SPECIAL TOPIC
CFTR trafficking and signaling in respiratory epithelium

Bruce R. Pitt

Department of Environmental and Occupational Health, Graduate School of Public Health, University of Pittsburgh, Pittsburgh, Pennsylvania 15238


    INTRODUCTION
TOP
INTRODUCTION
MOLECULAR CHAPERONES AND...
PROINFLAMMATORY PROFILE OF CF...
SERINE PROTEASE-SENSITIVE...
EDITOR'S NOTE
REFERENCES

RESEARCH IN CYSTIC FIBROSIS (CF) remains at the leading edge of investigations employing human genetics and molecular and cellular biology. In the 12 years since the discovery of the gene encoding the CF transmembrane conductance regulator (CFTR) Cl- channel (14), translation of such efforts clinically has been remarkable and includes 1) identification of almost 1,000 different mutations (http://www.genet.sickkids.on.ca/cftr), including 70% with a phenylalanine deletion (Delta F508) that has led to successful clinical detection of individuals with a family history (even before the completion of the human genome project) and 2) ~20 trials of human CF gene therapy involving dosing to the nose or airways (2, 22). Such rapid progress, however, belies the complexity of the CFTR gene and the pathophysiology of CF.

CFTR is a unique member of the ATP-binding cassette transporter gene family in that it 1) conducts Cl-, 2) transports substrates across membranes in a nonconductive fashion (e.g., facilitates ATP release), 3) regulates other ion channel proteins (e.g., positive regulation of outwardly rectifying Cl- channels or negative regulation of Na+ channels), and 4) regulates intracellular compartment acidification and protein processing (18). Such complexity has led these authors to categorize defects in CF respiratory epithelium as primary (dehydration associated with dysfunctional Cl- transport), secondary (hyperabsorption of Na+), or tertiary (enhanced binding of Pseudomonas aeruginosa; proinflammatory environment). Fundamental to such diversity is the awareness that very minor mutations (e.g., Delta F508, a single-amino acid deletion within a 1,480-amino acid protein) result in the profound pathological phenotype of CF (23). In the case of Delta F508, the missense mutation results in a mutant CFTR that is believed to fold improperly, and such defects in its biosynthesis and trafficking result in little or no surface CFTR. Other mutations in CFTR lead to genotypic or phenotypic changes that span the range from no CFTR in the membrane to CFTR that reaches the membrane but does not respond to appropriate stimuli or conduct Cl- (6). Adding to this complexity is a recent report describing tissue-specific changes in the impairment of Delta F508 processing (10). Some mutations overlap with PDZ-interacting domains on the COOH terminus of CFTR that are required for polarization of CFTR to the apical membrane of respiratory epithelium (13) and may underscore CFTR regulation of other ion channels by affecting the relationship between accessory proteins, linker proteins, and regulatory cofactors.

Accordingly, research into the biosynthesis and trafficking of wild-type and mutated CFTR is at the forefront of CF investigations. Although the defect in Cl- conductance is critical to the pathology of CF respiratory epithelia, contributions of alterations in vesicle trafficking, protein processing, and immune function clearly contribute to the pathogenesis and maintenance of CF (18). These efforts underlie the disparities between genotype and phenotype in CF and suggest that in addition to nucleotide repair (gene therapy, aminoglycoside-dependent restoration of readthrough of full-length CFTR), pharmacotherapy targeted toward improved biosynthesis of mutated CFTR may be a useful adjunct approach to the treatment of CF (24).


    MOLECULAR CHAPERONES AND BIOSYNTHESIS
OF CFTR
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INTRODUCTION
MOLECULAR CHAPERONES AND...
PROINFLAMMATORY PROFILE OF CF...
SERINE PROTEASE-SENSITIVE...
EDITOR'S NOTE
REFERENCES

The importance of providing mechanistic support for pharmacologically affecting the maturation of CF is placed in the context of our current molecular physiological understanding of the roles of various chaperones for CFTR by Brodsky (4) in this issue of the American Journal of Physiology-Lung Cellular and Molecular Physiology. The steps in proper folding of CFTR, quality control mechanism of endoplasmic reticulum trafficking, and requisite association with various chaperones are outlined by Brodsky and reference is made to three original articles in the Special Topic. The mechanisms by which sodium 4-phenylbutyrate may be a modulator of such chaperones are presented by Rubenstein and Lyons (17) and Choo-Kang and Zeitlin (5), and the in vivo efficacy of a chemical chaperone, trimethylamine oxide, in the restoration of defective Cl- conductance in the critical target tissue (intestinal epithelium) of CFTR null mutant mice is presented by Fischer et al. (7). This collective work strongly supports the concept that small molecules are useful alternative or adjunct therapies for disorders of protein folding (23, 24).


    PROINFLAMMATORY PROFILE OF CF EPITHELIA
TOP
INTRODUCTION
MOLECULAR CHAPERONES AND...
PROINFLAMMATORY PROFILE OF CF...
SERINE PROTEASE-SENSITIVE...
EDITOR'S NOTE
REFERENCES

A critical aspect of CF is chronic airway inflammation, and several groups have provided support that dysregulation of the inflammatory response is an intrinsic component of such a phenotype. An editorial by Blackwell et al. (1) accompanies the manuscript by Weber et al. (21) and highlights the central role of unregulated nuclear factor (NF)-kappa B in the proinflammatory cytokine profile of cells expressing mutated CFTR. In this regard, blocking the production of NF-kappa B may be an alternative rational therapeutic approach in CF therapy. In addition to enhanced cytokine (interleukin-8, tumor necrosis factor-alpha ) production, oxidative stress appears to be a component of CF epithelia. Previous in vitro work (9, 11) showed that CFTR is associated with the transport of glutathione (GSH). In the current Special Topic, Gao et al. (8) show that the Cl- channel-forming peptide N-K4-M2GlyR increased Cl- secretion and GSH efflux in a human CF airway epithelial cell line, suggesting that apical Cl- conductance (but not necessarily CFTR function per se) is coupled to GSH efflux. Velsor et al. (20) report in this issue that GSH was decreased in the epithelial lining fluid of CFTR-deficient mice and that an imbalance in antioxidant defense in CFTR-deficient mice was evident. These data extend previous observations that GSH was decreased in the airway fluid of humans with CFTR (16) and that aerosol delivery of GSH may be a useful therapy for CF (15). GSH and other cellular defense mechanisms may indeed represent a rational therapeutic strategy in CF.


    SERINE PROTEASE-SENSITIVE SODIUM HYPERABSORPTION IN HUMAN AIRWAY EPITHELIUM
TOP
INTRODUCTION
MOLECULAR CHAPERONES AND...
PROINFLAMMATORY PROFILE OF CF...
SERINE PROTEASE-SENSITIVE...
EDITOR'S NOTE
REFERENCES

As noted above, Na+ hyperabsorption is a critical secondary-like phenomenon in CF epithelia. Although the mechanism coupling such hyperabsorption to CFTR mutations remains unknown, an extracellular serine protease-mediated signaling pathway has been identified (19) that activates an amiloride-sensitive Na+ channel. In the Special Topic in this issue, Bridges et al. (3) show that human bronchial epithelial cells are sensitive to aprotinin. Using the Kunitz domain of aprotinin as a pharmacophore, they show that BAY 39-9437, a recombinant Kunitz-type serine protease inhibitor, decreased Na+ absorption in non-CF and CF human bronchial epithelia.


    EDITOR'S NOTE
TOP
INTRODUCTION
MOLECULAR CHAPERONES AND...
PROINFLAMMATORY PROFILE OF CF...
SERINE PROTEASE-SENSITIVE...
EDITOR'S NOTE
REFERENCES

This is the first of a series of special topics that will appear routinely in the American Journal of Physiology-Lung Cellular and Molecular Physiology. The purpose of these special calls is to highlight areas of significant interest in respiratory biology by soliciting input from a broad research community, fast tracking the submitted manuscripts through the review process, and publishing them under their own separate heading. In this case, a call for papers in "CFTR Trafficking and Signaling in Respiratory Epithelium" initially appeared in July 2000 (12). This led to the seven manuscripts and two accompanying editorials that appear in the current issue. This remarkable and timely turnaround in peer review is a testimony to the importance of the topic and the enthusiasm of the authors and reviewers to bring this information to our readership.


    FOOTNOTES

This special topic section is a collection of papers accepted under a special call for manuscripts by the Editor. See Journal web site for information about the next call.

Address for reprint requests and other correspondence: B. R. Pitt, Dept. of Environmental and Occupational Health, Graduate School Public Health, Univ. of Pittsburgh, Pittsburgh, PA 15238 (E-mail: brucep+{at}pitt.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


    REFERENCES
TOP
INTRODUCTION
MOLECULAR CHAPERONES AND...
PROINFLAMMATORY PROFILE OF CF...
SERINE PROTEASE-SENSITIVE...
EDITOR'S NOTE
REFERENCES

1.   Blackwell, TS, Stecenko AA, and Christman JW. Dysregulated NF-kappa B activation in cystic fibrosis: evidence for a primary inflammatory disorder. Am J Physiol Lung Cell Mol Physiol 281: L69-L70, 2001[Free Full Text].

2.   Boucher, RC. Status of gene therapy for cystic fibrosis lung disease. J Clin Invest 103: 441-445, 1999[Free Full Text].

3.   Bridges, RJ, Newton BB, Pilewski JM, Devor DC, Poll CT, and Hall RL. Na+ transport in normal and CF human bronchial epithelial cells is inhibited by BAY 39-9437. Am J Physiol Lung Cell Mol Physiol 281: L16-L23, 2001[Abstract/Free Full Text].

4.   Brodsky, JL. Chaperoning the maturation of the cystic fibrosis transmembrane conductance regulator. Am J Physiol Lung Cell Mol Physiol 281: L39-L42, 2001[Free Full Text].

5.   Choo-Kang, LR, and Zeitlin PL. Induction of HSP70 promotes Delta F508 CFTR trafficking. Am J Physiol Lung Cell Mol Physiol 281: L58-L68, 2001[Abstract/Free Full Text].

6.   Drumm, M. What happens to Delta F508 in vivo? J Clin Invest 103: 1369-1370, 1999[Free Full Text].

7.   Fischer, H, Fukuda N, Barbry P, Illek B, Sartori C, and Matthay MA. Partial restoration of defective chloride conductance in Delta F508 CF mice by the chemical chaperone trimethylamine oxide. Am J Physiol Lung Cell Mol Physiol 281: L52-L57, 2001[Abstract/Free Full Text].

8.   Gao, L, Broughman JR, Iwamoto T, Tomich JM, Venglarik CJ, and Forman HJ. Synthetic chloride channel restores glutathione secretion in cystic fibrosis airway epithelia. Am J Physiol Lung Cell Mol Physiol 281: L24-L30, 2001[Abstract/Free Full Text].

9.   Gao, L, Kim KJ, Yankaskas JB, and Forman HJ. Abnormal glutathione transport in cystic fibrosis airway epithelia. Am J Physiol Lung Cell Mol Physiol 277: L113-L118, 1999[Abstract/Free Full Text].

10.   Kalin, N, Claas A, Sommer M, Puchelle E, and Tummler B. Delta F508 CFTR protein expression in tissues from patients with cystic fibrosis. J Clin Invest 103: 1379-1389, 1999[Abstract/Free Full Text].

11.   Lindsell, P, and Hanrahan JW. Glutathione permeability of CFTR. Am J Physiol Cell Physiol 275: C323-C326, 1998[Abstract/Free Full Text].

12.   Malik, AB. Call for papers in "CFTR trafficking and signaling in respiratory epithelium." Am J Physiol Lung Cell Mol Physiol 279: L1, 2000[Free Full Text].

13.   Moyer, BD, Denton J, Karlson KH, Reynolds D, Wang S, Mickle JE, Milewski M, Cutting GR, Guggino WB, Li M, and Stanton BA. A PDZ-interacting domain in CFTR is an apical membrane polarization signal. J Clin Invest 104: 1353-1361, 1999[Abstract/Free Full Text].

14.   Riordan, JR, Rommens JM, Kerem B, Alon N, Rozmahel R, Grzelczak Z, Zielinski J, Lok S, Plavsic N, Chou JL, Drumm ML, Iannuzzi MC, Collins FS, and Tsui LC. Identification of the cystic fibrosis gene: cloning and characterization of complementary cDNA. Science 245: 1066-1073, 1989[ISI][Medline].

15.   Roum, JH, Borok Z, McElvaney NG, Grimes GJ, Bokser AD, Buhl R, and Crystal RG. Glutathione aerosol suppresses lung epithelial surface inflammatory cell-derived oxidants in cystic fibrosis. J Appl Physiol 87: 438-443, 1999[Abstract/Free Full Text].

16.   Roum, JH, Buhl R, McElvaney NG, Borok Z, and Crystal RG. Systemic deficiency of glutathione in cystic fibrosis. J Appl Physiol 75: 2419-2424, 1993[Abstract].

17.   Rubenstein, RC, and Lyons BM. Sodium 4-phenylbutyrate downregulates HSC70 expression by facilitating mRNA degradation. Am J Physiol Lung Cell Mol Physiol 281: L43-L51, 2001[Abstract/Free Full Text].

18.   Schweibert, EM, Benos DJ, Egan ME, Stutts MJ, and Guggino WB. CFTR is a conductance regulator as well as a chloride channel. Physiol Rev 79: S145-S166, 1999[Medline].

19.   Vallet, V, Chraibi A, Gaeggeler HP, Horisberger JD, and Rossier BC. An epithelial serine protease activates the amiloride sensitive sodium channel. Nature 389: 607-610, 1997[ISI][Medline].

20.   Velsor, LW, van Heeckeren A, and Day BJ. Antioxidant imbalance in the lungs of cystic fibrosis transmembrane conductance regulator protein mutant mice. Am J Physiol Lung Cell Mol Physiol 281: L31-L38, 2001[Abstract/Free Full Text].

21.   Weber, AJ, Soong G, Bryan R, Saba S, and Prince A. Activation of NF-kappa B in airway epithelial cells is dependent on CFTR trafficking and Cl- channel function. Am J Physiol Lung Cell Mol Physiol 281: L71-L78, 2001[Abstract/Free Full Text].

22.   Welsh, MJ. Gene transfer for cystic fibrosis. J Clin Invest 104: 1165-1166, 1999[Free Full Text].

23.   Welch, WJ, and Howard MB. Antagonists to the rescue. J Clin Invest 105: 853-854, 2000[Free Full Text].

24.   Zeitlin, PL. Novel pharmacologic therapies for cystic fibrosis. J Clin Invest 103: 447-452, 1999[Free Full Text].


Am J Physiol Lung Cell Mol Physiol 281(1):L13-L15
1040-0605/01 $5.00 Copyright © 2001 the American Physiological Society




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