Molecular mechanisms underlying the prevention of glucose-induced life span reduction by the polyphenol quercetin in the mev-1 mutant of Caenorhabditis elegans
Ageing is an inevitable process that is often accompanied by the development of mild hyperglycaemia. Besides this physiological type of hyperglycaemia, pathological, more pronounced hyperglycaemia is a hallmark of diabetes mellitus and responsible for its late-onset complications. Polyphenols are a group of secondary plant compounds with preventive effects against glucose-induced degenerative processes.In the present work the mev 1 mutant of the nematode Caenorhabditis elegans was used as model organisms to study the effects of glucose as well as protective effects of the polyphenol quercetin at the molecular level. The mev 1 mutant was used since it displays elevated mitochondrial superoxide generation, which is typical for diabetes, too.It was found that glucose in a concentration of 10 mM significantly reduced the life span measured under heat stress at 37 °&
#8202;C. Although glucose led to enhanced generation of reactive oxygen species (ROS) in mitochondria, the scavenging of ROS by ascorbic acid did not prolong life span. Further processes that are discussed to play a role in glucose-induced damage, such as protein carbonylation and formation of advanced glycation end products (AGEs) were not affected by 10 mM glucose. A substantial reduction of the P/O ratio pointed to a reduction of mitochondrial efficiency by glucose. To prove functional losings of mitochondrial proteins, the expression of key proteins of the mitochondrial unfolded protein response (UPRmt) was reduced by RNA interference (RNAi). For example, the knockdown of heat shock proteins HSP-6 and HSP-60, respectively, strongly shortened the life span of mev 1, yet glucose did not cause an additional reduction. It may be concluded that the inhibition of those proteins by glucose is the reason for the observed reduction of life span. Similar results were found after RNAi of proteins of the unfolded protein response in the endoplasmic reticulum (UPRER) or proteasomal protein degradation. Interestingly, RNAi against BEC 1, which is essential for the formation of autophagosomes, completely prevented glucose toxicity but did not affect life span in absence of glucose. It may be assumed that the inhibition of autophagy enhances proteasomal degradation.Apoptosis, on the contrary, does not seem to be relevant for the life span reduction by glucose. This is supported by the finding that RNAi against the Bcl homologue EGL 1 prevents the life span reduction by glucose but does not affect glucose-induced apoptosis.The plant polyphenol quercetin at a concentration of 1 µM prevented the glucose-induced life span reduction completely. RNAi against the identified targets of glucose with respect to life span finally inhibited the life span prolongation by quercetin as well. The Sirtuin SIR-2.1, the nuclear hormone receptor DAF 12 and its putative co-activator MDT 15 were identified as upstream regulators of UPRmt, UPRER, and proteasomal and autophagosomal degradation and are activated by quercetin.In conclusion, the present work demonstrates that enhanced glucose concentrations reduce the life span of the model organism C. elegans. Whereas the formation of ROS, mitochondrial dysfunction, protein carbonylation and generation of AGEs were ruled out as a cause, impairments of proteostasis were identified to be crucial. Quercetin at low micromolar concentrations completely reversed the life span reduction by glucose depending on SIR 2.1, DAF 12, and MDT 15. Those cellular regulators appear to activate the proteostasis network and thereby inhibit the accumulation of proteins damaged by glucose.
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