Detergents as intrinsic P-glycoprotein substrates and inhibitors

P-glycoprotein-ATPase modulation: the molecular mechanisms

P-glycoprotein-ATPase is an efflux transporter of broad specificity that counteracts passive allocrit influx. Understanding the rate of allocrit transport therefore matters. Generally, the rates of allocrit transport and ATP hydrolysis decrease exponentially with increasing allocrit affinity to the transporter. Here we report unexpectedly strong down-modulation of the P-glycoprotein-ATPase by certain detergents. To elucidate the underlying mechanism, we chose 34 electrically neutral and cationic detergents with different hydrophobic and hydrophilic characteristics. Measurement of the P-glycoprotein-ATPase activity as a function of concentration showed that seven detergents activated the ATPase as expected, whereas 27 closely related detergents reduced it significantly. Assessment of the free energy of detergent partitioning into the lipid membrane and the free energy of detergent binding from the membrane to the transporter revealed that the ratio, q, of the two free energies of binding determined the rate of ATP hydrolysis. Neutral (cationic) detergents with a ratio of q = 2.7 ± 0.2 (q > 3) followed the aforementioned exponential dependence. Small deviations from the optimal ratio strongly reduced the rates of ATP hydrolysis and flopping, respectively, whereas larger deviations led to an absence of interaction with the transporter. P-glycoprotein-ATPase inhibition due to membrane disordering by detergents could be fully excluded using (2)H-NMR-spectroscopy. Similar principles apply to modulating drugs.

Entrapment of epirubicin in poly(butyl cyanoacrylate) colloidal nanospheres by nanoprecipitation: formulation development and in vitro studies on cancer cell lines

This report describes the preparation of poly(butyl cyanoacrylate) nanospheres loaded with epirubicin by nanoprecipitation, their characterization and in vitro evaluation of the drug uptake and cytotoxicity on cancer cell lines. The epirubicin-loaded nanospheres were prepared by nanoprecipitation using presynthesized polymer and dextran 40 as a colloidal stabilizer at different pH and initial drug concentrations. The nanospheres were characterized for particle morphology, size distribution, zeta-potential and drug loading. Epirubicin-loaded particles with diameters around 350 nm were obtained. Drug loading depended on the pH and epirubicin concentration. Epirubicin was more cytotoxic when loaded in nanospheres. Drug release was studied by dialysis method. Cytotoxicity and drug uptake experiments were performed on HeLa and A549 cell lines. It was found that addition of polysorbate 80 could increase cytotoxicity. The cytotoxicity was found to correlate with the drug uptake by cells. The findings reported here demonstrate that epirubicin-loaded nanospheres of poly(butyl cyanoacrylate) can be successfully prepared by the nanoprecipitation approach as alternative to the well-known polymerization-based methods. It is found that the epirubicin-loaded nanospheres are more cytotoxic than the free drug to human carcinoma cell lines in vitro. The higher cytotoxicity of the obtained epirubicin formulations, compared with the free drug, is due to enhanced cellular internalization of epirubicin.

The cystic fibrosis transmembrane conductance regulator (CFTR): three-dimensional structure and localization of a channel gate

Cystic fibrosis affects about 1 in 2500 live births and involves loss of transmembrane chloride flux due to a lack of a membrane protein channel termed the cystic fibrosis transmembrane conductance regulator (CFTR). We have studied CFTR structure by electron crystallography. The data were compared with existing structures of other ATP-binding cassette transporters. The protein was crystallized in the outward facing state and resembled the well characterized Sav1866 transporter. We identified regions in the CFTR map, not accounted for by Sav1866, which were potential locations for the regulatory region as well as the channel gate. In this analysis, we were aided by the fact that the unit cell was composed of two molecules not related by crystallographic symmetry. We also identified regions in the fitted Sav1866 model that were missing from the map, hence regions that were either disordered in CFTR or differently organized compared with Sav1866. Apart from the N and C termini, this indicated that in CFTR, the cytoplasmic end of transmembrane helix 5/11 and its associated loop could be partly disordered (or alternatively located).

Influence of detergents on the activity of the ABC transporter LmrA

The ABC transporter LmrA from Lactococcus lactis has been intensively studied and a role in multidrug resistance was proposed. Here, we performed a comprehensive detergent screen to analyze the impact of detergents for a successful solubilization, purification and retention of functional properties of this ABC transporter. Our screen revealed the preference of LmrA for zwitterionic detergents. In detergent solution, LmrA purified with FC-16 was highly active with respect to ATPase activity, which could be stimulated by a substrate (rhodamine 123) of LmrA. Both, high ATPase activity and substrate stimulation were not detected for LmrA solubilized in DDM. Interestingly, reconstituted LmrA showed an opposite behavior, with a high basal ATPase activity and stimulation by rhodamine 123 for a DDM-reconstituted, but only low ATPase activity and no substrate stimulation for a FC-16 reconstituted sample.

Exploring the P-glycoprotein binding cavity with polyoxyethylene alkyl ethers

P-glycoprotein (ABCB1) moves allocrits from the cytosolic to the extracellular membrane leaflet, preventing their intrusion into the cytosol. It is generally accepted that allocrit binding from water to the cavity lined by the transmembrane domains occurs in two steps, a lipid-water partitioning step, and a cavity-binding step in the lipid membrane, whereby hydrogen-bond (i.e., weak electrostatic) interactions play a crucial role. The remaining key question was whether hydrophobic interactions also play a role for allocrit binding to the cavity. To answer this question, we chose polyoxyethylene alkyl ethers, C(m)EO(n), varying in the number of methylene and ethoxyl residues as model allocrits. Using isothermal titration calorimetry, we showed that the lipid-water partitioning step was purely hydrophobic, increasing linearly with the number of methylene, and decreasing with the number of ethoxyl residues, respectively. Using, in addition, ATPase activity measurements, we demonstrated that allocrit binding to the cavity required minimally two ethoxyl residues and increased linearly with the number of ethoxyl residues. The analysis provides the first direct evidence, to our knowledge, that allocrit binding to the cavity is purely electrostatic, apparently without any hydrophobic contribution. While the polar part of allocrits forms weak electrostatic interactions with the cavity, the hydrophobic part seems to remain associated with the lipid membrane. The interplay between the two types of interactions is most likely essential for allocrit flipping.

Multidrug resistance protein. Identification of regions required for active transport of leukotriene C4

Multidrug resistance protein (MRP) is a broad specificity, primary active transporter of organic anion conjugates that confers a multidrug resistance phenotype when transfected into drug-sensitive cells. The protein was the first example of a subgroup of the ATP-binding cassette superfamily whose members have three membrane-spanning domains (MSDs) and two nucleotide binding domains. The role(s) of the third MSD of MRP and its related transporters is not known. To begin to address this question, we examined the ability of various MRP fragments, expressed individually and in combination, to transport the MRP substrate, leukotriene C4 (LTC4). We found that elimination of the entire NH2-terminal MSD or just the first putative transmembrane helix, or substitution of the MSD with the comparable region of the functionally and structurally related transporter, the canalicular multispecific organic anion transporter (cMOAT/MRP2), had little effect on protein accumulation in the membrane. However, all three modifications decreased LTC4 transport activity by at least 90%. Transport activity could be reconstituted by co-expression of the NH2-terminal MSD with a fragment corresponding to the remainder of the MRP molecule, but this required both the region encoding the transmembrane helices of the NH2-terminal MSD and the cytoplasmic region linking it to the next MSD. In contrast, a major part of the cytoplasmic region linking the NH2-proximal nucleotide binding domain of the protein to the COOH-proximal MSD was not required for active transport of LTC4.