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P-Glycoprotein Transporter Expression on A549 Respiratory Epithelial Cells Is Positively Correlated With Intracellular Dexamethasone Levels
  1. Joanna S. Cohen, MD*†,
  2. Angela S. Benton, BA,
  3. Fisayo Nwachukwu, BA§,
  4. Tugba Ozedirne, BA,
  5. Stephen J. Teach, MD, MPH*†¶,
  6. Robert J. Freishtat, MD, MPH*†‡**
  1. From the *Division of Emergency Medicine, Children's National Medical Center; †Department of Pediatrics, School of Medicine and Health Sciences, George Washington University; ‡Centers for Genetic Medicine, Children's National Medical Center, Washington, DC; §Temple University, Philadelphia, PA; ∥University of California - Davis, Davis, CA; ¶Center for Clinical and Community Research, Children's National Medical Center; and **Department of Integrative Systems Biology, School of Medicine and Health Sciences, George Washington University, Washington, DC.
  1. Received June 29, 2010, and in revised form August 18, 2010.
  2. Accepted for publication August 18, 2010.
  3. Reprints: Joanna S. Cohen, MD, 111 Michigan Ave NW, Washington, DC 20010. E-mail: jcohen{at}
  4. Supported by The Board of Visitors, Children's National Medical Center, Washington, DC (to J.S.C.).


Background Several mechanisms of glucocorticoid resistance in asthma have been proposed. P-glycoprotein (P-gp), a ubiquitous efflux transport protein, is associated with variability in the disposition of many drugs and interindividual variability in drug treatment response. This study was undertaken to determine the effect of P-gp expression on glucocorticoid efflux from airway epithelial cells.

Hypothesis Decreasing respiratory epithelial P-gp expression in dexamethasone-exposed airway epithelial cells in vitro will increase intracellular dexamethasone concentration.

Methods A549 lung epithelial cells, transfected with small interfering RNA (siRNA) targeted at messenger RNA for the gene encoding P-gp, were exposed to 100-nM dexamethasone for 15 minutes. Transfection efficiency of siRNA, P-gp expression, and intracellular dexamethasone were measured with flow cytometry.

Results Cells transfected with both negative siRNA and siRNA targeted at P-gp exhibited a positive correlation of P-gp expression with intracellular dexamethasone. The mean ± SEM correlation coefficients were 0.78 ± 0.07 for cells transfected with negative siRNA and 0.79 ± 0.08 for cells transfected with siRNA targeted at P-gp.

Discussion Contrary to our hypothesis, the positive correlation between P-gp expression and intracellular dexamethasone suggests that P-gp is not a primary transporter of glucocorticoids from airway epithelial cells. Increased P-gp expression is unlikely to be an important mechanism of glucocorticoid resistance in asthma.

Key Words
  • asthma
  • glucocorticoids
  • P-glycoprotein
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