Prolonged treatment of cells with genistein modulates the expression and function of the cystic fibrosis transmembrane conductance regulator

BACKGROUND AND PURPOSE: Cystic fibrosis (CF) is caused by dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. In the search for new CF therapies, small molecules have been identified that rescue the defective channel gating of CF mutants (termed CFTR potentiators). Here, we investigate the long-term effects of genistein, the best-studied CFTR potentiator, on the expression and function of CFTR. EXPERIMENTAL APPROACH: We pre-treated baby hamster kidney (BHK) cells expressing wild-type or F508del-CFTR (the most common CF mutant) with concentrations of genistein that potentiate (30 microM) or inhibit (100 microM) CFTR function for 2 or 24 h at 37 degrees C before examining CFTR maturation, expression and single-channel activity. KEY RESULTS: Using the iodide efflux technique, we found that genistein pre-treatment failed to restore function to F508del-CFTR, but altered that of wild-type CFTR. Pre-treatment of cells with genistein for 2 h had little effect on CFTR processing, whereas pre-treatment for 24 h either augmented (30 microM genistein) or impaired (100 microM genistein) CFTR maturation. Using immunocytochemistry, we found that all genistein pre-treatments increased the localization of CFTR protein to the cell surface. However, following the incubation of cells with genistein (100 microM) for 2 h, individual CFTR Cl(-) channels exhibited characteristics of channel block upon channel activation. CONCLUSIONS AND IMPLICATIONS: Genistein pre-treatment alters the maturation, cell surface expression and single-channel function of CFTR in ways distinct from its acute effects. Thus, CFTR potentiators have the potential to influence CFTR by mechanisms distinct from their effects on channel gating.     

Targeting F508del-CFTR to develop rational new therapies for cystic fibrosis

The mutation F508del is the commonest cause of the genetic disease cystic fibrosis (CF). CF disrupts the function of many organs in the body, most notably the lungs, by perturbing salt and water transport across epithelial surfaces. F508del causes harm in two principal ways. First, the mutation prevents delivery of the cystic fibrosis transmembrane conductance regulator (CFTR) to its correct cellular location, the apical (lumen-facing) membrane of epithelial cells. Second, F508del perturbs the Cl(-) channel function of CFTR by disrupting channel gating. Here, we discuss the development of rational new therapies for CF that target F508del-CFTR. We highlight how structural studies provide new insight into the role of F508 in the regulation of channel gating by cycles of ATP binding and hydrolysis. We emphasize the use of high-throughput screening to identify lead compounds for therapy development. These compounds include CFTR correctors that restore the expression of F508del-CFTR at the apical membrane of epithelial cells and CFTR potentiators that rescue the F508del-CFTR gating defect. Initial results from clinical trials of CFTR correctors and potentiators augur well for the development of small molecule therapies that target the root cause of CF: mutations in CFTR.     

对羟基苯甲酸丁酯激活囊性纤维化跨膜电导调节因子氯离子通道开放

对20种挥发油类化合物中的对羟基苯甲酸丁酯(butyl-p-hydroxybenzoate, Bpb)的CFTR氯离子通道激活作用进行系统的分子药理学研究。利用稳定共表达人CFTR和对卤族元素碘离子高度敏感的荧光绿蛋白突变体(EYFP)的Fischer大鼠甲状腺 (FRT) 上皮细胞为筛选模型, 测定Bpb对CFTR介导的I^- 内流速度的影响。发现了Bpb对野生型CFTR的Cl^- 通道具有显著的激活作用; Bpb不能纠正 ?F508-CFTR蛋白胞内转运的障碍, 但却具有纠正其通道开放障碍的功能; Bpb对G551D突变型CFTR Cl^- 通道无激活作用。激活作用具有可逆和剂量依赖的特点, 初步机制分析结果表明, 它可能是通过与CFTR直接结合而发挥作用的。首次发现了Bpb对CFTR Cl^- 通道有激活作用, 为深入研究Bpb的药理学作用提供了新方向, 使其有可能成为治疗CFTR有关疾病的先导药物。     

Repairing mutated proteins – development of small molecules targeting defects in the cystic fibrosis transmembrane conductance regulator

Introduction: Cystic fibrosis (CF) is the most prevalent, recessively inherited, disease in the western world. It is characterized by gene mutations in CF transmembrane conductance regulator (CFTR), a transmembrane ion channel that is responsible for chloride secretion in the airway passages. Although much is known about the defects in CFTR and the consequences of these mutations, CF therapy currently focuses on the secondary outcomes and symptoms of the disease. However, developments in CFTR modulators may bring about new therapeutic options.

Areas covered: The authors discuss CFTR defects, as a molecular basis, before presenting and discussing CFTR modulators including correctors and potentiators. Specifically, the authors review promising CFTR modulators currently in preclinical and clinical development along with their medicinal chemistry and structure–activity relationships (SARs) and their in vitro and in vivo pharmacology.

Expert opinion: Although the development of CFTR-targeting agents has little access to structural information from crystal structures, several promising compounds have been discovered so far. Advanced virtual models of CFTR and high-throughput assays have helped the developmental programs. While Ivacaftor, the first of the CFTR potentiators, has now reached clinical use, CFTR corrector development has not been successful thus far. However, intense research of the mutation F508del, the mutation considered the most frequent in CF, could provide new causal treatment options in the future. Furthermore, the eventual synergy with multiple correctors may bring further success. CFTR modulators provide a new personalized therapeutic option where CF therapy is based on the mutations patients carry rather than by simply their symptoms.     

The therapeutic potential of the human cystic fibrosis transmembrane conductance regulator gene

The cloning of the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR), mutations of which are responsible for the clinical onset of cystic fibrosis (CF), along with progress in understanding the interplay between CFTR functions and the CF cellular phenotype have prompted many investigators to explore the therapeutic potential of CFTR gene delivery to airway cells in CF patients. In the last four years, a large number of Phase I clinical trials have been started. The results from the very first trials, although mixed, showed that it was possible to transfer and express the CFTR gene, and in certain cases restore the functional electrophysiological properties of the diseased CF cells. These initial trials have been fundamental in encouraging more basic research on vector design to improve the safety and efficiency of persistent CFTR gene transfer, and to introduce novel ways of administration and new techniques to assess the potential therapeutic efficacy of functional gene expression. New Phase I clinical trials based on novel protocol design were therefore initiated.     

New horizons in the treatment of cystic fibrosis

Cystic fibrosis (CF) is a lethal, recessive, genetic disease affecting approximately 1 in 2500 live births among Caucasians. The CF gene codes for a cAMP/PKA-dependent, ATP-requiring, membrane chloride ion channel, generally found in the apical membranes of many secreting epithelia and known as CFTR (cystic fibrosis transmembrane conductance regulator). There are currently over 1700 known mutations affecting CFTR, many of which give rise to a disease phenotype. Around 75% of CF alleles contain the ΔF508 mutation in which a triplet codon has been lost, leading to a missing phenylalanine at position 508 in the protein. This altered protein fails to be trafficked to the correct location in the cell and is generally destroyed by the proteasome. The small amount that does reach the correct location functions poorly. Clearly the cohort of patients with at least one ΔF508 allele are a major target for therapeutic intervention. It is now over two decades since the CF gene was discovered and during this time the properties of CFTR have been intensely investigated. At long last there appears to be progress with the pharmaco-therapeutic approach. Ongoing clinical trials have produced fascinating results in which clinical benefit appears to have been achieved. To arrive at this point ingenious ways have been devised to screen very large chemical libraries for one of two properties: (i) agents promoting trafficking of mutant CFTR to, and insertion into the membrane, and known as correctors or (ii) agents which activate appropriately located mutant CFTR, known as potentiators. The best compounds emerging from these programmes are then used as chemical scaffolds to synthesize other compounds with appropriate pharmaceutical properties, hopefully with their pharmacological activity maintained or even enhanced. In summary, this approach attempts to make the mutant CFTR function in place of the real CFTR. A major function of CFTR in healthy airways is to maintain an adequate airway surface liquid (ASL) layer. In CF the position is further confounded since epithelial sodium channels (ENaC) are no longer regulated and transport salt and water out of the airways to exacerbate the lack of ASL. Thus an additional possibility for treatment of CF is to use agents that inhibit ENaC either alone or as adjuncts to CFTR correctors and/or potentiators. Yet a further way in which a pharmacological approach to CF can be considered is to recruit alternative chloride channels, such as calcium-activated chloride channel (CaCC), to act as surrogates for CFTR. A number of P2Y(2) receptor agonists have been investigated that operate by increasing Ca(2+)(i) which in turn activates CaCC. Some of these compounds are currently in clinical trials. The knowledge base surrounding the structure and function of CFTR that has accumulated in the last 20 years is impressive. Translational research feeding from this is now yielding compounds that provide real prospects for a pharmacotherapy for this disease.     

Cystic fibrosis transmembrane conductance regulator and the outwardly rectifying chloride channel: a relationship between two chloride channels expressed in epithelial cells

1. Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) result in the primary defect observed in patients with cystic fibrosis. 2. The CFTR is a member of the ATPase-binding cassette (ABC) transporter family but, unlike other members of this group, CFTR conducts a chloride current that is activated by cAMP. 3. In epithelial cells, the cAMP-stimulated chloride current is conducted by both CFTR and the outwardly rectifying chloride channel (ORCC). 4. The present review summarizes the current knowledge of the properties of the two channels, as well as their relationship. Because the gene encoding the ORCC has not been identified, a discussion as to possible candidates for this chloride channel is included.     

Novel action of the chalcone isoliquiritigenin as a cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor: potential therapy for cholera and polycystic kidney disease

Overstimulation of cAMP-activated Cl(-) secretion can cause secretory diarrhea. Isoliquiritigenin (ISLQ) is a plant-derived chalcone that has a wide range of biological activities. The present study thus aimed to investigate the effect of ISLQ on cAMP-activated Cl(-) secretion in human intestinal epithelium, especially the underlying mechanism and therapeutic application. Short-circuit current analysis of human intestinal epithelial (T84) cell monolayers revealed that ISLQ dose-dependently inhibited cAMP-activated Cl(-) secretion with an IC(50) of approximately 20 μM. ISLQ had no effect on either basal short-circuit current or Ca(2+)-activated Cl(-) secretion. Apical Cl(-) current analysis of T84 cell monolayers indicated that ISLQ blocked mainly the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels, but not other unidentified cAMP-dependent Cl(-) channels. ISLQ did not affect intracellular cAMP levels or cell viability. ISLQ completely abolished the cholera toxin-induced transepithelial Cl(-) secretion in T84 cells and reduced the cholera toxin-induced intestinal fluid secretion in mouse closed loop models by 90%. Similarly, ISLQ completely inhibited the cAMP-activated apical Cl(-) current across monolayers of Madin-Darby Canine Kidney (MDCK) cells and retarded cyst growth in MDCK cyst models by 90%. This study reveals a novel action of ISLQ as a potent CFTR inhibitor with therapeutic potential for treatment of cholera and polycystic kidney disease.     

Cross talk of cAMP and flavone in regulation of cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel and Na+/K+/2Cl- cotransporter in renal epithelial A6 cells

We have reported that in renal epithelial A6 cells flavones stimulate the transepithelial Cl- secretion by activating the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel and/or the Na+/K+/2Cl- cotransporter. On the other hand, it has been established that cAMP activates the CFTR Cl- channel and the Na+/K+/2Cl- cotransporter. However, no information is available on the interaction between cAMP and flavones on stimulation of the CFTR Cl- channel and the Na+/K+/2Cl- cotransporter. To clarify the interaction between cAMP and flavones, we studied the regulatory mechanism of the CFTR Cl- channel and the Na+/K+/2Cl- cotransporter by flavones (apigenin, luteolin, kaempherol, and quercetin) under the basal and cAMP-stimulated conditions in renal epithelial A6 cells. Under the basal (cAMP-unstimulated) condition, these flavones stimulated the Cl- secretion by activating the Na+/K+/2Cl- cotransporter without any significant effects on the CFTR Cl- channel activity. On the other hand, these flavones diminished the activity of the cAMP-stimulated Na+/K+/2Cl- cotransporter without any significant effects on the CFTR Cl- channel activity. Interestingly, the level of the flavone-induced Cl- secretion under the basal condition was identical to that under the cAMP-stimulated condition. Based on these results, it is suggested that although both cAMP and flavones activate the Na+/K+/2Cl- cotransporter, these flavones have more powerful effects than cAMP on the Na+/K+/2Cl- cotransporter.     

The cystic fibrosis mutation G1349D within the signature motif LSHGH of NBD2 abolishes the activation of CFTR chloride channels by genistein

Cystic fibrosis (CF) is a common lethal genetic disease caused by autosomal recessive mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel that belongs to the ATP-Binding Cassette (ABC) family of transporters. The class III CF mutations G551D and G1349D are located within the "signature" sequence LSGGQ and LSHGH of NBD1 and NBD2, respectively. We have constructed by site-directed mutagenesis vectors encoding green fluorescent protein (GFP)-tagged wild-type (wt) CFTR or CFTR containing delF508, G551D, G1349D and G551D/G1349D to study their pharmacology after transient expression in COS-7 cells. We show that IBMX and the benzo[c]quinolizinium derivative MPB-91 stimulates the activity of G1349D-, G551D- and G551D/G1349D-CFTR only in the presence of cAMP-promoting agents like forskolin or cpt-cAMP. Similar half-maximal effective concentrations (EC(50)) of MPB-91 (22-36microM) have been determined for wt-, G551D-, G1349D- and G551D/G1349D-CFTR. The isoflavone genistein stimulates wild-type (wt)- and delF508-CFTR channel activity in a non-Michaelis-Menten manner. By contrast, the response of G1349D- and G551D-CFTR to genistein is dramatically altered. First, genistein is not able to stimulate G1349D- and G551D/G1349D-CFTR. Second, genistein stimulates G551D-CFTR without any inhibition at high concentration. We conclude from these results that whereas G551 in NBD1 is an important molecular site for inhibition of CFTR by genistein, the symmetrical G1349 in NBD2 is also one major site but for the activation of CFTR by genistein. Because both mutations alter specifically the mechanism of CFTR channel activation by genistein, we believe that the signature sequences of CFTR act as molecular switches that upon interaction with genistein turn on and off the channel.