ATP-binding cassette proteins

Role of P-glycoprotein-mediated secretion in absorptive drug permeability: An approach using passive membrane permeability and affinity to P-glycoprotein

It has been shown in vivo and in vitro that P-glycoprotein (P-gp) may be able to influence the permeability of its substrates across biological membranes. However, the quantitative contribution of the secretion process mediated by P-gp on the overall permeability of membranes has not been determined yet. In particular, observations need to be clarified in which substrates showing high affinity to P-glycoprotein, e.g., verapamil, apparently do not seem to be greatly influenced by P-gp in their permeability and consequently also with respect to their extent of GI-absorption after oral administration, whereas weaker substrates of P-gp, e.g., talinolol, have clearly shown P-gp-related absorption phenomena such as nonlinear intestinal permeability and bioavailability. Experiments with Caco-2 cell monolayers and mathematical simulations based on a mechanistic permeation model should aid in clarifying the underlying mechanism for these observations and quantifying the influence of passive membrane permeability and affinity to P-gp to the overall transmembrane drug flux. In addition, the concentration range of drug at which P-glycoprotein-mediated transport across the biological membrane is relevant should be examined. The permeability of various drugs in Caco-2 monolayers was determined experimentally and modeled using a combination of passive absorptive membrane permeability and a Michaelis-Menten-type transport process in the secretory direction. The passive permeabilities were experimentally obtained for the apical and basolateral membrane by efflux experiments using Caco-2 monolayers in the presence of a P-gp inhibitor. The Michaelis-Menten parameters were determined by a newly developed radioligand-binding assay for the quantification of drug affinity to P-gp. The model was able to accurately simulate the permeability of P-glycoprotein substrates, with differing passive membrane permeabilities and P-glycoprotein affinities. Using the outlined approach, permeability vs donor-concentration profiles were calculated, and the relative contribution of passive and active transport processes to the overall membrane permeability was evaluated. A model is presented to quantitatively describe and predict direction-dependent drug fluxes in Caco-2 monolayers by knowing the affinity of a compound to the exsorptive transporter P-gp and its passive membrane permeability. It was shown that a combination of high P-gp affinity with good passive membrane permeability, e.g., in the case of verapamil, will readily compensate for the P-gp-mediated reduction of intestinal permeability, resulting in a narrow range in which the permeability depends on the apical drug concentration. On the other hand, the permeability of compounds with low passive membrane permeability (e. g., talinolol) might be affected over a wide concentration range despite low affinity to P-gp.

Regulation of adenylyl cyclase isoforms by N-alkanols

We examined the effect of n-alkanols on adenylyl cyclase isoforms (types II and V) overexpressed in insect cells. Ethanol stimulated the type II isoform but not the type V isoform. Ethanol stimulated type II adenylyl cyclase greater than GTP gamma S, and the treatment of the membrane with GDP beta S or cholera toxin did not affect this stimulation. Other n-alkanols inhibited type V adenylyl cyclase activity in proportion to their lipophilic potency. In contrast, type II adenylyl cyclase was stimulated by weakly lipophilic n-alkanols and inhibited by strongly lipophilic n-alkanols. When solubilized membranes and purified preparations were used, all the n-alkanols inhibited type II adenylyl cyclase. Our data suggest that n-alkanols regulated adenylyl cyclase isoform-dependently. Stimulation of the type II isoform was independent from the interaction with Gs alpha but required the presence of an intact membrane structure. Our study may provide another step to understanding how membrane protein subtypes are differentially regulated by n-alkanols.

Soluble adenylyl cyclase from Spodoptera frugiperda (Sf9) cells. Purification and biochemical characterization

An insect ovarian cell, Spodoptera frugiperda (Sf9), has been widely used to express recombinant proteins, including adenylyl cyclase, as a host cell in the baculovirus expression system. We report the presence and characterization of a soluble adenylyl cyclase (sAC) distinct from a membrane-bound form of adenylyl cyclase (mAC) that is also present in Sf9 cells. sAC was purified 3,500-fold to near homogeneity; a single band at 25 kDa on SDS-polyacrylamide gel electrophoresis correlated well with adenylyl cyclase catalytic activity. The purified enzyme had a catalytic activity of 0.1 micromol/min.mg and the Km of 0.55 mM for the substrate ATP. In contrast to mAC, sAC was heat-stable. Enzymatic activity of sAC was not stimulated by forskolin and was inhibited by salts at high concentrations. sAC utilized both manganese- and magnesium-ATP as substrate. Di- or triphosphate-containing nucleotides, such as GTP and GDP, as well as pyrophosphate, noncompetitively inhibited sAC. Our data suggest that the physical and biochemical characteristics of sAC are different from those of mAC in Sf9 cells as well as from those of other known forms of adenylyl cyclase in animal cells; sAC in Sf9 cells may constitute a new member of adenylyl cyclase found in animals.

Functional evidence that transmembrane 12 and the loop between transmembrane 11 and 12 form part of the drug-binding domain in P-glycoprotein encoded by MDR1

Human MDR1 encodes an ATP-binding cassette transporter, P-glycoprotein, that mediates multiple drug resistance (MDR) to antitumor agents. It has been previously shown that photoaffinity drug-labeling sites reside within, or near, the last transmembrane loop of each cassette within P-glycoprotein (transmembrane domains (TM) 5-6 and 11-12). A genetic approach was used to determine if the drug-labeling site in the second cassette contains functionally important amino acids. Since human MDR3 is 77% identical to MDR1 but does not mediate MDR, the region from TM10 to the C terminus of MDR1 was replaced with the corresponding sequences from MDR3. The resultant chimeric protein was expressed but not functional. By using progressively smaller replacements, we show that replacements limited to TM12 markedly impaired resistance to actinomycin D, vincristine, and doxorubicin, but not to colchicine. The phenotype was associated with an impaired ability to photoaffinity label the chimeric P-glycoprotein with [125I]iodoaryl azidoprazosin. In contrast, replacement of the loop between TM11 and 12 appears to create a more efficient drug pump for actinomycin D, colchicine, and doxorubicin, but not vincristine. These results suggest that, similar to voltage-gated ion channels, amino acids within and immediately N-terminal to the last transmembrane domain of P-glycoprotein compose part of the drug-binding pocket and are in close proximity to photoaffinity drug-labeling domains.