Functional expression of sinusoidal and canalicular hepatic drug transporters in the differentiated human hepatoma HepaRG cell line.

Functional expression of both sinusoidal and canalicular hepatic drug transporters was investigated in the highly differentiated human hepatoma HepaRG cell line and also, for comparison, in primary human hepatocytes and in the hepatoma HepG2 cell line. Using RT-qPCR assays, differentiated HepaRG cells were found to display a pattern of transporter expression close to that found in primary human hepatocytes, i.e. they exhibit substantial mRNA levels of the influx transporters OCT1, OATP-B, OATP-C and NTCP, and of the secretion transporters MRP2, MRP3, BSEP and P-glycoprotein. By contrast, expression of influx transporters was not present or very weak in HepG2 cells. Drug transport assays allowed to detect functional activities of OCT1, OATPs/OAT2, NTCP, MRPs and P-glycoprotein in differentiated HepaRG cells as in primary human hepatocytes whereas HepG2 cells only showed notable MRP and P-glycoprotein activities. In addition, expression of canalicular transporters in HepaRG cells was found to be up-regulated by known inducers of transporters such as rifampicin, phenobarbital and chenodeoxycholate acting on P-glycoprotein, MRP2 and BSEP, respectively. HepaRG cells thus exhibit functional expression of both sinusoidal and canalicular drug transporters and have retained regulatory pathways controlling transporter levels. These data, associated with the known high expression of drug metabolizing enzymes in HepaRG cells, highlight the interest of such hepatoma cells for analysing hepatic drug detoxification pathways.     

Proteomic analysis for the impact of hypercholesterolemia on expressions of hepatic drug transporters and metabolizing enzymes

1. Our objective is to investigate the alterations of hepatic drug transporters and metabolizing enzymes in hypercholesterolemia. Male Sprague–Dawley rats were fed high-cholesterol chows for 8 weeks to induce hypercholesterolemia. Protein levels of hepatic drug transporters and metabolizing enzymes were analyzed by iTRAQ labeling coupled with LC TRIPLE-TOF.

2. Total 239 differentially expressed proteins were identified using proteomic analysis. Among those, protein levels of hepatic drug transporters (MRP2, ABCD3, OAT2, SLC25A12, SCL38A3, SLC2A2 and SLC25A5) and metabolizing enzymes (CYP2B3, CYP2C7, CYP2C11, CYP2C13, CYP4A2 and UGT2B) were markedly reduced, but the levels of CYP2C6 and CYP2E1 were increased in hypercholesterolemia group compared to control. Decreased expressions of drug transporters MRP2 and OAT2 were further confirmed by real time quantitative PCR (RT-qPCR) and western blot.

3. Ingenuity pathway analysis revealed that these differentially expressed proteins were regulated by various signaling pathways including nuclear receptors and inflammatory cytokines. One of the nuclear receptor candidates, liver X receptor alpha (LXRα), was further validated by RT-qPCR and western blot. Additionally, LXRα agonist T0901317 rescued the reduced expressions of MRP2 and OAT2 in HepG2 cells in hypercholesterolemic serum treatment.

4. Our present results indicated that hypercholesterolemia affected the expressions of various drug transporters and metabolizing enzymes in liver via nuclear receptors pathway. Especially, decreased function of LXRα contributes to the reduced expressions of MRP2 and OAT2.     

Bifendate treatment attenuates hepatic steatosis in cholesterol/bile salt- and high-fat diet-induced hypercholesterolemia in mice

Effects of bifendate, a synthetic intermediate of schisandrin C (a dibenzocyclooctadiene derivative), on liver lipid contents were investigated in experimentally-induced hypercholesterolemia in mice. Hypercholesterolemia was induced by either chronic administration of cholesterol/bile salt or feeding a high-fat diet containing cholesterol and/or bile salt. Hepatic and serum total cholesterol levels were significantly increased (42-268% and 23-124%, respectively) in cholesterol or high-fat diet-treated mice, when compared with control animals receiving vehicle or normal diet. Hepatic triglyceride level was increased (up to 108%), but serum triglyceride level was significantly reduced by 23-63% in hypercholesterolemic mice. Daily administration of bifendate (0.03-1.0 g/kg, i.g.) for 4 days decreased hepatic levels of total cholesterol (9-37%) and triglyceride (10-37%) in hypercholesterolemic mice. Supplementing the high-fat diet with bifendate (0.25%, w/w) caused decreases in hepatic total cholesterol (25-56%) and triglyceride (22-44%) levels following 7 or 14 days of experiment, respectively, when compared with animals fed with high-fat diet not supplemented with bifendate. While fenofibrate treatment decreased both hepatic and serum lipid levels in hypercholesterolemic mice, bifendate treatment did not reduce serum lipid levels. Bifendate and fenofibrate caused an increase (10-41% and 59-98%, respectively) in hepatic index of hypercholesterolemic mice. The results indicate that bifendate treatment can invariably decrease hepatic (but not serum) lipid levels in various mouse models of hypercholesterolemia.     

Down-regulation of hepatic transporters for BSP in rats with indomethacin-induced intestinal injury

Previous reports have demonstrated that an intestinal injury causes hypofunctions of the liver associated with down-regulations of cytochrome P450, but an influence on hepatic transporters remains unclear. Here, we tested hepatic transporter functions in a rat model of bowel injury using indomethacin (IDM). After administration of IDM (8.5 mg/kg, i.p., 3 d), the rats suffered the intestinal impairment indicated by a reduction of alkaline phosphatase activity in mucosa. In vivo pharmacokinetic experiments of bromosulfophthalein (BSP) showed that there was a reduction in its plasma elimination rate and cumulative biliary excretion in IDM-treated rats and systemic and biliary clearances reduced to nearly 50% of the control group. Protein expressions in plasma membrane and mRNA levels of organic anion transporting polypeptide 1b2 (Oatp1b2) and multidrug resistance-associated protein 2 (Mrp2), which play hepatic BSP uptake and biliary excretion, respectively, in the liver were significantly reduced following the IDM treatment. In portal plasma, the levels of proinflammatory cytokines were unchanged, while the level of nitric oxide metabolites (NO2- + NO3-) increased to 6.5-fold that of the control. The time-course on IDM treatment indicated that, firstly, intestinal injury was induced, the NO level increased, and the hepatic Oatp1b2 and Mrp2 expression began to fall followed by an increase in plasma ALT. In conclusion, IDM-induced injury to the small intestine causes the hypofunction of hepatic Oatp1b2 and Mrp2 independently on the hepatic impairment, and NO arising from bowel injury may be one of key factors for it through the remote effect.     

New targets in and potential treatments for cholesterol gallstone disease

Gallstone disease is very common among American Indians and Hispanics, and approximately 20 million patients are treated for this disease annually in the US. Bile acid receptor (nuclear farnesoid X receptor; FXR) knockout mice fed a lithogenic diet are more susceptible to gallstone disease than wild-type mice. The C57L mouse is also susceptible to gallstone formation when fed a lithogenic diet, and in this model, the small-molecule FXR agonist GW-4064 prevents the precipitation of cholesterol. Bile acids (eg, P-muricholic acid) and their derivatives are also being developed as FXR agonists. Fatty acid bile acid conjugates have the potential to prevent and reverse cholesterol crystallization. Furthermore, agents that increase the expression of selected hepatocyte bile acid transporters may also be useful in the treatment of gall bladder disease.     

ATP-binding cassette transporters A1 and G1, HDL metabolism, cholesterol efflux, and inflammation: important targets for the treatment of atherosclerosis

Atherosclerosis has been characterized as a chronic inflammatory response to cholesterol deposition in arteries. Plasma high density lipoprotein (HDL) levels bear a strong independent inverse relationship with atherosclerotic cardiovascular disease. One central antiatherogenic role of HDL is believed to be its ability to remove excessive peripheral cholesterol back to the liver for subsequent catabolism and excretion, a physiologic process termed reverse cholesterol transport (RCT). Cholesterol efflux from macrophage foam cells, the initial step of RCT is the most relevant step with respect to atherosclerosis. The ATP-binding cassette (ABC) transporters ABCA1 and ABCG1 play crucial roles in the efflux of cellular cholesterol to HDL and its apolipoproteins. Moreover, ABCA1 and ABCG1 affect cellular inflammatory cytokine secretion by modulating cholesterol content in the plasma membrane and within intracellular compartments. In humans, ABCA1 mutations can cause a severe HDL-deficiency syndrome characterized by cholesterol deposition in tissue macrophages and prevalent atherosclerosis. Disrupting Abca1 or Abcg1 in mice promotes accumulation of excessive cholesterol in macrophages, and physiological manipulation of ABCA1 expression affects atherogenesis. Here we review recent advances in the role of ABCA1 and ABCG1 in HDL metabolism, macrophage cholesterol efflux, inflammation, and atherogenesis. Next, we summarize the structure, expression, and regulation of ABCA1 and ABCG1. Finally, we give an update on the progress and pitfalls of therapeutic approaches that target ABCA1 and ABCG1 to stimulate the flux of lipids through the RCT pathway.     

Interaction of sirolimus and everolimus with hepatic and intestinal organic anion-transporting polypeptide transporters

The goal of this study was to assess the interaction of the mTOR inhibitors (ImTORs) sirolimus and everolimus with the human organic anion-transporting polypeptides (OATPs) expressed in hepatocytes and enterocytes by conducting uptake experiments using (i) transfected HEK293T cells, (ii) the hepatocyte-like HepaRG cell line and (iii) the enterocyte-like Caco-2 cell line. Sirolimus and everolimus inhibited in a dose-dependent manner the uptake of [3H]-estrone sulphate by OATP1A2 and OATP1B1 and that of mycophenolic acid 7-O-glucuronide (MPAG) by OATP1B3. ImTOR apparent 50% inhibitory concentrations (IC??) for OATPs were 11.9 μM (OATP1A2), 9.8 μM (OATP1B1) and 1.3 μM (OATP1B3) for sirolimus and 4.2 μM (OATP1A2), 4.1 μM (OATP1B1) and 4.3 μM (OATP1B3) for everolimus. No transport of sirolimus or everolimus by OATP1A2, OATP1B1 or OATP1B3 was observed in HEK-transfected cells and the OAT/OATP/MRP chemical inhibitor probenecid did not significantly decrease the uptake of sirolimus and everolimus in HepaRG and Caco-2 cells, but tended to increase their intracellular accumulation presumably through efflux inhibition. In conclusion, our data suggest that the major OATP transporters expressed in the liver and the intestine do not contribute to the pharmacokinetics of sirolimus and everolimus. However, ImTORs are inhibitors of these transporters.     

Interaction of sirolimus and everolimus with hepatic and intestinal organic anion-transporting polypeptide transporters

1. The goal of this study was to assess the interaction of the mTOR inhibitors (ImTORs) sirolimus and everolimus with the human organic anion-transporting polypeptides (OATPs) expressed in hepatocytes and enterocytes by conducting uptake experiments using (i) transfected HEK293T cells, (ii) the hepatocyte-like HepaRG cell line and (iii) the enterocyte-like Caco-2 cell line.

2. Sirolimus and everolimus inhibited in a dose-dependent manner the uptake of [³H]-estrone sulphate by OATP1A2 and OATP1B1 and that of mycophenolic acid 7-O-glucuronide (MPAG) by OATP1B3. ImTOR apparent 50% inhibitory concentrations (IC₅₀) for OATPs were 11.9 µM (OATP1A2), 9.8 µM (OATP1B1) and 1.3 µM (OATP1B3) for sirolimus and 4.2 µM (OATP1A2), 4.1 µM (OATP1B1) and 4.3 µM (OATP1B3) for everolimus.

3. No transport of sirolimus or everolimus by OATP1A2, OATP1B1 or OATP1B3 was observed in HEK-transfected cells and the OAT/OATP/MRP chemical inhibitor probenecid did not significantly decrease the uptake of sirolimus and everolimus in HepaRG and Caco-2 cells, but tended to increase their intracellular accumulation presumably through efflux inhibition.

4. In conclusion, our data suggest that the major OATP transporters expressed in the liver and the intestine do not contribute to the pharmacokinetics of sirolimus and everolimus. However, ImTORs are inhibitors of these transporters.

     

New molecular targets for the treatment of osteoarthritis

Osteoarthritis (OA) is a chronic degenerative joint disorder characterized by destruction of the articular cartilage, subchondral bone alterations and synovitis. Current treatments are focused on symptomatic relief but they lack efficacy to control the progression of this disease which is a leading cause of disability. Therefore, the development of effective disease-modifying drugs is urgently needed. Different initiatives are in progress to define the molecular mechanisms involved in the initiation and progression of OA. These studies support the therapeutic potential of pathways relevant in joint metabolism such as Wnt/β-catenin, discoidin domain receptor 2 or proteinase-activated receptor 2. The dysregulation in cartilage catabolism and subchondral bone remodeling could be improved by selective inhibitors of matrix metalloproteinases, aggrecanases and other proteases. Another approach would favor the activity of anabolic processes by using growth factors or regulatory molecules. Recent studies have also revealed the role of oxidative stress and synovitis in the progression of this disease, supporting the development of a number of inhibitory strategies. Novel targets in OA are represented by genes involved in OA pathophysiology discovered using gene network, epigenetic and microRNA approaches. Further insights into the molecular mechanisms involved in OA initiation and progression may lead to the development of new therapies able to control joint destruction and repair.     

New central targets for the treatment of obesity

The review focuses on the central neuronal circuits involved in energy homeostasis and the opportunities these offer for pharmacological intervention to decrease feeding behaviour and reduce weight. This article is based on the presentation ‘New central targets for the treatment of obesity’ (Sargent, British Pharmacological society, Clinical Section Symposium, December 2008).Central neuronal substrates controlling weight offer numerous opportunities for pharmacological intervention. These opportunities range from non-specific enhancement of monoamine signalling (triple reuptake inhibitors) to targeting specific monoamine receptor subtypes (5-HT_2c and 5-HT₆). The data reviewed suggest that these approaches will lead to weight loss; whether this is sufficient to produce clinically meaningful effect remains to be determined. Combination therapy targeting more than one mechanism may be a means of increasing the magnitude of the response. Preclinical studies also suggest that novel approaches targeting specific neuronal pathways within the hypothalamus, e.g. MCH₁ receptor antagonism, offer an opportunity for weight reduction. However, these approaches are at an early stage and clinical studies will be needed to determine if these novel approaches lead to clinically meaningful weight loss and improvements in co-morbid conditions such as diabetes and cardiovascular disorders.     

New targets for the treatment of vascular diseases and diabetes

This symposium was organized by Dan Flynn (Monsanto Life Sciences, USA) and Timothy M Willson (Glaxo Wellcome, USA). PPARs (peroxisome proliferator-activated receptors), are nuclear hormone receptors that govern glucose and lipid homeostasis. There are several subtypes of receptors that share activation by unsaturated fatty acids and work in combination with retinoic acid receptors (RXR), which were a topic covered in an earlier symposium. Two classes of chemistry were discussed at the symposium: (i) thiazolidinediones, that interact directly with PPARs and alter lipid metabolism; and, (ii) benzothiepines or benzothiazepines, that inhibit the ileal bile acid transporter (IBAT) and reduce cholesterol levels by increasing bile acid excretion.     

Segmental intestinal transporters and metabolic enzymes on intestinal drug absorption.

Recently, a physiologically-based, segregated flow model that incorporates separate intestinal tissue and flow to both a nonabsorptive and an absorptive outermost layer (enterocytes) was shown to better describe the observations on route-dependent morphine glucuronidation in the rat small intestine than a traditional physiologically-based model. These theoretical models were expanded, as the segmental segregated flow model and the segmental traditional model, to view the intestine as three segments of equal lengths receiving equal flows to accommodate heterogeneities in segmental transporter and metabolic functions. The influence of heterogeneity in absorptive, exsorptive, and metabolic functions on drug clearance, bioavailability (F), and metabolite formation after intravenous and oral dosing was examined for the intestine when the tissue was the only organ of removal. Simulations were performed for first-order conditions, when drug partitioned readily (flow-limited distribution) or less readily (membrane-limited distribution) into intestinal tissue, and for different gastrointestinal transit times. The intestinal clearance was found to be inversely related to the rate constant for absorption of a drug that was subjected to secretion and was positively correlated with the metabolic and secretory intrinsic clearances. F was positively correlated with the absorption rate constant but was inversely related to the metabolic and secretory intrinsic clearances. The gastrointestinal transit time decreased metabolite formation, increased clearance, and decreased F. The simulations further showed that a descending metabolic intrinsic clearance yielded a lower F and an ascending segmental distribution of metabolic intrinsic clearance yielded a higher F.     

Non-synaptic receptors and transporters involved in brain functions and targets of drug treatment

Beyond direct synaptic communication, neurons are able to talk to each other without making synapses. They are able to send chemical messages by means of diffusion to target cells via the extracellular space, provided that the target neurons are equipped with high-affinity receptors. While synaptic transmission is responsible for the ‘what’ of brain function, the ‘how’ of brain function (mood, attention, level of arousal, general excitability, etc.) is mainly controlled non-synaptically using the extracellular space as communication channel. It is principally the ‘how’ that can be modulated by medicine. In this paper, we discuss different forms of non-synaptic transmission, localized spillover of synaptic transmitters, local presynaptic modulation and tonic influence of ambient transmitter levels on the activity of vast neuronal populations. We consider different aspects of non-synaptic transmission, such as synaptic–extrasynaptic receptor trafficking, neuron–glia communication and retrograde signalling. We review structural and functional aspects of non-synaptic transmission, including (i) anatomical arrangement of non-synaptic release sites, receptors and transporters, (ii) intravesicular, intra- and extracellular concentrations of neurotransmitters, as well as the spatiotemporal pattern of transmitter diffusion. We propose that an effective general strategy for efficient pharmacological intervention could include the identification of specific non-synaptic targets and the subsequent development of selective pharmacological tools to influence them.     

Lomitapide for the treatment of hypercholesterolemia

Introduction: Homozygous familial hypercholesterolemia (HoFH) is a serious rare inherited condition that leads to extremely elevated levels of low density lipoprotein cholesterol (LDL-C), and predisposes affected individuals to high risk of atherosclerotic vascular disease. Traditional therapies are largely ineffective in managing the hypercholesterolemia in these patients; diet and regular LDL-apheresis are the mainstays of management. Lomitapide is an inhibitor of microsomal triglyceride transfer protein (MTP) that blocks the assembly of metabolic precursors of LDL particles. Lomitapide has been approved for use in the HoFH population.

Areas covered: This article explores the basic properties of lomitapide, including its pharmacodynamic, pharmacokinetic and metabolic profiles. It also reports the current market status of lomitapide and its close competitors. Trials of lomitapide are also briefly reviewed as well as the safety and tolerability of the drug.

Expert opinion: Lomitapide has been recently approved for use in HoFH, a population that has been traditionally very difficult to effectively manage. While lomitapide has some safety concerns, including gastrointestinal symptoms and potential hepatotoxicity, and has yet to prove long term efficacy on hard cardiovascular endpoints, it does represent an attractive treatment option for a small group of patients who, until now, had very limited available effective therapies.     

ATP-binding cassette transporters in macrophages: promising drug targets for treatment of cardiovascular disease

Numerous ATP-binding cassette (ABC) transporters are expressed in monocyte-derived macrophages and are subject to sterol-dependent regulation. ABCA1 has been identified as a key regulator of macrophage cholesterol efflux and HDL-mediated reverse cholesterol transport. Although the precise mechanisms of ABCA1 function are not completely understood, recent data suggest that the ABCA1 pathway regulates vesicular traffic, filipodia formation and lipid microdomains, thereby controlling susceptibility to atherosclerosis. Nuclear hormone receptors including LXR/RXR and PPAR/RXR heterodimers are recognized as direct or indirect regulators of ABCA1 expression and are discussed as potential targets for pharmacological intervention in cardiovascular disease. Future studies clarifying the processes involved in the ABCA1 pathway at the cellular level are expected to identify new and possibly more specific pharmaceutical targets.     

Lead nitrate-induced development of hypercholesterolemia in rats: sterol-independent gene regulation of hepatic enzymes responsible for cholesterol homeostasis

Changes in the gene expressions of hepatic enzymes responsible for cholesterol homeostasis were examined during the process of lead nitrate (LN)-induced development of hypercholesterolemia in male rats. Total cholesterol levels in the liver and serum were significantly increased at 3-72 h and 12-72 h, respectively, after LN-treatment (100 micromol/kg, i.v.). Despite the development of hypercholesterolemia, the genes for hepatic 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) and other enzymes (FPPS, farnesyl diphosphate synthase; SQS, squalene synthase; CYP51, lanosterol 14alpha-demethylase) responsible for cholesterol biosynthesis were activated at 3-24 h and 12-18 h, respectively. On the other hand, the gene expression of cholesterol 7alpha-hydroxylase (CYP7A1), a catabolic enzyme of cholesterol, was remarkably suppressed at 3-72 h. The gene expression levels of cytokines interleukin-1beta (IL-1beta) and TNF-alpha, which activate the HMGR gene and suppress the CYP7A1 gene, were significantly increased at 1-3 h and 3-24 h, respectively. Furthermore, gene activation of SREBP-2, a gene activator of several cholesterogenic enzymes, occurred before the gene activations of FPPS, SQS and CYP51. This is the first report demonstrating sterol-independent gene regulation of hepatic enzymes responsible for cholesterol homeostasis in LN-treated male rats. The mechanisms for the altered-gene expressions of hepatic enzymes in LN-treated rats are discussed.     

Rosuvastatin for the treatment of hypercholesterolemia

Rosuvastatin, a new hydrophilic 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor (statin), is approved as an adjunct to diet in patients with primary hypercholesterolemia, mixed dyslipidemia, or Fredrickson type IV hypercholesterolemia. Because of its increased affinity for the reductase, rosuvastatin reduces the low-density lipoprotein cholesterol (LDL) level more than atorvastatin, simvastatin, and pravastatin do, without additional adverse effects. In addition, cytochrome P450 isoenzymes do not extensively metabolize rosuvastatin, and inhibitors of these isoenzymes do not substantially affect it. Rosuvastatin could be a first-line option for patients requiring a reduction of 50% or more to reach the LDL goal of the National Cholesterol Education Program Adult Treatment Panel III. Rosuvastatin monotherapy may allow patients to achieve this LDL goal earlier, and it may help them avoid combination therapy or potential adverse effects of high-dose statin therapy. However, because cardiovascular disease morbidity and mortality data are lacking for rosuvastatin (but available for all other marketed statins) and because its postmarketing data are limited, rosuvastatin should be reserved for patients requiring an LDL reduction of 50% or less who cannot reach the recommended goal with other statins because of adverse effects, drug interactions, or cost.