Effect of CFTR modulator therapy on cystic fibrosis-related diabetes

BACKGROUND

Half of adults with cystic fibrosis (CF) develop CF-related diabetes (CFRD). CFRD contributes to worsened pulmonary function and malnutrition. We undertook this study to determine the effect of cystic fibrosis transmembrane regulator (CFTR) modulators on CRFD.

METHODS

We reviewed the medical records of adults with CF who followed in the CF clinic at Oklahoma University Medical Center. We collected data for age at diagnosis of CF and CFRD, CF mutations present, first date of ivacaftor therapy either alone or in combination, insulin use, pulmonary function, body mass index data, and home glucose monitoring results. Clinical resolution of CFRD was taken as discontinuation of routine insulin and resolution of high interstitial home glucose values.

RESULTS

We identified 69 adult CF patients, of whom 31 had CFRD. Among these 14 CFRD patients taking ivacaftor alone or in combination, four patients completely stopped using insulin. Another patient went from three times a day pre-prandial insulin to using insulin once a week. Home blood glucose and hemoglobin A1c values supported resolution of CFRD. Three patients continued to have hypoglycemia despite stopping insulin. No CFRD patient not taking CFTR modulators markedly changed the insulin regimen. Pulmonary function was preserved in those patients with resolved CFRD (FEV₁ +6.75% ±7.6), whereas it worsened in CFRD patients who either were not taking CFTR modulators (FEV₁ -2.09% ±3.9) or who had no response of CFRD status (FEV₁ -4.9% ±7.6).

CONCLUSIONS

About one-third of patients on CFTR modulator therapy had resolution or near resolution of CFRD.

1-BENZYLSPIRO[PIPERIDINE-4,1'-PYRIDO[3,4-b]indole] 'co-potentiators' for minimal function CFTR mutants

We previously identified a spiro [piperidine-4,1-pyrido [3,4-b]indole] class of co-potentiators that function in synergy with existing CFTR potentiators such as VX-770 or GLGP1837 to restore channel activity of a defined subset of minimal function cystic fibrosis transmembrane conductance regulator (CFTR) mutants. Here, structure-activity studies were conducted to improve their potency over the previously identified compound, 20 (originally termed CP-A01). Targeted synthesis of 37 spiro [piperidine-4,1-pyrido [3,4-b]indoles] was generally accomplished using versatile two or three step reaction protocols with each step having high efficiency. Structure-activity relationship studies established that analog 2i, with 6'-methoxyindole and 2,4,5-trifluorobenzyl substituents, had the greatest potency for activation of N1303K-CFTR, with EC50 ∼600 nM representing an ∼17-fold improvement over the original compound identified in a small molecule screen.

From Ivacaftor to Triple Combination: A Systematic Review of Efficacy and Safety of CFTR Modulators in People with Cystic Fibrosis

Over the last years CFTR (cystic fibrosis transmembrane conductance regulator) modulators have shown the ability to improve relevant clinical outcomes in patients with cystic fibrosis (CF). This review aims at a systematic research of the current evidence on efficacy and tolerability of CFTR modulators for different genetic subsets of patients with CF. Two investigators independently performed the search on PubMed and included phase 2 and 3 clinical trials published in the study period 1 January 2005-31 January 2020. A final pool of 23 papers was included in the systematic review for a total of 4219 patients. For each paper data of interest were extracted and reported in table. In terms of lung function, patients who had the most beneficial effects from CFTR modulation were those patients with one gating mutation receiving IVA (ivacaftor) and patients with p.Phe508del mutation, both homozygous and heterozygous, receiving ELX/TEZ/IVA (elexacaftor/tezacaftor/ivacaftor) had the most relevant beneficial effects in term of lung function, pulmonary exacerbation decrease, and symptom improvement. CFTR modulators showed an overall favorable safety profile. Next steps should aim to systematize our comprehension of scientific data of efficacy and safety coming from real life observational studies.

Cystic fibrosis transmembrane conductance regulator modulators reduce the risk of recurrent acute pancreatitis among adult patients with pancreas sufficient cystic fibrosis

BACKGROUND

Approximately 1 in 5 patients with pancreas sufficient cystic fibrosis (PS-CF) will develop acute pancreatitis (AP). It is not known whether ivacaftor alone or in combination with other CFTR (cystic transmembrane regulator) modulators (tezacaftor or lumacaftor) can reduce the risk of AP in patients with PS-CF and AP history.

METHODS

We retrospectively queried the CF registry at our institution for adult patients with PS-CF, a documented history of AP and initiation of CFTR modulators for pulmonary indications. Patient characteristics including demographics, CFTR genotype, pancreatitis risk factors, pancreatic exocrine function and other relevant laboratory, imaging parameters were obtained from the time of the sentinel AP episode through the follow-up period.

RESULTS

A total of 15 adult CF patients were identified with mean age of 44.1 years (SD ± 13.8). In the 24 months preceding CFTR modulator initiation, six of these patients had at least 1 episode of AP with median of 2 episodes [1.75, 2.5]. None of the patients had evidence of pancreatic calcifications or exocrine pancreas insufficiency at the time of CFTR modulator initiation. The mean duration of follow-up after CFTR modulator initiation was 36.7 months (SD ± 21.5). None of the patients who remained on CFTR modulators developed an episode of AP or required hospitalization for AP related abdominal pain during follow-up.

CONCLUSIONS

CFTR modulators, alone or in combination, substantially reduce the risk of recurrent AP over a mean follow-up period of 3 years in adult patients with PS-CF and a history of prior AP. These data suggest that any augmentation of CFTR function can reduce the risk of pancreatitis.

Physiological and pharmacological characterization of the N1303K mutant CFTR

BACKGROUND

N1303K, one of the common, severe disease-causing mutations in the CFTR gene, causes both defective biogenesis and gating abnormalities of the CFTR protein. The goals of the present study are to quantitatively assess the gating defects associated with the N1303K mutation and its pharmacological response to CFTR modulators including potentiators VX-770 and GLPG1837 and correctors VX-809, and VX-661.

METHODS

Gating behavior and pharmacological responses to CFTR potentiators were assessed using patch-clamp technique in the excised, inside-out mode. We also examined the effects of GLPG1837, VX-770, VX-809 and VX-661 on N1303K-CFTR surface expression using Western blot analysis.

RESULTS

Like wild-type (WT) CFTR, N1303K-CFTR channels were activated by protein kinase A-dependent phosphorylation, but the open probability (P_o) of phosphorylated N1303K-CFTR was extremely low (~0.03 vs ~0.45 in WT channels). N1303K mutants showed abnormal responses to ATP analogs or mutations that disrupt ATP hydrolysis and/or dimerization of CFTR's two nucleotide-binding domains (NBDs). However, the P_o of N1303K-CFTR was dramatically increased by GLPG1837 (~17-fold) and VX-770 (~8-fold). VX-809 or VX-661 enhanced N1303K-CFTR maturation by 2-3 fold, and co-treatment with GLPG1837 or VX-770 did not show any negative drug-drug interaction.

CONCLUSION

N1303K has a severe gating defect, reduced ATP-dependence and aberrant response to ATP analogs. These results suggest a defective function of the NBDs in N1303K-CFTR. An improvement of channel function by GLPG1837 or VX-770 and an increase of Band C protein by VX-809 or VX-661 support a therapeutic strategy of combining CFTR potentiator and corrector for patients carrying the N1303K mutation.

Cigarette smoke activates CFTR through ROS-stimulated cAMP signaling in human bronchial epithelial cells

Air pollution stimulates airway epithelial secretion through a cholinergic reflex that is unaffected in cystic fibrosis (CF), yet a strong correlation is observed between passive smoke exposure in the home and impaired lung function in CF children. Our aim was to study the effects of low smoke concentrations on cystic fibrosis transmembrane conductance regulator (CFTR) function in vitro. Cigarette smoke extract stimulated robust anion secretion that was transient, mediated by CFTR, and dependent on cAMP-dependent protein kinase activation. Secretion was initiated by reactive oxygen species (ROS) and mediated by at least two distinct pathways: autocrine activation of EP4 prostanoid receptors and stimulation of Ca2+ store-operated cAMP signaling. The response was absent in cells expressing the most common disease-causing mutant F508del-CFTR. In addition to the initial secretion, prolonged exposure of non-CF bronchial epithelial cells to low levels of smoke also caused a gradual decline in CFTR functional expression. F508del-CFTR channels that had been rescued by the CF drug combination VX-809 (lumacaftor) + VX-770 (ivacaftor) were more sensitive to this downregulation than wild-type CFTR. The results suggest that CFTR-mediated secretion during acute cigarette smoke exposure initially protects the airway epithelium while prolonged exposure reduces CFTR functional expression and reduces the efficacy of CF drugs.

Cystic fibrosis gene modifier SLC26A9 modulates airway response to CFTR-directed therapeutics

Cystic fibrosis is realizing the promise of personalized medicine. Recent advances in drug development that target the causal CFTR directly result in lung function improvement, but variability in response is demanding better prediction of outcomes to improve management decisions. The genetic modifier SLC26A9 contributes to disease severity in the CF pancreas and intestine at birth and here we assess its relationship with disease severity and therapeutic response in the airways. SLC26A9 association with lung disease was assessed in individuals from the Canadian and French CF Gene Modifier consortia with CFTR-gating mutations and in those homozygous for the common Phe508del mutation. Variability in response to a CFTR-directed therapy attributed to SLC26A9 genotype was assessed in Canadian patients with gating mutations. A primary airway model system determined if SLC26A9 shows modification of Phe508del CFTR function upon treatment with a CFTR corrector. In those with gating mutations that retain cell surface-localized CFTR we show that SLC26A9 modifies lung function while this is not the case in individuals homozygous for Phe508del where cell surface expression is lacking. Treatment response to ivacaftor, which aims to improve CFTR-channel opening probability in patients with gating mutations, shows substantial variability in response, 28% of which can be explained by rs7512462 in SLC26A9 (P = 0.0006). When homozygous Phe508del primary bronchial cells are treated to restore surface CFTR, SLC26A9 likewise modifies treatment response (P = 0.02). Our findings indicate that SLC26A9 airway modification requires CFTR at the cell surface, and that a common variant in SLC26A9 may predict response to CFTR-directed therapeutics.

Robust Stimulation of W1282X-CFTR Channel Activity by a Combination of Allosteric Modulators

W1282X is a common nonsense mutation among cystic fibrosis patients that results in the production of a truncated Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) channel. Here we show that the channel activity of the W1282X-CFTR polypeptide is exceptionally low in excised membrane patches at normally saturating doses of ATP and PKA (single channel open probability (PO) < 0.01). However, W1282X-CFTR channels were stimulated by two CFTR modulators, the FDA-approved VX-770 and the dietary compound curcumin. Each of these compounds is an allosteric modulator of CFTR gating that promotes channel activity in the absence of the native ligand, ATP. Although W1282X-CFTR channels were stimulated by VX-770 in the absence of ATP their activities remained dependent on PKA phosphorylation. Thus, activated W1282X-CFTR channels should remain under physiologic control by cyclic nucleotide signaling pathways in vivo. VX-770 and curcumin exerted additive effects on W1282X-CFTR channel gating (opening/closing) in excised patches such that the Po of the truncated channel approached unity (> 0.9) when treated with both modulators. VX-770 and curcumin also additively stimulated W1282X-CFTR mediated currents in polarized FRT epithelial monolayers. In this setting, however, the stimulated W1282X-CFTR currents were smaller than those mediated by wild type CFTR (3-5%) due presumably to lower expression levels or cell surface targeting of the truncated protein. Combining allosteric modulators of different mechanistic classes is worth considering as a treatment option for W1282X CF patients perhaps when coupled with maneuvers to increase expression of the truncated protein.

Impact of the CFTR-potentiator ivacaftor on airway microbiota in cystic fibrosis patients carrying a G551D mutation

Background

Airway microbiota composition has been clearly correlated with many pulmonary diseases, and notably with cystic fibrosis (CF), an autosomal genetic disorder caused by mutation in the CF transmembrane conductance regulator (CFTR). Recently, a new molecule, ivacaftor, has been shown to re-establish the functionality of the G551D-mutated CFTR, allowing significant improvement in lung function.

Objective and Methods

The purpose of this study was to follow the evolution of the airway microbiota in CF patients treated with ivacaftor, using quantitative PCR and pyrosequencing of 16S rRNA amplicons, in order to identify quantitative and qualitative changes in bacterial communities. Three G551D children were followed up longitudinally over a mean period of more than one year covering several months before and after initiation of ivacaftor treatment.

Results

129 operational taxonomy units (OTUs), representing 64 genera, were identified. There was no significant difference in total bacterial load before and after treatment. Comparison of global community composition found no significant changes in microbiota. Two OTUs, however, showed contrasting dynamics: after initiation of ivacaftor, the relative abundance of the anaerobe Porphyromonas 1 increased (p<0.01) and that of Streptococcus 1 (S. mitis group) decreased (p<0.05), possibly in relation to the anti-Gram-positive properties of ivacaftor. The anaerobe Prevotella 2 correlated positively with the pulmonary function test FEV-1 (r=0.73, p<0.05). The study confirmed the presumed positive role of anaerobes in lung function.

Conclusion

Several airway microbiota components, notably anaerobes (obligate or facultative anaerobes), could be valuable biomarkers of lung function improvement under ivacaftor, and could shed light on the pathophysiology of lung disease in CF patients.

A rapid membrane potential assay to monitor CFTR function and inhibition

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is an important regulator of ion transport and fluid secretion in humans. Mutations to CFTR cause cystic fibrosis, which is a common recessive genetic disorder in Caucasians. Involvement of CFTR has been noted in other important diseases, such as secretory diarrhea and polycystic kidney disease. The assays to monitor CFTR function that have been described to date either are complicated or require specialized instrumentation and training for execution. In this report, we describe a rapid FlexStation-based membrane potential assay to monitor CFTR function. In this assay, agonist-mediated activation of CFTR results in membrane depolarization that can be monitored using a fluorescent membrane potential probe. Availability of a simple mix-and-read assay to monitor the function of this important protein might accelerate the discovery of CFTR ligands to study a variety of conditions.

[News in cystic fibrosis]

The improvement over the last two decades in the treatment of cystic fibrosis led to an increase in life expectancy approaching 40 years at birth. Logically, the population of adult patients has been increasing and is currently 50% of patients followed in France. These therapeutic advances have justified the establishment in 2003 of a generalized neonatal screening for cystic fibrosis. The latest data of this screening show an incidence of CF of 1/5359 live births, far below the incidence of 1/2500 which was widely accepted twenty years ago. The performance of this screening is currently based on the dosage of trypsin immuno reactive, followed in case of exceeding the threshold of a search of the 30 most common mutations, can detect around 96% of 150 to 200 CF cases every year. Therefore, the possibility of a false negative of the screening cannot be excluded and evocative symptoms of cystic fibrosis, even for children born after 2003, will lead to prescribe a sweat test. While treatments available so far goal consequences of cystic fibrosis, a new therapeutic class to correct the functional defect of the mutated protein, called CFTR modulators, is emerging. Ivacaftor, leader of this new class, belonging to the category of "CFTR potentiator" got its access on the market in September 2012 for patients carrying the G551D mutation. New other molecules, named "CFTR correctors" which can have synergistic effect with ivacaftor and concern patients carrying the most common mutation--DF 508--are under development.

Nickel inhibits β-1 adrenoceptor mediated activation of cardiac CFTR chloride channels

Cardiac ventricular myocytes exhibit a protein kinase A-dependent Cl(-) current (ICl.PKA) mediated by the cystic fibrosis transmembrane conductance regulator (CFTR). There is conflicting evidence regarding the ability of the divalent cation nickel (Ni(2+)), which has been used widely in vitro in the study of other cardiac ionic conductances, to inhibit ICl.PKA. Here the action of Ni(2+) on ICl.PKA activated by β-adrenergic stimulation has been elucidated. Whole-cell patch-clamp recordings were made from rabbit isolated ventricular myocytes. Externally applied Ni(2+) blocked ICl.PKA activated by 1 μM isoprenaline with a log IC50 (M) of -4.107 ± 0.075 (IC50=78.1 μM) at +100 mV and -4.322 ± 0.107 (IC50=47.6 μM) at -100 mV. Thus, the block of ICl.PKA by Ni(2+) was not strongly voltage dependent. Ni(2+) applied internally via the patch-pipette was ineffective at inhibiting isoprenaline-activated ICl,PKA, but in the same experiments the current was suppressed by external Ni(2+) application, indicative of an external site of Ni(2+) action. In the presence of 1 μM atenolol isoprenaline was ineffective at activating ICl.PKA, but in the presence of the β2-adrenoceptor inhibitor ICI 118,551 isoprenaline still activated Ni(2+)-sensitive ICl.PKA. Collectively, these data demonstrate that Ni(2+) ions produce marked inhibition of β1-adrenoceptor activated ventricular ICl.PKA at submillimolar [Ni(2+)]: an action that is likely to involve an interaction between Ni(2+) and β1-adrenoceptors. The concentration-dependence for ICl.PKA inhibition seen here indicates the potential for confounding effects on ICl,PKA to occur even at comparatively low Ni(2+) concentrations, when Ni(2+) is used to study other cardiac ionic currents under conditions of β-adrenergic agonism.

Lubiprostone activates CFTR, but not ClC-2, via the prostaglandin receptor (EP(4))

The goal of this study was to determine the mechanism of lubiprostone activation of epithelial chloride transport. Lubiprostone is a bicyclic fatty acid approved for the treatment of constipation [1]. There is uncertainty, however, as to how lubiprostone increases epithelial chloride transport. Direct stimulation of ClC-2 and CFTR chloride channels as well as stimulation of these channels via the EP(4) receptor has been described [2-5]. To better define this mechanism, two-electrode voltage clamp was used to assay Xenopus oocytes expressing ClC-2, with or without co-expression of the EP(4) receptor or β adrenergic receptor (βAR), for changes in conductance elicited by lubiprostone. Oocytes co-expressing CFTR and either βAR or the EP(4) receptor were also studied. In oocytes co-expressing ClC-2 and βAR conductance was stimulated by hyperpolarization and acidic pH (pH = 6), but there was no response to the β adrenergic agonist, isoproterenol. Oocytes expressing ClC-2 only or co-expressing ClC-2 and EP(4) did not respond to the presence of 0.1, 1, or 10 μM lubiprostone in the superperfusate. Oocytes co-expressing CFTR and βAR did not respond to hyperpolarization, acidic pH, or 1 μM lubiprostone. However, conductance was elevated by isoproterenol and inhibited by CFTR(inh)172. Co-expression of CFTR and EP(4) resulted in lubiprostone-stimulated conductance, which was also sensitive to CFTR(inh)172. The EC(50) for lubiprostone mediated CFTR activation was ~10 nM. These results demonstrate no direct action of lubiprostone on either ClC-2 or CFTR channels expressed in oocytes. However, the results confirm that CFTR can be activated by lubiprostone via the EP(4) receptor in oocytes.

Synthesis of 4-thiophen-2'-yl-1,4-dihydropyridines as potentiators of the CFTR chloride channel

The gating of the CFTR chloride channel is altered by a group of mutations that cause cystic fibrosis. This gating defect may be corrected by small molecules called potentiators. Some 1,4-dihydropyridine (DHP) derivatives, bearing a thiophen-2-yl and a furanyl ring at the 4-position of the nucleus, were prepared and tested as CFTR potentiators. In particular, we evaluated the ability of novel DHPs to enhance the activity of the rescued DeltaF508-CFTR as measured with a functional assay based on the halide-sensitive yellow fluorescent protein. Most DHPs showed an effect comparable to or better than that of the reference compound genistein. The potency was instead significantly improved, with some compounds, such as 3g, 3h, 3n, 4a, 4b, and 4d, having a half effective concentration in the submicromolar range. CoMFA analysis gave helpful suggestions to improve the activity of DHPs.

Stimulation of salivary secretion in vivo by CFTR potentiators in Cftr+/+ and Cftr-/- mice

BACKGROUND

Physiologically, salivary secretion is controlled by cholinergic and adrenergic pathways but the role of ionic channels in this process is not yet clearly understood. In cystic fibrosis (CF), most exocrine glands failed to response to beta-adrenergic agonists.

METHODS

To determine the implication of CFTR in this process, we measured in vivo the salivary secretion of Cftr(+/+) and Cftr(-/-) mice in the presence of 2 water-soluble benzo[c]quinolizinium derivatives; MPB-07 a potentiator of CFTR Cl(-) channel and MPB-05 an inactive analogue. We also used genistein and its vehicle ethanol to confirm the implication of CFTR in salivary secretion.

RESULTS

We showed that subcutaneous injection of MPB-07 in the mice cheek enhanced in a dose dependent manner the isoprenaline-induced salivary secretion in Cftr(+/+) but not in Cftr(-/-) mice. By contrast, MPB-05 did not activate the salivary secretion in Cftr(+/+) mice. The CFTR activator genistein (50 microM) significantly potentiated the secretory response of Cftr(+/+) mice whereas its vehicle, ethanol, had no effect.

CONCLUSIONS

These results show for the first time in vivo pharmacological stimulation of salivary secretion by a water-soluble CFTR potentiator, MPB-07 and by the isoflavone, ethanol-soluble genistein and suggest that this chloride channel plays an important role in salivary gland physiology.

Emerging drug treatments for cystic fibrosis

Although the gene for cystic fibrosis was discovered in 1989, a definitive cure remains elusive for this deadly orphan disease. Advances in nutritional rehabilitation, antibiotics, mucolytics and delivery of care have improved survival to a median age of 37.5 years; however, the psychosocial, personal and financial burdens of this lifelong chronic illness remain considerable. The current portfolio of investigational therapeutics is explored here and placed in a context of therapeutic target and predicted clinical benefit. Partnership between large and small pharma, the Cystic Fibrosis Foundation and academia should be fostered to accelerate therapeutic development.

Molecular targeting of CFTR as a therapeutic approach to cystic fibrosis

One of the major challenges facing the pharmaceutical field is the identification of novel, 'druggable' targets common to distinct diseases that, despite their clinical diversity, share the same basic molecular defect(s) - thus, being termed 'horizontal diseases'. Membrane proteins constitute one of the largest families in the human genome and, given their major roles in cells and organisms, they are relevant to common human disorders such as cardiovascular disease and cancer, but also to rare genetic conditions such as cystic fibrosis (CF). Here, we review therapeutic approaches to correcting the basic defect in CF, which is caused mainly by the intracellular retention of a misfolded protein, and focus on various recent drug-discovery strategies for this important and paradigmatic disease. These strategies have possible applications in many membrane protein disorders, including other channelopathies. The mechanisms of action of potent and specific compounds, representing promising drug leads for CF pharmacotherapy, are explained and discussed.

On the discovery and development of CFTR chloride channel activators

Chloride channels play important roles in vital cellular signalling processes contributing to homeostasis in both excitable and non-excitable cells. Since 1987, more than ten ion channel genes have been identified as causing human hereditary diseases among them the genes for the voltage-dependent chloride channel ClC-1 (myotonia) and the cystic fibrosis transmembrane conductance regulator (CFTR) protein (cystic fibrosis). The CFTR gene was cloned in 1989 and its protein product identified as an ATP-gated and phosphorylation-regulated chloride channel during the following two years. Since then, searching for potent and specific small molecules able to modulate normal and mutated CFTR has become a crucial endpoint in the field for both our understanding of the physiological role that CFTR plays in epithelial cells and more importantly for the development of therapeutic agents to cure cystic fibrosis (CF). It is predicted that a pharmacological approach would help not only to restore the defective transport activity of mutant CFTR but also to correct the regulatory function of CFTR. This review describes the evolution of CFTR pharmacology and how during the last five years, high throughput screening assays have been developed to identify novel molecules, some of them probably constituting a reservoir of future therapeutic agents for CF.

Pharmacological induction of CFTR function in patients with cystic fibrosis: mutation-specific therapy

CFTR mutations cause defects of CFTR protein production and function by different molecular mechanisms. Mutations can be classified according to the mechanisms by which they disrupt CFTR function. This understanding of the different molecular mechanisms of CFTR dysfunction provides the scientific basis for the development of targeted drugs for mutation-specific therapy of cystic fibrosis (CF). Class I mutations are nonsense mutations that result in the presence of a premature stop codon that leads to the production of unstable mRNA, or the release from the ribosome of a short, truncated protein that is not functional. Aminoglycoside antibiotics can suppress premature termination codons by disrupting translational fidelity and allowing the incorporation of an amino acid, thus permitting translation to continue to the normal termination of the transcript. Class II mutations cause impairment of CFTR processing and folding in the Golgi. As a result, the mutant CFTR is retained in the endoplasmic reticulum (ER) and eventually targeted for degradation by the quality control mechanisms. Chemical and molecular chaperones such as sodium-4-phenylbutyrate can stabilize protein structure, and allow it to escape from degradation in the ER and be transported to the cell membrane. Class III mutations disrupt the function of the regulatory domain. CFTR is resistant to phosphorylation or adenosine tri-phosphate (ATP) binding. CFTR activators such as alkylxanthines (CPX) and the flavonoid genistein can overcome affected ATP binding through direct binding to a nucleotide binding fold. In patients carrying class IV mutations, phosphorylation of CFTR results in reduced chloride transport. Increases in the overall cell surface content of these mutants might overcome the relative reduction in conductance. Alternatively, restoring native chloride pore characteristics pharmacologically might be effective. Activators of CFTR at the plasma membrane may function by promoting CFTR phosphorylation, by blocking CFTR dephosphorylation, by interacting directly with CFTR, and/or by modulation of CFTR protein-protein interactions. Class V mutations affect the splicing machinery and generate both aberrantly and correctly spliced transcripts, the levels of which vary among different patients and among different organs of the same patient. Splicing factors that promote exon inclusion or factors that promote exon skipping can promote increases of correctly spliced transcripts, depending on the molecular defect. Inconsistent results were reported regarding the required level of corrected or mutated CFTR that had to be reached in order to achieve normal function.

A CFTR chloride channel activator prevents HrpN(ea)-induced cell death in Arabidopsis thaliana suspension cells

Erwinia amylovora is a necrogenic bacterium that causes fire blight of the Maloideae subfamily of Roseacae, such as apple and pear. It provokes necrosis in aerial parts of susceptible host plants and the typical hypersensitive reaction in non-host plants. The secreted harpin, HrpN ea, is able by itself to induce an active cell death in non-host plants. Ion flux modulations were shown to be involved early in such processes but very few data are available on the plasma membrane ion channel activities responsible for the pathogen-induced ion fluxes. We show here that HrpN ea induces cell death in non-host Arabidopsis thaliana suspension cells. We further show that two cystic fibrosis transmembrane conductance regulator modulators, glibenclamide and bromotetramisole, can regulate anion channel activities and HrpN ea-induced cell death.

Activating cystic fibrosis transmembrane conductance regulator channels with pore blocker analogs

Cystic fibrosis (CF) is caused by mutations that disrupt the surface localization and/or gating of the CF transmembrane conductance regulator (CFTR) chloride channel. The most common CF mutant is deltaF508-CFTR, which inefficiently traffics to the surfaces of most cells. The deltaF508 mutation may also disrupt the opening of CFTR channels once they reach the cell surface, but the extent of this gating defect is unclear. Here, we describe potent activators of wild-type and deltaF508-CFTR channels that are structurally related to 5-nitro-2-(3-phenylpropylamino)benzoate (NPPB), a negatively charged pore blocker that we show to have mixed agonistic activity (channel activation plus voltage-dependent pore block). These CFTR agonists include 1) an uncharged NPPB analog that stimulates channel opening at submicromolar concentrations without blocking the pore and 2) curcumin, a dietary compound recently reported to augment deltaF508-CFTR function in mice by an unknown mechanism. The uncharged NPPB analog enhanced the activities of wild-type and deltaF508-CFTR channels both in excised membrane patches and in intact epithelial monolayers. This compound increased the open probabilities of deltaF508-CFTR channels in excised membrane patches by 10-15-fold under conditions in which wild-type channels were already maximally active. Our results support the emerging view that CFTR channel activity is substantially reduced by the deltaF508 mutation and that effective CF therapies may require the use of channel openers to activate mutant CFTR channels at the cell surface.

Binding site of activators of the cystic fibrosis transmembrane conductance regulator in the nucleotide binding domains

The use of substances that could activate the defective chloride channels of the mutant cystic fibrosis transmembrane conductance regulator (CFTR) has been suggested as possible therapy for cystic fibrosis. Using epithelia formed by cells stably transfected with wildtype or mutant (G551D, G1349D) CFTR, we estimated the apparent dissociation constant, K(D), of a series of CFTR activators by measuring the increase in the apical membrane current. Modification of apparent K(D) of CFTR activators by mutations of the nucleotide-binding domains (NBDs) suggests that the binding site might be in these regions. The human NBD structure was predicted by homology with murine NBD1. An NBD1-NBD2 complex was constructed by overlying monomers to a bacterial ABC transporter NBD dimer in the "head-to-tail" conformation. Binding sites for CFTR activators were predicted by molecular docking. Comparison of theoretical binding free energy estimated in the model to free energy estimated from the apparent dissociation constants, K(D), resulted in a remarkably good correlation coefficient for one of the putative binding sites, located in the interface between NBD1 and NBD2.

Curcumin Stimulates Cystic Fibrosis Transmembrane Conductance Regulator Cl– Channel Activity

Compounds that enhance either the function or biosynthetic processing of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel may be of value in developing new treatments for cystic fibrosis (CF). Previous studies suggested that the herbal extract curcumin might affect the processing of a common CF mutant, CFTR-DeltaF508. Here, we tested the hypothesis that curcumin influences channel function. Curcumin increased CFTR channel activity in excised, inside-out membrane patches by reducing channel closed time and prolonging the time channels remained open. Stimulation was dose-dependent, reversible, and greater than that observed with genistein, another compound that stimulates CFTR. Curcumin-dependent stimulation required phosphorylated channels and the presence of ATP. We found that curcumin increased the activity of both wild-type and DeltaF508 channels. Adding curcumin also increased Cl(-) transport in differentiated non-CF airway epithelia but not in CF epithelia. These results suggest that curcumin may directly stimulate CFTR Cl(-) channels.

Strategies to investigate the mechanism of action of CFTR modulators

The malfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is associated with a wide spectrum of disease. In the search for modulators of CFTR, pharmaceutical agents have been identified that (i) act indirectly by regulating the protein kinases and phosphatases, which control CFTR, and (ii) interact directly with CFTR. Some agents modulate CFTR by altering the function of the nucleotide-binding domains (NBDs) that control channel gating, whereas others inhibit CFTR by preventing Cl- flow through the channel pore. Knowledge of CFTR modulators might lead to new understanding of the CFTR Cl- channel, its physiological role and malfunction in disease.

Increase in intracellular Cl- concentration by cAMP- and Ca2+-dependent stimulation of M1 collecting duct cells

In the lungs of cystic fibrosis (CF) patients, mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) lead to defective Cl- secretion and hyperabsorption of electrolytes. This may be a an important cause for the defective mucociliary clearance in CF lungs. Previous studies have suggested that inhibition of ENaC during activation of CFTR or by purinergic stimulation could be related to an increase in the intracellular [Cl-]i. This was examined in the present study using cultured mouse M1 collecting duct cells transfected with the chloride-sensitive enhanced yellow fluorescent protein YFP(V163S). Calibration experiments showed a linear decrease of YFP fluorescence intensity with increasing [Cl-]i (0-100 mM). Activation of CFTR by isobutyl-1-methylxanthine (IBMX, 100 microM) and forskolin (2 microM) increased [Cl-]i by 9.6+/-1.5 mM (n=35). Similarly, ATP (100 microM) increased [Cl-]i transiently by 9.5+/-2.2 mM (n=17). The increase in [Cl-]i was reduced by the Na+/K+/2 Cl- -cortransporter-1 (NKCC1) blocker azosemide (100 microM), the CFTR blocker SP-303 (50 microM), the blocker of Ca2+-activated Cl- channels DIDS (100 microM) or the ENaC blocker amiloride (10 microM). Changes in YFP(V163S) fluorescence were not due to changes in cell volume or intracellular pH. The present data thus demonstrate an increase in [Cl-]i following stimulation with secretagogues, which could participate in the inhibition of ENaC.

Identification of CFTR activators and inhibitors: chance or design?

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-activated Cl(-) channel expressed in various epithelial cells, and is a pharmacological target for activators and inhibitors. Activators are useful for the pharmacotherapy of cystic fibrosis, specifically for those mutations that affect CFTR protein by reducing its ability to stay in the open state. Conversely, inhibitors are potentially useful to treat secretory diarrhoea caused by enterotoxins, as the CFTR is the main route for Cl(-) flux in the intestine. Recently, a variety of potent modulators of the CFTR Cl(-) channel activity have been identified by high-throughput screening of a large collections of small molecules. The identification of CFTR activators and inhibitors with novel chemical scaffolds might help with the rational design of compounds with improved pharmacological properties.

2-(dialkylamino)-4H-1-benzopyran-4-one derivatives modify chloride conductance in CFTR expressing cells

Some 2-(diethylamino)-7-hydroxy-4H-1-benzopyran-4-one derivatives, potentially useful as activators of the cystic fibrosis transmembrane conductance regulator (CFTR), were prepared. The synthesized compounds were tested, together with others 2-(dialkylamino)-7-hydroxybenzopyran-4-one derivatives, by measuring their capacity to modify the kinetics of iodide influx in Fisher rat thyroid cells expressing wild type CFTR and the halide-sensitive yellow fluorescent protein. Among the tested compounds the dinitrile derivatives 8 and 9 are endowed with an activity comparable to the reference compound apigenin.

CFTR pharmacology and its role in intestinal fluid secretion

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-activated Cl(-) channel expressed in epithelial cells in the airways, pancreas, intestine and other fluid-transporting tissues. Cystic fibrosis is caused by mutations in the CFTR, resulting in impaired Cl(-) transport and plasma membrane targeting. CFTR is expressed in the lumenal membrane of enterocytes, where it functions as the principal pathway for secretion of Cl(-) and fluid in enterotoxin-induced secretory diarrheas such as cholera. Small-molecule CFTR inhibitors reduce enterotoxin-induced intestinal fluid secretion in animal models. CFTR inhibition might also reduce intestinal fluid losses in cholera and possibly in other infectious and non-infectious diarrheas.

High-affinity activators of cystic fibrosis transmembrane conductance regulator (CFTR) chloride conductance identified by high-throughput screening

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) protein that reduce cAMP-stimulated Cl(-) conductance in airway and other epithelia. The purpose of this investigation was to identify new classes of potent CFTR activators. A collection of 60,000 diverse drug-like compounds was screened at 10 microm together with a low concentration of forskolin (0.5 microm) in Fisher rat thyroid epithelial cells co-expressing human CFTR and a green fluorescent protein-based Cl(-) sensor. Primary screening yielded 57 strong activators (greater activity than reference compound apigenin), most of which were unrelated in chemical structure to known CFTR activators, and 284 weaker activators. Secondary analysis of the strong activators included analysis of CFTR specificity, forskolin requirement, transepithelial short-circuit current, activation kinetics, dose response, toxicity, and activation mechanism. Three compounds, the most potent being a dihydroisoquinoline, activated CFTR by elevating cellular cAMP, probably by phosphodiesterase inhibition. Fourteen compounds activated CFTR without cAMP elevation or phosphatase inhibition, suggesting direct CFTR interaction. The most potent compounds had tetrahydrocarbazol, hydroxycoumarin, and thiazolidine core structures. These compounds induced CFTR Cl(-) currents rapidly (<5 min) with K(d) down to 200 nm and were CFTR-selective, reversible, and nontoxic. Several compounds, the most potent being a trifluoromethylphenylbenzamine, activated the CF-causing mutant G551D, but with much weaker affinity (K(d) > 10 microm). When added for 10 min, none of the compounds activated DeltaPhe(508)-CFTR in transfected cells grown at 37 degrees C (with DeltaPhe(508)-CFTR trapped in the endoplasmic reticulum). However, after correction of trafficking by 48 h of growth at 27 degrees C, tetrahydrocarbazol and N-phenyltriazine derivatives strongly stimulated Cl(-) conductance with K(d) < 1 microm. The new activators identified here may be useful in defining molecular mechanisms of CFTR activation and as lead compounds in CF drug development.

Cell-based assay for high-throughput quantitative screening of CFTR chloride transport agonists

Drug discovery by high-throughput screening is a promising approach to develop new therapies for the most common lethal genetic disease, cystic fibrosis. Because disease-causing mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) protein produce epithelial cells with reduced or absent Cl(-) permeability, the goal of screening is to identify compounds that restore cell Cl(-) transport. We have developed a rapid, quantitative screening procedure for analysis of CFTR-mediated halide transport in cells with the use of a conventional fluorescence plate reader. Doubly transfected cell lines were generated that express wild-type or mutant CFTR together with a yellow fluorescent protein (YFP)-based halide sensor. CFTR function was assayed from the time course of cell fluorescence in response to extracellular addition of 100 mM I(-) followed by forskolin, resulting in decreased YFP fluorescence due to CFTR-mediated I(-) entry. Cell lines were chosen, and conditions were optimized to minimize basal halide transport to maximize assay sensitivity. In cells cultured on 96-well plastic dishes, the assay gave reproducible halide permeabilities from well to well and could reliably detect a 2% activation of CFTR-dependent halide transport produced by low concentrations of forskolin. Applications of the assay are shown, including comparative dose-dependent CFTR activation by genistein, apigenin, 8-cyclopentyl-1,3-dipropylxanthine, IBMX, 8-methoxypsoralen, and milrinone as well as activation of alternative Cl(-) channels. The fluorescence assay and cell lines should facilitate the screening of novel CFTR activators and the characterization of alternative Cl(-) channels and transporters.

Novel CFTR chloride channel activators identified by screening of combinatorial libraries based on flavone and benzoquinolizinium lead compounds

The flavonoid genistein and the benzo[c]quinolizinium MPB-07 have been shown to activate the cystic fibrosis transmembrane conductance regulator (CFTR), the protein that is defective in cystic fibrosis. Lead-based combinatorial and parallel synthesis yielded 223 flavonoid, quinolizinium, and related heterocyclic compounds. The compounds were screened for their ability to activate CFTR at 50 microm concentration by measurement of the kinetics of iodide influx in Fisher rat thyroid cells expressing wild-type or G551D CFTR together with the green fluorescent protein-based halide indicator YFP-H148Q. Duplicate screenings revealed that 204 compounds did not significantly affect CFTR function. Compounds of the 7,8-benzoflavone class, which are structurally intermediate between flavones and benzo[c]quinoliziniums, were effective CFTR activators with the most potent being 2-(4-pyridinium)benzo[h]4H-chromen-4-one bisulfate (UCcf-029). Compounds of the novel structural class of fused pyrazolo heterocycles were also strong CFTR activators with the most potent being 3-(3-butynyl)-5-methoxy-1-phenylpyrazole-4-carbaldehyde (UCcf-180). A CFTR inhibitor was also identified. The active compounds did not induce iodide influx in null cells deficient in CFTR. Short-circuit current measurements showed that the CFTR activators identified by screening induced strong anion currents in the transfected cell monolayers grown on porous supports. Compared with genistein, the most active compounds had up to 10 times greater potency in activating wild-type and/or G551D-CFTR. The activators had low cellular toxicity and did not elevate cellular cAMP concentration or inhibit phosphatase activity, suggesting that CFTR activation may involve a direct interaction. These results establish an efficient screening procedure to identify CFTR activators and inhibitors and have identified 7,8-benzoflavones and pyrazolo derivatives as novel classes of CFTR activators.

Pharmacological modulation of ion transport across wild-type and DeltaF508 CFTR-expressing human bronchial epithelia

Forskolin, UTP, 1-ethyl-2-benzimidazolinone (1-EBIO), NS004, 8-methoxypsoralen (Methoxsalen; 8-MOP), and genistein were evaluated for their effects on ion transport across primary cultures of human bronchial epithelium (HBE) expressing wild-type (wt HBE) and DeltaF508 (DeltaF-HBE) cystic fibrosis transmembrane conductance regulator. In wt HBE, the baseline short-circuit current (I(sc)) averaged 27.0 +/- 0.6 microA/cm(2) (n = 350). Amiloride reduced this I(sc) by 13.5 +/- 0.5 microA/cm(2) (n = 317). In DeltaF-HBE, baseline I(sc) was 33.8 +/- 1.2 microA/cm(2) (n = 200), and amiloride reduced this by 29.6 +/- 1.5 microA/cm(2) (n = 116), demonstrating the characteristic hyperabsorption of Na(+) associated with cystic fibrosis (CF). In wt HBE, subsequent to amiloride, forskolin induced a sustained, bumetanide-sensitive I(sc) (DeltaI(sc) = 8.4 +/- 0.8 microA/cm(2); n = 119). Addition of acetazolamide, 5-(N-ethyl-N-isopropyl)-amiloride, and serosal 4, 4'-dinitrostilben-2,2'-disulfonic acid further reduced I(sc), suggesting forskolin also stimulates HCO(3)(-) secretion. This was confirmed by ion substitution studies. The forskolin-induced I(sc) was inhibited by 293B, Ba(2+), clofilium, and quinine, whereas charybdotoxin was without effect. In DeltaF-HBE the forskolin I(sc) response was reduced to 1.2 +/- 0.3 microA/cm(2) (n = 30). In wt HBE, mucosal UTP induced a transient increase in I(sc) (Delta I(sc) = 15. 5 +/- 1.1 microA/cm(2); n = 44) followed by a sustained plateau, whereas in DeltaF-HBE the increase in I(sc) was reduced to 5.8 +/- 0. 7 microA/cm(2) (n = 13). In wt HBE, 1-EBIO, NS004, 8-MOP, and genistein increased I(sc) by 11.6 +/- 0.9 (n = 20), 10.8 +/- 1.7 (n = 18), 10.0 +/- 1.6 (n = 5), and 7.9 +/- 0.8 microA/cm(2) (n = 17), respectively. In DeltaF-HBE, 1-EBIO, NS004, and 8-MOP failed to stimulate Cl(-) secretion. However, addition of NS004 subsequent to forskolin induced a sustained Cl(-) secretory response (2.1 +/- 0.3 microA/cm(2), n = 21). In DeltaF-HBE, genistein alone stimulated Cl(-) secretion (2.5 +/- 0.5 microA/cm(2), n = 11). After incubation of DeltaF-HBE at 26 degrees C for 24 h, the responses to 1-EBIO, NS004, and genistein were all potentiated. 1-EBIO and genistein increased Na(+) absorption across DeltaF-HBE, whereas NS004 and 8-MOP had no effect. Finally, Ca(2+)-, but not cAMP-mediated agonists, stimulated K(+) secretion across both wt HBE and DeltaF-HBE in a glibenclamide-dependent fashion. Our results demonstrate that pharmacological agents directed at both basolateral K(+) and apical Cl(-) conductances directly modulate Cl(-) secretion across HBE, indicating they may be useful in ameliorating the ion transport defect associated with CF.

Pharmacology of CFTR chloride channel activity

Pharmacology of CFTR Chloride Channel Activity. Physiol. Rev. 79, Suppl.: S109-S144, 1999. - The pharmacology of cystic fibrosis transmembrane conductance regulator (CFTR) is at an early stage of development. Here we attempt to review the status of those compounds that modulate the Cl- channel activity of CFTR. Three classes of compounds, the sulfonylureas, the disulfonic stilbenes, and the arylaminobenzoates, have been shown to directly interact with CFTR to cause channel blockade. Kinetic analysis has revealed the sulfonylureas and arylaminobenzoates interact with the open state of CFTR to cause blockade. Suggestive evidence indicates the disulfonic stilbenes act by a similar mechanism but only from the intracellular side of CFTR. Site-directed mutagenesis studies indicate the involvement of specific amino acid residues in the proposed transmembrane segment 6 for disulfonic stilbene blockade and segments 6 and 12 for arylaminobenzoate blockade. Unfortunately, these compounds (sulfonylureas, disulfonic stilbenes, arylaminobenzoate) also act at a number of other cellular sites that can indirectly alter the activity of CFTR or the transepithelial secretion of Cl-. The nonspecificity of these compounds has complicated the interpretation of results from cellular-based experiments. Compounds that increase the activity of CFTR include the alkylxanthines, phosphodiesterase inhibitors, phosphatase inhibitors, isoflavones and flavones, benzimidazolones, and psoralens. Channel activation can arise from the stimulation of the cAMP signal transduction cascade, the inhibition of inactivating enzymes (phosphodiesterases, phosphatases), as well as the direct binding to CFTR. However, in contrast to the compounds that block CFTR, a detailed understanding of how the above compounds increase the activity of CFTR has not yet emerged.

Interrelationship between the Na+/glucose cotransporter and CFTR in Caco-2 cells: relevance to cystic fibrosis

Both the Na+-dependent glucose cotransporter (SGLT1) and the cystic fibrosis transmembrane conductance regulator (CFTR) modulate Na+ and fluid movement, although in opposite directions. Yet few studies have investigated a possible interrelationship between these two transporters. By using the Caco-2 human colon carcinoma cell line, we confirmed that the activities of these transporters increased with spontaneous differentiation to the enterocytic phenotype. We showed that SGLT1 was positively regulated by Cl- and that optimal activity of CFTR was dependent on the presence of glucose. We also demonstrated that inhibition of CFTR by glibenclamide or diphenylamine-2-carboxylate did not modify the activity of SGLT1 and inhibition of SGLT1 by phlorizin did not modify the activity of CFTR, although it resulted in inhibition of glycoconjugate synthesis. These results point to positive substrate-cross regulation of SGLT1 and CFTR and suggest that NaCl and glucose are important for not only Na+ absorption and fluid movement, but also for cAMP-dependent Cl- efflux, and glycoconjugate synthesis, functions that are known to be anomalous in cystic fibrosis.

Structural basis for specificity and potency of xanthine derivatives as activators of the CFTR chloride channel

1. On the basis of their structure, we compared the ability of 35 xanthine derivatives to activate the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel stably expressed in chinese hamster ovary (CHO) cells using the cell-attached patch clamp and iodide efflux techniques. 2. Activation of CFTR channels was obtained with 3-mono, 1,3-di or 1,3,7-tri-substituted alkyl xanthine derivatives (enprofylline, theophylline, aminophylline, IBMX, DPMX and pentoxifylline). By contrast, xanthine derivatives substituted at the C8- or N9-position failed to open CFTR channels. 3. The CFTR chloride channel activity was blocked by glibenclamide (100 microM) but not by DIDS (100 microM). 4. Activation of CFTR by xanthines was not mimicked by the calcium ionophore A23187, adenosine, UTP, ATP or the specific phosphodiesterase inhibitors rolipram, Ro 20-1724 and milrinone. In addition, we found no correlation between the effect of xanthines on CFTR and on the cellular cyclic AMP or ATP levels. 5. We then synthesized a series of 3,7-dimethyl-1-alkyl xanthine derivatives; among them, 3,7-dimethyl-1-propyl xanthine and 3,7-dimethyl-1-isobutyl xanthine both activated CFTR channels without increasing the intracellular cyclic AMP level, while the structurally related 3,7-dimethyl-1-(2-propenyl) xanthine and 3,7-dimethyl-1-(oxiranyl methyl) xanthine were inactive. 6. Our findings delineate a novel function for xanthine compounds and identify the molecular features that enable xanthine activation of CFTR. These results may be useful in the development of new molecules for studying the pharmacology of chloride channels.

cGMP stimulation of cystic fibrosis transmembrane conductance regulator Cl- channels co-expressed with cGMP-dependent protein kinase type II but not type Ibeta

In order to investigate the involvement of cGMP-dependent protein kinase (cGK) type II in cGMP-provoked intestinal Cl- secretion, cGMP-dependent activation and phosphorylation of cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels was analyzed after expression of cGK II or cGK Ibeta in intact cells. An intestinal cell line which stably expresses CFTR (IEC-CF7) but contains no detectable endogenous cGK II was infected with a recombinant adenoviral vector containing the cGK II coding region (Ad-cGK II) resulting in co-expression of active cGK II. In these cells, CFTR was activated by membrane-permeant analogs of cGMP or by the cGMP-elevating hormone atrial natriuretic peptide as measured by 125I- efflux assays and whole-cell patch clamp analysis. In contrast, infection with recombinant adenoviruses expressing cGK Ibeta or luciferase did not convey cGMP sensitivity to CFTR in IEC-CF7 cells. Concordant with the activation of CFTR by only cGK II, infection with Ad-cGK II but not Ad-cGK Ibeta enabled cGMP analogs to increase CFTR phosphorylation in intact cells. These and other data provide evidence that endogenous cGK II is a key mediator of cGMP-provoked activation of CFTR in cells where both proteins are co-localized, e. g. intestinal epithelial cells. Furthermore, they demonstrate that neither the soluble cGK Ibeta nor cAMP-dependent protein kinase are able to substitute for cGK II in this cGMP-regulated function.