A Dual Role of Autophagy in Disease Prevention and Drug Resistance

Abstract

Aging is associated with various diseases, such as neurodegenerative diseases. However, individual risk of developing these age-related diseases clearly varies due to differences in both genetic and environmental factors. Autophagy is a prosurvival mechanism that contributes to restore cellular homeostasis by eliminating excessive or damaged proteins, protein aggregates and organelles. Hence, autophagy is believed to be cytoprotective in diseases associated with protein aggregates. Interestingly, studies of autophagy deficient animal models have revealed that such animals develop various kinds of age-associated diseases including early-onset neurodegeneration. Hence, acceleration the autophagy process may be promising in disease prevention. Diet is a major environmental factor that strongly influences human health. Several epidemiological studies have indicated an inverse correlation between dietary intake of n-3 PUFAs and the risk of developing many pathologies. However, the disease-preventive mechanisms mobilized by n-3 PUFAs are not completely understood, and for several of the age-related diseases where n-3 PUFAs have a protective effect, loss of autophagy has the opposite outcome. We were therefore interested in exploring if there was a direct relation between n-3 PUFAs and increased autophagy in disease relevant cell models. In the context of age-related macular degeneration (AMD) we found that physiologically relevant doses of the n-3 PUFA docosahexaenoic acid (DHA) induced a transient increase in cellular reactive oxygen species (ROS) levels that activated the oxidative stress response regulator nuclear factor, erythroid derived 2, like 2 (NFE2L2). Simultaneously, a transient increase in intracellular protein aggregates containing sequestosome 1 (SQSTM1) and increased autophagy was observed. Pretreatment with DHA rescued the cells from cell cycle arrest induced by misfolded proteins or oxidative stress. Together, these results suggest that DHAinduced increase in endogenous antioxidants, and selective autophagy of misfolded proteins may be relevant to reduce the risk of developing aggregate-associated diseases like AMD. Interestingly, our results also emphasize that exogenous antioxidants may interfere with and counteract some of the putative positive effects of DHA, including the activation of NFE2L2. Most age-related diseases share an underlying low-grade inflammation that contributes to their development and progression. Despite increasing evidence of the established antiinflammatory effects of n-3 PUFAs, the underlying mechanisms are still largely unknown. Hence, we further investigated if the DHA-specific effects observed in epithelial cells also exist in macrophages, and whether this could contribute to the established anti-inflammatory effects of n-3 PUFAs. We report that DHA induced a transient increase in cytosolic SQSTM1-positive speckles and protein quality control also in macrophages. Further, DHA influenced the expression of some inflammatory genes. Especially the C-X-C motif chemokine 10 (CXCL10) in lipopolysaccharide (LPS)-activated human macrophages was repressed in a very potent manner. Surprisingly, SQSTM1 was found to be necessary for a proper induction of CXCL10 by LPS. Finally, we demonstrated that Tax1 (human T-cell leukemia virus type I) binding protein 1 (TAX1BP1) and tumor necrosis factor, alpha-induced protein 3 (TNFAIP3) associated with SQSTM1-positive speckles in response to DHA. TAX1BP1 and TNFAIP3 form a ubiquitin-editing complex known to downregulate inflammatory signaling, and we suggest that this could contribute to the DHA-induced repression of CXCL10 expression. From this, we propose a model that connects the autophagic cargo receptor SQSTM1 to the ubiquitin-editing machinery involved in the regulation of inflammatory signaling. Autophagy is a cytoprotective mechanism that also may be utilized by cancer cells to provide resistance towards various treatment regimes. Multiple myeloma is an incurable cancer of the antibody-producing plasma cells. Despite that proteasome inhibitors have improved treatment significantly; inherent and acquired drug resistance remains a problem. The mechanisms of proteasome inhibitor-induced cytotoxicity have remained elusive. Therefore, using myeloma cell lines and cancer cells isolated from patients, we wanted to examine whether autophagy protected the cells from cell death induced by proteasome inhibitors. Surprisingly, we found that inhibiting autophagy specifically potentiated the cytotoxicity of irreversible proteasome inhibitors. On the other hand, the reversible proteasome inhibitor bortezomib was highly dependent on intracellular glutathione (GSH) levels. Our work indicate that clinical studies combining irreversible- and reversible proteasome inhibitors with autophagy inhibitors and drugs reducing intracellular GSH levels, respectively, might be beneficial.