Abstract
CFTR (cystic fibrosis transmembrane conductance regulator) is expressed in the apical membrane of epithelial cells. Cell-surface CFTR levels are regulated by endocytosis and recycling. A number of adaptor proteins including AP-2 (μ2 subunit) and Dab2 (Disabled-2) have been proposed to modulate CFTR internalization. In the present study we have used siRNA (small interfering RNA)-mediated silencing of these adaptors to test their roles in the regulation of CFTR cell-surface trafficking and stability in human airway epithelial cells. The results indicate that μ2 and Dab2 performed partially overlapping, but divergent, functions. While μ2 depletion dramatically decreased CFTR endocytosis with little effect on the half-life of the CFTR protein, Dab2 depletion increased the CFTR half-life ~3-fold, in addition to inhibiting CFTR endocytosis. Furthermore, Dab2 depletion inhibited CFTR trafficking from the sorting endosome to the recycling compartment, as well as delivery of CFTR to the late endosome, thus providing a mechanistic explanation for increased CFTR expression and half-life. To test whether two E3 ligases were required for the endocytosis and/or down-regulation of surface CFTR, we siRNA-depleted CHIP [C-terminus of the Hsc (heat-shock cognate) 70-interacting protein] and c-Cbl (casitas B-lineage lymphoma). We demonstrate that CHIP and c-Cbl depletion have no effect on CFTR endocytosis, but c-Cbl depletion modestly enhanced the half-life of CFTR. The results of the present study define a significant role for Dab2 both in the endocytosis and post-endocytic fate of CFTR.