Docosahexaenoic acid-mediated protein aggregates may reduce proteasome activity and delay myotube degradation during muscle atrophy in vitro

Seung Kyun Shin,Jung Hoon Lee,Min Wook Lee,Sue Ah Noh,Min Jae Lee,In-Gyu Kim,Hak Joong Kim,Kwang Pyo Kim,Young Hoon Son,Ji Hyeon Kim

doi:10.1038/emm.2016.133

Abstract

Proteasomes are the primary degradation machinery for oxidatively damaged proteins that compose a class of misfolded protein substrates. Cellular levels of reactive oxygen species increase with age and this cellular propensity is particularly harmful when combined with the age-associated development of various human disorders including cancer, neurodegenerative disease and muscle atrophy. Proteasome activity is reportedly downregulated in these disease conditions. Herein, we report that docosahexaenoic acid (DHA), a major dietary omega-3 polyunsaturated fatty acid, mediates intermolecular protein cross-linkages through oxidation, and the resulting protein aggregates potently reduce proteasomal activity both in vitro and in cultured cells. Cellular models overexpressing aggregation-prone proteins such as tau showed significantly elevated levels of tau aggregates and total ubiquitin conjugates in the presence of DHA, thereby reflecting suppressed proteasome activity. Strong synergetic cytotoxicity was observed when the cells overexpressing tau were simultaneously treated with DHA. Antioxidant N-acetyl cysteine significantly desensitized the cells to DHA-induced oxidative stress. DHA significantly delayed the proteasomal degradation of muscle proteins in a cellular atrophy model. Thus, the results of our study identified DHA as a potent inducer of cellular protein aggregates that inhibit proteasome activity and potentially delay systemic muscle protein degradation in certain pathologic conditions.

Highlights

The ubiquitin–proteasome system (UPS) is the primary cellular proteolytic machinery that regulates the homeostasis of various regulatory proteins and eliminates damaged, misfolded and mutant proteins for protein quality control.[1]
docosahexaenoic acid (DHA) increases intracellular Reactive oxygen species (ROS) levels and decreases cellular UPS flux To examine the effects of DHA on cellular oxidative stress and its consequences, we initially examined HEK293 cell viability after treatment with DHA and menadione, another ROS inducer
DHA is reportedly cytotoxic in various cancer cells, we observed only the mild effects of DHA on HEK293 viability; whereas, menadione showed significant cytotoxicity at ~ 25 μM, DHA had no noticeable cytotoxicity at concentrations up to ~ 250 μM (Figure 1a)

Summary

Introduction

The ubiquitin–proteasome system (UPS) is the primary cellular proteolytic machinery that regulates the homeostasis of various regulatory proteins and eliminates damaged, misfolded and mutant proteins for protein quality control.[1] UPS substrates are covalently conjugated with ubiquitin through isopeptide bonds between the ε-amine in internal lysine residues and the C-terminal carboxylic group of ubiquitin. Intracellular oxidized proteins are rapidly recognized and degraded by cytosolic calpains,[4] lysosomal cathepsins[5] and the UPS in both ubiquitin-dependent and ubiquitin-independent manners to avoid their harmful consequences.[6] The proteasome, as the primary clearance system of oxidatively damaged proteins, is responsible for the degradation of ~ 90% of these proteins.[5]

Methods

Results

Conclusion