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Background information. CF (cystic fibrosis) is a disease caused by mutations within the CFTR (CF transmembrane conductance regulator) gene. The most common mutation, ΔF508 (deletion of Phe-508), results in a protein that is defective in folding and trafficking to the cell surface but is functional if properly localized in the plasma membrane. We have recently demonstrated that overexpression of the PDZ protein NHERF1 (Na+/H+-exchanger regulatory factor 1) in CF airway cells induced both a redistribution of ΔF508CFTR from the cytoplasm to the apical membrane and the PKA (protein kinase A)-dependent activation of ΔF508CFTR-dependent chloride secretion. In view of the potential importance of the targeted up-regulation of NHERF1 in a therapeutic context, and since it has been demonstrated that oestrogen treatment increases endogenous NHERF1 expression, we tested the hypothesis that oestrogen treatment can increase NHERF1 expression in a human bronchiolar epithelial CF cell line, CFBE41o−, with subsequent rescue of apical ΔF508CFTR chloride transport activity.Results. We found that CFBE41o− cells do express ERs (oestrogen receptors) in the nuclear fraction and that β-oestradiol treatment was able to significantly rescue ΔF508CFTR-dependent chloride secretion in CFBE41o− cell monolayers with a peak between 6 and 12 h of treatment, demonstrating that the ΔF508CFTR translocated to the apical membrane can function as a cAMP-responsive channel, with a significant increase in chloride secretion noted at 1 nM β-oestradiol and a maximal effect observed at 10 nM. Importantly, knock-down of NHERF1 expression by transfection with siRNA (small interfering RNA) for NHERF1 inhibited the β-oestradiol-dependent increase in ΔF508CFTR protein expression levels and completely prevented the β-oestradiol-dependent rescue of ΔF508CFTR transport activity.Conclusions. These results demonstrate that β-oestradiol-dependent up-regulation of NHERF1 significantly increases ΔF508CFTR functional expression in CFBE41o− cells. [ABSTRACT FROM AUTHOR]