Molecular mechanisms of oxidative stress-induced cancer cell death : hsp90 chaperone cleavage and consequences

Abstract

Previous works in our laboratory have shown that ascorbate-driven menadione-redox cycling leads to reactive oxygen species (ROS) formation and generates an oxidative stress that results in cancer cell death. This cell death is necrotic-like and the underlying mechanism involves inhibition of glycolysis (with subsequent ATP depletion) and impairment of calcium homeostasis. In this study, we wanted to determine whether additional mechanisms involved in survival signaling pathways were affected by asc/men-mediated effects. In addition to the impairment of MAPK pathways (deactivation of ERK1/2 and activation of p38), the main finding of this work is that asc/men provokes a cleavage of the hsp90 chaperone protein. This cleavage seems to be specific for cancer cells. The subsequent loss of its chaperone function leads to degradation of its client proteins including Bcr-Abl, mutated Bcr-Abl (T315I), Akt, RIP, c-Raf, and hTERT. The degradation of these proteins is accompanied by deactivation of downstream signaling pathways (i.e. MAPKinases) of these proteins. Hsp90 cleavage can be observed in living cells, in cell lysates and in both purified and recombinant proteins. The cleavage is likely provoked by a localized Fenton-type reaction, leading to a subsequent oxidation of the protein backbone, the formation of a protein radical, and a protein cleavage in a conserved sequence that is part of the N-terminal nucleotide-binding pocket of the protein. By inhibiting hsp90 function, asc/men induces the degradation of numerous oncogenic proteins, thereby impairing not only one but several signaling pathways that are important for cancer cells. Since asc/men does not rely on a classical pharmacological inhibition, which can be impaired by mutation of the target protein, its use as an adjuvant to classical cancer therapy could overcome some drug resistance. Indeed, asc/men is able to significantly delay the growth of tumors that are bearing wild-type or mutated forms of the Bcr-Abl protein. In conclusion, asc/men-induced oxidative stress leads to an hsp90 inhibition through a new mechanism that involves an N-terminal cleavage of the protein. This could represent an interesting and non-toxic adjuvant approach for the treatment of cancer.(SBIM 3) -- UCL, 201