TRAIL Resistance of Breast Cancer Cells Is Associated with Constitutive Endocytosis of Death Receptors 4 and 5

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and its agnostic antibodies, which are being evaluated clinically as anticancer therapies, selectively kill cancer cells through the death receptors DR4 and DR5. However, their therapeutic potential is limited by occurring resistance in tumor cells. Here, we compared the apoptotic response of a panel of six human breast cancer cell lines with recombinant human TRAIL and antibodies to DR4 or DR5. Despite their total mRNA and protein expression, TRAIL death receptors, with a higher frequency in DR4, are absent on cell surface in some cell lines. Loss of cell surface expression of DR4 or DR5 accounts for resistance to their corresponding antibody and, importantly, correlates with a decreased sensitivity to TRAIL. TRAIL resistance occurs when both receptors are absent on cell surface regardless of alterations in Bcl-2 family proteins or caspases. Furthermore, inhibition of endocytosis by pharmacologic inhibitors or disruption of clathrin-dependent endocytosis signaling components (adaptor protein 2 and clathrin) restores cell surface expression of the death receptors and sensitize TRAIL-resistant cells to TRAIL-induced apoptosis. DR4 endocytosis appears to be mediated by its cytoplasmic domain EAQC(337)LL. The results show that TRAIL death receptors undergo constitutive endocytosis in some breast cancer cells. Loss of cell surface expression of DR4 and DR5 could be evaluated as a biomarker for TRAIL resistance in breast tumors. Moreover, the clathrin-mediated endocytosis pathway could be a potential target for therapeutics to overcome tumor resistance to TRAIL receptor-targeted therapies. (Mol Cancer Res 2008;6(12):1861-71)