Autophagy is one of the main cytoprotective mechanisms that
cancer cells deploy to withstand the cytotoxic stress and survive the lethal damage induced by anti-
cancer drugs. However, under specific conditions, autophagy may, directly or indirectly, induce cell death. In our study, treatment of the Atg5-deficient DU145
prostate cancer cells, with the multi-
tyrosine kinase inhibitor,
sorafenib, induces mitochondrial damage, autophagy and cell death. Molecular inhibition of autophagy by silencing ULK1 and
Beclin1 rescues DU145 cells from cell death indicating that, in this setting, autophagy promotes cell death. Re-expression of Atg5 restores the lipidation of LC3 and rescues DU145 and MEF atg5-/- cells from
sorafenib-induced cell death. Despite the lack of Atg5 expression and LC3 lipidation, DU145 cells form autophagosomes as demonstrated by transmission and immuno-electron microscopy, and the formation of LC3 positive foci. However, the lack of cellular content in the autophagosomes, the accumulation of long-lived
proteins, the presence of GFP-RFP-LC3 positive foci and the accumulated p62
protein levels indicate that these autophagosomes may not be fully functional. DU145 cells treated with
sorafenib undergo a
caspase-independent cell death that is inhibited by the RIPK1 inhibitor,
necrostatin-1. Furthermore, treatment with
sorafenib induces the interaction of RIPK1 with p62, as demonstrated by immunoprecipitation and a proximity
ligation assay. Silencing of p62 decreases the RIPK1
protein levels and renders
necrostatin-1 ineffective in blocking
sorafenib-induced cell death. In summary, the formation of Atg5-deficient autophagosomes in response to
sorafenib promotes the interaction of p62 with RIPK leading to cell death by necroptosis.