Abstract
Autophagy is critical for maintaining cellular function via clearance of excess nutrients and damaged organelles. In pancreatic β-cells, it helps counter the endoplasmic reticulum (ER) stress that impairs insulin secretory capacity during Type 2 diabetes. Chronic exposure of β-cells to saturated fatty acids (FAs) such as palmitate stimulates ER stress and modulates autophagy, but the effects of unsaturated FAs such as oleate, which are also elevated during obesity, are less well understood. We therefore treated MIN6 cells and mouse islets for 8-48 h with either palmitate or oleate, and then monitored autophagic flux, signaling pathways, lysosomal biology, and phospholipid profiles. Compared with palmitate, oleate more effectively stimulated both autophagic flux and clearance of autophagosomes. The flux stimulation occurred independently of ER stress, nutrient-sensing (mTOR) and signaling pathways (protein kinases A, C, and D). Instead the mechanism involved the exchange factor directly activated by cAMP 2 (EPAC2). Oleate reduced cellular cAMP, and its effects on autophagic flux were reproduced or inhibited, respectively, by Epac2 knockdown or activation. Oleate also increased lysosomal acidity and increased phospholipid saturation, consistent with improved autophagosomal fusion with lysosomes. We conclude that a potent stimulation of autophagy might help explain the known benefits of unsaturated FAs in countering the toxicity of saturated FAs in β-cells during obesity and lipid loading.
Highlights
Autophagy is critical for maintaining cellular function via clearance of excess nutrients and damaged organelles
In many further experiments conducted over several years, with multiple batches and passages of cells, we established that palmitate was more variable in its responses than oleate, such that the unsaturated fatty acids (FAs) was confirmed as the more effective stimulator of autophagic flux overall
It is generally established that obesity and Type 2 diabetes (T2D) are associated with enhanced accumulation of autophagic markers [7,8,9, 15]
Summary
Autophagy is critical for maintaining cellular function via clearance of excess nutrients and damaged organelles. Macroautophagy (referred to as autophagy hereafter) is a critical degradative mechanism for maintaining cellular function and survival under both normal and stress conditions [1] It is characterized by formation of a double membrane structure, the autophagosome, which engulfs target proteins or organelles [2]. Several studies have pointed to an impairment of autophagic clearance in -cells during T2D, based on alterations in morphology, and accumulation of the autophagosomal marker LC3II [15, 16] The latter is generated by the lipidation of the precursor molecule LC3I as part of the proximal signaling cascade, so its accumulation could represent either enhanced flux through the upstream pathway, as well as a reduction in distal clearance. These results suggest that, regardless of what happens in frank diabetes, autophagic flux is activated
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