhibition of the sterol pathway with statin drugs has not yielded consistent results in patients. Here, we present a combination chemical genetic study to explore how the sterol and protein prenylation pathways work together to affect hepatitis C viral replication in a replicon assay. In addition to finding novel targets affecting viral replication, our data suggest that the viral replication is strongly affected by sterol pathway regulation. There is a marked transition from antagonistic to synergistic antiviral effects as the combination targets shift downstream along the sterol pathway. We also show how pathway regulation frustrates potential hepatitis C therapies based on the sterol pathway, and reveal novel synergies that selectively inhibit hepatitis C replication over host toxicity. In particular, combinations targeting the downstream sterol pathway enzymes produced robust and selective synergistic inhibition of hepatitis C replication. Our findings show how combination chemical genetics can reveal critical pathway connections relevant to viral replication, and can identify potential treatments with an increased therapeutic window.

PPARgamma1 and LXRalpha face a new regulator of macrophage cholesterol homeostasis and inflammatory responsiveness, AEBP1

Peroxisome proliferator-activated receptor γ1 (PPARγ1) and liver X receptor α (LXRα) are nuclear receptors that play pivotal roles in macrophage cholesterol homeostasis and inflammation; key biological processes in atherogenesis. The activation of PPARγ1 and LXRα by natural or synthetic ligands results in the transactivation of ABCA1, ABCG1, and ApoE; integral players in cholesterol efflux and reverse cholesterol transport. In this review, we describe the structure, isoforms, expression pattern, and functional specificity of PPARs and LXRs. Control of PPARs and LXRs transcriptional activity by coactivators and corepressors is also highlighted. The specific roles that PPARγ1 and LXRα play in inducing macrophage cholesterol efflux mediators and antagonizing macrophage inflammatory responsiveness are summarized. Finally, this review focuses on the recently reported regulatory functions that adipocyte enhancer-binding protein 1 (AEBP1) exerts on PPARγ1 and LXRα transcriptional activity in the context of macrophage cholesterol homeostasis and inflammation.

Synthetic LXR agonist attenuates plaque formation in apoE-/- mice without inducing liver steatosis and hypertriglyceridemia

Liver X receptors (LXRs) are important regulators of cholesterol and lipid metabolism. LXR agonists have been shown to limit the cellular cholesterol content by inducing reverse cholesterol transport, increasing bile acid production, and inhibiting intestinal cholesterol absorption. Most of them, however, also increase lipogenesis via sterol regulatory element-binding protein-1c (SREBP1c) and carbohydrate response element-binding protein activation resulting in hypertriglyceridemia and liver steatosis. We report on the antiatherogenic properties of the steroidal liver X receptor agonist N,N-dimethyl-3beta-hydroxy-cholenamide (DMHCA) in apolipoprotein E (apoE)-deficient mice. Long-term administration of DMHCA (11 weeks) significantly reduced lesion formation in male and female apoE-null mice. Notably, DMHCA neither increased hepatic triglyceride (TG) levels in male nor female apoE-deficient mice. ATP binding cassette transporter A1 and G1 and cholesterol 7alpha-hydroxylase mRNA abundances were increased, whereas SREBP1c mRNA expression was unchanged in liver, and even decreased in macrophages and intestine. Short-term treatment revealed even higher changes on mRNA regulation. Our data provide evidence that DMHCA is a strong candidate as therapeutic agent for the treatment or prevention of atherosclerosis, circumventing the negative side effects of other LXR agonists.

Selective partial agonism of liver X receptor alpha is related to differential corepressor recruitment

Classically, activated transcription by nuclear receptors (NRs) is due to a ligand-induced switch from corepressor- to coactivator-bound states. However, coactivators and corepressors recognize overlapping surfaces of liganded and unliganded NRs, respectively. Here we show that, at sufficiently high concentration, the NR corepressor (NCoR) influences the activity of the liver X receptor (LXR) even in the presence of a potent full agonist that destabilizes NCoR binding. Partial agonist ligands that less effectively dissociate NCoR from LXR are even more sensitive to NCoR levels, in a target gene-selective manner. Thus, differential recruitment of NCoR is a major determinant of partial agonism and selective LXR modulation of target genes.

PCSK9: an enigmatic protease

Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays a critical role in cholesterol metabolism by controlling the levels of low density lipoprotein (LDL) particles that circulate in the bloodstream. Several gain-of-function and loss-of-function mutations in the PCSK9 gene, that occur naturally, have been identified and linked to hypercholesterolemia and hypocholesterolemia, respectively. PCSK9 expression has been shown to be regulated by sterol regulatory element binding proteins (SREBPs) and statins similar to other genes involved in cholesterol homeostasis. The most critical finding concerning PCSK9 is that this protease is able to influence the number of LDL receptor molecules expressed on the cell surface. Studies have demonstrated that PCSK9 acts mainly by enhancing degradation of LDL receptor protein in the liver. Inactivation of PCSK9 in mice reduces plasma cholesterol levels primarily by increasing hepatic expression of LDL receptor protein and thereby accelerating clearance of circulating LDL cholesterol. The objective of this review is to summarize the current information related to the regulation and function of PCSK9 and to identify gaps in our present knowledge.