Proteasome inhibition by quercetin triggers macroautophagy and blocks mTOR activity

Abstract

The bioflavonoid quercetin has long been known to exert anti-tumor effects, although the underlying mechanisms remain unknown. Investigation of the potential interference of this anti-oxidant with the efficacy of cell stress-inducing anti-cancer drugs revealed extensive intracellular vacuolation induced by quercetin in epithelial cancer cells that led to cell cycle arrest and ensuing apoptosis. Accumulation of biomarkers of autophagy, including fluorescent autophagy markers and acidotropic dyes characterized these vacuoles as phagolysosomes. Prior to the formation of autophagosomes, an immediate and pronounced inhibition of the autophagy-controlling mTOR activity in quercetin-treated cancer cells occurred, accompanied by a marked reduction in the phosphorylation of the mTOR substrates 4E-BP1 and p70S6 kinase. Assessment of cellular proteasome activity revealed an effective and immediate inhibition of the activity of the proteasome by quercetin in cancer cells. In addition to the formation of autophagosomes, accumulation of poly-ubiquitinated protein aggregates was observed. Thus, proteasome inhibition by quercetin can be regarded as a major cause of quercetin-induced cancer cell death. These results suggest potential new applications for quercetin in cancer science and identify quercetin as an easy-to-handle agent to study proteasome activity, mTOR signaling and autophagy in human cancer cells for cell biological purposes.

Changes of intracellular Ca<sup>2+</sup> in quercetin-induced autophagy progression

“…Meanwhile, competitive crosstalk between the ubiquitin-proteasome system and autophagy has also been recently reported. For example, when quercetin inhibited proteasomal activity, polyubiquitinated protein aggregates were accumulated and autophagy was increase via marked reduction in the phosphorylation of the mTOR substrates [21,22]. These results suggested that quercetin can be an autophagy inducer.…”

Heat shock response and autophagy—cooperation and control

“…Pharmacologically induced proteasome inhibition upregulates autophagy in mouse cardiomyocytes, 196 human prostate cancer cells, 197 and human breast cancer cells. 198 In mouse fibroblasts, autophagy is activated by selective blockage of chaperone-mediated autophagy. 199 Similarly, knockdown of an essential autophagy gene (ATG5) results in upregulation of chaperone-mediated autophagy in mouse embryonic fibroblasts.…”

Crosstalk between autophagy and proteasome protein degradation systems: possible implications for cancer therapy

“…Also, inhibition of autophagic degradation can often lead to the accumulation of LC3-II proteins [94]; however, in all the above-cited papers rapidly activated autophagy after proteasome inhibition was observed [75,85]. The presence of a considerably increased number of autophagosomes at various stages was postulated as the most convincing proof of active ongoing autophagy after proteasome inhibition by different groups of proteasome inhibitors [24,88,91,[95][96][97].…”