The effect of Mg2+, nucleotides and ATPase inhibitors on the uptake of [3H]-cGMP to inside-out vesicles from human erythrocytes

Theoretical basis, experimental design, and computerized simulation of synergism and antagonism in drug combination studies

The median-effect equation derived from the mass-action law principle at equilibrium-steady state via mathematical induction and deduction for different reaction sequences and mechanisms and different types of inhibition has been shown to be the unified theory for the Michaelis-Menten equation, Hill equation, Henderson-Hasselbalch equation, and Scatchard equation. It is shown that dose and effect are interchangeable via defined parameters. This general equation for the single drug effect has been extended to the multiple drug effect equation for n drugs. These equations provide the theoretical basis for the combination index (CI)-isobologram equation that allows quantitative determination of drug interactions, where CI < 1, = 1, and > 1 indicate synergism, additive effect, and antagonism, respectively. Based on these algorithms, computer software has been developed to allow automated simulation of synergism and antagonism at all dose or effect levels. It displays the dose-effect curve, median-effect plot, combination index plot, isobologram, dose-reduction index plot, and polygonogram for in vitro or in vivo studies. This theoretical development, experimental design, and computerized data analysis have facilitated dose-effect analysis for single drug evaluation or carcinogen and radiation risk assessment, as well as for drug or other entity combinations in a vast field of disciplines of biomedical sciences. In this review, selected examples of applications are given, and step-by-step examples of experimental designs and real data analysis are also illustrated. The merging of the mass-action law principle with mathematical induction-deduction has been proven to be a unique and effective scientific method for general theory development. The median-effect principle and its mass-action law based computer software are gaining increased applications in biomedical sciences, from how to effectively evaluate a single compound or entity to how to beneficially use multiple drugs or modalities in combination therapies.

cGMP (guanosine 3′,5′-cyclic monophosphate) transport across human erythrocyte membranes

Human erythrocytes produce cGMP that can be eliminated by phosphodiesterases or active efflux transporters. The efflux can be studied under controlled conditions as ATP-dependent uptake into inside-out membrane vesicles. However, widely differing values for the transport rates have been reported. We have here examined factors that influence the uptake rates measured and thus may explain these discrepancies. Both the ionic composition of the buffer used during uptake and the mode of vesicle preparation were found to affect the observed transport rates. Furthermore it was apparent that different blood donors expressed on their erythrocytes different amounts of both MRP4 and MRP5, transporters that have been putatively linked to cGMP efflux across erythrocyte membranes. These differences in expression were reflected in differences in rates of cGMP uptake into inside-out erythrocyte membrane vesicles. Calculations based on the transport rates observed using vesicles suggest that efflux may be the principal means for eliminating cGMP from human erythrocytes.

Cyclic GMP transporters

The biokinetics of guanosine 3',5'-cyclic monophosphate (cGMP) is characterized by three distinct processes: synthesis by guanylate cyclases (GCs), conversion of cGMP to GMP by cyclic nucleotide phosphodiesterases (PDEs) and the excretion of unchanged cGMP by transport proteins in the cell membrane. Efflux is observed in virtually all cell types including cells which originate from brain. Studies of intact cells, in which metabolic inhibitors and probenecid reduced extrusion of cGMP and wherein cGMP was extruded against concentration gradients, indicated the existence of ATP requiring organic anion transport system(s). Functional studies of inside-out vesicles have revealed cGMP transport systems wherein translocation is coupled to hydrolysis of ATP. The extrusion of cGMP is inhibited by a number of unrelated compounds and this indicates that cGMP is substrate for multispecific transporters. Recent transfection studies suggest that members of the MRP (multidrug resistance protein) family; MRP4, MRP5 and MRP8 translocate cGMP across the cell membrane. Many of the MRPs have been detected in brain. In addition tertiary active transport by the organic anion transporter family has also been identified. At least one member (OAT1) shows relative high affinity for cGMP and is also expressed in brain. The biological significance of cGMP transporters has to be clarified. Their role in cGMP biokinetics, being responsible for one of the cellular elimination pathways, is well established. However, there is growing evidence that extracellular cGMP has effects on cell physiology and pathophysiology by an auto- or paracrine mechanism.

Reconstitution of ATP-dependent cGMP transport into proteoliposomes by membrane proteins from human erythrocytes

The cellular efflux of cGMP from human erythrocytes has previously been characterized in functional studies. The purpose of the present study was to find membrane proteins with the ability to restore ATP-dependent uptake of cGMP into proteoliposomes. Human erythrocyte membranes were solubilized with CHAPS (3-([3-cholamidopropyl]dimethylammonio)-1-propanesulfonate) and gel filtration gave three protein fractions with the ability to restore active transport. Only two of these fractions were retained on a lentil lectin column. By using these two purification steps, active transport was 11 times higher in the first fraction compared to the original material and SDS-PAGE showed the presence of proteins with sizes of 145 kDa and 165 kDa. The second fraction gave 20 times higher active transport after purification and comprised proteins with sizes of 145 kDa and 180 kDa. At present three members of the MRP (multi-resistance associated protein) family have been detected in human erythrocytes: MRPI, MRP4 and MRP5. The last two proteins have been shown to transport cyclic nucleotides. The present findings are compatible with MRP4 as the 145 kDa protein, MRP5 as the 165 kDa protein and MRP1 as the 180 kDa protein. However, the 145 kDa protein could also be SMRP (short multi-resistance protein), the gene splice variant of MRP5. Immunoprecipitation of MRP5 from CHAPS-solubilized extract reduced active transport and specific binding by about 45% and 40%, respectively. This shows that MRP5 is an important cGMP-transporting protein in human erythrocytes.

cGMP and glutathione-conjugate transport in human erythrocytes

The nature of cGMP transport in human erythrocytes, its relationship to glutathione conjugate transport, and possible mediation by multidrug resistance-associated proteins (MRPs) have been investigated. MRP1, MRP4 and MRP5 are detected in immunoblotting studies with erythrocytes. MRP1 and MRP5 are also detected in multidrug resistant COR-L23/R and MOR/R cells but at greatly reduced levels in the parent, drug sensitive COR-L23/P cells. MRP4 is detected in MOR/R but not COR-L23/R cells. Uptake of cGMP into inside-out membrane vesicles prepared by a spontaneous, one-step vesiculation process is shown to be by a low affinity system that accounts for more than 80% of the transport at all concentrations above 3 micro m. This transport is reduced by MRP inhibitors and substrates including MK-571, methotrexate, estradiol 17-beta-d-glucuronide, and S(2,4-dinitrophenyl)glutathione (DNP-SG) and also by glibenclamide and frusemide but not by the monoclonal Ig QCRL-3 that inhibits high-affinity transport of DNP-SG by MRP1. It is concluded that the cGMP exporter is distinct from MRP1 and has properties similar to those reported for MRP4. Furthermore the evidence suggests that the protein responsible for cGMP transport is the same as that mediating low-affinity DNP-SG transport in human erythrocytes.

Pharmacological characterization of the ATP-dependent low K(m) guanosine 3',5'-cyclic monophosphate (cGMP) transporter in human erythrocytes

The efflux pump for cGMP has been shown to be an ATP-energized multiorganic anion transporter. The present study was performed to extend the knowledge of the pharmacological characteristics of this efflux pump. Inside-out vesicles prepared from fresh blood were incubated with [3H]-cGMP (1 microM) with or without various concentrations of competitors for 120min at 37 degrees. The tested compounds could be divided in four groups: one with high affinity (K(i) < 5 microM), a second with moderate affinity (K(i): 5-50 microM), a third with low affinity (K(i): 0.1-5mM) and the fourth with extremely low or no affinity at all. With the mean K(i)-values given in parenthesis, the high affinity group consisted of mifepristone (0.2 microM), zaprinast (0.35 microM), dipyridamole (0.35 microM), estradiol 3-beta-glucuronide (0.42 microM), genistein (0.43 microM), estradiol 17-beta-glucuronide (0.47 microM), onapristone (1.3 microM), progesterone (1.7 microM) and sildenafil (3.6 microM). The inhibitors with medium affinity were estradiol (8 microM), sulfinpyrazone (13 microM), daunorubicin (23 microM), megestrol acetate (26 microM), doxorubicin (28 microM), 6-thioguanine (28 microM) and 6-thioguanosine-5'-monophosphate (32 microM). The low affinity group comprised 6-TIMP (220 microM), 6-methylmercaptopurine (MMP) (220 microM), vincristine (270 microM), medroxyprogesterone (680 microM), para-aminohippurate (PAH) (1.9mM) and taurocholate (2.2mM). No or minimal effect was seen in the presence of 6-mercaptopurine (6-MP), methotrexate, 9-(2-phosphonylmethoxyethyl)adenine and mitoxantrone. The cGMP transporter had a unique pharmacological profile, different from that of MRP1, but with some characteristics in common with MRP4 and MRP5.

Transport of cyclic nucleotides and estradiol 17-beta-D-glucuronide by multidrug resistance protein 4. Resistance to 6-mercaptopurine and 6-thioguanine

Human multidrug resistance protein 4 (MRP4) has recently been determined to confer resistance to the antiviral purine analog 9-(2-phosphonylmethoxyethyl)adenine and methotrexate. However, neither its substrate selectivity nor physiological functions have been determined. Here we report the results of investigations of the in vitro transport properties of MRP4 using membrane vesicles prepared from insect cells infected with MRP4 baculovirus. It is shown that expression of MRP4 is specifically associated with the MgATP-dependent transport of cGMP, cAMP, and estradiol 17-beta-D-glucuronide (E(2)17 beta G). cGMP, cAMP, and E(2)17 beta G are transported with K(m) and V(max) values of 9.7 +/- 2.3 microm and 2.0 +/- 0.3 pmol/mg/min, 44.5 +/- 5.8 microm and 4.1 +/- 0.4 pmol/mg/min, and 30.3 +/- 6.2 microm and 102 +/- 16 pmol/mg/min, respectively. Consistent with its ability to transport cyclic nucleotides, it is demonstrated that the MRP4 drug resistance profile extends to 6-mercaptopurine and 6-thioguanine, two anticancer purine analogs that are converted in the cell to nucleotide analogs. On the basis of its capacity to transport cyclic nucleotides and E(2)17 beta G, it is concluded that MRP4 may influence diverse cellular processes regulated by cAMP and cGMP and that its substrate range is distinct from that of any other characterized MRP family member.

Inhibition by guanosine cyclic monophosphate (cGMP) analogues of uptake of [(3)H]3',5'-cGMP without stimulation of ATPase activity in human erythrocyte inside-out vesicles

The cellular extrusion of guanosine 3',5'-cyclic monophosphate (3',5'-cGMP) is a unidirectional ATP-dependent process that is inhibited by probenecid, a non-selective transport inhibitor of organic anions. In the present study, various cGMP analogues were tested for their ability to inhibit 3',5'-cGMP efflux and stimulate the cGMP-selective ATPase in human erythrocytes. The difference in uptake of 1 microM [(3)H]3',5'-cGMP to inside-out vesicles in the presence and absence of 1 mM ATP at 37 degrees was defined as active transport. Two ATP-dependent components were detected for unlabelled 3',5'-cGMP (0.01--100 microM) with respective K(i) of 1.3 +/- 0.2 and 280 +/- 50 microM (mean +/- SEM, N = 3). The high-affinity transport was inhibited by the analogues with a typical pattern: Rp-monophosphorothioate guanosine 3',5'-cyclic monophosphate (Rp-cGMPS) > 3',5'-cGMP > 2'-O-monobutyryl guanosine 3',5'-cyclic monophosphate (O-mb-cGMP) approximately N(2)-monobutyryl guanosine 3',5'-cyclic monophosphate (N-mb-cGMP) > or = N(2),2'-O-dibutyryl guanosine 3',5'-cyclic monophosphate (Db-cGMP) approximately 8'-bromo guanosine 3',5'-cyclic monophosphate (Br-cGMP) approximately Guanosine 2',3'-cyclic monophosphate (2'3'-cGMP) > Sp-monophosphorothioate guanosine 3',5'-cyclic monophosphate (Sp-cGMPS). A concentration-dependent inhibition was found for the low-affinity transport, but no distinct order of potency was identified. Analysis according to Lineweaver--Burk of active [(3)H]3',5'-cGMP transport (0.2--2 microM) gave a K(m) value of 1.5 +/- 0.1 microM (mean +/- SEM, N = 3). The presence of 10 microM cGMP analogues did not change the ordinate intercept, but made the slopes steeper with a typical order: Rp-cGMPS > 3',5'-cGMP > N-mb-cGMP approximately O-mb-cGMP approximately db-cGMP approximately 8-Br-cGMP > 2',3'-cGMP > Sp-cGMPS. Only 3',5'-cGMP and 2',3'-cGMP were able to activate the cGMP-specific ATPase, 640 +/- 200% and 430 +/- 160% (mean +/- SEM, N = 5) above basal levels, respectively. The present data show that the binding is less selective than ATPase activation of the cellular cGMP transport system.

ATPase activity and transport by a cGMP transporter in human erythrocyte ghosts and proteoliposome-reconstituted membrane extracts

We previously described the [(3)H]cGMP-binding characteristics of a CHAPS-solubilized protein that we proposed to be a cGMP transporter. We now report the ATPase activity of the membrane-bound, solubilized and reconstituted form of a cGMP transporter. The membrane-bound protein of unsealed ghosts had a linear ATPase activity over a 120 min incubation period with optimal activity of about 400 pmol/mg/min. The apparent K(m) and V(max) for ATP were about 0.5 mM and 300 pmol/mg/min, respectively. When solubilized with CHAPS the specific activity of the protein was reduced to about 70 pmol/mg/min. Reconstitution of the CHAPS preparation into phospholipid bilayer using rapid detergent removal by Extracti-gel column resulted in proteoliposomes which had ATPase activity similar to that found in the erythrocyte membranes. The proteoliposomes displayed a linear ATP-dependent uptake of [(3)H]cGMP with an apparent K(m) value of 1. 0 microM. This low K(m)-uptake of [(3)H]cGMP in proteoliposomes was not affected by 10 microM of AMP, cAMP and GMP, but was completely abolished in the presence of the non-hydrolyzable ATP analogue, ATP-gamma-S. Some ATPase activation was also observed in the presence of 2 microM cAMP, but it is unclear whether this activity was coupled to the cGMP transporter. Our results show that the membrane protein responsible for cGMP transport has an ATPase activity and transports the cyclic nucleotide in the presence of ATP.

Leukotriene C(4) (LTC(4)) does not share a cellular efflux mechanism with cGMP: characterisation of cGMP transport by uptake to inside-out vesicles from human erythrocytes

The transport of cGMP out of cells is energy requiring and has characteristics compatible with an ATP-energised anion pump. In the present study a model with inside-out vesicles from human erythrocytes was employed for further characterisation of the cGMP transporter. The uptake of leukotriene C(4) (LTC(4)), a substrate for multidrug resistance protein (MRP), was concentration-dependently inhibited by the leukotriene antagonist MK571 (IC(50)=110+/-20 nM), but cGMP was unable to inhibit LTC(4) uptake. Oxidised glutathione (GSSG) and glutathione S-conjugates caused a concentration-dependent inhibition of [(3)H]cGMP uptake with IC(50) of 2200+/-700 microM for GSSG, 410+/-210 microM for S-(p-nitrobenzyl)glutathione and 37+/-16 microM for S-decylglutathione, respectively. Antioxidants such as reduced glutathione and dithiothreitol did not influence transport for concentrations up to 100 microM, but both inhibited cGMP uptake with approx. 25% at 1 mM. The cGMP pump was sensitive to temperature without activity below 20 degrees C. The transport of cGMP was dependent on pH with maximal activity between pH 8.0 and 8.5. Calcium caused a concentration-dependent inhibition with IC(50) of 43+/-12 microM. Magnesium gave a marked activation in the range between 1 and 20 mM with maximum effect at 10 mM. The other divalent cations, Mn(2+) and Co(2+), were unable to substitute Mg(2+), but caused some activation at 1 mM. EDTA and EGTA stimulated cGMP transport concentration-dependently with 50% and 100% above control at 100 microM, respectively. The present study shows that the cGMP pump has properties compatible with an organic anion transport ATPase, without affinity for the MRP substrate LTC(4). However, the blockade of the cGMP transporter by glutathione S-conjugates suggests it is one of several GS-X pumps.