Is there facilitated uptake of fatty acids by the liver? Interpretation and analysis of experimental data

Plasma Protein-Mediated Uptake and Contradictions to the Free Drug Hypothesis: A Critical Review

According to the free drug hypothesis (FDH), only free, unbound drug is available to interact with biological targets. This hypothesis is the fundamental principle that continues to explain the vast majority of all pharmacokinetic and pharmacodynamic processes. Under the FDH, the free drug concentration at the target site is considered the driver of pharmacodynamic activity and pharmacokinetic processes. However, deviations from the FDH are observed in hepatic uptake and clearance predictions, where observed unbound intrinsic hepatic clearance (CL_int,u) is larger than expected. Such deviations are commonly observed when plasma proteins are present and form the basis of the so-called plasma protein-mediated uptake effect (PMUE). This review will discuss the basis of plasma protein binding as it pertains to hepatic clearance based on the FDH, as well as several hypotheses that may explain the underlying mechanisms of PMUE. Notably, some, but not all, potential mechanisms remained aligned with the FDH. Finally, we will outline possible experimental strategies to elucidate PMUE mechanisms. Understanding the mechanisms of PMUE and its potential contribution to clearance underprediction is vital to improving the drug development process.

An examination of protein binding and protein-facilitated uptake relating to in vitro-in vivo extrapolation

As explained by the free drug theory, the unbound fraction of drug has long been thought to drive the efficacy of a molecule. Thus, the fraction unbound term, or f_u, appears in equations for fundamental pharmacokinetic parameters such as clearance, and is used when attempting in vitro to in vivo extrapolation (IVIVE). In recent years though, it has been noted that IVIVE does not always yield accurate predictions, and that some highly protein bound ligands have more efficient uptake than can be explained by their unbound fractions. This review explores the evolution of f_u terms included when implementing IVIVE, the concept of protein-facilitated uptake, and the mechanisms that have been proposed to account for facilitated uptake.

The Role of Extracellular Binding Proteins in the Cellular Uptake of Drugs: Impact on Quantitative In Vitro-to-In Vivo Extrapolations of Toxicity and Efficacy in Physiologically Based Pharmacokinetic-Pharmacodynamic Research

A critical component in the development of physiologically based pharmacokinetic-pharmacodynamic (PBPK/PD) models for estimating target organ dosimetry in pharmacology and toxicology studies is the understanding of the uptake kinetics and accumulation of drugs and chemicals at the cellular level. Therefore, predicting free drug concentrations in intracellular fluid will contribute to our understanding of concentrations at the site of action in cells in PBPK/PD research. Some investigators believe that uptake of drugs in cells is solely driven by the unbound fraction; conversely, others argue that the protein-bound fraction contributes a significant portion of the total amount delivered to cells. Accordingly, the current literature suggests the existence of a so-called albumin-mediated uptake mechanism(s) for the protein-bound fraction (i.e., extracellular protein-facilitated uptake mechanisms) at least in hepatocytes and cardiac myocytes; however, such mechanism(s) and cells from other organs deserve further exploration. Therefore, the main objective of this present study was to discuss further the implication of potential protein-facilitated uptake mechanism(s) on drug distribution in cells under in vivo conditions. The interplay between the protein-facilitated uptake mechanism(s) and the effects of a pH gradient, metabolism, transport, and permeation limitation potentially occurring in cells was also discussed, as this should violate the basic assumption on similar free drug concentration in cells and plasma. This was made because the published equations used to calculate drug concentrations in cells in a PBPK/PD model did not consider potential protein-facilitated uptake mechanism(s). Consequently, we corrected some published equations for calculating the free drug concentrations in cells compared with plasma in PBPK/PD modeling studies, and we proposed a refined strategy for potentially performing more accurate quantitative in vitro-to-in vivo extrapolations (IVIVEs) of toxicity (efficacy) at the cellular level from data generated in cell assays. Overall, this present study may help to optimize the human dose prediction in preclinical and clinical studies, while prescribing drugs with narrow therapeutic windows that are highly bound to extracellular proteins and/or highly ionized at the physiological pH. This may facilitate building a more accurate safety (efficacy) profile for such drugs.

Membrane binding proteins are the major determinants for the hepatocellular transmembrane flux of long-chain fatty acids bound to albumin

PURPOSE

The hepatic transmembrane flux of long-chain fatty acids (LCFA) occurs through passive and fatty acid transport protein facilitated processes from blood. The extent that these transport processes can be related to the unbound and protein-bound fractions of LCFA in blood is not clear.

METHODS

We used hepatocyte suspensions, hepatoma monolayers, and perfused rat livers to quantitate the transport of purified [(3)H]palmitate ([(3)H]PA) and 12-(N-methyl)-N-[(7-nitrobenz-2oxa-1,3-diazol-4yl-)amino]octadecanoicacid (12-NBDS) from solutions with a constant unbound LCFA concentration with varying bovine serum albumin (BSA) concentrations and in the presence and absence of antisera raised against cytosolic liver fatty acid binding protein (L-FABP).

RESULTS

In the absence of L-FABP antisera, using an unbound ligand concentration that was adjusted to remain constant at each BSA concentration, hepatocyte [(3)H]PA and 12-NBDS uptake rates increased linearly with an increase in BSA concentration (p < 0.0001). In the presence of L-FABP antisera, [(3)H]PA uptake showed a greater reduction in the presence of 100 muM BSA than 5 muM BSA. The calculated permeability surface area product (PS) confirmed that both unbound and bound fractions of LCFA contributed to the overall flux, but only the PS for the protein-bound fraction was reduced in the presence of L-FABP antisera. In situ rat liver perfusion studies showed that the only rate process for the disposition of [(3)H]PA in the liver inhibited by L-FABP antisera was that for influx, as defined by PS, and that it reduced PS in the perfused liver by 42%.

CONCLUSION

These results suggest that, at physiological albumin concentrations, most of the LCFA uptake is mediated from that bound to albumin by a hepatocyte basolateral membrane transport protein, and uptake of unbound LCFA occurring by passive diffusion contributes a minor component.

Assessing the cellular transmembrane electrical potential difference on the hepatic uptake of palmitate

Understanding the driving forces for the hepatic uptake of endogenous and exogenous substrates in isolated cells and organs is fundamental to describing the underlying hepatic physiology/pharmacology. In this study we investigated whether uptake of plasma protein-bound [3H]-palmitate across the hepatocyte wall is governed by the transmembrane electrical potential difference (PD). Uptake was studied in isolated hepatocytes and isolated perfused rat livers (IPL). Protein-binding and vasoactive properties of the different perfusates were determined using in vitro heptane/buffer partitioning studies and the multiple indicator dilution (MID) technique in the IPL, respectively. Altering hepatocyte PD by perfusate ion substitution resulted in either a substantial depolarization (-14 +/- 1 mV, n = 12, mean +/- S.E., substituting choline for Na+) or hyperpolarization (-46 +/- 3 mV, n = 12, mean +/- S.E., substituting nitrate for Cl-). Perfusate ion substitution also affected the equilibrium binding constant for the palmitate-albumin complex. IPL studies suggested that, other than with gluconate buffer, hepatic [3H]-palmitate extraction was not affected by the buffer used, implying PD was not a determinant of extraction. [3H]-Palmitate extraction was much lower (p < 0.05) when gluconate was substituted for Cl- ion. This work contrasts with that for the extraction of [3H]-alanine where hepatic extraction fraction was significantly reduced during depolarization. Changing the albumin concentration did not affect hepatocyte PD, and [3H]-palmitate clearance into isolated hepatocytes was not affected by the buffers used. MID studies with vascular and extravascular references revealed that, with the gluconate substituted buffer, the extravascular volume possibly increased the diffusional path length thus explaining reduced [3H]-palmitate extraction fraction in the IPL.

Hepatocyte [3H]-palmitate uptake: effect of albumin surface charge modification

The role of plasma proteins on the cellular uptake of lipophilic substrates has perplexed investigators for many years. We tested the hypothesis that an ionic interaction between the protein–ligand complex and hepatocyte surface may be responsible for supplying more ligand to the cell for uptake. The surface-charged groups on albumin were modified to yield proteins having a range of isoelectric points (ALB, ALBs, ALBm, ALBe had values of 4.8–5.0, 4.5–4.7, 3.0–3.5, 8.4–8.6, respectively). [3H]-Palmitate uptake studies were performed with adult rat hepatocyte suspensions using similar unbound ligand fractions in the presence of the different binding proteins. Mass spectrometry, isoelectric focusing (pI), and heptane : water partitioning were used to determine protein molecular weight, pI, and protein-palmitate equilibrium binding constant, respectively. Hepatocyte [3H]-palmitate clearance in the presence of ALBs and ALBm were significantly lower (p < 0.05) than ALB, whereas [3H]-palmitate clearance in the presence of ALBe was significantly higher (p < 0.05) than ALB. The data were consistent with the notion that ionic interactions between extracellular protein–ligand complexes and the hepatocyte surface facilitate the uptake of long-chain fatty acids.Key words: uptake, long-chain fatty acid, hepatic, protein, albumin, surface charge, facilitation, ionic interaction.

Palmitate uptake by neonatal rat myocytes and hepatocytes. Role of extracellular protein

The role of extracellular binding proteins in the rate of [3H]palmitate uptake by neonatal cardiac myocytes and hepatocytes was investigated using a model-independent approach. Binding proteins used in this study included alpha1-acid glycoprotein [isoelectric point (pI) approximately 2.7], conalbumin (pI approximately 6.4), lysozyme (pI approximately 11.0), albumin (pI approximately 4.9), and albumin which had been modified to yield proteins with pI values of 3.5, 4.7, 7.5 and 8.6. All uptake studies were conducted at similar unbound ligand fractions. There was a linear relationship between the rate of neonatal hepatocyte [3H]palmitate clearance and protein pI (r2 = 0.98). In contrast, there was an overall poor relationship between neonatal cardiac myocyte [3H]palmitate-clearance rate and protein pI (r2 = 0.48). However, the relationship improved when the data on [3H]palmitate-clearance were analyzed using only the modified albumins. The study indicates that an ionic interaction between extracellular proteins and the hepatocyte surface enhances the overall uptake of [3H]palmitate. This interaction may be limited to albumin for neonatal cardiac myocytes.

Saturable stimulation of fatty acid transport through model cytoplasm by soluble binding protein

To better define the role of soluble binding proteins in the cytoplasmic transport of amphipathic molecules, we measured the diffusional mobility of a fluorescent long-chain fatty acid, 12-N-methyl-(7-nitrobenz-2-oxa-1,3-diazol)aminostearate (NBD-stearate), through model cytoplasm as a function of soluble binding protein concentration. Diffusional mobilities were correlated with the partition of the fatty acid between membrane and protein binding sites. Cytoplasm was modeled as a dense suspension of liposomes, and albumin was used as a model binding protein. Albumin saturably increased NBD-stearate mobility through the membrane suspension approximately eightfold. Fatty acid mobility in the absence of albumin was identical to the mobility of the membrane vesicles (1.99 +/- 0.33 x 10(-8) cm(2)/s), whereas the mobility at saturating concentrations was identical to the mobility of albumin (1.65 +/- 0.12 x 10(-7) cm(2)/s). The protein concentration producing half-maximal stimulation of NBD-stearate diffusion (42.8 +/- 0.3 microM) was unexpectedly greater than that required to solubilize half of the NBD-stearate (17.9 +/- 3.0 microM). These results support a proposed mechanism for cytoplasmic transport of small amphipathic molecules in which aqueous diffusion of the protein-bound form of the molecule largely determines the transport rate. However, slow interchange of fatty acid between the binding protein and membranes also appears to influence the transport rate in this model system.

Skeletal muscle fatty acid transport and transporters

Long-chain fatty acids (LCFAs) are an important energy source for many tissues. The dogma that LCFAs are freely diffusible has been challenged. It is now known that LCFAs are transported into many tissues. Our studies have shown that LCFAs are also transported into skeletal muscle and into the heart. In recent years a number of putative fatty acid transport proteins have been identified. These are known as plasma membrane fatty acid binding protein (FABPpm, 43 kDa), fatty acid translocase (FAT, 88 kDa) and fatty acid transporter protein (FATP, 63 kDa). All three proteins are present in skeletal muscle and in the heart. The existence of an LCFA transport system in muscle may be essential 1) to facilitate the rapid and regulatable transport of LCFA to meet the metabolic requirements of working muscles and 2) to cope with an increase in circulating LCFAs in some pathological conditions (e.g. diabetes). There is now some evidence that metabolic changes and chronically increased muscle activity can increase the transport of LCFAs and increase the expression of putative LCFA transporters.

Binding of [3H]palmitate to BSA

Determination of the BSA-palmitate high-affinity binding constant (Ka) traditionally relied on the heptane-water partitioning technique. We used this technique to calculate Ka for the BSA-[3H]palmitate complex, to determine if Ka was independent of protein concentration, and to determine if the unbound [3H]palmitate concentration is constant at different BSA concentrations using constant BSA-to-palmitate molar ratios (range 1:1 to 1:4). After extensive extraction of non-[3H]palmitate radiolabeled substances, the heptane-to-buffer partition ratio, in the absence of BSA, was 702 +/- 19 (mean +/- SD, n = 6). This value was much lower than the predicted value of 1,376 and was highly dependent on which phase (organic or aqueous) initially contained the [3H]palmitic acid. The data were consistent with the notion of self-association of [3H]palmitate in the aqueous phase. Ka for the BSA-[3H]palmitate complex was determined to be similar (2.2 +/- 0.1) x 10(8) M-1 (mean +/- SD, P > 0.05) at all BSA concentrations studied. At each BSA-to-palmitate molar ratio, the equilibrium unbound ligand concentration was constant only at low BSA concentrations (<10 microM) and at low BSA-to-palmitate molar ratios (i.e., 1:1 and 1:2). At higher BSA concentrations and molar ratios, the unbound ligand concentration increased with an increase in protein concentration. Hepatocyte uptake using the manufacturer-supplied radiolabeled product was significantly higher than with the purified product, suggesting that a non-[3H]palmitate radiolabel is also a substrate for the uptake process.

Molecular mechanism of cellular uptake and intracellular translocation of fatty acids

The molecular mechanism of the transport of long-chain fatty acids across cellular membranes and the necessity and precise functioning of specific proteins in this process are still unclear. Various alternative mechanisms have been proposed. Studies with artificial phospholipid bilayers support the concept that fatty acids may enter and traverse the plasma membrane without the involvement of proteins. On the other hand, a number of membrane-associated fatty acid-binding proteins (FABPs) have been described which putatively function as acceptors for fatty acids released from albumin or from lipoproteins. Albumin binding proteins located at the outer cell surface could play an additional role in the delivery of fatty acids. The subsequent transmembrane translocation of fatty acids could take place by a membrane protein acting as a translocase, or by simple diffusion of fatty acids through either the phospholipid bilayer or a pore or channel formed by one or more membrane fatty acid transporters. At the inner side of the plasma membrane, the fatty acid is bound to a cytoplasmic FABP, which serves to buffer the intracellular aqueous fatty acid concentration. The direction of fatty acid migration through the plasma membrane most likely is governed by the transmembrane gradient of fatty acid concentration, assisted to some extent and in selected tissues by co-transport of sodium ions. The intracellular transport of fatty acids from the plasma membrane to the sites of metabolic conversion (oxidation, esterification) or subcellular target (signal transduction) is greatly facilitated by cytoplasmic FABPs. In conclusion, cellular uptake and intracellular translocation of long-chain fatty acids is a multi-step process that is facilitated by various membrane-associated and soluble proteins. The mechanism of cellular uptake of fatty acids probably involves both a passive and carrier-mediated transmembrane translocation.

Effect of binding protein surface charge on palmitate uptake by hepatocyte suspensions

1. Studies were directed at determining whether hepatocytes, isolated from female Sprague-Dawley rats, facilitate the uptake of protein-bound long-chain fatty acids. We postulated one form of facilitated uptake may occur through an ionic interaction between the protein-ligand complex and the cell surface. These interactions are expected to supply additional ligand to the cell for uptake. 2. The clearance rate of [3H]-palmitate in the presence of alpha 1-acid-glycoprotein (pI = 2.7), albumin (pI = 4.9) and lysozyme (pI = 11.0) was investigated. Palmitate uptake was determined in the presence of protein concentrations that resulted in similar unbound ligand fractions (= 0.03). The experimental clearance rates were compared to the theoretical predictions based upon the diffusion-reaction model. 3. By use of our experimentally determined equilibrium binding and dissociation rate constants for the various protein-palmitate complexes, the diffusion-reaction model predicted clearance rates were 4.9 microliters s-1/10(6) cells, 4.8 microliters s-1/10(6) cells and 5.5 microliters s-1/10(6) cells for alpha 1-acid-glycoprotein, albumin and lysozyme, respectively; whereas the measured hepatocyte palmitate clearance rates were 1.2 +/- 0.1 microliters s-1/10(6) cells, 2.3 +/- 0.3 microliters s-1/10(6) cells and 7.1 +/- 0.7 microliters s-1/10(6), respectively. 4. Hepatocyte palmitate clearance was significantly faster (P < 0.01) in the presence of lysozyme than albumin which was significantly faster than alpha 1-acid-glycoprotein (P < 0.01). The marked difference in clearance rates could not be explained by considering differences in solution viscosity. 5. Our results are consistent with the notion that ionic interactions between protein-ligand complexes and the cell surface facilitate the ligand uptake by decreasing the diffusional distance of the unbound ligand and/or by facilitating the protein-ligand dissociation rate.