Is there a temperature-dependent uptake of anandamide into cells?

Chicken embryo as animal model to study drug distribution to the developing brain

INTRODUCTION

Rodent models are routinely used to assess the safety and developmental toxicity of pharmaceuticals, along with analysis of their distribution. These models require sacrifice of parent females, have challenges in the estimation of the number of embryos and stage of development, and are expensive and time-consuming. In this study, we used fertilized chicken eggs as an alternative model to address drug distribution to the developing brain of two antiepileptic drugs, valproic acid (VPA) and lamotrigine (LTG) at two developmental stages.

METHODS

VPA or LTG was injected into the allantois of the egg on embryonic day 13 (E13) or E16. Whole chicken brains were harvested at time-points of 5 min to 24 h and the concentrations of the drugs determined using GC/MS and LC-MS/MS, for VPA and LTG, respectively.

RESULTS

VPA and LTG had distinct absorption and elimination phases and were found in the brain as early as 5-15 min after injection. Both drugs reached the brain in clinically relevant concentrations, with C_max 10-30% of the calculated concentration assuming uniform distribution throughout the egg. LTG concentrations were higher when injected at E13 compared to E16.

CONCLUSION

The chicken embryo model may be a suitable alternative animal model for preclinical drug distribution studies. It enables to easily approach antenatal development on an individual level, with a precise number of experimental animals, high reproducibility and low time and cost. Knowledge of the concentrations reaching the brain at different developmental stages with different drugs is important for the planning and interpretation of neurodevelopmental toxicity studies.

Immortalized Dorsal Root Ganglion Neuron Cell Lines

Pain is one of the most significant causes of suffering and disability world-wide, and arguably the most burdensome global health challenge. The growing number of patients suffering from chronic pain conditions such as fibromyalgia, complex regional pain syndrome, migraine and irritable bowel syndrome, not only reflect the complexity and heterogeneity of pain types, but also our lack of understanding of the underlying mechanisms. Sensory neurons within the dorsal root ganglia (DRG) have emerged as viable targets for effective chronic pain therapy. However, DRG's contain different classes of primary sensory neurons including pain-associated nociceptive neurons, non-nociceptive temperature sensing, mechanosensory and chemoreceptive neurons, as well as multiple types of immune and endothelial cells. This cell-population heterogeneity makes investigations of individual subgroups of DRG neurons, such as nociceptors, difficult. In attempts to overcome some of these difficulties, a limited number of immortalized DRG-derived cell lines have been generated over the past few decades. In vitro experiments using DRG-derived cell lines have been useful in understanding sensory neuron function. In addition to retaining phenotypic similarities to primary cultured DRG neurons, these cells offer greater suitability for high throughput assays due to ease of culture, maintenance, growth efficiency and cost-effectiveness. For accurate interpretation and translation of results it is critical, however, that phenotypic similarities and differences of DRG-derived cells lines are methodically compared to native neurons. Published reports to date show notable variability in how these DRG-derived cells are maintained and differentiated. Understanding the cellular and molecular differences stemming from different culture methods, is essential to validate past and future experiments, and enable these cells to be used to their full potential. This review describes currently available DRG-derived cell lines, their known sensory and nociceptor specific molecular profiles, and summarize their morphological features related to differentiation and neurite outgrowth.

Role of pannexin-1 in the cellular uptake, release and hydrolysis of anandamide by T84 colon cancer cells

The large pore ion channel pannexin-1 (Panx1) has been reported to play a role in the cellular uptake and release of anandamide (AEA) in the hippocampus. It is not known whether this is a general mechanism or limited to the hippocampus. We have investigated this pharmacologically using T84 colon cancer cells. The cells expressed Panx1 at the mRNA level, and released ATP in a manner that could be reduced by treatment with the Panx1 inhibitors carbenoxolone and mefloquine and the Panx1 substrate SR101. However, no significant effects of these compounds upon the uptake or hydrolysis of exogenously applied AEA was seen. Uptake by T84 cells of the other main endocannabinoid 2-arachidonoylglycerol and the AEA homologue palmitoylethanolamide was similarly not affected by carbenoxolone or mefloquine. Total release of tritium from [³H]AEA-prelabelled T84 cells over 10 min was increased, rather than inhibited by carbenoxolone and mefloquine. Finally, AEA uptake by PC3 prostate cancer and SH-SY5Y neuroblastoma cells, which express functional Panx1 channels, was not inhibited by carbenoxolone. Thus, in contrast to the hippocampus, Panx1 does not appear to play a role in AEA uptake and release from the cells studied under the conditions used.

Effects of temperature and PEG grafting density on the translocation of PEGylated nanoparticles across asymmetric lipid membrane

Plasma membrane internalization of nanoparticles (NPs) is important for their biomedical applications such as drug-delivery carriers. On one hand, in order to improve their half-life in circulation, PEGylation has been widely used. However, it may hinder the NPs' membrane internalization ability. On the other hand, higher temperature could enhance the membrane permeability and may affect the NPs' ability to enter into or exit from cells. To make full use of their advantages, we systematically investigated the effects of temperature and PEG density on the translocation of PEGylated nanoparticles across the plasma asymmetric membrane of eukaryotic cells, using near-atom level coarse-grained molecular dynamics simulations. Our results showed that higher temperature could accelerate the translocation of NPs across membranes by making lipids more disorder and faster diffusion. On the contrary, steric hindrance effects of PEG would inhibit NPs' translocation process and promote lipids flip-flops. The PEG chains could rearrange themselves to minimize the contacts between PEG and lipid tails during the translocation, which was similar to 'snorkeling effect'. Moreover, lipid flip-flops were affected by PEGylated density as well as NPs' translocation direction. Higher PEG grafting density could promote lipid flip-flops, but inhibit lipid extraction from bilayers. The consequence of lipid flip-flop and extraction was that the membranes got more symmetric.

Ketoconazole inhibits the cellular uptake of anandamide via inhibition of FAAH at pharmacologically relevant concentrations

Background

The antifungal compound ketoconazole has, in addition to its ability to interfere with fungal ergosterol synthesis, effects upon other enzymes including human CYP3A4, CYP17, lipoxygenase and thromboxane synthetase. In the present study, we have investigated whether ketoconazole affects the cellular uptake and hydrolysis of the endogenous cannabinoid receptor ligand anandamide (AEA).

Methodology/Principal Findings

The effects of ketoconazole upon endocannabinoid uptake were investigated using HepG2, CaCo2, PC-3 and C6 cell lines. Fatty acid amide hydrolase (FAAH) activity was measured in HepG2 cell lysates and in intact C6 cells. Ketoconazole inhibited the uptake of AEA by HepG2 cells and CaCo2 cells with IC₅₀ values of 17 and 18 µM, respectively. In contrast, it had modest effects upon AEA uptake in PC-3 cells, which have a low expression of FAAH. In cell-free HepG2 lysates, ketoconazole inhibited FAAH activity with an IC₅₀ value (for the inhibitable component) of 34 µM.

Conclusions/Significance

The present study indicates that ketoconazole can inhibit the cellular uptake of AEA at pharmacologically relevant concentrations, primarily due to its effects upon FAAH. Ketoconazole may be useful as a template for the design of dual-action FAAH/CYP17 inhibitors as a novel strategy for the treatment of prostate cancer.

Transport of endocannabinoids across the plasma membrane and within the cell

Endocannabinoids are readily accumulated from the extracellular space by cells. Although their uptake properties have the appearance of a process of facilitated diffusion, it is by no means clear as to whether there is a plasma membrane transporter dedicated to this task. Intracellular carrier proteins that shuttle the endocannabinoid anandamide from the plasma membrane to its intracellular targets such as the metabolic enzyme, fatty acid amide hydrolase, have been identified. These include proteins with other primary functions, such as fatty-acid-binding proteins and heat shock protein 70, and possibly a fatty acid amide hydrolase-like anandamide transporter protein. Thus, anandamide uptake can be adequately described as a diffusion process across the plasma membrane followed by intracellular carrier-mediated transport to effector molecules, catabolic enzymes and sequestration sites, although it is recognized that different cells are likely to utilize different mechanisms of endocannabinoid transport depending upon the utility of the endocannabinoid for the cell in question.

Fatty acid-binding proteins transport N-acylethanolamines to nuclear receptors and are targets of endocannabinoid transport inhibitors

N-acylethanolamines (NAEs) are bioactive lipids that engage diverse receptor systems. Recently, we identified fatty acid-binding proteins (FABPs) as intracellular NAE carriers. Here, we provide two new functions for FABPs in NAE signaling. We demonstrate that FABPs mediate the nuclear translocation of the NAE oleoylethanolamide, an agonist of nuclear peroxisome proliferator-activated receptor α (PPARα). Antagonism of FABP function through chemical inhibition, dominant-negative approaches, or shRNA-mediated knockdown reduced PPARα activation, confirming a requisite role for FABPs in this process. In addition, we show that NAE analogs, traditionally employed as inhibitors of the putative endocannabinoid transmembrane transporter, target FABPs. Support for the existence of the putative membrane transporter stems primarily from pharmacological inhibition of endocannabinoid uptake by such transport inhibitors, which are widely employed in endocannabinoid research despite lacking a known cellular target(s). Our approach adapted FABP-mediated PPARα signaling and employed in vitro binding, arachidonoyl-[1-(14)C]ethanolamide ([(14)C]AEA) uptake, and FABP knockdown to demonstrate that transport inhibitors exert their effects through inhibition of FABPs, thereby providing a molecular rationale for the underlying physiological effects of these compounds. Identification of FABPs as targets of transport inhibitors undermines the central pharmacological support for the existence of an endocannabinoid transmembrane transporter.

Inactivation of Anandamide Signaling: A Continuing Debate

Since the first endocannabinoid anandamide was identified in 1992, extensive research has been conducted to characterize the elements of the tightly controlled endocannabinoid signaling system. While it was established that the activity of endocannabinoids are terminated by a two-step process that includes cellular uptake and degradation, there is still a continuing debate about the mechanistic role of these processes in inactivating anandamide signals.

Pitfalls and solutions in assaying anandamide transport in cells

Nonspecific binding of anandamide to plastic exhibits many features that could be mistaken as biological processes, thereby representing an important source of conflicting data on the uptake and release of this lipophilic substance. Herein, we propose an improved method to assay anandamide transport, by using glass slides (i.e., coverslips) as physical support to grow cells. Although the results obtained using plastic do not differ significantly from those obtained using glass, the new procedure has the advantage of being faster, simpler, and more accurate. In fact, the lack of aspecific adsorption of anandamide to the glass surface yields a lower background and a higher precision and accuracy in determining transport kinetics, especially for the export process. Remarkably, the kinetic parameters of anandamide uptake obtained with the old and the new procedures may be similar or different depending on the cell type, thus demonstrating the complexity of the interference of plastic on the transport process. In addition, the novel procedure is particularly suitable for visualization and measurement of anandamide transport in intact cells by using a biotinylated derivative in confocal fluorescence microscopy.

Exploiting nanotechnologies and TRPV1 channels to investigate the putative anandamide membrane transporter

Background

Considerable efforts have been made to characterize the pathways regulating the extracellular levels of the endocannabinoid anandamide. However, none of such pathways has been so argued as the existence of a carrier-mediated transport of anandamide across the membrane. Apart from the lack of molecular evidence for such a carrier, the main reasons of this controversy lie in the methodologies currently used to study anandamide cellular uptake. Furthermore, the main evidence in favor of the existence of an “anandamide transporter” relies on synthetic inhibitors of this process, the selectivity of which has been questioned.

Methodology/Principal Findings

We used the cytosolic binding site for anandamide on TRPV1 channels as a biosensor to detect anandamide entry into cells, and exploited nanotechnologies to study anandamide membrane transport into intact TRPV1-overexpressing HEK-293 cells. Both fluorescence and digital holographic (DH) quantitative phase microscopy were used to study TRPV1 activation. Poly-ε-caprolactone nanoparticles (PCL-NPs) were used to incorporate anandamide, which could thus enter the cell and activate TRPV1 channels bypassing any possible specific protein(s) involved in the uptake process. We reasoned that in the absence of such protein(s), pharmacological tools previously shown to inhibit the “anandamide transporter” would affect in the same way the uptake of anandamide and PCL-NP-anandamide, and hence the activation of TRPV1. However, when masked into PCL-NPs, anandamide cellular uptake became much less sensitive to these agents, although it maintained the same pharmacokinetics and pharmacodynamics as that of “free” anandamide.

Conclusions

We found here that several agents previously reported to inhibit anandamide cellular uptake lose their efficacy when anandamide is prevented from interacting directly with plasma membrane proteins, thus arguing in favor of the specificity of such agents for the putative “anandamide transporter”, and of the existence of such mechanism.

The complications of promiscuity: endocannabinoid action and metabolism

In this review, we present our understanding of the action and metabolism of endocannabinoids and related endogenous molecules. It is clear that the interactions between the multiple endocannabinoid-like molecules (ECLs) are highly complex, both at the level of signal transduction and metabolism. Thus, ECLs are a group of ligands active at 7-transmembrane and nuclear receptors, as well as transmitter-gated and ion channels. ECLs and their metabolites can converge on common endpoints (either metabolic or signalling) through contradictory or reinforcing pathways. We highlight the complexity of the endocannabinoid system, based on the promiscuous nature of ECLs and their metabolites, as well as the synthetic modulators of the endocannabinoid system.

Structure-activity relationship of a series of inhibitors of monoacylglycerol hydrolysis--comparison with effects upon fatty acid amide hydrolase

A series of 32 heterocyclic analogues based on the structure of 2-arachidonoylglycerol (2-AG) were synthesized and tested for their ability to inhibit monoacylglycerol lipase and fatty acid amide hydrolase activities. The designed compounds feature a hydrophobic moiety and different heterocyclic subunits that mimic the glycerol fragment. This series has allowed us to carry out the first systematic structure-activity relationship study on inhibition of 2-AG hydrolysis. The most promising compounds were oxiran-2-ylmethyl (5Z,8Z,11Z,14Z)-icosa-5,8,11,14-tetraenoate (1) and tetrahydro-2H-pyran-2-ylmethyl (5Z,8Z,11Z,14Z)-icosa-5,8,11,14-tetraenoate (5). They inhibited cytosolic 2-oleoylglycerol (2-OG) hydrolysis completely (IC50 values of 4.5 and 5.6 muM, respectively). They also blocked, albeit less potently, 2-OG hydrolysis in membrane fractions (IC50 values of 19 and 26 muM, respectively) and anandamide hydrolysis (IC50 values of 12 and 51 muM, respectively). These compounds will be useful in delineating the importance of the cytosolic hydrolytic activity in the regulation of 2-AG levels and, hence, its potential as a target for drug development.

Inhibition of the cellular uptake of anandamide by genistein and its analogue daidzein in cells with different levels of fatty acid amide hydrolase-driven uptake

BACKGROUND AND PURPOSE

Genistein, a tyrosine kinase inhibitor used to block caveolae dependent endocytosis, reduces the cellular uptake of anandamide in RBL2H3 basophilic leukaemia cells. However, genistein is also a competitive inhibitor of fatty acid amide hydrolase, the enzyme responsible for anandamide hydrolysis. Here we have investigated whether inhibition of fatty acid amide hydrolase rather than inhibition of endocytosis is the primary determinant of genistein actions upon anandamide uptake.

EXPERIMENTAL APPROACH

Cellular uptake of anandamide, labelled in the arachidonoyl part of the molecule was assessed in four different cell lines using a standard method. Fatty acid amide hydrolase activity in homogenates and intact cells was measured using anandamide labelled in the ethanolamine part of the molecule.

KEY RESULTS

The fatty acid amide hydrolase inhibitor URB597 inhibited anandamide uptake into RBL2H3 cells and R3327 AT-1 prostate cancer cells, but not into 3T3-L1 preadipocytes or PC-3 prostate cancer cells. An identical pattern was seen with genistein. The related compound daidzein inhibited anandamide hydrolysis in homogenates and intact cells, and reduced its uptake into RBL2H3 and R3327 AT-1, but not PC-3 cells. Anandamide hydrolysis by cell homogenates was in the order RBL2H3 > R3327 AT-1 > PC-3 approximately 3T3-L1.

CONCLUSIONS AND IMPLICATIONS

The ability of genistein to inhibit anandamide uptake is mimicked by daidzein (which does not affect tyrosine kinase), and is only seen in cells that show sensitivity to URB597. This indicates that blockade of fatty acid amide hydrolase is the primary determinant of the effects of genistein on cellular anandamide uptake.

The cannabinoid system and its pharmacological manipulation--a review, with emphasis upon the uptake and hydrolysis of anandamide

Although cannabis has been used both recreationally and for medicinal purposes since ancient times, it was not until the 1990s that the receptors responsible for many of the actions of Delta(9)-tetrahydrocannabinol, the main psychoactive ingredient of cannabis, were cloned. Since then, our knowledge of the endogenous cannabinoid system, its physiology, pharmacology and therapeutic potential have expanded enormously. In the present review, the cannabinoid system is described, with particular emphasis on the mechanisms of removal and metabolism of the endocannabinoid signalling molecule anandamide. The current literature shows that cells can accumulate anandamide, and that this process can be disrupted pharmacologically, but that the nature of the mechanism(s) involved remains a matter of some debate. The main enzyme for the hydrolysis of anandamide, fatty acid amide hydrolase, is well characterized, and molecules selectively inhibiting this enzyme have potential therapeutic utility in a number of areas, in particular for the treatment of pain conditions.

The 'specific' tyrosine kinase inhibitor genistein inhibits the enzymic hydrolysis of anandamide: implications for anandamide uptake

BACKGROUND AND PURPOSE

The cellular uptake of anandamide is reduced by inhibitors of fatty acid amide hydrolase (FAAH) and by agents disrupting endocytotic mechanisms. However, it is not clear if these events occur over the same time frame and if they occur to the same extent in different cells. We have therefore investigated the effects of such compounds in three cell lines of different origins using different assay incubation times and temperatures.

EXPERIMENTAL APPROACH

FAAH activity and cellular uptake of anandamide was measured using anandamide, radio-labelled either in the ethanolamine or arachidonoyl part of the molecule.

KEY RESULTS

The FAAH inhibitor URB597 inhibited the uptake of anandamide into C6 glioma, RBL2H3 basophilic leukaemia cells and P19 embryonic carcinoma cells at incubation time 4 min. However, a time-dependent and temperature-sensitive residual uptake remained after URB597 treatment. The combination of progesterone and nystatin reduced the uptake, but also decreased the amount of anandamide retained by the wells. Genistein inhibited anandamide uptake in a manner that was not additive to that of URB597. However, genistein was a potent competitive inhibitor of FAAH (K(i) value 8 microM).

CONCLUSIONS AND IMPLICATIONS

The reduction of anandamide uptake by genistein can be explained by its ability to inhibit FAAH with a potency which overlaps that for inhibition of tyrosine kinase. The FAAH- resistant but time-dependent uptake of anandamide is seen in all three cell lines studied and is thus presumably a generally occurring process.