Autophagy facilitates the development of resistance to the tumor necrosis factor superfamily member TRAIL in breast cancer.

Autophagy, an important homeostatic cellular recycling mechanism, has emerged as a novel cytoprotective mechanism to increase tumor cell survival through escaping chemotherapy‑induced cell death. To explore whether autophagy plays a protective role in the resistance to the tumor necrosis factor‑related apoptosis‑inducing ligand (TRAIL), we evaluated the autophagy levels in TRAIL‑sensitive MDA‑MB‑231 breast cancer cell lines and in TRAIL‑refractory MDA‑MB‑231 cells before and after TRAIL treatment. After treatment with 40 ng/ml TRAIL, TRAIL‑sensitized MDA‑MB‑231 parental cells expressed higher level of LC3B protein and accumulated more autophagic vacuoles. Compared with TRAIL‑sensitive MDA‑MB‑231, MDA‑MB‑231 TRAIL‑refractory cells showed higher levels of the lipidated form of LC3B and decreased p62/SQSTM1 protein expression, characterizing the occurrence of increased autophagic flux in TRAIL‑refractory cells. Electron microscopy and monodansylcadaverine (MDC) autophagy‑specific fluorescence staining analyses also revealed that the accumulation of autophagic vacuoles was drastically higher in TRAIL‑refractory MDA‑MB‑231 parental cells. We demonstrated that chloroquine (CQ) and 2‑(4‑morpholinyl)‑8‑phenylchromone (LY294002) could effectively reduce TRAIL‑refractory breast cancer cell viability. Combination of TRAIL with CQ could effectively reverse the resistance of MDA‑MB‑231 TRAIL‑refractory cells to TRAIL. Knockdown of light chain 3 (LC3) expression via small interfering RNA (siRNA) similarly resulted in reduced TRAIL‑refractory cell proliferation and re‑sensitizing to TRAIL. This is the first report showing that breast cancer cells chronically exposed to TRAIL exhibit upregulation of the autophagic activity, indicating that autophagy efficiently protects breast cancer cells from TRAIL. Therapeutic targeting of autophagosome formation could be a novel molecular avenue to reduce the resistance of TRAIL in breast cancer.