Membrane function in cystic fibrosis. II. Methionine transport in normal and cystic fibrosis fibroblasts

Initial rate kinetics of methionine transport, time course of accumulation of methionine, and efflux of accumulated methionine were studied in three normal and four CF human diploid fibroblast strains. The range of apparent Km's was 12.7-32.1 micrometer for the CF strains and 18.3-39.2 micrometer for the normal strains. The range of apparent Vmax's was 6.69-9.22 nmole mg-1 min-1 for the CF strains and 5.59-7.87 nmole mg-1 min-1 for the normal strains. The patterns of accumulation and efflux are quite similar in all the strains studied except for WI-38, which showed somewhat higher efflux and lower accumulation than for others. There was no significant difference in the kinetic parameters of methionine transport between CF and normal skin fibroblasts, and methionine transport will not serve as a marker for cystic fibrosis in cultured fibroblasts.     

Membrane function in cystic fibrosis. I. Putrescine transport in normal and cystic fibrosis fibroblasts

Putrescine transport was examined in normal and cystic fibrosis fibroblasts. No differences were observed in accumulation pattern, kinetics of uptake, or efflux between CF and normal cells. In both growing and growth-arrested CF and normal fibroblasts, exogenously supplied putrescine remained unchanged for at least 60 min. Some differences were observed in the response of CF and normal cells to environmental (media) changes.This research was supported by a grant from the Cystic Fibrosis Foundation and by a grant from the National Institutes of Health, Training Grant (GM01316 11 GNC).     

Protein methylation in normal and cystic fibrosis fibroblasts

Human-cultured fibroblasts contain protein methylase activities. These activities were determined and the enzymatic products were identified after acid hydrolysis of the protein substrate for protein methylases I (arginine) and III (lysine) and by organic solvent extraction of the methanol produced by alkaline treatment of the protein substrate (for the protein methylase II). A methylation of histidine residues of proteins occurs in cultured fibroblasts. Protein methylase activities were unmodified in the cystic fibrosis fibroblasts as compared to the control cells.     

Membrane fluidity in normal and cystic fibrosis fibroblasts

We have observed distinct differences in the polarization of fluorescence and temperature dependent emission intensity of the highly fluorescent phospholipid derivative (1-acyl-2-(N-4-nitrobenzo-2-oxa-1,3-diazole)--aminocaproyl phosphatidylcholine (NBD-PC), when incorporated in the plasma membranes of normal and cystic fibrosis fibroblasts. Fluorescence polarization measurements indicate that the fluorochrome has a much higher degree of rotational mobility in cystic fibrosis fibroblasts as compared with normal cells. Temperature dependent transitions in the emission intensity of NBD-PC incorporated in normal fibroblasts are indicated at 17.7 and 21.2° C while the abnormal cell membranes apparently undergo transitions at 8.7 and 13.5° C. These differences might be due to changes in plasma membrane composition and/or organization, in the case of the cystic fibrosis cells.     

Actin and tubulin in normal and cystic fibrosis fibroblasts

Actin and tubulin contents of early passage, confluent human fibroblast cultures have been determined. Actin comprised 5.87 ± 0.81% of the total protein of IMR-90 fibroblasts which was not significantly different than the actin contents of two cystic fibrosis fibroblast cultures GM0142 and GM1348 (5.64 ± 0.90% of total protein). However, a significant difference between the amount of tubulin in IMR-90 fibroblasts (7.17 ± 0.25% of total protein) and the amount of tubulin in cystic fibrosis fibroblasts (4.51 ± 0.64% of total protein) was found.     

Taurine uptake by normal and cystic fibrosis fibroblasts

Taurine deficiency recently has been proposed to be clinically significant in cystic fibrosis (CF). Uptake of [14C]taurine by four cystic fibrosis (CF) and three control fibroblast lines was examined to determine whether a generalized defect in taurine transport could contribute to the deficiency. The time course of uptake was linear up to 20 h and was similar in both CF and control fibroblasts. Taurine was avidly retained after uptake, and the effect of metabolic (chlorpromazine) and competitive (hypotaurine, L-leucine) inhibitors was similar in both CF and control cells. In contrast, while taurine uptake in a calcium-free medium was impaired in both CF and control fibroblasts, the impairment was significantly less in CF cells. The findings suggest that a generalized abnormality in taurine transport is unlikely to be responsible for the taurine deficiency in CF.     

X-ray microanalysis of cAMP-induced ion transport in cystic fibrosis fibroblasts.

cAMP-induced ion transport in normal and cystic fibrosis (CF) fibroblasts was investigated by X-ray microanalysis. Stimulation with cAMP causes an increase in cellular Na content and a decrease in cellular Cl and K content. No significant difference in response between CF and normal cells was noted. In this respect, fibroblasts differ from epithelial cells, where cAMP-induced Cl- efflux blocked in CF patients. Isoproterenol produced similar changes in Na and K content as cAMP, but did not effect Cl content.     

Regulation of chloride transport in cultured normal and cystic fibrosis keratinocytes.

Cultured normal (N) cystic fibrosis (CF) keratinocytes were evaluated for their Cl(-)-transport properties by patch-clamp-, Ussing chamber- and isotopic efflux-measurements. Special attention was paid to a 32 pS outwardly rectifying Cl- channel which has been reported to be activated upon activation of cAMP-dependent pathways in N, but not in CF cells. This depolarization-induced Cl- channel was found with a similar incidence in N and CF apical keratinocyte membranes. However, activation of this channel in excised patches by protein kinase (PK)-A or PK-C was not successful in either N or CF keratinocytes. Forskolin was not able to activate Cl- channels in N and CF cell-attached patches. The Ca(2+)-ionophore A23187 activated in cell-attached patches a linear 17 pS Cl- channel in both N and CF cells. This channel inactivated upon excision. No relationship between the cell-attached 17 pS and the excised 32 pS channel could be demonstrated. Returning to the measurement of Cl- transport at the macroscopic level, we found that a drastic rise in intracellular cAMP induced by forskolin did in N as well as CF cells not result in a change in the short-circuit current (Isc) or the fractional efflux rates of 36Cl- and 125I-. In contrast, addition of A23187 resulted in an increase of the Isc and in the isotopic anion efflux rates in N and CF cells. We conclude that Cl(-)-transport in cultured human keratinocytes can be activated by Ca2+, but not by cAMP-dependent pathways.     

Glycosyl acceptors in intact and permeabilized normal and cystic fibrosis fibroblasts

Using cell permeabilization, a technique which allows addition of exogenously supplied radiolabeled sugar nucleotides to serve as direct glycosyl donors, oligosaccharide biosynthesis was examined in fibroblasts obtained from normal and cystic fibrosis (CF) subjects. Incubation of logarithmically growing cells with either radiolabeled leucine or xylose has indicated that there was a difference in the synthetic rate between the cell types. Protein synthesis in normal cells made permeable with 50 m?g/ml lysolecithin (LL) was demonstrated to be absent, and could not be induced to take place by adding exogenous components, including energy sources and amino acids, normally required for protein synthesis. Thus radiolabeled sugars were being added to peptide acceptors which were already present at the time of LL addition. Both permeable and intact fibroblasts were exposed to labeled UDP-xylose, UDP-galactose, and UDP-glucuronic acid, all donors of mucopolysaccharide precursors. The uptake of xylose into protein was the same for both normal and CF cells, but permeable CF fibroblasts incorporated statistically greater amounts of sugar from UDP-galactose and UDP-glucuronic acid. Intact CF cells were also labeled using these two sugar nucleotides. Trypan blue exclusion indicated CF and normal fibroblasts were equally intact. This and the fact that preincubation of CF cells with the appropriate cold sugar nucleotide eliminated the differences in incorporation between the normal and CF cells suggested that CF fibroblasts had more cell surface acceptor than the normal cells.     

Kinetic analysis of chloride efflux from normal and cystic fibrosis fibroblasts

Chloride permeability in 9 cystic fibrosis- and 11 normal-skin fibroblast lines has been investigated. Chloride efflux, under steady-state conditions, involves two intracellular compartments characterized by slow- and fast-rate constants of efflux. We show here that the fast rate constant in cystic fibrosis cells is reduced by 25% in comparison with controls. The data presented support recent studies indicating that isolated sweat glands and respiratory epithelia of patients suffering from cystic fibrosis have an unusual low permeability to chloride ions compared to control epithelia. It is concluded that variation in chloride transport can successfully be studied in cultured fibroblasts, which are not directly involved in the pathology of the disease.     

The response of chloride transport to cyclic AMP, calcium and hypotonic shock in normal and cystic fibrosis fibroblasts

It has widely been established that Cl- transport is defective in cystic fibrosis fibroblasts. In the present study, the effect of elevation of intracellular concentration of cyclic AMP and calcium on the efflux of Cl- from human fibroblasts has been investigated. Cyclic AMP analogs (8-bromo cAMP and dibutyryl cAMP) and a beta agonist (isoproterenol) induced only a weak stimulation (5-10%) of Cl- efflux. Conversely, elevation of cytoplasmic calcium concentration produced by addition of the Ca2+ ionophore A23187 in the efflux medium, did not affect Cl- efflux. Our data indicate that the response of Cl- efflux to elevation of cAMP and calcium is similar in normal and cystic fibrosis fibroblasts. Exposure to hypotonic medium induced a significant stimulation of Cl- efflux in fibroblasts from both normal and cystic fibrosis individuals. Substitution of Cl- in the medium by gluconate and the subsequent addition of furosemide did not inhibit the effect of hypotonicity, indicating the involvement of a conductive pathway for Cl- transport, which was insensitive to oligomicin C.     

Electrolyte transport in the epithelium of pulmonary segments of normal and cystic fibrosis lung.

The epithelium of pulmonary segments from trachea to aveoli actively transports electrolytes and allows osmotic movement of water to maintain the ionic environment in the airway lumen. Models of airway absorption and secretion depict the operation of transporters localized to apical or basolateral membrane. In many epithelia, a variety of electrolyte transporters operate in different combinations to produce absorption or secretion. This also applies to pulmonary epithelium of the large airways (trachea, main-stem bronchi), bronchioles, and alveoli. Na+ absorption occurs in all three pulmonary segments but by different transporters: apical Na+ channels in large airways and bronchioles; Na+/H+ exchange and Na+ channels in adult alveoli. The Na+ channels in each pulmonary segment share a sensitivity to amiloride, a potent inhibitory of epithelial Na+ channels. Fetal alveoli display spontaneous Cl- secretion, as do the large airways of some mammals, such as dog and bovine trachea. Cl- channels differ in conductance properties and in regulation by intracellular second messengers, osmolarity, and voltage mediate stimulated Cl- secretion. Electroneutral carriers, such as NaCl(K) cotransport, Cl-/HCO3- exchange, and Na+/HCO3- exchange, operate in large airways and alveoli during absorption and secretion. Abnormal ion transport in airways of cystic fibrosis (CF) patients is manifest as a reduced Cl- conductance and increased Na+ conductance. Isolation of the CF gene and identification of its product CFTR now allow investigations into the basic defect. Intrinsic to these investigations is the development of systems to study the function of CFTR and its relation to electrolyte transporters and their regulation.     

Differences in the incorporation of thymidine into DNA of normal and cystic fibrosis fibroblasts in vitro.

Although similar fractions of cells were in the S phase of the cell cycle, normal human skin fibroblasts were shown to incorporate more than twice the 3HTdR into their DNA in vitro than did cells obtained from individuals with cystic fibrosis (CF). Obligate heterozygotes incorporated an intermediate amount of the DNA precursor. Studied were initiated to determine the basis of the differential incorporation of 3HTdR among the genotypes. An analog of thymidine, BUdR, produced varied effects on the growth kinetics of the three genotypes. The growth of cells in BUdR resulted in a 50% increase in the population doubling times of all three genotypes, and caused the cell morphology to change from a spindle shape to one in which the cells became broadened and flat, with numerous cytoplasmic projections extending for distances of several cell diameters. The activities of thymidine kinase and the participation of the exogenous and de novo pathways in the synthesis of TMP were found to be approximately the same in all three genotypes. The data suggest that an alteration in the transport of thymidine into the cells may account for the differences in TdR incorporation into DNA, and this may be associated with other changes in cystic fibrosis that are apparently membrane associated.     

Plasma membrane lipids of human diploid fibroblasts from normal individuals and patients with cystic fibrosis.

The lipid composition of isolated plasma membranes of human skin fibroblasts is described for the first time. Plasma membranes from a number of strains of fibroblasts from patients with cystic fibrosis and matched normals were isolated by a recently described procedure and analysed for major phospholipid classes, cholesterol and fatty acids. No differences in the quantities of these compounds were detected between cells of the two different origins. The fetal calf serum used to supplement the growth medium contained relatively more palmitoleate and oleate but less stearate than the membranes. There were also no consistent differences between cystic fibrosis and normal membranes in terms of the fatty acid compositions of their individual phospholipid classes. Consistent with this lack of chemical change in the lipids of membranes of cystic fibrosis cells, the degree of fluorescence polarization of diphenylhexatriene, an index of fluidity, was also unchanged.     

Surface enzymes in cultured fibroblasts from cystic fibrosis patients.

Membrane function was examined in cultured cells from cystic fibrosis patients by assaying several enzymes on intact skin fibroblasts attached to culture dishes. This technique required few cells and minimized disruption of cellular organization. Comparison of enzyme activities of intact and broken cells showed that 12% of total glucose-6-phosphate dehydrogenase, a cytoplasmic enzyme, was measurable using intact cells, while all adenosine monophosphatase was measurable using intact cells. Alkaline paranitrophenylphosphatase activity was divided between the cell surface and interior. Substrate competition experiments indicated that substrate specificities for adenosine monophosphatase and paranitrophenylphosphatase activities were different. Adenosine monophosphatase activities of 2 control and 2 cystic fibrosis strains fluctuated similarly during the cell culture cycle. The apparent Km values relative to adenosine monophosphate were similar in all strains. A chromatographic fraction of serum from a cystic fibrosis patient that was inhibitory to oyster ciliary activity had no effect on adenosine monophosphatase activity of normal fibroblasts. Furthermore, fractions of media from cystic fibrosis homozygote and heterozygote fibroblast cultures were not inhibitory to adenosine monophosphatase activities of intact normal fibroblasts or of part iculate fractions prepared from them. In light of previous studies that showed that factors from cystic fibrosis serum of culture medium disrupted specific membrane activities, it is proposed that the cystic fibrosis factor interacts with the plasma membrane, interfering most conspicuously with the protein functions that are sensitive to changes in their membrane environment.     

Biochemical characterization of the cystic fibrosis transmembrane conductance regulator in normal and cystic fibrosis epithelial cells.

Affinity-purified polyclonal antibodies, raised against two synthetic peptides corresponding to the R domain and the C terminus of the human cystic fibrosis transmembrane conductance regulator (CFTR), were used to characterize and localize the protein in human epithelial cells. Employing an immunoblotting technique that ensures efficient detection of large hydrophobic proteins, both antibodies recognized and approximately 180-kDa protein in cell lysates and isolated membranes of airway epithelial cells from normal and cystic fibrosis (CF) patients and of T84 colon carcinoma cells. Reactivity with the anti-C terminus antibody, but not with the anti-R domain antibody, was eliminated by limited carboxypeptidase Y digestion. When normal CFTR cDNA was overexpressed via a retroviral vector in CF or normal airway epithelial cells or in mouse fibroblasts, the protein produced had an apparent molecular mass of about 180 kDa. The CFTR expressed in insect (Sf9) cells by a baculovirus vector had a molecular mass of about 140 kDa, probably representing a nonglycosylated form. The CFTR in epithelial cells appears to exist in several forms. N-glycosidase treatment of T84 cell membranes reduces the apparent molecular mass of the major CFTR band from 180 kDa to 140 kDa, but a fraction of the T84 cell CFTR could not be deglycosylated, and the CFTR in airway epithelial cell membranes could not be deglycosylated either. Moreover, wheat germ agglutinin absorbs the majority of the CFTR from detergent-solubilized T84 cell membranes but not from airway cell membranes. The CFTR in all epithelial cell types was found to be an integral membrane protein not solubilized by high salt or lithium diiodosalicylate treatment. Sucrose density gradient fractionation of crude membranes prepared from the airway epithelial cells, previously surface-labeled by enzymatic galactosidation, showed a plasma membrane localization for both the normal CFTR and the CFTR carrying the Phe508 deletion (delta F 508). The CFTR in all cases co-localized with the Na+, K(+)-ATPase and the plasma membrane calcium ATPase, while the endoplasmic reticulum calcium ATPase and mitochondrial membrane markers were enriched at higher sucrose densities. Thus, the CFTR appears to be localized in the plasma membrane both in normal and delta F 508 CF epithelial cells.     

Induction of alkaline phosphatase in cultured human fibroblasts. Comparison of normal cells and those from patients with cystic fibrosis.

The membrane glycoprotein enzyme, alkaline phosphatase was induced in cultured human fibroblasts by dibutyryl cyclic AMP, sodium butyrate, the serum glycoprotein fetuin, the Tamm-Horsfall urinary glycoprotein, and by a number of inhibitors of DNA synthesis. The uninduced basal enzyme activity increased at later stages of growth when the cells became confluent. Induction by dibutyryl cyclic AMP or fetuin was most effective when the agents were added after the cells had reached stationary phase and was maximal after at least two days of exposure. The levels of induction resulting from the addition of pairs of the agents, dibutyryl cyclic AMP, n-butyrate and fetuin were additive indicating that these have different modes of action. The inhibitors of DNA synthesis, cytosine arabinoside, hydroxyurea, and methothrexate were less effective inducers. Bromodeoxyuridine which also has non-DNA mediated effects induced to the same extent as dibutyryl cyclic AMP. Similar experiments with sex- and age-matched cell strains derived from patients with cystic fibrosis failed to detect differences in the levels of induction from those observed in normal cells. In addition, the combined inductive effects of Tamm-Horsfall glycoprotein, isoproterenol and theophylline, were similar with normal and cystic fibrosis cells.     

Altered intestinal chloride transport in cystic fibrosis

Sodium ion and chloride transport was studied in vitro in small intestinal and colonic tissue from patients with cystic fibrosis (CF) and from non-CF control subjects matched as to age and sex. Normal histological appearance and substantial response to mucosal glucose (5 mM, ileum) or mucosal amiloride (10(-5) M, colon) indicated normal tissue viability in both control and CF tissues. Electroneutral NaCl absorption was demonstrated in the small intestine of control subjects and CF patients. Small intestinal and colonic tissues of control subjects responded to four secretagogues (theophylline, 5 mM; prostaglandin E2, 10(-6) M; calcium ionophore (A23187), 10(-5) M; bethanechol, 5 x 10(-5) M), with electrogenic chloride secretion. The tissues of CF patients, however, did not respond to any of the test secretagogues. These studies demonstrate that an abnormality in chloride transport is present in the small intestinal and colonic epithelia of CF patients. Unlike airway epithelia, which secrete chloride in response to Ca ionophore, the intestinal epithelia of CF patients do not respond to either cAMP- or Ca-mediated secretagogues. This abnormality in intestinal electrolyte transport may play a role in the pathogenesis of meconium impactions in CF patients.     

alpha 2-Macroglobulin-proteinase complexes in cystic fibrosis serum. Analysis by monoclonal antibodies and receptor-mediated endocytosis by normal human fibroblasts

alpha 2-Macroglobulin complexed to proteinases activated during clotting of cystic fibrosis and control sera was quantitated with the complex-specific monoclonal antibody F2B2 . Similar amounts of alpha 2-macroglobulin complexes (between 40 and 90 micrograms/ml) were generated in cystic fibrosis and control sera. Endocytosis of the complexes by normal human fibroblasts was compared to the amount of complexes detected by the F2B2 -radioimmunoassay. Normal uptake was observed with 13 out of 14 cystic fibrosis sera. One cystic fibrosis serum showed strongly reduced endocytosis of the complexes. Complexes isolated from this serum on immobilized F2B2 failed to inhibit binding of purified alpha 2-macroglobulin-trypsin to its receptor, demonstrating deficient receptor-binding of these complexes. The low uptake complexes could not be distinguished from complexes isolated from control or other cystic fibrosis sera by isoelectric focusing, rate electrophoresis or SDS-polyacrylamide gel electrophoresis.