Erythrocyte membrane ATP binding cassette (ABC) proteins: MRP1 and CFTR as well as CD39 (ecto-apyrase) involved in RBC ATP transport and elevated blood plasma ATP of cystic fibrosis.

In addition to the better-known roles of the erythrocyte in the transport of oxygen and carbon dioxide, the concept that the red blood cell is involved in the transport and release of ATP has been evolving (J. Luthje, Blut 59, 367, 1989; G. R. Bergfeld and T. Forrester, Cardiovasc. Res. 26, 40, 1992; M. L. Ellsworth et al., Am. J. Physiol. 269, H2155, 1995; R. S. Sprague et al., Am. J. Physiol. 275, H1726, 1998). Membrane proteins involved in the release of ATP from erythrocytes now appear to include members of the ATP binding cassette (ABC) family (C. F. Higgins, Annu. Rev. Cell Biol. 8, 67, 1992; C. F. Higgins, Cell 82, 693, 1995). In addition to defining physiologically the presence of ABC proteins in RBCs, accumulating gel electrophoretic evidence suggests that the cystic fibrosis transmembrane conductance regulator (CFTR) and the multidrug resistance-associated protein (MRP1), respectively, constitute significant proteins in the red blood cell membrane. As such, this finding makes the mature erythrocyte compartment a major mammalian repository of these important ABC proteins. Because of its relative structural simplicity and ready accessibility, the erythrocyte offers an ideal system to explore details of the physiological functions of ABC proteins. Moreover, the presence of different ABC proteins in a single membrane implies that interaction among these proteins and with other membrane proteins may be the norm and not the exception in terms of modulation of their functions.

Cellular and biophysical evidence for interactions between adenosine triphosphate and P-glycoprotein substrates: functional implications for adenosine triphosphate/drug cotransport in P-glycoprotein overexpressing tumor cells and in P-glycoprotein low-level expressing erythrocytes.

P-glycoprotein is involved with the removal of drugs, most of them cations, from the plasma membrane and cytoplasm. Pgp is also associated with movement of ATP, an anion, from the cytoplasm to the extracellular space. The central question of this study is whether drug and ATP transport associated with the expression of Pgp are in any way coupled. We have measured the stoichiometry of transport coupling between drug and ATP release. The drug and ATP transport that is inhibitable by the sulfonylurea compound, glyburide (P. E. Golstein, A. Boom, J. van Geffel, P. Jacobs, B. Masereel, and R. Beauwens, Pfluger's Arch. 437, 652, 1999), permits determination of the transport coupling ratio, which is close to 1:1. In view of this result, we asked whether ATP interacts directly with Pgp substrates. We show by measuring the movement of Pgp substrates in electric fields that ATP and drug movement are coupled. The results are compatible with the view that substrates for Pgp efflux are driven by the movement of ATP through electrostatic interaction and effective ATP-drug complex formation with net anionic character. This mechanism not only pertains to drug efflux from tumor cells overexpressing Pgp, but also provides a framework for understanding the role of erythrocytes in drug resistance. The erythrocyte consists of a membrane surrounding a millimolar pool of ATP. Mammalian RBCs have no nucleus or DNA drug/toxin targets. From the perspective of drug/ATP complex formation, the RBC serves as an important electrochemical sink for toxins. The presence in the erythrocyte membrane of approximately 100 Pgp copies per RBC provides a mechanism for eventual toxin clearance. The RBC transport of toxins permits their removal from sensitive structures and ultimate clearance from the organism via the liver and/or kidneys.

Phase I trial of photodynamic therapy in the treatment of recurrent superficial transitional cell carcinoma of the bladder.

OBJECTIVES: A Phase I trial of photodynamic therapy (PDT) in the treatment of superficial transitional cell carcinoma (TCC) of the bladder was performed. METHODS: Twenty patients with recurrent superficial TCC of the bladder after receiving a mean of 2.6 (range 1 to 6) courses of intravesical therapy were treated with PDT. The photosensitizer Photofrin II dose was 1.5 or 2.0 mg/kg. A 630-nm intravesical red laser was used to activate the photosensitizer 2 days after administration of Photofrin II. A 0.01% intralipid solution was used as a bladder-filling medium to scatter light and achieve more homogeneous light distribution. Light doses from 5.1 to 25.6 J/cm2 (total dosage 1500 to 5032 J) were used to illuminate the bladder. RESULTS: Twenty patients underwent 21 treatments with PDT. Complications included asymptomatic reflux in 4 patients. One other patient, treated at the highest total light dose, experienced bladder contraction and fibrosis. Nine patients (45%) had no tumor evident at cystoscopy, on random biopsies, or in urinary cytology at the 3-month evaluation after treatment. Four patients remained without recurrent disease for 23 to 56 months. Sixteen of 20 (80%) patients experienced recurrence, and 8 of the 16 underwent cystectomy. CONCLUSIONS: An intravenous photosensitizer dose of 1.5 mg/kg Photofrin II followed by light energy in the range of 13 J/cm2 (total light dose 2500 to 3250 J) was defined as a safe treatment parameter and resulted in tumor responses. With present technologies, administration of PDT requires careful dosimetry.

Cystic fibrosis hetero- and homozygosity is associated with inhibition of breast cancer growth.

Cystic fibrosis (CF) is the most common lethal recessive genetic disease of the Caucasian population. Although reports of cancer frequency in CF have emphasized an elevated observed-to-expected ratio of 6.5 for digestive tract cancers, these studies also show a significantly decreased observed-to-expected ratio for other malignancies including breast cancer. The cystic fibrosis transmembrane conductance regulator (CFTR) functions as an ATP channel. We found that heterozygous and homozygous CFTR knockout mice had elevated blood ATP concentrations. Elevated extracellular ATP is known to inhibit tumor growth in vivo and in vitro. Using double mutant mice created by F2 generation crosses of CFTR knockout and nude mice, we observed reduced breast tumor implantability in CFTR homozygous nude animals. Decreased tumor growth rate was observed in both CFTR heterozygous and homozygous nude animals. Extracellular ATP reduced human breast tumor cell growth rate in vitro, and a breast tumor transfected with human CFTR that had high extracellular ATP concentrations in vitro correspondingly had a slower growth rate in vivo. The results suggest that both CFTR heterozygosity and homozygosity suppress breast cancer growth and that elevated extracellular ATP can account for this phenomenon.

Effect of intraperitoneal ATP on tumor growth and bone marrow radiation tolerance.

Transplants of a spontaneous murine fibrosarcoma (FSaII) treated with intraperitoneal ATP were studied in vitro, and in both C3H and nu/nu mice. Daily ATP treatment prolonged tumor volume doubling time in vivo and in vitro. Daily ATP treatments at the maximally tolerated dose (2 mmol/kg i.p.) did not significantly affect the pH or the PCr/Pi, or beta ATP/Pi ratios (measured by MRS). In contrast to the reduced tumor growth rate, there was no change in bone marrow recovery after whole body irradiation. ATP is minimally toxic to animals at active dose levels. It slows tumor growth rate without adversely affecting bone marrow radiation tolerance. ATP might therefore be useful as a biological modifier of chemotherapy or radiation therapy.

Basic fibroblast growth factor radioprotects bone marrow and not RIF1 tumor.

We examined the potential of bFGF to function as a radioprotector of bone marrow (BM). Total intravenous doses of bFGF ranged from 1 to 24 micrograms/mouse, in 2 divided doses. Whole body radiation (WBI) was given in a single fraction to C3H mice. Histologic observations were performed on femur BM at various times after bFGF (or placebo) treatment. Thigh radiation in thigh-tumor bearing mice was delivered in a single fraction. bFGF increased the LD50/30 of mice in a dose dependent fashion, with an apparent maximum protection obtained with > or = 6 micrograms given half 24 h and half 4 h before irradiation. BM histology shows prominent recovery of megakaryocytes and all cell lineages along with less loss in cellularity compared to control irradiated animals. No radioprotection of RIF1 tumors after bFGF was detected. These results indicate that bFGF may be a selective radioprotector of normal tissue.

The cystic fibrosis transmembrane conductance regulator is a dual ATP and chloride channel.

The cystic fibrosis transmembrane conductance regulator (CFTR) belongs to a superfamily of proteins implicated in the transport of ions, proteins, and hydrophobic substances. Recent studies have demonstrated that CFTR is a protein kinase A-sensitive anion channel regulated by ATP. In the present study, patch-clamp techniques were used to assess the role of CFTR in the transport of Cl- and ATP. The stable transfection of mouse mammary carcinoma cells, C127i, with the cDNA for human CFTR resulted in the appearance of a diphenylamine-2-carboxylate-inhibitable Cl- channel, which was activated by cAMP under whole-cell and cell-attached conditions and by protein kinase A plus ATP under excised, inside-out conditions. CFTR expression was also associated with the electrodiffusional movement of ATP as indicated by the cAMP activation of ATP currents measured under whole-cell conditions. In excised, inside-out patches, it was demonstrated that ATP currents were mediated by ATP-conductive channels, which were also activated by protein kinase A and blocked by the Cl- channel blocker diphenylamine-2-carboxylate under excised, inside-out conditions. Single-channel currents observed in the presence of asymmetrical Cl-/ATP concentrations indicated that the same conductive pathway was responsible for both ATP and Cl- movement. Thus, CFTR is a multifunctional protein with more than one anion transport capability and may modify signal transduction pathways for Cl- or other secretory processes by the selective delivery of nucleotides to the extracellular domain.

External ATP and its analogs activate the cystic fibrosis transmembrane conductance regulator by a cyclic AMP-independent mechanism.

The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl- channel activated by protein kinase A and regulated by ATP in a complex manner. We have applied patch-clamp techniques to C127i mouse mammary carcinoma cells transfected with human CFTR to assess the role of external ATP in the modulation of CFTR function. Extracellular ATP was sufficient to activate non-rectifying, Cl(-)-selective whole-cell currents in CFTR-transfected, but not mock-transfected cells. The ATP-mediated activation of CFTR was independent of protein kinase A since channel activation by ATP was preserved in cells that were (a) depleted of intracellular ATP, (b) incubated with the cAMP antagonist Rp-cAMPS, or (c) exposed to the protein kinase A inhibitor, 5-24 amide. In each of these conditions, 8-Br-cAMP was no longer capable of activating CFTR. The possibility that the extracellular ATP activation of Cl- currents in CFTR-expressing C127i cells was mediated by a P2-type purinergic receptor was supported by studies in which the effect of external ATP on the Cl- currents was mimicked by the ATP analogs, ATP gamma S and beta,gamma-methylene ATP, but not the uridine nucleotide, UTP. Single-channel analysis of ATP-activated Cl -currents under both cell-attached and excised, inside-out patch-clamp configurations indicated that this channel is only present in CFTR-transfected cells and indistinguishable from CFTR. External ATP also activated ATP currents in CFTR-transfected cells, a novel function of CFTR. These findings are consistent with the presence of a purinergic receptor signal transduction mechanism in C127i cells whose activation by external ATP is linked to the activation of CFTR in a cAMP-independent manner. The data provide additional support for the use of ATP and its analogs as alternative therapies in cystic fibrosis.

The biology of radioresistance: similarities, differences and interactions with drug resistance.

Cells and tissues have developed a variety of ways of responding to a hostile environment, be it from drugs (toxins) or radiation (summarized in Fig. 1). Three categories of radiation damage limitation are: (i) DNA repair (ii) changes in cellular metabolism (iii) changes in cell interaction (cell contact or tissue-based resistance; whole organism based resistance). DNA repair has been evaluated predominantly by the study of repair-deficient mutants. The function of the repair genes they lack is not fully understood, but some of their important interactions are now characterized. For example, the interaction of transcription factors with nucleotide excision repair is made clear by the genetic syndromes of xeroderma-pigmentosum groups B, D and G. These diseases demonstrate ultraviolet light sensitivity and general impairment of transcription: they are linked by impaired unwinding of the DNA required for both transcription and repair. The transfer of DNA into cells is sometimes accompanied by a change in sensitivity to radiation, and this is of special interest when this is the same genetic change seen in tumors. DNA repair has a close relationship with the cell cycle and cell cycle arrest in response to damage may determine sensitivity to that damage. DNA repair mechanisms in response to a variety of drugs and types of radiation can be difficult to study because of the inability to target the damage to defined sequences in vivo and the lack of a satisfactory substrate for in vitro studies. Changes in cellular metabolism as a result of ionizing radiation can impart radiation resistance, which is usually transient in vitro, but may be more significant in vivo for tissues or tumors. The mechanisms by which damage is sensed by cells is unknown. The detection of free radicals is thought likely, but distortion to DNA structure or strand breakage and a direct effect on membranes are other possibilities for which there is evidence. Changes in extracellular ATP occur in response to damage, and this could be a direct membrane effect. External purinergic receptors can then be involved in signal transduction pathways resulting in altered levels of thiol protection or triggering apoptosis. Changes in the functional level of proteins as a consequence of ionizing radiation include transcription factors, for example c-jun and c-fos; cell cycle arrest proteins such as GADD (growth arrest and DNA damage inducible proteins) and p53; growth factors such as FGF, PDGF; and other proteins leading to radioresistance.(ABSTRACT TRUNCATED AT 400 WORDS)

The multidrug resistance (mdr1) gene product functions as an ATP channel.

The multidrug resistance (mdr1) gene product, P-glycoprotein, is responsible for the ATP-dependent extrusion of a variety of compounds, including chemotherapeutic drugs, from cells. The data presented here show that cells with increased levels of the P-glycoprotein release ATP to the medium in proportion to the concentration of the protein in their plasma membrane. Furthermore, measurements of whole-cell and single-channel currents with patch-clamp electrodes indicate that the P-glycoprotein serves as an ATP-conducting channel in the plasma membrane. These findings suggest an unusual role for the P-glycoprotein.

Biopsy after external beam radiation therapy for adenocarcinoma of the prostate: correlation with original histological grade and current prostate specific antigen levels.

We obtained post-irradiation biopsies in 37 men with initially stage T3 prostatic adenocarcinoma treated by external beam radiotherapy. Eligibility for post-irradiation biopsy included no clinical local failure, interval since treatment of 24 months or more and no endocrine therapy. Of the 37 patients 23 (62%) had negative biopsies while 14 (38%) had positive biopsies. Of 23 patients whose original cancer was well or moderately differentiated 18 (78%) had negative biopsies, compared to only 5 of 14 (36%) of those with poorly differentiated cancer (p less than 0.03). Among 19 patients whose current serum prostate specific antigen (PSA) value is less than 2.5 ng./ml. 15 (79%) had negative biopsies, compared to only 4 of 14 (29%) with a PSA level of greater than 2.5 ng./ml. (p less than 0.02). These results strongly suggest that there is a low probability of positive post-irradiation biopsy regardless of its significance in men with a normal prostate by palpation and a normal serum PSA value. However, short followup since biopsy precludes analysis of the predictive value of post-irradiation biopsy on long-term local and distant disease status.

Development of intercellular communication during the epithelial reorganization of a renal cell line (LLC-PK1).

Junctional permeability determinations after microinjection of the fluorescent tracer, Lucifer Yellow CH, show that the cells in confluent monolayers of the renal epithelial cell lines LLC-PK1 and A6 are interconnected by intercellular junctions. This cell-to-cell communication network permits the fluorescent dye to diffuse from the microinjected cell into multiple adjacent neighboring cells. Cell-to-cell diffusion of the fluorescent dye was not observed at pH 6.0. Full recovery occurred, however, when the pH of the extracellular medium was adjusted to 7.4. To provide a sensitive index of the averaged efficacy of junctional communication, we measured the number of cells that survived ouabain treatment in a 50% mixture of wild and ouabain-resistant mutant LLC-PK1 cells. Electron probe microanalysis in uncoupled cells showed that ouabain treatment produced two populations of cells, with totally different intracellular Na+ and K+ content. Under this condition, only 50% of the population survived after 48 h of treatment. When ouabain treatment was initiated 24 h after plating, however, 100% survival was observed, and the cells contained uniform intracellular Na+ and K+ concentration. This finding is consistent with the theory that this protective effect is mediated through the presence of the functional communicating intercellular junctions. When ouabain was applied at different times after plating, full protection is reached by 2 h. The early development of cell-to-cell communication, which precedes the development of the occluding junctions and several transport systems by several hours, is consistent with the involvement of the intercellular junctions in the synchronization of the polarization process.

Preparation of individual human diploid fibroblasts and study of ion transport.

A method for analyzing individual mammalian cells with electron probe microanalysis has been developed using human diploid fibroblasts. Cells were grown on the same support that is used for experimental manipulations and analysis. Steady-state cation and anion concentrations and kinetic processes during experimental perturbations could be measured on populations of less than 1,000 cells. Human diploid fibroblasts in normal tissue culture medium had the following intracellular concentrations (in mM): K, 168; Na, 25.0; Cl, 51.2; P, 84.1; S, 16.5; Ca, 6.04; and Mg, 10.0. The ratios of K to Na were equivalent when measured in the nuclear or cytoplasmic area of the cells. Serum in the incubation medium was found to increase the cellular effective permeability to Na by a factor of 2.5, while leaving the effective permeability to K unchanged. When returned to control medium after 7 h of incubation in K-free medium, the cells recovered normal K/Na in less than 1 h. In some experiments the coupling ratio of the ouabain-inhibitable cellular transport of Na to K was 3:2 and the ratio of Cl to K was 1:2. The sum of intracellular content (Na + K) (an estimate of cellular volume) did not change when the cells were placed in K-free medium and increased by less than 30% after ouabain treatment. After 5-7 h of ouabain treatment or of incubation in K-free medium, long after the intracellular K had been replaced by Na, the cellular chloride content had not changed significantly.