© 2002 American Thoracic Society
No Evidence of Glucocorticosteroid-induced Apoptosis of Airway Epithelial Cells In VivoNO EVIDENCE OF GLUCOCORTICOSTEROID-INDUCED APOPTOSIS OF AIRWAY EPITHELIAL CELLS IN VIVOTo the Editor : In the November issue of AJRCCM, Dorscheid and colleagues (1) reported on "Apoptosis of airway epithelial cells induced by corticosteroids." In this in vitro study, various glucocorticoids induced apoptosis of airway epithelial cells (cultured primary cells and a cell line) in a time-dependent and concentration-dependent manner. The conclusion was drawn that "our data raise the possibility that glucocorticoids may have a deleterious role in the ongoing process of airway damage and the airways remodeling of chronic asthma." First of all, these in vitro results are in contrast to well-known clinical findings. Regular treatment with an inhaled corticosteroid restores a damaged airway epithelium and not the reverse (2). Inhaled corticosteroids are also so far the only antiasthma medication resulting in this normalization (3). An explanation of the findings of Dorscheid and colleagues (1) is probably found in the methodology that used high glucocorticoid concentrations and continuous incubation with steroids for 24 hours. For beclomethasone, triamcinolone, and budesonide, the apoptosis was significant at the concentrations equal to 3 µM (and higher) after continuous incubations for 24 hours. The authors speculate that concentrations up to 30 µM can be achieved at "the apical face of exposed central airway epithelial cells." However, the empirical data do not support this view (4–6). Tissue concentrations in the order of up to 0.1 µM can possibly be reached in the airways for a short time after inhalation of high doses (6). However, the tissue concentration declines quickly with time, and after some hours, concentrations in the order of 10-9–10-8 M are reported in the airway and lung tissue (4–6). In the article by Dorscheid and colleagues (1), dexamethasone was the most potent of the steroids tested and induced epithelial cell apoptosis at 0.3 µM after 24 hours incubation. However, even at the 10-times higher concentration (3 µM), dexamethasone did not have any effect at all after incubation for 2 hours but required 12 hours incubation to reach statistical significance. In in vitro studies, it is probable that short pulses of a steroid provide the best way to mimic the intermittent exposure of airway epithelial cells to inhaled steroids in vivo. In vitro studies may give important information but extrapolation of results to the clinical situation is difficult and requires caution.
a Lund, Sweden REFERENCES
We appreciate the letter from Drs. Miller-Larsson and Selroos regarding our article in the November issue of AJRCCM (1), and would like to address their points. There is evidence that corticosteroids are associated with in vivo epithelial damage. Benayoun and colleagues (2) found increased epithelial cell apoptosis (measured by increased activated caspase-3) in endobronchial biopsies of subjects with asthma when compared to normal subjects. This finding was further exaggerated in those subjects with asthma treated with corticosteroids. Chagani and colleagues (3) suggest that the extent of epithelial apoptosis corresponds to asthma severity. As yet, these are associations and do not demonstrate causality, but certainly raise the issue of a link between corticosteriod use and epithelial damage. We acknowledge that a study (4) demonstrates severely damaged epithelium to be "repaired" after treatment with inhaled corticosteroids; however, this study did not specifically identify epithelial cell apoptosis. Drs. Miller-Larsson and Selroos comment that at 2 hours, inhaled steroids are absent from lung tissue. As apoptosis is detected in vitro after 2 hours of exposure, they feel it is unreasonable to project this result onto the in vivo condition. We demonstrated in our article that glucocorticoid-induced apoptosis is a time-dependent process. As receptor activation may lead to an effect even after the ligand has been cleared, our results are not inconsistent with apoptosis in vivo, although "pulsed" steroids need to be investigated. Numerous in vitro studies of corticosteroid-induced eosinophil apoptosis examine a 24-hour time point (5). The benefits of corticosteroids on eosinophil apoptosis are accepted even with the same issue of steroid clearance and measured time points. Miller-Larsson and colleagues (6) demonstrated that budesonide has improved uptake from the airway and a prolonged duration of action in comparison to other steroids. Esmailpour and colleagues (7) demonstrated the persistence of budesonide for 16 to 21 hours after inhalation. Both fluticasone and budesonide have greater receptor affinity and a longer half-life than dexamethasone (8). These studies and the calculations put forth in our article, suggest that our concentration of dexamethasone reflects concentrations that may be achieved in vivo. Corticosteroid concentrations determined in whole lung tissue are unlikely to reflect the concentration at the apical surface. We agree that caution is necessary in extrapolating culture-based data to a clinical situation. Our data fairly raise the possibility that untoward damage may occur to the airway epithelium as a result of treatment with high-potency, prolonged-duration inhaled corticosteroids as therapy for asthma.
a University of British Columbia Vancouver, Canada REFERENCES
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expression in human asthmatic airways. Relationship with proliferation, apoptosis, and airway remodeling. Am J Respir Crit Care Med 2001;164:1487–1494.